Abstract:
Aspects of the invention are associated with the discovery of processes for converting methane (CH 4 ), present in a methane-containing feedstock that may be obtained from a variety of sources such as natural gas, to higher hydrocarbons (e.g., C 4   +  hydrocarbons) such as gasoline, diesel fuel, or jet fuel boiling-range hydrocarbons, which may optionally be separated (e.g., by fractionation) for use as transportation fuels, or otherwise as blending components for such fuels. Particular aspects of the invention are associated with advantages arising from maintaining reaction conditions that improve the yield of C 4   +  hydrocarbons. Further aspects relate to the advantages gained by integration of the appropriate reactions to carry out the methane conversion, with downstream separation to recover and recycle desirable components of the reaction effluent, thereby improving process economics to the extent needed for commercial viability.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. provisional application No. 62/238,201, filed Oct. 7, 2015, and to U.S. provisional application No. 62/310,889, filed Mar. 21, 2016, both of which applications are hereby incorporated by reference in their entireties. 
     
    
     FIELD OF THE INVENTION 
       [0002]    Aspects of the invention relate to processes and systems for the soft oxidation of methane (i.e., reaction of methane with a sulfur-containing compound as opposed to oxygen) to produce higher hydrocarbons, and more particularly C 4   +  hydrocarbons that may be used in transportation fuels or as chemicals. 
       DESCRIPTION OF RELATED ART 
       [0003]    The ongoing search for alternatives to crude oil, for the production of hydrocarbon fuels and specialty chemicals (e.g., petrochemical precursors such as olefins and aromatics), is increasingly driven by a number of factors. These include diminishing petroleum reserves, higher anticipated energy demands, and heightened concerns over greenhouse gas (GHG) emissions from sources of non-renewable carbon. In view of its abundance in natural gas reserves, methane has become the focus of a number of possible synthesis routes. Known commercial processes for converting natural gas into liquid fuels include Fisher-Tropsch (FT) synthesis and those involving the formation of methanol as an intermediate for subsequent dehydration, i.e., in methanol-to-gasoline (MTG) conversion. Whereas these methods are widely used and improve the economics of transporting natural gas over long distances, they nonetheless involve considerable complexity, capital expenditure, and multiple conversion steps. These known methods also suffer from poor selectively to gasoline boiling-range hydrocarbons and result in the production of carbon dioxide. Furthermore, both FT and MTG processes require pretreatment of the feedstock for H 2 S removal, in order to obtain acceptable catalyst stability. 
         [0004]    The oxidation of methane with O 2  to directly produce hydrocarbons and H 2 O, while studied extensively, has been met with a number of significant challenges. These include thermodyamically favorable reaction pathways that lead to further oxidation (“over oxidation”) of the desired hydrocarbons and oxygenates, resulting in substantial CO 2  formation. In addition, management of the highly exothermic oxidation reaction poses a number of practical problems in terms of process design. The catalytic, oxidative coupling of methane and other hydrocarbons to form higher hydrocarbons is described, for example in U.S. Pat. No. 5,043,505. 
         [0005]    In comparison, the free energy losses associated with the counterpart reactions using S 2  versus O 2  as a reactant with methane, including over oxidation reactions, are significantly lower. This has led to the characterization of sulfur-based methane conversion as “soft oxidation.” The study of various catalysts for the conversion of CH 4  and elemental sulfur to CS 2  and hydrocarbons is documented, for example, in Zhu, Q. et al. (N ATURE  C HEMISTRY , Vol. 5 (December 2012): 104-109). Other publications disclosing the production of CS 2  from methane and sulfur include U.S. Pat. No. 4,480,143; U.S. Pat. No. 4,543,434; U.S. Pat. No. 4,822,938; and U.S. Pat. No. 4,864,074, which also describe further processing steps to obtain higher hydrocarbons such as aromatics. See also Quann, R. J. et al. (I ND . E NG . C HEM . R ES ., Vol. 27(4) (1988): 565-570) and U.S. Pat. No. 4,451,685. The use of S 2  over O 2  has therefore been investigated as a route to hydrocarbon production, in which the product selectivity and process thermodynamics are more easily managed. In addition, methods for obtaining elemental sulfur as a necessary starting material are practiced industrially as the Claus process, or are otherwise known from, for example, Fukuda, K. et al. (I ND . E NG . C HEM . F UNDAM ., VOL 17(4) (1978): 243-248). Sulfur is also a less expensive oxidant than oxygen, since oxygen must be initially separated from nitrogen for use. 
         [0006]    More recently, the use of H 2 S, rather than elemental sulfur, has been investigated as the reactant for catalytically converting CH 4  to CS 2 . See Hosseini, H. el al. (I NTERNATIONAL  S CHOLARLY AND  S CIENTIFIC  R ESEARCH  &amp; I NNOVATION , Vol. 4(2) (2010): 198-201). An additional downstream, catalytic reaction of the CS 2 , as part of a two-step hydrogen sulfide methane (“HSM”) process for producing hydrocarbons, is discussed in Erekson, E. J. (Work Performed Under Contract No.: DE-AC22-93PC92114 (July 1996)). In order for processes that synthesize liquid hydrocarbons (e.g., gasoline and jet fuel) from methane to advance to the point of economic feasibility, a number of factors must be addressed, particularly in terms of product yields and process integration steps that limit the losses of valuable reactants and intermediates. 
       SUMMARY OF THE INVENTION 
       [0007]    Aspects of the invention are associated with the discovery of processes for converting methane (CH 4 ), present in a methane-containing feedstock, which may be obtained from a variety of sources such as natural gas, to higher hydrocarbons (e.g., C 4   +  hydrocarbons). These higher hydrocarbons include gasoline, diesel fuel, or jet fuel boiling-range hydrocarbons, which may optionally be separated (e.g., by fractionation) from liquid products of the processes. In addition to separation, or alternatively, these higher hydrocarbons or their separated fractions may be further reacted for use as (i) transportation fuels, (ii) blending components for such fuels, (iii) viscosity-reducing agents to enhance transportability of other hydrocarbon fractions, and/or (iv) specialty chemicals such as aromatic hydrocarbons (e.g., para-xylene). Particular aspects of the invention are associated with advantages arising from maintaining reaction conditions that improve the selectivity to, and/or yield of C 4   +  hydrocarbons over a given stage or reactor. For example, in the case of methane being predominantly reacted, such as converted to an intermediate (e.g., CS 2 ), in one reaction step or stage, conventional considerations regarding process design would suggest that the most efficient location for introduction of all of the methane-containing feedstock would be an inlet to this reaction step or stage, or at least a point upstream. of this reaction step or stage (i.e., without any intervening separation or reaction vessels, prior to the reaction step or stage). In contrast, according to embodiments of the invention, discussed in greater detail below, introducing the methane-containing feedstock at one or more other introduction locations has important implications with respect to influencing reaction selectivity and yield in other parts of the process, such as a reaction step or stage to convert the intermediate, produced in the first stage, to the C 4   +  hydrocarbons. In particular embodiments, at least part, and preferably substantially all, of the methane-containing feedstock is fed to an inlet of a reaction step or stage, or a point upstream of this reaction step or stage, which is not the reaction step or stage used predominantly to convert methane to an intermediate (e.g., CS 2 ). 
