PROCESS FOR PRODUCING A SUBSTITUTE NATURAL GAS

Description

[0001] This invention relates to a process for the production of fuel gases suitable for use as a substitute natural gas (SNG) from a synthesis gas.

[0002] SNG is a clean fuel which can be distributed with existing natural gas pipelines and facilities, and can be used as a substitute for natural gas in a wide range of applications.

[0003] Process to produce substitute natural gas (SNG) involves catalytic methanation of a synthesis gas comprising hydrogen and carbon oxides. By the reaction of methanation, the synthesis gas is converted to a product consisting of 95% or more of methane (CH₄) with small amounts of carbon dioxide, hydrogen and inerts. The synthesis gas may be obtained from coal or petcoke or biomass gasification. The methanation of the syngas involves the following, highly exothermic reactions:

        CO + 3H₂ → CH₄ + H₂O ΔH = minus 206 kJ/mol

        CO₂ + 4H₂ → CH₄ + 2H₂O ΔH = minus 165 kJ/mol

[0004] Typically the reactions are carried out in a methanation section comprising a plurality of adiabatic reactors operated in series with heat recovery and gas recirculation. Heat recovery and gas recirculation are used to keep the exothermic reactions under control and avoid an excessive temperature inside reactors, that may damage the reactor itself and/or the catalyst. Heat recovery may be provided by heat exchangers cooling the hot gas stream at the outlet of each reactor e.g. by producing high pressure steam. Recirculation is a further measure to control the reaction rate and the temperature inside the reactors, by dilution of the fresh synthesis gas fed to the first reactor with a portion of the reacted gas. The gas recirculation requires the provision of an appropriate compressor.

[0005] Various processes are known for producing SNG. One such process is described in US 4016189. Here the feed stream is treated in a single high temperature bulk methanator followed by treatment in a single low temperature trim methanator. In this process all of the fresh feed is fed to the bulk methanator where a large proportion of the carbon oxides are methanated to methane. Since the reaction is highly exothermic, a thermal mass is required to limit the temperature rise across the bulk methanator to an acceptable level. This thermal mass is supplied in the form of a recycle gas which is taken from downstream of the bulk methanator but prior to the trim methanator. The recycle stream is compressed prior to being fed upstream of the bulk methanator. The single stage of trim methanation described in US 4016189 is adequate to produce a low calorific gas with a methane content of 60%. This is below the required methane level for current SNG product specifications.

[0006] In general it should be noted that a bulk methanator is one which receives part or all of the synthesis gas feed, i.e. fresh synthesis gas feed to the plant. Thus a "bulk methanator" is a reactor in which a reactant gas comprising at least a portion of fresh synthesis gas is catalytically methanated. A trim methanator is one that does not receive any fresh synthesis gas feed and carries out trim methanation on a partially methanated gas stream, usually at lower temperature than in the bulk methanator, to produce a SNG product. Thus a "trim methanator" is a reactor in which a reactant gas, consisting of a partially methanated gas recovered from either a bulk methanator or a trim methanator, is catalytically methanated.

[0007] US 2013/055637 A1 discloses a method for processing a hydrocarbon, comprising: gasifying a feedstock within a gasifier to provide a raw syngas; processing the raw syngas within a purification system to provide a treated syngas; converting a first portion of the treated syngas into a first effluent in a first methanator; mixing the first effluent with a second portion of the treated syngas to provide a first mixed effluent; converting the first mixed effluent into a second effluent in a second methanator; mixing the second effluent with a third portion of the treated syngas to provide a second mixed effluent; converting the second mixed effluent into a third effluent in a third methanator. Modern SNG plants typically have two or more bulk methanators in series. For example, an alternative process is described in WO2012001401 (A1), which discloses providing a feed stream to a first and/or second and/or subsequent bulk methanator; subjecting that feed stream to methanation in the presence of a suitable catalyst; removing an at least partially reacted stream from the first bulk methanator and supplying it to the second and/or subsequent bulk methanator where it is subjected to further methanation; passing a product stream from the final bulk methanator to a trim methanator train where it is subjected to further methanation; removing a recycle stream downstream of the first, second or subsequent bulk methanator, and, in any order, passing it through a compressor, subjecting it to cooling and then supplying to a trim and/or recycle methanator for further methanation before being recycled to the first and/or second and/or subsequent methanator. A recycle methanator is one which is contained within the recycle loop returning a methanated gas stream to an upstream methanator and which does not receive any fresh synthesis gas feed.
Other methanation processes are described in GB2060686, CN102329671, CN102585949 and EP2110425.
Such processes are however designed around single synthesis gas feeds and where different feeds are available, separate, unconnected SNG production trains are used due to differences in gas composition and operating pressure.

[0008] Furthermore, whereas having two bulk methanators in series is useful for minimising the pressure drop over the plant, the process requires a higher product gas recycle and places a limitation on capacity due to the maximum size of the methanator vessels that may be fabricated. Therefore, currently for large-scale plants with higher capacities, reactors and equipment items inside the bulk methanation recycle gas loop have to be twinned, i.e. parallel sets of reactors and ancillary equipment have to be used. A large-scale SNG Plant may be considered to be one with a capacity that requires installation of at-least two bulk methanators in series with one or both of the bulk methanators also having parallel vessels due to the transportation and/or shop floor manufacturing limitations.

[0009] We have now surprisingly found that by increasing the number of bulk methanators and providing bulk methanation both inside and outside the recycle gas loop, higher capacities can be achieved without the need for parallel reactors and ancillary equipment items inside the recycle gas loop. Furthermore, we have realised that reactors outside the recycle gas loop can be fed with gas at lower pressure.

