Multistage Reforming With Interstage Aromatics Removal

The present invention relates to a process for catalytically reforming a gasoline boiling range hydrocarbonaceous feedstock. The reforming is conducted in multiple stages with aromatics separation between stages. At least one of the downstream reactors contains a supported noble-metal containing catalyst promoted with a promoter metal or a catalyst comprised of a Group VIII metal on a zeolite support, which zeolite is selected from type-X, type-Y, and type-L zeolites.

Catalytic reforming is a well established refinery process for improving the octane quality of naphthas or straight run gasolines. Reforming can be defined as the total effect of the molecular changes, or hydrocarbon reactions, produced by dehydrogenation of cyclohexanes, dehydroisomerization of alkylcyclopentanes, and dehydrocyclization of paraffins and olefins to yield aromatics; isomerization of n-paraffins; isomerization of alkylcycloparaffins to yield cyclohexanes; isomerization of substituted aromatics; and hydrocracking of paraffins which produces gas, and inevitably coke, the latter being deposited on the catalyst. In catalytic reforming, a multifunctional catalyst is usually employed which contains a metal hydrogenation-dehydrogenation (hydrogen transfer) component, or components, usually platinum, substantially atomically dispersed on the surface of a porous, inorganic oxide support, such as alumina. The support, which usually contains a halide, particularly chloride, provides the acid functionality needed for isomerization, cyclization, and hydrocracking reactions.

Reforming reactions are both endothermic and exothermic, the former being predominant, particularly in the early stages of reforming with the latter being predominant in the latter stages. In view thereof, it has become the practice to employ a reforming unit comprised of a plurality of serially connected reactors with provision for heating of the reaction stream from one reactor to another. There are three major types of reforming: semiregenerative, cyclic, and continuous. Fixed-bed reactors are usually employed in semiregenerative and cyclic reforming and moving-bed reactors in continuous reforming. In semiregenerative reforming, the entire reforming process unit is operated by gradually and progressively increasing the temperature to compensate for deactivation of the catalyst caused by coke deposition, until finally the entire unit is shut-down for regeneration and reactivation of the catalyst, In cyclic reforming, the reactors are individually isolated, or in effect swung out of line, by various piping arrangements. The catalyst is regenerated by removing coke deposits, and then reactived while the other reactors of the series remain on stream. The "swing reactors" temporarily replaces a reactor which is removed from the series for regeneration and reactivation of the catalyst, which is then put back in the series. In continuous reforming, the reactors are moving-bed reactors, as opposed to fixed-bed reactors, with continuous addition and withdrawal of catalyst and catalyst is regenerated in a separate regeneration vessel.

Through the years, many process variations have been proposed to improve such things as C.sub.5.sup.+ liquid (a relatively high octane product stream) yield and/or octane quality of the product stream from catalytic reforming. For example, if a product of high octane is desired, e.g., 100 or higher RON (research octane number), the severity of reforming must be increased. This can generally be accomplished by reducing the space velocity or increasing reaction temperature. While increased severity for obtaining a higher octane product is desirable, it has disadvantages. For example, high severity usually: (i) reduces the yield of C.sub.5.sup.+ as a percent of the naphtha feedstock; (ii) usually causes more rapid accumulation of coke on the catalyst, thus rapidly decreasing the activity of the catalyst and requiring more frequent regeneration.

Practice of the present invention results in a significantly higher yield of hydrogen and of C.sub.5.sup.+ liquid as a percent of the naphtha feedstock. This is achieved by conducting the reforming in multiple stages and separating an aromatics-rich (high octane) stream between stages. The separation is performed after reforming at low severity, in a first stage or stages, to convert most of the alkycyclohexanes and alkylcyclopentanes to aromatics with minimum conversion, especially cracking, of paraffins. The remaining paraffin-rich, or aromatics-lean stream is processed in the downstream stage, or stages, at relatively high severity and preferably at relatively low pressures.

While there are some references in the art teaching aromatics removal between and after reactors of a reforming process unit, none suggests aromatics removal, after low severity catalytic reforming using a multimetallic catalyst followed by relatively high severity reforming, at low pressures.

