Apparatus and method for promoting the recovery of effective gas in a synthesis gas purification process

Abstract

The present invention discloses an apparatus for recovering effective gas in a synthesis gas purification device. The apparatus comprises a flash tank, a pipeline connected to the flash tank for introducing a stream to be flashed, a pipeline for introducing stripping gas, and a pipeline for discharging circulating gas and wastewater, respectively, after flashing. The present invention also discloses a method for recovering effective gas in a synthesis gas purification process using the apparatus mentioned above, wherein the stripping gas is delivered to the flash tank during flashing. The stripping gas may be H2 obtained by pressure swing adsorption (PSA) having a purity of greater than 98 mol%, preferably greater than 99 mol%, more preferably greater than 99.8 mol%. Alternatively, the stripping gas may be purified synthesis gas containing greater than 80 mol% H2.

Description

Technology Field

[0001] The present invention relates to the field of gas purification, more particularly to the field of synthetic gas purification using rectisol. Background Technology

[0002] China, influenced by its energy structure, utilizes fossil fuel coal as its primary energy source. In the clean and efficient conversion and utilization of coal, the production of synthesis gas through coal gasification and the subsequent synthesis of ammonia, methanol, and other carbonyl compounds from the synthesis gas used as a feed gas are crucial linkages. Compared to synthesis gas produced via SMR (Steam Methane Reforming), crude synthesis gas produced through coal gasification contains large amounts of excess CO2 and small amounts of acid gases such as H2S and COS. These acid gases are detrimental to production, and among them, sulfides can cause catalyst poisoning during downstream production, so they must be removed and recovered. Lectizol, which uses cold methanol as an absorption solvent, utilizes methanol's excellent ability to dissolve acid gases at cryogenic temperatures to remove acid gases from the feed gas, and then desorbs the absorbed gas to recycle the methanol. Therefore, Lectizol is the most suitable solution for purifying crude synthesis gas obtained from coal gasification.

[0003] The effective gas in the crude synthesis gas contains CO and H2, and the ratio of these in the crude synthesis gas can be adjusted by a conversion reaction. After adjustment, the H2 content in the crude synthesis gas can be greater than 80%.

[0004] Conventional lectisol equipment includes an absorption column that scrubs crude synthesis gas using cryogenic methanol and absorbs impurities therein, a recovery device for recovering effective gas from the scrubbed cryogenic methanol, and a device for regenerating methanol. The literature (Ullmann's Encyclopedia of Industrial Chemistry, 6th Edition, Volume 15, on pages 399 et seq.) presents a lectisol method for purifying crude synthesis gas by scrubbing the gas with cryogenic methanol. Patent literature such as CN111246928B, CN1491882A, CN103845988A, and CN101812325 all describe methods and equipment for removing impurities or acid gases. Effective gas is generally recovered by sudden expansion (referred to as flash evaporation / flash). The recovered effective gas is recirculated back to the absorption column, and the unrecovered effective gas, especially CO, enters the subsequent stage along with flash drainage and can be released into the surrounding environment along with acid gas.

[0005] As higher requirements are imposed for environmental protection and energy conservation, and in particular, as the CO content released into the atmosphere is required to be less than 1000 ppm, higher requirements are imposed on the recovery of effective gas. The problem to be solved

[0006] Considering this, designing a new synthesis gas purification device suitable for lectizol to eliminate the aforementioned defects and flaws of the prior art becomes an urgent task for industry engineers. means of solving the problem

[0007] Summary of the Invention

[0008] To improve the recovery efficiency of effective gas, particularly CO, and to reduce the CO content in flash waste and system tail gas, the present invention discloses an apparatus for recovering effective gas in a synthesis gas purification device. The apparatus comprises a flash tank, a pipeline connected to the flash tank for feeding a stream to be flashed, a pipeline for feeding stripping gas, and a pipeline for discharging circulating gas and waste after flashing, respectively. In addition to the apparatus for recovering effective gas, the synthesis gas purification device further comprises at least an absorption column and a solvent regeneration device. For example, the pipeline for feeding stripping gas is connected to the outlet of the purified synthesis gas of the absorption column, wherein the purified synthesis gas contains more than 80 mol%, preferably more than 85 mol%, of H2. Alternatively, the pipeline for stripping gas may also be connected to an H2 source. The H2 source comprises a pressure swing adsorption (PSA) device, and the H2 has a purity of more than 98 mol%, preferably more than 99 mol%, more preferably more than 99.8 mol%.

[0009] In one aspect, an apparatus for recovering effective gas comprises a high-pressure flash tank and a low-pressure flash tank having different flash pressures, wherein the stream to be flashed comprises CO2-containing methanol and sulfur-containing methanol from an absorption column, and optionally comprises the wastewater after the high-pressure flash.

[0010] For example, a flash tank for flashing CO2-containing methanol includes a first high-pressure flash tank and a first medium-pressure flash tank, wherein the first high-pressure flash tank is connected to a pipeline for introducing CO2-containing methanol, a first high-pressure circulating gas, and a pipeline for discharging the first high-pressure waste, and the first medium-pressure flash tank is connected to a pipeline for introducing the first high-pressure waste, a pipeline for introducing stripping gas, and a pipeline for discharging the first medium-pressure circulating gas and the first medium-pressure waste.

[0011] As another example, a flash tank for flashing sulfur-containing methanol comprises a second high-pressure flash tank and a second medium-pressure flash tank, wherein the second high-pressure flash tank is connected to a pipeline for introducing sulfur-containing methanol, a pipeline for discharging a second high-pressure circulating gas and a second high-pressure waste, and the second medium-pressure flash tank is connected to a pipeline for introducing a second high-pressure waste, a pipeline for introducing stripping gas, and a pipeline for discharging a second medium-pressure circulating gas and a second medium-pressure waste. A device for recovering effective gas further comprises a pipeline for transferring scrubbing methanol and a first medium-pressure circulating gas to the second medium-pressure flash tank, respectively, wherein the scrubbing methanol enters the second medium-pressure flash tank at a position higher than the first medium-pressure circulating gas.

