The present invention relates to a plant and a process for treating natural gas to produce a liquid product.

It is well known that most oil wells also produce natural gas. At many oil wells natural gas is produced in relatively small quantities along with the oil. When the quantities of this associated gas are sufficiently large or the well is close to pre-existing gas transportation infrastructure, the gas can be transported to an off-site processing facility. When oil production takes place in more remote places it is difficult to introduce the associated gas into existing gas transportation infrastructure. In the absence of such infrastructure, the associated gas has typically been disposed of by flaring or re-injection. However, flaring the gas is no longer an environmentally acceptable approach, while re- injection can have a negative impact on the quality of the oil production from the field.

Gas-to-liquids technology can be used to convert the natural gas into liquid hydrocarbons and may follow a two-stage approach to hydrocarbon liquid production comprising syngas generation, followed by Fischer-Tropsch synthesis. In general, syngas (a mixture of hydrogen and carbon monoxide) may be generated by one or more of partial oxidation, auto-thermal reforming, or steam methane reforming. Where steam methane reforming is used, the reaction is endothermic and so requires heat, and a catalyst such as platinum/rhodium. The syngas is then subjected to Fischer-Tropsch synthesis. For performing Fischer-Tropsch synthesis the optimum ratio of hydrogen to carbon monoxide is about 2:1 , and steam reforming has a benefit of providing more than sufficient hydrogen for this purpose. As regards the Fischer-Tropsch process, a suitable catalyst uses cobalt on a ceramic support.

Such a process is described for example in WO 01/ 51 194 (AEA Technology) and WO 03/048034 (Accentus pic). Natural gas is primarily methane, but also contains small proportions of longer-chain hydrocarbons. In each case the natural gas is first subjected to a pre-reforming step in which the longer-chain hydrocarbons are converted to methane by reaction with steam, for example over a nickel catalyst at 400 °C. The various catalysts used in the gas-to-liquid process may be detrimentally affected by sulphur compounds in the natural gas. As described in WO 03/048034 the natural gas may therefore be subjected to a desulphurisation process before performing any of the chemical steps. According to the present invention there is provided a gas-to-liquids plant for treating natural gas, the plant comprising a synthesis-gas-producing catalytic reactor to produce a synthesis gas containing hydrogen and carbon monoxide, and a Fischer- Tropsch synthesis reactor for generating hydrocarbons, and producing a tail gas, the plant also comprising means to generate a hydrogen-rich gas stream from the synthesis gas or from the tail gas, a hydrogen-desulphurisation reactor and means to supply both the natural gas and the hydrogen-rich gas stream to the hydrogen-desulphurisation reactor during normal operation of the gas-to-liquids plant; wherein the plant also comprises a hydrogen production module for use at start-up of the plant.

The hydrogen production module provides hydrogen to the hydrogen- desulphurisation reactor during start-up, and once the plant has started up and is producing synthesis gas or is performing Fischer-Tropsch synthesis, the hydrogen production module can be switched off, and instead the hydrogen-rich gas stream be supplied to the desulphurisation reactor.

Provision of a hydrogen production module avoids the requirement to store hydrogen, and any associated hazards. The invention is particularly suitable in situations where storage of hydrogen should be avoided, or where obtaining a supply of hydrogen would otherwise be impractical. For example it is suitable in remote locations; and it is suitable in offshore plant for example on an oil rig or a gas rig, or on a floating production storage and offloading (FPSO) vessel. Where the plant is on or adjacent to water, and electrical power is available, the hydrogen production module may be a water electrolysis unit. For example in the case of an FPSO power is available for dynamic positioning, and in calm weather not all of the power is required for that purpose, so if the plant is started up in calm weather there is power available for electrolysis, and so for production of hydrogen.

As an alternative possibility the hydrogen production module may comprise a reformer, to which an oxygenate such as methanol or ethanol is supplied. Such an oxygenate can be safely stored as an aqueous solution, is commercially available, and is low in sulphur (e.g. < 1 ppm), so it may be used in this way to provide a stream of hydrogen. For example the reformer may perform steam reforming of methanol, providing the requisite heat by combustion of natural gas or methanol in air in adjacent flow channels in a reactor. It may be desirable also to subject the resultant gas mixture to a water gas shift reaction. This process produces a gas stream consisting almost exclusively of hydrogen and carbon dioxide. This can be used without further treatment in the hydrogen- desulphurisation reactor.

