Method and device for preparing natural gas from synthesis gas

Abstract

The present invention relates to the technical field of natural gas preparation, and specifically to a method and apparatus for preparing natural gas from synthesis gas. The method comprises: a multi-stage methanation step and a methanation heat recovery steam production step; wherein the multi-stage methanation step sequentially utilizes a primary adiabatic methanation reaction, a secondary adiabatic methanation reaction, and a composite methanation reaction to obtain natural gas; the methanation heat recovery steam production step utilizes the reaction heat generated by the multi-stage methanation step to obtain 9-12.5 MPa ultra-high-pressure superheated steam and 3-5 MPa medium-pressure superheated steam; wherein the composite methanation reaction sequentially comprises: an adiabatic methanation stage and an isothermal methanation stage. The method rationally utilizes the heat released by the methanation reaction to produce synthetic natural gas that meets Class I requirements while also producing ultra-high-pressure superheated steam and medium-pressure superheated steam as by-products.

Description

Technical Field

[0001] The present invention relates to the technical field of natural gas preparation, and in particular to a method for preparing natural gas from synthesis gas and a device for preparing natural gas from synthesis gas. Background Art

[0002] Currently, the main process for producing substitute natural gas from syngas is high-temperature methanation. High-temperature methanation technology involves the reaction of CO and CO2 with hydrogen in the presence of a high-temperature methanation catalyst to produce methane and water, releasing a large amount of reaction heat. The reactions that occur during the methane synthesis process mainly include:

[0003] CO+3H2→CH4+H2O+206.2kJ / mol(1);

[0004] CO2+4H2→CH4+2H2O+165kJ / mol(2);

[0005] CO+H2O→H2+CO2+41kJ / mol(3).

[0006] The development of high-temperature methanation technology can be traced back to the 1970s. Due to the oil crisis, the United States, the United Kingdom and other countries at that time began to conduct research on the application of alternative energy sources, including the research and development of high-calorific value city gas from coal and alternative natural gas technologies.

[0007] The current high-temperature methanation technology process has the following defects:

[0008] 1) Configuration of the reactor, superheater, and steam generator. In other words, there are two ways to configure the steam generator and superheater in the existing high-temperature methanation process. One is to first configure the steam generator after the No. 1 methanation reactor to produce steam, and then configure the superheater for superheating, which can produce medium-pressure steam. In the former, because the superheater is placed at the end, the steam pressure level and output will be affected. The other is to first configure the superheater after the No. 1 methanation reactor, and then configure the steam generator for steam production, which can produce high-pressure steam. In the latter, because the superheater is placed at the beginning, the outlet temperature of the No. 1 reactor is generally 620-670°C. If the by-product high-pressure or ultra-high-pressure steam is produced, the temperature is ≥500°C, making the design and manufacture of the steam generator and superheater difficult to achieve domestically.

[0009] 2) The process is complex. In other words, the existing process involves a large number of methanation reactors, a large number of intermediate heat recovery equipment, a long process, and high equipment investment. Summary of the Invention

[0010] The purpose of the present invention is to overcome the problems existing in the existing high-temperature methanation process, such as the large number of methanation reactors, complex process flow, high requirements for equipment and materials, and single quality of by-product steam (for example, medium-pressure steam). The present invention provides a new method for preparing natural gas from synthesis gas and an apparatus for preparing natural gas from synthesis gas. The method adopts a two-stage adiabatic and one-stage composite methanation process, rationally utilizes the heat released by the methanation reaction, and produces synthetic natural gas that meets Class I in GB / T 33445-2016 (coal-based synthetic natural gas), while also producing ultra-high-pressure superheated steam and medium-pressure superheated steam as by-products.

[0011] To achieve the above objectives, the present invention provides, in a first aspect, a method for preparing natural gas from synthesis gas, comprising: a multi-stage methanation step and a methanation heat recovery steam production step; wherein the multi-stage methanation step sequentially utilizes a primary adiabatic methanation reaction, a secondary adiabatic methanation reaction, and a composite methanation reaction to produce natural gas p1; and the methanation heat recovery steam production step utilizes the reaction heat generated in the multi-stage methanation step to produce 9-12.5 MPa ultra-high-pressure superheated steam s1 and 3-5 MPa medium-pressure superheated steam s2.

[0012] The composite methanation reaction includes, in sequence: an adiabatic methanation stage and an isothermal methanation stage.

[0013] Preferably, the method comprises the following steps:

[0014] (1) Pre-treating the synthesis gas 01 to obtain pre-treated synthesis gas 04, dividing the pre-treated synthesis gas 04-i into two streams, mixing the first pre-treated synthesis gas 04-i with the circulating gas 05, and performing the primary adiabatic methanation reaction to obtain the first high-temperature gas 06;

[0015] (2) exchanging heat between the first high-temperature gas 06 and the first ultra-high-pressure boiler water 07-i to obtain a first low-temperature gas 09 converted from the first high-temperature gas 06, and a first ultra-high-pressure saturated steam 08-i converted from the first ultra-high-pressure boiler water 07-i; exchanging heat between the first low-temperature gas 09 and the preheated high-pressure boiler water 010 to obtain a second low-temperature gas 011 converted from the first low-temperature gas 09 and an ultra-high-pressure boiler water 07 converted from the preheated high-pressure blast furnace water 010;

[0016] (3) The second low-temperature gas 011 and the second stream of pretreated synthesis gas 04-ii are mixed and subjected to the secondary adiabatic methanation reaction. The obtained second high-temperature gas 012 and the ultra-high-pressure saturated steam 08 are subjected to ultra-high-pressure steam superheating to obtain a third low-temperature gas 013 converted from the second high-temperature gas 012 and the ultra-high-pressure superheated steam s1 converted from the ultra-high-pressure saturated steam 08; the third low-temperature gas 013 is heat-exchanged with the medium-pressure saturated steam 014 to obtain a fourth low-temperature gas 015 converted from the third low-temperature gas 013 and the medium-pressure superheated steam s2 converted from the medium-pressure saturated steam 014;

[0017] (4) exchanging heat between the fourth low-temperature gas 015 and the second ultra-high-pressure boiler water 07-ii to obtain a fifth low-temperature gas 016 converted from the fourth low-temperature gas 015 and a second ultra-high-pressure saturated steam 08-ii converted from the second ultra-high-pressure boiler water 07-ii; exchanging heat between the fifth low-temperature gas 016 and the compressed circulating gas 017 to obtain a sixth low-temperature gas 018 converted from the fifth low-temperature gas 016 and the circulating gas 05 converted from the compressed gas 017;

[0018] (5) The sixth low-temperature gas 018 is divided into two streams, one process gas 018-i is subjected to heat exchange with high-pressure boiler water 019 and first desalted water 020-i respectively to obtain a gas-liquid mixture 021 converted from the process gas 018-i, the preheated high-pressure boiler water 010 converted from the high-pressure boiler water 019, and the first preheated desalted water 022-i converted from the first desalted water 020-i; wherein the gas-liquid mixture 021 is sequentially subjected to the first gas-liquid separation and compression to obtain the compressed circulating gas 017; the other process gas 018-ii is subjected to the composite methanation reaction, and the obtained third high-temperature gas is subjected to heat exchange with medium-pressure boiler water 023 to obtain sub-high-temperature gas 024 converted from the third high-temperature gas and the medium-pressure saturated steam 014 converted from the medium-pressure boiler water 023;

[0019] The sub-high temperature gas 024 preheats the synthesis gas 01, and the resulting seventh low temperature gas 025 is subjected to a second gas-liquid separation. The resulting gas is then heat exchanged with the second desalted water 020-ii to obtain second preheated desalted water 022-ii and an eighth low temperature gas 026. The eighth low temperature gas 026 is then water-cooled and subjected to a third gas-liquid separation to obtain the natural gas p1.

[0020] The ultra-high-pressure boiler water 07 includes the first ultra-high-pressure boiler water 07-i and the second ultra-high-pressure boiler water 07-ii; the ultra-high-pressure saturated steam 08 includes the first ultra-high-pressure saturated steam 08-i and the second ultra-high-pressure saturated steam 08-ii.

