A hot start system and method for a liquid expander in a low-temperature methanol washing system

Abstract

The present invention discloses a hot start system and method for a liquid expander in a low-temperature methanol washing system. A crude synthesis gas pipeline is connected to the inlet of an absorption tower. The outlet of the absorption tower is divided into two paths after passing through a cooler, one of which is connected to the inlet of a flash tank through a liquid throttle valve, and the other is divided into two paths after passing through a first stop valve. One of which is connected to the inlet of the liquid expander through a flow regulating valve, and the other is connected to the inlet of a circulating gas compressor through a cooling gas discharge valve. The outlet of the liquid expander is connected to the inlet of the flash tank through a third stop valve. The circulating gas outlet at the top of the flash tank is divided into two paths, one of which is connected to the inlet of the circulating gas compressor, and the other is connected to the pipeline between the liquid expander and the third stop valve through a second stop valve. The system and method realize a smooth hot start of the liquid expander and reduce the impact on the flow and purity of product gas and tail gas.

Description

Technical Field

[0001] The invention belongs to the technical field of low-temperature methanol washing, and relates to a hot starting system and method for a liquid expander in a low-temperature methanol washing system. Background Art

[0002] The low-temperature methanol scrubbing process was developed in the 1950s and is primarily used to purify acid gases. The low-temperature methanol scrubbing process uses low-temperature methanol as the absorption solvent. The process principle is that acid gases have extremely high solubility in the cold methanol solvent, which can greatly remove acid gases from the feed gas. The CO2 stream produced by the low-temperature methanol scrubbing process is a pure product gas free of sulfur compounds. The process also produces a concentrated H2S stream, which is supplied to the subsequent Claus unit. Compared with other purification methods, this process has the advantages of absorbing a wide variety of impurities, achieving high gas purification levels, good absorption selectivity, and being economical and energy-efficient. It is a relatively mature gas purification method. The low-temperature methanol scrubbing process system is complex and involves numerous equipment. The startup and shutdown of key equipment and the stability of operating parameters have a significant impact on the safe and stable operation of the low-temperature methanol scrubbing system and product purity. For example, patent 201711058770.1 "A system and method for hot starting a low-temperature liquid expander" provides a system and method for hot starting a low-temperature liquid expander, aiming to prevent the power equipment from entering an unstable operating condition due to improper operation during the process of switching the throttle valve to the liquid expander, and even causing abnormal shutdown of the power equipment and failure shutdown of the air separation system.

[0003] Numerous patents have been published regarding technologies that enhance energy conservation and operational stability in low-temperature methanol scrubbing systems. Patent 202123426360.2, "A Low-Temperature Methanol Washing Process Gas Excess Pressure Power Generation System," provides a low-temperature methanol scrubbing process gas excess pressure power generation system that converts process gas excess pressure into electricity. Patent 202210777778.8, "A Low-Temperature Methanol Washing Methanol Scrubber Inter-Sectional Propulsion Force Lifting Control Device," provides a low-temperature methanol scrubbing process gas excess pressure power generation system. This device utilizes a booster pump with a variable-frequency motor installed on the inter-section outlet pipes of the low-temperature methanol scrubbing tower to address the issue of liquid retention in the scrubbing tower. Patent 201720446770.8, "Anti-Overheating Low-Temperature Methanol Washing System Process Gas Circulation Device," provides an anti-overheating low-temperature methanol scrubbing process gas circulation device. By adding a cooling device to reduce the operating temperature of the circulating gas compressor, this device avoids shutdowns caused by overheating and ensures stable operation. Patent 202110901963.9 "A conversion gas treatment or improvement process and a low-temperature methanol washing device and process" provides a conversion gas treatment or improvement process and a low-temperature methanol washing device and process. By connecting multiple conversion gases in parallel with multiple sets of methanol washing devices, it solves the problem of high shutdown risk and high start-up and shutdown costs for one set of conversion gases corresponding to one set of low-temperature methanol washing processes, thereby improving the stability and risk resistance of the system.

