Manufacture of aromatic compounds
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of copending application Ser. No. 914,645, filed June 12, 1978. 
    
    
     FIELD OF THE INVENTION 
     The invention is concerned with manufacture of aromatic compounds having six (C 6  Ar) to eight (C 8  Ar) carbon atoms, namely benzene, toluene and xylenes (BTX). At the present time, the most valuable of these is p-xylene, which may be separated for use in synthesis of polyesters from mixed xylenes by fractional crystallization. Also highly valued is benzene for use is chemical raw material. Toluene is also valuable for varied uses as solvent, in chemical manufacture and as a high octane gasoline component. 
     According to the present invention, available raw materials such as reformed petroleum naphthas (reformate) are processed to yield p-xylene as the principal product and to produce toluene and benzene as substantially the sole aromatic by-products. 
     BACKGROUND OF THE INVENTION 
     Typically, p-xylene is derived from mixtures of C 8  aromatics separated from such raw materials as petroleum naphthas, particularly reformates, usually by selective solvent extraction. The C 8  aromatics in such mixtures and their properties are: 
     
         ______________________________________                                    
        Freezing  Boiling    Density Lbs./                                
        Point °F.                                                  
                  Point °F.                                        
                             U.S. Gal.                                    
______________________________________                                    
Ethylbenzene                                                              
          -139.0      277.0      7.26                                     
P-xylene    55.9      281.0      7.21                                     
M-xylene  -54.2       282.4      7.23                                     
O-xylene  -13.3       292.0      7.37                                     
______________________________________                                    
 
    
     Principal sources are catalytically reformed nahthas and pyrolysis distillates. The C 8  aromatic fractions from these sources vary quite widely in composition but will usually be in the range 10 to 32 wt. % ethylbenzene with the balance, xylenes, being divided approximately 50 wt. % meta, and 25 wt. % each of para and ortho. 
     Individual isomer products may be separated from the naturally occurring mixtures by appropriate physical methods. Ethylbenzene may be separated by fractional distillation although this is a costly operation. Ortho xylene may be separated by fractional distillation and is so produced commercially. Para xylene is separated from the mixed isomers by fractional crystallization. 
     As commercial use of para and ortho xylene has increased there has been interest in isomerizing the other C 8  aromatics toward an equilibrium mix and thus increasing yields of the desired xylenes, as by OCTAFINING. 
     In a typical plant for utilization of Octafining, a mixture of C 8  aromatics is introduced to an ethylbenzene tower wherein the stream is stripped of a portion of its ethylbenzene content, to an extent consistent with retaining all the xylenes in the feed stream without unduly expensive &#34;superfractionation&#34;. Ethylbenzene is taken overhead while a bottom stream, consisting principally of xylenes, together with a significant amount of ethylbenzene, passes to a xylene splitter column. The bottoms from the xylene splitter constituted by o-xylene and C 9  aromatics passes to the o-xylene tower from which o-xylene is taken overhead and heavy ends are removed. The overhead from the xylene splitter column is transferred to conventional crystallization separation. The crystallizer operates in the manner described in Machell et al., U.S. Pat. No. 3,662,013 dated May 9, 1972. 
     Because it&#39;s melting point is much higher than that of the other C 8  aromatics, p-xylene is readily separated in the crystallizer after refrigeration of the stream and a xylene mixture lean in p-xylene is transferred to an isomerization unit. The isomerization charge passes through a heater, is admixed with hydrogen and the mixture is introduced to the isomerizer. 
     Isomerized product from the isomerizer is cooled and passed to a high pressure separator from which separated hydrogen can by recycled in the process. The liquid product of the isomerization passes to a stripper from which light ends are passed overhead. The remaining liquid product constituted by C 8  +hydrocarbons is recycled in the system to the inlet of the xylene splitter. 
     It will be seen that the system is adapted to produce quantities of p-xylene from a mixed C 8  aromatic feed containing all of the xylene isomers plus ethylbenzene. The key to efficient operation for that purpose is in the isomerizer which takes crystallizer effluent lean in p-xylene and converts the other xylene isomers in part to p-xylene for further recovery at the crystallizer. 
     Among the xylene isomerization processes available in the art, Octafining was originally unique in its ability to convert ethylbenzene. Other xylene isomerization processes have required extremely expensive fractionation to separate that component of C 8  aromatic fractions. As will be seen from the table of properties above, the boiling point of ethylbenzene is very close to those of p and m-xylene. Complete removal of ethylbenzene from the charge is impractical. The usual expedient for coping with the problem was an ethylbenzene separation column in the isomerizer-separator loop when using catalyst other than those characteristic of Octafining. It will be seen that Octafining does not need this expensive auxiliary to prevent build up of ethylbenzene in the loop. This advantageous feature is possible because the Octafining catalyst converts ethylbenzene. 
     In Octafining, ethylbenzene reacts through ethyl cyclohexane to dimethyl cyclohexanes which in turn equilibrate to xylenes. Competing reactions are disporportionation of ethylbenzene to benzene and diethylbenzene, hydrocracking of ethylbenzene to ethane and benzene and hydrocracking of alkyl cyclohexanes. 
     A significant improvement arose with the introduction of catalysts such as zeolite ZSM-5 combined with a metal such as platinum as described in Morrison U.S. Pat. No. 3,856,872. At temperatures around 700°-800° F., ethylbenzene is converted by disporportionation over this catalyst to benzene and diethylbenzene. At higher temperatures and using a zeolite ZSM-5 catalyst of reduced activity, ethylbenzene and other single ring aromatics are converted by splitting off side chains of two or more carbon atoms as described in copending application Ser. No. 914,645, filed June 12, 1978. 