         [0008]    More specifically, by feeding at least a portion of the methane-containing feedstock to a reaction step or stage, or upstream of such reaction step or stage, predominantly for conversion of the intermediate to C 4   +  hydrocarbons, important reaction conditions may be established in this conversion, such as a desired methane partial pressure. By maintaining sufficient methane partial pressure, undesired reactions such as methane re-formation may be advantageously suppressed, leading to an increase in the selectivity to, and/or yield of C 4   +  hydrocarbons. Accordingly, the process may be operated with a sufficient methane partial pressure in a reaction step or stage predominantly to convert the intermediate to higher hydrocarbons, with, or possibly even without, feeding at least a portion of the methane-containing feedstock to this reaction step or stage, or upstream of this reaction step or stage. Advantageously, an increase in the yield of higher hydrocarbons, across a particular reaction step or stage of the process, reduces the amount of materials being recycled, as well as the amount of materials being heated to the substantial reaction temperatures needed to convert methane to an intermediate. Therefore, both process equipment costs and operating costs are reduced. 
         [0009]    Further aspects of the invention relate to the advantages gained by integration of the appropriate reactions to carry out the methane conversion, with downstream separation to recover and recycle desirable components of the reaction effluent, thereby improving process economics to the extent needed for commercial viability. According to one important aspect, H 2 S, which is a reactant with CH 4 , may be separated (together with unconverted CH 4 ) from the reaction effluent (e.g., separated from a vapor product of this effluent) and recycled. This leads to particular advantages if two or more reaction steps or stages in the overall process lead to the conversion of methane to higher hydrocarbons, and H 2 S is consumed in one reaction step or stage but produced in another reaction step or stage. In this case, the H 2 S may be continually recycled, and only very small rate of H 2 S addition is required to sustain the process, for example to make up for losses in a bleed (vent or purge) gas stream or in a net hydrogen production stream, and/or otherwise losses by dissolution in a liquid hydrocarbon product (e.g., comprising some or all of the higher hydrocarbons produced). 
         [0010]    Processes described herein therefore perform the “soft oxidation” of methane, i.e., at least one reaction step or stage of the process is predominantly to convert methane by reaction with sulfur or a sulfur-containing compound (e.g., H 2 S), in a reaction stage or step that leads to the overall conversion to higher hydrocarbons that may be a source of a variety of products. These products may include “drop in” gasoline and/or diesel fuel, or otherwise may include chemicals such as aromatic hydrocarbons (e.g., benzene, toluene, and/or xylenes), potentially having a higher value relative to hydrocarbon fuels. The processes may have a number of practical applications, including the conversion of stranded natural gas, for example if the process is made portable by mounting on a skid. Without access to a suitable source for conversion to value-added products, such stranded natural gas might otherwise be flared (combusted), with the accompanying generation of CO 2 . Accordingly, processes described herein can effectively monetize otherwise unusable sources of natural gas, with the added benefit of reducing greenhouse gas emissions. Moreover, if the methane-containing feedstock is obtained from a renewable resource (e.g., biomass), for example by hydropyrolysis as described in U.S. Pat. No. 8,915,981 assigned to Gas Technology Institute, then processes described herein may be used to provide renewable hydrocarbon-containing fuels, fuel blending components, and/or chemicals. The overall carbon footprint associated with the production of the higher hydrocarbons, e.g., based on a lifecycle assessment of their greenhouse gas (GHG) emissions, may be further reduced if at least a portion of the hydrogen product is combusted to provide some or all of the heating requirements of the process (e.g., by transferring combustion heat to the process recycle gas or to the methane-containing feedstock). By combusting hydrogen product, the process may be sustained, at least in terms of its heating requirements, without the release of CO 2  into the environment. 
         [0011]    Soft oxidation processes described herein may convert, in a first stage, substantially all of the methane in a methane-containing feedstock to reactive carbon disulfide, advantageously without the need for solid sulfur as a reactant. The processes may additionally include the conversion of carbon disulfide (CS 2 ) at economically favorable selectivity to C 4   +  hydrocarbons (i.e., higher hydrocarbons having four carbon atoms or more), in a second stage. Improvements in both selectivity and yield (the product of conversion and selectivity) of the C 4   +  hydrocarbons in the second stage may be achieved by suppressing or largely avoiding the undesirable re-formation of methane. Moreover, the processes may be advantageously operated without the release of any significant amounts of carbon dioxide, sulfur, and/or sulfur-containing compounds to the environment. As described above, the required sulfur, in the form of H 2 S, may be consumed and regenerated in first and second process stages, respectively, as well as recycled continuously without any significant overall consumption or production. In one sense, the H 2 S acts as a gas phase “catalyst,” that is consumed in the process to only a very minimal extent, e.g., as needed to replace trace amounts in gas and liquid products. Overall, therefore, in representative embodiments, (i) all or substantially all of the carbon of the methane, initially present in the methane-containing feedstock, is converted to higher hydrocarbons present in the liquid product, (ii) all or substantially all of the hydrogen, initially present in the methane-containing feedstock, is converted to H 2  present in a hydrogen product stream, or a net hydrogen production stream, as described herein, (iii) all or substantially all of the H 2 S, used as a reactant in a first stage, is regenerated in a second stage and recycled, and/or (iv) all or substantially all of methane that is not converted in a given pass through the first and second stages is recycled to extinction. 
         [0012]    According to particular processes, sulfur oxidation of methane in a first stage is combined with vapor phase hydrogenation/oligomerization of CS 2  in a second stage. Suppression of the undesirable re-formation of methane in the second stage may be achieved using second stage operating conditions that include a sufficient methane partial pressure. For example, methane partial pressure can be increased if all, or substantially all, of the methane-containing feedstock is introduced to an inlet to the second stage, or to a point of mixing with the effluent of the first stage. The increased methane partial pressure in the second stage, compared to a base-case operation in which all of the methane-containing feedstock is introduced to the first stage (e.g., at an inlet to a sulfur oxidation reactor used in this stage) where it is predominantly consumed, improves selectivity to C 4   +  hydrocarbons in the second stage, relative to this base-case operation. Process economics are thereby improved considerably, as recycle compressor power, heating and cooling duties, and equipment sizes, are reduced. Accordingly, disclosed herein are processes for the commercially viable production of hydrocarbon fuels from methane, using soft oxidation. 
         [0013]    These and other embodiments, aspects, and advantages relating to the present invention are apparent from the following Detailed Description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    A more complete understanding of the exemplary embodiments of the present invention and the advantages thereof may be acquired by referring to the following description in consideration of the accompanying figures, in which the same reference numbers are used to identify the same features. 
           [0015]      FIG. 1  depicts a flowscheme that illustrates a representative two-stage process as described herein. 
           [0016]      FIG. 2  depicts flowscheme that illustrates a separation stage that may be used in a process as described herein. 
