Articolo JCE Sul Cracking

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Many university chemistry teachers consider industrial Table 1. Ra w Materialsfor Ethylene Manufacture,USA (31chemistry to be uninteresting and often inexplicable.In mostuniversity courses this aspect of the subject receives little 1970 1975 lest.) 1980 (en.)attention, which is a pity since chemistry owes much .of its 50.9 54.0-49.0 40.0-35.0status as an academic subject to its industrial importance and Propane 33.9 21.5 11.5ignoring its applications presents a distorted view of the :y'd;Oh, and other 3.4 3.5 4.511.8 21.0-26.0 44.0-49.0subject. In reality, shying away from applied chemistry be- liquid feedstockscause of a supposed lack of intellectual coherence and appeal %total ethylene.is not justified; it can be discussed rationally a t a level ap-

Peter WisemanUniversity of ManchesterInstitute of Science and TechnologyP.O. Box 88Manchester M ~ OOD, United Kingdom

propriate t o university courses (1).This artide sets out to il-lustrate this by discussing an important topic in industrialorganic chemistry.

Ethylene by Naphtha CrackingFree radicals in action

Ethylene ManufaciureThe manufacture of ethylene by thermal cracking of hy-

drocarbon feedstocks is one of the cornerstones of the petro-chemical industry (2). It is carried out on an enormousscale-the current rates of ethylene production in both theUSA and Western Europe are about 11 million tons perannum-and is the basis for the manufacture of a very widerange of organic chemicals.Thermal cracking involves passing a mixture of feed, whichmay be ethane, propane, butane, or a liquid petroleum frac-tion, and steam, through a tube heated in a furnace, where itstemperature is raised to a level in the region of 750-900C.After a reaction period of 0.14.5 s the products are rapidlycwled to 300-400C to quench the reaction. The products areseparated by distillation.In Europe and Japan the feed most often used for ethylenemanufacture is naphtha, one of the products obtained in re-fineries by fractionally distilling crude oil. In the USA, ethaneand propane have been the major feeds, though for a numberof reasons, including diminishing availability of these mate-rials, there is now a marked trend towards the use of naphthaand other liquid feedstocks (see Tahle 1). roducts typicallyobtained in naphtha cracking are shbwn in Tahle 2.At first sight, this process has all the characteristics calcu-lated to discourage the chemistry teacher. I t uses an ill-definedraw material, puts i t through a brutal reaction system, andgives an extremely complicated set of products. What can onepossibly say about such a process?One can, in fact, say quite a lot.Mechanismot Ethylene Formailon

Naphtha is the name given to any fraction in the approxi-mate boiling range 25-185T obtained by the distillation of

Table 2. Typical Product Distribution in Na~hthaCrackins 14)Products wt %

This provides the initiation st ep for a free radical chain re-action.The radicals thus formed can undergo a series of @-scissionreactions, each giving rise to a molecule of ethylene

CHFH2CH2. --+ CHc + CH2=CH2and

CH8CH,CH,c~,. --t CH&H,. + CH,=CH,Hydrogen atom abstraction from a substra te molecule by amethyl or ethyl radical (or a hydrogen atom-see below)continues the reaction chain

Most molecules of substrate enter the reaction through hy-drogen-atom abstraction rather tban carbon-carbon cleavage.For both energetic and statistical reasons the radicals thusformed will be predominantly secondary radicals, and whenthese undergo @-scission, lkenes other tban ethylene will beproduced; e.g.crude oil. A "full range naphtha" covering the whole of thisrange contains mainly compounds having four to twelve car- CH,CH,CH,CH,CH.CH.~HCH,CH~bon atoms. The compounds, which will be numbered in thehundreds, will be the C4 to C12 alkanes, cycloalkanes,' andaromatics which were present in the original crude oil. Al-though naphthas contain alarge number of components, theindividual compounds have relatively simple structures, andthe reactions involved in cracking can be considered in termsof model alkanes, cycloalkanes, and aromatics.Consider n-nonane as a model straight-chain alkane.At thecracking temperature thermal cleavage of carbon-carbonbonds occurs readily; e.g.

+ H3CH&H,CH2CH2.+ CH2=CHCH&H,The primary radical then produced can undergo further @-scissions producing ethylene

CH,CH&H,CH&H< -+ CH,. + 2CH,=CH,I t has been indicated above that ethyl radicals carry out.hydrogen atom abstractions: they can also undergo carbon-hydrogen @-scission

CH3CH,. - H,=CH, + H .'The cycloalkanes n crude oil contain five- and six-memberedringsonly.


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Maxlrnlzlng Ethylene YieldIn naphtha cracking it is normally desirable to produce thehighest possible yield of ethylene. This is achieved by ad-iustine the process conditions to favor the ethylene-producing. .reactions described a h v eover the many other reactions whichcan occur in the system. From the reaction scheme discussedit can he seen that high ethylene yields will be favored by en-suring that primarv alkyl radicals, once formed, undergo 8-scission reactions to the maximum extent.Thus. t is desirable.-...-----~~~ ~to avoid termination and chain transfer reactions.

