GB2034351A - Upgrading naphtha fractions - Google Patents

Abstract

A naphtha fraction is upgraded to produce a product useful as a high-octane gasoline blending stock or as a source from which benzene, toluene and xylene can be recovered. The naphtha fraction (20) is fractionated (21) into a light fraction (25) boiling below methylcyclopentane and a heavy fraction (26) containing methylcyclopentane and higher boiling hydrocarbons. The heavy fraction is reformed (27) to produce a reformate stream having increased aromatics content and octane number and at least part (31) of the reformate stream is contacted with a ZSM-5-type zeolite catalyst (38) to produce a hydrocarbon effluent (39) enriched in aromatic hydrocarbons, from which can be separated various product fractions including a C5+ product stream (48) which may be used as a source of benzene, toluene and xylene, or as a high-octane gasoline blending stock for a gasoline pool. The light naphtha fraction (25) may be contacted with the ZSM-5-type zeolite (38) in admixture with the reformate (31). <IMAGE>

Description

SPECIFICATION Process for upgrading naphtha fractions This invention relates to a combination process for upgrading a naphtha fraction. In one aspect, the invention relates to a process combining reforming and aromatization over a ZSM-5-type zeolite, to produce a product useful as a high-octane gasoline blending stock or a source from which benzene, toluene and xylene can be recovered. In another aspect, the invention relates to a process combining reforming, isomerization and hydrocracking in the presence of a ZSM-5-type zeolite to produce a product useful as a highoctane gasoline blending stock and as a source for recovering benzene, toluene and xylene.

In view of the current concern over air pollution and environmental control, processes which will increase the octane number of gasoline while mimimizing or eliminating the need for additives are being sought.

One traditional way of increasing the octane of a naphtha fraction has been to subject it to catalytic reforming, usually over a platinumcontaining or bimetallic catalyst. In the reformer, naphthenes and paraffins are converted to aromatics, both reactions which substantially increase the octane number of the hydrocarbons involved. Naphthenes are reformed to aromatics with high selectivity.

However, the selectivity with which paraffins are converted to aromatics decreases with the number of carbon atoms per paraffin molecule. Only a minor fraction of C6 paraffins is converted to benzene. Other reactions which occur in the reformer are isomerization and cracking of paraffins. The cracking to C3hydrocarbons represents an irreversible yield loss, and the isomerization of paraffins mainly to singly branched paraffins is a reversible reaction in which a relatively high concentration of low-octane n-paraffins remain in thermal equilibrium with the branched isomers.

Thus, inclusion of C6 paraffinic hydrocarbons in a reformer feed is a less efficient use of the catalyst and reactor facilities than the inclusion of C6 naphthenes.

Another way of improving the octane number of hydrocarbon fractions is by contacting them with a ZSM-5 type of aluminosilicate zeolite catalyst to produce new aromatic rings from aliphatic compounds. For example, U.S.

Patent 3,761,389 teaches aromatization of a hydrocarbon fraction boiling within the range of C2 to 400"F with a ZSM-5 type of synthetic aluminosilicate zeolite catalyst, and U.S.

Patent 3,756,942 teaches aromatization of a feed consisting essentially of C5 + paraffins, olefins and/or naphthenes over a ZSM-5-type catalyst to produce a predominantly aromatic liquid and a light hydrocarbon gas. If the aromatization is performed at high temperature (e.g., about 538"C) and low pressure (e.g., about 1 atmosphere) without added H2, the light gas includes C2-C4 olefins as well as C,-C4 paraffins.

The art discloses several combinations of reacting a hydrocarbon stream over a reform ing catalyst and over a ZSM-5-type catalyst.