[0012] In another aspect, the present invention also discloses a method for recovering effective gas in a synthesis gas purification process applicable to the above-mentioned apparatus for recovering effective gas, wherein the method is characterized in that stripping gas is delivered to a flash tank during flashing. The effective gas contains CO and H2, the stripping gas contains H2 or purified synthesis gas, and the purified synthesis gas contains more than 80%, preferably more than 85%, of H2. The ratio of the amount of stripping gas (kmol / h) entering the flash tank to the amount of effective gas in the stream to be flashed is 35%-50%.

[0013] The circulating gas generated from flash evaporation is discharged from the flash tank, pressurized, and delivered to the inlet of the synthesis gas purification device.

[0014] In another aspect, in a method for recovering effective gas, a CO2 separation device is additionally provided, and wastewater after flash is discharged from a flash evaporation device and processed by a CO2 separation device to produce a CO2 stream, wherein the CO2 stream contains less than 1000 ppm of CO. Effects of the invention

[0015] Compared to the prior art, the technical solution of the present invention has the following advantages:

[0016] 1. It is highly suitable for lectizol technology, and the technical effect of the present invention can be achieved simply by introducing stripping gas into a conventional flash tank, and is characterized by simple modification of the device and low cost.

[0017] 2. In the prior art, generally, more CO and H2 are recovered by reducing the pressure of the flash tank, but if the flash pressure is reduced, more acidic gases, such as CO2, will become gas. To prevent additional CO2 from circulating into the purification system along with the effective gas during flash evaporation, more cryogenic methanol must be used in the flash tank to scrub and remove some of this CO2, which will increase the amount of methanol required by the entire purification device and the amount of energy consumed for cooling the methanol. However, the method of the present invention will not increase the amount of CO2 during flash evaporation and can avoid the above disadvantages.

[0018] 3. Purified synthesis gas can be used as stripping gas, which is convenient and cost-effective. Brief explanation of the drawing

[0019] The advantages and purpose of the present invention may be further understood through the following detailed description and the accompanying drawings. It will be recognized by those skilled in the art that the drawings and embodiments do not limit the present invention. Figure 1 is a partial schematic diagram of the synthesis gas purification device of Comparative Example 1. FIG. 2 is a partial schematic diagram of the synthesis gas purification device of Embodiment 1. FIG. 3 is a partial schematic diagram of a synthesis gas purification device of Embodiment 2. List of reference symbols: A-Cast Syngas Absorption Column; B-Flash Column; C-Lean Methanol Pump; D-Main Scrubbing Methanol Pump; E1-First Water Cooler; E2-Second Water Cooler; K1-First Circulating Gas Compressor; K2-Second Circulating Gas Compressor; V1-First Gas-Liquid Separation Tank; V2-Second Gas-Liquid Separation Tank; Section I-First Medium Pressure Flash Tank; Section II-Second Medium Pressure Flash Tank; Section III-Second High Pressure Flash Tank; Section IV-First High Pressure Flash Tank; 1-Cast Syngas; 2-Purified Syngas; 3, 3'-First Part of Stripping Gas; 4, 4'-Second Part of Stripping Gas; 5-Pre-Scrubbing Methanol; 6-CO2-Containing Methanol; 7-Sulfur-Containing Methanol; 8-Lean Methanol; 9-Main Scrubbing Methanol; 10-Second High Pressure Circulating Gas; 11-1st High-Pressure Circulating Gas; 12-1st Medium-Pressure Circulating Gas; 13-2nd Medium-Pressure Circulating Gas; 14-1st High-Pressure Drain; 15-2nd High-Pressure Drain; 16-1st Medium-Pressure Drain; 17-2nd Medium-Pressure Drain; 18-Scrubbing Methanol Specific details for implementing the invention

[0020] Detailed description of the embodiment

[0021] The technical solution of the present application will be clearly and completely described below with reference to the accompanying drawings to clearly illustrate the purpose, technical solution, and advantages of the present application. It is evident that the embodiments described herein are not all but some of the embodiments of the present application. Based on the embodiments of the present application, all other embodiments obtained by a person skilled in the art without creative effort will fall within the scope of protection of the present application.

[0022] It should be noted that in the description of this application, the terms “installation” and “connection” are to be understood broadly unless otherwise specified or defined. “Fixedly connected,” “fixed to,” or “immovably connected” means that the connection between two or more structural members is configured so as not to provide relative movement. Examples of fixed connections include welded connections, flanged connections, or bolted connections.

[0023] Furthermore, in this application, quantifiers similar to "a" are not intended to limit quantity but to describe technical features that did not appear in the prior text. Similarly, unless a word is a noun modified by a specific quantifier, it should be considered to include both singular and plural forms, and the technical solution may include both singular and plural technical features.

[0024] In this application, "at least one" is understood to mean one or more, and "many" is understood to mean two or more. "And / or" is used to describe the relationship between related objects, indicating three possible relationships. For example, "A and / or B" includes the following three cases: A only, B only, and both A and B, where A and B may be singular or plural. The character " / " generally implies that the related objects have an OR relationship. "At least one of the following" or a similar expression refers to any combination of item(s) including any combination of singular or plural forms. For example, at least one of a, b, or c means a, b, c, a and b, a and c, b and c, or a and b and c, where a, b, and c may be singular or plural.

[0025] The terms "top," "bottom," "upper," and "lower" refer to the orientation of the device in the operating state.

[0026] Synthesis gas contains CO and H2 and is generally produced through coal gasification or SMR (steam methane reforming) reactions. The ratio of CO to H2 in the produced crude synthesis gas can be adjusted through conversion reactions. Crude synthesis gas contains some impurities. Taking synthesis gas obtained through coal gasification as an example, it contains impurities such as CO2, H2S, COS, NH3, HCN, and heavy hydrocarbons. Due to its remarkable ability to remove sulfur, CO2, and related trace components, lectizol is a preferred purification process. The process generally comprises the following steps: 1) scrubbing the crude synthesis gas with cryogenic methanol in an absorption column; 2) recovering the effective gas (i.e., CO and H2) dissolved in the methanol during the scrubbing process in a device for recovering the effective gas; and 3) regenerating, cooling, and recirculating the methanol in a solvent regeneration device.

