US2527824A - Manufacture of xylene isomers - Google Patents

Description

Oct. 31, 1950 J. D. KEMP MANUFACTURE OF XYLENE ISOMERS 4 Sheets-Sheet 1 Filed June 13, 1947 IN VE N TOR Jacob D. Kemp KML vkvG kl A TTORNEVS Oct. 31, 1950 J. D. KEMP MANUFACTURE OF XYLENE ISOMERS Filed June 13, 1947 4 Sheets-Sheet 2 /N l/E N TOR Jacob D. Kemp wk MK WEWNEQQ A TTORNEVS 4 Sheets-Sheet 3 Filed June 13, 1947 META -XYLENE O-XYLENE C A a w Q I I\ A my 7 E fiwfioqmlm M E #365 7 n J \1 \ll w xQQQQxl 3 V $5855 b M n m M m C E a m m a w r 4 2952933 2 @l s M M M $59 33 w M c r\ N w A 3 8 p. 4 p- /R W. H 0 Q w 2 V N 2 m b w R 5 M U lll|lli x N 0 J y E 9 F M lNVE/VTOR Oct. 31, 1950 J. D. KEMP MANUFACTURE OF XYLENE xsoms 4 Sheets-Sheet 4 Filed June 13, 1947 N T L 0 m m W 5w m m E m w 1T .WRR hum mw A 0 C m R P w s m S 5 4 3 2 w. w. w. w. w. m.

aotwzotmoqomqwfi mom RS K2538 E5 utbmmw MOL. FRACTION OF HF SPECIFIC RATE CONSTANT vs MOL FRAcr/o/v CATALYST FOR ISOMER/ZA T/0/v lNVENTOR Jacob D. Kemp 5 M m m 0. w

EQRYNEmSQQ Q9 35 1255 28 HE uEBmQw MOL. FRACTION OF HF A T TOP/VEVS Patented Oct. 31, 1950'- UNITED STATES PATENT OFFICE 2,527,824 I MANUFACTURE or xncm: rsomms Jacob D. Kemp, Richmond, Calm, ossignor to Corporation, San Francisco, Calif., a corporation of Delaware Application June 13, 1947, Serial No. 754,573 4 Claims. (Cl. 260-668) California The present invention relates to catalytic iso merization of aromatic compounds containing a plurality of saturated side chains and more particularly to catalyzing the shifting of intact aliphatic groups (preferably without isomerization of the groups per se) along or around the benzene ring of an aromatic hydrocarbon. This type of isomerization of aliphatic-substituted aromatic compounds involves aliphatic to aromatic carbon to carbon bonds and is hereinafter termed isomerization at the ring or nucleus to distinguish from mere isomerization of aliphatic to aliphatic carbon bonds in an aliphatic chain or aromatic to aromatic carbon bonds in the cyclic structure of the ring itself.

An object of this invention is to produce commercially valuable isomeric aikyl aromatic compounds from other alkyl aromatic compounds having the same number of alkyl side chains.

Another object of this invention is the preparation of para dialkyl benzenes from meta and/or ortho dialkyl benzenes by an improved process.

A further object of this invention is to provide a new and improved process and apparatus for converting a polymethyl benzene to an isomer thereof and more particularly to isomerize meta and/or ortho xylenes to para xylene.

An additional object of this invention is to minimize disproportionation reactions in the isomerization of polyalkyl aromatic hydrocarbons. To furnish an improved method for selectively catalyzing isomerization while suppressing disproportionation of xylenes comprises another object of the invention.

It is also an object simultaneously to eliminate Research ethyl. benzene and avoid buildup of this impurity in a cyclic process for isomerization of xylenes containing ethyl benzene. Another object is to provide a new and useful reaction technique for effecting faster conversion of polyalkyl benzenes by isomerization.

Although the invention is particularly applicable to and finds its greatest present utility in the isomerization of dialkyl benzenes, more notably dialkyl benzenes in which at least one alkyl group is methyl, and especially xylenes, the invention in its broader aspects is applicable to other alkyl aromatic compounds. However, it has been found that methyl substituents are more difflcult to isomerize at the aromatic nucleus than are longer alkyl substituents, and it will be apparent in the light of the following disclosure that the invention is applicable to other aliphatic-substituted aromatics, and more desirably to dialkyl benzenes containing from 1 to 4 carbon atoms in each side chain. For the sake of simplicity the invention will be illustrated primarily by reference to the more diflicult isomerization, i. e., catalyzing the shift of methyl groups at the benzene ring as in the xylenes.

Flexibility and economical utilization of alkyl aromatic hydrocarbons introduce a need for an eflicient process for conversion of alkyl aromatic compounds from one isomeric form to another. Thus, orthoxylene is convertible by oxidation to phthalic anhydride and in some instances it may become desirable to increase the supply of orthoxylene for this or other uses by conversion of meta or para xylenes. 0n the other hand, meta and para xylenes are superior to orthoxylene as blending agents or additives for certain special motor fuels, and the supply of this superior blending agent could be enhanced by isomerization of orthoxylene. In still another situation, relatively pure para xylene is developing into an important chemical for manufacture of various derivatives, and in those xylene plants which specialize in production of the para isomer there is the need of a suitable process for converting ortho xylene or meta xylene or both to the para product. Pure meta xylene (as distinguished from mixtures of meta and para xylenes for gasoline blending) as yet has encountered no large market demand, but flexibility of industrial operations to meet any future requirements for this isomer makes it desirable to provide a process for isomerization of ortho or para xylenes or both to the meta isomer.

To appreciate the difliculties in the development of a process for isomerizing one or more of the xylenes, it should be recognized that a large number of reactions potentially may, and many actually do, occur simultaneously. One of the major problems therefore becomes the provision of a process which will promote selectivity in effecting the desired isomerization reaction or reactions while inhibiting or eliminating formation of undesired side reaction products. The following discussion is in reality an oversimplification of the problem but is illustrative of its nature.

Without reference to the mechanism of the reactions the main conversions for orthoxylene maybe written:

CH3 CH Ortho- Meta- CHI Ortho- Ba Para- Orthoxylene also tends to enter side reactions, such as disproportionation:

(a) CH: OH:

on. on.