[0004] Liquid expanders are used in low-temperature methanol washing systems to replace traditional liquid throttle valves. They not only meet the process's pressure reduction requirements but also recover throttling energy and reduce working fluid temperature, significantly improving the overall system's energy efficiency. Patent 201611044859.8, "Low-temperature Methanol Washing Energy Recovery and Reuse Device," describes a low-temperature methanol washing process that uses a hydraulic turbine to replace the throttle valve to generate electricity, with the converted electricity used to drive a screw chiller for refrigeration or a heat regeneration tower feed pump. However, it does not cover the expander startup process. Patent 201710396428.6, "A Low-temperature Methanol Washing Process for Efficient Recovery and Utilization of Pressure and Cold Energy," describes a low-temperature methanol washing process that uses a hydraulic turbine to replace the throttle valve and simultaneously adds a heat exchanger for efficient cold energy recovery and utilization. However, it does not connect the original throttle valve in parallel as a bypass and does not address the expander startup process. Patent 202210992689.5, "Low-temperature methanol washing, high-pressure methanol-rich power generation and grid-connected device and process," provides a low-temperature methanol washing, high-pressure methanol-rich power generation and grid-connected device and process. This process adds a bypass to the methanol washing tower liquid level control valve (decompression function). This bypass is equipped with a power generation inlet regulating valve, a hydraulic turbine, a generator, and a grid-connected cabinet. The turbine generator converts the energy lost during the decompression of the low-temperature methanol washing, high-pressure methanol-rich solution, into electricity for grid connection, reducing energy loss. The process also does not cover the expansion machine startup process.

[0005] A hot start of a liquid expander is essentially the process of switching the liquid throttle valve to the liquid expander throttle valve after the low-temperature methanol wash system has stabilized. This is a hot start. The outlet temperature and gas entrainment rate directly impact the safe and stable operation of subsequent systems. Excessively high temperatures and gas entrainment rates increase the circulating gas volume, which not only increases the load on the circulating gas compressor but also reduces the purity of the purified gas product. Excessively low temperatures and gas entrainment rates increase the flash liquid volume, increasing the load on the CO2 desorption tower and stripping tower. Temperatures and gas entrainment rates that are too high or too low can cause system fluctuations or even shutdowns. Effectively reducing the impact of the throttle valve and liquid expander switching process on the low-temperature methanol wash system is an urgent issue that needs to be addressed in the implementation and application of liquid expander technology.

[0006] like Figure 1As shown, in a low-temperature methanol washing system, a liquid expander and a high-pressure liquid throttle valve are installed in parallel. Typically, the liquid throttle valve is used for throttling, and then the liquid expander is switched to after the entire low-temperature methanol washing system is operating stably. The startup process of the liquid expander is the process of switching from the throttling of the high-pressure liquid throttle valve to the throttling of the liquid expander. Because the pressure reduction process using the liquid expander is a nearly isentropic process, the temperature drop is greater than the isenthalpic process using the throttle valve. In addition to the flow rate and pressure fluctuations during the switching process, temperature fluctuations can also affect subsequent processes. During this process, the flow control valve opening is gradually increased to gradually increase the flow rate. At the same time, the liquid throttle valve opening is gradually decreased to gradually reduce the flow rate, reducing the refrigeration compressor power to the power corresponding to the curve, thereby ensuring that the expander outlet temperature is consistent with the throttle valve outlet temperature. The coordinated regulation of the three ensures stable outlet working fluid temperature, pressure, and gas content. Otherwise, it will affect the subsequent processes. The temperature, pressure and gas content of the working fluid entering the flash tank will affect the flash evaporation effect. Too much or too little flash gas (i.e., circulating gas) will cause the circulating gas compressor to enter an unstable operating condition, or even cause a malfunction and shutdown, while affecting the flow and purity of the absorption tower product gas; the liquid phase flow at the bottom of the flash tank affects the flow and purity of the product gas and tail gas.

[0007] In the low-temperature methanol washing system, how to achieve a smooth hot start of the liquid expander and reduce the impact on the flow and purity of the product gas and exhaust gas is a key issue in the application and implementation of low-temperature liquid expander technology and needs to be solved urgently. However, no relevant public information has been found domestically and internationally. Summary of the Invention

[0008] The purpose of the present invention is to overcome the shortcomings of the above-mentioned prior art and provide a hot start system and method for a liquid expander in a low-temperature methanol washing system. The system and method achieve a smooth hot start of the liquid expander and reduce the impact on the flow and purity of the product gas and exhaust gas.