[0027] In the present invention, the circulating gas contains an effective gas obtained from a flash tank through flash evaporation. After the liquid within it is separated, the circulating gas is pressurized, cooled, and combined with crude synthesis gas, and then re-enters a synthesis gas purification device.

[0028] In this document, "column" refers to a hollow structure that enables complete contact and mass transfer between a gas and a liquid, and packing may be installed to facilitate gas-liquid contact as needed. The column is fluidly connected through a pipeline. Taking an absorption column as an example, it is erected vertically to the ground, and the sealed column shell accommodates three parts: a pre-scrubbing section closest to the ground, a desulfurization section positioned above the pre-scrubbing section, and a CO2 removal section positioned above the desulfurization section. Each section is equipped with packing to increase gas-liquid contact. The pre-scrubbing section primarily removes heavy hydrocarbons, NH3, HCN, etc., from crude synthesis gas to produce pre-scrubbing methanol. The desulfurization section primarily removes COS, H2S, etc., to produce sulfur-containing methanol. The CO2 removal section primarily removes CO2 to produce CO2-containing methanol. CO2-containing methanol and sulfur-containing methanol are sent to their respective flash tanks for flash evaporation to recover the effective gas inside.

[0029] CO2-containing methanol released from the CO2 removal section is sent to a flash column to recover the effective gas. The gas obtained from flash evaporation is used as part of the circulating gas, pressurized, and combined with crude synthesis gas, then fed back into the absorption column for purification. The first portion of the liquid obtained from flash evaporation enters the CO2 product gas tank and, after gas-liquid separation, generates the first CO2 stream, while the second portion of the liquid obtained from flash evaporation is further flashed to remove the CO2 within it and produce main scrubbing methanol. One portion of the main scrubbing methanol is sent to a reabsorption column for regeneration, and the other portion is sent to the absorption column as a scrubbing liquid.

[0030] The sulfur-containing methanol discharged from the desulfurization section is sent to a flash column to recover the effective gas. The gas obtained from flash evaporation is used as part of the circulating gas, pressurized and combined with crude synthesis gas, and then fed back into the absorption column for purification. The liquid obtained from flash evaporation enters a CO2 separation column to generate a second CO2 stream, and the bottom liquid then enters a reabsorption column. In the reabsorption column, CO2 is further removed through N2 stripping, and the discharged gas is a tail gas containing CO2. Most of the bottom liquid from the reabsorption column is sent to a thermal regeneration column, and a small stream from it is sent to the flash column as scrubbing methanol, which is used to remove CO2 from the gas obtained from flash evaporation.

[0031] The CO2 product gas tank and the CO2 separation column are both part of the CO2 separation device.

[0032] The pre-scrubbing methanol discharged from the pre-scrubbing section is sent to a pre-scrubbing flash tank to recover the effective gas within it. The gas obtained from flash evaporation is pressurized as part of the circulating gas and subsequently combined with the crude synthesis gas, and the resulting liquid is sent to a thermal regeneration column. All methanol liquid combined in the thermal regeneration column becomes dilute methanol after regeneration and cooling. The first CO2 stream and the second CO2 stream are combined to form a total CO2 stream. In some technical processes, the total CO2 stream and CO2 tail gas are discharged after being combined, and the CO content therein must meet the national standard, namely less than 1000 ppm.

[0033] Flash evaporation is a process of converting a liquid substance into a gaseous substance by rapidly evaporating it through rapid depressurization, utilizing the difference in boiling points of the substances under different pressures. If the liquid to be flashed is a mixture of components, substances with lower boiling points and higher volatility are primarily in the gaseous phase after flash evaporation. The flash tank serves to provide space for the rapid gasification of the fluid and gas-liquid separation, and packing or trays that aid in gas-liquid separation may be installed in the tank. Multiple flash tanks can be stacked together to form a flash column. Single-stage flash or multi-stage flash may be used for any multi-component liquid herein. Single-stage flash means that the multi-component liquid undergoes only a single rapid depressurization and requires only one flash tank. Multi-stage flash means that the multi-component liquid undergoes multiple rapid depressurizations at different end pressures, which will be realized in multiple flash tanks. In this invention, high-pressure flash and medium-pressure flash represent the relative relationship between flashes in two stages. The pressure in the high-pressure flash tank is higher than the pressure in the medium-pressure flash tank, but both are lower than the initial pressure of the multi-component liquid, i.e., the pressure in the absorption column. Optionally, the pressure range in the absorption column is 40-60 barg, the pressure range in the high-pressure flash tank is 30-40 barg, and the pressure range in the medium-pressure flash tank is 10-20 barg.

[0034] FIG. 1 is a schematic diagram of Comparative Example 1. A is a crude synthesis gas absorption column, and B is a flash column. A reabsorption column, a thermal regeneration column, a methanol-water separation column, etc., for desorbing and regenerating methanol after absorption are not illustrated as they are conventional techniques well known to those skilled in the art. The absorption column (A) includes a lower pre-scrubbing section, an intermediate desulfurization section, and an upper CO2 removal section. Cryogenic scrubbing methanol introduced from outside the column contains lean methanol (8) and main scrubbing methanol (9). The lean methanol is methanol that has been regenerated by the thermal regeneration column and then cooled, which contains no impurities such as CO2 and sulfides, and has a water content of less than 1%. The main scrubbing methanol comes from the reabsorption column, contains no impurities such as sulfides, and has a CO2 content of about 10%-15%. The input to the pre-scrubbing section of the absorption column is crude synthesis gas (1), preferably converted crude synthesis gas, where the molar content of H2 is greater than 80%. After scrubbing, pre-scrubbing methanol (5) is discharged from the bottom of the pre-scrubbing section, which contains impurities such as heavy hydrocarbons, NH3, and HCN. Sulfur-containing methanol (7) is discharged from the desulfurization section, which contains impurities such as COS and H2S. CO2-containing methanol (6) is discharged from the CO2 removal section, which is CO2-rich and contains, for example, 20%-50% CO2. Purified synthesis gas (2) is discharged from the top of the absorption column (A), where the ratio of CO to H2 is approximately the same as that of the crude synthesis gas. The operating pressure of the crude synthesis gas absorption column (A) is about 40 barg (gauge pressure).