[0021] A second aspect of the present invention provides an apparatus for preparing natural gas from synthesis gas, the apparatus comprising: a feed separator 1, a raw gas preheater 2, and a fine desulfurization tank 3 connected in sequence, for pretreating synthesis gas 01 to obtain pretreated synthesis gas 04 and separating it into two streams;

[0022] The first stream of pretreated synthesis gas 04-i is mixed with the circulating gas 05 from the circulating gas heat exchanger 12 and enters the 1# adiabatic methanation reactor 4. The obtained first high-temperature gas 06 enters the 1# ultra-high pressure steam generator 5 and exchanges heat with the first ultra-high pressure boiler water 07-i from the ultra-high pressure steam drum 23 to obtain the first low-temperature gas 09 and the first ultra-high pressure saturated steam 08-i; the first low-temperature gas 09 enters the 2# ultra-high pressure boiler water preheater 6 and exchanges heat with the preheated high-pressure boiler water from the 1# ultra-high pressure boiler water preheater 15. 010 is heat exchanged, and the obtained ultra-high pressure boiler water 07 enters the ultra-high pressure steam drum 23, and the obtained second low-temperature gas 011 is mixed with the second pretreated synthesis gas 04-ii and then enters the 2# adiabatic methanation reactor 7. The obtained second high-temperature gas 012 enters the ultra-high pressure steam high-temperature superheater 8 and the ultra-high pressure steam low-temperature superheater 9 in sequence, and exchanges heat with the ultra-high pressure saturated steam 08 from the ultra-high pressure steam drum 23 to obtain 9-12.5 MPa ultra-high pressure superheated steam s1 and the third low-temperature gas 013;

[0023] The third low-temperature gas 013 enters the medium-pressure steam superheater 10 and exchanges heat with the medium-pressure saturated steam 014 from the medium-pressure steam drum 14 to obtain 3-5 MPa medium-pressure superheated steam s2 and fourth low-temperature gas 015;

[0024] The fourth low-temperature gas 015 enters the 2# ultra-high-pressure steam generator 11 and exchanges heat with the second ultra-high-pressure boiler water 07-ii from the ultra-high-pressure steam drum 23 to obtain the fifth low-temperature gas 016 and the second ultra-high-pressure saturated steam 08-ii; the fifth low-temperature gas 016 exchanges heat with the compressed circulator 017 from the circulating gas compressor 18 in the circulating gas heat exchanger 12, and the obtained sixth low-temperature gas 018 is divided into two streams.

[0025] A stream of process gas 018-i sequentially enters the 1# ultra-high pressure boiler water preheater 15 and the 1# desalted water preheater 16 for heat exchange, and the resulting gas-liquid mixture 021 sequentially enters the circulating gas separator 17 and the circulating gas compressor 18 to obtain the compressed circulating gas 017;

[0026] Another stream of process gas 018-ii enters the 3# composite methanation reactor 13. The resulting third high-temperature gas exchanges heat with the medium-pressure boiler water 023 in the shell side. The resulting medium-pressure saturated steam 014 enters the medium-pressure steam drum 14. The resulting sub-high-temperature gas 024 enters the raw gas preheater 2 to preheat the synthesis gas 01. The resulting seventh low-temperature gas 025 enters the gas-liquid separator 19, the 2# desalted water preheater 20, the product water cooler 21, and the product separator 22 in sequence to obtain natural gas p1.

[0027] The ultra-high pressure boiler water 07 includes the first ultra-high pressure boiler water 07-i and the second ultra-high pressure boiler water 07-ii; the ultra-high pressure saturated steam 08 includes the first ultra-high pressure saturated steam 08-i and the second ultra-high pressure saturated steam 08-ii;

[0028] The 3# composite methanation reactor 13 includes an adiabatic methanation bed layer arranged on the top and an isothermal methanation bed layer arranged on the bottom.

[0029] Compared with the prior art, the present invention has the following advantages:

[0030] (1) The method provided by the present invention adopts a multi-stage methanation step and a methanation heat recovery steam production step, specifically adopting a two-stage adiabatic + one-stage composite high-temperature methanation process, which rationally utilizes the reaction heat generated by the multi-stage methanation reaction in a cascade manner. On the premise of obtaining Class I synthetic natural gas that meets GB / T 33445-2016 (coal-based synthetic natural gas), 9-12.5 MPa ultra-high-pressure superheated steam and 3-5 MPa medium-pressure superheated steam are simultaneously produced as by-products, thereby fully utilizing the reaction heat and improving the utilization rate of the reaction heat.

[0031] (2) Compared with the traditional high-temperature methanation reaction process which mostly adopts 3-6 adiabatic reactor processes, the number of reactors is large and the requirements for equipment materials are high; the present invention adopts a two-stage adiabatic + one-stage composite methanation process with a total of three reactors, which reduces the number of reactors and saves equipment investment. At the same time, it rationally utilizes the heat released by the methanation reaction, enriches the types of by-product steam, and adopts two ultra-high pressure steam superheaters in series to achieve domestic design and manufacturing. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 This is a schematic structural diagram of a device for preparing natural gas from synthesis gas provided by the present invention;

[0033] Figure 2 It is a partial structural diagram of a device for preparing natural gas from synthesis gas improved by the present invention.

[0034] Description of Reference Numerals

[0035] 1. Feed separator 2. Raw gas preheater 3. Fine desulfurization tank

[0036] 4. 1# adiabatic methanation reactor 5. 1# ultra-high pressure steam generator 6. 2# ultra-high pressure boiler water preheater 7. 2# adiabatic methanation reactor 8. ultra-high pressure steam high-temperature superheater 9. ultra-high pressure steam low-temperature superheater 10. medium-pressure steam superheater 11. 2# ultra-high pressure steam generator 12. circulating gas heat exchanger

[0037] 13. 3# composite methanation reactor 14. Medium-pressure steam drum 15. 1# ultra-high-pressure boiler water preheater 16. 1# desalted water preheater 17. Circulating gas separator 18. Circulating gas compressor 19. Gas-liquid separator 20. 2# desalted water preheater 21. Product water cooler 22. Product separator 23. Ultra-high-pressure steam drum 24. Boiler blowdown separator 25. Boiler blowdown cooler 26. Medium-pressure boiler water pump

[0038] p1, natural gas s1, ultra-high-pressure superheated steam s2, medium-pressure superheated steam 01, synthesis gas 02, synthesis gas after heat exchange 03, water for fine desulfurization

[0039] 04, pretreated synthesis gas 04-i, first stream of pretreated synthesis gas 04-ii, second stream of pretreated synthesis gas 05, circulating gas 06, first high-temperature gas 07, ultra-high pressure boiler water

[0040] 07-i, first ultra-high pressure boiler water 07-ii, second ultra-high pressure boiler water 08, ultra-high pressure saturated steam

[0041] 08-i, first ultra-high pressure saturated steam 08-ii, second ultra-high pressure saturated steam 09, first low temperature gas

[0042] 010, preheating high-pressure boiler water 011, second low-temperature gas 012, second high-temperature gas

[0043] 013. Third low-temperature gas 014. Medium-pressure saturated steam 015. Fourth low-temperature gas

[0044] 016, Fifth cryogenic gas 017, Compressed circulating gas 018, Sixth cryogenic gas

[0045] 018-i, one process gas 018-ii, another process gas 019, high-pressure boiler water

[0046] 020, desalted water 020-i, first desalted water 020-ii, second desalted water

[0047] 021. Gas-liquid mixture 022. Preheated desalted water 022-i. First preheated desalted water

[0048] 022-ii, Second preheated desalted water 023, Medium-pressure boiler water 024, Sub-high-temperature gas

[0049] 025. Seventh low-temperature gas 026. Eighth low-temperature gas 028. Boiler blowdown

[0050] 028-i, First Boiler Blowdown 028-ii, Second Boiler Blowdown 029, Blowdown Flash Steam 030, Blowdown Water 031, Steam Generator Blowdown DETAILED DESCRIPTION

[0051] The endpoints of the ranges and any values ​​disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoints of each range, the endpoints of each range and individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered to be specifically disclosed herein.

[0052] In the present invention, unless otherwise specified, the terms "first," "second," "third," ..., and "Nth" do not indicate a sequence of reactions or limitations on materials or steps; they are simply used to indicate that they are not the same material or step. For example, the terms "first" and "second" in "first desalted water" and "second desalted water" are simply used to indicate that they are not the same desalted water.

[0053] A first aspect of the present invention provides a method for preparing natural gas from synthesis gas, comprising: a multi-stage methanation step and a methanation heat recovery steam production step; wherein the multi-stage methanation step sequentially utilizes a primary adiabatic methanation reaction, a secondary adiabatic methanation reaction, and a composite methanation reaction to produce natural gas p1; and the methanation heat recovery steam production step utilizes the reaction heat generated in the multi-stage methanation step to produce 9-12.5 MPa ultra-high-pressure superheated steam s1 and 3-5 MPa medium-pressure superheated steam s2.

[0054] The composite methanation reaction includes, in sequence: an adiabatic methanation stage and an isothermal methanation stage.

[0055] In the present invention, unless otherwise specified, the pressure parameters all refer to gauge pressure. For example, the pressure of ultra-high pressure superheated steam s1 is 9-12.5 MPaG.

