US3387052A - Process for production of aromatic hydrocarbons - Google Patents

Description

June 4, 1968 G. E. ADDISON 3,387,052

PROCESS FOR PRODUCTION OF AROMATIC HYDROCARBONS Filed May ll, 1966 2 Sheets-Sheet 1 -I b t INVENTOR" a George E. Addison l: b a

.ATTOR/VEYS G. E. ADDISON 3,387,052

PROCESS FOR PRODUCTION OF AROMATIC HYDROCARBONS June 4, 1968 2 Sheets-Sheet Filed May 11, 1966 mm M m R m E on m M M m a V E T N a m g r R m A 0 8 mm H K F R 8 mm mm \w mm /|M.N \m m Q f v t E v vw/ Q .r. m t I H 4! I 6 x m N 9 w t 9 k 1 W 0| m /|m S N Sas United States Patent -Oifice 3,387,052 Patented June 4, 1968 3,387,052 PROCESS FOR PRODUCTION F ARGMATIC HYDROCARBUNS George E. Addison, Mount Prospect, Ill, assignor to Universal 0i! Products Company, Des Plaines, ill. a corporation of Delaware Fiied May 11, 1966, Ser. No. 549,288 8 Claims. (Cl. 260-668) ABSTRACT OF THE DISCLOSURE Process which comprises charging naphtha to a catalytic reforming unit to produce aromatics and hydrogen, recovering high purity aromatics from the resulting reformate, fractionating the resulting ramnate to provide a selected paraffinic fraction, and recycling the selected parafiinic fraction to the reformer for dehydrocyclization to provide additional aromatics. The selected paraffinic fraction is comprised of 0 hydrocarbons and has substantial freedom from polycyclic aromatics. Where the catalytic reformer comprises a series of reaction zones, the selected paraffinic fraction is preferably recycled only to the last reaction zone.

The present invention relates to the production of aromatic hydrocarbons from a selected petroleum fraction. More specifically, the present invention relates to a combination process for reforming a gasoline boiling range hydrocarbon wherein the production of aromatic hydrocarbons is enhanced While substantially reducing the production of paraflinic hydrocarbons.

In view of the profitability advantage in expanding into petrochemicals, the hydrocarbon processing industry has seen a world-wide trend away from emphasis upon the production of high octane gasoline by catalytic reforming, and a shift toward maximization of aromatics production. The maximization of aromatics has best been achieved commercially by the combination of catalytic reforming and solvent extraction wherein high purity aromatic hydrocarbons are produced and parafiinic rafiinate is recovered. The paraffinic raffinate normally finds utility as a solvent, or a blending component for jet fuel, or a gasoline blending component. The parafiinic rafiinate is not a generally desired refinery product, however, since the market for parafifinic solvent is limited and the relatively low boiling range of some paraifinic fractions will often limit their marketability as jet fuel components, it becomes necessary in many instances to blend the parafiinic raffinate into the gasoline pool. Since the octane rating of parafiinic components is relatively low, the refinery units which produce motor fuel must operate at more severe conditions in order to provide high octane gasoline components which will blend with low octane paraffinic components to provide the specification octane rating on the final gasoline blend. Catalyst life and product yields are reduced by increased severity on the catalytic units which produce high octane gasoline, and it is therefore apparent that paraifinic raffinate is an undesirable refinery product where no external market exists for it as-produced.

As used herein, the term raffinate refers to the hydrocarbon product, or any fraction thereof, which remains after the aromatic content of the original reformate product has been substantially removed by solvent extraction or other processing means.

In the present invention, paraffini raffinate production is reduced by recycling to the catalytic reformer. Similar combination processing has been undertaken in the petroleum industry, but whereas such processing has been technically feasible, it has not been commercially successful. Experience has shown that Where a paraifinic raflinate is recycled to the catalytic reformer, the recycle stream accumulates in volume requiring that at least a portion of such stream be removed in order to maintain a constant hydrocarbon inventory within the combination process. It has further been found that such combination processes require high operating severities which lead to increased rates of catalyst deactivation. In an eifort to overcome these handicaps, the further reforming of the paraflinic rafiinate stream has been attempted in separate reforming facilities external to the first reformer zone which operates upon the fresh charge. Such double reformer systems have not proven effective since in addition to the disadvantage of added capital and operating expense, such systems have not successfully overcome the technical handicaps noted herein.

It is therefore an objective of the present invention to enhance the production of aromatic hydrocarbons while minimizing the production of paraffinic raffinate. It is a further objective of the present invention to afford a means whereby the distribution between aromatic product and parafiinic product may be varied as required to supply the needs of a fluctuating market. It is a more specific objective of the present invention to afford a means whereby these ends are achieved while providing substantially improved catalyst life at a reduced capital and operating expense.

Therefore, in accordance with the practice of this invention, one embodiment thereof comprises contacting hydrocarbon feed stock containing naphthenic and paraffinic compounds with hydrogen in a reaction zone under conditions suitable to convert at least a portion of said naphthenes into aromatic compounds; separating the eflluent from said reaction zone into a hydrogen-rich product and a product containing said aromatic compounds; subjecting said aromatic-containing product to conditions suificient to produce a first paraflinic product and a product comprising monocyclic aromatics; recovering said monocyclic aromatic product; subjecting said first parafiinic product to conditions sufficient to produce at least an intermediate paraffinic product comprising hydrocarbons having at least eight carbon atoms per molecule and having substantial freedom from polycyclic aromatics; and recycling at least a portion of said intermediate paraffinic product to the reaction zone.

A modified embodiment of the present invention, comprises contacting a hydrocarbon feed stock containing naphthenic and parafiinic compounds with hydrogen in a reaction Zone under conditions suitable to convert at least a portion of said naphthenes into aromatic compounds; separating the efiluent therefrom into at least a hydrogen rich gas product and a product containing said aromatic compounds; subjecting said aromatic-containing product to conditions suificient to provide at least a light aromatic-containing fraction comprised of hydrocarbons containing from about six to about eight carbon atoms per molecule and a heavy aromatic-containing fraction comprised of hydrocarbons containing about nine or more carbon atoms per molecule; separating said light aromatic-containing fraction into a first parafiinic fraction and a light aromatic product; separating said heavy aromatic-containing fraction into a second paratiinic fraction and a heavy aromatic product; recovering said light aromatic product and said heavy aromatic product; subjecting said first paraffinic fraction and said second paraflinic fraction to conditions sufficient to provide at least an intermediate parafrinic product comprising hydrocarbons having at least eight carbon atoms per molecule, and having substantial freedom from polycyclic aromatics; and returning at least a portion of said intermediate paraffiuic product to the reaction zone wherein at least a portion of said product is converted to aromatic compounds.