       
    
    
       [0017]    The figures should be understood to present an illustration of the disclosure and/or principles involved. In order to facilitate explanation and understanding, simplified equipment is depicted in  FIGS. 1 and 2 , and these figures are not necessarily drawn to scale, such that some components and structures, as well details pertaining to their configurations, may be exaggerated. Valves, instrumentation, and other equipment and systems not essential to the understanding of the various aspects 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 converting a methane-containing feedstock to higher hydrocarbons, will have configurations and components determined, in part, by their specific use. 
       DETAILED DESCRIPTION 
       [0018]    Embodiments of the invention relate to a process for converting a methane-containing feedstock to higher hydrocarbons (e.g., C 4   +  hydrocarbons). Representative methane-containing feedstocks are gases comprising at least 50% (e.g., from 50% to more than 99%) CH 4 , with such gases typically comprising at least 75% (e.g., from 75% to more than 99%) CH 4 , and often comprising at least 90% (e.g., from 90% to more than 99%) CH 4 . Methane-containing feedstocks may include gaseous hydrocarbon impurities such as ethane and propane, as well as non-hydrocarbon impurities such as CO and CO 2 . Advantageously, because H 2 S is present in the process, the methane-containing feedstock may contain this sulfur-containing compound, without concerns relating to its detrimental effect as a catalyst poison in known processes, such as FT synthesis and MTG conversion, referenced above. Accordingly, in some embodiments, the methane-containing feedstock may include H 2 S in a concentration of at least 500 parts per million by volume (vol-ppm), at least 0.1% by volume (vol-%), or even at least 1 vol-%. 
         [0019]    An important methane-containing feedstock is natural gas, and particularly stranded natural gas, which, using known processes, cannot be economically upgraded to C 4   +  hydrocarbons. Other methane-containing feedstocks may be obtained from coal or biomass (e.g., char) gasification, from a biomass digester, or as effluents from biofuel production processes (e.g., pyrolysis processes and fatty acid/triglyceride hydroconversion processes). The methane may therefore be derived from a renewable carbon source. Other sources of methane-containing feedstocks include effluents of industrial processes such as steel manufacturing processes or non-ferrous product manufacturing processes. Further sources include effluents of petroleum refining processes, electric power production processes, chemical (e.g., methanol) production processes, and coke manufacturing processes. 
         [0020]    Processes described herein convert methane, in one or more reaction stages or steps, to higher hydrocarbons, which may be recovered (e.g., by condensation) into a liquid product. The higher hydrocarbons may also be further separated into desired fractions using one or more separation steps, such as on the basis of relative volatility (e.g., by a single vapor-liquid equilibrium stage of flashing or by multiple vapor-liquid equilibrium stages of distillation, either of which may optionally be performed with a stripping gas). A representative fraction is C 4   +  hydrocarbons, although this fraction may also be the entire liquid product recovered from a final (e.g., the second) reaction step or stage of the process, without further separation. Other representative fractions include C 4 -C 10  hydrocarbons, C 6 -C 10  hydrocarbons, and other fractions of the higher hydrocarbons produced from the process. Commercially relevant fractions, in the case of transportation fuels, include those comprising (i) predominantly, or substantially all, naphtha or gasoline boiling-range hydrocarbons (i.e., a gasoline fraction), (ii) predominantly, or substantially all, diesel fuel boiling-range hydrocarbons (i.e., a diesel fuel fraction), or (iii) predominantly, or substantially all, jet fuel boiling-range hydrocarbons (i.e., a jet fuel fraction). Naphtha or gasoline boiling-range hydrocarbons may have an initial boiling point (or “front-end”) temperature characteristic of C 5  hydrocarbons, for example from about 30° C. (86° F.) to about 40° C. (104° F.), with a representative value being 35° C. (95° F.) and a distillation end point temperature generally from 110° C. (230° F.) to about 149° C. (300° F.), and typically from about 121° C. (250° F.) to about 143° C. (290° F.), with a representative value being 130° C. (266° F.). Diesel fuel boiling-range hydrocarbons and jet fuel boiling-range hydrocarbons may have an initial boiling point temperature in the range from about 120° C. (248° F.) to about 160° C. (320° F.)), with a representative value being 149° C. (300° F.). The distillation end point temperature of diesel fuel boiling-range hydrocarbons is generally in the range from about 300° C. (572° F.) to about 400° C. (752° F.)), with a representative value being 370° C. (698° F.). These initial and end point temperatures, which are also characteristic of hydrocarbons in respective naphtha, gasoline, diesel fuel, and jet fuel fractions obtained from crude oil fractionation, may be measured according to ASTM D86, with the end point being the 95% recovery value. 
         [0021]    “Higher hydrocarbons,” relative to methane, include hydrocarbons having two or more carbon atoms, such ethane, propane, butane, etc. “C 4   +  hydrocarbons,” as understood in the art, refer to hydrocarbons having four or more carbon atoms, which are readily condensable. Of the C 4   +  hydrocarbons, C 4 -C 10  hydrocarbons are of particular interest for their use in transportation fuels, e.g., as a source of gasoline boiling-range hydrocarbons, diesel fuel boiling-range hydrocarbons, and jet fuel boiling-range hydrocarbons as described above. Of the C 4   +  hydrocarbons, C 6 -C 10  hydrocarbons are of particular interest for their use as chemical products, such as aromatic hydrocarbon products including benzene, toluene, xylenes, and alkylbenzenes. Desired fractions, from which the higher hydrocarbons (or from which larger fractions, such as C 6 -C 10  hydrocarbons) may be separated therefore include a purified benzene fraction, a purified toluene fraction, a purified xylene fraction (which may be further separated and/or isomerized to obtain a desired xylene isomer, e.g., para-xylene), and a purified alkylbenzene fraction. 
         [0022]    As used herein, the term “substantially all” means “at least 95%,” and the term “substantially complete” means “at least 95% complete.” The term “predominantly” means “at least 50%.” 
         [0023]    Representative processes comprise feeding at least a portion of the methane-containing feedstock to a hydrogenation/oligomerization reactor to suppress a methane re-formation reaction and thereby increase a selectivity to, and/or yield of, C 4   +  hydrocarbons (i.e., the C 4   +  hydrocarbon-containing fraction of the higher hydrocarbons, which may be all or substantially all of the higher hydrocarbons), in an oligomerization effluent of the hydrogenation/oligomerization reactor, which is obtained from oligomerization of CS 2 . The selectivity increase with respect to this reactor may, for example, be measured relative to a comparable base-case in which all of the methane-containing feedstock is fed to a sulfur oxidation reactor, upstream of the hydrogenation reactor. The selectivity to the C 4   +  hydrocarbons, with respect to the hydrogenation/oligomerization reactor, refers to the weight percentage of the carbon in CS 2 , fed to this reactor, which becomes converted to C 4   +  hydrocarbons in the effluent of this reactor. In representative embodiments, the selectivity to C 4   +  hydrocarbons in the hydrogenation/oligomerization reactor may be increased, relative to the base case, by at least 2% (e.g., from 2% to 35%), by at least 5% (e.g., from 5% to 30%), or by at least 8% (e.g., from 8% to 25%). As the conversion of CS 2  in the hydrogenation/oligomerization reactor is, in preferred embodiments, substantially complete, substantially all of the same increases in the yield (the product of conversion and selectivity) of C 4   +  hydrocarbons in the hydrogenation/oligomerization reactor, relative to the base case, may be realized. These increases in selectivity and yield are namely the differences (rather than percentages of increases) between selectivities and yields obtained for processes as described herein, and those obtained for the comparable base-cases. 
