US2954414A - Production of para xylene - Google Patents

Description

6 Claims. (Cl. 260-671) This invention relates to the production of an ethylbenzene-free xylene mixture. More particularly the invention relates to an ethyl-benzene free xylene mixture con- 7 taining substantially more than the equilibrium amount of para xylene. a 1

All three of the xylene isomers have now attained comtes atent ice The process is operable with small amounts of toluene relative to the polymethylbenzene or small amounts of polymethylbenzene relative to the toluene. However, in general, the process, is carried out using amolar ratio of toluene to polymethylbenzene of between about 5 and 0.2, i.e., between about '5 moles .of toluene to 1 mole of polymethylbenzene and 1 vmole of toluene to 5 moles of polymethylbenzene, More usually the. molar ratio of toluene to polymethylbenzene v.is between about 3 and 1.

The-process utilizes a liquid HF catalyst. The entire process is carried out under substantially anhydrous conditions in order to maintain the catalyst concentration; The liquid HF catalyst contains onthe order of not more than '5 weight percent of water. The commercial grade of anhydrous hydrofluoric acid which contains about 2 weight'percent of water is particularly suitable for use in the :process. The process is a liquid phase one ;and

sufficient pressure is maintained on the system to keep mercial importance as raw materials for the production of chemical products. Ethylbenzene has for manyyearsl now been important as a source material for the producl tion of styrene resins. vast amounts of ethylbenzene and xylene isomers in the The petroleum industry produces equilibrium mixture. Various solvent refining operations are now available which produce a mixture of xylenes and ethylbenzene essentially free of non-aromatic hydrocarbons.

However, it is a tedious and expensive operation to separate the four components in high purity,

.An object of theinvention is the preparation of ,a xylene mixture which is free of ethylbenzene. .Another object of theinvention is apreparation of the xylene mixture which contains more than the equilibrium amount of para xylene, which xylene is the most attractive commercially at this time, A further object is a process for upgrading higher polymethylbenzenes to xylenes, particularly para xylene. Other objects will become apparent in the course of the detailed description of the invention.

A xylene product fraction containingsubstantially more than the equilibrium amount of para xylene is obtainable by contacting, under substantially anhydrous conditions, toluene and a polymethylbenzene containing from 4 to 6 methyl groups with a liquid HF catalyst at a temperature between about 100 C. and 200C. for a time between about one minute and 10 hours, wherein the shorter times correspond to the higher temperatures.

The feed to the instant process contains as essentially the only aromatic constituents, toluene and at least one polymethylbenzene containing from 4 to 6 methyl groups. The polymethylbenzene may be any one of the tetramethylbenzene isomers or a mixture thereof, or pentamethylbenzene, or hexamethylbenzene, or a mixture of all of these polymethylbenzenes. Hexamethylbenzene is the preferred polymethylbenzene. The feed may contain, in addition to the defined toluene and polymethylbenzenes, hydrocarbons which are inert to the action of liquid HF at the conditions of operation of the process. Thus it is possible to use close-boiling mixtures of aromatic and parafiinic hydrocarbons containing said polymethylbenzene and/or close-boiling mixtures of toluene and paraflinic hydrocarbons as the feed to the process. The use of a wide boiling range feed is not desirable as the xylene product'may be contaminated with close boiling [non-aromatic hydrocarbons. It is preferred to utilize a feed consisting essentially of toluene and at least one of t e defined polymethylbenzenes.

the HF catalyst and the "feed in essentially the liquid state.

Quite small :amounts of. catalyst may be utilized with some conversion of thefeed to the desired xylene product. In general, the amount of catalyst used is between about 25 and 500 volume-percent, based on feed. More than this .amount may, under some conditions, be utilized. When operating with a feed consisting essentially of toluene and polymethylbenzene, and in order to obtain highconversions, it is preferred to use between about and 200 volume. percent of liquid'HF catalyst, based on feed.

The time of contacting in the reaction zone and the temperature maintained in said ,zonemust be closely related in order to obtain the desired xylene product in high yield and with a para xylene content substantially in excess of the equilibrium amount. Herein the equilibrium composition of a mixture of ortho xylene, meta xylene and para xylene is considered to be, ortho xylene, 20 mole percent, meta xylene, 60 mole percent, and para xylene, 20 mole percent. While the equilibrium concentration varies somewhat with the source of the xylene mixture and also the analytical procedure used, the above composition is in close agreement with published analysis of so-called equilibrium mixtures.

The process is carried out at a temperature between about 100 C. and about. 200 C. It is preferred-to operate between about C. and C.