[0056] The inventors of this invention have discovered that the core of high-temperature methanation technology lies in controlling the reaction depth and comprehensively utilizing the reaction heat. The reaction depth is primarily controlled by adjusting the amount of material entering the reactor and the circulating gas flow rate. The heat generated during the methanation reaction is primarily used to produce steam as a byproduct. Most existing technologies employ separate multi-stage adiabatic reactors and steam generators, with the recovered reaction heat used to produce high-pressure or medium-pressure steam. Placing a steam superheater before the waste heat boiler can also produce high-pressure steam. However, due to the high temperatures and pressures, the selection and manufacture of steam superheaters is difficult in China.

[0057] In response to the above problems, the present invention arranges an ultra-high-pressure steam superheater unit (i.e., an ultra-high-pressure steam high-temperature superheater and an ultra-high-pressure steam low-temperature superheater) after the 2# adiabatic methanation reactor, and the steam superheater adopts two special lined high and low-temperature heat exchangers (i.e., the steam superheater tube side adopts a two-tube side structure, ceramic fiber is used to isolate the inlet flange pipe and the inlet channel pipe, and an internal expansion joint is provided to solve the thermal expansion problem; the shell side adopts a single-shell side structure, and refractory castables are provided inside; the heat exchange tube is in a high-temperature and high-pressure hydrogen environment, and the material is selected from nickel-based alloy.) After being connected in series, ultra-high-pressure saturated steam of 9-12.5MPa is superheated, and both the design and manufacturing can be domestically produced; a 9-12.5MPa ultra-high-pressure steam generator is arranged after the 1# adiabatic methanation reactor; a 3-5MPa medium-pressure steam superheater is arranged after the ultra-high-pressure steam superheater unit, and 3-5MPa medium-pressure saturated steam is produced as a by-product in the 3# composite methanation reactor. Through the above process, the reaction heat is utilized in a step-by-step manner to produce 9-12.5MPa ultra-high-pressure superheated steam and 3-5MPa medium-pressure superheated steam.

[0058] By installing a composite methanation reactor, the current four- or five-stage process is shortened to three stages, with two adiabatic methanation reactors and one composite methanation reactor installed. The adiabatic methanation reactors control the reaction temperature by adjusting the recycle gas volume and adding steam, ensuring it does not exceed the catalyst's tolerance. The recycle gas is drawn off after the second adiabatic methanation reactor. A third composite methanation reactor is installed at the outlet of the second adiabatic methanation reactor. This reactor produces medium-pressure saturated steam as a byproduct, maximizing the methanation reaction equilibrium temperature within the reactor and increasing the outlet methane content. The outlet gas can be fed into a downstream natural gas liquefaction unit to produce LNG, or it can be dehydrated and dried before being fed into the natural gas pipeline network.

[0059] In some embodiments of the present invention, preferably, the method comprises the following steps:

[0060] (1) Pre-treating the synthesis gas 01 to obtain pre-treated synthesis gas 04, dividing the pre-treated synthesis gas 04-i into two streams, mixing the first pre-treated synthesis gas 04-i with the circulating gas 05, and performing the primary adiabatic methanation reaction to obtain the first high-temperature gas 06;

[0061] (2) exchanging heat between the first high-temperature gas 06 and the first ultra-high-pressure boiler water 07-i to obtain a first low-temperature gas 09 converted from the first high-temperature gas 06, and a first ultra-high-pressure saturated steam 08-i converted from the first ultra-high-pressure boiler water 07-i; exchanging heat between the first low-temperature gas 09 and the preheated high-pressure boiler water 010 to obtain a second low-temperature gas 011 converted from the first low-temperature gas 09 and an ultra-high-pressure boiler water 07 converted from the preheated high-pressure blast furnace water 010;

[0062] (3) The second low-temperature gas 011 and the second stream of pretreated synthesis gas 04-ii are mixed and subjected to the secondary adiabatic methanation reaction. The obtained second high-temperature gas 012 and the ultra-high-pressure saturated steam 08 are subjected to ultra-high-pressure steam superheating to obtain a third low-temperature gas 013 converted from the second high-temperature gas 012 and the ultra-high-pressure superheated steam s1 converted from the ultra-high-pressure saturated steam 08; the third low-temperature gas 013 is heat-exchanged with the medium-pressure saturated steam 014 to obtain a fourth low-temperature gas 015 converted from the third low-temperature gas 013 and the medium-pressure superheated steam s2 converted from the medium-pressure saturated steam 014;

[0063] (4) exchanging heat between the fourth low-temperature gas 015 and the second ultra-high-pressure boiler water 07-ii to obtain a fifth low-temperature gas 016 converted from the fourth low-temperature gas 015 and a second ultra-high-pressure saturated steam 08-ii converted from the second ultra-high-pressure boiler water 07-ii; exchanging heat between the fifth low-temperature gas 016 and the compressed circulating gas 017 to obtain a sixth low-temperature gas 018 converted from the fifth low-temperature gas 016 and the circulating gas 05 converted from the compressed gas 017;

[0064] (5) The sixth low-temperature gas 018 is divided into two streams, one process gas 018-i is subjected to heat exchange with high-pressure boiler water 019 and first desalted water 020-i respectively to obtain a gas-liquid mixture 021 converted from the process gas 018-i, the preheated high-pressure boiler water 010 converted from the high-pressure boiler water 019, and the first preheated desalted water 022-i converted from the first desalted water 020-i; wherein the gas-liquid mixture 021 is sequentially subjected to the first gas-liquid separation and compression to obtain the compressed circulating gas 017; the other process gas 018-ii is subjected to the composite methanation reaction, and the obtained third high-temperature gas is subjected to heat exchange with medium-pressure boiler water 023 to obtain sub-high-temperature gas 024 converted from the third high-temperature gas and the medium-pressure saturated steam 014 converted from the medium-pressure boiler water 023;

[0065] The sub-high temperature gas 024 preheats the synthesis gas 01, and the resulting seventh low temperature gas 025 is subjected to a second gas-liquid separation. The resulting gas is then heat exchanged with the second desalted water 020-ii to obtain second preheated desalted water 022-ii and an eighth low temperature gas 026. The eighth low temperature gas 026 is then water-cooled and subjected to a third gas-liquid separation to obtain the natural gas p1.

[0066] The ultra-high-pressure boiler water 07 includes the first ultra-high-pressure boiler water 07-i and the second ultra-high-pressure boiler water 07-ii; the ultra-high-pressure saturated steam 08 includes the first ultra-high-pressure saturated steam 08-i and the second ultra-high-pressure saturated steam 08-ii.

[0067] In the present invention, unless otherwise specified, the ultra-high pressure boiler water 07 is composed of the first ultra-high pressure boiler water 07-i and the second ultra-high pressure boiler water 07-ii; the ultra-high pressure saturated steam 08 is composed of the first ultra-high pressure saturated steam 08-i and the second ultra-high pressure saturated steam 08-ii.

[0068] In some embodiments of the present invention, preferably, the primary adiabatic methanation reaction, the secondary adiabatic methanation reaction and the adiabatic methanation stage are each independently carried out in the presence of a first methanation catalyst; and the isothermal methanation stage is carried out in the presence of a second methanation catalyst.

[0069] In the present invention, a wide range of options is available for the first and second methanation catalysts. Preferably, each of the first and second methanation catalysts independently comprises an alumina support and nickel and a rare earth element supported on the alumina support. The rare earth element includes, but is not limited to, Y and / or lanthanides.

[0070] In some embodiments of the present invention, preferably, based on the total weight of the first methanation catalyst, the nickel content is 15-22 wt%, and the rare earth element content is 1-5 wt%; based on the total weight of the second methanation catalyst, the nickel content is 20-25 wt%, and the rare earth element content is 1-5 wt%.

[0071] In some embodiments of the present invention, further preferably, the physical properties of the first methanation catalyst meet the following requirements: inlet temperature of 250-350°C; reaction temperature of 250-700°C; space velocity of 6000-11000h -1 ; Reaction pressure is 2-5MPa; The physical properties of the second methanation catalyst meet the following requirements: inlet temperature is 250-300°C; reaction temperature is 300-400°C; space velocity is 5000-10000h -1 ; The reaction pressure is 2-5MPa.

[0072] In the present invention, the sources of the first and second methanation catalysts can be selected from a wide range, as long as they meet the above-mentioned requirements. In the present invention, the first methanation catalyst can be prepared or purchased; the second methanation catalyst can be prepared or purchased.

[0073] In some embodiments of the present invention, preferably, in step (1), the conditions of the primary adiabatic methanation reaction include: pressure of 2-5.5 MPa; temperature of 590-690°C; volume space velocity of 5000-8000 h -1 .

[0074] In some embodiments of the present invention, preferably, the pressure of the synthesis gas 01 is 2.2-5.5 MPa; the temperature is 20-60°C; further preferably, the CO content in the synthesis gas 01 is 20-25% by volume, the CO2 content is 1.5-2% by volume, and the H2 content is 72-77% by volume.