For example: in U.S. Patent 3,729,409 there is described a process for upgrading a refor mate by contacting the reformate and hydrogen with a ZSM-5-type zeolite, to selectively crack the normal paraffins and to form and alkylate aromatic compounds. In U.S Patent 3,849,290 there is described a process for reforming a naphtha and then removing normal and singly branched hydrocarbons by selective cracking to leave an aromatics-enriched product. In these two processes, some cracking of the alkyl side chain on the aromatic ring occurs, resulting in production of unwanted light ends. In U.S. Patent 3,770,614 there is disclosed a process in which a reformate is fractionated and the light reformate fraction (C6 to 116 or 127"C) passed over a ZSM-5-type zeolite to alkylate mono-aromatics. In U.S.Patent 3,950,241 there is disclosed a process for upgrading naphtha by separating it into low- and highboiling fractions, reforming the low-boiling fraction, and combining the high-boiling naphtha with the reformate for contact with a ZSM-5-type catalyst to crack the paraffins.

Reacting heavy naphthas over ZSM-5-type catalysts in the absence of H2 and at high temperatures leads to rapid catalyst deactivation, while processing naphthenes over them leads primarily to cracking, which reduces liquid yields.

We have found it advantageous to separate a naphtha into a light fraction and a heavy fraction, reform the heavy fraction, and pass at least a portion of the reformate over ZSM-5-type catalyst to produce a C5 + product fraction enriched in aromatic hydrocarbons.

Thus in accordance with the invention there is provided a process for upgrading a naphtha hydrocarbon feedstock, which comprises: (1) separating said naphtha feedstock into a light naphtha fraction boiling below methylcyclopentane and containing C6 aliphatics and lower-boiling hydrocarbons, and a heavy naphtha fraction containing methylcyclopentane and higher-boiling hydrocarbons; (2) reforming said heavy naphtha fraction under reforming conditions to produce a reformate stream enriched in aromatics compared with said heavy naphtha fraction; (3) contacting at least a portion of said reformate stream with a ZSM-5-type zeolite catalyst at an elevated temperature and recovering from the ZSM-5 contacting step a hydrocarbon effluent enriched in aromatic hydrocarbons; and (4) separating from said effluent a C5 + product stream.

For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying draw ings in which Figures 1 and 2 are schematic block-type flow diagrams of preferred embodiments of the present invention in which each block represents one particular step or zone of the process.

Conventional items, for example pumps, compressors and miscellaneous valving, have been omitted from the drawings. Likewise, with respect to the piping throughout the process system, only the major streams, required to illustrate the relationships between the various stages are presented. Accordingly, various recycle lines and vent gas streams have also been omitted.

Referring to the flow diagram depicted in Fig. 1, a C5 to C5 straight-run naphtha obtained from an Alaskan North Slope crude oil is charged through line 20 into fractionator 21. The naphtha has an approximate composition of 55 volume percent paraffins, 35 volume percent naphthenes and 10 volume percent aromatics. In the fractionator, the naphtha is separated into a light fraction comprising C5 and C6 hydrocarbons boiling under about 66"C (150"F) and a heavy fraction including methylcyclopentane boiling above about 66"C (150"F). The light naphtha fraction, about 20 volume percent of the feed, is removed from fractionator 21 via line 25.The heavy naphtha fraction, about 80 volume percent of the feed, is charged via line 26 to reformer 27 in which it is reformed under conventional mild reforming conditions with a platinum-rhenium-chloride reforming catalyst (see, for example, U.S. Patent 3,415,737, incorporated herein by reference) to increase the aromatics content and octane number of the naphtha. The reformate, in a yield of approximately 90 LV% and substantially depleted of naphthenes, is passed to separation zone 30 via line 28. In separation zone 30, which may comprise one or more stages, hydrogen is recovered for recycle to the reformer (not shown), light gases are removed and one or more reformate fractions are recovered.

