c 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 10.1002/14356007.a28 433 Xylenes 1 Xylenes J¨ org Fabri, Veba AG, D¨ usseldorf, Federal Republic of Germany Ulrich Graeser, Veba ¨ Ol AG, Gelsenkirchen, Federal Republic of Germany Thomas A. Simo, Lurgi ¨ Ol, Gas, Chemie GmbH, Frankfurt, Federal Republic of Germany 1. Introduction ............... 1 2. Properties ................. 3 3. Occurrence and Raw Materials ... 6 3.1. Natural Occurrence .......... 6 3.2. Raw Materials for Xylene Produc- tion ..................... 6 4. Production, Separation, and Fur- ther Processing .............. 7 4.1. Production of C8 Aromatics ..... 7 4.2. Separation and Further Processing 9 4.2.1. Separation of the C8 Aromatics Frac- tion ..................... 9 4.2.2. Isomerization of the C8 Fraction ... 12 4.2.3. Combination of the Technologies in the Aromatics Complex ......... 12 5. Integration into Refinery and Petro- chemical Complexes .......... 13 5.1. Backward Integration into Petroleum Refining ........... 13 5.2. Forward Integration into Chemical Processing ................. 13 6. Economic Aspects ............ 13 7. Quality Specifications and Analysis 18 8. Storage, Transport, and Safety ... 18 9. Environmental Aspects and Toxicol- ogy ...................... 19 10. References ................. 20 1. Introduction The benzene homologues of general formula C 8 H 10 are generally known as mixed xylenes. The mixture of isomers with a boiling point range of 135 – 145 ◦ C mainly consists of the three isomeric dimethylbenzenes and ethylben- zene: With the exception of xylene production by disproportionation of toluene, the isomeric xylenes and ethylbenzene are always produced as a mixture in all production processes. How- ever, the relative proportions of the C 8 isomers often differ considerably (see Chap. 3). Because of their high knock resistance (see Chap. 2), xylenes are well suited to the produc- tion of motor fuels. In terms of quantity the pro- duction of gasoline exceeds that of BTX aro- matics (B = benzene, T = toluene, X = xylenes) quite considerably (in Western Europe in 1995 gasoline production was ca. 150×10 6 t and that of BTX aromatics ca. 13.7×10 6 t, of which ca. 2.7×10 6 t/a was mixed xylenes, 0.65×10 6 t/a o- xylene, and 1.4×10 6 t/a p-xylene) [1]. The aver- age aromatics content of motor fuels in Western Europe is ca. 38 % [2]. The close association with gasoline produc- tion strongly affects the economics of separating xylene mixtures, for example, for use in chemi- cal processes. In Western Europe the production/use bal- ance in 1995 was as follows [3]: Production from coal 36×10 3 t (ca. 1.3 %) from pyrolysis gasoline 300×10 3 t (ca. 11.1 %) from reformate 2165×10 3 t (ca. 80.0 %) by disproportionation 205×10 3 t (ca. 7.6 %) Total production 2706×10 3 t Use in extraction and chemical processing (incl. imports) 2563×10 3 t (ca. 88.4 %) solvents 306×10 3 t (ca. 10.6 %) as gasoline component 30×10 3 t (ca. 1 %) Total use 2899×10 3 t Domestic production 93.3 % Net impor 6.7 % The structure of world mixed-xylenes pro- duction capacity in 1994 was as follows:
The benzene homologues of general formulaC8H10 are generally known as mixed xylenes.The mixture of isomers with a boiling pointrange of 135 – 145 ◦C mainly consists of thethree isomeric dimethylbenzenes and ethylben-zene:
With the exception of xylene productionby disproportionation of toluene, the isomericxylenes and ethylbenzene are always producedas a mixture in all production processes. How-ever, the relative proportions of the C8 isomersoften differ considerably (see Chap. 3).
Because of their high knock resistance (seeChap. 2), xylenes are well suited to the produc-tion of motor fuels. In terms of quantity the pro-duction of gasoline exceeds that of BTX aro-matics (B = benzene, T = toluene, X = xylenes)quite considerably (in Western Europe in 1995
gasoline production was ca. 150×106 t and thatof BTX aromatics ca. 13.7×106 t, of which ca.2.7×106 t/a was mixed xylenes, 0.65×106 t/a o-xylene, and 1.4×106 t/a p-xylene) [1]. The aver-age aromatics content of motor fuels in WesternEurope is ca. 38% [2].
The close association with gasoline produc-tion strongly affects the economics of separatingxylene mixtures, for example, for use in chemi-cal processes.
In Western Europe the production/use bal-ance in 1995 was as follows [3]:
Productionfrom coal 36×103 t (ca. 1.3%)from pyrolysis gasoline 300×103 t (ca. 11.1%)from reformate 2165×103 t (ca. 80.0%)by disproportionation 205×103 t (ca. 7.6%)
Total production 2706×103 tUsein extraction and
chemical processing (incl.imports)
2563×103 t (ca. 88.4%)
solvents 306×103 t (ca. 10.6%)as gasoline component 30×103 t (ca. 1%)
Total use 2899×103 tDomestic production 93.3%Net impor 6.7%
The structure of world mixed-xylenes pro-duction capacity in 1994 was as follows:
2 Xylenes
Total capacity 23×106 tReformer 83%Disproportionation 10%Pyrolysis gasoline 6%Coal 1%
In Western Europe until the year 2000 a sig-nificant increase in the proportion of xylene usedin gasoline production is predicted, because thegrowth in production of xylene-containing re-formate may exceed its consumption in chem-ical processes. However, in other regions, suchas theUnited States, the reduction and limitationof the content of aromatics, and thus of xylenes,in gasoline is being discussed (see also Chap. 2and Section 5.2) [2,4].