CH: CH:

CH: CH: Ortbo- Xyleues Toluene Trimethyl benrenes Upon isomerization meta xylene may be correspondingly converted:

( OH: CH:

Meta- Ortho- CHI HI Ha Meta- Para- Likewise, metal xylene tends to enter side reactions illustrated by:

(a) CH: CH:

CH1 OH:

CH: H; CH: Meta- Xylenes Toluene Trimethyl bentenes Ortho Again there are side reactions:

CHl

C 1 CHI CH: Q I :1-(3H: i CH; CH: CH: Pars Xyleues Trimethi benzene Toluene Of course, each of the foregoing reaction products may further react with the xylene and with each other to introduce additional complications.

This invention involves catalysis of the foregoing desired .isomerization reactions with hydrogen fluoride. The catalytic action of hydrogen fluoride in isomerizations of this type is re- Para markable because it can be made highly selective for the desired shifting of substituents at the ring. Thus, the isomerization of xylenes by hydrogen fluoride catalysis may be effected with a minimum of side reactions such as disproportionation and formation of trimethylbenzene or toluene. The catalytic action of hydrogen fluoride also lowers the temperature at which isomerization at the ring will occur and thereby reduces or avoids undesirable losses by dealkylation, cracking and other reactions heretofore encountered in various high temperature thermal processes.

Hydrogen fluoride catalysis in conformance with this invention and discovery provides a valuable process for converting ortho xylene to a mixture of meta and para xylene isomers or for producing ortho and para isomers from meta xylene or for obtaining ortho and meta xylenes from the para isomer. In effecting these reactions, it has been found that 'isomerization is an equilibrium reaction, and the proportions of xylene isomers at equilibrium is about the same at any given temperature irrespective of the particular isomer or mixture undergoing conversion. These relative proportions in the present process ordinarily are approximately:

Per cent Orthoxylene l9 Paraxylene 21 Metaxylene However. it has been found that isomerizatlon of meta xylene to ortho or para xylenes, as well as conversion of either para or ortho xylene to the meta product are first order reactions with respect to the xylene being isomerized and that the specific reaction rate constant for conversion of meta to para is about twice that for the conversion of meta to ortho xylene. On the other hand conversion of ortho to para xylene has been found to occur in a stepwise fashion through the meta isomer. These discoveries make is possible to stop the reaction short of equilibrium and effect the following conversions selectively:

Meta-+para Parameta 0rthometa while substantially inhibiting though not avoiding the reactions:

Orthopara Paraortho Metaortho Such selectivity may be obtained by shortening the reaction period sufiiciently to utilize the differential reaction rates, for example, stopping the reaction by quenching to a temperature below about 200 to 250 F. prior to formation of about 60% of the equilibrium proportion of ortho xylene. The foregoing selective reactions may be further promoted by the presence of the undesired isomer in the reaction mixture. Thus:

Metapara (in the presence of ortho) Parameta (in the presence of ortho) Ortho-+meta (in the presence of para) In practicing this invention it is preferred to feed a mixture of xylenes containing at least one isomer ina proportion different from that represented by isomerization equilibrium or formed by the isomerization reaction. This gives a conversion corresponding to the difference in composition of feed and reaction product, which can be relatively large as desired. Conversion or isomerization by such differentials has the advantage of avoiding difli'cult and expensive purification treatments otherwise necessary to supply a feed stock of any given pure isomer. On the other hand, such differential conversion processes necessitate adequate suppression of side reactions and involve problems of build-up of impurities such as ethyl benzene.

The reaction product in the preferred isomerization by dilferential conversion will contain all three isomeric xylenes. In addition to accepting reduced conversions, it is preferred and usually necessary to accept considerably less than 100 per cent recovery of the desired isomer or isomers from any single pass isomerization product. Nevertheless high overall conversions and yields can be obtained by removing a substantial proportion of the desired isomer from the reaction product and again subjecting the remaining a maximum of about 10% of the xylenes in the reaction product is removable by crystallization which in turn signifies that only about l i J% conversion per pass can be obtained upon recycle through the isomerization zone. For 100% overall conversion of a xylene fraction to paraxylene about 10 passes are therefore required. It follows that if only 5% of the xylenes are converted to side reaction products, each time the reaction is efl'ected, the real loss in xylenes is not 5% but 33% in such a cyclic process. When it is recalled that, generally, a single pass or batch organic preparation would be considered excellent if only 5% loss to side reactions took place, it is evident that ordinarily highly acceptable standards for organic reactions are likely to introduce economically prohibitive losses in the isomerization of this invention.-

In accordance with this invention xylenes are isomerized at the benzene ring by a new and improved diflerential conversion process which is capable of converting either a single xylene isomer or a mixture of xylenes substantially entirely to any selected xylene isomer or mixture,

economically and with minimized loss to side reaction products or production of disproportionation products. Ethyl benzene impurity is eliminated simultaneously by disproportionation. Suitable reaction conditions will be apparent from the following discussion of process variables particularly applicable to the xylenes:

A temperature of above about 250 F. is operative, the upper operative temperature limit for isomerization being primarily determined by permissible pressures. Generally temperatures higher than 400 F. are not warranted and tend to introduce or increase side reactions. It is to be understood that less severe conditions, i. e., lower temperatures or lower contact times, or both, are acceptable, and even preferable, for isomerization of ethyl and higher molecular weight aliphatic side chains. Ethyl groups are much more easily isomerized at the aromatic nucleus than are methyl groups, for example, and temperatures below 250 F. will be found suitable ass-1,2024

for such higher alkyl chains. In the case of isomerization of methyl group; at the nucleus, 300-375 F. is desirable and a temperature of from about 310-350 F. has been found preferable for the isomerization of xylenes with anhydrous hydrogen fluoride. These higher temperatures also are preferred for xylenes containing ethyl benzene impurities to remove this component simultaneously by disproportionation.

