WO2011086345A1

WO2011086345A1 - Separation of gases

Info

Publication number: WO2011086345A1
Application number: PCT/GB2011/000024
Authority: WO
Inventors: Richard James Beavis; Jonathan Alec Forsyth
Original assignee: Bp Alternative Energy International Limited
Priority date: 2010-01-12
Filing date: 2011-01-10
Publication date: 2011-07-21

Abstract

A process is described for decarbonising a gas stream comprising carbon monoxide. The process comprises: (a) reacting the gas stream comprising carbon monoxide in the presence of at least one Water Gas Shift reaction catalyst to generate a shifted gas comprising carbon dioxide and hydrogen having a pressure of at least 50 bar; (b) cooling the shifted gas using at least one heat exchanger such that carbon dioxide condenses from the shifted gas and a two-phase gas-liquid mixture is formed; (c) separating the two phase mixture into separate liquid carbon dioxide and gaseous hydrogen-rich fractions and (d) warming and expanding the hydrogen-rich fraction using at least one heat exchanger to exchange heat with the shifted gas and a plurality of turbo-expanders capable of progressively recovering energy from the hydrogen rich fraction as mechanical work. Examples describe an integrated process for shifting raw synthesis gas stream and cryogenically separating the shifted product into component hydrogen rich and a liquid carbon dioxide streams. The process described is especially suitable for treating raw synthesis gas produced by a gasifier, steam methane reformer or autothermal reformer where the object is to recover a carbon-free hydrogen rich stream suitable for combustion in a hydrogen power station. The carbon dioxide recovered in liquid form can be conveniently pipelined offsite for underground storage in appropriate geological formations (for example depleted oil wells). The process can suitably form part of for example an Integrated Gasification Combined Cycle (IGCC).

Description

SEPARATION OF GASES

The present invention relates to a process for separating carbon dioxide a mixture of carbon dioxide and relatively non-condensable gases, for example separating carbon dioxide from hydrogen or from a mixture of hydrogen and other non-condensable gases. In a particular example described herein, the invention relates to an integrated process in which synthesis gas derived from for example the reforming of natural gas or the gasification of coal, petroleum coke or biomass is first shifted to increase its hydrogen and carbon dioxide content and then treated cryogenically to remove liquid carbon dioxide.

For environmental reasons it is becoming increasing desirable to generate power from carbonaceous feedstocks whilst minimising the emission of carbon dioxide into the atmosphere. Broadly speaking there are three approaches to achieving this outcome. In so called 'post-combustion' technologies the carbonaceous fuel is burnt in air and the carbon dioxide so generated removed by scrubbing the flue gases before they are vented.

Alternatively in 'oxyfuel' technologies the carbonaceous fuel is burnt in oxygen and the combustion products are continuously recycled to the burners after removal of water. Meanwhile a side-stream of relatively pure carbon dioxide is removed from the recycle and taken away for storage.

The final approach and the one with which the process of examples of the present invention are primarily concerned comprises the so-called 'pre-combustion' technologies. In these technologies the carbonaceous fuel is either gasified or reformed at high temperature in the presence of air or oxygen to produce raw synthesis gas (a mixture principally comprising the combustion products carbon monoxide, carbon dioxide, hydrogen, and steam together with the un-reacted components of the air (principally nitrogen)) and if the carbonaceous fuel is 'sour' impurities such as hydrogen sulphide, carbonyl sulphide and volatile lower mercaptans. The raw synthesis gas is then treated with steam in a 'shift' unit which catalytically converts the carbon monoxide and water into carbon dioxide and hydrogen by means of the 'Water Gas Shift' reaction. Typically this reaction is effected at a temperature in the range 150 to 500°C and at a pressure of up to 65 bar to produce a 'shifted' product with reduced content of or free of carbon monoxide and consisting essentially of carbon dioxide, steam, hydrogen and nitrogen which can then be first cooled and dried to remove water and then treated to remove carbon dioxide leaving hydrogen and nitrogen which can be burnt more cleanly to generate power.

A number of approaches to separating carbon dioxide from the shifted product have been proposed including inter alia the use of membranes which are permeable to hydrogen and optionally nitrogen, membranes which are permeable to carbon dioxide and solvent extraction processes. US Patent 3614872 describes a superatmospheric autorefrigeration process for separating hydrogen and carbon dioxide. In this process a gaseous feedstream at a pressure of 40 to 250 atmospheres is cooled so that 30 to 95% of the carbon dioxide is condensed by non-contact counterflow heat exchange with refrigerants and then separated from the uncondensed gases. The separated streams are themselves then separately employed as refrigerants to cool fractions of the feedstream. In the case of the

uncondensed gas stream it is taught that further cooling capacity may be generated by expansion at constant enthalpy across a single valve or by expansion at constant entropy so that the gas is made to drive a single expansion engine or the rotor of a single turbo-electric generator.

International Patent Application No. WO2010/012981 describes an improvement to such a process. The process described comprises in general terms first compressing and cooling the dried shifted product to a pressure and temperature at which carbon dioxide liquefies and thereafter separating the liquid carbon dioxide so generated from the other, relatively non-condensable, gases. Thereafter the separated components are returned to a temperature and pressure suitable for further use by a plurality of heat exchangers and turbo expanders integrated amongst themselves and with those used to cool the incoming shifted mixture so that the total energy across the whole process is managed for optimum efficiency. International Patent Application No. WO2010/012981 describes process configurations and in particular the use of compact, diffusion-bonded heat exchangers to reduce the number of heat exchangers needed thereby simplifying the demands on hardware and space. Furthermore by utilising a plurality of turboexpanders it is possible to carry out the whole process with improved energy efficiency and utilisation.

We have now found that by carrying out the Water Gas Shift reaction at a pressure in excess of 60 bar the shift reaction and the process of the type described in International Patent Application No. WO2010/01298 lean be beneficially integrated to substantially reduce or indeed eliminate the need to compress the shifted gas, for example dried shifted syngas, prior to cooling and the cryogenic separation of the carbon dioxide. Not only can this reduce the need for multiple compression and associated interstage cooling

immediately upstream of the cryogenic separator thereby improving the simplicity and efficiency of the overall process but also the conducting of the Water Gas Shift reaction at high pressure has been found to enable further advantages including better reaction kinetics, smaller vessels and/or improved heat recovery on account of the higher dew point temperature of the steam in some examples.