For example, U.S. Pat. No. 2,970,106 teaches reforming to a relatively high octane (99.9 RON) followed by two stage distillation to produce three different streams: a light, intermediate, and heavy boiling stream. The intermediate stream, which contains C.sub.7 and C.sub.8 aromatics, is subjected to permeation by use of a semipermeable membrane resulting in an aromatics-rich stream and an aromatics-lean stream, both of which are distilled to achieve further isolation of aromatics. It is also taught that the aromatics-lean stream from the permeation process may be combined with a low octane stream from hydroformate distillation and further hydroformed, or isomerized, to improve octane number. It is further taught that the total hydroformate may be processed using the permeation process. Partial or low severity reforming, followed by aromatics separation, followed by further reforming with a stream containing a significant fraction of the paraffins in the original feedstock is not suggested in U.S. Pat. No. 2,970,106. Operation of the first-stage at high octane (99.9 RON) would result in very high conversion of feed paraffins. For example a key paraffin, n-heptane and its various isomers, would be about 46 to 54% converted at 99.9 RON for a petroleum naphtha cut (185.degree./330.degree. F.) comprised of 59% paraffins, 27% naphthenes, and 14% aromatics, which percents are liquid volume percent on total paraffins, naphthenes and aromatics present in the feed. In accordance with the process of the present invention, conversion of the n-heptane and its various isomers would be only about 11 to 14% in the first reforming stage-thus allowing more selective (less paraffin cracking) conversion to aromatics in the lower pressure second-stage.

Also, U.S. Pat. No. 3,883,418 teaches reforming a feedstock in the presence of hydrogen over a bifunctional catalyst in a first stage to convert naphthenes to aromatics, followed by distillation of the first stage product to produce an intermediate boiling (120.degree.-260.degree. F.) material which is subjected to extractive distillation to produce an aromatics-rich extract and an aromatics-lean raffinate. The aromatics-lean, or paraffins-rich, raffinate is then reformed in the presence of steam over a steam-stable catalyst. Steam reforming employs a steam reaction atmosphere in the presence of a catalyst having a relatively low surface area aluminate support material. Reforming, in accordance with the present invention, employs a hydrogen reaction atmosphere, in the substantial absence of steam, and in the presence of a catalyst having a relatively high surface area support material, such as gamma alumina or a zeolitic material.

Further, U.S. Pat. No. 4,206,035 teaches a process similar to U.S. Pat. No. 3,883,418, except that solvent extraction is used to remove aromatics instead of extractive distillation, and the aromatics-lean fraction sent to steam reforming is restricted to carbon numbers between 5 and 9. Also, specific hydrogen to hydrocarbon ratios and steam to hydrocarbon ratios are required.

U.S. Pat. No. 2,933,455 teaches a catalytic reforming process wherein the entire feedstock is first fractionated. The resulting 140.degree. to 210.degree. F. and 260.degree. to 420.degree. F. fractions are reformed in the presence of hydrogen in parallel reformers. In the reforming of the 140.degree. to 210.degree. F. fraction, the reforming severity is set such that naphthenes are converted to benzene and toluene and the resulting reformate is treated to remove aromatics. The remaining stream, containing at least 80 percent paraffins (primarily those containing 6 and 7 carbon atoms) is blended with the heavy 260.degree. to 420.degree. F. fraction and reformed in a second reformer. This reference teaches restricting the hydrocarbons reformed prior to aromatics removal to only the light naphtha components which form C.sub.6 and C.sub.7 aromatics. In addition, it teaches further reforming of the light paraffin-rich stream remaining after aromatics removal, in admixture with a heavy feed which is rich in aromatics and naphthenes.

Further, U.S. Pat. No. 3,640,818 teaches a process wherein virgin and cracked naphthas are reformed in a first stage and the reaction stream passed to solvent extraction where aromatics are removed. The paraffin-rich raffinate is passed to second stage reforming, preferably at pressures the same or higher than the first stage.

