Xylenes

Xylenes

1

Xylenes ¨ u¨ sseldorf, Jorg Fabri, Veba AG, Dusseldorf, Ulrich Graeser, Veba Thomas A. Simo,

1. 2. 3. 3.1. 3.1. 3.2. 3.2. 4. 4.1. 4.1. 4.2. 4.2. 4.2.1. 4.2.1.

4.2.2. 4.2.2.

Federal Republic of Germany

¨ AG, Gelsenkirchen, Federal Republic of Germany Ol

¨ Gas, Chemie GmbH, Frankfurt, Federal Republic of Germany Lurgi Ol,

Introduction . . . . . . . . . . . . . . . Properties . . . . . . . . . . . . . . . . . Occu Oc curr rren ence ce and and Raw Raw Mate Materi rial alss . . . Natu Natura rall Oc Occu curr rren ence ce . . . . . . . . . . Raw Raw Mat Mater eria ials ls for for Xyl Xylen enee Pr Produc oduc-tion . . . . . . . . . . . . . . . . . . . . . Prod Produc ucti tion on,, Se Separ paratio ation n, and and Fururther Processing Processing . . . . . . . . . . . . . . Prod Produc ucti tion on of C8 Aromatics . . . . . Separa Sep aratio tion n and Furthe Furtherr Proce Processi ssing ng Separa Separatio tion n of of the the C8 Aromatics Fraction . . . . . . . . . . . . . . . . . . . . . Isomer Isomeriz izati ation on of of the the C8 Fraction . . .

1 3 6 6 6 7 7 9 9 12

1. Introduction The benzen benzenee homolo homologue guess of genera generall formul formulaa C8 H10 are generally known as mixed xylenes. The mixture mixture of isomer isomerss with with a boilin boiling g point point ◦ rang rangee of 135 – 145 145 C main mainly ly cons consis ists ts of the the three isomeric dimethylbenzenes and ethylbenzene:

With With the excep exceptio tion n of xylene xylene produc productio tion n by disproportionation of toluene, the isomeric xylenes and ethylbenzene are always produced as a mixture in all production processes. However, the relative proportions of the C 8 isomers often differ differ considerab considerably ly (see Chap. 3). Because of their high knock resistance (see Chap. 2), xylenes xylenes are well suited to the production of motor fuels. In terms of quantity the production of gasoline exceeds that of BTX aromati matics cs (B = benz benzen ene, e, T = tolu toluen ene, e, X = xyle xylene nes) s) quite considerably (in Western Europe in 1995 c 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim  10.1002/14356007. 10.1002/14356007.a28 a28 433

4.2.3. 4.2.3. 5. 5.1. 5.1. 5.2. 5.2. 6. 7. 8. 9. 10.

Combin Combinat ation ion of the Tec Techno hnolog logies ies in the Aromatics Complex . . . . . . . . . Inte Integr grat atio ion n into into Refin Refiner ery y and and Petr Petroochemical Complexes . . . . . . . . . . Back Backwa ward rd Inte Integr grat atio ion n into into Petroleum Petroleum Refining . . . . . . . . . . . Forwa Forward rd Integr Integrati ation on into into Chemi Chemical cal Processing . . . . . . . . . . . . . . . . . Economic Aspects . . . . . . . . . . . . Qual Qu alit ity y Spec Specifi ifica cati tion onss and and Anal Analys ysis is Stor Storag age, e, Trans ranspo port rt,, and and Safe Safety ty . . . Envir En vironm onment ental al Aspec Aspects ts and Toxicol oxicol-ogy . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . .

12 13 13 13 13 18 18 19 20

gasoline production was ca. 150 ×106 t and that of BTX aromatics ca. 13.7×106 t, of which ca. 2.7×106 t/a was mixed xylenes, 0.65 ×106 t/a oxyle xylene ne,, and and 1.4 1.4×106 t/a p-xylen -xylene) e) [1]. [1]. The aver aver-age aromatics content of motor fuels in Western Europe Europe is ca. ca. 38 % [2]. The close association association with gasoline production tion strongl strongly y affec affects ts the econom economics ics of separa separatin ting g xylene mixtures, for example, for use in chemical processes. In Western Europe the production/use balance in 1995 was as follows [3]: Production from coal from from pyroly pyrolysis sis gasoli gasoline ne from reformate by disp dispro ropo port rtio iona natio tion n Total production Use in extraction and chemical processing (incl. imports) solvents as gaso gasoli line ne comp compon onen entt Total use Domestic production ion Net impor

36×103 t 300×103 t 2165×103 t 205 205×103 t 2706×103 t

(ca. 1.3 %) %) (ca. 11.1 %) %) (ca. 80.0 %) %) (ca. 7.6 %) %)

2563×103 t

(ca. 88.4 %) %)

306×103 t 30×103 t 2899×103 t 93.3% 6.7 %

(ca. 10.6 %) %) (ca. 1 %)

The structure of world mixed-xylenes production capacity in 1994 was as follows:

2

Xylenes

Total capacity Reformer Disproportionation Pyrolysis gasoline Coal

23×106 t 83 % 10 % 6% 1%

In Western Europe until the year 2000 a significan nificantt increa increase se in the propor proportio tion n of xylene xylene used used in gasoline production is predicted, because the growth in production of xylene-containing reformate may exceed its consumption in chemical processes. However, in other regions, such as the United United States States,, the reduct reduction ion and limita limitatio tion n of the content of aromatics, and thus of xylenes, in gasoline is being discussed discussed (see also Chap. 2 and Section 5.2) [2,4]. The use of xylene xyleness in chemic chemical al proces processes ses brok brokee down down as foll follo ows (dat (dataa for for Weste estern rn Euro Europe pe 1995) [1]:  p-Xylene o-Xylene Ethylbenzene and miscellaneous

1761×103 t (68.7 (68.7 %) 3 650×10 t (25.4 (25.4 %) 152×103 t (5.9 (5.9 %)

Total

2563×103 t

The dominant importance of  p-xylene can be seen seen from from the the rela relati tive ve prop propor orti tion onss of the the isol isolat ated ed xylene isomers. p-Xylene is mainly oxidized to terephthalic acid, which can be esterified to dimethyl terephthalate (precursor for polyesters). -Xylen enee is oxid oxidiz ized ed to phth phthal alic ic anhy anhydr drid idee (pre (pre-o-Xyl cursor for plasticizers) and m-xylene to isophthalic acid (precursor for polyesters). Ethylbenzene zene is dehy dehydr drog ogen enat ated ed to styre styrene ne,, whic which h is conconverted to polystyrene and other polymers [5]. History [6]. Xylene was first discovered in crude wood spirit in 1850 by Cahours. The name xylene was derived from the Greek word wood). Xylene Xylene was detect detected ed in coalcoal xylon (= wood). tar in 1855 by Ritthausen and Church . In 1891 1891 ethy ethylb lben enze zene ne was was foun found d in hard hard coal coal-t -tar ar by and Moschner carried Noelting and Palmar, and out the first synthesis of pure ethylbenzene from ethylbenzenesulfonic acid. Between 1865 and 1869 Ernst and Fittig found that the “xylene” in coal-tar does not consist of a single single compou compound. nd. They They synthe synthesiz sized ed “xylene” from toluene and named it “methyltoluene.” They then established that some properties of “methyltoluene” differed from those of  coal-tar xylene, particularly with regard to the nitro compounds. Fittig et al. conclude concluded d that in

synthetic “methyltoluene” (  p-xylene) the second methyl group was in a different position from that in coal-tar xylene. Glinser and Fittig described the production of methyltol methyltoluene uene in 1865. Fittig obtained a hydroc hydrocarb arbon on by dry distil distillat lation ion of the calcium cium salt salt of mesi mesito toic ic acid acid whic which h rese resemb mble led d both both known known modific modificati ations ons of dimeth dimethylb ylbenz enzene ene,, (“xy(“xylene” and “methyltoluene”) but was not completely identical to either. The new hydrocarbon was named “isoxylene” ( m-xylene). This work  elucidated the structures of  p- and m-xylene and proved their occurrence in coal-tar. In a further experiment Fittig identified oxylene xylene as the third third isomer isomer,, which which was was discov discovere ered d in coal-tar by Jacobson in 1877. m- and p-xylene were initially separated on the basis basis of their their differ different ent chemic chemical al reacti reactivit vity y. mXylene reacts with concentrated sulfuric acid. -Xylenee is then then regen regenera erated ted from from the cryscrysm-Xylen talline m-xylenesulfonic acid using superheated steam in the presence of sulfuric acid. The fraction unaffected by sulfonation is then reacted with oleum. The sulfonic acid that crystallizes is cleaved to give pure p-xylene. For decades coke-oven tar and benzole were used as raw materials for production of aromatics. Worldwide their production was associated in location and quantity with coal and steel product ductio ion. n. The The plan plants ts for for extr extrac acti tion on and and refin refinin ing g of  coke-oven benzole, a mixture of light oil from coke-o coke-ove ven n tar and crude crude benzen benzene, e, obtain obtained ed from from coke-oven gas by oil scrubbing, were built ad jacent to coking plants. These plants were very small compared with those currently in operation. At that time the refining of crude benzole was was carrie carried d out exclu exclusi sive vely ly by washi washing ng with with conconcentrated centrated sulfuric sulfuric acid, acid, whereby whereby impurities, impurities, such as resin-forming compounds, nitrogen and oxygen compounds, and some of the sulfur compounds pounds,, were were remov removed ed as acid acid tar. tar. Howe Howeve ver, r, valuable substances, such as styrene, were also lost in the acid tar. The xylenes were in demand as solv solven ents ts and and were were used used as raw raw mate materi rial alss in the the chemical and pharmaceutical industries. However, production of the three isomers in pure form form was was comp compli lica cate ted d due due to thei theirr almo almost st iden identi ti-cal physical properties. The quantities available were too small for the development of industrial xylene chemistry. chemistry. m- and p-xylene could be separated from oxylene by distillation and thus o-xylene could

