IMPROVED CATALYST AND PROCESS FOR SWEETENING A SOUR HYDROCARBON STREAM

Abstract: A process for treating a sour hydrocarbon stream is improved by the use of a dipolar compound which has a positively charged atom and an electronegative group in the same structure. A particularly preferred dipolar compound is ephedrine. The dipolar compounds may be used in conjunction with a metal chelate and a basic solution either in a liquid-liquid process or a fixed bed process with substantially increased performance for oxidizing mercaptans which are found in the sour hydrocarbon stream.

Description: Processes for the treatment of a sour hydrocarbon fraction where the fraction is treated by contacting it with an oxidation catalyst and an alkaline agent in the presence of an oxidizing agent at reaction conditions have become well known and widely practiced in the petroleum refining industry. These processes are typically designed to effect the oxidation of offensive mercaptans contained in a sour hydrocarbon fraction to innocuous disulfides - a process commonly referred to as sweetening. The oxidizing agent is most often air. Gasoline, including natural, straight run and cracked gasolines, is the most frequently treated sour hydrocarbon fraction. Other sour hydrocarbon fractions which can be treated include the normally gaseous petroleum fraction as well as naphtha, kerosene, jet fuel, fuel oil, and the like. A commonly used continuous process for treating sour hydrocarbon fractions entails contacting the fraction with a metal phthalocyanine catalyst dispersed in an aqueous caustic solution to yield a doctor sweet product. The sour fraction and the catalyst containing aqueous caustic solution provide a liquid-liquid system wherein mercaptans are converted to disulfides at the interface of the immiscible solutions in the presence of an oxidizing agent--usually air. Sour hydrocarbon fractions containing more difficult to oxidize mercaptans are more effectively treated in contact with a metal chelate catalyst dispersed on a high surface area adsorptive support--usually a metal phthalocyanine on an activated charcoal. The fraction is treated by contacting it with the supported metal chelate catalyst at oxidation conditions in the presence of an alkaline agent. One such process is described in US-A-2 988 500. The oxidizing agent is most often air admixed with the fraction to be treated, and the alkaline agent is most often an aqueous caustic solution charged continuously to the process or intermittently as required to maintain the catalyst in the caustic-wetted state. The prior art shows that the usual practice of catalytically treating a sour hydrocarbon fraction containing mercaptans involves the introduction of alkaline agents, usually sodium hydroxide, into the sour hydrocarbon fraction prior to or during the treating operation, see US-A-3108081 and -4156641. The prior art also discloses that quaternary ammonium compounds can improve the activity of these catalytic systems, for example, see US-A--4290913 and -4337147. In these patents the catalytic composite comprises a metal chelate, an alkali metal hydroxide and a quaternary ammonium hydroxide dispersed on an adsorptive support. The prior art also discloses the use of other nitrogen-containing com- pounds as promoters for mercaptan sweetening. For example, US-A-4207173 discloses the use of guanidine as a promoter for mercaptan oxidation. Further, US-A-4 753 722 discloses a large number of nitrogen-containing compounds as promoters. These compounds are classified as heterocyclic compounds, substituted homocyclic compounds and aliphatic compounds. In contrast to this prior art, it has now been found that a betaine compound can greatly promote the oxidation of mercaptans in both liquid-liquid and fixed bed processes. There is no mention in the prior art that such dipolar compounds would be effective promoters for the oxidation of mercaptans. One embodiment of the present invention provides a process for treating a sour hydrocarbon fraction containing mercaptans comprising contacting the hydrocarbon fraction in the presence of an oxidizing agent with a basic solution containing 0.1 to 25 weight percent of an alkali metal hydroxide or ammonium hydroxide, and 0.1to 2,000 wt ppm of a metal chelate effective in oxidizing said mercaptans to disulfides. According to the invention from 0.1 to 400 ppm of a betaine having the general formula: (R')₃ N ⁺CH₂COO⁻ wherein R' is an alkyl, alkaryl, aralkyl or cycloalkyl group, is added to the basic solution. Preferably the basic solution contains from 0.1 to 10 weight percent of sodium hydroxide, or ammonium hydroxide, and preferably the betaine is present in a concentration from 1 to 100 ppm. Another embodiment of the present invention provides a process for treating a sour hydrocarbon fraction containing mercaptans by contacting the hydrocarbon fraction in the presence of an oxidizing agent and of a basic agent containing 0.1 to 25 weight percent of an alkali metal hydroxide or ammonium hydroxide with a catalyst effective in oxidizing the mercaptans to disulfides, which catalyst comprises an adsorbent support having dispersed thereon 0.1 to 25 weight percent of a metal chelate and 0.1 to 5 weight percent of the