Maritime green hydrogen methanol preparation system and method for realizing fresh water self-supply based on electrodialysis

Abstract

According to the offshore green hydrogen methanol production system and method based on waste heat coupling electrodialysis to achieve fresh water self-supply, in the system, a PEM electrolytic hydrogen production subsystem electrolyzes fresh water through renewable energy source electric power to produce high-pressure hydrogen and oxygen, and process waste heat is generated; the methanol synthesis subsystem is connected with the PEM electrolytic hydrogen production subsystem, so that hydrogen and carbon dioxide are synthesized into crude methanol under the action of a catalyst, the crude methanol is rectified and purified into a green methanol product, and meanwhile, reaction waste heat is generated; the multi-stage electrodialysis seawater desalination subsystem is connected with the PEM electrolytic hydrogen production subsystem, seawater is subjected to multi-stage electrodialysis desalination treatment by the multi-stage electrodialysis seawater desalination subsystem to generate fresh water, and the fresh water is buffered by a fresh water storage tank and then is fed back to the PEM electrolytic hydrogen production subsystem to form a fresh water self-sustaining closed loop; and the heat recovery cooling water circulation loop respectively exchanges heat with the PEM electrolytic hydrogen production subsystem and the methanol synthesis subsystem so as to recover waste heat.

Description

Technical Field

[0001] This invention relates to the interdisciplinary fields of comprehensive utilization of marine renewable energy, green chemical engineering, and water treatment technology, and in particular to a marine green hydrogen-to-methanol system and method based on waste heat coupled electrodialysis to achieve self-sufficiency in freshwater. Background Technology

[0002] Renewable energy sources, such as offshore wind and solar power, have become a key focus of global energy strategies due to their superior resource endowment, enormous development potential, and lack of land occupation. However, the inherent volatility, intermittency, and remote geographical location of these energy sources pose significant challenges to their efficient and stable grid connection and consumption. While traditional ultra-high voltage direct current (UHVDC) transmission technology can achieve long-distance power transmission, its construction cost, operation and maintenance difficulties, and energy loss remain key factors restricting its economic viability for deep-sea wind farms far from land. Meanwhile, existing electrochemical energy storage technologies (such as lithium-ion batteries) are still limited by energy density, cycle life, safety risks, and high capital investment when facing gigawatt-hour (GWh) level energy storage demands. Therefore, converting unstable electrical energy into stable chemical energy that is easy to store and transport is widely recognized as one of the most promising solutions, namely "Power-to-X" technology.

[0003] Among the many "Power-to-X" pathways, the "electro-hydrogen-ethanol" technology pathway has attracted much attention due to the unique advantages of its product, methanol (CH3OH). Methanol is liquid at room temperature and pressure, possessing advantages such as high energy density, convenient and safe storage and transportation, and good compatibility with existing petrochemical infrastructure. It can be used directly as a clean fuel in internal combustion engines or fuel cells, and it is also a platform chemical that can be used to produce high-value-added products such as olefins and aromatics. The core of this pathway lies in: first, using proton exchange membrane (PEM) electrolysis technology to convert fluctuating electricity into high-purity hydrogen (H2). PEM technology has advantages such as rapid start-up, wide load adjustment range, and fast dynamic response, which is highly compatible with the fluctuating characteristics of renewable energy; second, catalytically synthesizing the produced "green hydrogen" with captured carbon dioxide (CO2) to produce "green methanol."

[0004] Despite the enormous theoretical potential of the "electro-hydrogen-alcohol" pathway, its engineering and deployment in the harsh environment of offshore platforms still faces a series of pressing technical bottlenecks and systemic challenges:

[0005] 1. Water Resource Constraint: PEM electrolysis technology has extremely stringent requirements for the quality of feed water, necessitating the use of ultrapure deionized water with a resistivity at the megaohm level to protect the expensive proton exchange membrane and catalyst. Offshore platforms are typically water-scarce environments, relying entirely on land-based freshwater supplies. This not only significantly increases operating costs (OPEX) but also severely impacts the continuity and reliability of production under harsh sea conditions, constituting a fundamental constraint on the operation of the entire system.

[0006] 2. Energy Utilization Efficiency Bottleneck: Significant energy waste exists within the system. Both the PEM electrolysis process and the methanol synthesis reaction are highly exothermic. For example, the conversion efficiency of electrical energy to hydrogen in a PEM electrolyzer is approximately 60-70%, with the remaining 30-40% of the electrical energy lost as low-grade heat. The methanol synthesis reaction (CO2 + 3H2 → CH3OH + H2O) also releases a large amount of heat. In traditional designs, this heat is typically treated as waste heat and directly discharged into the marine environment through complex cooling systems. This not only significantly reduces the overall energy utilization efficiency (from initial electrical energy to final chemicals) but may also cause thermal pollution to the local marine ecosystem.

[0007] 3. Bottlenecks in Seawater Desalination Technology Coupling: While seawater desalination is an inevitable choice for solving water resource problems, there is a mismatch between traditional technologies and this system. Low-temperature multi-effect evaporation (LT-MED) technology, although capable of utilizing low-grade heat energy, suffers from slow start-up, poor dynamic response, and difficulty adapting to rapid load fluctuations in the hydrogen production system; furthermore, the equipment is bulky and complex. Reverse osmosis (RO) technology, while compact, places higher demands on pump energy consumption and the equipment's vibration and sway resistance due to its high-pressure operation; RO membranes are also extremely sensitive to feed water quality (such as SDI and turbidity), and the pretreatment system is complex. More importantly, these traditional desalination technologies have failed to form effective energy synergy with the hydrogen and methanol production units, existing merely as independent auxiliary units.

[0008] The information disclosed in the background section is only intended to enhance the understanding of the background of the present invention, and therefore may contain information that does not constitute prior art known to those skilled in the art. Summary of the Invention

[0009] In view of the above problems, the purpose of this invention is to overcome the defects of the prior art by providing a marine green hydrogen to methanol system and method based on waste heat coupled with electrodialysis to achieve self-sufficiency in freshwater. Utilizing the readily available seawater resources on the offshore platform, high-purity freshwater required for PEM electrolysis hydrogen production is produced economically and efficiently, achieving water resource self-sufficiency for the system. The large amount of medium- and low-grade waste heat generated during PEM electrolysis hydrogen production and methanol synthesis is effectively recovered and utilized in a cascade manner, avoiding energy waste and significantly improving the overall energy efficiency of the system. A seawater desalination technology adapted to the fluctuation characteristics of renewable energy and the hydrogen / methanol production unit is selected and optimized, and deeply coupled with the system's waste heat resources to achieve a synergistic effect of 1+1>2.

