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  • c 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim10.1002/14356007.a27 333

    Urea 1

    UreaJozef H.Meessen, DSM Stamicarbon, Geleen, The Netherlands (Chaps. 1 7)Harro Petersen, BASF Aktiengesellschaft, Ludwigshafen, Federal Republic of Germany (Chap. 8)

    1. Physical Properties . . . . . . . . . . 22. Chemical Properties . . . . . . . . . 33. Production . . . . . . . . . . . . . . . 43.1. Principles . . . . . . . . . . . . . . . . 43.1.1. Chemical Equilibrium . . . . . . . . 43.1.2. Physical Phase Equilibria . . . . . . 73.2. Challenges in Urea Production

    Process Design . . . . . . . . . . . . 83.2.1. Recycle of Nonconverted Ammonia

    and Carbon Dioxide . . . . . . . . . . 93.2.2. Corrosion . . . . . . . . . . . . . . . . 113.2.3. Side Reactions . . . . . . . . . . . . . 123.3. Description of Processes . . . . . . 133.3.1. Conventional Processes . . . . . . . . 133.3.2. Stripping Processes . . . . . . . . . . 133.3.2.1. Stamicarbon CO2-Stripping Process 133.3.2.2. Snamprogetti Ammonia- and

    Self-Stripping Processes . . . . . . . 163.3.2.3. ACES Process . . . . . . . . . . . . . 183.3.2.4. Isobaric Double-Recycle Process . 193.3.3. Other Processes . . . . . . . . . . . . 193.4. Efuents and Efuent Reduction 203.5. Product-Shaping Technology . . . 214. Forms Supplied, Storage, and

    Transportation . . . . . . . . . . . . 225. Quality Specications and

    Analysis . . . . . . . . . . . . . . . . . 23

    6. Uses . . . . . . . . . . . . . . . . . . . 247. Economic Aspects . . . . . . . . . . 248. Urea Derivatives . . . . . . . . . . . 258.1. Thermal Condensation Products

    of Urea . . . . . . . . . . . . . . . . . . 258.2. Alkyl- and Arylureas . . . . . . . . 258.2.1. Transamidation of Urea with

    Amines . . . . . . . . . . . . . . . . . . 258.2.2. Alkylation of Urea with Tertiary

    Alcohols . . . . . . . . . . . . . . . . . 268.2.3. Phosgenation of Amines . . . . . . . 268.2.4. Reaction of Amines with Cyanates

    (Salts) . . . . . . . . . . . . . . . . . . 278.2.5. Reaction with Isocyanates . . . . . . 278.2.6. Acylation of Ammonia or Amines

    with Carbamoyl Chlorides . . . . . . 278.2.7. Aminolysis of Esters of Carbonic

    and Carbamic Acids . . . . . . . . . . 278.3. Reaction of Urea and Its

    Derivatives with Aldehydes . . . . 288.3.1. -Hydroxyalkylureas . . . . . . . . . 288.3.2. -Alkoxyalkylureas . . . . . . . . . . 298.3.3. ,-Alkyleneureas . . . . . . . . . . 308.3.4. Cyclic Urea Aldehyde

    Condensation Products . . . . . . . . 319. References . . . . . . . . . . . . . . . 33

    Abbreviations:CRH, % critical relative humidityHS, kJ/mol integral heat of solutionm, mol/kg urea molality, moles of urea per

    kilogram of waterPO(s)H2 , Pa water vapor pressure of a satu-

    rated urea solutionPv , Pa vapor pressure

    Urea [57-13-6], CO(NH2)2, Mr 60.056,plays an important role in many biological pro-cesses, among others in decomposition of pro-teins. The human body produces 20 30 g ofurea per day.

    In 1828, Wohler discovered [1] that ureacan be produced from ammonia and cyanic acidin aqueous solution. Since then, research on the

    preparation of urea has continuously progressed.The starting point for the present industrial pro-duction of urea is the synthesis of Basaroff [2],in which urea is obtained by dehydration of am-moniumcarbamate at increased temperature andpressure:

    NH2COONH4CO(NH2)2 +H2O

    In the beginning of this century, urea was pro-duced on an industrial scale by hydration ofcyanamide, which was obtained from calciumcyanamide:

    CaCN2 +H2O+CO2CaCO3 +CNNH2

    CNNH2 +H2OCO(NH2)2

  • 2 Urea

    After development of the NH3 process (Haberand Bosch, 1913, Ammonia, Chap. 2.Ammonia, Chap. 3. Ammonia, Chap. 4.),the production of urea from NH3 and CO2,which are both formed in the NH3 synthesis,developed rapidly:

    2NH3 +CO2NH2COONH4

    NH2COONH4CO(NH2)2 +H2O

    At present, urea is prepared on an industrialscale exclusively by reactions based on this re-action mechanism.

    1. Physical Properties [3], [4]

    Pure urea forms white, odorless, long, thinneedles, but it can also appear in theform of rhomboid prisms. The crystal lat-tice is tetragonal scalenohedral; the axis ratioa : c=1 : 0.833. The urea crystal is anisotropic(noncubic) and thus shows birefringence. At20 C the refractive indices are 1.484 and1.602. Urea has an mp of 132.6 C; its heat offusion is 13.61 kJ/mol.

    Physical properties of the melt at 135 C fol-low: 1247 kg/m3Molecular volume 48.16m3/kmol 3.018mPa sKinematic viscosity 2.42106 m2/sMolar heat capacity, Cp 135.2 Jmol1 K1Specic heat capacity, cp 2.25 kJ kg1 K1Surface tension 66.3103 N/m

    In the temperature range 133 150 C, den-sity and dynamic viscosity of a urea melt can becalculated as follows:

    r= 1638.5 0.96T

    ln= 6700/T 15.311

    The density of the solid phase at 20 C is1335 kg/m3; the temperature dependence of thedensity is given by 0.208 kgm3 K1.

    At 240 400K, themolar heat capacity of thesolid phase is [5]

    Cp= 38.43 + 4.98 102T +7.05 104T 2

    8.61 107T 3

    The vapor pressure of the solid phase between56 and 130 C [6] can be calculated from

    lnPv= 32.472 11755/T

    Hygroscopicity. The water vapor pressure ofa saturated solution of urea in water PO(s)H2 inthe temperature range 10 80 C is given by therelation [7]

    lnP(s)H2O= 175.766 11552/T 22.679lnT

    By starting from the vapor pressure of purewaterPOH2 , the critical relative humidity (CRH) thencan be calculated as

    CRH =(P(s)H2O/PH2O

    )100

    The CRH is a threshold value, above which ureastarts absorbing moisture from ambient air. Itshows the following dependence on tempera-ture:

    25 C 76.5%30 C 74.3%40 C 69.2%

    At 25 C, in the range of 0 20mol of ureaper kilogram of water, the integral heat of solu-tion of urea crystals in waterHs as a functionof molality m is given by [8]:

    Hs= 15.351 0.3523m+2.327 102m2

    1.0106 103m3+1.8853 105m4

    Urea forms a eutectic mixture with 67.5wt%of water with a eutectic point at 11.5 C.

    The solubility of urea in a number of solvents,as a function of temperature is summarized inTable 1 [9], [10].

    2. Chemical Properties

    Upon heating, urea decomposes primarily toammonia and isocyanic acid. As a result, thegas phase above a urea solution contains a con-siderable amount of HNCO, if the isomerizationreaction in the liquid phase

    CO(NH2)2NH4NCONH3 +HNCO

  • Urea 3Table 1. Solubility of urea in various solvents (solubility in wt% of urea)

    Temperature, C

    Solvent 0 20 40 60 80 100

    Water 39.5 51.8 62.3 71.7 80.2 88.1Ammonia 34.9 48.6 67.2 78.7 84.5 90.4Methanol 13.0 18.0 26.1 38.6Ethanol 2.5 5.1 8.5 13.1

    has come to equilibrium [11]. In diluteaqueous solution, the HNCO formed hy-drolyzes mainly to NH3 and CO2. In amore concentrated solution or in a ureamelt, the isocyanic acid reacts further withurea, at relatively low temperature, to formbiuret (NH2 CONHCONH2), tri-uret (NH2 CONHCONHCONH2),and cyanuric acid (HNCO)3 [12]. Athigher temperature, guanidine [CNH(NH2)2],ammelide [C3N3(OH)2NH2], amme-line [C3N3OH(NH2)2], and melamine[C3N3(NH2)3] are also formed [13], [14].

    Melamine can also be produced from ureaby a catalytic reaction in the gas phase. To thisend, urea is decomposed into NH3 andHNCO atlow pressure, and subsequently transformed cat-alytically to melamine (Melamine and Gua-namines, Chap. 4.).

    Urea reacts with NOx, both in the gas phaseat 800 1150 C and in the liquid phase at lowertemperature, to form N2, CO2, and H2O. Thisreaction is used industrially for the removal ofNOx from combustion gases [15], [16].

    Reactions with Formaldehyde. Under acidconditions, urea reacts with formaldehyde toform among others, methyleneurea, as wellas dimethylene-, trimethylene-, tetramethy-lene-, and polymethyleneureas. These prod-ucts are used as slow-release fertilizer underthe generic name ureaform [17] (Fertilizers,Chap. 4.4.2.1.). The reaction scheme for theformation of methyleneurea is given below:

    Methyleneurea reacts with additional moleculesof formaldehyde to yield dimethyleneurea andother homologous products.

    The reactions of urea with formaldehyde un-der basic conditions are used widely for the pro-duction of synthetic resins (Amino Resins,Chap. 7.1.). As a rst step, methylolurea insteadof methyleneurea is formed:

    This product subsequently reacts with formal-dehyde to dimethylol urea, CO(NHCH2OH)2,and further polymerization products. Since ureais also the raw material for the production ofmelamine, from which melamine formalde-hyde resins are produced, it is the most impor-tant building block in the production of aminoresins.

    When urea is applied as fertilizer to soil, ithydrolyzes in the presence of the enzyme ureaseto NH3 and CO2, after which NH3 is bacteri-ologically converted into nitrate and, as such,absorbed by crops [17].

    3. Production

    3.1. Principles

    3.1.1. Chemical Equilibrium

    In all commercial processes, urea is producedby reacting ammonia and carbon dioxide at ele-vated temperature and pressure according to theBasaroff reactions:

  • 4 Urea

    2NH3 (l) + CO2 (l)NH2COONH4H = 117 kJ/mol (1)

    NH2COONH4NH2CONH2 +H2OH =+ 15.5 kJ/mol (2)

    A schematic of the overall process and the phys-ical and chemical equilibria involved is shownin Figure 1. In the rst reaction, carbon dioxideand ammonia are converted to ammonium carb-amate; the reaction is fast and exothermic. Inthe second rection, which is slow and endother-mic, ammonium carbamate dehydrates to pro-duce urea andwater. Sincemore heat is producedin the rst reaction than consumed in the second,the overall reaction is exothermic.