The use of xylenes in chemical processesbroke down as follows (data forWestern Europe1995) [1]:
p-Xylene 1761×103 t (68.7%)o-Xylene 650×103 t (25.4%)Ethylbenzene and 152×103 t (5.9%)miscellaneous
Total 2563×103 t
The dominant importance of p-xylene can beseen from the relative proportions of the isolatedxylene isomers. p-Xylene is mainly oxidized toterephthalic acid, which can be esterified to di-methyl terephthalate (precursor for polyesters).o-Xylene is oxidized to phthalic anhydride (pre-cursor for plasticizers) and m-xylene to isoph-thalic acid (precursor for polyesters). Ethylben-zene is dehydrogenated to styrene, which is con-verted to polystyrene and other polymers [5].
History [6]. Xylene was first discovered incrude wood spirit in 1850 by Cahours. Thename xylene was derived from the Greek wordxylon (=wood). Xylene was detected in coal-tar in 1855 by Ritthausen and Church. In1891 ethylbenzenewas found in hard coal-tar byNoelting and Palmar, and Moschner carriedout the first synthesis of pure ethylbenzene fromethylbenzenesulfonic acid.
Between 1865 and 1869 Ernst and Fittigfound that the “xylene” in coal-tar does not con-sist of a single compound. They synthesized“xylene” from toluene and named it “methyl-toluene.” They then established that some prop-erties of “methyltoluene” differed from those ofcoal-tar xylene, particularly with regard to thenitro compounds. Fittig et al. concluded that in
synthetic “methyltoluene” ( p-xylene) the sec-ond methyl group was in a different positionfrom that in coal-tar xylene.
Glinser and Fittig described the produc-tion of methyltoluene in 1865. Fittig obtaineda hydrocarbon by dry distillation of the cal-cium salt of mesitoic acid which resembled bothknownmodifications of dimethylbenzene, (“xy-lene” and “methyltoluene”) but was not com-pletely identical to either. The new hydrocarbonwas named “isoxylene” (m-xylene). This workelucidated the structures of p- andm-xylene andproved their occurrence in coal-tar.
In a further experiment Fittig identified o-xylene as the third isomer,whichwas discoveredin coal-tar by Jacobson in 1877.
m- and p-xylene were initially separated onthe basis of their different chemical reactivity.m-Xylene reacts with concentrated sulfuric acid.m-Xylene is then regenerated from the crys-talline m-xylenesulfonic acid using superheatedsteam in the presence of sulfuric acid. The frac-tion unaffected by sulfonation is then reactedwith oleum. The sulfonic acid that crystallizesis cleaved to give pure p-xylene.
For decades coke-oven tar and benzole wereused as raw materials for production of aromat-ics. Worldwide their production was associatedin location and quantity with coal and steel pro-duction. The plants for extraction and refining ofcoke-oven benzole, a mixture of light oil fromcoke-oven tar and crude benzene, obtained fromcoke-oven gas by oil scrubbing, were built ad-jacent to coking plants. These plants were verysmall compared with those currently in opera-tion. At that time the refining of crude benzolewas carriedout exclusively bywashingwith con-centrated sulfuric acid,whereby impurities, suchas resin-forming compounds, nitrogen and oxy-gen compounds, and some of the sulfur com-pounds, were removed as acid tar. However,valuable substances, such as styrene, were alsolost in the acid tar. The xylenes were in demandas solvents and were used as rawmaterials in thechemical and pharmaceutical industries. How-ever, production of the three isomers in pureformwas complicated due to their almost identi-cal physical properties. The quantities availablewere too small for the development of industrialxylene chemistry.
m- and p-xylene could be separated from o-xylene by distillation and thus o-xylene could
Xylenes 3
be obtained in limited purity. m- and p-xylene,however, could only be separated laboriously onthe basis of their different chemical properties.
With the development of the hydrogenativerefining of coke-oven benzole over sulfur-resistant catalysts under pressure, sulfuric acidrefining became uneconomical andwas replacedby a technically superior process (see →Ben-zene, Chap. 6.1.). Parallel to this developmentin process technology, aromatics production be-came independent of steelworks or coking plantsand centralization took place. Large refineriesand distillation plants provided a basis for mod-est chemical processing of the C8 aromaticsin the mid-1950s. Between 1955 and 1960 ca.30 000 t of xylenes were obtained from a cokeproduction level of (40 – 45)×106 t/a.
The fourth isomer, ethylbenzene, must beconsidered separately from the historical devel-opment of the isomeric xylenes. Its synthesisfrom benzene and ethylene in a Friedel – Craftsreaction solved the problem of raw materials atan early stage.
With the discovery of the catalytic dehydro-genation of naphthenes and the catalytic dehy-drocyclization of paraffins and isoparaffins onnoble metal catalysts and their bifunctional sys-tems (platforming and rheniforming processes),the rawmaterial basis for the development of xy-lene chemistry on a large scale was secured inEurope from about 1960. A large number of pro-cesses have now been developed: liquid – liquidextraction, azeotropic distillation, extractive dis-tillation, isomerization, crystallization, adsorp-tion, and absorption. These not only providedpure products on an industrial scale, but also per-mitted the separation of m- and p-xylene. Thusthe basis for a large-scale growth of polyesterfiber production was created. The residual m-xylene could be isomerized or oxidized to isoph-thalic acid,which had a limitedmarket. Themar-ket for o-xylene was provided by phthalic anhy-dride production.
2. Properties [6]
Physical Properties. Technical-grade xy-lene is a mixtrue of C8 aromatics, also knownas the A8 fraction, consisting of the three xyleneisomers, ethylbenzene, and depending on theorigin, varying amounts of nonaromatics which
boil in the same range (136 – 145 ◦C). Table 1lists some physical data for the aromatic compo-nents. Other data can be found in [7,8]. The rela-tive proportions of these components in a xylenefraction depend on its origin and also which pro-cess steps the xylene has already passed through(e.g., hydrogenation).