In accordance with the present invention, there is provided a process for catalytically reforming a gasoline boiling range hydrocarbon feedstock in the presence of hydrogen in a reforming process unit comprised of a plurality of serially connected reactors wherein each of the reactors contains a Group VIII metal-containing reforming catalyst composition, the process comprising:

(a) conducting the reforming two or more stages comprised of one or more reactors;

(b) separating at least a portion of the reaction stream between each stage into an aromatics-rich and an aromatics-lean stream;

(c) passing at least a portion of the aromatics-lean stream to the next downstream stage, in the substantial absence of heavy virgin naphtha; and

(d) conducting the reforming of one or more of the reactors of the downstream stages in the presence of a reforming catalyst selected from: (i) at least one noble metal, and a promoter metal selected from metals from Groups, IIA, IVA, IB, VIB, VIIB, and VII of the Periodic Table of the Elements, and an inorganic oxide support other than a zeolitic support; and (ii) a Group VII metal on a zeolitic support which zeolitic support is selected from type-X, type-Y, and type-L zeolites; and wherein at least one downstream reactor is operated in the substantial absence of steam, and at a pressure which is at least 25 psig lower than that of the first stage.

In a preferred embodiment of the present invention, the zeolite is a type-L zeolite.

In other preferred embodiments of the present invention, one or more of the downstream stages are operated such that gaseous products are not recycled.

In yet other preferred embodiments of the present invention, separation of the reaction stream into an aromatic-rich and an aromatics-lean stream is accomplished by permeation using a semipermeable membrane, adsorption, distillation, or extraction.

In another preferred embodiment of the present invention, separation of the product stream is accomplished by using solvent extraction and distillation.

In still other preferred embodiments of the present invention, the noble metal of both the upstream catalyst and the downstream catalyst is platinum, and the upstream catalyst is further defined as containing a chloride and alumina as the support material.

In yet another preferred embodiment of the present invention, the reforming process unit contains two stages, wherein the first stage is operated in semiregenerative mode and the second stage is operated in cyclic mode.

The sole FIGURE hereof depicts a simplified flow diagram of a preferred reforming process unit of the present invention. The reforming process unit is comprised of a first stage which includes a lead reactor and a first downstream reactor operated in semiregenerative mode, wherein the reaction stream of the first stage is separated into an aromatics-rich stream and an aromatics-lean stream. The aromatics-lean stream is passed to a second reforming stage which includes two serially connected downstream reactors operated in cyclic mode with a swing reactor.

Feedstocks which are suitable for reforming in accordance with the instant invention are any hydrocarbonaceous feedstocks boiling in the gasoline range. Non-limiting examples of such feedstocks include the light hydrocarbon oils boiling from about 70.degree. F. to about 500.degree. F., preferably from about 180.degree. F. to about 400.degree. F., for example straight run naphtha, heavy virgin naphtha, synthetically produced naphtha such as a coil or oil-shale derived naphtha, thermally or catalytically cracked naphtha, hydrocracked naphtha, or blends or functions thereof.

Referring to the FIGURE, a feedstock, which preferably is first hydrotreated by any conventional hydrotreating method to remove undesirable components such as sulfur and nitrogen, is passed to a first reforming stage represented by heater or preheat furnaces F.sub.1 or F.sub.2, and reactors R.sub.1 and R.sub.2, A reforming stage, as used herein, is any one or more reactors and its associated equipment (e.g., preheat furnaces etc.) separated from an immediately preceding or succeeding stage by the separation of aromatics from the reaction stream of the preceding stage. The feedstock is fed into heater, or preheat furnace, F.sub.1 via line 10 where it is heated to an effective reforming temperature. That is, to a temperature high enough to initiate and maintain dehydrogenation reactions, but not so high as to cause excessive hydrocracking. The heated feedstock is then fed, via line 12, into reactor R.sub.1 which contains a catalyst suitable for reforming. Reactor R.sub.1, as well as all other reactors in the process unit, is operated at reforming conditions. Typical reforming operating conditions that can be used for any of the reactors of any of the stages hereof are such that the reactor inlet temperature is from about 800.degree. to about 1200.degree. F.; the reactor pressure from about 30 psig to about 1,000 psig, preferably from about 30 psig to about 500 psig; a weight hourly space velocity (WHSV) of about 0.5 to about 20, preferably from about 1 to about 10; and a hydrogen to oil ratio of about 1 to 10 moles of hydrogen per mole of C.sub.5.sup.+ feed.