Xylenes be obtained in limited purity. m- and p-xylene, howe howeve verr, could could only only be separa separated ted labori laborious ously ly on the basis of their different chemical properties. With the development of the hydrogenative refin refinin ing g of coke coke-o -ove ven n benz benzol olee over over sulf sulfur ur-resistant catalysts under pressure, sulfuric acid refinin refining g became became unecon uneconomi omical cal and was was replac replaced ed by a technically superior process (see → Benzene, zene, Chap. 6.1.). Parallel Parallel to this developmen developmentt in process technology, technology, aromatics production became came indepe independe ndent nt of steel steelwo works rks or coking coking plants plants and centralization took place. Large refineries and distillation plants provided a basis for modest est chem chemic ical al proc proces essin sing g of the the C 8 aromatics in the mid-1950s. Between 1955 and 1960 ca. 30 000 t of xylenes xylenes were were obtained obtained from a coke production production level level of (40 (40 – 45)×106 t/a. The fourth fourth isomer isomer,, ethylb ethylbenz enzene ene,, must must be considered separately from the historical development of the isomeric xylenes. Its synthesis from benzene benzene and ethylene ethylene in a Friedel Friedel – Crafts reaction solved the problem of raw materials at an early stage. With the discovery of the catalytic dehydrogenation of naphthenes and the catalytic dehydrocyclization of paraffins and isoparaffins on noble metal catalysts and their bifunctional systems (platforming and rheniforming processes), the the raw raw mate materi rial al basis basis for for the the deve develo lopm pmen entt of xyxylene chemistry on a large scale was secured in Euro Europe pe from from abou aboutt 1960 1960.. A larg largee numb number er of proprocesses cesses have now been develope developed: d: liquid – liquid liquid extractio extraction, n, azeotropi azeotropicc distillatio distillation, n, extracti extractive ve distillation, isomerization, crystallization, adsorption, and absorption. These not only provided pure pure prod produc ucts ts on an indu industr stria iall scal scale, e, but but also also perpermitted mitted the separation separation of  m- and p-xylene. Thus the basis for a large-scale growth of polyester fiber production was created. The residual mxyle xylene ne coul could d be isom isomer eriz ized ed or oxid oxidiz ized ed to isop isophhthal thalic ic acid acid,, whic which h had had a limi limite ted d mark market et.. The The marmarket for o-xylene was provided by phthalic anhydride production.

2. Properties [6] echnical-grade rade xyPhysical Physical Propertie Properties. s. Technical-g lene is a mixtrue of C 8 aromatics, also known as the A8 frac fracti tion on,, cons consis isti ting ng of the the thre threee xyle xylene ne isomers, isomers, ethylbenz ethylbenzene, ene, and depending depending on the origin, varying amounts of nonaromatics which

3

boil boil in the the same same range range (136 – 145 145 ◦ C). Table 1 list listss some some phys physic ical al data data for for the the arom aromat atic ic comp compoonent nents. s. Othe Otherr data data can can be foun found d in [7,8]. [7,8]. The The rela rela-tive tive prop propor orti tion onss of thes thesee comp compon onen ents ts in a xyle xylene ne frac fracti tion on depe depend nd on its its orig origin in and and also also whic which h proprocess cess steps steps the the xyle xylene ne has has alre alread ady y pass passed ed thro throug ugh h (e.g., hydrogenation). The The vapo vaporr pres pressu sure re curv curves es of the the thre threee xyle xylene ne isom isomer erss and and ethy ethylb lben enze zene ne are are sho shown in Figu Figure re 1. The isomeric xylenes and ethylbenzene form azeotropic mixtures with water and numerous organic compounds (Table 2) [9–13]. The absorption properties of xylene are of  technical interest because of the significant differences in the solubilities of various gases as a function of temperature (see Table 3). Chemical Properties. Oxidation of the xylene isomers gives the corresponding aromatic dicarboxylic acids. Phthalic acid is produced industrially from o-xylene, isophthalic acid from m-xylene, and terephthalic acid from p-xylene (see → Phthalic Acid and Derivatives, → Terephthal phthalic ic Acid, Acid, Dimeth Dimethyl yl Terepht erephthal halate ate,, and Isophthalic Acid). The oxidat oxidation ion reacti reactions ons have have been been estabestablished as industrial processes in both the gas and liquid liquid phases phases.. Attemp Attempts ts have have been been made made to introduce the co-oxidation of  p-xylene with paraformaldehyde (Toray Industries) or acetaldehyde (Eastman Kodak) [5]. Ammonoxidation of  m- and p-xylenes -xylenes initially tially give givess the corres correspon pondin ding g phthal phthalic ic acid acid dinidinitriles, which are important raw materials for the production of isocyanates via reduction to the corresponding xylylenediamines. The dinitriles can also be hydrolyzed to the corresponding phthalic acids. However, this step has only limited industrial and economic importance. The nitration of  o- and m-xylenes provides a route to xylidines following hydrogenation of  the initially formed dimethylnitrobenzene isomers. mers. Xylidi Xylidines nes are used used as interm intermedi ediate atess in dye and rubber additive production, for example (→ Xylidines, Xylidines, Chap. 6.1.). Thecap The capaci acity ty of thexyl the xylene eneiso isomer merss to under undergo go isomerization and disproportionation reactions is also exploited exploited industriall industrially y (see Chap. 4). Sulfon Sulfonati ation on of  m-xylen -xylenee and subseq subsequen uentt decomposition of the sulfonic acid derivatives gives 3,5- and 2,4-xylenols, providing starting materials for insecticides, herbicides, etc.

4

Xylenes

Table 1. Physical data for xylene isomers and ethylbenzene

 M r bp at 1 bar, bar, ◦

o-Xylene

m-Xylene

p-Xylene

Ethylbe lbenzene

106.16 144.4

106.16 139.1

106.16 138.4

106.16 136.2

357.1

343.6

342.8

344.0

35.20 0.260 0.380 0.281 − 25.182

35.47 0.270 0.390 0.282 − 47.87

34.45 0.250 0.370 0.290 + 13 13.26

37.27 0.260 0.371 0.284 − 95.00

30.70 0.809 0.544 36.89

30.12 0.617 0.523 36.45

28.8 0.644 0.511 36.00

29.5 0.6783 0.494 36.05

41.25 40.84

41.24 40.83

41.24 40.84

41.34 40.94

C

Critical temperature, ◦

C b ar

Critical pressure Critical compressibility Critical molar volume

L/mol g/cm3

mp, ◦

◦

Surface tension at 15.6 C, Dynamic viscosity at 20 C, Thermal conductivity at 0 C, Enthalpy of evaporation at the bp and 1 bar, bar, Lower calorific value, Gas Liquid Density at 1 ba bar, at 25 C at 20 C Refractive index n20 D Heat of fusion, ◦

◦

C mN/m mPa · s kJ m 1 h kJ/mol −

1

−

K

kJ/g

g/cm3

◦ ◦

Explosion limits in air, lower upper Ignition temperature,

1

−

kJ/mol kJ/kg vol %

0.8760 0.8802 1.50449 13.78 129.8

0.8599 0.8642 1.49712 11.56 108.9

0.8567 0.8610 1.49575 17.02 160.4

1.0 5.3 – 7.6 496 496 – 502 502 ◦

C

◦

pressure curves of the xylene isomers and ethylbenzene ethylbenzene at 100 – 240 C Figure 1. Vapor pressure

0.8624 0.8670 1.49588 9.16 86.2

Xylenes

5

Table 2. Binary azeotropes

Component A

Aromatic

Azeotrope bp,

Water Methanol Butanol Isobutanol n-Hexanol Ethylene glycol 1,2-Propanediol Glycol 4-Heptanone Formic acid Acetic acid Propionic acid Phenol Nonane

m-xylene ethylbenzene p-xylene ethylbenzene ethylbenzene o-, m-, p-xylene o-xylene o-xylene o-, m-, p-xylene and ethylbenzene m-xylene m-xylene o-xylene o-, m-, p-xylene and ethylbenzene o-, m-, p-xylene and ethylbenzene m-xylene o-xylene

◦

C

Proportion of A, %

94.5 33.5 (80 mbar) 64.0 115.85 125.7 138 – 143 135.7 135.8 139 – 133 139 94 95.5 114 – 116 131 – 135 133 139

40 33 5 65 49 18 – 13 7 10 14 10 70 74 76 – 66 42 – 28 18 10

Table 3. Temperature dependence of the absorption coefficients of a technical-grade xylene (in m3 /m3 xylene, STP) T ,

◦

C

0 10 −20 −30 −40 −50 −

H2 S

CO2

CO

H2

NH3

Methane

Ethylene

Propene

25 35 50 72 110 174

1.53 1.62 1.71 1.85 2.01 2.21

0.30 0.33 0.36 0.39 0.44

0.013 0.017 0.02 0.024 0.029

8.0 12.4 18.6 27.7 42.2

0.6 0.68 0.78 0.92 1.08

4.0 4.4 4.9 5.5 6.3

24.0 33.0 46.5 66.0 95.0

Of the chemical properties of ethylbenzene as a component of the A 8 fraction, the most important is catalytic dehydrogenation to styrene, althou although gh ethylb ethylbenz enzene ene used used for this this purpos purposee mainly comes from other sources (alkylation of  benzene with ethylene).

of the xylene isomers [14,15] with the specification of a typical gasoline. A more detailed account of the overall much more more comple complex x intera interacti ctions ons with with the pool pool of gasogasoline components can be found in Section 5.1.

