Combination process for treatment of hydrocarbon streams containing mercapto compounds

Abstract

A FIRST HYDROCARBON STREAM CONTAINING A MERCAPTO COMPOUND IS TREATED TO REMOVE MERCAPTO COMPOUNDS THEREFROM WITHOUT ADDING ANY SUBSTANTIAL AMOUNTS OF UNDESIRED DISULFIDE COMPOUNDS THERETO AND A SECOND HYDROCARBON STREAM CONTAINING A MERCAPTO COMPOUND IS SIMULTANEOUSLY SWEETENED BY A COMBINATION PROCESS INVOLVING: A FIRST EXTRACTION STEP WHICH IS PERFORMED ON THE FIRST HYDROCARBON STREAM WITH A DISULFIDE-FREE ALKALINE STREAM, A SECOND EXTRACTION STEP WHICH IS PERFORMED ON THE SECOND HYDROCARBON STREAM, AN ALKALINE STREAM REGENERATION STEP, AND A COMBINATION SWEETENING AND DISULFIED-EXTRACTION STEP WHICH IS PERFORMED ON THE SECOND HYDROCARBON STREAM IN ORDER TO SWEETEN SAME WHILE SIMULTANEOUSLY PREPARING THE DISULFIDE-FREE ALKALINE STREAM FOR USE IN THE FIRST EXTRACTION STEP. KEY FEATURE OF THE RESULTING PROCESS IS THE USE OF THE SWEETENING STEP TO REMOVE MERCAPTIDE AND DISULFIDE COMPOUNDS FROM A PORTION OF THE REGENERATED ALKALINE STREAM SO THAT THE AMOUNT OF RE-ENTRY DISULFIDE COMPOUNDS THAT ARE CARRIED BACK INTO THE FIRST EXTRACTION STEP BY THE ALKALINE STREAM IS HELD TO A MINIMAL LEVEL.

Claims

COMBINATION PROCESS FOR TREATMENT OF HYDROCARBON STREAMS CONTAINING MERCAPTO COMPOUNDS Filed Jan. 22.v 1969 United States Patent Oihce" 3,574,093 Patented Apr. 6, 1971 U.S. Cl. 208-206 6 Claims ABSTRACT F THE DISCLOSURE A first hydrocarbon stream containing a mercapto compound is treated to remove mercapto compounds therefrom Without adding any substantial amounts of undesired disulfide compounds thereto and a second hydrocarbon stream containing a mercapto compound is simultaneously sweetened by a combination process involving: a first extraction step which is performed on the first hydrocarbon stream with a disulfide-free alkaline stream, a second extraction step which is performed on the second hydrocarbon stream, an alkaline stream regeneration step, and a combination sweetening and disulfide-extraction step` which is performed on the second hydrocarbon stream in order to sweeten same While simultaneously preparing the disulfide-free alkaline stream for use in the first extraction step. Key feature of the resulting process is the use of the sweetening step to remove mercaptide and disulfide compounds from a portion of the regenerated alkaline stream so that the amount of re-entry disulfide compounds that are carried back into the first extraction step by the alkaline stream is held to a minimal level. The subject of the present invention is a combination process directed towards the simultaneous treatment of two hydrocarbon streams containing mercapto compounds, one having a relatively low boiling range, and the second having a relatively high boiling range, to produce a first treated hydrocarbon stream which is substantially free of sulfur compounds and a second treated hydrocarbon stream which is substantially free of mercapto compounds. More specifically, the present invention provides an eflicient and economic solution to the problem of continuously treating, in a closed loop process, a hydrocarbon stream with an alkaline stream to remove mercapto compounds therefrom without increasing the amount of disulfide compounds contained in the treated distillate. The solution disclosed herein essentially involves a combination process wherein a second hydrocarbon stream containing mercapto compounds is first subjected to an extraction step and then is sweetened in a combination sweetening and disulfide extraction step which is designed to perform a final sweetening function on this second hydrocarbon stream while simultaneously extracting disulfide compounds from a partially regenerated alkaline stream to produce an alkaline stream which is substantially free of all sulfur compounds. This latter stream is subsequently used in the removal of mercapto compounds from the first hydrocarbon stream. The concept of the present invention developed from my efforts toward the solution of a substantial hydrocarbon stream treatment problem which has heretofore plagued application of extraction processes which attempt to continuously utilize an alkaline stream to completely extract .mercapto compounds from a relatively low boiling hydrocarbon stream without injecting sulfur compounds into the treated stream; that is, processes of the type that operate with a closed loop with respect to the alkaline stream. In this type of process, yan extraction step is coupled with a regeneration step and an alkaline stream is continuously recirculated therebetween. In the extraction step, the alkaline stream is used to extract mercapto compounds from the hydrocarbon stream, and the resulting fat alkaline stream is treated in the regeneration step to remove mercaptide compounds therefrom with continuous cycling of the alkaline stream between the extraction step and the regeneration step. The regeneration step is typically operated to produce disulfide compounds which are immiscible in the alkaline stream, and the major portion of which is typically separated