The vapor pressure curves of the three xyleneisomers and ethylbenzene are shown in Figure 1.
The isomeric xylenes and ethylbenzene formazeotropic mixtures with water and numerousorganic compounds (Table 2) [9–13].
The absorption properties of xylene are oftechnical interest because of the significant dif-ferences in the solubilities of various gases as afunction of temperature (see Table 3).
Chemical Properties. Oxidation of the xy-lene isomers gives the corresponding aromaticdicarboxylic acids. Phthalic acid is produced in-dustrially from o-xylene, isophthalic acid fromm-xylene, and terephthalic acid from p-xylene(see → Phthalic Acid and Derivatives, → Tere-phthalic Acid, Dimethyl Terephthalate, andIsophthalic Acid).
The oxidation reactions have been estab-lished as industrial processes in both the gasand liquid phases. Attempts have been madeto introduce the co-oxidation of p-xylene withparaformaldehyde (Toray Industries) or acetal-dehyde (Eastman Kodak) [5].
Ammonoxidation of m- and p-xylenes ini-tially gives the corresponding phthalic acid dini-triles, which are important raw materials for theproduction of isocyanates via reduction to thecorresponding xylylenediamines. The dinitrilescan also be hydrolyzed to the corresponding ph-thalic acids. However, this step has only limitedindustrial and economic importance.
The nitration of o- and m-xylenes providesa route to xylidines following hydrogenation ofthe initially formed dimethylnitrobenzene iso-mers. Xylidines are used as intermediates indye and rubber additive production, for exam-ple (→Xylidines, Chap. 6.1.).
The capacity of the xylene isomers to undergoisomerization and disproportionation reactionsis also exploited industrially (see Chap. 4).
Sulfonation of m-xylene and subsequentdecomposition of the sulfonic acid derivativesgives 3,5- and 2,4-xylenols, providing startingmaterials for insecticides, herbicides, etc.
4 Xylenes
Table 1. Physical data for xylene isomers and ethylbenzene
Surface tension at 15.6 ◦C, mN/m 30.70 30.12 28.8 29.5Dynamic viscosity at 20 ◦C, mPa · s 0.809 0.617 0.644 0.6783Thermal conductivity at 0 ◦C, kJm−1 h−1 K−1 0.544 0.523 0.511 0.494Enthalpy of evaporation at the bpand 1 bar,
Of the chemical properties of ethylbenzeneas a component of the A8 fraction, the most im-portant is catalytic dehydrogenation to styrene,although ethylbenzene used for this purposemainly comes from other sources (alkylation ofbenzene with ethylene).
Properties as a Motor Fuel Component.The properties of motor fuels are determinedby a large number of quality aspects whoseminimum requirements are mostly specified(see → Automotive Fuels). The value which isattributed to xylenes in blending of motor fu-els essentially depends on how the xylenes canbe combined individually with other availablefuel components to produce motor fuel qualitieswhich conform to market requirements. Typi-cally the high knock rating, particularly as ablend component, and the low vapor pressure ofxylenes are important factors affecting their useas motor fuel components. Table 4 compares theoctane number and vapor pressure values (RVP)
of the xylene isomers [14,15] with the specifi-cation of a typical gasoline.
A more detailed account of the overall muchmore complex interactionswith the pool of gaso-line components can be found in Section 5.1.
3. Occurrence and Raw Materials
3.1. Natural Occurrence
Practically the only natural source of xylenes ispetroleum. The concentration of xylenes variesconsiderably depending on location and geo-logical age of the crude oil. Table 5 shows acomparison of the C8 aromatics and the C6 –C8aromatics contents in various types of crude oil[16].
Although in a few cases petroleums (e.g., thatof South East Asian origin) can have an evenhigher aromatics content (up to 35%), the directisolation of xylenes is not economical.
6 Xylenes
Table 4. Properties of xylene isomers as motor fuel components compared with finished gasoline
o-Xylene m-Xylene p-Xylene Ethylbenzene Super unleaded(DIN 51 607/EN 228)
For the industrial extraction of xylenes, the oilfraction or coal used as the raw material is sub-jected to thermal or catalytic treatment in whicharomatics- and xylene-containing fractions areobtained. These conversion processes determinethe quantity of xylenes available and supplythem in a form that is enriched sufficiently tomake isolation and further processing economi-cal.
Raw Materials based on Petroleum.Reformates. Xylenes are mainly isolated
from reformates [17].Naphtha fractions are usedas the raw materials for the reforming process,which is now practically only carried out cat-alytically. Because of the sulfur sensitivity of thePt – Re catalysts, the naphthamust first be desul-furized. The composition of the reformates andthus the xylene content depends on how the re-forming process is carried out (see Section 4.1)and on the quality of the naphtha fraction used.The composition of the naphtha fraction, in par-ticular its naphthene and aromatics content, sig-nificantly affects the composition of the refor-mate [18].
Naphthenic crude oils give naphtha fractionswhich are particularly suitable for reforming andaromatics production. However, naphtha frac-tions produced by hydrocracking (see → OilRefining, Chap. 3.5.) also generally have high
naphthene contents [19] and are therefore suit-able starting materials for xylene production byreforming. Besides the composition, the boilingrange of the naphtha fraction affects the yield ofxylenes (see Table 6). A particularly high yieldcan be obtained when the naphtha fraction al-ready has a high proportion of the correspond-ing xylene precursors (e.g., C8 aromatics or C8naphthenes) in a suitable boiling range [20].
Pyrolysis Gasoline. The yield of pyrolysisgasoline and its xylene content is decisively af-fected by the raw materials used, and by themode of operation of the plant (see Chap. 4).In steam cracking very different types of rawmaterial can be used, which give rise to widelyvarying yields and compositions of the pyrolysisgasoline (Table 7) [21].
