c© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim10.1002/14356007.a01 097.pub2

Acetylene 1

Acetylene

Peter Passler, BASF Aktiengesellschaft, Ludwigshafen, Germany (Chap. 1, 3, 4.1, 4.2, 4.4.1, 5.1, 6.2 and6.3)

Werner Hefner, BASF Aktiengesellschaft, Ludwigshafen, Germany (Chap. 1, 3, 4.1, 4.2, 4.4.1, 5.1, 6.2 and6.3)

Klaus Buckl, Linde AG, Hollriegelskreuth, Germany (Chap. 2, 4.4.2, 5.1 and 7)

Helmut Meinass, Linde AG, Hollriegelskreuth, Germany (Chap. 5.1 and 5.2)

Andreas Meiswinkel, Linde AG, Linde Engineering Division, Pullach, Germany (Chap. 2, 4.4.2, 5.1, 5.2,6.1 and 7)

Hans-Jurgen Wernicke, Linde AG, Hollriegelskreuth, Germany (Chap. 6.1)

Gunter Ebersberg, Degussa-Huls AG, Marl, Germany (Chap. 4.3.1, 4.3.2 and 4.3.3)

Richard Muller, Degussa-Huls AG, Marl, Germany (Chap. 4.3.1 and 4.3.3)

Jurgen Bassler, Uhde GmbH, Dortmund, Germany (Chap. 4.3.4)

Hartmut Behringer, Hoechst Aktiengesellschaft, Werk Knapsack, Germany (Chap. 4.3.4)

Dieter Mayer, Hoechst Aktiengesellschaft, Pharma-Forschung, Toxikologie, Frankfurt, Germany (Chap. 8)

1. Introduction . . . . . . . . . . . . . . 12. Physical Properties . . . . . . . . . . 23. Chemical Properties . . . . . . . . . 53.1. Industrially Important Reactions 53.2. Other Reactions; Derivatives . . . 84. Production . . . . . . . . . . . . . . . 94.1. Thermodynamic and Kinetic

Aspects . . . . . . . . . . . . . . . . . 94.2. Partial Combustion Processes . . 104.2.1. BASF Process

(Sachsse-Bartholome) . . . . . . . . 114.2.2. Other Partial Combustion Processes 174.2.3. Submerged Flame Process . . . . . . 184.2.4. Partial Combustion Carbide Process 194.3. Electrothermic Processes . . . . . . 204.3.1. Production from Gaseous and/or

Gasified Hydrocarbons (Huls ArcProcess) . . . . . . . . . . . . . . . . . 21

4.3.2. Production from Liquid Hydrocar-bons (Plasma Arc Process) . . . . . 25

4.3.3. Production from Coal (Arc CoalProcess) . . . . . . . . . . . . . . . . . 26

4.3.4. Production from Calcium Carbide . 28

4.3.4.1. Wet Generators . . . . . . . . . . . . . 294.3.4.2. Dry Generators . . . . . . . . . . . . . 304.3.4.3. Acetylene Purification . . . . . . . . 324.4. Other Cracking Processes . . . . . 324.4.1. Thermal Cracking By Heat Carriers 324.4.2. Acetylene as a Byproduct of Steam

Cracking . . . . . . . . . . . . . . . . . 345. Safety Precautions,

Transportation, and Storage . . . 365.1. General Safety Factors and Safety

Measures . . . . . . . . . . . . . . . . 365.2. Acetylene Storage in Cylinders . . 426. Uses and Economic Aspects . . . . 446.1. Use in Metal Processing . . . . . . 446.2. Use as Raw Material in Chemical

Industry . . . . . . . . . . . . . . . . . 456.3. Competitive Position of Acetylene

as Chemical Feedstock . . . . . . . 467. Propyne . . . . . . . . . . . . . . . . . 478. Toxicology and Occupational

Health . . . . . . . . . . . . . . . . . . 509. References . . . . . . . . . . . . . . . 51

1. Introduction

Acetylene [74-86-2] is the simplest hydrocar-bon with a triple bond. In the days before oilgained widespread acceptance as the main feed-stock of chemical industry, acetylene was the

predominant building block of industrial organicchemistry. The calcium carbide process was thesole route for acetylene production until 1940,when thermal cracking processes using meth-ane and other hydrocarbons were introduced. Atfirst, these processes used an electric arc; then,

2 Acetylene

in the 1950s, partial oxidation and regenerativeprocesses were developed.

However, along with the expansion of the pe-troleum industry there was a changeover fromcoal chemistry to petrochemistry, in the 1940sin the United States and in the 1950s in Europe.As a consequence, acetylene lost its competitiveposition to the much cheaper and more read-ily available naphtha-derived ethylene and otherolefins. This competition between acetylene andethylene as feedstocks for chemical industry hasbeen much discussed in the 1960s and 1970s [1,2]. The few hopes, such as BASF’s contributionto the submerged flame process, Hoechst’s crudeoil cracking (HTP), or Huls’ plasma process,have not halted the clear trend toward ethyleneas a basic chemical.With the first oil price explo-sion in 1973, the development of crude crackingprocesses suffered a setback, and the new pro-cesses, such as the Kureha/Union Carbide pro-cess, DOW’s PCC process (PCC = partial com-bustion cracking), or theKureha/Chiyoda/UnionCarbide ACR process (ACR = advanced crack-ing reactor), raise little hope for a comebackof acetylene chemistry. Acetylene productionpeaked in the United States at 480000 t in the1960s, and in Germany at 350000 t in the early1970s [3]. Since then, acetylene production hasdecreased steadily. In both countries the losseswere principally in carbide-derived acetylene; infact, Germany has produced acetylene for chem-ical purposes almost exclusively from naturalgas and petrochemical sources since 1975.

All acetylene processes, including carbideprocesses, are high-temperature processes, re-quiring a large amount of energy. They differessentially only in the manner in which the nec-essary energy is generated and transferred. Theycan be classified into three groups: partial com-bustion processes, electrothermic processes, andprocesses using heat carriers. Finally, the use ofbyproduct acetylene from olefin plants is eco-nomically viable in many cases. For each groupof acetyleneprocesses several variants havebeendeveloped using various feedstocks and tech-niques. Today, only three processes remain forthe commercial production of acetylene: the cal-cium carbide route, in which the carbide is pro-duced electrically, the arc process, and the par-tial oxidation of natural gas. Other once popu-lar processes have become uneconomical as theprice of naphtha has increased.

Some processes were shelved in the exper-imental or pilot-plant stage as the importanceof acetylene declined. However, other new pro-cesses involving the use of coal, sulfur-con-taining crude oil, or residues as feedstocks foracetylene production are in the pilot-plant stage.

However, the position of acetylene in chem-ical industry may improve because of the vari-ety of valuable products to which acetylene canbe converted with known technology and highyields.

2. Physical Properties

Due to the carbon–carbon triple bond and thehigh positive energy of formation, acetylene isan unstable, highly reactive unsaturated hydro-carbon. The C – C triple bond and C – H σ bondlengths are 0.1205 and 0.1059 nm, respectively.For the electronic structure of acetylene and amolecular orbital description, see [4]. The acid-ity of acetylene (pKa = 25) permits the forma-tion of acetylides (see Section 3.2). For com-parison the pKa value of ethylene is 44 and thatof acetone 20.

Figure 1. Vapor pressure of acetylene [5, 6]

Acetylene 3

Under normal conditions acetylene is a col-orless, nontoxic but narcotic gas; it is slightlylighter than air. The main physical propertiesare listed inTable 1. The critical temperature andpressure are 308.32 K and 6.139 MPa. The triplepoint at 128.3 kPa is 192.4 K. The vapor pres-sure curve for acetylene is shown in Figure 1.The formation of acetylene is strongly endother-mic (∆H f = + 227.5 kJ/mol at 298.15 K).

Table 1. Physical properties of acetylene [5 – 8]

Molecular mass 26.0379Critical temperature 308.32 K (35.17 ◦C)Critical pressure 6.139 MPaCritical volume 0.113 m3/kmolTriple point 192.4 K (− 80.75 ◦C)Triple point pressure 128.3 kPaNormal sublimation point andnormal boiling point

189.15 K (− 84.0 ◦C)

Crystal transition point 133.0 K (− 140.15 ◦C)Enthalpy of transition 2.54 kJ/molDensity 760.2 kg/m3 (131 K)

764.3 kg/m3 (141 K)Density (liquid C2H2) 465.2 kg/m3 (273.15 K)Density (gaseous C2H2 at 1 bar) 1.095 kg/m3 (288.15 K)Molecular volume (0 ◦C, 1.013bar)

22.223 m3/kmol

Enthalpy ofvaporization(calculated)

10.65 kJ/mol (273.15 K)

Enthalpy of sublimation 21.168 kJ/mol (5.55 K)Enthalpy of formation 227.5 ± 1.0 kJ/mol (298.15 K)Gibbs free energy of formation 209.2 ± 1.0 kJ/mol (298.15 K)Entropy of formation 200.8 J mol−1 K−1 (298.15 K)Enthalpy of combustion −1255.6 kJ/mol (298.15 K)Higher heating value 50 400 kJ/kgLower heating value 48 700 kJ/kgVapor pressure 2.6633 MPa (273.15 K)Thermal conductivity 0.0184 W m−1 K−1 (0 ◦C,

1.013 bar)Heat capacity (ideal gas state) 43.990 J mol−1 K−1

(298.15 K)

Self-decomposition can be initiated whencertain pressure limits above atmospheric pres-sure are exceeded (for details see Section 5.1).

The crystalline structure of solid acetylenechanges at − 140.15 ◦C from a cubic to an or-thorhombic phase. The heat of reaction for thisphase change is 2.54 kJ/mol [7]; two differentvalues for the enthalpy of fusion are reported inthe literature [5, 6]. Figure 2 shows the densityof liquid and gaseous acetylene.

Details about flame properties, decomposi-tion, and safety measures are given in Chapter5.

Solubility coefficients of acetylene in organicsolvents are listed in Table 2 [9]. Further solu-bility data are available as Bunsen absorption

coefficients α (20 ◦C, m3 (STP) m−3 atm−1),as solubilities (g/kg of solvent), and for dif-ferent pressures (see [10]). The solubilities ofacetylene at infinite dilution are shown in Figure3 for water, methanol, DMF, and N-methyl-2-pyrrolidone (NMP) [872-50-4]. Figure 4 showsthe solubility of acetylene in acetone for var-ious partial pressures and temperatures. Whileat 20 ◦C and 1.013 bar 27.9 g (51.0 g) acety-lene can be dissolved in 1 kg acetone (DMF) at20.26 bar 689.0 g (628.0 g) can be dissolved inthe same amount of solvent [131]. The heat ofsolution depends on the concentration of acety-lene in the solvent: dissolving 0.5 kg of acety-lene in 1 kg of solvent generates 293 kJ for ace-tone and 335 kJ for DMF. For details on the in-fluence of water, of partial pressure, and devi-ations from Henry’s law, see [9, 10]. Only attemperatures above 25 ◦C and pressures < ca.10 bar the solubility of acetylene in acetone fol-lows Henry’s law. At higher pressures the solu-bility increases more rapidly than predicted byHenry’s law [132]. The temperature dependenceof the solubility of acetylene in DMF at infinitedilution is compared with those of ethylene andethane in Figure 5 (see also [9, 11] for selectiv-ities).

Figure 2.Density of acetylene vapor (at 1.013 bar) and liq-uid

4 Acetylene

Figure 3. Solubility of acetylene in various solvents at infi-nite dilution

Figure 4. Solubility of acetylene in acetone [9]

The solubility of acetylene in water at25 ◦C is 0.042 mol L−1 bar−1. Under pressureof acetylene (e.g., > 0.5 MPa at 0 ◦C) and attemperatures between 268 and 283 K, waxy hy-drates of the composition C2H2 · (H2O)∼5.8 areformed [9, 10]. The hydrates can block equip-ment; shock waves may initiate self-decompo-sition.

Liquid oxygen dissolves only traces of acety-lene (5.5 ppm at 90 K [12]); the solubilities ofethylene and ethane in oxygen are much higher(factor of 350 and 2280, respectively). The pre-purification of the process air in air separationplants with molecular sieves removes acetylene

to < 1 ppb provided there is no breakthrough ofcarbon dioxide. This fact guarantees a safe op-eration of the downstream equipment [13].

Figure 5. Solubility of C2 hydrocarbons in DMF at infinitedilution

Typical adsorption isotherms of acetylene areshown inFigure 6 formolecular sieves, activatedcarbon and silica gel at 25 ◦C [14]; additional in-formation for activated carbon is summarized in[15].

Figure 6.Adsorption isotherms for acetylene on 4A and 5Amolecular sieves, activated carbon, and silica gel at 25 ◦C[14]

Acetylene 5

Table 2. Solubility coefficients of C2H2 in various solvents (in mol kg−1 bar−1) [9]

Solvent C2H2 pressure, bar − 20 ◦C 25 ◦C

Methanol 0.98 1.979 0.569Ethanol 0.98 0.851 0.318n-Butanol 0.245 – 0.657 0.2371,2-Dichloroethane 0.4–1.05 0.569 0.218Carbon tetrachloride 0.98 0.164 0.075n-Hexane 6.90 0.523 0.264n-Octane 0.196 – 14.71 0.205 0.146 (0◦ C)Benzene 0.98 0.225Toluene 0.98 0.619 0.214Xylene (tech.) 0.98 0.528 0.1894-Methyl-1,3-dioxolan-2-one (propylene carbonate) 0.98 1.137 0.350Tri-n-butylphosphate 0–0.4 2.366 0.614Methyl acetate 0.98 2.912 0.878Triethylene glycol 0.98 0.205Acetone 0.98 4.231 1.069N-Methyl-2-pyrrolidone 0.98 5.687 1.319N,N-Dimethylformamide 0.98 5.096 1.501Dimethyl sulfoxide 0.98 1.001Ammonia 0.98 7.052 2.229

3. Chemical Properties

Because of its strongly unsaturated character andhigh positive free energy of formation, acety-lene reacts readilywithmany elements and com-pounds. As a result acetylene is used as raw ma-terial for a great variety of substances. Importantare addition reactions, hydrogen replacements,polymerization, and cyclization.

Acetylene is more susceptible to nucleophil-ic attack than, for instance, ethylene. In addi-tion, the polarized C-H bond makes acetyleneacidic (pKa = 25) [16]. Because of this acidity,acetylene is very soluble in basic solvents [17,18], forming hydrogen bonds with them [19].Therefore, the vapor pressures of such solutionscannot be described by Raoult’s law [20].

The development of the acetylene pressurereactions by W. Reppe (1892 – 1969), BASFLudwigshafen (Germany) [21 – 23] beganmod-ern acetylene chemistry. The most interestinggroups of reactions are vinylation, ethynylation,carbonylation, and cyclic and linear polymeriza-tion.

3.1. Industrially Important Reactions

Vinylation Reactions and Products [24].Vinylation is the addition of compounds with amobile hydrogen atom, such as water, alcohols,

thiols, amines, and organic and inorganic acids,to acetylene to form vinyl compounds chieflyused for polymerization.

The two types of vinylation reactions are het-erovinylation and the less usual C vinylation. Inthe former, the hydrogen atom originates fromthe heteroatoms O, S, and N, whereas C vinyla-tion occurs when the mobile hydrogen atom isdirectly bound to a carbon atom. Examples ofC vinylation are dimerization and trimerizationof acetylene, the synthesis of acrylonitrile fromacetylene and hydrogen cyanide, and the addi-tion of acetylene to unsaturated hydrocarbonswith activated hydrogen atoms, such as cyclo-pentadiene, indene, fluorene, and anthracene.

The first industrial vinylation products wereacetaldehyde, vinyl chloride, and vinyl acetate.Many other products followed.

Some examples of industrial vinylation pro-cesses are given below:

Acetaldehyde [75-07-0] (→ Acetaldehyde):

HC≡CH+H2O→CH3CHO

Catalyst: acidic solutions of mercury salts,such as HgSO4 in H2SO4 . Liquid-phase reac-tion at 92 ◦C.

Vinyl chloride [75-01-4] (→ ChlorinatedHydrocarbons):

HC≡CH+HCl→CH2 = CHCl

Catalyst: HgCl2 on coal. Gas-phase reactionat 150 – 180 ◦C.

6 Acetylene

Vinyl acetate [108-05-04] (→ Vinyl Es-ters):

HC≡CH+CH3COOH→CH2 = CHOOCCH3

Catalyst: cadmium, zinc, or mercury salts oncoal. Gas-phase reaction at 180 – 200 ◦C.

Vinyl ethers (→ Vinyl Ethers), conjecturedreaction steps:

where R is an alkyl group. Reaction temper-ature of 120 – 150 ◦C; pressure high enough toavoid boiling the alcohol used, e.g., 2 MPa withmethanol to produce methyl vinyl ether (acety-lene pressure reaction).

Vinyl phenyl ether [766-94-9], vinylationwith KOH catalyst:

Vinyl sulfides, KOH catalyst:

HC≡CH+RSH→CH2 = CH−S−R

Vinyl esters of higher carboxylic acids:

HC≡CH+R−COOH→RCOO−CH = CH2

Catalyst: zinc or cadmium salts. Liquid-phase reaction.

Vinyl amines, vinylation with zinc or cad-mium compounds as catalyst:

R1R2NH+HC≡CH→R1R2N−CH = CH2

where R1 and R2 are alkyl groups.N-Vinylcarbazole [1484-13-5], vinylation

of carbazole in a solvent, e.g., N-methyl-pyrrolidone, at 180 ◦C.