     These developments permit upgrading of Octafining reactors by substitution of the improved (ZSM-5) catalyst. 
     Another xylene isomerization system which has achieved widespread commercial use in low pressure operation in vapor phase. Temperatures employed are in the same range as for Octafining, in the neighborhood of 850° F. Pressures are only that required to equal pressure drop through the downstream recovery towers, heat exchanges and the like. For all practical purposes, this is an atmospheric pressure reaction with reactor inlet pressure of about 30 pounds per square inch, gauge. The catalyst is essentially silica-alumina, the acid amorpohous heterogeneous catalyst employed in a number of such acid catalyzed processes. Several advantages for that type of isomerization will be immediately apparent. 
     The unit cost of catalyst is drastically reduced by omission of platinum. At these low pressures, the reactor vessels are made of inexpensive steel and need no structural provision for resisting pressure stress. The process is practical without introduction of molecular hydrogen and needs no auxiliaries for manufacture and recycle of that gas. These features greatly reduce capital and operating costs and have made the low pressure process essentially competitive with Octafining despite the requirement for large vessels at low pressure and low space velocity and the operating disadvantages inherent in the process. 
     A primary drawback of low pressure vapor phase isomerization as practiced heretofore is its low tolerance for ethylbenzene in the charge. The catalyst will convert ethylbenzene only at high severities such that unacceptable loss of xylene occurs by disproportionation. 
     Low pressure isomerization as practiced heretofore accepts a further disadvantage in that the catalyst rapidly declines in activity due to deposition of &#34;coke&#34;, a carbonaceous layer masking the active sites of the porous silica-alumina catalyst presently conventional in this operation. The coke can be removed by burning with air to regenerate the activity of the catalyst. Continuity of operation is achieved by the well-known &#34;swing reactor&#34; technique employing two or more reactors, one of which is on stream while burning regeneration is conducted on a reactor containing spent catalyst which has lost activity by coke deposition. Cycles of two to four days are common practice using one reactor on stream for that period and then shifting to a freshly regenerated vessel. 
     Present commercial practice involves many large plants of both the Octafining and low pressure types in a loop of p-xylene separation and recycle of other isomers, together with such quantity of ethylbenzene as may be present, through isomerization and back to p-xylene recovery. The commercial options presently in use are high pressure isomerization with large quantities of hydrogen or low pressure (essentially atmospheric) isomerization with complicated cycling of a swing reactor and necessity for expensive distillation to remove ethylbenzene from the charge to some acceptable level, usually about 5%. 
     Substitution of zeolite ZSM-5 for the silica-alumina catalyst of low pressure isomerization results in an advantageous shift in the relative rates of disproportionation of ethylbenzene and xylenes. The so revised process as described in U.S. Pat. No. 4,101,596 is operated at lower temperature than conventional low pressure isomerization over silica-alumina. At these conditions, long on-stream periods are observed even at high levels of ethylbenzene in the charge permitting operation of the ethylbenzene tower at moderate, and less expensive, conditions to remove only a portion of the ethylbenzene in the charge. The so improved low pressure with zeolite ZSM-5 requires modification of the conventional low pressure plant to accommodate the lower temperature of operation. 
     A further alternative heretofore described is isomerization in liquid phase at a pressure adequate to maintain that phase. Highly active zeolite catalysts are effective under these conditions and demonstrate long cycle life, possibly because precursors of coke are dissolved by the reactant liquid and flushed from the reactor before deterioration to coke. See, for example, Wise, U.S. Pat. No. 3,377,400; Bowes et al., U.S. Pat. No. 3,578,723; and Haag et al., U.S. Pat. No. 3,856,871. 
     It is further known that zeolite ZSM-5 is a very effective catalyst for isomerization of xylenes. See Argauer et al., U.S. Pat. No. 3,790,471; Burress, U.S. Pat. No. 3,856,873; Morrison, U.S. Pat. No. 3,856,872; and Haag et al., supra. It should be noted that Burress describes a wide range of operating conditions and demonstrates effectiveness of the catalyst at (1) low temperature, high pressure and (2) high temperature, low pressure operation over zeolite ZSM-5. See also Bonacci et al. U.S. Pat. No. 3,957,621. On this state of the art, zeolite ZSM-5 can be expected to function effectively in low pressure, vapor phase isomerization, and indeed it does. That zeolite and the related zeolites are defined hereinafter by silica/alumina ratio, constraint index and crystal density. 
     SUMMARY OF THE INVENTION 
     The known low pressure isomerization process is effectively upgraded while operating in the design temperature range by using as catalyst a zeolite in the nature of ZSM-5 substantially reduced in activity as by dilution, steaming, very high silica/alumina ratio, base exchange with alkali metal, coking or the like. At these high temperatures of 800°-1000° F., the zeolite of reduced activity exhibits effective power for isomerization of xylene and for splitting off alkyl side chains of two or more carbon atoms from single ring aromatics at long on-stream periods. The disproportionation activity of the zeolite is severely depressed by the reduced acid activity, resulting in low losses of xylene by that mechanism. That lack of disproportionation activity impairs the capacity of the catalyst to handle aromatics of nine or more carbon atoms, e.g. trimethylbenzene, as practiced in some processes. It thus becomes necessary to remove from the recycle stream those components having more than eight carbon atoms to avoid excessive build-up in the system of C 9  and higher. The catalyst also has the capacity to crack paraffins in the charge to lower boiling compounds readily removable from recycle streams by fractionator normally present in the p-xylene recovery/isomerizer loop. 