    Figure 1. Physical and chemical equilibria in urea produc-tion

    Processes differ mainly in the conditions(composition, temperature, and pressure) atwhich these reactions are carried out. Tradition-ally, the composition of the liquid phase in thereaction zone is expressed by two molar ratios:usually, the molar NH3 : CO2 and the molarH2O : CO2 ratios. Both reect the compositionof the so-called initial mixture [i.e., the hypo-thetical mixture consisting only of NH3, CO2,andH2O if both Reactions (1) and (2) are shiftedcompletely to the left].

    First attempts to describe the chemical equi-librium of Reactions (1) and (2) were madeby Frejacques [19]. Later descriptions of thechemical equilibria can be divided into regres-sion analyses of measurements [20], [21] andthermodynamically consistent analyses of theequilibria [20], [22]. As far as the most impor-tant consequences of these equilibria on ureaprocess design are concerned, the methods cor-respond closely to each other: The achievableconversion per pass, dictated by the chemicalequilibrium as a function of temperature, goes

    through a maximum (Figs. 2 and 3). This ef-fect is usually attributed to the fact that the am-monium carbamate concentration as a functionof temperature goes through a maximum. Thismaximum in the ammonium carbamate concen-tration can be explained, at least qualitatively, bythe respective heat effects of Reactions (1) and(2).However, thismechanismcannot explain theobserved conversion maximum fully and quan-titatively; other contributing mechanisms havebeen suggested [23].

    Figure 2. Carbon dioxide conversion at chemical equilib-rium as a function of temperatureNH3 : CO2 ratio = 3.5mol/mol (initial mixture); H2O: CO2ratio = 0.25mol/mol (initial mixture)

    Figure 3. Ammonia conversion at chemical equilibrium asa function of temperatureNH3 : CO2 ratio = 3.5mol/mol (initial mixture); H2O: CO2ratio = 0.25mol/mol (initial mixture)

    The inuence of the composition of the ini-tial mixture on the chemical equilibrium can beexplained qualitatively by Reactions (1) and (2)and the law of mass action:

  • Urea 5

    1) Increasing the NH3 : CO2 ratio (increasingthe NH3 concentration) increases CO2 con-version, but reducesNH3 conversion (Figs. 4and 5).

    2) Increasing the amount of water in the ini-tial mixture (increasing the H2O : CO2 ratio)results in a decrease in both CO2 and NH3conversion (Figs. 6 and 7).In these cases, too, a full quantitative descrip-

    tion cannot be derived simply from the law ofmass action and Reactions (1) and (2). Other,not yet fully understood reaction mechanismsprobably contribute to the chemical equilibriato a minor extent.

    Figure 4. Carbon dioxide conversion at chemical equilib-rium as a function of NH3 : CO2 ratioT= 190 C; H2O : CO2 ratio = 0.25mol/mol (initial mix-ture)

    Figure 5. Ammonia conversion at chemical equilibrium asa function of NH3 : CO2 ratioT = 190 C; H2O : CO2 ratio = 0.25mol/mol (initial mix-ture)

    Figure 6. Carbon dioxide conversion at chemical equilib-rium as a function of H2O : CO2 ratioT = 190 C;NH3 : CO2 ratio = 3.5mol/mol (initialmixture)

    Figure 7. Ammonia conversion at chemical equilibrium asa function of H2O : CO2 ratioT = 190 C;NH3 : CO2 ratio = 3.5mol/mol (initialmixture)

    In Figures 2, 4, and 6, the conversion at chem-ical equilibrium is expressed as CO2 conversion,that is, the amount of CO2 in the initial mixtureconverted into urea (plus biuret), if no changesoccur in overall NH3, CO2, and H2O concen-trations in the liquid phase. This way of repre-senting the chemical equilibrium is consistentwith the presentation usually found in the tradi-tional urea literature. However, it is based on thearbitrary choice of CO2 as the key component.Historically, this may be justied by the fact thatin early urea processes, CO2 conversion wasmore important than NH3 conversion. For thepresent generation of stripping processes, how-ever, giving a higher weight to CO2 conversionis not justied. Comparing, e.g., Figs. 4 and 5,

  • 6 Urea

    shows that an arbitrary choice of one of the twofeedstock components as yardstick to evaluateoptimum reaction conversion can easily lead tofaulty conclusions.

    Figure 8. Urea yield in the liquid phase at chemical equi-librium as a function of temperatureNH3 : CO2 ratio = 3.5mol/mol (initial mixture); H2O: CO2ratio = 0.25mol/mol (initial mixture)

    Figure 9. Urea yield in the liquid phase at chemical equi-librium as a function of H2O : CO2 ratioT= 190 C;NH3 : CO2 ratio = 3.5mol/mol (initialmixture)

    Ultimately, project economics (investmentand consumptions)will dictate the choice of pro-cess parameters in the reaction section. With-out going into such time- and place-dependenteconomic considerations, one can argue that theurea yield (i.e., the concentration of urea in theliquid phase) is a better tool for judging opti-mum process parameters than CO2 or NH3 con-version. Figure 8 illustrates that urea yield as

    a function of temperature also goes through amaximum; the location of this maximum is ofcourse composition dependent. Figure 9 againshows the detrimental effect of excess water onurea yield; thus, one of the targets in designinga recycle system must be to minimize water re-cycle.

    Figure 10 shows that the urea yield as a func-tion of NH3 : CO2 ratio reaches a maximumsomewhat above the stoichiometric ratio (2 : 1).This is one of the reasons that all commercialprocesses operate at NH3 : CO2 ratios above thestoichiometric ratio. Another important reasonfor this can be found from the physical phaseequilibria in the NH3 CO2 H2O urea sys-tem.

    Figure 10. Urea yield in the liquid phase at chemical equi-librium as a function of NH3 : CO2 ratioT= 190 C; H2O : CO2 ratio = 0.25mol/mol (initial mix-ture)

    3.1.2. Physical Phase Equilibria

    In urea production, the phase behavior of thecomponents under synthesis conditions is im-portant. In all commercial processes, conditionsare such that pressure and temperature are wellabove the critical conditions of the feedstocksammonia and carbon dioxide; i.e., both compo-nents are in the supercritical state. The chemi-cal interaction between NH3 and CO2 (mainlythe formation of ammonium carbamate) resultsin a strongly azeotropic behavior of the bi-nary system NH3 CO2. An approach to thedescription of the phase equilibria if urea andwater are added to the NH3 CO2 system wasgiven by Kaasenbrood and Chermin [24]. If

  • Urea 7

    a less volatile solvent C (water) is added to anazeotropic system AB (NH3 CO2) at a pres-sure where both components A and B are super-critical, then the T X liquid and gas planes forthe ternary system thus formed assume a spe-cial shape owing to the peculiar path describedby the boiling points of the changing solutions(Fig. 11). Sections through the liquid plane forconstant solvent content are analogous to the liq-uid line for the binary system. The liquid planefor the ternary systems appears as a ridge in theT X space. If the peak points of this ridge arelinked up, the top ridge line is obtained. Thepoints on this line do not have the same A : Bratio as the maximum for the binary azeotrope,because A and B are not soluble in solvent C tothe same extent. The A : B ratio changes and theboiling point increases as the percentage of Cincreases.

    Figure 11. Liquid gas equilibrium in a ternary systemwith binary azeotrope at constant pressureThe system AB forms a binary azeotrope; C is a solventfor both A and B. The pressure is such that both A and Bare supercritical, whereas the pressure is below the criticalpressure of C.

    Analogous to the description of Figure 11, theequilibria in the NH3 CO2 H2O urea sys-temunder urea synthesis conditions showamax-imum in temperature at a given pressure as afunction ofNH3 : CO2 ratio.A full description ofthe phase equilibria in this system is even morecomplex than the aforementioned hypotheticalA B C system, since the solid liquid (S L)and solid gas (S G) equilibria interfere withthe liquid gas (L G) equilibria.

    The strongly azeotropic behavior of theNH3 CO2 system, and the associated temper-ature maximum (or pressure minimum) in theternary and quaternary systems with water andurea, are of practical importance in the real-ization of commercial urea processes. Carbondioxide is less soluble than ammonia in waterand urea melts. As a result, the pressure gradi-ent at constant temperature is much steeper onthe CO2-rich side of the top ridge line. More-over, this difference in solubility also causes thepressure minimum (or temperature maximum)to shift toward higher NH3 : CO2 ratios as theamount of solvent (water and urea) increases.Practically, this means that in order to achieverelatively low pressures at a given temperature,the NH3 : CO2 ratio in all commercial processesis chosen well above the stoichiometric ratio(2 : 1). In some processes, this ratio is chosen onthe pressureminimum (on the top ridge line, i.e.,at a ratio of ca. 3 : 1), whereas in other processesan even greater excess of ammonia is used.

    3.2. Challenges in Urea ProductionProcess Design

    Like any process design, a urea plant design hasto full a number of criteria. Most importantitems are product quality, feedstocks andutilitiesconsumptions, environmental aspects, safety, re-liability of operation and a low initial invest-ment. Since the urea process already has half acentury of commercial scale history, it will beclear that compromises between the aforemen-tioned, partly conicting, criteria are well estab-lished. Also resulting from the age of urea pro-cess design is the observation that a process canonly be successful if acceptable and competingsolutions to all of these criteria can be combinedinto one process design. Apart from applyingstraightforward normal engineering approaches,

  • 8 Urea

    the challengeofnding anoptimumsynergybet-ween partly conicting criteria, focuses in ureaplant design essentially on a few peculiarities:

    1) The thermodynamic limit on the conver-sion per pass through the urea reactor, com-bined with the azeotropic behavior of theNH3 CO2 system, necessitates a cunningrecycle system design.

    2) The intermediate product ammonium carba-mate is extremely corrosive. A proper com-bination of process conditions, constructionmaterials, and equipment design is thereforeessential.

    3) Theoccurrenceof two side reactions hydro-lysis of urea and biuret formation must beconsidered.

    3.2.1. Recycle of Nonconverted Ammoniaand Carbon Dioxide

    The description of the chemical equilibria inSection 3.1.1 indicates that the conversion of thefeedstocks NH3 and CO2 to urea is limited. Animportant differentiator between processes is theway these nonconverted materials are handled.

    Once-Through Processes. In the very rstprocesses, nonconverted NH3 was neutralizedwith acids (e.g., nitric acid) to produce ammoni-um salts (such as ammonium nitrate) as coprod-ucts of urea production. In this way, a relativelysimple urea process scheme was realized. Themain disadvantages of the once-through pro-cesses are the large quantity of ammonium saltformed as coproduct and the limited amount ofoverall carbon dioxide conversion that can beachieved. A peculiar aspect of this historic de-velopment is a partial revival of these com-bined urea ammonium nitrate production fa-cilities (UAN plants, see Section 3.3.3).

