Combination process for producing LPG and aromatic rich material from naphtha

Abstract

Processing of reformate product of catalytic reforming to produce significant yields of LPG and BTX as well as aromatic enriched reformate with a ZSM-5 type crystalline zeolite is described.

Description

BACKGROUND OF THE INVENTION

Reforming of hydrocarbons is a widely used process in petroleum technology for upgrading hydrocarbon fractions such as naphthas, gasolines and kerosines to improve the antiknock characteristics thereof. Hydrocarbon fractions suitable for upgrading by reforming are composed of normal and branched paraffins, naphthenic hydrocarbons and even some aromatic hydrocarbons. During reforming a multitude of reactions take place including dehydrogenation, isomerization, dehydrocyclization, hydrocracking, and combinations thereof to yield a product of increased aromatics content and branched chain hydrocarbons. Thus in reforming it is desired to dehydrogenate the naphthenic hydrocarbons to produce aromatics, cyclize straight chain paraffins to form naphthenes, to convert C.sub.5 ring compounds to C.sub.6 ring compounds which are dehydrogenated to form aromatics, isomerize normal and branched paraffin hydrocarbons to yield higher octane branched chain hydrocarbons and effect a controlled hydrocracking of hydrocarbon constituents which are of undesired octane characteristics.

Normal and slightly branched paraffin hydrocarbons found in the above hydrocarbon fractions are generally of low octane rating. Highly branched-chain paraffin hydrocarbons, on the other hand, are characteristic of higher octane ratings. Therefore, one object of reforming is to effect isomerization of the normal and slightly branched chain paraffins to higher octane products by any one of the aforementioned reactions. The production of aromatics during reforming is accomplished by one or more of the above identified reactions leading to the production of naphthenes which are then dehydrogenated to aromatics such as benzene, toluene and xylene. One method for producing aromatics involves the isomerization of alkyl cyclopentanes to form cyclohexanes which thereafter are dehydrogenated to aromatics.

Ever since the concept of catalytic reforming was developed and commercially adopted, the refiner has been concerned with improving upon the selectivity of the product obtained and thus has strived to reduce yields of carbon and normally gaseous product materials since such materials represent a loss in desired liquid product. Thus small improvement in product selectivity has been gained with difficulty since there is a limit to the quantity of normally liquid constituents of desired octane rating that can be produced from a given charge. Consequently increases in product selectivity are viewed with considerable interest particularly if the selectivity increases can be associated with products of economic interest to the refiner. It has been found that the selectivity of a particular product slate or composition can be considerably enhanced by following the concepts and sequence of steps comprising this invention.

THE INVENTION

This invention relates to a method and combination of processing steps for effecting a selective conversion and a rearrangement of petroleum hydrocarbon constituents to form aromatic enriched products and improve yields of LPG materials. In one aspect the present invention is concerned with one or more methods for selectively conducting chemical reactions with an arrangement of catalytic compositions possessing selective reaction properties with respect to different hydrocarbon components existing in the naphtha boiling range material. In yet another aspect the present invention relates to effecting a selective catalytic conversion of hydrocarbon components comprising, normal and isoparaffin hydrocarbon components in a sequence of hydrogenating conversion steps maintained under operating conditions selected to obtain products rich in aromatics and LPG material. More specifically, the invention is concerned with an arrangement and sequence of catalytic reactions designed to manipulate the reaction of hydrocracking, dehydrogenation, isomerization and dehydrocyclization to improve upon the yields of LPG products and aromatic components readily separated by distillation.

The present invention is concerned with contacting a relatively wide boiling range naphtha hydrocarbon material boiling in the range of C.sub.5 hydrocarbons up to about 380.degree. or 400.degree.F. under selective reforming conditions in the presence of a platinum type reforming catalyst. In this reforming operation the conditions employed lead to the production of relatively low octane branched and normal paraffin compounds which are available for conversion and production of additional LPG products. The reforming catalyst may be relied upon to hydrocrack these low octane compounds formed during the reforming operation but it is preferred that the reformate product comprising any C.sub.5 and higher boiling normal paraffin constituents be subjected to a selective zeolite hydrocracking operation designed to convert low boiling normal paraffins to LPG product. Thus the present invention includes the selective cracking of low and high boiling normal paraffin components comprising the naphtha boiling material processed in the combination of catalytic contact steps comprising this invention. It includes reforming a naphtha charge under reforming conditions providing normal and branched chain hydrocarbons component along with reactions of dehydrogenation and dehydrocyclization comprising catalytic reforming. This relationship between normal and branched chain hydrocarbon components provides normal paraffin constituents suitable for conversion to LPG material.

