A method and system for reducing water content in a low-temperature methanol washing system

Abstract

The present invention provides a method and system for reducing the water content of a low-temperature methanol washing system. The method for reducing the water content of a low-temperature methanol washing system comprises: using a methanol-water separation tower to distill the water-containing methanol obtained by low-temperature methanol washing to remove the water contained therein, wherein the water-containing methanol from the methanol washing system for removing CO2 and acid gas is fed from the top plate of the methanol-water separation tower, and the rich methanol is flashed to remove the dissolved gas using a first methanol-water flash tank, and the gas is discharged from the top, and the rich methanol discharged from the bottom enters the middle part of the methanol-water separation tower. The method for reducing the water content of a low-temperature methanol washing system of the present invention can effectively reduce the water content of circulating methanol, increase operational stability, and reduce equipment corrosion.

Description

Technical Field

[0001] The present invention relates to the field of coal chemical gas purification, and in particular to a method and system for reducing the water content of a low-temperature methanol washing system. Background Art

[0002] The raw gas of coal chemical plants contains saturated water. Water and methanol are miscible. After washing with methanol, the water in the gas will dissolve in methanol, and the purified gas basically contains no water. Therefore, the water in the raw gas is removed by low-temperature methanol washing to obtain water-free purified synthesis gas that meets downstream needs.

[0003] After water is dissolved in methanol, a methanol-water separation tower is added to separate the water in the methanol and discharge it out of the system, thereby maintaining the water content in the system at a low level.

[0004] Figure 1 A schematic diagram of a conventional low-temperature methanol washing system is shown. The system comprises:

[0005] (1) A heat exchanger, which is used to cool the raw gas. In the heat exchanger, the raw gas sprayed with a small amount of methanol is heat-exchanged with the purified gas from the top of the absorption tower and the CO2 product gas from the top of the desorption tower to cool the raw gas; the purified gas and CO2 product gas after the heat exchange are collected, wherein the methanol is sprayed to prevent freezing;

[0006] (2) a raw gas separator, which is used to separate the rich methanol and gas, and is fed with the raw gas cooled by the heat exchanger, and the raw gas is discharged from the top and the separated rich methanol is discharged from the bottom;

[0007] (3) an absorption tower, which uses methanol to absorb and purify acidic gases such as water, CO2 and H2S in the raw gas. The raw gas from the raw gas separator is fed from the lower part of the absorption tower, and the lean methanol from the bottom of the thermal regeneration tower is fed from the top plate of the absorption tower. The lean methanol is countercurrently contacted with the raw gas to absorb the water, CO2 and H2S in the raw gas. The purified gas is discharged to a collection device after heat exchange through a heat exchanger. Sulfur-free rich methanol is discharged from the middle part of the absorption tower, and sulfur-containing rich methanol is discharged from the bottom of the absorption tower;

[0008] (4) Sulfur-free methanol flash tank, the sulfur-free rich methanol from the middle of the absorption tower is fed into the sulfur-free methanol flash tank for flash treatment, the gas discharged from the top is compressed and enters the heat exchanger together with the raw gas for recycling, and the sulfur-free rich methanol is discharged from the bottom;

[0009] (5) Sulfur-containing methanol flash tank, the sulfur-containing methanol from the bottom of the absorption tower is fed into the sulfur-containing methanol flash tank for flash treatment, the gas discharged from the top is compressed and enters the heat exchanger together with the feed gas for recycling, and the sulfur-containing methanol discharged from the bottom enters the middle of the concentration tower;

[0010] (6) a desorption tower, which receives the sulfur-free rich methanol from the bottom of the sulfur-free flash tank at an upper position and performs CO2 desorption, discharges CO2 product gas from the top of the desorption tower, and discharges the sulfur-free rich methanol and the sulfur-containing rich methanol from the middle and bottom of the desorption tower respectively;

[0011] (7) a concentration tower, which is used to further desorb carbon dioxide to relatively concentrate acidic gases such as hydrogen sulfide, feed sulfur-rich methanol from the bottom of the desorption tower from the lower part of the concentration tower, feed sulfur-rich methanol from the bottom of the sulfur-containing methanol flash tank from the middle part of the concentration tower, and feed sulfur-free rich methanol discharged from the middle part of the desorption tower from the upper part of the concentration tower, feed nitrogen from the lower part of the concentration tower, and discharge a mixed gas containing a small amount of acidic gases such as H2S, CO2 and N2 from the top of the concentration tower to the tail gas washing tower;