The reaction product of reactor R.sub.1 is fed to preheat furnace F.sub.2 via line 14, then to reactor R.sub.2 via line 16. The reaction product from the first stage is sent to cooler K.sub.1 via line 18 where it is cooled to condense the liquid to a temperature within the operating range of the aromatics separation unit. This temperature will generally range from about 100.degree. to about 300.degree. F. The cooled reaction product is then fed to separator S.sub.1 via line 20 where a lighter gaseous stream is separated form a heavier liquid stream. The gaseous stream, which is hydrogen-rich, is recycled, via line 22, to line 10 by first passing it through compressor C.sub.1 to increase its pressure to feedstock pressure. Of course, during startup, the unit is pressured-up with hydrogen from an independent source until enough hydrogen can be generated in the first stage, or stages, for recycle. It is preferred that the first stage be operated in semiregenerative mode.

The liquid fraction from separator S.sub.1 passed via line 24, through pressure reduction valve 25, to aromatics separation unit A where aromatic materials are separated, thus resulting in an aromatics-rich and an aromatics-lean stream. The terms "aromatic-rich" and "aromatics-lean" as used herein refer to the level of aromatics in the liquid fraction reaction stream after aromatics separation relative to the level of aromatics prior to separation. That is, after a reaction stream is subjected to an aromatics separation technique two fractions result. One fraction has a higher level of aromatics relative to the stream before separation and is thus referred to as the aromatics-rich fraction. The other fraction is, of course, the aromatics-lean fraction which can also be referred to as the paraffin-rich fraction. Aromatics separation can be accomplished by extraction, extractive distillation, distillation, adsorption, and by permeation through a semipermeable membrane, or by any other appropriate aromatics or paraffins removal process. Preferred are use of a semipermeable membrane, extraction and distillation. More preferred are extraction and distillation.

Both the aromatics-rich and the aromatics-lean streams will also contain paraffin and naphthenic material. The aromatics-rich stream, because of the relatively high level of aromatic components, has a relatively high octane value. Such a high octane stream, which exits the separation unit via line 26, can be used as a high octane blending stock or its can be used as a source of raw material for chemical feedstock. The aromatics-lean stream exits the separation unit via line 28 where it is mixed with the hydrogen rich gaseous product of the first stage via line 29, which passes from the separator and through pressure reduction valve 27, then to a second reforming stage by passing it through furnace F.sub.3 via line 30 where it is heated to reforming temperatures.

The heated aromatics-lean stream from furnace F.sub.3 is introduced into reactor R.sub.3 via liner 32. It will be noted that this invention excludes the introduction of heavy virgin naphtha into the downstream stages. By heavy virgin naphtha, we mean a virgin, or straight run, naphtha having an initial boiling point of at least about 250.degree. F. Such a naphtha usually contains components boiling above about 300.degree. F. Non-reformed heavy virgin naphtha is to be avoided in the downstream stages because it will act to increase catalyst deactivation at the conditions at which the downstream stages are operated. The reaction stream from reactor R.sub.3 is then passed to furnace F.sub.4 via line 34 then to reactor R.sub.4 via lie 36. Reactors R.sub.3 and R.sub.4 also contain a reforming catalyst which may or may not be the same as the catalyst composition used in the first reforming stage. Furthermore, any reactor, or portion thereof, of any stage may contain a reforming catalyst different than that of any other reactor so long as at least one reactor of a downstream stage containsl (i) a catalyst comprised of at least one noble metal, and a promoter metal, and an inorganic support; or (ii) a Group VIII metal on a zeolite support which is selected from type-X, type-Y, and type-L zeolites. Product from reactor R.sub.4 is passed to cooler K.sub.2 via line 38 where it is cooled and sent via line 40 to separator S.sub.2 where it is separated into a liquid stream 42 and a hydrogen-rich make-gas stream 44 which is passed through compressor C.sub.2 after which it leaves the process unit or can be recycled to the process unit. It is preferred that the second stage be operated in cyclic mode with swing reactor R.sub.5, regeneration gas heater F.sub.5, compressor C.sub.3, and cooler K.sub.3. The second stage, as well as any additional downstream stage, is operated at a pressure at least 25 psig lower than the first stage, more preferably at a pressure less than about 200 psig total pressure, and most preferably less than 100 psig total pressure. While the FIGURE shows only two reactors on oil for both stages, it is understood that any number of reactors can be used. Of course, economics will dictate the number of reactors and stages employed commercially.