A platinum type reforming catalyst including bimetallic and non-bimetallic reforming catalysts and those comprising platinum or palladium in combination with another Group VIII metal component such as rhenium, iridium, ruthenium and osmium promoted with a halogen will indiscriminately effect hydrocracking under elevated temperature reforming conditions of the normal and branched paraffin components comprising the hydrocarbon material in the reforming operation. Thus employing a platinum type reforming catalyst under controlled isomerizing and hydrocracking severity conditions may be relied upon to produce LPG type products or products more easily converted to LPG products. That is, hydrocracking reactions performed with platinum reforming catalysts are more usually rate controlled reactions wherein, for example, a normal C.sub.8 hydrocarbon will crack more easily than a C.sub.7 hydrocarbon or a lower carbon number paraffin and thus a high severity reforming operation would be required to crack, for example, a C.sub.5 paraffin. However, such a high severity non-selective hydrocracking operation with the platinum reforming catalyst is undesirable since cracking of branched C.sub.8 and C.sub.7 hydrocarbons will be accomplished before cracking of normal hexane. This will result in cracking desired high octane branched chain hydrocarbons. Furthermore, such an operation produces an undesired mixture of light gases particularly comprising C.sub.1 and C.sub.2 hydrocarbons rather than C.sub.3 and C.sub.4 hydrocarbons. On the other hand, using the small pore selective hydrocracking catalyst described herein, cracking the lower boiling C.sub.5 and C.sub.6 paraffins more effectively for the production of LPG products. Thus by maintaining a selective balance in rate control and equilibrium controlled hydrocracking reactions with the different catalysts described herein and particularly suitable for this purpose, an improved overall yield of LPG products can be obtained along with an aromatic rich product by the present invention.

Crystalline aluminosilicate conversion catalysts identified with the prior art which are not selective within the limits defined herein or those particularly known as methane producers rather than producers of propane and butane are of little interest in pursuing the concepts of this invention. Furthermore, high methane producing cyrstalline aluminosilicate catalysts generally small pore crystalline zeolites promoted with Zn, Cd and Hg or other hydrocracking catalyst compositions which non-selectively produce gaseous streams rich in methane are of little interest for practicing the concept of this invention unless they can be controlled by operating conditions to exclude the undesirable production of light gaseous hydrocarbon constituents particularly methane and ethane.

In the interest of convenience to a better understanding of the concepts of the present invention the platinum type of reforming catalyst used will be referred to as catalyst A hereinafter, and the described selective crystalline aluminosilicate hydrocracking catalyst relied upon particularly for the production of LPG gases will be referred to hereinafter as catalyst B.

The platinum type reforming catalyst, catalyst A, selected for use in the sequence of process steps of this invention may be selected from any one of a number of known prior art reforming catalysts suitable for accomplishing the results desired. These catalysts include generally, for example, alumina as the carrier material for one or more hydrogenation-dehydrogenation components distributed thereon with the alumina being in either the eta, chi, gamma or mixed forms thereof. The alumina carrier is promoted with, for example, one or more Group VIII metal components either with or without an acidic promoter such as silica, boron or a halogen. The platinum type of reforming catalyst is intended to include platinum, palladium, osmium, iridium, ruthenium, rhenium and mixtures thereof deposited on an alumina containing carrier or support with the alumina components generally being in an amount up to about 95% by weight. Other components such as magnesium, zirconium, thorium, vanadium and titanium may also be combined or distributed in the alumina carrier. The platinum type catalyst may also include various amounts of halogen such as chlorine or fluorine in amounts ranging from about 0.1 up to about 10%; usually not more than 5 or 6%. The platinum reforming catalysts described may be one of those described in the prior art as homogeneous mixtures of metal components, alloys, and metal halide complexes thereof. A bimetal catalyst composition suitable for the reforming operation of this invention may be platinum combined with either rhenium, ruthenium, osmium or iridium and an alumina carrier promoted with chlorine to provide desired acid activity.