     By reason of this combination of activities, the catalyst may be installed in conventional low pressure isomerization equipment with elimination and/or reduced service of some auxiliary equipment normally used in preparation of charge stock. For example, the normal design charge stock is a C 8  aromatic fraction from which much of the ethylbenzene has been removed by precision fractionation. Thus a petroleum derived reformate would be distilled to remove components of lower boiling point than C 8  aromatics, again distilled for removal of C 9  and higher. The resultant C 8  fraction would then be extracted with solvent to separate aromatics from paraffins and the C 8  aromatics then again distilled by the expensive technique necessary to separation of at least a large part of the ethylbenzene. 
     The manner in which some of this equipment may be retired from service is indicated in the annexed drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The single FIGURE of the drawing is a diagrammatic representation of the principal elements in a conventional plant for preparation of p-xylene by the low pressure isomerization technique. Processing equipment which may be retired from service or reduced in service load for operation according to the invention are indicated by enclosure in broken lines, marked &#34;optional&#34;. 
    
    
     DESCRIPTION OF SPECIFIC EMBODIMENTS 
     The essence of the invention is the versatile and efficient isomerization reactor which can also serve for selective dealkylation of single ring aromatic compounds to remove alkyl side chains of two or more carbon atoms. That reactor is characterized by a zeolite catalyst having a silica/alumina ratio of at least 12, a constraint index as hereinafter defined between about 1 and about 12 and a reduced acid activity also defined below. The reactor is maintained at a temperature of 800° to 1000° F. and a pressure below about 100 pounds per square inch, guage (psig), preferably only sufficient pressure to overcome pressure drop through downstream processing equipment, say below 50 psig. In some embodiments, discharge at the end of the processing train may be held below atmospheric by connection to suction of a suitable pump, thus maintaining the reactor at or below one atmosphere. It has been shown that reduced pressure results in lower xylene loss and improved catalyst properties when employing silica-alumina amorphous catalyst. 
     As shown in the annexed drawing, the reactor of this invention may be incorporated in a reaction train through which a petroleum naphtha is processed for manufacture of p-xylene. Alternatively, the process of the invention may be used in a facility charging purchased C 8  aromatics or mixed xylenes. In the embodiment here illustrated, a light petroleum naphtha supplied by line 10 is charged to a reformer 11 operated to convert naphthenes to aromatics by dehydrogenation under hydrogen pressure over a catalyst of platinum supported on alumina. 
     The aromatic rich reformate product of reformer 11 is transferred by line 12 to fractionator 13 operated to take compounds of seven or less carbon atoms overhead, leaving a bottom fraction of eight or more carbon atoms which may be transferred by line 14 to fractionator 15. Alternatively, the bottoms of column 13 may pass directly to the solvent extraction stage or to the low pressure reactor, both presently to be described. Assuming utilization of fractionator 15, a bottoms fraction at line 16 is constituted by hydrocarbons of nine or more carbon atoms, rich in aromatics. The fraction in line 16 may be used as a component of high octane motor fuel or processed to convert its aromatic content to a mixture of benzene, toluene and xylenes (BTX). 
     The eight carbon atoms overhead of fractionator 15 is shown transferred by line 17 to a solvent extraction stage 18 wherein paraffins are separated by use of a suitable solvent in known manner. Alternatively, the C 8  overhead of fractionator 15 may be passed directly to the low pressure reactor. The C 8  aromatic fraction prepared by the solvent extraction stage 18 is preferably transferred by line 19 to a stripper 20 for removal of ethylbenzene. The degree of ethylbenzene removal, if practiced, may be accommodated to the desires of the operator, since the low pressure reactor of this invention can tolerate considerable amounts of ethylbenzene, which is thereby converted to benzene. 
     In any event, fresh feed to the isomerizer loop is constituted by bottoms of stripper 20 supplied through line 21, by bottoms from fractionator 13, by overhead of fractionator 15 or by the C 8  aromatic fraction from solvent extraction stage 18. Each of these feeds will contain the xylenes generated in reformer 11 in association with more or less paraffins and/or aromatics boiling in the xylene range or higher. If the feed is taken from ethylbenzene stripper 20 bottoms, it is supplied directly to a p-xylene separator 22 mixed with a recycle stream presently to be described. In separator 22, p-xylene is separated from the mixture by any of the known techniques. The residue from separator 22 passes by line 23 to low pressure reactor 24 containing catalyst consisting essentially of low activity zeolite having a silica/alumina ratio greater than 12 and a constraint index of 1-12. 
     Under the low pressure, high temperature conditions prevailing in reactor 24, that catalyst has capacity (1) isomerize xylenes thereby restoring the mixed xylenes of the feed from separator 22 to generate addition p-xylene, (2) to remove alkyl chains of two or more carbon atoms from single ring aromatics, leaving methyl groups to thereby generate BTX, and (3) to crack paraffins to lower boiling compounds which can be removed in the downstream fractionators. The invention therefor contemplates adding to the feed for reactor 24 streams which contain paraffins and/or aromatics of more than eight carbon atoms. Such streams are provided by bottoms of fractionator 13 or overhead of fractionator 15 which may be solvent extracted, if desired, by equipment like solvent extraction stage 18. Similarly, EB tower 20 may be dispensed with. One such variant is illustrated in the drawing by broken line as feed pipe 25 to transfer tower 13 bottoms to the feed for reactor 24. 
     The effluent of reactor 24 in line 26 contains the three xylenes in proportions approaching the thermodynamic equilibrium value together with conversion products from reaction of ethylbenzene and higher boiling alkyl aromatics as well as a portion of unreacted ethylbenzene and higher. The by-products will include benzene, toluene and xylenes derived by reactions of such compounds as ethylbenzene, methylethylbenzene, dimethylethylbenzene, etc. The reaction mixture is fractionated in column 27 to take compounds of five carbon atoms and less overhead, benzene and toluene as a side stream and bottoms substantially constituted by aromatics of eight or more carbon atoms. That bottoms fraction passes by line 28 to fractionator 29 to remove aromatics of nine or more carbon atoms as bottoms. The overhead fraction of eight carbon atom aromatics is transferred by line 30 as feed to separator 22. 