therefrom in a settling step. In many cases, however, it is desired to remove substantially all disulfide compounds from the alkaline streams and complete separation of disulfide compounds from the alkaline stream in a settling step is not feasible because of the high dispersion of these compounds throughout the alkaline solution. Accordingly, the art has resorted to a number of sophisticated techniques in order to coalesce the disulfide compounds and effect their removal from the regenerated alkaline stream. One technique that has been utilized involves the use of a coalescing agent such as steel wool in order to spring disuldes from the regenerated alkaline solution. Another technique which has been widely utilized involves the use of one or more stages of a naphtha wash in order to extract disulfide compounds from this alkaline solution. This latter technique has been Widely utilized in the art, but it has the disadvantage of requiring a separate train of vessels and separators with attendent piping and instrumentation, which hardware adds a substantial increment of cost in the economics of the overall process. As is well known to those skilled in the art, there are certain low boiling range hydrocarbon streams for which it is absolutely critical that the amount of sulfur compounds contained therein be held to a very low level. In many cases, this requirement is expressed as a limitation on the total amount of sulfur that can be tolerated in the treated stream-typically, the requirement is for a sulfur content less than 50 wt. p.p.m. calculated as elemental sulfur, and, more frequently, the requirement is less than l0 Wt. p.p.m. sulfur. Accordingly, when a mercapto compound extraction process of the type described above is designed to meet these stringent sulfur limitations, it is essential that the amount of so-called re-entry sulfur, (which is the sulfur in the form of disulfide or mercaptide compounds contained in the regenerated alkaline solution) be held to an extremely low level in order to avoid contamination of the extracted stream with disulfide sulfur. For example, in the sweetening of a hydrocarbon stream containing C3 and C4 olefins and about 750 wt. p.p.m. mercaptan sulfur, an extraction process can easily be designed to produce a treated hydrocarbon distillate having about =5 wt. p.p.m. mercaptan sulfur; however, without special treatment of the regenerated alkaline solution utilized, by one of the techniques mentioned above, the total sulfur content of the treated hydrocarbon stream will be about 50 wt. parts per million because of disulfide compounds which are returned to the extraction step via the alkaline stream where they are transferred to the treated hydrocarbon stream. In general, the reasons why it is desired to produce treated hydrocarbon streams of extremely low sulfur content are well known to those skilled in the art. One example is commonly encountered in the treatment of C3, C4, and C3 plus C4 feedstocks for alkylation units wherein it has been determined that by desulfurizing these feedstocks to the lowest possible level, the quality and quantity of the alkylate yield is improved and the amount of acid consumed in the alkylation unit is sharply reduced. Similarly, in plants designed to polymerize light olefins, the principal effect of removing substantially all sulfur compounds from the feedstock is a dramatic increase in the stability of the polymerization catalyst. Other examples are associated with various petrochemical processes where the presence of sulfur in the feed stream to the process `adversely affects the activity, selectivity, or stability of the catalyst utilized, where sulfur increases the amount of reagents utilized, where sulfur causes corrosion problems, etc. Regardless of the reason why it is desired to maintain the total sulfur content of the treated hydrocarbon stream at a very low level, it is clear that there is a substantial demand in the art for a simple and economic procedure designed not only to extract mercapto compounds from a relatively low boiling stream, but to insure that the amount of reentry sulfur acquired by the treated stream during the treatment procedure is held to an extremely low level In many refineries where hydrocarbon streams containing mercapto compounds are treated to remove mercapto compounds therefrom, there are a great number of hydrocarbon streams containing mercapto compounds of diverse boiling range which rnust be similarly treated. In general, it is only the relatively low boiling streams that are required to be substantially free of all sulfur compounds as .explained above. For the relatively high boiling 'streams such as gasoline and kerosene fractions, the presence of disulfide compounds in the treated streams is not considered to be detrimental. In fact, these last streams are often sweetened by subjecting the stream to conditions resulting in the conversion of mercapto compounds to disulfide compounds which are allowed to remain in the treated hydrocarbon stream. I have now discovered a convenient, simple, and economic 'solution