[0010] Accordingly, the invention provides a process for producing a substitute natural gas comprising the steps of: feeding a first synthesis gas feed stream comprising hydrogen, methane, carbon monoxide and/or carbon dioxide in parallel to two or more bulk methanators in a first bulk methanation zone comprising a first bulk methanator and a final bulk methanator, feeding a second synthesis gas feed stream comprising hydrogen, carbon monoxide and/or carbon dioxide to one or more bulk methanators in a second bulk methanation zone comprising a first bulk methanator, each methanator containing a methanation catalyst such that the feed streams are at least partially methanated, dividing the methanated gas stream recovered from the final bulk methanator in the first bulk methanation zone into a first portion and a second portion, recirculating the first portion in a recirculation loop to the first bulk methanator of the first bulk methanation zone to dilute the first synthesis gas feed stream fed to said first bulk methanator, and feeding the second portion to the first bulk methanator of the second bulk methanation zone to dilute the second synthesis gas feed stream fed to said first bulk methanator, wherein the feed pressure of the second synthesis gas feed stream is lower than the feed pressure of the first synthesis gas feed stream and the difference in pressure between the first and second feed streams is at least the pressure drop through the first bulk methanation zone.

[0011] The invention further comprises a methanation system for converting first and second synthesis gas feed streams into substitute natural gas using the first and second methanation zones, said methanation system being adapted to operate according to the claimed process. Accordingly the invention includes a methanation system comprising a first synthesis gas feed stream supply configured to supply a first synthesis gas feed stream comprising hydrogen, methane, carbon monoxide and/or carbon dioxide at a first feed pressure in parallel to two or more bulk methanators in a first bulk methanation zone comprising a first bulk methanator, and a final bulk methanator, a second synthesis gas feed stream supply configured to supply a second synthesis gas feed stream comprising hydrogen, carbon monoxide and/or carbon dioxide at a feed pressure lower than the first synthesis gas feed pressure by at least the pressure drop through the first methanation zone, to one or more bulk methanators in a second bulk methanation zone comprising a first bulk methanator, each methanator containing a methanation catalyst, wherein dividing means are provided downstream of the final bulk methanator in the first bulk methanation zone that divide the methanated gas stream recovered from the final bulk methanator in the first bulk methanation zone into a first portion and second portion, wherein a recirculation loop is connected to the dividing means so that the first portion may be recirculated to the first bulk methanator of the first methanation zone to dilute the first synthesis gas feed stream fed to said first bulk methanator, and wherein the dividing means are connected to the second methanation zone so that the second portion may be fed to the first bulk methanator of the second methanation zone to dilute the second synthesis gas feed stream fed to said first bulk methanator.

[0012] Compared to prior art processes, the present invention offers lower recycle flow and power consumption and higher capacities can be achieved without installing parallel equipment items. Such a process offers significant capital savings over prior art processes. The present invention also process offers a more flexible arrangement and the plant is able to utilise feed streams with different pressures and different methane contents. Simplification of the design also offers lower design and installation costs compared to prior art processes.

[0013] The first and second feed streams are synthesis gases comprising hydrogen, carbon dioxide and carbon monoxide. Other gases such as nitrogen and/or methane and /or higher hydrocarbons may also be present in the feed stream. The synthesis gas feed streams have not been subjected to methanation but may contain <15mole% methane. The feed stream may be formed from the gasification of carbonaceous feedstocks, such as coal or petcoke or biomass using conventional techniques. Alternatively, the feed stream mixture may be prepared by mixing a hydrogen-containing gas mixture with a carbon dioxide-containing gas mixture. The hydrogen containing gas mixture may be a synthesis gas or may be a gas stream containing hydrogen.

[0014] The first and second feed streams may have the same or different compositions.

[0015] In the present process, the feed pressure of the second feed stream is lower than the feed pressure of the first feed stream and the difference in pressure between the streams is at least the pressure drop through the first bulk methanation zone, i.e. the second stream pressure is the same as or lower than the pressure of the methanated gas recovered from the final bulk methanator in the first bulk methanation zone. The pressure of the methanated gas recovered from the final bulk methanator in the first bulk methanation zone is lower than the feed pressure of the first stream, i.e. there is a pressure drop through the first bulk methanation zone. This is because of the resistance to flow of the first feed gas through the catalyst within the methanators and the pipework connecting them. The pressure of the first feed stream may be in the range 5-80 bar abs, preferably 15-80 bar abs. The pressure drop through the first bulk methanation zone may be 3 to 10 bar or higher. Hence, the difference in pressure between the first and send streams may be in the range 3-15 bar, for example 3-10 bar. Preferably, the second stream pressure is the same as the pressure of the methanated gas recovered from the final bulk methanator in the first bulk methanation zone. The pressure of the second feed stream may therefore be adjusted if necessary using conventional means to provide the desired pressure. Where the difference in pressure between the first and second feed streams is larger than the pressure drop through the first bulk methanation zone, the pressure of the methanated gas recovered from the first methanation zone may be lowered using conventional means before feeding it to the first bulk methanator of the second bulk methanation zone.

[0016] In particular, in the present invention, methanators inside the recirculation loop can be operated with a first synthesis gas feed stream having a higher pressure, such as a synthesis gas from a block coal gasifier, and methanators outside the recirculation loop can be operated with a second synthesis gas feed stream having a lower pressure, such as a synthesis gas from a dust coal gasifier. In addition to the pressure difference, such feed streams may differ in their compositions, e.g. their methane contents. The present process is able to cater for these compositional differences as well.