[0010] A marine green hydrogen-to-methanol system based on waste heat coupled with electrodialysis to achieve self-sufficiency in freshwater includes,

[0011] The PEM electrolysis hydrogen production subsystem uses renewable energy electricity to electrolyze fresh water to produce high-pressure hydrogen and oxygen, and generates waste heat from the process.

[0012] The methanol synthesis subsystem is connected to the PEM electrolysis hydrogen production subsystem to synthesize crude methanol from hydrogen and carbon dioxide under the action of a catalyst and then purify it into a green methanol product by distillation, while generating waste heat from the reaction.

[0013] A multi-stage electrodialysis seawater desalination subsystem is connected to the PEM electrolysis hydrogen production subsystem. The multi-stage electrodialysis seawater desalination subsystem desalinates seawater through multi-stage electrodialysis to produce fresh water. The fresh water is buffered in a fresh water storage tank and then fed back to the PEM electrolysis hydrogen production subsystem to form a self-sustaining closed loop for fresh water.

[0014] The heat recovery cooling water circulation loop exchanges heat with the PEM electrolysis hydrogen production subsystem and the methanol synthesis subsystem to recover waste heat. The heat recovery cooling water circulation loop is also connected to the ED feed preheater to transfer the recovered waste heat to the seawater entering the multi-stage electrodialysis seawater desalination subsystem for feed preheating.

[0015] In the aforementioned marine green hydrogen to methanol system based on waste heat coupled electrodialysis for self-sufficiency in freshwater, the PEM electrolysis hydrogen production subsystem includes,

[0016] PEM electrolyzers produce high-pressure hydrogen and oxygen by electrolyzing fresh water.

[0017] The electrolysis feed pump measures and pumps the freshwater from the freshwater storage tank to the first mixer.

[0018] The first mixer mixes fresh water from the fresh water storage tank with circulating water from the PEM electrolyzer.

[0019] An oxygen-water separator, connected to the PEM electrolyzer, separates the circulating water entrained in the oxygen.

[0020] A hydrogen-water separator, connected to the PEM electrolyzer, to separate circulating water entrained in hydrogen gas.

[0021] A second mixer, connected to the oxygen-water separator and the hydrogen-water separator, collects the circulating water separated from hydrogen and oxygen. The second mixer is connected to the first mixer via a heat exchanger to transfer heat from the circulating water to a cooling circuit. This cooling circuit includes an electrolyzer cooling water pump, a heat exchanger, and a third mixer. The electrolyzer cooling water pump drives an independent cooling circuit for heat exchange.

[0022] The product cooling circuit includes a product cooling water pump, a hydrogen heat exchanger, and an oxygen heat exchanger. Cooling water driven by the product cooling water pump flows sequentially through the hydrogen heat exchanger and the oxygen heat exchanger to cool the hydrogen separated by the hydrogen-water separator and the oxygen separated by the oxygen-water separator.

[0023] In the marine green hydrogen to methanol system based on waste heat coupled electrodialysis to achieve self-sufficiency in freshwater, the temperature of both the product cooling circuit and the cooling water in the cooling circuit rises to 70-80°C.

[0024] In the offshore green hydrogen to methanol system based on waste heat coupled electrodialysis to achieve self-sufficiency in freshwater, the PEM electrolyzer is a high-pressure electrolyzer with an outlet hydrogen pressure of not less than 6 MPa, and the effective volume of the freshwater storage tank is not less than 10 m³, so as to ensure that the PEM electrolysis hydrogen production subsystem can continue to operate for 24–48 hours during the shutdown of the seawater desalination subsystem.

[0025] In the aforementioned marine green hydrogen-to-methanol system based on waste heat coupled electrodialysis for freshwater self-sufficiency, the methanol synthesis subsystem includes:

[0026] A hydrogen storage tank connected to the hydrogen-water separator of the PEM electrolysis hydrogen production subsystem to buffer and stabilize the hydrogen;

[0027] A circulating gas compressor, which compresses circulating gas;

[0028] The raw material mixer is connected to a hydrogen storage tank, a carbon dioxide gas source, and a circulating gas compressor to mix hydrogen, carbon dioxide, and circulating gas to form raw materials;

[0029] A methanol synthesis reactor connected to the feed mixer to synthesize crude methanol from hydrogen and carbon dioxide in the presence of a catalyst;

[0030] A gas-liquid separation and purification unit, connected to the methanol synthesis reactor, is provided for gas-liquid separation and purification of crude methanol. The gas-liquid separation and purification unit includes...

[0031] The high-pressure flash evaporator performs preliminary gas-liquid separation on the crude methanol. The high-pressure flash evaporator, via a distributor that controls the ratio of recirculated gas to purge gas, feeds the recirculated gas into the recirculated gas compressor.

[0032] A medium-pressure flash tank, connected to a high-pressure flash tank via a pressure reducing valve, is used to remove dissolved gases from crude methanol liquid.

[0033] A distillation column, connected to the medium-pressure flash tank, is used to purify crude methanol to the desired specifications of methanol product.

[0034] Waste heat cooling circuit, which includes,

[0035] Waste heat recovery cooling water pump,

[0036] A raw material preheater that preheats the raw material;

[0037] A first waste heat recovery heat exchanger is connected to the methanol synthesis reactor and the third mixer to recover heat to the cooling water in the third mixer.

[0038] A second waste heat recovery heat exchanger is connected to the first waste heat recovery heat exchanger and the high-pressure flash tank to transfer heat.

[0039] A distillation feed preheater is located between the medium-pressure flash tank and the distillation column and is connected to the feed preheater to preheat the liquid entering the distillation column.

[0040] A distillation condenser is connected to the distillation column to condense the methanol vapor at the top of the column.

[0041] In the marine green hydrogen to methanol system based on waste heat coupled electrodialysis to achieve self-sufficiency in freshwater, the mixed feed gas enters the feed preheater and exchanges heat countercurrently with the high-temperature reaction products (250-280°C) from the methanol synthesis reactor, and is preheated to an initial temperature of 220°C.

[0042] In the offshore green hydrogen to methanol system based on waste heat coupled electrodialysis to achieve self-sufficiency in freshwater, the methanol synthesis reactor is a shell-and-tube isothermal reactor. The tube side is filled with Cu / ZnO / Al2O3 low-temperature methanol synthesis catalyst, and the shell side is circulated with an independent cooling medium to control the reaction bed temperature. The H2 / CO2 molar ratio is 3.0-3.2.

[0043] In the aforementioned marine green hydrogen-to-methanol system based on waste heat coupled electrodialysis for self-sufficiency in freshwater, the multi-stage electrodialysis seawater desalination subsystem includes,

[0044] The ED feed preheater is connected to the third mixer.

[0045] The pretreatment unit includes an ultrafiltration membrane filtration device and a chemical softening or nanofiltration device to pretreat seawater.

