CAPACITY MlLLlONSOF LBSIYR.
I4
i POSSIBLE FUTURE INSTALLATION 1s
I5
5 IO
25
I
T
n
PRESENT INSTALLAT ION S
I 40
DATE
A STAFF-INDUSTRY COLLABORATIVE REPORT HARRY W. HAINES, JR., Associate Editor in collaboration with
J. M. POWERS and R. B. BENNETT
p-Xylene from Petrol-
Humble Oil & Refining Co., Baytown, Tex.
C
OWIMERCIAL fractionation of xylenes was launched in the
United States during 1945 when Oronite Chemical turned t o abundant petroleum-derived stocks for its phthalic anhydride production via the oxidation of o-xylene. iMeanwhile, another significant development under way simultaneously in England and the United States was soon to create an unprecedented demand for the para isomer. A polyester fiber had been produced as early as 1931; the first one was described that year a t an AMERICAN CHEMICAL SOCIETY meeting by Wallace H. Carothers and Julian W. Hill ( 3 , 19). It had little commercial importance because of its low melting point and poor hydrolytic stability; Carothers then became interested in high molecular weight linear aliphatic polyamides (nylon). His brilliant work, a few years later, caused so much excitement that polyesters were largely neglected. T h e turning point for polyester fibers came after Carothers’ untimely death in 1940, when all of his papers in the high polymer field were collected and published (84). From reading this book, J. R. Whinfield and J. T. Dickson of the Calico Printers’ Aesociation in England decided t o investigate symmetrical aromatic polyesters. This work, supported by Imperial Chemical Industries, led to the synthesis of polyethylene terephthalate from terephthalic acid and ethylene glycol. It is known in England as Terylene (10)and in the United States as Dacron.
1096
D u Pont attempted t o produce polyethylene terephthalate during the mid-thirties, but was unsuccessful in its application of nylon manufacturing methods. Word of the British discovery did not reach the United States until the middle of 1944 because of wartime secrecy, and details were not available until early 1945. D u Pont’s work on polyester chemistry began in earnest during 1944 : company chemists soon synthesized the compound on their own, by procedures nearly identical t o those of Whinfield and Diclrson. T h e company now makes three products from this compound: Dacron fiber, Mylar film, and Cronar photographic film base (6, 6, 19). T h e immediate success of polyester fibers caused world-wide production of p-xylene, the basic raw material for making terephthalic acid, to increase a t a rapid pace. Since 1949, Imperial Chemical Industries has operated a pioneer polymer plant a t Huddersfield and a spinning plant a t Hillhouse, England. Output from these experimental plants is approximately 2,000,000 pounds per year. Construction on the first large scale Terylene unit as Wilton has been completed and production began in January of this year (6, 9, 16). When a t full capacity, the unit will have an output of 11,000,000 pounds. By the end of the year a second unit of equal capacity will be completed, and by mid-1956 production is expected to reach 22,000,000 pounds. The $56-million plant is manufacturing filament yarn and staple fiber.
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vol. 47, No. 6
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PLANT PROCESSES-n-Xvlene
Table 1.
Distribution of
Catalytic Conversion Process
CS Alkylbenzenes in Products from Catalytic Reforming Processes Per Cent b y Volume Temp.,
O
F.
