Method for preparing methanol by recovering carbon dioxide with low carbon emission and high efficiency

Abstract

The application discloses a method for preparing methanol by recycling carbon dioxide with low carbon emission. The method realizes carbon dioxide resource utilization at normal temperature and pressure, and has low raw material cost and equipment cost. The selectivity of a gas separation membrane is used to realize effective separation and reuse of unreacted hydrogen on an anode, unreacted carbon dioxide and by-product hydrogen on a cathode in electrochemical hydrogenation, realize high utilization rate of carbon dioxide and hydrogen, realize effective separation and reuse of unreacted formic acid in normal temperature and pressure formic acid preparation by using a rectification technology, and realize high formic acid utilization rate. The application solves core bottlenecks such as low reaction efficiency and competitive adsorption of side reactions, the device can be quickly started and stopped, has high flexibility and low production operation cost, flexibly selects product types and product proportions, and significantly improves the competitiveness of the process and market adaptability.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of chemical industry, and particularly relates to a method for preparing methanol by efficiently recovering carbon dioxide with low carbon emission. BACKGROUND

[0002] In order to respond to climate change and realize green and low-carbon development, China needs to recycle carbon dioxide by recycling carbon dioxide resources. However, simply recycling carbon dioxide will only increase the cost of enterprises without economic benefits, and it is necessary to convert carbon dioxide into high-value products for recycling to realize the sustainable development of carbon dioxide recycling. A large amount of flue gas is generated during the production of a refinery, which is the main way for the refinery to emit carbon dioxide into the atmosphere. At present, an economical and sustainable recycling method of carbon dioxide is urgently needed, which can not only realize low carbon emission but also increase enterprise benefits. Methanol is an important organic raw material in chemical production and has high economic value. It can be used as fuel, to produce olefins and to prepare other chemical products. Using carbon dioxide in flue gas to produce methanol by hydrogenation is an important method for efficiently recycling carbon dioxide. In recent years, the demand for methanol in industry has been increasing, and the global demand for methanol in 2021 was about 100 million tons.

[0003] Carbon monoxide hydrogenation to synthesize methanol is an atomically economic method. At present, carbon monoxide catalytic hydrogenation can be used to synthesize methanol in industry, but this synthesis method has many by-products, and it needs to be heated and pressurized for operation. The device has a short running period, and the start-up, shutdown and maintenance time is long. For methanol production by hydrogenation of carbon dioxide, carbon dioxide needs to be reduced to carbon monoxide under heating and pressurization operation conditions, and then carbon monoxide is used to catalytically hydrogenate methanol in two steps, which greatly improves the device requirements and operation cost of methanol production, and is not conducive to the market price fluctuation of methanol.

[0004] Carbon dioxide electrochemical catalytic hydrogenation to produce methanol is a synthesis method combining electrochemistry and catalysis, and is also a more energy-saving and lower-carbon synthesis method. Electrochemical hydrogenation mainly uses electric energy, and all reactions are carried out under mild operating conditions. The overpotential generated by electric current on the cathode can significantly improve the activity of adsorbed hydrogen, and the catalyst is optimized to control the current density and reduce the reaction hydrogen pressure requirement. According to the experimental results, the adsorbed hydrogen activity generated by the current density of 0.005 A / cm 2 at normal pressure is equivalent to pressurizing hydrogen to about 5 MPa, so the electrochemical hydrogenation method can realize normal temperature and normal pressure reaction. Electrochemical hydrogenation also does not need a large amount of heat energy input, thereby eliminating the need for any reducing agent.

[0005] However, since electrochemical hydrogenation units still require carbon dioxide as a reactant and hydrogen as a proton source and current transfer medium, the costs of carbon dioxide and hydrogen directly affect the production and operating costs of electrochemical carbon dioxide hydrogenation units. Refinery flue gas, emitted into the atmosphere as waste gas, contains 5-20 vol% carbon dioxide, which is often difficult to utilize. Hydrogen is a renewable resource in refineries, typically produced by reforming and hydrogen production units and consumed in hydrogenation units. The purity of hydrogen after the reaction is often significantly reduced; it can be recycled after further purification using devices such as pressure swing adsorption (PSA). However, the PSA tail gas still contains 5-30 vol% hydrogen, which is often difficult to utilize and is usually burned and emitted as tail gas.

[0006] The above analysis reveals that effectively utilizing low-purity waste carbon dioxide and low-purity waste hydrogen from refineries is key to reducing the cost of carbon dioxide hydrogenation to methanol systems. In electrochemical hydrogenation reactions, carbon dioxide produces hydrogen as a byproduct during the cathode reaction, and unreacted hydrogen at the anode is also discharged as tail gas. Therefore, electrochemical hydrogenation units are complex devices that consume, process, and generate hydrogen; however, existing technologies have not yet developed systems or processes optimized to address this characteristic.

[0007] Patent CN111559956A discloses a carbon dioxide conversion system and its operating method for a coal-to-methanol process. It employs both a carbon dioxide reactor containing a catalyst and a syngas reactor to produce methanol using carbon dioxide and syngas as feedstocks in the presence of hydrogen. This method offers advantages such as on-site conversion of carbon dioxide to methanol and high methanol yield, but suffers from high hydrogen production costs.

[0008] Patent CN108265145A discloses a method and system for utilizing carbon dioxide-rich blast furnace gas. It employs a fixed-bed reactor containing a catalyst, using carbon dioxide-rich gas and hydrogen-rich gas as raw materials. The hydrogen-carbon molar ratio of the raw gas is 2–3, the pressure is 3–7 MPa, the temperature is 220–300℃, and the space velocity is 4000–15000 hr. -1 Methanol can be prepared under certain conditions. However, the equipment requirements and costs are high.

[0009] Patent CN103922487A discloses a method for producing methanol from wastewater through carbon dioxide reduction. This method uses a microbial electrolysis cell containing a proton exchange membrane to convert carbon dioxide into methanol with electrical assistance and a catalyst. However, the conversion rate and reaction efficiency are low.