     The heavy aromatics in the bottoms of column 29 are not advantageously recycled to the reactor as in some other processes. The lack of transalkylation activity in the catalyst of this invention renders it incapable of generating substantial amounts of xylenes by reaction of, e.g. trimethylbenzene. Column 29 bottoms may be subjected to reaction with toluene in a separate heavy aromatics reactor to produce xylenes and the reaction product added to the loop at line 26 as described in application Ser. No. 945,279, filed Sept. 25, 1978 or may be diverted to motor fuel blending. 
     The reactor 24 contains a crystalline alumino-silicate (zeolite) catalyst of relatively low acid activity. That catalyst promotes a reaction course which is unique at temperatures upwards of 800° F. Ethylbenzene in the charge is selectively cracked to benzene at little or no conversion of xylenes. Two or more carbon atom chains on other aromatics undergo like conversion. The two types of conversion are decoupled such that, for the first time, reaction severity is not a compromise to achieve effective ethyl aromatic conversion at &#34;acceptable&#34; loss of xylene. This characteristic of the process renders unnecessary the preliminary distillation to separate at least some of the ethyl benzene and C 9  + aromatics from the feed stream as practiced in prior processes. It has been further found that the present process has capability to convert paraffin hydrocarbons. This makes it possible to dispense with the expensive extraction step conventionally applied to a fraction of catalytically reformed naphthas in the manufacture and recovery of xylenes. In taking advantage of this feature, the feed stream at line 25 will contain the C 8  + aromatics of a reformate or the like together with the paraffins of like boiling range, nonanes and heavier. The paraffins in the charge are hydrocracked to lighter paraffins which will come off column 27 in much greater quantity than that resulting from conversion of ethylbenzene. 
     Particularly preferred catalysts for reactor 24 are those zeolites having a constraint index within the approximate range of 1 to 12. Zeolites characterized by such constraint indices induce profound transformations of aliphatic hydrocarbons to aromatic hydrocarbons in commercially desirable yields and are generally highly effective in conversion reactions involving aromatic hydrocarbons. These zeolites retain a degree of crystallinity for long periods in spite of the presence of steam at high temperature which induces irreversible collapse of the framework of other zeolites, e.g. of the X and A type. Furthermore, carbonaceous deposits when formed, may be removed by burning at higher than usual temperatures to restore activity. In many environments the zeolites of this class exhibit very low coke forming capability, conducive to very long times on stream between burning regenerations. 
     An important characteristic of the crystal structure of this class of zeolites is that it provides constrained access to, and egress from the intracrystalline free space by virtue of having a pore dimension greater than about 5 Angstroms and pore windows of about a size such as would be provided by 10-membered rings of oxygen atoms. It is to be understood, of course, that these rings are those formed by the regular disposition of the tetrahedra making up the anionic framework of the crystalline aluminosilicate, the oxygen atoms themselves being bonded to the silicon or aluminum atoms at the centers of the tetrahedra. Briefly, the preferred type zeolites useful in this invention possess, in combination, a silica to alumina mole ratio of at least about 12; and a structure providing constrained access to the crystalline free space. 
     In one embodiment, the desired low activity is achieved by unusually high silica/alumina ratio, greater than 200, preferably above 500. 
     The silica to alumina ratio referred to may be determined by conventional analysis. This ratio is meant to represent, as closely as possible, the ratio in the rigid anionic framework of the zeolite crystal and to exclude aluminum in the binder or in cationic or other form within the channels. Such zeolites, after activation, acquire an intracrystalline sorption capacity for normal hexane which is greater than that for water, i.e. they exhibit &#34;hydrophobic&#34; properties. It is believed that this hydrophobic character is advantageous in the present invention. 
     The type zeolites useful in this invention freely sorb normal hexane and have a pore dimension greater than about 5 Angstroms. In addition, the structure must provide constrained access to larger molecules. It is sometimes possible to judge from a known crystal structure whether such constrained access exists. For example, if the only pore windows in a crystal are formed by 8-membered rings of oxygen atoms, then access by molecules of larger cross-section than normal hexane is excluded and the zeolite is not of the desired type. Windows of 10-membered rings are preferred, although, in some instances, excessive puckering or pore blockage may render these zeolites ineffective. Twelve-membered rings do not generally appear to offer sufficient constraint to produce the advantageous conversions, although puckered structures exist such as TMA offretite which is a known effective zeolite. Also, structures can be conceived, due to pore blockage or other cause, that may be operative. 
     Rather than attempt to judge from crystal structure whether or not a zeolite possesses the necessary constrained access, a simple determination of the &#34;constraint index&#34; may be made by passing continuously a mixture of an equal weight of normal hexane and 3-methylpentane over a sample of zeolite at atmospheric pressure according to the following procedure. A sample of the zeolite, in the form of pellets or extrudate, is crushed to a particle size about that of coarse sand and mounted in a glass tube. Prior to testing, the zeolite is treated with a stream of air at 1000° F. for at least 15 minutes. The zeolite is then flushed with helium and the temperature adjusted between 550° F. and 950° F. to give an overall conversion between 10% and 60%. The mixture of hydrocarbons is passed at 1 liquid hourly space velocity (i.e., 1 volume of liquid hydrocarbon per volume of zeolite per hour) over the zeolite with a helium dilution to give a helium to total hydrocarbon mole ratio of 4:1. After  20 minutes on stream, a sample of the effluent is taken and analyzed, most conveniently by gas chromotography, to determine the fraction remaining unchanged for each of the two hydrocarbons. 
