Patent Publication Number: US-2012043259-A1

Title: Extraction of Mercaptans in the Absence of Oxidation Catalyst

Description:
FIELD OF THE INVENTION 
     The invention relates to processes for treating hydrocarbon streams comprising mercaptan compounds and molecular oxygen, in which at least a portion of the mercaptan compounds are extracted into a liquid phase (e.g., an aqueous sodium hydroxide solution) that is essentially free of circulating oxidation catalyst. 
     DESCRIPTION OF RELATED ART 
     The removal of mercaptan compounds from various hydrocarbon-containing refinery process streams has been of significant importance historically. Mercaptan compound removal not only overcomes offensive odor problems, but also addresses various other issues associated with the use of these streams, including corrosion, burning, catalyst poisoning, undesired side reactions, etc. Conventional methods for mercaptan removal may be broadly divided between those for either (i) removal of mercaptan compounds and their derivatives, or (ii) oxidation of mercaptan compounds into less objectionable derivatives, namely disulfides, without attempting to remove these oxidation products. Methods of the first and second types are normally referred to as “extraction” and “sweetening” processes, respectively. In the case of the first type, the prior art of mercaptan extraction with alkaline solutions is described, for example, in U.S. Pat. No. 2,853,432; U.S. Pat. No. 2,921,020; U.S. Pat. No. 2,988,500; and U.S. Pat. No. 3,408,287. These references exemplify the practice of catalytic oxidation of an aqueous alkaline stream, having been used to extract mercaptan compounds from a hydrocarbon feed. Oxidation is carried out in the presence of air and is followed by phase separation of the oxidation zone effluent, from which the excess air is vented. The above references also describe the removal of the alkaline stream from the phase separation zone and its recirculation to the extraction zone. 
     Extraction methods rely on the slight acidity of mercaptan compounds, which form mercaptide salt derivatives in the presence of a strong base. In the case of sodium hydroxide as an extraction medium, for example, the mercaptide-forming reaction proceeds as follows: 
     
       
         
         
             
             
         
       
     
     The mercaptide salts have a very high solubility in the basic solution. This is especially true in the case of mercaptide compounds derived from low molecular weight mercaptan compounds (i.e., having relatively small “R” hydrocarbon radical groups in the formula RSH), such as those present in refinery gas streams, liquefied petroleum gas (LPG), or light gasoline fractions. These hydrocarbon feeds are therefore ideal for treatment with an aqueous extraction medium that is a strong base, for example a caustic solution, to selectively extract unwanted mercaptan compounds. Extraction processes are normally carried out with the hydrocarbon and caustic solution (e.g., aqueous sodium hydroxide) being contacted counter-currently in an extraction tower (extractor) having trays and/or packing material to ensure efficient contacting between phases. 
     The caustic solution extraction step, used to extract mercaptan compounds from the hydrocarbon feed, is coupled with a caustic solution regeneration step. The mercaptide rich caustic solution exiting the extractor is normally sent to a steam heater that maintains a suitable temperature in a regenerator or oxidizer, typically having an upflow configuration. Air is typically injected into the mercaptide rich caustic solution as a source of oxygen for the subsequent catalytic oxidation, which converts the mercaptide sulfur compounds to corresponding disulfide compounds and regenerates the caustic solution, according to the following reaction: 
     
       
         
         
             
             
         
       
     