[0009] To achieve the above-mentioned object, the hot start system of the liquid expander in the low-temperature methanol washing system of the present invention comprises a crude synthesis gas pipeline, an absorption tower, a cooler, a liquid throttle valve, a flash tank, a first stop valve, a flow regulating valve, a liquid expander, a cooling gas discharge valve, a circulating gas compressor, a third stop valve, a second stop valve, a desorption tower and a gas stripping tower;

[0010] The crude synthesis gas pipeline is connected to the inlet of the absorption tower. The outlet of the absorption tower is divided into two paths after passing through the cooler, one of which is connected to the inlet of the flash tank through the liquid throttle valve, and the other is divided into two paths after passing through the first stop valve, one of which is connected to the inlet of the liquid expander through the flow regulating valve, and the other is connected to the inlet of the circulating gas compressor through the cooling gas discharge valve. The outlet of the liquid expander is connected to the inlet of the flash tank through the third stop valve. The circulating gas outlet at the top of the flash tank is divided into two paths, one of which is connected to the inlet of the circulating gas compressor, and the other is connected to the pipeline between the liquid expander and the third stop valve through the second stop valve. The liquid outlet at the bottom of the flash tank is connected to the inlet of the desorption tower and the inlet of the gas stripping tower.

[0011] The top of the desorption tower is provided with a CO2 product gas outlet, the liquid outlet at the bottom of the desorption tower is connected to the inlet of the gas stripping tower, the top of the gas stripping tower is provided with a tail gas outlet, and the top of the absorption tower is provided with a purified gas outlet.

[0012] The bottom outlet of the gas stripping tower is connected to the external heat regeneration tower.

[0013] It also includes a refrigeration compressor, the outlet of which is connected to the cold working medium inlet of the cooler.

[0014] The utility model also includes a generator, and the liquid expander is connected to the generator.

[0015] The hot start method of the liquid expander in the low-temperature methanol washing system of the present invention comprises the following steps:

[0016] 1) Perform reverse precooling of the liquid expander;

[0017] 2) Filling the liquid expander with liquid;

[0018] 3) Coordinately adjust the opening of the flow control valve, the opening of the liquid throttle valve, and the power of the refrigeration compressor until the speed of the liquid expander is increased to the grid-connected power generation speed;

[0019] 4) Load the liquid expander to the design load.

[0020] The specific process of step 1) is:

[0021] Open the second stop valve, flow regulating valve and cooling gas discharge valve to allow the low-temperature circulating gas output from the flash tank to enter the second stop valve, liquid expander, flow regulating valve and cooling gas discharge valve in sequence, and then be led to the circulating gas compressor through the cooling gas discharge valve for circulation recovery until the temperature in the liquid expander approaches the working medium temperature of the liquid throttle valve.

[0022] The specific process of step 2) is:

[0023] Open the first stop valve and the third stop valve, adjust the opening of the flow control valve, and introduce the low-temperature methanol-rich medium upstream of the liquid expander into the inlet pipe, volute, nozzle, impeller and diffuser of the liquid expander in sequence.

[0024] The specific process of step 3) is:

[0025] Obtain a curve showing the power of the refrigeration compressor changing with the flow rate of the liquid expander during the startup of the liquid expander;

[0026] The opening of the flow control valve is gradually increased, while the opening of the liquid throttle valve is gradually reduced, and the power of the refrigeration compressor is reduced to the power corresponding to the curve, so that the working fluid temperature, pressure and gas content at the outlet of the liquid expander remain stable until the speed of the liquid expander is increased to the grid-connected power generation speed.

[0027] The specific operations of step 4) are:

[0028] Reduce the opening of the liquid throttle valve and increase the opening of the flow control valve at the same time, reduce the power of the refrigeration compressor to the power corresponding to the curve, and gradually increase the load of the liquid expander until the load of the liquid expander reaches the design value.