Only on using higher boiling hydrocarbonsfeedstocks can appreciable xylene yields andconcentrations in pyrolysis gasoline be attained.Therefore, in general, isolation of xylenes is onlyeconomical when using these types of raw ma-terials.
Raw Materials based on Coal. The isola-tion of xylenes from coke-oven tar, coke-ovenbenzole, or from hydrocarbon mixtures pro-duced by hydrogenation of coal is of minor im-portance [17] (see also Section 4.1). The effectof the composition of the coal is described in[22]. Processes in which xylene-rich aromaticsfractions can be produced from LPG or metha-nol are described in Chapter 4.
Xylenes 7
Table 6. Dependence of reformate composition on the boiling range of reformer feedstock (Kuwaitnaphtha) [20]
Table 7. Effect of feedstocks on the yields of pyrolysis gasoline and C8 aromatics from steam cracking (conditions: very high severity withC2 and C3 recycle)
Feedstock Yield of pyrolysis gasoline(C5 – 200 ◦C), %
Yield of C8 aromatics, % Proportion of C8 aromatics inpyrolysis gasoline, %
Ethane 1.7Propane 6.6Butane 7.1 0.4 5.6Medium-range naphtha 18.7 1.8 9.6Atmospheric gas oil 18.4 2.2 12.0Vacuum gas oil 19.3 1.9 9.8
4. Production, Separation, andFurther Processing
4.1. Production of C8 Aromatics
Catalytic Reforming. Besides the nature ofthe raw materials, process engineering parame-ters also significantly determine the compositionof the reformate and its content of C8 aromat-ics [23]. The design of the catalytic reformer(see → Oil Refining, Chap. 3.4.) [24], its modeof operation, and the catalyst used are also veryimportant. The three following types of catalyticreformer are in common use:
1) Semiregenerative reformer, in which the no-ble metal catalyst is periodically regeneratedduring a production shutdown (typically ev-ery 6 to 12 months)
2) Fully regenerative reformer, in which instal-lation of an additional swing reactor allowsthe alternating regeneration of one reactorwhile the other continues to operate
3) Continuously regenerative reformer whosefluidized bed catalyst is continuously regen-erated during operation
Operating conditions of the different types ofreformer are listed in Table 8 [25].
Since at least six reactions take place in paral-lel during the reforming process, ofwhich essen-tially only two lead directly to the formation ofaromatics, establishing optimal operation con-ditions is very important. The reactions whichgive aromatics (the dehydrogenation of naph-thenes and the dehydrocyclization of paraffins)are favored by comparatively high reactor tem-peratures and low pressures (see Fig. 2). At thesame time a simultaneous increase in undesiredside reactions (hydrocracking and coke forma-tion) must be reckoned with, so that while in-creasing the severity leads to an increase in thearomatics content of the reformate, it also de-creases reformate yield [26]. Dealkylation reac-tions are also of specific importance for xyleneproduction. They can lead to decomposition ofthe xylenes formed, giving toluene, or benzeneand methane, particularly at high temperatures.
A further control parameter for xylene pro-duction is the space velocity (LHSV). If through-put is lowered to increase the residence time inthe reactor, reformates with high aromatics andxylene contents can be produced [25]. The ad-justment of the operating parameters to give amaximum aromatics concentration in the refor-mate is known as aromizing. Within certain lim-its the quality of the reformate can also be in-
8 Xylenes
Table 8. Typical operating conditions of various reformer types [25]
fluenced by the choice of catalyst. Besides con-ventional catalysts, which essentially only con-tain Pt as the active substances, bimetallic andmultimetallic catalyst systems, additionally con-taining Re, Ge, Ir, or Pb, are used. For aromat-ics production the selectivity for C6+ aromaticsis of general importance, while the suppressionof dealkylation reactions is particularly impor-tant for xylene production. The activity of thecatalyst can be increased by regular addition ofchlorinated hydrocarbons.Major suppliers of re-forming catalysts designed for xylene produc-tion include Acreon, Criterion, Exxon, Katale-una, and UOP [27].
Figure 2.Variation of the selectivity of aromatics productionin catalytic reforming with operating pressure
Steam Cracking. In contrast to the reform-ing process, the xylenes in the pyrolysis gaso-line produced by steam cracking are regardedas byproducts of ethylene and propene produc-tion. However, their isolation can improve theeconomics of the process under certain circum-stances. Although the operating conditions arenot adjusted to maximize the xylene yield insteam cracking, the effect of severity on xyleneformation is significant. Increasing the sever-ity – i.e., raising the reaction temperature, lower-
ing the residence time, and reducing the partialpressure by increasing the quantity of steam –significantly decreases the yield of pyrolysisgasoline [28], but increases the concentration ofxylene (see Table 9) [29].
Synthesis Processes. In the Cyclar processdeveloped by UOP/BP, propane and butane areconverted into aromatics-rich gasoline fractionsby cyclization over zeolite catalysts [30,31].On using propane the gasoline fraction containsca. 17.3wt% C8 aromatics, and with butaneca. 19.8wt%. The xylene yield is ca. 15wt%based on the total starting material. A pilot plantfor the Cyclar process has been commissionedat the BP refinery at Grangemouth, Scotland.For economic reasons, however, this process hasnot been put into industrial-scale operation. Thesame applies to the Mobil Oil process for syn-thesizing aromatics from alcohols using ZSM5catalysts [31].