         [0024]    Particular processes may further comprise recycling a recycle gas stream comprising both CH 4  and H 2 S to a sulfur oxidation reactor positioned upstream of the hydrogenation/oligomerization reactor. The recycle gas stream may comprise at least a portion, and preferably substantially all, of an H 2 S/CH 4  stream that is separated from a vapor product of the oligomerization effluent of the hydrogenation/oligomerization reactor. The processes may otherwise, but preferably in addition, comprise recycling, to the hydrogenation/oligomerization reactor, at least a portion of a hydrogen product stream that is separated from the vapor product of the oligomerization effluent. 
         [0025]    Further embodiments of the invention relate to a process for converting a methane-containing feedstock to higher hydrocarbons (e.g., C 4   +  hydrocarbons), in which the process comprises continuously recycling H 2 S in an H 2 S recycle loop. This H 2 S recycle loop may be defined by (i) a recycle gas stream, comprising both CH 4  and H 2 S, to a sulfur oxidation reactor, (ii) a sulfur oxidation effluent to a hydrogenation/oligomerization reactor, (iii) a hydrogenation/oligomerization effluent to a separation stage for condensing at least a portion of the higher hydrocarbons (e.g., as a liquid hydrocarbon product), and (iv) an H 2 S/CH 4  stream that is separated, in the separation stage, from a vapor product of the effluent of the hydrogenation reactor. The recycle gas stream comprises at least a portion of the H 2 S/CH 4  stream, thereby completing the loop. Advantageously, as described above, the continuous recycle of H 2 S in the H 2 S recycle loop maintains this valuable sulfur-containing compound, which serves as a carrier of the sulfur for sulfur oxidation (i.e., soft oxidation) of methane. Sulfur losses, as well as the requirements for handling H 2 S (which is both corrosive and toxic), are thereby minimized. According to representative embodiments, for example, sulfur is added to the process (e.g., added to the H 2 S recycle loop at any of the streams (i), (ii), (iii), and/or (iv) defining this loop, as described above) at a makeup rate of less than 2000 grams (e.g., from 2 grams to less than 2000 grams) S per million grams of the C 4   +  hydrocarbons produced. In preferred embodiments, the makeup rate is less than 1000 grams (e.g., from 2 grams to less than 1000 grams), less than 500 grams (e.g., from 2 grams to less than 500 grams), or even less than 100 grams (e.g., from 2 grams to less than 100 grams) S per million grams of the C 4   +  hydrocarbons produced. This makeup rate, in terms of grams of elemental sulfur (S) added per million parts of the C 4   +  hydrocarbons produced, may also be equivalently expressed in terms of “parts by weight S per million parts by weight of the C 4   +  hydrocarbons.” 
         [0026]    According to any of the processes described herein, a sufficient methane partial pressure in the hydrogenation/oligomerization reactor, or in the second stage generally, may be maintained such that the undesirable re-formation of methane is suppressed, thereby increasing selectivity to C 4   +  hydrocarbons in this reactor or stage. Such methane partial pressure may be maintained, for example, by introducing at least a portion, and preferably substantially all, of the methane-containing feedstock to the second stage of the process, or more particularly, to an inlet to the hydrogenation/oligomerization reactor. At least a portion (e.g., at least 50%), or substantially all, of the methane-containing feedstock may otherwise, or in addition, be introduced to the sulfur oxidation effluent, or namely a point of mixing with the sulfur oxidation effluent. A representative methane partial pressure in the second stage, or more particularly in the hydrogenation/oligomerization reactor, sufficient to obtain the C 4   +  hydrocarbon selectivity and yield improvements described herein, is at least 10 kilopascals (10 kPa), for example from 10 kPa to 4.5 MPa or from 250 kPa to 4.5 MPa. This methane partial pressure may be at least 20 kPa (e.g., from 20 kPa to 3.5 MPa or from 500 kPa to 3.5 MPa), or at least 35 kPa (e.g., from 35 kPa to 3 MPa or from 1 MPa to 3 MPa). 
         [0027]    According to any of the processes described herein, for example as a result of maintaining sufficient methane partial pressure in the hydrogenation/oligomerization reactor, or in the second stage generally, the selectivity to C 4   +  hydrocarbons may be at least 35%, for example from 35% to 95%. This selectivity may be at least 45% (e.g., from 45% to 70%), or at least 50% (e.g., from 50% to 65%). The same percentages, and ranges of percentages, apply to the yields of C 4   +  hydrocarbons in the hydrogenation/oligomerization reactor, or in the second stage generally, in view of the conversion of CS 2  in this reactor or stage being complete, or substantially complete. 
       First Reaction Stage 
       [0028]    In representative embodiments, a first reaction stage is used to perform sulfur oxidation, such that this stage may alternatively be referred to as a sulfur oxidation stage. This stage may comprise one or more sulfur oxidation reactors, in which CH 4  in the methane-containing feedstock is reacted with H 2 S to form CS 2  according to the reaction: 
         [0000]      2H 2 S +CH 4 →CS 2 +4H 2   (1).
 
         [0029]    In a preferred embodiment, the first reaction stage comprises a single sulfur oxidation reactor. The CH 4  may be fed to the sulfur oxidation stage in a recycle gas comprising recycle CH 4  and recycle H 2 S. Amounts of H 2 S needed to sustain the process, for example to provide a makeup rate of sulfur to compensate for steady-state losses of the sulfur-containing compound as described above, may be introduced to this recycle gas in the form of H 2 S that is generated from an H 2 S-precursor, such as an organic sulfide (e.g., dimethyl disulfide, DMDS) or even CS 2 , which decomposes at elevated temperatures and in a hydrogen atmosphere, to form the reactant H 2 S. For example, DMDS decomposes to form H 2 S and CH 4  in the recycle gas, according to the reaction: 
         [0000]      CH 3 S 2 CH 3 +3H 2 →2 CH 4 +2 H 2 S  (2).
 
         [0030]    An H 2 S-precursor may also be used to provide an initial H 2 S charge rate that is significantly higher, relative to the makeup rate at steady state. The initial charge rate can establish a concentration of H 2 S in the recycle gas, during a startup period that precedes the introduction (feeding) of the methane-containing feedstock to the process. According to alternative embodiments, the H 2 S or an H 2 S-precursor may be introduced at various introduction locations described herein, such as the possible feedstock introduction locations, described below. Suitable H 2 S precursors are preferably organic sulfur-containing liquids, such as DMDS, that facilitate handling of the process sulfur requirements. 