In reforming operations it is known that as the reforming severity is increased to achieve higher and higher product octane number, the octane number increase is obtained primarily by way of paraffin aromatization and 5 carbon ring aromatization. At the relative high severity conditions required for paraffin to aromatic dehydrocyclization reactions to become important, these are accompanied by progressive and non-selective elimination by hydrocracking of remaining paraffins to light gaseous products thus increasing octane number at the expense of substantial liquid volume loss. It is, therefore, preferred to selectively control the reforming operation severity to restrict the chemical reactions encountered therein to minimize the production of low octane liquid and undesired gaseous component in favor of producing branched chain hydrocarbons in an aromatic enriched product of relatively high octane rating. Accordingly, the method and combination of process steps herein described provide significant and unusual benefits by adjusting the reaction mechanisms to implement and improve the production of LPG products as well as high octane aromatic products without unusual sacrifice of yield of liquid products.

The operating conditions employed in the process combination of this invention and particularly those of the reforming operation with type A catalysts are those conditions which promote dehydrogenation of naphthenes along with reactions associated with paraffin isomerization which establish a relationship between normal paraffins to branched paraffins and include operating temperatures selected from the range of about 800.degree.F to about 1000.degree.F and preferably from 850.degree.F up to about 980.degree.F., liquid hourly space velocity in the range of about 0.1 to about 10, preferably about 0.5 to about 5; a pressure in the range of about atmospheric up to about 600 p.s.i.g. and preferably about 100 to about 400 p.s.i.g. and a hydrogen to hydrocarbon ratio of about 0.5 to about 20 and preferably about 1 to 10.

The type B catalysts referred to herein are members of a special class of zeolites exhibiting some unusual properties. These zeolites induce profound transformations of aliphatic hydrocarbons to aromatic hydrocarbons in commercially desirable yields and are generally highly effective in alkylation, isomerization, disproportionation and other conversion reactions involving aromatic hydrocarbons. Although they have unusually low alumina contents, i.e., high silica to alumina ratios, they are very active even with silica to alumina ratios exceeding 30. This activity is surprising since catalytic activity of zeolites is generally attributed to framework aluminum atoms and cations associated with these aluminum atoms. These zeolites retain their crystallinity for long periods in spite of the presence of steam even at high temperatures which induce irreversible collapse of the crystal 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 intra-crystalline 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 zeolites useful in type B catalysts in this invention possess, in combination: a silica to alumina ratio of at least about 12; and a structure providing constrained access to the crystalline free space.

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. Although zeolites with a silica to alumina ratio of at least 12 are useful, it is preferred to use zeolites having higher ratios of at least about 30. Such zeolites, after activation, acquire an intracrystalline sorption capacity for normal hexane which is greater than that for water, i.e., they exhibit "hydrophobic" properties. It is believed that this hydrophobic character is advantageous in the present invention.

The zeolites useful as B catalysts in this invention freely sorb normal hexane and have a pore dimension greater than about 5 Angstroms. In addition, their structure must provide constrained access to some 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 substantially excluded and the zeolite is not of the desired type. Zeolites with windows of 10-membered rings are preferred, although excessive puckering or pore blockage may render these zeolites substantially ineffective. Zeolites with windows of 12-membered rings do not generally appear to offer sufficient constraint to produce the advantageous conversions desired in the instant invention, although 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 "constraint index" may be made by continuously passing a mixture of equal weight of normal hexane and 3-methylpentane over a small sample, approximately 1 gram or less, 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.degree.F for at least 15 minutes. The zeolite is then flushed with helium and the temperature adjusted between 550.degree.F and 950.degree.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 catalyst 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 chromatography, to determine the fraction remaining unchanged for each of the two hydrocarbons.

The "constraint index" is calculated as follows: ##EQU1##

The constraint index approximates the ratio of the cracking rate constants for the two hydrocarbons. Catalysts suitable for the present invention are those which employ a zeolite having a constraint index from 1.0 to 12.0. Constraint Index (CI) values for some typical zeolites including some not within the scope of this invention are: CAS C.I. ______________________________________ ZSM-5 8.3 ZSM-11 8.7 TMA Offretite 3.7 ZSM-12 2 Beta 0.6 ZSM-4 0.5 H-Zeolon 0.5 REY 0.4 Amorphous Silica-alumina 0.6 Erionite 38 ______________________________________

The class of zeolites defined herein is exemplified by ZSM-5, ZSM-11, ZSM-12, ZSM-21 and other similar materials. Recently issued 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.