[0075] In the present invention, the source of the syngas has a wide range of options, as long as the syngas meets the above-mentioned requirements. Preferably, the syngas is selected from the purified gas at the outlet of the acid gas removal unit and / or the green hydrogen provided by the green hydrogen device.

[0076] In the present invention, the pretreatment is intended to optimize the physical properties of the syngas, that is, to increase the temperature of the syngas and reduce the sulfur content of the syngas. Preferably, after the pretreatment, the syngas O4 has a temperature of 185-190°C, a sulfur content of 2-30 ppb, and a pressure of 2-5.5 MPa.

[0077] In some embodiments of the present invention, the volume flow ratio of the first pretreated syngas O4-i to the second pretreated syngas O4-ii is preferably 0.7-1.2:1. In the present invention, maintaining a volume flow ratio within this range allows for control of the outlet temperature of the second adiabatic methanation reactor; a larger volume flow ratio results in a lower outlet temperature. Furthermore, the reaction load of the first and second adiabatic methanation reactors can be adjusted.

[0078] In some embodiments of the present invention, preferably, the volume flow ratio of the first pretreated synthesis gas 04-i and the circulating gas 05 is 0.4-0.8:1, for example, 0.4:1, 0.5:1, 0.6:1, 0.8:1, and any value in the range consisting of any two values.

[0079] In some embodiments of the present invention, preferably, the pressure of the circulating gas 05 is 2-5.5 MPa and the temperature is 270-350°C.

[0080] In some embodiments of the present invention, preferably, the pressure of the first high-temperature gas 06 is 2-5.5 MPa and the temperature is 590-690°C.

[0081] In some embodiments of the present invention, preferably, the pretreatment process includes: subjecting the synthesis gas 01 to a fourth gas-liquid separation, exchanging heat with the sub-high temperature gas 024, mixing the heat-exchanged synthesis gas 02 obtained with fine desulfurization water 03 and desulfurizing the mixture to obtain the pretreated synthesis gas 04.

[0082] In some embodiments of the present invention, preferably, the temperature of the synthesis gas O2 after heat exchange is 200-240°C.

[0083] In the present invention, the water used for fine desulfurization is deoxygenated boiler water and does not contain sulfur. Preferably, the temperature of the water used for fine desulfurization 03 is 240-280°C.

[0084] In some embodiments of the present invention, preferably, the volume flow ratio of the synthesis gas 02 after heat exchange to the water for fine desulfurization 03 is 3000-3500:1, for example, 3000:1, 3100:1, 3200:1, 3500:1, and any value in the range consisting of any two values.

[0085] In some embodiments of the present invention, preferably, in step (2), the volume flow ratio of the first high-temperature gas O6 to the first ultra-high-pressure boiler water O7-i is 120-140:1, for example, 120:1, 125:1, 130:1, 140:1, or any value within a range consisting of any two values. In the present invention, satisfying the volume flow ratio within the above range can meet the by-production of high-pressure saturated steam at a level of 9-12.5 MPa. Changes in the volume flow rate of the first high-temperature gas O6 will affect the production of high-pressure steam.

[0086] In some embodiments of the present invention, preferably, the temperature of the ultra-high pressure saturated steam 08 is 320-326° C. and the pressure is 9-12.5 MPa.

[0087] In some embodiments of the present invention, preferably, the volume ratio of the first ultra-high pressure saturated steam 08-i and the second ultra-high pressure saturated steam 08-ii is 2.1-2.5:1, for example, 2.1:1, 2.2:1, 2.3:1, 2.5:1, and any value in the range consisting of any two numerical values.

[0088] In some embodiments of the present invention, preferably, the temperature of the ultra-high pressure boiler water 07 is 240-280°C; further preferably, the volume flow ratio of the first ultra-high pressure boiler water 07-i and the second ultra-high pressure boiler water 07-ii is 2.1-2.5:1, for example, 2.1:1, 2.2:1, 2.3:1, 2.5:1, and any value in the range consisting of any two numerical values.

[0089] In some embodiments of the present invention, preferably, in step (3), the conditions of the secondary adiabatic methanation reaction include: pressure of 2-5.5 MPa; temperature of 590-690°C; volume space velocity of 5000-8000 h -1 .

[0090] In some embodiments of the present invention, preferably, the second high-temperature gas 012 has a pressure of 2-5.5 MPa and a temperature of 590-690°C.

[0091] In some embodiments of the present invention, preferably, the process of superheating the ultra-high pressure steam includes: superheating the second high temperature gas 012 and the ultra-high pressure saturated steam 08 in ultra-high pressure steam high temperature and ultra-high pressure steam low temperature, respectively.

[0092] In the present invention, the process of ultra-high-pressure steam high-temperature superheating includes: superheating the second high-temperature gas 12 and the low-temperature ultra-high-pressure saturated steam from the ultra-high-pressure steam low-temperature superheater 9 with ultra-high-pressure steam at high temperature to obtain a process gas converted from the second high-temperature gas 12, and 9-12.5MPa ultra-high-pressure superheated steam s1 converted from the low-temperature ultra-high-pressure saturated steam; the process of ultra-high-pressure steam low-temperature superheating includes: superheating the process gas from the ultra-high-pressure steam high-temperature superheater 8 and the ultra-high-pressure saturated steam 08 from the ultra-high-pressure steam drum 23 with ultra-high-pressure steam at low temperature to obtain a third low-temperature gas 013 converted from the process gas, and low-temperature ultra-high-pressure saturated steam converted from the ultra-high-pressure saturated steam 08.

[0093] In some embodiments of the present invention, the volume flow ratio of the third low-temperature gas 013 to the intermediate-pressure saturated steam 014 is preferably 14-16:1, for example, 14:1, 14.5:1, 15:1, 16:1, or any other value within a range consisting of any two of these values. In the present invention, meeting this volume flow ratio can produce 3-5 MPa of intermediate-pressure superheated steam as a by-product. Changes in the volume flow of the third low-temperature gas 013 can affect the production of intermediate-pressure steam.

[0094] In some embodiments of the present invention, preferably, the temperature of the medium-pressure saturated steam 014 is 234-252° C. and the pressure is 3-5 MPa.

[0095] In some embodiments of the present invention, preferably, in step (4), the volume flow ratio of the fourth low-temperature gas 015 to the second ultra-high-pressure boiler water 07-ii is 300-320:1, for example, 300:1, 310:1, 315:1, 320:1, or any value within a range consisting of any two values. In the present invention, when the volume flow ratio fluctuates within this range, it can meet the requirement of producing ultra-high-pressure saturated steam at a grade of 9-12.5 MPa; at the same time, changes in the volume flow of the fourth low-temperature gas 015 will affect the production of high-pressure steam.

[0096] In some embodiments of the present invention, preferably, in step (5), the volume flow ratio of one process gas 018-i to another process gas 018-ii in the sixth low-temperature gas is 2.3-2.5:1, for example, 2.3:1, 2.4:1, 2.5:1, and any value within a range consisting of any two values. In the present invention, adjusting the volume flow ratio within the above range can not only control the outlet temperature of the 1# adiabatic methanation reactor, but also control the methane content in the product gas. When the above volume flow ratio is larger, the amount of circulating gas is larger, the power consumption of the compressor is greater, the outlet temperature of the methanation reactor is lowered, and the methane content in the product gas is higher.

[0097] In some embodiments of the present invention, preferably, the volume flow ratio of the process gas 018-i, the high-pressure boiler water 019 and the first desalted water 020-i is 4100-4300:65-85:30-45.

[0098] In some embodiments of the present invention, preferably, the temperature of the high-pressure boiler water 019 is 150-180° C. and the pressure is 17-18 MPa.

[0099] In some embodiments of the present invention, preferably, the temperature of the preheated high-pressure boiler water (010) is 220-250°C.

[0100] In some embodiments of the present invention, preferably, the conditions of the adiabatic methanation stage and the isothermal methanation stage independently include: pressure of 2-5.5 MPa; temperature of 220-300°C; volume space velocity of 5000-8000 h -1 The top of the 3# composite methanation reactor is insulated. This layer of catalyst reacts adiabatically, raising the bed temperature. The heated gas then enters the isothermal bed below the reactor to continue reacting. This arrangement allows the methanation reaction to approach equilibrium, increasing the methane content in the product gas.

[0101] In some embodiments of the present invention, preferably, the pressure of the sub-high temperature gas 024 is 2-5.5 MPa and the temperature is 220-300°C.

[0102] In some embodiments of the present invention, preferably, the temperature of the medium-pressure boiler water 023 is 110-120° C. and the pressure is 3.7-5.2 MPa.