[0012] (8) A thermal regeneration tower, which is used to remove acidic gases such as H2S. The sulfur-rich methanol from the bottom of the concentration tower is fed to the middle of the thermal regeneration tower. The lean methanol vapor from the top of the methanol-water separation tower enters the lower part of the thermal regeneration tower after cooling. The acidic gases containing H2S and other gases are discharged from the top and enter the Claus recovery system. The lean methanol discharged from the bottom is divided into three parts, one part is used to spray the raw gas, one part enters the upper part of the absorption tower, and the other part enters the methanol-water separation tower from the top tray to continuously remove moisture from the system;

[0013] (9) a methanol-water separation tower, which removes water from the aqueous methanol from the thermal regeneration tower and the rich methanol from the bottom of the raw gas separator, feeds the lean methanol from the bottom of the thermal regeneration tower to the top tray of the methanol-water separation tower, feeds the rich methanol from the bottom of the raw gas separator to the middle of the methanol-water separation tower, feeds the tail gas washing water from the bottom of the tail gas washing tower to the middle of the methanol-water separation tower, discharges waste water containing a small amount of methanol from the bottom, and discharges lean methanol steam from the top; and

[0014] (10) A tail gas washing tower is used to remove a small amount of residual methanol in the tail gas so that the tail gas meets the emission standards. Desalted water is fed into the tail gas washing tower from a feed port located on the top plate of the tail gas washing tower, and tail gas from the top of the concentrator is fed into the tail gas washing tower from a feed port located on the bottom plate. The purified tail gas is discharged from the top, and tail gas washing water is discharged from the bottom.

[0015] The system operates smoothly by removing water from the methanol circulating in the system in the methanol water separation tower. The methanol water separation tower continuously removes water from the methanol to keep the water content of the methanol in the system at a low level.

[0016] However, when the moisture content in the raw gas is too high, the system has poor moisture removal effect, and there is a lot of residual moisture in the system during initial startup, which leads to a high methanol-water content in the system, the reflux methanol from the thermal regeneration tower has a high water content and cannot play a reflux role. The water content in the top gas phase that balances it is too high, which brings water back to the system. Once the amount of water brought back to the system is greater than the amount of water discharged from the system, the water amount in the system cannot be reduced, forming a vicious circle. The water content will continue to rise, affecting normal operation and aggravating equipment corrosion. Summary of the invention

[0017] In view of the above problems existing in the prior art, the present invention provides a system and method for reducing the water content of a low-temperature methanol washing system. The system for reducing the water content of a low-temperature methanol washing system according to the present invention has the advantages of improving the methanol-water separation effect, quickly and efficiently separating the water in the system, reducing the water content in the methanol of the system, and stabilizing the methanol-water content in the system at a relatively low level.

[0018] According to a first aspect of the present invention, there is provided a method for reducing the water content of a low-temperature methanol washing system, comprising:

[0019] The aqueous methanol obtained by low-temperature methanol washing is distilled using a methanol-water separation tower to remove the water contained therein, methanol is discharged from the top of the methanol-water separation tower, and waste water is discharged from the bottom of the methanol separation tower; wherein,

[0020] Feeding aqueous methanol that has passed through a methanol washing system for removing CO2 and acidic gases from a feed port located at the top tray of a methanol-water separation tower, preferably, the aqueous methanol that has passed through a methanol washing system for removing CO2 and acidic gases is methanol from the bottom of a thermal regeneration tower;

[0021] flashing the rich methanol from the bottom of the raw gas separator using a first methanol-water flash tank to remove dissolved gas, discharging gas from the top, and the rich methanol discharged from the bottom enters the middle of the methanol-water separation tower; and

[0022] Optionally, a second methanol-water flash tank is used to flash the tail gas washing water to remove dissolved gases therein, the gas discharged from the top enters the tail gas absorption tower, and the tail washing water discharged from the bottom enters the middle of the methanol-water separation tower.

[0023] Before entering the methanol-water separation tower, the methanol-rich and / or tail gas washing water from the raw gas separator tank is flashed through a methanol-water flash tank to remove the gas dissolved therein, thereby reducing the gas entering the methanol-water separation tower, reducing the amount of gas phase at the top of the methanol-water separation tower, and thereby reducing the amount of water returned to the system, while reducing the gas phase load of the methanol-water separation tower and improving the stability of the operation.

[0024] Preferably, anhydrous methanol is introduced into the methanol water separation tower; when anhydrous methanol is introduced, the anhydrous methanol is fed into the methanol water separation tower from a feed port located at the top tray of the methanol water separation tower; the aqueous methanol that has passed through a methanol wash system for removing CO2 and acidic gases is fed into the methanol water separation tower from a feed port located 3 to 8 trays below the top tray.

[0025] Preferably, the tail gas washing water is fed from a feed inlet located at 1 / 3 to 2 / 3 of the total number of plates of the methanol-water separation tower.

[0026] The addition of anhydrous methanol reduces the water content in the methanol vapor at the top of the methanol-water separation tower, thereby reducing the water content of the methanol refluxed to the absorption tower, effectively ensuring the safe operation of the device.