It will also be appreciated that during operation of the gas-to-liquid plant the Fischer-Tropsch synthesis reaction forms water and some oxygenates, so that oxygenates can be obtained from the aqueous phase output from the Fischer-Tropsch reactor, once the plant is in operation. They may be separated by distillation into a more concentrated form. Hence on the next occasion that the plant is started up, a supply of oxygenates can be already available.

According to another aspect of the present invention there is provided a gas-to- liquids plant for treating natural gas, the plant comprising a synthesis-gas-producing catalytic reactor to produce a synthesis gas containing hydrogen and carbon monoxide, and a Fischer-Tropsch synthesis reactor for generating hydrocarbons, and producing a tail gas, the plant also comprising means to generate a hydrogen-rich gas stream from the synthesis gas or from the tail gas, a hydrogen-desulphurisation reactor and means to supply both the natural gas and the hydrogen-rich gas stream to the hydrogen- desulphurisation reactor during normal operation of the gas-to-liquids plant; wherein the synthesis-gas producing catalytic reactor is provided with a catalyst bed which is in part sacrificial. In one such plant the synthesis-gas producing catalytic reactor comprises a pre- reformer, in which C2+ hydrocarbons are converted to methane; and the pre-reformer is provided with a catalyst bed in which is in part sacrificial.

One manner in which such a partly sacrificial catalyst bed may be provided is that the catalyst bed is provided in the form of successive layers, the first layer being the sacrificial layer. It is also envisaged that the catalyst in the sacrificial layer may be replaced when it is spent (or contaminated). For example at least the first layer may be in the form of a cartridge, so it can be readily replaced. The term 'catalyst bed' in this specification refers to a catalyst on a support, whatever the nature of the support, and so encompasses a catalyst on walls of a flow channel, or on a substrate located within a flow channel, or on pellets within a flow channel. In each case the reactant gases would flow through the flow channel and so flow past the catalyst.

The term "sacrificial" is used in the sense that the plant can continue to operate satisfactorily despite the destruction or inactivity of part of the catalyst bed; that part of the catalyst bed has been sacrificed to enable the remainder of the catalyst bed to function normally. The catalyst bed is therefore larger than would be required for treating a sulphur- free gas stream. However it will be appreciated that during normal operation the gas stream is subjected to desulphurisation, so that the sacrificial nature of the catalyst bed is primarily required only during start-up. In a further aspect of the present invention there is provided a gas-to-liquids plant for treating natural gas, the plant comprising a synthesis-gas-producing catalytic reactor to produce a synthesis gas containing hydrogen and carbon monoxide, and a Fischer- Tropsch synthesis reactor for generating hydrocarbons, and producing a tail gas, the plant also comprising means to generate a hydrogen-rich gas stream from the synthesis gas or from the tail gas, a hydrogen-desulphurisation reactor and means to supply both the natural gas and the hydrogen-rich gas stream to the hydrogen-desulphurisation reactor during normal operation of the gas-to-liquids plant; wherein the plant is arranged to feed an oxygenate such as methanol or ethanol into the synthesis-gas producing catalytic reactor, rather than natural gas, during start-up.

It will be appreciated that the present invention enables the gas-to-liquid plant to start-up without having a detrimental effect due to sulphur poisoning of any of the catalysts.

The invention will now be further and more particularly described, by way of example only, and with reference to the accompanying drawings, in which:

Figure 1 shows a schematic flow diagram of a gas-to-liquid plant and associated equipment, including a desulphurisation system; and

Figure 2 shows a schematic flow diagram of the desulphurisation system of the plant of figure 1 . 1 . Gas-to-Liquid Plant Overview

The invention relates to a chemical process for converting natural gas (primarily methane) to longer chain hydrocarbons. It is suitable for treating associated gas, which is natural gas that is produced along with crude oil, and is then separated from the crude oil. The first stage of the chemical process involves the formation of synthesis gas. This may be achieved for example by steam reforming, by a reaction of the type:

H ₂ 0 + CH ₄ → CO + 3 H ₂ (1 )

This reaction is endothermic, and may be catalysed by a rhodium or platinum/rhodium catalyst in a first gas flow channel. The heat required to cause this reaction may be provided by catalytic combustion of a gas such as methane or hydrogen, which is exothermic, in an adjacent channel, or by heat exchange with exhaust gases from a separate combustion reactor. The combustion may be catalysed by a palladium catalyst in an adjacent second gas flow channel in a compact catalytic reactor. In both cases the catalyst may be on a stabilised-alumina support which forms a coating typically less than 100 μηη thick on a metallic substrate. Alternatively, the catalyst may be applied to the walls of the flow channels or may be provided as pellets within the flow channel. The heat generated by the combustion would be conducted through the metal sheet separating the adjacent channels.

