Food-grade carbon dioxide acid gas purification system and method

Abstract

The invention relates to the technical field of acid gas removal, and discloses a food-grade carbon dioxide acid gas purification system and method. The system comprises a primary cooling liquid separation unit, an absorption unit, a secondary cooling liquid separation unit, a pressure recovery unit, a rectification unit and an adsorption unit, the first-stage cooling and liquid separation unit is used for sequentially performing first-stage cooling and first-stage liquid separation on raw material gas; the absorption unit comprises a desulfurization section and a decarbonization section; the desulfurization section is used for desulfurizing the primary cooling feed gas; the second-stage cooling and liquid separation unit is used for sequentially performing second-stage cooling and second-stage liquid separation on the desulfurization raw material gas; the decarburization section is used for decarburizing the secondary cooling feed gas; the pressure recovery unit comprises a pressure reducing valve and a pressure recovery device; the pressure reducing valve is used for reducing pressure of carbon dioxide recovery liquid; the pressure recovery device is used for recovering pressure generated by pressure reduction; the rectification unit is used for rectifying the reduced-pressure carbon dioxide recovery liquid; the adsorption unit is used for performing temperature swing adsorption on the rectified carbon dioxide liquid. The system can reduce the energy consumption of the system and obtain food-grade carbon dioxide.

Description

Technical Field

[0001] This invention relates to the field of acid gas removal, and more specifically to a food-grade carbon dioxide acid gas purification system and method. Background Technology

[0002] Coal gasification plays a crucial role in coal chemical industry. It refers to the process of reacting raw coal with a gasifying agent (air, steam, oxygen, carbon dioxide, or a mixture) under high-temperature conditions to produce syngas. Syngas produced by coal gasification is widely used in the synthesis of ammonia, methanol from coal, and acetic anhydride, and is also a raw material for the synthesis of various chemical products such as liquid fuels. However, some impurities are inevitably generated during coal gasification. Therefore, gas purification is a vital part of the coal chemical industry. Low-temperature methanol washing, as a process that can simultaneously achieve decarbonization and desulfurization, has the advantages of high efficiency and economy and is widely used in the gas purification sector of coal chemical industry.

[0003] The low-temperature methanol washing process utilizes physical absorption, taking advantage of the extremely high solubility of cold methanol for acidic gases under high pressure to remove acidic gas impurities from the feed gas. Based on the differences in solubility of different gases, CO2 and H2S are removed from the methanol solution separately, yielding a pure CO2 product gas that is almost sulfur-free, while the concentrated H2S is sent to a sulfur recovery unit.

[0004] Currently, in common low-temperature methanol washing processes, the CO2 product gas obtained through low-pressure desorption typically has a pressure of around 0.1 MPaG. After compression, it is sent to subsequent processes, a process that consumes a significant amount of compression work. Furthermore, the CO2 concentration of this product gas is approximately 98.5 mol%, which is insufficient in purity, making it difficult to find suitable users. Currently, some plants, unable to find suitable users for this product gas, combine it with their tail gas emissions, increasing the plant's carbon emissions. Summary of the Invention

[0005] The purpose of this invention is to overcome the problems of high energy consumption and low purity of carbon dioxide in the existing low-temperature methanol washing purification process for acidic gases containing carbon dioxide. This invention provides a food-grade carbon dioxide acidic gas purification system and method. This system reduces the amount of raw gas that needs to be cooled and separated in two stages, thereby reducing the energy consumption of the system. At the same time, it can obtain high-purity and high-pressure food-grade carbon dioxide.

[0006] To achieve the above objectives, the first aspect of the present invention provides a food-grade carbon dioxide acid gas purification system, wherein the system includes a primary cooling and liquid separation unit, an absorption unit, a secondary cooling and liquid separation unit, a pressure recovery unit, and a distillation unit that are interconnected by pipelines.

[0007] The primary cooling and liquid separation unit is used to sequentially perform primary cooling and primary liquid separation on the raw gas to obtain primary cooled raw gas and primary condensate. The raw gas contains carbon dioxide and hydrogen sulfide.