Properties as a Motor Fuel Component. The properties of motor fuels are determined by a larg largee numb number er of qual qualit ity y aspe aspect ctss whos whosee minimu minimum m requir requireme ements nts are mostly mostly specifi specified ed (see → Automotive Fuels). The value which is attributed to xylenes in blending of motor fuels essentially depends on how the xylenes can be combined individually with other available fuel components to produce motor fuel qualities which conform to market requirements. Typically the high knock rating, particularly as a blend component, and the low vapor pressure of  xylenes are important factors affecting their use as moto motorr fuel fuel comp compon onen ents. ts. Table able 4 comp compar ares es the the octane octane number number and vapor vapor pressu pressure re value valuess (RVP) (RVP)

3. Occurrence Occurrence and Raw Materials 3.1. Natural Occurrence Practically the only natural source of xylenes is petroleum. The concentration of xylenes varies considerab considerably ly depending depending on location location and geological age of the crude oil. Table 5 shows a comparison of the C 8 aromatics and the C 6 – C8 aromatics contents in various types of crude oil [16]. Alth Althou ough gh in a few few case casess petr petrol oleu eums ms (e.g (e.g., ., that that of South East Asian origin) can have an even high higher er arom aromat atic icss cont conten entt (up (up to 35 %), %), the the dire direct ct isolation of xylenes is not economical.

6

Xylenes

Table 4. Properties of xylene isomers as motor fuel components compared with finished gasoline

RON (pure) MON (pure) RON (typical blending value) MON (typical blending value) RVP, bar (pure)

o-Xylene

m-Xylene

p-Xyle Xylen ne

Ethy Ethylb lben enze zene ne

107.4 101.5 120 103 0.028

117.5 11 1 10 145 124 0.025

116.4 109 146 127 0.04

113 98 124 107 0.045

Super uper unlea nleade ded d (DIN (DIN 51 607/EN228) 607/EN228)

95 ∗ 85 ∗ 0.9 (Winter) ∗∗ 0.7 (Summer) ∗∗

Minimum values. ∗∗ Maximum values. ∗

Table 5. Aromatics content of various crude oils [15]

C8 aromatics, wt % C6 – C8 aromatics, wt %

Libya

Louisiana Gulf

West Texas

Venezuela

Nigerian

Iranian

0.56 1.0

0.50 1.10

1.10 1.79

1.10 1.85

1.47 2.50

1.05 1.80

3.2. Raw Materials for Xylene Production For the industrial extraction of xylenes, the oil fraction or coal used as the raw material is sub jected to thermal or catalytic treatment in which aromatics- and xylene-containing fractions are obtained. obtained. These conver conversion sion processes processes determine determine the quanti quantity ty of xylene xyleness avail availabl ablee and supply supply them in a form that is enriched sufficiently to make isolation and further processing economical. Raw Materials based on Petroleum. Xylene ness are are main mainly ly isol isolat ated ed  Reformates. Xyle from from reform reformate atess [17]. [17]. Naphth Naphthaa fracti fractions ons are used used as the raw materials for the reforming process, which is now practically only carried out catalytic alyticall ally y. Becaus Becausee of the sulfur sulfur sensit sensitiv ivity ity of the Pt – Re cata cataly lyst sts, s, the the naph naphth thaa must must first first be desu desullfurized. The composition of the reformates and thus the xylene content depends on how the reforming forming process process is carried out (see Section 4.1) and on the quality of the naphtha fraction used. The composition of the naphtha fraction, in particular its naphthene and aromatics content, significantly affects the composition of the reformate [18]. Naphthenic crude oils give naphtha fractions which which are partic particula ularly rly suitab suitable le for reform reforming ing and aromatics production. However, naphtha fractions produced by hydrocrack hydrocracking ing (see → Oil Refining, Refining, Chap. 3.5.) also generally have have high

naphthene contents [19] and are therefore suitable starting materials for xylene production by reforming. Besides the composition, the boiling range of the naphtha fraction affects the yield of  xylenes (see Table 6). A particularly high yield can be obtained when the naphtha fraction already has a high proportion of the corresponding xylene precursors (e.g., C8 aromatics or C 8 naphthenes) in a suitable boiling range [20]. yield of pyrol pyrolysi ysiss Pyrolysis Pyrolysis Gasoline. The yield gasoline and its xylene content is decisively affected by the raw materials used, and by the mode mode of operat operation ion of the plant plant (see (see Chap. Chap. 4). In steam cracking very different types of raw material can be used, which give rise to widely varyi varying ng yields yields and compos compositi itions ons of the pyrol pyrolysi ysiss gasoline (Table 7) [21]. Only on using higher boiling hydrocarbons feedstocks feedstocks can appreciab appreciable le xylene xylene yields yields and concentra concentrations tions in pyrolysis pyrolysis gasoline gasoline be attained. attained. Theref Therefore ore,, in genera general, l, isolat isolation ion of xylene xyleness is only only economical when using these types of raw materials. Raw Materials based on Coal. The isolation of xylenes from coke-oven tar, coke-oven benzol benzole, e, or from from hydroc hydrocarb arbon on mixtur mixtures es proproduced by hydrogenation of coal is of minor importance [17] (see also Section 4.1). The effect of the composition of the coal is described in [22]. Processes in which xylene-rich aromatics fractions can be produced from LPG or methanol are described in Chapter 4.

Xylenes

7

Table 6. Dependence of reformate composition on the boiling range of reformer feedstock (Kuwaitnaphtha) [20]

Naphtha boiling range,

Feedstock composition, composition, wt % Paraffins Naphthenes Aromatics Aromatics composition of reformate, reformate, wt % Benzene Toluene C8 aromatics C9+ aromatics

◦

C

60 – 160

107 – 160

90 – 160

69.6 19.5 10.9

62.2 21.2 16.6

64.2 22.2 13.6

9.3 21.7 20.8 8.8

1.6 19.0 34.3 15.2

5.2 25.1 26.2 11.2

Table 7. Effect of feedstocks on the yields of pyrolysis gasoline and C8 aromatics from steam cracking (conditions: very high severity with C2 and C3 recycle)

Feedstock

Yield of pyrolysis gasoline (C5 –200 C), %

Yield of C8 arom aromat atic ics, s, %

Prop roporti ortion on of C8 aromatics in pyrolysis gasoline, %

0.4 1.8 2.2 1.9

5.6 9.6 12.0 9.8

◦

Ethane Propane Butane Medium-range naphtha Atmospheric gas oil Vacuum gas oil

1.7 6.6 7.1 18.7 18.4 19.3

4. Production, Production, Separation, Separation, and Further Processing 4.1. Production Production of C8 Aromatics Catalytic Reforming. Besides the nature of  the raw materials, process engineering parameters ters also also signifi significan cantly tly determ determine ine the compos compositi ition on of the reformate and its content of C 8 aromatics [23]. The design of the catalytic reformer (see → Oil Refining, Refining, Chap. 3.4.) [24], [24], its mode of operation, and the catalyst used are also very import important ant.. The three three follo followin wing g types types of cataly catalytic tic reformer are in common use:

1) Semiregenerative Semiregenerative reformer, reformer, in which the noble metal catalyst is periodically regenerated during a production shutdown (typically every 6 to 12 months) 2) Fully regenerative regenerative reformer, reformer, in which installation of an additional swing reactor allows the alternating regeneration of one reactor while the other continues to operate 3) Continuousl Continuously y regenera regenerativ tivee reformer reformer whose fluidized bed catalyst is continuously regenerated during operation Oper Operat atin ing g cond condit itio ions ns of the the dif differe ferent nt type typess of  reformer are listed in Table 8 [25].

Sinc Sincee at leas leastt six reac reacti tion onss take take plac placee in para paralllel during during the reform reforming ing proces process, s, of which which essenessentially only two lead directly to the formation of  aromatics, establishing optimal operation conditions is very important. The reactions which give aromatics (the dehydrogenation of naphthenes and the dehydrocyclization of paraffins) are favored by comparatively high reactor temperatures peratures and low pressures pressures (see Fig. 2). At the same time a simultaneous increase in undesired side reactions (hydrocracking and coke formation) must be reckoned with, so that while increasing the severity leads to an increase in the aromatics content of the reformate, it also decreases reformate yield [26]. Dealkylation reactions are also of specific importance for xylene production. They can lead to decomposition of  the xylenes formed, giving toluene, or benzene and methane, particularly at high temperatures. A further control parameter for xylene productio duction n is thespa the space ce veloc velocity ity (LHSV) (LHSV).. If throug throughhput is lowered to increase the residence time in the reactor, reformates with high aromatics and xylene contents can be produced [25]. The ad justment of the operating parameters to give a maximum aromatics concentration in the reformate mate is kno known as arom aromiz izin ing. g. Withi ithin n cert certai ain n limlimits the quality of the reformate can also be in-

8

Xylenes

Table 8. Typical operating conditions of various reformer types [25]

Parameter

Pressure, MPa Operating temperature, LHSV ∗, h 1

◦

C

−

∗

Semiregenerative reformer

Fully regenerative reformer

Continuously regenerative reformer

1.5 – 2.5 510 – 540 2 – 3.5

0.7 – 1.5 510 – 540 3.5 – 4

1 510 – 540 1.5 – 4

Liquid hourly space velocity.

fluenced by the choice of catalyst. Besides conventional catalysts, which essentially only contain Pt as the active substances, bimetallic and multimetal multimetallic lic catalyst catalyst systems, systems, additiona additionally lly containing Re, Ge, Ir, or Pb, are used. For aromatics production the selectivity for C 6+ aromatics is of general importance, while the suppression of dealkylation reactions is particularly important for xylene production. The activity of the catalyst can be increased by regular addition of  chlori chlorinat nated ed hydroc hydrocarb arbons ons.. Major Major suppli suppliers ers of reforming catalysts designed for xylene production include Acreon, Criterion, Exxon, Kataleuna, and UOP [27].

ing the residence residence time, and reducing reducing the partial partial pressu pressure re by increa increasin sing g the quantity quantity of steam steam – signifi significan cantly tly decrea decreases ses the yield yield of pyrol pyrolysi ysiss gasoline [28], but increases the concentration of  xylene (see Table 9) [29]. Synthesis Processes. In the Cyclar process developed by UOP/BP, propane and butane are converted into aromatics-rich gasoline fractions by cycli cyclizat zation ion over over zeolit zeolitee cataly catalysts sts [30, [30, 31]. 31]. On using propane the gasoline fraction contains ca. 17.3wt% C 8 aromatics, aromatics, and with butane ca. ca. 19.8 wt %. The xyle xylene ne yiel yield d is ca. 15 wt % base based d on the the tota totall start startin ing g mate materi rial al.. A pilo pilott plan plantt for the Cyclar process has been commissioned at the BP refinery at Grangemouth, Scotland. For For econom economic ic reason reasons, s, howe howeve ver, r, this this proces processs has not been put into industrial-scale operation. The same applies to the Mobil Oil process for synthesizing thesizing aromatics aromatics from alcohols using ZSM5 catalysts [31].