[0029] The present invention has the following beneficial effects:

[0030] In the hot start system and method of the liquid expander in the low-temperature methanol washing system described in the present invention, during specific operation, the outlet of the liquid expander is connected to the inlet of the flash tank via the third stop valve, and the circulating gas outlet at the top of the flash tank is divided into two paths, one of which is connected to the inlet of the circulating gas compressor, and the other is connected to the pipeline between the liquid expander and the third stop valve via the second stop valve. During startup, a reverse flow method is adopted, so that the low-temperature circulating gas output from the flash tank enters the second stop valve, the liquid expander, the flow regulating valve and the cooling gas discharge valve in sequence, and is then guided to the circulating gas compressor through the cooling gas discharge valve for recycling and recovery. This allows the low-temperature liquid expander to be stably hot started in the currently operating low-temperature methanol washing system, thereby stabilizing the operating parameters of the circulating gas compressor, the desorption tower and the gas stripping tower, avoiding fluctuations in the flow and purity of the product gas and tail gas caused by fluctuations in the circulating gas compressor, the absorption tower, the desorption tower and the gas stripping tower due to fluctuations in the feed flow of the flash tank, and even system shutdown. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 It is a structural diagram of the prior art;

[0032] Figure 2 It is a structural diagram of the present invention.

[0033] Among them, 1 is the liquid throttle valve, 2 is the liquid expander, 3 is the circulating gas compressor, 4 is the generator, 5 is the absorption tower, 6 is the desorption tower, 7 is the gas stripping tower, 8 is the cooler, 9 is the first stop valve, 10 is the third stop valve, 11 is the cooling gas discharge valve, 12 is the second stop valve, 13 is the circulating gas outlet, 14 is the refrigeration compressor, 15 is the flow regulating valve, 16 is the flash tank, 17 is the purified gas outlet, 18 is the CO2 product gas outlet, and 19 is the tail gas outlet. DETAILED DESCRIPTION

[0034] In order to enable those skilled in the art to better understand the solutions of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only embodiments of a part of the present invention, not all embodiments, and are not intended to limit the scope of the present invention. In addition, in the following description, descriptions of well-known structures and technologies are omitted to avoid unnecessary confusion of the concepts disclosed in the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work should fall within the scope of protection of the present invention.

[0035] The accompanying drawings illustrate schematic diagrams of the structures of the disclosed embodiments of the present invention. These figures are not drawn to scale; for the purpose of clarity, some details are exaggerated and some details may be omitted. The shapes of the various regions and layers shown in the figures, as well as their relative sizes and positional relationships, are merely exemplary and may deviate in practice due to manufacturing tolerances or technical limitations. Those skilled in the art may design regions / layers with different shapes, sizes, and relative positions as needed.

[0036] refer to Figure 2 The hot start system of the liquid expander in the low-temperature methanol washing system of the present invention includes a liquid throttle valve 1, a liquid expander 2, a circulating gas compressor 3, a generator 4, an absorption tower 5, a desorption tower 6, a gas stripping tower 7, a cooler 8, a first stop valve 9, a third stop valve 10, a cooling gas discharge valve 11, a second stop valve 12, a flow regulating valve 15, a flash tank 16 and a refrigeration compressor 14;

[0037] The crude synthesis gas pipeline is connected to the inlet of the absorption tower 5. The outlet of the absorption tower 5 is divided into two paths after passing through the cooler 8, one of which is connected to the inlet of the flash tank 16 through the liquid throttle valve 1, and the other is divided into two paths after passing through the first stop valve 9, one of which is connected to the inlet of the liquid expander 2 through the flow regulating valve 15, and the other is connected to the inlet of the circulating gas compressor 3 through the cooling gas discharge valve 11. The outlet of the liquid expander 2 is connected to the inlet of the flash tank 16 through the third stop valve 10. The circulating gas outlet 13 at the top of the flash tank 16 is divided into two paths, one of which is connected to the inlet of the circulating gas compressor 3, and the other is connected to the pipeline between the liquid expander 2 and the third stop valve 10 through the second stop valve 12.