Contact time for a given conversion in the isomerization reaction is a function of temperature, catalyst concentration and feed concentration. Ordinarily, a minimum of about one minute will be required and the maximum contact time is primarily determined by permissible plant size, although the danger of increasing relatively slow side reactions is increased by inordinately long contacts. Generally thirty minutes is a maximum, and five to twenty minutes is presently regarded as optimum for isomerization of xylenes with anhydrous hydrogen fluoride in the proportions and at the temperatures herein recommended. I

Catalyst concentration in the reaction mixture may be varied widely. Although not critical to operativeness of the isomerization reactions, it has been discovered that by adjusting reaction conditions remarkable and unpredictable improvements may be obtained and that the isomerization may be eifected as a homogeneous liquid phase reaction. To eflect such a homogeneous reaction rapidly, relatively high proportions of hydrogen fluoride are utilized, for example, irom about to about volume per cent of anhydrous hydrogen fluoride based on the total reaction mixture. Smaller amounts are operable, but at least 40 per cent by volume is desirable for xylene isomerization, depending upon temperature and pressure. The maximum proportion of catalyst is determined primarily by acceptable plant size, but more than per cent by volume based on total reaction mixture ordinarily is not warranted.

Commercial anhydrous hydrogen fluoride may contain up to about 0.5% water and such anhydrous hydrogen fluoride is preferred as a catalyst, although small amounts of water, for example, 2 to 5% are not precluded. Water inthe hydrogen fluoride catalyst reduces the activity of the catalyst by decreasing the mol fraction of hydrogen fluoride, and consequently reduces either the reaction rate, or the capacity of the system for hydrocarbon if the amount of aqueous catalyst is increased to yield an equivalent HF mol fraction. An additional loss of 35% in activity was found for hydrogen fluoride containing 2 to 4% water when compared to anhydrous hydrogen fluoride at equivalent mol fractions of HF. This may be compensated for by raising the temperature of operation. The invention embraces processes in which hydrogen fluoride is the effective catalyst and does not preclude the presence of auxiliary catalytic agents. The term the effective catalyst" used in the claims with reference to hydrogen fluoride is intended to mean that hydrogen fluoride is present in an amount and concentration suflicient to act as the sole catalyst necessary to carry out the reaction. This terminology is intended to cover not only isomerizations at the aromatic nucleus where hydrogen fluoride is 'the only catalytic agent present, but also such isomerization reactions where, in addition to hydrogen fluoride, other catalytic materials are added in amounts such that in the ab sence of'hydrogen fluoride satisfactory catalysil of the reaction would not be effected.

The presence of a hydrocarbon diluent miscible with the reaction mixture selectively promotes the isomerization reaction, that is, the same fractional conversions of meta or ortho xylene to para xylene, for example, are obtained in diluted solutions as in more concentrated ones under otherwise comparable conditions. By contrast second order reactions, such as disproportionation, are selectively reduced or inhibited by dilution of the reaction mixture. Additionally, it has been found that by using toluene as a diluent, losses by disproportionation of xylenes are reduced over and above the reduction obtained with substantially inert diluents, such as cyclohexane. For these reasons, it is preferred to eflect the isomerization reaction in the presence of a diluent, most desirably toluene with xylene isomerization; and for other dialkyl benzenes, the preferred diluent is a mono-alkyl benzene in which the alkyl group is like an alkyl group of the dialkyl benzene being isomerized. Suitable proportions of diluent usually are in excess of and desirably in the order of 50% by volume based on the total feed.

Pressure is not critical except that it shoul be sufficiently high to maintain reactants and catalyst in liquid phase. In order to maintain catalyst activity or at least to keep its reduction 1 to a controlled minimum, the hydrocarbon feed and diluent desirablyshould be dried prior to contacting with the hydrogen fluoride catalyst. Any suitable drying process may be adopted, for

example, contacting with bauxite or other porous adsorbents for water.

The principal impurities in xylene fractions here contemplated are of two types: (a') aromatic hydrocarbons boiling in the same range as the xylenes or forming constant boiling mixtures therewith; (b) unsulfonatable hydrocarbons normally designated paraflinic and likewise boiling in the same range or forming constant boiling mixtures with the xylenes. In a cyclic differential isomerization process, such impurities must be dealt with even though present only in minor proportions in the feed, since repeated recycling of the isomerized fraction (with removal of the desired isomer only) allows and causes a buildup in the concentration of such impurities. In the present process the ethyl benzene impurity is disproportionated selectively during the isomerization reaction and converted to benzene with formation of ethyl toluene and ethyl xylenes,

each of which can be eliminated readily by simple fractional distillation. Inordinant increase in ethyl benzene concentration is thus prevented in the presently preferred cyclic process for differentially isomerizing mixed xylenes. Parafllnic impurities on the other hand desirably are eliminated if present. Various methods are feasible such as selective solvent extraction and azeotropic distillation, but the so-called solvent distillation or perhaps more accurately designated extractive distillation is presently preferred.

In the drawing, Fig. 1 is a schematic flow sheet of a process and apparatus illustrating only the major unit operations in the combination process of this invention;

Fig. 2 is a more detailed flow'sheet of the isomerization stage of a process and apparatus for xylene conversion;

Fig. 3 illustrates a combination of isomerization and separation stages adapted to produce xylene isomers in any desired relative proportion;

Figs. 4 and 5 illustrate the effects of proportion of hydrogen fluoride catalyst on specific rate constants for isomerization and disproportlonation.

For purposes of simplicity and to avoid incorporation of unnecessary details of 1'18. 2. the isomerization unit of Figs. 1 and 3 is shown in block diagram.

Referring to Fig. 1, polyalkyl benzene feed is introduced into the system by inlet line l0,'and for purposes of simplicity the process will be illustrated with reference to a xylene feed. Examples of suitable xylene feeds are meta xylene, para xylene, mixtures of meta and para xylene or mixtures of meta, para and ortho xylenes in which the desired isomer is present in less than the relative proportion formed upon completion of the isomerization reaction. In the production of para xylene, for example, the feed should contain less than 14% para xylene based on the xylene content of the feed. In the preferred process, about 10% or less of the xylene isomers in the feed is para xylene. The xylene feed ordinarily contains impurities principally parafilns and ethyl benzene.