above-defined betaine. Preferably the metal chelate is a cobalt phthalocyanine which is present in a concentration from 0.1 to 10 weight percent of the catalyst, and preferably the betaine is present in a concentration from 0.1 to 3 weight percent of the catalyst. A further embodiment of the invention provides a catalyst effective for oxidizing mercaptans present in a sour hydrocarbon fraction comprising an adsorbent support having dispersed thereon from 0.1 to 10 weight percent of a metal chelate and 0.1 to 5 weight percent of the above-defined betaine. This invention relates to improved processes and catalysts for treating a sour hydrocarbon fraction. The process comprises contacting a sour hydrocarbon fraction in the presence of an oxidizing agent with a catalyst. The catalyst can be present either in a liquid phase (liquid-liquid sweetening) or as a solid phase (fixed bed sweetening). The liquid-liquid process comprises contacting the sour hydrocarbon fraction with a basic solution containing a metal chelate and the betaine. The basic solution is an aqueous solution containing from 0.1 to 25 weight percent, preferably from 0.1 to 10 weight percent, and most preferably from 0.5 to 7 weight percent of an alkali metal hydroxide or ammonium hydroxide. Of the alkali metal hydroxides, sodium and potassium hydroxides are preferred, although lithium hydroxide, rubidium hydroxide and cesium hydroxide may also be used. The metal chelate employed in the practice of this invention can be any of the various metal chelates known to the art as effective in catalyzing the oxidation of mercaptans contained in a sour petroleum distillate, to disulfides or polysulfides. The metal chelates include the metal com- pounds of tetrapyridinoporphyrazine described in US-A-3980582, e.g., cobalt tetrapyridinoporphyrazine; prophyrin and metaloporphyrin catalysts as described in US-A-2966453, e.g., cobalt tetraphenylporphyrin sulfonate; corrinoid catalysts as described in US-A-3252892, e.g., cobalt corrin sulfonate; chelate organometallic catalysts such as described in US-A-2918426, e.g., the condensation product of an aminophenol and a metal of Group VIII; or the metal phthalocyanines as described in US-A-4290913. The metal phthalocyanines which can be employed in the basic solution to catalyze the oxidation of mercaptans generally include magnesium phthalo-cyanine, titanium phthalocyanine, hafnium phthalocyanine, vanadium phthalo-cyanine, tantalum phthalocyanine, molybdenum phthalocyanine, manganese phthalocyanine, iron phthalocyanine, cobalt phthalocyanine, platinum phthalo-cyanine, palladium phthalocyanine, copper phthalocyanine, silver phthalo-cyanine, zinc phthalocyanine, tin phthalocyanine, and the like. Cobalt phthalo-cyanine and vanadium phthalocyanine are particularly preferred. The ring sub stituted metal phthalocyanines are generally employed in preference to the unsubstituted metal phthalocyanine (see US-A-4290913), with the sul- fonated metal phthalocyanine being especially preferred, e.g., cobalt phthalo-cyanine monosulfate, cobalt phthalocyanine disulfonate, etc. The sulfonated derivatives may be prepared, for example, by reacting cobalt, vanadium or other metal phthalocyanine with fuming sulfuric acid. While the sulfonated derivatives are preferred, it is understood that other derivatives, particularly the carboxylated derivatives, may be employed. The carboxylated derivatives are readily prepared by the action of trichloroacetic acid on the metal phthalo-cyanine. The concentration of metal chelate in general and metal phthalo-cyanine in specific in the basic solution can vary from 0.1 to 2000 wt. ppm and preferably from 50 to 800 weight ppm. Regardless of the betaine actually used, it is desirable that it be present in the basic solution in a concentration from 0.1 to 400 ppm, preferably from 1 to 100 ppm and most preferably from 3 to 20 ppm. Sweetening of the sour hydrocarbon fraction is effected by oxidation of mercaptans. Accordingly, an oxidizing agent is necessary for the reaction to proceed. Air is a preferred oxidizing agent, although oxygen or other oxygen-containing gases may be used. At least a stoichiometric amount of oxygen (relative to the concentration of mercaptans) is required to oxidize the mercaptans to disulfides, although an excess amount of oxygen is usually employed. In some cases the sour hydrocarbon fraction may contain entrained air or oxygen in sufficient concentration to accomplish the desired sweetening, but generally it is preferred to introduce air into the reaction zone. Sweetening of the sour hydrocarbon fraction may be effected in any suitable manner well known in the art and may be in a batch or continuous process. In a batch process the sour hydrocarbon fraction is introduced into a reaction zone containing the basic solution which contains the metal chelate and the betaine. Air is introduced therein or passed therethrough. Preferably the reaction zone is equipped with suitable stirrers or other mixing devices to obtain intimate mixing. In a continuous process the basic solution containing the metal chelate catalyst and the betaine is passed countercurrently or concurrently with the sour hydrocarbon fraction in the presence of a continuous stream of air. In a