It is also to be understood that the FIGURE hereof sets forth a preferred mode of practicing the instant invention and as such, many variations of the process scheme illustrated in the FIGURE can be practiced and still be within the scope of the invention. For example, at least a portion of the reaction stream from the second stage can be recycled through the aromatics separation unit between the stages or it can be separated in an aromatics separation unit following the second stage and the resulting aromatics-lean stream recycled to the second stage reactors. Further, a three stage reforming process can be employed with an aromatics separation unit between stages one and two as well as an aromatics separation unit following the third stage with the resulting aromatics-lean stream from this third aromatics separation unit recycled to the reactors of the third stage. Also, the same aromatics-separation unit can be used to produce an aromatic-rich and aromatics-lean stream from more than one reactor.

Catalysts suitable for use herein include both monofunctional and bifunctional catalysts. The bifunctional reforming catalysts are typically comprised of a hydrogenation-dehydrogenation function and an acid function. The acid function, which is important for isomerization, is thought to be associated with a material of the porous, adsorptive, refractory oxide type, which serves as the support, or carrier, for the metal component. The metal component is usually one or more Group VIII noble metals, to which is generally attributed the hydrogenation-dehydrogenation function. Preferably, the Group VIII noble metal is platinum.

The catalyst used in this invention, preferably the non-zeolitic catalyst of the downstream stages, may also contain a promoter metals selected from Groups IIA, such as gallium; IVA, such as tin; IB, such as copper; VIB, such as chromium; VIIB, such as rhenium; and VIII, such as iridium, of the Periodic Table of the Elements. The promoter metal can be present in the form of an oxide, sulfide, or elemental stage in an amount from about 0.01 to about 5 wt. % preferably from about 0.1 to about 3 wt. %, and more preferably from about 0.2 to about 3 wt. %, calculated on an elemental basis, and based on the total weight of the catalyst composition. The Periodic Table referred to herein is the one found in the inside front cover of Perry's Chemical Engineer's Handbook, Perry and Green, McGraw-Hill Book, Co., Sixth Edition, 1984.

Monofunctional catalysts suitable for use herein include zeolites comprised of a hydrogenation-dehydrogenation function on a large pore zeolite support. The hydrogenation-dehydrogenation function is provided by one or more Group VIII metals, preferably a Group VIII noble metal. Large-pore zeolites, as referred to herein, are defined as zeolites having an effective pore diameter of about 6-15 Angstroms. Preferred large-pore zeolites include zeolite-X, zeolite-Y, zeolite-L, and those zeolites isostructural to said zeolites. The term, type, will be used herein to define a particular zeolite and those isostructural to it. For example, type-L zeolites would include zeolite-L itself and those zeolites isostructural to it. Naturally occurring large-pore zeolites, such as faujasite and mordenite, are also suitable for use herein. If a zeolite catalyst is used herein it is preferred to use it in one or more of the downstream reactors.

Large pore zeolites usually contain an exchangeable cation which may be one or more metals selected from the group consisting of alkali and alkaline-earth metals. Preferably, the exchangeable cation comprises one or more alkali metals, particularly potassium, which can be partially or substantially fully exchanged with one or more alkaline-earth metals. The preferred alkaline-earth metals are magnesium, calcium, barium, and strontium. Cation exchange may also be effected with zinc, nickel, manganese, cobalt, copper, lead, and cesium. The most preferred alkaline-earth metal is barium.