    Conventional Recycle Processes. Once-through processes were soon replaced by total-recycle processes, where essentially all of thenonconverted ammonia and carbondioxidewererecycled to the urea reactor. In the rst genera-tion of total-recycle processes, several licensorsdeveloped schemes in which the recirculationof nonconverted NH3 and CO2 was performedin two stages. Figure 12 is a typical ow sheet

    of these, now called conventional, processes.The rst recirculation stage was operated atmedium pressure (18 25 bar); the second, atlow pressure (2 5 bar). The rst recircula-tion stage comprises at least a decompositionheater (d), in which carbamate decomposes intogaseous NH3 and CO2, while excess NH3 evap-orates simultaneously. The off-gas from thisrst decomposition step was subjected to recti-cation (e), from which relatively pure NH3 (atthe top) and a bottom product consisting of anaqueous ammonium carbamate solution wereobtained. Both products are reycled separatelyto the urea reactor (c). In these processes, allnonconverted CO2 was recycled as an aqueoussolution, whereas the main portion of noncon-verted NH3 was recycled without an associatedwater recycle. Because of the detrimental effectof water on reaction conversion (see Figs. 6,7, and 9), achieving a minimum CO2 recycle(and thus maximum CO2 conversion per re-action pass) was much more important thanachieving a low NH3 recycle. All conventionalprocesses therefore typically operate at highNH3 : CO2 ratios (4 5mol/mol) to maximizeCO2 conversion per pass. Although some ofthese conventional processes, partly equippedwith ingenious heat-exchanging networks, havesurvived until now (see Section 3.3.1), theirimportance decreased rapidly as the so-calledstripping processes were developed.

    Stripping Processes. In the 1960s, theStamicarbon CO2-stripping process was de-veloped, followed later by other processes (seeSection 3.3.2). Characteristic of these processesis that the major part of the recycle of both non-converted NH3 and nonconverted CO2 occursvia the gas phase, such that none of these re-cycles is associated with a large water recycleto the synthesis zone. Another characteristicdifference between conventional and strippingprocesses in terms of the recycle scheme, canbe found in the way heat is supplied to therecirculation zones. The energy balance of theconventional processes is shown in Figure 13. Inthis rst-generation urea process, the heat sup-plied to the urea synthesis solution was usedonly once; therefore, this type of process can bereferred to as an N = 1 process. Such a processrequired about 1.8 t of steam per tonne of urea.

  • Urea 9

    Figure 12. Typical ow sheet of a conventional urea planta) CO2 compressor; b) High-pressure ammonia pump; c) Urea reactor; d)Medium-pressure decomposer; e) Ammonia carba-mate separation column; f) Low-pressure decomposer; g) Evaporator; h) Prilling; i) Desorber (wastewater stripper); j) Vacuumcondensation section

    Figure 13. Conceptual diagram of the heat balance of aconventional urea processHeat to each subsequent heater is supplied in the form ofsteam; the heat is used only once (N = 1).

    The energy balance of a stripping plant isshown in Figure 14. As in conventional plants,heat must be supplied to the urea synthesis solu-tion to decompose unconverted carbamate and toevaporate excess ammonia and water. However,a distinct difference in the heat balance with re-spect to the conventional process is that only theheat in the rst heater (the high-pressure strip-per) is imported. This heat is recovered in a high-pressure carbamate condenser (unconverted am-monia and carbon dioxide are condensed to formammonium carbamate) and reused in the low-pressure heaters. The heat supplied is effectivelyused twice; thus, the term N = 2 process is used.The average energy consumption of the strip-ping process is 0.8 1.0 t of steam per tonne ofurea.

    Figure 14. Conceptual diagram of the heat balance of astripping plantHeat supplied to the rst heater (the stripper) is recoveredin the rst condenser (high-pressure carbamate condenser)and subsequently used again in the low-pressure heaters (de-composers and water evaporators); the heat is effectivelyused twice (N = 2).

    In the 1980s, some processes were describedthat aim at a greater reduction of energy con-sumption by a further application of thismultipleeffect to N = 3 (Fig. 15) [2529]. As can be seenfrom Figures 14 and 15, the steam requirementfor process heating is reduced in these types ofprocesses. However, whether the total energyconsumption for the process is also reduced isdoubtful, if the full capabilities of a N = 2 typeof process are exploited and if the total energysupply scheme, including the energy supply tothe carbon dioxide compressor drive, are takeninto consideration [30]. Moreover, it seems thatthe emphasis in urea technology now is shift-ing from low energy consumption toward other

  • 10 Urea

    factors, such asmore durable constructionmate-rials, more modern process control systems, andsimple process design [31].

    Figure 15. Further heat integration of a stripping plant inconceptual formHeat supplied to the rst heater (the stripper) is effectivelyused three times (N = 3).

    3.2.2. Corrosion [32]

    Urea synthesis solutions are very corrosive.Generally, ammonium carbamate is consideredthe aggressive component. This follows from theobservation that carbamate-containing productstreams are corrosive whereas pure urea solu-tions are not. The corrosiveness of the synthe-sis solution has forced urea manufacturers to setvery strict demands on the quality and composi-tion of construction materials. Awareness of theimportant factors in material selection, equip-ment manufacture and inspection, technologi-cal design and proper operations of the plant,together with periodic inspections and nonde-structive testing are the key factors for safe op-eration for many years.

    Role of Oxygen Content. Since the liquidphase in urea synthesis behaves as an electrolyte,the corrosion it causes is of an electrochemi-cal nature. Stainless steel in a corrosive mediumowes its corrosion resistance to the presence ofa protective oxide layer on the metal. As long asthis layer is intact, the metal corrodes at a verylow rate. Passive corrosion rates of austeniticurea-grade stainless steels are generally between

  • Urea 11

    less steels (e.g., containing 25wt% chromium,22wt%nickel, and2wt%molybdenum) appearto be much less sensitive to this critical temper-ature than 316 L types of steel.

    Sometimes, the NH3 : CO2 ratio in synthesissolutions is also claimed to have an inuence onthe corrosion rate of steels under urea synthesisconditions. Experiments have showed that underpractical conditions this inuence is not measur-able because the steel retains passivity. Sponta-neous activation did not occur. Only with elec-trochemical activation could 316 L types of steelbe activated at intermediateNH3 : CO2 ratios.Atlow and high ratios, 316 L stainless steel couldnot be activated. The higher-alloyed steel type25Cr 22Ni 2Mo showed stable passivity, irre-spective of the NH3 : CO2 ratio, even when acti-vated electrochemically. Of course, these resultsdepend on the specic temperature and oxygencontent during the experiments.

    Material Selection. Corrosion resistance isnot the only factor determining the choice ofconstruction materials. Other factors such asmechanical properties, workability, and weld-ability, as well as economic considerations suchas price, availability, and delivery time, also de-serve attention.

    Stainless steels that have found wide use arethe austenitic grades AISI 316 L and 317 L. Likeall Cr-containing stainless steels,AISI 316 Land317 L are not resistant to the action of suldes.Hence it is imperative in plants using the 316 Land 317 L grades in combination with CO2 de-rived from sulfur-containing gas, to purify thisgas or the CO2 thoroughly.

    In stripping processes, the process conditionsin the high-pressure stripper aremost severewithrespect to corrosion.

    In the Stamicarbon CO2-stripping process, ahigher-alloyed, but still fully austenitic stainlesssteel (25Cr 22Ni 2Mo)was chosen as construc-tion material for the stripper tubes. This choiceensures better corrosion resistance than 316 L or317 L types ofmaterial but still maintains the ad-vantages of workability, weldability, reparabil-ity, and the cheaper price of stainless steel-typematerials.

    In the Snamprogetti stripping processes, ti-tanium usually is chosen for this critical appli-cation, although mechanically bonded bimetal-lic 25Cr 22Ni 2Mo zirconium tubes have also

    been suggested to improve corrosion resistance[37], [38].

    In the ACES process, duplex alloys(ferritic austenitic) are used as constructionmaterial for the stripper tubes.

    3.2.3. Side Reactions [39]

    Three side reactions are of special importance inthe design of urea production processes:

    Hydrolysis of ureaCO(NH2)2 +H2ONH2COONH4 2NH3 +CO2 (3)

    Biuret formation from urea:2CO(NH2)2NH2CONHCONH2 +NH3 (4)

    Formation of isocyanic acid from urea:CO(NH2)2NH4NCONH3 +HNCO (5)

    All three side reactions have in common thedecomposition of urea; thus, the extent to whichthey occur must be minimized.

    The hydrolysis reaction (3) is nothing but thereverse of urea formation. Whereas this reac-tion approaches equilibrium in the reactor, inall downstream sections of the plant the NH3and CO2 concentrations in urea-containing so-lutions are such that Reaction (3) is shifted to theright. The extent to which the reaction occurs isdetermined by temperature (high temperaturesfavor hydrolysis) and reaction kinetics; in prac-tice, this means that retention times of urea-con-taining solutions at high temperatures must beminimized.

    The biuret reaction (4) also approaches equi-librium in the urea reactor [20], [22]. The highNH3 concentration in the reactor shifts Reaction(4) to the left, such that only a small amount of bi-uret is formed in the reactor. In downstream sec-tions of the plant, NH3 is removed from the ureasolutions, thereby creating a driving force forbiuret formation. The extent to which biuret isformed is determined by reaction kinetics; there-fore, the practical measures to minimize biuretformation are the same as described above forthe hydrolysis reaction.

    Reaction (5) shows that formation of iso-cyanic acid from urea is also favored by lowNH3 concentrations. This reaction is especially

  • 12 Urea

    relevant in the evaporation section of the plant.Here, low pressures are applied, resulting in atransfer of NH3 and HNCO into the gas phaseand, consequently, low concentrations of theseconstituents in the liquid phase. Together withthe relatively high temperatures in the evapora-tors, this shifts Reaction (5) to the right. Theextent to which this reaction occurs is again de-termined by kinetics. The HNCO removed viathe gas in the evaporators collects in the processcondensate from the vacuum condensers, wherelow temperatures shift Reaction (5) to the left,again forming urea. As a result of this mecha-nism of chemical entrainment, attempts to min-imize entrainment from evaporators with physi-cal (liquid gas) separation devices are destinedto be unsuccessful.

    3.3. Description of Processes3.3.1. Conventional Processes

    As explained in Section 3.2, conventional pro-cesses have generally been replaced by strippingprocesses.Only twoconventional processesmaystill have some importance in the near future.