According to an aspect of the present invention there is provided a process for separating a raw synthesis gas stream into a hydrogen rich stream and a liquid carbon dioxide stream which process comprises:

(1) reacting a raw synthesis gas comprising carbon dioxide, hydrogen, carbon

monoxide and steam with optionally further steam at high pressure and at a temperature in the range 175 to 400°C in the presence of at least one Water Gas Shift reaction catalyst under conditions which generate a shifted gas comprising carbon dioxide, hydrogen and steam having a pressure in the range 50 to 150 bar;

(2) cooling and drying the shifted gas to remove water therefrom:

(3) further cooling the dried shifted gas in at least one heat exchanger to a temperature at which carbon dioxide condenses and a two-phase gas-liquid mixture is formed;

(4) separating the two phase mixture into separate liquid carbon dioxide and gaseous hydrogen-rich fractions in a fractionation unit and

(5) warming and expanding the hydrogen-rich fraction using at least one heat

exchanger to exchange heat with the shifted gas in stage (3) and a plurality of turbo-expanders capable of progressively recovering energy from the hydrogen rich fraction as mechanical work.

As described herein, in some examples, other pressures and/or temperature ranges may be used. For example, in some cases, higher pressures may be employed for the shift reaction, for example pressures up to about 250 bar. In some examples, lower reaction temperatures may be used, for example greater than 145 degrees C, for example 155 degrees C or more.

The fractionation unit may be of any type suitable for effecting the separation of the two phase mixture. For example the apparatus for separating the two phase mixture may include a series of phase separators having coolers and pressure reducers therebetween.

In stage (1) of the process of the present invention raw synthesis is treated with steam under catalytic conditions which promote the exothermic Water Gas Shift reaction:

CO + H₂0 - CC-2 + ¾

thereby creating a 'shifted' gas rich in carbon dioxide and hydrogen. The exact composition of the raw synthesis gas treated will depend upon how it is manufactured but in general it will consist of carbon monoxide, carbon dioxide, hydrogen and steam together with the un-reacted components of air (oxygen, nitrogen and the like) and, if sour, sulphurous impurities such as hydrogen sulphide, carbonyl sulphide and the lower mercaptans. Suitable sources of raw synthesis gas include those produced by the autothermal or steam reforming of natural gas or methane and the gasification or partial oxidation of carbonaceous sources such as heavy oil, shale oil coal, lignite, coke or petroleum coke. In the case where the raw synthesis gas has been prepared by e.g. the reforming of natural gas, or steam reforming of methane the raw synthesis gas may further contain impurities such as methane and ethane. Typically the raw synthesis gas is generated by these processes at a pressure in the range up to 60bar.

The degree of shift which can be effected by the Water Gas Shift reaction is limited by the equilibrium partial pressures of each component (reactants and products) at a given temperature and maximum yields of carbon dioxide are achieved by working at low temperatures and high partial pressures of steam. However at low temperatures the kinetics of the reaction are slow and the steam has a tendency to condense and form 'dew' necessitating the use of a two stage process to achieve the desired outcome. In the first or 'high temperature shift' stage of such an approach raw synthesis gas is passed though a bed of a first Water Gas Shift reaction catalyst at a relatively higher temperature in the range 270 to 510°C and at a high partial pressure of steam. The catalyst used in such a bed (which may either be fixed or fluidised) is suitably a transition metal oxide for example one based on the oxides of iron, copper or cobalt. Preferably the transition metal oxide catalyst is a multi-functional mixed metal oxide catalyst e.g. a iron/chromium/copper oxide, a cobalt/vanadium oxide, a cobalt/molybdenum oxide, a copper/zinc oxide or other alternatives which are commercially available from suppliers such as Johnson Matthey, Sud Chemie. The catalyst may also contain one or more precious metals such as platinum or palladium. If the raw synthesis gas is sour the catalyst must also be sulphur tolerant. Generally the high temperature shift stage will generate a wet partially shifted synthesis gas containing an equilibrium amount of carbon monoxide of the order of 1 to 8 mol%. Thereafter the partially shifted synthesis gas is cooled before being further treated in a second or 'low temperature shift' stage. In some cases, water might be removed from the partially shifted gas at this stage. In this stage the cooled partially shifted gas and steam is passed though a bed of a second Water Gas Shift reaction catalyst operating at a temperature in the range 175 to 320°C and at a lower partial pressure of steam. Although this is likewise typically a transition metal oxide catalyst it may be different from that used in the high temperature shift in which case it is suitably formulated to be more active at lower temperatures. Working at this lower temperature such a catalyst is able to reduce the carbon monoxide level further generating a wet fully shifted synthesis (the shifted gas) gas containing an equilibrium amount of typically less than 4 mol% carbon monoxide.

It is a feature of aspects of the present invention that the Water Gas Shift reaction is carried out at a higher pressure that is conventionally used in industrial scale operations (typically up to 50bar). Suitably a pressure regime is chosen so that the shifted gas leaving the Water Gas Shift unit is at a pressure in the range 50 to 150 bar, preferably 60 bar to 120 bar, more preferably 60 to 95 bar, even more preferably 65 to 90 bar and most preferably 70 to 80 bar. This can be achieved in a number of ways. For example certain types of gasifier are able to produce raw synthesis gas at a pressure in excess of 50 bars in which case the raw synthesis gas can be fed to the shift unit without the need for further compression. Generally speaking however and in particular for lower pressure raw synthesis gas stream, for example those produced by natural gas reformers where the pressure is typically in the range 20 to 40 bar the raw synthesis gas needs to be compressed in accordance with one or other of two strategies. Under the first it is compressed to the desired pressure upstream of the high temperature stage of the Water Gas shift reaction unit so that consequently both the high temperature and low temperature stages are carried out high pressure. Since increasing pressure has no significant on the Water Gas Shift reaction equilibrium but does increase the rate of the reaction this has the advantage that lower catalyst contact times and smaller vessels can be used in both stages. Also since the steam can be added to the inlet of the high temperature stage in the form of pumped water, which may be later evaporated, the amount of gas to be compressed can be kept to a minimum. However the downside is that by increasing the reactor pressure the tendency of the steam to condense increases may make it desirable to work either at a higher temperature or to employ use a lower partial pressure of steam in the reactor. Since both of these effects work to increase the amount of residual carbon monoxide present at equilibrium care needs to be taken in some examples to ensure that sufficient carbon monoxide conversion occurs on the one hand without risking the formation of water in either the high or low temperature stage which will be detrimental to the integrity of the catalyst.