U.S. application, Ser. No. 358,192, filed May 7, 1973, now abandoned, the entire contents of which are incorporated herein by reference, describes a zeolite composition, and a method of making such, designated as ZSM-21 which is useful in this invention. Recent evidence has been adduced which suggests that this composition may be composed of two different zeolites, one or both of which are the effective material insofar as the catalyst for this invention is concerned.

The specific zeolites described, when prepared in the presence of organic cations, are substantially 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.degree.F for 1 hour, for example, followed by base exchange with ammonium salts followed by calcination at 1000.degree.F in air. The presence of organic cations in the forming solution may not be absolutely essential to the formation of this special 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 zeolite by base exchange with ammonium salts followed by calcination in air at about 1000.degree.F for from about 15 minutes to about 24 hours.

Natural zeolites may sometimes be converted to this type zeolite by various activation procedures and other treatments such as base exchange, steaming, alumina extraction and calcination, alone or in combinations. Natural minerals which may be so treated include ferrierite, brewsterite, stilbite, dachiardite, epistilbite, heulandite and clinoptilolite. The preferred crystalline aluminosilicates are ZSM-5, ZSM-11, ZSM-12 and ZSM-21, with ZSM-5 particularly preferred.

The zeolites used as catalysts in this invention may be in the hydrogen form or they may be base exchanged or impregnated to contain ammonium or a metal cation complement. It is desirable to calcine the zeolite after base exchange. The metal cations that may be present include any of the cations of the metals of Groups I through VIII of the periodic table. However, in the case of Group IA metals, the cation content should in no case be so large as to substantially eliminate the activity of the zeolite for the catalysis being employed in the instant invention. For example, a completely sodium exchanged H-ZSM-5 appears to be largely inactive for shape selective conversions required in the present invention.

In a preferred aspect of this invention, the zeolites useful as catalysts herein 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 catalysts of this invention are those using zeolites having a constraint index as defined above of about 1 to 12, a silica to alumina ratio of at least about 12 and a dried crystal density of not substantially 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 1000 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 "Proceedings of the Conference on Molecular Sieves, London, April 1967," published by the Society of Chemical Industry, London, 1968. When the crystal structure is unknown, the crystal framework density may be determined by classical pyknometer 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, seems to be important as the locus of catalytic activity.

Crystal framework densities of some typical zeolites including some which are not within the preview of this invention 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 ______________________________________

The heavier reformate fraction is overwhelmingly aromatic in composition, the aromatics comprising at least about 80 volume percent, preferably at least about 90 volume percent, thereof. At least a portion, and preferably all, of this heavier reformate is contacted with a distinct mass of type B catalyst under conditions conducive to substantially eliminating any aliphatic components thereof. These conditions also induce aromatic isomerization, disproportionation, alkylation, dealkylation, transalkylation and side chain splitting so as to produce a product which is substantially benzene and methyl substituted benzenes up to about C.sub.10. BTX preferably predominates in this product.

The light reformate comprises substantial proportions of aliphatic as well as aromatic components. It is contacted with a separate and distinct mass of type B catalyst under conditions conducive to cracking out the lower octane aliphatic components to produce LPG components and to simultaneously increase the proportion of aromatics at least partially by alkylation of existing aromatics with fragments produced by such cracking.

The two separate and distinct type B catalyst conversions may be carried out under substantially similar or widely different reaction conditions depending upon the exact compositions of the light and heavier reformate fractions and upon the desired product distribution. It has been found, however, that if the reformate is split in the manner set forth hereinabove, each of the type B catalyst conversion performs its intended function admirably whereas if the reformate is split differently, these conversion are substantially less efficient at producing the desired product slate of high quality, aromatic enriched gasoline, high quality substantially aliphatic free aromatics concentrate, and large quantities of LPG.