[0103] In some embodiments of the present invention, preferably, the temperature of the medium-pressure saturated steam 014 is 234-252° C. and the pressure is 3-5 MPa.

[0104] In some embodiments of the present invention, preferably, the desalted water 020 includes the first desalted water 020-i and the second desalted water 020-ii; further preferably, the temperature of the desalted water 020 is 25-35°C.

[0105] In some embodiments of the present invention, preferably, the preheated desalted water 022 includes the first preheated desalted water 022-i and the second preheated desalted water 022-ii; further preferably, the temperature of the preheated desalted water 022 is 110-120°C.

[0106] In some embodiments of the present invention, preferably, the physical properties of the natural gas p1 meet the following requirements: pressure of 2-5.5 MPa; temperature of 35-45°C; CH4 content of 93-96% by volume; high calorific value ≥35 MJ / m 3 , H2 content ≤3.5×10 - 2 mol%, CO2 content ≤2×10 -2 mol%, H2S content ≤1mg / m 3 , CO content ≤ 0.15×10 -2 mol%, NH3 content ≤50×10 -6 mol%, solid particle content ≤1mg / m 3 That is to say, the synthesis gas produced by the method provided by the present invention meets the technical indicators of Class I synthetic natural gas in GB / T33445-2016 (synthetic natural gas from coal).

[0107] In some embodiments of the present invention, preferably, the method further comprises: mixing the first boiler wastewater 028-i and the second boiler wastewater 028-ii generated by the high-pressure boiler water 019 and the medium-pressure boiler water 023, and then performing wastewater separation to obtain wastewater flash steam 029 and wastewater 030; and recycling the steam generator wastewater 031 obtained by cooling the wastewater 030 for circulating water.

[0108] The second aspect of the present invention provides a schematic structural diagram of a device for preparing natural gas from synthesis gas. Figure 1-2 As shown by Figure 1-2It can be seen that the device comprises: a feed separator 1, a raw gas preheater 2 and a fine desulfurization tank 3 connected in sequence, for pre-treating the synthesis gas 01 and separating the obtained pre-treated synthesis gas 04 into two streams;

[0109] The first stream of pretreated synthesis gas 04-i is mixed with the circulating gas 05 from the circulating gas heat exchanger 12 and enters the 1# adiabatic methanation reactor 4. The obtained first high-temperature gas 06 enters the 1# ultra-high pressure steam generator 5 and exchanges heat with the first ultra-high pressure boiler water 07-i from the ultra-high pressure steam drum 23 to obtain the first low-temperature gas 09 and the first ultra-high pressure saturated steam 08-i; the first low-temperature gas 09 enters the 2# ultra-high pressure boiler water preheater 6 and exchanges heat with the preheated high-pressure boiler water from the 1# ultra-high pressure boiler water preheater 15. 010 is heat exchanged, and the obtained ultra-high pressure boiler water 07 enters the ultra-high pressure steam drum 23, and the obtained second low-temperature gas 011 is mixed with the second pretreated synthesis gas 04-ii and then enters the 2# adiabatic methanation reactor 7. The obtained second high-temperature gas 012 enters the ultra-high pressure steam high-temperature superheater 8 and the ultra-high pressure steam low-temperature superheater 9 in sequence, and exchanges heat with the ultra-high pressure saturated steam 08 from the ultra-high pressure steam drum 23 to obtain 9-12.5 MPa ultra-high pressure superheated steam s1 and the third low-temperature gas 013;

[0110] The third low-temperature gas 013 enters the medium-pressure steam superheater 10 and exchanges heat with the medium-pressure saturated steam 014 from the medium-pressure steam drum 14 to obtain 3-5 MPa medium-pressure superheated steam s2 and fourth low-temperature gas 015;

[0111] The fourth low-temperature gas 015 enters the 2# ultra-high-pressure steam generator 11 and exchanges heat with the second ultra-high-pressure boiler water 07-ii from the ultra-high-pressure steam drum 23 to obtain the fifth low-temperature gas 016 and the second ultra-high-pressure saturated steam 08-ii; the fifth low-temperature gas 016 exchanges heat with the compressed circulator 017 from the circulating gas compressor 18 in the circulating gas heat exchanger 12, and the obtained sixth low-temperature gas 018 is divided into two streams.

[0112] A stream of process gas 018-i sequentially enters the 1# ultra-high pressure boiler water preheater 15 and the 1# desalted water preheater 16 for heat exchange, and the resulting gas-liquid mixture 021 sequentially enters the circulating gas separator 17 and the circulating gas compressor 18 to obtain the compressed circulating gas 017;

[0113] Another stream of process gas 018-ii enters the 3# composite methanation reactor 13. The resulting third high-temperature gas exchanges heat with the medium-pressure boiler water 023 in the shell side. The resulting medium-pressure saturated steam 014 enters the medium-pressure steam drum 14. The resulting sub-high-temperature gas 024 enters the raw gas preheater 2 to preheat the synthesis gas 01. The resulting seventh low-temperature gas 025 enters the gas-liquid separator 19, the 2# desalted water preheater 20, the product water cooler 21, and the product separator 22 in sequence to obtain natural gas p1.

[0114] The ultra-high pressure boiler water 07 includes the first ultra-high pressure boiler water 07-i and the second ultra-high pressure boiler water 07-ii; the ultra-high pressure saturated steam 08 includes the first ultra-high pressure saturated steam 08-i and the second ultra-high pressure saturated steam 08-ii;

[0115] The 3# composite methanation reactor 13 includes an adiabatic methanation bed layer arranged on the top and an isothermal methanation bed layer arranged on the bottom.

[0116] In some embodiments of the present invention, preferably, Figure 1-2 As shown, in the third composite methanation reactor 13, the height ratio of the adiabatic methanation bed to the isothermal methanation bed is 1:26-79, for example, 1:26, 1:30, 1:40, 1:50, 1:60, 1:70, 1:79, or any other value within a range consisting of any two values. In the present invention, the catalyst in the upper adiabatic methanation bed reacts under adiabatic conditions, raising the bed temperature. The heated gas then enters the lower isothermal methanation bed to continue reacting. This arrangement allows the composite methanation reaction to approach equilibrium and increases the methane content in the product gas.

[0117] In some embodiments of the present invention, preferably, in the 3# composite methanation reactor (13), the height of the adiabatic methanation bed is ≤300 mm, for example, 100 mm, 200 mm, 300 mm, and any value in the range consisting of any two values, preferably 100-300 mm.

[0118] In some embodiments of the present invention, preferably, Figure 1-2 As shown, the device also includes: a boiler waste separator 24 connected to the ultra-high pressure steam drum 23 and the medium pressure steam drum 14, which is used to mix the first boiler waste 028-i and the second boiler waste 028-ii generated by the high pressure boiler water 019 and the medium pressure boiler water 023 and then perform waste separation to obtain waste flash steam 029 and waste water 030.

[0119] In some embodiments of the present invention, preferably, Figure 1-2As shown, the device further includes: a boiler wastewater cooler 25 connected to the boiler wastewater separator 24, for cooling the wastewater 030, and recycling the steam generator wastewater 031 for circulating backwater.

[0120] The present invention will be described in detail below through examples.

[0121] Based on the total weight of the first methanation catalyst, the content of the alumina carrier is 80 wt %, the content of nickel is 19 wt %, and the content of the rare earth element is 1 wt %.

[0122] Based on the total weight of the second methanation catalyst, the content of the alumina support is 75 wt %, the content of nickel is 23 wt %, and the content of the rare earth element is 2 wt %.

[0123] The sources of synthesis gas are the purified gas from the outlet of the acid gas removal unit and the green hydrogen provided by the green hydrogen device; the CO content in the synthesis gas is 15-25% by volume, the CO2 content is 0.5-2% by volume, and the H2 content is 65-80% by volume.

[0124] Example 1

[0125] Plants for preparing natural gas from synthesis gas Figure 1-2 As shown;

[0126] A method for preparing natural gas from synthesis gas, which is carried out in the above-mentioned device, comprises:

[0127] (1) Synthesis gas 01 (pressure of 5.11 MPa, temperature of 31°C) is separated into gas and liquid in feed separator 1, and then heat-exchanged with sub-high temperature gas 024 from 3# composite methanation reactor 13 in raw gas preheater 2. The heat-exchanged synthesis gas 02 (temperature of 220°C) is mixed with fine desulfurization water 03 (temperature of 258°C) at a volume flow ratio of 3223:1 and desulfurized in fine desulfurization tank 3 to obtain pretreated synthesis gas 04 (temperature of 209°C, sulfur content of 4 ppb), which is divided into two streams. The volume flow ratio of the first pretreated synthesis gas 04-i to the second pretreated synthesis gas 04-ii is 0.75:1.