[0017] In the methanation process it is desirable that for a feed stream containing carbon monoxide, carbon dioxide and hydrogen for x mols/hr of carbon monoxide and y mols/hr carbon dioxide, and z mols/hr hydrogen; z is about (3x + 4y). The upstream adjustment of the feed stream composition may be achieved using known methods, such as by employing one or more water-gas shift stages and/or a stage of acid gas removal (AGR).

[0018] It may be desirable, in order to prevent catalyst poisoning, to subject the first and second feed streams to a desulphurisation step prior to the methanation process. For example the feed stream mixtures may be passed over separate beds of a particulate zinc oxide desulphurisation material. Suitable inlet temperatures for desulphurisation are in the range 100-300°C. A particularly effective zinc oxide desulphurisation material is Puraspec_JM™ 2020, available from Johnson Matthey PLC. In addition, should the first and/or second feed stream mixtures contain unsaturated compounds (e.g. dienes or acetylenes) that might present coking problems on the methanation catalysts, these maybe removed by hydrogenation over a suitable hydrogenation catalyst, such as a copper catalyst. Oxygen and organic sulphur compounds may also be removed using a suitable catalyst or sorbent, such as a copper catalyst, upstream of the first bulk methanator.

[0019] The methanation catalyst used in bulk methanators is desirably a nickel- or ruthenium-methanation catalyst, preferably a particulate nickel-containing methanation catalyst, more preferably a precipitated Ni catalyst with a Ni content in the range 35 to ≥ 50% by weight. Particularly suitable methanation catalysts are Katalco™ CRG-S2R and Katalco™ CRG-S2CR available from Johnson Matthey PLC. The same or different methanation catalyst may be present in the first, second and/or subsequent methanation reactors in each of the first and second bulk methanation zones. The methanation catalyst may be in the form of pellets or extrudates, but may also be a foam, monolith or coating on an inert support. Particulate methanation catalysts are preferred such that the feed stream is preferably passed over a fixed bed of particulate methanation catalyst disposed within each methanator. Suitable particulate catalysts are pellets or extrudates with a diameter or width in the range 2-10 mm and an aspect ratio, i.e. length /diameter or width in the range 0.5 to 4. The flow through the catalyst in the first, second and one or more subsequent bulk methanators may be axial-flow, radial flow or axial-radial flow.

[0020] The bulk methanators in the first and/or second bulk methanation zones may contain another type of catalyst in addition to the methanation catalyst. For example, a water-gas shift catalyst and/or a methanol synthesis catalyst may be included upstream of the methanation catalyst in one or more of the bulk methanators. Suitable water-gas shift catalysts include those based on iron, copper and cobalt/molybdenum. Suitable methanol synthesis catalysts include those based on copper/zinc oxide/alumina.

[0021] The methanation catalyst may be operated at an inlet temperature in the range 200-450°C, preferably 200-350°C, more preferably 300-350°C. The inlet temperature may be achieved by applying heat exchange to the feed streams with a suitable heating medium. In one embodiment the feed stream heating may be done using hot product gas recovered from the final bulk methanator or the final trim methanator using a suitable gas-gas interchanger. Where the methanators are operated adiabatically, the exit temperatures may be in the range 450-750°C, preferably 500-650°C and more preferably 550-650°C. The gas hourly space velocity (GHSV) of the feed stream mixtures through the catalyst beds may be in the range 2000 to 20000hr^-1.

[0022] The first bulk methanation zone comprises a first bulk methanator, a final bulk methanator and optionally one or more bulk methanators in between the first and final bulk methanators, hence the first bulk methanation zone may comprise a second bulk methanator and optionally one or more further bulk methanators. Hence two, three, four or more bulk methanators may be employed in the first bulk methanation zone, i.e. N may be in the range 2-10, preferably 2-4, where N is the number of bulk methanators in the first bulk methanation zone.

[0023] The second bulk methanation zone comprises a first bulk methanator. This may be the only bulk methanator in the second bulk methanation zone, in which case it may be described as the first and final bulk methanator in the second bulk methanation zone. However the second bulk methanation zone may comprise one or more additional methanators such that it comprises, a first bulk methanator, a final bulk methanator and optionally one or more bulk methanators in between the first and final bulk methanators, hence the second bulk methanation zone may comprise a second bulk methanator and optionally one or more further bulk methanators. Hence one, two, three, four or more bulk methanators may be employed in the second bulk methanation zone, i.e. N may be in the range 1-10, preferably 2-4, where N is the number of bulk methanators in the second bulk methanation zone.

[0024] In one preferred arrangement, the number, N, of bulk methanators in the first bulk methanation zone is 3 and the number of bulk methanators in the second bulk methanation zone is 2.

[0025] The first feed stream is fed in parallel to the bulk methanators in the first bulk methanation zone. Where two or more bulk methanators are present in the second bulk methanation zone, the second feed stream is desirably fed in parallel to each bulk methanator in the second bulk methanation zone.

[0026] The bulk methanators in the first bulk methanation zone are desirably connected in series. In this way the feed gas to the second and each subsequent bulk methanator, if present, may be diluted with a methanated gas recovered from the previous bulk methanator. Where there are two or more bulk methanators in the second bulk methanation zone, the bulk methanators may also desirably be connected in series. Hence where there are two or more bulk methanators in each bulk methanation zone, preferably in each methanation zone the methanators are connected in series.

