Abstract:
Process for increasing the hydrogen content of a synthesis gas containing one or more sulphur compounds, the synthesis gas including hydrogen, carbon oxides and steam, and having a ratio defined as R=(H 2 −CO 2 )/(CO+CO 2 )≦0.6 and a steam to carbon monoxide ratio ≦1.8, including the steps of (i) heating the synthesis gas; (iii) subjecting at least a portion of the heated synthesis gas to a first stage of water-gas shift in a first shift vessel containing a first sulphur-tolerant water-gas shift catalyst that is cooled in heat exchange with boiling water, to form a pre-shifted gas stream; and (iii) forming a shifted gas stream by subjecting at least a portion of the pre-shifted gas stream to a second stage of water-gas shift in a second shift vessel containing a second sulphur-tolerant water-gas shift catalyst that is cooled in heat exchange with a gas stream including the synthesis gas.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a process for increasing the hydrogen content of a synthesis gas, in particular increasing the hydrogen content of a synthesis gas generated from a carbonaceous feedstock. 
     2. Description of the Related Art 
     Synthesis gas, also termed syngas, comprising hydrogen and carbon oxides (CO and CO 2 ) may be generated by a gasification of carbonaceous feedstocks such as coal, petroleum coke or other carbon-rich feedstocks using oxygen or air and steam at elevated temperature and pressure. Generally, the resulting synthesis gas is hydrogen deficient and to increase the concentration of hydrogen, it is necessary to subject the raw synthesis gas to the water-gas-shift reaction by passing it, in the presence of steam, over a suitable water-gas shift catalyst at elevated temperature and pressure. The CO 2  that is formed may then be removed in a downstream gas washing unit to give a hydrogen rich product gas. The synthesis gas generally contains one or more sulphur compounds and so must be processed using sulphur-tolerant catalysts, known as “sour shift” catalysts. The reaction may be depicted as follows;
 