In the most specific embodiments of the present invention, the desired ends are achieved wherein the reaction zone is comprised of multiple stage contact sections and the portion of said intermediate paraflinic product which is recycled is returned only to the last of said multiple stage contact sections.

The foregoing embodiments are illustrated by FIG- URE I and FIGURE 11 which are schematic flow diagrams of the inventive process. FIGURE I is illustrative of the present invention in its broad embodiment while FIGURE II comprises a more specific application of the present invention.

The charge stocks which may be reformed in accordance with the present invention comprise gasoline boiling range hydrocarbons containing naphthenes, paraffins, and aromatics with only minor amounts of olefins being present. Suitable hydrocarbon charge may be a straight run gasoline or a natural gasoline, or it may be a refined gasoline such as a thermal cracked gasoline or a hydrocracked gasoline, etc, or it may comprise any combination thereof. The gasoline may be a full boiling range gasoline fraction having an initial boiling point of from about 50 F. to about 100 F. and an end boiling point of from about 375 F. to about 425 F, or it may be a selected fraction thereof. The usual fraction to be processed in the present invention will be a selected naphtha fraction having an initial boiling point of from about 150 F. to about 250 F. and an end boiling point of from about 350 F. to about 425 F. Where such fraction comprises substantial sulfur or nitrogen-containing hydrocarbons, or where substantial olefinic hydrocarbons are included, hydrogen pretreatment may be necessary in order to profeet the reforming catalyst from loss of activity.

Hydrogen pretreatment of contaminated hydrocarbon charge stocks is well known in the art of hydrocarbon processing, and a typical method is shown in U.S. Letters Patent No. 2,878,180. Any hydrocarbon charge stock containing more than about 10.0 parts per million (p.p.m.) by weight of sulfur and/or more than about 1.0 ppm. of nitrogen and/or more than about 1.0 volume percent of olefinic hydrocarbons should be treated. Hydrogen pretreatment will also serve to remove trace quantities of arsenic, lead, copper, nickel, vanadium, tungsten, and other metals which may be present in untreated hydrocarbon fractions and which may be detrimental to noble metal reforming catalysts.

The basic processing techniques of catalytic reforming and a preferred catalyst are indicated in U.S. Patents No. 2,479,109 and 2,479,110 wherein the catalyst comprises alumina, platinum and halogen. Reforming is accomplished at a temperature in the range of from about 600 F. to 1100 F; at a pressure in the range of from about 100 p.s.i.g. to 1000 p.s.i.g.; at a liquid hourly space velocity in the range of from about 0.5 to 10.0; and in the presence of from about 0.5 to 10.0 moles of hydrogen per mole of hydrocarbon. The hydrogen is normally present as the major component of a hydrocarbon-containing gas which is circulated with the charge stock at a rate of from about 500 to 13,000 standard cubic feet of gas per barrel of liquid charge stock.

As understanding of the reaction mechanisms occurring within the reforming zone has increased, it has become possible to adjust operating techniques and to modify catalyst composition in order to enhance the specific reaction desired. It has been determined that catalytic reforming is characterized by four specific chemical reactions: (1) the dehydrogenation of naphthenic hydrocarbons to produce the corresponding aromatic derivative; (2) the dehydrocyclization of parafiinic hydrocarbons to produce corresponding aromatic hydrocarbons; (3) the hydrocracking of high molecular weight hydrocarbons; and (4) the isomerization of normal parafiinic hydrocarbons to produce branched chain isomers of equal molecular weight. Each of these four reaction mechanisms upgrade low octane hydrocarbons to high octane hydrocarbons, but as the automotive manufacturers have increased engine compression ratios, it has become necessary to adjust operating techniques in order to control the reaction mechanisms selectively to maximize octane with minimum loss of liquid product yield. It has been determined that the dehydrogenation of naphthenes to aromatics is promoted by operating at lower pressure levels; that dehydrocyclization of paraffins to aromatics is promoted by low pressure and high temperature; that hydrocracking of paraffins is promoted by high pressure, high temperature, and high residence time of the charge stock on the catalyst; and that isomerization of parafiins is promoted by intermediate temperature and a catalyst containing a much higher halogen content than normally employed. The reforming catalyst is, therefore, composited in a manner to effect the desired balance between the competing reactions, and a preferred catalyst is comprised of 0.375 wt. percent platinum, 0.350 wt. percent fluorine, 0.900 -wt. percent chlorine, and 98.375 wt. percent alumina.

The catalytic reforming unit of the present invention is maintained at operating conditions to enhance the dehydrogenation of naphthenes and the dehydrocyclization of paraffins in order to maximize the production of both aromatics and hydrogen, maximum hydrogen production also being desired since it is consumed elsewhere in many petroleum refinery and petrochemical complexes. The production of aromatic hydrocarbons is enhanced by catalytic reforming at a temperature in the range of from about 850 F. to 1050" F., and at a pressure in the range of from about 100 p.s.i.g. to 400 p.s.i.g. when the end boiling point of the charge stock is about 350 F. However, when the end point of the charge stock is about 400 F. or more, the preferred pressure is about 500 p.s.i.g. in order to maintain catalyst stability. Charge stocks having end points of about 400 F. comprise higher molecular weight hydrocarbons which have a greater tendency to hydrocrack. The hydrocracking mechanism forms carbonium ions, olefinic fragments, and carbon. The olefinic fragments become saturated with hydrogen in-part, but some of the fragments will polymerize with carbonium ions to form polycyclic aromatics. The carbon produced is retained upon the catalyst surface and some of the polycyclic aromatics are so retained, thus detrimentally effecting catalyst activity and selectivity. The processing of such higher molecular weight charge stocks at 500 p.s.i.g. gives a higher partial pressure of hydrogen which is to be preferred since it retards carbon formation and enhances saturation of the olefinic fragments thus retarding the formation of polycyclic aromatics.