Vinylation of ammonia, complex Co and Nisalts as catalysts, reaction temperature of 95 ◦C:

Vinylation of acid amides, potassium salt ofthe amide as catalyst:

HC≡CH+RCO−NH2→RCO−NH−CH = CH2

N-Vinyl-2-pyrrolidone [88-12-0], vinylationof 2-pyrrolidone with the potassium salt of thepyrrolidone as catalyst.

Acrylonitrile [107-13-1], C-vinylation ofHCN in aqueous hydrochloric acid with CuCland NH4Cl catalyst:

HC≡CH+HCN→H2C = CH−CN

Ethynylation Reactions and Products[25]. Ethynylation is the addition of carbonylcompounds to acetylene with the triple bond re-maining intact. Reppe found that heavy metalacetylides (see Section 3.2), especially the cop-per(I) acetylide of compositionCu2C2 · 2 H2O ·2 C2H2 , are suitable catalysts for the reactionof aldehydes with acetylene. Alkaline catalystsare more effective than copper acetylide for theethynylation of ketones. The generalized reac-tion scheme for ethynylation is:

HC≡CH+RCOR′→HC≡C−C (OH)RR′

where R and R′ are alkyl groups or H.The most important products from ethynyla-

tion are propargyl alcohol and butynediol.Propargyl alcohol, 2-propyn-1-ol [107-19-

7] (→ Alcohols, Aliphatic):

CH≡CH+HCHO→HC≡CCH2OH

Catalyst: Cu2C2 · 2 H2O · 2 C2H2 .Butynediol, 2-butyne-1,4-diol [110-65-6]

(→ Butanediols, Butenediol, and Butynediol):

HC≡CH+2 HCHO→HOCH2C≡CCH2OH

Catalyst: Cu2C2 · 2 H2O · 2 C2H2.Other examples of ethynylation are the re-

actions of aminoalkanol and secondary amineswith acetylene:

Acetylene 7

Carbonylation Reactions and Products[26]. Carbonylation is the reaction of acetyleneand carbon monoxide with a compound havinga mobile hydrogen atom, such as water, alco-hols, thiols, or amines. These reactions are cat-alyzed bymetal carbonyls, e.g., nickel carbonyl,Ni(CO)4 [13463-39-3]. Instead ofmetal carbon-yls, the halides ofmetals that can form carbonylscan also be used.

Acrylic acid [79-10-7] (→ Acrylic Acidand Derivatives):

HC≡CH+CO+H2O→CH2 = CH−COOH

The reaction of acetylene with water or alco-hols and carbon monoxide using Ni(CO)4 cata-lyst was first reported byW. Reppe [26]. If wateris replacedby thiols, amines, or carboxylic acids,then thioesters of acrylic acid, acrylic amides, orcarboxylic acid anhydrides are obtained.

Ethyl acrylate [140-88-5] (→ Acrylic Acidand Derivatives):

Catalyst: nickel salts. Reaction temperature:30 – 50 ◦C. The process starts with the stoichio-metric reaction (1); afterwards, most of the acry-late is formed by the catalytic reaction (2). Thenickel chloride formed in the stoichiometric re-action (1) is recovered and recycled for carbonylsynthesis.

Hydroquinone [123-31-9] is formed in asuitable solvent, e.g., dioxane, at 170 ◦C and 70MPa [27]. The catalyst is Fe(CO)5 :

Hydroquinone is formed at 0 – 100 ◦C and 5 –35 MPa if a ruthenium carbonyl compound isused as catalyst [28]:

Bifurandiones: The reaction of acetylene andCO in the presence of octacarbonyldicobalt,(CO)3Co – (CO)2 – Co(CO)3 [10210-68-1],forms a cis–trans mixture of bifurandione. Thereaction is carried out under pressure (20 – 100MPa) at temperatures of about 100 ◦C [29]:

New aspects of suchCO insertion reactions havebeen reported [30].

Cyclization and Polymerization of Acety-lene. In the presence of suitable catalysts, acety-lene can reactwith itself to formcyclic and linearpolymers.

Cyclization wasfirst observed byBerthelot,who polymerized acetylene to a mixture of aro-matic compounds including benzene and naph-thalene. In 1940, Reppe synthesized 1,3,5,7-cy-clooctatetraene [629-20-9] with a 70% yield atan only slightly elevated pressure:

Reaction temperature of 65 – 115 ◦C, pressureof 1.5 – 2.5 MPa, Ni(CN)2 catalyst.

The reaction is carried out in anhydrous tetra-hydrofuran. The byproducts are mostly benzene(about 15%), chain oligomers of acetylene ofthe empirical formulasC10H10 andC12H12 , anda black insoluble mass, called niprene after thenickel catalyst.

If dicarbonylbis(triphenylphosphine)nickel[13007-90-4], Ni(CO)2[(C6H5)3P]2 , is used ascatalyst, the cyclization products are benzene(88% yield) and styrene (12% yield). The re-action is carried out in benzene at 65 – 75 ◦Cand 1.5 MPa [31, 32].

Linear polymerization of acetylene occurs inthe presence of a copper (I) salt such as CuCl inhydrochloric acid. Reaction products are viny-lacetylene, divinylacetylene, etc. [33]:

HC≡CH+HC≡CH→H2C = CH−C≡CH

A particular polymerization product, knownas cuprene, is formedwhen acetylene is heated to225 ◦C in contact with copper sponge. Cupreneis chemically inert, corklike in texture, and yel-low to dark brown.

Polyacetylene [34, 35] is formed withZiegler–Natta catalysts, e.g., a mixture of trieth-

8 Acetylene

ylaluminum, Al(C2H5)3 , and titanium tetrabu-toxide, Ti(n-OC4H9)4 , at 10−2 to 1 MPa:

Polymerization can be carried out in an auxiliaryinert liquid, such as an aliphatic oil or petroleumether. The monomer can also be copolymerizedin the gas phase.

Polyacetylene is a low-density sponge-likematerial consisting of fibrils with diame-ters of 20 – 50 nm. The ratio cis- to trans-polyacetylene depends on the reaction tempera-ture.

Polyacetylene doped with electron acceptors(I2 , AsF5), electron donors (Na, K), or protonicdopants (HClO4 , H2SO4) is highly conductiveand has the properties of a one-dimensionalmetal [35].

3.2. Other Reactions; Derivatives

Metal Acetylides [36]. The hydrogen atomsof the acetylene molecule can be replaced bymetal atoms (M) to yieldmetal acetylides.Alkaliand alkaline-earth acetylides can be prepared viathe metal amide in anhydrous liquid ammonia:

C2H2+MNH2→MC2H+NH3

The direct reaction of the acetylene with amolten metal, such as sodium, or with a finelydivided metal in an inert solvent, such as xylene,tetrahydrofuran, or dioxane, at a temperature ofabout 40 ◦C, is also possible:

2 M+C2H2→M2C2+H2

The very explosive copper acetylides, e.g.,Cu2C2 · H2O, can be obtained by reaction ofcopper(I) salts with acetylene in liquid ammoniaor by reaction of copper(II) salts with acetylenein basic solution in the presence of a reducingagent such as hydroxylamine. Copper acetylides

can also form from copper oxides and other cop-per salts. For this reason copper plumbing shouldbe avoided in acetylene systems.

Silver, gold, and mercury acetylides, whichcan be prepared in a similar manner, are alsoexplosive.

In sharp contrast to the highly explosiveCu2C2 · H2O, the catalyst used for the synthesisof butynediol, Cu2C2 · 2 H2O · 2 C2H2 , is notas sensitive to shock or ignition.

Halogenation. The addition of chlorine toacetylene in thepresenceofFeCl3 yields 1,1,2,2-tetrachloroethane [79-34-5], an intermediate inthe production of the solvents 1,2-dichloroeth-ylene [540-59-0], trichloroethylene [79-01-6],and perchloroethylene [127-18-4].

Bromine and iodine can also be added toacetylene. The addition of iodine to acetylenestops with formation of 1,2-diiodoethylene.

Hydrogenation. Acetylene can be hydro-genated, partly or completely, in the presence ofPt, Pd, or Ni catalysts, giving ethylene or ethane.

Organic Silicon Compounds [37, 38]. Theaddition of silanes, such as HSiCl3 , can be car-ried out in the liquid phase using platinum orplatinum compounds as catalysts:

HC≡CH+HSiCl3→CH2 = CH−SiCl3

Oxidation. At ambient temperature acetyleneis not attacked by oxygen; however, it can formexplosivemixtureswith air or oxygen (seeChap.5). The explosions are initiated by heat or ig-nition. With oxidizing agents such as ozone orchromic acid, acetylene gives formic acid, car-bon dioxide, and other oxidation products. Thereaction of acetylene with dilute ozone yieldsglyoxal.

Hydrates. At temperatures below ca. 15 ◦C,under pressure, hydrates of the compositionC2H2 · 6 H2O are formed (see Section 2).

Chloroacetylenes [39].Monochloroacetylene, HC≡CCl, Mr 60.49,

bp −32 to −30 ◦C, a gas with nauseating odorthat irritates the mucousmembranes, is obtainedby reaction of 1,2-dichloroethylene with alco-holic NaOH in the presence of Hg(CN)2. It ig-nites in the presence of traces of oxygen. In air itexplodes violently. Chloroacetylene is very poi-sonous.

Acetylene 9

Dichloroacetylene, ClC≡CCl,Mr 94.93,mp− 66 to − 64.2 ◦C, a colorless oil of unpleas-ant odor, explodes in the presence of air or onheating. It is obtained from acetylene in stronglyalkaline potassium hypochlorite solution [40] orby reaction of trichloroethylene vaporwith caus-tic alkali.

4. Production

4.1. Thermodynamic and KineticAspects

The production of acetylene fromhydrocarbons,e.g.,

2 CH4�C2H2+3 H2 ∆H (298 K)

= 376.4 kJ/mol

requires very high temperatures and very shortreaction times. Themain reasons for the extremeconditions are the temperature dependence ofthe thermodynamic properties (molar enthalpyof formation, ∆H f , and molar free energy offormation, ∆Gf ) of the hydrocarbons; the posi-tion of the chemical equilibria under the reactionconditions; and the kinetics of the reaction.

Table 3. Standard molar enthalpies of formation and Gibbs freeenergy of formation at 298 K

∆Hf (kJ/mol) ∆Gf (kJ/mol)C (s) 0 0H2 (g) 0 0CH4 (g) − 74.81 − 50.82C2H2 (g) +226.90 +209.30C2H4 (g) + 52.30 + 68.15C2H6 (g) − 84.64 − 32.90C3H6 (g) + 20.43 + 62.75C3H8 (g) −103.90 − 23.48n-C4H10 (g) −126.11 − 17.10

Thermodynamic data relevant to the hydro-carbon–acetylene system are shown in Table 3and Figure 7. These data show clearly that atnormal temperatures acetylene is highly unsta-ble compared to the other hydrocarbons. How-ever, Figure 7 also shows that the free energyof acetylene decreases as temperature increases,whereas the free energies of the other hydro-carbons increase. Above about 1230 ◦C, acety-lene is more stable than the other hydrocar-bons. The temperature at which the acetyleneline intersects an other line in Figure 7 is higher

the shorter the chain length of the hydrocar-bons. Acetylene production from methane re-quires higher reaction temperatures than produc-tion from heavier hydrocarbons.

Figure 7. Gibbs free energy of formation per carbon atomof several hydrocarbons as a function of temperature

The equilibrium curve for the methane reac-tion as a function of temperature (Fig. 8) showsthat acetylene formation only becomes apparentabove 1000 K (730 ◦C). Therefore, a very largeenergy input, applied at high temperature, is re-quired.

Figure 8. Equilibrium curve for the methane cracking reac-tion, 2 CH4 � C2H2 + 3 H2

However, even at these high temperaturesacetylene is still less stable than its componentelements, carbon und hydrogen (see Fig. 7). In

10 Acetylene

fact, the large difference in free energy betweenacetylene and its component elements favors thedecompositionof acetylene to carbon andhydro-gen up to temperatures of about 4200 K.

C2H2→2 C (s)+H2 (g)−∆Gf (298 K)

= −209.3 kJ/mol

Thus cracking and recombination of thehydrocarbons and decomposition of acetylenecompete. To achieve reasonable acetylene yieldsand to avoid the thermodynamically favorabledecomposition into the elements, rapid quench-ing of acetylene produced in the cracking reac-tion is necessary. In practice, the residence timeat high temperature is between 0.1 and 10 ms.

Higher temperatures also increase the rate ofconversion of acetylene to byproducts. Again,the residence time must be sufficiently short toprevent this.

In the case of cracking by partial oxidation,the combustion reaction of the hydrocarbon sup-plies the energy necessary for the production ofacetylene from the other part of the hydrocarbonfeed:

CH4+O2→CO+H2+H2O ∆H (298 K)

= −277.53 kJ/mol

CO+H2O→CO2+H2 ∆H (298 K)

= −41.19 kJ/mol

From these reaction enthalpies, the amountof oxygen needed to produce the high reactiontemperature can be calculated. Therefore, in ad-dition to the short residence time, the correctmethane : oxygen ratio, which also determinesthe reaction temperature, is essential to obtaingood acetylene yields.

4.2. Partial Combustion Processes

In this group of processes, part of the feedis burnt to reach the reaction temperature andsupply the heat of reaction. The necessary en-ergy is produced where it is needed. Almost allcarbon-containing raw materials can be used asfeedstocks: methane, ethane, natural gas liquids(NGL), liquefied petroleum gas (LPG), naphtha,vacuum gas oil, residues, and even coal or coke.Natural gas is especially suitable because it isavailable in many parts of the world. Only under

the conditions of acetylene synthesis can meth-ane be transformed into another hydrocarbon ina single process step, and this is the essentialreason for using the thermodynamically unfa-vorable acetylene synthesis.

Thepartial combustionprocesses for light hy-drocarbons, frommethane to naphtha, all followsimilar schemes. The feed and a certain amountof oxygen are preheated separately and intro-duced into a burner. There they pass through amixing zone and a burner block into the reac-tion zone, where they are ignited. On leavingthe reaction zone the product mixture is cooledrapidly, either by water or oil. Cooling by wateris easier, and more common, but it is thermallyless efficient than cooling by oil. Alternatively,the gases can be cooled with light hydrocarbonliquids, which leads to additional acetylene andethylene formation between 1500 and 800 ◦C.These processes are usually called two-step pro-cesses.

Burner design is very important for all partialcombustion processes. The residence time of thegas in the reaction zone must be very short, onthe order of a few milliseconds, and it shouldbe as uniform as possible for all parts of thegas. Flow velocity within the reaction zone isfixed within narrow limits by the requirementsof high yield and the avoidance of preignition,flame separation from the burner block, and cokedepositions. A survey of the processes operatingaccording to these principles is given in [7, 41].Only the BASF process is described here in de-tail, because it is the most widely used processfor the partial combustion of natural gas.

The submerged flame process, SFP, was de-veloped by BASF with the aim of producingacetylene from crude oil or its heavy fractions,and thus to be independent of the more expen-sive refined oil products used in olefin chem-istry. One unit of this kind was built in Italy, butit became uneconomic and was shut down aftera year of operation [42]. Nevertheless, the pro-cess is described in some detail below becauseof its simple cracking section, because of thesimultaneous formation of acetylene and ethyl-ene, and because of its high thermal efficiencyand its high degree of carbon conversion (per-haps of even greater importance in the future).

The partial combustion carbide process, alsodeveloped by BASF, uses coke, oxygen, andlime as feed. It was developed in the 1950s to

Acetylene 11

reestablish the competitive position of carbidein the face of the new acetylene processes ona petrochemical–natural gas basis. Some atten-tion is given here to the basics of this process,although it has never gone beyond the pilot-plantstage. When petrochemical feedstocks becomescarce, this process may have a place in a futurecoalbased chemistry because it has a higher de-gree of carbon conversion and a higher thermalefficiency than the electric carbide process.

All these acetylene processes based on partialcombustion yield a number of byproducts, suchas hydrogen and/or carbon monoxide, whichmay cause problems if acetylene is the onlyproduct desired. Within a complex chemicalplant, however, these may be converted to syn-thesis gas, pure hydrogen, and pure CO andcan actually improve the economics of acety-lene production.

4.2.1. BASF Process (Sachsse-Bartholome)

The BASF process for the production of acety-lene fromnatural gas has been known since 1950[43]. Worldwide, some 13 plants used this pro-cess in 1983, a total capacity of about 400000t/a. All use a water quench, except the plant inLudwigshafen (Germany) operated with an oilquench [44].

The basic idea of partial combustion involvesa flame reaction on a premixed feed of hydrocar-bon and oxygen. In this way the rate of hydro-carbon conversion is made independent of thegas-mixing rate, which is governed by diffusion.Only then can the residence time in the reactionzone be mademuch smaller than the average de-cay time of acetylene. The separate preheatingof the reactants to the highest temperature pos-sible before introduction into the burner reducesthe consumption of oxygen and the hydrocarbonwithin the burner. It also causes a higher flamepropagation speed and therefore a higher massflow within the acetylene burner.

The smallest, but most important, part of apartial oxidation acetylene plant is the burner,Figure 9. Its design is nearly identical in the twoprocess variants (i.e., oil and water quench).

At the top of the burner, the preheated reac-tants, (600 ◦C in the case of methane) must bemixed (c) so rapidly that there are no domainswith a high oxygen concentration. Such domains

cause preignition before the reactants are intro-duced into the reaction zone (g). In fact, the re-action mixture ignites after an induction timedepending on the hydrocarbon used as feed andon the preheat temperature, on the order of a fewtenths of a second. The maximum preheat tem-perature is lower for higher hydrocarbons thanfor methane. Backmixing of the gas between themixing and the reaction zones is avoided by thediffuser (e), a tube which connects the mixingzone and the burner block (f). Because of itssmooth surface and the small opening angle thereaction feed is decelerated gently and backmix-ing does not occur.

