Process for Reforming a Hydrocarbon Fraction With a Limited C.sub.9 + Content

1. Field of the Invention

The process of this invention provides for reforming of a hydrocarbon stream having a limited C.sub.9 + hydrocarbons content. The improved process is beneficial for any of several purposes, including the upgrading of motor gas (mogas) pools, or enhancing the yield of aromatic compounds in petrochemical operations.

2. Description of Material Information

Hydrocarbons can be subjected to a variety of processes, depending upon the product or products desired, and their intended purposes. A particularly significant process for treating hydrocarbons is that of reforming.

In hydrocarbon conversion, the reforming process is generally applied to fractions in the C.sub.6 -C.sub.11 range. The light fractions are unsuitable because they crack to lighter gases at reforming conditions; the heavier fractions cause higher coking rates (deposition of carbon on the catalyst), and therefore accelerate deactivation of the catalyst.

A variety of reactions occur as part of the reforming process. Among such reactions are dehydrogenation, isomerization, and hydrocracking. The dehydrogenation reactions typically include dehydroisomerization of alkylcyclopentanes to aromatics, dehydrogenation of paraffins to olefins, dehydrogenation of cyclohexanes to aromatics, and dehydrocyclization of paraffins and olefins to aromatics. Reforming processes are especially useful in petrochemical operations for upgrading mogas pool octane value, and in petrochemical operations for enhancing aromatics yield.

Different types of catalysts are used for conducting the reforming of hydrocarbon streams. One means of categorizing the type of catalysts so used is by designating them as "monofunctional" and "bifunctional" catalysts.

Monofunctional catalysts are those which accomplish all of the reforming reactions on one type of site - usually, a catalytically active metal site; these catalysts are monofunctional by virtue of lacking an acidic site for catalytic activity. Examples of monofunctional catalysts include the large pore zeolites, such as zeolites L, Y, and X and the naturally occurring faujasite and mordenite, wherein the exchangeable cation comprises a metal such such as alkali or alkaline earth metal; such catalysts also comprise one or more Group VIII metals providing the catalytically active metal sites, with platinum being a preferred Group VIII metal. Exchange of the metallic exchangeable cation of the zeolite crystal with hydrogen will provide acidic sites, thereby rendering the catalyst bifunctional.

A bifunctional catalyst is rendered bifunctional by virtue of also including acidic sites for catalytic reactions in addition to catalytically active metal sites. Included among conventional bifunctional reforming catalysts are those which comprise metal oxide support acidified by a halogen, such as chloride, and a Group VIII metal. A preferred metal oxide is alumina, and a preferred Group VIII metal is platinum.

The suitability of monofunctional and bifunctional catalysts for reforming varies according to the hydrocarbon number range of the fraction.

Both bifunctional and monofunctional catalysts are equally well suited for the naphthenes, or saturated cycloalkanes.

Monofunctional catalysts are particularly suited for reforming the C.sub.6 -C.sub.8 hydrocarbons. However, it has been discovered that the presence of dimethylbutanes, the lowest boiling of the C.sub.6 isomers, in the hydrocarbon fraction treated over monofunctional catalyst is commercially disadvantageous for two reasons.

As one reason, because of the reaction mechanism associated with monofunctional catalysts, they are not facile for dehydrocyclyzing dimethylbutanes to benzene. Instead, such catalysts crack a large portion of the dimethylbutanes to undesirable light gases.

As the second reason, dimethylbutanes have the highest octane rating among the non-aromatic C.sub.6 hydrocarbons, and are therefore of the most value in the mogas pool. Subjecting dimethylbutanes to catalytic activity renders them unavailable for upgrading the octane value of the mogas pool to the extent that they are cracked.

This discovery is the subject of a concurrently filed application, entitled PROCESS FOR REFORMING A DIMETHYLBUTANE-FREE HYDROCARBON FRACTION, 175570 this application is incorporated herein in its entirety by reference thereto.