[0027] The feed amount of anhydrous methanol is determined according to the water content of lean methanol discharged from the top of the methanol-water separation tower. When the water content of the lean methanol discharged exceeds a predetermined value, anhydrous methanol is fed or the feed amount of anhydrous methanol is increased. A valve is used to control whether anhydrous methanol is introduced, and whether the methanol passing through the methanol washing system for removing CO2 and acidic gas is fed from the feed port at the top tray or from the feed port at the 3-8th tray below the top tray.

[0028] Preferably, when the water content of the methanol vapor discharged from the top of the methanol-water separation tower exceeds 0.5 mol%, anhydrous methanol is fed, and the weight ratio of the amount of the fed anhydrous methanol to the methanol passed through the methanol washing system for removing CO2 and acidic gases is 1:2 to 1:1.

[0029] Preferably, the methanol washing system is used to purify the raw coal gas, and the aqueous methanol in the methanol washing system after removing CO2 and acidic gases is the aqueous methanol obtained after absorbing and purifying impurities in the raw coal gas and removing acidic gases such as CO2 and H2S, and its water content is preferably 1wt% to 2wt%.

[0030] Preferably, the water content of the methanol vapor discharged from the top of the methanol-water separation tower is 0.1 mol% to 0.5 mol%, and the methanol content in the water discharged from the bottom of the methanol-water separation tower is less than 0.01 wt%.

[0031] Preferably, the operating conditions of the methanol-water separation tower are: 0.23-0.26 MPaG, 100-140°C.

[0032] Preferably, the working conditions of the first methanol-water flash tank are: operating pressure 0.4-0.8 MPaG, temperature 30-70°C.

[0033] Preferably, the working conditions of the second methanol-water flash tank are: operating pressure 0.4-0.8 MPaG, temperature 70-120°C.

[0034] According to a second aspect of the present invention, there is provided a system for reducing the water content of a low-temperature methanol washing system, comprising:

[0035] A methanol-water separation tower, which is used to separate water and methanol, is provided with feed ports at the top plate and 3 to 8 plates below the top plate, respectively, for feeding anhydrous methanol or aqueous methanol obtained by low-temperature methanol washing; a rich methanol feed port, which is used to feed rich methanol from the bottom of the first methanol flash tank; a tail gas washing water feed port, which is used to feed tail gas washing water, and the feed port is located at 1 / 3 to 2 / 3 of the total number of plates of the methanol-water separation tower; a methanol vapor outlet, which is used to discharge dehydrated methanol vapor, and the methanol vapor outlet is located at the top of the methanol-water separation tower; and a wastewater outlet, which is used to discharge wastewater, and the wastewater outlet is located at the bottom of the methanol-water separation tower;

[0036] a first methanol-water flash tank, which is used to remove gas from the rich methanol in the raw gas separator tank, and is connected to the bottom of the raw gas separator tank through a pipeline to feed the rich methanol from the raw gas separator tank, the top of the first methanol-water flash tank is connected to the middle of the concentration tower to feed the separated gas to the concentration tower, and the bottom of the first methanol flash tank is connected to the middle of the methanol-water separation tower through a pipeline to transport the rich methanol at the bottom of the first methanol flash tank to the methanol-water separation tower; and

[0037] An optional second methanol-water flash tank is used to remove gas from the tail gas washing water from the tail gas washing tower, and is connected to the bottom of the tail gas washing tower through a pipeline to feed the tail gas washing water from the tail gas washing tower. The top of the second methanol-water flash tank is connected to the bottom of the tail gas washing tower to circulate the separated gas to the tail gas washing tower.

[0038] In the present invention, the role and beneficial effects of adding the first methanol-water flash tank and / or the second methanol-water flash tank are as follows:

[0039] The rich methanol from the raw gas separator may carry a large amount of CO2. The newly added methanol-water flash tank can effectively remove the CO2 gas dissolved in the rich methanol and prevent too much gas from entering the methanol-water separation tower. Too much gas entering the methanol-water separation tower will increase the operating load and the amount of gas at the top. The increase in the amount of gas will increase the water content of the circulating methanol, which is not conducive to the control of the water content of methanol in the system.

[0040] The washing water of the tail gas washing tower is also flashed through the second methanol water flash tank before entering the methanol water separation tower. The separated gas phase, mainly CO2 and water, is returned to the tail gas before the tail gas washing. At the same time, the gas phase entering the methanol water separation tower can be reduced, the operating load can be reduced, the amount of top gas phase can be reduced, and the amount of water entering the front system can be controlled.