The time .of contacting at any particular temperature is determined not only by the amount of polymethyl benzene converted, but also by the composition of the xylene product obtained. At low times, the amount of para xylene present is maximized but the amount of polymethylbenzene converted is quite low. As the temperature is increased, the time for maximum conversion and simultaneously maximized para xylene content of the xylene fraction approach each other. The process is operated at about 100 C. for a maximum timeof about 10 hours, thereby maximizing both conversion and para xylene content. At about 200 C., the maximum contacting time is about 1 minute. At a temperature of about 120 C.,-the maximum contacting time is about "5 hours. And at about 150 C., the maximum contacting time is about 10 minutes. Thus itis apparent that an inverse relationship exists between temperature of-contacting and maximum time of contacting at that temperature, namely, the higher the temperature of contacting, the lower the corresponding maximum time of contacting.

The results obtainable .by the process of the invention and comparative data are set out in thetable which forms a part of this description.

The data presented in the table "were obtained-by contactin'g commercial grade anhydrous hydrofluoric acid and toluene and polymethylbenzene in an autoclave provided with a mechanical stirrer. The contents of the autoclave were contacted for a predetermined period of time at a predetermined temperature. At the completion of the contacting time, the contents of the autoclave were withdrawn into a vessel filled with ice and water. The aqueous layer and the hydrocarbon layer were separated. The hydrocarbon product was neutralized and water washed prior to analysis. The reaction product mixture .Was separated into close-boiling fractions by means of laboratory column providing about 30 theoretical distillation plates. The close-boiling fractions were then analyzed by infrared or ultra-violet techniques to determine the isomer distribution.

tion contained 59 mole percent of para xylene.

- low under these conditions.

amount of non-usable trimethylbenzene was also quite Table Run No 1 2 3 4 5 6 7 8 Polymethylbenzene Type Penta- Penta- Penta- Penta- Penta- Hexa- Tetra- Tri- Toluene/PMB, molar ratio- 2 2 2 2 2 4 2 2 HF, Vol. Percent on H0 100 100 100 100 100 110 110 110 Temperature, C 120 120 150 150 190 (165) 150 150 Time, Minutes--- 60 300 60 1O 60 10 10 Reactor Pressure, p s i 280 500 550 1050 Reaction Product Distribution, mol percent: T "one 30 29 39 52. 2 61 Xylene ca 1 26- 16- 38- 19. 7 4

Ortho- 6 12 6 27 5 11 Meta- 39 30 36 34 Para- 55 53 64 33 59 55 Trimethylbenzene T1 23- 26- 7- 34- 7. 4 6 61 56 67 56 71 64 1,2,3- 6 8 6 8 6 8 1,3,5- 33 36 28 36 23 28 Tetramethylbenzene ca 1 19 19 21 8 11. 6 29 4s 60 47 41 4s 1,2,3,5- 50 46 45 47 53 53 3 6 5 6 6 4 Pentamethylhen Pnn 3 Tr 17 Tr 5.4 0 Hexamethylbenzene 0 0 0 0 0 3. 7 0

In runs 1 through 5, the feed consisted of high purity nitration grade toluene and technical grade pentamethylbenzene. These runs were carried out at the same toluene to pentamethylbenzene ratio of 2, using about equal volumes of liquid HF catalyst and feed. In run 1, which was carried out at a temperature of 120 C. for a time of 60 minutes, there was some conversion, as is evidenced by the presence of about 1 mole percent of xylene tetratained about the equilibrium distribution of isomers.

Runs 3 and 4 show the effect of time at the higher temperature of 150 C. At 60 minutes time, in runNo. 3, all the pentamethylbenzene was converted. 'It was found that no significant improvement in xylene yield was obtained and the xylene isomer distribution was essentially the same as that of run No. 2, namely, more than 50% of the xylene product was the para isomer. In runNo. 4, wherein only about half of the pentamethylbenzene was converted in the 10 minute contacting time, the yield of xylene was reduced by about one-third as compared with run N0. 3; it is noteworthy that the para xylene content of the xylene product was at 64 mole percent or better than 3 times the theoretical," whereas the meta xylene content was only about one-half of the theoretical.

In run No. '5, the effects of high temperature (190 C.) and longer time (60 minutes) were observed. In this run, all of the pentamethylbenzene was converted. The yield of xyzlene was increased to 38% of the mixture, but the para xylene content of the xylene product dropped to 33 mole percent with both meta and ortho xylene showing yield increases at the expense of para xylene. It is of interest that the yield-of tetramethyl- Run No. 7 was carried out using as a feed tolueneand an equilibrium mixture of tetramethylbenzene isomers.