Ethylbenzene Actual Calcd. 11.0 11.5
A $2O-million plant is under construction at Millhaven, Ontario, by Canadian Industries Ltd., t o manufacture 11,000,000 pounds per year of Terylene from dimethyl terephthalate (6, 6, 12, IS). Hercules Powder will supply dimethyl terephthalate, 12,000,000 pounds per year, from its $4-million Burlington, N. J., plant which is to receive p-xylene from Sinclair Chemicals' 10,000,000-pound plant a t Marcus Hook, Pa. Imperial Chemical Industries has world rights for Terylene production, except in the United States. I n recent months Imperial Chemical Industries has completed negotiations with a number of European firms, whereby Terylene will be manufactured under license in Italy, France, Western Germany, and the Netherlands (6, 9 ) , b u t Calico Printers' Association still retains the basic patents and collects royalties. Terylene's counterpart in the United States, Dacron, has been manufactured by D u Pont under Calico Printers' Association license since 1950. Rapid growth of Dacron has encouraged several petroleum companies into p-xylene production and others are actively engaged in planning for future output ( 4 , 5 , 7, I i , 12, 15-16, 2 7 ) . The first commercial United States supply of p-xylene was manufactured by Oronite Chemical Co. in 1950. Annual capacity in the United States increased from 2,000,000 pounds in that year t o an estimated 70,000,000 pounds in 1954, but there has been little improvement in the recovery processes; all companies use low temperature fractionation, although different in operating details from the process pioneered by Standard Oil of California a t Richmond, Calif., for production of the material marketed by Oronite.
p-Xylene Actual Calcd.
*l.O
m-Xylene Actual Calcd. 11.0 4~5.0
13.5
+Xylene Actual Calcd.
11.0
f3.5
A survey (26) made b y Ebasco Services, Inc., in 1952, estimated t h a t total U. S. xylenes production would reach 133,000,000 gallons per year by 1953 and would increase t o 175,000,000 gallons by 1955. These predictions were obviously somewhat optimistic in the light of recent production figures. Assuming, however, that xylenes capacity will continue t o grow a t its present rate, total production should be well over 140,000,000 gallons per year by 1956. Assuming further that 10% of the xylenes is
130 I20
I10 100
-.-.-
PETROLEUM
90
80 70
60 50
4 0
30 20 10 0
'
1940 41
Figure
"
'
~
'
42 4 3 44 45 46 4 7
1.
l 48
' 49
' 5 0 51
l 52
~ 53
~ 54
~
55
United States xylenes production
Domestic p-xylene production may reach 100,000,000 pounds by 1960
Prior t o 1940, xylenes were produced mainly from coal tar distillation, but the major portion now comes from petroleum. In 1954, the United States produced 121,000,000 gallons of xylenes (30) of which more than 90% was from petroleum conversion processes, as indicated in Figure 1. The synthesis of xylenes is accomplished by catalytic reforming of virgin naphtha stocks containing major concentrations of naphthenic hydrocarbons. Naphthenes are dehydrogenated t o yield the corresponding aromatic compounds which may be separated from paraffinic and olefinic compounds in the product by solvent extraction or extractive distillation (25). The concentrated xylenes contain the four Cs aromatic compounds and small amounts of toluene and Cg aromatics. Although xylenes are produced by a wide variety of feed stocks and by a number of different catalytic reforming processes, the composition of the product mixture does not vary greatly. This is illustrated in Table I by data (21) on different types of catalytic conversion processes which compare analytical results with the distribution of C g alkylbenzenes calculated from thermodynamic equilibria. Comparable data from platinum catalyst reforming indicates a similar isomer distribution. The calculated value for p-xylene varies from 20.6 to 21.5% for a temperature range of 800" t o 975' F., but the determined value is always lower. Equilibrium concentrations of the Cs alkylbenzenes foi the temperature range of 200' t o 1600' K. have been calculated by Taylor and others (20).
lune 1955
recoverable as pxylene, the total amount of p-xylene available for chemical raw material use would be approximately 100,000,000 pounds per year, which coincides closely with the present estimated production capacity for 1960. This estimate is based only on the amount from xylenes recovered by fractional crystallization; i t does not allow for improved recovery processes in the future that may give higher yields.
low temperature crystallization i s the standard recovery method
The boiling points and freezing points of aromatic compounds generally found in a mixture of commercial xylenes are listed in Table 11. With the exception of toluene, the boiling points are very close together. Separation of toluene by distillation may be accomplished readily, and separation of o-xylene is done commercially, although with more difficulty. Separation of ethylbenzene or m-xylene from p-xylene b y distillation is highly impractical because of the small difference in their boiling points. . The freezing point of p-xylene is a great deal higher than t h a t of its associated isomers and use is made of this physical property for carrying out the separation of p-xylene from its mixtures. Although several variations of crystallization procedures are known, fundamentally all utilize the same principle-low temperature chilling and separation of the crystals by mechanical devices. The first known commercial process for separation of p-xylene by low temperature crystallization was operated by
INDUSTRIAL AND ENGINEERING CHEMISTRY
1091
l
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT
t
I
LL
t W W z
J
> X I
a
Y m 0
.