     The disulfide reaction products are immiscible in the caustic solution and may therefore be subjected to phase separation, for example in a settling zone. Accordingly, the effluent from the oxidation zone is sent to a disulfide separator, where spent air (as a gas phase), the disulfide conversion products (as an oil phase), and the regenerated caustic solution (as an aqueous phase) are separated. The regenerated caustic solution may then be returned to the extraction zone to supply all or a part of the lean aqueous extraction medium (containing essentially no extracted mercaptide compounds). 
     For some low boiling range hydrocarbon streams, including hydrocarbon containing gas streams such as fuel gas streams, the reduction of total sulfur to very low levels, for example less than about 10 parts per million (ppm), or even less than 5 ppm, by weight, is critical. However, the sulfur reduction that can be attained is directly related to the extractable mercaptan content of the hydrocarbon feed. Accordingly, there is an ongoing need in the art for hydrocarbon treatment processes, particularly with respect to light hydrocarbon feed streams such as refinery gases, LPG, and light gasoline, in which mercaptan compounds contained in these streams are extracted with the highest extent of removal possible, in order to achieve the current, strict limitations on total sulfur. 
     SUMMARY OF THE INVENTION 
     Embodiments of the invention are directed to processes for the extraction of mercaptan compounds into an aqueous extraction medium, and especially those processes in which the extraction is accompanied by regeneration of the aqueous extraction medium by oxidizing mercaptide compounds (i.e., the salt forms of the extracted mercaptan compounds) in the aqueous extraction medium to their corresponding disulfide compounds. Aspects of the invention are associated with the discovery that small amounts of oxygen, (e.g., dissolved oxygen) present in hydrocarbon feeds that are conventionally subjected to extraction for removal of mercaptan compounds, can detrimentally oxidize these mercaptan compounds to disulfide compounds, such that they are no longer efficiently extracted into an aqueous phase, for example an aqueous sodium hydroxide solution. This oxidation within the extraction zone, which adversely affects the overall mercaptan compound removal efficiency of the extraction process, occurs if oxidation catalyst is present in this zone, for example as liquid phase oxidation catalyst that is recirculated in the aqueous extraction medium according to conventional processes. 
     Therefore, oxidation catalyst present in the circulating aqueous extraction medium (e.g., caustic solution such as an aqueous sodium hydroxide solution) promotes, in the extraction zone, the reaction of mercaptan compounds present in the hydrocarbon feed with oxygen, resulting in the formation of disulfide compounds. Disulfide compounds formed in the extraction zone (or extraction column), unlike mercaptan compounds, are not easily extractable from the hydrocarbon feed and therefore remain in the treated hydrocarbon product. This results in reduced overall sulfur removal efficiency and provides a treated hydrocarbon product having a higher content of total sulfur content than would otherwise be possible if the formation of disulfide compounds were prevented. 
     To address this problem, process according to embodiments of the invention do not include the addition of oxidation catalyst to the aqueous extraction medium that circulates from the extraction zone to the oxidation zone. Rather, the oxidation catalyst is present in the process as a fixed bed of a catalytically active metal or metal compound on a solid support. This solid catalyst can be used to replace all or a portion of the conventional carbon rings or other mechanical devices, used in the oxidation zone in conventional systems to improve contacting between liquid phase oxidation catalyst, added oxygen, and mercaptan compounds extracted into the circulating aqueous extraction medium. Eliminating oxidation catalyst in this circulating stream advantageously prevents the oxidation of mercaptan compounds in the extraction zone, which is detrimental to the overall process performance for the reasons discussed above. Sulfur, in the form of mercaptan compounds entering the extraction zone with the hydrocarbon feed, remains in that form in the substantial absence of oxidation catalyst. This is the desired form of sulfur in the extraction zone, as it is readily extractable into the lean aqueous extraction medium. 