[0128] The first stream of pretreated synthesis gas 04-i and circulating gas 05 (temperature 302°C, pressure 5.07 MPa) are mixed at a volume flow ratio of 0.42:1 and enter the first adiabatic methanation reactor 4 for a primary adiabatic methanation reaction (temperature 628°C, pressure 4.99 MPa, volume space velocity 6000 h -1 ), obtaining the first high-temperature gas 06 (temperature of 628°C, pressure of 4.99 MPa);

[0129] (2) The first high-temperature gas 06 and the first ultra-high-pressure boiler water 07-i are heat exchanged in the 1# ultra-high-pressure steam generator 5 at a volume flow ratio of 126:1 to obtain the first low-temperature gas 09 (temperature of 335°C) converted from the first high-temperature gas 06, and the first ultra-high-pressure saturated steam 08-i converted from the first ultra-high-pressure boiler water 07-i; the first low-temperature gas 09 and the preheated high-pressure boiler water 010 (temperature of 235°C) from the 1# ultra-high-pressure boiler water preheater 15 are heat exchanged in the 2# ultra-high-pressure boiler water preheater 6 to obtain the second low-temperature gas 011 (temperature of 304°C) converted from the first low-temperature gas 09 and the ultra-high-pressure boiler water 07 (temperature of 258°C) converted from the preheated high-pressure blast furnace water 010, and the ultra-high-pressure boiler water 07 enters the ultra-high-pressure steam drum 23;

[0130] (3) The second low-temperature gas 011 and the second pretreated synthesis gas 04-ii are mixed and introduced into the 2# adiabatic methanation reactor 7 for a secondary adiabatic methanation reaction (temperature of 626°C, pressure of 4.93MPa). The obtained second high-temperature gas 012 (temperature of 626°C, pressure of 4.89MPa) and ultra-high-pressure saturated steam 08 are superheated in the ultra-high-pressure steam high-temperature superheater 8 and the ultra-high-pressure steam low-temperature superheater 9 in turn to obtain a third low-temperature gas 013 (temperature of 470°C) converted from the second high-temperature gas 012 and a third low-temperature gas 014 (temperature of 470°C) converted from the above-mentioned second high-temperature gas 012. The ultra-high-pressure saturated steam 08 is converted into the ultra-high-pressure superheated steam s11 (temperature of 535°C and pressure of 11.5 MPa); the third low-temperature gas 013 and the medium-pressure saturated steam 014 (temperature of 242°C and pressure of 3.35 MPa) are heat exchanged in the medium-pressure steam superheater 10 at a volume flow ratio of 15:1 to obtain the fourth low-temperature gas 015 (temperature of 461°C) converted from the third low-temperature gas 013 and the medium-pressure superheated steam s21 (temperature of 380°C and pressure of 3.2 MPa) converted from the medium-pressure saturated steam 014;

[0131] (4) The fourth low-temperature gas 015 and the second ultra-high-pressure boiler water 07-ii are heat exchanged in the second ultra-high-pressure steam generator 11 at a volume flow ratio of 306:1 to obtain a fifth low-temperature gas 016 (with a temperature of 354°C) converted from the fourth low-temperature gas 015 and a second ultra-high-pressure saturated steam 08-ii converted from the second ultra-high-pressure boiler water 07-ii; the fifth low-temperature gas 016 and the compressed circulating gas 017 are heat exchanged in the circulating gas heat exchanger 12 to obtain a sixth low-temperature gas 018 (with a temperature of 280°C) converted from the fifth low-temperature gas 016 and a circulating gas 05 converted from the compressed gas 017;

[0132] (5) Splitting the sixth low-temperature gas 018 into two streams, with the volume ratio of one process gas 018-i to the other process gas 018-ii being 2.4:1;

[0133] A stream of process gas 018-i is heat exchanged with high-pressure boiler water 019 (temperature of 161°C and pressure of 17.4 MPa) and first desalted water 020-i, respectively, to obtain a gas-liquid mixture 021 converted from the stream of process gas 018-i, the preheated high-pressure boiler water 010 (temperature of 235°C) converted from the high-pressure boiler water 019, and the first preheated desalted water 022-i converted from the first desalted water 020-i. The gas-liquid mixture 021 is sequentially subjected to a first gas-liquid separation in a circulating gas separator 17 and compressed in a circulating gas compressor 18 to obtain the compressed circulating gas 017.

[0134] Another process gas 018-ii enters the 3# composite methanation reactor 13 for methanation reaction (temperature 280°C; pressure 4.75 MPa; volume space velocity 5000 h -1 ), the obtained third high-temperature gas is heat exchanged with medium-pressure boiler water 023 (temperature of 115°C and pressure of 4 MPa) to obtain sub-high-temperature gas 024 (temperature of 260°C and pressure of 4.75 MPa) converted from the third high-temperature gas and medium-pressure saturated steam 014 (temperature of 251°C and pressure of 4 MPa) converted from the medium-pressure boiler water 023;

[0135] The sub-high-temperature gas 024 enters the 2nd raw material preheater 2 to preheat the above-mentioned synthesis gas 01. The obtained seventh low-temperature gas 025 (with a temperature of 165°C) enters the gas-liquid separator 19 for a second gas-liquid separation. The obtained gas is then heat-exchanged with the second desalted water 020-ii in the 2nd desalted water preheater 20 to obtain the second preheated desalted water 022-ii and the eighth low-temperature gas 026 (with a temperature of 90°C). The eighth low-temperature gas 026 is then water-cooled in the product water cooler 21 and subjected to a third gas-liquid separation in the product separator 22 to obtain natural gas p11.

[0136] Among them, ultra-high pressure boiler water 07 (temperature of 258°C) includes first ultra-high pressure boiler water 07-i and second ultra-high pressure boiler water 07-ii with a volume flow ratio of 2.3:1; ultra-high pressure saturated steam 08 (temperature of 323°C; pressure of 11.65 MPa) includes first ultra-high pressure saturated steam 08-i and second ultra-high pressure saturated steam 08-ii with a volume flow ratio of 2.3:1;

[0137] The desalted water 020 (temperature is 40° C.) includes the first desalted water 020-i and the second desalted water 020-ii; the preheated desalted water 022 (temperature is 114° C.) includes the first preheated desalted water 022-i and the second preheated desalted water 022-ii.

[0138] Among them, the first boiler wastewater 028-i generated by the high-pressure boiler water 019 in the ultra-high-pressure steam drum 23 is mixed with the second boiler wastewater 028-ii generated by the medium-pressure boiler water 023 in the medium-pressure steam drum 14, and then enters the boiler wastewater separator 24 for wastewater separation to obtain wastewater flash steam 029 and wastewater 030. The wastewater 030 enters the boiler wastewater cooler 25 for wastewater cooling, and the obtained steam generator wastewater 031 is reused for circulating water.

[0139] Among them, in the 3# composite methanation reactor 13, the height ratio of the adiabatic methanation bed layer to the isothermal methanation bed layer is 1:79, and the height of the adiabatic methanation bed layer is 100 mm;

[0140] The physical properties of natural gas p11 are as follows: pressure 4.62 MPa; temperature 40°C; CH4 content 93.5% by volume, meeting the requirements of Class I synthetic natural gas;

[0141] The energy consumption of the process flow of Example 1 is shown in Table 1.

[0142] Comparative Example 1

[0143] The synthesis gas of Example 1 was subjected to a methanation process according to CN201610942197.X;

[0144] The energy consumption of the process flow of Comparative Example 1 is shown in Table 1.

[0145] Comparative Example 2

[0146] The synthesis gas of Example 1 was processed according to the traditional four-stage adiabatic process (the process commonly used in the methanation devices currently in operation in China);

[0147] The energy consumption of the process flow of Comparative Example 2 is shown in Table 1.

[0148] Comparative Example 3

[0149] The apparatus of Example 1 is different in that the 3# composite methanation reactor 13 is replaced by an isothermal methanation reactor;

[0150] According to the method of Example 1, except that the method is carried out in the above-mentioned device, ultra-high pressure superheated steam ds13, medium pressure superheated steam ds23 and natural gas dp13 are obtained;

[0151] Among them, the temperature of ultra-high pressure superheated steam ds13 is 535℃ and the pressure is 11.5MPa;

[0152] Among them, the temperature of medium-pressure superheated steam ds23 is 380°C and the pressure is 3.2MPa;

[0153] The physical properties of natural gas dp13 are as follows: pressure of 4.54 MPa; temperature of 40°C; and CH4 content of 93.2% by volume, meeting the requirements of Class I synthetic natural gas.

[0154] The energy consumption of the process flow of Comparative Example 3 is shown in Table 1.