It is known in the art to employ split feed reforming processes, wherein fractions of different hydrocarbon number range are separated out of a hydrocarbon feed, and subjected to different reforming catalysts. U.S. Pat. No. 4,594,145 discloses a process wherein a hydrocarbon feed is fractionated into a C.sub.5 - fraction, and a C.sub.6 + fraction; in turn, the C.sub.6 + fraction is fractionated into a C.sub.6 fraction and a C.sub.7 + fraction. The C.sub.7 + fraction is subjected to catalytic reforming, employing a catalyst most broadly disclosed as comprising platinum on an acidic alumina carrier. The C.sub.6 fraction is subjected to catalytic aromatization with a catalyst most broadly disclosed as comprising a Group VIII noble metal and a non-acidic carrier, with the preferred embodiment being platinum on potassium type L zeolite, which is monofunctional.

At column 3, lines 54-64, it is indicated that the C.sub.6 fraction advantageously contains at least 10 vol. % of C.sub.7 + hydrocarbons, with a general range of 10-50% by volume, and a preferable range of 15-35%. In Example 1, the C.sub.6 fraction is indicated to contain 3.2% C.sub.5 hydrocarbons, 72.7% C.sub.6 hydrocarbons, and 24.1% C.sub.7 + hydrocarbons. There is no disclosure or suggestion of limiting the proportion of C.sup.9 + hydrocarbons in the C.sub.6 fraction to less than 10% by volume of the fraction

As previously indicated, the monofunctional catalysts are particularly suited for reforming the C.sub.6 -C.sub.8 hydrocarbons, other than the dimethylbutane isomers. It has been discovered that the presence of more than about 10% by volume of C.sub.9 + hydrocarbons in the fraction treated with monofunctional catalyst will significantly inhibit catalytic activity.

In the process of this invention, the hydrocarbon fraction treated with monofunctional catalyst is limited to not more than about 10% by volume of C.sub.9 + hydrocarbons. This fraction preferably comprises not more than about 3%, and most preferably, not more than about 1% by volume C.sub.9 + hydrocarbons. The inventive process therefore provides benefits not taught by or disclosed in the prior art.

3. Definition of Terms

As used herein in the context of hydrocarbon or naphtha feeds, the terms "light fraction" and "heavy fraction" define the carbon number range of the hydrocarbons comprising the indicated fraction These terms are used in a relative manner; a "heavy fraction" is defined in reference to the carbon number range of its corresponding "light" fraction, and visa versa.

Specifically, a "light" fraction is a C.sub.6 fraction, a C.sub.7 fraction, a C.sub.8 fraction, a C.sub.6 -C.sub.7 fraction, a C.sub.7 -C.sub.8 fraction, a C.sub.6 -C.sub.8 fraction, or a fraction consisting essentially of C.sub.6 and C.sub.8 hydrocarbons; further, it is understood that, unless otherwise indicated, dimethylbutanes present in a light fraction amount to not more than about 10%, preferably about 3%, and, most preferably, no dimethylbutanes.

Further, a light fraction preferably comprises not more than about 10%, and, most preferably, not more than 2% by volume C.sub.5 - hydrocarbons. Of course, as discussed in detail herein, a light fraction also comprises, by volume, not more than about 10%, preferably not more than about 3%, more preferably, not more than about 1%, and, most preferably, no, or essentially no C.sub.9 + hydrocarbons.

C.sub.6 and C.sub.7 feeds will contain very little C.sub.9 content. It is the light fractions containing C.sub.8 hydrocarbons for which C.sub.9 + removal is critical

A "heavy" fraction comprises a range of hydrocarbons wherein the lowest carbon number compound is one carbon number higher than the highest carbon number compound of the corresponding light fraction.

Accordingly, when the light fraction is C.sub.6, the corresponding heavy fraction is C.sub.7 +. When the light fraction is C.sub.6 -C.sub.7 or C.sub.7, the corresponding heavy fraction is C.sub.8 +. When the light fraction is C.sub.8, C.sub.7 -C.sub.8, C.sub.6 -C.sub.8, or a fraction consisting essentially of C.sub.6 and C.sub.8 hydrocarbons, the corresponding heavy fraction is C.sub.9 +.