In addition to, or other than by ion exchange, the alkaline-earth metal can be incorporated into the zeolite by synthesis or impregnation.

The zeolite containing catalysts used in the practice of this invention can also contain one or more inorganic refractory oxides which may be utilized as a carrier to bind the large pore zeolites. Preferred inorganic oxides include clays, alumina, and silica. Most preferred is alumina.

Included among the catalysts suitable for use herein are those disclosed in U.S. Pat. Nos. 4,595,668; 4,645,586; 4,636,298; 4,594,145; and 4,104,320. The disclosures of all of these patents are incorporated herein by reference.

The type-L zeolites, which are most preferred for use in the instant invention, can be defined as synthetic zeolites which crystallize in the hexagonal system with a characteristic x-ray diffraction pattern obtained from CuK.alpha. radiation with the major d(.ANG.) big values as set out below:

For purposes of this invention, type-L zeolites have the general formula:

where M designates at least one exchangeable cation, n represents the valence of M, y is any value from 0 to about 9, and x is any value between 5.2 and 6.9. Zeolite L is thoroughly described in U.S. Pat. No. 3,216,789, which is incorporated herein by reference. The variable x may be outside the disclosed range provided the x-ray diffraction pattern of the zeolite is substantially the same as that of zeolite L. Thus, type-L zeolites with SiO.sub.2 /Al.sub.2 O.sub.3 ratios less than 5.2 and greater than 6.9 are applicable to this invention. Preferably, the SiO.sub.2 /Al.sub.2 O.sub.3 ratio may vary between about 2 and 50. For example, one method of reducing the SiO.sub.2 /Al.sub.2 O.sub.3 ratio involves leaching some of the SiO.sub.2 with an alkaline metal hydroxide, e.g. KOH, to produce type L zeolite useful in this invention. Zeolite L has channel shaped pores undulating from about 7 to about 13 .ANG. in diameter and may occur in the form of cylindrical crystals with a diameter of at least 0.5 micron in an aspect ratio of at least 0.5 (as described, e.g., in UK application 82-14147, the entirety of which is incorporated herein by reference), as well as in other sizes and shapes.

Type-L zeolites are conventionally prepared such that M in the above formula is potassium. See, e.g., U.S. Pat. Nos. 3,216,789 and 3,867,512. The potassium may be ion exchanged, as is well known, by treating the zeolite in an aqueous solution containing other cations. It is difficult, however, to exchange more than 75% of the original potassium cations because some cations occupy sites in the zeolite structure which are nearly inaccessible. At least 75% of the exchangeable cations are selected from lithium, sodium, potassium, rubidium, cesium, calcium, and barium. More preferably, the cation is sodium, potassium, rubidium, or cesium, and most preferably it is potassium. Optionally, the exchangeable cations may consist of mixtures of the abovenamed Group IA cations or mixtures of a Group IA cation and barium or calcium cations. These mixtures of cations may be achieved for example, by treating the zeolite L with an aqueous solution containing a rubidium and/or cesium salt and then washing to remove excess ions. This ion exchange treatment can be repeated to effect further ion exchange, although to a lesser degree.

Group VIII metals suitable for use herein include nickel and the noble metals platinum, palladium, iridium, ruthenium, rhodium, and osmium. Preferred are the noble metals, and more preferred is platinum. It is also preferred that the catalyst compositions has a relatively high surface area, for example, about 100 to 400 m.sup.2 /g.

The bifunctional catalysts will preferably contain a halide component which contributes to the necessary acid functionality of the catalyst. This halide component may be fluoride, chloride, iodide, bromide, or mixtures thereof. Of these, fluoride, and particularly chloride, are preferred. Generally, the amount of halide is such that the final catalyst composition will contain from about 0.1 to about 3.5 wt. %, preferably about 0.5 to about 1.5 wt. % of halogen calculated on an elemental basis.