     The rich aqueous extraction medium exiting the extraction zone contains the extracted mercaptan compounds (now in the form of their corresponding mercaptide salts). This rich aqueous extraction medium then flows through a fixed bed of solid catalyst contained in the oxidation zone, normally after being combined with air or other oxygen-containing reactant. The conditions, catalyst, and oxygen in the oxidation zone promote the conversion of the extracted mercaptan compounds to disulfide compounds, which can be phase separated in a downstream settling zone. If desired, a wash oil can also be added to the rich aqueous extraction medium (e.g., in the combined oxidation feed containing the added oxygen) prior to its entry to the fixed bed of oxidation catalyst in the oxidation zone. Otherwise, such an oil may be added to the oxidation zone effluent, upstream of the settling zone. The wash oil then reports to the oil phase containing the disulfide compounds. Regenerated aqueous extraction medium is recovered in the aqueous phase from this zone and is generally recycled back to the extraction zone for additional contacting with the hydrocarbon feed, in the absence of any circulating oxidation catalyst. 
     Accordingly, embodiments of the invention are directed to processes for treating a hydrocarbon feed, namely a feed containing at least some hydrocarbon compounds, but generally comprising a majority of hydrocarbons, typically comprising at least about 80% hydrocarbons, and often comprising at least about 95% hydrocarbons. The hydrocarbon feed also comprises one or more mercaptan compounds of the formula RSH, wherein R is a hydrocarbon radical, resulting from the removal a hydrogen atom from a terminal carbon atom of a straight-chain, branched-chain, or cyclic (e.g., cycloparaffinic or aromatic) hydrocarbon, optionally having one or more carbon-carbon double bonds and/or one or more carbon-carbon triple bonds. Typically, R is a straight-chain or branched-chain hydrocarbon radical having no unsaturated carbon-carbon double bonds (i.e., an alkyl radial) and having from 1 to 8 carbon atoms (i.e., a methyl or ethyl radical, or a straight-chain or branched-chain propyl, butyl, pentyl, hexyl, heptyl, or octyl radical). The contribution of mercaptan compounds in the hydrocarbon feed to the total sulfur content (i.e., the mercaptan sulfur content of the feed) is generally from about 10 parts per million (ppm) to about 1000 ppm, and typically from about 20 ppm to about 500 ppm, by weight. The hydrocarbon feed further comprises at least some molecular oxygen (O 2 ), generally present as oxygen that may be gaseous molecular oxygen, particularly in the case of a gas phase hydrocarbon feed (e.g., fuel gas). Otherwise, the molecular oxygen may be dissolved in a liquid hydrocarbon feed, or even present as a combination of gaseous and dissolved O 2 . The total amount of oxygen, whether present as gaseous or dissolved O 2 , is generally in the range from about 1 ppm to about 500 ppm, and often from about 1 ppm to about 100 ppm, by weight, based on the hydrocarbon feed. 
     Representative hydrocarbon feed stream comprise refinery gases including fuel gas, comprising predominantly methane, that is contaminated with mercaptan compounds. Other representative hydrocarbon feed streams comprise liquefied petroleum gas (LPG) or a hydrocarbon fraction, such as a light gasoline fraction having a distillation end point temperature generally from about 138° C. (280° F.) to about 216° C. (420° F.), and often from about 138° C. (280° F.) to about 160° C. (320° F.). Such light gasoline fractions may be obtained, for example, from the product fractionation section of a fluid catalytic cracking (FCC) process. FCC involves the cracking of higher molecular weight hydrocarbon feeds, such as straight run vacuum gas oil, to lower molecular weight hydrocarbons in the absence of added hydrogen that can convert sulfur-containing compounds such as mercaptan compounds to H 2 S. Products of FCC therefore generally have a mercaptan sulfur content within the ranges as discussed above. 