H 2 O+CO           H 2 +CO 2  

     This reaction is exothermic, and conventionally it has been allowed to run adiabatically, with control of the exit temperature governed by feed gas inlet temperature and composition. 
     Furthermore, where it is required that only fractional shift conversion is needed to achieve a target gas composition, this is conventionally achieved by by-passing some of the synthesis gas around the reactor. 
     Side reactions can occur, particularly methanation, which is usually undesirable. To avoid this, the shift reaction requires considerable amounts of steam to be added to ensure the desired synthesis gas composition is obtained with minimum formation of additional methane. The cost of generating steam can be considerable and therefore there is a desire to reduce the steam addition where possible. 
     WO2010/106148 discloses a process to prepare a hydrogen rich gas mixture from a halogen containing gas mixture comprising hydrogen and at least 50 vol. % carbon monoxide, on a dry basis, by contacting the halogen containing gas mixture with water having a temperature of between 150 and 250 DEG C. to obtain a gas mixture poor in halogen and having a steam to carbon monoxide molar ratio of between 0.2:1 and 0.9:1 and subjecting said gas mixture poor in halogen to a water-gas shift reaction wherein part or all of the carbon monoxide is converted with the steam to hydrogen and carbon dioxide in the presence of a catalyst as present in one fixed bed reactor or in a series of more than one fixed bed reactors and wherein the temperature of the gas mixture as it enters the reactor or reactors is between 190 and 230 degrees C. The space velocity in the water-gas shift reactor is preferably between 6000-9000 h −1 . In the single Example a space velocity of 8000 hr −1  was used. Because this process operates at a low steam to CO ratio and at low inlet temperature it is limited in utility to certain types of gasifier and requires a relatively high catalyst volume. Therefore there is a need for a process operating at a low steam to CO ratio that requires less catalyst and which has broader utility. 
     WO2010/013026 discloses a process for increasing the hydrogen content of a synthesis gas containing one or more sulphur compounds, comprising the steps of (i) heating the synthesis gas and (ii) passing at least part of the heated synthesis gas and steam through a reactor containing a sour shift catalyst, wherein the synthesis gas is heated by passing it through a plurality of tubes disposed within said catalyst in a direction co-current to the flow of said synthesis gas through the catalyst. The resulting synthesis gas may be passed to one or more additional reactors containing sour shift catalyst to maximise the yield of hydrogen production, or used for methanol production, for the Fischer-Tropsch synthesis of liquid hydrocarbons or for the production of synthetic natural gas. While effective, we have found that in some cases with a cooled first shift reactor that the catalyst temperature profile may be too high, leading to undesirable side-reactions. 
     SUMMARY OF THE INVENTION 
     We have found that the disadvantages of the previous processes may be overcome using a pre-shift stage operated in heat exchange with boiling water in combination with a downstream gas-cooled shift vessel. 
     Accordingly, the invention provides a process for increasing the hydrogen content of a synthesis gas containing one or more sulphur compounds, said synthesis gas comprising hydrogen, carbon oxides and steam, and having a ratio, R, defined as R=(H 2 −CO 2 )/(CO+CO 2 )≦0.6 and a steam to carbon monoxide ratio ≦1.8, comprising the steps of (i) heating the synthesis gas, (ii) subjecting at least a portion of the heated synthesis gas to a first stage of water-gas shift in a first shift vessel containing a first sulphur-tolerant water-gas shift catalyst that is cooled in heat exchange with boiling water, to form a pre-shifted gas stream, and (iii) forming a shifted gas stream by subjecting at least a portion of the pre-shifted gas stream to a second stage of water-gas shift in a second shift vessel containing a second sulphur-tolerant water-gas shift catalyst that is cooled in heat exchange with a gas stream comprising the synthesis gas. 
     In the present invention the synthesis gas comprising hydrogen and carbon oxides and containing one or more sulphur compounds may be produced by any method although it is particularly suited to synthesis gas produced by gasification of a carbonaceous feedstock at elevated temperature and pressure. Any known gasification technology may be used. The carbonaceous feedstock may be coal, petroleum coke or another carbon-rich feedstock. Preferably the carbonaceous feedstock is a coal. In coal gasification, a coal powder or aqueous slurry may be partially combusted in a gasifier in a non-catalytic process using oxygen or air and in the presence of steam at pressures up to about 85 bar abs and exit temperatures up to about 1450° C., preferably up to about 1400° C., to generate a raw synthesis gas comprising hydrogen and carbon oxides (carbon monoxide and carbon dioxide) and containing one or more sulphur compounds such as hydrogen sulphide and carbonyl sulphide. 
     The R ratio, defined as R=(H 2 −CO 2 )/(CO+CO 2 ), in the synthesis gas feed is ≦0.6 and preferably is in the range 0.1 to 0.6, more preferably 0.2 to 0.6. R may readily be calculated from the molar quantities of the components in the synthesis gas feed. 
     Before the synthesis gas is subjected to the water-gas shift reaction, it is preferably cooled, optionally filtered and then washed to remove particulates such as coal ash. 
     The synthesis gas comprises one or more sulphur compounds, such as hydrogen sulphide. In order that the water-gas shift catalysts remain suitably sulphided, the sulphur content of the synthesis gas fed to the water-gas shift catalyst is desirably &gt;250 ppm. 
     If the synthesis gas does not contain enough steam for the water-gas shift process, steam may be added to the synthesis gas, for example by live steam addition or saturation or a combination of these. Steam may be added to the synthesis gas before or after heating in the second vessel. The steam to carbon monoxide ratio (i.e. molar ratio) of the synthesis gas mixture fed to the first water-gas shift catalyst should be ≦1.8 and preferably is in the range 0.2 to 1.8, more preferably 0.7 to 1.8. In some embodiments, it may be desirable to operate with a ratio in the range 0.95 to 1.8. 
     The water-gas shift catalyst used in the shift vessels may be any suitably stable and active sulphur-tolerant water-gas shift catalyst. The synthesis gas contains one or more sulphur compounds and so the water-gas shift catalyst should remain effective in the presence of these compounds. In particular so-called “sour shift” catalysts may be used, in which the active components are metal sulphides. Preferably the water-gas shift catalyst comprises a supported cobalt-molybdenum catalyst that forms molybdenum sulphide in-situ by reaction with hydrogen sulphide present in the synthesis gas stream. The Co content is preferably 2-8% wt and the Mo content preferably 5-20% wt. Alkali metal promoters may also be present at 1-10% wt. Suitable supports comprise one or more of alumina, magnesia, magnesium aluminate spinel and titania. The catalysts may be supplied in oxidic form, in which case they require a sulphiding step, or they may be supplied in a pre-sulphided form. Particularly preferred sour shift catalysts are supported cobalt-molybdate catalysts such as KATALCO™ K8-11 available from Johnson Matthey PLC, which comprises about 3% wt. CoO and about 10% wt. MoO 3  supported on a particulate support containing magnesia and alumina. 
     It is desirable to adjust the temperature of the synthesis gas so that the temperature within the first water-gas shift vessel is maintained within suitable operating conditions. For instance, after the synthesis gas is washed, thereby significantly cooling it, it may be advantageous to preheat the synthesis gas passing to the vessel. A suitable heat exchanger can be placed on the feed synthesis gas stream. According to the particular details of the process, suitable media for heat exchange with the inlet gas may be, for example, another gas stream at a different temperature, steam or water. Furthermore, using such a heat exchanger, with a bypass provided around it, gives the ability to control the inlet temperature to the catalyst bed, independently of variation in other parameters. 
     In the present invention, at least part of the temperature adjustment of the synthesis gas before it is fed to the first shift vessel includes heating it by passing the synthesis gas through heat exchange apparatus, such as a plurality of tubes, coils or plates, disposed within the second catalyst bed. The synthesis gas is at a lower temperature than the reacting pre-shifted gas stream and accordingly the synthesis gas acts as a cooling medium thereby removing heat from the second catalyst bed. A preferred temperature for the synthesis gas fed to the heat exchange apparatus within the second catalyst bed is in the range 150 to 250° C. The synthesis gas is heated as it passes through the heat exchange apparatus in the second vessel. The heated synthesis gas recovered from the heat exchange apparatus in the second vessel may be further heated or cooled to provide the desired inlet temperature for the first shift vessel. 
     The inlet temperature for the first bed of water-gas shift catalyst may be in the range 190 to 350° C., preferably 200 to 330° C. 