In one embodiment of the present invention, for maximizing BTX production, the naphtha feed is preferably a C5 to C5 straight-run naphtha, and all of the C3 + reformate is passed via line 31, combined with the light naphtha fraction in line 25 and passed into ZSM reaction zone 38. In the ZSM reaction zone, the normal and lightly branched paraffins undergo aromatization, and aromatics, particularly xylenes, are isomerized. The ZSM reaction zone is operated at reaction conditions including a temperature of 538"C (1000"F), no added hydrogen, and an LHSV of 2 V/V/Hr.The effluent from ZSM reaction zone is passed via line 39 to separation zone 40 wherein a C5 + product stream in an amount of about 70 weight percent of the combined feed in line 25 and rich in aromatics is separated and sent via line 48 for recovery of benzene, toluene and xylene values therefrom. The C4 - component of the effluent is preferably separated into an H2/Ct/C2 fraction which is removed via line 42 and a C3/C4 fraction, in an amount of about 1 8 weight percent of the combined feed, which may be removed from the process via line 47 but which is preferably recycled via line 46 to ZSM reaction zone 38. The C3/C4 fraction may contain propylene and butenes in addition to propane and butane.

In another embodiment of the present invention, the naphtha feed is preferably a C5 to C5 straight-run naphtha, and the reformer effluent is separated in separation zone 30 into a C3 to 135"C - (275"F -) fraction which is sent via line 31 to the ZSM reaction zone operated at conditions to form and isomerize aromatics, and a 135"C + (275"F +) fraction rich in xylenes which may be passed via line 32 to a conventional xylene isomerization zone, such as that described in U.S.

Patent 3,948,758, incorporated herein by reference. When the feed is a full-boiling-range naphtha, a C6 + fraction may be removed from separation zone 30 via line 33 and used as a high-octane (research octane number of about 11 6 Clear) gasoline blending stock.

Advantages of this embodiment include a longer ZSM catalyst life because higher-endpoint hydrocarbons which tend to coke the ZSM catalyst are not passed over it and a more efficient use of the ZSM catalyst and reactor because feeding the C5 + aromatics to the ZSM reaction zone would not contribute to higher aromatics yields and would reduce catalyst life.

In yet another embodiment of the present invention, particularly useful for the production of high-octane gasoline, a full-boilingrange straight-run naphtha is the feedstock, the reformer effluent is separated in separation zone 30 into a light reformate fraction (C3 to 104"C or 220"F), usually about 70 volume percent of the reformate, which is passed via line 31 to the ZSM reaction zone, and a heavy reformate fraction (1 0C + or 220"F + ), usually having a research octane number of about 106 (Clear), which is sent to a gasoline pool (not shown) via line 32. The light reformate fraction in line 31 is combined with the light naphtha fraction in line 25 and preferably also with a recycle C3/C4 stream, and passed in contact with a ZSM-5-type zeolite catalyst at reaction conditions previously mentioned. The effluent from zone 38 is passed via line 39 to separation zone 40, from which a C5 + fraction having a research octane numbr of about 11 6 (Clear) is removed via line 48 and combined with the heavy reformate fraction in line 32 to form a highoctane gasoline blending stock having a research octane number of at least 90 (Clear), and preferably 95 and still more preferably at least 100 or more.

An advantage of this embodiment is that in splitting the reformate at 104"C (220"F), C7 paraffins having a low octane number are included in the feedstock to the ZSM reaction zone in which they will undergo aromatization, but most of the toluene fraction is excluded since feeding it to the ZSM reaction zone would not increase aromatic yields.

Another advantage of this embodiment is that the reformer is used for what it does efficiently and in high yield: dehydrocyclization of naphthenes, while C6 - paraffins are aromaticized over the ZSM catalyst to form highoctane aromatic compounds rather than cracked to C4 - in the reformer, with the attendant yield loss.