[0008] The absorption unit includes a desulfurization section and a decarbonization section;

[0009] The desulfurization section is used to desulfurize the primary cooling raw gas to obtain desulfurized raw gas;

[0010] The secondary cooling and liquid separation unit is used to sequentially perform secondary cooling and secondary liquid separation on the desulfurization feed gas to obtain secondary cooled feed gas and carbon dioxide recovery liquid.

[0011] The decarbonization section is used to decarbonize the secondary cooling raw gas to obtain purified raw gas;

[0012] The pressure recovery unit includes a pressure reducing valve and a pressure recovery device;

[0013] The pressure reducing valve is used to reduce the pressure of the carbon dioxide recovery liquid to obtain a reduced-pressure carbon dioxide recovery liquid;

[0014] The pressure recovery device is used to recover the pressure generated during decompression.

[0015] The distillation unit is used to distill the reduced-pressure carbon dioxide recovery liquid to obtain distilled carbon dioxide liquid;

[0016] The adsorption unit is used to perform temperature-switched adsorption on the distilled carbon dioxide liquid to obtain food-grade carbon dioxide product gas.

[0017] A second aspect of the present invention provides a method for purifying food-grade carbon dioxide acid gas, wherein the method includes the following steps:

[0018] (1) The raw gas is subjected to primary cooling and primary liquid separation in sequence to obtain primary cooled raw gas and primary condensate. The raw gas contains carbon dioxide and hydrogen sulfide.

[0019] (2) Desulfurize the primary cooling raw gas to obtain desulfurized raw gas;

[0020] (3) The desulfurization raw gas is subjected to secondary cooling and secondary liquid separation in sequence to obtain secondary cooled raw gas and carbon dioxide recovery liquid;

[0021] (4) The secondary cooling raw gas is decarbonized to obtain purified raw gas;

[0022] (5) The carbon dioxide recovery liquid is subjected to reduced pressure to obtain reduced-pressure carbon dioxide recovery liquid;

[0023] (6) The reduced pressure carbon dioxide recovery liquid is distilled to obtain distilled carbon dioxide liquid;

[0024] (7) The distilled carbon dioxide liquid is subjected to temperature-switched adsorption to obtain food-grade carbon dioxide product gas.

[0025] The system provided by this invention introduces a two-stage cooling and liquid separation unit, a distillation unit, and a pressure recovery unit into the traditional low-temperature methanol washing system. These units work together to perform two cooling separations on the feed gas, achieving the liquefaction and separation of carbon dioxide in the desulfurization feed gas. This reduces the amount of feed gas that needs to be depurified through secondary cooling, thereby reducing the consumption of steam, nitrogen stripping gas, electricity, and other utilities in the purification system. At the same time, it reduces the size of the entire system equipment and pipelines, thus significantly reducing the investment in the equipment.

[0026] The system provided by this invention uses a two-stage cooling and liquid separation unit coupled with a distillation unit to obtain distilled carbon dioxide liquid. The distilled carbon dioxide liquid has a high concentration and high pressure. After further coupling with an adsorption unit to remove methanol through temperature-switched adsorption, food-grade carbon dioxide product gas can be obtained. This food-grade carbon dioxide product gas can be sold directly, reducing the carbon emissions of the device and has high promotional value.

[0027] The system provided by this invention has a pressure recovery unit that can directly recover the pressure generated during the decompression process of carbon dioxide recovery liquid, thereby achieving further energy recovery and further reducing the energy consumption of the device.

[0028] In the preferred embodiment of the system provided by the present invention, the heat source required by the reboiler in the distillation unit comes from the heat regeneration unit provided by the system itself. The reboiler does not need to introduce an additional heating medium and can directly extract heat from the system, thereby reducing the energy consumption of the device.