[0010] CN202110056471.4 provides a petrochemical tail gas efficient resource utilization method coupled with double membrane separation and electrochemical hydrogen pump hydrogenation. The petrochemical tail gas containing H2 and CO2 is enriched with H2 and CO2 by a double membrane separator, and then is used as anode and cathode of an electrochemical hydrogen pump hydrogenation reactor respectively to participate in CO2 hydrogenation at normal pressure. The cathode obtains formic acid hydrogen energy fuel carrier product, and the anode outlet low hydrogen tail gas is mixed with the retentate gas of the double membrane separation device and then enters the CO2 membrane separation device. The concentrated CO2 is returned to the double membrane separation device for recycling, and the inert gas is discharged from the retentate side of the CO2 membrane separator. The present application couples separation-reaction unit, optimizes operation conditions, synergizes, efficiently realizes the resource utilization of petrochemical tail gas, reduces CO2 emission, and has great economic and social benefits. SUMMARY

[0011] In view of the deficiencies of the prior art, the present application provides a method and device for low-carbon emission and efficient recovery of carbon dioxide to methanol. The method is characterized in that the system provided by the present application can realize the resource utilization of carbon dioxide at normal temperature and pressure, and the raw material cost and equipment cost are low; the selectivity of the gas separation membrane is used to realize the effective separation and reuse of the unreacted hydrogen gas at the anode, the unreacted carbon dioxide and the byproduct hydrogen gas at the cathode of the electrochemical hydrogenation, realize high carbon dioxide and hydrogen utilization, solve the core bottlenecks of low reaction efficiency and side reaction competitive adsorption, maximize the formic acid yield, realize the effective separation and reuse of unreacted formic acid for methanol production at normal temperature and pressure by rectification technology, realize high formic acid utilization, maximize the methanol yield, realize high conversion rate and maximize the product value, and the whole system realizes no material consumption except raw gas through material mutual supply; the device can realize quick start and shutdown, has high flexibility and low production operation cost, can flexibly select product type and product ratio, realizes the maximization of product value and the minimization of operation cost, significantly improves the competitiveness of the process, and has better market adaptability.

[0012] According to a first aspect of the present application, the present application provides a method for low-carbon emission and efficient recovery of carbon dioxide to methanol.

[0013] A method for low-carbon emission and efficient recovery of carbon dioxide to methanol, comprising the following steps:

[0014] (1) The low-purity hydrogen gas raw material is mixed with the recycled hydrogen-rich gas, and then enters the anode of the electrochemical hydrogenation device to dissociate hydrogen, to obtain protons, unreacted hydrogen and hydrocarbon gas contained in the low-purity hydrogen gas raw material as anode tail gas;

[0015] (2) The acid gas passes through a low-temperature methanol washing device to obtain a first CO2-rich gas and a sulfur-containing gas, and the first CO2-rich gas is mixed with a second CO2-rich gas and then enters the cathode of the electrochemical hydrogenation device;

[0016] (3) The protons obtained in step (1) are transferred to the cathode of the electrochemical hydrogenation device, and hydrogenation reaction is carried out on the hydrogenation catalyst in the cathode with CO2 to obtain a product containing formic acid; the product containing formic acid is separated in the first gas-liquid separation device to obtain cathode tail gas and a generated liquid;

[0017] (4) The anode tail gas obtained in step (1) is separated in the hydrogen purification membrane separation device to obtain a first hydrogen-rich gas; the cathode tail gas obtained in step (3) is introduced into the double-membrane gas separator to obtain a second hydrogen-rich gas, a second CO2-rich gas and a double-membrane separation tail gas;

[0018] (5) The generated liquid obtained in the first gas-liquid separation device is introduced into the formic acid disproportionation device to obtain a product, which is separated in the second gas-liquid separation device to obtain a disproportionation liquid and a gas; the gas obtained in the second gas-liquid separation device is introduced into the double-membrane gas separator; the disproportionation liquid obtained in the second gas-liquid separation device is introduced into the fractionation device to obtain methyl formate, methanol and formic acid;

[0019] (6) The sulfur-containing gas obtained in the low-temperature methanol washing device in step (2) is introduced into the wet-process sulfuric acid device to obtain sulfuric acid.

[0020] Further, part of the sulfuric acid obtained in step (6) is introduced into the formic acid disproportionation device as a catalyst.

[0021] Further, the hydrogen concentration of the low-purity hydrogen raw material is generally 15-30 vol%. Further, the membrane separation tail gas obtained in the hydrogen purification membrane separation device is discharged and can be used as fuel gas for combustion to provide energy, and the carbon dioxide produced by combustion can be used as a cathode reaction raw material.

[0022] Further, the carbon dioxide concentration of the acid gas is generally 10-60 vol%. The acid gas is used as a carbon dioxide raw material source, and it is directly introduced into the low-temperature methanol washing device to obtain purified carbon dioxide (i.e., the first CO2-rich gas), and the carbon dioxide is directly introduced into the cathode of the electrochemical hydrogenation device to carry out carbon dioxide hydrogenation reaction.

[0023] Further, the low-temperature methanol washing device in step (2) mainly functions to absorb acid gas by using low-temperature methanol, and to separate and purify carbon dioxide, separate sulfur-containing gas and recycle and regenerate the methanol solvent by increasing the temperature and reducing the pressure. The obtained sulfur-containing gas is further introduced into the wet-process sulfuric acid device to produce high-purity sulfuric acid to fully utilize the sulfur resources in the flue gas. The operating conditions of the low-temperature methanol washing device are conventional knowledge in the art. For example, the operating pressure of the desulfurization tower and the decarbonization tower in the low-temperature methanol washing device is 3-4 MPa, and the operating temperature is -45 to -55°C. The methanol solvent loss in the low-temperature methanol washing device is 2-5% of the carbon dioxide production, and the low-temperature methanol washing device can obtain purified carbon dioxide with a purity of 85-99 vol%.