Referring to the flow diagram depicted in Fig. 2, an Arabian straight-run naphtha fraction boiling in the range of C5-193 C (380"F) is charged through line 21 into fractionator 22. The naphtha has an approximate composition of 66 volume percent paraffins, 21 volume percent naphthenes and 1 3 volume percent aromatics. In the fractionator, the naphtha is separated at about 66"C (150"F) into a light naphtha fraction comprising C5 and C6 hydrocarbons boiling under about 66"C (150"F) and a heavy naphtha fraction boiling above about 66"C (150"F) which includes most of the methylcyclopentane.The light naphtha fraction, about 10 volume percent of the feed, is removed from the fractionator 22 via line 24. The heavy naphtha fraction, about 90 volume percent of the feed, is charged via line 26 into reformer 27 in which it is reformed under conventional reforming conditions with a platinum-rhenium-chloride reforming catalyst (see, for example, U.S. Patent 3,415,737, incorporated herein by reference).

The reformer acts to increase the aromatics content and octane number of the naphtha.

The reformate, having a research octane number of about 98, in a yield of approximately 83 liquid volume percent and substantially depleted of naphthenes, is passed to separation zone 29 via line 28. In separation zone 29, which may comprise one or more stages, hydrogen is recovered for recycle to the refor mer (not shown), C4 - light gases are re- moved and the C5 + reformate is split into two fractions. The first fraction is a C5 - 77'C (170"F) light reformate which is removed from the separator via line 32. The light reformate has a research octane number of about 71 and is about 10 liquid volume percent of the total reformate.

The light reformate in line 32 is combined with the light straight-run naphtha fraction having a research octane number of about 65 in line 24 and passed as a composite stream via line 24 into isomerization zone 50, where it is contacted in the presence of hydrogen (supply line not shown) with a hydrocarbon isomerization catalyst. The isomerization conditions in zone 50 include a temperature of approximately 150"C (302"F), a pressure of 1 5 atmospheres, a liquid hourly space velocity of 1.5 V/V/Hr and a hydrogen to hydrocarbon mol ratio of 3. The resulting isomerizate is withdrawn in line 51 and passed to separator 52 wherein it is separated into a C4 fraction and a liquid hydrocarbon fraction in about a 95 liquid volume percent yield and having a research octane number of about 79.

The C4 - fraction is withdrawn from the separator 1 52 via line 53 and the hydrogen component therein may be separated and recycled to isomerization zone 50, if desired. The Cs + fraction from separator 52 is withdrawn via line 56 and passed to a gasoline pool as a high-octane gasoline blending stock having a research octane number of at least 95 (clear).

Referring again to separator 29, the portion of the reformate boiling above 77"C (170"F) is passed via line 1 33 to ZSM conversion zone 34. About 90 liquid volume percent of the reformate leaves separator 29 via line 33 and this heavy reformate fraction has a research octane number of approximaely 100.

In the ZSM reaction zone, the heavy reformate is contacted with an H-ZSM-5 catalyst comprising 50% zeolite and 50% Catapal matrix.

ZSM reaction conditions include a temperature of 343"C (650"F), a pressure of 28 atmospheres (400 psig), a hydrogen to hydrocarbon mol ratio of 5, and a liquid hourly space velocity of 2 V/V/Hr. The ZSM reactor is operated at reaction conditions to crack out paraffins, leaving mainly aromatics in the effluent, and to isomerize the xylenes as well as generally upgrade the octane of the effluent.

The effluent from the ZSM reactor is withdrawn via line 35 and is charged to separator 36. A C4 - fraction is withdrawn from the top of the separator via line 37 and a C5 + fraction is removed from separator 36 via line 38. The C5 + fraction has a reseach octane number of approximately 113, contains approximately 45 volume percent BTX and represents about 85 volume percent of the feed to the ZSM reaction zone. The C6 + fraction in line 38 may be sent via line 40 to form a part of the high-octane gasoline pool, or some or all of it may be sent via line 39 to a BTX separation step, and if desired the BTX-depleted stream may be returned to the gasoline pool.