[0041] In the conventional process, the reflux methanol from the bottom of the thermal regeneration tower enters the top plate of the methanol-water separation tower. When the methanol-water content in the system is high, the water content in the reflux methanol from the thermal regeneration tower is high and cannot play a reflux role. The water content in the top gas phase that is balanced with it is also high, bringing water back to the system. Once the amount of water brought back to the system is greater than the amount of water discharged from the system, the amount of water in the system cannot be reduced, forming a vicious circle. The water content will continue to rise, affecting normal operation and aggravating equipment corrosion.

[0042] Preferably, when anhydrous methanol is introduced, anhydrous methanol is fed from the feed port located at the top tower plate, and hydrous methanol is fed from the feed port located at the 3 to 8 tower plates below the top tower plate; when anhydrous methanol is not introduced, hydrous methanol is fed from the feed port located at the top tower plate, and the feed port located at the 3 to 8 tower plates below the top tower plate is closed.

[0043] The system for reducing the water content of a low-temperature methanol washing system according to the present invention is a part of the low-temperature methanol washing system.

[0044] According to a third aspect of the present invention, there is provided a low-temperature methanol washing system, comprising:

[0045] 1) A heat exchanger, which is used to cool the raw gas. In the heat exchanger, the raw gas sprayed with a small amount of methanol is heat-exchanged with the purified gas from the top of the absorption tower and the CO2 product gas from the top of the desorption tower to cool the raw gas; the purified gas and CO2 product gas after the heat exchange are collected;

[0046] 2) A raw gas separator, which is used to separate the rich methanol and gas. The raw gas cooled by the heat exchanger is fed into the separator, and the raw gas is discharged from the top and the rich methanol is discharged from the bottom;

[0047] 3) An absorption tower, which uses methanol to absorb and purify acidic gases such as water, CO2 and H2S in the raw gas. The raw gas from the raw gas separator is fed from the lower part of the absorption tower, and the lean methanol from the bottom of the thermal regeneration tower is fed from the top plate of the absorption tower, so that the lean methanol is countercurrently contacted with the raw gas to absorb the water, CO2 and H2S in the raw gas. The purified gas is discharged to a collection device after heat exchange through a heat exchanger, and sulfur-free rich methanol is discharged from the middle of the absorption tower, and sulfur-containing rich methanol is discharged from the bottom of the absorption tower;

[0048] (4) Sulfur-free methanol flash tank, the sulfur-free rich methanol from the middle of the absorption tower is fed into the sulfur-free methanol flash tank for flash treatment, the gas discharged from the top is compressed and enters the heat exchanger together with the raw gas for recycling, and the sulfur-free rich methanol is discharged from the bottom;

[0049] (5) Sulfur-containing methanol flash tank, the sulfur-containing rich methanol from the bottom of the absorption tower is fed into the sulfur-containing flash tank for flash treatment, the gas discharged from the top is compressed and enters the heat exchanger together with the feed gas for recycling, and the sulfur-containing rich methanol discharged from the bottom enters the middle of the concentration tower;

[0050] 6) a desorption tower, which receives the sulfur-free rich methanol from the bottom of the sulfur-free flash tank at the upper part and desorbs CO2, and the CO2 discharged from the top of the desorption tower is discharged to a collection device after heat exchange through a heat exchanger, and the sulfur-free rich methanol and the sulfur-containing rich methanol are discharged from the middle and bottom of the desorption tower respectively;

[0051] 7) A concentration tower, which is used to further desorb carbon dioxide to relatively concentrate acidic gases such as hydrogen sulfide, feed sulfur-rich methanol from the bottom of the desorption tower to the lower part of the concentration tower, feed sulfur-rich methanol from the bottom of the sulfur-containing methanol flash tank from the middle part of the concentration tower, feed gas from the top of the first methanol-water flash tank from the middle part of the concentration tower, and feed sulfur-free rich methanol discharged from the middle of the desorption tower from the top plate of the concentration tower, feed nitrogen from the bottom plate of the concentration tower, and discharge a mixed gas containing a large amount of CO2, a small amount of H2S and N2 from the top of the concentration tower to the tail gas washing tower;

[0052] 8) Thermal regeneration tower, which further removes acidic gases such as H2S in methanol. The sulfur-rich methanol from the bottom of the concentration tower is fed to the middle of the thermal regeneration tower. The lean methanol vapor from the top of the methanol-water separation tower enters the lower part of the thermal regeneration tower after cooling. Acidic gases such as H2S are discharged from the top and enter the Claus recovery system. The lean methanol discharged from the bottom is divided into three parts, one part is used for spraying the raw gas, one part enters the absorption tower through the feed port located at the top tray, and the other part enters the methanol-water separation tower to continuously remove moisture from the system;