     If desired, the heated synthesis gas recovered from the heat exchange apparatus in the second vessel may be divided into first and second streams, with the first stream passed over the first bed of shift catalyst and the second stream by-passing the first bed of shift catalyst, thereby forming a catalyst by-pass stream. In addition or alternatively, it may be desirable, upstream of the catalyst of the second shift vessel, to divide the synthesis gas into first and second streams, with the first stream fed to the second shift vessel where it is heated, and the second stream by-passing the second shift vessel, thereby forming a vessel by-pass stream. The catalyst by-pass stream may if desired be combined with the vessel by-pass stream, thereby forming a combined by-pass stream. The combined by-pass stream is preferably ≦40% by volume of the total synthesis gas feed. 
     The by-pass stream may be fed to one or more of the pre-shifted gas stream, the shifted gas stream, or separately to downstream processes. Utilising a vessel by-pass around the second shift stage or a combined by-pass stream around both first and second stage shift vessels is useful when it is desired to precisely control the overall extent of CO conversion for the total synthesis gas feed, e.g. for making a shifted synthesis gas product of a specific R ratio, as for methanol synthesis. Especially useful is a use of a second vessel by-pass, because this also allows better control of the gas flow so that the temperature profile in the pre-shift vessel is unaffected by control of the extent of CO conversion. 
     If desired, a by-pass stream may be subjected to a carbonyl sulphide (COS) hydrolysis step by passing the stream over a COS hydrolysis catalyst, such as a particulate alumina or titania based catalyst, disposed in a suitable vessel. In this step, the COS in the by-pass stream is hydrolysed by steam to form H 2 S, which may be easier to remove in downstream processes. In such a COS hydrolysis step, essentially no water-gas shift reaction takes place. 
     The synthesis gas and steam mixture is passed at elevated temperature and pressure, preferably temperatures in the range 190 to 420° C. more preferably 200 to 400° C., and pressure up to about 85 bar abs, over the first bed of water-gas shift catalyst. The flow-rate of synthesis gas containing steam may be such that the gas hourly space velocity (GHSV) through the first bed of sulphur-tolerant water-gas shift catalyst may be ≧6000 hour −1 , but is preferably ≧12,500 hour −1 , more preferably ≧15,500 hour −1 , most preferably ≧17,500 hour −1 , and especially ≧20,000 hour −1 . 
     The water-gas shift reaction occurs, consuming carbon monoxide and steam and forming carbon dioxide and hydrogen. Under the conditions, only a portion of the carbon monoxide and steam are consumed and so the pre-shifted gas stream comprises hydrogen, carbon monoxide, carbon dioxide and steam that may be further reacted in the one or more further stages of water-gas shift. It is desirable to convert only 10 to 40% (by moles) of the carbon monoxide present in the synthesis gas to carbon dioxide over the first bed of water-gas shift catalyst disposed in the first shift vessel. The first shift vessel may thus be termed a pre-shift vessel. 
     The pre-shift vessel operates in heat exchange with boiling water, typically boiling water under pressure thereby generating steam at a suitable pressure for use in the water-gas shift or downstream or upstream processes. The water may be fed to tubes, coils or plates disposed within the first catalyst bed. An axial or radial flow vessel may be used. In a preferred embodiment, the pre-shift vessel is a radial flow steam raising vessel. Such vessels typically comprise a plurality of vertical tubes and are known for use in advanced methanol synthesis processes, for example the second reactor (11) described in FIG. 1 of US2004/0162357 A1. The boiling water is at a lower temperature than the reacting synthesis gas stream and accordingly the water passing though the tubes acts as a cooling medium thereby removing heat from the first catalyst bed. 
     It may be advantageous to have heat exchange apparatus, e.g. tubes, for cooling the reacting gases disposed only in part of the first catalyst bed, for instance in one embodiment the reacting gases flow radially outwards from a central distributor to an outer annular collector through the catalyst bed. The catalyst bed is subdivided into two zones; an inner annular zone with no cooling tubes where the shift reaction takes place adiabatically, and an outer annular zone with cooling tubes where heat is removed by boiling water in the tubes. Such radial flow reactors are described, for example, in FIG. 3 of the aforesaid US2004/0162357 A1 
     In addition to the cooling effect of the boiling water in the pre-shift vessel, some further cooling of the pre-shifted gas may be desirable before passing the pre-shifted gas stream to the second stage of water-gas shift. 
     At least a portion of the pre-shifted synthesis gas from the reactor containing the first bed of sulphur-tolerant water-gas shift catalyst is fed to the second water-gas shift stage. In the second stage the pre-shifted gas stream is further reacted over a second bed of sulphur-tolerant water-gas shift catalyst. The second bed of catalyst may be the same or different to the first bed but is preferably also a supported cobalt-molybdenum water-gas shift catalyst. The bed is cooled by heat exchange with a gas stream comprising the synthesis gas fed to the first shift vessel. The gas stream comprising the synthesis gas may be passed through tubes, coils or plates. Axial or radial flow vessels may be used. Axial flow vessels comprising a plurality of vertical tubes though which the gas stream comprising synthesis gas flows are preferred. Where tubes are used, the synthesis gas may be passed through the tubes in the second shift vessel in a direction that is counter-current or co-current to the flow of the pre-shifted gas stream through the vessel. Axial, co-current flow is preferred. Where tubes are used, heat transfer enhancement devices may be used inside the tubes, for example, core-rods or structures that increase the turbulence of the flowing gas within the tubes. 
     If desired, additional steam may be added to the pre-shifted gas stream before the second stage of water-gas shift. 
     The second shift vessel is preferably operated at a temperature in the range 250 to 420° C., more preferably 340-400° C. The gas hourly space velocity in the second bed of sulphur-tolerant water-gas shift catalyst may be ≧5000 h −1 , preferably ≧6000 h −1  and is more preferably in the range 6000 to 12000 h −1 , most preferably 6000 to 10000 h −1 . 
     The resulting shifted gas stream from the second water-gas shift vessel may be used in downstream processes for the production of methanol, dimethylether (DME), Fischer-Tropsch (FT) liquids or synthetic natural gas (SNG). Where a higher degree of water-gas shift is required, for example when making hydrogen or a low carbon content fuel for combustion in a gas turbine, additional water-gas shift steps may be performed. In such cases, one or more further water-gas shift stages, which may be uncooled or cooled and operated in series or parallel, may be used. Preferably one or two further stages of adiabatic water-gas shift are used in series, with optional cooling before each stage, to maximise CO conversion in the shifted gas stream. 
     The present invention has a number of distinct advantages over the prior art processes. Heat generation in each of the first two shift stages is less and therefore it is easier to control the peak temperature in each bed, and thus minimise the formation of by-products. As a result of using the cooled second shift vessel, a more optimal water-gas shift reaction profile is followed without the use of excessive heat transfer area. This permits a reduction in the total volume of catalyst or a greater CO conversion achieved with the same catalyst volume. Vessel and/or tube inlet temperatures can be varied over a wide range in order to accommodate varying catalyst reaction activity, without the risk of high peak temperatures. 
     In the present invention, the feed synthesis gas is preheated, while cooling the catalyst in the second shift reactor. This is in contrast to a process scheme with sequential adiabatic reactors, where the whole feed gas stream must be heated in a separate heat exchanger to the inlet temperature to the first shift reactor. Hence, the use of the water-cooled reactor and gas-cooled reactor in this invention reduces the equipment count by combining the preheating duty within the shift reactors. 
     The process of the present invention does not rely on having a very low H2O/CO ratio in the feed gas to limit the theoretical equilibrium CO conversion and associated temperature rise. It is also applicable to a wide range of gasifier types, including those with a radiant cooling and quench section, which therefore have a higher, involuntary water content and are unsuitable for utilising the ‘steam deficient’ shift methodology set out in the aforesaid WO2010/106148. 
     In order to generate a hydrogen-rich synthesis gas the process preferably further comprises the steps of:
         (i) cooling a shifted gas stream or a mixture of the shifted gas stream and a bypass stream, to below the dew point to condense water,   (ii) separating the resulting condensate therefrom to form a dry gas stream,   (iii) feeding the dry gas stream to a gas-washing unit operating by means of counter-current solvent flow, to produce a product synthesis gas and   (iv) collecting the product synthesis gas from the washing unit.       