[0035] Sulfur-containing methanol and CO2-containing methanol still contain some CO and H2 (e.g., 0.5%–1%), and some of this effective gas needs to be recovered through flash evaporation. Flash evaporation is implemented in a flash tank at a pressure lower than the pressure of the liquid to be flashed, preferably the pressure in the flash tank is approximately half the pressure of the liquid to be flashed. Flash evaporation can be completed in a single or multiple flashes. In this embodiment, a multi-stage flash is adopted. Sulfur-containing methanol and / or CO2-containing methanol discharged from the absorption column (A) is first flashed in a high-pressure flash tank at a pressure of about 20 barg, and the resulting flash gas is separated, pressurized, and cooled as a circulating gas, and then combined with the crude synthesis gas. The liquid obtained from the flash evaporation is sent to a medium-pressure flash tank at a pressure of about 9.3 barg for flashing again. After CO2 is removed from the flash gas obtained by scrubbing methanol, the remaining effective gas is separated, pressurized, and cooled as a circulating gas, and then combined with the crude synthesis gas. Upon further separation of the liquid obtained from flash evaporation, a CO2 stream containing more than 98% CO2 is obtained. Specifically, CO2-containing methanol (6) and sulfur-containing methanol (7) are depressurized to the pressure of the high-pressure flash tank by a pressure reducing valve and then injected from the top into the first high-pressure flash tank (Section IV) and the second high-pressure flash tank (Section III). In the first high-pressure flash tank (Section IV), the lighter-component gas having a lower boiling point obtained from the flash evaporation of CO2-containing methanol (6), whose main components are H2 and CO, is discharged from the top outlet as the first high-pressure circulating gas (11), and the remaining liquid after flash is discharged from the bottom of the tank as the first high-pressure drain (14).In the second high-pressure flash tank (Section III), a lighter-component gas with a lower boiling point obtained from the flash evaporation of sulfur-containing methanol (7), whose main components are H2 and CO, is discharged from the top outlet as the second high-pressure circulating gas (10), and the remaining liquid after flash is discharged from the bottom of the tank as the second high-pressure drain (15). The first high-pressure circulating gas (11) and the second high-pressure circulating gas (10) are combined and supplied to the second gas-liquid separation tank (V2), and the gas obtained after separation consists mainly of H2, CO and CO2. This is compressed to about 40 barg in the second circulating gas compressor (K2), cooled to 40-42°C by the second water cooler (E2), combined with the crude synthesis gas (1) at the front end of the absorption column (A), and then re-entered the absorption column (A) for purification.

[0036] The first high-pressure drain (14) is depressurized in a pipeline by a depressurization device, such as a depressurization valve, to the pressure of the first medium-pressure flash tank (Section I), and then injected from above into the first medium-pressure flash tank (Section I). In the first medium-pressure flash tank (Section I), a light-component gas with a lower boiling point obtained from the flash evaporation of the first high-pressure drain (14) consists mainly of H2 and CO and is discharged from the top outlet as the first medium-pressure circulating gas (12), and the remaining liquid after flash is discharged from the bottom of the tank as the first medium-pressure drain (16). The second high-pressure drain (15) is depressurized in a pipeline by a depressurization device, such as a depressurization valve, to the pressure of the second medium-pressure flash tank (Section II), and then injected from above into the second medium-pressure flash tank (Section II). Above the inlet of the stream, the first medium-pressure circulating gas (12) and scrubbing methanol (18) are introduced into the second medium-pressure flash tank from bottom to top, respectively. The second high-pressure drain (15) is flashed in the second medium-pressure flash tank (Section II), and the gas obtained from the flash evaporation consists mainly of H2 and CO, as well as a small amount of CO2. Scrubbing methanol (18) is optionally obtained from cryogenic sulfur-containing methanol at the bottom of the reabsorption column. Since this does not contain CO2, it can help to further scrub and remove CO2 from the gas obtained from the flash evaporation and the first medium-pressure circulating gas (12). The scrubbed flash gas is discharged from the top outlet as the second medium-pressure circulating gas (13). The remaining liquid after flashing exits from the bottom of the tank as the second medium-pressure drain (17). The first medium-pressure flash tank (Section I), the second medium-pressure flash tank (Section II), the second high-pressure flash tank (Section III), and the first high-pressure flash tank (Section IV) are assembled from top to bottom to form a flash column.

[0037] The second medium-pressure circulating gas (13) enters the first gas-liquid separation tank (V1), and the gas obtained after separation consists mainly of H2, CO, and trace amounts of CO2. This is compressed to the pressure of the high-pressure flash tank, i.e., about 40 barg, in the first circulating gas compressor (K1), cooled to 40-42°C by the first water cooler (E1), and then supplied to the second gas-liquid separation tank (V2), where it is mixed with the first and second high-pressure circulating gases. After being compressed and cooled through the above-mentioned process, the separated gas is combined with the crude synthesis gas (1) at the front end of the absorption column (A) and then re-enters the absorption column (A) for purification.