Unless specifically stated otherwise, the C.sub.5 - fraction is understood to include C.sub.6 dimethylbutane isomers. As stated above, the light fraction is understood essentially to exclude the C.sub.6 dimethylbutane isomers.

It is further understood that particular fractions are not necessarily comprised exclusively of hydrocarbons within the stated carbon number range of the fraction. Other hydrocarbons may also be present. Accordingly, a fraction of particular carbon number range may contain up to 15 percent by volume of hydrocarbons outside the designated hydrocarbon number range, subject to the limitation that the light fraction does not contain more than about 10% by volume of C.sub.9 + hydrocarbons.

The invention pertains to a process for reforming a hydrocarbon fraction containing about 10 volume percent or less C.sub.9 + hydrocarbons. Preferably, the reforming is conducted under reforming conditions, in the presence of a monofunctional catalyst. The hydrocarbon fraction is preferably selected from a group of fractions consisting of a C.sub.6 fraction, a C.sub.7 fraction, a C.sub.8 fraction, a C.sub.6 -C.sub.7 fraction, a C.sub.7 -C.sub.8 fraction, a C.sub.6 -C.sub.8 fraction, or a fraction consisting essentially of C.sub.6 and C.sub.8 hydrocarbons. The most preferred fraction is a C.sub.6 -C.sub.8 fraction.

Preferably, the monofunctional catalyst comprises a large-pore zeolite and at least one Group VIII metal; the Group VIII metal may be platinum, and the large-pore catalyst may be zeolite L. The monofunctional catalyst may further comprise an alkaline earth metal, with suitable alkaline earth metals including barium, magnesium, strontium, cesium and calcium. Also suitable are zinc, nickel, manganese, cobalt, copper, and lead.

The invention further pertains to a process wherein a first fraction of a hydrocarbon feed is separated into a light fraction, comprising not more than about 10% by volume C.sub.9 + hydrocarbons, and a heavy fraction; the light fraction is thereafter reformed under reforming conditions, in the presence of a monofunctional catalyst. In this process, the hydrocarbon feed preferably comprises a C.sub.5 -C.sub.11 fraction.

The heavy fraction comprises a range of hydrocarbons wherein the lowest carbon number hydrocarbon is one carbon number higher than the highest carbon number hydrocarbon of the light fraction.

The light fraction, as indicated, comprises not more than about 10% by volume C.sub.9 + hydrocarbons. In one embodiment, the light fraction is selected from the group consisting of a C.sub.6 fraction, a C.sub.7 fraction, a C.sub.8 fraction, a C.sub.6 -C.sub.7 fraction, a C.sub.7 -C.sub.8 fraction, a C.sub.6 -C.sub.8 fraction and a fraction consisting essentially of C.sub.6 and C.sub.8 hydrocarbons. The preferred light fraction in this embodiment is a C.sub.6 -C.sub.8 fraction. The hydrocarbon feed may be separated into the first fraction, comprising a C.sub.5 - fraction, and a second fraction, comprising a C.sub.6 + fraction, prior to separation of the first fraction into light and heavy fractions.

In another embodiment of the process of the invention, the light fraction may be selected from the group consisting of a C.sub.7 fraction, a C.sub.8 fraction, and a C.sub.7 -C.sub.8 fraction. The preferred light fraction in this embodiment is a C.sub.7 -C.sub.8 fraction. The hydrocarbon feed may be separated into the first fraction, comprising. a C.sub.7 + fraction, and a second fraction, comprising a C.sub.6 - fraction, prior to separation of the first fraction into light and heavy fractions

The monofunctional catalyst of the process of the invention preferably comprises a large-pore zeolite and at least one Group VIII metal. Preferably, the large-pore zeolite is zeolite L, and the Group VIII metal is platinum. The monofunctional catalyst may further comprise an alkaline earth metal selected from the group consisting of magnesium, calcium, barium, cesium, and strontium.