[0053] 9) A methanol-water separation tower, which is used to remove water from aqueous methanol, wherein feed ports are respectively arranged at the top tray and 3 to 8 trays below the top tray. When anhydrous methanol is introduced, anhydrous methanol is fed from the feed port at the top tray, and aqueous methanol from the bottom of the thermal regeneration tower is fed from the feed port at the 3 to 8 trays below the top tray. When anhydrous methanol is not introduced, aqueous methanol from the thermal regeneration tower is fed from the feed port at the top tray, and the feed port at the 3 to 8 trays below the top tray is closed; a methanol-rich feed port, which is used to feed the methanol from the first methanol The rich methanol at the bottom of the flash tank is arranged in the middle of the methanol water separation tower; and a tail gas washing water feed port is arranged in the middle of the methanol water separation tower, and the tail gas washing water from the second methanol water flash tank is received from the feed port; a methanol vapor outlet is arranged at the top of the methanol water separation tower, and the dehydrated methanol vapor is discharged from the outlet; and a waste water outlet is arranged at the bottom of the methanol water separation tower to discharge waste water, wherein the methanol content in the waste water is less than 0.01wt%; wherein the operating conditions of the methanol water separation tower are: 0.23-0.26MPaG, 100-140℃;

[0054] 10) A first methanol-water flash tank, which receives the rich methanol from the bottom of the raw gas separator and performs a flash operation to remove the gas dissolved therein, and feeds the rich methanol that has undergone the flash treatment to the middle of the methanol-water separation tower, and feeds the gas discharged from the top to the middle of the concentration tower;

[0055] 11) A tail gas washing tower, which is used to remove methanol from the tail gas, feeds gas from the top of the concentration tower from the feed port located at the bottom plate, feeds desalted water from the feed port located at the top plate, discharges tail gas washing water from the bottom, and discharges tail gas from the top,

[0056] 12) A second methanol flash tank is used for flashing the tail gas washing water from the tail gas washing tower to remove the dissolved gas therein. The tail gas washing water from the tail gas washing tower is received. After the flash treatment, the tail gas washing water at the bottom of the second methanol flash tank is fed into the methanol water separation tower through the tail gas washing water feed port of the methanol water separation tower. The gas discharged from the top enters the tail gas washing tower together with the gas from the top of the concentration tower.

[0057] In the present invention, a feed inlet for introducing anhydrous methanol is provided at the top tray of the methanol-water separation tower to prevent a vicious cycle caused by excessively high water content in the system.

[0058] Normally, if the water content in the system is not high, there is no need to introduce anhydrous methanol.

[0059] When the water content in the system is high, anhydrous methanol is introduced, the water content in the top of the methanol-water separation tower and the top gas phase in equilibrium with it will decrease, the water content brought into the system before the gas phase will decrease, and the water content of the lean methanol will decrease, effectively ensuring the safe operation of the device.

[0060] In the present invention, by introducing anhydrous methanol into the top of the methanol-water separation tower, the water content of the gas phase at the top of the methanol-water separation tower is reduced, and the amount of water returned to the system is reduced, especially when the water content of the system is high, the system water can be effectively discharged; in addition, the two aqueous solutions entering the methanol-water separation tower are first flashed to remove the dissolved gas, reducing the gas entering the methanol-water separation tower, reducing the amount of gas phase at the top of the methanol-water separation tower, and then reducing the amount of water returned to the system, while reducing the gas phase load of the methanol-water separation tower and increasing the stability of the operation. The long-term operation of the present application scheme ensures that the water content of the lean methanol entering the absorption tower is less than 0.5 mol%, and the water content of some devices can be less than 0.1 mol%. BRIEF DESCRIPTION OF THE DRAWINGS

[0061] Figure 1 A schematic diagram of a low-temperature methanol washing system in the prior art;

[0062] Figure 2 The schematic diagram of a low-temperature methanol washing system includes a system for reducing the methanol-water content of the low-temperature methanol system according to the present invention. DETAILED DESCRIPTION

[0063] The following will describe in detail the device and system for reducing the water content in methanol of a low temperature methanol system according to the present invention in conjunction with the accompanying drawings. It should be noted that the following embodiments are only used to describe the present invention, but not to limit the scope of the present invention.

[0064] It should be noted that the structures of the absorption tower, desorption tower, concentration tower, thermal regeneration tower, methanol-water separation tower and tail gas washing tower in the low-temperature methanol washing system described herein are well known to those skilled in the art and will not be described in detail. In addition, the low-temperature methanol washing system also includes conventional heat exchangers, valves, pumps and heaters, etc. The functions and effects of these components are well known to those skilled in the art. In order to save space, they are not shown in the drawings and will not be expanded in detail.

[0065] The terms "rich methanol" and "lean methanol" are used to distinguish the water content in methanol. "Rich methanol" refers to methanol with a higher water content, while "lean methanol" refers to methanol with a lower water content.