[0024] Further, the hydrogen purification gas separation membrane device is a conventional membrane separation device in the art, and the main function is to separate and purify hydrogen in the anode tail gas of the electrochemical hydrogenation device. The structure and operating conditions of the hydrogen purification gas separation membrane are conventional knowledge in the art. For example, the gas separation membrane device is generally composed of 1-3 stages of hydrogen membrane separation assemblies connected in series, a vacuum pump is used to vacuumize the permeate gas outlets of the hydrogen membrane separation assemblies, so that the permeate gas outlets of the hydrogen membrane separation assemblies are in a negative pressure state with an absolute pressure of 10-15 kPa. The purity of the purified hydrogen obtained on the permeation side is generally 60-90 vol%.

[0025] Further, in step (4), the membrane material of the hydrogen purification membrane separation device is selected from at least one of polyimide, polysulfone, cellulose acetate, polyetherimide, and preferably polyimide.

[0026] Further, the tail gas of the double-membrane gas separator obtained in step (4) enters the hydrogen purification membrane separation device. The membrane material of the double-membrane gas separator can be selected from polyimide, polysulfone, cellulose acetate, polyetherimide, etc., and preferably polyimide; the carbon dioxide purification separation membrane material can be selected from polyethylene oxide, poly-4-methyl-1-pentene, polydimethylsiloxane, etc., and preferably polyethylene oxide.

[0027] Further, the double-membrane gas separation device adopts a conventional double-membrane separator. The double-membrane gas separator mainly realizes the separation and purification of carbon dioxide, and simultaneously realizes the separation and purification of hydrogen by using the bidirectional enrichment effect, wherein the purified hydrogen-rich gas returns to the anode of the electrochemical hydrogenation device for reaction, the purified carbon dioxide-rich gas enters the cathode of the electrochemical hydrogenation device for reaction, and the separation tail gas is used as fuel gas for combustion to provide energy, and the combustion produces carbon dioxide which can be used as a cathode reaction raw material. A typical double-membrane gas separation device is a membrane separation assembly enriched with CO2 and hydrogen on both sides of the raw material gas, and a vacuum pump is used to vacuumize the permeate gas outlets of the CO2 and hydrogen membrane separation assemblies, so that the permeate gas outlets of the membrane separation assemblies are in a negative pressure state with an absolute pressure of 10-15 kPa.

[0028] Further, the reaction temperature of the cathode of the electrochemical hydrogenation device is 40-80°C, the reaction pressure is atmospheric pressure, the electrolyte can be a liquid electrolyte or a solid electrolyte, and preferably a proton exchange membrane polymer solid electrolyte, and the proton exchange membrane is preferably a perfluorosulfonic acid membrane.

[0029] Further, a part of the methanol obtained in step (5) is recycled back to the low-temperature methanol washing device, and the weight fraction of the recycled methanol in the fractionated methanol is 5-10%; preferably, at least a part of the fractionated formic acid enters a formic acid disproportionation methanol device for disproportionation reaction.

[0030] Further, the sulfuric acid production device produces sulfur trioxide by burning and catalytic oxidation of the sulfur-containing gas, and dissolves the sulfur trioxide in water to produce sulfuric acid, the concentration of the sulfuric acid product can reach 97%-98%, and the sulfuric acid production device adopts a two-stage conversion process to maximize the production of high-value-added sulfuric acid, part of which is used as an acidic additive for the formic acid disproportionation methanol production device to reduce production cost, and the rest is sold as a product.

[0031] Further, the first gas-liquid separation device mainly separates the carbon dioxide, hydrogen and carbon monoxide mixed gas from the formic acid. The obtained liquid is formic acid, which is introduced into the formic acid disproportionation methanol production device, and the obtained hydrogen, carbon dioxide and carbon monoxide mixed gas is used as the cathode tail gas of the electrochemical hydrogenation device and directly introduced into the double-membrane gas separator. The operating temperature of the first gas-liquid separation device is normal temperature, and the operating pressure is normal pressure.

[0032] Further, the operation of the second gas-liquid separation device is familiar to those skilled in the art. For example, the separation operation temperature is generally normal temperature, and the operation pressure is generally normal pressure.

[0033] Further, the operating conditions of the formic acid methanol production device are as follows: the reaction temperature is preferably 40-60℃, and the reaction pressure is generally normal pressure. Under the conditions, the formic acid can be prepared into methanol by hydrogenation.

[0034] Further, the fractionation device mainly separates the formic acid, methanol and methyl formate, and mainly uses methods such as rectification, extraction and extraction for separation, and the rectification method is preferred; the separation temperature of the formic acid is preferably 100-105℃, the separation temperature of the methanol is preferably 65-70℃, and the separation temperature of the methyl formate is preferably 30-35℃.

[0035] According to a second aspect of the present application, the present application provides a low-carbon-emission device for recovering carbon dioxide to produce methanol.

[0036] A low-carbon-emission device for recovering carbon dioxide to produce methanol, comprising:

[0037] A low-temperature methanol washing device for washing and purifying the acid gas to obtain a first CO2-rich gas and a sulfur-containing gas;

[0038] An electrochemical hydrogenation device, comprising a cathode and an anode; the anode is used for dissociating hydrogen-containing gas to produce protons; the cathode is provided with a CO2 hydrogenation catalyst bed for catalyzing the hydrogenation reaction of protons and CO2 to obtain a formic acid product;

[0039] A hydrogen purification gas separation membrane device for separating the anode tail gas of the electrochemical hydrogenation device into a first hydrogen-rich gas and a fuel gas;

[0040] The first gas-liquid separation device is used for separating the formic acid product into formic acid generating liquid and cathode tail gas, and discharging the generating liquid and the cathode tail gas out of the first gas-liquid separation device;

[0041] The double-membrane gas separation device is used for separating the cathode tail gas into a second hydrogen-rich gas and a second CO2-rich gas and a second tail gas.

[0042] The formic acid methanolization device is used for methanolization of the formic acid under the action of a catalyst to obtain a methanolization product.

[0043] The second gas-liquid separation device is used for gas-liquid separation of the methanolization product to obtain a methanolization generating liquid and a methanolization tail gas.