Termination-CH,. + .CH- - CHtCHr--Chain Transfer-CH%. + CBHm- --CH3 + C,H,*. (secondary)Since these are himolecular, their rates may he reducedcompared to those of the unimolecular @-scission eactions byoperating at low partial pressure. This also has a favorableeffect on the positions of the equilibria of the alkene-formingreactions. Table 3 illustrates the effect of pressure on productdistribution in cracking. In cracking processes low partialnressures are achieved hv usine steam dilution. a weieht toweight steam to feed ratioin therange 0 .448 being ty$allyused in nanhtha crackine.2 The effect of steam ratio on eth-ylene yield is illu str ate dh Tahle 4.The other main fador hv which control is achieved over theproduct distribution is by the reaction temperature. Theinitiation. B-scission, and termination reactions have activa-tion energies of app;oximately 350.45, and 0 kJ mole-' re-spectively (7).Thus, increasing the temperature increases theri te constants of the init iatioAac tions and toa lesser extentth e &scission reactions, hut has no effect on tha t of the ter-mination reactions. This has a two-fold effect on the nro-~~~ ~~~ ~ ~~~~~ ~duction of ethylene. Firstly, at higher temperatures a higherproportion of t he substrate molecules enter th e reaction viacleavage to two primary radicals, rather than by hydrogenatom abstraction to give a secondary radical, which can onlygive an alkene other than ethylene as th e product of its [email protected], the ethylene-producing &scission re-actions are favored over the termination reactions.Durine the past 25 vears developments in furnace desimand t u h ~ m e ~ l l u r g yave allowed>racking temperatures iohe raised from about 750 to over 900C, and maximum oh-tainable ethylene yields have increased from about 20 to over30%.Effeci of Feedstock CornDosltlon

Naphtha does not of course consist solely of normal alkanes,hut contains in addition branched-chain alkanes, cycloal-kanes, and aromatic hydrocarbons. It can readily he seen fromthe mechanism discussed above that these will give loweryields of ethylene than normal alkanes. For example, considera tvpical reaction path in the cracking of a model branched-chain alkane, 2-methyloctane .

20peratingst reduced absolute pressure would involve engineeringdifficulties and is potentially hazardous-air could bedrawn into thesystem through leaks.Also, the steam he lp reduce carbon depositionin the furnace tubes: C+ HzO+CO + Hz.

Table 3. Effect of Pressure on Cracking of n-Hexadecane 15)Mole/100 mole hexsdecene reacfednProducts 1 2

4 16.5 17.4CH . 50.9 23.3

C,H6 1.7 0.9C,H, 20.1 19.1C2'm 3.3 13.8C s H t ~ 14.8 14.3C P , , 1.0 8.1C. and above 92.9 111.9

dcondit ions for o l umn 1:Temperature SOO'C: errure I a tm ;c o n ~ e ~ s i o n2.4%. FOT column 2: 5 0 0 ' ~ ; 1 a tm . 47.5%.Table 4. Steam Dilution and Ethylene Yieldin Nallhtha Cracking (61H,O/naphtha ratio 0.6 1.2Yield C,H, (wt%) 31.0 34.5

outlet temperature8 4 0 ~ ~ ;esidence time 0.35 r.

secondary reactions I

- everityLVariation01prduct dishibutionwith MCking severity in naphma cracking.Theshaded region represents high severihlcracking, operated to obtain maximumelhyiene yield.

One molecule of ethvlene is formed compared with two froman analogous sequence of reactions involving n-nonane.Similar ronsiderations aodv to cvcloalkanes. In this case.i t can also be seen that the &htio;of conjugated dienes c&he expectedV o l m 54. Number 3,March 1977 1 155


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Naphthas containing a high proportion of cycloalkanes givehieher vields of butadiene than predominantly paraffinic.naphthas.Aromatic hydrocarbons undergo cracking essentially onlyin side chains, the aromatic ring being relatively stable undercracking conditions.It c anbe seen from the above discussion that for maximumethylene yield the most desirable naphtha feed is one con-taining a high proportion of normal alkanes.SecondaryReactions

The products formed in th e reactions discussed above canundergo further 'secondary reactions' which are complex andless well characterized than the primary cracking reactions.Some of these, e.g. the cracking of propene and higher alkenes,can give rise to further production of ethylene. Among sug-gested routes from propene to ethylene areCH&H=CH, --c CH,. + .CH=CH,CHP=CH. + RH -+ CH2=CH, + R.

andH. + CH,CH=CH, + CH,CH,CH,.

CH,CH,CH,. - H,. + CH,=CH,Other secondary reactions result in the consumption of eth-ylene and other alkenes and the formation of dienes, aromatichydrocarbons, and high-molecular-weight carbonaceousmaterials known as coke. Consequently, as the severity ofcracking, tha t is the extent t o which the overall reaction isallowed to proceed, is increased, the product distributionchanges as indicated in the figure.

The variation in ethy1ene:propyIene ratio with crackingseverity allows considerable flexibility in cracker operationsin response to market demands. When,maximum ethyleneyield is required, cracking is operated a t high severity,' i.e. inthe shaded region in the figure.At higher levels than this, cokeformation in the furnace tubes becomes a problem, and in anycase there is little further t o he gained in ethylene yield.

(1) See, for example: (a) Reuben, B. G.. and Bumtall. M. L., "The Chemical Ee,m~my:Langmsn Group Ltd., London, 1973: lbl Todder, J. M., Nechustal, A,. and dubb,A.H. , "BasieOrganicChemistry, Part5 Induateal Pn d u ~ . " h h nWileyand &,ns Ltd..Landan. 1975; lcl Wiseman. P. . "A n lntruduction to Industr ial O~ aniech emistry :Applied Seienee Publishers Ltd., London, 1972.(21 Bsba ,T. B.,and Kennedy,J. R.,Chrm.Ewf., 3, (11,116 (19761.(3) Taylor ,E.D.,Chem.En~.Propr, 8, (121.61 lL9lZi.(41 Zdonik. S. B., Bassler, E. J., and Hallce, L. P.. Hydroeorbon Proees. 53. (2). 13,."g,,\.m,.,.

151 Voho,H.H.,and Gmd,G. M. ,J Amar Chsm.Soe., 71,593 (19491.(61 Europeon Chomieal Neus, 27, (6841.39 (25th . April 1975).(71 ReE ( lb ) , p.41.

156 I Jwmal of ChemicalEducation

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Articolo JCE Sul Cracking