     Representative processes according to the invention comprise contacting (e.g., in a countercurrent extraction column such as a gas/liquid or liquid/liquid extractor) the hydrocarbon feed with a lean aqueous extraction medium (e.g., an aqueous sodium hydroxide solution having a concentration from about 5% to about 20% by weight). The contacting results in both a treated hydrocarbon product and a rich aqueous extraction medium comprising extracted mercaptide compounds, resulting from the mercaptide-forming reaction discussed above. The extracted mercaptan compound derivatives are therefore generally the corresponding mercaptide salt derivatives, such as sodium mercaptide compounds. The treating processes further comprise passing the rich aqueous extraction medium and an oxygen-containing reactant (e.g., air), as a combined oxidation feed, to a catalytic oxidation zone comprising a fixed bed of solid oxidation catalyst. The presence of the oxygen-containing reactant and solid oxidation catalyst converts at least a portion of the extracted mercaptan compounds (as mercaptide salts) to disulfide compounds, which are contained in an oxidation zone effluent. According to particular embodiments, the processes may also comprise separating, from the oxidation zone effluent, a regenerated extraction medium (e.g., as a liquid aqueous phase), a disulfide rich byproduct (e.g., as a liquid oil phase), and an off gas such as spent or excess air (e.g., as a gas phase). The processes may further comprise recycling at least a portion of the regenerated aqueous extraction medium to provide at least a portion of the lean aqueous extraction medium. Incremental fresh extraction medium may be added to, and/or spent extraction medium withdrawn from, the extraction medium recirculation (or recycle) loop. 
     Advantageously, and as explained in greater detail below, the lean extraction medium is essentially free of any circulating oxidation catalyst, whether it be the same catalyst (having the same catalytically active metal or metal compound) as used in the fixed bed or another type of oxidation catalyst (having a different catalytically active metal or metal compound). This prevents the unwanted catalytic oxidation of mercaptan compounds prior to their extraction, which, as discussed above, reduces overall sulfur extraction efficiency due to the relative lack of affinity of the oxidized, disulfide compounds for the extraction medium. Accordingly, in further representative embodiments, the treated hydrocarbon product is essentially free of disulfide compounds, such that the contribution of disulfide compounds in the treated hydrocarbon product to the total sulfur content is (i.e., the treated hydrocarbon product comprises disulfide sulfur in an amount of) generally less than about 5 ppm, typically less than about 3 ppm, and often less than about 1 ppm, by weight. Likewise, due to the high affinity of the mercaptan compounds in the hydrocarbon feed for the aqueous extraction medium, the treated hydrocarbon product comprises mercaptan sulfur, and even total sulfur, in these same amounts as discussed with respect to disulfide sulfur. 
     These and other aspects and embodiments associated with the present invention are apparent from the following Detailed Description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The FIGURE depicts the basic flow scheme of a representative hydrocarbon feed treatment process for extracting mercaptan compounds in the feed with a lean aqueous extraction medium, followed by regenerating the extraction medium in a catalytic oxidation zone. 
       The FIGURE is to be understood to present an illustration of the invention and/or principles involved. Details including pumps, heaters and heat exchangers, valves, instrumentation, and other items not essential to the understanding of the invention are not shown. As is readily apparent to one of skill in the art having knowledge of the present disclosure, processes for treating hydrocarbon feeds to remove mercaptan sulfur, according to various other embodiments of the invention, will have configurations and components determined, in part, by their specific use. 
     