1098
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 47, No. 6
PLANT PROCESSES-p-Xylene
Table II.
Boiling Point and Freezing Point of C7 and Cs Alkylbenzenes ( 7 )
Compound
Boiling Point, F.
Freezing Point, F.
By reference to the solubility curves, it is obvious that on chilling this mixture, the saturation temperature for p-xylene (15.8%) is reached before the saturation value for o-xylene or m-xylene is encountered. On continued cooling; the solubility of p-xylene will be reached causing more crystallization of pure p-xylene until the next isomer, o-xylene, begins to precipitate as a binary eutectic. Further cooling will result in cocrystallization of m-xylene as a ternary eutectic when its saturation temperature is reached. Accordingly, the amount of pure p-xylene recoverable from this typical mixture is limited by the eutectic
I. G. Farbenindustrie as early as 1937 ( 6 ) . The method used by I. G. Farbenindustrie, however, was discontinuous and apparently was employed on a small scale; known processes operating in the U. S. are continuous. Eutectic point limits p-xylene yield to 10% of the xylenes
In the initial development work on low temperature crystallization processes, solubility-temperature relationship of the isomers in the xylenes had to be determined. Kravchenko (29) had calculated eutectic points in the quaternary system of ethylbenzene, o-xylene, m-xylene, and p-xylene, assuming the mixture t o be an ideal solution, but the calculated results were slightly in errur as later shown by Pitzer and Scott ( 2 6 ) who employed more precise values for the heat of fusion of the Cg aromatics. During development of the process used a t Humble's Baytown Refinery, the solubility-temperature relationships were calculated and these data are presented in Figure 3. In calculating the solubility of individual C, isomers, the Van't Hoff equation, dlnx dT- = RAH~ was 2 integrated and theoretical composition curves were developed for various saturated solution temperatures T, where X is the mole fraction in solution and AH is the heat of fusion. The integration m-as carried out assuming that the difference in specific heats of the solid and liquid phases remained constant in the temperature range under consideration. Values for the heats of fusion employed in these calculations are given in Table 111.
Table 111.
Heat of Fusion of
CSAlkylbenzenes ( 7 ) Heat of Fusion, Calories per Mole
Compound Ethylbenzene o-Xylene m-Xylene p-Xylene
2190 3260 2766
4090
Figure 3.
I
Experimental data obtained by various workers (26, 68) measuring the freezing points of p-xylene blends in other Cs aromatics were plotted in Figure 4 t o test the reliability of calculated solubilities. The experimental points are in close agreement. B y means of these solubility curves, Figure 3, it is also possible to determine the temperature a t which eutectic formation will begin. In typical xylenes mixtures, either m-xylene or o-xylene may be encountered as the first eutectic depending on the composition of the mixture. A typical commercial xylenes mixture, where the concentration of o-xylene is 20% and m-xylene is 39.6%, is given in Table IV.
Table IV.