The gas mixture produced by the steam/methane reforming is then used to perform a Fischer-Tropsch synthesis to generate a longer chain hydrocarbon, that is to say: n CO + 2n H ₂ → (CH ₂ ) _n + n H ₂ 0 (2) which is an exothermic reaction, occurring at an elevated temperature, typically between ^' \ 90°C and 280 °C, for example 230 °C, and an elevated pressure typically between 1 .8 MPa and 2.6 MPa (absolute values), for example 2.5 MPa, in the presence of a catalyst such as iron, cobalt or fused magnetite, with a potassium promoter. Whilst Fe based catalysts can be used, metallic Co promoted with precious metals such as Pd, Pt, Ru or Re doped to 1 wt% are preferred when operating at lower temperatures as they have enhanced stability to oxidation. The active metals are impregnated to 10-40 wt% into refractory support materials such as Ti0 ₂ , Al ₂ 0 ₃ or Si0 ₂ which may be doped with rare earth and transition metal oxides to improve their hydrothermal stability.

It will be appreciated from the equations above that, if steam/methane reforming is used to produce the synthesis gas, there is an excess of hydrogen. A hydrogen-rich gas stream can therefore be separated either from the synthesis gas stream before performing Fischer-Tropsch synthesis, or from the tail gases that remain after performing Fischer- Tropsch synthesis. Such a separation may use a membrane separator.

Referring to figure 1 , there is shown a gas-to-liquid plant 10 of the invention. A natural gas feed 5 consists primarily of methane, but with small proportions of other gaseous hydrocarbons, hydrocarbon vapours, and water vapour. The gas feed 5 may for example be at a pressure of 4.0 MPa (40 atmospheres) and 35 °C, following sea water cooling from an initial temperature of 90 °C, and may constitute associated gas from a well producing crude oil.

The natural gas feed 5 is supplied to a pretreatment system 25, in which it is subjected to treatment which may comprise one or more of the following: changing its pressure; changing its temperature; and removing impurities such as sulphur using a desulphurisation system 85 (shown in figure 2). It is then mixed with steam in a mixer 26.

2. Making Synthesis Gas

The gas/steam mixture, preferably at a temperature of about 450 °C, is then fed into a catalytic steam/methane reformer 30; the first section of the reformer 30 may be a pre- reformer in which any ethane or higher hydrocarbons are converted to methane. The reformer 30 consists of a compact catalytic reactor formed from a stack of plates defining two sets of channels arranged alternately. One set of channels are for the reforming reaction, and contain a reforming catalyst on removable corrugated metal foil supports, while the other set of channels are for the provision of heat. In a modification the pre- reformer and the reformer are separate reactors.

In this embodiment the heat is provided using a separate burner 32, the exhaust gases from the burner 32 at about 850 °C being passed through the reformer 30 in counter- current to the flow of the steam/methane mixture. The reaction channels of the reformer 30 may contain a nickel catalyst in an initial part of the channel, of length between 100 and 200 mm, for example 150 mm, out of a total reaction channel length of 600 mm. In the first part of the channel, where the nickel catalyst is present, pre-reforming takes place, so any higher hydrocarbons will react with steam to produce methane. The remainder of the length of the reaction channels contains a reformer catalyst, for example a

platinum/rhodium catalyst, where the steam and methane react to form carbon monoxide and hydrogen.

The heat for the steam/methane reforming reaction in the reformer 30 is provided by combustion of a fuel gas from a fuel header 34 in a stream of combustion air. In this example the fuel gas is primarily hydrogen. The combustion air is provided by a blower 36 and is preheated in a heat exchanger 38, taking heat from the hot exhaust gases from the combustion after they have passed through the reformer 30. In addition a mixture of steam and alcohol vapour 40 is introduced into the combustion air upstream of the burner 32. After passing through the heat exchanger 38 the exhaust gases may be vented through a stack 39.

A mixture of carbon monoxide and hydrogen at above 800 °C emerges from the reformer 30, and is quenched to below 400 °C by passing it through a steam-raising heat exchanger 42 in the form of a thermosiphon. The heat exchanger 42 is a tube and shell heat exchanger, the hot gases passing through the tubes, and with inlet and outlet ducts communicating with the shell at the top and bottom, and communicating with a steam drum 44. The steam drum 44 is about half full of water, and so water circulates through natural convection between the heat exchanger 42 and the steam drum 44. The resulting steam from the steam drum 44 is supplied to the mixer 26 through a control valve 46.