     The shifted gas stream may be subjected to these steps alone to form a dry shifted gas stream, or as a mixture with a bypass stream. Alternatively, a bypass stream may be separately subjected to these steps to form a dry un-shifted by-pass stream, which is fed to the same or a separate gas washing unit. Where the dry un-shifted gas is fed to the same gas washing unit, preferably this un-shifted stream is fed to the gas washing unit such that the solvent flowing through said unit contacts first with the dry un-shifted synthesis gas and then the dry shifted gas stream. 
     The cooling step may be performed by heat exchange, e.g. with cold water, to cool the gases to below the dew point at which steam condenses. The resulting condensates, which comprise water and some contaminants, are separated. 
     The gases may be further cooled and dried, e.g. by means of chilled solvent, and then fed to a gas-washing unit operating by means of counter-current solvent flow. In the gas-washing unit, also known as an acid-gas removal (AGR) unit, a solvent suitable for the dissolution/absorption of carbon dioxide flows counter-current to gas flowing through the unit and dissolves/absorbs carbon dioxide present in the gas stream. A small quantity of other gas components in the gas stream, particularly carbon monoxide, will also be co-absorbed. Contaminants present in the gas stream that may poison downstream catalysts, e.g. sulphur compounds such as H 2 S &amp; COS, may also be removed to differing extents. Using AGR, CO 2  levels may be reduced to below 5 mole %, on a dry gas basis. 
     Suitable solvents for absorbing CO 2  are physical solvents, including methanol, other alcohol or glycol products, such as glycols or polyethylene glycol ethers, and propylene carbonate, and chemical solvents, such as activated alkanolamines. Methanol is the preferred solvent where a downstream catalyst is being used. Methanol may be used at temperatures in the range −30 to −70° C. and at elevated pressures up to about 75 bar abs. 
     A gas-washing unit may comprise, for example, a column having a solvent inlet near the top and a solvent outlet near the bottom, down which a solvent suitable for the dissolution/absorption of carbon dioxide flows over one or more perforate trays or packing. The gases passing up through the column contact the solvent and carbon dioxide is dissolved/absorbed. The gases may leave the column near the top via a synthesis gas outlet. The synthesis gas is cold and may be used to cool the feed gases to the gas-washing unit using suitable heat exchange means such as a spiral wound heat exchanger. In one embodiment, the dry by-pass synthesis gas mixture and dry shifted gas stream are fed separately to the unit, with the separate feeds arranged such that that the solvent contacts first with the dry by-pass synthesis gas mixture and then the dry shifted gas stream. This is in contrast to previous processes, where a synthesis gas mixture is fed to a gas-washing unit so that the solvent contacts the gas mixture in one stage. We have found that by separately feeding the two different gas streams to the unit such that that the solvent contacts first with the dry gas mixture and then the dry shifted gas stream, the efficiency of the process is improved, which offers the potential for reduced CO co-absorption and an increased potential for methanol or liquid hydrocarbon production from a given quantity of synthesis gas. 
     The process is desirably operated such that the synthesis gas collected from the gas-washing unit has an R ratio suited to the downstream use, such as methanol or DME production, FT hydrocarbon production or SNG production. For the production of methanol or hydrocarbons, the desired stoichiometry ratio, R, of the product synthesis gas is preferably in the range 1.4 to 2.5. For generating synthetic natural gas (SNG) the range is preferably in the range 2.8 to 3.3. Alternatively, the sour shift reactor, additional downstream sour shift stage or stages, and gas-washing stage may be operated such that the synthesis gas collected from the gas-washing unit is hydrogen rich, with minimal CO and CO 2  content, where this is desirable. Such hydrogen-rich gas streams may be used in ammonia synthesis, for hydrogenation purposes, for chemicals synthesis or power generation by combustion in a gas turbine with or without additional hydrocarbon fuels. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
       The invention is further illustrated by reference to the accompanying drawings in which; 
         FIG. 1  is a depiction of one embodiment according to the present invention suitable for feed from a gasifier having a radiant cooling and quench section producing a steam-containing synthesis gas with a steam:CO ratio in the range 1.3-1.4 and operating with a radial flow steam raising pre-shift vessel and co-current flow through tubes disposed within the second vessel, and 
         FIG. 2  is a depiction of a further embodiment suitable for feed from gasifier producing a steam-containing synthesis gas with a steam:CO ratio in the range 0.20-0.30 and operating with a radial flow steam raising pre-shift vessel and co-current flow through tubes disposed within the second vessel. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In  FIG. 1 , a synthesis gas  110  containing one or more sulphur compounds and steam with a steam:CO ratio in the range 1.3-1.4 is fed to a distributor  112  disposed within a second sour shift vessel  114 . The distributor is connected to a plurality of tubes  116  that pass vertically through a bed of particulate Co/Mo sour shift catalyst  118 . The synthesis gas is able to pass from the distributor vertically through the tubes where it is heated thereby cooling the reactant gases in the catalyst bed  118 . The tubes are connected to a collector  120  at the other end of the tubes that collects heated synthesis gas. 
     The heated synthesis gas is fed via line  122  to heat exchanger  124  where its temperature is adjusted to the desired inlet temperature. The temperature adjusted synthesis gas is fed from exchanger  124  via line  126  to a radial-flow pre-shift vessel  128 . The radial flow pre-shift vessel comprises a central distribution zone  130 , a first fixed bed of particulate sulphided Co/Mo sulphur-tolerant water-gas shift catalyst  132  disposed around the central distribution zone  130 , and a peripheral collection zone  134  between the outside of the catalyst bed  132  and the internal wall of the vessel  128 . Catalyst restraining means are used to define the central and peripheral zones ( 130 ,  134 ). A plurality of vertical tubes  136  pass through the catalyst bed  132 . The tubes  136  are fed with boiling water via line  138  from steam drum  140  to which the heated boiling water is returned via line  142 . The synthesis gas flows from the central distribution zone  130  radially through the catalyst bed  132  to the peripheral collection zone  134 . The bed of catalyst  132  is cooled in heat exchange with the boiling water  138  passing through the tubes  136 . The synthesis gas containing steam reacts over the catalyst to form carbon dioxide and hydrogen. 
     The pre-shifted gas stream is recovered from the vessel  128  via line  144  and passed through heat exchanger  146  where it is cooled. 
     The cooled pre-shifted gas stream is then fed via line  148  to the inlet of the second water-gas shift vessel  114  containing the second fixed bed of particulate sulphided Co/Mo sulphur-tolerant water-gas shift catalyst  118 . If desired, additional steam may be added to the pre-shifted gas mixture  148  upstream of vessel  114  (not shown). The pre-shifted gas mixture is passed over the water-gas shift catalyst  118  further increasing the hydrogen content of the synthesis gas. The bed of catalyst  118  is cooled in heat exchange with the synthesis gas  110  passing through the tubes  116  in a direction co-current to the flow of pre-shifted gas stream through the vessel  114 . A hydrogen-enriched shifted gas stream is recovered from the outlet of the second vessel  114  via line  150 . 