Typical Composition of Solvent Xylenes
Component p-Xylene n-Xylene o-Xylene Ethylbenzene Toluene Others (paraffins, naphthenes, Cs aromatics)
June 1955
Wt. % 15.8 39.6 20.0 18.6
3.5 2.5
Solubility-temperature relationships C5 alkylbenzenes
of
point and operations carried out for the production of p-xylene must be a t a temperature level above this point. Material balances readily indicate t h a t the yield of p-xylene from this mixture will be t O . l % a t -98" F., the eutectic point for o-xylene and p-xylene in this mixture. Distillation of the xylenes mixture to reduce the o-xylene content will increase this yield slightly by reducing the temperature at which the first eutectic will be encountered. Undercutting t o remove o-xylene may be practiced for this purpose, although Humble Oil does not employ this technique. Humble Oil 8, Refining Co. uses a two-stage process at Baytown
Humble constructed a large scale low temperature crystallization pilot plant in 1951 at its Baytown, Tex., refinery for the development of the p-xylene process (4, l l ) . This pilot unit was capable of producing 10,000 to 20,000 pounds per month of 95% p-xylene from a solvent xylenes feed stock. The purpose of the unit was threefold:
INDUSTRIAL AND ENGINEERING CHEMISTRY
1099
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT such as scraping with a spring loaded blade in order to achieve adequate heat transfer rates. Standard of California's process eliminates the necessity for scraping interior surfaces of the chiller. A4dditionalequipment required for separation of the internal refrigerant, however, tends to counteract the economic advantages of Standard's simplified chiller. Referring again to the flow, the slurry is held a t -95' F. for a sufficient length of time to permit a close approach to equilibrium for p-xylene crystallization, thereby realizing maximum p-xylene recovery; also, the crystals tend to grow in size in the agitated holding tank ( 8 8 , 22E), making the crystals more amenable t o
Close-up view of scraped surface chillers To obtain engineering and process data for use in the preparation of a firm and economical plant design; To demonstrate the operability of certain types of equipment which might be incorporated in the plant; To prepare large p-xylene samples for evaluation and experimental use by potential customers. Based on pilot plant information, a commercial plant was designed, and construction was completed in 1953. This plant was designed for the production of 25,000,000 pounds per year (226 barrels per day) of p-xylene, employing a two-stage crystallisation and centrifugation technique. One feature of this process is that no internal refrigerants are used. In this respect, the Humble process differs from that of Standard of California, which utilizes an internal refrigerant. The solvent xylenes feed is produced a t Baytown by hydroforming virgin naphtha, fractionating the produrt for a xylenes distillate, and sulfur dioxide extracting the distillate for high purity xylenes. A recent analysis of solvent xylenes is shown in Table IV. Briefly, the process involves crystallization of p-xylene from solvent xylenes a t -95' F., followed by separation of a n 80% p-xylene concentrate by centrifuging: this concentrate is melted, recrystallized a t 0' F., and again centrifuged to produce a 95% finished p-xylene product. The plant flow plan is shown in Figure 2. First crystallization stage produces 80% purity p-xylene
I n the first stage the solvent xylenes feed (15.8% p-xylene) is pumped through a drier containing alumina for removal of water and is precooled to -35" F. with cold filtrate from the first centrifugation step in a conventional shell-and-tube heat exchanger ( S E ) . This precooled feed then is chilled stepwise in two scraped surface double-walled heat exchangers ( I Q E )which are refrigerated with ethylene on the shell side. This external refrigeration method of chilling the feed requires no extraneous solvent handling facilities or equipment for stripping refrigerant from t h e filtrate and the finished product. Ethylene for refrigeration is supplied by an electrically driven compressor (11E). Since the wall of the scraped surface chiller is below the crystallization temperature of the flowing xylenes, p-xylene crystals deposit on the cold wall and must be removed by some means 1100