4.2. Separation and Further Processing
4.2.1. Separation of the C8 AromaticsFraction
The reformates used to produce C8 aromaticscan be processed such that C8 and heavier aro-matics are produced in the following approxi-mate proportions [31]:
Besides the 38% aromatics, 51 – 52% naph-tha fractions and 10 – 11% gaseous products(hydrogen, fuel gases, etc.) are formed. This op-timized result involves the following operations:
1) Toluene is dealkylated to benzene.2) m-Xylene is isomerized to o- and p-xylenes.3) Ethylbenzene is transalkylated to xylenes.
Xylenes 9
Table 9. Influence of severity in steam cracking on the yield and composition of pyrolysis gasoline (naphtha feedstock) [28]
Figure 3. Flow sheet of the ethylbenzene distillationa) Pre-column; b) Ethylbenzene columns
If the toluene is subjected to transalkylationinstead of thermal dealkylation, the yield ofxylenes can be further increased at the expenseof benzene. The structure of a modern aromaticscomplex, which produces o- and p-xylene in thehighest possible yield, is based on a combinationof a series of these processes.
The ethylbenzene is mainly converted toxylenes because obtaining ethylbenzene fromreformate is energy intensive. The main sourceof ethylbenzene, which is almost exclusivelydehydrogenated to styrene, is therefore now themore economical alkylation of benzenewith eth-ylene (see → Ethylbenzene).
Distillative removal of ethylbenzene, whichfor styrene production must be toluene-free,from reformates can take place after removal oftoluene from the sump of the toluene column.Figure 3 shows a flow sheet of the ethylbenzenedistillation. Other products obtained by this sep-aration are the xylene and C9 aromatics. Themajor separation problem is caused by a boilingpoint difference of only 2 ◦C between ethylben-zene and p-xylene. The distillative separation ofthis mixture requires high reflux ratios (1 : 80 to1 : 120) with 300 – 360 effective plates. A yieldof ca. 95% ethylbenzene with > 99.5wt% pu-rity is achieved under these conditions [32].
Figure 4. Flow sheet showing separation of o-xylenea) Xylene splitter; b) o-Xylene column
The distillative separation of o-xylene fromthe C8+ stream is also difficult. Although theboiling point difference is only ca. 5 ◦C, therequired o-xylene purity (min. 98%) can beachieved with 120 – 150 effective plates and areflux ratio of 1 : 10 to 1 : 15, with a yield of> 95%. Figure 4 shows a flow sheet for thisseparation. A further increase in o-xylene puritycan be achieved by subsequent extractive dis-tillation. Another method of separation is crys-tallization, mainly used to obtain p-xylene. Thisprocess utilizes the crystallization behavior of p-xylene near the eutectic point of a C8 aromaticsmixture. Depending on the composition of themixture, this lies between −60 and −68 ◦C (seeFig. 5).
10 Xylenes
Figure 5.Melting behavior of the C8 aromatics in their mix-tures
Figure 6. p-Xylene crystallization by indirect refrigerationa) Drier; b) Precooler; c) Scraped-surface crystallizer; d) Re-frigeration plant; e) Filter; f) Centrifuge; g) Mixer
The Krupp –Koppers process (Fig. 6) is atypical example of the crystallization process.The actual separation process is a fractionalcrystallization in which the product of the firstseparation step in the filter (ca. 65% of the p-xylene from the starting mixture) is mixed withhighly concentrated p-xylene and is isolated inhigh purity in a centrifuge. This process and thatof Phillips Petroleum [33] operate using indirectrefrigeration.
Direct contact refrigeration involves down-ward flow of the initially cooled liquid in thevertical crystallization vessel. This in turn di-rectly cools the remainder of the contents of thevessel (see Fig. 7).
Figure 7. p-Xylene crystallization by direct contact refrig-erationa) Drying; b) Crystallization; c) Centrifuge; d)Melting tank;e) Desorber; f) Scraped-surface crystallizer; g) Mixing tank
The cooling agent can be evaporating ethyl-ene (Maruzen Oil Co.) [34]. Chevron Researchand Development Corp., Sohio/BP, and ArcoTechnology use other process designs [35].
In the Parex process (UOP) [36] p-xylene isobtained in high purity from the isomer mixtureby adsorption on amolecular sieve. The attachedfinishing column removes lighter components(raffinate consisting of ethylbenzene and a xy-lenemixture low in p-xylene) at the head. The p-xylene constitutes the bottomproduct of this col-umn. Adsorption – desorption on the solid bed iscarried out in such a way that a moving bed issimulated (Fig. 8). The rotary valve brings seg-ments of the filled adsorption chamber into con-tact with the feed and with the desorbent in sucha way that part of the molecular sieve adsorbswhile another part is desorbed. The rotary valveswitches the streams in the direction of flow inthe bed. This simulates the movement of the ad-sorbent in a direction opposite to that of the liq-uid. The adsorbent exhibits high selectivity to-wards p-xylene compared with the other C8 aro-matics, especially ethylbenzene. Ethylbenzeneis therefore one of the most suitable desorbentsor eluents; this is kept in circulation in the extractcolumn. The heavy part of the raffinate can alsobe used as the desorbent. The nonadsorbed C8aromatics leave the raffinate column at the head
Xylenes 11
Figure 8. Parex processa) Adsorbent chamber; b) Rotary valve; c) Extract column; d) Raffinate column
and are then subjected to isomerization (Sec-tion 4.2.2).
The Parex process is a variant of the Sor-bex group of processes. The first process whichused this principle of the simulated moving bedwas the Molex process developed in 1964 forthe separation of n-paraffins from isoparaffinsand cyclic hydrocarbons. Other modifications ofthe Sorbex processes are used to separate olefinsand paraffins in C3 –C18 hydrocarbon mixtures(Olex process) and for separating carbohydrates(Sarex process).