[0044] The fractionation device is used for fractionating the methanolization generating liquid into methanol, methyl formate and formic acid, and the fractionation device further comprises a feed pipeline for feeding the formic acid to the methanolization device and a feed pipeline for feeding a part of the methanol to the rectisol device.

[0045] The wet-process acid device is used for converting the sulfur-containing gas obtained by the rectisol device into sulfuric acid, discharging the generated sulfuric acid and feeding a part of the sulfuric acid to the methanolization device.

[0046] Further, the raw gas of the rectisol device is cooled to -25~-35℃ and enters the desulfurization tower, the rich methanol liquid of the desulfurization tower is separated from the sulfur-containing gas through heat regeneration, the regenerated lean methanol liquid is returned to the desulfurization tower for recycling, and the desulfurized gas is warmed to -25~-30℃ in the change section and enters the decarbonization tower.

[0047] Further, the rich methanol liquid of the decarbonization tower is separated from the purified carbon dioxide through multi-stage vacuum flash evaporation, the regenerated lean methanol liquid is returned to the decarbonization tower for recycling, and the methanol solvent loss of the rectisol device is 2~5% of the carbon dioxide production.

[0048] Further, in the hydrogen purification gas separation membrane device, the permeate gas of each stage enters the inlet of the next stage hydrogen membrane separation assembly through a vacuum pump, the retentate gas of the first stage hydrogen membrane separation assembly is discharged as fuel gas, the retentate gas of each stage hydrogen membrane separation assembly is returned to the inlet of the previous stage hydrogen membrane separation assembly for recycling separation, and the permeate gas of the last stage hydrogen membrane separation assembly is the purified hydrogen.

[0049] Further, the formic acid disproportionation methanol device is composed of 10-20 parallel intermittent stirred tanks, the reaction time of formic acid in each intermittent stirred tank is preferably 16-24 hours, the intermittent reaction tank feed is replaced at intervals of 1-2 hours, the intermittent stirred tank contains formic acid aqueous solution, acid additive and catalyst, the formic acid concentration is preferably 10-14 mol / L, the acid additive is preferably sulfuric acid and phosphoric acid, preferably sulfuric acid, the molar ratio of sulfuric acid to formic acid is preferably 0.7-0.9, the catalyst is preferably iridium catalyst, ruthenium catalyst and molybdenum catalyst, preferably iridium catalyst; the stirring speed is preferably 1400-1600 revolutions per minute.

[0050] Further, the low-temperature methanol washing device is connected to the inlet of the wet-process acid device through the sulfur-containing gas outlet, and is connected to the cathode inlet of the electrochemical hydrogenation device through the carbon dioxide-rich gas outlet. The cathode outlet of the electrochemical hydrogenation device is connected to the inlet of the first gas-liquid separator. The gas phase outlet of the first gas-liquid separator is connected to the inlet of the double-membrane gas separator, and the liquid phase outlet is connected to the inlet of the formic acid disproportionation methanol device. The carbon dioxide-rich gas outlet of the double-membrane gas separator is connected to the cathode inlet of the electrochemical hydrogenation device, and the hydrogen-rich gas outlet is connected to the anode inlet of the electrochemical hydrogenation device. The anode outlet of the electrochemical hydrogenation device is connected to the inlet of the hydrogen gas purification gas separation membrane device, and the hydrogen-rich gas outlet is connected to the anode inlet of the electrochemical hydrogenation device. The outlet of the wet-process acid device is connected to the inlet of the formic acid disproportionation methanol device. The outlet of the formic acid disproportionation methanol device is connected to the inlet of the second gas-liquid separation device; the gas phase outlet of the second gas-liquid separation device is connected to the inlet of the double-membrane gas separator, and the liquid phase outlet is connected to the inlet of the fractionation device; the formic acid outlet of the fractionation device is connected to the inlet of the formic acid disproportionation methanol device; and the methanol outlet of the fractionation device is connected to the methanol solvent feed inlet of the low-temperature methanol washing device.

[0051] The present inventors have found through research that the side reactions of hydrogen, carbon monoxide and water generated by the cathode of the electrochemical hydrogenation device have an adsorption site competition relationship with the carbon dioxide hydrogenation reaction. In order to improve the carbon dioxide hydrogenation rate, it is necessary to increase the current density to promote the generation of adsorbed hydrogen by protons in the cathode, but higher adsorbed hydrogen concentration will promote side reactions to generate more hydrogen. In conventional electrochemical hydrogenation devices, this side reaction will have a serious impact, causing the carbon dioxide hydrogenation reaction rate to no longer increase when the current density is increased to a certain extent, i.e., the reaction efficiency of the reactor reaches a limit, and further increasing the current density will only produce more byproduct hydrogen; the conventional electrochemical hydrogenation device does not separate and purify the carbon dioxide in the cathode tail gas for reuse, so the carbon dioxide can only pass through once, and therefore a high carbon dioxide conversion rate must be pursued; the conventional electrochemical hydrogenation device uses high-purity hydrogen as a raw material, so the byproduct hydrogen from the cathode becomes the core of increasing operating costs and reducing reaction efficiency.

[0052] In the present application, the problem of hydrogen by-product of cathode is no longer the key to restrict the reaction efficiency of electrochemical hydrogenation device. The present application solves the problem through process optimization design, the core of which is the efficient separation and reuse of carbon dioxide and hydrogen. Firstly, the present application uses low-purity hydrogen as raw material, so the raw material is almost cost-free. On the other hand, the separation and recovery of carbon dioxide and hydrogen in the cathode tail gas are realized through a double-membrane gas separator, so that carbon dioxide and hydrogen can be enriched and returned to the cathode and anode of the electrochemical hydrogenation device respectively, thereby improving the total conversion rate of carbon dioxide and hydrogen through cyclic reaction. Based on the above two advantages, low-cost carbon dioxide hydrogenation to produce methanol can be realized.

