Method for co-producing liquefied natural gas (LNG) and hydrogen by using methanol device

Abstract

According to the method for co-producing LNG and hydrogen by using the methanol device, methanol purge gas and purified synthesis gas gasified from the methanol device are subjected to heat exchange and heated to 220-300 DEG C, methanation reaction is carried out through a methanation unit in a single pass under the action of a methane synthesis catalyst, and methanation product gas is obtained; according to the method, methanation product gas is cooled to 50-100 DEG C through heat exchange and enters a cryogenic separation unit after being dehydrated, LNG and non-condensable gas are obtained through a cryogenic separation process, the non-condensable gas enters a PSA unit, and hydrogen and fuel gas are obtained through a pressure swing adsorption process under the action of an adsorbent. According to the method for co-producing the LNG and the hydrogen by utilizing the methanol device, the problem that the H2 component of the methanol purge gas is not fully utilized in the existing methanol purge gas recovery process is solved, and the LNG and the hydrogen can be co-produced by utilizing H2, CO, CO2 and CH4 components in the methanol purge gas.

Description

Technical Field

[0001] The present invention belongs to the technical field of carbon neutrality and carbon reduction, and specifically relates to a method for co-producing LNG and hydrogen using a methanol device. Background Art

[0002] In 2023, my country's methanol production capacity will be about 107 million tons, and the output will be 84.4 million tons. my country's methanol plants mainly use coal as raw material. During the methanol synthesis process, a large amount of purge gas with H2, CO, CO2, CH2 and N2 as the main components will be produced. The different contents of inert components in the purge gas result in different purge gas volumes for methanol. At present, industrially operated methanol plants mainly use membrane separation + PSA technology to recover effective gas and return it to the inlet of the synthesis tower. The remaining CO / CO2 / CH4 and other components in the purge gas are directly sent to the boiler as fuel, and the effective carbon components are not fully utilized.

[0003] Patent CN202010698135.5 discloses a system and method for recycling methanol purge gas from a methanol to olefins unit. The purge gas is separated by membrane and PSA, and the permeate gas obtained is returned to the inlet of the methanol unit, and the product gas hydrogen is sent to the methanol to olefins system. The process flow is long and the operation is complicated.

[0004] Patent 202222213780.0 discloses a methanol purge gas recovery and utilization system, in which most of the purge gas is recovered and utilized through a membrane hydrogen extraction device, the permeate gas is returned to the methanol device for synthesis, a part of the permeate gas is mixed with the purge gas as a co-burning gas for the acid flare device, and a part is sent to the boiler device to produce steam.

[0005] Patent CN202011203961.4 discloses a methanol purge gas recovery and utilization system and process, which allows the purge gas to undergo a single methanol synthesis reaction to replace the traditional membrane separation process. No compressor is required, and the CO and CO2 in the purge gas are converted into methanol for recovery.

[0006] Patent CN201710844259.8 discloses a methanol purge gas pressure energy recovery system and method, which transfers the pressure energy of high-pressure tail gas to the permeate gas, ensuring the required pressure for the permeate gas to be sent to the downstream, thereby reducing the pressure behind the membrane. This means that the pressure difference before and after the membrane in the membrane separation device is increased, the hydrogen separation effect is improved, and the pressure energy of the high-pressure tail gas can be recovered and utilized.

[0007] Although there are many studies on the resource utilization of methanol purge gas in methanol plants and a large number of related patents have been published, these studies mainly focus on membrane separation, PSA optimization or integration, and the production of more methanol. The hydrogen-carbon modulus ratio in methanol purge gas is usually 6.5, and the molar number of hydrogen is 6.5 times that of carbon. There is an excess of hydrogen in the methanol purge gas. When only the purge gas is used for methanation, there will still be a large amount of excess hydrogen at the export, and the hydrogen component cannot be maximized, resulting in energy waste. Summary of the invention

[0008] The purpose of the present invention is to provide a method for co-producing LNG and hydrogen using a methanol device. The method solves the problem that the H2 component of the methanol purge gas is not fully utilized in the existing methanol purge gas recovery process, and can fully utilize the H2, CO, CO2 and CH4 components in the methanol purge gas to co-produce LNG and hydrogen.