Figure 4. Measured vs. calculated p-xylene solubility purification in a subsequent centrifuging operation. The slurry is charged from the holding tank into semicontinuous batch centrifugal filters ( I E )for separation of the crystals from the filtrate or mother liquor. The crude crystals, containing about 80% p-xylene, are melted a t 75' F. in a melt tank (10E) and are pumped t o the second stage feed storage tank ( 4 E ) . The filtrate goes to a surge tank (6%) and is pumped t o refinery solvents storage after heat exchange with the fresh feed. All piping and process equipment operating a t -20' F. or below in the first stage is constructed of stainless steel and low nickel steel. Secondary crystallization increases product purity to better than 95%
In the second stage operation, the 80% p-xylene concentrate produced in the first stage is precooled to 45" F. with propane in a conventional shell-and-tube heat exchanger (%E). This concentrate then is charged as a liquid to a scraped surface exchanger
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 41, No. 6
PLANT PROCESSES-p-Xylene ( I S E ) , refrigerated with propane. T h e dual service compressor t h a t supplies t h e first stage operation with ethylene ah0 provideq propane for the second stage operation. Concentrate leaving the scraped surface exchanger enters the holding tank ( 7 E ) a t 0" F. Recrystallization a t a relatively high temperature produces second stage cryFtals which are very large in comparison t o t h e first stage crystals; consequentlv, special crystallization techniques are not necessary to attain high purities in t h e final centrifuging step. Crystals fr0.m the centrifuge ( I E ) are melted at 75" F. in a melt tank ( Q E )and then pumped t o final product storage. T h e second stage filtrate, which contains about 42% p-xylene, goes t o a surge tank (5E) and is recycled to t h e first stage for further p-xylene removal, as a result t h e yield of p-xylene f r o n the feed is determined by the yield realized in t h e first stage. Processing the feed in two stages at different sluiry temperature? permits close control of product quality at a high purity level and, a t t h e same time, allows the realization of maximum p-xylene yield. Ordinarily, 9591, purity product is produced, but the flexibility of t h e second stage operation permits the attainment of purities as high as 98%, with some reduction in yield or plant production capacity. Product quality depends largely on crystallization techniques
Crystals m a y appear a t -47' F. with solvent xylenes feed containing 15.8% p-xylene; supersaturation occurs with continued
June 1955
cooling, causing crystal growth if nuclei are present. For maximum p-xylene recovery, it is desirable t o operate at t h e lowest possible temperature without, eutectic interference. T h e crystallization mechanism is rather complex, involving t h e phenomena of diffusion. nucleation, and crystal growth occurring simultaneously Factors exerting an influence on crystallization include t h e concentration of t h e crystallizable p-xylene, supersaturation, impurities, and t h e natiire of the remaining solvent. I n general, high supersaturation tends t o give many nuclei, while a t a low supersaturation large crystals are produced a t a slow rate of growth. Nucleation m a y be controlled t o some extent by the addition of seed crystals, and crystal growth may be regulated b y t h e degree of supersaturation. These effects are controlled t o a large degree b y t h e stepwise manner of ccnducting crystallization in the first stage of t h e process. A cold stage microscope has been used t o observe crystals produced in the first and second stage slurries With this equipment, it was possible t o estimate the crystal size and shape as well as actually make photographs of the crystals. During the first fitage of crystallization, in which t h e xylenes mixture is precooled t o near t h e crystallization point in conventional exchangers and then is chilled stepwise t o the operating temperature of about -95' F., crystals as illustrated in Figure 5 are produced. T h e primary concentrate produced in the first stage (SOTo p-xylene) is completely melted a t about 75" F. and then recrystallized a t 0' F. These large crystals illustrated in Figure 5 , B differ markedly from t h e needlelike monoclinic type produced in the first stage, Figure 5 , A . The average second stage
INDUSTRIAL AND ENGINEERIVG CHEMISTRY
1101
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT obtain a dry cake, and discharge t h e cake from the centrifuge in a few seconds. As stated previously the concentration of p-xylene in t h e final product is readily controllable between 95 and 98% b y adjustment of operating variables. Concentrations above 9870 are attainable; however, purities in excess of 95% are not justified b y current economics. Inspections of a typical product, are shown in Table V. Supply of p-xylene adequate for at least three more years