The advantage to this flow scheme of the present invention is that it allows the isomerization zone, the reforming zone and the ZSM reaction zone each to be operated with the feedstocks that are converted most effectively therein. The C5-66 C portion of the straightrun naphtha is more advantageously isomerized than reformed, because the lower temper atures in the isomerization zone favor production of more highly branched paraffins with a resulting higher octane. In the reformer, C5 and C6 paraffins would suffer a yield loss due to cracking as well as having a relatively high equilibrium amount of low-octane normal and slightly branched paraffins.Thus, the process of the present invention provides for sending the portion of the naphtha containing C6 + naphthenes (such as methylcyclopentane) and higher-boiling hydrocarbons to the reforming zone where the naphthenes are efficiently converted to aromatics, while the light naphtha containing C6 and most of the C6 paraffins is sent to the isomerization zone.

The advantage of splitting the reformate into a light fraction (boiling from C5 to 77"C) is that this fraction may be isomerized to further increase the octane number with little decrease in yield. The heavy reformate containing benzene and higher-boiling hydrocarbons is advantageously contacted with the ZSM catalyst at cracking conditions to remove any C, + paraffins and leave a product stream concentrated in aromatics such as benzene, toluene and xylene.

Feedstocks suitable for use in the process of the present invention include full-boiling-range naphtha hydrocarbon materials boiling in the range of C6 hydrocarbons up to about 1 75'C (347"F) or 200"C (392"F) which contain lowoctane paraffinic C6 and C6 components and preferably are C6 to C6 straight-run naphthas.

The C6 naphthenes in these naphthas are excellent reformer feedstock components, for they are efficiently converted to aromatics.

The paraffins are undesirable as components of a gasoline pool because of their low research octane numbers, usually under about 70 (Clear, ASTM Method). The C7 + paraffins are suitable reformer feed components, but the C6 - paraffins are marginal because of the yield loss when they are cracked and the equilibrium amount of relatively low-octane normal and singly branched paraffins which remain after isomerization.

The reformer of the present invention is a conventional one in which the feedstock is contacted with a platinum-containing reforming catalyst, preferably a bimetallic catalyst such as platinum-rhenium-chloride on alumina, under reaction conditions such as a temperature from 427 to 552to (800-1025"F), preferably from 454-538"C (850-1000"F), a pressure from atmospheric to 50 atmospheres or higher, preferably from 6.8 to 40 atmospheres, a liquid hourly space velocity from 0.1 to 10, preferably from 0.5 to 5, and a hydrogen to hydrocarbon mol ratio from 0.5 to 20, and preferably from 1 to 10.During reforming a multitude of reactions takes place, including dehydrogenation, isomerization, dehydrocyclization, hydrocracking, and combinations thereof to yield a product having an increased content of aromatics and branched-chain hydrocarbons. The reformer is especially efficient when used to convert naphthenes to aromatics.

At least a portion of the reformate is passed over a ZSM-5-type catalyst in a ZSM reaction zone operated at conditions including an elevated temperature. Broadly, the temperature will be from 343 to 649"C (650" to 1200"F) and the pressure will be from 0.5 to 55 atmospheres and a liquid hourly space velocity from 0.1 to 20 and preferably 1 to 5.

In the preferred embodiment in which the ZSM reaction zone is operated at conditions to crack paraffins and isomerize C6 aromatics in the feed, the preferred reaction conditions include a temperature from 371-482"C (700-900"F), a hydrogen pressure from 1.7 to 55 atmospheres, and a hydrogen to hydrocarbon mol ratio from 1 to 10 and preferably 3 to 8.

In the preferred embodiment in which the light naphtha fraction is combined with at least a portion of the reformate and optionally a recycle or externally supplied C3/C4 stream, the reaction zone is preferably maintained under reaction conditions promoting aromatization of paraffinic compounds, such as a temperature from 454 to 566"C (850 to 1050"F) and more preferably from 500 to 540"C (932 to 1004"F), a pressure from 0.5 to 35 atmospheres or higher, more preferably from 1 to 10 atmospheres and still more preferably 1 to 5 atmospheres, and preferably in the absence of added hydrogen.