The aromatics separation Zone of the present invention may be characterized by a solvent extraction technique, or an aromatics solid adsorption technique, or an extractive distillation technique, or a fractional crystallization technique. A preferred separation method is described by US. Patent No. 2,730,558. A particularly preferred solvent for separating aromatic hydrocarbons from non-aromatic hydrocarbons is a mixture of water and one or more hydrophilic organic solvents. Such a combination solvent may have its solubility regulated by varying the water content. Thus, by adding more water to the solvent, the solubility of all components in the hydrocarbon mixture is reduced, but the solubility difference between components (selectivity) is increased. The net effect is to decrease the number of contacting stages required to achieve a given purity of product, or to increase the resulting purity of product where the number of contacting stages is held constant. Because of the resulting reduction in solubility due to the increased water content, the throughput of the combination solvent must be increased in order to dissolve the solute at the same production rate. Suitable hydrophilic organic solvents for this process include alcohols, glycols, aldehydes, glycerine, phenol, etc. Particularly preferred solvents are diethylene glycol, triethylene glycol, dipropylene glycol, tripropylene glycol, and mixtures thereof containing from about 2% to weight of water. In classifying hydrocarbon and hydrocarbon-type compounds according to increasing solubility in such mixed solvent, it is found that paraflins are least soluble, followed in increasing order of solubility by naphthenes, olefins, diolefins, acetylenes, sulfurcontaining hydrocarbons, nitrogen-containing hydrocarbons, and aromatic hydrocarbons. It may thus be seen that the ideal charge to such a solvent extraction process is one consisting essentially of paraflins and aromatics, and that since catalytic reformates contain only minor amounts of naphthenes and olefins, reformates are well suited to such an aromatics extraction procedure.

Aromatic hydrocarbons differ in their relative solubility in the solvent in that solubility is a function of normal boiling point, with the lighter aromatics being more soluble than the heavier aromatics. Similarly, the solubility of non-aromatic hydrocarbons also decreases with increasing normal boiling point. Thus, in operation of a single extraction system upon a full boiling range catalytic reformate, the low molecular weight aromatics may be extracted to recover high purity benzene, toluene, ethylbenzene, and xylenes, with little or no contamination by naphthenes and paraffins, but the paraffin-rich rafiinate will contain substantial heavy alkylaromatics and polycyclic aromatics. However, if the extraction conditions are modified to recover not only the light aromatics but the heavy aromatics as well, substantial contamination by non-aromatics will occur in the aromatic product. The lower molecular weight paraffins have solubilities comparable to the higher molecular Weight aromatics and these paraflins are indiscriminately dissolved in the selective solvent. The maximum effective recovery of pure aromatics and paraffin-rich raffinate is therefore not technically feasible without a double extraction system. In such a system, the aromatic-containing hydrocarbon feed is fractionated to provide a light fraction and a heavy fraction. The light fraction, comprised of aromatics containing from six to eight carbon atoms per molecule, is charged to one extraction system and the heavy fraction, comprised of aromatics containing about eight or more carbon atoms per molecule, is charged to a second extraction system. Since the light paraffins are concentrated in the light fraction, recovery of the heavy aromatics in the second extraction system is accomplished without solubility interference from such paraflins.

The split betwen the light and heavy fractions will vary in accordance with the concentration and composition of the aromatics contained within the hydrocarbon feed. In some instances the first extraction system will recover benzene and toluene, while the second extraction system will recover the heavier aromatics. In other cases the first extraction system will recover benzene, toluene, ethylbenzene, and xylenes, while the second extraction system may recover aromatics containing nine or more carbon atoms per molecule. It is readily apparent that such a double extraction system will entail increased capital and operating expense. Because of the economic considerations involved, it is therefore customary to extract maximum benzene, toluene, ethylbenzenes, and xylenes in a single extraction system and allow substantial amounts of the heavier aromatics to remain in the paraflinic rafinate.

In the present invention, the paraffin-rich rafiinate is distilled in a fractionation zone to provide a selected par atlinic raffinate stream which is recycled to the catalytic reformer.

The total paraffin-rich ralfinate is first fractionated to remove all hex'anes and heptanes which are recovered as a by-product stream. Although hexanes and heptanes may be isomerized and dehydrocyclicized in the catalytic reforming zone, it has been determined that these light paraflins are not effectively reformed in the presence of higher molecular Weight paraffins. The primary reason for this is that the reforming catalyst will selectively dehydrocy-clicize the heavier parafiins at less severe operating conditions, and operating severities required to dehydrocyclicize hexanes and heptanes cannot be approached without resulting in 'an excessive yield loss due to undesired hydrocracking of other hydrocarbons. In addition, the octanes and heavier parafiins are not only dehydrocyclicized to form aromatics, but they are also hydrocracked to form lighter paraflins, including hexanes and heptanes. Thus, it has been found that the net result of recycling a raffinate comprising hexane and heavier parafiins is that the recycle stream accumulates hexanes and heptanes and provision must, therefore, be made for removing this accumulation. This may be accomplished by drawing off a portion of the recycle stream, but since such a drag stream would also remove octane and heavier hydrocarbons, the preferred method is to continually remove hexanes and heptanes by fractionation.

The resulting deheptanized paraffinic rafiinate stream must be further fractionated to remove all polycyclic aromatics. The polycyclic aromatics may be removed in the aromatics separation zone by adjustment of the solvent composition, use of a high solvent circulation rate, and provision of a greater number of contact stages, but such changes in operation cannot produce a completely aromatic-free rafiinate and a completely parafiin-free aromatic product without entailing prohibitive capital and operating expenses. Since polycyclic aromatics have higher boiling points than the paraffins having an equal number of carbon atoms, the most economical and the preferred method of removing polycyclic aromatics from the deheptanized parafiinic rafiinate is by removing them as a bottoms product in the rafiinate fractionation zone. The polycyclic aromatics must be removed from the raffinate being recycled to the reformer since failure to remove the polycyclic aromatics will cause them to accumulate in the recycle stream. It must also be noted that such an accumulation of polycyclic aromatics has been found to be a primary cause of acceleration in the rate of loss of reforming catalyst activity. Certain species of such hydrocarbons tend to be retained on the cat 'alyst surface without being effectively reacted in any manner and the catalytically active sites become effectively shielded from the material being processed. The net result is that catalyst activity and selectivity are prematurely destroyed. It is, therefore, the practice of the present invention to remove the polycyclic aromatics from the paraffinic rafiinate by fractionating the raffinate to an end boiling point of about 350 F. or less.