The burner block (f) consists of a water-cooled steel plate with a large number of smallchannels. The flow velocity through these chan-nels is substantially higher than the flame propa-gation speed, so that the flame below the burnerblock cannot backfire into the diffuser. Thelower side of the burner block has small open-ings between the channels through which addi-tional oxygen is fed into the reactionmixture. Atthese openings small flames form and initiate theflame reaction. The strong turbulence below theburner block stabilizes the flame.

Under unfavorable conditions the flame mayappear above the burner block. In this casethe oxygen feed must be shut off immediatelyand replaced by nitrogen. This extinguishes thepreignition before it can cause any damage tothe equipment. Such preignitions can result froma momentary shift in the oxygen : hydrocarbonratio or the entrainment of small particles of py-rophoric iron formed from rust in the preheaters.

As mentioned above, the hot gas leaves thereaction chamber within a few milliseconds andpasses through sprays of water or oil, which coolthe gas almost instantaneously, to about 80 ◦C inthe case of water or 200 – 250 ◦C in the case ofoil. The quench system consists of a set of noz-zles that are fed by three annular tubes below thereaction chamber.

The concentrations of the major constituentsof the crackedgas dependon theoxygen : hydro-carbon ratio in the feed as shown in Figure 10.As the oxygen supply is increased, the acetyleneconcentration increases until it passes through asmooth maximum. At the same time there is anincrease in the volume of the cracked gas. Thusmaximumacetylene production is attainedwhena little more oxygen is used than the amount

12 Acetylene

Figure 9. BASF acetylene burnerA) The burner: a) Oxygen: b) Hydrocarbon; c) Mixer; d) Concrete lining; e) Diffuser; f) Burner block; g) Reaction chamber;h) Rupture disk; i) Quench-medium inlet; j) Quench rings; k) Quench chamber; l) Manual scraper; m) Cracked-gas outlet;n) Quench-medium outlet. B) The burner block

Acetylene 13

required for maximum acetylene concentrationin the cracked gas. This is clear from the con-sumption of natural gas per ton of acetylene pro-duced and the reduction in unconverted meth-ane. When the oxygen : hydrocarbon ratio is toolow, the reaction time is insufficient for com-plete conversion of oxygen, and the cracked gascontains free oxygen. Free oxygen can be tol-erated only up to a certain concentration. Whenthe oxygen : hydrocarbon ratio is too high, theincreased velocity of flame propagation exceedsthe flow velocity in the channels of the burnerblock, leading to preignitions.

Figure 10. Burner characteristicsa) Burner block; b) Reaction chamber; c) Flame front;d) Quench-medium inlet

Coke deposits in the reaction chamber haveto be removed from time to time with a manualor an automatic scraper. Normally, a burner pro-duces 25 t of acetylene per day from natural gasand 30 t per day from liquid feedstocks.

Acetylene Water Quench Process (AWP),Soot Removal (Fig. 11). After quenching withwater the cracked gas leaves the burner (b) at 80– 90 ◦C. A certain amount of soot is formed in

the reaction chamber in spite of the very shortreaction time.When natural gas is used as a feed-stock, the soot is 50 kg per ton of acetylene, withLPG feedstock it is 250 kg, and with naphthait is 350 kg. The soot is partly removed fromthe gas by the quench, then by washing withrecirculated water in a cooling column (c), andby passing the gas through an electrofilter (d).After cooling and soot removal, the gas has apressure slightly above atmospheric, a tempera-ture of about 30 ◦C, and a soot content of about1 mg/m3. The water effluents from the quenchsystem, the cooling column, and the electrofiltercarry the washed-out soot. Some gas remainsattached to the soot, causing it to float whenthe soot-containing water flows slowly throughbasin decanters (e). The upper soot layer, whichcontains 4 – 8 wt% of carbon, depending on thefeedstock, is scraped off the water surface andincinerated.

AcetyleneOilQuenchProcess (AOP), SootRemoval (Fig. 12). In this process the crackedgas is quenched with oil sprays and leaves theburner at 200 – 250 ◦C. The oil absorbs the heatfrom the gas and then passes through waste heatboilers before returning to the quench. The sen-sible heat of the cracked gas represents morethan 15% of the heating value of the feedstock.The pressure of the generated steam depends onthe process configuration and can reach 15 bar(1.5 MPa).

Unlike the water quench process, where thescraped coke deposits sink to the bottom of thequench chamber and are easily removed, in theoil quench the coke deposits do not settle imme-diately. In order to prevent plugs in the quenchnozzles a mill pump (d) is installed immediatelyunderneath the burner column.

The coke and soot content in the quench cir-cuit is kept near 25%by sending a fraction of thecoke-containing oil to externally heated, stirredkettles (coker (e)). In the kettles the volatile mat-ter evaporates very quickly, leading to fluidiza-tion of the coke bed. The vapor is returned tothe burner column, while the soot is agglomer-ated. A fine-grained coke is withdrawn from thebottom of the coker.

Because of the cracking losses in the quencha certain amount of quench oil has to be addedcontinuously to the process. This makeup oil isat least 0.15 to 0.3 t per ton of acetylene, depend-

14 Acetylene

Figure 11. Acetylene water quench process (AWP)a) Preheaters; b) Acetylene burner; c) Cooling column; d) Electrofilter; e) Soot decanter; f) Cooling tower

Figure 12. Acetylene oil quench process (AOP)a) Preheaters; b) Acetylene burner; c) Burner column; d) Mill pump; e) Coker; f) Decanter; g) Final cooler

ing on the stability of the oil used.When residualoil from steam crackers is used, it can be desir-able to add up to 1 t of oil per ton of acetylene,because the excess oil is partially converted tolight aromatic hydrocarbons.

The cracked gas leaving the quench is cooledin a burner column (c), where there are ad-ditional oil circuits for the production of 3-bar steam and for boiler feedwater preheat. At

the top of the column a small amount of alow-boiling oil (BTX = benzene, toluene, andxylene) is added to prevent deposit-formingaromatics (mainly naphthalene) from passingdownstream into other parts of the plant. Thecracked gas, which has to be compressed beforeseparation, is cooled further (g) by water. At thisstage most of the BTX condenses and is sepa-rated from the water in a large decanter (f).

Acetylene 15

Table 4 shows the cracked gas compositionsfor the BASF acetylene oil quench processwhen natural gas, liquid petroleum gas (LPG),or naphtha is used as feedstock. The waterquench process gives very similar compositions.The relative amounts of hydrogen and carbonmonoxide formed depend on the hydrogen : car-bon ratio of the feedstock used. Evenwhennaph-tha is used, almost no ethylene forms. This isbecause the reaction takes place above 1200 ◦Cwhere the formation of ethylene is thermody-namically impossible. Only a prequench withadditional naphtha or LPG produces additionalacetylene and ethylene at intermediate tempera-tures, as in the case of two-step processes. Thehigher hydrocarbons require a somewhat lowerreaction temperature than methane and have aless endothermic heat of reaction: oxygen con-sumption per ton of acetylene is lower for thehigher hydrocarbons in spite of the lower pre-heating temperature.

Comparison of Oil Quench and WaterQuench Processes. The advantage of the oilquench process is obvious: the heat recovery inthe form of steammakes the overall thermal effi-ciency in relation to primary energy input ratherhigh. If the thermal efficiency for the productionof electricity is 33%, over 70% of the net heat-ing value of the overall primary energy input isrecovered in the form of products and steam. Acomparison between the oil quench and waterquench (see Table 6) shows that the oil quenchrequires a net heating value input of 300 – 330GJ per ton of acetylene, of which 82 GJ (27 –25%) is lost, whereas the water quench requiresa 288 GJ input, of which 113 GJ (39%) is lost.

Acetylene Recovery. Liquid acetylene is adangerous product, even at low temperatures.Separation of the cracked gas by cryogenic pro-cesses such as those used in olefin productionis clearly ruled out. One exception to this ruleis the acetylene recovery unit of the submergedflame process (Section 4.2.3) [45], in which allhydrocarbons except methane are condensed at−165 ◦C. Otherwise, acetylene is recovered byselective absorption into a solvent. This proce-dure is economical only when the cracked gas iscompressed. The upper limit for the pressure isdetermined by the danger of explosions, and as

a rule the partial pressure of acetylene should bekept below 1.4 bar (0.14 MPa).

The solubility of acetylene in the solventsused is between 15 and 35 m3 (STP) per cu-bic meter of solvent under process conditions.The dissolved gas is recovered by depressuriz-ing the solvent and by vapor stripping at highertemperatures. All solvents used commercially,N-methylpyrrolidone (NMP), methanol, ammo-nia, and dimethylformamide (DMF), are misci-ble with water. They are recovered from the gasstreams leaving the plant bywater scrubbing anddistillation.

The kinetics of acetylene formation alwayslead to the formation of higher homologues ofacetylene as byproducts [46], mainly diacety-lene, but also methylacetylene, vinylacetylene,and others. These compounds polymerize veryeasily andmust be removed from the cracked gasas soon as possible. Because they aremuchmoresoluble in the solvents than acetylene, scrubbingthe cracked gas with a small amount of solventbefore it enters the acetylene recovery stages issufficient.

Absorption Section (Fig. 13). Acetylene re-covery is illustrated here by the BASF pro-cess. N-Methylpyrrolidone is used to separatethe cracked gas into three streams:

1) Higher homologues of acetylene and aromat-ics, the most soluble part of the cracked gas.(This is a small stream of gas, which is dilutedwith crude synthesis gas for safety reasonsand is used as fuel.)

2) Product acetylene, less soluble than the higheracetylenes, but much more soluble than theremainder of the gas

3) Crude synthesis gas (off-gas), mainly hydro-gen and carbon monoxide

In the prescrubber (b) the cracked gas isbrought into contact with a small amount ofsolvent for removal of nearly all the aromaticcompounds and C4 and higher acetylenes ex-cept vinylacetylene. This is done after the com-pression of the gas if screw compressors are usedbut before compression if turbo compressors areused because turbo compressors cannot toleratedeposits on their rotors. In the main scrubber(d) the gas is brought into contact with a muchlarger amount of N-methylpyrrolidone (NMP),which dissolves all the acetylene, the remaininghomologues, and some carbon dioxide. Crude

16 Acetylene

Table 4. BASF acetylene oil quench process, cracked gas composition (vol%)

Component * Raw material (∆H, kJ/mol)

Methane LPG Naphtha(400) (325) (230)

H2 56.5 46.4 42.7CH4 5.2 5.0 4.9C2H4 0.3 0.4 0.5C2H2 7.5 8.2 8.8C3+

∗∗ 0.5 0.6 0.7CO 25.8 35.0 37.9CO2 3.2 3.4 3.5O2 0.2 0.2 0.2Inerts balance

* Dry gas, water, and aromatic compounds condensed out;** Hydrocarbons with three or more carbon atoms.

Figure 13. BASF acetylene process — N-methylpyrrolidone absorption sectiona) Compressor; b) Prescrubber; c) Acetylene stripper; d) Main scrubber; e) Stripper; f) Vacuum column; g) Vacuum stripper;h) Side column; i) Condenser; j) Vacuum pumps

synthesis gas (off-gas) leaves at the top of thecolumn.

TheNMPsolution is degassed in several stepsin which the pressure is reduced and the tem-perature increased. The stripper (e) operates atpressures and temperatures slightly above ambi-ent. In this tower, the solution is put in contactwith a countercurrent gas stream from the sub-sequent degassing step (f). This leads to the evo-lution of carbon dioxide, the least soluble of thedissolved gases, at the top of the stripper. Thecarbon dioxide is recycled to the suction sideof the compression and thereby is shifted intothe crude synthesis gas. The acetylene productis withdrawn as a side stream from the stripper.

The N-methylpyrrolidone solution is then com-pletely degassed (f) in two further steps at 110– 120 ◦C, first at atmospheric, then at reducedpressure. Vinylacetylene, methylacetylene, andexcess process water are withdrawn as bleedstreams from the vacuum column (f). The watercontent of the solvent is controlled by the reboil-ing rate in the vacuum column. At the bottom ofthe vacuumcolumn, degassing is completed, andthe solvent is cooled and returned to the mainscrubber (d).

The small amount of solvent from the pre-scrubber (b) is stripped with crude synthesis gasfor recovery of the dissolved acetylene, the over-head gas being recycled to the suction side of

Acetylene 17

the compressor. The solvent is then degassedcompletely in the vacuum stripper (g), a columnwhich also accepts the bleed stream from thevacuum column (f) containing the excess pro-cesswater togetherwith some higher acetylenes.The overhead vapor of the vacuum stripper con-tains the higher acetylenes, water, and someNMP vapor. In a side column (h) the NMP isrecovered by scrubbing with a small amountof water, which is recycled to the main solventstream. The gas is cooled (i) by direct contactwith water from a cooling circuit to condensemost of the water vapor. The higher acetylenesare diluted with crude synthesis gas before theyenter and after they leave the vacuum pump (j).The diluted higher acetylenes, which are nowat a pressure slightly above atmospheric, can beused as fuel gas, e.g., for soot incineration.

In order to minimize the polymer content ofthe solvent, about 2% of the circulating flow iswithdrawn continuously from the vacuum strip-per circuit and distilled under reduced pressure,leaving the polymers as a practically dry cakefor disposal.

The acetylene product from the process asdescribed above has a purity of about 98.4%,the remainder consisting mainly of propadiene,methylacetylene, and nitrogen. For most appli-cations the purity is increased to 99.7% byscrubbingwith sulfuric acid and sodiumhydrox-ide solutions. Table 5 compares the composi-tions of crude and purified acetylene. Table 6compares the consumption and product yieldsper ton of acetylene for the oil quench processwith those for the water quench process.

Table 5. Purity of the acetylene from the BASF process

Component Crudeacetylene, vol%

Purified acetylene,vol%

Acetylene ca. 98.42 99.70Propadiene 0.43 0.016Propyne 0.75 tracesVinylacetylene 0.05 01,3-Butadiene 0.05 0Pentanes 0.01 0.01Carbon dioxide ca. 0.10 0Nitrogen ca. 0.30 0.30

4.2.2. Other Partial Combustion Processes

The main features of the BASF process de-scribed in detail above are common to all partialoxidation processes. Therefore only the differ-

ences between the BASF acetylene burner andburners used in theMontecatini and the SBApro-cesses [41, 47] are described. These two pro-cesses have also attained some importance. Thedetails of the acetylene recovery process dependon the properties of the solvent, but here too thebasic principles are the same for all processes.

Montecatini Process . The Montecatiniburner [48] has the same main components asthe BASF burner: mixing unit, gas distributor,reaction chamber, and quench. The essentialdifference is the pressure for acetylene synthe-sis, which can be as high as several bar. Thissaves compression energy, improves heat recov-ery from the quench water, which is obtainedat 125 ◦C, and is claimed to make soot removaleasier because the cracked gas is scrubbed withwater above 100 ◦C. Although it is well known[7] that acetylene decomposition is acceler-ated under pressure at high temperatures (>1000 ◦C), the acetylene yield is comparableto that obtained at atmospheric pressure be-cause of the short residence time in the reactor.Methanol is used at cryogenic temperatures foracetylene recovery. The main steps of the gasseparation are absorption of higher acetylenesand of aromatics, absorption of acetylene, strip-ping of coabsorbed impurities, and desorptionof acetylene.

SBA Process (of the Societe Belge del’Azote). The SBA burner [49] has the samemain components as the other processes. How-ever, it has a telescope-like reaction chamberand a device for shifting the quench up anddown. Thus it is possible to adjust the length ofthe reaction zone for optimum residence timeat any throughput. The walls of the reactionchamber are sprayed with demineralized wa-ter to prevent coke deposits. This eliminates theneed to scrape the reaction chamber periodically.Acetylene recovery is carried out with severalscrubbing liquids—kerosene, aqueous ammo-nia, caustic soda, and liquid ammonia, each withits own circuit. After soot is separated from thegas in an electrofilter, higher hydrocarbons areabsorbed in kerosene or gas oil. Carbon dioxideis scrubbed in two steps, first with aqueous am-monia and then with caustic soda solution. Theacetylene product is absorbed into anhydrous

18 Acetylene

Table 6. BASF acetylene process, consumption and product yields per ton of acetylene

Consumption and product yields Oil quench Water quenchFeed and energy requirementsNatural gas, 36 000 kJ/m3(STP) (LHV) * 5833 m3 = 210 GJ 5694 m3 = 205 GJOxygen, 0.55 kWh/m3(STP) ** 3400 m3 = 20.4 GJ 3400 m3 = 20.4 GJFuel gas = 12.0 GJ = 18.0 GJResidue oil minimum (surplus) 0.3 (1.0) t = 12.0 (40.0) GJSulfuric acid 160 kg 160 kgSodium hydroxide 5 kg 5 kgN-Methylpyrrolidone 5 kg 5 kgElectric energy ** 3200 kWh = 34.9 GJ 3100 kWh = 33.8 GJSteam, 4 bar 5.0 t = 11.7 GJ 4.5 t = 10.5 GJ

Energy input 301.0 (329.0) GJ 287.7 GJProduct yieldsAcetylene, 48650 kJ/kg (LHV) 1.0 t = 48.6 GJ 1.0 t = 48.6 GJCrude synthesis gas, 12100 kJ/m3(STP) (LHV) 10600 m3 = 128.3 GJ 10150 m3 = 122.8 GJCoke (with residue surplus), 35 500 kJ/kg 0.3 (0.46) t = 10.7 (16.3) GJ –BTX (with residue surplus), 40 250 kJ/kg (LHV) 0.05 (0.12) t = 2.0 (4.8) GJ –Naphthalenes (with residue surplus), 38770 kJ/kg (LHV) 0.0 (0.41) t = – (15.9) GJ –Steam (up to 15 bar) 13.0 (14.0) t = 30.3 (32.6) GJ 1.5 t = 3.5 GJ

Energy output 219.9 (246.5) GJ 174.9 GJ

Thermal efficiency 73.0 (74.9)% 60.8%

Energy losses, absolute per ton acetylene 81.1 (82.5) GJ 112.8 GJ

* If the natural gas contains inerts and higher hydrocarbons, the required input will remain approximately the same on a heating value basis(LHV= low heating value), but the cracked gas analyses and the crude synthesis gas analyses will differ slightly.** Thermal efficiency of electricity production is assumed to be 33%.

ammonia and must be scrubbed with water af-ter desorption. All the ammonia–water mixturesare separated in a common distillation unit. Thisrecovery scheme leads to exact separation of thevarious cracked gas components.