     The shifted gas stream is cooled in heat exchangers  152  and  154  and the cooled shifted synthesis gas stream fed via line  156  to a third water-gas shift vessel  158  containing a third particulate bed of sulphur-tolerant Co/Mo water-gas shift catalyst  160 . The shifted gas stream containing steam further reacts over the catalyst  160  to form carbon dioxide and hydrogen. The third vessel is operated adiabatically without cooling and the exothermic reactions heat the resulting shifted gas stream. The shifted gas stream is recovered from the third water-gas shift vessel  158  and passed via line  162  to heat exchanger  164  where it is cooled. The cooled shifted gas stream is then fed via line  166  to a fourth water-gas shift vessel  168  containing a fourth particulate bed of sulphur-tolerant Co/Mo water-gas shift catalyst  170 . The shifted gas stream containing steam further reacts over the catalyst  170  to form carbon dioxide and hydrogen. The fourth vessel is operated adiabatically without cooling and the exothermic reactions heat the resulting shifted gas stream. 
     The shifted gas stream is recovered from the fourth water-gas shift vessel  168  via line  172  and passed through heat exchanger  174 , and optionally further heat exchangers (not shown) to cool the gas below the dew point and so condense the remaining steam. The cooled shifted stream is fed via line  176  to separator  178  in which the condensate is separated from the hydrogen rich shifted gas stream. The dry hydrogen-rich shifted gas stream is recovered from separator  178  via line  180  and the condensate via line  182 . The condensate may be used to generate steam for use in the process. The dry hydrogen-rich shifted gas stream  180  may be used in downstream processing or sent to a gas washing unit (not shown) to recover CO2 and H2S and generate a hydrogen rich gas stream product. The carbon dioxide recovered from such processes may be used in carbon-capture and storage (CCS) processes or in enhanced oil recovery (EOR) processes. 
     In an alternative embodiment, by utilising the collector  120  as the distributor and vice-versa, the synthesis gas  110  may be fed through the tubes  116  in a direction counter-current with the flow of pre-shifted gas through the second water-gas shift vessel  114 . 
     In  FIG. 2  the process is modified by steam addition to the synthesis gas before and after heating in the second shift vessel. Accordingly, a synthesis gas  210  containing one or more sulphur compounds with a steam:CO ratio in the range 0.20-0.30 is heated in heat exchanger  212  and mixed with steam from line  214 . The steam in line  214  is provided by a boiler-feed water supply  216  heated by heat exchanger  218 . Additional steam is supplied to the synthesis gas steam mixture via line  220 . 
     The combined synthesis gas and steam mixture is fed via line  222  to a distributor  224  disposed within a second sour shift vessel  226 . The distributor is connected to a plurality of tubes  228  that pass vertically through a bed of particulate Co/Mo sour shift catalyst  230 . The synthesis gas steam mixture is able to pass from the distributor vertically through the tubes where it is heated thereby cooling the reactant gases in the catalyst bed  230 . The tubes are connected to a collector  232  at the other end of the tubes that collects heated synthesis gas. 
     The heated synthesis is recovered from the vessel  226  via line  234  and mixed with a further amount of steam from line  236 . The synthesis gas steam mixture is passed to heat exchanger  238  where its temperature is adjusted to the desired inlet temperature. The temperature adjusted synthesis gas is fed from exchanger  238  via line  240  to a radial-flow pre-shift vessel  242 . The radial flow pre-shift vessel comprises a central distribution zone  244 , a first fixed bed of particulate sulphided Co/Mo sulphur-tolerant water-gas shift catalyst  246  disposed around the central distribution zone  244 , and a peripheral collection zone  248  between the outside of the catalyst bed  246  and the internal wall of the vessel  242 . Catalyst restraining means are used to define the central and peripheral zones ( 244 ,  248 ). A plurality of vertical tubes  250  pass through the catalyst bed  246 . The tubes  250  are fed with boiling water via line  252  from steam drum  254  to which the heated boiling water is returned via line  256 . The synthesis gas flows from the central distribution zone  244  radially through the catalyst bed  246  to the peripheral collection zone  248 . The bed of catalyst  246  is cooled in heat exchange with the boiling water  252  passing through the tubes  250 . The synthesis gas containing steam reacts over the catalyst to form carbon dioxide and hydrogen. 
     The pre-shifted gas stream is recovered from the vessel  242  via line  258  and passed through heat exchanger  260  where it is cooled. 
     The cooled pre-shifted gas stream is then fed via line  262  to the inlet of the second water-gas shift vessel  226  containing the second fixed bed of particulate sulphided Co/Mo sulphur-tolerant water-gas shift catalyst  230 . The pre-shifted gas mixture is passed over the water-gas shift catalyst  230  further increasing the hydrogen content of the synthesis gas. The bed of catalyst  230  is cooled in heat exchange with the synthesis gas/steam mixture  222  passing through the tubes  228  in a direction co-current to the flow of pre-shifted gas stream through the vessel  226 . A hydrogen-enriched shifted gas stream is recovered from the outlet of the second vessel  226  via line  264 . 
     The shifted gas stream is cooled in heat exchangers  266  and  268  and the cooled shifted synthesis gas stream fed via line  270  to a third water-gas shift vessel  272  containing a third particulate bed of sulphur-tolerant Co/Mo water-gas shift catalyst  274 . The shifted gas stream containing steam further reacts over the catalyst  274  to form carbon dioxide and hydrogen. The third vessel is operated adiabatically without cooling and the exothermic reactions heat the resulting shifted gas stream. The shifted gas stream is recovered from the third water-gas shift vessel  272  and passed via line  276  to heat exchanger  278  where it is cooled. The cooled shifted gas stream is then fed via line  280  to a fourth water-gas shift vessel  282  containing a fourth particulate bed of sulphur-tolerant Co/Mo water-gas shift catalyst  284 . The shifted gas stream containing steam further reacts over the catalyst  284  to form carbon dioxide and hydrogen. The fourth vessel is operated adiabatically without cooling and the exothermic reactions heat the resulting shifted gas stream. 
     The shifted gas stream is recovered from the fourth water-gas shift vessel  282  via line  286  and passed through heat exchanger  288 , and optionally further heat exchangers (not shown) to cool the gas below the dew point and so condense the remaining steam. The cooled shifted stream is fed via line  290  to separator  292  in which the condensate is separated from the hydrogen rich shifted gas stream. The dry hydrogen-rich shifted gas stream is recovered from separator  292  via line  294  and the condensate via line  296 . The condensate may be used to generate steam for use in the process. The dry hydrogen-rich shifted gas stream  294  may be used in downstream processing or sent to a gas washing unit (not shown) to recover CO2 and H2S and generate a hydrogen rich gas stream product. The carbon dioxide recovered from such processes may be used in carbon-capture and storage (CCS) processes or in enhanced oil recovery (EOR) processes. 
     In an alternative embodiment, by utilising the collector  232  as the distributor and vice-versa, the synthesis gas/steam mixture  222  may be fed through the tubes  228  in a direction counter-current with the flow of pre-shifted gas through the second water-gas shift vessel  226 . 
     The invention is further illustrated by reference to the following calculated Examples. 
     Example 1 (Comparative) 
     The calculated mass balance below is for the use of three adiabatic sour shift reactors in series to carry out a high degree of shift (&gt;90%) on a feed gas with a steam:CO ratio of 1.35 and an R ratio of 0.37. The results were as follows; 
     