[0046] At least two stages of electrodialysis membrane stacks connected in series are connected to the pretreatment unit, and each stage of the electrodialysis membrane stack is equipped with an independent DC power supply.

[0047] A freshwater collection tank, which connects the electrodialysis membrane stack and the freshwater storage tank.

[0048] In the marine green hydrogen to methanol system based on waste heat coupled electrodialysis to achieve self-sufficiency of freshwater, the ED feed preheater is a plate or shell-and-tube heat exchanger, and the temperature of the preheated seawater is 40–50°C.

[0049] The method for achieving self-sufficiency in freshwater through a marine green hydrogen-to-methanol system based on waste heat coupled with electrodialysis includes the following steps:

[0050] a. Utilize offshore renewable energy power to drive the PEM electrolysis hydrogen production subsystem to electrolyze water and produce hydrogen, and recover the waste heat generated during the electrolysis process through a cooling circuit;

[0051] b. The hydrogen and carbon dioxide are catalytically synthesized into green methanol in the methanol synthesis subsystem, and the waste heat generated in the reaction process is recovered through a preheating and cooling circuit.

[0052] c. The waste heat from step a and step b is centrally transported to the ED feed preheater through a heat recovery cooling water circulation loop to preheat the pretreated seawater from the ocean.

[0053] d. The preheated seawater is sent into the multi-stage electrodialysis seawater desalination subsystem for multi-stage desalination under the action of a DC electric field to produce fresh water;

[0054] e. The produced freshwater is stored in a freshwater storage tank and used as feedwater to supply the PEM electrolysis hydrogen production subsystem, realizing the system's self-sufficient freshwater cycle and energy cascade utilization.

[0055] Compared with the prior art, the present invention has the following advantages:

[0056] (1) Complete self-sufficiency in freshwater has been achieved, which fundamentally guarantees the production stability and operational economy of offshore platforms. This invention utilizes electrodialysis technology to convert inexhaustible seawater into high-quality production water on-site, completely eliminating the dependence on expensive and unreliable land-based freshwater replenishment and solving the core bottleneck restricting the development of the offshore "electric-hydrogen-alcohol" pathway;

[0057] (2) This invention achieves efficient cascaded utilization of energy, significantly improving the overall energy efficiency of the system. It creatively transforms the low-grade waste heat generated by the two major chemical units, which would otherwise be discharged into the sea, into an "effective energy input" that reduces the power consumption of the seawater desalination unit. Based on the principle of electrodialysis, increasing the influent temperature effectively reduces the resistance of the solution and membrane stack, thereby significantly reducing the power consumption of the electrodialysis process (by up to 20-30%) while achieving the same desalination rate. This "waste-to-treasure" energy cascaded utilization method greatly improves the overall energy conversion efficiency from initial renewable electricity to the final methanol product.

[0058] (3) Deep synergy and optimization between technologies have been achieved, resulting in superior overall system performance. The electrodialysis (ED) technology selected in this invention has a faster dynamic response speed and a wider load adjustment range compared to traditional thermal methods (MED) and membrane methods (RO), and can better adapt to the volatility of renewable energy power generation. Through multi-stage series design and with an independent power supply for each stage, the operating parameters can be finely adjusted according to the different salinity conditions of each stage to achieve global energy consumption optimization. The system uses waste heat for preheating instead of directly driving phase change, resulting in more reasonable energy matching, avoiding additional energy upgrade equipment such as heat pumps, simplifying the system, and reducing investment (CAPEX) and maintenance costs (OPEX).

[0059] (4) A green, low-carbon, and environmentally friendly production process has been achieved. The entire system uses renewable energy as the sole external energy input and carbon dioxide as a chemical raw material, realizing the recycling of carbon resources. The internal closed-loop design of energy and water minimizes the extraction of external resources and the emission of waste and heat into the environment, fully complying with the concepts of sustainable development and circular economy.

[0060] The above description is merely an overview of the technical solution of the present invention. In order to make the technical means of the present invention clearer and more understandable, so that those skilled in the art can implement it according to the contents of the specification, and in order to make the above and other objects, features and advantages of the present invention more obvious and understandable, specific embodiments of the present invention are described below. Attached Figure Description

[0061] Various other advantages and benefits of the present invention will become apparent to those skilled in the art upon reading the detailed description of the preferred embodiments below. The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. It is obvious that the drawings described below are merely some embodiments of the invention, and those skilled in the art can obtain other drawings based on these drawings without any inventive effort. Furthermore, the same reference numerals denote the same parts throughout the drawings.

[0062] In the attached diagram:

[0063] Figure 1 This is a schematic diagram of the system flow of a preferred embodiment of a marine energy integrated utilization system that couples PEM electrolysis hydrogen production, methanol synthesis and electrodialysis seawater desalination provided by the present invention. The diagram shows in detail the equipment connection relationship between the three core subsystems in the system—PEM electrolysis hydrogen production subsystem, methanol synthesis subsystem and multi-stage electrodialysis seawater desalination subsystem—as well as the paths of key material flow and energy flow.

[0064] PEM electrolysis hydrogen production subsystem

[0065] 101: Product cooling water pump, used to drive a closed cooling water circuit to cool the hydrogen and oxygen products;

[0066] 102: Hydrogen heat exchanger, used to cool high-temperature hydrogen gas while transferring its heat to product cooling water;

[0067] 103: Oxygen heat exchanger, used to cool high-temperature oxygen while transferring its heat to product cooling water;

[0068] 104: Freshwater storage tank, used for storing and buffering high-purity freshwater produced in Zone 300;

[0069] 105: Electrolysis feed pump, used to accurately measure and pump fresh water from storage tanks to the electrolysis system;

[0070] 106: First mixer, used to mix fresh fresh water with circulating water from the electrolyzer;

[0071] 107: PEM electrolyzer, the core hydrogen production unit of the system;

[0072] 108: Oxygen-water separator, used to separate liquid water entrained in oxygen;

[0073] 109: Hydrogen-water separator, used to separate liquid water entrained in hydrogen gas;

[0074] 110: Second mixer, used to collect the circulating water separated from hydrogen and oxygen;

[0075] 111: Heat exchanger, used to transfer the heat of the circulating cooling water in the high-temperature electrolytic cell to a separate cooling water circuit;

[0076] 112: Electrolytic cell cooling water pump, used to drive an independent closed cooling water circuit to indirectly cool the electrolytic cell;

[0077] 113: The third mixer is used to combine the two streams of hot water from the product cooling circuit and the electrolytic cell cooling circuit to form a total hot water flow to the heat recovery unit;

[0078] Methanol synthesis subsystem:

[0079] 201: Hydrogen storage tank, high-pressure vessel, used for buffering and stabilizing the supply of hydrogen from Zone 100;