[0066] The term "top of a column" refers to the top of the corresponding column. The term "middle of a column" refers to the position of an intermediate tray among a plurality of trays in a column. The term "upper part of a column" refers to the position below the top tray and above the intermediate tray; the term "lower part of a column" refers to the position below the intermediate tray and above the bottom tray; the term "bottom of a column" refers to the very bottom of a column.

[0067] Example 1

[0068] The system for reducing the water content of low-temperature methanol of the present invention is a part of the low-temperature methanol washing system of raw coal gas.

[0069] The low temperature methanol washing system for raw coal gas includes:

[0070] 1) A heat exchanger is used to cool the raw coal gas. In the heat exchanger, the raw coal gas sprayed with a small amount of methanol is heat-exchanged with the purified gas from the top of the absorption tower and the CO2 product gas from the top of the desorption tower to cool the raw coal gas. The purified gas and CO2 product gas after the heat exchange are collected. The methanol is sprayed to prevent freezing.

[0071] 2) Raw gas liquid separator, which is used to separate rich methanol and gas. It feeds the raw gas cooled by the heat exchanger, discharges the raw gas from the top and discharges rich methanol from the bottom;

[0072] 3) An absorption tower, which uses methanol to absorb and purify acidic gases such as water, CO2 and H2S in the raw coal gas. The raw coal gas from the raw gas separator is fed from the lower part of the absorption tower, and the lean methanol from the bottom of the thermal regeneration tower is fed from the top plate of the absorption tower, so that the lean methanol is countercurrently contacted with the raw coal gas to absorb the water, CO2 and H2S in the raw coal gas. The purified gas is discharged to a collection device after heat exchange through a heat exchanger, and sulfur-free rich methanol is discharged from the middle of the absorption tower, and sulfur-containing rich methanol is discharged from the bottom of the absorption tower. The components of the raw coal gas are: H2: 50.8 mol%, CO2: 42.31 mol%, CO: 5.98 mol%, N2: 0.4 mol%, H2O: 0.22 mol%, H2S: 0.19 mol%, Ar: 0.07 mol%, CH4: 0.03 mol%.

[0073] 4) Sulfur-free methanol flash tank, the sulfur-free rich methanol from the middle of the absorption tower is fed into the sulfur-free methanol flash tank for flash treatment, the gas discharged from the top is compressed and enters the heat exchanger together with the raw coal gas for recycling, and the sulfur-free rich methanol is discharged from the bottom;

[0074] 5) Sulfur-containing methanol flash tank, the sulfur-containing rich methanol from the bottom of the absorption tower is fed into the sulfur-containing flash tank for flash treatment, the top exhaust gas is compressed and enters the heat exchanger together with the feed gas for recycling, and the sulfur-containing rich methanol discharged from the bottom enters the middle of the concentration tower;

[0075] 6) a desorption tower, which receives the sulfur-free rich methanol from the bottom of the sulfur-free flash tank at the upper part and desorbs CO2, and the CO2 discharged from the top of the desorption tower is discharged to a collection device after heat exchange through a heat exchanger, and the sulfur-free rich methanol and the sulfur-containing rich methanol are discharged from the middle and bottom of the desorption tower respectively;

[0076] 7) A concentration tower, which is used to further desorb carbon dioxide to relatively concentrate acidic gases such as hydrogen sulfide, feed sulfur-rich methanol from the bottom of the desorption tower to the lower part of the concentration tower, feed sulfur-rich methanol from the bottom of the sulfur-containing methanol flash tank from the middle part of the concentration tower, feed gas from the top of the first methanol-water flash tank from the middle part of the concentration tower, and feed sulfur-free rich methanol discharged from the middle of the desorption tower from the top plate of the concentration tower, feed nitrogen from the bottom plate of the concentration tower, and discharge a mixed gas containing a large amount of CO2, a small amount of H2S and N2 from the top of the concentration tower to the tail gas washing tower;

[0077] 8) Thermal regeneration tower, which further removes acidic gases such as H2S in methanol. The sulfur-rich methanol from the bottom of the concentration tower is fed to the middle of the thermal regeneration tower. The lean methanol vapor from the top of the methanol-water separation tower enters the lower part of the thermal regeneration tower after cooling. Acidic gases such as H2S are discharged from the top and enter the Claus recovery system. The lean methanol discharged from the bottom is divided into three parts, one part is used for spraying the raw gas, one part enters the absorption tower through the feed port located at the top tray, and the other part enters the methanol-water separation tower to continuously remove moisture from the system;