[0038] The liquid outlet at the bottom of the flash tank 16 is connected to the inlet of the desorption tower 6 and the inlet of the gas stripping tower 7. The top of the desorption tower 6 is provided with a CO2 product gas outlet 18. The liquid outlet at the bottom of the desorption tower 6 is connected to the inlet of the gas stripping tower 7. The top of the gas stripping tower 7 is provided with an exhaust gas outlet 19. The bottom outlet of the gas stripping tower 7 is connected to the external thermal regeneration tower. The top of the absorption tower 5 is provided with a purified gas outlet 17.

[0039] In this embodiment, the outlet of the refrigeration compressor 14 is connected to the cold working medium inlet of the cooler 8 , and the liquid expander 2 is connected to the generator 4 .

[0040] The principle of the present invention is:

[0041] Draw a curve showing the power of the refrigeration compressor 14 as a function of the flow rate of the liquid expander 2 during the startup of the liquid expander 2. Assume that the outlet enthalpy of the liquid throttle valve 1 is h J,out , the outlet enthalpy of liquid expander 2 is h LE,out , the flow rate of liquid expander 2 is M LE The refrigeration coefficient of the refrigeration compressor 14 is COP, and the power of the refrigeration compressor 14 before the liquid expansion machine 2 is started is P0. Then the enthalpy difference between the liquid expansion machine 2 and the outlet of the liquid throttle valve 1 is △h=h LE,out -h J,out , the cooling capacity difference Q between the outlet of liquid throttle valve 1 and liquid expander 2 Ref =M LE *△h=M LE *(h LE,out -h J,out ), the power change of the refrigeration compressor 14 ΔP=Q Ref / COP=M LE *(h LE,out -h J,out ) / COP, the power of the refrigeration compressor 14 P=P0+ΔP=P0+M LE *(h LE,out -h J,out) / COP. Reverse precooling and forward filling of the liquid expander 2 and pipeline are performed. The flow control valve 15 is then gradually opened, while the liquid throttle valve 1 is gradually closed. The power of the refrigeration compressor 14 is slowly reduced to the power corresponding to the curve to maintain the stability of the outlet working fluid temperature, pressure, and gas content until the liquid expander 2 reaches the grid-connected power generation speed. The opening of the flow control valve 15 is then increased, while the opening of the liquid throttle valve 1 is simultaneously reduced. The power of the refrigeration compressor 14 is reduced to the power corresponding to the curve, gradually increasing the load of the liquid expander 2 until the operating parameter requirements are met. The specific operating steps are as follows:

[0042] 1) Precooling before starting the liquid expander 2:

[0043] The low-temperature circulating gas is drawn out from the flash tank 16 and reversely enters the diffuser pipe, impeller, nozzle, volute and inlet pipe fittings of the liquid expander 2 in sequence, and finally is led to the circulating gas compressor 3 through the cooling gas discharge valve 11 for circulation recovery until the temperature in the liquid expander 2 approaches the working medium temperature of the liquid throttle valve 1. The reverse cooling process is completed, the cooling gas discharge valve 11 and the second stop valve 12 are closed, and then liquid filling is carried out to introduce the low-temperature methanol-rich medium into the inlet pipe, volute, nozzle, impeller and diffuser pipe of the liquid expander 2 in sequence.

[0044] 2) Start liquid expander 2:

[0045] Gradually increase the opening of the flow control valve 15, close the opening of the liquid throttle valve 1, and at the same time reduce the power of the refrigeration compressor 14 to the power corresponding to the curve, so that the working medium temperature, pressure and gas content at the outlet of the liquid expander 2 are stable, and the speed of the liquid expander 2 is gradually increased to reach the grid-connected power generation speed.

[0046] 3) Increase the load:

[0047] By continuing to increase the opening of the flow control valve 15, closing the opening of the liquid throttle valve 1, and reducing the power of the refrigeration compressor 14 to the power corresponding to the curve, until the expected technical indicators are reached, the entire hot start process is completed.