[0053] The present application greatly reduces the process cost through process improvement. On the one hand, the reaction can be carried out under normal pressure and mild conditions of 40-60℃; on the other hand, the problem of external high-purity hydrogen is solved by generating hydrogen through formic acid self-decomposition. Based on the above two advantages, low-cost formic acid hydrogenation to produce methanol can be realized.

[0054] Compared with the prior art, the present application has the following advantages:

[0055] 1. The present application solves the problems of high hydrogen cost, high reaction temperature and pressure in the conventional carbon dioxide hydrogenation to produce methanol. On the one hand, the present application uses low-purity hydrogen (typical by-product gas of hydrogen production device) as raw material, which is almost valueless. Through optimization of catalyst and proton exchange membrane, the carbon dioxide hydrogenation to produce formic acid under normal temperature and pressure is realized by using the principle of electrochemical hydrogenation reaction. On the other hand, hydrogen is generated by formic acid self-decomposition, which is almost cost-free. Through optimization of acidic additive and catalyst, and design of the molar ratio of acidic additive to formic acid, the formic acid hydrogenation to produce methanol under normal temperature and pressure is realized. The carbon dioxide hydrogenation to produce methanol under normal temperature and pressure is realized, the product value is maximized, the hydrogen cost is minimized, and the competitiveness of the process is significantly improved.

[0056] 2. The present application solves the problems of high material consumption and low conversion rate in the conventional carbon dioxide hydrogenation to produce methanol. On the one hand, the present application realizes the recycling and reuse of carbon dioxide and hydrogen in the cathode tail gas through the bidirectional enrichment function of the double-membrane gas separator, thereby improving the conversion efficiency of carbon dioxide and hydrogen. Through the fractionation device, the formic acid is recycled and utilized, thereby improving the conversion rate of formic acid hydrogenation to produce methanol. Through the sulfuric acid produced by sulfur-containing gas wet method, the acidic additive of formic acid disproportionation to produce methanol device is supplemented, and the methanol solvent lost in the low-temperature methanol washing device is supplemented by the product methanol. The high utilization rate of carbon dioxide and hydrogen is realized, the product yield is maximized, the competitiveness of the process is significantly improved, and the whole system realizes no material consumption except raw gas.

[0057] 3、The process system adopted by the present application has lower equipment cost and higher flexibility. The present application adopts a vacuum pump and a membrane separation device, so that the purification of hydrogen and carbon dioxide can be realized at normal temperature and pressure, avoiding the problem that the conventional purification separation must work in a high pressure environment, thereby reducing the equipment investment; through the carbon dioxide conversion reaction at normal temperature and pressure and the formic acid disproportionation methanol production reaction, the pressure grade of the process main body separation and the reaction part is all normal pressure, thereby significantly reducing the equipment investment; since the prices of formic acid, methanol and methyl formate fluctuate, the conventional process cannot flexibly select products, thereby limiting the production benefit, the present application can flexibly control the start and stop of the formic acid disproportionation methanol production device, and flexibly select the product type and the proportion of each product. The product value is maximized and the equipment cost is minimized, thereby significantly improving the competitiveness of the process and the market adaptability. BRIEF DESCRIPTION OF DRAWINGS

[0058] Figure 1 The present application is a process flow diagram for the carbon dioxide hydrogenation methanol production.

[0059] Among them, each number mark corresponds to: 1-acid gas; 2-low purity hydrogen; 3-low temperature methanol washing device; 4-sulfur-containing gas; 5-first CO2-rich gas; 6-second CO2-rich gas; 7-electrochemical hydrogenation device cathode; 8-electrochemical hydrogenation device anode; 9-methanol solvent; 10-wet acid making device; 11-sulfuric acid additive; 12-formic acid disproportionation methanol production device; 13-first gas-liquid separation device; 14-anode tail gas; 15-hydrogen purification gas separation membrane device; 16-circulating hydrogen-rich gas; 17-circulating formic acid; 18-second gas-liquid separator device; 19-cathode tail gas; 20-fractionation device; 21-disproportionation tail gas; 22-double membrane gas separation device; 23-tail gas; 24-sulfuric acid; 25-methyl formate; 26-methanol; 27-fuel gas. DETAILED DESCRIPTION

[0060] The method of the present application will be described in detail below in combination with specific examples. In the examples, Aspen HYSYS software is used for simulation and calculation.

[0061] Example 1

[0062] The method of the present application for low-carbon emission and efficient recovery of carbon dioxide to produce methanol comprises a low-temperature methanol washing device 3, an electrochemical hydrogenation device, a hydrogen purification gas separation membrane device 15, a first gas-liquid separation device 13, a double membrane gas separator 22, a formic acid disproportionation methanol production device 12, a fractionation device 20, and a wet acid making device 10.

[0063] The low-temperature methanol washing device 3 is connected with the wet-process acid making device 10 at the outlet of the sulfur-containing gas, and is connected with the electrochemical hydrogenation device cathode 7 at the outlet of the carbon dioxide-rich gas; the electrochemical hydrogenation device cathode 7 is connected with the first gas-liquid separator 13 at the outlet of the cathode; the first gas-liquid separator 13 is connected with the double-membrane gas separator 22 at the gas phase outlet, and is connected with the formic acid disproportionation methanol device 12 at the liquid phase outlet; the double-membrane gas separator 22 is connected with the electrochemical hydrogenation device cathode 7 at the carbon dioxide-rich gas outlet, and is connected with the electrochemical hydrogenation device anode 8 at the hydrogen-rich gas outlet; the electrochemical hydrogenation device anode 8 is connected with the hydrogen gas purification gas separation membrane device 15 at the outlet of the anode, and is connected with the electrochemical hydrogenation device anode 8 at the hydrogen-rich gas outlet; the wet-process acid making device 10 is connected with the formic acid disproportionation methanol device 12 at the outlet; the formic acid disproportionation methanol device 12 is connected with the second gas-liquid separation device 18 at the outlet; the second gas-liquid separation device 18 is connected with the double-membrane gas separator 22 at the gas phase outlet, and is connected with the fractionation device 20 at the liquid phase outlet; the fractionation device 20 is connected with the formic acid disproportionation methanol device 12 at the formic acid outlet; and the fractionation device 20 is connected with the low-temperature methanol washing device 3 at the methanol outlet.