    Urea Technologies Inc. (UTI) Heat Recy-cle Process (see Fig. 16). Ammonia (containingpassivating air), recycled carbamate, and about60% of the feed CO2 are charged to the top of anopen-ended coil reactor (c) operating at 210 bar.Ammoniumcarbamate is formedwithin the coil,exits the coil at the bottom, and then ows upand around itthe exothermic heat of carbamateformation in the coil driving the endothermic de-hydration of carbamate to urea outside the coil.The reactor is claimed to achieve a uniform tem-perature prole in this way. In the reactor, a rel-ative high NH3 : CO2 ratio (4.2 : 1) is applied.The reactor efuent is depressurized and sub-cooled, and the ashed gases are released beforethe rst decomposer (f ). Gases leaving the rstdecomposer separator (g) are mixed with about40% of the feed carbon dioxide and partiallycondensed in the heat recovery section (i k).The two combined gas streams are then furthercondensed and form the carbamate recycle ow.Urea as an 86 88% solution is concentratedby evaporation (i) before granulation or prilling.The process is applied in eight small-scale andtwo medium-scale plants.

    MTC (Mitsui Toatsu Corporation) Con-ventional Processes of Toyo Engineering Cor-poration. The conventional processes devel-oped by Toyo Engineering Corporation (TEC)were successfully commercialized until themid-1980s. The continuous evolution of these pro-cesses is reected in their sequential nomencla-ture:

    TR A Total-Recycle A ProcessTR B Total-Recycle B ProcessTR C Total-Recycle C ProcessTR CI Total-Recycle C Improved ProcessTR D Total-Recycle D Process

    Partial-recycle versions of these processeswere also realized. These TEC MTC conven-tional processes were applied in more then70 plants. However, the licensor of these pro-cesses has announced a stripping process (theACES process; see Section 3.3.2.3); this proba-blymeans the end of theTECMTCconventionalprocess line.

    3.3.2. Stripping Processes

    3.3.2.1. Stamicarbon CO2-Stripping Process(Figs. 17 and 18)

    The synthesis stage of the Stamicarbon processconsists of a urea reactor (c), a stripper for un-converted reactants (d), a high-pressure carba-mate condenser (e), and a high-pressure reactoroff-gas scrubber (f ). To realize maximum ureayield per pass through the reactor at the stipu-latedoptimumpressure of 140 bar, anNH3 :CO2molar ratio of 3 : 1 is applied. The greater part ofthe unconverted carbamate is decomposed in thestripper, where ammonia and carbon dioxide arestripped off. This stripping action is effected bycountercurrent contact between the urea solutionand fresh carbon dioxide at synthesis pressure.Low ammonia and carbon dioxide concentra-tions in the stripped urea solution are obtained,such that the recycle from the low-pressure recir-culation stage (h, j) isminimized.These lowcon-centrations of both ammonia and carbon dioxidein the stripper efuent can be obtained at rela-tively low temperatures of the urea solution be-cause carbon dioxide is only sparingly solubleunder such conditions.

  • Urea 13

    Figure 16. UTI heat recycle urea processa) CO2 compressor; b) High-pressure ammonia feed pump; c) Urea reactor with internal coil; d) Air compressor; e) Liquiddistributor (used as rst ash vessel); f ) First decomposer; g) First separator; h) Second decomposer; i) Urea concentrator;j) Carbamate heater; k) Ammonia heater; l) High-pressure carbamate recycle pumpCW=Cooling water

    Figure 17. Stamicarbon CO2-stripping urea process (The process suitable for combination with a granulation plant is shownhere; combination with prilling is also possible.)a) CO2 compressor; b) Hydrogen removal reactor; c) Urea reactor; d) High-pressure stripper; e) High-pressure carbamatecondenser; f) High-pressure scrubber; g) Low-pressure absorber; h) Low-pressure decomposer and rectier; i) Pre-evaporator;j) Low-pressure carbamate condenser; k) Evaporator; l) Vacuum condensation section; m) Process condensate treatmentCW=Cooling water; TCW=Tempered cooling water

  • 14 Urea

    Figure 18. Functional block diagram of the StamicarbonCO2-stripping urea process

    Condensation of ammonia and carbon diox-ide gases, leaving the stripper, occurs in the high-pressure carbamate condenser (e) at synthesispressure. As a result, the heat liberated from am-monium carbamate formation is at a high tem-perature. This heat is used for the productionof 4.5-bar steam for use in the urea plant it-self. The condensation in the high-pressure carb-amate condenser is not effected completely. Re-maining gases are condensed in the reactor andprovide the heat required for the dehydration ofcarbamate, as well as for heating the mixture toits equilibrium temperature. In an improvementto this process, the condensation of off-gas fromthe stripper is carried out in a prereactor, wheresufcient residence time for the liquid phase isprovided. As a result of urea and water forma-tion in the condensing zone, the condensationtemperature is increased, thus enabling the pro-duction of steam at a higher pressure level [40].

    The feed carbon dioxide, invariably originat-ing from an associated ammonia plant, alwayscontains hydrogen. To avoid the formation of ex-plosive hydrogen oxygen mixtures in the tailgas of the plant, hydrogen is catalytically re-moved from the carbon dioxide feed (b). Apartfrom the air required for this purpose, additionalair is supplied to the fresh carbon dioxide inputstream. This extra portion of oxygen is neededto maintain a corrosion-resistant layer on thestainless steel in the synthesis section. Beforethe inert gases, mainly oxygen and nitrogen,are purged from the synthesis section, they arewashed with carbamate solution from the low-pressure recirculation stage in the high-pressurescrubber (f ) to obtain a low ammonia concen-tration in the subsequently purged gas. Further

    washing of the off-gas is performed in a low-pressure absorber (g) to obtain a purge gas thatis practically ammonia free. Only one low-pres-sure recirculation stage is required due to the lowammonia and carbon dioxide concentrations inthe stripped urea solution. Because of the idealratio between ammonia and carbon dioxide inthe recovered gases in this section, water dilu-tion of the resultant ammonium carbamate is at aminimum despite the low pressure (about 4 bar).As a result of the efciency of the stripper, thequantities of ammonium carbamate for recycleto the synthesis section are also minimized, andno separate ammonia recycle is required.

    The urea solution coming from the recircu-lation stage contains about 75wt% urea. Thissolution is concentrated in the evaporation sec-tion (k). If the process is combinedwith a prillingtower for nal product shaping, the nal mois-ture content of urea from the evaporation sec-tion is ca. 0.25wt%. If the process is combinedwith a granulation unit, the nal moisture con-tent may vary from 1 to 5wt%, depending ongranulation requirements. Higher moisture con-tents can be realized in a single-stage evaporator,whereas lowmoisture contents are economicallyachieved in a two-stage evaporation section.

    When urea with an extremely low biuret con-tent is required (at amaximumof 0.3wt%), pureurea crystals are produced in a crystallizationsection. These crystals are separated from themother liquor by a combination of sieve bendsand centrifuges and are melted prior to nalshaping in a prilling tower or granulation unit.

    The process condensate emanating from wa-ter evaporation from the evaporation or crys-tallization sections contains ammonia and urea.Before this process condensate is purged, ureais hydrolyzed into ammonia and carbon diox-ide (l), which are stripped off with steam andreturned to urea synthesis via the recirculationsection. This process condensate treatment sec-tion can produce water with high purity, thustransforming this wastewater treatment intothe production unit of a valuable process con-densate, suitable for, e.g., cooling tower or boilerfeedwater makeup. Since the introduction of theStamicarbon CO2-stripping process, some 125units have been built according to this processall over the world.

  • Urea 15

    Figure 19. Functional block diagram of the Snamprogetti self-stripping process

    3.3.2.2. Snamprogetti Ammonia- andSelf-Stripping Processes [4147]

    In the rst generation of NH3- and self-strippingprocesses, ammoniawas used as stripping agent.Because of the extreme solubility of ammonia inthe urea-containing synthesis uid, the stripperefuent contained rather large amounts of dis-solved ammonia, causing an ammonia overloadin downstream sections of the plant. Later ver-sions of the process abandoned the idea of us-ing ammonia as stripping agent; stripping wasachieved only by supply of heat (thermal orself-stripping). Even without using ammoniaas a stripping agent, the NH3 : CO2 ratio inthe stripper efuent is relatively high, so therecirculation section of the plant requires anammonia carbamate separation section, as inconventional processes (see Fig. 19).

    The process uses a vertical layout in the syn-thesis section. Recycle within the synthesis sec-tion, from the stripper (h) via the high-pressurecarbamate condenser (f ), through the carbamateseparator (e) back to the reactor (b), is main-tained by using an ammonia-driven liquid liq-uid ejector (c) [43], [45] (see Fig. 20). In the re-actor,which is operated at 150 bar, anNH3 : CO2molar feed ratio of ca. 3.5 is applied. The strip-per is of the falling lm type [46]. Since strip-ping is achieved thermally, relatively high tem-peratures (200 210 C) are required to obtain areasonable stripping efciency. Because of this

    high temperature, stainless steel is not suitableas construction material for the stripper from acorrosion point of view; titanium and bimetalliczirconium stainless steel tubes have been used[37], [38].

    Off-gas from the stripper is condensed in akettle-type boiler (f ) [44]. At the tube side ofthis condenser the off-gas is absorbed in recy-cled liquid carbamate from themedium-pressurerecovery section. The heat of absorption is re-moved through the tubes, which are cooled bythe production of low-pressure steam at the shellside. The steam produced is used effectively inthe back end of the process.

    In the medium-pressure decomposition andrecirculation section, typically operated at18 bar, the urea solution from the high-pressurestripper is subjected to the decomposition ofcarbamate and evaporation of ammonia (i). Theoff-gas from this medium-pressure decomposeris rectied. Liquid ammonia reux is applied tothe top of this rectier ( j); in this way a top prod-uct consisting of pure gaseous ammonia, and abottom product of liquid ammonium carbamateare obtained. The pure ammonia off-gas is con-densed (k) and recycled to the urea synthesissection. To prevent solidication of ammoniumcarbamate in the rectier, some water is addedto the bottom section of the column to dilute theammonium carbamate below its crystallizationpoint. The liquid ammonium carbamate watermixture obtained in this way is also recycled to

  • 16 Urea

    Figure 20. Schematic of the Snamprogetti self-stripping processa) CO2 compressor; b) Urea reactor; c) Ejector; d) High-pressure ammonia pump; e) Carbamate separator; f) High-pressurecarbamate condenser; g) High-pressure carbamate pump; h) High-pressure stripper; i) Medium-pressure decomposer and rec-tier; j) Ammonia carbamate separation column; k) Ammonia condenser; l) Ammonia receiver; m) Low-pressure ammoniapump; n) Ammonia scrubber; o) Low-pressure decomposer and rectier; p) Low-pressure carbamate condenser; q) Low-pres-sure carbamate receiver; r) Low-pressure off-gas scrubber; s) First evaporation heater; t) First evaporation separator; u) Secondevaporation heater; v) Second evaporation separator; w) Wastewater treatment; x) Vacuum condensation sectionCW=Cooling water

    the synthesis section. The purge gas of the am-monia condensers is treated in a scrubber (n)prior to being purged to the atmosphere.