to the problem of providing an extraction process using a continuously regenerated alkaline stream for a relatively low boiling hydrocarbon stream containing a mercapto compound where the amount of sulfur compounds (i.e., disulfide and mercaptides) carried into the extraction zone by the alkaline recycle stream is to be minimize-d. In essence, my solution involves a combination process wherein both a relatively low boiling hydrocarbon stream and a relatively high boiling hydrocarbon stream are simultaneously treated in a series of interconnected and interdependent steps, and the high boiling stream is used to purify an alkaline recycle stream which is subsequently used to extract mercapto compounds from the low boiling stream. More specifically, my invention involves the use of the higher boiling hydrocarbon stream in a combination final sweetening and extraction step to sweeten this stream while simultaneously removing residual disulfide compounds and unreacted mercapto compounds from the alkaline stream used in the extraction step on the lower boiling hydrocarbon stream. Some of the broad advantages associated with this solution to the sulfur re-entry problem are: (l) it eliminates a separate purification lstep on the regenerated alkaline stream which is to be recycled to the extraction step to which the lower boiling stream is being charged; (2) it simplifies the resulting process by combining in a single zone an extraction step with a final sweetening step; and, (3) it minimizes the amount of unreacted mercaptides contained in the alkaline recycle stream charged to the extraction step operating on the lower boiling stream, thereby substantially increasing the efiiciency of this extraction step. `It is, accordingly, an object of the present invention to provide a combination process for extracting mercapto compounds from a relatively low boiling hydrocarbon stream with a continuously recirculated alkaline stream, while simultaneously sweetening a relatively high boiling hydrocarbon stream without contaminating the treated low boiling hydrocarbon stream with sulfur compounds. Another object of the present invention is to provide a simple method for the continuous purification of an alkaline stream utilized in the extraction of mercapto compounds from a relatively low boiling range hydrocarbon stream. Yet another object is to provide a solution to the problem of minimizing re-entry sulfur in an extraction process which operates on a relatively low boiling hydrocarbon stream and utilizes a continuously recirculated alkaline stream. In brief summary, my invention is a combination process for treating a first hydrocarbon stream containing a mercapto compound and having a relatively low boiling range to produce a treated hydrocarbon stream which is substantially free of sulfur compounds and for simultaneously sweetening a second hydrocarbon stream containing a mercapto compound and having a relatively high boiling range. The first hydrocarbon stream is contacted with a first alkaline recycle stream which is substantially free of sulfur compounds, in the first step of my process, at extraction conditions selected to produce a treated hydrocarbon stream which is substantially free of sulfur compounds and a first alkaline extract stream containing a mercaptide compound. In the second step, the second hydrocarbon stream is contacted with a second alkaline recycle stream at extraction condition selected to produce a hydrocarbon stream of reduced mercapto compound content and a second alkaline extract stream containing a mercaptide compound. In the third step, a mixture of the first extract stream, the second extract stream, and a rst air stream are contacted with a first catalyst at oxidizing conditions effective to form an efliuent stream containing N2, disulfide compounds, and an alkaline solution. Thereafter, in the fourth step, the efiiuent stream from the third step is separated into a gas stream containing N2, an oil stream containing disulfide compounds, and an alkaline stream containing a minor amount of disulfide compounds. A first portion of the alkaline stream from the fourth step is recovered, in the fifth step, and passed to the second step as the second alkaline recycle stream. In the sixth step, a mixture of the remaining portion of the alkaline stream from the fourth step, the hydrocarbon stream from the second step, and a second air stream is contacted with a second catalyst at oxidizing conditions effective to form an efliuent stream containing N2, disulfide compounds, alkaline solution, and treated hydrocarbons. In the seventh step, the efiiuent stream from the sixth step is separated into a gas stream containing N2, a sweetened hydrocarbon stream which is substantially free of mercapto compounds but contains a minor amount of disulfide compounds, and an alkaline stream which is substantially free of mercaptide and disulfide compounds. In the final step, the alkaline stream from the seventh step 1s passed to the first step as the first alkaline recycle stream. Other objects and embodiments of the present