The indicated heavy fraction may also be reformed under reforming conditions, in the presence of a bifunctional catalyst. Preferably, this bifunctional catalyst comprises a Group VIII metal, and a metal oxide support provided with acidic sites. The preferred metal oxide support is alumina, and the preferred Group VIII metal of the bifunctional catalyst is platinum. The bifunctional catalyst may further comprise at least one promoter metal selected from the group consisting of rhenium, tin, germanium, iridium, tungsten, cobalt, rhodium, and nickel.

FIG. 1 is a graph showing the effect of C.sub.9 + content on the performance of the monofunctional catalyst.

FIG. 2 is a schematic representation of the process of the invention as adapted for petrochemical operations; and

FIG. 3 is a schematic representation of the process of the invention as adapted for refinery operations.

The catalyst employed in reforming of the hydrocarbon light fraction is a monofunctional catalyst, providing a single type of reactive site for catalyzing the reforming process.

Preferably, this monofunctional catalyst comprises a large-pore zeolite charged with one or more Group VIII metals, e.g., platinum, palladium, iridium, ruthenium, rhodium, osmium, or nickel. The preferred of these metals are the Group VIII noble metals, including rhodium, iridium, and platinum. The most preferred such metal is platinum.

Large-pore zeolites, as referred to herein, are defined as zeolites having an effective pore diameter of about 6-15 Angstroms. Among the large-pore zeolites suitable for the monofunctional catalysts are zeolite X, zeolite Y, and zeolite L, as well as such naturally occurring zeolites as faujasite and mordenite. The most preferred large-pore zeolite is zeolite L.

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

The most preferred of alkaline earth metals 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 monofunctional catalyst may further comprise one or more of an inorganic oxide, which may be utilized as a carrier to bind the large-pore zeolite containing the Group VIII metal. Suitable such inorganic oxides include clays, alumina, and silica, the most preferred being alumina.

Included among the monofunctional catalysts suitable for use in the process of this invention 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 these patents are incorporated herein in their entirety, by reference thereto.

The bifunctional catalyst of the inventive process is a conventional reforming catalyst, comprising a metal oxide support provided with acidic sites, and a Group VIII metal. Suitable metal oxides include alumina and silica, with alumina being preferred. The acidic sites are preferably provided by the presence of a halogen, such as chlorine.

The preferred Group VIII metal is platinum. One or more additional promoter elements, such as rhenium, tin, germanium, cobalt, nickel, iridium, rhodium, ruthenium, may also be included.

Each of the monofunctional and bifunctional catalysts is utilized under reforming conditions conventional for the particular catalyst. Reforming with either or both of the catalysts may be carried out of the presence of hydrogen.

As previously stated, the inclusion of too great a C.sub.9 + content in a fraction catalyzed by the monofunctional catalyst will adversely affect the performance of the catalyst. For instance, 10 per cent by volume or more of C.sub.9 + hydrocarbons will significantly inhibit catalytic activity.

The effect of higher C.sub.9 + content on catalytic activity is shown from reforming feeds having compositions as indicated in Table 1.

Table 1 indicates that Feedstock A is about 17.5 liquid volume % C.sub.9 +, as opposed to about 8.5 liquid volume % C.sub.9 + for Feedstock B.

The adverse effect of higher C.sub.9 + concentrations on the catalytic activity of PtKL (a monofunctional catalyst comprising platinum mounted on potassium zeolite L) is shown in FIG. 1. Specifically, FIG. 1 compares the aromatics yield, measured by weight percent plotted against hours on oil, resulting from catalyzation of feeds comprising 17.5 and 8.5 liquid volume % of C.sub.9 + hydrocarbons, respectively.