[0027] The portions of the feed streams fed to the bulk methanators in each bulk methanation zone may be the same or different. Where there is only one bulk methanator in the second bulk methanation zone, 100% by volume of the second feed stream is fed to that bulk methanator. Where there are two or more bulk methanators in each bulk methanation zone, the portion of the feed streams fed to the first bulk methanators in each bulk methanation zone may be in the range 10vol% to 60vol% of the first or second feed stream, the exact value being adjusted to control the methanator isotherm. However, it will be understood that the split of feed between the methanators will depend on the number of bulk methanators, the operating conditions and the feed composition.

[0028] In each bulk methanator, the hydrogen reacts with carbon dioxide and carbon monoxide to form methane. A portion of the hydrogen in the feed stream typically remains unreacted because there is an equilibrium limitation on the extent of conversion.

[0029] Whereas the present process is particularly suited for adiabatic operation of the bulk methanators, if desired cooling may be applied to one or more methanation catalyst beds by passing a coolant, such as a portion of a feed stream, through one or more heat exchange devices disposed within the catalyst. The coolant flow may be arranged co-current or countercurrent to the flow of reacting gases passing through the methanators.

[0030] In order to prevent overheating of the catalyst and unwanted side reactions it is desirable to adjust the temperature of the partially methanated gas mixture recovered from the first and subsequent bulk methanators before mixing it with the feed streams. This may be performed by passing the partially methanated gas mixture through one or more heat exchangers, such as a shell and tube heat exchanger fed with water under pressure as the cooling medium.

[0031] A recirculation loop is used to provide a partially methanated gas to the first bulk methanator in the first bulk methanation zone to dilute the portion of the first feed stream fed to it. The re-circulation loop may be configured using known methods such as using a recycle compressor or by using a steam ejector. A steam ejector may also add steam to the process to dilute the feed stream or provide steam for water-gas shift. Preferably the recycle loop comprises a compressor for the re-circulated gas stream and a pre-heater for heating the diluted gas stream fed to the first bulk methanator. This preheater may be a gas-gas interchanger fed with a hot methanated gas stream, e.g. a product gas stream from a final bulk methanator in first methanation zone or from a bulk methanator in second zone methanation zone or a trim methanator.

[0032] In both bulk methanation zones, the volume ratio between the total diluted gas flow entering the first bulk methanator, and the feed stream fed to the first bulk methanator may be between 1.5 and 7, with the exact value depending on the feed stream composition and pressure.

[0033] In both bulk methanation zones, steam may be added at the inlet of at least the first bulk methanator to further dilute the feed stream. Hence, if desired steam may be added to the feed stream to at least one of the bulk methanators in each bulk methanation zone.

[0034] A methane-containing substitute natural gas product may be recovered from the final bulk methanator of the second bulk methanation zone. If desired, the methane-containing substitute natural gas product may be subjected to further processing including subjecting it to one or more further stages of methanation in a trim methanation zone. Trim methanators may be used to produce high-specification substitute natural gases. The trim methanator zone may comprise one or more, e.g. 1 to 4, particularly 1 or 2, trim methanators. Where more than one trim methanator is present, they will generally be located in series and be fed with a gas mixture consisting of a methanated gas stream and optionally steam. The inlet temperature for trim methanators may be in the range 200-300°C, preferably 230-280°C. Where more than one trim methanator is used, they may be operated at the same temperature or the temperature may be lower in the second and any subsequent trim methanator(s) than in the first trim methanator. Otherwise the trim methanation zone may be operated using the same catalysts and catalyst arrangements as the bulk methanation zones.
A fully methanated substitute natural gas product may be recovered from the final trim methanator, if used. The fully methanated gas may be subjected to one or more further SNG
preparation stages such as drying to remove water and/or carbon dioxide removal. The drying may be performed by cooling the product gas stream to below the dew point and collecting the liquid condensate, optionally with polishing over a suitable desiccant such as molecular sieves or silica gel. CO₂-removal, if required, may be accomplished using solvent- or amine-wash techniques known in the art.
The invention is further illustrated by reference to the accompanying drawings in which; Figure 1 is a depiction of a flow sheet of one embodiment according to the present invention.

[0035] It will be understood by those skilled in the art that the drawings are diagrammatic and that further items of equipment such as feedstock drums, pumps, vacuum pumps, compressors, gas recycling compressors, temperature sensors, pressure sensors, pressure relief valves, control valves, flow controllers, level controllers, holding tanks, storage tanks and the like may be required in a commercial plant. Provision of such ancillary equipment forms no part of the present invention and is in accordance with conventional chemical engineering practice.

[0036] One embodiment of the present invention is illustrated in Figure 1. In Figure 1, a first desulphurised synthesis gas feed stream comprising hydrogen, methane, carbon monoxide and/or carbon dioxide is fed in line 110 to a first bulk methanation zone which consists of three bulk methanators 114, 116, & 118, each containing a bed of particulate methanation catalyst. A second desulphurised synthesis gas feed stream comprising hydrogen, carbon monoxide and/or carbon dioxide and having a lower feed pressure than the first feed stream 110 is fed in line 112 to a second bulk methanation zone which consists of two bulk methanators 120 and 122, each containing a bed of particulate methanation catalyst.

[0037] The second feed stream 112 is at a lower pressure than the first feed stream 110. The pressure difference between the first feed stream 110 and the second feed stream 112 is the same as the pressure drop through the first bulk methanation zone.