Additional Remarks. The Montecatini andSBA processes can also be operated with two-stage burners. A prequench with light hydrocar-bons cools the cracked gas to about 800 ◦C. Af-ter a residence time at this intermediate temper-ature the gas is cooled down with water. In thisway the heat content of the hot gases is used forfurther cracking of hydrocarbons to yield extraacetylene and olefins. The presence of additionalcomponents in the cracked gas requires moreprocess steps in the gas separation units.

4.2.3. Submerged Flame Process

The submerged flame process (SFP) of BASFattracted considerable interest up to 1973 as apartial combustion process for the production ofacetylene, ethylene, C3, and C4 hydrocarbons,and synthesis gas from feedstock of crude oil

and residues, such as Bunker C oil and vacuumresidue [44, 45]. Although it was abandoned atthe end of 1973, the need to make the most eco-nomic use of raw materials has renewed interestin this process [50].

Oxygen compressed to 16 bar (1.6 MPa)feeds a flame that is submerged in the oil. The oilsurrounding the flame is partially burnt to obtainthe necessary reaction temperature and also actsas the quenching medium. This process differsfrom the partial oxidation processes using nat-ural gas and lighter hydrocarbons in five mainrespects:

1) Crude oil can be gasified without the forma-tion of residues, and the process can be oper-ated under certain conditions with heavy fueloil.

2) All the soot formed is consumed when crudeoil feedstock is used, eliminating all the prob-lems associated with the storage, disposal, orutilization of acetylene soot.

3) The heat of reaction is removed by steam gen-eration at 8 bar (0.8 MPa).

4) The process is operated at 9 bar (0.9 MPa) sothat the oxygen is the only compressed stream.

Acetylene 19

The cracked gas is formed at a pressure suffi-cient for economic separation.

5) The design of the cracking unit is greatly sim-plified because the reaction feed, fuel, andquenching medium are identical.

The process is described in detail in the lit-erature cited; therefore, only general overviewsof the cracking unit (Fig. 14) and the separa-tion unit (Fig. 15) are shown here. The capaci-ties of a submerged flame burner for acetyleneand ethylene are 1 t/h and 1.15 t/h, respectively.To produce these quantities, 5000 m3 (STP) ofoxygen and 8 – 10 t of oil are required per hour.The cracked gas shows the following averagecomposition (vol%, the components grouped asstreams leaving the separation unit):

Main productsAcetylene 6.2Ethylene 6.5

Crude synthesis gasCarbon monoxide 42.0Hydrogen 29.0Methane 4.0Inerts 0.6

Other hydrocarbonsEthane 0.5Propane 0.1Propene 1.2Propadiene, propyne 0.71,3-Butadiene 0.5Other C4 and C5+ *

hydrocarbons1.5

RemainderCarbon dioxide 7.0Hydrogen sulfide 0.05 – 0.5Carbon oxide sulfide 0.03 – 0.3

* C5+ , five or more carbons

Unlike all other processes the submergedflame process uses low temperatures (–165 ◦C)to separate the off-gas, consisting of carbonmonoxide, hydrogen, and methane, from theC2 and higher hydrocarbons. On account ofthe acetylene in the condensed phase, exten-sive decomposition tests have been carried out.Whereas the cracking unit (Fig. 14) and theamine scrubbing unit have been tested by Soc.Ital. Serie Acetica Sintetica, Milan, on a com-mercial scale, the remaining purification units(Fig. 15) have not. However, the experience ob-tained with a pilot plant indicates that major dif-ficulties are not to be expected.

The submerged flame process may becomecompetitive because of its ability to use crude

oil and especially residues and because of itslow losses on the primary energy input.

Figure 15. Submerged flame process — purification unit

4.2.4. Partial Combustion Carbide Process

Calcium carbide production from lime and coalrequires a large high-temperature heat input (seeSection 4.3.4). In the thermal process some ofthe coal must be burnt to attain the necessary re-action temperature and supply the heat of reac-tion. The thermal carbide processwas developedby BASF [7, 51] from 1950 to 1958 to elimi-nate the input of electrical energy necessary inthe classic carbide process. Starting in 1954, alarge pilot plant, with a nominal carbide capac-ity of 70 t/d, was operated, but in 1958 the moreeconomical petrochemical acetylene productionhalted further development. Carbide productionis just one way of converting coal chemically;othermethods include pyrolysis, hydrogenation,and gasification. The question arises as to theconditions under which a thermal carbide pro-cess using oxygen can compete with the electriccarbide process. The biggest drawback of car-bide production in a shaft furnace (Fig. 16) com-pared to the electric carbide process is the lack ofcommercial-scale operational experience. Spe-cific disadvantages are greater susceptibility to

20 Acetylene

Figure 14. Submerged flame process (SFP) — cracking unita) Reactor; b) Oil cooler; c) Steam generator; d) Oil recycle pump; e) Scrubber; f) Naphtha cooler; g) Naphtha separator;h) Naphtha pump; i) Spray cooler; j) Separating vessel; k) Recycle-water pump; l) Recycle-water cooler

disruption because of plugging of the furnacefeed, more stringent specifications for the rawmaterials, more handling of solids, and the largeamount of byproduct. There are two main ad-vantages:

1) A thermal efficiency of about 50% versusabout 30% for the electrothermal process ifthe thermal efficiency of electricity produc-tion is 33%

2) Carbon monoxide production, which is desir-able because carbon monoxide can be con-verted to synthesis gas by the water-gas shiftreaction (→ Gas Production)

Table 7. Partial combustion carbide process, consumption andproduct yields per ton acetyleneRaw materialsCoke, dry (88% C) 5700 kgLime (92% CaO) 3140 kgOxygen (98%) 3560 m3 (STP) 5090 kgTotal consumption 13 930 kg

Products

Carbide (80.5%) 2850 kg∧= 1000 kg acetylene

Carbon monoxide 7980 m3

(STP)9975 kg

(CO 95.5, H2 2.0, N2 2.0,CO2 0.5 vol%)Dust 900 kgLosses 205 kgTotal products 13 930 kg

If the carbon monoxide is converted to syn-thesis gas and the electrical energy is producedfrom fossil fuels, production costs are about one

third lower for the thermal process than for theelectrical process [52] based on the pilot-plantconsumption data (Table 7).

4.3. Electrothermic Processes

Because calcium carbide is produced elec-trothermally, the production of acetylene fromthis material also is discussed in this group ofprocesses (Section 4.3.4).

Electrothermic processes have the followingadvantages over partial oxidation:

– The energy requirement for the formation ofacetylene can be made independent of the hy-drocarbons used as feedstock.

– Hydrocarbon consumption can be reduced by50%.

– Provided that electrical energy is availableunder favorable conditions (nuclear power,hydroelectric power, cheap coal) and/or theavailability of hydrocarbons is limited, elec-trothermic processes are more economical.

In the case of acetylene formation, theelectric-arc process offers optimal conditions forthe endothermic reaction at high temperatures.

The development of the electric-arc processfor cracking light hydrocarbons to acetylene be-gan in 1925 in Germany. The acetylene was tobe used as feedstock for butadiene production.

Acetylene 21

Figure 16. Partial combustion carbide processa) Carbide furnace; b) Refractory brick lining; c) Charging hopper; d) Gas outlet; e) Oxygen jet; f) Tapping burner; g) Tappingchute; h) Bogey; i) Cyclone; j) Washing column; k) Desintegrator; l) Compressor

In 1940, the first commercial plant was put onstream at Chemische Werke Huls in Marl, Ger-many. TheHuls process has since been improved, and the capacity raised, but it retaired the orig-inal principles [53].

Feedstock for electric-arc processes may begaseous or liquid hydrocarbons or even solidssuch as coal. The design of the arc furnace andthe purification section for the cracked productshave to be adapted to the different feedstocks.For gaseous or gasified hydrocarbons the clas-sical one-step process is used: the arc burns di-rectly in the gas being cracked. For liquid andsolid feeds, a one- or two-step process may beused. In the two-step process hydrogen is firstheated in the arc furnace, and then liquid or solidfeed is injected into the hydrogen plasma [54].Figure 17 shows both types of arc furnaces. Be-cause of hydrogen formation during the crackingreaction, the arc burns in a hydrogen atmospherein both processes. The conductivity and the highrate of ion–electron recombination for hydrogenmean that arcs above a certain length cannot beoperated with alternating current at normal fre-quency and high voltage. All commercial plantstherefore run on direct current.

4.3.1. Production from Gaseous and/orGasified Hydrocarbons (Huls Arc Process)

The plant for the Huls arc process includes thearc furnace section itself (Fig. 17 A), which isoperated at a pressure of 1.2 bar, and a low andhigh pressure purification system.

Arc Furnace. A cathode, a vortex chamber,and an anode make up the arc furnace. Cath-ode and anode are water-jacketed tubes of car-bon steel 0.8 m and 1.5 m long, respectively, andwith inner diameters of 150 and100 mm, respec-tively. The arc burns between cathode and anodewith a length of about 1.2 m and with a currentof 1200 A. The cathode is connected to the high-voltage side of the rectifier (7.1 kV) and electri-cally isolated from the other parts of the furnace.Between cathode and anode is the vortex cham-ber. The gas is injected into it tangentially at aspecific velocity to stabilize the arc by creatinga vortex. The arc burns in the dead zone, and thestriking points of the arc on the electrodes areforced into a rapid rotation so that they only burnfor fractions of amillisecond at one point, whichgives the electrodes a lifetimeup to 1000 h.Tem-peratures reach 20000 ◦C in the center of the arc.Because of the tangential flow of the gas, the arcis surrounded by a sharply decreasing coaxialtemperature field, and the temperatures at thewall of the electrode are only 600 ◦C. Thermallosses are therefore limited to less than 10% ofthe electrical power input of 8.5 MW.

The residence time of the gas in the arc fur-nace is a few milliseconds. In this interval, thehydrocarbons are cracked, mainly into acety-lene, ethylene, hydrogen, and soot. At the endof the arc furnace, the gases are still at a tem-perature of about 1800 ◦C. The high heat con-tent of this gas can be exploited for additionalethylene production by means of a prequenchwith liquid hydrocarbons. This lowers the tem-perature to about 1200 ◦C. Because acetylene

22 Acetylene

Figure 17. Huls electric-arc furnaces for gaseous, liquid, and solid feedA) One-step process; B) Two-step process

rapidly decomposes into soot and hydrogen atthese temperatures, the gases must be quenchedimmediately with water to about 200 ◦C, i.e., aquench rate of 106 ◦C/s must be achieved.

The specific energy requirement (SER) andthe acetylene yield depend on the geometry anddimensions of the furnace and electrodes, thevelocity distribution of the gas, and the kind ofhydrocarbon to be cracked. Once the furnacehas been designed, only the hydrocarbons canbe varied.

Process Without Prequench. Figure 18shows acetylene and ethylene yields and thespecific energy requirement (SER) of varioussaturated hydrocarbons under constant condi-tions without prequench. Methane shows thehighest SER and acetylene yield, but the low-est ethylene yield. As the chain length is in-creased, both acetylene yield and SER decline,corresponding to the declining heat of acetyleneformation from the various hydrocarbons.

Normally, pure hydrocarbons are not avail-able. The results obtained from mixtures of hy-drocarbons can be expressed as a function of the

carbon number, which is the number of moles ofcarbon atoms bound in hydrocarbons per moleof the gaseousmixture. Figure 19 shows specificamounts of acetylene, ethylene, and hydrogenformed and of hydrocarbon consumed as a func-tion of carbon number. This function enables theHuls process to be optimized within certain lim-its, for example, for hydrogen output in relationto acetylene production.

Figure 18. Acetylene yield, ethylene yield, and energyconsumption for various hydrocarbons in the Huls arcprocess

Acetylene 23

Figure 19. Specific values for acetylene and hydrogenformation

Process with Prequench. Cracking in theprequench section is essentially an ultraseveresteam cracking process. The kind and amount ofhydrocarbons used for the prequench can be var-ied. Figure 20 shows the specific product yield

for different prequench rates for feeding meth-ane to the arc furnace and propane to the pre-quench. Acetylene and hydrogen yields are un-affected, whereas ethylene shows a slight max-imum and declines when the temperature is notsufficient at a given residence time. Propeneshows a steady increase, and the C3 : C2 ratiois below 0.25. The relative ethylene yield fromvarious hydrocarbons is as follows: ethane 100,propane 75, n-butane 72, isobutane 24, 1-butene53.

Figure 20. Specific product yield for different prequenchrates

Oil Quench. Because the gas temperature ofthe furnace gas after prequench is on the order of

Figure 21. Process with oil quench systema) Heat recovery; b) Arc furnace; c) Oil recovery; d) Separation of medium-boiling compounds; e) Separation of low-boilingcompounds; f) Oil regeneration

24 Acetylene

1200 ◦C, an oil quench system has been devel-oped to regain about 80% of the sensible heatcontent of the furnace gas as steam by heat ex-change. The soot–oil mixture formed can be up-graded to a sulfur- and ash-free high-grade pe-troleum coke. Figure 21 shows the Huls systemwith oil quench.

The Purification System. The process ofpurification depends on the type of the quenchsystem. In the case of water quenching, 80%of the carbon black is removed by cyclones asdry carbon black, the remaining 20% as soot inwater-operated spray towers. In a combined oil–water scrubbing system, aromatic compoundsare removed and benzene, toluene, and xylene(BTX) are recovered in a distillation process.

Figure 22 shows the principle separation andpurification steps for the furnace gas. The gasleaves the first three purification sections witha carbon black content of 3 mg/m3 and is com-pressed by four-stage reciprocating compressorsto 19 bar (1.9 MPa). The gas is washed in tow-ers with water in a countercurrent flow. At thebottom of the tower, the water is saturated withacetylene, whereas the overhead gas containsless than 0.05 vol% acetylene. The acetylene–water solution is decompressed in four stages.Gas from the first decompression stage returnsto the compressor to improve selectivity. Thelast two stages operate at 0.2 and 0.05 bar (20and 5 kPa). The gas still contains about 10 vol%of higher acetylenes, which are removed by acryogenic process. The higher acetylenes are liq-uefied, diluted with flux oil, stripped, and re-turned to the arc furnace together with spenthydrocarbon. A more selective solvent such asN-methylpyrrolidone or dimethylformamide ispreferred to the water wash.

Linde and Huls have designed an appropriatepurification system. Hydrogen and ethylene areseparated bywell-known technology, such as thecryogenic process or pressure-swing adsorption(see → Adsorption).

Process Data. Huls operates its plant with amixture of natural gas, refinery gas, and lique-fied petroleum gas. The carbon number varies

between one and two. Table 8 shows a typicalanalysis of feed gas and cracked gas.

Table 8. Typical analysis of feed and cracked gas

Feed gas, vol% Cracked gas, vol%C2H2 0.4 15.5C3H4 1.4 0.4C4H2 1.2 0.3C4H4 1.7 0.4C2H4 0.8 6.9C3H6 3.6 1.0Allene 0.4 0.2C4H8 1.0 0.2C4H6 0.9 0.2C5H6 0.6 0.2C6H6 0.5 0.5CH4 64.6 13.8C2H6 7.5 0.4C3H8 3.6 0.3C4H10 4.6 1.0C5H12 0.5 0.1H2 4.5 57.6CO 0.5 0.6O2 0.1 0.0N2 1.6 0.4

The Huls plant has 19 arc furnaces, the num-ber operated depending on the electricity supply.The arc furnaces can be started up and shut downimmediately. Two large gas holders provide astorage volume of 350000 m3 so that the purifi-cation section operates on permanent load andthere is a dependable supply of products, even ifthe arc furnace section is operated at higher orlower load.

The plant has an annual capacity of 120000t acetylene, 50000 t ethylene, 400× 106 m3

(STP) hydrogen, 54000 t carbon black and soot,and 9600 t aromatic compounds. The energyconsumption is 1.5× 106 MW h/a.

Specific data for consumption of hydrocar-bons and energy and the production of byprod-ucts per ton of acetylene produced are as follows:

Hydrocarbons to the arc furnace 1.8 tHydrocarbons for prequench 0.7 tEnergy for the arc furnace 9800 kW hEnergy for gas purification 2500 kW hEthylene 0.42 tHydrogen 3300 m3 (STP)Carbon black and soot 0.45 tAromatics 0.08 tResidue 0.12 tHeating gas 0.12 t

Acetylene 25

Figure 22. Principal separation and purification steps for the furnace gas of the Huls arc process

4.3.2. Production from LiquidHydrocarbons (Plasma Arc Process)

Two different plasma furnaces, each with theappropriate reactor for the cracking of liquidhydrocarbons, were developed by Hoechst andChemische Werke Huls in close cooperation.Both units were tested on an industrial scale ata power level of 8 – 10 MW [55]. However, nei-ther process has actually come into use for acety-lene production on account of the economics.