[0038] The first medium-pressure drain (16) and the second medium-pressure drain (17) are both rich in CO2, and streams containing more than 98% CO2 are obtained through further separation. Whether it is used as a CO2 product or released as a tail gas, the CO content therein must be below the national standard, generally less than 1000 ppm. In this comparative example, to reduce the CO content in the drain, the pressure of the medium-pressure flash tank is reduced so that more CO2 can enter the gas obtained from flash evaporation along with CO. To maintain the CO2 content in the circulating gas at less than 50%, the CO2 in the flash gas is scrubbed and removed using scrubbing methanol.

[0039] FIG. 2 is a schematic diagram of Embodiment 1. A is a crude synthesis gas absorption column, and B is a flash column. A reabsorption column, a thermal regeneration column, a methanol-water separation column, etc., for desorbing and regenerating methanol after absorption are not illustrated as they are conventional technologies well known to those skilled in the art. The absorption column (A) includes a lower pre-scrubbing section, an intermediate desulfurization section, and an upper CO2 removal section. Cryogenic scrubbing methanol introduced from outside the column includes lean methanol (8) and main scrubbing methanol (9). The lean methanol is methanol regenerated by the thermal regeneration column, which does not contain impurities such as CO2 and sulfides, and has a water content of less than 1%. The main scrubbing methanol comes from the reabsorption column, does not contain impurities such as sulfides, and has a CO2 content of about 10%-15%. The input to the pre-scrubbing section of the absorption column is crude synthesis gas (1), preferably converted crude synthesis gas, where the molar content of H2 is greater than 80%, preferably greater than 85%. After scrubbing, pre-scrubbing methanol (5) is discharged from the bottom of the pre-scrubbing section, which contains impurities such as heavy hydrocarbons, NH3, and HCN. Sulfur-containing methanol (7) is discharged from the desulfurization section, which contains impurities such as COS and H2S. CO2-containing methanol (6) is discharged from the CO2 removal section, which is CO2-rich and contains, for example, 20%–50% CO2. Purified synthesis gas (2) is discharged from the top of the absorption column (A), where the ratio of CO to H2 is approximately the same as that of the crude synthesis gas. The operating pressure of the crude synthesis gas absorption column (A) is about 40 barg (gauge pressure).

[0040] Sulfur-containing methanol and CO2-containing methanol still contain some CO and H2 (e.g., 0.5%–1%), and some of this effective gas needs to be recovered through flash evaporation. Flash evaporation is implemented in a flash tank at a pressure lower than the pressure of the liquid to be flashed, preferably the pressure in the flash tank is approximately half the pressure of the liquid to be flashed. Flash evaporation can be completed in a single or multiple flashes. In this embodiment, a multi-stage flash is adopted. Sulfur-containing methanol and / or CO2-containing methanol discharged from the absorption column (A) is first flashed in a high-pressure flash tank at a pressure of about 20 barg, and the resulting flash gas is separated, pressurized, and cooled as a circulating gas, and then combined with the crude synthesis gas. The liquid obtained from the flash evaporation is sent to a medium-pressure flash tank at a pressure of about 11.1 barg for flashing again. After CO2 is removed from the flash gas obtained by scrubbing methanol, the remaining effective gas is separated, pressurized, and cooled as a circulating gas, and then combined with the crude synthesis gas. Upon further separation of the liquid obtained from flash evaporation, a CO2 stream containing more than 98% CO2 is obtained. Specifically, CO2-containing methanol (6) and sulfur-containing methanol (7) are depressurized to the pressure of the high-pressure flash tank by a pressure reducing valve and then injected from the top into the first high-pressure flash tank (Section IV) and the second high-pressure flash tank (Section III). In the first high-pressure flash tank (Section IV), the lighter-component gas having a lower boiling point obtained from the flash evaporation of CO2-containing methanol (6), whose main components are H2 and CO, is discharged from the top outlet as the first high-pressure circulating gas (11), and the remaining liquid after flash is discharged from the bottom of the tank as the first high-pressure drain (14).In the second high-pressure flash tank (Section III), the lighter-component gas with a lower boiling point obtained from the flash evaporation of sulfur-containing methanol (7) is mainly composed of H2 and CO and is discharged from the top outlet as the second high-pressure circulating gas (10), and the remaining liquid after flash is discharged from the bottom of the tank as the second high-pressure drain (15). The first high-pressure circulating gas (11) and the second high-pressure circulating gas (10) are combined and supplied to the second gas-liquid separation tank (V2), and the gas obtained after separation is mainly composed of H2, CO and CO2. This is compressed to about 40 barg in the second circulating gas compressor (K2), cooled to 40-42°C by the second water cooler (E2), combined with the crude synthesis gas (1) at the front end of the absorption column (A), and then re-entered the absorption column (A) for purification.

[0041] The first high-pressure drain (14) is depressurized in the pipeline by a depressurization device, such as a depressurization valve, to the pressure of the first medium-pressure flash tank (Section I), and then injected from above into the first medium-pressure flash tank (Section I). The first portion (3) of the stripping gas is introduced below the stream inlet and over the packing or tray of the tank. In the first medium-pressure flash tank (Section I), the light-component gas having a lower boiling point obtained from the flash evaporation and stripping of the first high-pressure drain (14) consists mainly of H2 and CO and is discharged from the top outlet as the first medium-pressure circulating gas (12), and the remaining liquid after flash is discharged from the bottom of the tank as the first medium-pressure drain (16). The second high-pressure drain (15) is depressurized in the pipeline by a depressurization device, such as a depressurization valve, to the pressure of the second medium-pressure flash tank (Section II), and then injected from above into the second medium-pressure flash tank (Section II). Above the inlet of the stream, the second portion of the stripping gas (4), the first medium-pressure circulating gas (12), and the scrubbing methanol (18) are each introduced from bottom to top into the second medium-pressure flash tank. The second high-pressure drain (15) is flashed in the second medium-pressure flash tank (Section II), and the lighter-component gas with a lower boiling point obtained from flash evaporation and stripping, which consists mainly of H2 and CO, is discharged from the top outlet as the second medium-pressure circulating gas (13), and the remaining liquid after flash is discharged from the bottom of the tank as the second medium-pressure drain (17). The scrubbing methanol (18) is optionally obtained from sulfur-containing methanol at the bottom of the reabsorption column. Since it does not contain CO2, it can help to further remove CO2 from the flash gas and the first medium-pressure circulating gas (12). The first part (3) and the second part (4) of the stripping gas are both obtained from purified synthesis gas (2), for example, synthesis gas containing about 85.8% H2 and about 11.1% CO.Stripping gas is introduced into each medium-pressure flash tank at a similar flow rate, which is approximately 35%–50% of the flow rate of the effective gas in the stream to be flashed. The first medium-pressure flash tank (Section I), the second medium-pressure flash tank (Section II), the second high-pressure flash tank (Section III), and the first high-pressure flash tank (Section IV) are assembled from top to bottom to form a flash column.