    
    
     DETAILED DESCRIPTION 
     The invention is associated with processes for the extractive removal of mercaptan compounds from hydrocarbon feeds, in which the contact between oxidation catalyst with oxygen and mercaptan in the feed is reduced or eliminated. This prevents the oxidation of mercaptan compounds to disulfides from occurring in the extraction zone, thereby overcoming separation inefficiencies associated with the inability to extract disulfides in this zone. The invention advantageously relies on the use of a fixed bed of catalyst in the oxidation or regeneration zone, which is coupled to the extraction zone. 
     Hydrocarbon feeds suitable for mercaptan compound removal, by treatment in an extraction zone, include refinery gases such as fuel gas, propane-butane mixtures, such as liquefied petroleum gas (LPG), and other hydrocarbon containing streams having representative boiling ranges up to and including those characteristic of middle distillates. Included in this description are refinery gases such as those derived from fluidized catalytic cracking (FCC) plant gas condensation units, straight-run or cracked gasolines including light gasoline fractions, jet fuels, fuel oils, kerosenes, and blends of these materials. The hydrocarbon treatment process may also be used to remove mercaptan compounds from solvents, alcohols, aldehydes, etc. In general, suitable hydrocarbon feeds may be characterized as containing normally liquid hydrocarbon compounds having boiling points under about 343° C. (650° F.). Often, however, as in the case of fuel gas, the hydrocarbon feed may be all or at least partially in the gas phase. 
     Accordingly, representative processes for extracting mercaptan compounds from a hydrocarbon feed involve contacting it with a recirculating aqueous extraction medium, such as an alkaline solution, that is essentially free of circulating oxidation catalyst, particularly a conventional, homogeneous liquid phase oxidation catalyst. The aqueous extraction medium is often a caustic solution comprising water and an alkaline reagent (e.g., a metal hydroxide such as sodium hydroxide or potassium hydroxide). Sodium hydroxide may be used in concentrations ranging generally from about 1% to about 50%, and typically from about 5% to about 25%, by weight. Optionally, in addition to the alkaline reagent, the aqueous extraction medium may further include an agent that enhances the solubility of the mercaptan compounds, typically methanol or ethanol, although other agents such as a phenol, cresol or butyric acid may be used. 
     Conditions in the extraction zone may vary depending on such factors as the nature of the hydrocarbon feed being treated and its mercaptan compound content. In general, the extraction may be performed at a temperature above about 15° C. (59° F.) and at a pressure that ensures operation with a desired phase (either gas or liquid phase) of the hydrocarbon feed. With very light (i.e., low boiling) material in the feed, such as methane that is the predominant component of mercaptan compound contaminated fuel gas, the extraction is normally performed with a vapor phase hydrocarbon feed. The pressure may range from atmospheric up to 69 barg (1000 psig) or more. Representative extraction conditions include a temperature from about 20° C. (68° F.) to about 121° C. (250° F.) and a pressure from about 3.4 barg (50 psig) to about 10 barg (150 psig). With respect to the operating pressure in the extraction zone, a second consideration is the amount of oxygen dissolved in the rich aqueous extraction medium (containing extracted mercaptide compounds) in the downstream oxidation zone. If practical, the oxidation zone is operated at substantially the same pressure as the extraction zone, accounting for the normal process flow pressure drop between these zones. Increasing oxidation zone pressure directionally favors increased dissolved oxygen concentration. 
     For mercaptan compound extraction from the hydrocarbon feed, the volumetric ratio of the lean aqueous extraction medium (prior to extracting mercaptan compounds) to the hydrocarbon feed will vary depending on the mercaptan compound content of the feed. This ratio ranges generally from about 1:100 to about 1:1, and typically from about 1:20 to about 1:4, although other ratios may be desirable. In the case of an LPG hydrocarbon feed, for example, the volumetric rate of flow of the lean aqueous extraction medium is typically from about 2% to about 3% of the volumetric rate of flow of the hydrocarbon feed. In the case of a light straight run naphtha hydrocarbon feed, this value may increase up to about 20%. In terms of efficiency of the extraction zone operation, generally at least about 95%, typically at least about 98%, and often at least about 99%, of the mercaptan compounds of the hydrocarbon feed are extracted, as mercaptide compounds, into the rich aqueous extraction medium exiting the extraction zone (e.g., counter-current gas/liquid or liquid/liquid extractor comprising multiple contacting trays or packing material). 
     The rich aqueous extraction medium, obtained from the extraction zone, can be regenerated by the catalytic oxidation of the extracted mercaptan compounds to disulfide compounds, according to the oxidation reaction discussed above. Oxidation is carried out over solid oxidation catalyst. In particular, oxidation occurs by passing an oxygen-containing reactant together with the rich aqueous extraction medium, as a combined oxidation feed, to an oxidation zone containing one or more fixed beds of solid oxidation catalyst. The oxygen-containing reactant and rich aqueous extraction medium are normally mixed downstream of the extraction zone and upstream of the oxidation zone. Good mixing ensures a close approach to the equilibrium level, under oxidation zone operating conditions, of dissolved oxygen in the combined oxidation feed. 