         [0031]    Suitable conditions in the first stage, e.g., sulfur oxidation reactor conditions, may include a temperature from 1000° C. to 1200° C., and typically from 1050° C. to 1150° C., and a total absolute pressure from 350 kPa to 6 MPa, and typically from 350 kPa to 4 MPa. These conditions may also include sufficient hydrogen partial pressure to maintain catalyst activity, by preventing side reactions that lead to coke formation. Representative hydrogen partial pressures in the first stage are from 100 kPa to 3.5 Mpa, and typically from 100 kPa to 2.5 MPa. 
         [0032]    By having a substantial molar excess of H 2 S in the first stage, conversion of CH 4  to CS 2  may be at least 90% in this stage, for example the conversion is typically at least 95% and often at least 98%. Conditions in the first stage may therefore include a molar ratio of H 2 S to CH 4  in the recycle gas, or otherwise in the combination of the recycle gas and any other gas stream (e.g., a portion of the methane-containing feedstock) that is fed to the first stage, from 1:1 to 4:1, and typically from 2.5:1 to 4:1 (i.e., in excess of the stoichiometric ratio according to reaction (1) above). Stated otherwise, the conditions may include a first stage inlet H 2 S/CH 4  molar ratio or sulfur oxidation reactor inlet H 2 S/CH 4  molar ratio in these ranges. As a result of high conversion in the first stage, the methane partial pressure in the sulfur oxidation effluent (i.e., the effluent of the first stage prior to being mixed with any portion of the methane-containing feedstock that would increase the methane partial pressure in the resulting, combined stream) may be low, for example from 0 kPa to less than 10 kPa. 
         [0033]    A sulfur oxidation reactor in the first stage may contain a sulfur oxidation catalyst comprising a sulfur oxidation active metal, or a compound of a sulfur oxidation active metal, wherein the sulfur oxidation active metal is selected from the group consisting of Pd, Mo, Cr, Ce, Pt, Ni, Rh, W, and Li. Combinations of these metals and/or metal compounds may also be used. Normally, in view of the significant concentration of H 2 S to which the sulfur oxidation catalyst is exposed, the sulfur oxidation active metal may be in its sulfided form, i.e., the sulfur oxidation catalyst may contain a metal sulfide compound of any one or more of these sulfur oxidation active metals. The sulfur oxidation active metal(s) may be supported on a suitable support material that is refractory to the conditions in the sulfur oxidation reactor. Representative support materials include alumina, silica, titania, and zirconia. Specific examples of sulfur oxidation catalysts include Pd or PdS that is supported on zirconia (Pd/ZrO 2  or PdS/ZrO 2 ); Pt, Ni, or Rh that is supported on alumina (Pt/Al 2 O 3 , Ni/Al 2 O 3 , or Rh/Al 2 O 3 ); MoS 2 ; PdS; Cr 2 S 3 ; CeS; WS 2 ; and LiS 2 . Preferred catalysts for use in the sulfur oxidation reactor include Pd/ZrO 2  and MoS 2 . 
         [0034]    The conversion of methane by soft oxidation to CS 2 , occurring in the first-stage, is endothermic. Process heat, which is supplied at the very high temperatures described above for the first stage, may be obtained from the combustion of at least a portion of a hydrogen product of the process, and, according to more particular embodiments, at least a portion (e.g., all or substantially all), of a net hydrogen production stream, as described herein. The combustion of this readily available product is useful in locations lacking an accessible utility for transporting the net hydrogen produced for a more valuable end use (e.g., to a refinery). In a representative embodiment, at least 80% of the heat required in the first stage is provided from hydrogen combustion. Alternatively, if all of the heat required in the first stage is provided in this manner, according to preferred embodiments, then advantageously no additional heat is required, i.e., the process may be operated with no external source of heat, such as external fuel, and with no emission of CO 2 . 
         [0035]    A sulfur oxidation reactor in the first stage is normally subjected to severe operating conditions, including the temperatures and pressures as described above, in addition to a high partial pressure of hydrogen sulfide, for example generally greater than 350 kPa. Representative construction materials for the sulfur oxidation reactor will therefore require resistance to corrosion under these first stage operating conditions. A vessel of the first stage reactor may comprise, for example, an alloy of iron, chromium, and aluminum, in which chromium and aluminum are present in amounts by weight of the alloy of 20%-30% and 4-7.5%, respectively. A vessel of the first stage reactor may alternatively comprise an alloy of nickel, cobalt, and chromium, and optionally other alloyed elements. For example, according to one such alloy, cobalt, chromium, silicon, manganese, titanium, and carbon are present in amounts of at least 29%, at least 28%, at least 2.75%, at least 0.5%, and least 0.5% and at least 0.05%, respectively, be weight of the alloy, together with nickel. According to another embodiment, a vessel of the first stage reactor may comprise an alloy having a large proportion (e.g., greater than 50% by weight of the alloy) of niobium or of molybdenum. Pure niobium or molybdenum may also be used. According to yet another embodiment, a vessel of the first stage reactor may comprise a highly temperature-resistant alloy, in order to provide sufficient mechanical strength, and this alloy may optionally be plated, on a surface facing the interior of the vessel, with a noble metal such as platinum or palladium for corrosion resistance. According to still another embodiment, a vessel of the first stage reactor may comprise a corrosion-resistant inner shell, facing the interior of the vessel, that is capable of resisting the corrosive atmosphere and high temperature of the first stage, and an outer shell, toward or facing the exterior of the vessel, of sufficient mechanical strength to contain the pressure in the first stage. 
       Second Reaction Stage 
       [0036]    In representative embodiments, a second reaction stage is used to perform oligomerization of the CS 2  that is produced in the first stage, according to reaction (1) above. Because oligomerization occurs in conjunction with hydrogen consumption, the second stage may alternatively be referred to as a “hydrogenation/oligomerization” stage. This stage may comprise one or more hydrogenation/oligomerization reactors, in which CS 2  in the effluent from the first stage (e.g., a sulfur oxidation effluent) is reacted with H 2  to form higher hydrocarbons (—[CH 2 ]—) according to the reaction: 
         [0000]      CS 2 +3H 2 →[−CH 2 ]−+2H 2 S  (3).
 
         [0037]    In a preferred embodiment, the second reaction stage comprises a single hydrogenation/oligomerization reactor. Also, according to other preferred embodiments as described above, the methane partial pressure in the second stage (e.g., at an inlet to a hydrogenation/oligomerization reactor) may be increased by feeding at least a portion, and preferably substantially all, of the methane-containing feedstock to an inlet of the second stage or to a point of mixing with the sulfur oxidation effluent. Therefore, the combined second stage feed, including the sulfur oxidation effluent being fed to the second stage, together with any portion of the methane-containing feedstock that is co-fed to the second stage or upstream of the second stage, may include methane at a concentration of at least 5 vol-%, such as from 5 vol-% to 50 vol-%. Typically, this concentration is at least 7 vol-% (e.g., from 7 vol-% to 35 vol-%), and often at least 10 vol-% (e.g., from 10 vol-% to 25 vol-%). Conditions in the second stage may therefore include these volume percentages of methane at the inlet to a hydrogenation/oligomerization reactor. Representative volume percentages of H 2 , H 2 S, and CS 2  at the inlet to this reactor are, respectively, 45 to 70 vol-%, 8 to 25 vol-%, and 10 to 25 vol-%. Representative methane partial pressures in the second stage, and accompanying increases in selectivity to —[CH 2 ]—, are described above. These advantages may be associated with suppression of undesired re-formation of methane, according to the reverse of reaction (1) above, occurring in the second stage. 