[0038] The first bulk methanator 114, the second bulk methanator 116 and third bulk methanator 118 are each fed with a portion of the first feed stream 110 by lines 124, 126, and 128 respectively. The fourth bulk methanator 120 and fifth bulk methanator 122 are each fed with a portion of the second feed stream 112 by lines 130, and 132 respectively. The feed streams are methanated in the bulk methanators 114, 116, 118, 120 & 122. The methanated gas stream from the first bulk methanator 114 in the first bulk methanation zone is passed in line 134 to heat exchanger 136 where it is cooled before being added via line 138 to the feed stream 126 to the second bulk methanator 116. The methanated gas stream from the second bulk methanator 116 is passed in line 140 to a heat exchanger 142 where it is cooled before being added via line 144 to the feed stream 128 to the third bulk methanator 118. The methanated gas stream from the third bulk methanator 118 is passed in line 146 to a heat exchanger 148 where it is cooled. A portion of the cooled stream from the heat exchanger 148 is passed in a recycle loop in line 152 to a compressor 154. The compressed methanated gas from the compressor 154 is passed via line 156 to dilute the feed stream fed to the first bulk methanator 114. If desired the compressed methanated gas may be heated to a suitable methanation inlet temperature in a heat exchanger (not shown). The remaining portion of the methanated gas stream from heat exchanger 148 is passed via line 150 to dilute the portion of the second feed stream fed to the second bulk methanation zone first bulk methanator 120. The methanated gas stream from the first bulk methanator of the second bulk methanation zone 120 is passed in line 158 to a heat exchanger 160 where it is cooled before being added via line 162 to dilute the feed stream to the second bulk methanator of the second bulk methanation zone 122. The product from the second bulk methanator of the second bulk methanation zone 122 is removed in line 164 and passed through heat exchanger 166 where it is cooled. It is then passed in line 168 to one or more subsequent trim methanators (not shown). The product SNG is withdrawn from the trim methanator and then is cooled and dried.

[0039] Depending on the feed composition and the operating conditions, it may be necessary or desirable to remove water from the methanated gas recovered from the third bulk methanator 118. This can be conveniently done before the compressor.

[0040] Steam may be added in line 124 or 130. This will only be required with some feed compositions and operating conditions.

[0041] The invention is further illustrated by reference to the following Example.

Example 1

[0042] This example is based on a production capacity of 1,000,000 Nm³/h. The process is fed with a first synthesis gas feed stream comprising hydrogen, carbon oxides and methane at a pressure of 3.6 MPa (abs). The first desulphurised feed stream composition is as follows;

	vol%
Water	0.92
Hydrogen	66.66
Carbon Monoxide	20.19
Carbon Dioxide	1.43
Methane	10.18
Nitrogen & Argon	0.17
Ethane	0.36
Propane	0.09

[0043] The process is fed with a second synthesis gas feed stream comprising hydrogen, carbon oxides at a pressure of 2.8 MPa (abs). The second desulphurised feed stream composition is as follows;

	vol%
Water	0.10
Hydrogen	74.76
Carbon Monoxide	23.40
Carbon Dioxide	1.20
Methane	0.03
Nitrogen & Argon	0.51

[0044] The product specification is as follows;

	vol%
Hydrogen	< 2%
Carbon Dioxide	< 1%
Methane	> 95%

[0045] In a comparative process, 4 trains operating in parallel with 2 bulk methanators in series are required for the first higher pressure feed stream and 2 trains operating in parallel with 2 bulk methanators in series are required for the second lower pressure feed stream. The reactors/equipment items inside the bulk methanation recycle loop are twinned due to manufacturing and transportation limitations. Thus the 6 trains would have 24 bulk methanator reactor vessels and 6 compressors. The required recycle gas flow is approximately 4 x 23,300 kmol/h and recycle compressor shaft power is approximately 4 x 3,800 kW for high methane feed stream. The required recycle gas flow is approximately 2 x 33,000 kmol/h and recycle compressor shaft power is approximately 2 x 8,500 kW for low methane feed stream.

[0046] In a process according to the flow sheet depicted in Figure 1, 4 trains operating in parallel with 5 bulk methanators in series are required, with 3 methanators placed inside the recycle loop and 2 methanators placed outside the recycle loop. Bulk methanators placed inside the recycle gas loop are fed with the first feed stream and bulk methanators placed outside the recycle gas loop are fed with the second feed stream.

[0047] The equipment count is reduced and the required catalyst volumes remain the same as current processes. The required recycle gas flow is approximately 4 x 13,550 kmol/h and recycle compressor shaft power is approximately 4 x 3000 kW. Instead of 6 trains, the proposed process would require 4 trains having 20 bulk methanation vessels and 4 compressors.

[0048] The following Table sets out the operation of this flow sheet for one train using Katalco™ CRG-S2R, and Katalco™ CRG-S2CR.

Stream Number	110	124	134	126	140	128	146	156	112	130	158	132	164
Temperature (°C)	190	320	620	320	620	320	620	365	175	320	621	320	621
Pressure (MPa abs)	3.08	3.03	2.97	2.94	2.88	2.84	2.78	3.04	2.66	2.65	2.600	2.53	2.48
Vapour Flow (kNm3/h)	544.2	428.7	378.0	560.0	495.8	732.7	649.3	303.6	273.7	444.2	393.8	543.2	482.1
Composition (mol%)
H2O	0.92	11.83	20.36	14.04	21.80	15.05	22.87	16.33	0.10	16.50	24.41	17.72	25.72
H2	66.66	35.73	20.05	35.20	20.66	35.53	21.17	22.97	74.76	36.17	22.19	36.66	22.87
CO	20.19	7.17	1.65	7.67	1.65	7.64	1.66	1.80	23.40	7.74	1.73	7.69	1.75
CO2	1.43	3.57	3.82	3.04	3.98	3.15	4.10	4.45	1.20	3.29	4.31	3.46	4.47
CH4	10.18	41.32	53.82	39.63	51.61	38.22	49.91	54.14	0.03	35.96	46.96	34.05	44.70
N2	0.17	0.27	0.31	0.26	0.30	0.26	0.29	0.32	0.51	0.35	0.40	0.43	0.48
Ethane	0.36	0.11	-	0.12	-	0.12	-	-	-	-	-	-	-
Propane	0.09	-	-	0.03	-	0.03	-	-	-	-	-	-	-