Preferably, the platinum group metal will be present on the catalyst in an amount from about 0.01 to about 5 wt. %, calculated on an elemental basis, of the final catalytic composition. More preferably the catalyst comprises from about 0.1 to 2 wt. % platinum group component, especially about 0.1 to 2 wt. % platinum. Other preferred platinum group metals include palladium, iridium, rhodium, osmium, ruthenium and mixtures thereof.

As previously mentioned, aromatics removal can be accomplished by extraction, extractive distillation, distillation, adsorption, by use of semipermeable membrane or any other appropriate method for the removal of aromatics or paraffins. Preferred are extraction and distillation.

Semipermeable membranes suitable for use herein as those which are compatible with the reaction stream and which preferentially permeate the aromatic components of the feed stream at an adequate and sustainable rate. Non-limiting examples of membranes which meet these requirements include those made from polyurea, polyurethane, and polyurea/urethanes.

The membranes used in the practice of the present invention may be cast in any thickness, membranes ranging in thickness of from about 0.1 to about 50 microns, preferably from about 0.1 to about 20 microns, and more preferably from about 0.1 to about 10 microns.

The separation techniques used herein with membranes could include either perstraction or pervaporation. Perstraction involves the selective dissolution of particular components contained in a mixture into the membrane, the diffusion of those components through the membrane and the removal of the diffused components from the downstream side of the membrane by use of a liquid sweep stream. In the perstractive separation of aromatics from non-aromatics, the aromatic molecules present in the stream dissolve into the membrane film due to similarities between the membrane solubility parameter and those of the aromatic species in the stream. The aromatics then permeate (diffuse) through the membrane and are swept away by a sweep liquid which is low in aromatics content. This keeps the concentration of aromatics at the permeate side of the membrane film low and maintains the concentration gradient which is responsible for the permeation of the aromatics through the membrane.

The sweep liquid is low in aromatics content so as not to itself decrease the concentration gradient. The sweep liquid is preferably a saturated hydrocarbon liquid with a boiling point much lower or much higher than that of the permeated aromatics. This is to facilitate separation, as by simple distillation. Suitable sweep liquids, therefore, would include, for example, C.sub.3 to C.sub.6 saturated hydrocarbons.

The perstraction process is run at a temperature 40.degree.-100.degree. C., preferably as low as practical to enhance membrane stability and life.

The choice of pressure is not critical since the perstraction process is not dependent on pressure, but on the ability of the aromatic components in the feed to dissolve into and migrate through the membrane under a concentration driving force. Consequently, any convenient pressure may be employed which pressure is determined by the hydrodynamics and configuration of the permeator used. Lower pressures are preferred to avoid undesirable compaction, if the membrane is supported on a porous backing, or rupture of the membrane, if it is not.

If C.sub.3 or C.sub.4 sweep liquids are used at 25.degree. C. or above in the liquid state, the pressure must be increased to keep them in the liquid phase.

Pervaporation, by comparison, is run at generally higher temperatures than perstraction to enhance aromatics permeation and relies on vacuum on the permeate side to evaporate the permeate from the surface of the membrane and maintain the concentration gradient driving force which drives the separation process. As in perstraction, the aromatic molecules present in the stream dissolve into the membrane film, permeate (diffuse) through said film and emerge on the permeate side where the aromatic molecules are removed by the vacuum generating equipment. Pervaporative separation of aromatics from non-aromatics of the reformate streams of the present invention are preformed at an effective temperature. That is, at a temperature that is not so high as to cause physical damage to the membrane or to result in an undesirable loss of selectivity. This temperature will usually range from about 80.degree. to 120.degree. C. Vacuum on the order of about 1-50 mm Hg is pulled on the permeate side. The vacuum stream containing the permeate is cooled to condense the highly aromatic permeate.

The membrane itself may be in any convenient form utilizing any convenient permeator design. Thus, sheets of membrane material may be used in spiral wound or plate and frame permeators. Tubes or hollow fibers of membranes may be used in bundled configurations. Feed can be processed either in the internal space of the tubes or fibers or the outside of the tubes or fibers. The sweep liquid, in the perstraction case, or the vacuum, in the pervaporation case, will be in the space opposite the feed.