As is evident from FIG. 1, increasing the C.sub.9 + content of the feed from 8.5 to 17.5 liquid volume % severely decreases the effectiveness of the catalyst in producing aromatics. Therefore, notwithstanding, as above indicated, that reference to a particular fraction is not limited to hydrocarbons of the specific carbon members or isomers designated, it is understood that the C.sub.9 + content of light fractions, as defined herein, is about 10 liquid volume % or less; more generally, the C.sub.9 + hydrocarbon content of the light fractions is small enough so as not to inhibit significantly the activity of the monofunctional catalyst.

FIGS. 2 and 3, discussed below, illustrate the utilization of the process of the invention in petrochemical and refinery operations, respectively. It is noted that these two embodiments are provided merely by way of example, not limitation, and demonstrate two particular methods for utilizing the process of the invention.

This Example, which demonstrates the application of the process of the invention to petrochemical operations, is described with reference to the flow diagram of FIG. 1, and the various hydrocarbon streams and units identified therein. Unless otherwise specifically stated, the percent proportions herein are by volume.

A crude oil stream is subjected to rough separation in a pipe still (not shown) to produce a naphtha feed stream, which is fed from the pipe still directly into distillation tower 1. The naphtha feed stream comprises a C.sub.5 -C.sub.11 fraction of hydrocarbons, and contains 50% paraffins, 33% naphthenes, and 17% aromatics.

Distillation tower 1 is a 50 tray distillation tower. The condenser, provided at the top of the tower, is operated at 120.degree. F. and 45 psia, with a reflux ratio of about 0.8. The reboiler, provided at the bottom of distillation tower 1, is operated at 290.degree. F., and at a pressure of 55 psia.

In distillation tower 1, this C.sub.5 -C.sub.11 fraction is separated into a C.sub.5 - fraction and a C.sub.6 + fraction. The C.sub.5 - fraction contains 14% C.sub.6 hydrocarbons, with the remainder being C.sub.5 - hydrocarbons 10% of the C.sub.6 hydrocarbons are dimethylbutanes; the dimethylbutanes which split off with the C.sub.5 - hydrocarbons in this fraction comprise 85% of the dimethylbutanes present in the C.sub.5 -C.sub.11 fraction prior to this separation.

This C.sub.5 - fraction, including the indicated C.sub.6 portion, is removed overhead from distillation tower 1. This fraction may be blended directly into the mogas pool. Alternatively, this fraction may be sent to isomerization unit 2, wherein its octane value is upgraded, and may thereafter be sent to the mogas pool.

The C.sub.6 + fraction from distillation tower is fed into distillation tower 3, and separated into a C.sub.6 -C.sub.8 fraction and a C.sub.9 + fraction. Because, as discussed previously, excessive C.sub.9 + content interferes with the activity of the monofunctional catalyst, a sharp cut is made between the C.sub.8 and C.sub.9 hydrocarbons.

Tower 3 may comprise 50 trays, with the condenser, at the top of the tower, operated at 190.degree. F. , 25 psia, and a reflux ratio of 2.5; the reboiler, at the bottom of the tower, may be operated at 320.degree. F. and 35 psia.

The C.sub.6 -C.sub.8 fraction obtained from distillation tower 3 as embodied above contains 1% C.sub.5 - hydrocarbons, 28% C.sub.6 hydrocarbons, 32% C.sub.7 hydrocarbons, 35% C.sub.8 hydrocarbons, and 4% C.sub.9 + hydrocarbons; the C.sub.9 + fraction contains 9% C.sub.8 -hydrocarbons, 48% C.sub.7 -C.sub.9 hydrocarbons, 29% C.sub.10 hydrocarbons, and 14% C.sub.11 hydrocarbons.

Where tower 3 comprises 44 trays, with the condenser operated at 240.degree. F. , 25 psia, and a reflux ratio of 2.0, and the reboiler operated at 400.degree. F. and 40 psia, the resultant C.sub.6 -C.sub.8 fraction comprises only 0.4% C.sub.9 + hydrocarbons.