[0048] refer to Figure 2 The method for starting the liquid expander in the low-temperature methanol washing system of the present invention comprises the following steps:

[0049] First, reversely pre-cool and fill the liquid expander 2 and pipeline with gas, then gradually increase the opening of the flow control valve 15, while gradually reducing the opening of the liquid throttle valve 1, and reduce the power of the refrigeration compressor 14 to the power corresponding to the curve, so that the working medium temperature and gas content at the outlet of the liquid expander 2 remain stable until the speed of the liquid expander 2 is increased to the grid-connected power generation speed, continue to increase the opening of the flow control valve 15, while reducing the opening of the liquid throttle valve 1, reduce the power of the refrigeration compressor 14 to the power corresponding to the curve, and gradually increase the load of the liquid expander 2 until the operating parameter requirements are met. The specific operating steps are:

[0050] 1) Perform reverse precooling of the liquid expander 2.

[0051] First, open the second stop valve 12, the flow regulating valve 15 and the cooling gas discharge valve 11, so that the low-temperature circulating gas drawn out from the flash tank 16 enters the second stop valve 12, the liquid expander 2, the flow regulating valve 15 and the cooling gas discharge valve 11 in sequence, and then is led to the circulating gas compressor 3 through the cooling gas discharge valve 11 for circulation and recovery until the temperature in the liquid expander 2 approaches the working medium temperature of the liquid throttle valve 1.

[0052] 2) Fill with liquid.

[0053] Open the first stop valve 9 and the third stop valve 10, adjust the opening of the flow control valve 15 to 15%, and introduce the low-temperature methanol-rich medium upstream of the liquid expander 2 into the inlet pipe, volute, nozzle, impeller and diffuser of the liquid expander 2 in sequence.

[0054] 3) Coordinately adjust the opening of the flow control valve 15, the opening of the liquid throttle valve 1 and the power of the refrigeration compressor 14.

[0055] Gradually increase the opening of the flow control valve 15 and gradually decrease the opening of the liquid throttle valve 1 at the same time, reduce the power of the refrigeration compressor 14 to the power corresponding to the curve, so that the working medium temperature, pressure and gas content at the outlet of the liquid expander 2 remain stable until the speed of the liquid expander 2 is increased to the grid-connected power generation speed;

[0056] 4) Load the liquid expander 2 to the design index.

[0057] Reduce the opening of the liquid throttle valve 1, and at the same time increase the opening of the flow control valve 15, reduce the power of the refrigeration compressor 14 to the power corresponding to the curve, so that the load of the liquid expander 2 gradually increases, and the load of the liquid expander 2 reaches the design value, completing the hot start of the liquid expander 2.

[0058] It should be noted that the present invention realizes the automatic hot start of the liquid expander 2 and the load adjustment of the refrigeration compressor 14, making the entire switching process efficient, accurate and smooth.

[0059] In addition, the cooling gas selected by the present invention is a low-temperature circulating gas having the same temperature as the working medium of the liquid throttle valve 1, thereby avoiding the gasification phenomenon caused by pre-cooling of the methanol-rich liquid and failures caused by insufficient pre-cooling and over-cooling that may be caused by other pre-cooling gases; the cooling gas selected by the present invention is an internal circulating gas, which does not introduce impurity gas, does not consume any product gas, and has no additional emissions, has little impact on the output and purity of upstream and downstream product gases and tail gas, and has low investment and operating costs.

[0060] The above is only a preferred specific embodiment of the present invention, and does not limit the scope of protection of the present invention. CO2 product gas or tail gas can also be used as cooling gas, and the corresponding exhaust positions are CO2 analysis tower 6 and gas stripping tower 7. Any equivalent modifications made by those skilled in the art in accordance with the contents of the patent of the present invention should be covered within the scope of protection of the patent of the present invention.

[0061] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the field should understand that the specific implementation methods of the present invention can still be modified or replaced by equivalents. Any modification or equivalent replacement that does not depart from the spirit and scope of the present invention should be covered by the scope of protection of the claims of the present invention.