     The &#34;constraint index&#34; is calculated as follows: ##EQU1## 
     The constraint index approximates the ratio of the cracking rate constants for the two hydrocarbons. Zeolites suitable for the present invention are those having a constraint index in the approximate range of 1 to 12. Constraint Index (CI) values for some typical zeolites are: 
     
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CAS                 C.I.                                                  
______________________________________                                    
ZSM-5               8.3                                                   
ZSM-11              8.7                                                   
ZSM-12              2                                                     
ZSM-38              2                                                     
ZSM-35              4.5                                                   
TMA Offretite       3.7                                                   
Beta                0.6                                                   
ZSM-4               0.5                                                   
H-Zeolon            0.4                                                   
REY                 0.4                                                   
Amporphous Silica-Alumina                                                 
                    0.6                                                   
Erionite            38                                                    
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     It is to be realized that the above constraint index values typically characterize the specified zeolites but that such are the cumulative result of several variables used in determination and calculation thereof. Thus, for a given zeolite depending on the temperatures employed within the aforenoted range of 550° F. to 950° F., with accompanying conversion between 10% and 60%, the constraint index may vary within the indicated approximate range of 1 to 12. Likewise, other variables such as the crystal size of the zeolite, the presence of possible occluded contaminants and binders intimately combined with the zeolite may affect the constraint index. It will accordingly be understood by those skilled in the art that the constraint index, as utilized herein, while affording a highly useful means for characterizing the zeolites of interest is approximate, taking into consideration the manner of its determination, with probability, in some instances, of compounding variables extremes. 
     While the above experimental procedure will enable one to achieve the desired overall conversion of 10 to 60% for most catalyst samples and represents preferred conditions, it may occasionally be necessary to use somewhat more severe conditions for samples of very low activity, such as those having a very high silica to alumina ratio. In those instances, a temperature of up to about 1000° F. and a liquid hourly space velocity of less than one, such as 0.1 or less, can be employed in order to achieve a minimum total conversion of about 10%. 
     The class of zeolites defined herein is exemplified by ZSM-5, ZSM-11, ZSM-12, ZSM-35, ZSM-38 and other similar materials. U.S. Pat. No. 3,702,886 describing and claiming ZSM-5 is incorporated herein by reference. 
     ZSM-11 is more particularly described in U.S. Pat. No. 3,709,979, the entire contents of which are incorporated herein by reference. 
     ZSM-12 is more particularly described in U.S. Pat. No. 3,832,449, the entire contents of which are incorporated herein by reference. 
     ZSM-35 is more particularly described in U.S. Pat. No. 4,016,245, the entire contents of which are incorporated herein by reference. 
     ZSM-38 is more particularly described in U.S. Pat. No. 4,046,859, the entire contents of which are incorporated herein by reference. 
     The specific zeolites described, when prepared in the presence of organic cations, are catalytically inactive, possibly because the intracrystalline free space is occupied by organic cations from the forming solution. They may be activated by heating in an inert atmosphere at 1000° F. for one hour, for example, followed by base exchange with ammonium salts followed by calcination at 1000° F. in air. The presence of oganic cations in the forming solution may not be absolutely essential to the formation of this type zeolite; however, the presence of these cations does appear to favor the formation of this special type of zeolite. More generally it is desirable to activate this type catalyst by base exchange with ammonium salts followed by calcination in air at about 1000° F. for from about 15 minutes to about 24 hours. 
     Natural zeolites may sometimes be coverted to this type zeolite catalyst by various activation procedures and other treatments such as base exchange, steaming, alumina extraction and calcination, in combinations. Natural minerals which may be so treated include ferrierite, brewsterite, stilbite, dachiardite, epistilbite, heulandite, and clinoptilolite. The preferred crystalline alumino-silicate are ZSM-5, ZSM-11, ZSM-12, ZSM-35, and ZSM-38, with ZSM-5 or its metal containing variant particularly preferred. 
     In a preferred aspect of this invention, the zeolites herreof are selected as those having a crystal framework density, in the dry hydrogen form, of not substantially below about 1.6 grams per cubic centimeter. It has been found that zeolites which satisfy all three of these criteria are most desired. Therefore, the preferred zeolites of this invention are those having a constraint index as defined above of about 1 to about 12, a silica to alumina ratio of at least about 500 and a dried crystal density of not less than about 1.6 grams per cubic centimeter. The dry density for known structures may be calculated from the number of silicon plus aluminum atoms per 100 cubic Angstroms, as given, e.g. on page 19 of the article on Zeolite Structure by W. M. Meier. This paper, the entire contents of which are incorporated herein by reference, is included in &#34;Proceedings of the Conference on Molecular Sieves, Longon, April 1967&#34;, published by the Society of Chemical Industry, London, 1968. When the crystal structure is unknown, the crystal framework density may be determined by classical pykometer techniques. For example, it may be determined by immersing the dry hydrogen form of the zeolite in an organic solvent which is not sorbed by the crystal. It is possible that the unusual sustained activity and stability of this class of zeolites is associated with its high crystal anionic framework density of not less than about 1.6 grams per cubic centimeter. This high density, of course, must be associated with a relatively small amount of free space within the crystal, which might be expected to result in more stable structures. This free space, however, is important as the locus of catalytic activity. 