The ZSM-5-type zeolite itself is known in the art per se, and is exemplified by ZSM-5, ZSM-8, ZSM-11 and ZSM-35 and other similar materials. ZSM-5-type zeolites, described in U.S. Patents 3,702,886, 3,729,409 and 3,770,614, describe the ZSM-5 preparation, composition and use as well as related information and are incorporated herein by reference. The H-ZSM-5 or Zn-H-ZSM-5 forms of the ZSM-5-type zeolites are preferred for use herein and may be obtained by conventional base and/or ionexchange methods well known in the art. It is especially beneficial with respect to the catalyst life and coke formation for the ZSM-5 zeolite to have a silica-to-alumina mol ratio from 40 to 160, and preferably from 60 to 120.

The catalyst of the ZSM reaction zone may be in any convenient form, that is, as required for conventional fixed, fluid or slurry usage.

Preferably, the ZSM-5-type is a fixed-bed type with the zeolite being composited with an inorganic binder or matrix such as alumina, silica, silica-alumina mixtures, naturally occurring and conventionally processed clays, e.g., kaolin and the like, as well as silica-magnesia, silica-zirconia, etc., and the mixtures of any of them. The composite is preferably prepared by mixing the binder or matrix in the form of a gel or a cogel with the zeolite, followed by shaping or extruding to the desired form and size customary for the intended use. The relative proportions of zeolite and binder may vary widely, from 5% to 95% by weight, with preferably 35% to 80% and more preferably about 65% of the composition being zeolite.

The preferred binder is alumina.

The isomerization zone is used to isomerize C5 and C6 paraffins in the light naphtha and light reformate fractions. Any suitable light paraffi nic hydrocarbon isomerization catalyst and method may be used and numerous suitable methods have been described in the prior art and descriptions of representative methods are given in an article entitled "Advances in Isomerization" by P. A. Lawrence et al, Proceedings of the Seventh World Petroleum Congress, Volume IV, pp. 135-145, Elsevier Publishing Company (1967). Other examples of isomerization processes for paraffins appear in the following U.S. Patents: 2,834,823, 3,190,939, 3,527,835, 3,577,479, 3,578,725, and 3,789,082.

In a preferred embodiment, the isomerization is carried out using as the catalyst a chlorided composite of platinum dispersed upon porous alumina (e.g., see U.S. Patent 3,789,082, Example 1). In another preferred embodiment, the catalyst employed is a composite of palladium and ultra-stable Y crystalline aluminosilicate molecular sieve in the H form (see, for example, U.S. Patent 3,293,192). This catalyst is prepared by any suitable method. For example an aqueous solution of the palladium salt is admixed with acid-peptized alumina hydrogel and, thereafter, the palladium is gravimetrically precipitated in a finely divided form by admixing a minor amount of 1,2,3-benzotriazole in hydrogel (see for example U.S. Patent 3,978,001).

Next, the H-Y sieve is ad-mixed with the hydrogel and the resulting composite is shaped, dried and calcined for use. Sufficient amounts of the components are used to provide on a dry basis for each 100 parts by weight a composite containing alumina, ultrastable Y-sieve and palladium in an amount of 35, 65 and 0.3 parts, respectively. The relative amounts of these components may be varied widely as in the conventional practice, and yet the catalyst will be effective for isomerizing the feed herein. The isomerization of the C5-C6 paraffinic hydrocarbon feed under hydrocarbon isomerizing conditions per se is not considered as inventive.