mixed type process, the reaction zone contains the basic solution, metal chelate and betaine, and gasoline and air are continuously passed therethrough and removed generally from the upper portion of the reaction zone. For specific examples of apparatus used to carry out a liquid/liquid process, see US-A-4019869, -4201626 and -4234544. In general the process is usually effected at ambient temperatures, although elevated temperatures may be employed and generally will be in the range of from 38 to 204°C (100° to 400°F), depending upon the pressure utilized therein, but usually below that at which substantial vaporization occurs. Pressures of up to 6890 kPa (1,000 psi) or more are operable although atmospheric or substantially atmospheric pressures are suitable. The process may also be carried out by contacting the sour hydrocarbon fraction with a catalyst comprising a metal chelate and a betaine dispersed on an adsorbent support. This is referred to as fixed bed sweetening. The adsorbent support which may be used in the practice of this invention can be any of the well known adsorbent materials generally utilized as a catalyst support or carrier material. Preferred adsorbent materials include the various charcoals produced by the destructive distillation of wood, peat, lignite, nutshells, bones, and other carbonaceous matter, and preferably such charcoals as have been heat-treated or chemically treated or both, to form a highly porous particle structure of increased adsorbent capacity, and generally defined as activated carbon or charcoal. Said adsorbent materials also include the naturally occurring clays and silicates, e.g., diatomaceous earth, fuller's earth, kieselguhr, attapulgus clay, feldspar, montorillonite, halloysite, kaolin, and the like, and also the naturally occurring or synthetically prepared refractory inorganic oxides such as alumina, silica, zirconia, thoria, boria, etc., or combinations thereof like silica-alumina, silica-zirconia, alumina-zirconia, etc. The adsorbent support should be insoluble in, and otherwise inert to, the petroleum distillate at the alkaline reaction conditions existing in the treating zone. Charcoal, and particularly activated charcoal, is preferred because of its capacity for metal chelates, and because of its stability under treating conditions. The metal chelates which can be deposited on the support are the ones that have been described above for the liquid-liquid process. Likewise, the betaines are the same as described above The metal chelate component and betaine can be dispersed on the adsorbent support in any conventional or otherwise convenient manner. The components can be dispersed on the support simultaneously from a common aqueous or alcoholic solution and/or dispersion thereof or separately and in any desired sequence. The dispersion process can be effected utilizing conventional techniques whereby the support in the form of spheres, pills, pellets, granules or other particles of uniform or irregular size or shape, is soaked, suspended, dipped one or more times, or otherwise immersed in an aqueous or alcoholic solution and/or dispersion to disperse a given quantity of the betaine and metal chelate components. Typically, the betaine will be present in a concentration of 0.1 to 5 weight percent of the catalyst and preferably from 0.1 to 3 weight percent. In general, the amount of metal chelate and metal phthalocyanine in particular which can be adsorbed on the solid adsorbent support and still form a stable catalyst is up to 25 weight percent of the catalyst. A lesser amount in the range of from 0.1 to 10 weight percent of the catalyst generally forms a suitably active catalyst. One preferred method of preparation involves the use of a steam-jacketed rotary dryer. The adsorbent support is immersed in the impregnating solution and/or dispersion containing the desired components contained in the dryer and the support is tumbled therein by the rotating motion of the dryer. Evaporation of the solution in contact with the tumbling support is expedited by applying steam to the dryer jacket. In any case, the resulting composite is allowed to dry under ambient temperature conditions, or dried at an elevated temperature in an oven, or in a flow of hot gases, or in any other suitable manner to vield a suitable catalyst. An alternative and convenient method for dispersing the betaine and metal chelate components on the solid adsorbent support comprises predisposing the support in a sour hydrocarbon fraction treating zone or chamber as a fixed bed and passing a metal chelate and betaine solution and/or dispersion through the bed in order to form the catalytic composite in situ. This method allows the solution and/or dispersion to be recycled one or more times to achieve a desired concentration of the betaine and metal chelate components on the adsorbent support. In still another alternative method, the adsorbent support may be predisposed in said treating zone or chamber, and the zone or chamber thereafter filled with the solution and/or dispersion to soak the support for a predetermined period. Processes for sweetening a sour hydrocarbon fraction using a fixed bed catalyst are described in the prior art. Specifically, temperature and pressure