. In this run, the xylene product contained mole percent of para xylene. This run also illustrates the point that as the number of methyl groups on the benzene ring in creases, the easier it is to carry out the transfer of methyl groups to the toluene acceptor.

Run No. 8 was carried out to ascertain the effect of using trimethylbenzene as a methyl group donor. these conditions there was only a small amount of conversion. The xylene product contained only 28% of para xylene. This is in very marked contrast with the results of runs 3 and 4 at C. using pentamethylbenzene as the donor, and with run No. 7, using tetramethylbenzene as the donor. With pentamethylbenzene as the donor, even at the high temperature of C. and the prolonged contacting time of 60 minutes, run 5 shows that more para xylene was present in the xylene product when pentamethylbenzene is the donor.

In all cases, the tetramethylbenzene product obtained from the reaction product mixture had come very close to the theoretical equilibrium composition of: 1,2,4,5- tetramethylbenzene, 45%, 1,2,3,5 -tetramethylbenzene, 50%, and l,2,3,4-tetramethylbenzene, 5%. In all cases, the trimethylbenzene product approached the equilibrium composition. Thus it can be seen by inspection of the data contained in the table that by the process of the invention high yields of xylene can be obtained by the transe alkylation reaction of toluene and the defined polymethylbenzene, which xylene product contains substantially more than the equilibrium amount of para xylene and .can contain as much as 50 mole percent and even more of the para xylene.

The process is further described in connection with the annexed figure which forms a part of this specification. The figure has been extremely simplified in-order, to show only the essential features. In the figure, feed from source 11 is passed by way of line 12 into reactor 13. The feed may be a natural mixture of toluene and other methylbenzenes such as is obtainable by the methyl 5 chloride alkylation of benzene or toluene, or the feed may consist of the fractionated product of this alkylation with the xylene and trimethylbenzene fractions being recycled to the alkylation step to produce the necessary tetramethylbenzene, etc. Or the feed may consist of tetramethylbenzene obtained by various reactions, such as the disproportionation of trimethylbenzene, or a mixture obtained from a natural source such as a petroleum catalytic reformate. In this instance, the feed has been obtained by the fractionation of the product from the alkylation of benzene with methyl chloride with the feed containing only toluene, tetramethylebenzenes, pentamethylbenzenes and hexamethylbenzenes. Additional toluene may be added from source 15 by way of valved line 15a. In this instance, the molar ratio of toluene to tetramethylbenzene, pentamethylbenzene, and hexamethylbenzene present in line 12 prior to entry into reactor 13 is 2. Reactor 13 herein is a vessel provided with turbine-type stirrers 14a, 14b, and 140, driven by motor 16. Reactor 13 is provided with means for maintaining the temperature of the contents at the desired temperature. The feed and the HF catalyst are both heated to about the reaction temperature by means not shown prior to their entry into reactor 13. In this illustration, reactor 13 is maintained at 150 C. and the holding time of the feed and catalyst is 18 minutes.

The liquid HF catalyst from source 17 is introduced by way of line 18 into reactor 13 at a point near the entry of the contents from line 12. Commercial grade anhydrous hydrofluoric acid containing 98% HP is utilized as the catalyst. Precautions are taken, if necessary, to dehydrate the feed in order to avoid dilution of the HF catalyst appreciably below the 98% point. In this illustration, 120 volume percent of HF catalyst is used, based on toluene and polymethylbenzene introduced by way of line 12 into reactor 13.

A reaction product mixture consisting of unreacted toluene, polymethylbenzene containing 3 to 6 methyl groups, and xylene product along with liquid HF catalyst are withdrawn from an upper point of reactor 13 and passed by way of line 21 into separator 22. Separator 22 operates to form a lower acid phase and an upper hydrocarbon phase. The lower acid phase is withdrawn from separator 22 by Way of line 23 and is passed to line 18 for reuse in reactor 13. Periodically it is desirable to withdraw the catalyst from the system and replace it with fresh catalyst. Used catalyst is withdrawn by way of lines 23 and 24.