[0080] 202: Raw material mixer, used to uniformly mix hydrogen, carbon dioxide and recycle gas;

[0081] 203: Raw material preheater, used to preheat cold raw materials entering the reactor using the heat of the reaction products;

[0082] 204: Methanol synthesis reactor, the core methanol production unit of the system;

[0083] 205: Second waste heat recovery heat exchanger, used to recover the medium and low grade heat of the reaction products to an independent cooling water circuit;

[0084] 206: High-pressure flash tank, used for preliminary gas-liquid separation;

[0085] 207: Diverter, used to control the ratio of recirculated air to purge air;

[0086] 208: Circulating gas compressor, used to compensate for pressure loss in the circulating loop;

[0087] 209: Pressure reducing valve (throttle valve), used to reduce the pressure of crude methanol liquid;

[0088] 210: Medium-pressure flash tank, used to remove dissolved gases from crude methanol liquid;

[0089] 211: Distillation feed preheater, used to preheat the liquid entering the distillation column using the heat of the reaction products;

[0090] 212: Distillation column, used to purify crude methanol to the required specifications;

[0091] 213: Distillation condenser, used to condense methanol vapor at the top of the column;

[0092] 214: Waste heat recovery cooling water pump, used to drive an independent closed cooling water circuit to recover waste heat from the methanol synthesis system;

[0093] 215: First waste heat recovery heat exchanger, used to recover the medium and low grade heat of the reaction products to an independent cooling water circuit;

[0094] Multi-stage electrodialysis seawater desalination subsystem:

[0095] 301: Seawater pump used to draw water from the ocean and provide the initial pressure required by the system;

[0096] 302: ED feed preheater, the energy exchange hub of the present invention, used to preheat seawater using waste heat;

[0097] 303: Pretreatment unit, used for filtering, softening, and other treatments of raw seawater;

[0098] 304, 305, 306, 307: These represent the first, second, third, and fourth stage electrodialysis (ED) membrane stacks, respectively.

[0099] 308: Freshwater mixer / collection tank, used to collect qualified freshwater from each stage of production.

[0100] The present invention will be further explained below with reference to the accompanying drawings and embodiments. Detailed Implementation

[0101] Specific embodiments of the invention will now be described in more detail with reference to the accompanying drawings. While specific embodiments of the invention are shown in the drawings, it should be understood that the invention can be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided to enable a more thorough understanding of the invention and to fully convey the scope of the invention to those skilled in the art.

[0102] It should be noted that certain terms are used in the specification and claims to refer to specific components. Those skilled in the art will understand that different terms may be used to refer to the same component. This specification and claims do not distinguish components based on differences in terminology, but rather on differences in function. The terms "comprising" or "including" used throughout the specification and claims are open-ended and should be interpreted as "comprising but not limited to." The following descriptions are preferred embodiments for carrying out the invention; however, these descriptions are for the purpose of understanding the general principles of the specification and are not intended to limit the scope of the invention. The scope of protection of this invention is determined by the appended claims.

[0103] To facilitate understanding of the embodiments of the present invention, further explanations and descriptions will be provided below with reference to the accompanying drawings and specific embodiments. The accompanying drawings do not constitute a limitation on the embodiments of the present invention.

[0104] To better understand, such as Figure 1 The illustrated marine green hydrogen-to-methanol system based on waste heat coupled electrodialysis for freshwater self-sufficiency includes,

[0105] The PEM electrolysis hydrogen production subsystem uses renewable energy electricity to electrolyze fresh water to produce high-pressure hydrogen and oxygen, and generates waste heat from the process.

[0106] The methanol synthesis subsystem is connected to the PEM electrolysis hydrogen production subsystem to synthesize crude methanol from hydrogen and carbon dioxide under the action of a catalyst and then purify it into a green methanol product by distillation, while generating waste heat from the reaction.

[0107] A multi-stage electrodialysis seawater desalination subsystem is connected to the PEM electrolysis hydrogen production subsystem. The multi-stage electrodialysis seawater desalination subsystem desalinates seawater through multi-stage electrodialysis to produce fresh water. The fresh water is buffered in fresh water storage tank 104 and then fed back to the PEM electrolysis hydrogen production subsystem to form a self-sustaining closed loop of fresh water.

[0108] The heat recovery cooling water circulation loop exchanges heat with the PEM electrolysis hydrogen production subsystem and the methanol synthesis subsystem to recover waste heat. The heat recovery cooling water circulation loop is also connected to the ED feed preheater 302 to transfer the recovered waste heat to the seawater entering the multi-stage electrodialysis seawater desalination subsystem 300 for feed preheating.

[0109] In a preferred embodiment of the marine green hydrogen to methanol system based on waste heat coupled electrodialysis for freshwater self-sufficiency, the PEM electrolysis hydrogen production subsystem includes,

[0110] PEM electrolyzer 107 produces high-pressure hydrogen and oxygen by electrolyzing fresh water.

[0111] The electrolysis feed water pump 105 measures and pumps the fresh water from the fresh water storage tank 104 to the first mixer.

[0112] The first mixer 106 mixes fresh water from the fresh water storage tank 104 with circulating water from the PEM electrolyzer 107.

[0113] An oxygen-water separator 108 is connected to the PEM electrolyzer 107 to separate circulating water entrained in oxygen.

[0114] A hydrogen-water separator 109 is connected to the PEM electrolyzer 107 to separate circulating water entrained in hydrogen gas.

[0115] The second mixer 110 is connected to the oxygen-water separator 108 and the hydrogen-water separator 109 to collect the circulating water separated from hydrogen and oxygen. The second mixer 110 is connected to the first mixer 106 via a heat exchanger to transfer the heat of the circulating water to a cooling circuit. The cooling circuit includes an electrolyzer cooling water pump 112, a heat exchanger, and a third mixer 113. The electrolyzer cooling water pump 112 drives an independent cooling circuit for heat exchange.

[0116] The product cooling circuit includes a product cooling water pump 101, a hydrogen heat exchanger 102, and an oxygen heat exchanger 103. Cooling water driven by the product cooling water pump 101 flows sequentially through the hydrogen heat exchanger 102 and the oxygen heat exchanger 103 to cool the hydrogen separated by the hydrogen-water separator 109 and the oxygen separated by the oxygen-water separator 108.

[0117] In the preferred embodiment of the marine green hydrogen to methanol system based on waste heat coupled electrodialysis to achieve self-sufficiency in freshwater, the temperature of both the product cooling circuit and the cooling water in the cooling circuit is raised to 70-80°C.