The scheme of the plasma generator used byHuls is shown in Figure 17 B. The unit consistsof three parts: the arc furnace, the reactor, and thequench system.The arc burns over a length of 1.6m at 7 kV d.c. and 1.2 kA, resulting in a powerinput of 8.5 MW. It is stabilized by hydrogeninjected tangentially through the vortex cham-ber. The thermal efficiency of the furnace is ca.88%of the electrical power input. The hydrogenplasma jet passing through the anode nozzle hasan energy density of 3.5 kW h/m3(STP), corre-sponding to an average temperature of 3500 K.The liquid hydrocarbons (e.g., crude oil) to becracked are injected into the cylindrical reactorto achievegoodmixingwith the plasma jet and toavoid the formation of carbonaceous deposits onthe wall. Within several milliseconds the hydro-

carbons are heated and cracked to acetylene, eth-ylene, hydrogen, soot, and other byproducts be-fore the mixture is quenched with oil to 300 ◦C.The acetylene ratio can be adjusted by varyingthe residence time. By operation of an oil quenchwith the high-boiling residue of the crude oil,80% of the sensible heat content of the crackedgas can be recovered as steam. The soot is takenupby the quenchoil and is removed from the sys-tem as an oil–soot dispersion having 20% sootconcentration. The unconverted vaporized frac-tions of the oil are condensed in oil scrubbers at alower temperature, simultaneously cleaning thegas of the aromatic components and fine soot.These oil fractions are recycled to the reactorand the quench system, respectively.

Tests were carried out with a variety of hy-drocarbons from propane to naphtha, but mainlywith crude oil and residue oils. The cracking re-sults depend on the chemical nature of the feed.Consumptionfigures andyields for various feed-stocks are given in [55]. For high-boiling petro-leum fractions, the acetylene and ethylene yieldsincrease with the content of low-boiling com-ponents in the feed (see Fig. 23). Consumptionand byproduct yields per ton of acetylene for aLibyan crude oil are summarized below:

26 Acetylene

ConsumptionCrude oil consumed 3.5 tPower (d.c.) 10500 kW h

ByproductsEthylene 0.46 tHydrogen (99.9%) 1100 m3 (STP)Fuel gas 0.74 tSoot – oil mixture (20% soot) 1.2 t

Hoechst used a high-intensity three-phasea.c. arc furnace at 1.4 kV and 4.2 kA, giving apower input of 10 MW [55]. The thermal effi-ciency was 90%. Because of the high amper-age the graphite electrodes had to be replen-ished continually. The generator was lined withgraphite. Different reactor designs for ethylene:acetylene ratios of 0.5 and 1.0 were developedby varying the mixing intensity of the hydrogenplasma jet with the liquid hydrocarbon. The testswere carried outwith naphtha feed (see Table 9).The cracked gas was quenched with residue oil,in a manner similar to that described in the Hulsprocess.

Figure 23. Acetylene and ethylene yields as a function ofthe low-boiling components

The acetylene concentration in the Huls pro-cess and the Hoechst process was ca. 14 vol%so that in principle the same acetylene separa-tion process can be used as described above forthe arc process.

4.3.3. Production from Coal (Arc CoalProcess)

Numerous laboratory tests for the conversionof coal to acetylene using the arc or plasmaprocesses have been carried out since the early

1960s [56]. The results can be summarized asfollows:

– Acetylene yields up to 30% can be obtained.– Because of the rapid heating of the coal inthe plasma jet, a higher total gas yield can beachieved than is indicated by the volatiles ofthe coal measured under standard conditions.

– Hydrogen (instead of argon) plasma gas con-siderably increases the acetylene yield.

The AVCO arc furnace (Fig. 24) consists of awater-cooled tungsten-tip cathode and a water-cooled anode [57]. The arc is stabilized by amagnetic field surrounding the anode, forcingthe anode striking point of the arc to rotaterapidly and so avoiding burnthrough. The driedand finely ground coal is injected by means ofa hydrogen gas flow around the cathode. Addi-tional gas without coal is introduced around thecathode and at the anode as a sheath. On pass-ing the arc zone the coal particles are heatedup rapidly. The volatiles are released and arecracked to acetylene and byproducts, leaving aresidue of fine coke particles covered with soot.After a residence time of some milliseconds thegas–coke mixture is quenched rapidly with wa-ter or gases. The use of a prequench system simi-lar to that of theHuls arc process was also tested.The system pressure can be varied between 0.2and 1.0 bar (20 and 100 kPa).

Figure 24. Principal scheme of the AVCO plasma furnacefor the pyrolysis of coal

Acetylene 27

Table 9. Consumption and yield per ton of acetylene for the Hoechst arc process and naphtha feed for different reactor designs

Low ethylene yield High ethylene yieldConsumptionNaphtha 1.92 t 2.50 tQuench oil 0.53 t 0.63 tEnergy(2-phase a.c.) 9300 kW h 10500 kW h

ByproductsEthylene 0.5 t 0.95 tHydrogen 1450 m3 (STP) 1500 m3 (STP)Soot–oilmixture(20% soot) 0.75 t 1.00 t

Huls’ pilot plant uses the same plasma fur-nace as for the crude oil cracking, but with 500kW of power [58]. The dried and ground coalis injected into the plasma jet, and the coal iscracked to acetylene and byproducts in the re-actor. The reactor effluent can be prequenchedwith hydrocarbons for ethylene production or isdirectly quenched with water or oil. Char andhigher boiling components are separated by cy-clones and scrubbers, respectively. The problemin the reactor design is to achieve thorough andrapid mixing of the coal with the plasma jet andto avoid forming carbonaceous deposits on thewall. Smaller amounts of deposits can be re-moved by periodic wash cycles with water. Op-eration times of 2.5 h by AVCO and 5 h by Hulshave been reported.

Figure 25. Effect of the energy density of the plasma on thecracking of coal (AVCO)

Experiments published by Huls and AVCOshow that at the optimal residence time the en-

ergy density of the plasma jet, the specific power,and the pressure all greatly affect the acetyleneyield (Fig. 25 and Fig. 26). Other parameters af-fecting the yield are the amounts of volatiles inthe coal and the particle size. The lowest figuresfor the specific energy consumption publishedby AVCO are of the order of 27 – 37% based onwater-free coal.

Figure 26.Acetylene yield and specific energy requirementas a function of pressure (Huls)

In addition to acetylene, the exit gas containsconsiderable amounts of CO, depending on theoxygen content of the coal.Because nitrogen andsulfur are present in the coal, other byproductsare HCN, CS2, COS, and mercaptans. The gasseparation system is therefore designed accord-ingly [59]. Depending on the hydrogen contentof the coal, the process is either self-sufficient inhydrogen or has a slight surplus. The total gasyield of the coal based on a volatile content inthe coal of 33% is up to 50%. Thus 50% of thecoal remains as char. Tests with a view to usingthis char in the rubber industry have been unsuc-

28 Acetylene

cessful so far. Thus the char can be used only forgasification or as a fuel.

In all the processes under development, theproduction of ethylene from coal requires sev-eral process steps (Fig. 27), resulting in a highcapital demand for a production plant. In con-trast, the acetylene production from coal arcpyrolysis is straightforward, leading to lower in-vestment costs. Demonstration units on a higherpower level are therefore scheduled by bothAVCO and Huls.

4.3.4. Production from Calcium Carbide

At present, the generation of acetylene from cal-ciumcarbide (→ CalciumCarbide) is of primaryimportance forwelding and for the production ofcarbon for batteries. The particular raw-materialsituation and the use of special processes are twocommon reasons for continuing to use acetylenegenerated from carbide in the chemical industry.

The reaction of calcium carbide and water toform acetylene and calcium hydroxide is highlyexothermic:

CaC2+2 H2O→C2H2+Ca(OH)2 ∆ H = −129 kJ/mol

The acetylene generator used for commercialproduction must therefore be designed to allowdissipation of the heat of reaction. In the event ofinadequate heat dissipation, for example, whengasification proceeds with insufficient water,the carbide may become red-hot. Under certaincircumstances (including increasing pressure),this may cause the thermodynamically unstableacetylene to decompose into carbon and hydro-gen. (For safety precautions see Chap. 5.) Car-bide for the production of acetylene is used inthe following grain sizes (mm): 2 – 4, 4 – 7, 7– 15, 15 – 25, 25 – 50, 50 – 80. This classifica-tion is virtually identical in most countries: DIN53922 (Germany); BS 642:1965 (United King-dom); JIS K 1901 – 1978 (Japan); Federal Spec-ification 0−6–101 b/GEN CHG NOT 3 (UnitedStates). In addition, grain size 0 – 3 is used forthe dry generation of acetylene.

Pure calcium carbide has a yield numberof 372.66. This means that the gasification of1 kg of carbide yields 372.66 L acetylene at15 ◦C and 1013 mbar (101.3 kPa). Commer-cially available carbide has a yield number of260 – 300.

A distinction is made between two groupsof acetylene generators (with continuous ratesof production greater than 10 m3 acetylene perhour): the wet type and the dry type.

In wet generators, the acetylene is convertedwith a large water excess. In most cases, a limeslurry containing 10 – 20 wt% calcium hydrox-ide is obtained. The heat of reaction increasesthe temperature of the generator water and is re-moved from the reactor with the lime slurry.

In dry generators, the water mixed with thecarbide is just sufficient for chemical reactionand for dissipating the heat of reaction. The cal-cium hydroxide is obtained in the form of a dry,easily pourable powder having a residual mois-ture content of 1 – 6%. The heat of reaction isdissipated by evaporation of part of the generatorwater.

Generators are classified according to theirworking pressure as either low or medium pres-sure. This classification is governed by the reg-ulations concerning acetylene plants and cal-cium carbide storage facilities (Acetylenverord-nung) [60] issued by Deutscher Acetylenauss-chuß (German acetylene committee) and the as-sociated technical rules for acetylene plants andcalcium carbide storage facilities TRAC (Tech-nische Regeln fur Acetylenanlagen und Calci-umcarbidlager) [61]. The Acetylenverordnungand TRAC constitute a comprehensive set ofrules for handling acetylene. Recommendationsin other countries (e.g., United States) deal onlywith some aspects, such as safety precautions[62].

Low-pressure generators are designed for amaximum allowable working pressure of 0.2bar. They must be rated for an internal pressureof at least 1 bar. Lower pressure ratings are pos-sible if proof is given in each particular case thatthe generator can withstand the expected stress(maximum working pressure, water filling, agi-tator, etc.; TRAC 201).

Medium-pressure generators have a maxi-mum allowable working pressure of 1.5 bar.Theymust be rated for an internal pressure of 24bar.A design pressure of 5 bar sufficeswheneverthe generators are equipped with rupture disksof a defined size and specified response pressure(3 – 4.5 bar, TRAC 201).

Acetylene 29

Figure 27. Alternative routes from coal to ethylene and acetylene

4.3.4.1. Wet Generators

Wet generators are used primarily for the pro-duction of small amounts of acetylene, e.g., forwelding purposes. Wet generators work by oneof three different principles [63]:

1) The carbide-to-water principle, where thecarbide is mixed with a large excess of waterat a rate corresponding to the gas withdrawalrate. Most generators work by this principle.

2) The water-to-carbide principle (drawer typegenerators), where water is added at a con-trolled rate to the carbide, which is held in areplaceable container (drawer).

3) The contact principle (basket generators),where the carbide, which is held in a bas-ket, is immersed into the generatorwater. Thistype is designed so that the water drifts offthe basket as a result of the gas pressure atlow gas withdrawal rates and, conversely, re-turns to the basket when gas withdrawal ratesincrease.

Medium-Pressure Generators. The MesserGriesheimMF1009 is a typical carbide-to-watergenerator (Fig. 28).

The carbide skip (a) is filled with carbide ofthe 4 – 7 grain size. The skip is connected bygas-tight gates to the hopper (b) and is purged ofair by nitrogen or acetylene. The carbide dropsinto the hopper (b) and is fed continuously bythe feeding system (c) to the gasification cham-ber (d). The gasification chamber contains waterup to a level defined by the generator capacityand is equipped with an agitator (e) for whirlingthe lime slurry. The heat evolved in gasificationheats the generator water. For continuous op-eration the water temperature must not exceed90 ◦C; therefore, fresh water is admitted contin-uously to the gasification chamber. If the definedwater level is exceeded, the slurry valve (f), con-trolled by a float, opens, allowing the excess wa-ter and the lime slurry to be discharged from thegenerator.

The acetylene generated collects above thewater and is withdrawn. The feeding system (c)is controlled by the gas pressure, i.e., the rate at

30 Acetylene

which carbide drops into the gasification cham-ber varies directly with the rate of gas with-drawal.

Figure 28.Medium-pressure wet generatora) Carbide skip; b) Hopper; c) Feeding system;d) Gasification chamber; e) Agitator; f) Slurry valve,g) Safety device

The carbide stock in the hopper (b) is suffi-cient for about one hour, but the skip (a) can berefilled with carbide and replaced on the hop-per so that continuous operation is possible. Thewet generator described has a continuous hourlyoutput of 75 m3 of acetylene. The skip holds1000 kg of carbide.

Low-Pressure Generators. The workingprinciple of the low-pressure carbide-to-watergenerators is very similar to that of the medium-pressure carbide-to-water generator describedabove. In most cases, a downstream acetyleneholder, normally of the floating gas bell type,is provided. In contrast to the medium-pressuregenerator, in which the carbide feed rate is con-trolled by the acetylene gas pressure, the feedrate in the low-pressure generator is controlledby the position of the bell in the acetylene holder,i.e., by the gas quantity.

Products. The acetylene generated in the wetgenerator can be used for welding, often with-out further purification. In certain cases, coke-or gravel-filled purifiers or a wet scrubber areconnected downstream from the generator forseparating solid or liquid particles. Before it is

fed to a synthesis unit, the acetylene must bepurified chemically (see Section 4.3.4.3).

The lime slurry formed is fed into pits. Here,the calcium hydroxide settles in the form of alime dough containing 35 – 75 wt% water (wetlime, carbide lime dough). This dough is usedas carbide lime.

4.3.4.2. Dry Generators

Dry generators are mainly used for the produc-tion of large quantities of acetylene for chemicalsynthesis.

Compared to the wet generator, the primaryadvantage of the dry generator is that the drycalcium hydroxide formed as a byproduct canbe used in other processes more easily, morecheaply, and in a more diversified way than thelime slurry obtained in the wet generator [64].Moreover, lime recycling into the carbide pro-duction process is only possible with dry cal-cium hydroxide.

A high gasification rate and the elimination ofthe risk of overheating were originally the mostimportant criteria for the design of dry genera-tors. Early designs of dry generators worked bycontinuous renewal of the reaction surfaces ofcoarse carbide with the aid of rotating drums,blades, vibrating screens, and similar equip-ment.

Typical examples are the early generator ofShawinigan Chemicals [65] and the Piesteritzdry generator [66].

Although a number of factors affect the gasi-fication rate of carbide, e.g., density, porosity,and crystalline structure, above all it is the spe-cific surface that affects the carbide gasificationrate the most. Hence, dry generators work withfinely ground carbide (0 – 3 mm), which gasi-fies in a fraction of the time needed for coarsecarbide. The result is a high space–time yield.

A typical application of this principle is thelarge-scale Knapsack dry generator, which wasdeveloped at the Knapsack works of Hoechst.This type of generator is used worldwide and isdescribed in more detail below (Fig. 29).

Carbide of the grain size 0 – 3 falls from thechain conveyor (a) into the subdivided feed bin(b). The chain conveyor is loaded with materialfrom the carbide bin. Because of the recirculat-ing stream of carbide the feed bin (b) is full at

Acetylene 31

all times. The carbide layer in the feed bin (b)acts as a gas seal between the generator and thecarbide conveying system.

Figure 29. Knapsack dry generatora) Chain conveyor; b) Feed bin; c) Star wheel; d) Carbidefeed screw; e) Generator; f) Lime lockhopper; g) Lime dis-charge screw; h) Lime scraper; i) First scrubbing tower;k) Second scrubbing tower; l) Dip seal

The carbide is fed to the generator (e) via thestar wheel (c) and the carbide feed screw (d).The largest generator of this kind built to datehas a diameter of 3.5 m and an overall height ofapproximately 8.0 m. The generator has up to 13circular trays. These are so designed as to leavealternate annular gaps on the shell side and at thecentral agitator shaft. The agitator shaft movesstirrer paddles across the trays.

The carbide first reaches the uppermost traywhere the generator water is also admitted. Thereaction mixture consisting of carbide, water,and calcium hydroxide is pushed by the stir-rer paddles towards the outer edge, drops onto the second tray, returns towards the center,etc. When it reaches the last tray, the carbidehas been fully gasified. The calcium hydroxide,which still contains up to 6% water, drops intothe lime lockhopper (f). Here, a lime layer twometers deep serves as the gas closure between

generator and lime conveying system. The limeis withdrawn continuously.

The gas leaving the generator through thelime scraper consists of 25% acetylene and ca.75% water vapor. The water vapor is the re-sult of dissipating the major portion of the re-action heat. Depending on the generator load,up to several hundred kilograms of lime hydratedust are carried along with the acetylene. Thelime scraper (h) retains the major portion of thisdust and returns it to the generator. The remain-der is sent together with the gas into the firstscrubbing tower (i). Here, lime slurry is sprayedinto the hot acetylene gas (ca. 90 ◦C) to scrubout the lime dust; part of the water vapor con-denses because of the simultaneous cooling. Inthe second scrubbing tower (k), the acetyleneis sprayed with atomized water to cool the gasbelow 40 ◦C; additional water vapor condenseshere. Any ammonia still present in the gas is alsoremoved.