       
         
               
               
               
               
             
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                   
               
               
                   
                 Reactor 1 
                 Reactor 2 
                 Reactor 3 
               
             
          
           
               
                   
                 In 
                 Out 
                 In 
                 Out 
                 In 
                 Out 
               
               
                   
               
             
          
           
               
                 Mol fraction 
                   
                   
                   
                   
                   
                   
               
               
                 H2O 
                 0.38685 
                 0.19883 
                 0.19883 
                 0.14571 
                 0.14571 
                 0.12987 
               
               
                 CO 
                 0.28383 
                 0.09503 
                 0.09503 
                 0.04190 
                 0.04190 
                 0.02606 
               
               
                 CO2 
                 0.08589 
                 0.27468 
                 0.27468 
                 0.32784 
                 0.32784 
                 0.34369 
               
               
                 COS 
                 0.00015 
                 0.00006 
                 0.00006 
                 0.00002 
                 0.00002 
                 0.00001 
               
               
                 H2S 
                 0.00545 
                 0.00555 
                 0.00555 
                 0.00559 
                 0.00559 
                 0.00559 
               
               
                 Argon 
                 0.00583 
                 0.00583 
                 0.00583 
                 0.00583 
                 0.00583 
                 0.00583 
               
               
                 N2 
                 0.00665 
                 0.00665 
                 0.00665 
                 0.00665 
                 0.00665 
                 0.00665 
               
               
                 NH3 
                 0.00127 
                 0.00127 
                 0.00127 
                 0.00127 
                 0.00127 
                 0.00127 
               
               
                 CH4 
                 0.00075 
                 0.00114 
                 0.00114 
                 0.00116 
                 0.00116 
                 0.00116 
               