[0049] The catalyst volumes are as follows;

Bulk Methanator	114	116	118	120	122
Catalyst Bed Diameter (mm)	4190	5070	5800	4455	4925
Catalyst volume (m3)	33	43	57	34	42

Claims

1. A process for producing a substitute natural gas comprising the steps of: feeding a first synthesis gas feed stream comprising hydrogen, methane, carbon monoxide and/or carbon dioxide in parallel to two or more bulk methanators in a first bulk methanation zone comprising a first bulk methanator and a final bulk methanator wherein the bulk methanators in the first bulk methanation zone are connected in series, feeding a second synthesis gas feed stream comprising hydrogen, carbon monoxide and/or carbon dioxide to two or more bulk methanators in a second bulk methanation zone comprising a first bulk methanator, wherein the bulk methanators in the second bulk methanation zone are connected in series, each bulk methanator containing a methanation catalyst such that the feed streams are at least partially methanated, dividing the methanated gas stream recovered from the final bulk methanator in the first bulk methanation zone into a first portion and a second portion, recirculating the first portion in a recirculation loop to the first bulk methanator of the first bulk methanation zone to dilute the first synthesis gas feed stream fed to said first bulk methanator, and feeding the second portion to the first bulk methanator of the second bulk methanation zone to dilute the second synthesis gas feed stream fed to said first bulk methanator, wherein the feed pressure of the second synthesis gas feed stream is lower than the feed pressure of the first synthesis gas feed stream and the difference in pressure between the first and second feed streams is at least the pressure drop through the first bulk methanation zone.

2. A process according to claim 1, wherein the first feed stream is a desulphurised synthesis gas obtained from a gasifier delivering gas at a higher pressure than second feed stream and the second feed stream is a desulphurised synthesis gas obtained from a gasifier delivering gas at a pressure lower than first feed stream.

3. A process according to claim 1 or claim 2, wherein the first feed stream is a desulphurised synthesis gas obtained from block coal gasifier and the second feed stream is a desulphurised synthesis gas obtained from a dust coal gasifier.

4. A process according to any one of claims 1 to 3, wherein the methanation catalyst is operated at an inlet temperature in the range 200-450 °C.

5. A process according to any one of claims 1 to 4, operated at a pressure in the range 5-80 bar abs.

6. A process according to any one of claims 1 to 4, wherein the number of bulk methanators in the first bulk methanation zone is in the range 2-10 and the number of bulk methanators in the second bulk methanation zone is in the range 2-10.

7. A process according to any one of claims 1 to 6, wherein the number, N, of bulk methanators in the first bulk methanation zone is in the range 2-4, and the number of bulk methanators in the second bulk methanation zone is in the range 2-4.

8. A process according to any one of claims 1 to 7, wherein two or more bulk methanators are present in the second bulk methanation zone and the second feed stream is fed in parallel to each bulk methanator in the second bulk methanation zone.

9. A process according to any one of claims 1 to 8, wherein two or more bulk methanators are present in each bulk methanation zone and the portion of the feed stream fed to the first methanator in each bulk methanation zone is in the range 10vol% to 60vol% of the feed stream.

10. A process according to any one of claims 1 to 9, wherein the re-circulation loop comprises a compressor for the re-circulated gas stream and a pre-heater for heating the diluted gas stream fed to the first bulk methanator in the first bulk methanation zone.

11. A process according to any one of claims 1 to 10, wherein steam is added to the feed stream to at least one of the bulk methanators in each bulk methanation zone.

12. A process according to any one of claims 1 to 11, further comprising subjecting a product gas from the second bulk methanation zone to further methanation in one or more trim methanators.

13. A process according to claim 12, further comprising subjecting a product gas from a final trim methanator to a drying step.

14. A methanation system for converting first and second feed streams into substitute natural gas using first and second methanation zones, said methanation system being adapted to operate according to the process of any one of claims 1 to 13.