4.2.2. Isomerization of the C8 Fraction
The yield of o- and p-xylenes can be maximizedbyusing catalytic conversion processes. The Iso-mar process (Fig. 9) [37] enriches the p-xylenein the Parex raffinate by re-establishment of theequilibrium. The acidicmetal-containing zeolitecatalyst used here also isomerizes ethylbenzeneselectively to xylene isomers in their equilibriumratio [38]. Since hydrogenolysis on the metalcomponents must be guaranteed to maintain theactivity of this bifunctional catalyst, the Isomarprocess is operatedwith a certain partial pressureof hydrogen in the catalytic reactor. Hydrogen isseparated from the effluent from the Isomar re-actor and then recycled together with fresh hy-drogen. The liquid product is separated in a de-heptanizer column. A C7 stream is obtained atthe head, which can be recycled to the startingreformate. The bottom product of this column
is a p-xylene-enriched stream which, after claytreatment to remove unsaturated compounds isfed to the Parex unit to obtain the newly formedp-xylene.
Figure 9. Isomar processa) Heater; b) Reaction column; c) Hydrogen separation;d) Recycle compressor; e) Deheptanizer column
In the Tatoray process (Fig. 10) toluene andthe C9 (A9) bottom fractions from the A9 col-umn (o-xylene splitter column) are mainly con-verted into xylene isomers by disproportiona-tion of the toluene and transalkylation of the C9aromatics (→Toluene, Chap. 3.2.), affording anewly formed A6 –A8 fraction from A9+. Thistechnology is the product of cooperation bet-ween UOP and Toray Industries. Commercialapplications have been in operation since 1978.Since then the catalyst has been improved sev-
12 Xylenes
eral times with regard to activity, thermal stabil-ity, and life span [37].
The Tatoray process consists of a reactorfilled with solid catalyst to which are attacheda cooling unit, a gas – liquid separation unit, aunit for distillation of the light components andone for clay treating to remove olefinic impuri-ties. As in the Isomar process a partial pressureof hydrogen is maintained in the reactor. By pro-cessing different feed qualities, which can be in-fluenced both by the different recycle rates and avarying charge of fresh reformate fractions, thecomposition of the product can be controlled.
Other proven disproportionation processesare the LTD process of Mobil Research of ArcoTechnology. Alkylation of toluene to increasethe xylene yield has been developed by Mo-bil Chemical (Mobil toluene-to-p-xylene pro-cess = MTPX) and allows p-xylene concentra-tions of ca. 94% to be achieved [39]. Mobil hasannounced that it will be using its new MTPXprocess in two facilities in the United States.
4.2.3. Combination of the Technologies inthe Aromatics Complex
The maximization of the yields of o- and p-xylenes by combining isomerization, transalkyl-ation, disproportionation, and adsorptive extrac-tion of p-xylene is exemplified by the UOPtechnology complex (Fig. 11) [37]. In this com-plex all processes which are of industrial and
commercial importance in this respect are re-presented. The center of the complex is theParex process combined with Isomar technol-ogy. Tatoray technology is also used for con-version of the toluene and the C9+ aromaticsfraction (A9) [40].
Naphtha is used as the feedstock in this com-plex (cf. Fig. 12). It is obtained by conventionalcrude oil distillation, sometimes also from anevaporated BTX fraction. The reformer in thiscase is the UOP –CCR Platformer, which al-lows particularly high yields of aromatics tobe obtained and also provides the hydrogen forsubsequent processes, such as the Isomar andTatoray processes, or for naphtha hydrogen-ation. The Sulfolane process is a frequently usedliquid – liquid extraction, developed by Shellto obtain benzene and toluene from the refor-mate and from its C5 –C7-fraction (top prod-uct from the deheptanizer; see → Benzene;→Toluene,Chap. 3.). The bottomproducts fromthe reformate deheptanizer and from the tolueneand post-Isomar deheptanizer columns are fedto the Parex unit. Consequently the productsof the complex are o- and p-xylene, benzene,aromatics-free raffinate, and the C9+ aromatics.
5. Integration into Refinery andPetrochemical Complexes
5.1. Backward Integration intoPetroleum Refining
Usually both the production and the separationand isomerization of the xylenes are integratedinto refinery and petrochemical complexes [41,42]. There are many advantages to this type ofincorporation [43]:
1) Availability of a very favorable raw materialbase
2) Improved economics, especially in the useof byproduct streams unavoidably produced,such as pyrolysis gasoline or raffinates
3) Use of common infrastructure (energy,steam)
4) Possibilities for further processing of xyleneson site
5) Alternative use as fuel components if eco-nomical
Xylenes 13
Figure 11. Flow sheet of the UOP aromatics complexa) Debutanizer; b) Platformate deheptanizer; c) Splitter; d) Clay tower; e) Benzene column; f) Toluene column; g) A9 column;h) Xylene stripper; i) o-Xylene re-run; j) Parex finishing column k) Isomerate deheptanizer
Figure 12. Integration of xylene production into petrochemical refineriesFCCU=Fluid catalytic cracking unit
14 Xylenes
Figure 13.Most important further processing steps and downstream products of xylene isomers and ethylbenzene
Table 10.Western European producers and product capacities for xylene isomers and primary downstream products if produced at the samesite (1994, in 103 t/a)
Figure 14. Development of prices of naphtha and xylenes in Western Europe
Figure 15. Development of quantities of o- and p-xylene and primary downstream products produced in Western Europe
The integration of xylene production into thestructure of a petrochemical refinery is shownschematically in Figure 12.
The separation of xylenes and other aromat-ics from reformate or pyrolysis gasoline fre-quently leaves a gap in the boiling curve of themotor fuels produced in the refinery. This phe-nomenon is typical of petrochemical refineriesand is known as the gap fuel characteristic. Tocomplywithmotor fuel specifications it is some-times necessary to add alternative fuel compo-nents in the same boiling range [44]. With re-gard to a possible future limitations on the to-tal aromatics content of motor fuels, as is beingdiscussed in the United States [4], the isolationof xylenes and other aromatics could, however,
also help to fulfill the specification requirements[45].