[0064] The low-purity hydrogen gas with a concentration of 20vol% is used as the hydrogen raw material, the molar flow rate is 2662 kg / h, the temperature is 40℃, and the pressure is 150kPa, and the hydrogen gas is directly introduced into the electrochemical hydrogenation device anode for hydrogen dissociation and proton transmission; the unreacted hydrogen gas in the electrochemical hydrogenation device anode and the hydrocarbon gas contained in the hydrogen raw material are used as the anode tail gas, the hydrogen gas is obtained by treating the tail gas through the hydrogen gas purification gas separation membrane device, the purity of the hydrogen gas is 88vol%, the hydrogen gas is introduced into the electrochemical hydrogenation device anode for reaction, the separated tail gas is used as fuel gas for combustion to provide energy, and the combustion produces carbon dioxide which can be used as the cathode reaction raw material.

[0065] The main function of the hydrogen gas purification gas separation membrane device 15 is to separate and purify the hydrogen gas in the electrochemical hydrogenation device anode tail gas, and the purified hydrogen gas is introduced into the electrochemical hydrogenation device anode for reaction.

[0066] The acid gas with a carbon dioxide concentration of 56vol% is used as the carbon dioxide raw material, the molar flow rate is 3094 kg / h, the temperature is 40℃, and the pressure is 250kPa, and the acid gas is directly introduced into the low-temperature methanol washing device to obtain purified carbon dioxide, the concentration of the purified carbon dioxide is 97.8vol%, and the purified carbon dioxide is directly introduced into the electrochemical hydrogenation device cathode for carbon dioxide hydrogenation reaction.

[0067] The main function of the low-temperature methanol washing device 3 is to absorb the acid gas by using low-temperature methanol, to separate and purify the carbon dioxide, to separate the sulfur-containing gas, and to recycle and utilize the methanol solvent by increasing the temperature and reducing the pressure. The sulfur-containing gas is introduced into the wet-process acid making device to produce high-purity sulfuric acid, and the sulfur resources in the flue gas can be fully utilized.

[0068] The wet-process sulfuric acid plant 10 produces sulfur trioxide by burning and catalytic oxidation of sulfur-containing gas, which is dissolved in water to produce sulfuric acid with a concentration of 96.8%. The sulfuric acid product is used in a two-stage conversion process to maximize the production of high-value-added sulfuric acid. Seventy percent of the sulfuric acid is used as an acidic additive in the methanol synthesis device, reducing production costs, and the remaining 30% is sold as a product.

[0069] The electrochemical hydrogenation device is composed of an anode 8, an electrolyte, and a cathode 7. The main function of the anode is to dissociate hydrogen gas in contact with the electrolyte into protons through a potential difference and the action of a catalyst. The dissociated protons move from the anode to the cathode through the electrolyte under the action of the potential difference. The protons entering the cathode become adsorbed hydrogen, which reacts with diffused carbon dioxide raw material or undergoes a side reaction to combine with itself to generate hydrogen gas.

[0070] The hydrogen gas that does not participate in the reaction in the anode of the electrochemical hydrogenation device and the hydrocarbon gas contained in the raw material are discharged as anode tail gas. The hydrogen gas and carbon monoxide produced by the cathode side reaction enter the first gas-liquid separation device together with unreacted carbon dioxide and hydrogenation products.

[0071] The first gas-liquid separation device 13 mainly separates carbon dioxide, hydrogen, and carbon monoxide mixed gas from formic acid. The obtained liquid is formic acid, which is introduced into the formic acid disproportionation methanol synthesis device. The mixture gas of hydrogen, carbon dioxide, and carbon monoxide is used as the cathode tail gas of the electrochemical hydrogenation device and directly enters the double-membrane gas separator.

[0072] The double-membrane gas separator 22 mainly separates and purifies carbon dioxide and simultaneously separates and purifies hydrogen gas using a two-way enrichment effect. The purified hydrogen-rich gas has a purity of 80.3 vol%, which is returned to the anode of the electrochemical hydrogenation device for reaction. The purified carbon dioxide-rich gas has a purity of 86.9 vol% and is introduced into the cathode of the electrochemical hydrogenation device for reaction. The separation tail gas is used as fuel gas for combustion to provide energy, and the combustion produces carbon dioxide, which can be used as cathode reaction raw material.

[0073] The formic acid disproportionation methanol synthesis device 12 uses a catalyst to decompose part of the formic acid into hydrogen and carbon dioxide in a mild aqueous solution. The un-decomposed formic acid reacts with hydrogen to produce methanol through catalytic hydrogenation. Part of the methanol reacts with formic acid to produce formic acid methyl ester as a byproduct. The single conversion rate of formic acid is 47.8%. The unreacted formic acid and hydrogen, intermediate product carbon dioxide, main product methanol, and byproduct formic acid methyl ester are introduced into the second gas-liquid separation device.

[0074] The second gas-liquid separation device 18 mainly realizes the separation of the carbon dioxide and hydrogen mixed gas and the formic acid, methanol and formic acid methyl ester mixed liquid. The obtained carbon dioxide and hydrogen mixed gas is introduced into the double membrane gas separator 22 as tail gas to realize the separation and purification of carbon dioxide and hydrogen for reuse. The obtained liquid is the formic acid, methanol and formic acid methyl ester mixed liquid, which is introduced into the fractionation device.

[0075] The fractionation device 20 mainly realizes the separation of formic acid, methanol and formic acid methyl ester. After being treated by the fractionation device, formic acid, methanol and formic acid methyl ester are obtained. The formic acid is introduced into the formic acid disproportionation methanol device for recycling to realize high conversion rate of formic acid. 5.9% of the methanol is introduced into the low-temperature methanol washing device to supplement the loss of methanol solvent, and the remaining 94.1% of the methanol and all the formic acid methyl ester are sold as products.