[0009] The technical solution adopted by the present invention is a method for co-producing LNG and hydrogen using a methanol device, comprising the following steps: S1: The methanol purge gas and the purified synthesis gas from coal gasification of the methanol unit are heated to 220°C-300°C by heat exchange, and a methanation reaction is carried out in a single pass through a methanation unit under the action of a methane synthesis catalyst to obtain a methanation product gas; S2: The methanogenic product gas is cooled to 50°C-100°C by heat exchange, and then enters the cryogenic separation unit after dehydration, and LNG and non-condensable gas are obtained through the cryogenic separation process; S3: The non-condensable gas is introduced into the PSA unit, and hydrogen is obtained through a pressure swing adsorption process under the action of the adsorbent.

[0010] The present invention is also characterized in that: In step S1, the volume ratio of the purified synthesis gas to the methanol purge gas is 0.05-0.35.

[0011] The methanation reaction in the methanation unit in step S1 adopts a temperature-controlled methanation reactor, the operating pressure of the temperature-controlled methanation reactor is 3.0MPaG-9.5MPaG, the operating temperature is 350℃-480℃, and the reaction space velocity is 3000h -1 -20000h -1 .

[0012] The carbon concentration at the inlet of the temperature-controlled methanation reactor is stabilized at 15% to 22%.

[0013] The temperature-controlled methanation reactor is a multi-stage structure. In step S1, the purified synthesis gas enters the temperature-controlled methanation reactor in a manner of being divided into multiple streams.

[0014] The methane synthesis catalyst in step S1 adopts a non-precious metal methanation catalyst, which has one or two of molybdenum and nickel as main active components, at least two of yttrium, lanthanum, cerium, aluminum, iron and manganese as auxiliary components, and at least two of aluminum oxide, silicon oxide, magnesium oxide, calcium oxide, ZSM5 molecular sieve and BETA molecular sieve as carriers.

[0015] The adsorbent of the PSA unit in step S3 is any one of activated alumina, carbon molecular sieve, silica gel or 5A / 3A molecular sieve.

[0016] In step S3, the operating temperature of the PSA unit is 30°C-80°C, the adsorption pressure is 0.5MPaG-7.5MPaG, and the desorption pressure is 0.02MPaG-0.20MPaG.

[0017] The beneficial effects of the present invention are: 1. The method for co-producing LNG and hydrogen by utilizing a methanol device provided by the present invention introduces purified synthesis gas, and the purified synthesis gas is a carbon-rich gas, so that the hydrogen-carbon modulus ratio at the inlet of a temperature-controlled methanation reactor can be adjusted, and the carbon content at the inlet of the temperature-controlled methanation reactor can be further improved, so as to maximize the utilization of H2, CO, CO2 and CH4 components and increase the production of LNG. When the methanol device is in operation, the CO / CO2 at the inlet of the methanation reactor fluctuates greatly. An online carbon concentration analysis detector is provided at the inlet of the temperature-controlled methanation reactor to monitor the inlet carbon concentration in real time, so as to automatically adjust the feed amount of the purified synthesis gas, so that the inlet carbon concentration entering the temperature-controlled methanation reactor is stabilized at 15% to 22%, and the components such as H2, CH4, CO and CO2 in the purge gas can be fully utilized to further co-produce LNG and hydrogen, thereby solving the problem that the H2 component of the methanol purge gas is not fully utilized in the existing methanol purge gas recovery process, increasing the supply of natural gas, being able to adapt to fluctuations under different operating conditions in the early, middle and late stages of the methanol device, and being flexible in operation.

[0018] 2. The method of co-producing LNG and hydrogen using a methanol device provided by the present invention completely converts them into methane by temperature-controlled methanation, has a high CO / CO2 single-pass conversion rate, reduces the carbon concentration at the outlet, and does not require a separate CO / CO2 separation device. At the same time, the process of the present invention does not require a separate circulating gas compressor, has low comprehensive energy consumption, is simple in the entire process, and has mild reaction conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 A schematic diagram of a process flow of the present invention in which methanol purge gas is introduced into purified synthesis gas and then divided into shares and enters a single temperature-controlled methanation reactor to co-produce LNG and hydrogen; Figure 2The figure is a schematic diagram of the process flow of the present invention in which the methanol purge gas is introduced into the purified synthesis gas and then divided into shares and enters the multi-stage temperature-controlled methanation reactor to co-produce LNG and hydrogen. DETAILED DESCRIPTION

[0020] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0021] Example 1 The method of co-producing LNG and hydrogen by using a methanol device proposed in this embodiment is as follows: Figure 1 As shown, the methanol purge gas produced by the methanol unit is 22000Nm 3 / h, with a composition of H2=76.81%, CH4=2.49%, CO=5.92%, CO2=1.24%, N2=10.71%, Ar=2.23%, methanol=0.57%, temperature 40℃, pressure 7.5MPaG, and at the same time, 4600Nm2 of purified synthesis gas from coal gasification of methanol unit was introduced. 3 / h, the composition is H2=68%, CH4=0.45%, CO=24.5%, CO2=5.0%, N2=1.5%.