The resulting paraffinic fraction, comprising octanes and heavier hydrocarbons and being substantially free of polycyclic aromatics, is recycled to the catalytic reforming zone as the selected paraffinic raffinate stream. By specific exclusion of hexanes, heptanes, andpolycyclic aromatics from the recycle rafiinate stream, the

problems noted above which have been experienced in similar combination processing are eliminated.

The dehydrogenation of the naphthenic hydrocarbons of the fresh charge stock to form aromatic hydrocarbons in the catalytic reforming zone is a highly endothermic reaction, and the hydrocarbon and hydrogen mixed stream must be intermittently reheated in order to maintain the mixture at effective reaction temperatures. The catalytic reforming zone is, therefore, comprised of several reactor vessels containing the reforming catalyst and reheating is provided between reactors. Since the amount of endothermic reaction will vary in accordance with the concentration of naphthenes in the hydrocarbon charge stream, the number of reheating applications will vary. Thus, it is normal for 'a catalytic reformer to be provided with three reactors and two interheaters, but where the naphthene content of the charge stock is in excess of 45 volume percent, at least three reheatings are normally required to maintain adequate temperature on the catalyst and thus at least four reactors must be provided.

In a more specific embodiment of the present invention, the selected paraflinic raffinate may be sent only to the last of the several reactors whenever the naphthene content of the paraffinic raffinate is low. Since substantial naphthene dehydrogenation occurs in the preceding reactors, the endothermicity causes the average catalyst temperature in these vessels to be substantially below what is required for effective dehydrocyclization of the predominantly paraffinic recycle refiinate. The amount of dehydrocyclization which may occur in the preceding reactors may be accomplished upon the parafiinic hydrocarbons which are introduced into the catalytic reformer by the fresh charge stock and the presence of additional parafiins from the selected parafiinic rafiinate is unnecessary and may be, in fact, undesirable. The naphthene and paraffins have comparable adsorption rates upon the catalyst and it is advantageous to minimize the competition between these hydrocarbon types for the active catalyst sites. The dehydrogenation of naphthenes to form aromatics is the selective reaction which is promoted by the catalyst and it is a relatively clean reaction, whereas the dehydrocyclization of parafiins is not as clean in that it is accompanied by hydrocracking with the resulting deposition of carbon upon the catalyst. Thus, catalyst activity may be detrimentally effected by combining the selected paraffinic rafiinate with the fresh charge stock and passing it through all reactors when the naphthene content of the selected raffinate stream is less than 10.0 volume percent.

In addition, it must be noted that the selected parafiinic raffinate stream will contain olefinic hydrocarbons and that their concentration may be substantial in a high severity reforming operation. Since olefins are more easily adsorbed upon the catalyst surface than either naphthenes or paraffins, the balance of catalyst selectivity between competing reaction mechanisms may be detrimentally upset by excessive olefins. Thus, by introducing the raffinate into the entire series of reactors, the dehydrogenation of naphthenes to form aromatics will be retarded, and excessive naphthene dehydrogenation reaction will be shifted into the last reactor of the series. Further, the olefins are known to polymerize to produce higher molecular weight parafiins, alkyl aromatics, and polycyclic aromatics. Certain of these polymerization products and more specifically certain polycyclic aromatics, are known to be adsorbed upon the catalyst surface where they effectively mask the active sites. Such polymerization will occur to some extent in all reactors since olefins as well as carbonium ions are produced as intermediate reactants in the reforming reactions, but normally this polymerization is concentrated in the last reactor where the temperature is more severe. Thus, introduction of excessive olefins into the first reactors of the series will cause accelerated catalyst degradation and such result is minimized by intro ducing the selected parafiinic raffinate only to the last reactor where polymerization products have a greater chance of being hydrocracked due to the higher temperature level which is maintained.

Therefore, in the practice of the present invention it is a preferred embodiment to recycle the selected paraffinic rafiinate only to the last of the several reforming reactors whenever the selected paraffinic rafiinate contains less than 10 volume percent of naphthenes, or when it contains more than 1.0 volume percent of olefins.

The process of the present invention may be more clearly understood by reference to the accompanying drawings which illustrate the various embodiments thereof.

Referring now to FIGURE 1, a charge stock comprising parafiinic and naphthenic hydrocarbons enters the process of the present invention via line 1 whereby it enters a catalytic reforming zone at a pressure in the range of from about p.s.i.g. to 500 p.s.i.g. The charge stock is admixed with a hydrogen-containing gas which enters line 1 by means of line 18. In addition, a selected paraffinic raffinate stream may enter line 1 by means of line 30 as will be further noted hereinbelow, but in the instant embodiment, line 30 is isolated by appropriate valving and no hydrocarbon stream is thereby introduced into line 1. The resulting mixture of charge stock and hydrogen-containing gas passes by means of line 1 into heater 2 wherein its temperature is raised to a level in the range of from about 850 F. to 1050" F. The heated mixture leaves via line 3 and enters a first catalytic reforming reactor 4 containing a suitable catalyst comprising aluminum, platinum, and chlorine wherein a substantially endothermic catalytic reaction occurs due to the dehydrogenation of naphthenes to form aromatic hydrocarbons and hydrogen. The resulting mixture passes via line 5 into heater 6 wherein the temperature of the stream is again raised to a level in the range of from about 850 F. to 1050 F. Leaving by means of line 7, the stream enters a second reforming reactor 8 wherein a further endothermic catalytic reaction occurs due to the further dehydrogenation of naphthenes. The resulting stream leaves via line 9 wherein it is mixed with the selected parafiinic raffinate stream which enters via line 31 and which is to be specified hereinbelow. The mixed stream then enters heater 10 wherein the temperature is again raised to a level in the range of from about 850 F. to 1050 F. The heated mixture leaves via line 11 and enters a third reforming reactor 12 wherein substantial catalytic hydrocrackin-g and dehydrocyclization of parafiins occurs. The final effluent leaves via line 13 and upon cooling enters a phase separator 14, wherein a hydrogencontaining gas phase and a liquid hydrocarbon phase are separated. The hydrogen-containing gas phase is Withdrawn via line 15 and the net hydrogen-containing gas which is generated within the catalytic reforming zone is withdrawn therefrom via line 16 as a product stream. The balance of the hydrogen-containing gas passes via line 15 to a recycle compressor 17 by means of which the gas is compressed. The compressed gas is discharged from compressor 17 via line 18 and enter line .1 wherein it is admixed with the hydrocarbon charge stock as previously noted.