Alternatively the high temperature stage can be operated at a conventional pressure and the partially shifted gas compressed after cooling, possibly with water removal before being treated in the low temperature stage. This has the further advantages of there being less material to compress (since much of the steam has been removed) and therefore being less energy intensive. However compression of this wet stream may also introduce its own dew point problems and in any event compression increases the temperature of the partially shifted synthesis gas. For this reason it may be desirable to condense out the majority of the water in the partially shifted synthesis gas before compression and feeding the required amount of condensate back into the inlet of the low temperature shift stage. In doing the condensate can be used to remove any excess heat generated during the compression whilst at the same time providing steam for the low temperature shift reaction.

The shifted gas leaving the low temperature shift stage may still contain significant amounts of steam and therefore may need to be dried before being subjected to stage (3) of the process. This may be effected by cooling the shifted gas significantly below the dew point temperature of the stream at the relevant pressure and recovering the water in a separator. The separator suitably consists of one or more condensers or knock-out pots arranged in series which remove most of the water followed by polishers containing e.g. molecular sieves or equivalent desiccants. Thereafter the dried shifted gas may be further compressed to bring it to the pressure required to carry out stage (3). As mentioned previously this is not however essential and depending upon the efficiency of carbon dioxide removal required in stage (4) it may be possible to use the shifted gas in stage (3) without any further compression whatsoever. Preferably however to maximise the efficiency of carbon dioxide separation the dried shifted gas is compressed to a pressure in the range 80 to 250 bar, preferably 100 to 200 bar before it is cooled in stage (3). Such compression can be effected by a single compressor or a series thereof for example with interstage heat exchangers designed to remove the heat generated by the compression. In the case where the raw synthesis is 'sour' and contains a significant quantity of the sulphurous impurities mentioned above, the gases will at some stage usually need to be desulphurised. In theory this desulphurisation can be carried out upstream of the high temperature Water Gas Shift reaction catalyst, downstream of the low temperature Water Gas Shift Reaction catalyst, between the two catalysts or in stage (4) of the process of the present invention by capturing it in the liquid carbon dioxide. Preferably however the desulphurisation is done either between the high and low temperature Water Gas Shift reaction catalysts or downstream of the low temperature Water Gas Shift reaction catalysts. If the first of these two approaches is used the low temperature Water Gas Shift reaction catalyst does not need to be resistant to sulphur which can have a considerable economic benefit. Suitably the desulphurisation is carried out by subjecting the gases to solvent extraction (e.g. a Selexol unit). The recovered sulphurous impurities may then be further treated in a Claus plant and the elemental sulphur so recovered for example stored underground. Whilst desulphurisation of the raw synthesis gas can in theory be effected upstream of the high temperature stage this is generally considered sub-optimal because of the need to cool and reheat the raw synthesis gas either side of the desulphuriser which operates at a very low temperature relative to that at which the raw synthesis gas is generated.

In stage (3) of the process of the present invention, the shifted gas may now be at a pressure in the range 50 to 250 bar in some examples, and may then be cooled until some preferably most of the carbon dioxide contained therein liquefies. Typically this involves passing the shifted gas though at least one heat exchanger in which the shifted gas enters into heat exchange relationship with the cold separated liquid carbon dioxide and/or hydrogen rich fractions generated in stage (4) below thereby allowing these latter fractions to be warmed back towards their final desired temperature. In particular it is preferred that the flows of the shifted gas, the cold separated liquid carbon dioxide and the hydrogen rich fractions are configured so as to flow though at least one or at least one array of multichannel diffusion bonded and/or micro-channel heat exchangers thereby cooling the shifted gas mixture to the operating temperature of the fractionation unit. Examples of such heat exchangers are described for example in EP 0212878 and WO 2004/017008 the contents of which are incorporated by reference herein.

In stage (4) the cooled shifted gas (now a two-phase mixture of liquid carbon dioxide and a non-condensable hydrogen rich gas) may be separated into its component parts in a fractionation unit. The exact temperature and pressure required to achieve this will depend on exactly how selective this fractionation unit is required to be but preferably the two- phase mixture is prevented from becoming supercritical and/or that the carbon dioxide is prevented from freezing out. In practical terms, this may mean ensuring that at one extreme the temperature and pressure of the mixture should not exceed both the critical temperature and critical pressure of carbon dioxide (T_c=31.4°C and P_c=73.9bar). At the other extreme it may be necessary to ensure that the temperature of the mixture should not fall below the triple point temperature of carbon dioxide (T_t= -56°C). Within these boundary conditions it may be desirable that the operating temperature of the fractionation unit is at least 20° C below the boiling point of carbon dioxide at the operating pressure in order to obtain efficient separation.

It will be appreciated by one of ordinary skill that the thermodynamic constraints referred to above relate to ideal systems and that the shifted gases being treated herein may exhibit significant deviation from ideality potentially giving rise to an elevation of the triple point temperature. To allow for this possibility it is generally preferred that the temperature of the fractionation unit should be at least 3°C preferably at least 5°C above the theoretical triple point temperature of carbon dioxide. In practical terms and for the mixtures described herein this generally means operating the fractionation unit at a temperature in the range -25 to -53°C and preferably in the range -40 to -50°C. At the same time the pressure should preferably be in the range 50 to 250bar, preferably 100 to 200bar as mentioned above.

The fractionation unit used in the process of the present invention may be for example a conventional gas-liquid separator adapted to work at the high pressures and low temperatures set out above. In such vessels the gaseous hydrogen rich fraction is typically taken off overhead and the liquid carbon dioxide removed at or near the bottom. The pressure drop across the fractionator is typically no more than between 0.1 and 0.5bar.

After separation and before stage (5) the hydrogen rich fraction may optionally be fed to a scrubber where it is contacted with preferably a continuously fed and continuously removed stream of cold alcohol in order to extract residual carbon dioxide therefrom. This is typically effected by continuously contacting a stream of the hydrogen rich fraction with the cold alcohol stream under conditions which cause intimate and turbulent mixing of the two for example by counter-current mixing or by sparging the hydrogen rich fraction through the solvent. Under these conditions the residual carbon dioxide dissolves in the alcohol and is removed from the system by way of the effluent from the scrubber. By effecting this contacting at the high pressure and low temperature characteristic of the fractionation unit a significant part of the residual carbon dioxide can be caused to be absorbed by and to dissolve in the alcohol in accordance with Henry's law. The thermodynamic driving force behind this absorption process, which is enhanced at high pressures, works synergistically with the increased capacity of the solvent at low temperature to hold proportionately more carbon dioxide making a highly efficient system. In particular it is more efficient than the alternative i.e. conventional use of a Rectisol or Selexol treatment carried out at much lower pressures after the hydrogen rich fraction has been returned to or near to its final desired state.