The operating conditions selected for processing the light reformate fraction boiling below about 240.degree.F particularly include an operating pressure within the range of 200 to 1000 psig; a temperature within the range of about 500.degree.F to 800.degree.F; a volume hourly space velocity in the range of about 1 to 4 and a hydrogen to hydrocarbon ratio within the range of about 1 to 10 to 1. Hydrogen consumption is about 100 or more SCF/B depending on the charge composition and operating conditions selected. Processing the heavy reformate fraction is preferably accomplished under more severe operating conditions including a temperature within the range of about 750.degree. to 900.degree.F; a space velocity (vol. basis) in the range of about 0.5 to 2; a hydrogen to hydrocarbon ratio in the range of 1-20/1 and a pressure within the range of about 200 to 1000 psig. Hydrogen consumption for the more severe operation is within the range of 200-800 SCF/B.

BRIEF DESCRIPTION OF THE DRAWING

The drawing is diagrammatic sketch in elevation of the process combination comprising the separation of reformate product of catalytic reforming into a light and heavy reformate product fractions which are thereafter separately processed over type B catalyst under lower and higher severity conditions as aforesaid to particularly produce LPG, high octane gasoline and a benzene-toluene-xylene rich fraction.

Referring now to the drawing, by way of example, a C.sub.5 .sup.+ full boiling range product of naphtha reforming is charged to the process by conduit 2. The naphtha feed to a catalytic reforming operation may be one boiling in the range of C.sub.5 hydrocarbon up to about 380.degree. or 400.degree.F. Reforming of naphtha boiling range hydrocarbons is well known in the prior art as discussed above and is a part of this invention to the extent that the material processed is a reformate product of catalytic reforming comprising a C.sub.5 .sup.+ reformate. The reformate charge introduced by conduit 2 to stabilizer 4 is separated in the stabilizer to recover toluene enriched C.sub.6 .sup.+ reformate material from the bottom thereof by conduit 8. C.sub.5 and lower boiling components are recovered from the top of the stabilizer by conduit 10 and then passed to gas recovery equipment not shown to separate LPG materials from other gasiform materials.

A portion of the material withdrawn by conduit 8 from the bottom of the stabilizer is passed to gasoline pool equipment. The remaining portion of the C.sub.6 .sup.+ material is passed by conduit 12 to a second separation zone 14, hereinafter sometimes referred to as a reformate splitter. In separation zone 14, conditions are maintained to separate the C.sub.6 .sup.+ reformate into an overhead fraction comprising some toluene and primarily lower boiling components withdrawn therefrom by conduit 16. A C.sub.7 .sup.+ toluene-xylene rich fraction is withdrawn from the bottom portion of separation zone 14 by conduit 18. Separation of the C.sub.6 .sup.+ reformate in zone 14 is accomplished to provide a cut point between the fractions within the range of about 200.degree. to about 240.degree.F.

The overhead fraction comprising primarily C.sub.6 and lower boiling components withdrawn by conduit 16 is passed with added hydrogen to a catalyst zone 20 containing a ZSM-5 type of crystalline zeolite conversion catalyst. In zone 20, the fraction comprising toluene and lower boiling components is subjected to processing conditions comprising a pressure of about 400 psig at a start of run temperature of about 550.degree.F. The reactant material is passed in contact with the catalyst at a volume hourly space velocity of about 2 and a hydrogen to hydrocarbon ratio of at least 1/1. In zone 20 the light reformate comprising aromatic and C.sub.7 minus saturated hydrocarbons are processed to particularly produce LPG materials comprising propane and butane and some alkyl aromatics comprising toluene. The product effluent of zone 20 is then passed by conduit 22 to a high pressure separation zone 24. In high pressure separator 24, a separation is made which permits the recovery of C.sub.2 and lower boiling material comprising hydrogen from an upper portion of the zone by conduit 26. Some of this material may be withdrawn by conduit 28 and used as fuel. The remaining portion of the material in conduit 26 is combined with makeup hydrogen added by conduit 30, compressed in compressor 32 to raise the pressure of this recycle stream about 50 pounds before recycle by conduit 34 to zone 20.

A product material comprising C.sub.3 and higher boiling components including benzene and toluene separated in zone 24 is withdrawn essentially as a liquid stream from the lower portion of separation zone 24 by conduit 36 and passed in its entirety to separation zone 4 wherein the C.sub.5 and lower boiling components are removed as overhead material and processed as above briefly discussed.