[0078] 9) A methanol water separation tower, which is used to remove water from methanol. The methanol water separation tower has 56 trays, and feed ports are respectively arranged at the 56th tray and the 52nd tray. When anhydrous methanol is introduced, anhydrous methanol is fed from the feed port located at the 56th tray, and water-containing methanol from the bottom of the thermal regeneration tower is fed from the feed port located at the 52nd tray. When anhydrous methanol is not introduced, water-containing methanol from the bottom of the thermal regeneration tower is fed from the feed port located at the 56th tray, and the feed port located at the 52nd tray is closed. A rich methanol is provided at the 28th tray of the methanol water separation tower. A methanol feed port is provided to feed the rich methanol from the bottom of the first methanol flash tank; and a tail gas washing water feed port is provided at the 28th layer of the tower plate, and the tail gas washing water from the second methanol water flash tank is fed from the feed port; a methanol vapor outlet is provided at the top of the methanol water separation tower, and the dehydrated methanol vapor is discharged from the outlet; and a waste water outlet is provided at the bottom of the methanol water separation tower to discharge waste water, wherein the methanol content in the waste water is less than 0.01wt%; wherein the operating conditions of the methanol water separation tower are: 0.23-0.26MPaG, 100-140℃;

[0079] 10) A first methanol-water flash tank receives the rich methanol from the bottom of the raw gas separator and performs a flash operation to remove the gas dissolved therein, and feeds the rich methanol that has undergone the flash treatment to the middle of the methanol-water separation tower, and feeds the gas discharged from the top to the middle of the concentration tower. The operating conditions of the first methanol-water flash tank are: 0.5 MPaG, 52°C;

[0080] 11) A tail gas washing tower, which is used to remove methanol from the tail gas, feeds gas from the top of the concentration tower from the feed port located at the bottom plate, feeds desalted water from the feed port located at the top plate, discharges tail gas washing water from the bottom, and discharges tail gas from the top,

[0081] 12) A second methanol flash tank is used for flashing the tail gas washing water from the tail gas washing tower to remove the dissolved gas therein. It receives the tail gas washing water from the tail gas washing tower. After flash treatment, the tail gas washing water at the bottom of the second methanol flash tank is fed into the methanol water separation tower through the tail gas washing water feed port of the methanol water separation tower. The gas discharged from the top enters the tail gas washing tower together with the gas from the top of the concentrator. The operating conditions of the second methanol water flash tank are: 0.45MPaG, 113°C.

[0082] The methanol-water separation tower of a low-temperature methanol washing device supporting a methanol unit with an annual output of 2 million tons has a water content of 0.815koml / h (water content of 0.22mol%) in the top gas phase of the methanol-water separation tower without adding anhydrous methanol, of which the water content of the rich methanol fed to the methanol-water separation tower through the first methanol-water flash tank is 25.19mol%. After running for 72 hours, the water content of the methanol fed to the absorption tower is 1.57mol%.

[0083] When the amount of anhydrous methanol added is 8t / h, the weight ratio of anhydrous methanol to the water-containing methanol from the bottom of the thermal regeneration tower is 1:1; the calculated value of water content in the gas phase at the top of the tower is 0.352koml / h (water content is 0.064mol%), of which the water content in the rich methanol fed to the methanol-water separation tower via the first methanol-water flash tank is 25.13mol%. After running for 72 hours, the water content in the methanol fed to the absorption tower is 0.47mol%.

[0084] Comparative Example 1

[0085] Except that the first methanol-water flash evaporator and the second methanol-water flash evaporator are not provided, and anhydrous methanol is not introduced, the operation is carried out in the same manner as in Example 1. Among them, the amount of water carried by the gas phase in the methanol-water separation tower is 1.368koml / h (water content is 3.72mol%). The water content is much higher than that in Example 1. The high water content affects the absorption effect of methanol on CO2 and H2S, and the purification index is not easy to guarantee. At the same time, the circulation volume increases, and consumption is increased, which is not conducive to the operation of the device; at the same time, the high water content aggravates the corrosion of the equipment and affects the service life of the device. The device that does not adopt the scheme of this application needs to replace a large amount of methanol to reduce the water content in the circulating methanol.

Claims

1. A method for reducing the water content of a low-temperature methanol washing system, comprising: The aqueous methanol obtained by washing with low-temperature methanol is distilled using a methanol-water separation tower to remove the water contained therein, methanol is discharged from the top of the methanol-water separation tower, and waste water is discharged from the bottom of the methanol-water separation tower; wherein, Feeding aqueous methanol that has passed through a methanol wash system for removing CO2 and acid gas from a feed inlet located at the top tray of a methanol-water separation tower; Using a first methanol-water flash tank to flash the rich methanol from the bottom of the raw gas separator to remove dissolved gas, discharging gas from the top, and the rich methanol discharged from the bottom enters the middle of the methanol-water separation tower; and The tail gas washing water is flashed using the second methanol-water flash tank to remove the dissolved gas therein, the gas discharged from the top enters the tail gas absorption tower, and the tail washing water discharged from the bottom enters the middle of the methanol-water separation tower. Anhydrous methanol is introduced into the methanol water separation tower. When the anhydrous methanol is introduced, the anhydrous methanol is fed into the methanol water separation tower from the feed port located at the top tower plate of the methanol water separation tower; the aqueous methanol that has passed through the methanol washing system for removing CO2 and acidic gas is fed into the methanol water separation tower from the feed port located 3 to 8 tower plates below the top tower plate.