As here shown an extractive distillation is utilized to effect removal of paraflins contained in the feed. Details of a suitable process are exempiified in U. S. Patent No. 2,215,915 to Cope et al. issued September 24, 1940. In accordance with this process, the xylene passes by way of xylene inlet line l0 through heater II to extractive distillation column I2 where the downwardly flowing selective solvent, introduced in the top of the column by solvent line i3, serves to carry the extracted xylenes downwardly in liquid phase. Heater coil i4 boils paraiiinic components of the mixture out of the liquid phase solvent, and these boiling vapors are repeatedly fractionated and contacted with selective solvent to separate any xylenes as the vapors progress upwardly through the distillation column I2. A suitable condenser and reflux is provided in paraflin discharge line it at the top of column l2, and the discharged parafiins pass through line IT to storage.

Liquid phase solvent together with dissolved xylenes and ethyl benzene or other aromatic impurities present in the original xylene fraction flow from the bottom of column 12 through discharge line l8 to fractionator is where the dissolved aromatics are separated from the higher boiling selective solvent by conventional fractionatnn procedure. The solvent is recycled from the bottom of this fractionator by way of discharge conduit 2i to extractive distillation column i2. Aromatic hydrocarbons vaporized in fractionator 1% flow by outlet line 22 through condenser 23 to reflux drum 24, from which a portion may be returned by way of valve-controlled reflux line 26 to fractionator IQ for purposes of improving separation of the selective solvent from the xylene fraction, when and if they boil sufliciently close to warrant such an expedient. The main portion of the purified xylene from reflux drum 24 passes through valvecontrolled header 21 to the isomerization stage 28.

The presently preferred form of isomerization unit is illustrated in Fig. 2 later to be described. However, the invention in its broader aspects is not limited to the particular isomerization stage there shown, since features for selectively promoting isomerization while suppressing disproportionation of the desired polyalkyl benzenes are applicable to other types of isomerization units, where hydrofluoric acid catalysis is utilized cycling rimarily trimethyl benzenes.

of catalyst flows from the isomerization unit' through product discharge line 32 to diluent fractionator 33 where the desirably more volatile diluent is distilled as overhead and returned through diluen'tlrecycle line 34 to isomerization feed header -21. Fractionator 33 is provided with a condenser 36 and condensate drum 31 in order that a portion of the diluent may be returned as reflux through valve-controlled line 38 'or withdrawn when desired by means of valve-controlled line 39.

I Isomerized hydrocarbons stripped of .diluent flow from the bottom of fractionator 33 by way of line 40 to a purification unit 4| comprising a fractionating column provided with an overhead condenser 42 and condensate drum 43. The desired isomerizates are distilled overhead in frac- -tionator M, a portion thereof being refluxed through valve-controlled line 44, and the remainder passed through valve-controlled line 46 to isomer separation stage 41. Bottoms from fractionator 41 comprise principally higher boiling.

disproportionation products of ethyl benzene such as ethyl toluene and ethyl xylenes, when the isomerization feed is xylenes diluted with toluene. Also, some trimethyl benzenes are formed from the xylenes. reaction, reduce loss of xylenes by side reactions and suppress formation of, disproportionation products from the xylenes, it has been found that these heavier impurities should be recycled at least in part to the isomerization feed as shown by valve-controlled heavy recycle line 48. Further, to facilitate and expedite control of the isomerization reaction in relation to side reactions, the heavier recycle fraction may be separated, for example, into a C9 and C10 fraction by conveying at least part thereof through valve-controlled line 49 to fraotionating column where Ca alkyl benzenes are taken overhead and C removed as bottoms, whereupon the relative proportions of C9 and C10 recycle can be altered andcontrolled to give optimum suppression of side reactions. Thus, one may wish to remove from the system primarily those disproportionation products containin an ethyl group (for example, ethyl toluene or ethyl xylene or both) while re- Independent recycle lines 52' and 53 are provided for this purpose.

Returning now to the separation of isomers in the stage 41, the desired isomeric product is removed by way' of line 54, and the remaining isomers preferably are returned to the isomerization stage by way of recycle line 56. The return'of these remaining isomers serve to promote the production of the desired isomer. Thus, recycle of ortho and meta xylenes, for example, suppresses further formation of these components during isomerization and promotes the conversion of xylenes in the feed to the para isomer:

To further favor the isomerization i to recycling all ortho and metayxylenes, the entire isomerization thus can be devoted to the production of the para product. Likewise, the ortho or the meta isomer may beselected as a desired product and the corresponding remaining isomers recycled.

The precise separation-procedure utilized in stage 41 will depend uponthe isomer or isomers to be obtained as a final product. In the production' of para xylene, for example, ortho xylene, meta xylene, a mixture of ortho and meta. Ora mixture of xylenes containing a smaller proportion of para than is present in the isomerized reaction product, will be fed to the isomerization stage. Separation of para xylene product in stage 41 desirably is effected by selective crystallization, although other methods may be utilized. such as selective sulfonation of the meta xylene isomer, separation of an unsulfonated oil layer from the sulfonation reaction mixture and fractional distillation of the para from the ortho xylenes in the unsulfonated oil. Crystallization of para xylene is effected, for example, by chilling the xylene mixture to a temperature of from at least about F, and preferably to about F. with separation of the crystals while so chilled. When the isomeric xylene fraction is free of other hydrocarbon contaminants, temperatures below -6'1 F. serve to introduce other isomeric xylenes as impurities in the crystallized para xylene fraction. For additional purification the separated para xylene crystals may be washed with a suitable solvent. such as isopentane and/or melted and subjected toa second crystallization operaion.

Ortho xylene can be produced in separation stage 41 by close distillation preferably by superfractionation to yield an ortho xylene product of and higher purity as a bottom fraction. However, for some purposes, a"10-90% or more desirably a 85-95% ortho xylene rich product is acceptable. The'ortho xylene poor overhead from the superfractionation will then be recycled to the isomerization stage when and if the only desired somer is the ortho product.

Production of meta xylene as the desired isomer requires a different separation in stage 41 and the presently preferred method for recovering meta xylene product involves selective sulfo'nation of the isomerization reaction product to selectively form a meta xylene sulfonic acid phase and a supernatant oil layer containing predom-' inantly ortho and para xylenes. These phases are separated by any suitable method, and the meta xylene sulfonic acid layer is converted back to meta xylene by hydrolysis. The meta xylene desirably is removed from the hydrolysis reaction zone by distillation, and residual sulfonic acids may be hydrolyzed to recover any ortho and para xylenes carried thereby. Suitable conditions for separation and recovery of meta xylene byselective sulfonation and selective hydrolysis are illustrated by the following.