The acetylene leaves the generator via the dipseal (l). It still contains certain impurities in theform of sulfur and phosphorus compounds.

The Knapsack dry generator is suitable for acarbide throughput of 15 t/h, corresponding toan acetylene quantity of 3750 m3/h. During thisprocess, about 17.5 t of calcium hydroxide perhour are obtained. The pressure in this low-pres-sure generator amounts to approximately 1.15bar (115 kPa).

The dry generator of Shawinigan Chemicals,Montreal [67], also processes finely ground car-bide and has a variety of applications. It consistsof several superimposed troughs. Carbide andwater are fed into the uppermost trough. The re-acting mixture, which is constantly kept in mo-tion by blades, flows over a weir onto the troughbelow, etc. At the uppermost trough, water isadmitted at such a rate that carbide-free calciumhydroxide can be withdrawn at the lowermosttrough. The generated acetylene is purified intwo scrubbing towers and cooled.

The calciumhydroxide formed (carbide lime)has a wide range of applications, e.g., in thebuilding industry (for preparing mortar, cement,etc.), in the chemical industry (for neutralizationand for recycling to the carbide furnace), in agri-culture (as fertilizer), and for water purificationand waste water treatment [64].

32 Acetylene

4.3.4.3. Acetylene Purification

During the gasification of carbide with water,gaseous compounds become mixed with theacetylene, and these must be removed becausethey have a harmful effect on the downstreamchemical synthesis processes. The impurities aremainly sulfur and phosphorus compounds. Theycan be removed by one of the following purifi-cation processes.

In the first process, dilute chlorinated wateris used as the oxidizing agent. The chlorine con-centration of the water is limited to 1.5 g/L toprevent the formation of unstable chlorine com-pounds, which present an explosion hazard. Thechlorine scrubbing step is followed by a caus-tic soda scrubber to remove the hydrogen chlo-ride formed during the oxidation process. Thedisadvantage of this purification process is thatconsiderable quantities of scrubbing water areproduced.

The second process uses 98% sulfuric acidas the oxidizing agent [68]. Because very smallquantities of sulfuric acid are admitted, it is diffi-cult to dissipate the heats of absorption and reac-tion. Heating the acetylene results in increasedformation and settling of polymerization prod-ucts in the purification stage. For this reason thegas requires additional cooling in the event thatthe acetylene contains appreciable quantities ofimpurities. Moreover, it is recommended that asecond scrubbing tower be kept on standby ifa high onstream factor is desired (e.g., 91%

∧=8000 h/a operating time).

The sulfuric acid scrubber is followed by acaustic soda scrubber, in which the sulfur diox-ide formed during oxidation is removed.

The main advantage of this purificationmethod is that virtually no waste water is ob-tained. The small amount of polluted, highlyconcentrated sulfuric acid can be used, for ex-ample, in fertilizer plants.

These two purification processes yield thefollowing acetylene purities (by volume):

Acetylene > 99.5%Sulfur, as H2S < 10 ppmPhosphorus, as H3P < 10 ppm

As a result of the extremely good sorptionproperties of the concentrated sulfuric acid, verypure acetylene can be expected.

4.4. Other Cracking Processes

4.4.1. Thermal Cracking By Heat Carriers

Well-known processes using heat carriers, suchas the Wulff and Hoechst high-temperaturepyrolysis (HTP) processes, are no longer usedbecause they require refined petrochemical feed-stocks such as naphtha and liquid petroleum gas.TheWulff process uses refractorymaterial as theheat carrier, whereas the Hoechst HTP processuses hot combustion gases.

Newer processes, which are able to convertcrude and heavy distillates into olefins and con-siderable amounts of acetylene, are still in thepilot-plant stage. These processes include theadvanced cracking reactor process developedby Kureha, Chiyoda, and Union Carbide, usinghigh-temperature superheated steam, andDow’spartial combustion cracking process, using hotcombustion gases produced from oxygen andfuel oil as the heat carrier.

Figure 30. High-temperature pyrolysis (HTP) process

Acetylene 33

Wulff Process [69 70, p. 58]. This process isbased on indirect heat transfer, an approach fun-damentally different from the partial-oxidationand electric-arc processes. The hydrocarbonfeed is cracked in refractory ovens previouslyheated by combustion gas. After cracking, theproducts are quenched outside the reactor. Sootformation is a serious problem because the feedcannot be heated as rapidly as in the partial-oxidation or arc processes. This problem can bediminished by using a feed with a higher hydro-gen:carbon ratio. However, methane is not suit-able because of the high temperature and highheat of reaction required, resulting in a low con-version rate. Thus the best feed for the Wulffprocess is ethane or propane.Hoechst High-Temperature Pyrolysis

(HTP) Process (Fig. 30) [1 70, p. 55, 71]. Thisis a two-stage process. In the first stage, heatis produced in the burner by the combustion ofresidual cracked gas from the acetylene recoverysection (CO, H2, CH4) with oxygen. Immedi-ately after combustion, the temperature is about2700 ◦C; this is moderated to about 2300 ◦Cby the injection of steam before the reactoris entered. In the second stage, the feedstocknaphtha is injected, and the adiabatic crack-ing reaction takes place. A final temperature ofabout 1300 ◦C is reached: This determines thecracked gas composition. By varying the feed

rate of naphtha the acetylene–ethylene ratio canbe altered from 30 : 70 to 70 : 30. However,thermodynamic and economic considerationsshow that the optimum ratio is 40 : 60.

After a reaction time of a few millisecondsthe cracked gas is quenched to approximately250 ◦C by the injection of cracked oil from theprocess. The oil absorbs heat from the crackedgas and is passed through waste heat boilers,raising the steam pressure. No soot is formed inthis process, even when crude oil is used as feedbecause of the high steam content of the carriergas.

After the oil crisis of 1973 the process be-came uneconomical in spite of its high thermalefficiency, and in 1976 it was shut down after15 years of operation. However, one unit is stillrunning in Czechoslovakia.

Kureha, Chiyoda, Union Carbide Ad-vanced Cracking Reactor (ACR) Process[72]. To avoid dependence on oil refineries orgas processors for the supply of feedstocks, pro-cesses for directly cracking crude oil have beendeveloped by various companies for the produc-tion of olefins. Some of these processes operateat reaction temperatures intermediate betweenthose of the usual crack processes for olefins andthose for acetylene. The ACR process (Fig. 31)uses amulti-port burner to produce a heat carrier

Figure 31. Advanced cracking reactor process (ACR)a) Crude distillation column; b) Burner; c) Advanced cracking reactor; d) Ozaki quench cooler; e) Oil gasoline fractionator;f) Compressor; g) Acid gas removal column; h) Gas separator

34 Acetylene

gas of 2000 ◦C by the combustion of H2 – CH4mixtures with oxygen in the presence of steampreheated to 800 ◦C.

The oil to be cracked is introduced throughnozzles into the stream of carrier gas and passesinto an advanced cracking reactor, where thereaction takes place adiabatically at 5 bar (0.5MPa). The initial temperature is 1600 ◦C; the fi-nal temperature at the exit of the reactor is 700– 900 ◦C after a residence time of 10 – 30 ms.The cracked gas is quenched by oil in an Ozakiquench cooler (Fig. 32), where steam produc-tion up to 120 bar (12 MPa) is possible. Thisparticular boiler designwas developed for a highheat transfer rate without coke formation on theexchanger surfaces. Yields reported for Arabianlight crude oil are 11.2 wt%hydrogen andmeth-ane, 40.7 wt% olefins, and 4.2 wt% acetylene.The acetylene yield is about ten times higherthan in usual olefin processes.

Figure 32. Ozaki quench cooler

Dow Partial Combustion Cracking (PCC)Process [72]. The basic idea of this process is toreduce coking and soot formation considerablywhen heavy feeds are cracked and when hydro-gen is present in the reaction mixture. The PCCprocess (Fig. 33), which accepts crude oil andheavy residue as feedstock, attains a high partialpressure of hydrogen in the reaction zone by re-

cycling the quench oil (produced in the process)to the burner where it is partially oxidized toyield synthesis gas. Thus there is no need to finda use for the quench oil as in the case of the ACRprocess. Starting from residual oil boiling above343 ◦C, yields are given as 12.4 wt% methane,about 38 wt% alkenes, and 2.5 wt% acetylene.This is seven to eight times more acetylene thanthat obtained from a steam cracker, but less thanthe acetylene yield of the ACR process, becauseof a residence time in the reaction zone which isthree to ten times longer.

Figure 33. Dow partial combustion cracking process(PCC)a) Reactor; b) Quench boiler; c) Quench column;d) Stripper; e) Decanter

4.4.2. Acetylene as a Byproduct of SteamCracking

In a steam cracker saturated hydrocarbons areconverted to olefinic products such as ethyleneand propene. Besides these desired components,acetylene and many other products are formedin the cracking process (Fig. 36). The concentra-tion of acetylene depends on the type of feed, theresidence time, and temperature (cracking sever-ity: expressed as conversion or propene/ethyl-ene ratio P/E). Typical data are given in Ta-ble 10. The acetylene concentration in the off-gas from the furnace varies between 0.25 and1.35 wt%. In certain cases of propane and bu-tane cracking the the raw gas can contain up to2.1 wt% acetylene with corresponding very lowamounts of propyne and propadiene. The corre-sponding content of acetylene in the C2 frac-

Acetylene 35

Table 10. Yields of unsaturated components (wt%) in raw gas from steam cracking

Feedstock Cracking severity Acetylene Propyne Propadiene

Ethane 65% convers. 0.4 – 0.50 0.04 0.02LPG 90% convers. 0.65 – 1.35 0.63 0.35Full-range naphtha P/E: 0.4 0.9 – 1.05 0.81 0.54Full-range naphtha P/E: 0.53 0.5 – 0.70 0.68 0.50Full-range naphtha P/E: 0.65 0.25 – 0.42 0.46 0.38Atmospheric gas oil P/E: 0.55 0.40 0.34 0.29Hydrocracker residue P/E: 0.55 0.50 0.36 0.31

Figure 34. Acetylene recovery process [77]

tion is about 0.4 – 2.5 wt%. An ethylene plantproducing 400 000 t/a ethylene produces 4500 –11 000 t/a acetylene. The acetylene is removedby catalytic selective hydrogenation or solventextraction. Today the dominating acetylene re-moval process is selective hydrogenation.

Acetylene Hydrogenation. Most ethyleneplants are equipped with a hydrogenation unit.Acetylene is converted selectively to ethylene ona Pd-doped catalyst. Whereas in the past mainlyNi catalysts were used, today Pd catalysts aredopedwithothermetals such as silver to improveselectivity [73]. Typical process conditions aretemperatures of about 40 – 120 ◦C, pressures of15 – 40 bar and space velocities of 1000 – 120000 kg L−1 h−1. Depending on the type of feedand the plant, there are several process options:

1) Front-end hydrogenation (C2− stream con-taining H2, CO, methane, C2H2, C2H4 andC2H6)

2) Raw-gas hydrogenation (hydrogenation be-fore C2/C3 separation, stream containing H2,CO and all hydrocarbons of the raw gas after

cracked gas compression).This variant of front-end hydrogenation isonly used in gas crackers where the contentof C3+ hydrocarbons is comparable low.

3) Tail-end hydrogenation (pure C2 stream con-taining C2H2, C2H4, and C2H6; separate ad-dition of an equimolar amount of hydrogen isnecessary)

Typical specifications for acetylene content inthe ethylene product are < 2 ppm with a ten-dency to further reduction to < 1 ppm.

General aspects of the process and the cata-lyst requirements are reviewed in [73 – 76] (seealso → Ethylene, Chap. 5.3.2.2).

Acetylene Recovery. Acetylene is extractedfrom the C2 fraction of the steam cracker. Thesolvent must fulfil the following criteria:

– Melting point lower than the dew point of thefeed gas

– High solubility of acetylene at a temperaturenear the dew point of the C2 fraction

– High acetylene selectivity ([9, 11])– High chemical and thermal stability

36 Acetylene

– No foaming tendency due to traces of hydro-carbons

– Low toxicity– Low vapor pressure at the operating temper-ature

After testing many solvents, including DMF,NMP, and acetone, the most suitable solvent forsuch a process proved to be DMF. The solubil-ity of acetylene as a function of temperature isshown in Figure 5.

The process for the recovery of high-purityacetylene is shown in Figure 34 [77]. Thegaseous C2 mixture, consisting of ethylene,ethane, and acetylene, is fed to the acetylene ab-sorber; the gas stream is contacted with coun-terflowing lean DMF at a pressure of 0.8 – 3.0MPa. The process is suitable for the full pres-sure range prevailing in any of the known eth-ylene processes. The entire acetylene and someof the ethylene and ethane are dissolved by thesolvent. Water and carbon monoxide decreasethe solubility of acetylene. Furthermore, watercauses hydrolysis of DMF, resulting in addi-tional formation of carbon monoxide. Entrain-ment of DMF at the top of the column is avoidedby a reflux stream. The purifiedC2 fraction, con-taining < 1 ppm of acetylene, is fed to the C2splitter. The rich solvent stream is sent to theethylene stripper, which operates slightly aboveatmospheric pressure. Ethylene and ethane arestripped off and recycled to the first stage ofthe cracked gas compression. Any acetylene en-trained with the overhead gas is recovered bywashing with cold solvent at the top of the strip-per. In the acetylene stripper, pure acetylene isisolated from the top of the column. After cool-ing and heat recovery, the acetylene-free solventis recycled to the absorber and ethylene stripper.The acetylene product has a purity of > 99.8%and a DMF content of less than 50 ppm and isavailable at 10 kPa and ambient temperature.

The material balance and the utilities con-sumption of an acetylene recovery unit are listedin Tables 11 and 12. At present, more than 112000 t/a of petrochemical acetylene from twelveolefinplantsworldwide is recoveredby this tech-nology. With a big drop in the total acetylenemarket in 2001, themarket decreased on averageby 6.8% per year from 1998 to 2003, and onlya marginal growth rate is expected until 2009(max. 2%per year) [135]. For this reason during

the last fiveyears (2006) nonewacetylene recov-ery facilities have been erected. Only some ex-isting plants were expanded. In North America,for example, there are currently only two majorcompanies producing acetylene as byproduct ofethylene production, i.e., DowChemical in Taft,LA, and Petroment at Varennes in Canada [136].Figure 35 shows an industrial plantwith a designcapacity of 14 400 t/a of high-purity acetylene.

Table 12. Consumption of utilities for an acetylene recoveryprocess operating on the C2 fraction from a plant producing 400000 t/a C2H4 (Linde) [78]DMF, kg/h 1.3Heating steam, t/h 3.9Cooling water, m3/h 100Electrical energy, kW 125Refrigerant, GJ/h 6.3Quench water, GJ/h 3.1Plot area, m×m 15× 40

A material balance for ethylene plant outputsincluding acetylene extraction or hydrogenationis shown in Figure 36 [79]. The economic eval-uation shows that petrochemical acetylene re-mains attractive even if the price of ethylene isdoubled. It is economical to retrofit acetylene ab-sorption in an existing olefin plant equippedwitha catalytic hydrogenation. Isolation of acetyleneobtained as an unavoidable byproduct of ethyl-ene production is the economically most attrac-tive route to cover acetylene demand.

Asimilar process is available for propyne (seeChap. 7)

5. Safety Precautions,Transportation, and Storage

General literature is given in [10, 80, 81].

5.1. General Safety Factors and SafetyMeasures

Decomposition and Combustion. Acety-lene is thermodynamically unstable under nor-mal conditions. Decomposition into carbon andhydrogen can achieve temperatures of about3100 ◦C, but due to formation of other prod-ucts, the temperature reached adiabatically is2800 – 2900 ◦C. The decomposition can be ini-tiated by heat of reaction, by contact with a hotbody, by an electrostatic spark, by compression

Acetylene 37

Figure 35. Acetylene recovery plant (name plate capacity: 14 400 t/a of high-purity acetylene)

Table 11.Material balance (mol%) for an acetylene recovery process operating on the C2 fraction from a plant producing 400 000 t/a C2H4(Linde) [78]

Gas to absorber Purified gas Recycle gas Product C2H2

Methane trace traceAcetylene 2 1 ppm 4 99.8Ethylene 82 83.5 85.7 0.2Ethane 16 16.5 10 traceC3 trace trace 0.3 traceDMF 1 ppm trace traceTemperature, K 252 249 255 258Pressure, MPa 2 1.98 0.11 0.12Flow rate, kmol/h 2186 2126 17.5 52.5

38 Acetylene

Figure 36. Material balances of a 300 000 t/a ethylene plant equipped with either C2 hydrogenation or acetylene extraction(all rates in kg/h, the numbers in parentheses are for the solvent extraction process) [79]* Chemical grade

heating, or by a shock wave. The decompositioninduced by heating the wall of the container orpipe is very sensitive to the pressure, the sizeand shape of the container or the diameter of thepipe, the material of the container, and tracesof impurities or other components. Solid parti-cles such as rust, charcoal, alumina, and silicacan lower the ignition temperature compared toclean steel pipe.

Decomposition gives rise to different scenar-ios:

– Working range I (deflagration): a flame pro-duced by decomposition or combustion andpropagates at a velocity below the velocityof sound into the unconverted gas (pressurerises simultaneously in front of and behindthe flame front)

– Working range III (detonation): the flamepropagates at ultrasonic velocity into the un-converted gas (shock wave between low pres-sure in the unconverted gas and high pressurein the converted gas)

– Working range II (intermediate between I andIII): often the propagation velocity of a de-flagration is not constant (it increases withincreasing density, temperature, and turbu-lence), and therefore a change from deflagra-tion to detonation is observed

As consequence, design criteria for pipingand other components are proposed for the dif-

ferent working ranges and depend on the diam-eter of the pipe. Limit lines for deflagration anddetonation are given in [82] on the basis of thework of Sargent [83]. An extended Sargent di-agram is shown in Figure 37.