     Crystal framework densities of some typical zeolites are: 
     
         ______________________________________                                    
             Void          Framework                                      
Zeolite      Volume        Density                                        
______________________________________                                    
Ferrierite   0.28 cc/cc    1.76 g/cc                                      
Mordenite    .28           1.7                                            
ZSM-5, -11   .29           1.79                                           
Dachiardite  .32           1.72                                           
L            .32           1.61                                           
Clinoptilolite                                                            
             .34           1.71                                           
Laumontite   .34           1.77                                           
ZSM-4 (Omega)                                                             
             .38           1.65                                           
Heulandite   .39           1.69                                           
P            .41           1.57                                           
Offretite    .40           1.55                                           
Levynite     .40           1.54                                           
Erionite     .35           1.51                                           
Gmelinite    .44           1.46                                           
Chabazite    .47           1.45                                           
A            .5            1.3                                            
Y            .48           1.27                                           
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     When synthesized in the alkali metal form, the zeolite is conveniently converted to the hydrogen form, generally by intermediate formation of the ammonium form as a result of ammonium ion exchange and calcination of the ammonium form to yield the hydrogen form. In addition to the hydrogen form, other forms of the zeolite wherein the original alkali metal has been reduced to less than about 1.5 percent by weight may be used. Thus, the original alkali metal of the zeolite may be replaced by ion exchange with other suitable ions of Groups IB to VIII of the Periodic Table, including, by way of example, nickel, copper, zinc, palladium, calcium or rare earth metals. 
     In practicing the desired conversion process, it may be desirable to incorporate the above described crystalline aluminosilicate zeolite in another material resistant to the temperature and other conditions employed in the process. Such matrix materials include synthetic or naturally occurring substances as well as inorganic materials such as clays, silica and/or metal oxides. The latter may be either naturally occurring or in the form of gelatinous precipitates or gels including mixtures of silica and metal oxides. Naturally occurring clays which can be composited with the zeolite include those of the montmorillonite and kaolin families, which families include the sub-bentonites and the kaolins commonly known as Dixie, McNamee-Georgia and Florida clays or others in which the main mineral constituent is halloysite, kaolinite, dickite, nacrite or anauxite. Such clays can be used in the raw state as originally mined or initially subjected to calcination, acid treatment or chemical modification. 
     In addition to the foregoing materials, the zeolites employed herein may be composited with the porous matrix material, such as alumina, silica-alumina, silica-magnesia, silica-zirconia, silica-thoria, silica-berylia, silica-titania as well as ternary compositions, such as silica-alumina-thoria, silica-alumina-zirconia, silica-alumina-magnesia and silica-magnesia-zirconia. The matrix may be in the form of a cogel. The relative proportions of zeolite component and inorganic oxide gel matrix may vary widely with the zeolite content ranging from between about 1 to about 99 percent by weight and more usually in the range of about 5 to about 80 percent by weight of the composite. 
     The invention utilizes zeolites of the type described, limited however to those forms which are of relatively low acid activity. It has been found that, as activity of these zeolites is reduced, the capacity to catalyze disproportionation declines without substantial decline in the capacity to catalyze isomerization of xylenes at temperatures above about 800° F. The invention takes advantage of that unique characteristic to achieve the processing advantage that isomerization is decoupled from ethylbenzene conversion which now proceeds by dealkylation in the presence of the low activity zeolite. A significant consequence of these catalytic properties is that recycle of toluene and trimethylbenzene to the reactor is generally undesirable. The lack of disproportionation activity means that these methylbenzenes will not be converted to significant amounts of xylenes. Hence recycle of these unreactive species results in undesirable build-up in the loop of diluent materials. 
     The low acid activity of the catalyst is attainable in any of several ways or a combination of these. A preferred alternative is to form the zeolite at high silica/alumina ratio above 200, preferably above 500. Very high dilution with an inert matrix is also effective. For example, composites of more active form of zeolite ZSM-5 with alumina at a ratio of 5 parts of zeolite with 95 parts of the inert matrix provides a suitable catalyst as described in our application Ser. No. 795,046, filed May 9, 1977, and now abandoned the entire contents of which are incorporated herein by reference. 
     Activity of these zeolites may be reduced to levels suited to practice of the invention by thermal treatment or steam at high temperature as described in application Ser. No. 585,025, filed May 22, 1975, now U.S. Pat. No. 4,105,537 and in U.S. Pat. No. 3,965,209, respectively. Zeolites employed in such severe reactions are aromatization of paraffins and olefins lose activity to an extent which makes them suitable for use in the process of this invention. See U.S. Pat. No. 3,960,978 for fuller discussion of this manner of deactivated zeolite. Another method for reducing activity is to provide basic cations such as sodium at a significant proportion of the cationic sites of the zeolite. That technique is described in U.S. Pat. No. 3,899,544. 
     In many cases, steaming will be the preferred manner of reducing activity of the zeolite catalyst. That catalyst preparation step may be conducted in situ by passing steam at suitable temperature, generally 1000° F. or higher through catalyst in reactor 24 for a period of several hours until the desired reduction in disproportionation activity is achieved. 
     By whatever means the reduced acid activity is achieved, the activity may be measured in terms of disproportionation activity. A suitable test for the purpose involves contacting xylenes in any convenient mixture or as a single pure isomer over the catalyst at 900° F., 0 psig and liquid hourly space velocity (LHSV) of 5. Suitable catalysts for use in the process of the invention will show a single pass loss of xylenes (by disproportionation) of less than 2 weight percent, preferably less than one percent. Catalysts which have been employed show losses in the neighborhood of 0.5 percent. It is this very low rate if disproportionation at very high levels of ethylbenzene conversion to benzene (about 30%) that provides the advantage of the new chemistry of aromatics processing characteristic of the invention. That lack of disproportionation (and transalkylation generally) activity also dictates withdrawal of compounds boiling above and below eight carbon atom aromatic compounds. For example, toluene and trimethyl benzene are converted to very little, if any, extent and become diluents which occupy reactor space to no advantage. Small amounts of such diluents can be tolerated, such as those present by reason of &#34;sloppy&#34; fractionation, but withdrawal to at least a major extent is important to efficient operation. 