4.2. Separation and Further Processing 4.2.1. Separation of the C 8 Aromatics Fraction ariation of the selectivit selectivity y of aromatics aromatics production production Figure Figure 2. Variation in catalytic reforming with operating pressure

Steam Cracking. In contrast to the reforming process, the xylenes in the pyrolysis gasoline produced by steam cracking are regarded as byproducts of ethylene and propene production. However, their isolation can improve the economics of the process under certain circumstances. Although the operating conditions are not adjusted to maximize the xylene yield in steam cracking, the effect of severity on xylene formation formation is significant. significant. Increasing Increasing the severseverity – i.e., i.e., raisin raising g the reacti reaction on temper temperatu ature, re, lower lower--

The reformates used to produce C 8 aromatics can be processed such that C 8 and heavier aromatics are produced in the following approximate proportions [31]: Benzene  p-Xylene o-Xylene C9+ aromatics

13.2 % 16.5 % 8.3 % 0.9 %

Beside Besidess the 38 % aromat aromatics ics,, 51 – 52 % naphnaphtha tha frac fracti tion onss and and 10 – 11 % gase gaseou ouss prod produc ucts ts (hydrogen, fuel gases, etc.) are formed. This optimize timized d result result invo involv lves es the follo followin wing g operat operation ions: s: 1) Toluene is dealkylated dealkylated to benzene. benzene. 2) m-Xylene is isomerized to o- and p-xylenes. 3) Ethylbenzene is transalkylated transalkylated to xylenes.

Xylenes

9

Table 9. Influence of severity in steam cracking on the yield and composition of pyrolysis gasoline (naphtha feedstock) [28]

Seve Severi rity ty ( % Ethy Ethyle lene ne))

Xyle Xylene+ ne+ ethy ethylb lben enze zene ne (all (all products), products), wt %

Xylene Xylene + ethylbenzen ethylbenzenee in pyrolysis pyrolysis gasoline, gasoline, wt %

Pyrolysis benzene yield, wt %

Low (24.4) Medium (28.5) High (33.4)

0.9 1.6 1.7

3.5 7.2 10.7

24.9 22.6 16.3

Figure 3. Flow sheet of the ethylbenzene distillation a) Pre-column; b) Ethylbenzene columns

If the toluene is subjected to transalkylation instea instead d of therma thermall dealk dealkyla ylatio tion, n, the yield yield of  xylenes can be further increased at the expense of benz benzen ene. e. The The stru struct ctur uree of a mode modern rn arom aromat atic icss complex, which produces o- and p-xylene in the highes highestt possib possible le yield, yield, is based based on a combin combinati ation on of a series of these processes. The ethylb ethylbenz enzene ene is mainly mainly conve converte rted d to xylenes xylenes because because obtaining obtaining ethylbenz ethylbenzene ene from reformate is energy intensive. The main source of ethylbenz ethylbenzene, ene, which which is almost almost exclusi exclusively vely dehydrogenated to styrene, is therefore now the more more econom economica icall alkyla alkylatio tion n of benzen benzenee with with ethethylene (see → Ethylbenzene). Distillative removal of ethylbenzene, which for styren styrenee produc productio tion n must must be toluen toluene-f e-free ree,, from reformates can take place after removal of  toluene from the sump of the toluene column. Figure 3 shows a flow sheet of the ethylbenzene distil distillat lation ion.. Other Other produc products ts obtain obtained ed by this this sepseparation are the xylene and C 9 aromatics. The major separation problem is caused by a boiling point difference of only 2 ◦ C between ethylbenzene and p-xylene. The distillative separation of  this mixture requires high reflux ratios (1 : 80 to 1 : 120) with 300 – 360 effecti effective ve plates. plates. A yield of ca. 95 % ethylbenzen ethylbenzenee with > 99.5 99.5 wt % pupurity is achieved under these conditions [32].

Figure 4. Flow sheet showing separation of  o-xylene a) Xylene splitter; b) o-Xylene column

The distillative separation of  o-xylene from the C8+ stream is also difficult. Although the boilin boiling g point point differ differenc encee is only only ca. 5 ◦ C, the required o-xyl -xylen enee puri purity ty (min (min.. 98 %) can can be achie achieve ved d with with 120 – 150 effecti effective ve plates plates and a reflux ratio of 1 : 10 to 1 : 15, with a yield of  Figure re 4 sho shows a flow flow shee sheett for for this this > 95 %. Figu separation. A further increase in o-xylene purity can be achieved by subsequent extractive distillation. tillation. Another method of separation separation is crystallization, mainly used to obtain p-xylene. This proces processs utiliz utilizes es the crysta crystalli llizat zation ion behav behavior ior of  p  pxylene near the eutectic point of a C 8 aromatics mixture. Depending on the composition of the mixture, this lies between −60 and −68 ◦ C (see Fig. Fig. 5). 5).

10

Xylenes

Melting ing beha behavio viorr of the the C8 aromati aromatics cs in their their mixmixFigure Figure 5. Melt tures

Figure 6. p-Xylene crystallization by indirect refrigeration a) Drier; Drier; b) Precool Precooler; er; c) Scraped Scraped-sur -surfac facee crysta crystalli llizer zer;; d) Refrigeration plant; e) Filter; f) Centrifuge; g) Mixer

The The Krup Krupp p – Koppe oppers rs proc proces esss (Fig (Fig.. 6) is a typical example of the crystallization process. The actua actuall separa separatio tion n proces processs is a fracti fractiona onall crystallization in which the product of the first separa separatio tion n step step in the filter (ca. 65 % of the pxylene from the starting mixture) is mixed with highly concentrated p-xylene and is isolated in high high puri purity ty in a cent centri rifu fuge ge.. This This proc proces esss and and that that of Philli Phillips ps Petrol Petroleum eum [33] [33] operat operatee using using indire indirect ct refrigeration. Direct contact refrigeration involves downward flow of the initially cooled liquid in the vertical crystallization vessel. This in turn directly cools the remainder of the contents of the vessel vessel (see Fig. Fig. 7).

-Xylene crystallizat crystallization ion by direct contact refrigFigure 7. p-Xylene eration a) Drying; Drying; b) Crysta Crystalli llizat zation; ion; c) Centrif Centrifuge; uge; d) Meltin Melting g tank; tank; e) Desorber; f) Scraped-surface crystallizer; g) Mixing tank 

The cooling agent can be evaporating ethylene (Maruzen Oil Co.) [34]. Chevron Research and Developm Development ent Corp., Sohio/BP Sohio/BP,, and Arco Technology use other process designs [35]. In the Parex process (UOP) [36] p-xylene is obtained in high purity from the isomer mixture by adsorp adsorptio tion n on a molecu molecular lar sieve sieve.. The attach attached ed finishing column removes lighter components (raffinate consisting of ethylbenzene and a xylene lene mixt mixtur uree low low in p-xyl -xylen ene) e) at the the head head.. The The pxylene xylene consti constitut tutes es the bottom bottom produc productt of this this colcolumn. umn. Adso Adsorp rpti tion on – deso desorp rpti tion on on the the soli solid d bed bed is carried out in such a way that a moving bed is simulated simulated (Fig. 8). The rotary valve brings brings segments of the filled adsorption chamber into contact with the feed and with the desorbent in such a way that part of the molecular sieve adsorbs while another part is desorbed. The rotary valve switches the streams in the direction of flow in the bed. This simulates the movement of the adsorbent in a direction opposite to that of the liquid. The adsorbent exhibits high selectivity towards  p-xyl -xylen enee comp compar ared ed with with the the othe otherr C 8 aromatics, especially ethylbenzene. Ethylbenzene is therefore therefore one of the most suitable desorbents desorbents or elue eluent nts; s; this this is kept kept in circ circul ulat atio ion n in the the extr extrac actt column. The heavy part of the raffinate can also be used as the desorbent. The nonadsorbed C 8 aromatics leave the raffinate column at the head

Xylenes

11

Figure 8. Parex process a) Adsorbent chamber; b) Rotary valve; c) Extract column; d) Raffinate column

and are then subjected to isomerization (Section 4.2.2). The Parex process is a variant of the Sorbex group of processes. The first process which used this principle of the simulated moving bed was the Molex process developed in 1964 for the separation of  n-paraffins from isoparaffins and cycli cyclicc hydroc hydrocarb arbons ons.. Other Other modific modificati ations ons of  the the Sorb Sorbex ex proc proces esse sess are are used used to sepa separa rate te olefi olefins ns and paraffins in C 3 – C18 hydrocarbon mixtures (Olex process) and for separating carbohydrates (Sarex process).

is a p-xylene-enriched stream which, after clay treatment to remove unsaturated compounds is fed to the Parex unit to obtain the newly formed  p-xylene.