         [0038]    Alternatively or in conjunction with reaction (3) above, the formation of higher hydrocarbons may occur through formation of intermediate methanethiol (CH 3 SH), according to the reactions: 
         [0000]      CH 4 +CS 2 +H 2 →2CH 3 SH+H 2 S  (4) and
 
         [0000]      2CH 3 SH→−[CH 2 ]−+2H 2 S  (5).
 
         [0039]    Suitable conditions in the second stage, e.g., hydrogenation/oligomerization reactor conditions, may include a temperature from 250° C. to 500° C., and typically from 350° C. to 400° C. The total absolute pressure and hydrogen partial pressure in the second stage may be within the same ranges as described above with respect to the first stage (e.g., a total absolute pressure from 350 kPa to 6 MPa, and typically from 350 kPa to 4 MPa, and a hydrogen partial pressure from 100 kPa to 3.5 Mpa, and typically from 100 kPa to 2.5 MPa). Preferably, the total absolute pressure in the second stage is lower than that of the first stage, such that process flow from the first to the second stage can be maintained without intermediate compression. The pressure drop from the first stage to the second stage is typically a nominal value (e.g., from 35 to 350 kPa), associated with head losses through process equipment. As in the first stage, elevated hydrogen partial pressure is preferred in the second stage (e.g., in the hydrogenation/oligomerization reactor) to minimize catalyst coking and thereby maintain catalyst activity. Other conditions in the second stage may include a molar ratio of H 2  to CS 2  in the combined second stage feed, including the sulfur oxidation effluent being fed to the second stage, together with any portion of the methane-containing feedstock that is co-fed to the second stage or upstream of the second stage (e.g., any portion fed to an inlet of the second stage or to a point of mixing with the sulfur oxidation effluent) from 1:1 to 10:1, and typically from 3:1 to 5:1. Accordingly, conditions in the second stage may include a second stage inlet H 2 /CS 2  molar ratio or hydrogenation/oligomerization reactor inlet H 2 /CS 2  molar ratio, within these ranges. In this regard, it can be appreciated that any co-fed, methane-containing feedstock normally will not appreciably impact this H 2 /CS 2  molar ratio. 
         [0040]    A hydrogenation/oligomerization reactor in the second stage may contain a hydrogenation/oligomerization catalyst comprising a hydrogenation/oligomerization active metal, or a compound of a hydrogenation/oligomerization active metal, wherein the hydrogenation/oligomerization active metal is selected from the group consisting of Co, Ga, Ni, and Mo. Combinations of these metals and/or metal compounds may also be used. Normally, in view of the significant concentration of H 2 S to which the hydrogenation/oligomerization is exposed, the hydrogenation/oligomerization active metal may be in its sulfided form, i.e., the hydrogenation/oligomerization catalyst may contain a metal sulfide compound of any one or more of these hydrogenation/oligomerization active metals. The hydrogenation/oligomerization active metal(s) may be supported on a suitable support material that is refractory to the conditions in the hydrogenation/oligomerization reactor and/or otherwise lends desired catalytic activity (e.g., acidity). Representative support materials include zeolitic and non-zeolitic molecular sieves, examples of which are, respectively, ZSM-5 and AMS-1B borosilicate. These materials are described, respectively, in U.S. Pat. No. 3,702,886 and U.S. Pat. No. 4,514,516. Specific examples of hydrogenation/oligomerization catalysts include Co that is supported on ZSM-5, in combination with MoS 2  (i.e., Co/ZSM-5+MoS 2 ); Ga that is supported on ZSM-5 (Ga/ZSM-5); and Co that is supported on AMS-1B borosilicate, in combination with MoS 2  (i.e., Co-AMS-1B/borosilicate+MoS 2 ). 
       Separation Stage 
       [0041]    Higher hydrocarbons (e.g., C 4   +  hydrocarbons) may be recovered from the second stage effluent (e.g., the hydrogenation/oligomerization reactor effluent) by condensing all, or substantially all, of these hydrocarbons into a liquid product and separating, from this liquid product, a vapor product comprising H 2  and H 2 S present in the second stage effluent (i.e., comprising second stage H 2  and second stage H 2 S). The condensing may be performed by simply cooling the second stage effluent, for example to a temperature of 30° C. or less, and more typically 25° C. or less, for example to a temperature between 10° C. and 25° C., characteristic of process cooling water. Alternatively, a chiller or chilled adsorber may be used to achieve lower temperatures, for example between −5° C. and 10° C. The condensing may involve a single vapor-liquid equilibrium stage of separation, for example by being performed in a flash drum, or otherwise multiple vapor-liquid equilibrium stages of separation in a single vessel (e.g., in the case of a stripper) or multiple vessels, such as in the case of a secondary knockout drum for removing higher hydrocarbons that may be carried (e.g., by entrainment) into a vapor phase of a primary flash drum. Alternative to, or in combination with, the use of a secondary knockout drum, such entrainment may be reduced using a suitable coalescer in an upper section of the primary flash drum. 
         [0042]    The separated vapor product, following condensation of higher hydrocarbons, may then be further separated to provide a hydrogen product stream that is enriched in H 2  concentration, relative to the vapor product, and an H 2 S/CH 4  stream that is depleted in H 2  concentration, relative to the vapor product. This H 2 /H 2 S separation may be performed using a sour gas pressure swing adsorber (PSA) that may also preferentially separate not only methane, but other non-condensable gases (e.g., ethane) into the H 2 S/CH 4  stream. According to a representative separation by PSA, the concentration of H 2 S in the hydrogen product stream is less than 10 ppm (e.g., from 0.1 ppm to less than 10 ppm) and recovery of H 2 S in the H 2 S/CH 4  stream is greater than 99% (e.g., from 99% to 99.999%). For a given adsorbent, the degree of H 2 S removal from the hydrogen product and degree of recovery of H 2 S in the H 2 S/CH 4  stream can be varied by manipulating operating parameters, such as the number of separation stages. 