It is preferred to use about two molsof sulfuric acid (96-98%) for each moi of meta xylene in the mixture being treated, although these proportions are not critical. Weaker acid may be utilized down to about 75%, though more acid and higher temperature. or greater time, or both, are-required. The temperature of selective sulfonation ranges from about 80 F. to about F. with a corresponding variation in the rate of sulfonation. The selective hydrolysis preferably is carried out by first heating the separated sulfonic acid mixture up to a temperature such that. by-

the hydrolysis is effected by heating the resulting sulfonic acid solution in the presence of water or steam at a temperature of from about 250-300 F. Meta xylene vapors are thus selectively distilled off from the reaction mixture at these temperatures toyield fractions of 90% and higher purity. Residual hydrocarbon sulfonic acids remaining in the still bottoms are finally hydrolyzed by raising the temperature of the reaction mixture to about 360460 F., collecting and condensing hydrocarbon vapors therefrom to form a second para rich xylene fraction which desirably is recycled to isomerization.

Provision has been made for removal of parafflnic impurities in two stages for those cases where complete elimination of these impurities in the feed is difficult or unduly expensive. In this two-stage operation a major portion of the paraflins is eliminated from the feed as previously described. In a cyclic operation parafliins eventually may build up; however, by reason of continued return of small quantities of paraflins from the isomer separation to the isomerization stage, this build-up is controlled by a second stage separation effected as here shown by diverting at least a portion of the recycle stock from recycle line 56 by way of a purification return line 51 through heat exchanger 58 to extractive distillation column l2. Thus, paraflln build-up can be eliminated entirely or controlled to a given level by regulating the proportion or frequency of diversion of recycle xylenes to extractive distillation -column i2.

As previously noted, ethyl benzene build-up is prevented by disproportionation to form either ethyl toluene or ethyl xylene, or both, as desired. Build-up of disproportionation product or products of ethyl benzene may be controlled by diverting a portion of selected heavy recycle streams. Thus, C9 disproportionation products may be removed from overhead distillate in frac-.

tionator by withdrawal through valve-controlled line 59 and disproportionation of the ethyl benzene to ethyl xylene suppressed by recycling C10 disproportionation products to isomerization stage through line 52. However, it may be found desirable to remove a portion of these heavier products by way of valve-controlled line 6| while recycling both the Ca and heavier disproportionation products to isomerization.

The preferred isomerization system of Fig. 2 likewise will be illustrated and described in connection with the .lsomerization of xylenes. Xylene feed from header 21 is diluted with toluene introduced through valve-controlled line 29 and the mixture flows through heater 66 and mixer 61 to isomerization reactor 68. Hydrogen fluoride catalyst is supplied from storage and passes from valve-controlled inlet 69 through heater H and is blended with the hydrocarbon feed in mixer 61. v

A single phase homogeneous reaction mixture can be formed with hydrogen fluoride catalyst by mixing and blending under suitable conditions.

The preferred expedient for production of a single phase is maintenance of temperature above the point of miscibility of hydrocarbon with hydrogen fluoride while retaining suflicient pressure to retain the hydrogen fluoride catalyst in liquid phase. Desirably, relatively large proportions of hydrol2 gen fluoride catalyst are utilized, for example, at least 40% by volume of the entire reaction mixture and more desirably -80% by volume. Ratios as hishas ten parts anhydrous hydrogen i e to One Part of hydrocarbon in the reaction mixture may be utilized; and in general, solutions are preferred in which hydrogen fluoride is the major component and hydrocarbons fed to the reaction zone are minor components. The blended single phase solution formed in mixer 6'! is maintained as such throughout isomerization reaction zone 88.

Although the isomerization process is operative where agitation is effected in the reaction zone, the process is effected more efliciently and the reaction rate is increased by utilizing an agitated blending zone of relatively small volume in combination with a quiescent reaction zone, where homogeneous liquid phase conditions are maintained for a relatively long residence time as compared with the mixing or blending zone. Thus, in the preferred isomerization process hydrocarbonfeed and hydrogen fluoride catalyst are continuously passed through the relatively small mixer 61 into relatively large single phase liquid reaction zone 68, where a differential in hydrocarbon composition is maintained through the reaction zone from inlet to outlet by preventing substantlal dilution of reaction mixture at the inlet with reaction mixture or hydrocarbons from the outlet of the reaction zone. This feature of maintaining a differential in hydrocarbon com position despite the fact that only a single liquid phase reaction mixture is utilized yields several advantages and new results including: Reductron of loss from relatively slow side reactions and increase in the overall conversion rate in the reaction zone, and elimination of hazards attending operation of agitators in large volume of hydrogen fluoride reactor vessels inherent in possible failure of agitator bearings and sealing devices.

In reactor 68, the single phase hydrogen fluoride-hydrocarbon mixture is maintained at temperatures above 250 F. and under suflicient pressure to insure liquid phase operation. The rate of flow through the reactor is controlled so that turbulence is kept at a minimum and prevention of mixing of inlet feed with outlet is'omerizate is assured by baiiie plates 12.

In order to effect initial separation of hydrogen fluoride catalyst and hydrocarbon isomerizate, the reaction mixture is withdrawn through outlet conduit l3 and cooled by heat exchanger 14 to a temperature substantially below that of complete miscibility,-for example, F., whereupon a hydrocarbon phase saturated with hydrogen fluoride catalyst and a hydrogen fluoride catalyst phase saturated with hydrocarbon are formed.

This two-phase system is separated m settler l6 and the upper oil phase withdrawn to outlet TI and passed to the hydrocarbon recovery system. The hydrogen fluoride catalyst phase passes from the bottom of settler '16 through outlet 18 and is recycled through valve-controlled hydrogen fluoride return line 19 to heater H and mixer 61 where it is blended with incoming hydrocarbon feed as previously described.