    The urea solution from the medium-pressuredecomposer is subjected to a second lowpressuredecomposition step (o). Here, further decompo-sition of ammonium carbamate is achieved, sothat a substantially carbamate-free aqueous ureasolution is obtained. Off-gas from this low-pres-sure decomposer is condensed (p) and recycledas an aqueous ammonium carbamate solution tothe synthesis section via the medium-pressurerecovery section.

    Concentrating the urea water mixture ob-tained from the low-pressure decomposer is per-formed in a single or double evaporator (s v),depending on the requirements of the nishingsection. Typically, if prilling is chosen as the -nal shaping procedure, a two-stage evaporator isrequired, whereas in the case of a uidized-bedgranulator a single evaporation step is sufcientto achieve the required nal moisture contentof the urea melt. In some versions of the pro-cess, heat exchange is applied between the off-gas from the medium-pressure decomposer andthe aqueous urea solution to the evaporation sec-

    tion. In this way, the consumption of low-pres-sure steam by the process is reduced.

    The process condensate obtained from theevaporation section is subjected to a desorption hydrolysis operation to recover the urea and am-monia contained in the process condensate.

    Up to now, about 70 plants have been de-signed according to the Snamprogetti ammonia-and self-stripping processes.

    Figure 21. Functional block diagram of the ACES ureaprocess

  • Urea 17

    Figure 22. Schematic of the ACES processa) Urea reactor; b) High-pressure ammonia pump; c) CO2 compressor; d) Stripper; e) High-pressure carbamate condensers;f) High-pressure scrubber; g) High-pressure carbamate pump; h) Medium-pressure absorber; i) Medium-pressure decom-poser; j) Low-pressure decomposer; k) Low-pressure absorber; l) Evaporators; m) Process condensate stripper; n) Hydrolyzer;o) Prilling tower; p) Granulation section; q) Surface condensersCW=Cooling water

    3.3.2.3. ACES Process [29], [48], [49]The ACES (i.e., Advanced Process for Costand Energy Saving) process has been devel-oped by Toyo Engineering Corporation. Its syn-thesis section consists of the reactor (a), strip-per (d), two parallel carbamate condensers (e),and a scrubber (f ) all operated at 175 bar (seeFigs. 21 and 22).

    The reactor is operated at 190 C and anNH3 : CO2 molar feed ratio of 4 : 1. Liquidammonia is fed directly to the reactor, whereasgaseous carbon dioxide after compression is in-troduced into the bottom of the stripper as astripping aid. The synthesis mixture from thereactor, consisting of urea, unconverted ammo-nium carbamate, excess ammonia, and water, isfed to the top of the stripper. The stripper hastwo functions. Its upper part is equipped withtrays where excess ammonia is partly separatedfrom the stripper feed by direct countercurrentcontact of the feed solution with the gas comingfrom the lower part of the stripper. This prestrip-

    ping in the top is said to be required to achieveeffective CO2 stripping in the lower part. In thelower part of the stripper (a falling lm heater),ammoniumcarbamate is decomposed and the re-sulting CO2 and NH3 as well as the excess NH3are evaporated by CO2 stripping and steam heat-ing. The overhead gaseous mixture from the topof the stripper is introduced into the carbamatecondensers (e). Here, two units in parallel are in-stalled, where the gaseous mixture is condensedand absorbed by the carbamate solution comingfrom the medium-pressure recovery stage. Heatliberated in the high-pressure carbamate con-densers is used to generate low-pressure steamin one of the condensers and to heat the ureasolution from the stripper after the pressure isreduced to about 19 bar in the shellside of thesecond carbamate condenser. The gas and liq-uid from the carbamate condensers are recycledto the reactor by gravity ow. The urea solutionfrom the stripper, with a typical NH3 contentof 12wt%, is puried further in the subsequent

  • 18 Urea

    medium- and low-pressure decomposers (i, j),operating at 19 and3 bar, respectively.Ammoniaand carbon dioxide separated from the urea solu-tion here are recovered through stepwise absorp-tion in the low- and medium-pressure absorbers(h, k). Condensation heat in the medium-pres-sure absorber is transferred directly to the aque-ous urea solution feed in the nal concentrationsection. In this nal concentration section (l), thepuried urea solution is concentrated further ei-ther by a two-stage evaporation up to 99.7% forurea prill production or by a single evaporationup to 98.5% for urea granule production. Watervapor formed in the nal concentrating sectionis condensed in surface condensers (q) to formprocess condensate. Part of this condensate isused as an absorbent in the recovery sections,whereas the remainder is puried in the processcondensate treatment section by hydrolysis andsteam stripping, before being discharged fromthe urea plant.

    The highly concentrated urea solution is -nally processed either through the prilling tower(o) or via the urea granulator (p). Instead of con-centration via evaporation, the ACES processcan also be combined with a crystallization sec-tion to produce urea with low biuret content.

    Until now, the ACES process has been usedin seven urea plants.

    3.3.2.4. Isobaric Double-Recycle Process[26], [28], [50]

    The isobaric double-recycle (IDR) strippingpro-cess, developed byMontedison, is characterizedby recycle of most of the unreacted ammoniaand ammonium carbamate in two decomposersin series, both operating at the synthesis pres-sure. A high molar NH3 : CO2 ratio (4 : 1 to5 : 1) in the reactor is applied. As a result ofthis choice of ratio, the reactor efuent containsa relatively high amount of nonconverted am-monia. In the rst, steam-heated, high-pressuredecomposer, this large quantity of free ammo-nia is mainly removed from the urea solution.Most of the residual ammonia, as well as someammonium carbamate, is removed in the secondhigh-pressure decomposer where steam heatingand CO2 stripping are applied. The high-pres-sure synthesis section is followed by two lower-pressure decomposition stages of traditional de-

    sign, where heat exchange between the condens-ing off-gas of the medium-pressure decomposi-tion stage and the aqueous urea solution to thenal concentration section improves the over-all energy consumption of the process. Probablybecause of the complexity of this process, it hasnot achieved great popularity so far. The IDRprocess or parts of the process are used in fourrevamps of older conventional plants.

    3.3.3. Other Processes

    Urea AmmoniumNitrate (UAN)Produc-tion. Mixtures of urea (mp 133 C) and ammo-nium nitrate (mp 169 C) with water have a eu-tectic point at 26.5 C [51]. As a result solu-tions with high nitrogen content can be madewith solidication temperatures below ambienttemperature. These mixtures, called UAN solu-tions, are used as liquid nitrogen fertilizers.

    UANsolutions can bemade bymixing the ap-propriate amounts of solid urea and solid ammo-niumnitratewithwater or, alternatively, in a pro-duction facility specially designed to produceUAN solutions. In this latter category the Stami-carbon CO2-stripping technology is especiallysuitable. In a partial-recycle version of this pro-cess, unconverted ammonia emanating from thestripped urea solution and from the reactor off-gas is neutralized with nitric acid. The ammo-nium nitrate solution thus formed and the ureasolution from the synthesis section are mixed toyield a product solution with the desired nitro-gen content (32 35wt%) directly. Such a plantdesignated for the production of UAN solutionsis cheaper than the separate production of ureaand ammonium nitrate in investment and in op-erating costs, because evaporation, nal productshaping for both urea and ammonium nitrate,and wastewater treatment sections are not re-quired.

    Integrated Ammonia Urea Production.Both feedstocks required for urea production,ammonia and carbon dioxide, are usually ob-tained from an ammonia plant. Since an ammo-nia plant is a net heat (steam) producer and aurea plant is a net heat (steam) consumer, it isnormal practice to integrate the steam systems ofboth plants. Since both processes usually containa process condensate treatment section where

  • Urea 19

    volatile components are removed by steam strip-ping, the advantages of combining these sectionshave been explored [5254]. Several attemptsfor further integration of mass streams of bothprocesses have been published [5559]. Despitethe claimed reduction in both capital and rawmaterial cost, these highly integrated processschemes have not gained acceptance mainly be-cause of their increased complexity.

    3.4. Efuents and Efuent Reduction

    Gaseous Efuents. There are potentiallytwo sources for air pollution from a urea plant:(1) gaseous ammonia emission from continuousprocess vents, and (2) urea dust and ammoniaemissions from the nishing section (prilling orgranulation).

    GaseousEmissions fromProcessVents. Non-condensable gases enter the urea process as con-taminants in the raw materials, as process airintroduced for corrosion protection, and as airleaking into the vacuum sections of the process.At places where these noncondensable gases arevented, proper measures should be taken to min-imize ammonia losses. The present state of theart allows reduction of these losses to

  • 20 Urea

    the pre-desorber and the desorber operate atlow pressure (1 5 bar), low-pressure steam asproduced in the urea synthesis section can beused as stripping agent. The combination ofpre-desorption and countercurrent operation ofthe hydrolyzer ensures that the chemical equilib-rium of the hydrolysis reaction does not limit theminimum achievable urea content in wastewaterto concentrations

  • Urea 21

    Kaltenbach Thuring, and Montedison. Pangranulation processes have also beendeveloped,for example, by Norsk Hydro and the TennesseeValleyAuthority (TVA).More recently spouted-bed and uidized-bed granulation techniqueswere introduced. A spouted-bed technique wasdeveloped by Toyo Engineering Corporation;however, the uidized-bed technology fromHy-droAgri seems to bemost successful at this time.

    All granulation processes require the additionof formaldehyde or formaldehyde-containingcomponents. Also common to all granulationtechniques is that they yield products with largerdiameters compared to prilling. However, theircapabilities in this respect differ from each otherto quite an extent.

    Although the improvements brought aboutby granulation are beyond doubt, prilling tech-niques still have a place because of the lowerinvestment and lower variable costs associatedwith prilling compared to granulation.

    4. Forms Supplied, Storage, andTransportation

    Forms Supplied. Urea may be supplied ei-ther in solid form or as a liquid. For liquid com-pound fertilizers, urea is a favorite ingredient.It is generally used in combination with ammo-nium nitrate as an aqueous solution to obtainliquids containing 32 35wt% nitrogen. Thesesolutions are designated as UAN-32 to UAN-35.

    The solid forms are generally classied asgranular or prilled products, because of the dif-ferences in handling properties. Prilled productis considered less suitable for bulk transporta-tion because prills have lower crushing strength,a lower shock resistance, and a higher cakingtendency than granules. Because of this, prilledproducts are usually marginally cheaper thangranulated product. Granulated product usuallyalso has a larger diameter (2.0 2.5mm) thanprills (1.5 2.0mm),makinggranulesmore suit-able for bulk blending to produce compound fer-tilizers.

    Special Grades. Themajority of urea is des-ignated as fertilizer grade; however, some spe-cial forms have found limited application:

    Technical Grade. Technical-grade ureashould be without additions; color, ash-, and

    metal content are sometimes also specied. Forurea used to produceurea formaldehyde resins,its content of pH-controlling trace componentsis important. Because of this, technical-gradeurea at present is mostly traded as a performanceproduct, rather than being bound to narrow spec-ication limits. The tness of the product for useis judged by application-specic tests.