         [0043]    The liquid product, into which the higher hydrocarbons (e.g., C 4   +  hydrocarbons) are condensed, may be further separated to remove impurities such as dissolved H 2 S and/or to resolve any of the various fractions described above, including gasoline boiling-range hydrocarbons, diesel fuel boiling-range hydrocarbons, and jet fuel boiling-range hydrocarbons, which may be used as end products or otherwise as blending components. For example, such gasoline, diesel fuel, and/or jet fuel fractions may be blended with a viscous hydrocarbon-containing liquid, comprising relatively higher molecular weight hydrocarbons and/or having a relatively higher viscosity and boiling point range, to obtain a blended liquid stream having a viscosity lower than that of the viscous hydrocarbon-containing liquid. Further separation of the higher hydrocarbons may be performed using a single vapor-liquid equilibrium stage of separation, but such separation is more preferably performed using multiple vapor-liquid equilibrium stages of separation, for example in one or more stripper and/or distillation columns. In a particular embodiment, a portion of the methane-containing feedstock is added to a stripper column to remove residual H 2 S that is dissolved in the liquid product, prior to fractionation of the liquid product in a distillation column to obtain desired fractions, including those described herein. 
       Representative Embodiment 
       [0044]    The flowscheme of  FIG. 1  illustrates a representative two-stage process, for the conversion of methane in a methane-containing feedstock to higher hydrocarbons. The illustrated process comprises feeding a recycle gas stream  27 , comprising recycle CH 4  and recycle H 2 S, to a sulfur oxidation stage or reactor  100 . First stage heater  15  is used to obtain the high temperatures, described above, as needed to perform sulfur oxidation in this stage or reactor. A least a portion, and preferably substantially all, of the recycle CH 4  is converted by reaction with the recycle H 2 S, to provide a sulfur oxidation effluent  11  comprising CS 2 . As described above, the H 2 S is normally provided to sulfur oxidation stage or reactor  100  with recycle gas stream  27 , at a molar excess of CH 4 , and preferably even in an excess of the stoichiometric (2:1 H 2 S:CH 4 ) molar ratio according to reaction (1) above, in order to ensure that CH 4  is the limiting reagent and thereby promote its conversion to CS 2 . The illustrated process further comprises feeding at least a portion of, and preferably substantially all, of sulfur oxidation effluent  11  to a second stage or reactor  200  (e.g., a hydrogenation/oligomerization stage or reactor), preferably following cooling in sulfur oxidation effluent cooler  45  to obtain the temperatures described above, as needed to perform hydrogenation/oligomerization in this stage or reactor. Following conversion of at least a portion of the CS 2  to C 4   +  hydrocarbons, a second stage effluent  13  (e.g., a hydrogenation/oligomerization effluent, for example an effluent of a hydrogenation/oligomerization reactor) is provided. Second stage effluent  13  comprises the C 4   +  hydrocarbons, together with second stage H 2  and second stage H 2 S, which are also contained in second stage effluent  13 . The illustrated process further comprises introducing second stage effluent  13  to separation stage  300 , following cooling in second stage effluent cooler  25 , to perform various separations as described above. These may include condensing, from second stage effluent  13 , at least a portion, and preferably substantially all, of the C 4   +  hydrocarbons in this stream into a liquid product  19  that is separated from vapor product  17 , comprising at least a portion, and preferably substantially all, of the second stage H 2  and the second stage H 2 S contained in second stage effluent  13 . 
         [0045]    The illustrated process further comprises separating at least a portion, and preferably substantially all, of vapor product  17  to provide a hydrogen product stream  33 . This separation is performed in vapor product separation stage  400 , which may include, for example, one or more vessels housing an adsorbent (e.g., in the case of separation by pressure swing adsorption (PSA)) or one or more vessels housing a membrane or multiple membranes. A first portion of hydrogen product stream  33  may be removed from the process as a net hydrogen production stream  47 , and a second portion (i.e., a recycle portion) of hydrogen product stream  33 , may be recycled to the process, using hydrogen recycle compressor  35 , as a hydrogen recycle stream  49 . Hydrogen product stream  33  is enriched in H 2  (i.e., has a higher H 2  concentration) relative to vapor product  17 . Separating vapor product  17 , in vapor product separation stage  400 , also provides an H 2 S/CH 4  stream  27 ′ that is depleted in H 2  (i.e., has a lower H 2  concentration) relative to vapor product  17 . At least a portion, and preferably substantially all, of H 2 S/CH 4  stream  27 ′ forms all or part of recycle stream  27 . Stated otherwise, recycle gas stream  27  comprises at least a portion, and preferably substantially all, of H 2 S/CH 4  stream  27 ′. For example, according to the illustrated process, the portion  27 ″ of H 2 S/CH 4  stream  27 ′ that is not removed in bleed stream  51 , is fed to H 2 S/CH 4  recycle compressor  55  and forms recycle gas stream  27 . According to some embodiments, bleed stream  51  may optionally be used, intermittently or continuously, to limit the accumulation of non-condensable gases in recycle gas stream  27 , such as hydrocarbons (e.g., ethane) produced in the process and/or impurities (e.g., CO, CO 2 ) entering the process in the methane-containing feedstock. 
         [0046]    All, substantially all, or a portion, of hydrogen recycle stream  49  may be introduced as a second stage hydrogen-containing reactant stream  21  to second stage or reactor  200  for sustaining the hydrogen/oligomerization occurring in this stage, as described above. Also, an H 2 S-precursor decomposition stream  23  may optionally be fed, as a portion of hydrogen recycle stream  49 , to H 2 S-precursor decomposition stage  500 . At this stage, an H 2 S-precursor stream  29  (e.g., comprising DMDS or other H 2 S-precursor as described above) is contacted with hydrogen that is contained in H 2 S-precursor decomposition stream  23 , to a provide makeup H 2 S stream  31 , which is fed to the process at a makeup rate to compensate for minor losses of H 2 S (e.g., contained in bleed stream  51  and in net hydrogen production stream  47 , and/or dissolved in liquid product  19 ), as described above. 
         [0047]    Certain advantages are gained, as described above and according to particular embodiments of the invention, by introducing the methane-containing feedstock at one or more feedstock introduction locations in the process, other than entirely to the sulfur oxidation stage and/or a point upstream of the sulfur oxidation stage. According to the illustrated process, possible feedstock introduction locations for methane-containing feedstock  10  include, (i) an inlet  12  to the sulfur oxidation stage or reactor  100 , (ii) an inlet  14  to the second stage or reactor  200 , (iii) a point of mixing  16  with the sulfur oxidation effluent  11 , (iv) a point of mixing  18  with the second stage effluent  13 , (v) a point of mixing  20  with the vapor product  17 , and/or (vi) a point of mixing  22  with the recycle gas stream  27 , which may comprise substantially all of the H 2 S/CH 4  stream  27 ′ (or a non-bleed portion  27 ″ thereof). Other feedstock introduction locations can include a point of mixing  24  with the second stage hydrogen-containing reactant stream  21  and/or even separation stage  300 . For example, a methane-containing feedstock introduction location at separation stage  300  may be suitable for stripping H 2 S from condensed higher hydrocarbons, to provide liquid product  19  with reduced H 2 S content and a stripper off gas  41  that may be added to H 2 S/CH 4  stream  27 ′. According to preferred embodiments, the one or more feedstock introduction locations includes inlet  14  to the second stage or reactor  200  and/or point of mixing  16  with the sulfur oxidation effluent  11 . 