     At the reaction conditions of this invention, metallic catalysts components such as nickel, platinum and the like are not used. The process is conducted in the absence of hydrogen at essentially atmospheric pressure. Experiments conducted to evaluate dual function catalysts of nickel and low activity zeolite ZSM-5 resulted in rapid loss of activity. Ethylbenzene conversion dropped from 27% to 12% in three days on stream charging 15% ethylbenzene and 85% mixed xylenes at atmospheric pressure, 880° F., 12 WHSV over extrudate of 0.7% nickel with zeolite ZSM-5 of 500 silica/alumina ratio. 
     The stability of the system here disclosed to paraffin was demonstrated with steamed ZSM-5 extrudate which had been on stream 22 days converting a mixture of ethylbenzene and xylenes with little loss in activity. Addition of 1.2% nonane to the charge had no effect on aging after an additional eight days on stream. 
     Little difference in process chemistry is found with ZSM-5 catalyst of low activity achieved by techniques other than steaming, though tests of these showed higher aging rates as measured by the temperature increase required to maintain constant ethylbenzene conversion. A catalyst of ZSM-5 having a silica/alumina ratio of 1600 aged at a rate of about 2° F. per day. A bulk diluted catalyst of 2% ZSM-5 with 98% alumina matrix aged at a rate of about 5° F. per day. 
     In many operations to prepare charge for p-xylene manufacture, the C 8  reformate cut is extracted with Sulfolane (dioxytetrahydrothiophene) to separate paraffins. The aromatic extract can contain traces of the selective solvent which can be detrimental to some catalysts. After 22 days on stream, steamed ZSM-5 was exposed to charge containing 290 ppm Sulfolane (80 ppm Sulfur) for 22 hours with no measurable aging effect. This was followed by 725 ppm Sulfolane (200 ppm Sulfur) for 21 hours, which resulted in about 13° F. of aging. By way of contrast, addition of 360 ppm Sulfolane (100 ppm Sulfur) in the process of U.S. Pat. No. 4,101,596 resulted in 50°-100° F. of aging after 24 hours. 
     EXAMPLE 1 Steamed ZSM-5 Catalyst 
     A long term experimental run was conducted by passing a simulated isomerizer charge at 12 parts by weight of charge per part by weight of catalyst per hour (WHSV) and atmospheric pressure, no hydrogen, over extrudate of H ZSM-5 of 70 silica/alumina ratio in 35% alumina binder which had been steamed 16 hours at 1050° F. with 100% steam at atmospheric pressure. Measurements of reaction conditions and analysis of products were made at intervals over a period of 776 hours. Representative data are set out in Table 1. The term &#34;light paraffins&#34; includes the total of all paraffin products having eight or less carbon atoms. 
     During the period of the run, the charge stock contained 0.1% each of toluene and C 9  aromatics, 9.9-10.1% ethylbenzene, 68.6-68.8% m-xylene, 9.1-9.4% p-xylene and 11.7-12.1% o-xylene. During a final period of the run, 1.2% nonane was added to the charge as described above. Temperature during the run varied between 860° and 882° F. 
     It was in the course of this run that sulfolane was added in amounts equal to 80 ppm sulfur from 530 to 554 to 575 hours. 
     
                       TABLE 1                                                     
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Low Pressure Isomerization-Steamed ZSM-5                                  
______________________________________                                    
Time on     3      99     411  553.sup.(1)                                
                                    575.sup.(2)                           
                                         742.sup. (3)                     
                                              776                         
Stream, Hrs.                                                              
Temp., °F.                                                         
            860    862    863  863  862  862  862                         
Products,                                                                 
wt %                                                                      
Light                                                                     
paraffins   0.36   0.74   1.05 1.02 0.52 0.86 0.89                        
Benzene     2.82   3.24   3.25 3.15 2.87 2.86 2.83                        
Toluene     0.88   0.75   0.69 0.68 0.63 0.61 0.55                        
EB          4.96   5.29   5.40 5.46 5.90 5.83 5.81                        
m-xylene    47.24  47.49  47.66                                           
                               47.82                                      
                                    48.16                                 
                                         47.66                            
                                              47.86                       
p-xylene    20.67  20.67  20.43                                           
                               20.39                                      
                                    20.67                                 
                                         20.39                            
                                              20.32                       
o-xylene    20.21  20.00  19.78                                           
                               19.76                                      
                                    19.74                                 
                                         19.51                            
                                              19.49                       
C.sub.9.sup.+ par.                                                        
            0.00   0.00   0.00 0.00 0.00 0.85 0.86                        
C.sub.9.sup.+ arom                                                        
            2.17   1.85   1.75 1.72 1.39 1.43 1.39                        
EB                                                                        
conversion,                                                               
wt %        50.98  47.62  46.53                                           
                               45.94                                      
                                    42.32                                 
                                         41.11                            
                                              42.31                       
______________________________________                                    
 .sup.(1) 80 ppm S in charge                                              
 .sup.(2) 200 ppm S in charge                                             
 .sup.(3) 1.2% nonane in charge                                           
 
    
     EXAMPLE 2 High Silica ZSM-5 
     A high temperature, low pressure run for isomerization of xylenes mixed with ethylbenzene was conducted with extruded H-ZSM-5 of 1600 silica/alumina ratio at atmospheric pressure in the absence of hydrogen. The charge contained 0.1% toluene, 9.8% of p-xylene, 4.4% o-xylene, 0.2% C 9   +  paraffins and 0.3% C 9  aromatics. Other reaction conditions and the products of reaction are shown in Table 2. 