[0118] In the preferred embodiment of the marine green hydrogen to methanol system based on waste heat coupled electrodialysis to achieve self-sufficiency in freshwater, the PEM electrolyzer 107 is a high-pressure electrolyzer with an outlet hydrogen pressure of not less than 6 MPa, and the effective volume of the freshwater storage tank 104 is not less than 10 m³, so as to ensure that the PEM electrolysis hydrogen production subsystem can continue to operate for 24–48 hours during the shutdown of the seawater desalination subsystem.

[0119] In a preferred embodiment of the offshore green hydrogen-to-methanol system based on waste heat coupled electrodialysis for freshwater self-sufficiency, the methanol synthesis subsystem includes,

[0120] Hydrogen storage tank 201 is connected to the hydrogen-water separator 109 of the PEM electrolysis hydrogen production subsystem to buffer and stabilize the hydrogen.

[0121] The circulating gas compressor 208 compresses circulating gas;

[0122] The raw material mixer 202 is connected to the hydrogen storage tank 201, the carbon dioxide gas source and the circulating gas compressor 208 to mix hydrogen, carbon dioxide and circulating gas to form raw materials.

[0123] A methanol synthesis reactor 204 is connected to the feed mixer 202 to synthesize crude methanol from hydrogen and carbon dioxide under the action of a catalyst.

[0124] A gas-liquid separation and purification unit, connected to the methanol synthesis reactor 204, is provided for gas-liquid separation and purification of crude methanol. The gas-liquid separation and purification unit includes...

[0125] The high-pressure flash evaporator 206 performs preliminary gas-liquid separation on the crude methanol. The high-pressure flash evaporator 206 feeds the recirculated gas into the recirculated gas compressor 208 via a distributor 207 that controls the ratio of recirculated gas to purge gas.

[0126] The medium-pressure flash tank 210 is connected to the high-pressure flash tank 206 via the pressure reducing valve 209 to remove dissolved gases from the crude methanol liquid.

[0127] Distillation column 212, which is connected to the medium-pressure flash tank 210 to purify crude methanol to the desired specifications of methanol product;

[0128] Waste heat cooling circuit, which includes,

[0129] Waste heat recovery cooling water pump 214,

[0130] Raw material preheater 203, which preheats the raw material;

[0131] A first waste heat recovery heat exchanger 215 is connected to the methanol synthesis reactor 204 and the third mixer 113 to recover heat to the cooling water in the third mixer 113.

[0132] The second waste heat recovery heat exchanger 205 is connected to the first waste heat recovery heat exchanger 215 and the high-pressure flash tank 206 to transfer heat.

[0133] A distillation feed preheater 211 is located between the medium-pressure flash tank 210 and the distillation column 212 and is connected to the feed preheater 203 to preheat the liquid entering the distillation column 212.

[0134] A distillation condenser 213 is connected to the distillation column 212 to condense the methanol vapor at the top of the column.

[0135] In a preferred embodiment of the marine green hydrogen to methanol system based on waste heat coupled electrodialysis to achieve self-sufficiency in freshwater, the mixed feed gas enters the feed preheater 203 and exchanges heat countercurrently with the high-temperature reaction products at 250-280°C from the methanol synthesis reactor 204, and is preheated to an initial temperature of 220°C.

[0136] In a preferred embodiment of the marine green hydrogen to methanol system based on waste heat coupled electrodialysis to achieve self-sufficiency in freshwater, the methanol synthesis reactor 204 is a shell-and-tube isothermal reactor. The tube side is filled with Cu / ZnO / Al2O3 low-temperature methanol synthesis catalyst, and the shell side is circulated with an independent cooling medium to control the temperature of the reaction bed. The H2 / CO2 molar ratio is 3.0-3.2.

[0137] In a preferred embodiment of the marine green hydrogen to methanol system based on waste heat coupled electrodialysis for self-sufficiency in freshwater, the multi-stage electrodialysis seawater desalination subsystem includes,

[0138] ED feed preheater 302 is connected to the third mixer 113.

[0139] Pretreatment unit 303 includes an ultrafiltration membrane filtration device and a chemical softening or nanofiltration device to pretreat seawater.

[0140] At least two stages of electrodialysis membrane stacks are connected in series to the pretreatment unit 303, and each stage of the electrodialysis membrane stack is equipped with an independent DC power supply.

[0141] Freshwater collection tank 308 is connected to the electrodialysis membrane stack and freshwater storage tank 104.

[0142] In the preferred embodiment of the marine green hydrogen to methanol system based on waste heat coupled electrodialysis to achieve self-sufficiency of freshwater, the ED feed preheater 302 is a plate or shell-and-tube heat exchanger, and the temperature of the preheated seawater is 40–50°C.

[0143] The method for achieving self-sufficiency in freshwater through a marine green hydrogen-to-methanol system based on waste heat coupled with electrodialysis includes the following steps:

[0144] a. Utilize offshore renewable energy power to drive the PEM electrolysis hydrogen production subsystem to electrolyze water and produce hydrogen, and recover the waste heat generated during the electrolysis process through a cooling circuit;

[0145] b. The hydrogen and carbon dioxide are catalytically synthesized into green methanol in the methanol synthesis subsystem, and the waste heat generated in the reaction process is recovered through a preheating and cooling circuit.

[0146] c. The waste heat from step a and step b is centrally transported to the ED feed preheater 302 through a heat recovery cooling water circulation loop to preheat the pretreated seawater from the ocean.

[0147] d. The preheated seawater is sent into the multi-stage electrodialysis seawater desalination subsystem for multi-stage desalination under the action of a DC electric field to produce fresh water;

[0148] e. The produced fresh water is stored in fresh water storage tank 104 and used as raw water to circulate to the PEM electrolysis hydrogen production subsystem, realizing the system's self-sufficient fresh water circulation and energy cascade utilization.

[0149] This invention utilizes the low-grade waste heat generated during PEM electrolysis for hydrogen production and methanol synthesis to preheat the feed seawater entering a multi-stage electrodialysis seawater desalination subsystem. The preheated seawater undergoes efficient multi-stage desalination under an electric field, producing high-purity freshwater for the PEM electrolysis hydrogen production subsystem. Through cascaded energy utilization and closed-loop material recycling, this invention significantly reduces the energy consumption of seawater desalination, achieves freshwater self-sufficiency, and improves overall energy efficiency and economics. It provides an innovative solution for the large-scale, high-value-added conversion of marine renewable energy.

[0150] In one embodiment, in step c, heat is extracted from the PEM electrolysis hydrogen production subsystem and the methanol synthesis subsystem through independent closed cooling water circulation loops, with two or more streams of hot water carrying waste heat serving as the heat source for preheater 302. In step d, independent and adjustable voltages or currents are applied to each stage (ED1, ED2, ED3, ED4) of the multi-stage electrodialysis membrane stack to adapt to different salinity conditions within each stage, thereby optimizing the overall desalination energy consumption.