Figure 37. Detonability limits of acetylene [83, 88]A) Deflagration limit; B) Detonation limitDetonation limits: a) Thermal ignition in a plain pipe(a1 melting wire, 20 – 80 J; a2 detonator cup, ≈2400 J);b) Thermal ignition plus orifice; c) Ignition by chemicalreaction in a shock wave; d) Range of possible quasi-detonation depending on ignition energy of shock wave;x) and y) Limiting ignition pressure for thermal ignitionwith melting Pt wire and with detonator cup, respectively

Acetylene 39

The limits are influenced by the method ofignition (e.g., melting wire or a detonator cap).Changing the method of ignition from a meltingPtwire (Reppe) to the explodingwire ignitor (ig-nition energy ca. 70 J) used by BAM resulted inlower stability pressures for acetylene mixtures[84] and pure acetylene [85] (Table 13). A de-scription of the experimental setup can be foundin [134].

Mixtures of acetylene with methane havehigher stability pressures than those with nitro-gen or hydrogen. The phlegmatizing influence offoreign gases increases in the order H2 < N2 <CO2 < NH3, which corresponds to the heat ca-pacities of these gases [134]. For C2H2 –C2H4mixtures the stability pressure riseswith increas-ing ethylene fraction. However, a significant ef-fect can be observed only at C2H2 contents <50% [134]. Further information on the effect ofadditional gases on acetylene decomposition isgiven in [86]. The dependence of stability pres-sure on the energy of the ignition source has ledto ongoing discussion about its relevance for in-dustrial design and operations [87].

Figure 38. Decomposition pressure versus ignition energyfor unsaturated hydrocarbons

Figure 38 shows the decomposition pressureof acetylene, propyne, and propadiene as func-tion of ignition energy. This relationship is thebasis for the safe design of processes for the re-covery of acetylenic components (see Section4.4.2 and Chap. 7). Additional investigationshave been published on the dependence of de-flagration pressure on the flow in pipes [88] andthe decomposition of high-pressure acetylene inbranched piping [89]. Solid acetylene is not crit-ical with regard to decomposition, provided it isthe only material involved [7]. In liquid oxygen,

solid acetylene can readily ignite on mechanicalimpact and react violently [90]. Recommenda-tions for equipment used in gas welding and cut-ting technology, such as rubber hoses, safety de-vices, and flame arresters, are given in [91 – 93].

Combustion of Acetylene in Oxygen (Air).The reaction of acetylene and oxygen at 25 ◦Cand 1 bar to form water and CO2 generates1255.6 kJ/mol. Temperatures of around3100 ◦Ccan be reached. Figure 39 shows flame temper-atures and flame front velocities for mixtures ofoxygen with hydrocarbons [94].

Figure 39. Flame temperatures and ignition velocities ofacetylene–oxygen mixtures and mixtures of other hydro-carbons with oxygen

Acetylene allows the highest temperaturesand flame front velocities to be attained. Themaximum temperature is very sensitive to themixing ratio, which also determines whether areducing, neutral, or oxidizing flame exists (Fig.40).

Fundamental safety data for acetylene–airand acetylene–oxygen mixtures are listed in Ta-ble 14. At atmospheric pressure and 25 ◦C mix-tures of 2.4 – 93.0 vol% acetylene in oxygenare explosive; the possibility of self-decompo-sition at high acetylene must also be taken intoaccount. A large explosion at Acetylene Ser-vices Company (ASCO) in the USA in 2005was caused by accumulation of acetylene in thewater-pipe system. Most probably the contactof acetylene with the hot surface of a propaneheater caused the explosion [95]. A comparison

40 Acetylene

Table 13. Stability pressure (bar) of acetylene and acetylene mixtures for two methods of ignition and pressure increase [84, 85]

Mixture Ignition method pex/pa

Reppe BAM (BAM)

100% C2H2 1.4 0.890% C2H2/10% N2 1.8 1.0 8.690% C2H2/10% CH4 2.1 1.0 7.290% C2H2/10% H2 1.6 0.9 6.950% C2H2/50% N2 9.0 3.6 6.350% C2H2/50% CH4 14.7 12.950% C2H2/50% H2 4.7 2.5 5.5* pex/pa: ratio of maximum pressure to pressure before ignition.

of C2H2 deflagration and C2H2 –O2 explosionwith TNT explosion is given in [96].

Figure 40. Chemical composition of an oxygen–acetyleneflame at its tip versus mixing ratio

Table 14. Fundamental safety data for acetylene–air andacetylene–oxygen mixtures

Air Oxygen

Lower flammability limit, vol% 2.5 2.4Upper flammability limit, vol% 82 93Flame temperature* , K 2863 3343Flame front velocity, m/s 1.46 7.6Increase of pressure (deflagration) 11 50Detonation velocity, m/s 2300 2900* Stoichiometric mixture.

Handling of Acetylene. For pure acetylenethe prescribed safety instructions, for example,the Technische Regeln fur Acetylenanlagen undCalciumcarbidlager (Technical regulations foracetylene plants and calcium carbide depots),TRAC, [97], have to be strictly followed. The for-

mer Acetylenverordnung (Decree about acety-lene) [53] in Germany has been replaced bythemore generalBetriebssicherheitsverordnung(Decree about safety in plants) [97] as of January1, 2003. However, it is not possible to formulategeneral safety instructions for the great varietyof chemical processes with acetylene as reactioncomponent under diverse reaction conditions.

Both handling acetylene and experimentswith it necessitate critical examination ofsources of possible danger. The literature citedcan only serve as an aid to decisions on precau-tions. The safety regulations mentioned abovehave been determined in experiments with well-defined apparatus dimensions (length, diame-ter, geometry). For other dimensions they canonly serve as an indication of explosive behav-ior and should not be considered as rigid limits.The development of economical chemical pro-cesses involving acetylene at elevated pressuresor under other hazardous conditions calls fordecomposition tests for the crucial stages wheredecomposition could occur. This must be donein close cooperation with official testing insti-tutions, such as the Bundesanstalt fur Materi-alprufung (BAM) in Germany.

In general, the following rules should be ob-served in handling acetylene:

– Temperature and pressure must be selected soas to avoid liquefaction of acetylene.

– Reactions of acetylene in solvents or with liq-uid reaction components must be carried outat such acetylene concentrations that explo-sive decomposition of the acetylene in the liq-uid phase cannot occur. In many cases thiscondition is fulfilled at an acetylene loadingbelow 100 m3 (STP) per cubic meter of sol-vent. Higher loadings are only permitted ifadditional precautions are taken, such as fill-ing the volumes containing the liquid with

Acetylene 41

steel packings. The formation of a separategas phase has to be avoided.

– The technical rules (such as TRAC [97]) arevalid for pure gaseous acetylene. If an inertgas, such as nitrogen, is added to the acety-lene, higher acetylene partial pressures arepermitted.

– In the design of apparatus the partial pres-sure of acetylene should be selected so that theminimum distance to the decomposition limitis about 20%. The apparatus should be de-signed to withstand pressures (1) 12-fold theinitial pressure for pure acetylene systems or(2) the initial pressure plus 12-fold the acety-lene partial pressure for mixtures and solu-tions.

– Formation of hydrates (see Chap. 2) underpressure must be avoided because this leadsto obstructions in the apparatus and pipelines.Themeltingpoint of these compounds is in therange 0 –13 ◦C; therefore, pressurized acety-lene containing water has to be kept above15 ◦C.In addition to the measures for building con-

struction, electrical installations, fire protection,purging, and leak detection, acetylene plantsand distribution systems are providedwith flametraps and flashback and release valves and locks[98, 99]. Flame traps consist either of tubes im-mersed into water-filled cylinders (wet trap) orcylinders filled with a packing of high surfacearea to decelerate the decomposition. A wet ar-rester, which is used for an acetylene distributingline, is shown in Figure 41. Suitable materialsfor dry-trap packings are sinter metals, ceramicbeads (e.g., Raschig rings), bundles of smalltubes, and corrugated metal foils [100, 101].

Tapping points for acetylene distributionunits which meet the German TRAC rules in-clude a nonreturn valve to avoid intrusion of airfrom downstream, a sinter-metal flame trap, anda thermo- or pressure-sensitive spring lock. Thelast closes if a flame is stopped by the trap butstill burns outside of the flame trap. Detailed in-formation is given in [98].

Transportation in Pipelines. Acetylene isoccasionally transported in pipelines. Figure 42shows the safety components of an acetylenepipeline betweenBurghausen andGendorf, Fed-eral Republic of Germany. The pipeline was op-erated until 1976 without incident. Its length

is 8 km, and the pipes are 300 mm in diame-ter. Design pressure was 100 bar, although op-eration pressure was only 2 bar at the inlet and1.25 bar at the outlet. The pipeline was providedwith rupture disks, which open to atmospherein case of decomposition. Quick closing valvesare initiated simultaneously to protect both up-stream and downstream equipment. At each end,part of the pipeline is filled with tube bundlesto stop propagation of any acetylene decom-position. The flame traps consist of 600-mm-diameter U’s filled with Raschig rings.

Figure 41. Hydraulic flame trap for acetylene lines (UnionCarbide), [102]

Figure 42. The 8 km acetylene pipeline from the Marathonrefinery, Burghausen, to Farbwerke Hoechst, Gendorf, Fed-eral Republic of Germany [103]a) Compressor; b) Control points; c) Automatic quick-closing valves; d) Rupture disks to atmosphere; e) Flametraps; f) Pipeline

42 Acetylene

A report [102] is available on an acetylene-decomposition event in a pipeline system,whichdemonstrates the need for safety measures.Instead of transportation of pure acetylene,pipeline transportation of acetylene solutionsin acetone was proposed as safer [103]. In theUnited States, transportation of acetylene solu-tions in liquid ammonia was considered for ex-isting ammonia distribution systems [104].

Hazardous Acetylene Traces in Low-Tem-perature Processes. Acetylene is the most dan-gerous component in gas mixtures processed inlow-temperature plants. In air separators, for ex-ample, acetylene can be suspended in liquid oxy-gen as a solid or as a segregated liquid phase thatis quite unstable and tends to uncontrollable andviolent decomposition. (The solubility of acety-lene in liquid oxygen is low, see Chap. 2) There-fore much attention must be given to checkingfor and removing acetylene in low-temperatureseparation plants.

Normally air contains some acetylene, up to0.3 mL/m3. In industrial areas, especially in theproximity of petrochemical plants, higher con-centrations (up to 1 mL/m3) can occur. Withoutany measures, acetylene in the feed air of an airseparator would be enriched in the cold sectionof the unit.

In modern air separators alternating molecu-lar sieve adsorbers are used. The adsorbers obeythe following breakthrough sequence:

To avoid acetylene breakthrough the adsor-ber is operated for sufficient CO2 removal andis regeneratedwhen theCO2 concentration at theadsorber outlet starts to increase. Further detailsare given in [105 – 109].

Another possible way to remove acetyleneand other combustible air contaminants is cat-alytic oxidation prior to the separation [107,110]. This method, being expensive, is rarelyused.

All air separators are provided with rou-tine analysis systems for acetylene (→ Oxygen).Routine analysis concentrates on the liquid oxy-gen of the main condenser [111].

The removal of acetylene in cracked gas sepa-ration is treated in Section 4.4.2. The processingof other acetylene-containing gases, e.g., coke-oven gas, by low-temperature separation is de-scribed in [112].

5.2. Acetylene Storage in Cylinders

Because of its tendency to deflagrate or to deto-nate, acetylene cannot be compressed and storedin gas cylinders like other gases. At the end ofthe 19th century attempts to store acetylene un-der high pressure or liquefied led to fatal deto-nations in the USA, Paris, and Berlin.

For desensitizing, acetylene stored in gascylinders is dissolved in a solvent in which theacetylene is very soluble. This solvent is dis-persed in a porous solid that completely fills thegas cylinder. As well as giving better solventdistribution the porous material arrests any localacetylene decomposition induced, for instance,by flashback.

Figure 43. Total pressure of acetylene solution in acetoneas a function of acetylene : acetone ratio and temperature[7]

Acetone and dimethylformamide are the pre-ferred solvents for acetylene in cylinders. An ad-vantage of dimethylformamide is its lower va-por pressure, resulting in lower solvent lossesduring acetylene discharge. A disadvantage isits higher toxicity. The total pressure of anacetone-containing acetylene cylinder depends

Acetylene 43

on the acetylene : acetone ratio and on tempera-ture as is shown in Figure 43. Deviations fromthe plotted curves resulting in higher pressuresare caused by the porous filling of the gas cylin-der, which absorbs acetone, changing the effec-tive acetylene : acetone ratio [113].

Impurities in the acetylene decrease the dis-solving capacity of the acetone. Figure 44 showsthe effect of moisture on acetylene solubility. Asa result, acetylene produced from calcium car-bide and water has to be dried.

Figure 44. Solubility of acetylene in water-containing ace-tone at 25 ◦C and pC2H2 = 1 bar (0.1 MPa) [114]

Calcium carbide-based acetylene containsfurther impurities that have to be scrubbed out toavoid decreased solubility in acetone. Examplesare divinyl sulfide and phosphine: 1 wt%divinylsulfide in the acetone reduces the acetylene sol-ubility from 35 g/kg to 31 g/kg at 20 ◦C and anacetylene pressure of 0.1 MPa. Further values,also for phosphine, are given in [114, 115]. Theimpurities have to be scrubbed out to residualconcentrations of 0.5 g of phosphorus and 0.1 gof sulfur per cubic meter of acetylene.

During acetylene production from calciumcarbide, disperse calcium hydroxide (0.1 – 1.0µm) is produced. This contaminates the productgas. The calcium hydroxide present in acetylenefilled into acetone-containing gas cylinders cat-alyzes aldol condensation of the solvent and re-duces the solubility for acetylene:

2 (CH3)2CO→(CH3)2COHCH2COCH3

diacetonealcohol

Therefore, the solids content of calcium car-bide-based acetylene for filling acetone-con-taining gas cylinders must be kept below 0.1mg/m3 [114, 116].

The gas cylinders have to be filled withdefinite amounts of acetylene and solvent.Commercial-grade, seamless gas cylinderswhich meet specified standards (in Germany,DIN 4664) may be filled with the amounts listedin Table 15.

The amounts are fixed by regulations for han-dling pressurized gases [118]. Correspondingregulations in theUnited States have been issuedby the Department of Transportation [119].

To determine maximum acetylene filling ofgas cylinders, extensive ignition, impact, andheating tests have been worked out [120, 121].

Figure 45. Porous silica material Linde M1 for acetylenecylinders (magnification 1 : 10000)

The porous material in the acetylene gascylinders must satisfy the following require-ments: no interaction with the cylinder mate-rial, acetylene, or acetone and suitable mechan-ical properties, such as sufficient impact re-sistance. Suitable materials include pumiceouscompounds, silica, charcoal, asbestos fiber, and

44 Acetylene

Table 15. Permitted acetylene and acetone filling of seamless gas cylinders (satisfying German standards and safety rules) [117]

Gas cylinder Acetone filling, kg Acetylene filling, kg

Vol. of gas Outer Length, Minimum Maximum

cylinder, diameter,L mm mm General Exceptional3 140 300 0.789 0.8625 0.9375 0.47255 140 460 1.315 1.4375 1.5625 0.787510 140 850 2.630 2.8750 3.125 1.57520 204 810 5.260 5.750 6.250 3.15027 204 1040 7.101 7.7625 8.4375 4.252540 204 500 10.520 11.50 12.50 6.3040 229 1210 10.520 11.50 12.50 6.30

Table 16. Examples of approved silica-based porous materials in German acetylene cylinders [122] (for seamless 40-L cylinders satisfyingDIN 4664, Blatt 2)Material Approved filling

Type Origin a Acetylene b, kg Acetone c, kg Acetylene : acetone,kg/kg

Maximum pressure d,bar

Linde M1 Linde, Munich 8.0 12.7 0.63 19AGA 2 AGA, Hamburg 8.0 12.4 0.645 19SIAD 2 SIAD, Sabbio 8.0 12.4 0.645 19a Approval only when porous filling is prepared at place of origin;b Maximum;c Desired value;d Gage, at 15 ◦C.

alkaline carbonates. The porosity of these ma-terials varies between 70 and 80% [114, 122].Modern monolithic materials are made prefer-ably from silica, lime, and glass fiber. Themixtures are suspended in water to obtain apasty material which is filled into the gas cylin-ders. The material is hardened at about 200 ◦Cand subsequently dried and activated at 350 –400 ◦C. A porosity of about 90% is obtained.

Any porous material to be used for acety-lene cylinders has to be examined and approvedby competent authorities. The examination in-cludes the determination of themaximum acety-lene and solvent filling, the maximum fillingpressure, and ignition and impact testing. Ta-ble 16 lists three porous materials approved foruse in Germany. Figure 45 shows a photographof “Linde M1” magnified 1 : 10000, clearly re-vealing the porous structure of such materials.

Methods for examining the materials havebeen standardized by CPI (Commission Perma-nente Internat. de l’Acetylene, de la SoudureAutogene et des Industries qui s’y rattachent,Paris) and ISO (International Organization forStandardization) [122].