[0076] The hydrogen purification gas separation membrane device 15 is a 1-3 level series hydrogen membrane separation assembly, preferably a 2-level hydrogen membrane separation assembly. The low-pressure permeate gas outlet of each hydrogen membrane separation assembly is in a vacuum state with an absolute pressure of 10 kPa, and after being vacuumized by a vacuum pump, it enters the next level hydrogen membrane separation assembly. The retentate gas of the first level membrane separation assembly is discharged as fuel gas, and the retentate gas of the other hydrogen membrane separation assemblies returns to the inlet of the previous level hydrogen membrane separation assembly. The low-pressure permeate gas of the last level hydrogen membrane separation assembly is the purified hydrogen.

[0077] For the hydrogen separation membrane material, a membrane material with high solubility and internal diffusion coefficient for hydrogen molecules is selected. Polyimide, polyetherimide or cellulose acetate and other membrane materials have higher hydrogen selectivity and permeation rate. The membrane material is preferably polyimide material.

[0078] The double membrane gas membrane separator 22 performs bidirectional enrichment on the mixed gas containing hydrogen and carbon dioxide, enriches hydrogen and carbon dioxide in the two sides of the membrane permeate gas respectively, realizes efficient separation, and the retentate gas is discharged as tail gas. The separation principle of the double membrane gas membrane separator is consistent with the separation mechanism of the membrane separation assembly, but its characteristic is that a set of membrane assembly is simultaneously installed with hydrogen selective and carbon dioxide selective membrane materials.

[0079] In the double membrane gas separator 22, the hydrogen selective membrane material is selected to be the same as the membrane material of the hydrogen purification gas separation membrane device, and the carbon dioxide selective membrane material is preferably polyethylene oxide, polydimethylsiloxane, etc., preferably polyethylene oxide.

[0080] The desulfurization tower and decarbonization tower in the low-temperature methanol washing unit 3 operate at a pressure of 3.4 MPa and a temperature of -50°C. The raw gas temperature drops to -30°C before entering the desulfurization tower. The methanol-rich liquid in the desulfurization tower is thermally regenerated to separate sulfur-containing gases, and the regenerated lean methanol liquid is returned to the desulfurization tower for recycling. The desulfurized gas is reheated to -30°C through a variable temperature section before entering the decarbonization tower. The methanol-rich liquid in the decarbonization tower is separated into purified carbon dioxide through multi-stage vacuum flash evaporation, and the regenerated lean methanol liquid is returned to the decarbonization tower for recycling. The methanol solvent loss of the low-temperature methanol washing unit is 3% of the carbon dioxide production.

[0081] The anode 8 and cathode 7 in the electrochemical hydrogenation device are respectively composed of a flow channel, a diffusion layer, and a catalyst layer. The flow pattern of the flow channel can be one of parallel flow channel, serpentine flow channel, cross flow channel, and point flow channel, with the anode and cathode preferably being serpentine flow channels. In the carbon dioxide electrochemical hydrogenation device of the present invention, the operating temperature of the carbon dioxide electrochemical hydrogenation reaction is 60°C, and the operating pressure is atmospheric pressure; the diffusion layer is a metal mesh between the catalyst layer and the flow channel layer; the catalyst layer is an alloy catalyst such as copper, platinum, or palladium; the electrolyte of the electrochemical hydrogenation device can be a liquid electrolyte or a solid electrolyte, preferably a proton exchange membrane polymer solid electrolyte, which can achieve hydrogen ion transfer through efficient transfer of hydrated protons. The proton exchange membrane of the electrochemical hydrogenation device is preferably a perfluorosulfonic acid membrane.

[0082] The wet sulfuric acid production device 10 converts sulfur-containing and hydrogen-containing combustible substances in sulfur-containing gas into sulfur dioxide and water vapor under excess air conditions. The high-temperature process gas containing sulfur dioxide and water vapor is cooled down to the catalyst activity temperature and then introduced into the converter to catalytically oxidize sulfur dioxide into sulfur trioxide. The sulfur trioxide and water vapor are directly fed into the sulfuric acid vapor condenser to generate sulfuric acid vapor. As the temperature decreases, it gradually condenses to produce liquid sulfuric acid, which is then purified by distillation.

[0083] The formic acid disproportionation to methanol apparatus 12 consists of 12 batch stirred tanks connected in parallel. The reaction time of formic acid in each batch stirred tank is preferably 16 hours, and the feed to the batch stirred tank is changed every 2 hours. The batch stirred tank contains an aqueous formic acid solution, an acidic additive, and a catalyst. The formic acid concentration is preferably 14 mol / L. The acidic additive is preferably sulfuric acid or phosphoric acid, with sulfuric acid being preferred. The molar ratio of sulfuric acid to formic acid is preferably 0.9. The catalyst is preferably an iridium catalyst, a ruthenium catalyst, or a molybdenum catalyst, with an iridium catalyst being preferred. The stirring speed is preferably 1500 rpm. The reaction temperature is preferably 60℃.

[0084] The fractionation device 20 mainly uses methods such as distillation, extraction, and extraction for separation, with distillation being the preferred method. The separation temperature of formic acid is preferably 105℃, the separation temperature of methanol is preferably 70℃, and the separation temperature of methyl formate is preferably 35℃.

[0085] The final methanol yield was 55.47%, the methyl formate yield was 33.79%, the carbon dioxide conversion rate was 98.04%, and the hydrogen utilization rate was 95.26%. Compared with existing technologies, the method of this invention has advantages such as low reaction temperature and pressure, high carbon dioxide conversion rate, high hydrogen utilization rate, high methanol yield, low overall cost, and low carbon emissions.