invention encompass details about particular input hydrocarbon streams, catalysts for use in the oxidation steps thereof, rnechamcs associated with each of the essential steps thereof, and preferred operating conditions for each of the steps thereof. These are hereinafter disclosed in the following discussion of each of the steps of the present invention which is presented in conjunction with a discussion of the attached drawing. Before considering in detail the various ramifications of the present invention, it is convenient to define several of the terms and phrases used herein. The term sweetening denotes the process of treating a hydrocarbon stream containing a mercapto compound with an oxidizing agent at conditions designed to effect the oxidation of mercaptans to disulfides. The term mercapto compound is used here to describe the sulfhydryl group-containing compounds such as the alkyl mercaptans and the aryl mercaptans. The term mercaptide compound denotes a metallic salt of a mercapto compound. The term disulde compound denotes a compound having the structure R-S- S-R where R and R' are any suitable alkyl or aryl group. The expression closed-loop is used herein with reference to the alkaline streams which are continuously recirculated within the process in a closed circuit. The lower boiling hydrocarbon stream which is treated by the process of the present invention is preferably a hydrocarbon stream containing mercapto compounds that are highly soluble in common alkaline streams such as an aqueous solution of sodium hydroxide. For example, hydrocarbon streams containing C3, C4, and C3-|C4 hydrocarbons are typical of the lower boiling hydrocarbon streams that can be treated by the method of the present invention. In general, the mercapto compounds contained in these hydrocarbon streams are soluble in common alkaline streams, and are present in an amount of about 50 to about 10,000 wt. p.p.m. of the stream, calculated as elemental sulfur. On the other hand, hydrocarbon streams boiling in the range of C5 and higher, contain mercapto compounds that are not completely soluble in common alkaline streams, and, thus, these latter streams would very rarely be used as the lower boiling hydrocarbon input stream. The requirements for the higher boiling hydrocarbon input stream for use in the present invention essentially involve its capability to dissolve disulfide compounds. In general, any hydrocarbon stream boiling in the gasoline range or above is suitable, and, typically, this will include gasoline fractions, including cracked gasoline, straight-run gasoline, natural gasoline, or mixtures thereof, naphthas, jet fuels, kerosines, etc. In most cases, the preferred higher boiling hydrocarbon input stream is a gasoline fraction primarily because these are commonly available and the presence of disulfide compounds therein is not detrimental. Typically, this hydrocarbon stream will contain about 100 to about 50,000 wt. ppm. of mercapto sulfur, calculated on an elemental sulfur basis. The alkaline solution utilized in the present invention may comprise any alkaline reagent known to have the capability to extract lower molecular weight mercapto compounds from relatively low boiling hydrocarbon streams. A preferred alkaline solution generally comprises an aqueous solution of an alkali metal hydroxide, such as sodium hydroxide, potassium hydroxide, etc. Similarly, aqueous solutions of alkaline earth hydroxides may be utilized if desired. A particularly preferred alkaline solution for use in the present invention is an aqueous solution of about 1 to about 50% by weight of sodium hydroxide with particularly good results obtained with aqueous solutions having about 4 to about 25 wt. percent sodium hydroxide. Suitable mercapto compound solubility increasing agents known to the art such as alcohols, alkylphenols, dialkylsulfoxides, etc., may be added to the alkaline solution if desired; however, excellent results are obtained with just aqueous solutions of sodium hydroxide. The catalyst utilized in the regeneration step and the sweetening step of the present invention may be any suitable catalyst known to the hydrocarbon treating art which is effective to accelerate the oxidation of mercapto compounds or mercaptides to disulfide compounds. A particularly preferred catalyst for use in the present invention comprises a metal phthalocyanine. These include cobalt phthalocyanine, vanadium phthalocyanine, iron phthalocyanine, copper phthalocyanine, nickel phthalocyanine, chromium phthalocyanine, etc. The metal phthalocyanine per se is, in general, not highly active for the oxidation of mercapto compounds and, accordingly, it is generally a good practice to utilize a polar derivative thereof. Particularly preferred polar derivatives are the sulfonated derivatives such as the mono-sulfo derivative, the disulfo derivative, the tri-sulfo derivative, and the tetra-sulfo derivative. These derivatives may be obtained from any suitable source or may be prepared by one of two general methods: first, the metallic phthalocyanine compound can be