The upper hydrocarbon layer in separator 22 is withdrawn and passed by way of line 26 into distillation zone 27. Distillation zone 27 is shown schematically herein. There is taken overhead a fraction consisting of toluene and HF which was dissolved in the recation product mixture. The overhead toluene (HF) fraction is passed by way of line 31 to line 12 for use in reactor 13. A trimethylbenzene fraction (Me B) is withdrawn from the distillation zone by way of line 32 and is passed to storage. This trimethylbenzene fraction may be recycled to the methyl chloride alkylation or a disproportionation operation to provide additional feed to the process, or it may be sold for use as a chemical intermediate. A hottoms fraction is withdrawn by way of line 33. This bottoms fraction consists of tetramethylbenzene, pentamethylbenzene and unconverted hexamethylbenzene. The bottoms fraction is recycled by way of line 33 to line 12 for reuse in reactor 13.

The product xylene fraction is withdrawn from the distillation zone by way of line 34. This product xylene fraction contains about 65 mole percent of para xylene. As this fraction contains no ethylbenzene, it is an excellent feed for a fractional crystallization operation sub= sequent to a superfractionation operation to remove most of the ortho xylene present. a

It is to be understood that the illustrative embodiment set out in the figure has been purposely simplified to eliminate pumps, valves, heat exchangers and other items which may be readily added thereto by those skilled in this art, following the teachings of the description herein. Other methods of carrying out the contacting and the sep aration may be readily divised by those skilled in this art. It is not intended that this illustrative embodiment limit the scope of the invention claimed herein.

Thus having described the invention, what is claimed l. A process which comprises contacting, under substantially anhydrous conditions, a feed consisting essentially of toluene and at least one polymethylbenzene containing from 4 to 6 methyl groups, said toluene and said polymethylbenzene being present in a molar ratio of toluene to polymethylbenzene of between about 5 and 0.2 with liquid HF catalyst, said catalyst being present in an amount between about 25 and 500 volume percent, based on said feed, at a temperature between about C. and 200 C. for a time between about 1 minute and 10 hours, wherein the shorter times correspond to the higher temperatures, separating a hydrocarbon reaction product mixture from catalyst and recovering from said reaction product mixture a xylene product containing substantially more than the theoretical equilibrium amount of para xylene.

2. The process of claim 1 wherein said polymethylbenzene is tetramethylbenzene.

3. The process of claim 1 wherein said polymethylbenzene is pentamethylbenzene.

4. The process of claim 1 wherein said polymethylbenzone is hexamethylbenzene.

5. A process which comprises contacting, under substantially anhydrous conditions, feed consisting essentially of (a) toluene, and (b) a polymethylbenzene selected from the class consisting of tetramethylbenzene, pentamethylbenzene, and hexamethylbenzene, in a molar ratio of toluene to polymethylbenzene of between about 3 and l, with liquid HF catalyst in an amount between about 100 and 200 volume percent, based on said feed, at a temperature between about C. and C. fora maximum time of between about 10 minutes at about 150 C. and about 5 hours at about 120 (2., wherein the longer maximum times correspond to the lower temperatures, separating hydrocarbon reaction product mixture from HF and recovering from said reaction product mixture a xylene product containing at least about 50 mole percent of para xylene.

6. The process of claim 5 wherein said polymethylbenzene is hexamethylbenzene.

References Cited in the file of this patent UNITED STATES PATENTS Passino Mar. 19, 1946

Claims (1)

1. A PROCESS WHICH COMPRISES CONTACTING, UNDER SUBSTANTIALLY ANHYDROUS CONDITIONS, A FEED CONSISTING ESSENTIALLY OF TOLUENE AND AT LEAST ONE POLYMETHYLBENZENE CONTAINING FROM 4 TO 6 METHYL GROUPS, SAID TOLUENE AND SAID POLYMETHYLBENZENE BEING PRESENT IN A MOLAR RATIO OF TOLUENE TO POLYMETHYLBENZENE OF BETWEEN ABOUT 5 AND 0.2 WITH LIQUID HF CATALYST, SAID CATALYST BEING PRESENT IN AN AMOUNT BETWEEN ABOUT 25 AND 500 VOLUME PERCENT, BASED ON SAID FEED, AT A TEMPERATURE BETWEEN ABOUT 100* C. AND 200*C. FOR A TIME BETWEEN ABOUT 1 MINUTE AND 10 HOURS, WHEREIN THE SHORTER TIMES CORRESPOND TO THE HIGHER TEMPERATURES, SEPARATING A HYDROCARBON REACTION PRODUCT MIXTURE FROM CATALYST AND RECOVERING FROM SAID REACTION PRODUCT MIXTURE A XYLENE PRODUCT CONTAINING SUBSTANTIALLY MORE THAN THE THEORETICAL EQUILIBRIUM AMOUNT OF PARA XYLENE.

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