When conducting this scrubbing it is preferred that the alcohol solvent used is selected from methanol, ethanol, the isomers of propanol and low molecular weight glycols and glycol ethers formed by oligomerisation of ethylene or propylene glycol. The alcohol solvent chosen should be one which will not freeze under the operating conditions of the scrubber. Since it is preferred that this scrubbing is conducted immediately after stage (4) with no intermediate treatment of the hydrogen rich fraction the operating temperature and pressure of the scrubber should preferably be the same as or substantially the same as those of the fractionation unit. However the temperature and pressure ranges disclosed above for the fractionation unit are applicable mutatis mutandis to the scrubber irrespective of whether any treatment of the hydrogen rich fraction has occurred between the fractionation unit and the scrubber. It will be appreciated however that the scrubber may work most efficiently when the cold alcohol solvent is fed to the scrubber at or close to the latter' s operating temperature.

The effluent solvent from the scrubber may be passed to a treater where fresh solvent may be regenerated by distillation and overhead removal of carbon dioxide in gaseous form. Thereafter the regenerated solvent can be cooled and recycled to the scrubber. The gaseous carbon dioxide so liberated can thereafter be either disposed of or liquefied and combined with the main carbon dioxide stream before doing so.

In stage (5) of the process of the present invention the hydrogen rich fraction is warmed and decompressed in order to restore it to the temperature and pressure required for its further utilisation. In order to seek to improve the efficiency of energy utilisation of the process , this stage is preferably effected by passing this fraction through a plurality of turboexpanders and associated interstage heat exchangers arranged in series. In a preferred example, in each turboexpander the hydrogen rich fraction is expanded isentropically progressively reducing its pressure and progressively releasing expansion energy which in turn drives a turbine capable of recovering this energy as mechanical work. Typically, and depending on the pressure of the hydrogen rich fraction after fractionation, the process of the present invention may suitably employ from two to eight turboexpanders arranged in series preferably from two to six. The turbo expanders themselves are preferably arranged so that they drive a common shaft. The mechanical work generated can if desired be used elsewhere in the process thereby minimising overall energy usage. At the same time the expansion of the hydrogen rich fraction causes it to cool and the cooling capacity generated can be used in the interstage coolers to cool warmer streams especially those involved in stage (3). In a preferred embodiment these interstage coolers are integrated into a single or array of multichannel heat exchangers though which the incoming shifted gas of stage (3) also flows in order to manage the cooling capacity of the whole system as efficiently as possible. In performing these series of expansions and coolings it is generally important not to let the temperature of the expanded hydrogen rich fraction after each turboexpansion fall below the triple point temperature of carbon dioxide in order to prevent progressive blockage of the transfer line between each turboexpander and interstage cooler by build-up over time of frozen, carbon dioxide derived from any small amounts still remaining in this fraction. Once the hydrogen rich fraction has been reduced to its desired temperature and pressure it can be used for its chosen duty. One or more of these features of the expansion system may be applied to any aspect of the invention described herein.

Subsequent treatment of the liquid carbon dioxide recovered in stage (4) will depend to a certain extent on what is to be done with it. It may for example be piped or tankered offsite for underground storage. In this case it may be desirable to liquefy any further gaseous carbon dioxide recovered in the optional scrubbing stage and combine it with the material recovered in stage (4). The liquid carbon dioxide may if desired be warmed by passing it through the multichannel heat exchanger to utilise its cooling capacity too. It is preferred some examples that little or no expansion of the liquid carbon dioxide occurs downstream of the fractionation unit so that its pressure is maintained at or above 50 bar after the fractionation unit.

It will be apparent that the process of the present application can manifest itself as an integrated shift and separation plant employing the process described above. Accordingly there is further provided, in further aspect of the present invention, a gas shift and separation plant for converting raw synthesis gas into separate streams consisting of respectively liquid carbon dioxide and a hydrogen rich fraction which plant comprises;

(a) at least one high pressure Water Gas Shift reaction unit for converting raw synthesis gas into a shifted gas comprising carbon dioxide, hydrogen and steam.

(b) a drying unit in which the shifted gas is cooled and water is removed

(c) a cooling system for cooling said dried shifted gas to a temperature at which carbon dioxide condenses and a two-phase gas-liquid mixture is formed said cooling system further comprising at least one heat exchanger in which relatively warm dried shifted gas is cooled against the relatively cool liquid carbon dioxide fraction and/or the relatively cool gaseous hydrogen rich fraction generated by the fractionation unit;

(d) a fractionation unit for separating the two-phase mixture generated in said cooling system into separate liquid carbon dioxide and gaseous hydrogen rich fractions and

(e) an expansion system for warming and expanding the separated gaseous hydrogen rich fraction comprising a plurality of turboexpanders for progressively recovering energy from the gaseous hydrogen rich fraction as mechanical work and at least one means for supplying cooling capacity to at least one of the heat exchangers in the cooling system.

The shift and separation plant described above may typically form part of a larger integrated complex for example an Integrated Gasification Combined Cycle (IGCC) or similar hydrogen power plants which include the additional step of burning the hydrogen (preferably diluted with nitrogen) in the burners of a gas turbine.

It will also be appreciated that aspects of the invention find application to gas streams other than synthesis gas. Thus a further aspect of the invention provides a process for decarbonising a gas stream comprising carbon monoxide, which process comprises:

(a) reacting the gas stream comprising carbon monoxide in the presence of at least one Water Gas Shift reaction catalyst to generate a shifted gas comprising carbon dioxide and hydrogen having a pressure of at least 50 bar; (b) cooling the shifted gas using at least one heat exchanger such that carbon dioxide condenses from the shifted gas and a two-phase gas-liquid mixture is formed;

(c) separating the two phase mixture into separate liquid carbon dioxide and gaseous hydrogen-rich fractions and

(d) warming and expanding the hydrogen-rich fraction using at least one heat exchanger to exchange heat with the shifted gas and a plurality of turbo-expanders capable of progressively recovering energy from the hydrogen rich fraction as mechanical work.

In examples of the invention a pressure regime may chosen so that the shifted gas leaving the Water Gas Shift unit is at a pressure of at least 60 bar, for example at least 65 bar, for example at least 70 bar. The pressure may be not more than 150 bar, for example not more than 120 bar, for example not more than 95 bar, for example not more than 80 bar. These pressure ranges may be applied to any of the aspects or examples of the invention described herein. The pressure may be for example in the range 50 to 150 bar, for example 60 bar to 120 bar, for example 60 to 95 bar, for example 65 to 90 bar, for example 70 to 80 bar.