[0022] The methanol purge gas and the purified synthesis gas are methanated to heat up to 220°C, the methanol purge gas passes through the temperature-controlled methanation reactor in a single pass, and the purified synthesis gas enters the temperature-controlled methanation reactor in batches. The methanol purge gas and the purified synthesis gas undergo methanation reaction in the temperature-controlled methanation reactor. The temperature of the reactants during the reaction is controlled at 350°C, and the pressure is the methanol purge gas pressure. After the reaction, the CO content in the methanation product gas is ≤50ppm, and the CO2 content is ≤80ppm.

[0023] The methanation product gas is then heat exchanged and cooled to 100°C. After dehydration, it passes through a cryogenic separation unit for a cryogenic separation process to obtain 2.9 tons / year of non-condensable gas and LNG products. The non-condensable gas enters the PSA unit for a pressure swing adsorption process to obtain 80 million cubic meters / year of high-purity hydrogen with a purity of more than 99.9%.

[0024] Example 2 The method of co-producing LNG and hydrogen by using a methanol device proposed in this embodiment is as follows: Figure 1 As shown, the methanol purge gas produced by the methanol unit is 22000Nm 3 / h, with a composition of H2=76.81%, CH4=2.49%, CO=5.92%, CO2=1.24%, N2=10.71%, Ar=2.23%, methanol=0.57%, temperature 40℃, pressure 7.5MPaG, and at the same time, 3800Nm2 of purified synthesis gas from coal gasification of methanol unit was introduced. 3 / h, the composition is H2=68%, CH4=0.45%, CO=24.5%, CO2=5.0%, N2=1.5%.

[0025] The methanol purge gas and the purified synthesis gas are methanated to heat up to 300°C, the methanol purge gas passes through the temperature-controlled methanation reactor in a single pass, and the purified synthesis gas enters the temperature-controlled methanation reactor in batches. The methanol purge gas and the purified synthesis gas undergo methanation reaction in the temperature-controlled methanation reactor. The temperature of the reactants during the reaction is controlled at 480°C, and the pressure is the methanol purge gas pressure. After the reaction, the CO content in the methanation product gas is ≤60ppm, and the CO2 content is ≤120ppm.

[0026] The methanation product gas is then heat exchanged and cooled to 50°C. After dehydration, it passes through a cryogenic separation unit for a cryogenic separation process to obtain 2.5 tons / year of non-condensable gas and LNG products. The non-condensable gas enters the PSA unit for a pressure swing adsorption process to obtain 90 million cubic meters / year of high-purity hydrogen with a purity of more than 99.9%.

[0027] Example 3 The method of co-producing LNG and hydrogen by using a methanol device proposed in this embodiment is as follows: Figure 1 As shown, the methanol purge gas produced by the methanol unit is 22000Nm 3 / h, with a composition of H2=76.81%, CH4=2.49%, CO=5.92%, CO2=1.24%, N2=10.71%, Ar=2.23%, methanol=0.57%, temperature 40℃, pressure 7.5MPaG, and at the same time, 4600Nm2 of purified synthesis gas from coal gasification of methanol unit was introduced. 3 / h, the composition is H2=68%, CH4=0.45%, CO=24.5%, CO2=5.0%, N2=1.5%.

[0028] The methanol purge gas and the purified synthesis gas are methanated to heat up to 250°C, the methanol purge gas passes through the temperature-controlled methanation reactor in a single pass, and the purified synthesis gas enters the temperature-controlled methanation reactor in batches. The methanol purge gas and the purified synthesis gas undergo methanation reaction in the temperature-controlled methanation reactor. The temperature of the reactants during the reaction is controlled at 400°C, and the pressure is the methanol purge gas pressure. After the reaction, the CO content in the methanation product gas is ≤50ppm, and the CO2 content is ≤100ppm.

[0029] The methanation product gas is then heat exchanged and cooled to 70°C. After dehydration, it passes through a cryogenic separation unit for a cryogenic separation process to obtain 2.9 tons / year of non-condensable gas and LNG products. The non-condensable gas enters the PSA unit for a pressure swing adsorption process to obtain 80 million cubic meters / year of high-purity hydrogen with a purity of more than 99.9%.