The C.sub.6 -C.sub.8 fraction taken overhead from tower 3 is fed into reactor 4, which contains the monofunctional reforming catalyst. The catalyst comprises potassium zeolite L, with 28% by weight alumina binder and 0.6% by weight platinum. Reforming is conducted in the presence of hydrogen gas; reactor 4 is operated at 850.degree.-900.degree. F., 1.5 WHSV, 160 psig, and a hydrogen to hydrocarbon mole ratio of 4. The product which results from this reforming contains 10% benzene, 14% toluene, 16% xylenes, 38% C.sub.5 -C.sub.8 paraffins and naphthenes and the remainder light gases and hydrogen.

The effluent from reactor 4 is fed into flash drum 5, operated at 110.degree. F. and approximately 115 psig. Therein, a crude separation between C.sub.4 - light gases and a C.sub.5 + fraction, with the C.sub.5 + fraction retaining about 2% of the C.sub.4 fraction, and further containing 98% or more of the effluent aromatics.

A stream including the C.sub.4 - fraction and hydrogen from flash drum 5 is recycled as needed to reactor 4; -the excess of this stream is removed from the process system, with byproducts being recovered therefrom.

The C.sub.5 + effluent from flash drum 5 is then fed into distillation tower 6. Distillation tower 6, comprising 30 trays, functions as a reformate stabilizer. The condenser is operated at 190.degree. F. and 100 psia; the reboiler, at 300.degree. F. and 105 psia.

As opposed to the crude separation conducted in flash drum 5, a sharp cut 6 is effected in distillation tower 6 between the C.sub.4 - and C.sub.5 + fractions. The resultant C.sub.5 + fraction contains, by volume, 2% C.sub.5 - hydrocarbons, 17% benzene, 22% toluene, 27% xylenes, and 32% C.sub.6 -C.sub.8 paraffins and naphthenes.

The C.sub.9 + fraction from distillation tower 3 is fed into conventional reformer 7, which contains a bifunctional catalyst comprising, by weight, 0.3% platinum, 0.3% rhenium, 0.8% chlorine, and 98.6% alumina. Reformer 7 is operated at 850.degree.-980.degree. F., 1.5 WHSV, 300 psig, and a recycled gas rate of 2.0 kSCFH/Bbl of feed. As in reformer 4, reforming is conducted in the presence of hydrogen.

Reformer 7 is operated at conditions predetermined to result in a product having an octane of 103. This product contains, by volume, 18% hydrogen, 21% C.sub.5 - hydrocarbons, 1% benzene, 3% other C.sub.6 hydrocarbons (excluding benzene), 1% toluene, 2% other C.sub.7 hydrocarbons, 9% xylenes, 3% other C.sub.8 hydrocarbons, 39% C.sub.9 + aromatics, and 3% other C.sub.9 + hydrocarbons.

This product is fed as effluent to flash drum 8 and distillation tower 9, which operate in the same manner with regard to reformer 7 as flash drum 5 and distillation tower 6 perform with reactor 4. In flash drum 8, a crude separation is effected between the C.sub.4 - light gases and a C.sub.5 + effluent; after this crude separation, the C.sub.5 + effluent retains about 2% of the C.sub.4 - hydrocarbons. The C.sub.4 - fraction thus separated is recycled with hydrogen, as needed, to reformer 7, with excess removed from the process system for recovery of valuable by-products. The C.sub.5 + effluent is fed from flash drum 8 into distillation tower 9, which comprises 30 trays. The condenser, in the top section of this tower, is operated at 190.degree. F. and 100 psia; the reboiler, in the bottom section, is operated at 300.degree. F. and 105 psia.