2. The method for reducing the water content of a low-temperature methanol washing system according to claim 1, wherein: The aqueous methanol that passes through the methanol wash system for removing CO2 and acid gases is the methanol from the bottom of the thermal regeneration tower.

3. The method for reducing the water content of a low-temperature methanol washing system according to claim 1, wherein: The tail gas washing water is fed into the methanol water separation tower from the feed port located at 1 / 3 to 2 / 3 of the total number of plates.

4. The method for reducing the water content of a low-temperature methanol washing system according to claim 1, wherein: When the water content of the methanol vapor sent out from the top of the methanol-water separation tower exceeds 0.5 mol%, anhydrous methanol is introduced, and the weight ratio of the introduced methanol to the water-containing methanol passing through the methanol washing system for removing CO2 and acid gases is 1:2 to 1:

1.

5. The method for reducing the water content of a low-temperature methanol washing system according to claim 4, wherein: The methanol washing system is used to purify the raw coal gas. The aqueous methanol in the methanol washing system after removing CO2 and acidic gases is the aqueous methanol obtained after absorbing and purifying the impurities in the raw coal gas and removing acidic gases such as CO2 and H2S.

6. The method for reducing the water content of a low-temperature methanol washing system according to claim 5, wherein: The water content of the obtained aqueous methanol is 1 wt% to 2 wt%.

7. The method for reducing the water content of a low-temperature methanol washing system according to any one of claims 1 to 6, wherein: The water content of the methanol vapor discharged from the top of the methanol water separation tower is 0.1 mol% to 0.5 mol%, and the methanol content in the water discharged from the bottom of the methanol water separation tower is less than 0.01%wt.

8. The method for reducing the water content of a low-temperature methanol washing system according to any one of claims 1 to 6, wherein: The operating conditions of the methanol-water separation tower are: 0.23-0.26 MPaG, 100-140°C.

9. The method for reducing the water content of a low-temperature methanol washing system according to any one of claims 1 to 6, wherein: The working conditions of the first methanol-water flash tank are: operating pressure 0.4-0.8MPaG, temperature 30-70°C.

10. The method for reducing the water content of a low-temperature methanol washing system according to any one of claims 1 to 6, wherein: The working conditions of the second methanol-water flash tank are: operating pressure 0.4-0.8MPaG, temperature 70-120°C.

11. A system for reducing the water content of a low-temperature methanol washing system, comprising: A methanol-water separation tower, which is used to separate water and methanol, is provided with feed ports at the top plate and 3 to 8 plates below the top plate, respectively, for feeding anhydrous methanol or aqueous methanol obtained by low-temperature methanol washing; a rich methanol feed port, which is used to feed rich methanol from the bottom of the first methanol flash tank; a tail gas washing water feed port, which is used to feed tail gas washing water, and the feed port is located at 1 / 3 to 2 / 3 of the total number of plates of the methanol-water separation tower; a methanol vapor outlet, which is used to discharge dehydrated methanol vapor, and the methanol vapor outlet is located at the top of the methanol-water separation tower; and a wastewater outlet, which is used to discharge wastewater, and the wastewater outlet is located at the bottom of the methanol-water separation tower; A first methanol-water flash tank, which is used to remove gas from the rich methanol in the raw gas separator tank, and is connected to the bottom of the raw gas separator tank through a pipeline to feed the rich methanol from the raw gas separator tank, the top of the first methanol-water flash tank is connected to the middle of the concentration tower to feed the separated gas to the concentration tower, and the bottom of the first methanol flash tank is connected to the middle of the methanol-water separation tower through a pipeline to transport the rich methanol at the bottom of the first methanol flash tank to the methanol-water separation tower; and a second methanol water flash tank, which is used to remove gas from the tail gas washing water from the tail gas washing tower, and is connected to the bottom of the tail gas washing tower through a pipeline to feed the tail gas washing water from the tail gas washing tower, and the top of the second methanol water flash tank is connected to the bottom of the tail gas washing tower to circulate the separated gas to the tail gas washing tower, When anhydrous methanol is introduced, anhydrous methanol is fed from a feed port located at the top tray, and aqueous methanol obtained by low-temperature methanol washing is fed from a feed port located 3 to 8 trays below the top tray; When anhydrous methanol was not introduced, aqueous methanol was fed from the feed port located at the top tray, and the feed ports located at trays 3 to 8 below the top tray were closed.

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