[0030] Example 4 The method of co-producing LNG and hydrogen by using a methanol device proposed in this embodiment is as follows: Figure 2 As shown, the methanol purge gas produced by the methanol unit is 22000Nm 3 / h, with a composition of H2=76.81%, CH4=2.49%, CO=5.92%, CO2=1.24%, N2=10.71%, Ar=2.23%, methanol=0.57%, temperature 40℃, pressure 7.5MPaG, and at the same time, 4600Nm2 of purified synthesis gas from coal gasification of methanol unit was introduced. 3 / h, the composition is H2=68%, CH4=0.45%, CO=24.5%, CO2=5.0%, N2=1.5%.

[0031] The methanol purge gas and the purified synthesis gas are methanated to heat up to 310°C, the methanol purge gas enters the temperature-controlled methanation reactor I, and the purified synthesis gas enters the temperature-controlled methanation reactor II and the temperature-controlled methanation reactor III in batches, where a methanation reaction occurs. The temperature of the reactants during the reaction is controlled at 410°C, and the pressure is the methanol purge gas pressure. After the reaction, the CO content in the methanation product gas is ≤70ppm, and the CO2 content is ≤80ppm.

[0032] The methanation product gas is cooled to 60°C through heat exchange and then dehydrated and passed through a cryogenic separation unit for a cryogenic separation process to obtain 18,000 tons / year of non-condensable gas and LNG products. The non-condensable gas enters the PSA unit for a pressure swing adsorption process to obtain 90 million cubic meters / year of high-purity hydrogen with an accuracy of more than 99.9%.

Claims

1. A method for co-producing LNG and hydrogen using a methanol device, characterized in that: The following steps are involved: S1: The methanol purge gas and the purified synthesis gas from coal gasification of the methanol unit are heated to 220°C-300°C by heat exchange, and a methanation reaction is carried out in a single pass through a methanation unit under the action of a methane synthesis catalyst to obtain a methanation product gas; S2: The methanogenic product gas is cooled to 50°C-100°C by heat exchange, and then enters the cryogenic separation unit after dehydration, and LNG and non-condensable gas are obtained through the cryogenic separation process; S3: The non-condensable gas is introduced into the PSA unit, and hydrogen is obtained through a pressure swing adsorption process under the action of the adsorbent.

2. The method for co-producing LNG and hydrogen using a methanol device according to claim 1, characterized in that: In step S1, the volume ratio of the purified synthesis gas to the methanol purge gas is 0.05-0.

35.

3. The method for co-producing LNG and hydrogen using a methanol device according to claim 2, characterized in that: The methanation reaction in the methanation unit in step S1 adopts a temperature-controlled methanation reactor, the operating pressure of the temperature-controlled methanation reactor is 3.0MPaG-9.5MPaG, the operating temperature is 350°C-480°C, and the reaction space velocity is 3000h -1 -20000h -1 .

4. The method for co-producing LNG and hydrogen using a methanol device according to claim 3, characterized in that: The carbon concentration at the inlet of the temperature-controlled methanation reactor is stabilized at 15% to 22%.

5. The method for co-producing LNG and hydrogen using a methanol device according to claim 4, characterized in that: The temperature-controlled methanation reactor is a multi-stage structure, and the purified synthesis gas in step S1 enters the temperature-controlled methanation reactor in a manner of being divided into multiple streams.

6. The method for co-producing LNG and hydrogen using a methanol device according to claim 1, characterized in that: The methane synthesis catalyst in step S1 adopts a non-precious metal methanation catalyst, which contains one or two of molybdenum and nickel as main active components, at least two of yttrium, lanthanum, cerium, aluminum, iron and manganese as auxiliary components, and at least two of aluminum oxide, silicon oxide, magnesium oxide, calcium oxide, ZSM5 molecular sieve and BETA molecular sieve as carriers.

7. The method for co-producing LNG and hydrogen using a methanol device according to claim 1, characterized in that: The adsorbent of the PSA unit in step S3 is any one of activated alumina, carbon molecular sieve, silica gel or 5A / 3A molecular sieve.

8. The method for co-producing LNG and hydrogen using a methanol device according to claim 1, characterized in that: The operating temperature of the PSA unit in step S3 is 30° C.-80° C., the adsorption pressure is 0.5 MPaG-7.5 MPaG, and the desorption pressure is 0.02 MPaG-0.20 MPaG.

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