     As an oxygen-containing reactant, air is preferably the source of oxygen required for regeneration of the aqueous extraction medium. Other than air, possible sources of oxygen include pure oxygen, oxygen-enriched air, or nitrogen-enriched air. The air or other oxygen source is normally injected into the rich aqueous extraction medium prior to the oxidation zone at a rate which ensures oxygen is present in excess of the stoichiometric requirement for the mercaptan compound oxidation. The rate of oxygen-containing reactant may therefore be sufficient to supply oxygen at generally from about 100% to about 500%, and typically from about 100% to about 200%, of the stoichiometric requirement. That is, the oxygen-containing reactant comprises oxygen in an amount, in these percentage ranges, of the required stoichiometric amount for oxidation of the extracted mercaptan compounds. 
     The solid oxidation catalyst generally comprises one or more metals or metal compounds, having catalytic oxidation activity, and a solid support or carrier onto which the metal(s) and/or compound(s) is/are disposed. Representative metals having catalytic oxidation activity include cobalt, nickel, molybdenum, vanadium, and/or tungsten. Representative metal compounds include compounds of these metals, for example their corresponding phthalocyanine disulfonates. Support or carrier materials should be highly absorptive and capable of withstanding the alkaline environment of the rich aqueous extraction medium. Activated charcoals are suitable, and either animal or vegetable charcoals may be used. Generally, the metal, whether or not in the form of a metal compound, is present in the solid oxidation catalyst in an amount from about 0.1% to 2.0% by weight. Suitable catalysts include those used in conventional mercaptan compound oxidation processes, as described, for example, in U.S. Pat. No. 2,988,500, U.S. Pat. No. 3,108,081, and U.S. Pat. No. 3,148,156. The use of a fixed bed of solid oxidation catalyst in the oxidation zone also ensures good mixing between the components of the combined oxidation feed. 
     Representative conditions in the catalytic oxidation zone include a pressure generally from atmospheric pressure to about 69 barg (1000 psig), and typically from about 0 barg (0 psig) to about 21 barg (300 psig). As mentioned above, in representative embodiments the pressures used in the extraction and catalytic oxidation zones are substantially the same. When operating at near atmospheric pressure, the temperature of the catalytic oxidation zone may range from ambient to about 93° F. (200° F.). When operating at elevated pressures, the temperature of this zone may vary range from ambient to about 204° F. (400° F.). Representative catalytic oxidation zone temperatures are in the range from about 20° C. (68° F.) to about 121° C. (250° F.), for example from about 38° C. (100° F.) to about 79° C. (175° F.). 
     Subsequent to (downstream of) the oxidation zone, the disulfide compounds, as products of the catalytic oxidation of the extracted mercaptan compounds, are subsequently phase separated in a phase separation zone such as a settler. The disulfide compounds and regenerated solution reside, respectively, in oil and aqueous phases. Excess (or unconsumed) oxygen and other components of the oxygen-containing reactant, as well as some water vapor, are also separated in this zone as an off gas. The phase separation zone is therefore sized to allow the denser aqueous phase, namely the regenerated aqueous extraction medium, to separate by gravity from the less dense oil phase that is rich in disulfide compounds. According to the oxidation reaction given above, these disulfide compounds result from the oxidation of the extracted mercaptide compounds in the oxidation zone. The desired phase separation may be aided by coalescing materials, such as wire mesh, in the phase separation zone. Normally, an average residence time exceeding 90 minutes used in the phase separation zone. By carrying out the phase separation, the disulfide compounds are substantially removed as a byproduct, while all or a portion of the aqueous extraction medium (e.g., after concentration) is recycled to provide at least a portion of the lean aqueous extraction medium for use in the extraction zone. 
     To promote the separation of excess oxygen, nitrogen, and water vapor into the off gas from the phase separation zone, it is generally desirable to operate this zone at the minimum pressure that other design considerations allow. If it is necessary to operate the extraction zone at a relatively high pressure, then the use of an intermediate pressure in the phase separation zone can avoid some utility expenses associated with re-pressurizing the regenerated aqueous extraction medium. The pressure in the phase separation zone (e.g., the settler pressure) may therefore range from atmospheric pressure to about 21 barg (300 psig) or more, although a pressure in the range of from about 0.7 barg (10 psig) to about 3.4 barg (50 psig) is preferred. The temperature in this zone ranges generally from about 10° C. (50° F.) to about 121° C. (250° F.), and often from about 27° C. (80° F.) to about 54° C. (130° F.). 