Ansprüche

1. Verfahren zur Herstellung eines Ersatz-Erdgases, umfassend folgende Schritte: Einspeisen eines ersten Synthesegaseinspeisstroms, umfassend Wasserstoff, Methan, Kohlenmonoxid und/oder Kohlendioxid, parallel zu zwei oder mehr Bulk-Methanisierern in einer ersten Bulk-Methanisierungszone, umfassend einen ersten Bulk-Methanisierer und einen letzten Bulk-Methanisierer, wobei die Bulk-Methanisierer in der ersten Bulk-Methanisierungszone in Serie verbunden sind, Einspeisen eines zweiten Synthesegaseinspeisstroms, umfassend Wasserstoff, Kohlenmonoxid und/oder Kohlendioxid in zwei oder mehr Bulk-Methanisierern in einer zweiten Bulk-Methanisierungszone, umfassend einen Bulk-Methanisierer, wobei die Bulk-Methanisierer in der zweiten Bulk-Methanisierungszone in Serie verbunden sind, jeder Bulk-Methanisierer einen Methanisierungskatalysator enthält, so dass die Einspeisströme zumindest teilweise methanisiert werden, wobei der aus dem letzten Bulk-Methanisierer in der ersten Bulk-Methanisierungszone gewonnene methanisierte Gasstrom in einen ersten Abschnitt und einen zweiten Abschnitt unterteilt wird, der erste Abschnitt in einer Rückführungsschleife zu dem ersten Bulk-Methanisierer der ersten Bulk-Methanisierungszone zurückgeführt wird, um den ersten Synthesegaseinspeisstrom zu verdünnen, der dem ersten Bulk-Methanisierer zugeführt wird, und Einspeisen des zweiten Abschnitts in den ersten Bulk-Methanisierer der zweiten Bulk-Methanisierungszone, um den dem ersten Bulk-Methanisierer zugeführten zweiten Synthesegaseinspeisstrom zu verdünnen, wobei der Einspeisdruck des zweiten Synthesegaseinspeisstroms niedriger ist als der Einspeisdruck des ersten Synthesegaseinspeisstroms und die Druckdifferenz zwischen dem ersten und dem zweiten Einspeisstrom mindestens dem Druckabfall durch die erste Bulk-Methanisierungszone entspricht.

2. Verfahren nach Anspruch 1, wobei der erste Einspeisstrom ein entschwefeltes Synthesegas ist, das aus einem Vergaser gewonnen wird, der Gas mit einem höheren Druck als der zweite Einspeisstrom liefert, und der zweite Einspeisstrom ein entschwefeltes Synthesegas ist, das aus einem Vergaser gewonnen wird, der Gas mit einem niedrigeren Druck als der erste Einspeisstrom liefert.

3. Verfahren nach Anspruch 1 oder Anspruch 2, wobei der erste Einspeisstrom ein entschwefeltes Synthesegas aus einem Blockkohlevergaser und der zweite Einspeisstrom ein entschwefeltes Synthesegas aus einem Staubkohlevergaser ist.

4. Verfahren nach einem beliebigen der Ansprüche 1 bis 3, wobei der Methanisierungskatalysator bei einer Eintrittstemperatur im Bereich von 200-450 °C betrieben wird.

5. Verfahren nach einem beliebigen der Ansprüche 1 bis 4, betrieben bei einem Druck im Bereich von 5-80 bar abs.

6. Verfahren nach einem beliebigen der Ansprüche 1 bis 4, wobei die Anzahl der Bulk-Methanisierer in der ersten Bulk-Methanisierungszone im Bereich von 2 bis 10 und die Anzahl der Bulk-Methanisierer in der zweiten Bulk-Methanisierungszone im Bereich von 2 bis 10 liegt.

7. Verfahren nach einem beliebigen der Ansprüche 1 bis 6, wobei die Anzahl N der Bulk-Methanisierer in der ersten Bulk-Methanisierungszone im Bereich von 2 bis 4 und die Anzahl der Bulk-Methanisierer in der zweiten Bulk-Methanisierungszone im Bereich von 2 bis 4 liegt.

8. Verfahren nach einem beliebigen der Ansprüche 1 bis 7, wobei zwei oder mehr Bulk-Methanisierer in der zweiten Bulk-Methanisierungszone vorhanden sind und der zweite Einspeisstrom parallel zu jedem Bulk-Methanisierer in der zweiten Bulk-Methanisierungszone zugeführt wird.

9. Verfahren nach einem beliebigen der Ansprüche 1 bis 8, wobei zwei oder mehr Bulk-Methanisierer in jeder Bulk-Methanisierungszone vorhanden sind und der Anteil des dem ersten Methanisierer in jeder Bulk-Methanisierungszone zugeführten Einspeisstroms im Bereich von 10 Vol.-% bis 60 Vol.-% des Einspeisstroms liegt.

10. Verfahren nach einem beliebigen der Ansprüche 1 bis 9, wobei die Rückführungsschleife einen Kompressor für den zurückgeführten Gasstrom und einen Vorwärmer zum Erhitzen des dem ersten Bulk-Methanisierer in der ersten Bulk-Methanisierungszone zugeführten verdünnten Gasstroms umfasst.

11. Verfahren nach einem beliebigen der Ansprüche 1 bis 10, wobei dem Einspeisstrom in mindestens einem der Bulk-Methanisierer in jeder Bulk-Methanisierungszone Dampf hinzugefügt wird.

12. Verfahren nach einem beliebigen der Ansprüche 1 bis 11, ferner umfassend die weitere Methanisierung eines Produktgases aus der zweiten Bulk-Methanisierungszone in einem oder mehreren Trim-Methanisierern.

13. Verfahren nach Anspruch 12, ferner umfassend das Unterziehen eines Produktgases aus einem Endtrimm-Methanisierer einem Trocknungsschritt.

14. Methanisierungssystem zum Umwandeln eines ersten und eines zweiten Einspeisstroms in Ersatz-Erdgas unter Verwendung einer ersten und einer zweiten Methanisierungszone, wobei das Methanisierungssystem angepasst ist, um nach dem Verfahren eines der Ansprüche 1 bis 13 betrieben zu werden.