[0151] In one embodiment, the system organically integrates the PEM electrolysis hydrogen production subsystem, the methanol synthesis subsystem, and the multi-stage electrodialysis seawater desalination subsystem through a closed-loop energy and material flow integration network. A heat recovery and utilization loop is constructed with the ED feed preheater 302 as the energy exchange hub. Specifically, the system utilizes an independent, closed cooling water circulation loop to capture and collect the waste heat that would otherwise be discarded from the PEM electrolyzer 107 and hydrogen heat exchanger 102, oxygen heat exchanger 103 of the PEM electrolysis hydrogen production subsystem, and the first and second waste heat recovery heat exchangers of the methanol synthesis subsystem, forming a medium-low temperature hot water flow. This hot water flow is then guided to the ED feed preheater 302, where its low-grade heat energy is transferred to the pretreated cold seawater feed that is about to enter the multi-stage electrodialysis ED membrane stacks 304-307. This invention constructs a material circulation loop using fresh water as the medium. The multi-stage electrodialysis seawater desalination subsystem utilizes the recovered waste heat for preheating, and then efficiently desalinates seawater under the drive of a DC electric field, producing high-purity freshwater that meets or even exceeds the requirements of PEM electrolysis. This freshwater is stored in freshwater storage tank 104 and is stably supplied as raw material to the PEM electrolysis hydrogen production subsystem, thus forming a self-sufficient closed-loop freshwater supply system.

[0152] In one embodiment, the heat recovery cooling water circulation loop includes a primary cooling loop and an entertainment cooling loop.

[0153] In a preferred embodiment, the PEM electrolyzer 107 is a modular unit with a single-unit power of not less than 1 MW, and its hydrogen production capacity under rated operating conditions is not less than 200 Nm³ / h. To maximize the synergistic effect with the downstream methanol synthesis unit and reduce the total system energy consumption, the electrolyzer is designed for high-pressure operation, with an outlet hydrogen pressure of not less than 6 MPa. This design allows the produced hydrogen to directly meet the pressure requirements of the methanol synthesis reactor 204 without or with only a small-scale pressurization, thus significantly saving the investment and operating costs of the energy-intensive main hydrogen compressor in the traditional low-pressure electrolysis path. The DC power consumption of the electrolyzer is strictly controlled below 4.5 kWh / Nm³ to ensure the energy conversion efficiency from electricity to hydrogen.

[0154] The system's freshwater supply comes from freshwater storage tank 104, which is designed with a capacity of at least 10 cubic meters. This is sufficient to ensure that the electrolyzer can continue to operate for 24 to 48 hours even during short-term maintenance of the seawater desalination subsystem or when it is shut down due to extreme sea conditions, greatly improving the system's operational reliability. The electrolysis feedwater pump uses a high-precision metering pump to ensure that ultrapure water with a resistivity of not less than 15 MΩ·cm is accurately supplied to the electrolyzer.

[0155] The heat generated during the electrolysis process is recovered through two parallel closed cooling water cycles:

[0156] Product cooling circuit: Cooling water driven by product cooling water pump 101 flows sequentially through hydrogen heat exchanger 102 and oxygen heat exchanger 103. The high-temperature (70-80°C) and high-pressure (6 MPa) hydrogen and oxygen from oxygen-water separator 108 and hydrogen-water separator 109 are cooled to about 40°C here for easy subsequent storage and processing.

[0157] Electrolyzer Cooling Circuit: The heat inside the PEM electrolyzer 107 is first carried away by its own circulating cooling water, typically deionized water. This hot water exchanges heat with an independent cooling water circuit driven by the electrolyzer cooling water pump 112 in heat exchanger 111. This design avoids contamination of the expensive electrolyzer core by impurities that may be present in the external cooling water through indirect heat exchange.

[0158] The cooling water that has completed its heat absorption task has now reached a temperature of about 70-80°C. It then merges in the third mixer 113 to form a hot water stream with a stable total flow rate, carrying all the recoverable waste heat from the entire 100 zone. This hot water is then sent to the ED feed preheater 302 to become the first heat source for heating the seawater.

[0159] The methanol synthesis subsystem is the core of achieving high-density, liquefied hydrogen storage and conversion. Its design revolves around a low-temperature, low-pressure synthesis route and is tightly integrated with the upstream hydrogen source and downstream heat recovery unit. High-pressure hydrogen gas, with a pressure not lower than 6 MPa, first enters the hydrogen storage tank 201. The effective volume of this tank is designed to be not less than 100 cubic meters to smooth out peak and valley fluctuations in hydrogen production caused by renewable energy fluctuations, providing a stable and continuous hydrogen supply to the methanol synthesis reactor. Externally supplied, purified carbon dioxide, after being pressurized to the same pressure level as the hydrogen by a compressor (not shown in the diagram), is then thoroughly mixed with hydrogen and recirculated gas from the recirculated gas compressor 208 in the feed mixer 202 at a stoichiometric H2 / CO2 molar ratio preferably of 3.0-3.2.

[0160] The mixed cold feed gas enters the feed preheater 203, where it undergoes countercurrent heat exchange with the high-temperature reaction products (250-280°C) exiting the methanol synthesis reactor 204, preheating itself to the required initial reaction temperature of approximately 220°C. The methanol synthesis reactor 204 is preferably a shell-and-tube isothermal reactor. The tube side is filled with a highly efficient Cu / ZnO / Al2O3 low-temperature methanol synthesis catalyst, while the shell side is circulated with an independent cooling medium such as heat transfer oil or high-pressure saturated water to precisely control the reaction bed temperature, maintain optimal catalyst activity and selectivity, and simultaneously remove a large amount of reaction heat.

[0161] After the reaction products are initially cooled by the feed preheater 203, the low-to-medium grade heat they carry is further recovered through the first and second waste heat recovery heat exchangers. An independent closed cooling water circulation driven by the waste heat recovery cooling water pump 214 is responsible for carrying away this heat, forming another stream of hot water at a temperature of about 70-80°C, which is sent to the ED feed preheater 302 to become the second heat source for heating the seawater.

[0162] The crude product, after being sufficiently cooled to approximately 40°C, enters the high-pressure flash tank 206. Unreacted H2 and CO2 in the gas phase are drawn from the top, and after the circulating pressure drop is compensated by the recirculating gas compressor 208 (approximately 0.2-0.5 MPa), most of it returns to the feed mixer 202. The liquid phase, containing methanol, water, and dissolved gases, is drawn from the bottom, and after being depressurized to approximately 1.5-2.0 MPa by the pressure reducing valve 209, most of the dissolved gases are removed in the medium-pressure flash tank 210. The degassed crude methanol liquid can be selectively preheated by the distillation feed preheater 211 before entering the distillation column 212 to reduce the energy consumption of the reboiler in the distillation column. Finally, in the distillation column 212, through standard distillation operations, a methanol product with a purity of not less than 99.5 wt% is obtained.