     A representative process for treating a hydrocarbon feed comprising a mercaptan compound and oxygen is illustrated in the FIGURE. According to this embodiment, a hydrocarbon feed  10  such as fuel gas, naphtha, or light gasoline containing a relatively high amount of mercaptan compounds (e.g., from about 20 ppm to about 500 ppm by weight sulfur as mercaptan sulfur), enters a lower section of an extraction zone  100 . Hydrocarbon feed  10  is then contacted in a countercurrent manner in this zone with lean aqueous extraction medium  12  (e.g., comprising some or all of regenerated extraction medium  25  from disulfide separator  300 ). In particular, hydrocarbon feed  10  passes upward against downwardly flowing lean aqueous extraction medium  12 , which is substantially free of mercaptan compounds and sulfur compounds in general. Contacting in extraction zone  100  provides rich aqueous extraction medium  14  comprising mercaptide compounds (i.e., the mercaptide salt derivatives of the extracted mercaptan compounds) and treated hydrocarbon product  16  having a significantly reduced level of total sulfur relative to hydrocarbon feed  10 . 
     Depending on whether the hydrocarbon feed is in the gas phase or liquid phase, gas/liquid or liquid/liquid extraction in extraction zone  100  transfers essentially all mercaptan compounds in hydrocarbon feed  10  into rich aqueous extraction medium  14  as mercaptide compounds according to the mercaptide-forming reaction described above. Advantageously, the lean aqueous extraction medium  12  is essentially free of circulating oxidation catalyst that has been found to adversely affect mercaptan compound extraction efficiency, by promoting mercaptan compound oxidation to disulfides in the presence of dissolved oxygen in hydrocarbon feed  10 . As discussed above, this conversion to disulfides directly impacts the extraction efficiency, since, whereas the mercaptan compounds are readily extracted into the aqueous extraction medium, the disulfide compounds have little or no affinity for the aqueous phase and remain in treated hydrocarbon product  16 . The phrase “essentially free of circulating oxidation catalyst” refers to no liquid phase oxidation catalyst being added to the aqueous extraction medium circulation loop. Since oxidation zone  200  contains a fixed bed of solid oxidation catalyst, it is possible that trace amounts of this catalyst may be leeched into this loop. However, any catalytic oxidation metal, for example cobalt, nickel, molybdenum, vanadium, or tungsten, or otherwise a combination of two or more of these metals, is generally present in the aqueous extraction medium (lean or rich) in an amount of less than about 10 ppm by weight, and typically less than about 5 ppm by weight, and often less than about 1 ppm by weight. The content of catalytic oxidation metals of these types is independent of the type(s) of metal(s) or metal compound(s) used in the fixed bed of solid oxidation catalyst. 
     Rich aqueous extraction medium  14  is mixed with an oxygen-containing reactant  18  (e.g., air) to provide combined oxidation feed  15  to catalytic oxidation zone  200 . Oxidation zone  200 , which conventionally contains only contacting devices such as carbon (e.g., machined graphite) rings, comprises, according to aspects of the present invention, a fixed bed of solid oxidation catalyst. Contact between the mercaptide compounds in rich aqueous extraction medium  14  and this catalyst oxidizes essentially all of these mercaptide compounds to disulfide compounds within oxidization zone  200 , comprising at least one oxidation reactor, according to the catalytic oxidation reaction shown previously. Oxidation zone effluent  20  is passed into a settling zone  300  which functions as a phase separation zone. In particular, settling zone  300  separates, from oxidation zone effluent  20 , regenerated aqueous extraction medium  25  (as an aqueous phase), disulfide rich organic product  24  (as an oil phase), and off gas  22  (e.g., spent or excess air as a gas phase). All or a portion of regenerated aqueous extraction medium  25  is recycled to provide at least a portion of lean aqueous extraction medium  12  entering extraction zone  100 . 
     Overall, aspects of the invention are directed to processes for treating a hydrocarbon feed, or a feed comprising hydrocarbons, and further comprising mercaptan compounds and oxygen (e.g., as free gas phase molecular oxygen and/or dissolved liquid phase molecular oxygen). The processes comprise extracting at least a portion of the mercaptan compounds in a rich aqueous extraction medium, as mercaptide compounds, that is essentially free of circulating oxidation catalyst. According to specific embodiments, the hydrocarbon feed comprises a hydrocarbon fraction obtained from a fluid catalytic cracking (FCC) process and the extracting step comprises contacting the hydrocarbon feed with a lean aqueous extraction medium comprising sodium hydroxide having a concentration from about 5% to about 20% by weight to provide a treated hydrocarbon product that is essentially free of disulfide compounds 
     In view of the present disclosure, it will be seen that several advantages may be achieved and other advantageous results may be obtained. Those having skill in the art, with the knowledge gained from the present disclosure, will recognize that various changes could be made in the above processes without departing from the scope of the present invention. Mechanisms used to explain theoretical or observed phenomena or results, shall be interpreted as illustrative only and not limiting in any way the scope of the appended claims.