The first stage of the recovery system for hydrocarbon isomerizate is a hydrogen fluoride stripper 8| in which hydrogen fluoride is fractionated and removed as overhead through outlet line 82, condenser 83 to reflux drum 84. A portion of the hydrogen fluoride overhead may be recycled to the stripping column 81 by means of valve-controlled reflux line 98 and the remainder passes by way of line 81 to catalyst recycle. Hydrocarbon isomerizate stripped of hydrogen fluoride flows from the bottom of stripper 8| by way of outlet line 88 to fractionator 89 where hydrocarbons boiling below the xylenes are removed as overhead. These lower boiling hydrocarbons, comprising principally toluene diluent with some benzene which may be formed during the isomerization reaction flow as overhead through outlet 9|, condenser 92 to condensate drum 93. A portion of the condensate is refluxed to column 89 by way of valve-controlled line 94, and the remainder is passed through valve-controlled line 95 and may be returned directly to feed through valve-controlled line 96 to serve as a reaction diluent or through valve-controlled line 91 to benzene still 98. In the latter instance the benzene and toluene are separated, benzene being removed as overhead to storage through the usual reflux system. Toluene is taken as fractionator bottoms and recycled via line 99 to xylene feed conduit 21.

Xylene and heavier hydrocarbon fractions flow from the bottom of .fractionating column 89 by way of conduit IM to fractionating column I02 for the distillation of xylene from higher boiling reaction products. The isomerized xylenes pass as overhead through outlet conduit I03, condenser I04 and reflux drum I05 to xylene isomer separation unit 4'! by way of isomerizate outlet line I08. A portion of the xylenes is returned as reflux from drum I05 through valvecontrolled line M1 to improved fractionation in column I02.

C9 and heavier alkyl aromatic hydrocarbons formed during the isomerization are withdrawn from the bottom of fractionator I02. These fractions may be recycled in total by way of valve-controlled line i08, or more desirably at least a portion thereof removed by way of valve-controlled line I09 for a separation of C9 and C10 alkyl aromatics and controlled recycle to xylene feed line 21 of the C9 and C10 fractions as previously described.

Water in the hydrogen fluoride catalyst decreases the catalyst activity and the mol fraction of hydrogen fluoride in the catalyst. It is therefore desirable to dehydrate all feed stocks entering the isomerization system. Even with the most eflicient dehydration methods, the hydrogen fluoride catalyst tends to pick up water from the hydrocarbon feed or other sources. Provision is made for maintaining hydrogen fluoride in the desired concentration 5 and for removal of water as an aqueous hydrogen fluoride solution by distillation. As here shown, valve-controlled line IIO serves for withdrawal of hydrogen fluoride catalyst from settler outlet product may flow through outlet header I2I to an ortho xylene recovery unit including fractionating columns I22 and I23, a meta xylene recovery unit and a para xylene recovery unit I28. The meta xylene recovery stage comprises selective sulionation and selective hydrolysis units I24, I28 and I21, and as here shown the para xylene is recovered in selective crystallization stage I28. The operation of this combination process can be adapted to treatment of any xylene mixture for the production or any one or more of the desired isomers in any relative proportions which may be specified. The types of operations may be classified in accordance with the feed stock available as follows: a

Case I.-Feed is mixture of ortho, meta and These feeds may be two or three xylene component mixtures. When the three xylenes are present, the two minor components may be present in an equilibrium ratio (with respect to each other) or in a ratio other than equilibrium.

In each of the foregoing major cases. it may be desired to operate the process to yield:

(a) Primarily ortho xylene and apportioned meta and para xylenes;

(b) Primarily meta xylene and apportioned ortho and para xylenes;

(c), Primarily para xylene and apportioned ortho and meta xylenes.

In the production of xylenes as in Case I, when the process of Fig. 3 is operated to yield primarily ortho xylene, the xylene feed admitted by way of inlet header I29 passes through valvecontrolled line I3I to fractionating column I22 of the ortho xylene recovery unit by way of fractionator inlet I32. Meta and para xylene are distilled overhead and ortho xylene flows as a bottoms fraction by way of line I33 to still I23.

Any C9 heavier hydrocarbons present are reline III. The withdrawn hydrogen fluoride passes to fractionating column III where substantially anhydrous hydrogen fluoride may be taken off as overhead through outlet conduit II2, cooler II3 to reflux drum H4. The purified hydrogen fluoride then passes by way of valve-controlled moved in this latter still and the ortho xylene P is passed by way of outlet I36 to storage.

The meta and para rich overhead from ortho xylene fractionating column I22 flows to hydrogen fluoride isomerization unit I20 by way of valve-controlled line I31. Hydrofluoric acid catalyst is admitted to the isomerization zone and recovered from the isomerized hydrocarbons as previously disclosed in connection with the specific description of Fig. 2. Hydrocarbon isomerizate passes through outlet I2I and is again recycled to ortho xylene recovery column I22. A selected portion of the meta-para rich xylene fraction from ortho xylene fractionator I22 is diverted by way of valve-controlled line I38 to meta and para recovery units.

As here shown, this diverted meta para rich fraction is apportioned between the para xylene and the meta xylene recovery units by way of valve-controlled lines I39 and HI respectively. Thus. a portion of flows from line I39 to selective crystallization unit I28 by way of inlet header I42, and a second portion flows from line I to selective sulionatlon unit I24 by way of inlet I43.

A para xylene fraction is removed by selective crystallization in unit I28 and passes to storage through outlet I44. The remaining mother liquor flows by way of recycle outlet I 46 to any of three destinations as desired. First, it may be returned to valve-controlled line I41 through ortho xylene recycle header I48 to inlet I32 of the ortho xylene recovery unit. Secondly, when the production of meta xylene is to be increased, crystallization mother liquor is passed through meta xylene recycle line I49 to inlet I43 of the meta xylene recovery system. Finally, crystallization mother liquor may flow by way of valvecontrolled conduit II through isomerization recycle line I52 to isomerization unit I20.