5.2. Forward Integration into ChemicalProcessing
Apart from their use as solvents, the individualxylene isomers can be regarded as intermedi-ates which are further processed in a series ofreactions to give consumer products [5]. Whileimmediate subsequent synthesis steps, such asoxidation of o- and p-xylenes to phthalic an-hydride and terephthalic acid, can be integratedinto petrochemical complexes to some extent,
16 Xylenes
further processing steps are mainly carried outat purely chemical sites.
The most important further processing stepsand downstream products of the various xyleneisomers and of ethylbenzene are shown sche-matically in Figure 13.
6. Economic Aspects
ParametersAffectingEconomics. The eco-nomics of xylene production and isolation areessentially determined by two parameters [3]:
1) The price difference between the proceeds ofxylene production and the cost of the naphthaused (see Fig. 14)
2) The value relative to that when used as a mo-tor fuel component
The first aspect is the most important whentargeted xylene production by reforming naph-tha without the possibility of the alternative usein motor fuel is considered. However, the sec-ond aspect determines the economics of the sep-aration of xylene mixtures, which can originatefrom reformate or pyrolysis gasoline, if the alter-native option of use as a motor fuel componentis available. Besides these aspects, other techni-cal parameters, such as the performance of thereformer, the efficiencyof the separating and iso-merization plants, or the quality of the naphtha,affect the overall economics.
The naphtha/xylene price difference is alsocontrolled byother, overriding factors.While theprice of naphtha is closely linked to that of pe-troleum, the proceeds of xylene production areprincipally determined by the price and demandfor downstream products.
From the historical and expected future de-velopment of the quantities of direct down-stream products of p-xylene produced in West-ern Europe (see Fig. 15) [3], it may be de-duced that conversion to purified terephthalicacid (PTA) will be increasingly favored over di-methyl terephthalate (DMT). On balance thisleads to an increased overall demand for p-xylenewhichmay essentially be attributed to theincreasing demand for poly(ethylene terephthal-ate) resins for the production of plastic bottles.
In the past the primary downstream productof o-xylene, phthalic anhydride, was produced
in significant quantities by oxidation of coal-tar naphthalene [3]. Now phthalic anhydride ismainly produced from o-xylene so the consump-tions of these products are directly associated.They are only increasing slightly because of therelatively low growth rate of plasticizer produc-tion.
The main downstream product from ethyl-benzene, styrene, is mainly converted to poly-styrene and ABS elastomers. Correspondingly,the demand for these products determines thedemand for ethylbenzene. Most of the ethylben-zene, however, is not producedwithin the frame-work of xylene production, but from benzeneand ethylene [46].
The economic importance of m-xylene andits primary downstream product, isophthalicacid, is comparatively low. Regional demand formixed xylenes in 1995 broke down as follows(total demand: 16×106 t):
Far East 44%North America 28%Western Europe 15%Eastern Europe 7%South America 3%Middle East 3%
Producers. The most important producersand their production capacities for the variousxylene isomers in Western Europe are listed inTable 10 [47]. Only in a few cases are additionalcapacities for conversion to phthalic anhydrideor terephthalic acid and dimethyl terephthalateavailable at the same production site. The largestcapacities for mixed xylenes are in North Amer-ica and the Asia/Pacific region. Production ca-pacities (in 103 t) worldwide for mixed xylenesfollow [48]:
North AmericaUnited StatesAmoco Chemical 680Exxon Chemical 625Chevron Chemical 573Koch 550Phillips Chemical 416Lyondell 314Fina 311Sun 270BP Chemicals 198Shell Chemical 198Ashland Chemical 182Mobil Chemical 152Coastal 109South Western Refining 98
Specifications for xylenes are generally laid by aparticular company, depending on the intendedapplication. Properties relevant to specificationare determined by standard procedures [49].
Some important methods of determinationfor xylene follow:
1) Distillation range (e.g., DIN 51 761,ASTMD850)
2) Hydrogen sulfide and sulfur dioxide content(e.g., ASTMD853)
3) Thiol sulfur (e.g., DIN 51 765)4) Density (e.g., DIN 51 757, ASTMD891)5) Flash point (e.g., DIN 51 755, ASTMD56)6) Color (e.g., ASTMD156, ISO 6271)7) Bromine consumption (e.g., DIN 51 774)8) Purity/composition (e.g., ASTMD1016;
GLC determination)9) Residue on evaporation (e.g., EN 5).
Typical specifications for o- and p-xylenesare given in Tables 11 and 12.
Table 11. Typical product specification for o-xylene
Method Value
o-Xylene (min.), wt% GC 98.0C9 and higher aromatics(max.), wt%
GC 0.5
Nonaromatics (max.), wt% GC 0.5Density at 15 ◦C, kg/m3 DIN51 757 882 – 885APHA color (max.) ISO 6271 10Copper corrosion (max.) DIN 51 759 1Hydrogen sulfide and sulfurdioxide (max.)
ASTMD853 free of H2S andSO2
Acidity DIN 51 558T.1 not detectableStart of boiling (min.), ◦C DIN51 761 143.0End of boiling (max.), ◦C DIN51 761 145.0Residue on evaporation(max.), mg/kg
The high purity requirement for p-xylene aredue to the quality requirements for the produc-tion of pure terephthalic acid. Provided that fur-ther processing takes place via dimethyl tere-phthalate, sometimes somewhat lower degreesof purity can be tolerated. For m-xylene, whichis produced in comparatively small quantities,only purities of 95.4wt%min. are usually re-quired.