reacted with a fuming sulfuric acid; or, second, the phthalocycanine compound can be synthesized from a sulfo-substituted phthalic anhydride or equivalent thereof. The preferred phthalocyanine catalyst can be used in the present invention in one of two modes. First, it can be utilized in a water soluble form or a form which is capable of forming a stable emulsion in water. Second, the phthalocyanine catalyst can be utilized as a combination of a phthalocyanine compound with a suitable carrier material. In the first mode, the catalyst is present as a dissolved or suspended solid in the alkaline stream which is charged to the regeneration step or to the final sweetening step. In this mode, the preferred catalyst is cobalt or vanadium phthalocyanine disulfonate which is typically utilized in an amount of about 5 to about 1,000 wt. p.p.m. of the alkaline stream. In the second mode of operation, the catalyst is preferably utilized as a fixed bed of particles of a composite of the phthalocyanine compound with a suitable carrier material. The carrier material should be insoluble or substantially unaffected by the alkaline stream or hydrocarbon stream under the conditions prevailing in the various steps of the process. Activated charcoals are particularly preferred because of their high adsorptivity under these conditions. The amount of the phthalocyanine compound combined with the carrier material is preferably about 0.1 to about 2.0 wt. percent of the final composite. Additional details as to alternative carrier materials, methods of preparation, and the preferred amount of catalytic components for the preferred phthalocyanine catalyst for use in this second mode are given in the teachings of U.S. Pat. 3,108,081. It is important to note that the catalyst must be utilized in the same mode for both the regeneration step and the sweetening step. That is, if the homogeneous mode of operation is used in the regeneration step, it must be used in the sweetening; similarly, if the heterogeneous catalyst mode is used, it must be used in both steps. This invention will be further described with reference to the attached drawing which is a schematic outline of the process under discussion. The attached drawing is merely intended as a general representation of a preferred flow scheme with no intent to give details about vessels, heaters, condensers, pumps, compressors, valves, process control equipment, etc., except where a knowledge of these devices is essential to the understanding of this invention or would not be self-evident to one skilled in the art. Referring now to the attached drawing, a first hydrocarbon stream having a relatively low boiling range enters the process via line 1 and is charged to the lower region of first extraction zone 2. This first hydrocarbon stream is typically, in a pretreatment step, given a prewash with a dilute solution of sodium hydroxide in order to remove hydrogen sulfide. A typical example of a first hydrocarbon stream is an LPG stream having a specific gravity of API at 60 F. and a mercapto compound content of 510 wt. ppm., calculated as elemental sulfur. Extraction zone 2 is typically a vertically positioned tower containing suitable contacting means such as baffles pans, trays, Raschig Rings, and the like contacting means, designed to effect intimate contact between the two liquid streams charged thereto. Also charged to extraction zone 2 is a first alkaline recycle stream which is substantially free of sulfur compounds and which enters the upper region of zone 2 via line 6. As explained hereinbefore, this stream is preferably an aqueous solution of sodium hydroxide having a weight percent sodium hydroxide of about 5 to about 40 with excellent results obtained with `about a 15 wt. percent solution. The origin of this rst alkaline recycle stream and the procedure by which mercaptide and disulfide compounds are removed therefrom will be explained below. The function of extraction zone 2 is to achieve intimate contact between the descending alkaline stream and the ascending first hydrocarbon stream. This zone is preferably operated at a temperature of about 25 to about 200 F. with excellent results obtained at a temperature of about 100 to about 150 F. Likewise, the pressure utilized within zone 2 is generally selected to maintain the first hydrocarbon stream in liquid phase, and may range from ambient up to about 300i p.s.i.g. For an LPG stream the pressure is preferably about 140 to about 175 p.s.i.g. The volume loading of the alkaline stream relative to the rst hydrocarbon stream is preferably about l to about 30 vol. percent of the hydrocarbon stream with excellent results obtained for an LPG type stream when the alkaline stream is introduced into zone 2 in an amount of about 5% of the hydrocarbon stream. A treated hydrocarbon stream is withdrawn from the upper region of zone 2 via line 3 and passed to first separating zone 4 which is preferably maintained at conditions corresponding to those used in zone 2. Similarly, a first alkaline extract stream containing mercaptide cornpounds is withdrawn