In the meta xylene recovery unit the metapara rich feed admitted by way of inlet I43 is selectively sulfonated in reactor I24 and the reaction mixture allowed to settle into an upper unsulfonated oil phase and a lower meta xylene sulfonic acid phase. The sulfonic acid phase passes through line I53 to selective hydrolysis unit I26 where entrained or loosely held ortho and para xylenes are initially distilled off. The meta xylene sulfonic acids are then selectively hydrolyzed and meta xylene removed by steam distillation. Meta xylene so recovered is passed through outlet line I54 to storage. Frequently xylene sulfonic acid impurities rich in para xylene remain in the acid layer and additional xylenes may be recovered by hydrolysis in final stage I21 to free a para-rich fraction. This para-rich fraction is removed through line I56 and may be blended with unsulfonated meta and para xylenes from selective sulfonation stage I24. These two xylene fractions are passed to header I 51 and may flow to any one of three destinations as desired. Thus, the para-rich xylene fraction from selective sulfonation may be returned from header I51 through valvecontrolled line I48 to ortho xylene recovery fractionator I22 by way of inlet I32. But more usually the ortho xylene content of the pararich fraction will not be sufficiently large to justify this procedure and the para-rich fraction more desirably flows from header I51 by way of recycle line I58 to inlet I42 of para xylene crystallization unit I28. However, if the demand for para xylene does not warrent this recovery, the para-rich fraction may alternately be passed through header I51 by way of valve-controlled line I59 and isomerization recycle line I52 to isomerization unit I20.

The foregoing description concerning the operation of the process of Fig. 3 has been with respect to Case I (a), that is, feeding an equilibrium mixture of xylenes to recover primarily otho xylene and apportioned meta and para xylenes. If para xylene recovery is the primary object, the feed may be passed directly from inlet I29 through valve-controlled conduit I6I to selective crystallization unit I28, para xylene crystals removed, and the mother liquor apportioned between isomerization recycle and the ortho and meta xylene recovery units respectiveiy. Likewise, if meta xylene is the primary product, the feed may flow directly to selective sull'onation unit I24 by way of valve-controlled line I62, and the unsulfonated oil layer apportioned between recycle to isomerization and ortho and para recovery units. Again, when the feed to the system contains more ortho xylene than in the equilibrium mixture (Case II) more 16 para xylene than in the equilibrium mixture (Case III), or more meta xylene than in the equilibrium mixture (Case IV), these respective feeds will be passed, first, to the isomerization unit unless the particular isomer desired as a primary product is the same as that contained in excess in the feed.' In the latter event the feed will pass first to the recovery unit for the isomer present in excess.

In order to guide those skilled in the art in the practice oi! the invention and to illustrate the directional efiect of various significant variables in the process, illustrative data are given in the following examples and in the graphs of Figs. 4 and 5. The term railinate as used throughout the specification and claims denotes the hydrocarbon phase separated from the hydrogen fluoride acid phase after removal from the isomerization reaction zone. This raflinate of the subject reaction product contains substantially the whole or the isomerization product and is exclusive of the acid phase which contains the hydrogen fluoride catalyst together with small amounts of catalyst complex.

EXAMPLES! Feed Railinate Toluene 50, 1 50, Ethyl benzene. 0.3 l. Ortho xyiene 0. 3 5. Meta xylene... 48. 8 33. Para xyiene 0. 5 9. Bot 0.0 0.

The xylene fraction recovered from the reaction mixture contains 10.8% ortho xylene, 70.4% meta xylene and 18.8% para xylene. Approximately of maximum para xylene was thus produced and only a very small percentage of the xylenes was disproportionated. The foregoing compositions are given in volume per cent.

EXAMPLE2 Isomerization was effected in a batch. reactor without stirring at a reaction temperature maintained within the range of from 300-332 F. and a contact time of seven minutes. Maximum pressure was 560 pounds gauge, and the weight ratio of xylene to anhydrous hydrogen fluoride catalyst was 0.3. Volume ratio of toluene diluent to xylene was 1:1. Following is a tabulation of approximate composition of feed and raflinate:

Feed Rafiinate Toluene 50, 1 4B, 6 Ethyl benzene 0. 3 0. 9 Ortho xylene 0.3 3. 5 Meta xylene..- 48. 8 39. 6 Pam xylene 0. 5 7. l Bottoms 0. 0 0. 3

The composition of the xylene fraction recovered from the reaction mixture contains 7.0%

' ortho xylene, 78.8% meta xylene and 14.2% para Feed Prod.

Food Prod.

rized in a continuous procr reactor tor the provision unae'itated reaction zone. give exemplary data. under tions PM Prod.

xylenee were home TABLE I ma Prod.

ess utilizing a. tubule. of a, quiescent or Tables I, II, and III- various reaction condi TABLE II pure ery small per- Isomerization of :rylen-cs by HI -tubular reactor fii'i''oa'II proportionslsomerization of xylene: by HF-tubular reactor or the xylenes was disproportionated.

Gen

l eneion,

y ylen n toms.

of Max. P Xylenes Dispropor e Temperature, F Contact Tim Vol. Per Diluent- Vol. Per Ce sfllnete" Pha HF Extract:

Grams.-..- Bromine N Meta X Para X Bot Xylene Frect Ortho. Meta- Para Per Cent Per Cent tion, k.

Non.-Fizuree.appearlng as subscripts in the above table are observed analytical values of doubtful accuracy Approximately 68% 1-0! maximum Example No.--.--

Operating Conditions:

Compositions, Vol. Per Cent:

Specific Rate 0 Specific Rate 0 The foregoing eom-poations are given in volume xylene. r v xylene was produced and only a v centage Per cent.

.Flgures appearing as subscripts in the above table are observed analytical values of doubtful accuracy.

Non:

- toms boiling above TABLE 111 lsomerizution of xylene: by til-tubular reactor Enmple No Operating Conditions:

Temperature, F

, Min

Vol. Per Cent HF in Beactor... Water A dded, Wt. Per Cent oi HF Cet- 2. None 2. lb None 2. 0| None None Gui-Bottoms Toluene 6. 0-47. 6

ram Bromine No..

Prod.

Feed

mo Feed Com tions, Vol. Per Cent:

Meta Xy ene 97. l

Para Xylene.

Bottom: 0.3. Xylene Fraction, Vol. Per Cent:

Ortbo. Mata.