A11 unstabilized hydrocarbon product, comprising substantial aromatics, leaves phase separator 14 via line 19 and passes into a first fractionation zone 20 which may be comprised of one or more fractionating columns as required to effect the desired separations. The unstabilized hydrocarbon product is fractionated to remove dissolved gases and light non-aromatic hydrocarbons. A gaseous product comprising hydrogen, methane, ethane and propane leaves fractionation zone 20 via line 21 while a first light non-aromatic hydrocarbon product comprising butanes and pentanes is withdrawn by means of line 22. An aromatic-rich stream comprising hexane and heavier hydrocarbons is withdrawn via line 23 and passed into an aromatics separation zone 24 which preferably is comprised of a solvent extraction process as previously noted. The high purity aromatic product stream is withdrawn via line and it may be sent to subsequent distillation means for fractionation into its separate aromatic components.

A paraflinic rafiinate stream, containing heavy alkylaromatics and polycyclic aromatics, leaves the aromatic separation zone 24 by means of line 26 and enters a raffinate fractionation zone 27. This fractionation zone 27 will be comprised of one or more fractionating columns as may be required to effect the distillation of the raftinate to remove hexanes andheptanes and to remove polycyclic aromatics. A second light hydrocarbon product stream comprising hexanes and heptanes is withdrawn from fractionation zone 27 by means of line 28. This paraflinic product stream will also contain traces of benzene and toluene as well as amounts of olefins and naphthenes which contain from six to about seven carbon atoms per molecule. A heavy hydrocarbon product stream, containing heavy alkylaromatics and polycyclic aromatics as well .as heavy paraffins, is Withdrawn via line 29.

The selected parafiinic rafiinate stream, comprised of paraifins having eight or more carbon atoms per molecule and having substantial freedom from polycyclic aromatics, is withdrawn from fractionation zone 27 via line 30. This selected paraifinic rafiinate stream may be introduced into line 1 by means of line as previously noted hereinabove. Since this stream is principally comprised of parafiins and since it contains some olefins, for reasons previously discussed, the preferred embodiment of the inventive process is not to return this stream to line 1. The preferred embodiment is to isolate line 30 from line 1 with appropriate valving, and withdraw this paraifinic raffinate stream from line 30 by means of line 31. The selected paraffinic rafiinate stream passes through line 31 and is injected into line 9 as the stream specified hereinabove, wherein it is mixed with the effluent leaving reactor 8. The mixed stream enters reactor 12 by means previously noted and the paraifinic hydrocarbons which have been recycled from the raffinate fractionation zone 27 are catalytically dehydrocyclized in reactor 12 to produce additional aromatic hydrocarbons.

Where the hydrocarbon charge stock of the inventive process has a high end boiling point, say from about 375 F. to 425 F., a considerable volume of aromatics will be produced having nine or more carbon atoms per molecule. For reasons previously set forth, such aromatics cannot be effectively recovered without a double aromatics extraction system. Such a processing combination as applied to the present invention is illustrated by the diagram of FIGURE 2.

Referring to FIGURE 2, a charge stock comprising paraffinic and naphthenic hydrocarbons enters the inventive process via line 1 whereby it enters a catalytic reforming zone at a pressure in the range of from about 160 p.s.i.g. to 500 p.s.i.g. The charge stock is admixed with a hydrogen-containing gas which enters line l by means of line 18. In addition, a selected paraifinic raffinate stream may enter line 1 by means of line 31 as will be further noted hereinbelow, but in the instant embodiment line 31 is isolated with appropriate valving and no hydrocarbon stream is thereby introduced into line 1. The resulting mixture of charge stock and hydrogen-containing gas passe by means of line 1 into heater 2 wherein its temperature is raised to a level in the range of from about 850 F. to 1050 F. The heated mixture leaves via line 3 and enters a first catalytic reforming reactor 4 containing a suitable catalyst wherein a substantially endothermic catalytic reaction occurs due to the dehydrogenation of naphthenes to form aromatic hydrocarbons and hydrogen. The resulting mixture passes via line 5 into heater 6 wherein the temperature of the stream is again raised to a level in the range of from about 850 F. to 1050" F. Leaving by means of line 7, the stream enters a second reforming reactor 8 wherein a further endothermic catalytic reaction occurs due to the further dehydrogenation of naphthcnes. The resulting stream leaves via line 9 wherein it is mixed with the selected parafiinic raifinate stream which enters via line 32 and which is to be specified hereinbelow. The mixed stream then enters heater 10 wherein the temperature is again raised to a level in the range of from about 850 F. to 1050 F. The heated mixture leaves via line 11 and enters a third reforming reactor 12 wherein substantial catalytic hydrocracking and dehydrocyclization of parafiins occurs. The final efiluent leaves via line 13, and upon cooling, enters a phase separator 14, wherein a hydrogen-containing gaseous-vapor phase and a liquid hydrocarbon phase are separated. The hydrogen-containing gaseous-vapor phase is withdrawn via line 15 and the net hydrogen-containing gas which is generated within the catalytic reforming zone is withdrawn therefrom via line 16 as a product stream. The balance of the hydrogen-containing gas passes via line 15 to a recycle compressor 17 by means of which the gas is compressed. The compressed gas is discharged from compressor 17 via line 18 and enters line 1 wherein it is admixed with the hydrocarbon charge stock as previously noted.

An unstabilized hydrocarbon product, comprising substantial aromatics, leaves phase separator 14 via line 19 and passes into a first fractionation zone 29 which may be comprised of one or more fractionating columns as required to effect the desired separations. The unstabilizcd hydrocarbon product is fractionated to remove dissolved gases and light non-aromatic hydrocarbons. A gaseous product comprising hdrogen, methane, ethane, and propane leaves fractionation zone 20 via line 21 while a first light non-aromatic hydrocarbon product comprising butanes and pentanes is withdrawn by means of line 22. A first aromatic-containing stream comprised of hydrocarbons having from six to-abc-ut eight carbon atoms per molecule is withdrawn via line 24 and passed into a first aromatics separation zone 25. A second aromatic-containing stream comprised of hydrocarbons having about nine or more carbon atoms per molecule is withdrawn via line 23 and passed into a second aromatics separation zone 33. Both aromatics separation zones may be characterized by the solvent extraction processing method previously set forth.