Distillation tower 9, like distillation tower 6, functions as a reformate stabilizer; in tower 9, a sharp cut is effected between the C.sub.5 + effluent and the C.sub.4 - fraction remaining therein. The resultant C.sub.5 + fraction contains, by volume, 2% C.sub.4 - hydrocarbons, 6% C.sub.5 hydrocarbons, 4% C.sub.6 hydrocarbons (excluding benzene), 1% benzene, 3% C.sub.7 hydrocarbons (excluding toluene), 2% toluene, 14% xylenes, 5% other C.sub.8 hydrocarbons, 4% other C.sub.9 hydrocarbon, 38% C.sub.9 aromatics, 1% C.sub.10 + hydrocarbons (excluding aromatics), and 20% C.sub.10 + aromatics.

As discussed with regard to Example 2, at this point in a refining operation, the C.sub.5 + effluent from stabilizer 9 can be sent directly to the mogas pool. However, Example 1 pertains to petrochemical operations, wherein the objective is to maximize aromatics production.

Accordingly, the C.sub.5 + effluent from distillation tower 9 is fed to distillation tower 10, which comprises 30 trays. The top section of the this tower, the condenser, is operated at 260.degree. F. , and 30 psia; the bottom, the reboiler, at 430.degree. F. and 50 psia.

In distillation tower 10, this C.sub.5 + effluent is separated into a C.sub.6 -C.sub.8 fraction, which comprises substantially all of the desirable light aromatic components of the C.sub.5 + effluent, and a C.sub.9 + fraction. Specifically, the indicated. C.sub.6 -C.sub.8 fraction comprises, by volume, 1% benzene, 26% toluene, 44% xylene, 2% C.sub.9 + aromatics, and 27% C.sub.6 -C.sub.10 + non-aromatic hydrocarbons. The C.sub.9 + fraction comprises 1% xylenes, 64% C.sub.9 aromatics, 34% C.sub.10 + aromatics, and 1% other C.sub.9 hydrocarbons.

This C.sub.9 + fraction is sent directly to the mogas pool for blending, and the C.sub.6 -C.sub.8 fraction is combined with the C.sub.5 + effluent from distillation tower 6.

This combined stream can be fed directly to aromatics extraction unit 12. More preferably, it is fed to distillation tower 11, comprising 25 trays. The condenser, in the upper section of tower 11, is operated at 200.degree. F. and 30 psia. the reboiler, in the lower section, is operated at 300.degree. F. and 35 psia.

Distillation tower 11 is employed to remove the C.sub.6 paraffins from the feed to be provided to aromatics extraction unit 12, thereby concentrating the aromatics in this feed. Specifically, in distillation tower 11, a C.sub.6 paraffin and naphthene fraction, comprising, by volume, 1% dimethylbutane, 39% 2-methyl pentane, 51% 3-methyl pentane, 3% cyclohexane, and 6% methyl cyclopentane is separated from a higher-boiling fraction, comprising benzene through the C.sub.8 hydrocarbons.

The C.sub.6 fraction from distillation tower 11 is particularly suitable as a feed for monofunctional catalyst reactor 4, and is recycled to this reactor. The fraction comprising benzene through C.sub.8 hydrocarbons, which largely comprises aromatics, is fed to aromatics extraction unit 12.

Aromatics extraction unit 12 utilizes a solvent selective for aromatics, such as sulfolane, to extract the aromatics from the non-aromatics, the latter being primarily paraffins. The resulting non-aromatic raffinate is recycled to the feed entering monofunctional catalyst reactor 4, thereby enhancing aromatics yield.

The aromatic extract from aromatics extraction unit 12 is fed to distillation tower 13, and separated therein into benzene, toluene and xylenes. Distillation tower 13 may be a single tower, or a series of towers, depending upon the purity of the products desired

As a single tower, distillation tower 13 comprises 40 trays. The condenser, at the top of the tower, is operated at 195.degree. F. and 20 psia; benzene issues from the top of the tower. Toluene issues from the tower as a side stream at tray 21, which is operated at 255.degree. F. and 25 psia. Xylene issues from the bottom of the tower, where the reboiler is located, and which is operated at 305.degree. F. and 30 psia.