4.2.2. Isomerizati Isomerization on of the C 8 Fraction

The yield of  o- and p-xylenes can be maximized by using using cataly catalytic tic conve conversio rsion n proces processes. ses. TheIsoThe Isomar process process (Fig. 9) [37] enriches enriches the p-xylene in the Parex raffinate by re-establishment of the equilibriu equilibrium. m. The acidic acidic metal-cont metal-containin aining g zeolite zeolite catalyst used here also isomerizes ethylbenzene select selectiv ively ely to xylene xyleneiso isomer merss in their their equili equilibri brium um ratio [38]. Since hydrogenolysis on the metal components must be guaranteed to maintain the activity of this bifunctional catalyst, the Isomar proces processs is operat operated ed with with a certa certain in partia partiall pressu pressure re of hydr hydrog ogen en in the the cata cataly lyti ticc reac reacto torr. Hydr Hydrog ogen en is separated from the effluent from the Isomar reactor and then recycled together with fresh hydrogen. The liquid product is separated in a deheptanizer column. A C 7 stream is obtained at the head, which can be recycled to the starting reformate. The bottom product of this column

Figure 9. Isomar process a) Heater; Heater; b) Reacti Reaction on column; column; c) Hydrogen Hydrogen separat separation ion;; d) Recycle compressor; e) Deheptanizer column

In the Tatoray atoray process (Fig. 10) toluene and the C9 (A9 ) bottom fractions from the A 9 column (o-xylene splitter column) are mainly converted into xylene isomers by disproportionation of the toluene and transalkylation of the C 9 aromatics (→ Toluene, oluene, Chap. 3.2.), affording affording a newly formed A 6 – A8 fraction from A9+ . This technology is the product of cooperation between UOP and Toray Industries. Commercial applications have been in operation since 1978. Since then the catalyst has been improved sev-

12

Xylenes

eral times with regard to activity, thermal stability, and life span [37].

Figure 10. Tatoray process a) Hydroge Hydrogen n recycl recyclee compres compressor sor;; b) Cataly Catalyticreacto ticreactor; r; c) PrePreheate heater; r; d) Hydro Hydrogen gen remo remova val; l; e) Ligh Lightt ends ends remo remova val; l; f) Clay Clay tower

The Tatoray atoray proces processs consist consistss of a reacto reactorr filled with solid catalyst to which are attached a coolin cooling g unit, unit, a gas – liquid liquid separat separation ion unit, unit, a unit for distillation of the light components and one for clay treating to remove olefinic impurities. As in the Isomar process a partial pressure of hydr hydrog ogen en is main mainta tain ined ed in the the reac reacto torr. By proprocessi cessing ng diff differ eren entt feed feed qual qualit itie ies, s, whic which h can can be ininfluen fluence ced d both both by the the diff differ eren entt recy recycl clee rate ratess and and a varying charge of fresh reformate fractions, the composition of the product can be controlled. Other proven proven disproporti disproportionat onation ion processes processes are the LTD process of Mobil Research of Arco Technology. Alkylation of toluene to increase the xylene yield has been developed by Mobil Chemical Chemical (Mobil (Mobil toluene-to toluene-to-- p-xylene process cess = MTPX) MTPX) and allow allowss p-xylene concentrations of ca. 94 % to be achieved achieved [39]. Mobil has announced that it will be using its new MTPX process in two facilities in the United States.

4.2.3. Combination of the Technologies in the Aromatics Complex

The maximization of the yields of  o- and pxylenes xylenes by combining combining isomerizat isomerization, ion, transalky transalkyllation, ation, disproporti disproportionati onation, on, and adsorptiv adsorptivee extracextraction tion of  p-xyl -xylen enee is exem exempl plifi ified ed by the the UOP UOP technology technology complex complex (Fig. 11) [37]. In this complex all processes which are of industrial and

commercial importance in this respect are repres presen ente ted. d. The The cent center er of the the comp comple lex x is the the Parex process combined with Isomar technology. Tatoray technology is also used for conversion of the toluene and the C 9+ aromatics fraction (A9 ) [40]. Naphtha is used as the feedstock in this complex plex (cf. Fig. 12). It is obtained obtained by conventio conventional nal crude oil distillation, sometimes also from an evaporated BTX fraction. The reformer in this case case is the the UOP UOP – CCR Plat Platfo form rmer er,, whic which h alallows lows partic particula ularly rly high high yields yields of aromat aromatics ics to be obtained and also provides provides the hydrogen hydrogen for subsequent processes, such as the Isomar and Tatoray atoray proces processes, ses, or for naphth naphthaa hydrog hydrogenenatio ation. n. The The Sulfo Sulfola lane ne proc proces esss is a freq freque uent ntly ly used used liquid liquid – liquid liquid extra extracti ction, on, deve develop loped ed by Shell Shell to obtain benzene and toluene from the reformate and from its C5 – C7 -fraction (top product from from the dehept deheptani anizer zer;; see → Benzene; → Toluene oluene,, Chap. Chap. 3.). 3.). The bottom bottom produc products ts from from the the refo reform rmat atee dehe dehept ptan aniz izer er and and from from the the tolu toluen enee and post-Isomar deheptanizer columns are fed to the Pare Parex x unit. unit. Conseq Consequen uently tly the produc products ts of the complex are o- and p-xylene, benzene, aromatics-free raffinate, and the C 9+ aromatics.

5. Integration into Refinery and Petrochemical Complexes 5.1. Backward Integration into Petroleum Petroleum Refining Usually Usually both the production production and the separation separation and isomerization of the xylenes are integrated into refinery and petrochemical complexes [41, 42]. There are many advantages to this type of  incorporation [43]: 1) Availability of a very very favorable favorable raw material base 2) Improved Improved economics, economics, especially especially in the use of byproduct streams unavoidably unavoidably produced, such as pyrolysis gasoline or raffinates 3) Use Use of comm common on infra infrast stru ruct ctur uree (ene (energ rgy y, steam) 4) Possib Possibili ilitie tiess for furthe furtherr proces processin sing g of xylene xyleness on site 5) Altern Alternati ative ve use as fuel fuel compon component entss if ecoeconomical

Xylenes

13

Figure 11. Flow sheet of the UOP aromatics complex a) Debutanizer; b) Platformate deheptanizer; c) Splitter; d) Clay tower; e) Benzene column; f) Toluene column; g) A 9 column; h) Xylene stripper; i) o-Xylene re-run; j) Parex finishing column k) Isomerate deheptanizer

Figure 12. Integration of xylene production into petrochemical refineries FCCU = Fluid catalytic catalytic cracking cracking unit 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 same site (1994, in 103 t/a)

Location

Producer

Geel, Belgium Gonfreville, France Schwedt, Germany Gelsenkirchen, Germany Heide, Germany, Wesseling, Germany Godorf, Germany Priolo, Italy Sarroch, Italy Milan, Italy Botlek, The Netherlands Oporto, Portugal Alqeciras, Spain Wilton, United Kingdom

Amoco Total PCK Ruhr Oel RWE/DEA RWE/DEA Shell Enichem Saras Sisas EXXON Petrogal Cepsa Interquisa ICI

o-Xylene

p-Xylene

m-Xylen -Xylenee

Ethylb Ethylbenz enzene ene from C8 aromatics

Phthalic Terephthalic Dimethyl anhydride acid terephthalate

30 90 35 45

105 25 105

12 44 100 60 60

110 75 140 70

105

165

42 40

115 34 330

250 62

10

70 50 50

45 50

20 50

200 580

70

Xylenes

15

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 the structure of a petrochemical refinery is shown schematically in Figure 12. The separation of xylenes and other aromatics from from reform reformate ate or pyrol pyrolysi ysiss gasoli gasoline ne frefrequently leaves a gap in the boiling curve of the motor fuels produced in the refinery. This phenomenon is typical of petrochemical refineries and is known as the gap fuel characteristic. To comply comply with with motor motor fuel fuel specifi specificat cation ionss it is somesometimes necessary to add alternative fuel components in the same boiling range [44]. With regard to a possible future limitations on the total aromatics content of motor fuels, as is being discussed in the United States [4], the isolation of xylenes and other aromatics could, however,

also also help help to fulfill fulfill the specifi specificat cation ion requir requireme ements nts [45].

5.2. Forward Integration into Chemical Processing Apart from their use as solvents, the individual xylene isomers can be regarded as intermediates which are further processed in a series of  reactions reactions to give give consumer consumer products [5]. While immediate subsequent synthesis steps, such as oxidation of  o- and p-xylenes to phthalic anhydride and terephthalic acid, can be integrated into petrochemical complexes to some extent,

16

Xylenes

further processing steps are mainly carried out at purely chemical sites. The most important further processing steps and downstream products of the various xylene isomers and of ethylbenzene are shown schematically in Figure 13.

6. Economic Economic Aspects ecoParameter Parameterss Affecting Affecting Economics. Economics. The economics of xylene production and isolation are essentially determined by two parameters [3]: 1) The The pric pricee diff differ eren ence ce betw betwee een n the the proc procee eeds ds of  xyle xylene ne prod produc ucti tion on and and the the cost cost of the the naph naphth thaa used used (see Fig. 14) 2) The value relative to that when used as a motor fuel component The first aspect is the most important when targeted xylene production by reforming naphtha without the possibility of the alternative use in motor fuel is considered. However, the second ond aspe aspect ct dete determ rmin ines es the the econ econom omic icss of the the sepseparation of xylene mixtures, which can originate from from reform reformate ate or pyrol pyrolysi ysiss gasoli gasoline, ne, if the alter alter-native option of use as a motor fuel component is available. Besides these aspects, other technical parameters, such as the performance of the reform reformer er,, the effici efficienc ency y of the separa separatin ting g and isoisomerization plants, or the quality of the naphtha, affect the overall economics. The naphtha/xylene price difference is also contro controlle lled d by other other,, overr overridi iding ng factor factors. s. While While the price of naphtha is closely linked to that of petroleum, the proceeds of xylene production are principally determined by the price and demand for downstream products. From the historical and expected future develop velopmen mentt of the quanti quantitie tiess of direct direct downdownstream products of  p-xylene produced in Western ern Euro Europe pe (see (see Fig. Fig. 15) 15) [3], [3], it may may be dededuced that conversion to purified terephthalic acid (PTA) will be increasingly favored over dimethyl methyl terephtha terephthalate late (DMT). On balance balance this leads leads to an increa increased sed overa overall ll demand demand for pxylene xylene which which may essent essential ially ly be attrib attribute uted d to the increasing increasing demand demand for poly(ethyle poly(ethylene ne terephthal terephthal-ate) resins for the production of plastic bottles. In the past the primary downstream product of  o-xylene, phthalic anhydride, was produced

in significant quantities by oxidation of coaltar naphthalene [3]. Now phthalic anhydride is mainly mainly produced produced from o-xylen -xylenee so the consum consumpptions of these products are directly associated. They are only increasing slightly because of the relatively low growth rate of plasticizer production. The main downstream product from ethylbenzene, styrene, is mainly converted to polystyrene and ABS elastomers. Correspondingly, the demand for these products determines the demand for ethylbenzene. Most of the ethylbenzene, zene, howe howeve ver, r, is not produc produced ed within within the frameframework of xylene production, but from benzene and ethylene [46]. The economic importance of  m-xylene and its primar primary y downs downstre tream am produc product, t, isopht isophthal halic ic acid, acid, is compar comparati ative vely ly low low. Region Regional al demand demand for mixed xylenes in 1995 broke down as follows (total demand: 16 ×106 t): Far East North America Western Europe Eastern Europe South America Middle East