Claims

1. A hot start method for a liquid expander in a low-temperature methanol washing system, characterized in that: A hot start system for a liquid expander in a low-temperature methanol washing system, comprising a crude synthesis gas pipeline, an absorption tower (5), a cooler (8), a liquid throttle valve (1), a flash tank (16), a first stop valve (9), a flow regulating valve (15), a liquid expander (2), a cooling gas discharge valve (11), a circulating gas compressor (3), a third stop valve (10), a second stop valve (12), a desorption tower (6), and a gas stripping tower (7); The crude synthesis gas pipeline is connected to the inlet of the absorption tower (5), and the outlet of the absorption tower (5) is divided into two paths after passing through the cooler (8), one of which is connected to the inlet of the flash tank (16) through the liquid throttle valve (1), and the other is divided into two paths after passing through the first stop valve (9), one of which is connected to the inlet of the liquid expander (2) through the flow regulating valve (15), and the other is connected to the inlet of the circulating gas compressor (3) through the cooling gas discharge valve (11). The outlet of the liquid expander (2) is connected to the inlet of the flash tank (16) through the third stop valve (10). The circulating gas outlet (13) at the top of the flash tank (16) is divided into two paths, one of which is connected to the inlet of the circulating gas compressor (3), and the other is connected to the pipeline between the liquid expander (2) and the third stop valve (10) through the second stop valve (12). The liquid outlet at the bottom of the flash tank (16) is connected to the inlet of the desorption tower (6) and the inlet of the gas stripping tower (7); The following steps are involved: 1) performing reverse precooling of the liquid expander (2); 2) filling the liquid expander (2); 3) Coordinately adjusting the opening of the flow control valve (15), the opening of the liquid throttle valve (1), and the power of the refrigeration compressor (14) until the speed of the liquid expander (2) is increased to the grid-connected power generation speed; 4) Loading the liquid expander (2) to the design index; The specific process of step 1) is: The second stop valve (12), the flow regulating valve (15) and the cooling gas discharge valve (11) are opened, so that the low-temperature circulating gas output from the flash tank (16) enters the second stop valve (12), the liquid expander (2), the flow regulating valve (15) and the cooling gas discharge valve (11) in sequence, and then is led to the circulating gas compressor (3) through the cooling gas discharge valve (11) for recycling until the temperature in the liquid expander (2) approaches the working medium temperature of the liquid throttle valve (1); The specific process of step 2) is: Open the first stop valve (9) and the third stop valve (10), adjust the opening of the flow control valve (15), and introduce the low-temperature methanol-rich medium upstream of the liquid expander (2) into the inlet pipe, volute, nozzle, impeller and diffuser pipe of the liquid expander (2) in sequence; The specific process of step 3) is: Obtaining a curve showing a change in power of the refrigeration compressor (14) versus flow rate of the liquid expander (2) during the startup of the liquid expander (2); The opening of the flow regulating valve (15) is gradually increased, while the opening of the liquid throttle valve (1) is gradually decreased, and the power of the refrigeration compressor (14) is reduced to the power corresponding to the curve, so that the working medium temperature, pressure and gas content at the outlet of the liquid expander (2) remain stable until the speed of the liquid expander (2) is increased to the grid-connected power generation speed; The specific operations of step 4) are: The opening of the liquid throttle valve (1) is reduced, while the opening of the flow control valve (15) is increased, and the power of the refrigeration compressor (14) is reduced to the power corresponding to the curve, so that the load of the liquid expander (2) is gradually increased, and the load of the liquid expander (2) reaches the design value.

2. The hot start method of the liquid expander in the low-temperature methanol washing system according to claim 1, characterized in that: The top of the desorption tower (6) is provided with a CO2 product gas outlet (18), the liquid outlet at the bottom of the desorption tower (6) is connected to the inlet of the stripping tower (7), the top of the stripping tower (7) is provided with a tail gas outlet (19), and the top of the absorption tower (5) is provided with a purified gas outlet (17).

3. The hot start method of the liquid expander in the low-temperature methanol washing system according to claim 1, characterized in that: The bottom outlet of the gas stripping tower (7) is connected to the external heat regeneration tower.

4. The hot start method of the liquid expander in the low-temperature methanol washing system according to claim 1, characterized in that: It also includes a refrigeration compressor (14), the outlet of the refrigeration compressor (14) is connected to the cold working medium inlet of the cooler (8).

5. The hot start method of the liquid expander in the low-temperature methanol washing system according to claim 1, characterized in that: It also includes a generator (4), and the liquid expander (2) is connected to the generator (4).

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