Claims

1. A method for producing methanol from carbon dioxide with low carbon emissions and high efficiency, comprising the following steps: (1) After the low-purity hydrogen feedstock is mixed with the circulating hydrogen-rich gas, it enters the anode of the electrochemical hydrogenation device to dissociate the hydrogen and obtain protons, unreacted hydrogen and hydrocarbon gases contained in the low-purity hydrogen feedstock as anode tail gas. (2) After the acid gas passes through the low-temperature methanol washing device, the first CO2-rich gas and the sulfur-containing gas are obtained. The first CO2-rich gas and the second CO2-rich gas are mixed and then enter the cathode of the electrochemical hydrogenation device. (3) In step (1), the protons are transferred to the cathode of the electrochemical hydrogenation device and react with CO2 on the cathode catalyst to obtain a hydrogenation product containing formic acid. The formic acid hydrogenation product is separated in the first gas-liquid separation unit to obtain cathode tail gas and product liquid; (4) The anode tail gas obtained in step (1) enters the hydrogen purification membrane separation device for separation to obtain the first hydrogen-rich gas; the cathode tail gas obtained in step (3) enters the double membrane gas separator to obtain the second hydrogen-rich gas, the second CO2-rich gas and the double membrane separation tail gas. (5) The product obtained from the first gas-liquid separation unit enters the formic acid disproportionation methanol production unit, and the product enters the second gas-liquid separation unit. After separation, disproportionated liquid and gas are obtained; the gas obtained from the second gas-liquid separation unit enters the double membrane gas separator; the disproportionated liquid obtained from the second gas-liquid separation unit enters the fractionation unit to obtain methyl formate, methanol and formic acid; (6) The sulfur-containing gas obtained from the low-temperature methanol washing unit in step (2) enters the wet acid production unit to obtain sulfuric acid; The hydrogen concentration of the low-purity hydrogen raw material is 15-30 vol; the purity of the first hydrogen-rich gas is 60-90 vol.

2. The method according to claim 1, characterized in that, A portion of the sulfuric acid obtained in step (6) is fed into the formic acid disproportionation methanol production unit as a catalyst.

3. The method according to claim 1, characterized in that, The membrane separation tail gas obtained from the hydrogen purification membrane separation unit is discharged externally and burned as fuel gas. The combustion produces carbon dioxide, which is used as a raw material for the cathode reaction.

4. The method according to claim 1, characterized in that, The carbon dioxide concentration of the acidic gas in step (2) is 10~60 vol.

5. The method according to claim 1, characterized in that, In step (2), the operating pressure of the desulfurization tower and the decarbonization tower in the low-temperature methanol washing device is 3~4MPa, and the operating temperature is -45~-55℃.

6. The method according to claim 1, characterized in that, The hydrogen purification membrane separation device described in step (4) consists of 1 to 3 stages of hydrogen membrane separation components connected in series, with the permeate outlet of each stage of hydrogen membrane separation components having an absolute pressure of 10 to 15 kPa.

7. The method according to claim 1, characterized in that, The reaction temperature at the cathode of the electrochemical hydrogenation device is 40~80℃, and the reaction pressure is atmospheric pressure.

8. The method according to claim 1, characterized in that, A portion of the methanol obtained in step (5) is recycled back to the low-temperature methanol washing unit, and the recycled methanol accounts for 5% to 10% of the weight fraction of the methanol obtained from fractionation.

9. The method according to claim 1, characterized in that, At least a portion of the formic acid obtained in step (5) is fed into the formic acid disproportionation methanol production unit for disproportionation reaction.

10. The method according to claim 1, characterized in that, The first gas-liquid separator operates at room temperature and at atmospheric pressure; the second gas-liquid separator operates at room temperature and at atmospheric pressure.

11. The method according to claim 1, characterized in that, The operating conditions of the formic acid disproportionation to methanol apparatus are: reaction temperature of 40~60℃ and reaction pressure of atmospheric pressure.

12. The method according to claim 1, characterized in that, In the fractionation apparatus described in step (5), the separation temperature of formic acid is 100~105℃, the separation temperature of methanol is 65~70℃, and the separation temperature of methyl formate is 30~35℃.

13. A device for producing methanol from carbon dioxide with low carbon emissions, characterized in that, include: A low-temperature methanol washing unit is used to wash and purify acidic gases to obtain a first CO2-rich gas and a sulfur-containing gas. An electrochemical hydrogenation device includes a cathode and an anode; the anode is used to dissociate hydrogen-containing gas to generate protons; the cathode is provided with a CO2 hydrogenation catalyst bed to catalyze the hydrogenation reaction of protons and CO2 to obtain a product containing formic acid; A hydrogen purification gas separation membrane device is used to separate the anode tail gas of an electrochemical hydrogenation unit into a first hydrogen-rich gas and fuel gas; The first gas-liquid separation device is used to separate the formic acid-containing product into a formic acid-containing generating liquid and cathode tail gas, and to discharge the generating liquid and cathode tail gas from the first gas-liquid separation device. A dual-membrane gas separation device is used to separate cathode tail gas into a second hydrogen-rich gas, a second CO2-rich gas, and a second tail gas. A formic acid disproportionation to methanol unit is used to disproportionate formic acid under the action of a catalyst to obtain disproportionation products; The second gas-liquid separation device is used to separate the disproportionation products into gas and liquid, and to obtain the disproportionation product liquid and the disproportionation tail gas. A fractionation unit is used to fractionate the disproportionation product into methanol, methyl formate, and formic acid; the fractionation unit also includes a feed line for feeding formic acid into the disproportionation unit and a feed line for feeding a portion of the methanol into a low-temperature methanol washing unit. The wet sulfuric acid production unit is used to convert sulfur-containing gas obtained from the low-temperature methanol washing unit into sulfuric acid, and to remove the generated sulfuric acid and feed a portion of the sulfuric acid to the disproportionation unit.

14. The apparatus according to claim 13, characterized in that, The anode and cathode are respectively composed of a flow channel, a diffusion layer, and a catalyst layer. The flow channel pattern is one of parallel flow channel, serpentine flow channel, cross flow channel and point flow channel. The diffusion layer is a metal mesh between the catalyst layer and the flow channel layer. The catalyst is at least one of copper, platinum and palladium alloy catalyst.

15. The apparatus according to claim 13, characterized in that, The formic acid disproportionation to methanol unit consists of 10 to 20 batch stirred tanks connected in parallel, each batch stirred tank containing an aqueous formic acid solution, acidic additives, and a catalyst.

Images