Discharging acetylene from a gas cylinderleads to acetone losses because the partial pres-sure of acetone at 15 ◦C ranges from 0.14 bar

at 15 bar total pressure to 0.18 bar at 1 bar totalpressure. Solvent loss has to be replaced whenan acetylene cylinder is reloaded. Further detailsconcerning acetylene cylinders, porous materi-als, and valves can be found in [131].

6. Uses and Economic Aspects

6.1. Use in Metal Processing

Acetylene has many applications in the process-ing ofmetals and othermaterials. This is becauseof the high flame temperature and propagationvelocity resulting in high energy densities andrapid heat transfer to the piece being worked.Examinations at Linde showed that for extremeoblique sections the efficiency using acetylene isup to 50% higher than using propane as fuel gas[133]. The properties of an oxyacetylene flamegiven here supplement those in Section 5.1.

The temperature profile of an oxyacetyleneflame consists of a hotter primary flame and ascattered secondary flame. The highest flametemperature is at the tip of the primary flame(Fig. 46). For material processing the primaryflame is the more important.

Acetylene 45

Figure 46. Temperature profile of an oxyacetylene flame

The heating efficiency of the primary flameis the product of the volume-based heat releasedby the primary flame and the propagation ve-locity. This is plotted in Figure 47 for the oxy-acetylene flame and some other flames. The heattransferred in welding is generated by radiation,convection, and thermal conduction (see Table17). The heat transfer is promoted by a high tem-perature gradient between flame and workpiece.

Figure 47. Heating efficiency of acetylene–oxygen mix-tures and mixtures of other hydrocarbons with oxygen* based on area of primary flame cone

Oxidizing, neutral, or reducing (carburizing)flames can be obtained by varying the oxygen :acetylene ratio (Fig. 40). For steel, alumina, andcopperwelding usually neutral or slightly reduc-ing flames are used, whereas oxidizing flamesare preferred for brass welding, cutting, pick-ling, and surface hardening [124]. Acetylene isburned with oxygen in single torches or in bun-dles of torches, the chief components of whichare the connections for acetylene and oxygen,regulating valves, a mixing chamber (usually ofthe injection type), a flashback protection ele-

ment, and a nozzle adapted to the specific appli-cations [125, 126].

Oxyacetylene flames are used in welding,cutting, brazing, soldering, surfacing, flamespraying, heating, hardening, straightening,cleaning, pickling, rust removal, and decar-bonizing.

Acetylene–air flames are occasionally usedfor tin brazing, hot air welding of thermoplas-tics, glassworking, and paint removal [125],although the convenience and safety of fu-els such as propane or butane has displacedacetylene in those applications. Soft and hardsoldering, flame hardening, and flame temper-ing are important applications for the softeracetylene–air flame. For acetylene–air mixtures,self-aspirating Bunsen-type and acetylene–compressed air burners are used.

The different uses of oxyacetylene andacetylene–air flames in metal working, the pro-cedures, and the equipment are comprehensivelydescribed in [7] and [127]; other sources of in-formation for oxyacetylene flame properties inwelding are [123, 124], and [128].

6.2. Use as Raw Material in ChemicalIndustry

Because of the diversity of acetylene chemistry(see Section 3.1), acetylene has been used as astarting material for a great variety of industri-ally important products. These are summarized,together with their applications, in Figure 48.

Between 1960 and 1970, when worldwideacetylene production peaked, most of the prod-ucts listed in Figure 48were produced via acety-lene. The competition between acetylene and theolefins since the 1970s (see Section 6.3) has re-sulted in substitution of ethylene and propene foracetylene, especially in the production of acetal-dehyde and acrylonitrile. At present, acetyleneis used mainly for the production of vinyl chlo-ride, vinyl acetate, and other vinyl esters; acrylicacid; acetylene black; and acetylenic chemicalssuch as 1,4-butynediol and acetylenic alcohols.For the acetylenic chemicals the acetylene routeis either the only commercial production pro-cess available or the predominant process. Vinylchloride, vinyl acetate, and acrylic acid, for-merly themain products from acetylene, are pro-

46 Acetylene

Table 17. Heat transfer in welding [123]

Gas Welding temperature, K

temperature, 800 1200 1600

K QS QK QS QK QS QK

1000 2.3 13.42000 3.8 55.4 3.4 37.0 2.3 18.53000 4.0 83.2 3.8 68.0 3.6 50.4

QS, heat transfer by radiation (kJ cm−2 h−1);

QK, heat transfer by convection (kJ cm−2h−1) at a gas velocity of 50 m/s

Figure 48. Acetylene as a starting material for industrial products

duced today mainly from ethylene and propene[3].

6.3. Competitive Position of Acetyleneas Chemical Feedstock

Today, acetylene plays an important role only inthe production of the acetylenic chemicals. Thefact that acetylene production has not decreasedfurther seems to indicate that the competitionfrom the olefins is no longer as strong as it was.The main reason for this is that European olefinchemistry depends on refinery products, whichhave become more expensive than natural gas,the main feedstock for acetylene. Another con-tributing factor is that acetylene is produced onlyin old plants, which have low capital costs.

In addition, process improvements, such as anincrease in thermal efficiency and optimum useof byproducts by other plants, can make acety-lenemore competitive. The position of acetylenein chemical industry may be advanced becauseof the variety of valuable products to which itcan be converted in high yieldswith known tech-nology. Acetylene must compete with ethylenefor the production of vinyl chloride and vinylacetate, and for the production of acrylic acidand its esters it must compete with propene. TheStanford Research Institute [3] has investigatedthis question in detail. The results, which takeinto account both capital investment (25% re-turn) and specific consumption figures for thevarious processes, show that the prices at which

Acetylene 47

Figure 49. Acetylene–methanol plant

the alternate routes are competitive can be ex-pressed by the following equations:

for vinyl chloride A= 1.10 E + 0.42for vinyl acetate A= 1.23 E + 0.40for acrylic acid A= 1.74 P + 0.23

where A is the acetylene price,E is the ethyleneprice, and P is the propene price, all in $/kg. Forexample, if ethylene costs 0.65 $/kg, acetylenecan cost 1.15 $/kg for the production of vinylchloride and 1.21 $/kg for the production ofvinyl acetate. If propene costs 0.49 $/kg, acrylicacid can be profitably produced from acetyleneif it costs 1.08 $/kg or less.

Acetylene prices of 1.08 to 1.21 $/kg or lesscan be reached in a new plant only with optimalintegration of energy and byproducts within anintegrated chemical plant. Figure 49 shows theflow sheet of a 100 000 t/a gas-based partial ox-idation acetylene plant in a chemical complex.The required oxygen facility and methanol plantbased on acetylene synthesis gas (off-gas) areincluded. The main products of the complex areacetylene and methanol. The acetylene processis operated with improved quenching technol-ogy, allowing a high proportion of energy tobe regained in the form of steam. The aromaticresidue oil froma steamcracker is converted intohigh-purity coke and light aromatics.

A production cost estimate based on power,feedstock, and product prices roughly corre-sponding tomarket prices in 1982 shows that low

production costs for acetylene are possible un-der the conditions described, in spite of the rel-atively high natural gas price of 5.5 $/106 BTU(0.021 $/kW h), which in heating value termscomes very close to that of crude oil (0.022$/kW h). The difference in production costs bet-ween acetylene from natural gas and ethylenefrom naphtha (price 333 $/t, corresponding to0.028 $/kW h) is so slight that acetylene fromnew plants can once again compete with ethyl-ene for certain syntheses, provided that there isa difference between the costs of natural gas andnaphtha. This was nearly 0.007 $/kW h in 1982in Germany. A similar calculation for the acety-lene production by the Huls arc process basedon an ethylene price of 600 $/t and a hydrogenprice 40% above the heating value results inthe same acetylene product value at 0.038 $/kWh for electrical power. Lower electrical energycosts favor the arc process.

7. Propyne

Propyne [74-99-7],methylacetylene, is obtainedin cracking processes mostly as a byproduct to-getherwith its isomer propadiene [463-49-0], al-lene. Typical concentrations depend on the feed-stock and the cracking conditions (Table 10) andvary between 0.3 and 0.8 wt%. The correspond-ing figures for propadiene are 0.3 – 0.6 wt%.

48 Acetylene

Figure 50. Vapor pressure curves of propyne and propadi-ene [129]

Figure 51. Solubility of propyne in various solvents at infi-nite dilutiona) Water; b) Methanol; c) N-methylpyrrolidone; d) DMF

Pure propyne is a colorless, nontoxic, flam-mable gas. The important physical properties ofpropyne are listed in Table 18 [5, 6, 129]. Vapor

pressure curves of propyne and propadiene in-cluding melting and critical points are shown inFigure 50 [129]. Table 19 summarizes solubil-ity coefficients of propyne for various solvents[9]; further data are available in [10]. The sol-ubility of propyne in various solvents is shownin Figure 51; the solubilities of the C3 hydrocar-bons in DMF are plotted in Figure 52 for infinitedilution.

Figure 52. Solubility of the C3 hydrocarbons in DMF atinfinite dilutiona) Propyne; b) Propadiene; c) Propene; d) Propane

The equilibrium between the two isomers ofC3H4 is reached in the presence of catalysts (forexampleAl2O3/3, SiO2, activated carbon, andγ-Al2O3/Na2CO3) [130, 131]. At 270 ◦C the equi-librium mixture contains 82% propyne, and at5 ◦C, 91.1% propyne. For calculated data, see[132]. This equilibrium is important for the in-dustrial propyne recovery process.

The decomposition pressure as a function ofthe ignition energy for propyne (and propadi-ene) is plotted in Figure 38. The lower and upperflammability limits of propyne in air are about2.3 and 16.8 vol%. For propadiene only rangesare available: 1.7 – 2.5 and 12 – 17 vol% [133].

Production. Propyne and propadiene can berecovered from cracked gas by solvent extrac-

Acetylene 49

Table 19. Solubility coefficients of propyne in various solvents (mol kg−1 bar−1)[9]

Solvent C3H4 pressure, bar − 20 ◦C 25 ◦CMethanol 0.1 9.099 1.8651,2-Dichloroethane 0.098 – 0.196 3.276 (0 ◦C) 1.546Carbon tetrachloride 0.25 4.732 0.842n-Octane 0.098 3.412Toluene 0.25/1.0 8.644 2.047Xylene (technical). 0.49/0.98 9.782 1.6834-Methyl-1,3-dioxolan-2-one 0.49/1.0 8.644 1.183Triethylene glycol 1.0 0.400 (30 ◦C)Acetone 0.6/1.0 35.03 4.186N-Methyl-2-pyrrolidone ≤ 0.78 6.597 (0 ◦C) 2.502DMF ≤ 0.15 12.512 3.003Water 1.0 0.071Ammonia ≤ 0.04 20.018 4.436

Figure 53. Propyne recovery process (Linde, Shell [77])MA: methylacetylene; PD: propadiene

tion. The process is outlined in Figure 53 [79,134] and is similar to the recovery of acetylene(see Chap. 4.4.2). This process was developedby Linde and Shell and is ready for use [70].Nevertheless, until today (2006) it has not beenused in an industrial application. An importantstep is the catalytic isomerization of propadieneto propyne in the liquid phase on a catalyst [131].

The bottom product from the propene/pro-pane splitter is routed to depropanizer II for re-moval of traces ofC4+ hydrocarbons originatingfrom the feed and the isomerization reactor efflu-ent. The overhead is sent to the absorption col-umn. The propane (and some propene) is routedback to the cracking furnace. Propyne (MA) andpropadiene (PD) are stripped off in the corre-sponding columns. The propadiene fraction isconverted to propyne in the isomerization reac-

tor and recycled to the feed. Traces of solvent inthe pure propyne are removed by cooling. Theproduct can be sent directly to methyl methacry-late (MMA) unit, where MMA is manufacturedby reaction of propyne with carbon monoxideand an alcohol in the presence of a Pd-basedcarboxylation catalyst [131, 135].

Usually propyne and propadiene are unde-sirable byproducts of the steam cracking pro-cess. The crude C3 fraction can contain upto 6 mol% MAPD, whereas the MAPD con-tent of chemical-grade propene typically has tobe < 20 ppm and that of polymerization-gradepropene < 2 ppm. To this end MA and PD areremoved by selective hydrogenation in the liquidor gas phase. Typical conditions are pressures of1.5 – 5 MPa and inlet temperatures of 20 – 100◦C. The vapor-phase processes are declining in

50 Acetylene

importance. Today mainly liquid or compositephase are in use as they have some advantages:

– Operation at lower temperatures (higher se-lectivity)

– Lower operating costs (no vaporization of thefeed and recondensation of the product)

– Regeneration of reactor in situ (removal ofpolymers)

– Lower frequency of catalyst regeneration– For high concentration of C3H4 in the feed,two reactors are often sufficient; the gas-phaseprocess requires more reactors

– Control of conversion by injection of hydro-gen (no excess hydrogen) reduces investmentcost

– Control of temperature by use of vaporizationminimizes risk of runaway

Table 18. Physical properties of propyne [5, 6, 129]

Molecular mass 40.065Critical temperature 402.39 K (129.24 ◦C)Critical pressure 5.626 MPaCritical volume 0.1635 m3/kmolMelting point 170.45 K (− 102.7 ◦C)Dipole moment 2.61× 10−30 C · mDensity (liquid) 638.92 kg/m3 (273.15 K)Normal boiling point 249.97 K (− 23.18 ◦C)Enthalpy of vaporization (273.15 K) 20.765 kJ/mol (273.15 K)Molar volume 0.05962 m3/kmol (249.91 K)Enthalpy of formation 185.5 ± 1.0 kJ/mol

(298.15 K)Gibbs free energy of formation 193.8 ± 1.0 kJ/mol

(298.15 K)Entropy of formation 248.4 J mol−1 K−1

(298.15 K)Enthalpy of combustion 1938.943 kJ/mol (298.15 K)Heat capacity (constant pressure) 59.842 J mol−1 K−1

(273.15 K)Viscosity (liquid) 1.7500× 10−4 Pa · s

(273.15 K)Viscosity (gaseous) 8.3300× 10−6 Pa · s

(293.15 K)Thermal conductivity (liquid) * 0.14560 W m−1 K−1

(233.45 K)Thermal conductivity (gas) * 0.014310 W m−1 K−1

(273.15 K)Surface tension * 1.47× 10−2 N/m (273.15 K)* Predicted or estimated.

As in the case of acetylene hydrogenation,Pd catalysts are used. Several catalyst manufac-turers offer special catalysts for C3 hydrogen-ation. The selectivity in case of liquid-phase hy-drogenation to propene is about 60 – 70% anddepends on the concentration of C3H4 in the C3feed. For better control of reaction heat, someproduct is normally recycled to the feed. The cy-cle time of the catalyst is very sensitive to con-

taminants (and to byproduct formation in gas-phase processes), especially in the first reactor.A reactivation procedure is necessary when thecatalyst loses its activity. In contrast to acetylenehydrogenation, the requirements on MAPD hy-drogenation are not that strict. MAPD going tothe propane - propylene separation (C3 splitter)mainly ends up in the bottom product propanewhich is usually recycled to the cracking fur-naces. This means that there is another purifi-cation step, which can remove MAPD from thepropene product. As propyne and propadiene aredifferent chemical compounds they are hydro-genatedwith different reaction rates. In contrast,many reports in literature about hydrogenationonly consider a simplified model with a com-bined MAPD fraction and do not discriminatebetween both isomers.

Stabilized Propyne–Propadiene Mixtures.As a replacement for acetylene, stabilizedpropyne–propadiene mixtures are availablecommercially. Trade names are Tetrene orMAPP gas. These mixtures are stabilized bypropane, propene, and/or butane and are usedfor metal cutting, welding, hardening, and braz-ing. The flame properties are closer to those ofpropane–propenemixtures. Therefore stabilizedC3H4 mixtures have not yet won a large-scalemarket. Further information is available in [136].InEurope, thosemixtures are available only veryrarely. In Europe, mixtures with the trade nameMAPP gas have a rather lowMAPD content ac-tually and mainly consist of propene, propane,and butane. Only in the US the original MAPPmixture has some importance.

8. Toxicology and OccupationalHealth

Pure acetylene is a simple asphyxiant. Whengenerated fromcalciumcarbide, acetylene is fre-quently contaminatedwith arsine, hydrogen sul-fide, or phosphine, and exposure to this impureacetylene has often resulted in serious conse-quences. Commercial acetylene no longer con-tains these impurities and is therefore less harm-ful [137].

The lowest published lethal concentration forrats is 9 vol% [138]. Dogs are less sensitive: 80

Acetylene 51

vol% acetylene in the air is necessary to pro-duce a narcosis accompanied by an increasedblood pressure and a decreased pulse frequency(stimulation of vasomotor and vagus centers)[139]. In humans, the inhalation of air containing10 vol% acetylene has a slight intoxicating ef-fect, marked intoxication occurs at 20 vol%, in-coordination at 30 vol%, and unconsciousnesswithin 5 min on exposure to 35 vol%. Inhaling35 vol.% for 5 – 10 min or 10 vol% for 30 –60 min is lethal. Symptoms of intoxication areexcitement, coma, cyanosis, weak and irregularpulse, and memory failure [140 – 142].

There is no evidence that repeated exposureto tolerable levels of acetylene has effects dele-terious to health [143]. Inhalation of air with33 vol% of acetylene by humans led to uncon-sciousness within 6 min, but when the experi-ment was repeated within the week the suscep-tibility to acetylene decreased: 9 min were re-quired on the second exposure and more than33 min on the third exposure to produce uncon-sciousness [139].

Acetylene does not irritate the mucous mem-branes [137]. Neither threshold limit value(TLV) nor a MAK has been established. Thestandard air concentration allowed for OSHAand NIOSH is 2500 ppm [141].

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