         [0048]    The flowscheme of  FIG. 2  illustrates a representative separation stage  300 , for processing second stage effluent  13 . According to this illustrated embodiment, second stage effluent  13  is fed to primary flash drum  310  to separate, in a vapor-liquid equilibrium separation stage, flash drum overhead vapors  53  from flash drum bottoms liquid  59 . A flash drum overhead vapor compressor  65  may be used to re-compress flash drum overhead vapors  53 , prior to introduction to secondary knockout vessel  330 . The overhead fraction from secondary knockout vessel  330  may be removed from separation stage  300  as vapor product  17 , and the bottoms fraction  57  from secondary knockout vessel  330  may be combined with flash drum bottoms liquid  59  and introduced as condensed higher hydrocarbons  61  to product stripper  320 , used to separate gases, including dissolved H 2 S, from condensed higher hydrocarbons  61  and provide both liquid product  19  and stripper off gas  41 , described above, which may be removed from separation stage  300 . Product stripper  320  may be used to perform multiple vapor-liquid equilibrium separation stages, and at least a portion of the methane-containing feedstock  10  may optionally be added to product stripper  320  to facilitate the desired separation of H 2 S into stripper off gas  41 . Liquid product  19  may be fed to distillation column  355 , used to perform multiple vapor-liquid equilibrium separation stages and thereby resolve desired product fractions as described above, for example gasoline fraction  37  and diesel fuel fraction  39 . 
       Overall Process 
       [0049]    As is apparent from the combination of first stage and second stage reactions (1) and (3) above, processes described herein may be used to perform an overall reaction, with continuous recycle of H 2 S in a recycle gas stream, of: 
         [0000]      CH 4 →—[CH 2 ]—+H 2   (6),
 
         [0000]    whereby the process converts substantially all of the carbon in methane to higher hydrocarbons and also converts substantially all of the hydrogen in methane to a net hydrogen production stream. Whereas the “per-pass” yield of higher hydrocarbons over a given stage (e.g., the second stage) may be limited by undesired reactions, such as the re-formation of methane as described above, the overall yield of the process may be at least 95% and may approach 100% if, in the recycle gas, H 2 S is continually recycled and CH 4  is recycled to extinction. As described above, process economics are significantly improved by increasing the per-pass selectivity to higher hydrocarbons (—[CH 2 ]—) in the second stage, leading to a reduced requirement for recycle gas circulation, which in turn beneficially reduces both capital (e.g., process equipment) and operating (e.g., utility) costs. Representative processes, in which methane is converted to higher hydrocarbons, advantageously transfer carbon and chemical energy in the methane-containing feedstock, of a relatively low bulk density, to a liquid product containing higher hydrocarbons, of a relatively high bulk density that can be more easily transported than the methane-containing feedstock. The first stage and second stage reactions (1) and (3) above may be performed in a single vessel (e.g., in separate zones within a vessel), although they are typically performed in separate vessels, or reactors, that may reside in separate stages of the processes in which specific and different conditions are maintained to promote the desired sulfur oxidation and hydrogenation/oligomerization. 
         [0050]    Processes as described herein may provide a number of products, such as a purified CS 2 -containing product stream, recovered as a portion of the sulfur oxidation effluent, or a bleed stream, as described above, comprising light (non-condensable) hydrocarbons, such as ethylene and propylene, which are valuable, although not condensed into the liquid product and not useful as liquid hydrocarbon fuel. 
         [0051]    Methane conversion to liquid fuels, as described herein, confers a very significant logistical benefit, since liquid fuels, because of their relatively greater bulk density, are far easier to transport over long distances than gaseous fuels. As a result, processes described herein allow for the economical use of supplies of “stranded” gas, such as remote natural gas wells or streams of renewable methane-containing gas from biomass digesters. According to particular embodiments, light hydrocarbon liquids obtained from processes described herein (e.g., gasoline, jet fuel, and/or diesel fuel fractions) may be blended with higher molecular weight hydrocarbons, such as those contained in crude oil. The resulting mixture may be less viscous than the higher molecular weight hydrocarbons would be in the absence of blending, thereby facilitating transport of the blend, particularly in the context of pipeline operations. 
         [0052]    If all, or substantially all, of the carbon supplied to the process is transferred to the liquid product, and this carbon is of biological origin (with the possible exception of carbon in an H 2 S-precursor such as DMDS that is needed to supply the process with sulfur), and the combustion of hydrogen is sufficient to meet the energy needs of the process, then the process provides a means whereby methane from renewable sources can be converted to a liquid product, and particularly liquid product fractions as described herein, without emitting carbon dioxide. That is, representative fractions, such as a gasoline fraction, a diesel fuel fraction, and/or a jet fuel fraction, may be produced with no or with negligible carbon footprint, based on a lifecycle assessment of the greenhouse gas (GHG) emission value, according to U.S. government accounting practices. The lifecycle greenhouse gas emission value may be measured based on CO 2  equivalents (e.g., grams (g) of CO 2 -equivalents/megajoule (MJ) of energy or pounds (lb) of CO 2  equivalents/million BTU (mmBTU of energy, wherein 1 g CO 2 -eq./MJ is about 2.33 lb CO 2 -eq./mmBTU), as measured according to guidelines set forth by the Intergovernmental Panel on Climate Change (IPCC) and the U.S. federal government. Lifecycle assessment (LCA) values of emissions in terms of CO 2  equivalents, from raw material cultivation (in the case of plant materials) or raw material extraction (in the case of fossil fuels) through fuel combustion, can be calculated using SimaPro 7.1 software and IPCC GWP 100a methodologies. 
         [0053]    Processes as described herein may also be used to obtain other valuable product streams, for example from the vapor product recovered downstream of the second stage. Otherwise, ethylene and other olefins may be separated and recovered from the liquid product and/or from the H 2 , H 2 S, CH 4 , and other non-condensable gases recycled in the recycle gas to the first stage. Ethylene and other olefins may therefore be enriched in a separate product stream. Another desired product stream may comprise CS 2 , for example a portion of this intermediate that is produced in the first stage and diverted to prevent its entry to the second stage. Once separated from the other process vapors, a product stream comprising CS 2  (e.g., enriched in CS 2  relative to the sulfur oxidation effluent) may comprise a separate product stream of the process. 
         [0054]    Overall, aspects of the invention are directed to processes and systems for converting methane in a methane-containing feedstock to higher hydrocarbons, which may be of value as transportation fuels. Such processes and systems may advantageously exhibit improved process economics compared to known processes, by virtue of improving reaction selectivity to desired end products and/or recycling valuable materials, as described throughout the present disclosure. Those having skill in the art, with the knowledge gained from the present disclosure, will recognize that various changes can be made to these processes in attaining these and other advantages, without departing from the scope of the present disclosure. As such, it should be understood that the features of the disclosure are susceptible to modification, alteration, changes, or substitution without departing from the scope of this disclosure. The specific embodiments illustrated and described herein are for illustrative purposes only, and not limiting of the invention as set forth in the appended claims.