               
                 H2 
                 0.22333 
                 0.41095 
                 0.41095 
                 0.46403 
                 0.46403 
                 0.47986 
               
               
                 Flow kgmols/hr 
                 31849.7 
                 31825.0 
                 31825.0 
                 31823.0 
                 31823.0 
                 31823.0 
               
               
                 T deg C. 
                 238 
                 439 
                 220 
                 277 
                 220 
                 237 
               
               
                 P bar abs. 
                 63.8 
                 63 
                 61.8 
                 61.3 
                 60.7 
                 60.2 
               
               
                   
               
             
          
         
       
     
     There is a large volume of catalyst in the first reactor, with a high exit temperature (about 440° C.), giving the potential for undesirable side reactions, including methanation. This situation is exacerbated when the catalyst is new and more active. 
     Example 2 
     This is an example of the invention according to  FIG. 1 , based on the same feed flow and composition and shift conversion duty as Example 1. The GHSV in the water-cooled pre-shift vessel ( 128 ) is 161000 h −1  and the GHSV in the gas-cooled converter ( 114 ) is 7900 h −1 . 
     This process overcomes the problem of the high temperature seen in Example 1. The peak temperatures in shift stages 1 and 2 are about 370° C. and 375° C. respectively. Due to the fact that the temperature profile in stage 2 follows better that which is optimal for high shift reaction rate, the combined catalyst volume for reactors 1 and 2 in this case is actually about 14% less than the volume of catalyst in reactor 1 in Example 1, whereas the CO conversion is actually slightly higher. The results were as follows: 
     
       
         
               
               
               
             
               
               
             
               
               
               
               
               
               
               
             
               
               
               
             
               
               
             
               
               
               
               
               
               
             
           
               
                   
               
             
             
               
                   
                 Reactor 1 
                 Reactor 2 
               
             
          
           
               
                   
                 Stream No 
               
             
          
           
               
                   
                 126 
                 144 
                 148 
                 150 
                 110 
                 122 
               
               
                   
                 In 
                 Out 
                 In Cat 
                 Out Cat 
                 In tubes 
                 Out tubes 
               
               
                   
               
               
                 Mol fraction 
                   
                   
                   
                   
                   
                   
               
               
                 H2O 
                 0.38685 
                 0.30550 
                 0.30550 
                 0.18967 
                 0.38685 
                 0.38685 
               
               
                 CO 
                 0.28383 
                 0.20255 
                 0.20255 
                 0.08640 
                 0.28383 
                 0.28383 
               
               
                 CO2 
                 0.08589 
                 0.16730 
                 0.16730 
                 0.28340 
                 0.08589 
                 0.08589 
               
               
                 COS 
                 0.00015 
                 0.00001 
                 0.00001 
                 0.00004 
                 0.00015 
                 0.00015 
               
               
                 H2S 
                 0.00545 
                 0.00559 
                 0.00559 
                 0.00557 
                 0.00545 
                 0.00545 
               
               
                 Argon 
                 0.00583 
                 0.00583 
                 0.00583 
                 0.00583 
                 0.00583 
                 0.00583 
               
               
                 N2 
                 0.00665 
                 0.00665 
                 0.00665 
                 0.00665 
                 0.00665 
                 0.00665 
               
               
                 NH3 
                 0.00127 
                 0.00127 
                 0.00127 
                 0.00127 
                 0.00127 
                 0.00127 
               
               
                 CH4 
                 0.00075 
                 0.00077 
                 0.00077 
                 0.00091 
                 0.00075 
                 0.00075 
               
               
                 H2 
                 0.22333 
                 0.30453 
                 0.30453 
                 0.42027 
                 0.22333 
                 0.22333 
               
               
                 Flow kgmols/hr 
                 31849.7 
                 31848.0 
                 31848.0 
                 31839.0 
                 31849.7 
                 31849.7 
               
               
                 T deg C. 
                 321 
                 368 
                 350 
                 370.5 
                 218 
                 321 
               
               
                 P bar abs. 
                 63.8 
                 62.8 
                 62.5 
                 62 
                 63.9 
                 63.8 
               
             
          
           
               
                   
                 Reactor 3 
                 Reactor 4 
               
             
          
           
               
                   
                 Stream No 
               
             
          
           
               
                   
                   
                 156 
                 162 
                 166 
                 172 
               
               
                   
                   
                 In 
                 Out 
                 In 
                 Out 
               
               
                   
               
               
                   
                 Mol fraction 
                   
                   
                   
                   
               
               
                   
                 H2O 
                 0.18967 
                 0.14516 
                 0.14516 
                 0.12942 
               
               
                   
                 CO 
                 0.08640 
                 0.04187 
                 0.04187 
                 0.02612 
               
               
                   
                 CO2 
                 0.28340 
                 0.32794 
                 0.32794 
                 0.34369 
               
               
                   
                 COS 
                 0.00004 
                 0.00002 
                 0.00002 
                 0.00001 
               
               
                   
                 H2S 
                 0.00557 
                 0.00558 
                 0.00558 
                 0.00559 
               
               
                   
                 Argon 
                 0.00583 
                 0.00583 
                 0.00583 
                 0.00583 
               
               
                   
                 N2 
                 0.00665 
                 0.00665 
                 0.00665 
                 0.00665 
               
               
                   
                 NH3 
                 0.00127 
                 0.00127 
                 0.00127 
                 0.00127 
               
               
                   
                 CH4 
                 0.00091 
                 0.00093 
                 0.00093 
                 0.00093 
               
               
                   
                 H2 
                 0.42027 
                 0.46475 
                 0.46475 
                 0.48049 
               
               
                   
                 Flow kgmols/hr 
                 31839.0 
                 31838.0 
                 31838.0 
                 31838.0 
               
               
                   
                 T deg C. 
                 230 
                 278 
                 220 
                 237 
               