In examples of the invention, the process includes the step of compressing the gas stream upstream of the water gas shift reaction. For example, the process may include compressing the gas stream to a pressure of at least 50 bar upstream of the water gas shift reaction.

The gas stream may comprise synthesis gas, although other gas streams including carbon monoxide may be used in applications of the present invention.

The gas stream may include steam. The process may include the step of adding water, or additional water to the gas stream. The water may for example be added in the form of steam.

The process may further include the step of removing water from the shifted gas prior to the condensing of the carbon dioxide.

In examples of the present invention, the temperature of the water gas shift reaction is chosen to be above the dew point. For example, the temperature of the reaction may be chosen to be about 10 degrees C above the dew point. Thus, in some examples, the reaction in the presence of the at least one Water Gas Shift reaction catalyst may be carried out at a temperature of at least 155 degrees C. For example, the reaction may be carried out at a temperature of at least 175 degrees C, for example at a temperature in the range of from 175 to 400 degrees C. In another example, where the pressure of the reaction is about 100 bar, the reaction temperature may be for example about 170 degrees C, for example 171 degrees C.

The water gas shift reaction may be carried out as a two-step process, for example in which the gas stream is sequentially treated with high temperature and low temperature Water Gas Shift reaction catalysts.

The pressure of the gas stream may be selected in dependence on system parameters. In some systems, it may be advantageous for a high pressure to be used. For example, the gas stream may be compressed to a pressure of more than 100 bar, for example 150 bar or more, or 250 bar or more. The gas stream may for example be compressed to a pressure in the range 50 to 150 bar, for example in the range 50 to 100 bar before treatment with the Water Gas Shift reaction catalyst.

These temperature and/or pressure conditions may be applied in relation to any aspect or example described herein, as appropriate.

The compression may occur upstream of all of the Water Gas Shift reaction catalysts, and/or between catalysts of a multi-step Water Gas Shift reaction. For example, the gas stream may be compressed to a pressure of at least 50 bar, preferably in the range 50 to 100 bar, before treatment with the low temperature Water Gas Shift reaction catalyst.

The shifted gas may be compressed prior to condensation of the carbon dioxide, for example to a pressure in the range of from 50 to 250 bar.

A partially shifted gas may be produced by the high temperature Water Gas Shift reaction catalyst, and the partially shifted gas may be cooled and treated to remove condensed water before any further compression of the gas. A part of the condensed water which is removed before compression may be returned to the partially shifted synthesis gas after compression.

The water gas shift reaction may be carried out at a pressure in the range of from 65 to 90 bar, for example at a pressure in the range of from 70 to 80bar.

The gas stream may include sulphurous impurities, and sulphurous impurities may be removed downstream of the Water Gas Shift reaction catalyst.

Where a multi-step Water Gas Shift reaction is used, the sulphurous impurities may be removed between the steps. For example, the sulphurous impurities may be removed between high temperature and low temperature Water Gas Shift reaction catalysts. The liquid carbon dioxide fraction may be maintained at a pressure of at least 50 bar after separation.

The separation of the two-phase mixture is carried out at a temperature in the range of from -40 to -50°C and/or at a pressure in the range of from 100 to 200bar.

Between the separation of the two-phase mixture and the expansion of the hydrogen- rich fraction, the hydrogen rich fraction may be scrubbed to remove carbon dioxide present therein.

Steam may be present in the water gas shift reaction and the temperature and partial pressure of any steam present may be such as to substantially prevent the formation of dew when gas is in contact with the Water Gas Shift reaction catalyst.

The process may be integrated with a hydrogen-fed power generation system.

According to a further aspect of the invention, there is provided apparatus for use in a method of decarbonising a gas stream comprising carbon monoxide, the apparatus comprising:

(a) a Water Gas Shift reactor for receiving the gas stream and for reacting the gas stream in the presence of at least one Water Gas Shift reaction catalyst to generate a shifted gas comprising carbon dioxide and hydrogen having a pressure of at least 50 bar;

(b) a cooling system including at least one heat exchanger for cooling the shifted gas such that carbon dioxide condenses from the shifted gas and a two-phase gas-liquid mixture is formed;

(c) separation apparatus for separating the two phase mixture into separate liquid carbon dioxide and gaseous hydrogen-rich fractions and

(d) an expansion system including at least one heat exchanger for exchanging heat with the shifted gas and a plurality of turbo-expanders for progressively recovering energy from the hydrogen rich fraction as mechanical work.

The apparatus may further include a compressor arranged upstream of the Water Gas Shift reactor for compressing the gas stream to a pressure of at least 50bar.

Also provided by the invention is a method being substantially as herein described, optionally having reference to one or more of the accompanying figures. Also provided by the invention is an apparatus being substantially as herein described, optionally having reference to one or more of the accompanying figures.

The features of the invention described herein may be provided in any appropriate combination. Features of one aspect of the invention may be applied to other aspects of the invention as appropriate. In particular, features of apparatus aspects may be applied to method aspects an vice versa.

The present invention will now be illustrated by way of example only having reference to the following Examples. In relation to these Examples:

Figure 1 shows a schematic view of stages (1) and (2) of the process of the present invention. In this embodiment the raw synthesis gas is compressed upstream of the high temperature shift unit.

Figure 2 shows another schematic view of stages (1) and (2) of the process of the present invention. In this embodiment the partially shifted synthesis gas is compressed upstream of the low temperature shift unit.

Figure 3 shows yet another schematic view of stages 1 and 2 of the process of the present invention. This embodiment is similar to that shown in Figure 1 except that desulphurisation is carried out before any Water Gas Shift reaction takes place.

Figure 4 shows a schematic view of stages (3) to (5) of the process of the present invention which can be used in association with any of the schemes set out in Figures 1 to 3.