[0163] The multi-stage electrodialysis seawater desalination subsystem is crucial for ensuring the self-sufficiency of water resources in the entire system. Its design fully utilizes the system's waste heat and employs advanced multi-stage electrodialysis technology. Seawater is pumped in by seawater pump 301 and first enters the pretreatment unit 303. Considering the sensitivity of the electrodialysis ion exchange membrane to pollutants, this unit includes at least one stage of precision filtration and one stage of ultrafiltration (UF) membrane filtration with a pore size no larger than 0.1 micrometers to thoroughly remove suspended solids, colloids, and microorganisms from the seawater, ensuring that the pollutant density index (SDI) of the produced water is less than 1. For areas with high hardness, a chemical softening unit, such as adding lime and soda ash or a nanofiltration (NF) membrane unit, can be added after ultrafiltration to preferentially remove 80-90% of calcium and magnesium ions, eliminating the risk of scaling on the ED membrane stack at the source.

[0164] The clean seawater, after rigorous pretreatment, enters the ED feed preheater 302. Here, its temperature is raised from the natural water temperature of 15°C to 40-50°C by the waste hot water from zones 100 and 200.

[0165] Preheated seawater is fed into a desalination system consisting of at least four series-connected electrodialysis membrane stacks (304-307). Each stack comprises alternating anion exchange membranes (AEMs) and cation exchange membranes (CEMs), forming numerous desalination and concentrate chambers. Driven by independent DC power supplies, ions in the desalination chambers migrate to adjacent concentrate chambers under the influence of an electric field. A key feature of this system is its multi-stage differentiated operation.

[0166] The first-stage electrodialysis ED membrane stack 304 can use a higher current density to achieve the maximum desalination flux when facing feed water with the highest salinity.

[0167] In subsequent electrodialysis ED membrane stacks 305-307, as the salinity of the freshwater chamber decreases and the resistance increases, the operating voltage and current density are gradually reduced to avoid overcurrent phenomena such as water decomposition and to maintain each stage operating within the optimal energy consumption range.

[0168] Ultimately, the total dissolved solids (TDS) content of the freshwater flowing from the fourth-stage electrodialysis ED membrane stack 307 is controlled below 10 ppm. After being collected by the mixer / collection tank 308, it is sent to the freshwater storage tank 104. Meanwhile, the concentrated brine flowing from each stage's concentrate chamber is collected and discharged. The overall system's freshwater production / influent seawater volume ratio can be optimized to 40-50% based on operating parameters. Through the above implementation, the three subsystems of this invention are tightly coupled together, forming a highly efficient, self-sustaining, and environmentally friendly comprehensive marine energy utilization platform.

[0169] Furthermore, this invention systematically solves the dual bottlenecks of water resource dependence and energy waste in the marine "electricity-hydrogen-alcohol" pathway by constructing an integrated system with waste heat recovery as the link, multi-stage electrodialysis as the core, and freshwater closed-loop as the goal. Its core technological functions are as follows: First, by efficiently recovering the low-to-medium temperature waste heat generated during PEM electrolysis for hydrogen production and methanol synthesis through a closed cooling water loop, this waste heat is used to heat the pretreated seawater to 40–50°C in the ED feed preheater, significantly reducing the solution and membrane stack resistance in the electrodialysis process, thereby reducing energy consumption by 20–30% under the same desalination rate, achieving a highly efficient conversion of "waste heat" into "energy saving"; Second, by employing a multi-stage series electrodialysis membrane stack equipped with independent power supplies, a differentiated operation strategy is implemented—high current density for efficient desalination in the front stage, and gradually reducing the current density in the subsequent stages to avoid water decomposition and energy consumption increases, achieving overall energy consumption optimization; Third, by using a pretreatment process combining ultrafiltration and softening / nanofiltration, SDI<1 is effectively controlled and calcium and magnesium ions are removed, preventing membrane fouling and scaling, and ensuring long-term stable operation of the system; Simultaneously, high-pressure PEM electrolysis directly produces 6 Hydrogen with a pressure of over MPa is used to significantly reduce or eliminate hydrogen compression energy consumption, improving overall energy efficiency. Finally, the produced high-purity freshwater (TDS < 10 ppm) is recycled for electrolysis via a buffer storage tank, forming a fully self-sustaining freshwater closed loop. The storage tank capacity can support continuous operation of the system for 24–48 hours when the desalination unit is shut down, significantly enhancing its anti-interference capability. In summary, this invention achieves efficient, stable, and high-value-added conversion of offshore renewable energy into green methanol, achieving multiple technical goals including water resource self-sufficiency, energy cascade utilization, system synergistic optimization, and environmental friendliness.

[0170] Although embodiments of the present invention have been described above in conjunction with the accompanying drawings, the present invention is not limited to the specific embodiments and application fields described above. The specific embodiments described above are merely illustrative and guiding, and not restrictive. Those skilled in the art can make many other modifications based on the guidance of this specification and without departing from the scope of the claims of the present invention, and all of these modifications are within the scope of protection of the present invention.

Claims

1. A marine green hydrogen-to-methanol system based on waste heat coupled with electrodialysis to achieve self-sufficiency in freshwater, characterized in that, It includes, The PEM electrolysis hydrogen production subsystem uses renewable energy electricity to electrolyze fresh water to produce high-pressure hydrogen and oxygen, and generates waste heat from the process. The methanol synthesis subsystem is connected to the PEM electrolysis hydrogen production subsystem to synthesize crude methanol from hydrogen and carbon dioxide under the action of a catalyst and then purify it into a green methanol product by distillation, while generating waste heat from the reaction. A multi-stage electrodialysis seawater desalination subsystem is connected to the PEM electrolysis hydrogen production subsystem. The multi-stage electrodialysis seawater desalination subsystem desalinates seawater through multi-stage electrodialysis to produce fresh water. The fresh water is buffered in a fresh water storage tank and then fed back to the PEM electrolysis hydrogen production subsystem to form a self-sustaining closed loop for fresh water. The heat recovery cooling water circulation loop exchanges heat with the PEM electrolysis hydrogen production subsystem and the methanol synthesis subsystem to recover waste heat. The heat recovery cooling water circulation loop is also connected to the ED feed preheater to transfer the recovered waste heat to the seawater entering the multi-stage electrodialysis seawater desalination subsystem for feed preheating.