The heavy C.sub.7 .sup.+ reformate material, enriched with C.sub.7 .sup.+ components from the separator 24, withdrawn from the lower portion of separation zone 14 is a toluene-xylene rich stream comprising C.sub.7 .sup.+ paraffins, having at most about 0.1 weight percent C.sub.7 paraffins, and boiling above about 225.degree.F which mixture is passed in a desired amount by conduit 38 to a second ZSM-5 zeolite catalytic conversion zone 40. A portion of this toluene-xylene enriched reformate in conduit 18 may be withdrawn as a product stream for use as desired. In zone 40, the herein identified C.sub.7 .sup.+ reformate is processed in the presence of added hydrogen which may come from conduit 28 over a ZSM-5 type zeolite under relatively more severe processing conditions designed to particularly crack paraffin components and convert the aromatic containing reformate to a light aromatics mixture comprising (BTX) benzene, toluene and xylene by a combination of dealkylation and disproportionation reactions. In this catalytic conversion operation it is proposed to employ an operating pressure of about 400 psig and a start of run temperature of about 750.degree.F. A hydrogen to hydrocarbon ratio of about 4/1 is used with a reactant volume space velocity of about 1. It is particularly important that the processing conditions of zone 40 be selected to particularly crack the paraffins to lower boiling components comprising LPG products and disproportionate charged aromatics to form a mixture of benzene, toluene, and aromatics since these materials are of a boiling range easily separated by simple distillation.

The product effluent of the aromatic forming zone 40 is passed by conduit 42 to a high pressure separation zone 44. In separation zone 44 a hydrogen rich gaseous stream is separated and recycled by conduit 46 to conduit 38 communicating with zone 40. Hydrogen rich make up gas is added to the recycled gas by conduit 48. The remaining product separated in zone 44 comprising light aromatics and lower boiling components of cracking are passed by conduit 50 to a separation zone 52 maintained at a pressure of about 100 psig and a temperature of about 450.degree.F. In separation zone 52 a separation is made between light aromatics comprising benzene and higher boiling aromatics and reaction components boiling below about benzene. Thus a light aromatic rich fraction is withdrawn by conduit 54 from zone 52 for further processing by distillation and other means not shown into desired components.

Materials generally lower boiling than benzene and comprising hexane and lower boiling components is withdrawn from separator 52 by conduit 56 for recycle to stabilizer 4 as by conduit 36.

An important aspect of this invention resides in flashing the product of shape selective conversion of light reformate, preferably in a high pressure separator, to split it into a gas and a higher octane gasoline liquid. The liquid is recycled to the stabilizer along with the full range reformate. Thus the heavier reformate from the splitter is enriched with C.sub.7 .sup.+ components (predominantly C.sub.9 .sup.+ alkyl benzenes) from this higher octane gasoline liquid product.

It is most important that a portion of the toluene enriched C.sub.6 .sup.+ reformate material from the stabilizer be drawn down out of the system. This product is excellent gasoline.

Having thus generally described the invention and presented a specific example in support thereof, it is to be understood that no undue restrictions are to be imposed by reason thereof except as defined by the following claims.

Claims

We claim:

1. A method for redistributing the paraffin-aromatic components of a reformate product of catalytic reforming comprising C.sub.5 and higher boiling hydrocarbons which comprises,

separating C.sub.5 and lower boiling components of catalytic reforming from C.sub.6 and higher boiling materials in a first separation zone,

separating the C.sub.6 and higher boiling material in a second separation zone into a first fraction comprising some C.sub.7 components and lower boiling paraffin and aromatic components and a second fraction comprising C.sub.7 and higher boiling paraffins and aromatic components,

passing said first fraction in contact with a first crystalline zeolite catalyst having properties for cracking paraffins to form LPG materials and redistribute the ratio between benzene and toluene components in the feed, separating product material of said first crystalline zeolite conversion operation into gaseous material comprising C.sub.2 and lower boiling material from material comprising C.sub.3 and higher boiling components, passing the C.sub.3 and higher boiling material to said first separation zone,

passing a portion of said second fraction comprising C.sub.7 and higher boiling paraffins and aromatics in contact with a separate second mass of crystalline zeolite catalyst having properties for cracking said paraffins and disproportionating aromatic to form a mixture of benzene, toluene and xylene; separating the product of said second zeolite catalyst conversion operation to recover a hydrogen rich stream, a stream rich in benzene, toluene and xylene aromatics and an intermediate product stream lower boiling than said aromatic rich stream, and recycling said intermediate product stream to said first separation zone.