Para

. P re a r Gama Specific Rate Constant, m-ip, k1 Specific Rate Constant, Disproportiouation, I:- .00096 1 The Anhydrous" HF used in these exgeriments contained 034.4% H10.

Nora.--Figures appearing as subscripts In Table I, Examples 3, 4 and 5 illustrate the benefits of successive increases in the relative proportion of hydrogen fluoride catalyst, and Example '7 in Table II reveals that formation of botthe xylene is reduced approximately two-thirds by toluene as a diluent. Examples 8 and 9 establish that reduction of catalyst content from 55 to 40% greatly reduces the specific reaction rate. Analogous benefits from increase in proportion of hydrogen fluoride catalyst are obtained at higher temperatures as illustrated by Examples 10 and 11. The eflect of small amounts of water in the hydrogen fluoride catalyst are revealed in Table III, Examples 12-15 inclusive. Note that in Example 15, decrease in catalyst activity due to the presence of water is largely mitigated by a mild increase in temperature. Suppression of the formation of disproportionation products boiling above the xylenes is illustrated in Example 16, wherein the isomerization was efiected in the presence of 5% of Co and higher boiling bottoms fraction as well as in the presence of toluene. The data show that disproportionation in this example was low, since the bottoms fraction increased only by .64% despite the fact that the temperature was sufllciently high to increase relative disproportionation rates.

Fig. 4 is a graph of specific rate constants (in) of isomerization versus mol fraction of hydrogen fluoride for reactions carried out at three different temperatures. Fig. 5 is a corresponding plot oi specific rate constants (kn) for disproportionation versus mol fraction of hydrogen fluoride at the same three temperatures. These graphs are for xylene isomerization and disproportionation respectively. It will be noted that the specific rate constant for isomerization is in the order of ten iold greater than the specific rate constant for disproportionation in the process of this invention and that the isomerization reaction rate increases greatly with mol traction of hydrogen fluoride catalyst.

the above table are observed analytical values of doubtful accuracy.

The applicability of hydrogen fluoride as a catalyst for isomerization of dialkyl benzenes other than dimethyl benzene is illustrated by the data in Table IV:

TABLE IV Isomerization of para-cymene with HF Example No i7 512 (33.3 vol.

toluene) Operating Conditions:

Temp., F Contact time, Min-" Pressure, p. s. i. g-

lea-203 10 146 Product Product Water washed product,

Vol. r cent:

To uene Ortho Cymene Meta Cymene Para 0 mane Materialba ce, Wt. per

cent:

Per cent Hydrocarbon in Raflinate Pha e Per cent H drocarboninAci Phase..

Attention is directed to the fact that this isomerization is effected under milder conditions with the propyl group of cymene than is required for the methyl groups of the xylenes.

Although hydrogen fluoride alone is preierred as a catalyst, at present, it is to be understood that activators or modifiers of the hydrogen fluoride are not outside the broader aspects of the invention. For example, temperatures required for isomerization may be reduced below 250 F. by addition of an activator, such as BF: in small amounts. to the hydrogen fluoride catalyst.

It is readily apparent from the foregoing description that various modifications of the process and apparatus can be made within the spirit of the present invention and the scope of the amended claims. In the drawings various details. such as pumps, valves, pressuring means, coolers and heat exchangers have in some instances been omitted. Any suitable form of apparatus incorporating these necessary details as well as utilizing appropriate corrosion-resistant metals and alloys can be supplied in obvious manner by those skilled in the art.

I claim: v

1. A continuous process for isomerizing a xylene comprising the steps of blending a xylene charging stock and at least by volume of toluene with anhydrous liquid hydrogen fluoride, at a temperature of 250 to 400 F., the proportion of said hydrogen fluoride being equal to 40% to 90% of the total liquid mixture thus forming a single liquid phase blend, thereafter immediately passing said blend through a reaction zone in nonturbulent flow, whereby the mixture downstream is substantially unmixed with the upstream reaction mixture, maintaining the temperature at about 250 to 400 F. in said reaction zone, cooling the reaction product mixture eflluent from the reaction zone to separate a hydrocarbon phase and an acid phase. separating the hydrocarbon phase from the acid phase, fractionally distilling the hydrocarbon phase to separate a toluene fraction, a xylene fraction, and a higher boiling fraction comprising side reaction products and returning at least a portion of the toluene fraction and 01' the higher boiling fraction to the blending step together with further quantities of liquid xylenes and hydrofluoric acid.

2. The process as defined in claim 1 wherein the temperature in the reaction zone is maintained in the range about 310 to 350 F.

3. The process as defined in claim 1 wherein the proportion 01' hydrogen fluoride employed is equal to to per cent by volume of the total liquid mixture.

4. The process as defined in claim 1 wherein the temperature in the reaction zone is maintained in the range about 310 to 350 F. and the proportion of hydrogen fluoride employed is equal to 60 to 85 per cent by volume of the total liquid mixture.

JACOB D. KEMP.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,282,231 Mattox May 5, 1942 2,396,965 Passino Mar. 19, 1946 2,416,184 Lee et al Feb. 18, 1947 2,425,559 Passino et a1 Aug. 12, 1947

Claims (1)

1. A CONTINUOUS PROCESS FOR ISOMERIZING A XYLENE COMPRISING THE STEPS OF BLENDING A XYLENE CHARGING STOCK AND AT LEAST 10% BY VOLUME OF TOLUENE WITH ANHYDROUS LIQUID HYDROGEN FLUORIDE, AT A TEMPERATURE OF 250* TO 400*F., THE PROPORTION OF SAID HYDROGEN FLUORIDE BEING EQUAL TO 40% TO 90% OF THE TOTAL LIQUID MIXTURE THUS FORMING A SINGLE LIQUID PHASE BLEND, THEREAFTER IMMEDIATELY PASSING SAID BLEN THROUGH A REACTION ZONE IN NONTURBULENT FLOW, WHEREBY THE MIXTURE DOWNSTREAM IS SUBSTANTIALLY UNMIXED WITH THE UPSTREAM REACTION MIXTURE, MAINTAINING THE TEMPERATURE AT ABOUT 250* TO 400*F. IN SAID REACTION ZONE, COOL-

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