8. Storage, Transport, and Safety
Instructions for ensuring safe storage, trans-fer, and transport of flammable liquids can befound in the technical rules for flammable liq-uids (TRbF), drawn up by the German commit-tee for flammable liquids [50,51]. For xylene thefollowing TRbF numbers are important:
TRbF 001: General, form, and application of the rulesTRbF 100: General safety requirementsTRbF 111: Filling stations, emptying stationsTRbF 112: Gasoline stationsTRbF 120/121: Tanks in a fixed locationTRbF 131/1 and 2: Piping and tubingTRbF 141: Tanks on vehiclesTRbF 142: Tank containersTRbF 143: Movable vesselsTRbF 180: Operating instructions
The flammability limits for xylenes in air arebetween 1 and 8%. Because the flash point ofxylene mixtures is below 21 ◦C, they are mostlyassigned to hazard classA I. The flash pointsof pure xylenes (not including ethylbenzene)are, however, > 21 ◦C so they may be declaredas hazard classA II. For storage and transportxylenes must be labeled as readily flammableand harmful [51,52]. For transport within Ger-many xylenes are assigned to class 3, cipher 31 cof the GGVS/GGVE regulations (regulations
governing transport of hazardous goods by roadand rail) [53].
9. Environmental Aspects andToxicology
Environmental Aspects. In Germanyxylenes are assigned to water hazard class 2(WGK2) [55]. The solubility of xylenes in wa-ter is low (ca. 0.14 g/L). Because of the com-paratively low vapor pressure (7 – 8mbar at20 ◦C), the danger of vapor emissions in air isrelatively low. However, xylenes can react withother air pollutants to give environmentally dam-aging products. This applies particularly to theUV-catalyzed photooxidation of xylenes by ni-trogen oxides. However, compared with someother hydrocarbons, the reactivity of xylenes iscomparatively low, with a reaction rate of ca.2×10−9 min−1 [55]. For example, disubstitut-ed internal olefins have a reaction rate of ca.50×10−9 min−1.
Toxicology. Exposure to xylene is possiblethrough inhalation of vapors and resorptionthrough the skin. The MAK value is 100 ppm(ca. 440mg/m3) for all three isomers [54].
General Activity. Xylene exhibits acute pre-narcotic and narcotic activity. Chronic exposureto xylene leads to disturbance of the CNS (e.g.,headaches, sleep disturbance) [56] and dam-age to the blood picture (dyspepsia). Providedthat there is no long-term chronic overexposure,these effects are reversible. Besides developinga certain tolerance, frequent exposure to xylenecan also lead to habituation or even addiction(solvent abuse).
Acute Toxicity. The LD50, LC50, and TCLovalues for oral administration and inhalationvary widely, depending on the animal investi-gated and the isomer composition. The follow-ing values give an indication of the toxicity ofxylene isomermixtures [54,57]: LD50 (rat, oral)4300mg/kg, LDL0 (rat, i.p.) 2000mg/kg, TCLo(humans, inhalation) 200 ppm.
A very high exposure to xylene of ca.10 000 ppm caused by an accident led to lungedema and subsequent death in one person [58].In other cases severe damage to the CNS, kid-neys, and liver were observed.
Xylenes 19
Irritant Effects. On repeated applicationxylenes can cause irritation of the respiratorypassages and mucous membranes of the eye.Frequent skin contact can lead to blister forma-tion and dermatitis [57,59–62].
Subchronic and Chronic Toxicity. At a con-centration range of 100 to over 1000 ppm xy-lene in inhaled air, damage to the CNS with dis-turbance of balance or slowing of reactions isobserved. Inhalation of xylene vapors can alsocause nausea and headaches [63–65].
A change in the blood picture is also fre-quently observed. In ca. 10% of persons whohad been exposed to xylene vapors in concen-trations of up to ca. 100 ppm for ca. 5 years,a leucocyte level of < 4500mm−3 was estab-lished [66]. A decrease in the immunobiologicalactivity has occasionally been observed [58].
Carcinogenicity, Mutagenicity, and Embry-otoxicity. The assessment of the carcinogenicityof xylenes is not consistent [67–69]. While inone study no indications of carcinogenicity orcocarcinogenicity were found, another investi-gation indicated that xylenes act as tumor pro-moters for skin tumors in rats.
None of the three xylene isomers showedmu-tagenicity in the Ames test [70]. However, slightmutagenicity was detected in a recessive lethaltest on drosophila [71].
In animal experiments long-term exposurethrough inhalation of xylenes causes smallchanges to fetuses [72].
Pharmacokinetics andMetabolism. All threexylene isomers are resorbed in the same way.Various investigations have shown that ca. 60 –70% of the xylene reaching the organism viathe lungs is retained [73–76]. This percentageremains approximately constant over the wholeexposure period. The ratio of the concentrationin the air in the alveoli (mg/m3) and in the blood(mg/kg) changes with the degree of bodily ac-tivity. When the body is at rest the ratio is ca.15 : 1 and whenmoving ca. 30 – 40 : 1 [77]. Xy-lene can be resorbed at a rate of ca. 2.5 (0.7 –4.3)mg/m3 per minute through intact skin [78].
Xylenes are deposited rapidly in body fat(up to ca. 5%) and remain there for hours af-ter exposure. The half-life in fat deposits is ca.0.5 – 1.0 h [78]. The metabolism of the individ-ual xylene isomers is identical. The main bio-transformation pathway initially involves oxi-dation to methylbenzoic acid, which forms the
corresponding methylhippuric acid by conjuga-tion with glycine (A) [74,79–81]. The methyl-hippuric acid can be excreted rapidly via the kid-neys. Another, less favored biotransformationpathway involves the hydroxylation of the xy-lene on the aromatic ring, forming xylenols (B)[73].
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Xylenols → Cresols and XylenolsXylidenesulfonic Acids → Benzenesulfonic Acids and Their Derivatives