Where distillation tower 13 is embodied as two towers in series, benzene issues from the top of the first tower in the series, and a mixture of toluene and xylenes issues from the bottom. This mixture is fed into the second tower in the series, with toluene taken off from the top of this tower, and xylenes from the bottom.

The first tower in this series comprises 22 trays, with the condenser, at the top of the tower, being operated at 195.degree. F. and 20 psia, and the reboiler, at the bottom of the tower, being operated at 275.degree. F. and 25 psia. The second tower comprises 20 trays, with the top of the tower being operated at 232.degree. F. and 1.5 psia, and the bottom being operated at 285.degree. F. and 25 psia.

As an optional preferred embodiment, to maximize the production of aromatics, especially benzene, the toluene stream from distillation tower 13 may be fed to unit 14, which is either a toluene hydrodealkylation (TDA) unit, or a toluene disproportionation (TDP) unit. The TDA unit produces 80% benzene and 20% light gases, i.e., methane and ethane. The TDP unit produces 50% benzene and 50% xylenes, primarily paraxylenes The benzene produced in these units is fed into the benzene stream exiting overhead from distillation tower 13.

Example 2, which demonstrates the application of the process of the invention to the enhancement of mogas octane pools in refinery operations, is described with reference to the flow diagram of FIG. 2, and the various hydrocarbon streams and units identified therein. The embodiment illustrated in FIG. 2 is substantially similar to that illustrated in FIG. 1. The primary difference is that the process used for enhancing mogas production is considerably simplified over that for maximizing aromatics yield; the former process lacks the aromatics extraction steps, which are included in the process solely for the purpose of maximizing the referred-to aromatics yield.

One difference between the two embodiments of the process is the cut point utilized in distillation tower 1. In refinery mogas octane pool operations, the production of excessive benzene in the monofunctional catalyst reactor can be undesirable due to benzene concentration restrictions on mogas. Accordingly, as shown in FIG. 2, the cut point in distillation tower 1 is raised, so that not only the dimethylbutanes, but a substantial portion of the other C.sub.6 isomers, are sent overhead as well.

Specifically, the overhead stream comprises, by volume, 3% n-butane, 9% i-butane, 17% n-pentane, 16% i-pentane, 1% cyclopentane, 17% n-hexane, 2% dimethyl butanes, 10% 2-methyl pentane, 8% 3-methyl pentane, 6% methyl cyclopentane, 5% cyclohexane, 5% benzene, and 1% C.sub.9 isomers. This stream is sent either directly to the mogas pool, or to isomerization unit 2.

Accordingly, the bottoms stream from distillation tower 1 comprises primarily the C.sub.7 + hydrocarbons; specifically, this fraction comprises, by volume, 1% C.sub.6 - hydrocarbons, 25% C.sub.7 hydrocarbons, 31% C.sub.8 hydrocarbons, 25% C.sub.9 hydrocarbons, 13% C.sub.10 hydrocarbons, 5% C.sub.11 + hydrocarbons.

Rather than the C.sub.6 -C.sub.8 light fraction fed to monofunctional catalyst reactor 4 in the embodiment of FIG. 1, the light fraction resulting from distillation tower 3 in the embodiment of the FIG. 2 is a C.sub.7 -C.sub.8 fraction. Specifically, this fraction comprises, by volume, 2% C.sub.6 hydrocarbons, 44% C.sub.7 hydrocarbons, 49% C.sub.8 hydrocarbons, and 5% C.sub.9 + hydrocarbons.

Processing units 4-9 are identical for the embodiments of both FIGS. 1 and 2. However, in the refinery operation of FIG. 2, the C.sub.5 + effluent from distillation towers 6 and 9 is sent directly to the mogas pool, rather than to the aromatics extraction steps specified in the petrochemical operation illustrated in FIG. 1.

Finally, although the invention has been described with reference to particular means, materials, and embodiments, it should be noted that the invention is not limited to the particulars disclosed, and extends to all equivalents within the scope of the claims.