The gas mixture, which is a form of synthesis gas, may be subjected to further cooling (not shown). It is then subjected to compression using two successive

compressors 50, preferably with cooling and liquid-separation stages (not shown) after each compressor 50. The compressors 50 raise the pressure to about 2.5 MPa (25 atm).

It will be appreciated from equation (1 ) above that the ratio of hydrogen to CO produced in this way is about 3:1 , whereas the stoichiometric requirement is about 2:1 , as is evident from equation (2). The high-pressure synthesis gas is therefore passed by a hydrogen-permeable membrane 52 to remove excess hydrogen. This hydrogen is supplied to the fuel header 34, and is the principal fuel gas. 3. Fischer-Tropsch Synthesis and Product Treatment

The stream of high pressure carbon monoxide and hydrogen is then heated to about 200 °C in a heat exchanger 54, and then fed to a catalytic Fischer-Tropsch reactor 55, this again being a compact catalytic reactor formed from a stack of plates as described above; the reactant mixture flows through one set of channels, while a coolant flows through the other set. The coolant is circulated by a pump 56 and through a heat exchanger 58. The Fischer-Tropsch reaction takes place at about 210 ^q C, and the coolant is circulated at such a rate that the temperature varies by less than 10 K on passage through the reactor 55.

The reaction products from the Fischer-Tropsch synthesis, predominantly water and hydrocarbons such as paraffins, are cooled to about 70 °C to condense the liquids by passage through a heat exchanger 60 and fed to a separating chamber 62 in which the three phases water, hydrocarbons and tail gases separate. The aqueous phase contains water with about 1 -2% oxygenates such as ethanol and methanol which are formed by the Fischer-Tropsch synthesis. Some of the aqueous phase from the separating chamber 62 is treated by steam stripping 63 to separate the oxygenates (marked "alcohol") to leave clean water that may be discharged to waste. The separated oxygenates, which are at an oxygenate concentration of about 80%, may be stored for subsequent use, as described below. The remainder of the aqueous phase is fed as process water through the heat exchanger 58, and hence through a pressure-drop valve 64 into a stripper tank 66. In the stripper tank 66 the aqueous phase boils, typically at a pressure of about 1 .0 MPa (10 atm), the liquid phase being fed from the bottom of the stripper tank 66 into the steam drum 44, while the vapour phase, which contains steam and the bulk of the oxygenates, provides the stream 40 that is introduced into the combustion air through a control valve 68.

The hydrocarbon phase from the separating chamber 62 is the longer-chain hydrocarbon product. The vapour and gas phase from the separating chamber 62 is fed through two successive cooling heat exchangers 70, the second of which cools the vapours to ambient temperature. Any liquids that condense on passage through the first heat exchanger 70 are fed back into the separating chamber 62. The output from the second heat exchanger 70 is fed into a phase separating chamber 72, where the water and light hydrocarbon product liquid separate. The remaining vapour phase, which is at the same pressure as the Fischer-Tropsch reactor 55, is then passed through a heat exchanger 74 to a throttle valve 76 followed by a phase separating vessel 78. As the gas passes through the throttle valve 76 it expands into a lower pressure region adiabatically, with no significant heat input from the surroundings. Consequently, in accordance with the Joule Thomson effect, the gas is cooled considerably. The liquids that emerge from the phase separating pressure 78 contain water and light hydrocarbon product. The gases that emerge from the phase separating vessel 78, which are the tail gases from the Fischer-Tropsch process, are passed back through the heat exchanger 74 to cool the in-flowing gases and, optionally, through a hydrogen permeable membrane (not shown). Part of the tail gas may be fed back into the synthesis gas stream upstream of the first compressor 50. At least part of the tail gas is fed into the fuel header 34, to ensure that there is no excessive build-up of methane in the Fischer-Tropsch reactor 55.

The fuel header 34 not only provides the fuel for the burner 32, but also supplies fuel via a fuel compressor 80 to a gas turbine 82. Indeed compressed fuel gas may also be supplied to other equipment (not shown) that does not form part of the plant 10. The gas turbine 82 may be arranged to provide electrical power for operating the plant 10. As indicated by a broken line in the figure, in this example the electrical power generated by the gas turbine 82 is used to power the compressors 50. Alternatively the gas turbine 82 may be coupled directly to drive the compressors 50.