44 % 28 % 15 % 7% 3% 3%

most import important ant produc producers ers Producers. The most and their production capacities for the various xylene isomers in Western Europe are listed in Table 10 [47]. Only in a few cases are additional capacities for conversion to phthalic anhydride or terephthalic acid and dimethyl terephthalate avail availabl ablee at the same same produc productio tion n site. site. The larges largestt capa capaci citi ties es for for mixe mixed d xyle xylene ness are are in Nort North h Amer Amer-ica and the Asia/Pacific region. Production capacities (in 10 3 t) worldwide for mixed xylenes follow [48]: North America United States Amoco Chemical Exxon Chemical Chevron Chemical Koch Phillips Chemical Lyondell Fina Sun BP Chemicals Shell Chemical Ashland Chemical Mobil Chemical Coastal South Western Refining

680 625 573 550 416 314 311 270 198 198 182 152 109 98

Xylenes Citgo OxyChem Phibro Energy Marathon Oil Uno-Ven Canada Kemtec Sunoco Petro-Canada Novacor acor Chem Chemic ical alss Shell Canada Imperial Oil South America  Argentina Pasa YPF  Brazil Petr Petroq oqui uimi mica ca do Nord Nordes este te Petroquimica do Sul Petrobras Petroquimica Uni¨ao Chile Corpoven Europe  Belgium Finaneste Germany DEA Mineraloel Deutsche Shell Ruhr Oel PK Schwedt  Italy EniChem Edison Spa The Netherlands Exxon Chemical Total Portugal Petrogal Spain Petresa United Kingdom ICI Asia/Pacific  Japan Idem Idemit itssu Petr Petro ochem chemic ical al Tonen Chemical Nikko Mitsubishi Oil Showa Shell Sekiyu Nippo ippon n Petro etroch chem emic ical alss Koa Oil Fuji Oil General Seikyu Maruz aruzen en Petro etroch chem emic ical al Kashima Oil Nippo ippon n Steel teel Chemi hemica call Mitsubishi Kagaku Mitsui Petrochemical Kyushu Aromatics South Korea Yukong Honam Oil Ssangyoung Oil Daelim Industrial

68 66 45 43 33 91 86 73 59 (tolu toluen enee –xylen –xylenee mixt mixtur ure) e) 32 14

80 80 250 250 68 25 96 50

35 242 200 175 138 200 185 285 200 185

Singapore Sing ingapor aporee Petr etroche ochemi mica call Shell hell East Easter ern n Chemi hemica cals ls Taiwan Chin Chines esee Petr Petrol oleu eum m Corp Corp..

17

30 20 750 750

7. Quality Specifications Specifications and Analysis Spec Specifi ifica cati tion onss for for xyle xylene ness are are gene genera rall lly y laid laid by a particular company, depending on the intended application. Properties relevant to specification are determined by standard procedures [49]. Some important important methods methods of determinat determination ion for xylene follow: 1) Disti stillation range (e. (e.g., DIN 51 761, ASTM ASTM D 850) 850) 2) Hydrogen Hydrogen sulfide sulfide and sulfur dioxide dioxide content (e.g., (e.g., ASTM ASTM D 853) 853) 3) Thiol Thiol sulfu sulfurr (e.g., (e.g., DIN DIN 51 765) 765) 4) Dens Densit ity y (e.g. (e.g.,, DIN 51 757, 757, AST ASTM M D 891) 891) 5) Flas Flash h poin pointt (e.g. (e.g.,, DIN 51 755, 755, ASTM ASTM D 56) 56) 6) Color Color (e.g., (e.g., ASTM ASTM D 156, 156, ISO ISO 6271) 6271) 7) Bromin Brominee consump consumptio tion n (e.g., (e.g., DIN 51 774) 774) 8) Pur Purit ity/ y/co comp mpos osit itio ion n (e.g (e.g., ., ASTM ASTM D 1016 1016;; GLC determination) 9) Residue Residue on evapora evaporation tion (e.g., (e.g., EN 5). Typical specifications for o- and p-xylenes are given in Tables 11 and 12.

216 345

550 550 540 510 420 370 340 340 250 190 190 170 170 120 115 115 100 100 90 500 365 360 95

Table 11. Typical product specification for o-xylene

Method o-Xylene (min.), wt % C9 and higher aromatics (max.), (max.), wt % Nonaromatics (max.), wt % Density at 15 C, kg/m3 APHA color (max.) Copper corrosion (max.) Hydrogen sulfide and sulfur dioxide (max.) Acidity Start of boiling (min.), C End of boiling (max.), C Residue on evaporation (max.), mg/kg ◦

◦

◦

GC GC GC DIN 51 51 75 757 ISO 62 6271 DIN 51 5 1 75 7 59 ASTMD 853 DIN 51 51 55 558 T. T.1 DIN 51 51 76 761 DIN 51 51 76 761 EN 5

Value 98.0 0.5 0.5 882 – 88 8 85 10 1 free of H2 S and SO2 not detectable 143.0 145.0 20

18

Xylenes governing transport of hazardous goods by road and rail) [53].

Table 12. Typical specification for p-xylene

 p-Xylene (min.), wt % C8 aromatics (max.), wt % Other aromatics (max.), wt % Nonaromatics Density at 15 C, kg/m3 Copper corrosion (max.) Total sulfur (max.), ppm Acidity Distillation range, C mp, C ◦

◦

◦

Method

Value

GC GC GC GC DIN 51 51 75 757 DIN 5 1 70 709 DIN 5 1 76 768 DIN 51 51 55 558 T. T.1 DIN 51 761

99.4 0.55 0.05 0 865 – 86 866 0 10 – 15 not detectable 1 13.1

The high purity requirement for p-xylene are due to the quality requirements for the production of pure terephthalic acid. Provided that further processing takes place via dimethyl terephthalate, sometimes somewhat lower degrees of purity can be tolerated. For m-xylene, which is produced in comparatively small quantities, only only puriti purities es of 95.4 95.4 wt % min. min. are are usua usuall lly y rerequired.

9. Environmental Aspects and Toxicology Enviro Envi ronm nmen enta tall Aspe Aspect cts. s. In Germany xyle xylene ness are are assig assigne ned d to wate waterr haza hazard rd clas classs 2 (WGK 2) [55]. The solubility solubility of xylenes in water is low low (ca. (ca. 0.14 0.14 g/L). g/L). Because Because of the comcompara parati tive vely ly low low vapor apor pres pressu sure re (7 – 8 mbar mbar at ◦ 20 C), the danger of vapor emissions in air is relatively low. However, xylenes can react with other other air pollut pollutant antss to give give envir environm onment entall ally y damdamaging products. This applies particularly to the UV-catalyzed photooxidation of xylenes by nitrogen oxides. However, compared with some other hydrocarbons, the reactivity of xylenes is comparatively low, with a reaction rate of ca. 2×10−9 min−1 [55]. For example, disubstituted internal olefins have a reaction rate of ca. 50×10−9 min−1 .

8. Storage, Transport, and Safety Instru Instructi ctions ons for ensuri ensuring ng safe safe storag storage, e, transtransfer, and transport of flammable liquids can be found in the technical rules for flammable liquids (TRbF), drawn up by the German committee tee for for flamm flammab able le liqu liquid idss [50,51] [50,51].. For For xyle xylene ne the the following TRbF numbers are important: TRbF00 TRbF001: 1: TRbF100 F100: TRb TRbF 111: 11: TRbF 11 1 12: TRbF12 TRbF120/ 0/12 121: 1: TRbF131 TRbF131/1 /1 and and 2: TRbF 14 1 41: TRbF 14 1 42: TRbF 14 1 43: TRbF180 F180:

Gene Genera ral, l, form form,, and and appl applic icat atio ion n of the the rule ruless General safety requirements Filli illin ng stat statio ions ns,, emp emptyin tying g stat statio ions ns Gasoline stations Tanks anks in a fixed fixed loca locati tion on Pipi Piping ng and and tubi tubing ng Tanks on vehicles Tank containers Movable vessels Opera erating ing ins instructio tions

The flammability limits for xylenes in air are betwee between n 1 and 8 %. Because Because the flash flash point point of  ◦ xylene mixtures is below 21 C, they are mostly assi assign gned ed to haza hazard rd clas classs A I. The The flash flash poin points ts of pure xylenes (not including ethylbenzene ethylbenzene)) ◦ are, however, > 21 C so they may be declared as hazard hazard class class A II. For storag storagee and transpo transport rt xylenes must be labeled as readily flammable and harmful [51,52]. For transport within Germany many xyle xylene ness are are assi assign gned ed to clas classs 3, ciph cipher er 31 c of the GGVS/GGVE GGVS/GGVE regulatio regulations ns (regulati (regulations ons

Toxicology. Exposure to xylene is possible throug through h inhala inhalatio tion n of vapor vaporss and resorp resorptio tion n throug through h the skin. skin. The MAK value value is 100 ppm 3 (ca. (ca. 440 mg/m mg/m ) for all three isomers [54]. General Activity. Activity. Xylene exhibits acute prenarcotic and narcotic activity. activity. Chronic exposure to xylene leads to disturbance of the CNS (e.g., headac headaches hes,, sleep sleep distur disturban bance) ce) [56] [56] and damdamage to the blood picture (dyspepsia). Provided that there is no long-term chronic overexposure, these effects are reversible. Besides developing a certain tolerance, frequent exposure to xylene can also lead to habituation or even addiction (solvent abuse).  Acute Toxicity. oxicity. The LD50 , LC50 , and TCLo value valuess for oral oral admini administr strati ation on and inhala inhalatio tion n vary widely, depending on the animal investigated gated and the isomer isomer composition composition.. The followfollowing values give an indication of the toxicity of  xylene xylene isomer isomer mixtur mixtures es [54, [54, 57]: 57]: LD50 (rat, (rat, oral) oral) 4300 4300 mg/k mg/kg, g, LDL LDL0 (rat (rat,, i.p.) i.p.) 2000 2000 mg/k mg/kg, g, TCLo TCLo (humans, (humans, inhalation) inhalation) 200 ppm. A very ery high high expo xposure sure to xyle xylene ne of ca. 10 000 ppm caused caused by an accide accident nt led to lung lung edema and subsequent death in one person [58]. In other cases severe damage to the CNS, kidneys, and liver were observed.