[0042] The second medium-pressure circulating gas (13) enters the first gas-liquid separation tank (V1), and the gas obtained after separation consists mainly of H2, CO, and trace amounts of CO2. This is compressed to the pressure of the high-pressure flash tank, i.e., about 40 barg, in the first circulating gas compressor (K1), cooled to 40-42°C by the first water cooler (E1), and then supplied to the second gas-liquid separation tank (V2), where it is mixed with the first and second high-pressure circulating gases. After being compressed and cooled through the above-mentioned process, the separated gas is combined with the crude synthesis gas (1) at the front end of the absorption column (A) and then re-enters the absorption column (A) for purification.

[0043] The first medium-pressure drain (16) and the second medium-pressure drain (17) are both rich in CO2, and streams containing more than 98% CO2 are obtained through further separation. Whether it is used as a CO2 product or released as a tail gas, the CO content therein must be below the national standard, generally less than 1000 ppm. In this embodiment, since the synthesis gas is used as a stripping gas in the medium-pressure flash tank, CO can be changed from liquid to gas under the higher pressure of the medium-pressure flash tank, which satisfies the requirement for a low CO content in the flashed drain. Compared to Comparative Example 1, the flash tank has a higher pressure, less CO2 enters the gas obtained from the flash evaporation, and the amount of scrubbing methanol for scrubbing and removing additional CO2 from the flash gas is correspondingly reduced. That is, while ensuring a reduction in CO content in the wastewater, Embodiment 1 uses less scrubbing methanol to ensure that the CO2 content in the circulating gas is lower than 50%, thereby reducing the energy consumption of the entire purification device. This technical effect can also be achieved with stripping gas similarly introduced into the high-pressure flash tank, for example, when a single-stage flash process is adopted, even if only a high-pressure flash tank is present.

[0044] FIG. 3 is a schematic diagram of Embodiment 2. A is a crude synthesis gas absorption column, and B is a flash column. A reabsorption column, a thermal regeneration column, a methanol-water separation column, etc., for desorbing and regenerating methanol after absorption are not illustrated as they are conventional technologies well known to those skilled in the art. The absorption column (A) includes a lower pre-scrubbing section, an intermediate desulfurization section, and an upper CO2 removal section. Cryogenic scrubbing methanol introduced from outside the column includes lean methanol (8) and main scrubbing methanol (9). The lean methanol is methanol regenerated by the thermal regeneration column, which does not contain impurities such as CO2 and sulfides, and has a water content of less than 1%. The main scrubbing methanol comes from the reabsorption column, does not contain impurities such as sulfides, and has a CO2 content of about 10%-15%. The input to the pre-scrubbing section of the absorption column is crude synthesis gas (1). After scrubbing, pre-scrubbed methanol (5) is discharged from the bottom of the pre-scrubbing section, which contains impurities such as heavy hydrocarbons, NH3, and HCN. Sulfur-containing methanol (7) is discharged from the desulfurization section, which contains impurities such as COS and H2S. CO2-containing methanol (6) is discharged from the CO2 removal section, which is CO2-rich and contains, for example, 20%-50% CO2. The purified synthesis gas (2) is discharged from the top of the absorption column (A). The operating pressure of the crude synthesis gas absorption column (A) is about 40 barg (gauge pressure).

[0045] Sulfur-containing methanol and CO2-containing methanol still contain some CO and H2 (e.g., 0.5%–1%), and some of this effective gas needs to be recovered through flash evaporation. Flash evaporation is implemented in a flash tank at a pressure lower than the pressure of the liquid to be flashed, preferably the pressure in the flash tank is approximately half the pressure of the liquid to be flashed. Flash evaporation can be completed in a single flash or multiple flashes. In this embodiment, a multi-stage flash is adopted. Sulfur-containing methanol and / or CO2-containing methanol discharged from the absorption column (A) is first flashed in a high-pressure flash tank at a pressure of about 20 barg, and the resulting flash gas is separated, pressurized, and cooled as a circulating gas, and then combined with the crude synthesis gas. The liquid obtained from the flash evaporation is sent to a medium-pressure flash tank at a pressure of about 12.5 barg for flashing again, and the resulting flash gas is separated, pressurized, and cooled as a circulating gas, and then combined with the crude synthesis gas. The liquid obtained from flash evaporation is further separated by a CO2 separation device, after which a CO2 stream containing more than 98% CO2 is obtained. Specifically, CO2-containing methanol (6) and sulfur-containing methanol (7) are depressurized to the pressure of a high-pressure flash tank by a pressure reducing valve and then injected from the top into a first high-pressure flash tank (Section IV) and a second high-pressure flash tank (Section III). In the first high-pressure flash tank (Section IV), a lighter-component gas having a lower boiling point obtained from the flash evaporation of CO2-containing methanol (6), whose main components are H2 and CO, is discharged from the top outlet as a first high-pressure circulating gas (11), and the remaining liquid after flash is discharged from the bottom of the tank as a first high-pressure drain (14).In the second high-pressure flash tank (Section III), the lighter-component gas with a lower boiling point obtained from the flash evaporation of sulfur-containing methanol (7) is mainly composed of H2 and CO and is discharged from the top outlet as the second high-pressure circulating gas (10), and the remaining liquid after flash is discharged from the bottom of the tank as the second high-pressure drain (15). The first high-pressure circulating gas (11) and the second high-pressure circulating gas (10) are combined and supplied to the second gas-liquid separation tank (V2), and the gas obtained after separation is mainly composed of H2, CO and CO2. This is compressed to about 40 barg in the second circulating gas compressor (K2), cooled to 40-42°C by the second water cooler (E2), combined with the crude synthesis gas (1) at the front end of the absorption column (A), and then re-entered the absorption column (A) for purification.