Most conveniently, for the instant process, the membrane is used in a hollow fiber configuration with the feed introduced on the inside of the fiber and vacuum pulled on the outside of the hollow fiber to sweep away the permeated species, thereby maintaining a concentration gradient. The permeated aromatics-rich stream is condensed and collected, as a product. The retenate, or aromatics-lean stream, continues on to the next reforming stage.

Solvent extraction can be carried out by any suitable known technique in the art. Solvents gradually suited for such purposes include triethlene glycol, diethlene glycol, phenol, liquid sulfur dioxide, and sulfolane. Preferred in sulfolane. A more comprehensive list of solvents suitable for use herein can be found in U.S. Pat. No. 3,640,818, which is incorporated herein by reference. The extraction may be carried out in a tower filled with suitable packing material, such as earthenware, glass, etc. A fractionating column is also an effective means for ensuring efficient extraction of the aromatics with the selective solvent.

By practice of the present invention, reforming is conducted more efficiently and results in increased hydrogen and C.sub.5.sup.+ liquid yields. That is, the reactors upstream of aromatics separation are operated at conventional reforming temperatures and pressures while the reactors downstream of the aromatics removal, because of the removal of a substantial portion of feed as an aromatics-rich stream, can be operated at lower pressures, for example at pressures as low as from about 30 to about 100 psig. In addition, because of the removal of this aromatics-rich stream, the reactors downstream to its removal can be operated without recycling hydrogen-rich make-gas. That is, the downstream reactors can be operated in once-through hydrogen-rich gas mode because a sufficient amount of hydrogen is generated in the downstream reactors, that when combined with the hydrogen-rich gas from the reactors of the previous stage, there is an adequate amount of hydrogen to sustain the reforming reactions taking place in the downstream reactors.

The pressure drop in the downstream reactors can be reduced by operating in the once-through hydrogen-rich gas mode, thereby allowing for a smaller product-gas compressor (C.sub.2 in the FIGURE) than would otherwise be required. Furthermore, operating in a once-through hydrogen-rich gas mode also eliminates the need for a recycle gas compressor to circulate the hydrogen-rich make-gas in the downstream reactors.

Further, as previously discussed, practice of the present invention allows for a dual mode of operation wherein the stage upstream of aromatics separation can be operated in semiregenerative mode and the stage downstream of aromatics separation can be operated in cyclic mode. The frequency of regeneration of the downstream stage is decreased because the aromatics-lean stream is less susceptible to coking when compared with a conventional reforming reaction stream. A still further benefit of the instant invention is the fact that two octane streams are produced. The aromatics-rich stream is exceptionally high in octane number, for example, up to about 108 RON, or higher, and the octane number of the product stream from the downstream stage is flexible depending on the octane requirements for gasoline blending. These two independent octane streams allow for increased flexibility.

Another benefit of the present invention is that by operating the downstream reactors at lower octane severity, one is able to achieve lower coking rates, and thus longer catalyst life between regenerations. This lower severity also results in less undesirable polynuclear aromatic side products. An additional benefit of the present invention is that the aromatics-rich stream can be more easily separated into high value chemicals feedstocks such as benzene, toluene, and xylene.

The present invention will be more fully understood, and appreciated by reference to the following examples based on computer model predictions and are presented for illustrative purposes and not intended to define the scope of the invention.

Two sets of experiments were generated by a computer model of the reforming process. Both sets were based on use of a conventional platinum on alumina reforming catalyst, such as one comprised of 0.3 wt. % Pt and 0.3 wt. % Re on alumina. The first set, Comparative Example A and Example 1, are conducted at relatively high pressures whereas the second set, Comparative Example B and Example 2, are conducted at relatively low pressures. The feed for the first set of data is a 185.degree./330.degree. F. cut petroleum naphtha comprised of 59 vol. % paraffins, 27 vol. % naphthenes, and 14 vol. % aromatics. The feed for the second set of data is also a 185.degree./330.degree. F. cut petroleum naphtha, but it is comprised of 50 vol. % paraffins, 38 vol. % naphthenes, and 12 vol. % aromatics. The table below sets forth reaction conditions and predicted results.