A
B
Figure 5. Large crystals produced in second stage slurry differ markedly from needlelike monoclinic type in first stage A. B
First s t a g e crystals
Second s t a g e crystals
crystal size is estimated at 200 b y 360 microns. Complete recrystallization produces large rectangular-type crystals which can be centrifuged readily t o t h e desired purity. Filtration or centrifugation could be employed t o separate crystals from t h e liquor, but automatic semicontinuous basket centrifuges ar4 used b y Humble because of their high Feparating force, solids handling capacity, and ability t o produce a high purity product. T h e known factors t h a t influence t h e p-xylene cake purity or t h a t control the liquid drainage in a centrifuging operation are t h e viscosity, t h e interfacial tension between liquid and solid phase, t h e surface area of t h e solid phase, t h e free opening between t h e solid particles, separating force, and t h e distance t h a t the liquid must travel or cake thickness. T h e factors of viscosity and interfacial tension are dependent only on t h e operating temperature since no additional solvent was used. T h e effect of temperature on the viscosity of t h e solvent xylenes a t low temperatures is given in Figure 6 which shows t h a t t h e viscosity increases from about 0.6 centipoise a t room temperature t o about 5.2 centipoise a t -95’ F. T h e size of t h e p-xylene crystals can be increased to some extent, as outlined previously, and t h e centrifugation prccess would be improved because of the smaller surface area of crystals retaining liquor and because of t h e larger size of t h e intercrystal openings. I n operating the centrifuges, t h e slurry containing t h e p-xylene crystals is automatically charged, and the crystals which are retained on a screen are spun t o proper dryness. When t h e cake is dry, a knife rises automatically t o cut and remove t h e crystals from t h e centrifuge bowl. T h e cycle is then repeated, the entire cycle beiqg controlled automatically with electric timers. For maximum capacity, it is desirable t o charge the centrifuge with sldrry a t a high rate, spin for as short a period as is feasible t o
Estimates made in 1952 of the potential growth rate indicate p-xylene requirements may reach a level of almost 100,000,000 pounds per year b y 1960 ( 2 5 ) . Although there were serious setbacks t o t h e synthetic fiber industry during 1952 and early 1953, which may delay the growth of Dacron demand, some recovery occurred during the latter part of 1953 (18) and during 1954. D u Pont is now pushing market outlets for Mylar film and Cronar photographic film base, in order t o take up the slack in Dacron demand. I n one application, Stromberg Carlson boosted its production of insulated telephone switchboard wire 35 t o 46Y0 by changing over t o Mylar film as a primary insulating material. During February of this year, D u Pont announced t h a t paper had been successfully made for t h e first time from synthetic fibers, using Dacron, Orlon, and nylon. Du.Pont does not plan t o manufacture synthetic fiber papers, although t h e company is making details of the work available t o t h e paper industry.
Figure 6.
Effect of temperature on viscosity of solvent xylenes Ubbelohde viscometer
Table V.
Typical p-Xylene Product
Inspections Freezing ooint. O C. p-xylene- wt. % Paraffin Lydrocarbon, vol. % Bromine No., c.g.s./cm. Acid ‘wash color
Test Results 1 1 53
95.7
Nil
0.02 (1
Sweet
0 6 Nil Bright and clear 30 0,001
+
1102
It appears t h a t the productive capacity of p-xylene in the United States will more than supply domestic requirements until a t least 1958; another 15,000,000 or 20,000,000 pounds could easily come on stream from two major oil companies t o meet all requirements until 1960. D u Pont recently lowered its estimates of Dacron consumption, indicating t h a t nylon and Dacron may go as high a s 6% of the total textile market sometime after 1960, compared with the 370 of 1954. Meanwhile, per capita fiber consumption in the United States has declined slightly from 40 pounds per year in 1950 t o about 32 pounds in 1954. D u Pont
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 4’2, No. 6