from zone 2 via line 7 and passed to regeneration zone 9. In first separating zone 4, any entrained alkaline solution that is present in the treated first hydrocarbon stream is separated therefrom, and a treated hydrocarbon stream is withdrawn via line S. For the LPG stream given as an example, the treated hydrocarbon stream withdrawn via line 5 will typically have a mercapto sulfur content of about 5 parts per million and because of the minimization of the sulfur compound content of the alkaline stream utilized in zone 2 'by the present invention the total sulfur content of this stream will be less than parts per million. The entrained alkaline solution which is separated in zone 4 is withdrawn therefrom via line 6 and returned to extraction zone 2. A second hydrocarbon stream enters the process via line 25 and is charged to the lower region of second extraction zone 16. This stream will also be typically prewashed with a dilute caustic solution in order to remove HZS therefrom. As explained hereinbefore, this stream may be any relatively high boiling hydrocarbon stream such as a gasoline fraction or a kerosine fraction. An example of a suitable second hydrocarbon stream is a straight-run naphtha having a boiling range of about 175 F. to about 300 F., a specific gravity of about 55 API at 60 F. and a mercapto compound sulfur content of about 300 parts per million. Also charged to zone 16 is a second alkaline recycle stream which enters the zone via line and the source of which is explained below. This stream preferably contains the same wt. percent sodium hydroxide as the first alkaline recycle stream mentioned above. Zone 16 contains suitable means for achieving intimate contact between the ascending hydrocarbon stream and the descending alkaline stream. It is preferably operated at approximately the same temperature and pressure conditions as disclosed above for zone 2. In general, for the higher boiling hydrocarbon streams it is preferred to use a slightly higher liquid-liquid loading of alkaline stream to hydrocarbon stream in zone 16 relative to that used in zone 2. For instance, for the LPG stream given as a specific example in the discussion of zone 2, a 5% ratio gives excellent results, whereas for the straight-run naphtha, given as an example here, excellent results are obtained with a volume ratio. Because the mercapto compounds contained in the higher boiling hydrocarbon stream are not completely soluble in the alkaline stream, the treated hydrocarbon stream withdrawn from the top of zone 16 via line 18 will generally not be sweet-that is, it will generally contain about 10 to 100 p.p.m. of residual mercapto compound sulfur. Accordingly, it is necessary to give this partly treated stream a final sweetening treatment, and the partially treated hydrocarbon stream Withdrawn from zone 16 via line 18 is passed to sweetening zone 19. A second alkaline extract stream containing mercaptide compounds is withdrawn from the lower region of zone 16 via line 17, and commingled with the first alkaline extract stream at the junction of line 17 with line 7. The resulting mixture is typically passed to a heating means not shown in the attached drawing wherein its temperature is raised about 10 to about 50 F. For example, when the extraction zones 2 and 16 are run at a temperature of about to about 120 F., the resulting mixture will be heated to a temperature of about F. The resulting heated mixture is then combined with a first air stream at the junction of line 8 with line 7. The amount of air commingled with the heated mixture at this point is ordinarily at least the stoichiometric amount necessary to oxidize mercaptides contained in this combined stream to disulfide compounds. That is to say, the amount of air will be sufiicient to react about 0.25 mole of O2 per mole of mercaptide compound contained in the combined alkaline stream. In general, it is a good practice to operate with sufficient excess air to insure that the reaction goes to completion. The resulting mixture of air and alkaline solution is passed via line 7 to regeneration zone 9. The function of zone 9 is to regenerate the alkaline solution by oxidizing the mercaptide compounds to disulfides; as pointed out hereinbefore this regeneration step is preferably performed in the presence of a phthalocyanine catalyst which may be present as a solution or emulsion in the alkaline stream or as a fixed bed of phthalocyanine compound combined with a carrier material. When the catalyst is present in zone 9 as a solution or emulsion in the alkaline stream, a suitable packing material will be utilized in order to effect intimate contact between the catalyst, the mercaptides, and oxygen-a particularly preferred contacting means is carbon Raschig Rings. On the other hand, in the other mode of operation, zone 9 will preferably contain a fixed bed of 1030 mesh particles comprising a combination of cobalt phthalocyanine monosulfonate with activated carbon. Zone 9 is preferably operated at a temperature corresponding to the entering rich alkaline solution which is typically