    Low-Biuret Grade. A maximum biuret con-tent up to 1.2wt% is considered acceptable fornearly all fertilizer applications of urea. Onlyfor the relative small market segment of foliarspray to citrus crops is a lower biuret content(max. 0.3%) desirable.

    Feed Grade. Some urea is also used directlyas a feed component for cattle. Urea used for thispurpose should be free of additions. Feed-gradeurea is supplied in the form of microprills witha mean diameter of about 0.5mm.

    Slow-Release Grades. Studies show thatonly 50 60% of fertilizer nitrogen appliedto soil is usually recovered by crop plants. Sev-eral attempts have been made to increase thispercentage by slowing the release of fertilizerto the ground via coating or additions [71].

    Urea Supergranules. Granulated productwith a very large diameter (up to 15mm) hasfound limited application for deep placement inwetland rice [72] and forest fertilization.

    Storage. The shift frombagged to bulk trans-port and storage of prilled and granulated ureahas called for warehouse designs in which largequantities of urea can be stored in bulk. Thesewarehouses should be designed in such a waythat the product suffers little degradation.Degra-dation may result from: (1) segregation of nes;(2) disintegration; and (3) absorption, loss, ormigration of water.

    Segregation of nes can be avoided throughuniform product spreading during pouring. Dis-integration can be minimized by:1) Providing the product pouring system with a

    pouring height adjuster2) Design of product-friendly reclaiming

    systems, because reclaiming the product bymeans of payloaders and tractor shovels in-variably leads to product disintegrationCaking and subsequent product disintegra-

    tion at unloading are known to result from wa-ter absorption. What is not commonly known,

  • 22 Urea

    however, is that excessive drying of the prod-uct during storage also leads to a higher cak-ing tendency and that migration of water fromwarm product in the bulk of a pile to the cold sur-face leads to crust formation. Thus, attempts todecrease water absorption through refrigerationor air conditioning, dehumidication, or spaceheating may cause the air in the warehouse tobecome too dry or may result in too great a tem-perature difference between the product and thesurrounding air. Instead, the warehouse (espe-cially the roof) should be airtight and thoroughlyinsulated. The caking tendency of urea can bereduced by addition of small amounts of form-aldehyde (up to 0.6wt%) to the urea melt or byaddition of surfactants to the solid product.

    Transportation. Urea prills and granules aretransported bybulk transport in trucks, ships, railcars, etc. Towithstand numerous and rapid load-ing and unloading operations, product for bulktransport should have a high initial physical sta-bility. Great demands are made, especially onthe shock resistance of the product, e.g., at sea-port loading andunloading facilities. In addition,a number of good housekeeping rules shouldbe adhered to:

    1) Do not load or unload during rain2) Make sure that themeans of transport is clean

    and dry3) Close the ships hold when rain is imminent4) Do not replace the air above the product or

    ventilate the holds5) Cover the product (e.g., by polyethylene

    sheeting) during prolonged transport6) Product should be spread rather then poured

    solely from one point to prevent dust coningdue to segregation

    7) Restrict the pouring height to avoid unnec-essary disintegration

    Liquid Fertilizer Transport. Liquid fertil-izers are transported by tank cars, railway tanks,ships, and pipelines. Although liquid fertilizer isgenerally accepted as themost economic form todistribute, the solid form is still the most popularby far. Distribution of large quantities of liquidfertilizer requires a complex infrastructure andis limited at present to large farm units in devel-oped countries. Transport and storage of UANsolutions in carbon steel lines and tanks require

    the addition of a corrosion inhibitor to the solu-tion.

    5. Quality Specications andAnalysis

    Typical quality specications for fertilizer-gradeurea are summarized in Table 2. The capabilitiesof a modern urea plant are better than the typicaltrade data given in this table.

    Table 2. Typical product specications for fertilizer-grade urea

    Specication Prilledproduct

    Granulated product

    Nitrogen content, wt% min. 46 min. 46Biuret content, wt% max. 1 max. 1Water content, wt% max. 0.3 max. 0.25Crushing strength, bar 20 25 30 60Shock resistance, wt% min. 85 100Product size1.0 2.4mm, wt% 90 951.6 4.0mm, wt% 95

    Bulk density (loose), kg/m3 730 750 790

    The total nitrogen content is usually de-termined by digesting urea with sulfuric acidtoyield ammonium sulfate. The ammonia con-tent is then determined by distillation and titra-tion. Alternatively, the total N content may bedetermined by the Kjeldahl method or by usinga method based on hydrolyzing urea with ureasefollowed by titration of the ammonia formed.

    The water content is usually determinedwith Karl Fischer reagent (Gas Production,Chap. 8.2.2.3.).

    Biuret is determined by the formation of aviolet-colored complex of biuret with copper(II)sulfate in an alkaline medium and subsequentmeasurement of the absorbance of the coloredsolution at 546 nm.

    Crushing strength is dened as the force re-quired per unit cross-sectional area of a granuleto crush the granule or, if it is not crushed, theforce atwhich it is deformedby0.1mm.A singlegranule is subjected to a force that is increasedat a constant rate, the force at breakage (or at0.1-mm deformation) being recorded.

    The shock resistance of granules is denedas the weight percentage of a sample that isnot crushed when subjected to a specied shockload. To determine shock resistance, a sampleof prills or granules is shot against a metal plate

  • Urea 23

    by means of compressed air under normalizedconditions. The amount of nondamaged productthat remains after the test is determined.

    The granulometry of the product is measuredby conventional sieve techniques.

    6. UsesUrea is used for soil and leaf fertilization (morethan 90% of the total use); in the manufactureof urea formaldehyde resins; in melamine pro-duction; as a nutrient for ruminants (cattle feed);and in miscellaneous applications.

    Soil and Leaf Fertilization. Worldwide,urea has become themost important nitrogenousfertilizer. Urea has the highest nitrogen contentof all solid nitrogenous fertilizers; therefore, itstransportation costs per tonne of nitrogen nutri-ent are lowest. Urea is highly soluble in waterand thus very suitable for use in fertilizer so-lutions (e.g., foliar feed fertilizers). Urea isalso used as a raw material for the productionof compound fertilizers. Compound fertilizersmay be produced by mixing in urea melts orurea solutions before shaping the compoundfertilizers or by mixing solid urea prills or gran-ules with other fertilizers (bulk blending). In thelatter case, the product sizes must match to pre-vent segregation of the products during furtherhandling.

    Urea Formaldehyde Resins (AminoResins, Chap. 7.1.). A signicant proportionof urea production is used in the preparationof urea formaldehyde resins. These syntheticresins are employed in the manufacture of adhe-sives, molding powders, varnishes, and foams.They are also used to impregnate paper, textiles,and leather.

    Melamine Production. At present, nearlyall melamine production is based on urea asa feedstock (Melamine and Guanamines,Chap. 4.). Since ammonia is formed as a co-product in melamine production from urea (seeChap. 1), integration of the urea and melamineproduction processes is benecial.

    Feed for Cattle and other Ruminants. Be-cause of the activity of microorganisms in their

    cud, ruminants can metabolize certain nitrogen-containing compounds, such as urea, as proteinsubstitutes. In the United States this capabilityis exploited on a large scale. In Western Europe,by contrast, not much urea is used in cattle feed.

    Other Uses. On a smaller scale, urea is em-ployed as a rawmaterial or auxiliary in the phar-maceutical industry, the fermenting and brewingindustries, and the petroleum industry. Urea canbe used for the removal of NOx from ue gases.Urea is also used as a solubilizing agent for pro-teins and starches, and as a deicing agent forairport runways.

    7. Economic AspectsThe predicted growth in demand for urea un-til 1997 is slightly more than 3% per year,bringing the total urea demand in 1997 to some89106 t/a. Most of the growth will occur inAsia, with China and India in the lead. A lit-tle more than 7% of the worldwide demand forurea is from industry, in which Europe takes aleading role, ahead of North America and theindustrialized countries of Asia.

    The predicted growth in capacity up to 1997will be around 1.5% per year, resulting ina total installed capacity of some 100106 tby 1997. Between 1994 and 1997, new plantswill account for an increase in capacity of ca.6.5106 t, whereas closure of old plants will re-sult in a reduction of the installed capacity bysome 2106 t. The worlds capacity utilizationcan thus be calculated to be ca. 89%.

    Because of geopolitical changes during theearly 1990s, different economic laws have be-come valid in the former Soviet Union and East-ern Europe, causing those countries to supply totheworldmarket large amounts of prilled urea atprices far below the cost tomany producers [e.g.,sales prices, free on board (FOB) Black Sea $ 75per tonne in 1993]. This situation is not expectedto last long, although FOB prices at Black Seaports will generally remain lower compared toother places. In the 1990s urea prices have shownlarge uctuations. For instance, in 1994 1995lows of $ 100 per tonne and highs of $ 250 pertonne have been reported. Predictions of futuredevelopments of urea prices are therefore highlyspeculative.

  • 24 Urea

    To cope with tough competition on the worldmarket, producers have the tendency to buildplants with very high single line capacities(2000 t/d or more), in which operating reliabil-ity is of extreme importance. Uninterrupted op-erating periods of more than one year are oftenachieved.

    Furthermore, producers are increasinglyshifting production facilities (both new plantsand relocations) to places where natural gas isplentiful and cheap. Europe seems to have lostits competitive edge in export markets due to itsexpensive feedstocks.

    The investment costs (in 106 $) for a presentstate-of-the-art total-recycle urea plant are esti-mated to be:

    1000-t/d plant (single line) 431500-t/d plant (single line) 522000-t/d plant (single line) 62

    8. Urea Derivatives

    Barbituric acids and derivatives, seeHypnotics, Chap. 5.1.

    8.1. Thermal Condensation Products ofUrea

    Thermolysis of urea gives biuret, triuret, andcyanuric acid, and in a special process,melamineis produced.

    Biuret [108-19-0], imidodicarbonic di-amide, H2NCONHCONH2, mp 193 C, is pro-duced by heating urea in inert hydrocarbons at110 125 C or in the melt at 127 C [73], [74].

    Triuret [556-99-0], diimidotricarbonic di-amide, H2NCONHCONHCONH2, mp 231 C,is produced by decomposing urea in a thin lm at120 125 C [75]. It is also obtained by treating2mol of urea with 1mol of phosgene in tolueneat 70 80 C [76], [77].

    Cyanuric acid [108-80-5], 1,3,5-triazine-2,4,6 (1H,3H,5H )-trione (1), is formed on heat-ing urea in the presence of zinc chloride, sulfurylchloride, or chlorosulfuric acid [78].

    Melamine [108-78-1], 1,3,5-triazine-2,4,6-triamine (2) is produced industrially from urea[79] (Melamine and Guanamines, Chap. 4.).