conditions are the same as stated for the liquid-liquid process described above. The prior art also discloses (see US-A-4033860 and -4337147) that the hydrocarbon fraction can be treated in the presence of a basic agent, usually an alkaline agent. Thus, a supported catalyst is typically initially saturated with an aqueous solution of an alkaline agent (as described above) and the alkaline agent thereafter passed in contact with the catalyst bed continuously or intermittently as required, admixed with the sour hydrocarbon fraction. An aqueous ammonium hydroxide solution (as described above) may be used in place of the alkaline solution. The aqueous solution may further contain a solubilizer to promote mercaptan solubility, e.g., alcohol, and especially methanol, ethanol, n-propanol, isopropanol, etc., and also phenols, cresols, and the like. The solubilizer, when employed, is preferably methanol, and the alkaline solution may suitably contain from 2 to 10 volume percent thereof. Examples of specific arrangements to carry out the treating process may be found in US-A-4490246 and -4753722. The following examples are presented in illustration of this invention and are not intended as undue limitations on the generally broad scope of the invention as set out in the appended claims. COMPARATIVE EXAMPLE 1 A stirred contactor which consisted of a cylindrical glass container measuring 89 mm (3.5 in) in diameter by 152 mm (6 in) high and which contained 4 baffles that are at 90° angles to the side walls was used. An air driven motor was used to power a paddle stirrer positioned in the center of the apparatus. When turning, the stirrer paddles passed within 12.7 mm (1/2 in) of the baffles. This resulted in a very efficient, pure type of mixing. To the above apparatus there were added 50 milliliters of an 8% aqueous sodium hydroxide solution which contained 30 weight ppm of a caustic soluble tetrasulfonated cobalt phthalocyanine and 200 milliliters of isooctane which contained 1,300 weight ppm of mercaptan sulfur as n-octylmercaptan. To this mixture 20 weight ppm of a mixture of quaternary ammonium compounds composed of alkyldimethylbenzyl ammonium chloride and dialkylmethylbenzyl ammonium chloride obtained from the Mason Chemical Co. as Maquat FL-76, was added and the mixture was stirred. Periodically stirring was stopped and a sample was withdrawn from the isooctane layer with a pipette. These samples were analyzed for mercaptan by titration and are presented in the second column of Table 1. EXAMPLE 1 The test described in Comparative Example 1 was carried out in accordance with the present invention with a fresh sample of isooctane, cobalt phthalocyanine and alkaline solution, but instead of the quaternary ammonium compound, 20 weight ppm of a betaine having the structural formula obtained from Aldrich Chemical Co. was added. These results are also pre- sented in the third column of Table 1. The data clearly show the superior promotion effect of betaine.

Claims: A process for treating a sour hydrocarbon fraction containing mercaptans comprising contacting the hydrocarbon fraction in the presence of an oxidizing agent with a basic solution containing 0.1 to 25 weight percent of an alkali metal hydroxide or ammonium hydroxide, and 0.1 to 2,000 wt ppm of a metal chelate effective in oxidizing said mercaptans to disulfides, characterized in that from 0.1 to 400 ppm of a betaine having the general formula: (R')₃N ⁺CH₂COO⁻ wherein R' is an alkyl, alkaryl, aralkyl or cycloalkyl group, is added to the basic solution. A process according to Claim 1 characterized in that the basic solution contains from 0.1 to 10 weight percent of sodium hydroxide, or ammonium hydroxide. A process according to Claim 1 or 2 characterized in that the betaine is present in a concentration from 1 to 100 ppm. A process for treating a sour hydrocarbon fraction containing mercaptans by contacting the hydrocarbon fraction in the presence of an oxidizing agent and of a basic agent containing 0.1 to 25 weight percent of an alkali metal hydroxide or ammonium hydroxide with a catalyst effective in oxidizing the mercaptans to disulfides, characterized in that the catalyst comprises an adsorbent support having dispersed thereon 0.1 to 25 weight percent of a metal chelate and 0.1 to 5 weight percent of a betaine having the general formula: (R')₃N ⁺CH₂COO⁻ wherein R' is an alkyl, alkaryl, aralkyl or cycloalkyl group. A process according to Claim 4 characterized in that the metal chelate is a cobalt phthalocyanine which is present in a concentration from 0.1 to 10 weight percent of the catalyst. A process according to Claim 4 or 5 characterized in that the betaine is present in a concentration from 0.1 to 3 weight percent of the catalyst. A catalyst effective for oxidizing mercaptans present in a sour hydrocarbon fraction comprising an adsorbent support having dispersed thereon from 0.1 to 25 weight percent of a metal chelate and 0.1 to 5 weight percent of a betaine having the general formula: (R')₃N ⁺CH₂COO⁻ wherein R' is an alkyl, alkaryl, aralkyl or cycloalkyl group.

Inventors: BRICKER, JEFFERY C., FRAME, ROBERT R., POLLASTRINI, SHEILA L.
Applicants: UOP
Application year: 1990
Main classification: C10G27
IPC codes: B01J31, C10G27