2. The method of claim 1 wherein hydrogen rich gas is passed to each zeolite catalyst conversion zone.

3. The method of claim 1 wherein a C.sub.6 .sup.+ reformate fraction enriched in toluene is withdrawn from said first separation zone.

4. The method of claim 1 wherein a toluene-xylene rich fraction is withdrawn from said second separation zone.

5. The method of claim 1 wherein a hydrogen rich gas is separated from the product of each zeolite catalyst conversion operation and is recycled to the catalyst operation.

6. The method of claim 1 wherein LPG product material of the process combination is concentrated in the overhead product withdrawn from the first separation zone.

7. The method of claim 1 wherein the temperatures relied upon in the first zeolite catalyst conversion zone are within the range of 500.degree.F. to 800.degree.F.

8. The method of claim 1 wherein the temperatures and operating conditions relied upon in the other zeolite catalyst conversion zone are more severe than those used in the first catalyst conversion operation.

9. The method of claim 1 wherein the formation of a benzene, toluene, xylene rich mixture is particularly promoted by the operating conditions relied upon in the second zeolite catalyst operation.

10. The method of claim 1 wherein the zeolite catalyst is preferably a ZSM-5 type crystalline zeolite.

11. A process of upgrading petroleum reformate to an aromatics concentrate comprising benzene, toluene and xylene (BTX), liquifiable petroleum gas (LPG) comprising C.sub.3 and C.sub.4 components, and high octane gasoline comprising:

splitting at least a C.sub.6 .sup.+ portion of said reformate into C.sub.7 .sup.+ and C.sub.7 .sup.- fractions having minimum and maximum boiling points respectively of about 200.degree. to 240.degree.F;

contacting said C.sub.7 .sup.+ fraction with a crystalline aluminosilicate zeolite having a silica to alumina ratio of at least 12 and a constraint index of 1 to 12 in a first conversion zone;

converting said C.sub.7 .sup.+ fraction in said first conversion zone at at least about 500.degree.F. to a product comprising C.sub.4 .sup.- gas and a substantially aromatic liquid concentrate;

contacting said C.sub.7 .sup.- fraction with a crystalline aluminosilicate zeolite, having the ability to crack substantially only paraffinic components of said C.sub.7 .sup.- fraction to C.sub.4 .sup.- gas, in a second conversion zone;

converting said C.sub.7 .sup.- fraction in said second conversion zone at at least about 700.degree.F. to a product comprising C.sub.4 .sup.- gas and an aromatics enriched liquid product; and

recovering LPG from said C.sub.4 .sup.- gas, said aromatics concentrate, and high octane gasoline comprising said aromatics enriched liquid.

12. A process as claimed in claim 11 including admixing a portion of said aromatics concentrate with said reformate prior to said splitting.

13. A process as claimed in claim 11 including deleting a C.sub.5 .sup.- fraction from said reformate and splitting the remaining C.sub.6 .sup.+ fraction.

14. A process as claimed in claim 11 including separating C.sub.3 and C.sub.4 components from said C.sub.4 .sup.- gas.

15. A process as claimed in claim 11 including splitting a portion of the C.sub.6 .sup.+ portion of said reformate.

16. A process as claimed in claim 11, including admixing said aromatics enriched liquid with said reformate prior to said splitting.

17. A process as claimed in claim 11 including admixing said aromatics enriched liquid, a portion of said aromatics concentrate and said reformate; and splitting said admixture into said C.sub.7 .sup.+ and C.sub.7 .sup.- fractions.

18. A process as claimed in claim 11 including utilizing the same zeolite respectively in both of said conversions.

19. A process as claimed in claim 18 wherein said zeolite is a ZSM-5.

20. A process as claimed in claim 11 wherein said C.sub.7 .sup.- conversion is carried out at about 500.degree. to 800.degree.F., about 200 to 1000 psig, about 1 to 4 LHSV and about 2 to 10 to 1 hydrogen to hydrocarbon ratio; and wherein said C.sub.7 .sup.+ conversion is carried out at about 750.degree. to 900.degree.F., about 200 to 1000 psig, about 0.5 to 2 LHSV and about 3 to 20 to 1 hydrogen to hydrocarbon ratio.