Revendications

1. Procédé de production d'un gaz naturel substitut comprenant les étapes de: alimentation d'un premier courant d'alimentation de gaz de synthèse comprenant de l'hydrogène, du méthane, du monoxyde de carbone et/ou du dioxyde de carbone en parallèle vers deux ou plus de deux réacteurs de méthanisation en masse dans une première zone de méthanisation en masse comprenant un premier réacteur de méthanisation en masse et un réacteur de méthanisation en masse final, dans lequel les réacteurs de méthanisation en masse de la première zone de méthanisation en masse sont raccordés en série, alimentation d'un deuxième courant d'alimentation de gaz de synthèse comprenant de l'hydrogène, du monoxyde de carbone et/ou du dioxyde de carbone vers deux ou plus de deux réacteurs de méthanisation en masse dans une deuxième zone de méthanisation en masse comprenant un premier réacteur de méthanisation en masse, dans lequel les réacteurs de méthanisation en masse de la deuxième zone de méthanisation en masse sont raccordés en série, chaque réacteur de méthanisation en masse contenant un catalyseur de méthanisation de sorte que les courants d'alimentation sont au moins partiellement méthanisés, division du courant de gaz méthanisé récupéré à partir du réacteur de méthanisation en masse final de la première zone de méthanisation en masse pour donner une première partie et une deuxième partie, recirculation de la première partie dans une boucle de recirculation vers le premier réacteur de méthanisation en masse de la première zone de méthanisation en masse afin de diluer le premier courant d'alimentation de gaz de synthèse alimenté vers ledit premier réacteur de méthanisation en masse, et alimentation de la deuxième partie vers le premier réacteur de méthanisation en masse de la deuxième zone de méthanisation en masse afin de diluer le deuxième courant d'alimentation de gaz de synthèse alimenté vers ledit premier réacteur de méthanisation en masse, dans lequel la pression d'alimentation du deuxième courant d'alimentation de gaz de synthèse est inférieure à la pression d'alimentation du premier courant d'alimentation de gaz de synthèse et la différence de pression entre les premier et deuxième courants d'alimentation correspond au moins à la baisse de pression dans la première zone de méthanisation en masse.

2. Procédé selon la revendication 1, dans lequel le premier courant d'alimentation est un gaz de synthèse désulfuré obtenu à partir d'un réacteur de gazéification fournissant du gaz à une pression supérieure à celle du deuxième courant d'alimentation et le deuxième courant d'alimentation est un gaz de synthèse désulfuré obtenu à partir d'un réacteur de gazéification fournissant du gaz à une pression inférieure à celle du premier courant d'alimentation.

3. Procédé selon la revendication 1 ou 2, dans lequel le premier courant d'alimentation est un gaz de synthèse désulfuré obtenu à partir d'un réacteur de gazéification à blocs de charbon et le deuxième courant d'alimentation est un gaz de synthèse désulfuré obtenu à partir d'un réacteur de gazéification à poussière de charbon.

4. Procédé selon l'une quelconque des revendications 1 à 3, dans lequel le catalyseur de méthanisation est utilisé à une température d'entrée située dans la plage comprise entre 200 et 450°C

5. Procédé selon l'une quelconque des revendications 1 à 4, mis en oeuvre à une pression située dans la plage comprise entre 5 et 80 bars abs.

6. Procédé selon l'une quelconque des revendications 1 à 4, dans lequel le nombre de réacteurs de méthanisation en masse dans la première zone de méthanisation en masse se situe dans la plage comprise entre 2 et 10 et le nombre de réacteurs de méthanisation en masse dans la deuxième zone de méthanisation en masse se situe dans la plage comprise en 2 et 10.

7. Procédé selon l'une quelconque des revendications 1 à 6, dans lequel le nombre, N, de réacteurs de méthanisation en masse dans la première zone de méthanisation en masse se situe dans la plage comprise entre 2 et 4, et le nombre de réacteurs de méthanisation en masse dans la deuxième zone de méthanisation en masse se situe dans la plage comprise entre 2 et 4.

8. Procédé selon l'une quelconque des revendications 1 à 7, dans lequel deux ou plus de deux réacteurs de méthanisation en masse sont présents dans la deuxième zone de méthanisation en masse et le deuxième courant d'alimentation est alimenté en parallèle vers chaque réacteur de méthanisation en masse de la deuxième zone de méthanisation en masse.

9. Procédé selon l'une quelconque des revendications 1 à 8, dans lequel deux ou plus de deux réacteurs de méthanisation en masse sont présents dans chaque zone de méthanisation en masse et la partie du courant d'alimentation alimenté vers le premier réacteur de méthanisation de chaque zone de méthanisation en masse se situe dans la plage comprise entre 10 % en volume et 60 % en volume du courant d'alimentation.

10. Procédé selon l'une quelconque des revendications 1 à 9, dans lequel la boucle de recirculation comprend un compresseur destiné au courant de gaz recirculé et un dispositif de préchauffage permettant de chauffer le courant de gaz dilué alimenté vers le premier réacteur de méthanisation en masse de la première zone de méthanisation en masse.

11. Procédé selon l'une quelconque des revendications 1 à 10, dans lequel de la vapeur est ajoutée au courant d'alimentation destiné à au moins un des réacteurs de méthanisation en masse de chaque zone de méthanisation en masse.

12. Procédé selon l'une quelconque des revendications 1 à 11, comprenant en outre la soumission d'un gaz produit issu de la deuxième zone de méthanisation en masse à une méthanisation ultérieure dans un ou plusieurs réacteurs de méthanisation de finissage.

13. Procédé selon la revendication 12, comprenant en outre la soumission d'un gaz produit issu d'un réacteur de méthanisation de finissage final à une étape de séchage.

14. Système de méthanisation pour convertir des premier et deuxième courants d'alimentation en un gaz naturel substitut à l'aide de première et deuxième zones de méthanisation, ledit système de méthanisation étant conçu pour fonctionner conformément au procédé selon l'une quelconque des revendications 1 à 13.

Drawing