    8.2. Alkyl- and Arylureas

    Most simple substituted alkylureas are crys-talline products. Tetramethyl- and tetraethylureaand some cyclic ureas are liquids. Alkylatedand arylated ureas are used in the production ofplant protection agents, in pharmaceutical anddye chemistry, as plasticizers, and as stabiliz-ers. Alkylureas and polyalkyleneureas are usedas additives in the production of aminoplastics.

    Various processes can be used for the produc-tion of substituted ureas, the most important ofwhich are listed in Table 3.

    Table 3. Production processes for substituted ureas

    Starting materials Product

    Urea and amines ureas substituted at one or bothnitrogen atoms and cyclic ureas

    Urea and tertiary alcohols ureas substituted at onenitrogen atom

    Phosgene and amines ureas substituted at onenitrogen atom

    Isocyanates and NH3 or amines ureas substituted at one or bothnitrogen atoms

    Carbamoyl chloride and NH3 oramines

    ureas substituted at one or bothnitrogen atoms

    Esters of carbonic or carbamicacids

    ureas substituted at one or bothnitrogen atoms

    Urea and aldehydes or ketones ureas substituted at one or bothnitrogen atoms and cyclic ureas

    8.2.1. Transamidation of Urea with Amines

    The transamidation of urea with amines is oneof the most important industrial production pro-cesses for substituted ureas. Monosubstituted

  • Urea 25

    ureas are produced by condensation of urea witha sufciently basic amine in a 1 : 1 molar ratioin the melt at 130 150 C [80]. Symmetricallydisubstituted ureas can be produced from 2molof amine and 1mol of urea at 140 170 C [81].Instead of the amines, amine salts can also bereacted with urea in the melt or by prolongedboiling in aqueous solution [82].

    Monomethylurea [598-50-5],CH3NHCONH2,mp 102 C, is produced indus-trially by passing monomethylamine into a ureamelt [83]. Monomethylurea is used for the syn-thesis of theobromine.

    Phenylurea [64-10-8], C6H5HNCONH2,mp 147 C, can be produced by heating an aque-ous solution of aniline hydrochloride and urea toits boiling point [84].

    SymmetricN,N -dimethylurea [96-31-1],CH3HNCONHCH3, mp 105 C, is used for thesynthesis of caffeine by the Traube method andfor the production of formaldehyde-free easy-care nishing agents for textiles [84].

    Hexamethylenediurea [2188-09-2],H2NCONH(CH2)6NHCONH2, is obtained byheating a mixture of hexamethylenediaminewith an excess of urea at 130 140 C, withelimination of ammonia [85], [86].

    N,N -Diphenylurea [102-07-8], carban-ilide, C6H5HNCONHC6H5, mp 238 C, can beproduced in high yields by heating 2mol of ani-linewith 1mol of urea in glacial acetic acid [86],[87].

    Polyalkyleneureas can be obtained by treat-ing di-, tri-, and tetraalkylenamines in con-centrated aqueous solution with urea at ca.110 C. They are used, for example, to modifymelamine formaldehyde impregnating resinsand binders for derived timber products. Exam-ples of polyalkyleneureas include the following:

    1,2-Ethylenediurea,H2NCONHCH2CH2 HNCONH2, mp198 C1,3-Propylenediurea,H2NCONHCH2CH2CH2 HNCONH2,mp 186 C

    Diethylenetriurea, H2NCONHCH2CH2 N(CONH2) CH2CH2 HNCONH2,mp 215 CDipropylenetriurea, H2NCONH (CH2)3 N(CONH2) (CH2)3 HNCONH22-Hydroxypropylene-1,3-diurea,H2NCONHCH2CH(OH) CH2 HNCONH2,mp 147 C

    Cycloalkyleneureas. 2-Imidazolidinone[120-93-4], ethyleneurea (3), mp 131 C [88],[89]; 2-oxohexahydropyrimidine [65405-39-2],propyleneurea (4), mp 260 265 C [90]; and2-oxo-5-hydroxyhexahydropyrimidine, 5-hy-droxypropyleneurea (5), are produced indus-trially in the melt above 180 C, preferablyat 200 230 C, by condensation of 1,2-eth-ylenediamine or 1,3-propylenediamine withurea and elimination of ammonia. These cy-cloalkyleneureas are used in the form of theirN,N -dihydroxymethyl compounds for easy-care nishes for cellulose-containing textiles[91].

    8.2.2. Alkylation of Urea with TertiaryAlcohols

    Urea can be alkylated with tertiary alcohols inthe presence of sulfuric acid [92], [93].

    tert-Butylurea, (CH3)3CHNCONH2,mp 182 C, is produced by treating urea with2mol of tert-butanol in the presence of ca. 2molof concentrated sulfuric acid at 20 25 C withice cooling [9294].

    8.2.3. Phosgenation of Amines

    Symmetrically disubstituted ureas are producedin good yields by passing phosgene into so-lutions of amines in aromatic hydrocarbons[95]. In some cases, aqueous solutions or sus-pensions of amines can also be reacted with

  • 26 Urea

    phosgenes [96]. The phosgenation of mixedaliphatic aromatic amines is carried out indus-trially in the presence of sodium hydroxide at40 60 C [97]. For the production of substan-tive dyes, aminosulfone or aminocarboxylic acidgroups are bonded by means of phosgenation[98]

    2RNH2 +COCl2RNHCONHR+2HCl

    Tetramethylurea [632-22-4],(CH3)2NCON(CH3)2, bp 156.5 C, is producedfrom dimethylamine and phosgene and is usedas an aprotic solvent [99].

    Asymmetric Diphenylurea,(C6H5)2NCONH2,mp189 C, is produced fromdiphenylamine, phosgene, and ammonia:

    COCl2 + (C6H5)2NH+3NH3 (C6H5)2NCONH2 + 2NH4Cl

    Symmetric dimethyldiphenylurea,mp 121 127 C, is obtained by treating phos-gene with monomethylaniline and sodium hy-droxide.

    Symmetric dialkyldiarylureas are used underthe name Centralite as plasticizers and stabiliz-ers for nitrocellulose and propellants [97].

    8.2.4. Reaction of Amines with Cyanates(Salts)

    The salts of aliphatic or aromatic amines reactwith potassiumcyanate at 20 60 C to give sub-stituted ureas in high yields [100], [101].

    R NH2HX+KNCO+H2ORHNCONH2 +KX+OH

    4-Ethoxyphenylurea [150-69-6] (dulcin) isproduced from potassium cyanate and p-phenetidine hydrochloride in aqueous solutionat room temperature [100]. This production

    method can be applied to aminosulfonic andaminocarboxylic acids, whereby the betaine-like salts formed by these acids react with potas-sium cyanate to give ureas that are substituted atonly one NH2 group [101]. Sulfonamides reactwith potassium cyanate to give potassium saltsof the corresponding sulfonylureas, from whichthe sulfonylureas are obtained by acidication[102]:

    8.2.5. Reaction with Isocyanates

    Symmetrically disubstituted ureas can also beproduced from isocyanates by prolonged heat-ing in aqueous solution [103], [104]:

    When ammonia or primary or secondaryamines are reacted with isocyanates, the corre-sponding substituted ureas are obtained in al-most quantitative yield [104106]. This processis particularly suitable for the production of un-symmetrically substituted ureas [105], [106].

    8.2.6. Acylation of Ammonia or Amineswith Carbamoyl Chlorides

    Ammonia and primary or secondary amines re-act with carbamoyl chlorides to give the corre-sponding urea derivatives in good yields [107].

    Tetraphenylurea, (C6H5)2NCON(C6H5)2,[108] is obtained in quantitative yield by heat-ing diphenylcarbamoyl chloride with diphenyl-amine.

    8.2.7. Aminolysis of Esters of Carbonic andCarbamic Acids

    Esters of carbonic and carbamic acids (carbon-ates and carbamates) react with amines at ele-

  • Urea 27

    vated temperatures to give symmetrically disub-stituted ureas [109112].

    8.3. Reaction of Urea and Its Derivativeswith Aldehydes

    8.3.1. -Hydroxyalkylureas

    The industrial production of -hy-droxyalkylureas is limited to the addition offormaldehyde or glyoxal to urea, monoalky-lureas, symmetrical dialkylureas, and cyclicureas. It involves acid- or base-catalyzed addi-tions that are generally equilibrium reactions[113130].

    Urea can bond with up to 4mol of formalde-hyde. However, only monohydroxymethyl- andN,N -dihydroxymethylurea can be isolated inpure form [131], [132]. Tri- and tetrahydroxy-methylureas are formed only as nonisolableintermediates, for example, in the synthesis oftrimethoxymethylurea (6), [133], [134]; N,N -dialkoxymethyl-4-oxomethyltetrahydro-1,3,5-oxadiazine (7) [135], [136]; and N,N -dihy-droxymethyl-2-oxo-5-alkyltetrahydro-1,3,5-triazines (8) [137140].

    N,N -Dihydroxymethylurea [140-95-4],HOCH2HNCONHCH2OH [131], [132] is pro-duced industrially by charging 2mol of formal-dehyde per mole of urea to a stirred vessel. Thesolution is neutralized with triethanolamine.Urea is added with cooling, and the temper-ature must not exceed 40 C in this slightlyexothermic reaction. After a few hours the re-action mixture is cooled to room temperatureand dihydroxymethylurea crystallizes out. Theproduct is dried in a spray-drying tower.

    Hydroxymethyl derivatives of cyclic ureascan be produced by reaction of these substanceswith formaldehyde in an alkalinemedium [141].

    Hydroxymethyl derivatives of urea and cyclicureas (9)(15) are used in easy-care nishes fortextiles [91].

    The addition of higher aldehydes to urea,mono- and symmetrically disubstituted ureas,and cyclic ureas generally gives unstable -hy-droxyalkyl compounds. Electron-withdrawingand electron-donating substituents next to the-hydroxyalkyl group affect the stability of thesecompounds and also their ability to undergo con-densations.

    For example, the chloral urea derivatives(16) exhibit considerable differences in reactiv-ity compared with the N-hydroxymethyl (17)and N--hydroxyethyl compounds (18). Thesedifferences are exemplied by a decrease in theH-acidity of the OH groups [142]. Chloral com-pounds (16) can form alkali-metal salts, whereasthe corresponding salts of N-hydroxymethyl(17) andN--hydroxyethyl compounds (18) are

  • 28 Urea

    unknown. Condensation of chloral compoundswith nucleophiles is possible only under extremereaction conditions. However, N--hydroxy-ethyl compounds can be converted smoothlywith alcohol into N--alkoxyethyl compoundsin basic and sometimes even in neutral me-dia. The reactivity of N-hydroxymethyl com-pounds lies between that of the chloral and N--hydroxyethyl compounds. The 4-hydroxycy-cloalkyleneureas (cyclic N-hemiacetals), whichare obtained by treating suitable aldehydes withureas, are stable. For example, 2-oxo-4,5-dihy-droxyimidazolidines are formed by cyclizationof urea, or its mono- or symmetrically disubsti-tuted derivatives, with glyoxal [143145]:

    N,N-Dihydroxymethyl-2-oxo-4,5-dihy-droxyimidazolidine (20) is produced byhydroxymethylation of 4,5-dihydroxy-2-oxoimidazolidine (19) in weakly acidic toweakly alkaline aqueous solution or, more ele-gantly, by direct reaction of urea with glyoxaland formaldehyde in the appropriate molar ratioinweakly acidic to neutral solution at 40 80 C,sometimes in the presence of catalytically activebuffers [130].