[0046] The first high-pressure drain (14) is depressurized in the pipeline by a depressurization device, such as a depressurization valve, to the pressure of the first medium-pressure flash tank (Section I), and then injected into the first medium-pressure flash tank (Section I) from above. The first portion (3') of the stripping gas is introduced below the stream inlet and over the packing or tray of the tank. In the first medium-pressure flash tank (Section I), the light-component gas having a lower boiling point obtained from the flash evaporation and stripping of the first high-pressure drain (14) consists mainly of H2 and CO and is discharged from the top outlet as the first medium-pressure circulating gas (12), and the remaining liquid after flash is discharged from the bottom of the tank as the first medium-pressure drain (16). The second high-pressure drain (15) is depressurized in the pipeline by a depressurization device, such as a depressurization valve, to the pressure of the second medium-pressure flash tank (Section II), and then injected into the second medium-pressure flash tank (Section II) from above. Above the inlet of the stream, the second portion (4') of the stripping gas, the first medium-pressure circulating gas (12), and the scrubbing methanol (18) are each introduced from bottom to top into the second medium-pressure flash tank. The second high-pressure drain (15) is flashed in the second medium-pressure flash tank (Section II), and the lighter-component gas with a lower boiling point obtained from flash evaporation and stripping, which consists mainly of H2 and CO, is discharged from the top outlet as the second medium-pressure circulating gas (13), and the remaining liquid after flash is discharged from the bottom of the tank as the second medium-pressure drain (17). The scrubbing methanol (18) is optionally obtained from sulfur-containing methanol at the bottom of the reabsorption column. Since it does not contain CO2, it can help to further remove CO2 from the flash gas and the first medium-pressure circulating gas (12).The first portion (3') and the second portion (4') of the stripping gas are both H2, which can be provided by an H2 source, for example, a pipeline network; or can be obtained through a pressure swing adsorption (PSA) device with a purity of more than 98 mol%, preferably more than 99 mol%, more preferably more than 99.8 mol%.

[0047] The amount of stripping gas introduced into each medium-pressure flash tank is similar to one another, which is approximately 35%–50% of the amount of effective gas in the stream to be flashed. The first medium-pressure flash tank (Section I), the second medium-pressure flash tank (Section II), the second high-pressure flash tank (Section III), and the first high-pressure flash tank (Section IV) are assembled from top to bottom to form a flash column.

[0048] The second medium-pressure circulating gas (13) enters the first gas-liquid separation tank (V1), and the gas obtained after separation consists mainly of H2, CO, and trace amounts of CO2. This is compressed to the pressure of the high-pressure flash tank, i.e., about 40 barg, in the first circulating gas compressor (K1), cooled to 40-42°C by the first water cooler (E1), and then supplied to the second gas-liquid separation tank (V2), where it is mixed with the first and second high-pressure circulating gases. After being compressed and cooled through the above-mentioned process, the separated gas is combined with the crude synthesis gas (1) at the front end of the absorption column (A) and then re-enters the absorption column (A) for purification.

[0049] The first medium-pressure drain (16) and the second medium-pressure drain (17) are both CO2-rich, and streams containing more than 98% CO2 are obtained through further separation. Whether it is used as a CO2 product or released as a tail gas, the CO content therein must be below the national standard, generally less than 1000 ppm. In this embodiment, since hydrogen is used as a stripping gas in the medium-pressure flash tank, CO can be changed from liquid to gas under the higher pressure of the medium-pressure flash tank, which satisfies the requirement for a low CO content in the flashed drain. Compared to Comparative Example 1, the flash tank has a higher pressure, less CO2 enters the gas obtained from flash evaporation, and the amount of scrubbing methanol for scrubbing and removing additional CO2 from the flash gas is correspondingly reduced. That is, while ensuring a reduction in CO content in the wastewater, Embodiment 2 uses less scrubbing methanol to ensure that the CO2 content in the circulating gas is lower than 50%, thereby reducing the energy consumption of the entire purification device. This technical effect can also be achieved with stripping gas similarly introduced into the high-pressure flash tank, for example, when a single-stage flash process is adopted, even if only a high-pressure flash tank is present.

[0050] Since the boiling point of a substance decreases as its pressure decreases, in the flash evaporation process, depressurization allows more substances with high boiling points to be gasified, thereby reducing the amount of the substance in the liquid phase. In the prior art, the content of the effective gas in the gas obtained from flash evaporation and the liquid after flash is generally controlled by adjusting the flash pressure. However, the effect of reducing the flash pressure on components other than the effective gas, particularly CO2, is not considered. If the flash pressure is reduced, more CO2 will vaporize along with CO and H2 and enter the gas obtained after flash. To prevent additional CO2 from entering the circulating gas and ultimately being mixed into the crude synthesis gas, scrubbing methanol must be introduced into the flash tank to remove CO2 from the gas obtained from flash evaporation. The higher the CO2 content in the gas after flash, the more cryogenic scrubbing methanol is required, and the more energy is consumed by the refrigerators of the entire purification system.