A product stream of high purity light aromatics containing benzene, toluene, ethylbenzene, and xylenes is removed from aromatics separation zone 25 by means of line 26, and a product stream of high purity heavy aromatics containing about nine or more carbon atoms per molecule is removed from aromatics separation zone 33 by means of line 34. These aromatic product streams may be sent to subsequent distillation means for fractionation into the separate aromatic product components.

A light parafiinic raffinate stream comprising hydrocarbons having from six to about eight carbon atoms per molecule leaves the first aromatics separation zone 25 via line 27 and is mixed with a heavy parafiinic ratfinate stream comprising hydrocarbons having about nine or more carbon atoms per molecule which leaves the second aromatics separation zone 33 via line 35. The two mixed rafiinate streams pass via line 27 and enter a raflinate fractionation zone 28. This fractionation zone 28 will be com prised of one or more fractionating columns as may be required to effect the distillation of the raflinate to remove hexanes and heptanes and to remove polycyclic aromatics. A second light hydrocarbon product stream comprising hexanes and heptanes is withdrawn from fractionation zone 28 by means of lines 29. This parafrinic product stream will also contain traces of benzene and toluene as well as amounts of olefins and naphthenes which contain from six to about seven carbon atoms per molecule. A heavy hydrocarbon product stream, containing heavy alkylaromatics and polycyclic aromatics, as well as heavy paraffins. is withdrawn via line 30.

The selected paraffinic raffinate stream, comprised of parafrins having eight or more carbon atoms per molecule and having substantial freedom from polycyclic aromatics, is withdrawn from fractionation zone 28 via line 31. This selected paraifinic raffinate stream may be introduced into line 1 by means of line 31 as previously noted hereinabove. Since this stream is principally comprised of paraffins, and since it contains some olefins, for reasons previously discussed, the preferred embodiment of the inventive process is not to return this stream to line 1. The preferred embodiment is to isolate line 31 from line 1 with appropriate valving, and withdraw this paraffinic raffinate stream from line 31 by means of line 32. The selected paraffinic rafiinate stream passes through line 32 and is injected into line 9 as the stream specified hereinabove, wherein it is mixed with the efiluent leaving reactor 8. The mixed stream enters reactor 12 by means previously noted and the paraffinic hydrocarbons which have been recycled from the raflinate fractionation zone 28 are catalytically dehydrocyclicized in reactor 12 to produce additional aromatic hydrocarbons.

In the foregoing generalized narrations of the inventive process, no operating conditions have been indicated for the fractionation zones or the aromatic separation zones. Such conditions will of necessity vary in accordance with the composition of the hydrocarbon streams being processed, as well as with the physical characteristics of the processing equipment utilized. Those skilled in the art will readily ascertain the operating conditions which may be required to effect the physical separations of the hydrocarbon components therein,

The advantages of the inventive process will be most effectively illustrated by the following examples. Example 1 illustrates the yields of aromatic products and parafiinic raifinates which are obtained by catalytic reforming of a specific hydrocarbon charge stock in combination with solvent extraction of the aromatics wherein no recycle of the paraiiinic ratiinate is practiced. Example 2 illustrates the yields of aromatic products and paraffinic rafiinate products which are obtained by catalytic reforming of the same hydrocarbon charge stock in combination with solvent extraction of the aromatics, wherein the selected parafiinic rafiinate is recycled to the catalytic reformer in accordance with the practice of the present invention.

Example 1 A full boiling range naphtha having an initial boiling point of 160 F. and an end boiling point of 375 F. is charged to a catalytic reformer at a liquid hourly space velocity of 1.4. The naphtha has a gravity of 590 API and has a volumetric composition of 60 vol. percent paraffins, 30 vol. percent naphthenes, and vol. percent aromatics. The charge stock is processed with 7.5 moles of hydrogen per mole of hydrocarbon at a pressure of 400 p.s.i.g. in the presence of a reforming catalyst comprised of alumina, platinum, and chlorine. The catalyst is conained in three reactors with the first reactor containing of the catalyst, the second reactor containing of the catalyst, and the third reactor containing of the catalyst. The naphtha is charged to the process at the rate of 12,000 barrels per stream day (b.p.s.d.) and is heated therein to provide that the mixed hydrocarbon and hydrogen stream enters each reactor at a temperature of 965 F. Upon cooling and separation of the efiluent into a gas phase and a liquid phase, the gas phase is recirculated to the reactor section in accordance with the art of catalytic reformin and a portion of the gas phase is withdrawn as a net product. The unstabilized liquid phase is passed to a fractionation section wherein an aromaticrich stream comprised of hydrocarbons having six or more carbon atoms per molecule is recovered at a rate of 8270 b.p.s.d. This stream is charged to a solvent extraction system wherein 4902 b.p.s.d. of high purity aromatic hydrocarbons are recovered for further processing, and 3368 b.p.s.d. of paratlinic raflinate are recovered and sent to product storage. The aromatic product is passed to an aromatic fractionation system wherein it is separated to 12 provide high purity aromatic component products comprising 420 b.p.s.d. of benzene, 1032 b.p.s.d. of toluene, 1638 b.p.s.d. of mixed ethylbenzene and xylenes, and 1812 b.p.s.d. of aromatics containing nine or more carbon atoms per molecule.