Example 1

In Figure 1 raw synthesis gas from a coal gasifier scrubbed of particulate matter and having a composition consisting of 49.6 mol% carbon monoxide, 1.2 mol% carbon dioxide, 23.6 mol% hydrogen, 18.1 mol% steam, 4.6 mol% nitrogen and hydrogen sulphide and at a temperature of 160°C and a pressure of 37 bar is fed via line 1 to a heat exchanger El where it is cooled to 50°C. Thereafter it is fed via line 2 to a phase separatorAl where gas is removed overhead via line 4 and water from the bottom via line 3. The raw synthesis gas is then fed to a compressor CI and a heat exchanger E2 where the temperature and pressure of the raw synthesis gas is adjusted to 250°C and 75 bar. The compressed gas is next fed via lines 6 and 9 together with heated water (originating from lines 7 and 8 and heat exchanger E3) to a high temperature Water Gas Shift reactor A2 in which the raw synthesis gas is contacted with a fixed bed of particulate high temperature Water Gas Shift reaction catalyst, (ex Johnson atthey) at a contact time sufficient to reach the equilibrium conditions for the Water Gas Shift reaction at the reaction temperature (c.450°C). The amount of water fed to A2 is controlled so that steam is generated and so that the partial pressure of the steam therein is less than that required to cause the formation of dew. A stream of partially shifted synthesis gas containing steam, hydrogen, carbon dioxide and 6 to 7 mol% carbon monoxide is then removed from A2 via line 10 and cooled in one or a series of heat exchangers E4 to a temperature of 220°C. The. cooled partially shifted synthesis gas is next passed via line 11 to a low temperature Water Gas Shift reactor A3 where it is contacted with a fixed bed of particulate low temperature Water Gas Shift reaction catalyst, (ex Johnson Matthey) at a contact time sufficient to reach the equilibrium conditions for the Water Gas Shift reaction at this lower temperature. Once again the amount of steam present in A3 is controlled to prevent the formation of dew. A stream of wet shifted gas containing less than 3% carbon monoxide is then removed via line 12 and cooled to 40°C in heat exchanger E5 after which any condensed water is removed in a knock-out pot (not shown). The effluent from E5 is then fed via line 13 to a desulphurisation unit A4 (typically a Selexol unit) where sulphurous impurities are removed by solvent extraction and taken away via line 14. The

desulphurised shifted gas is then further dried by passing through a bed of molecular sieve (not shown) before being finally fed via line 15 to stages (3) to (5) of the process of the present invention as described in Figure 4 and Example 4 below.

Example 2

In Figure 2 raw synthesis gas from an oxygen-blown autothermal reformer having the composition 12.7 mol% carbon monoxide, 7.2 mol% carbon dioxide, 42.9% hydrogen, 35.6 mol% steam. 0.5 mol% nitrogen and 1.2 mol% methane and at a temperature of 900°C and a pressure of 31 bar is cooled to around 350°C in a heat exchanger El before being fed via line 2 to a high temperature Water Gas Shift reactor Al in which the raw synthesis gas is contacted with a fixed bed of particulate high temperature Water Gas Shift reaction catalyst, (ex Johnson Matthey) at a contact time sufficient to reach the equilibrium conditions for the Water Gas Shift reaction at the reaction temperature (450°C). The amount of water in the reactor may be controlled so that the partial pressure of steam in the reactor is less than that required to cause the formation of dew at its operating temperature and pressure. A stream of wet partially shifted synthesis gas containing 4 to 5 mol% carbon monoxide is then removed via line 3 and cooled in one or a series of heat exchangers E2 to a temperature of 50°C. The cooled partially shifted synthesis gas is then passed first to a separator A2 where condensed water is recovered via line 6 and then via lines 5 and 7 to one or a series of compressors CI with (optional) interstage heat exchangers and a final heat exchanger E3 where the raw synthesis gas is heated and its pressure increased to 70 bar before being fed via line 11 to a low temperature Water Gas Shift reactor A3. It will be appreciated by one of ordinary skill that some of or all of the recovered water from A2 via line 6 can be returned to the compressed partially shifted synthesis gas stream in line 1 1 order to cool it back down to the operating temperature of the next stage and generate steam. Alternatively as shown herein fresh heated water is added via lines 9 and 10 and heat exchanger E5 to line 11 upstream of the inlet to A3. In A3 the wet partially shifted synthesis gas is contacted with a fixed bed of particulate low temperature Water Gas Shift reaction catalyst, (ex Johnson Matthey) at a contact time determined to be sufficient for the gas composition to reach the required approach to equilibrium conditions for the Water Gas Shift reaction at this lower temperature. Once again the amount of steam present should be less than that necessary to cause the formation of dew in A3. A stream of wet shifted gas containing less than 0.7% carbon monoxide is then removed from A3 and cooled to 40°C in heat exchanger E4 after which any condensed water is removed in a knock out pot (not shown).

If appropriate, for example in a case in which there is a sulphur presence, the effluent from E4 is then fed via line 13 to a desulphurisation unit A4 (typically a Selexol unit) where sulphurous impurities are removed by solvent extraction and taken away via line 14. If there is no sulphur present, this step would not be required and the unit A4 would be omitted.

The treated shifted gas is then passed via line 15 and dehydration unit containing molecular sieve (not shown) to stages (3) to (5) of the process of the present invention as described in Figure 4 and Example 4 below.

While this Example 2 shows the gas being cooled prior to compression, in other examples, this cooling might not be carried out. Thus there may or may not be cooling prior to compression before the first (or only) shift reactor and/or between shift reactors. The removal of water from the gas stream prior to compression will reduce the power required for compression, but in some examples, such energy saving will need to be balanced with an assessment of the energy relating to the steam required for the shift reaction. Example 3

In Figure 3 scrubbed raw synthesis gas from a coal gasifier having the same composition as Example 1 and at a temperature of 160°C and a pressure of 37 bar is fed via line 1 to a heat exchanger El where it is cooled to 50°C. Thereafter it is fed via line 2 to a drier Al where dried gas is removed overhead via line 4 and water from the bottom via line 3. The dried raw synthesis gas is then fed to a compressor CI and a heat exchanger E2 where the temperature and pressure of the raw synthesis gas is adjusted to 50°C and 75 bar. The effluent from E2 is then fed via line 6 to a desulphurisation unit A2 (typically a Selexol unit) where sulphurous impurities are removed by solvent extraction and taken away via line 8. The desulphurised raw synthesis gas is then passed via line 7 to heat exchanger E3 where it is heated before passed together with heated water and/or steam (originating from lines 10 and 1 1 and heat exchanger E4) via lines 9 and 12 to a high temperature Water Gas Shift reactor A3 in which the raw synthesis gas is contacted with a fixed bed of particulate high temperature Water Gas Shift reaction catalyst, (ex Johnson Matthey) at a contact time sufficient to reach the equilibrium conditions for the Water Gas Shift reaction at the reaction temperature (c.450°C). The amount of water fed to A3 is controlled so that steam is generated and so that the partial pressure of the steam therein is less than that required to cause the formation of dew. A stream of partially shifted synthesis gas containing steam, hydrogen, carbon dioxide and 6 to 7 mol% carbon monoxide is then removed from A3 via line 13 and cooled in one or a series of heat exchangers E5 to a temperature of 220°C. The. cooled partially shifted synthesis gas is next passed via line 14 to a low temperature Water Gas Shift reactor A4 where it is contacted with a fixed bed of particulate low temperature Water Gas Shift reaction catalyst, (ex Johnson Matthey) at a contact time determined to be sufficient for movement of the gas composition towards equilibrium conditions for the Water Gas Shift reaction at this lower temperature. Once again the amount of steam present in A4 is controlled to prevent the formation of dew. A stream of wet shifted gas containing less than 3% carbon monoxide is then removed via line 15 and cooled to 40°C in heat exchanger E6 after which any condensed water is removed in a further knock-out pot (not shown). The desulphurised shifted gas is then further dried by passing through a bed of molecular sieve (not shown) before being finally fed via line 16 to stages (3) to (5) of the process of the present invention as described in Figure 4 and Example 4 below. Example 4