     EXAMPLE 3 Bulk Diluted ZSM-5 
     In similar fashion an extruded catalyst of 2 wt% H-ZSM-5 having a silica/alumina ratio of 70 with 98% alumina was used for conversion of a charge containing 0.2% toluene, 10.3% ethylbenzene, 67.9% m-xylene, 9.4% p-xylene, 12.1% o-xylene and 0.1% C 9  aromatics. Data are repeated in Table 2. 
     
                       TABLE 2                                                     
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Low Pressure Isomerization                                                
          Example 2, High                                                 
                        Example 3,                                        
          Silica ZSM-5  Bulk Diluted %                                    
______________________________________                                    
Time on     3      199    340  348  5    145  222                         
Stream, Hrs.                                                              
WHSV        8.00   8.00   8.00 8.00 5.00 6.50 5.00                        
Temp., °F.                                                         
            900    900    899  912  871  868  875                         
Products,                                                                 
wt %                                                                      
Light                                                                     
paraffins   1.11   0.75   0.81 1.12 0.54 0.57 0.07                        
Benzene     4.04   2.96   2.55 2.92 2.82 1.51 1.34                        
Toluene     1.41   0.74   0.62 1.25 0.86 0.27 0.31                        
EB          4.15   5.42   5.86 5.39 6.30 7.99 8.14                        
m-xylene    45.62  48.08  50.59                                           
                               48.15                                      
                                    49.33                                 
                                         53.47                            
                                              52.40                       
p-xylene    20.27  20.39  20.34                                           
                               20.70                                      
                                    20.68                                 
                                         19.21                            
                                              19.76                       
o-xylene    20.18  20.18  18.27                                           
                               19.40                                      
                                    18.36                                 
                                         16.41                            
                                              17.34                       
C.sub.9.sup.+ par.                                                        
            0.00   0.00   0.00 0.00 0.00 0.00 0.00                        
C.sub.9.sup.+ arom                                                        
            3.16   1.47   0.96 1.05 1.10 0.56 0.65                        
EB                                                                        
conversion                                                                
wt %        57.65  44.69  40.20                                           
                               45.00                                      
                                    38.83                                 
                                         22.42                            
                                              20.97                       
______________________________________                                    
 
    
     EXAMPLE 4 Low Pressure Conversion of Heavy Reformate 
     The effect of adding reformate to the charge was demonstrated by experiments in which heavy reformate was converted at atmospheric pressure and high temperature in contact with steamed zeolite ZSM-5. The catalyst was extrudate H-ZSM-5 of 70 silica/alumina ratio. That catalyst was steamed in the reactor at 900° F. and 50 psig for 16 hours. During a first period, the charge stock was commercial reformate fractionated to prepare a fraction boiling above 305° F., essentially the components of nine or more carbon atoms. Composition of that heavy reformate is shown in Table 3, &#34;charge A&#34;. During a later period, the charge stock was a blend of the heavy reformate with simulated isomerizer loop recycle of eight carbon atoms aromatics. Analysis of that blend is shown as &#34;charge B&#34; in Table 3. Processing conditions and product distribution are reported in Table 4. As before, paraffins of eight or less carbon atoms are aggregated as &#34;light paraffins&#34;. The data taken at 293 and 297 hours were for charge A. Charge B was used at 302 and 306 hours. 
     
                       TABLE 3                                                     
______________________________________                                    
Charge Stocks for                                                         
C.sub.9.sup.+ Reformate Reaction Over Steamed H-ZSM-5                     
Product Dist., Wt. %                                                      
                Charge A     Charge B                                     
______________________________________                                    
Toluene                      0.1                                          
Ethylbenzene    0.9          8.4                                          
m-xylene        3.9          53.9                                         
p-xylene        1.6          10.8                                         
o-xylene        1.9          11.1                                         
C.sub.9.sup.+ par.                                                        
                2.3          0.4                                          
C.sub.9 aromatics                                                         
                58.9         8.8                                          
C.sub.10 aromatics                                                        
                22.2         5.0                                          
C.sub. 11 --C.sub.12 aromatics                                            
                5.3          1.2                                          
Naphthlene                   .2                                           
C.sub.13.sup.+ &#39;s                                                         
                3.0                                                       
______________________________________                                    
 
    
     
                       TABLE 4                                                     
______________________________________                                    
C.sub.9.sup.+ Reformate Reaction Over Steamed H-ZSM-5                     
______________________________________                                    
Temp., °F.                                                         
             900.00   862.00   861.00 901.00                              
Pressure, psig                                                            
             1.00     1.00     1.00   0.00                                
WHSV         10.00    10.00    10.00                                      
H2/HC        0.00     0.00     0.00   0.00                                
Time on Stream, Hrs.                                                      
             293.00   297.00   302.00 306.00                              
Product Dist., Wt. %                                                      
Light paraffins                                                           
             5.44     5.17     1.41   1.62                                
Benzene      2.98     2.72     2.62   3.39                                
Toluene      9.59     8.76     1.87   2.39                                
Ethylbenzene 1.66     1.73     5.61   4.97                                
m-xylene     5.74     5.56     42.05  41.39                               
p-xylene     2.52     2.48     18.6   17.50                               
o-xylene     2.81     2.43     16.75  16.97                               
C.sub.9.sup.+ par.                                                        
             0.22     0.23     0.00   0.00                                
C.sub.9.sup.+ aromatics                                                   
             69.07    70.91    12.32  11.77                               
Benzene Made 2.98     2.72     2.62   3.39                                
Toluene Made 9.59     8.96     1.77   2.39                                
C.sub.8 Ar Made                                                           
             4.45     3.90     0.47   0.08                                
Wt. % C.sub.9.sup.+ Conv.                                                 
             22.74    20.68    18.94  22.57                               
______________________________________