               
                   
                 P bar abs. 
                 61.4 
                 60.4 
                 60.1 
                 59.1 
               
               
                   
               
             
          
         
       
     
     Example 3 
     This is an example of the invention according to  FIG. 2 . The syngas feed (steam:CO=0.21 and R=0.45) is preheated and some steam is added before it is fed to the tubes of the second shift reactor, where it flows co-current to the reactant gas flow, thereby cooling it. The heated gas from the tubes is then cooled and mixed with further steam to give a steam:CO ratio of approximately 1.1 and then passes to the first steam raising pre-shift reactor. The pre-shifted gas is cooled before passing to the second (cooled) reactor. The shifted gas from the second shift stage flows to reactors 3 and 4 with cooling before each stage, such that a CO conversion of ˜92% overall is achieved. The GHSV in the water-cooled pre-shift vessel ( 242 ) is 58000 h −1  and the GHSV in the gas-cooled converter ( 226 ) is 5600 h −1 . The results were as follows; 
     
       
         
               
               
               
               
             
               
               
             
               
               
               
               
               
               
               
               
             
               
               
               
             
               
               
             
               
               
               
               
               
               
             
           
               
                   
               
             
             
               
                   
                 Syngas feed 
                 Reactor 1 
                 Reactor 2 
               
             
          
           
               
                   
                 Stream No 
               
             
          
           
               
                   
                   
                 240 
                 258 
                 262 
                 264 
                 222 
                 234 
               
               
                   
                 210 
                 In 
                 Out 
                 In Cat 
                 Out Cat 
                 In tubes 
                 Out tubes 
               
               
                   
               
               
                 Mol fraction 
                   
                   
                   
                   
                   
                   
                   
               
               
                 H2O 
                 0.11546 
                 0.39488 
                 0.29524 
                 0.29524 
                 0.15146 
                 0.25311 
                 0.25311 
               
               
                 CO 
                 0.52476 
                 0.35899 
                 0.25975 
                 0.25975 
                 0.11584 
                 0.44310 
                 0.44310 
               
               
                 CO2 
                 0.01937 
                 0.01325 
                 0.11292 
                 0.11292 
                 0.25678 
                 0.01636 
                 0.01636 
               
               
                 COS 
                 0.00064 
                 0.00044 
                 0.00001 
                 0.00001 
                 0.00004 
                 0.00054 
                 0.00054 
               
               
                 H2S 
                 0.00744 
                 0.00509 
                 0.00552 
                 0.00552 
                 0.00548 
                 0.00628 
                 0.00628 
               
               
                 Argon 
                 0.00891 
                 0.00609 
                 0.00609 
                 0.00609 
                 0.00609 
                 0.00752 
                 0.00752 
               
               
                 N2 
                 0.05371 
                 0.03674 
                 0.03674 
                 0.03674 
                 0.03674 
                 0.04535 
                 0.04535 
               
               
                 NH3 
                 0.00303 
                 0.00207 
                 0.00207 
                 0.00207 
                 0.00207 
                 0.00256 
                 0.00256 
               
               
                 CH4 
                 0.00037 
                 0.00025 
                 0.00026 
                 0.00026 
                 0.00031 
                 0.00031 
                 0.00031 
               
               
                 H2 
                 0.26633 
                 0.18219 
                 0.28140 
                 0.28140 
                 0.42517 
                 0.22488 
                 0.22488 
               
               
                 Flow kgmols/hr 
                 19880 
                 29060.0 
                 29060.0 
                 29060.0 
                 29057.0 
                 23544.0 
                 23544.0 
               
               
                 T deg C. 
                 145 
                 335 
                 385 
                 340 
                 378 
                 175 
                 338 
               
               
                 P bara 
                 38.9 
                 38.6 
                 37.8 
                 37.5 
                 36.5 
                 38.9 
                 38.8 
               
             
          
           
               
                   
                 Reactor 3 
                 Reactor 4 
               
             
          
           
               
                   
                 Stream No 
               
             
          
           
               
                   
                   
                 270 
                 276 
                 280 
                 286 
               
               
                   
                   
                 In 
                 Out 
                 In 
                 Out 
               
               
                   
               
               
                   
                 Mol fraction 
                   
                   
                   
                   
               
               
                   
                 H2O 
                 0.15146 
                 0.08185 
                 0.08185 
                 0.06536 
               
               
                   
                 CO 
                 0.11584 
                 0.04624 
                 0.04624 
                 0.02977 
               
               
                   
                 CO2 
                 0.25678 
                 0.32639 
                 0.32639 
                 0.34288 
               
               
                   
                 COS 
                 0.00004 
                 0.00003 
                 0.00003 
                 0.00002 
               
               
                   
                 H2S 
                 0.00548 
                 0.00549 
                 0.00549 
                 0.00551 
               
               
                   
                 Argon 
                 0.00609 
                 0.00609 
                 0.00609 
                 0.00609 
               
               
                   
                 N2 
                 0.03674 
                 0.03674 
                 0.03674 
                 0.03674 
               
               
                   
                 NH3 
                 0.00207 
                 0.00207 
                 0.00207 
                 0.00207 
               
               
                   
                 CH4 
                 0.00031 
                 0.00031 
                 0.00031 
                 0.00031 
               
               
                   
                 H2 
                 0.42517 
                 0.49477 
                 0.49477 
                 0.51124 
               
               
                   
                 Flow kgmols/hr 
                 29057.0 
                 29057.0 
                 29057.0 
                 29057.0 
               
               
                   
                 T deg C. 
                 190 
                 269.5 
                 190 
                 209 
               
               
                   
                 P bara 
                 35.9 
                 34.9 
                 34.6 
                 33.7 
               
               
                   
               
             
          
         
       
     
     The peak temperature in each of the first two reactors is about 385° C. and 380° C. respectively.