2. The marine green hydrogen to methanol system based on waste heat coupled electrodialysis for freshwater self-sufficiency as described in claim 1, characterized in that, Preferably, the PEM electrolysis hydrogen production subsystem includes, PEM electrolyzers produce high-pressure hydrogen and oxygen by electrolyzing fresh water. The electrolysis feed pump measures and pumps the freshwater from the freshwater storage tank to the first mixer. The first mixer mixes fresh water from the fresh water storage tank with circulating water from the PEM electrolyzer. An oxygen-water separator, connected to the PEM electrolyzer, separates the circulating water entrained in the oxygen. A hydrogen-water separator, connected to the PEM electrolyzer, to separate circulating water entrained in hydrogen gas. A second mixer, connected to the oxygen-water separator and the hydrogen-water separator, collects the circulating water separated from hydrogen and oxygen. The second mixer is connected to the first mixer via a heat exchanger to transfer heat from the circulating water to a cooling circuit. This cooling circuit includes an electrolyzer cooling water pump, a heat exchanger, and a third mixer. The electrolyzer cooling water pump drives an independent cooling circuit for heat exchange. The product cooling circuit includes a product cooling water pump, a hydrogen heat exchanger, and an oxygen heat exchanger. Cooling water driven by the product cooling water pump flows sequentially through the hydrogen heat exchanger and the oxygen heat exchanger to cool the hydrogen separated by the hydrogen-water separator and the oxygen separated by the oxygen-water separator.

3. The marine green hydrogen to methanol system based on waste heat coupled electrodialysis for freshwater self-sufficiency as described in claim 2, characterized in that, The product cooling circuit and the cooling water temperature in the cooling circuit both rise to 70-80°C.

4. The marine green hydrogen to methanol system based on waste heat coupled electrodialysis for freshwater self-sufficiency as described in claim 2, characterized in that, The PEM electrolyzer is a high-pressure type with an outlet hydrogen pressure of not less than 6 MPa. The effective volume of the freshwater storage tank is not less than 10 m³, so as to ensure that the PEM electrolysis hydrogen production subsystem can continue to operate for 24–48 hours during the shutdown of the seawater desalination subsystem.

5. The marine green hydrogen to methanol system based on waste heat coupled electrodialysis for freshwater self-sufficiency as described in claim 2, characterized in that, The methanol synthesis subsystem includes, A hydrogen storage tank connected to the hydrogen-water separator of the PEM electrolysis hydrogen production subsystem to buffer and stabilize the hydrogen; A circulating gas compressor, which compresses circulating gas; The raw material mixer is connected to a hydrogen storage tank, a carbon dioxide gas source, and a circulating gas compressor to mix hydrogen, carbon dioxide, and circulating gas to form raw materials; A methanol synthesis reactor connected to the feed mixer to synthesize crude methanol from hydrogen and carbon dioxide in the presence of a catalyst; A gas-liquid separation and purification unit, connected to the methanol synthesis reactor, is provided for gas-liquid separation and purification of crude methanol. The gas-liquid separation and purification unit includes... The high-pressure flash evaporator performs preliminary gas-liquid separation on the crude methanol. The high-pressure flash evaporator, via a distributor that controls the ratio of recirculated gas to purge gas, feeds the recirculated gas into the recirculated gas compressor. A medium-pressure flash tank, connected to a high-pressure flash tank via a pressure reducing valve, is used to remove dissolved gases from crude methanol liquid. A distillation column, connected to the medium-pressure flash tank, is used to purify crude methanol to the desired specifications of methanol product. Waste heat cooling circuit, which includes, Waste heat recovery cooling water pump, A raw material preheater that preheats the raw material; A first waste heat recovery heat exchanger is connected to the methanol synthesis reactor and the third mixer to recover heat to the cooling water in the third mixer. A second waste heat recovery heat exchanger is connected to the first waste heat recovery heat exchanger and the high-pressure flash tank to transfer heat. A distillation feed preheater is located between the medium-pressure flash tank and the distillation column and is connected to the feed preheater to preheat the liquid entering the distillation column. A distillation condenser is connected to the distillation column to condense the methanol vapor at the top of the column.

6. The marine green hydrogen to methanol system based on waste heat coupled electrodialysis for freshwater self-sufficiency as described in claim 5, characterized in that, The mixed feed gas enters the feed preheater and undergoes countercurrent heat exchange with the high-temperature reaction products (250-280°C) from the methanol synthesis reactor, thus preheating itself to an initial temperature of 220°C.

7. The marine green hydrogen to methanol system based on waste heat coupled electrodialysis for freshwater self-sufficiency as described in claim 1, characterized in that, The methanol synthesis reactor is a shell-and-tube isothermal reactor. The tube side is filled with a Cu / ZnO / Al2O3 low-temperature methanol synthesis catalyst, and the shell side is circulated with an independent cooling medium to control the reaction bed temperature. The H2 / CO2 molar ratio is 3.0-3.

2.

8. The marine green hydrogen to methanol system based on waste heat coupled electrodialysis for freshwater self-sufficiency as described in claim 1, characterized in that, The multi-stage electrodialysis seawater desalination subsystem includes... The ED feed preheater is connected to the third mixer. The pretreatment unit includes an ultrafiltration membrane filtration device and a chemical softening or nanofiltration device to pretreat seawater. At least two stages of electrodialysis membrane stacks connected in series are connected to the pretreatment unit, and each stage of the electrodialysis membrane stack is equipped with an independent DC power supply. A freshwater collection tank, which connects the electrodialysis membrane stack and the freshwater storage tank.

9. The marine green hydrogen to methanol system based on waste heat coupled electrodialysis for freshwater self-sufficiency as described in claim 1, characterized in that, The ED feed preheater is a plate or shell-and-tube heat exchanger, and the temperature of the preheated seawater is 40–50°C.

10. A method for a marine green hydrogen-to-methanol system based on waste heat coupled electrodialysis to achieve self-sufficiency in freshwater, as described in any one of claims 1-9, characterized in that, It includes the following steps: a. Utilize offshore renewable energy power to drive the PEM electrolysis hydrogen production subsystem to electrolyze water and produce hydrogen, and recover the waste heat generated during the electrolysis process through a cooling circuit; b. The hydrogen and carbon dioxide are catalytically synthesized into green methanol in the methanol synthesis subsystem, and the waste heat generated in the reaction process is recovered through a preheating and cooling circuit. c. The waste heat from step a and step b is centrally transported to the ED feed preheater through a heat recovery cooling water circulation loop to preheat the pretreated seawater from the ocean. d. The preheated seawater is sent into the multi-stage electrodialysis seawater desalination subsystem for multi-stage desalination under the action of a DC electric field to produce fresh water; e. The produced freshwater is stored in a freshwater storage tank and used as feedwater to supply the PEM electrolysis hydrogen production subsystem, realizing the system's self-sufficient freshwater cycle and energy cascade utilization.

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