in the range of about 100 to about 150 F. The pressure used in zone 9 is generally substantially less than that utilized in the extraction zones. For instance, in a typical embodiment wherein extraction zone 2 is run at p.s.i.g. and extraction zone 16 at 110 p.s.i.g., zone 9 is preferably operated at about 50 p.s.i.g. For the preferred catalyst, the residence time Within oxidizing zone 9 is ordinarily relatively loW-for example, in the homogeneous mode of operation a residence time of about 5 to about 20 minutes ordinarily gives excellent results, and in the heterogeneous mode of operation with the fixed bed of catalyst, a liquid hourly space velocity of about 0.5 to about l0 is satisfactory. Regardless of which mode zone 9 is operated in, an effluent stream containing N2, disulfide compounds, and alkaline solution is withdrawn therefrom via line 10 and passed to third separating zone 11 which is prefera-bly operated at the conditions used in zone 9. In many cases, this eliiuent stream will also contain unreacted oxygen and a very minor amount of unreacted mercaptide compounds. In zone 11 the effluent stream is allowed to separate into a gas phase which is withdrawn via line 12 and discharged from the process, a disulfide phase which is substantially immiscible with the alkaline phase and is withdrawn from the process via line 13, and an alkaline phase which is withdrawn via line 14. In general, the complete coalescence of the disulfide compound into a separate phase is extremely difiicult to achieve without the aid of suitable coalescing agents such as a bed of steel Wool, sand, glass, etc. In addition, a relatively high residence time of about 0.5 to 2 hours is typically used within zone 11 in order to further facilitate this phase separation. Despite these precautions, the regenerated alkaline stream which is withdrawn via line 14 inevitably contains minor amounts of disulfide compounds and mercaptide compounds, In fact, the amount of sulfur present in this regenerated stream is ordinarily in the range of 25 to about 200 p.p.m., calculated as elemental sulfur. It is, of course, understood that these numbers refer to equilibrium levels of disulfide cornpounds which can build up in this alkaline stream during the course of a prolonged recycle operation. In accordance with the present invention, the regenerated alkaline solution is divided into two portions at the junction of line 15 with line 14. The first portion is returned via line 15 to second extraction zone 16 as the second alkaline recycle stream which was referred to hereinbefore. rPhe remaining portion of the regenerated alkaline stream is passed via line 14 to the junction with line 18 wherein it is commingled with the partially treated hydrocarbon stream withdrawn from zone 16 via line 18. The resulting mixture is then admixed with a second air stream at the junction of line 27 with line 18. The amount of air injected at this point is selected to provide oxygen in substantial excess of that required to oxidize the residual mercapto compounds, contained in the hydrocarbon stream from zone 16, to disulfide compounds. The resulting mixture is then passed via line 18 into sweetening zone 19. Sweetening zone 19 will contain either a fixed bed of suitable packing material such as carbon Raschig Rings when the preferred catalyst is utilized in the form of a suspension or solution in the alkaline stream. In the heterogeneous mode of operation of sweetening zone 19, it will preferably contain a fixed bed of l to 30 mesh particles comprising a combination of phthalocyanine compound with a carrier material. A particularly preferred catalyst for this latter mode of operation is l0 to 30 mesh particles of cobalt phthalocyanine monosulfonate combined with an activated carbon carrier material in an amount such that the catalyst contains about 1.0 wt. percent of the phthalocyanine compound. Regardless of which mode the catalyst is used in, sweetening zone 19 is preferably operated at a temperature and pressure corresponding to that utilized Within zone 16. The time of contact in zone 19 between the liquid stream and the catalyst is generally adjusted to reduce the mercapto compound content of the treated stream to the desired low level. For the homogeneous catalyst mode of operation a contact time of about .05 to 1 hour is generally preferred. Likewise, for the iheterogeneous mode of operation, a liquid hourly space velocity of about 0.5 to about 10 is preferred. The function of sweetening zone 19 is not only to oxidize residual mercaptide compounds contained in the hydrocarbon stream from zone 16, but also to oxidize any residual mercaptide compounds contained in the regenerated alkaline stream from zone 11 and to extract disulfide compounds from this last alkaline stream. Accordingly, this zone is preferably operated so that intimate contact Ibetween the hydrocarbon stream and the alkaline stream charged thereto is achieved therein. Pursuant to this end, any suitable mixing means may be used on the mixture of alkaline solution with the hydrocarbon stream either