    Compound 20 and its derivative in which theOH groups are partly acetalized with metha-nol are used as formaldehyde-free cross-linkingagents for easy-care nishes for cellulose-con-taining textiles.

    2-Oxo-4-hydroxyhexahydropyrimidinesalso belong to the group of -hy-droxyalkylureas. These compounds can be pro-duced industrially by cyclocondensation of ureawith active enolizable aldehydes (see Section8.3.4).

    8.3.2. -Alkoxyalkylureas

    Condensation of N-hydroxymethylureaswith alcohols to give N-alkoxymethylureas(ureidoalkylation of alcohols) is of great in-dustrial importance. Pure hydroxymethylureasand an excess of alcohol are reacted in the pres-ence of catalytic amounts of acid. The natureand quantity of the acid catalyst depend on thereactivity of the N-hydroxymethyl compound,its stability to hydrolysis, and the formation ofbyproducts and polycondensation products. Atelevated temperature the reaction can be carriedout under weakly acidic conditions, whereby theequilibrium position must be adjusted by vari-ation of the concentration and the molar ratios[91], [146], [147]. Sometimes, ureidomethyla-tion of alcohols is better at room temperature inthe presence of strong acids.

    The alcohol-modied urea formaldehydecondensation products are used as resins forheat- or acid-curing coatings. Besides the water-soluble or almost solvent-free resins, which arebecoming increasingly important for environ-mental reasons, a wide range of aminoplasticresins for coatings exist that are readily sol-uble in common paint solvents. To converturea formaldehyde resins to resins that are sol-uble in organic solvents, the highly polar N-hy-droxymethyl groups obtained in the initial reac-tion between urea and formaldehyde are acetal-ized with alcohols, mainly butanol and isobu-tanol, as well as ethanol and methanol or theirmixtures.

    The alcohol-modied urea formaldehyderesins are produced industrially by passingaliphatic alcohols into aqueous solutions of ureaand formaldehyde or solutions of hydroxymeth-ylated ureas in the presence of small quantities ofacid at 90 100 C so that the water formed andexcess alcohol distill off. The molar ratios varybetween 2 and 4mol of formaldehyde and 2 and5mol of alcohol per mole of urea. The processis carried out in a reactor equipped with an ade-quately dimensioned heat exchanger, a vacuumpump, a distillation column, and for alcohols thatare sparingly soluble in water, a water separator.

    Some 4-hydroxycycloalkyleneureas are so re-active that they can be converted into the N--alkoxy compounds (21), (22) even in a neutralmedium by heating with alcohol [148]:

  • Urea 29

    To shift the equilibrium in the N--ureidoalkylation of alcohols in the directionof the N--alkoxyalkyl compounds, the waterformed during the reaction must be removed.In industrial production processes an aprotic en-trainer (e.g., an aromatic) is used. In the ure-idomethylation of alcohols that are immiscibleor sparinglymiscible with water, an excess of al-cohol is used andwater is removed by azeotropicdistillation.

    Higher-boiling alcohols can also beureidomethylated by transacetalization of theN-methoxymethyl compounds.

    Polymerizable compounds are obtainedby ureidoalkylation of unsaturated alcohols(e.g., allyl alcohol [149], [150]). Tetraallyl-oxymethyltetrahydroimidazo[4,5-d]imidazole-2,5(1H,3H )-dione (23) has achieved impor-tance as a polymerizable coating component[151], [152]:

    8.3.3. ,-Alkyleneureas

    ,-Alkyleneureas are obtained by condensa-tion of urea or its derivatives with aldehydes in aweakly acidic medium. The aldehyde group rstadds to the urea to form an -hydroxyalkylurea,which then reactswith a secondmolecule of ureaor with another -hydroxyalkylurea to give lin-ear or branched ,-alkyleneureas:

    Reaction of equimolar quantities of ureaand formaldehyde in acidic solution givespolymethyleneureas as a result of stepwise ure-idomethylations:

    Kadowaki obtained polymethyleneureascontaining up to ve urea groups joined bymethylene bridges by means of a stepwise syn-thesis [135]. Because of their extreme insolubil-ity, no higher polymethyleneureas have yet beenisolated. Polymethyleneureas are used as slow-release nitrogen fertilizers, for example.

    Isobutylidenediurea (24), which is spar-ingly soluble in water, is obtained by conden-sation of urea with isobutyraldehyde in a molarratio of 2 : 1 in a weakly acidic medium:

    Isobutylidenediurea is also a slow-release ni-trogen fertilizer used in various special fertil-

  • 30 Urea

    izer formulations. Isobutylidenediurea is pro-duced by a continuous process. According to apatent published byMitsubishi Chemical Indus-tries [153], urea is charged continuously with ascrew feed via a belt weigher and is reacted witha stoichiometric quantity of isobutyraldehyde inthe presence of semiconcentrated sulfuric acidin a mixer. In the last section of the mixer thereaction product is neutralized by injecting di-lute aqueous potassium hydroxide solution. Theproduct is dried by using plate driers and pro-cessed by sieving, ltering, and grinding.

    8.3.4. Cyclic Urea Aldehyde CondensationProducts

    Almost all cyclizations of urea and its deriva-tives with aldehydes involve an - or a vinyl-ogous ureidoalkylation [113115], [146]. If theurea bears a nucleophilic substituent on the sec-ond nitrogen atom, cyclocondensation occurs[154]. Saturated and unsaturated cyclic ureaswith ve, six, seven, or eight ring atoms, bicyclicand polycyclic heterocycles (with both uncon-densed rings and rings anellated in the 1,2- or1,3-position), and spiro compounds can be pro-duced this way [154], [155].

    Industrially important reactions are those ofureawith formaldehyde, acetaldehyde, isobutyr-aldehyde, and their mixtures. Treatment of ureawith formaldehyde in a molar ratio of 1 : 4

    gives an equilibrium mixture of hydroxymeth-yl derivatives. On ureidomethylation of a hy-droxymethyl group bonded to the second ni-trogen atom of the urea, cyclocondensationto hydroxymethylated 4-oxotetrahydro-1,3,5-oxadiazines (25) occurs [135], [136].

    4-Oxo-3,5-dialkoxymethyltetrahydro-1,3,5-oxadiazines are used as cross-linkingagents for easy-care nishing of textiles. Thesecompounds can be obtained directly by conden-sation of urea with formaldehyde and alcohols[135], [136]:

    The hydroxymethyl and methoxymethylderivatives of 2-oxo-5-alkylhexahydro-1,3,5-triazines have achieved importance as cross-linking agents for easy-care nishing ofcellulose-containing fabrics. These compoundsare obtained by cyclizing ureidomethylation ofurea with formaldehyde and a primary amine[137140], [154]:

    Bicyclic Ureas. 4,5-Dihydroxy- or 4,5-dialkoxyimidazolidin-2-ones can be con-verted into bicyclic ureas, such as tetra-hydroimidazo[4,5-d]-imidazole-2,5(1H,3H )-dione (26), by means of a double -ureidoalkyl-ation with urea:

  • Urea 31

    Tetrahydroimidazo[4,5-d]imidazole-2,5(1H, 3H )-dione [496-46-8], acetylenediurea(26), can be obtained directly by condensationof glyoxal with excess urea in an acidic medium[156], [157]. A solution of urea is acidied topH

  • 32 Urea

    The 2-oxo-4-hydroxy-5,5-dimethylhexa-hydropyrimidines are cyclic N-hemiacetals,whose OH groups can undergo nucleophilicsubstitutions similar to those undergone by N-hydroxymethylureas. 2-Oxo-4-hydroxy- and2-oxo-4-alkoxy-5,5-dialkylhexahydropyrimi-dines are used in the form of their N,N -dihy-droxymethyl compounds in noncrease nishesfor cellulose-containing textiles [91], [165].

    Preparation of N,N-Dihydroxymethyl-2-oxo-4-hydroxy(methoxy)-5,5-dimethylhexa-hydropyrimidine. Urea is treated with form-aldehyde in the molar ratio 1 : 1 at pH >9 and50 60 C in the presence of an excess of meth-anol. Isobutyraldehyde is added in the pres-ence of a strong mineral acid to bring aboutcyclocondensation. The reaction mixture is ren-dered alkaline, and hydroxymethylation is car-ried out with formaldehyde.

    2-Oxo-4-hydroxyhexahydropyrimidines(32), which are not or only mono-substituentedin the 5-position, react with ureas to give 2-oxo-4-ureidohexahydropyrimidines (32).

    2-Oxo-4-ureido-6-methylhexahydropy-rimidine [1129-42-6], crotonylenediurea (33),is obtained either by condensation of urea withcrotonaldehyde in the presence of acid [166] orby the industrially more straightforward routeinvolving condensation of urea with acetal-dehyde in the molar ratio 1 : 1 in the pres-ence of acid [167], [168]. 2,7-Dioxo-4,5-di-methyldecahydropyrimido[4,5-d]pyrimidine isformed as a byproduct in the second route [168170].

    2-Oxo-4-ureido-6-methylhexahydropyrimi-dine is produced industrially in a continuousprocess employing a stirred tank cascade. A70% urea solution is treated with acetalde-hyde at a molar ratio of 1 : 1 in the presenceof a catalytic quantity of 75% sulfuric acid.The exothermic reaction is kept at 38 60 Cby controlled cooling. The pH is initially keptabove 3 and in the nal reactors below 2 by ad-dition of sulfuric acid. Average residence timein the cascade is 40min. In the last stirred tank,the reaction mixture is neutralized with aqueouspotassium hydroxide solution. Drying is carriedout in a spray tower.

    2-Oxo-4-ureido-6-methylhexahydropyrimi-dine is used as a slow-release nitrogen fertilizer[171]. This fertilizer is characterized by its ex-treme insolubility in water and is therefore notwashed out of the soil by rain or irrigation. Itdecomposes as a result of acid hydrolysis in-duced by humic acids during the growth periodof plants, bringing about mineralization of thenitrogen it contains.

    2-Oxo-4-hydroxy-5,5-dimethylhexahy-dropyrimidyl-N,N-bisneopentals can be pro-duced from urea, formaldehyde, and isobutyr-aldehyde in acid-catalyzed cyclo- and linearcondensati