4. Desulphurisation

It will be appreciated that in the above-described plant 10 a hydrogen-rich gas stream is available in the fuel header 34, the hydrogen being obtained partly from the hydrogen-permeable membrane unit 52 and partly from the tail gas from the Fischer- Tropsch synthesis. There is also a supply of oxygenates ("alcohol"), primarily methanol, obtained by steam stripping 63 of the aqueous phase obtained from the Fischer-Tropsch synthesis. Referring now to figure 2 there is shown a hydrogen-desulphurisation system 85 which forms part of the pretreatment system 25. The natural gas 5, during normal operation of the plant 10, is mixed at 86 with a hydrogen-rich gas such that the hydrogen corresponds to about 2% of the flow rate of natural gas; this may be provided from the fuel header 34, through a valve 84. This gas mixture is then passed through a catalytic reactor 87 in which any mercaptans in the natural gas react to form hydrogen sulphide. The gas mixture is then passed through a hydrogen chloride absorber 88, and then through two absorber beds 90 of zinc oxide arranged in series. The absorber beds 90 absorb any hydrogen sulphide from the gas mixture, and the remaining purified natural gas can then be supplied to the mixer 26.

The concentration of hydrogen sulphide in the gas mixture leaving the first absorber bed 90 is monitored, and if this rises above a threshold value, indicating that the absorptive capacity of the first absorber bed 90 is reaching exhaustion, then the flow direction through the absorber beds 90 is reversed. This has the consequence that the second absorber bed 90 now becomes the first absorber bed; the absorber bed 90 that had been the first absorber bed can now be taken out of the flow path, the zinc oxide replaced, and then reconnected into the flow path.

This hydrogen-desulphurisation system 85 enables the level of sulphur-containing compounds in the remaining natural gas to be kept to no more than a few parts per billion, and so minimises poisoning of the catalysts in the reformer 30 or the Fischer-Tropsch reactor 55.

At start-up of the plant 10 the hydrogen-rich gas is not available. The catalysts that are most likely to be poisoned are those in the reformer 30, and in particular the pre- reformer section, as this is the first catalyst exposed to the natural gas. Particularly in the case where the pre-reformer is a separate reactor, the pre-reformer may be oversized for the expected gas flow, that is to say with an excessive quantity of pre-reformer catalyst, so that even when poisoned there is sufficient catalytic activity.

As an alternative, at start-up, instead of supplying steam and natural gas 5 to the reformer 30, a steam/alcohol mixture may be supplied to the reformer 30. This may for example be produced by vaporising aqueous methanol solution, and if the plant 10 has previously operated a suitable aqueous solution would be the alcohol obtained by steam- stripping 63 the aqueous phase produced by Fischer-Tropsch synthesis. The methanol, particularly that produced as a byproduct of Fischer-Tropsch synthesis, can be

substantially free of sulphur compounds, so poisoning of the catalysts is prevented.

Another possibility is to provide an aqueous alcohol stream to a dedicated reformer 92 so as to produce a gas mixture of hydrogen and carbon monoxide which can be supplied to the mixer 86 through a valve 94. The reformer 92 is similar to the reformer 30 in consisting of a compact catalytic reactor formed from a stack of plates defining two sets of channels arranged alternately. The channels of one set are for the reforming reaction, and contain a reforming catalyst on removable corrugated metal foil supports, while the channels of the other set are for the provision of heat. In this case the heat may be obtained by performing combustion of methane/air or alcohol/air mixture in the second set of channels, providing a combustion catalyst in those channels. This dedicated reformer 92 provides sufficient hydrogen to enable the catalytic reactor 87 and the absorber beds 90 to remove sulphur-containing compounds. Once the plant 10 is in operation, the hydrogen- rich gas stream from the fuel header 34 may be again supplied to the mixer 86, so the valve 94 can be closed, and operation of the dedicated reformer 92 cease.

Another possibility is to supply water to an electrolysis cell 95 which is connected to a source 96 of electricity. In the case of a FPSO, this may be powered by the positioning power supply for the FPSO. Hydrogen is produced by electrolysis at a cathode 97 and may be supplied to the mixer 86 through a valve 98; oxygen produced at an anode 99 may be discharged into the atmosphere. The electrolysis cell 95 is required to produce sufficient hydrogen to enable the catalytic reactor 87 and the absorber beds 90 to remove sulphur-containing compounds. Once the plant 10 is in operation, the hydrogen-rich gas stream from the fuel header 34 may be again supplied to the 86, so the valve 98 can be closed, and operation of the electrolysis cell 95 cease.