[0155] Comparative Example 4

[0156] The apparatus is as in Example 1, except that the 3# composite methanation reactor 13 is replaced by the 3# adiabatic methanation reactor;

[0157] According to the method of Example 1, except that the method is carried out in the above-mentioned device to obtain ultra-high pressure superheated steam ds14 and natural gas dp14;

[0158] Among them, the temperature of ultra-high pressure superheated steam ds14 is 535℃ and the pressure is 12.5MPa;

[0159] The physical properties of natural gas dp14 are as follows: pressure of 4.42 MPa; temperature of 40°C; CH4 content of 87.2% by volume, meeting the requirements of Class I synthetic natural gas.

[0160] The energy consumption of the process flow of Comparative Example 4 is shown in Table 1.

[0161] Table 1

[0162]

[0163]

[0164] Table 1

[0165]

[0166] Note: - (minus sign) indicates output.

[0167] The data in Table 1 show that, compared to Comparative Examples 1-4, Example 1, using a two-stage adiabatic + one-stage composite process, produces comparable ultra-high-pressure steam as a conventional four-stage adiabatic process. It also produces medium-pressure saturated steam as a byproduct, while the other processes produce relatively little ultra-high-pressure steam. In terms of waste heat utilization, power consumption, and water consumption, the adiabatic process and the adiabatic + composite process are comparable.

[0168] The preferred embodiments of the present invention are described in detail above, but the present invention is not limited thereto. Within the scope of the technical concept of the present invention, various simple variations of the technical solution of the present invention may be made, including combining the various technical features in any other appropriate manner. These simple variations and combinations should also be regarded as the contents disclosed by the present invention and belong to the present invention.

Claims

1. A method for preparing natural gas from synthesis gas, characterized in that: The method comprises: a multi-stage methanation step and a methanation heat recovery steam production step; wherein the multi-stage methanation step sequentially utilizes a primary adiabatic methanation reaction, a secondary adiabatic methanation reaction, and a composite methanation reaction to produce natural gas (p1); and the methanation heat recovery steam production step utilizes the reaction heat generated in the multi-stage methanation step to produce 9-12.5 MPa ultra-high-pressure superheated steam (s1) and 3-5 MPa medium-pressure superheated steam (s2); Wherein, the composite methanation reaction includes in sequence: an adiabatic methanation stage and an isothermal methanation stage; The method comprises the following steps: (1) pre-treating the synthesis gas (01) to obtain a pre-treated synthesis gas (04) divided into two streams, the first pre-treated synthesis gas (04-i) and the circulating gas (05) are mixed and subjected to the first-stage adiabatic methanation reaction to obtain a first high-temperature gas (06); (2) exchanging heat between the first high-temperature gas (06) and the first ultra-high-pressure boiler water (07-i) to obtain a first low-temperature gas (09) converted from the first high-temperature gas (06) and a first ultra-high-pressure saturated steam (08-i) converted from the first ultra-high-pressure boiler water (07-i); exchanging heat between the first low-temperature gas (09) and the preheated high-pressure boiler water (010) to obtain a second low-temperature gas (011) converted from the first low-temperature gas (09) and ultra-high-pressure boiler water (07) converted from the preheated high-pressure boiler water (010); (3) The second low-temperature gas (011) and the second stream of pretreated synthesis gas (04-ii) are mixed and subjected to the secondary adiabatic methanation reaction, and the obtained second high-temperature gas (012) and the ultra-high-pressure saturated steam (08) are subjected to ultra-high-pressure steam superheating to obtain a third low-temperature gas (013) converted from the second high-temperature gas (012) and the ultra-high-pressure superheated steam (s1) converted from the ultra-high-pressure saturated steam (08); the third low-temperature gas (013) and the medium-pressure saturated steam (014) are subjected to heat exchange to obtain a fourth low-temperature gas (015) converted from the third low-temperature gas (013) and the medium-pressure superheated steam (s2) converted from the medium-pressure saturated steam (014); (4) exchanging heat between the fourth low-temperature gas (015) and the second ultra-high-pressure boiler water (07-ii) to obtain a fifth low-temperature gas (016) converted from the fourth low-temperature gas (015) and a second ultra-high-pressure saturated steam (08-ii) converted from the second ultra-high-pressure boiler water (07-ii); exchanging heat between the fifth low-temperature gas (016) and the compressed circulating gas (017) to obtain a sixth low-temperature gas (018) converted from the fifth low-temperature gas (016) and the circulating gas (05) converted from the compressed circulating gas (017); (5) The sixth low-temperature gas (018) is divided into two streams, one process gas (018-i) is heat-exchanged with high-pressure boiler water (019) and the first desalted water (020-i) respectively, to obtain a gas-liquid mixture (021) converted from the process gas (018-i), the preheated high-pressure boiler water (010) converted from the high-pressure boiler water (019), and the first preheated desalted water (022-i) converted from the first desalted water (020-i); wherein the gas-liquid mixture (021) is sequentially subjected to the first gas-liquid separation and compression to obtain the compressed circulating gas (017); the other process gas (018-ii) is subjected to the composite methanation reaction, and the obtained third high-temperature gas is heat-exchanged with medium-pressure boiler water (023) to obtain a sub-high-temperature gas (024) converted from the third high-temperature gas and the medium-pressure saturated steam (014) converted from the medium-pressure boiler water (023); wherein the sub-high temperature gas (024) preheats the synthesis gas (01), and the obtained seventh low temperature gas (025) is subjected to a second gas-liquid separation, and the obtained gas is heat exchanged with a second desalted water (020-ii) to obtain a second preheated desalted water (022-ii) and an eighth low temperature gas (026), and the eighth low temperature gas (026) is sequentially subjected to water cooling and a third gas-liquid separation to obtain the natural gas (p1); The ultra-high-pressure boiler water (07) includes the first ultra-high-pressure boiler water (07-i) and the second ultra-high-pressure boiler water (07-ii); and the ultra-high-pressure saturated steam (08) includes the first ultra-high-pressure saturated steam (08-i) and the second ultra-high-pressure saturated steam (08-ii).

2. The method according to claim 1, wherein The primary adiabatic methanation reaction, the secondary adiabatic methanation reaction and the adiabatic methanation stage are each independently carried out in the presence of a first methanation catalyst; the isothermal methanation stage is carried out in the presence of a second methanation catalyst; Wherein, the first methanation catalyst and the second methanation catalyst each independently comprise: an alumina carrier and nickel and rare earth elements supported on the alumina carrier.

3. The method according to claim 2, wherein: Based on the total weight of the first methanation catalyst, the nickel content is 15-22 wt%, and the rare earth element content is 1-5 wt%. Based on the total weight of the second methanation catalyst, the nickel content is 20-25 wt%, and the rare earth element content is 1-5 wt%. And / or, the physical properties of the first methanation catalyst meet the following requirements: inlet temperature of 250-350°C; reaction temperature of 250-700°C; space velocity of 6000-11000h -1 ; Reaction pressure is 2-5MPa; The physical properties of the second methanation catalyst meet the following requirements: inlet temperature is 250-300°C; reaction temperature is 300-400°C; space velocity is 5000-10000h -1 ; The reaction pressure is 2-5MPa.

4. The method according to claim 1, wherein In step (1), the conditions for the first-stage adiabatic methanation reaction include: pressure of 2-5.5 MPa; temperature of 590-690°C; volume space velocity of 5000-8000 h -1 ; and / or, the synthesis gas (01) has a pressure of 2.2-5.5 MPa and a temperature of 20-60° C.; and / or, in the synthesis gas (01), the CO content is 15-25% by volume, the CO2 content is 0.5-2% by volume, and the H2 content is 65-80% by volume; and / or, the temperature of the pretreated synthesis gas (04) is 180-220° C., the sulfur content is 2-30 ppb, and the pressure is 2.2-5.5 MPa; and / or, the volume flow ratio of the first pretreated synthesis gas (04-i) to the second pretreated synthesis gas (04-ii) is 0.7-1.2:1; and / or, the volume flow ratio of the first pretreated synthesis gas (04-i) to the circulating gas (05) is 0.4-0.8:1; And / or, the pressure of the circulating gas (05) is 2-5.5 MPa; the temperature is 270-350°C; And / or, the pressure of the first high-temperature gas (06) is 2-5.5 MPa; the temperature is 590-690°C; And / or, the pretreatment process includes: subjecting the synthesis gas (01) to a fourth gas-liquid separation and then exchanging heat with the sub-high temperature gas (024); mixing the heat-exchanged synthesis gas (02) obtained with fine desulfurization water (03) and performing desulfurization to obtain the pretreated synthesis gas (04).

5. The method according to claim 4, wherein The temperature of the synthesis gas (02) after the heat exchange is 200-240°C; The temperature of the refined desulfurization water (03) is 240-280°C; The volume flow ratio of the synthesis gas (02) after heat exchange to the water for fine desulfurization (03) is 3000-3500:

1.