The dry shifted gas from any of Examples 1, 2 or 3, consisting mainly of carbon dioxide, hydrogen and nitrogen and at a pressure of 70 to 85 bar, for example 80 bar and a temperature of 40°C, is first fed to a series of compressors C2, C3 and C4 and associated interstage heat exchangers E2,E3 and E4 where the dry shifted gas is compressed and cooled to a pressure of 175 bar and 40°C before being passed via line SI to a multichannel array of diffusion bonded heat exchangers ex Heatric UK (shown schematically as LNG- 100) where it is cooled against a plurality of cold process streams derived from the decompression system (see below) thereby generating a two phase gas/liquid stream S2 having a pressure of 172 bar and a temperature of about -50°C. This stream is passed directly to gas-liquid separator vessel F360 where a non-condensable hydrogen rich gas (mainly hydrogen and nitrogen) is separated from liquid carbon dioxide. The hydrogen rich gas is removed overhead as stream S2V and fed at the same temperature and pressure via line IN to a series of turboexpanders EXl, EX2, EX3 and EX4 where it is

progressively expanded isentropically to lower pressure. The person skilled in the art will understand that isentropic expansion of this gas stream will result in it being cooled.

Accordingly the hydrogen-rich gas exits EXl at a pressure of 1 12 bar and a temperature of -50°C and is routed through the multichannel heat exchanger LNG-100 where it is heat exchanged with the high pressure gas stream SI up to a temperature of -30°C and then passed to turboexpander EX2 (via line 2N) where it is expanded yet again to form stream 2T at a pressure of 75bar and a temperature of -50°C.

At the same time a liquid carbon dioxide stream S2L is withdrawn from the bottom of F360 and is flashed across valve VLV-109 thereby generating a further two phase stream 18 that is passed to flash vessel F150. A hydrogen-rich gas stream S2LV is withdrawn from the top of this vessel and combined with stream 2T to form combined vapour stream 2TM at point Ml . Gas stream 2TM is then passed through the multichannel heat exchanger LNG-100 thereby again cooling stream SI . The hydrogen-rich gas stream 3N that exits the multichannel heat exchanger LNG-100, now at a temperature of -30°C is then passed to turboexpander EX3 where it is expanded once again to a pressure of 48 barg and a temperature of -50°C (line 3T). This stream is warmed yet again to -30°C in LNG- 100 before being expanded one last time in turboexpander EX4 to a pressure of 31 bar and a temperature of -50°C. Finally the gas stream is passed through LNG-100 one last time and optionally a series of other heat exchangers (not shown) where it is warmed to a final temperature of 25°C. The warm hydrogen-rich gas stream then leaves the plant via line 29.

Meanwhile the liquid carbon dioxide stream is withdrawn from the bottom of F 150 and passed through LNG-100 where it too is warmed against stream SI and optionally other heat exchangers before being exported offsite.

After contact with the gaseous second component in scrubber A3, the methanol solvent, now rich in carbon dioxide, is removed an fed via line 26 to the head of a stripper column A2 in which the carbon dioxide and the methanol are separated. A hydrogen carbon dioxide rich gas stream is then removed overhead via line 20. Lean methanol is then returned to A3 via line 25. and A2 is provided with a reboiler serviced by lines 21 and 22 to maintain the methanol at the correct temperature.

It will be understood that the present invention has been described above purely by way of example, and modification of detail can be made within the scope of the invention. For example, the number of shift reactors used can be varied. While examples are described in which two shift reactors are used, it is envisaged that additional shift reactors could be used. In other examples, a compressor, or series of compressors could be placed between one or more of the shift reactors in series.

Each feature disclosed in the description, and (where appropriate) the claims and drawings may be provided independently or in any appropriate combination.

Claims

1. A process for decarbonising a gas stream comprising carbon monoxide, which process comprises:

(a) reacting the gas stream comprising carbon monoxide in the presence of at least one Water Gas Shift reaction catalyst to generate a shifted gas comprising carbon dioxide and hydrogen having a pressure of at least 50 bar;

(b) cooling the shifted gas using at least one heat exchanger such that carbon dioxide condenses from the shifted gas and a two-phase gas-liquid mixture is formed;

(d) warming and expanding the hydrogen-rich fraction using at least one heat

exchanger to exchange heat with the shifted gas and a plurality of turbo-expanders capable of progressively recovering energy from the hydrogen rich fraction as mechanical work.

2. A process according to claim 1, further including the step of compressing the gas stream upstream of the water gas shift reaction.

3. A process according to claim 2, including compressing the gas stream to a pressure of at least 50 bar upstream of the water gas shift reaction.

4. A process according to any preceding claim, wherein the gas stream comprises synthesis gas.

5. A process according to any preceding claim, further including the step of adding water to the gas stream.

6. A process according to any preceding claim, further including the step of removing water from the shifted gas prior to the condensing of the carbon dioxide.

7. A process according to any preceding claim, wherein the reaction in the presence of the at least one Water Gas Shift reaction catalyst is carried out at a temperature of at least 155 degrees C.

8. A process according to any preceding claim, wherein the water gas shift reaction is carried out as a two-step process in which the gas stream is sequentially treated with high temperature and low temperature Water Gas Shift reaction catalysts.

9. A process according to any preceding claim, wherein the gas stream is compressed to a pressure in the range 50 to 150 bar, before treatment with the Water Gas Shift reaction catalyst.

10. A process as claimed in claim 8 wherein the gas stream is compressed to a pressure of at least 50 bar, preferably in the range 50 to 100 bar, before treatment with the low temperature Water Gas Shift reaction catalyst.