before, during, or after passage of the combined stream through zone 19. An eiuent stream comprising nitrogen, disulfide compounds, alkaline solution, and treated hydrocarbon is thereafter withdrawn from zone 19 and passed to second separating zone 21. The function of zone 21 is to separate this efiiuent stream into three phases: a gas phase containing nitrogen and unreacted oxygen which is removed from the system via line 22, a hydrocarbon phase containing a minor amount of disulfide compound but which typically contains less than p.p. m. of mercapto compound sulfur which is withdrawn via line 26 as a sweetened hydrocarbon stream, and an alkaline stream which 10 is substantially free of both mercaptide and disulfide compounds which is withdrawn via line 23. This last alkaline stream will typically contain less than l0 p.p.m. of sulfur compounds, calculated as elemental sulfur, and is consequently substantially free of sulfur compounds. In accordance with the present invention at least a portion of this last alkaline stream lis passed via lines 23, 24, and 6 to extraction zone 2 as the rst alkaline recycle stream. Moreover, a second portion may be passed via lines 23, 14, and 18 back to zone 19, if desired, in order to adjust the amount of disulfide compounds injected into this zone. Accordingly, the process is operated in the manner described for a substantial period of time, and it is determined to provide an efficient and simple solution to the problem of minimization of re-entry sulfur injected into zone 2 via the tfirst alkaline recycle stream. I claim as my invention: 1. A combination process for treating a first hydrocarbon stream of C3 or C4 constituents containing a mercapto compound to produce a first treated hydrocarbon stream which is substantially free of sulfur compounds, and for simultaneously sweetening a second hydrocarbon stream boiling in the gasoline range and containing a mercapto compound, said process comprising the steps of: (l) contacting said first hydrocarbon stream with a first aqueous, alkaline recycle stream which is substantially free of sulfur compounds, in a first extraction zone, at conditions selected to produce said first treated hydrocarbon stream which is substantially free of sulfur compounds and a first aqueous, alkaline .extract stream containing a mercaptide compound; (2) contacting said second hydrocarbon stream with a second aqueous, alkaline recycle stream, in a second extraction zone, at conditions selected to produce a product hydrocarbon stream of reduced mercapto compound content and a second aqueous, alkaline extract stream containing a mercaptide compound; (3) contacting a mixture of the first extract stream, the second extract stream, and a first air stream with a phthalocyanine catalyst at oxidation conditions elective to form an efiiuent stream containing N2, disulfide compounds, and an aqueous, alkaline solution; (4) separating the effluent stream from step (3) into a gas stream containing N2, a stream containing disulfide compounds, and an aqueous, alkaline stream containing a minor amount of disulfide compounds; (5) passing a first portion of the aqueous, alkaline stream from step (4) to step (2) as said second recycle stream; (6) contacting a mixture of the remaining portion of the aqueous, alkaline stream from step (4), the product hydrocarbon stream from step (2) and a second air stream with a phthalocyanine catalyst at oxidizing conditions effective to form an effluent stream containing N2, disulfide compounds, alkaline solution and said treated product hydrocarbon; (7) separating the efiiuent stream from step (6) into a gas stream containing N2 said second treated hydrocarbon stream, which is substantially free of mercapto compounds and contains a minor amount of disulfide compounds, and an aqueous, alkaline stream which is substantially free of mercaptides and disulfide compounds; and, (8) passing at least a portion of the alkaline stream from step (7) to step (l) as the first recycle stream. 2. A combination process as defined in claim 1 wherein said first and second alkaline recycle streams are aqueous solutions of an alkali metal hydroxide. 3. A combination process as defined in claim 1 wherein said first and second catalysts are a metallic phthalocyanine disulfonate. 4. A combination process as defined in claim 1 wherein said first and second catalysts are sulfo-substituted cobalt phthalocyanine. l 1 l 2 5. A combination process as dened in claim 1 Whereir References Cited the rst and second catalysts are xedbeds of solid par- UNITED STATES PATENTS lfieles comprising a phthalocyanine compound combined 2,921,020 1/1960 Urban et al 208 206 Wlth a came mteral' 3,409,543 11/1968 Urban et a1. 208-206 6. A combmatlon process as dened 1n clalm 1 wherein 5 3,449,239 6/1969 Moore 208 206 the rst and second catalysts are xed beds of solid parn' ticles comprising cobalt phthalocyanne monosulfonate DELBERT E- GANTZ, Primary EXamIleI combined with an activated carbon carrier material. G, J, CRASANAKIS, Assistant Examiner

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