6. The method according to claim 1, wherein In step (2), the volume flow ratio of the first high-temperature gas (06) to the first ultra-high-pressure boiler water (07-i) is 120-140:1; And / or, the temperature of the ultra-high pressure saturated steam (08) is 320-326°C; the pressure is 9-12.5 MPa; and / or, the volume ratio of the first ultra-high pressure saturated steam (08-i) to the second ultra-high pressure saturated steam (08-ii) is 2.1-2.5:1; and / or, the temperature of the ultra-high pressure boiler water (07) is 240-280°C; And / or, the volume flow ratio of the first ultra-high pressure boiler water (07-i) to the second ultra-high pressure boiler water (07-ii) is 2.1-2.5:

1.

7. The method according to claim 1, wherein In step (3), The conditions of the secondary adiabatic methanation reaction include: pressure of 2-5.5 MPa; temperature of 590-690°C; volume space velocity of 5000-8000 h -1 ; And / or, the second high-temperature gas (012) has a pressure of 2-5.5 MPa and a temperature of 590-690°C; And / or, the process of superheating the ultra-high pressure steam comprises: superheating the second high temperature gas (012) and the ultra-high pressure saturated steam (08) in sequence by superheating the ultra-high pressure steam at high temperature and superheating the ultra-high pressure steam at low temperature; and / or, the volume flow ratio of the third low-temperature gas (013) to the medium-pressure saturated steam (014) is 14-16:1; And / or, the temperature of the medium-pressure saturated steam (014) is 234-252°C; and the pressure is 3-5 MPa.

8. The method according to claim 1, wherein In step (4), The volume flow ratio of the fourth low-temperature gas (015) to the second ultra-high-pressure boiler water (07-ii) is 300-320:1; In step (5), In the sixth low-temperature gas, the volume flow ratio of one process gas (018-i) to the other process gas (018-ii) is 2.3-2.5:1; and / or, the volume flow ratio of the process gas (018-i), the high-pressure boiler water (019) and the first desalted water (020-i) is 4100-4300:65-85:30-45; And / or, the temperature of the high-pressure boiler water (019) is 150-180°C; the pressure is 17-18MPa; and / or, the temperature of the preheated high-pressure boiler water (010) is 220-250°C; And / or, the conditions of the adiabatic methanation stage and the isothermal methanation stage independently include: pressure of 2-5.5 MPa; temperature of 220-300°C; volume space velocity of 5000-8000 h -1 ; And / or, the pressure of the sub-high temperature gas (024) is 2-5.5 MPa; the temperature is 220-300°C; And / or, the temperature of the medium-pressure boiler water (023) is 110-120°C; the pressure is 3.7-5.2MPa; And / or, the temperature of the medium-pressure saturated steam (014) is 234-252°C; the pressure is 3-5MPa; And / or, the desalted water (020) includes the first desalted water (020-i) and the second desalted water (020-ii), and the temperature of the desalted water (020) is 25-35°C; And / or, the preheated desalted water (022) includes the first preheated desalted water (022-i) and the second preheated desalted water (022-ii), and the temperature of the preheated desalted water (022) is 110-120°C.

9. The method according to any one of claims 1 to 8, wherein: The physical properties of the natural gas (p1) meet the following requirements: pressure of 2-5.5 MPa; temperature of 35-45°C; CH4 content of 93-96% by volume; high calorific value ≥35 MJ / m 3 , H2 content ≤3.5×10 -2 mol%, CO2 content ≤2×10 -2 mol%, H2S content ≤1mg / m 3 , CO content ≤ 0.15×10 -2 mol%, NH3 content ≤50×10 -6 mol%, solid particle content ≤1mg / m 3 ; And / or, the method further comprises: mixing the first boiler blowdown (028-i) and the second boiler blowdown (028-ii) generated by the high-pressure boiler water (019) and the medium-pressure boiler water (023), performing blowdown separation to obtain blowdown flash steam (029) and blowdown water (030), cooling the blowdown water (030), and recycling the steam generator blowdown (031) obtained as circulating water.

10. A device for preparing natural gas from synthesis gas, characterized in that: The device comprises: a feed separator (1), a raw gas preheater (2) and a fine desulfurization tank (3) connected in sequence, and is used to pretreat the synthesis gas (01) and separate the obtained pretreated synthesis gas (04) into two streams; The first stream of pretreated synthesis gas (04-i) and the circulating gas (05) from the circulating gas heat exchanger (12) are mixed and then enter the 1# adiabatic methanation reactor (4). The obtained first high-temperature gas (06) enters the 1# ultra-high-pressure steam generator (5) and exchanges heat with the first ultra-high-pressure boiler water (07-i) from the ultra-high-pressure steam drum (23) to obtain the first low-temperature gas (09) and the first ultra-high-pressure saturated steam (08-i); the first low-temperature gas (09) enters the 2# ultra-high-pressure boiler water preheater (6) and exchanges heat with the preheated high-pressure boiler water from the 1# ultra-high-pressure boiler water preheater (15). (010) for heat exchange, the obtained ultra-high pressure boiler water (07) enters the ultra-high pressure steam drum (23), and the obtained second low temperature gas (011) is mixed with the second pre-treated synthesis gas (04-ii) and then enters the 2# adiabatic methanation reactor (7), and the obtained second high temperature gas (012) enters the ultra-high pressure steam high temperature superheater (8) and the ultra-high pressure steam low temperature superheater (9) in sequence, and exchanges heat with the ultra-high pressure saturated steam (08) from the ultra-high pressure steam drum (23) to obtain 9-12.5 MPa ultra-high pressure superheated steam (s1) and the third low temperature gas (013); The third low-temperature gas (013) enters the medium-pressure steam superheater (10) and exchanges heat with the medium-pressure saturated steam (014) from the medium-pressure steam drum (14) to obtain 3-5 MPa medium-pressure superheated steam (s2) and a fourth low-temperature gas (015); The fourth low-temperature gas (015) enters the 2# ultra-high-pressure steam generator (11) and exchanges heat with the second ultra-high-pressure boiler water (07-ii) from the ultra-high-pressure steam drum (23) to obtain a fifth low-temperature gas (016) and a second ultra-high-pressure saturated steam (08-ii); the fifth low-temperature gas (016) exchanges heat with the compressed circulating gas (017) from the circulating gas compressor (18) in the circulating gas heat exchanger (12), and the obtained sixth low-temperature gas (018) is divided into two streams. A stream of process gas (018-i) sequentially enters the 1# ultra-high pressure boiler water preheater (15) and the 1# desalted water preheater (16) for heat exchange, and the resulting gas-liquid mixture (021) sequentially enters the circulating gas separator (17) and the circulating gas compressor (18) to obtain the compressed circulating gas (017); Another stream of process gas (018-ii) enters the 3# composite methanation reactor (13), and the obtained third high-temperature gas is heat-exchanged with the medium-pressure boiler water (023) in the shell side, and the obtained medium-pressure saturated steam (014) enters the medium-pressure steam drum (14). The obtained second high-temperature gas (024) enters the raw gas preheater (2) to preheat the synthesis gas (01), and the obtained seventh low-temperature gas (025) enters the gas-liquid separator (19), the 2# desalted water preheater (20), the product water cooler (21) and the product separator (22) in sequence to obtain natural gas (p1); Wherein, the ultra-high pressure boiler water (07) includes first ultra-high pressure boiler water (07-i) and second ultra-high pressure boiler water (07-ii); the ultra-high pressure saturated steam (08) includes the first ultra-high pressure saturated steam (08-i) and the second ultra-high pressure saturated steam (08-ii); The 3# composite methanation reactor (13) comprises an adiabatic methanation bed layer arranged on the top and an isothermal methanation bed layer arranged on the bottom.

11. The device according to claim 10, wherein In the 3# composite methanation reactor (13), the height ratio of the adiabatic methanation bed layer to the isothermal methanation bed layer is 1:26-79; and / or, in the 3# composite methanation reactor (13), the height of the adiabatic methanation bed is ≤300 mm; And / or, the device further comprises: a boiler blowdown separator (24) connected to the ultra-high-pressure steam drum (23) and the medium-pressure steam drum (14), for mixing the first boiler blowdown (028-i) and the second boiler blowdown (028-ii) generated by the high-pressure boiler water (019) and the medium-pressure boiler water (023), and then performing blowdown separation to obtain blowdown flash steam (029) and blowdown water (030); And / or, the device further comprises: a boiler wastewater cooler (25) connected to the boiler wastewater separator (24), for cooling the wastewater (030), and recycling the steam generator wastewater (031) as circulating water.

12. The device according to claim 11, wherein In the 3# composite methanation reactor (13), the height of the adiabatic methanation bed is 100-300 mm.

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