PLANT PROCESSES-p-Xylene foresees little change in total fiber consumption well into 1960, with a possible maximum gain of 5%. Aside from the market outlook for p-xylene, which over the long term period looks favorable, technological advances in processing have been rather slow. Phillips Chemical began production of p-xylene in December 1953 a t its new plant near Big Spring, Tex., t h e first process t o produce a 98% product in commercial quantities. T h e method is reported t o have other applications for liquids which are difficult to separate ( 7 ) . Hercules Powder manufactures dimethyl terephthalate from p-xylene using a new liquid phase air oxidation process, based on esterification of the monobasic p-toluic acid produced in the first stage and subsequent oxidation of the methyl toluate in the second stage. The process is licensed by the California Research Corp., which holds the basic U. S. patent ($2). Hercules is understood t o have made significant improvements in the process during the past two years in a development program estimated to have cost $1,500,000. The same process that converts p-xylene t o dimethyl terephthalate can produce dimethyl isophthalate from a feed of mixed xylenes (70 t o 80% meta, the remainder para). Hercules is still operating a pilot plant producing 120,000 pounds per year of dimethyl isophthalate from meta and mixed xylenes. The only large scale plan for pure meta conversion is Oronite’s $10-million plant for isophthalic acid production now under construction a t Standard of California’s refinery in Richmond, Calif. (12, 15). Unlike Hercules, Oronite plans t o market the acid rather than the dimethyl ester. The first large commercial supplies of technirally pure (95+%) m-xylene in the United States will also be offered by Oronite on completion of this new plant. Richfield Oil Co. is operating a pilot plant unit a t its Watson, Calif., refinery for the production of phthalic acids by the oxidation of xylenes, but the company’s plans for commercial developments are as yet unknown. Significant increases in the demand for p-xylene could bring about technical changes in processing, if the demand should exceed either the market for ortho and meta isomers, or the market for mixed xylenes as a solvent. One alternative would be isomerization of the mixed xylenes leaving the crystallization process for recycling and additional p-xylene recovery. Another possibility is the use of solid compound formation (15, 171, a California Research Corp. technique designed t o boost para recovery through t h e use of carbon tetrachloride. Possible production of terephthalic acid from butylene has been pointed out in some German work on reactions catalyzed with lithium aluminum hydride. I n this process, yields of over 50% p-xylene are obtained with ethylbenzene and o-xylene as the only other products. Since separation of these materials is much easier than separating the three isomeric xylenes, this process, in which the hydride is a “true” catalyst and is not consumed, has real commercial possibilities. Recent European work, in which terephthalic acid is derived by isomerization of phthalic anhydride, opens an attractive direct route t h a t eliminates the p-xylene recovery step, but this process has not been employed on a commercial scale in the United States. Acknowledgment
The authors are grateful to Humble Oil &- Refining Co. for the opportunity t o prepare and publish this information. Acknowledgment is made, also, of the technical contributions of R. F. Pfennig and R. A. Speed of Humhle, and others who assisted in the development of this process. Other helpful technical assist-
June 1955
ance was obtained from Esso Research and Engineering Co. through a research contract. literature cited
(1) American Petroleum Institute Research Project 44, “Selected Values of Physical and Thermodynamic Properties of Hydrocarbons and Related Compounds,” Carnegie Institute of Technology, Pittsburgh, Pa., 1953. (2) British Intelligence Objectives Subcommittee, Final Report No. 1146, Item No. 22, H.M. Stationary Office, London, England. (3) Carothers, W. H., and Hill, J. W., J. Am. Chem. Soc., 54, 1579 (1932). (4) Chem. E n g . , 59, 219 (April 1952). (5) Ihid., 62, 118, 120 (April 1955). (6) Chem. Ens. X t w s , 32, 128 (1954). (7) Ihid., p. 223. (8) Ihid., p. 3718. (9) I h i d . , 33, 512-13 (1955). (10) Chem. Inds., 66, 521-6 (April 1950). (11) Chem. W e e k , 70, 35 (April 19, 1952). (12) I h i d . , 73, 73-4, 77-8 (Oct. 17, 1953). (13) Ibid., 74, 64-5 (Jan. 30, 1954). (14) I b i d . , pp. 16-7 (Feb. 20, 1954). (15) Ihid., 75, 78-9 (Jan. 15, 1955). (16) Ibid., p. 30 (Feb. 12, 1955). (17) Egan, C. J., and Luthy, R. V., IND.ENG.CHEM.,47, 250-3 (1955). (18) IND.ENG.CHERI.,46, 31 A, 35 A (January 1954). (19) Izard, E. F., Chem. Eng. News, 32, 3724-8 (1954). (20) J. Research iVat2. B u r . Standards, 37, 95-122 (1946). (21) Ibid.. 39. 303-8 (1947).