Claims (11)

1. A process for upgrading a naphtha hydrocarbon feedstock, which comprises: (1) separating said naphtha feedstock into a light naphtha fraction boiling below methylcyclopentane and containing C6 aliphatics and lower-boiling hydrocarbons, and heavy naphtha fraction containing methylcyclopentane and higher-boiling hydrocarbons; (2) reforming said heavy naphtha fraction under reforming conditions to produce a reformate stream enriched in aromatics compared with said heavy naphtha fraction; (3) contacting at least a portion of said reformate stream with a ZSM-5-type zeolite catalyst at an elevated temperature and recovering from the ZSM-5-type contacting step a hydrocarbon effluent enriched in aromatic hydrocarbons; and (4) separating from said effluent a C5 + product stream.

2. A process according to Claim 1, wherein step (3) comprises separating said reformate into a light reformate fraction containing C5 and C6 paraffinic hydrocarbons and a heavy reformate fraction containing benzene and higher-boiling hydrocarbons which heavy reformate fraction is said portion of the reformate passed into contact with the ZSM-5type zeolite catalyst at conditions selected to effect cracking of normal and lightly branched hydrocarbons as well as isomerization of xylenes, including a temperature of from 371 to 482"C, a pressure of from 1.5 to 55 atmospheres, and a hydrogen-to-hydrocarbon mol ratio from 1 to 15, and further comprises (5) passing said light reformate fraction together with said light naphtha fraction into contact with a hydrocarbon isomerization catalyst under isomerization conditions including a temperature of from 50 to 300"C and a pressure of from 0.5 to 100 atmospheres and recovering the resultant isomerized product comprising C5 + hydrocarbons having a higher proportion of multi-branched paraffins than the feed to the isomerization catalyst.

3. A process according to Claim 2, wherein at least a portion of said C5 + product stream from said ZSM-5 contacting step is combined with said isomerized product to form a gasoline blend having a research octane number of at least 95 (Clear).

4. A process accordinf to Claim 2, wherein benzene, toluene and xylene values are recovered from at least a portion of said C5 + product stream from said ZSM-5 contacting step and the resulting benzene, toluene and xylene-deficient stream is combined with the C5 + portion of said isomerized product to form a hydrocarbon blend having a research octane number of at least 95 (Clear).

5. A process according to Claim 1, wherein in step (3) said reformate portion is combined with said light fraction and the ZSM-5 contacting conditions include a temperature of from 343 to 649"C and a pressure of frrom 0.5 to 55 atmospheres.

6. A process according to Claim 5, wherein the naphtha feedstock is a C5 to C6 straight-run naphtha and all of said reformate is contacted with said ZSM-5-type zeolite catalyst, and said C5 + product stream is sent for recovery of the benzene, toluene and xylene components therein.

7. A process according to Claim 5, wherein said reformate is separated into a first fraction boiling just below xylene and a second fraction containing xylene and higherboiling components, said first fraction is said portion which is contacted with said ZSM-5type zeolite catalyst, said C5 + product stream is sent for recovery of the benzene, toluene and xylene components thereof, and said second fraction is isomerized to optimize the xylene content thereof.

8. A process according to Claim 5 and further comprising separating a C3/C4 fraction from said aromatics-enriched hydrocarbon effluent and recycling said C3/C4 fraction to contact said ZSM-5-type zeolite catalyst.

9. A process according to Claim 5, 6, 7 or 8, wherein said ZSM-5-type zeolite catalyst is in the Zn-H-ZSM-5 form.

10. A process according to Claim 5, 6, 7, 8 or 9, wherein the contacting of the ZSM-5type catalyst is effected at a temperature of from 500 to 540"C and a pressure of from 1 to 5 atmospheres, and in the absence of added hydrogen.

11. A process according to Claim 1, 2, 3 or 4, wherein said ZSM-5-type zeolite catalyst is in the H-ZSM-5 form.

1 2. A process for upgrading a naphtha hydrocarbon feedstock substantially as hereinbefore described with reference to Fig. 1 of the accompanying drawings.

1 3. A process for upgrading a naphtha hydrocarbon feedstock substantially as hereinbefore described with reference to Fig. 2 of the accompanying drawings.