[0051] To control the CO content in the medium-pressure flash wastewater to less than 1000 ppm, Table 1 comparing the parameters in Comparative Example 1, Embodiment 1, and Embodiment 2 is prepared below. Comparative Example 1 did not use any stripping gas; Embodiment 1 adopted purified synthesis gas with an H2 content of 85.75 mol%; and Embodiment 2 adopted H2 with a purity close to 100 mol%. The amount of stripping gas includes the total stripping gas delivered to the first and second medium-pressure flash tanks, and the distribution ratio of the stripping gas in the two medium-pressure flash tanks is 30:50. Dilute methanol is a methanol stream with a methanol content of over 99% and a temperature lower than -50 °C. This is a methanol stream with the highest purity and extremely low temperature in the entire purification device, and its amount is used here to indicate the consumption of methanol in the entire device. Accordingly, the energy consumption of the refrigerator represents the amount of cooling that must be replenished during normal operation, and this is an important indicator of the operating cost (Opex) of the lectizol plant.

[0052] Table 1 Comparison of Parameters in the Synthesis Gas Purification Process

[0053]

[0054] As can be seen from the table above, in order to obtain the same low CO content from medium-pressure flash waste, a lower flash pressure is required when stripping gas is not used. Consequently, the flash gas contains more CO2, more scrubbing methanol is required to remove additional CO2, and the consumption of lean methanol by the entire purification device and the energy consumption of the refrigerator are higher than in the two cases where stripping gas is used. When stripping gas is used, the higher the H2 content, the higher the allowable flash pressure, the lower the CO2 content entering the flash gas along with the effective gas, the less lean methanol is used by the synthesis gas purification device, and the less energy is consumed by the refrigerator. Pure H2 entails high costs. If the synthesis gas generated by the purification device is used as stripping gas, costs will be controlled, and energy savings and emission reductions will be realized.

[0055] As described in the present invention, modifying an existing flash column with a solution for introducing stripping gas is very convenient and feasible.

[0056] The embodiments described above are merely preferred embodiments of the invention and are used to illustrate the technical solutions of the invention, rather than to limit the invention. Unless otherwise noted, each aspect or embodiment defined herein may be combined with any other aspect(s) or embodiment(s). In particular, any feature indicated as preferred or advantageous may be combined with any other feature indicated as preferred or advantageous. Any technical solution obtainable by a person skilled in the art through logical analysis, reasoning, or limited experimentation based on the concept of the invention will fall within the scope of the invention.

Claims

Claim 1 An apparatus for recovering effective gas in a synthesis gas purification device, characterized by comprising a flash tank, a pipeline connected to the flash tank for introducing a stream to be flashed, a pipeline for introducing stripping gas, and a pipeline for discharging circulating gas and wastewater after flashing, respectively. Claim 2 An apparatus for recovering effective gas according to claim 1, characterized in that the synthesis gas purification device comprises at least an absorption column, an apparatus for recovering effective gas, and a solvent regeneration device. Claim 3 An apparatus for recovering effective gas according to claim 1, wherein a pipeline for introducing stripping gas is connected to the outlet of purified synthesis gas of an absorption column, wherein the purified synthesis gas contains more than 80 mol%, preferably more than 85 mol%, of H2. Claim 4 An apparatus for recovering effective gas according to claim 1, characterized in that a pipeline for introducing stripping gas is connected to an H2 supply source. Claim 5 An apparatus for recovering effective gas according to paragraph 2, optionally comprising a high-pressure flash tank and a low-pressure flash tank having different flash pressures, wherein the stream to be flashed comprises CO2-containing methanol and sulfur-containing methanol from an absorption column and optionally the wastewater thereof after the high-pressure flash. Claim 6 An apparatus for recovering effective gas according to claim 5, wherein a flash tank for flashing CO2-containing methanol comprises a first high-pressure flash tank and a first medium-pressure flash tank, wherein the first high-pressure flash tank is connected to a pipeline for introducing CO2-containing methanol, a first high-pressure circulating gas, and a pipeline for discharging a first high-pressure drain, and the first medium-pressure flash tank is connected to a pipeline for introducing a first high-pressure drain, a pipeline for introducing stripping gas, and a pipeline for discharging a first medium-pressure circulating gas and a first medium-pressure drain. Claim 7 An apparatus for recovering effective gas according to claim 5, wherein a flash tank for flashing sulfur-containing methanol comprises a second high-pressure flash tank and a second medium-pressure flash tank, wherein the second high-pressure flash tank is connected to a pipeline for introducing sulfur-containing methanol, a pipeline for discharging a second high-pressure circulating gas and a second high-pressure drain, and the second medium-pressure flash tank is connected to a pipeline for introducing a second high-pressure drain, a pipeline for introducing stripping gas, and a pipeline for discharging a second medium-pressure circulating gas and a second medium-pressure drain. Claim 8 An apparatus for recovering effective gas according to claim 7, further comprising a pipeline for transferring scrubbing methanol and a first medium-pressure circulating gas to a second medium-pressure flash tank, wherein the scrubbing methanol enters the second medium-pressure flash tank at a position higher than the first medium-pressure circulating gas. Claim 9 A method for recovering effective gas in a synthesis gas purification process applicable to an apparatus for recovering effective gas according to claim 1, characterized in that the stripping gas is transferred to a flash tank during the flash. Claim 10 A method according to claim 9, characterized in that the effective gas contains CO and H2, the stripping gas contains H2 or purified synthesis gas, and the purified synthesis gas contains more than 80 mol%, preferably more than 85 mol%, of H2. Claim 11 A method according to claim 10, characterized in that H2 as a stripping gas is obtained by pressure swing adsorption with a purity of more than 98 mol%, preferably more than 99 mol%, more preferably more than 99.8 mol%. Claim 12 A method according to claim 10, characterized in that the circulating gas is discharged from the flash tank, pressurized, and delivered to the inlet of the synthesis gas purification device. Claim 13 A method according to claim 9, wherein a CO2 separation device is additionally provided, and wastewater after flash is processed by the CO2 separation device to produce a CO2 stream, wherein the CO2 stream contains less than 1000 ppm of CO. Claim 14 A flashing method according to claim 9, characterized in that the ratio of the amount of stripping gas (kmol / h) entering the flash tank to the amount of effective gas (kmol / h) in the stream to be flashed is 35%-50%.

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