Xylenes repeat ated ed appl applic icat atio ion n  Irritant Effects. On repe xylenes can cause irritation of the respiratory passages and mucous membranes of the eye. Frequent skin contact can lead to blister formation and dermatitis dermatitis [57, 59–62]. 59–62]. Subchronic and Chronic Toxicity. At a concentra centratio tion n range range of 100 to over over 1000 ppm xylene in inhaled air, damage to the CNS with disturbance of balance or slowing of reactions is observed. Inhalation of xylene vapors can also cause nausea and headaches [63–65]. A change in the blood picture is also frequentl quently y observ observed. ed. In ca. 10 % of person personss who had been exposed to xylene vapors in concentrat tratio ions ns of up to ca. 100 100 ppm ppm for for ca. ca. 5 year years, s, −3 a leucocyte level of  < 4500 4500 mm was established lished [66]. [66]. A decrea decrease se in the immuno immunobio biolog logica icall activity has occasionally been observed [58]. Carcinogenicity, Mutagenicity, and Embryassessmen mentt of the carcin carcinoge ogenic nicity ity otoxicity. The assess of xylenes is not consistent [67–69]. While in one study no indications of carcinogenicity or cocarcinogenicity were found, another investigation indicated that xylenes act as tumor promoters for skin tumors in rats. None None of the the thre threee xyle xylene ne isom isomer erss show showed ed mumutage tageni nici city ty in the the Ames Ames test test [70] [70].. Howe Howeve verr, slig slight ht mutagenicity was detected in a recessive lethal test on drosophila [71]. In animal animal experime experiments nts long-term long-term exposure exposure thro throug ugh h inha inhala lati tion on of xyle xylene ness caus causes es smal smalll changes to fetuses [72]. three Pharmaco Pharmacokinet kinetics ics and Metabolis Metabolism. m. All three xylene isomers are resorbed in the same way. Various investigations have have shown that ca. 60 – 70 % of the xylene xylene reachin reaching g the organi organism sm via the lungs is retained [73–76]. This percentage remains approximately constant over the whole exposure exposure period. The ratio of the concentration concentration 3 in the air in the alveoli (mg/m ) and in the blood (mg/kg) changes with the degree of bodily activity. When the body is at rest the ratio is ca. 15 : 1 and and when hen movi moving ng ca. 30 – 40 : 1 [77] [77].. XyXylene lene can be resorbed resorbed at a rate rate of ca. 2.5 (0.7 – 3 4.3) 4.3) mg/m mg/m per minute through intact skin [78]. Xylene Xyleness are deposit deposited ed rapidl rapidly y in body body fat (up to ca. 5 %) and remain remain there there for hours after exposure. The half-life in fat deposits is ca. 0.5 – 1.0 h [78]. The metabo metabolis lism m of the indiv individidual xylene isomers is identical. The main biotransformation pathway initially involves oxidation to methylbenzoic acid, which forms the

19

corresponding methylhippuric acid by conjugation with glycine (A) [74,79–81]. The methylhipp hippur uric ic acid acid can can be excr excret eted ed rapi rapidl dly y via via the the kidkidneys. neys. Another Another,, less favored favored biotransfor biotransformatio mation n pathway involves the hydroxylation of the xylene on the aromatic ring, forming xylenols (B) [73].

10. References 1. Parpinelli Technon: Technon: “West “West European Petrochemical Industry,” Industry,” Aromatics Report and Tables Tables,, Mailand 1995, pp. 18, 31, 38. 2. J. S. McArraghe McArragher: r: Paper Paper to the Institut Institutee of  Petroleum Conference 16 Oct. 1991: “Making Cleaner Fuels in Europe – Their Need and Cost,” Institute of Petroleum, London 1991. 3. Chem Systems Systems Ltd.: Ltd.: “Petroleum “Petroleum and Petrochemicals Petrochemicals Economics,” Economics,” annual reports 1 and 2, London, London, Aug. 1995. 4. Octane Week, July 19 (1993) 6. 5. K. Weiss Weisserme ermel, l, H. J. Arpe: Arpe: Industrielle Organische Chemie, 3rd ed., Verlag Chemie, Weinheim einheim 1988, pp. 407, 426. 6. Ullmann, 4th ed., 24, 526. 7. C. L. Yaws, aws, Chem. Eng. (N.Y.) 82 (1975) no. 15, 113 –122; –122; 82 (197 (1975) 5) no. no. 20, 20, 73 – 81. 81. 8. Beilstein, 3rd ed., suppl., suppl., vol. 5, 2nd part (1964 (1964)) 807 –823 –823,, 823 – 845, 845, 845 845 – 864, 864, ¨  ¨  776 – 807. W. W. Berghoff Berghoff:: Erd olverarbeitung und Petrolchemie, VEB Deutscher Verlag f ¨ f ur u¨ r Grundstof Grundstoffindust findustrie, rie, Leipzi Leipzig g 1968, pp. 80 – 81. W. L. Nelson: Petroleum Refinery Engineering, 4th ed., McGraw-Hill, McGraw-Hill, Toronto Toronto 1958. H. J. V. Winkler: Der Steinkohlenteer und seine  Aufarbeitung, Verlag Gl¨ Gluckauf, u¨ ckauf, Essen 1951, pp.70–71. 9. F. D. Rossin Rossinii et et al.: al.: Selected Values of Physical and Thermodynamic Properties of   Hydrocarbons and Related Compounds, Carnegie Press, Pittsburgh, PA, 1953.

20

Xylenes

10. L. H. Hors Horsle ley y, Anal. Chem. 19 (1947) (1947) no. 8, 508–600. 11. L. H. Hors Horsle ley y, Anal. Chem. 21 (1949) (1949) no. 7, 831–873. 12. L. H. Hors Horsle ley y, Adv. Chem. Ser. 35 (1962). suppl., vol. 1, 2nd part 13. Beilstein, 3rd ed., suppl., (1964) 811 (o-Xylol), 828 (m-Xylol), 849 (p-Xylol), 782 (Ethylbenzol). 14. K. Owen, Owen, T. T. Coley: Coley: Automotive Fuels  Handbook, Society of Automotive Engineers, Warrendale arrendale,, PA, PA, 1990, pp. 564 – 565. 15. F. E. Condon Condon,, J. Am. Chem. Soc. 70 (1948) 1963. 16. H. G. Franck, Franck, J. W. Stadelho Stadelhofer fer in: in: Industrielle  Aromatenchemie, Springer Verlag, Berlin 1987 1987,, pp. pp. 60 – 61. 61. 17. Parpinelli Technon: Technon: “West “West European Petrochemical Industry”, Aromatics Report and Tables, Tables, Mailand 1993, p. 231. 18. J. A. Weisz Weiszmann: mann: “The “The UOP Platform Platforming ing Process,” Process,” in R. A. Meyers (ed.): Handbook of  Petroleum Refining Processes, Chap. Chap. 3, McGraw-Hil McGraw-Hill, l, London 1986, p. 6. 19. D. G. Tajbl Tajbl:: “UOP HC Unibon Unibon Process Process for for Hydrocracking,” Hydrocracking,” in R. A. Meyers (ed.):  Handbook of Petroleum Refining Processes, Chap. Chap. 2, Mc-Graw-Hil Mc-Graw-Hill, l, London 1986, p. 41. 20. A. Chauvel, Chauvel, G. G. Lefebre Lefebre in: Petrochemical Processes, vol. 1, Editions Technip IFP, IFP, Paris 1989, 1989, p. 176. 176. 21. A. Chauvel, Chauvel, G. G. Lefebre Lefebre in: Petrochemical Processes, vol. 1, Editions Technip IFP, IFP, Paris 1989, 1989, p. 130. 130. 22. H. G. Franck, Franck, J. W. Stadelho Stadelhofer fer in: in: Industrielle  Aromatenchemie, Springer Verlag, Berlin 1987 1987,, pp. pp. 35 – 57. 57. 23. J. A. Weisz Weiszmann: mann: “The “The UOP Platform Platforming ing Process,” Process,” in R. A. Meyers (ed.): Handbook of  Petroleum Refining Processes, Chap. Chap. 3, McGraw-Hil McGraw-Hill, l, London London 1986, 1986, pp. pp. 11 – 13. 24. N. L. Gilsd Gilsdorf, orf, A. P. Furano, Furano, R. H. Rachf Rachford, ord, R. J. Schmidt, Schmidt, D. L. York: York: “Platformi “Platforming ng Technology for Aromatics Production,” in: Petrochemical Technology: Building Success through Teamwork and Technology, UOP Technology Conference, Des Plaines, Ill., 1992 1992,, pp. pp. 20 – 21. 21. 25. A. Chauvel, Chauvel, G. G. Lefebre Lefebre in: Petrochemical Processes, vol. 1, Editions Technip IFP, IFP, Paris 1989, 1989, pp. pp. 170 –172. –172. 26. J. A. Weisz Weiszmann: mann: “The “The UOP Platform Platforming ing Process, Process,”” in R. A. Meyers (ed : ): Handbook of  Petroleum Refining Processes, Chap. Chap. 3, McGra McGraw-H w-Hill ill,, London London 1986, 1986, pp. 6 – 9. 27. Oil Gas J., 2nd Oct. Oct. (1995) (1995) 35 – 37.