Example 2 The full boiling range naphtha defined in Example 1 is charged to a catalytic reforming unit at a rate of 12,000 b.p.s.d. and a liquid hourly space velocity of 1.3 in the presence of 7.5 moles of hydrogen per mole of hydrocarbon at a pressure of 400 p.s.i.g. The reactor section is composed of three reaction vessels containing a catalyst comprised of alumina, platinum and chlorine distributed in the reactors with 20% of the catalyst contained in the first reactor, 30% in the second, and 50% in the third. A selected paratfinic rafiinate stream is introduced into the reactor section at a rate of 1187 b.p.s.d and is processed through the third reactor only. The full boiling range naphtha, in conjunction with the selected paraffinic raffinate, amounts to a combined feed of 13,187 b.p.s.d. yielding an effective liquid hourly space velocity of 1.4. Sufficient heating means are provided to assure that the combined hydrogen and hydrocarbon stream entering each reactor is introduced therein at a temperature of 965 F. The final efiiuent leaving the third reactor is cooled and separated into a gas phase and a liquid phase. The gas phase is recirculated through the reaction vessels in accordance with the art of catalytic reforming and a portion of the gas is withdrawn as a net product.

The unstabilized liquid phase is passed to a fractionation section wherein an aromatic-rich stream comprised of hydrocarbons having six or more carbon atoms per molecule is recovered at a rate of 8980 b.p.s.d. This stream is then passed to a solvent extraction system wherein 5456 b.p.s.d. of high purity aromatic hydrocarbons are withdrawn, and 3524 b.p.s.d. of paraifinic raffinate are withdrawn. The aromatic product is passed to an aromatic fractionation system wherein it is separated to provide high purity aromatic component products comprising 448 b.p.s.d. of benzene, 1108 b.p.s.d. of toluene, 2084 b.p.s.d. of ethylbenzene and mixed xylenes, and 1816 b.p.s.d. of aromatics containing nine or more carbon atoms per molecule. The parafiinic raffinate stream is passed to a raffinate fractionation section wherein the rafiinate is separated to provide 2156 b.p.s.d. of a light rafiinate product containing hydrocarbons having from six to seven carbon atoms per molecule, 181 b.p.s.d. of a heavy ratfinate product containing about 53.0 vol. percent of polycyclic aromatics and heavy alkylbenzenes, and 1187 b.p.s.d. of a selected parafiinic raffinate stream comprised of hydrocarbons having at least eight carbon atoms per molecule and having substantial freedom from polycyclic aromatics. The selected paraflinic rafiinate stream, having an initial boiling point of 225 F. and an end boiling point of 320 F., ocntains 9.3 vol. percent naphthenes and 0.5 vol. percent olefins. This selected rafiinate stream is sent back to the catalytic reformer and introduced into the hydrogenhydrocarbon stream which feeds the third reaction vessel as has been previously noted hereinbefore.

The foregoing Example 1 provides an indication of the yields of aromatic hydrocarbons and of paraflinic rafiinate obtained when the subject full boiling range naphtha is catalytically reformed followed by extraction of aromatics in accordance with the current processing art. The foregoing Example 2 provides an indication of the yields of aromatic hydrocarbons and of paraifinic rafiinate obtained when the subject naphtha is catalytically reformed and solvent extracted in accordance with the present invention whereby the selected parafiinic raffinate fraction is recycled to the catalytic reforming zone. The advantage of the present invention is most readily ascertained by referring to Table A below wherein the results indicated in the examples are tabulated for ease of comparison. For reliability of comparison, the solvent extraction efi1- ciences' in recovering the individual aromatic components were held the same in the two processing examples.

TABLE A.-PROCESSING 12,000 B.P.S.D. OF FULL BOILING RANGE NAPHTHA Example 1- Example 2- Reforming Reforming and and Extraction Extraction without with Raflinate Recycle Recycle Daily Production, b.p.s.d. Aromatic Products:

Benzene 420 448 Toluene l, 032 l, 108 Total C3 Aromat 1, 638 2, 084 Aromatics 1, 812 1, 816

Total Aromatics 4, 902 5, 456

Parafiinic Rafl'mate Products: N Light Rafiinate .1 2, 156 Heavy Rafimate... 181

Total Rafiinato 3, 368 2, 337

Product Yields, Volume Percent of Naphtha Feed Aromatics Yields:

Benzene 3. 50 3. 73 Toluene 8. 60 9. 23 Total 03 Aromatics 13. 65 17. 34 0 Aromatics 2 15.10 15.13

Total Aromatics 40. 85 45. 43

Parafiinic Raffinate Yields:

Light Raflinate 17. 96 Heavy Rafllnate 1. 50

Total Ratfinate 28. 07 19. 46

1 C is defined as having eight carbon atoms per molecule. 2 0 is defined as having nine or more carbon atoms per molecule.

It will be noted that the inventive process has the advantage of increasing aromatic hydrocarbon production and yields while decreasing the production and yields of parafiinic rafiinate. As indicated in Table A above, the present invention resulted in an increase in the yield of total aromatics from 40.85 vol. percent to 45.43 vol. percent of the naphtha charge stock, while decreasing the yield of raflinate from 28.07 vol. percent to 19.46 vol. percent. The yield of benzene increased from 3.50 vol. percent to 3.73 vol. percent for a net increase of 0.23 vol. percent, and the yield of toluene increased from 8.60 vol. percent to 9.23 vol. percent for a net increase of 0.63 vol. percent. The yield of mixed ethylbenzene and Xylenes increased from 13.65 vol. percent to 17.34 vol. percent for a net increase of 3.69 vol. percent, while the commercially less desirable aromatics containing nine or more carbon atoms per molecule increased from 15.10 vol. percent to 15.13 vol. percent for a net increase of only 0.03 vol. percent. The total aromatic yield was thus increased 4.58 vol. percent, while the yield of paraifinic raflinate was decreased 8.61 vol. percent.

Additional advantages to the present invention may be summarized. Thus by fractionating the raflinate to provide the selected paraflinic raffinate fraction for recycle to the catalytic reformer, no accumulation of hexanes and heptanes occurs within the process while the reforming catalyst is protected from an accelerated decline in activity. By sending the selected paraffinic raifinate to the last of the several reforming reactors when the naphthene content of the selected raffinate is less than 10.0 vol. percent, or when the olefin content exceeds 1.0 vol. percent, the reforming catalyst is most effectively used to balance the desired chemical reactions and the catalyst is protected from premature deactivation. In addition, by elimination of the recycled paramnic raifinate from the front of the reforming zone, the size of the prior heaters, reactors, and process lines may be reduced and thereby the capital and operating expenses are reduced. Since the selected paraffiuic rafimate is obtained by fractionation to meet deslred specifications, flexibility is inherent in the inventive process. The operation of the raflinate fractionation zone may be adjusted as markets for paraffinic rafiinate products vary, provided only that the paraffinic rafiinate fraction which is recycled to the catalytic reforming zone be comprised of hydrocarbons having at least eight carbon atoms per molecule and that this recycle stream has substantial freedom from polycyclic aromatics. Other advantages inherent to the inventive process may be readily determined by those skilled in the art of hydrocarbon processing.