11. A process according to any preceding claim wherein the shifted gas is compressed prior to condensation of the carbon dioxide.

12. A process according to claim 1 1, wherein the shifted gas is compressed to a pressure in the range of from 50 to 250 bar prior to condensation of the carbon dioxide.

13. A process as claimed in claim 8 or any of claims 9 to 12 when dependent on claim 8, in which a partially shifted gas is produced by the high temperature Water Gas Shift reaction catalyst and the partially shifted gas is cooled and treated to remove condensed water before being compressed.

14. A process as claimed in claim 13 wherein a part of the condensed water which is removed before compression is returned to the partially shifted synthesis gas after compression.

15. A process according to any preceding claim wherein the water gas shift reaction is carried out at a pressure in the range of from 65 to 90 bar.

16. A process according to claim 15, wherein the water gas shift reaction is carried out at a pressure in the range of from 70 to 80bar.

17. A process as claimed in any preceding claim, wherein the gas stream includes a sulphurous impurities, and sulphurous impurities are removed downstream of the Water Gas Shift reaction catalyst.

18. A process as claimed in any preceding claim, wherein the gas stream includes sulphurous impurities and sulphurous impurities are removed upstream of the Water Gas Shift reaction catalyst.

19. A process according to claim 17 or claim 18 when dependent on claim 8, wherein the sulphurous impurities are removed between the high temperature and low temperature Water Gas Shift reaction catalysts.

20. A process according to any preceding claim wherein the liquid carbon dioxide fraction is maintained at a pressure of at least 50 bar after separation.

21. A process according to any preceding claim wherein the separation of the two- phase mixture is carried out at a temperature in the range of from -40 to -50°C.

22. A process according to any preceding claim wherein the separation of the two- phase mixture is carried out at a pressure in the range of from 100 to 200bar.

23. A process according to any preceding claim wherein between the separation of the two-phase mixture and the expansion of the hydrogen-rich fraction, the hydrogen rich fraction is scrubbed to remove carbon dioxide present therein.

24. A process according to any preceding claim, wherein steam is present in the water gas shift reaction and the temperature and partial pressure of steam present is such as to substantially prevent the formation of dew when gas is in contact with the Water Gas Shift reaction catalysts.

25. A process according to any preceding claim, the process being integrated with a hydrogen-fed power generation system.

26. Apparatus for use in a method of decarbonising a gas stream comprising carbon monoxide, the apparatus comprising:

(c) separation apparatus for separating the two phase mixture into separate liquid

carbon dioxide and gaseous hydrogen-rich fractions and

27. Apparatus according to claim 26, further including a compressor arranged upstream of the Water Gas Shift reactor for compressing the gas stream to a pressure of at least 50bar.

28. A process for separating a raw synthesis gas stream into a hydrogen rich stream and a liquid carbon dioxide stream which process comprises:

(1) reacting a raw synthesis gas comprising carbon dioxide, hydrogen, carbon monoxide and steam with optionally further steam at high pressure and at a temperature in the range 175 to 400°C in the presence of at least one Water Gas Shift reaction catalyst under conditions which generate a shifted gas comprising carbon dioxide, hydrogen and steam having a pressure in the range 50 to 150 bar;

(2) cooling and drying the shifted gas to remove water therefrom:

(5) warming and expanding the hydrogen-rich fraction using at least one heat

29. A process as claimed in claim 28 characterised in that stage (1) comprises a two- step process in which the raw synthesis gas is sequentially treated with high temperature and low temperature Water Gas Shift reaction catalysts.

30. A process as claimed in claim 29 characterised in that the raw synthesis gas is compressed to a pressure in the range 50 to 100 bar before treatment with the high temperature Water Gas Shift reaction catalyst.

31. A process as claimed in claim 29 characterised in that the raw synthesis gas is compressed to a pressure in the range 50 to 100 bar before treatment with the low temperature Water Gas Shift reaction catalyst.

32. A process as claimed in claim 28 characterised in that the dried shifted gas is compressed to a pressure in the range. 50 to 250 bar before being treated in stage (3)

33. A process as claimed in 29 characterised in that a partially shifted synthesis gas is produced by the high temperature Water Gas Shift reaction catalyst and the partially shifted synthesis gas is cooled and treated to remove condensed water before being compressed.

34. A process as claimed in claim 33 characterised that a part of the condensed water which is removed before compression is returned to the partially shifted synthesis gas after compression.

35. A process as claimed in claim 28 characterised that stage (1) is carried out at a pressure in the range 65 to 90 bar.

36. A process as claimed in claim 35 characterised that stage (1) is carried out at a pressure in the range 70 to 80bar.

37. A process as claimed in anyone of claims 29 to 36 characterised in that the raw synthesis gas is sour and that sulphurous impurities are removed either downstream of the low temperature Water Gas Shift reaction catalyst or between of the high temperature and low temperature Water Gas Shift reaction catalysts.

38. A process as claimed in claim 28 characterised in that the liquid carbon dioxide is maintained at a pressure of greater than 50 bar after the fractionation unit.

39. A process as claimed in claim 28 characterised in that stage (4) is carried out at a temperature in the range -40 to -50°C and at a pressure in the range 100 to 200bar.

40. A process as claimed in claim 28 characterised in that between stage (4) and (5) the hydrogen rich fraction is scrubbed to remove any residual carbon dioxide present therein.

41. A process as claimed in claim 29 characterised in that the temperature and partial pressure of steam present in stage (1) is such as to prevent the formation of dew when the raw synthesis gas or partially shifted gas is in contact with the Water Gas Shift reaction catalysts.

42. A gas shift and separation plant for converting raw synthesis gas into separate streams consisting of respectively liquid carbon dioxide and a hydrogen rich fraction characterised in that it comprises;

(b) a drying unit in which the shifted gas is cooled and water is removed

(c) a cooling system for cooling said dried shifted gas to a temperature at which carbon dioxide condenses and a two-phase gas-liquid mixture is formed said cooling system further comprising at least one or at least one array of multi-channel diffusion bonded and/or micro-channel heat exchangers in which relatively warm dried shifted gas is cooled against the relatively cool liquid carbon dioxide fraction and or the relatively cool gaseous hydrogen rich fraction generated by the fractionation unit;

(e) an expansion system for warming and expanding the separated gaseous hydrogen rich fraction comprising a plurality of turboexpanders for progressively recovering energy from the hydrogen rich fraction as mechanical work and at least one means for supplying cooling capacity to at least one of the heat exchangers in the cooling system.