28. W. Keim, A. Behr, Behr, G. Schmitt Schmitt in: Grundlagen der Industriellen Chemie, Verlag Salle-Sauerl¨ Salle-Sauerlander, a¨ nder, Frankfurt 1986, p. 129. 29. A. Chauvel, Chauvel, G. Lefebre Lefebre in: in: Petrochemical Processes, vol. 1, Editions Technip IFP, IFP, Paris 1989, 1989, p. 156. 156. 30. C. D. Gosling, Gosling, G. L. Gray Gray, P. P. J. Kuchar Kuchar,, L. Sullivan: “Produce BTX from LPG,” in:  Building Success through Teamwork Teamwork and  Technology, UOP Technology Conference, Des Plain Plaines, es, Ill., Ill., 1992, 1992, pp. 1 – 17. 31. H. G. Franck, Franck, J. W. Stadelho Stadelhofer fer in: in: Industrielle  Aromatenchemie, Springer Verlag, Berlin 1987, 1987, pp. pp. 88 – 91. 91. 32. E. C. Haun, Haun, M. W. Golem, Golem, P. P. P. Piotrowski, Piotrowski, S. S. Sapuntzakis: “The Modern Aromatics Complex and Revamp Options for Existing Plants,” UOP Technology Conference, Des Plaines, Plaines, Ill. 1988. 33. J. G. Jenkin Jenkins, s, Oil Gas J. 63 (1965) Jan. 18, 78–86. 34. N. E. Ocke Ockerbl rbloom oom,, Hydrocarbon Process 50 (1971) July, 113. 35. J. Morrison, Morrison, Oil Gas Int. 10 (1970) (1970) no. 12, 67–69; Hydrocarbon Process. 60 (1981) Nov., 240. 36. Chevro Chevron n Res., Res., US US 3 219 722, 722, 1965. 1965. ARCO ARCO Techn., Hydrocarbon Process. 60 (1981) Nov., 239. 37. J. A. Johns Johnson, on, A. A. P. Furfar Furfaro, o, S. H. Hobbs Hobbs,, R. G. Kabza: “Advances in para-Xylene Recovery,” UOP Technology Conference, Des Plaines, Ill. 1988. ¨  ¨  Erdgas Kohle 111 38. A. K. Abou Aboul-J l-Jhei heit, t, Erd ol (1995). 39. Chemical Week, Aug. 30, 1995, 1995, p. 18. 40. P. J. Kuchar, Kuchar, O. Y. Lin, V. V. Zukauskas, D. Brkic: “Isomar and Tatoray Aromatics Production Flexibility,” UOP Technology Conference, Des Plaines, Ill. 1988. 41. L. Grim Grimm, m, Erd¨  ol Erdgas 108 (1992) (1992) no. 9, 355. 42. H. Fort Forth, h, Oel 19 (1981 (1981)) no. 7, 172 172 – 178. 178. 43. B. Hnat: Hnat: Wirtschaftspolitische Studien, vol. vol. 87, Verlag Vandenhoek and Ruprecht, Gotti o¨ tting ngen en – Zuric u¨ rich h 1992, 1992, pp. 69 – 71. 71. 44. K. Owen, Owen, T. T. Coley: Coley: Automotive Fuels  Handbook, Society of Automotive Engineers Inc., Warren Warrendale dale,, PA, PA, 1990, pp. 144 – 145. 45. UOP: “The “The Clean Clean Air Act And The Refining Refining Industry,” Des Plaines, Ill., 1 Sep. 1991, p. 17.UOP: 17.UOP: “Process Solutions Solutions for Reformulated Gasoline,” Des Plaines, Ill., 1992.

Xylenes 46. Parpinelli Technon: Technon: “West “West European Petrochemical Industry,” Industry,” Aromatics Report and Tables, Tables, Mailand Mailand 1993, p. 19. 47. Parpinelli Technon: Technon: “West “West European Petrochemical Industry,” Industry,” Aromatics Report and Table Tables, s, Mailand Mailand 1993, 1993, pp. 229 – 311. 48. Chemical Week, Oct. Oct. 11, 1995, 1995, p. 58. 49. H. G. Franck, Franck, J. W. Stadelho Stadelhofer fer in: in: Industrielle  Aromatenchemie, Springer Verlag, Berlin 1987, 1987, p. 133. 133. 50. VbF/TRbF VbF/TRbF,, vol. 5 (Regulati (Regulations), ons), 16. Lfg., June June 1989. 51. 51. Kuhn, u¨ hn, Birett – Merkbl¨ Merkblatter a¨ tter Gef ahrliche a¨ hrliche Arbeitsstoffe, 49th suppl., 6/90 – X03, X06, X07, X08, X19, 1990. 52. BArbBl ArbBl,, no. no. 9/1990 9/1990,, pp. pp. 65 ff. ff. 53. Unfallme Unfallmerkbla rkblatt tt f ur u¨ r den Straßentransport, MED-Verlagsgesellschaft, Landsberg, edition edition 5/87. 54. MERCK Sicherheitsdatenbank Sicherheitsdatenbank MS-Safe, MS-Safe, 1 Jul. 1991. ¨  ¨  Ergas 55. J. Fabri, A. Reglitzky, Reglitzky, M. Voisey Voisey,, Erd ol, 107 (199 (1991) 1) no. no. 1, 24 24 – 29. 29. 56. K. Savolainen, Savolainen, V. V. Riihim¨ Riihimaki, a¨ ki, E. Vaheri, M. Linnoila, Scand. J. Work Environ. Health 6 (1980) 94. 57. M. Wolf, Arch. Ind. Health 14 (1956) 387. 58. R. Morley Morley, D. W. Eccleston, Eccleston, C. P. Douglas, Douglas, W. E. J. Greville Greville,, D. J. Scott, Scott, J. Anderson, Anderson, Brit.  Med. J. 3 (1970) 442. 59. E. Brown Browning ing in: in: Toxicity and Metabolism of   Industrial Solvents, Elsevier, Elsevier, Amsterdam 1956. 60. H. W. Gerar Gerarde: de: Toxicology and Biochemistry of Aromatic Hydrocarbons, Elsevier, Amsterdam 1965. 61. E. Schmidt, Schmidt, Arch. Gewerbepath. Gewerbehyg. 15 (1956) 37. 62. W. Matthau Matthaus, s, Klin. Monatsbl. Augenheilk. 144 (1964) 713.

21

63. K. Savolainen, Savolainen, V. V. Riihim¨ Riihimaki, a¨ ki, A. M. Sepp¨ Seppal¨ a¨ lainen, a¨ inen, M. Linnoila, Int. Arch. Occup.  Environ.  Environ. Health 45 (1980) 105. 64. F. Gamberale, G. Annwall, M. Hultengren, Scand. J. Work Environ. Health 4 (1978) 204. 65. K. Savolai Savolainen, nen, M. Linnavuo Linnavuo,, Acta Pharmacol. Toxicol. 44 (1979) 315. 66. V. I. Boik Boiko, o, Gig. Tr. Prof. Zabol. 1970, 1970, no. 6, 23. 67. A. Pound, Pound, N. Engl. J. Med. 67 (1968) 88. 68. A. Pound, Pound, Pathology 2 (1970) 269. 69. I. Berenblum Berenblum,, Cancer Res. 1 (1941) 44. 70. R. P. Bos, Bos, R. M. E. Broun Brouns, s, R. van van Doorn Doorn,, J. L. G. Theuws, Theuws, P. P. T. Henderson, Henderson, Mutat. Res. 88 (1981) 273. 71. M. Donner, Donner, Mutat. Res. 74 (1980) 171. 72. I. Krotov Krotov, N. Chebotar, Chebotar, Gig. Tr. Prof. Zabol. 1972, 1972, no. 16, 40. 73. V. Sedivec, Sedivec, J. Flek, Int. Arch. Occup. Environ.  Health 37 (1976) 205. ¨ 74. I. Astrand Astrand,, I. Engstr Engstrom, ¨ P. Ovrum, Scand. J. Work Environ. Environ. Health 4 (1978) 185. 75. V. Riihim Riihim¨aki, a¨ ki, P. Pf affli, a¨ ffli, K. Savolainen, K. Pekari, Scand. J. Work Environ. Health 5 (1979) 217. 76. W. Senczuk, J. Orlowski, Orlowski, Br. J. Ind. Med. 35 (1978) 50. 77. I. Engstr Engstr¨om, o¨ m, R. Bjurstr¨ Bjurstrom, o¨ m, Scand. J. Work   Environ.  Environ. Health 4 (1978) 195. 78. K. Engstr Engstr¨om, o¨ m, K. Husman, V. Riihim¨ Riihimaki, a¨ ki, Int.  Arch. Occup. Environ. Environ. Health 39 (1977) 181. 79. H. Bray, Bray, B. Humphris, W. W. Thorpe, Biochem. J. 85 (1949) 241. 80. H. Bray, Bray, B. Humphris, W. W. Thorpe, Biochem. J. 87 (1950) 395. 81. G. Bienik, T. Wilczok, Wilczok, Br. J. Ind. Med. 38 (1981) 304.

Xylenols → Cresols and Xylenols Xylidenesulfonic Acids → Benzenesulfonic Acids and Their Derivatives