Although the discussion of the present invention has been oriented to the maximization of aromatic hydrocarbons as specific products, it must be realized that the present invention is not so limited. Since aromatic hydrocarbons are high in octane, the inventive process is equally useful in providing a means of producing high octane gasoline. Thus, the aromatics produced could be blended in whole or in part with any or all of the other liquid product streams leaving the process in any proportions which may be required to result in a gasoline of the desired quality. However, the manner in which the product streams leaving the inventive process are subsequently utilized has no bearing on the broadness of the present invention.

I claim as my invention:

1. Process for the production of aromatic hydrocarbons which comprises the steps of:

(a) contacting a hydrocarbon feed stock containing naphthenic and paraffinic compounds with hydrogen in a reaction zone under conditions suitable to convert at least a portion of said naphthenes into aromatic compounds;

(b) separating the eflluent therefrom into a hydrogenrich gas product and a product containing said aromatic compounds;

(c) subjecting said aromatic-containing product to conditions sufiicient to produce a first paraflinic product containing polycyclic aromatic compounds and a product comprising high-purity monocyclic aromatic hydrocarbons;

(d) recovering said high-purity monocyclic aromatic product;

(e) separating said first parafiinic product into at least a second paraflinic product comprising hydrocarbons having at least eight carbon atoms per molecule and having substantial freedom from said polycyclic aromatic compounds; and

(f) returning at least a portion of said second paraflinic product to said reaction zone wherein at least a portion of said product is converted to aromatic compounds.

2. Process for the production of aromatic hydrocarbons which comprises the steps of (a) contacting a hydrocarbon feed stock containing naphthenic and paraffinic compounds with hydrogen in a reaction zone under conditions suitable to convert at least a portion of said naphthenes into aromatic compounds;

( b) separating the effiuent therefrom into at least a hydrogen-rich gas product and a product containing said aromatic compounds;

(c) subjecting said aromatic-containing product to conditions sufficient to provide a light aromatic-containing fraction comprised of hydrocarbons containing from about six to about eight carbon atoms per molecule and a heavy aromatic-containing fraction comprised of hydrocarbon containing about nine or more carbon atoms per molecule;

((1) separating said light aromatic-containing fraction into a first paraffinic fraction and a light high-purity monocyclic aromatic product;

(e) separating said heavy aromatic-containing fraction into a second paraffinic fraction containing polycyclic aromatic compounds and a heavy high-purity monocyclic aromatic product;

(f) recovering said light monocyclic aromatic product and said heavy monocyclic aromatic product;

(g) subjecting said first paraffinic fraction and said second paraffinic fraction to conditions sufiicient to 15 provide a third paraffinic fraction comprising hydrocarbons having at least eight carbon atoms per molecule, and having substantial freedom from said polycyclic aromatic compounds; and

(h) returning at least a portion of said third paraffinic fraction to said reaction zone wherein at least a portion of said fraction is converted to aromatic compounds.

3. Process of claim 1 wherein said second paratfinic product has an end boiling point not greater than 350 F.

4. Process of claim 2 wherein said third paraffinic fraction has an end boiling point not greater than 350 F.

5. Process for the production of aromatic hydrocarbons which comprises the steps of:

(a) contacting a hydrocarbon feed stock containing naphthenic and parafiinic compounds with hydrogen in a reaction zone containing a series of contact sections maintained under conditions suitable to convert at least a portion of said naphthenes into aromatic compounds;

(b) separating the efiiuent therefrom into a hydrogenrich gas product and a product containing said aromatic compounds;

(c) subjecting said aromatic-containing product to conditions sufficient to produce a first paratfinic product containing polycyclic aromatic compounds and a product comprising high-purity monocyclic aromatic hydrocarbons;

(d) recovering said high-purity monocyclic aromatic product;

(e) separating said first paraffinic product into at least a second parafiinic product comprising hydrocarbons having at least eight carbon atoms per molecule and having substantial freedom from said polycyclic aromatic compounds; and

(f) returning at least a portion of said second paraffinic product to the last of said contact sections wherein at least a portion of said product is converted to aromatic compounds.

6. The process of claim 5 wherein said second paraffinic product has an end boiling point not greater than about 350 F.

7. Process for the production of aromatic hydrocarbons which comprises the steps of:

(a) contacting a hydrocarbon feed stock containing 16 naphthenic and parafiinic compounds with hydrogen in a reaction zone containing a series of contact sections maintained under conditions suitable to convert at least a portion of said naphthenes into aromatic compounds;

(b) separating the eflluent therefrom into at least a hydrogen-rich gas product and a product containing said aromatic compounds;

(c) subjecting said aromatic-containing product to conditions sufiicient to provide a light aromaticcontaining fraction comprised of hydrocarbons containing from about six to about eight carbon atoms per molecule and a heavy aromatic-containing fraction comprised of hydrocarbons containing about nine or more carbon atoms per molecule;

(d) separating said light aromatic containing fraction into a first paraffinic fraction and a light highpurity aromatic product;

(e) separating said heavy aromatic-containing fraction into a second parafiinic fraction containing polycyclic aromatic compounds and a heavy high-purity monocyclic aromatic product;

(f) recovering said light monocyclic aromatic product and said heavy monocyclic aromatic product;

g) subjecting said first paraffinic fraction and said second parafiinic fraction to conditions sufficient to provide a third paraflinic fraction comprising hydrocarbons having at least eight carbon atoms per molecule, and having substantial freedom from polycyclic aromatic compounds; and

(h) returning at least a portion of said third paraffinic fraction to the last of said series of contact sections wherein at least a portion of said fraction is converted to aromatic compounds.

8. The process of claim 7 wherein said third paraffinic fraction has an end boiling point not greater than about 350 F.

References Cited UNITED STATES PATENTS 2,870,226 1/1959 Deanesly 260-668 2,909,477 10/1959 Muller 20865 2,915,455 12/1959 Donaldson 208-65 ABRAHAM RIMENS, Primary Examiner.

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