[0029] The method provided by this invention employs a direct coupling of two-stage cooling, two-stage liquid separation, and distillation to achieve the liquefaction and separation of carbon dioxide in the desulfurization feed gas, reducing the content of gases requiring further impurity removal, decreasing the amount of absorbent circulating, and lowering energy consumption. Furthermore, by combining carbon dioxide liquid temperature-switching adsorption, methanol is adsorbed and removed to obtain high-purity and high-pressure food-grade carbon dioxide product gas for sale, reducing the carbon emissions of the equipment and possessing high value for promotion and application.

[0030] The method provided by this invention can directly recover the pressure generated during the decompression process of carbon dioxide recovery liquid, thereby achieving further energy recovery and reducing energy consumption. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of the system used in Embodiment 1 of the present invention;

[0032] Figure 2 This is a schematic diagram of the system used in Comparative Example 1 of the present invention.

[0033] Explanation of reference numerals in the attached figures

[0034] Figure 1 middle,

[0035] 1- Raw material gas; 2- Contains primary cooling raw material gas and A mixture of primary coolant; 3- Primary cooling feed gas; 4- Desulfurization feed gas;

[0036] 5-A mixture containing secondary cooling feed gas and carbon dioxide recovery liquid; 6-Carbon dioxide recovery liquid;

[0037] 7 - Secondary cooling feed gas; 8 - Purified feed gas;

[0038] 9-Purify the product gas; 10-Distill carbon dioxide liquid;

[0039] 11-Carbon dioxide product gas; 12-Distillation impurity gas;

[0040] 13-High-temperature heat source; 14-Low-temperature heat source;

[0041] 15-Semi-lean methanol; 16-Lean methanol;

[0042] 17 - Low-purity carbon dioxide; 18 - Exhaust gas;

[0043] 19 - Low-purity carbon dioxide product gas; 20 - Waste gas;

[0044] 21 - Primary condensate; 22 - Refrigerant inlet;

[0045] 23 - Refrigerant outlet; 24 - Carbon-rich methanol;

[0046] 25-Sulfur-rich methanol; 26-Food-grade carbon dioxide product gas;

[0047] C1 - Absorption column; C2 - Distillation column;

[0048] E1 - Primary cooler; E2 - Secondary cooler;

[0049] E3 - Reboiler; S1 - Flash evaporation unit;

[0050] S2 - Methanol thermal regeneration unit; S3 - Adsorption unit;

[0051] S4 - Pressure recovery device; V1 - Primary separatory tank;

[0052] V2 - Secondary separatory tank.

[0053] Figure 2 middle,

[0054] 1- Raw material gas; 2- Contains primary cooled raw material gas and primary cooled raw material gas. A mixture of liquids; 3 - Primary cooling feed gas; 4 - Primary cooling fluid;

[0055] 5-Semi-lean methanol; 6-Lean methanol;

[0056] 7- Carbon-rich methanol; 8- Purified feedstock gas;

[0057] 9-Purify the product gas; 10-Sulfur-rich methanol;

[0058] 11-Exhaust gas; 12-Waste gas;

[0059] 13 - Low-purity carbon dioxide; 14 - Low-purity carbon dioxide product gas;

[0060] C1 - Absorption tower; E1 - Primary cooler;

[0061] E2 - Secondary cooler; V1 - Primary separator tank;

[0062] S1 - Flash evaporation unit; S2 - Methanol thermal regeneration unit. Detailed Implementation

[0063] The embodiments of the technical solution of the present invention are described in detail below. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of actually identical structures may be omitted. This is to avoid making the following description unnecessarily lengthy and to facilitate understanding by those skilled in the art. Furthermore, the following description is provided to enable those skilled in the art to fully understand the present invention and is not intended to limit the subject matter described in the claims.

[0064] The "range" disclosed in this invention is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of the particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is understood that ranges of 60-110 and 80-120 are also expected. Furthermore, if minimum range values ​​1 and 2 are listed, and if maximum range values ​​3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this invention, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0065] Unless otherwise specified, all embodiments and optional embodiments of the present invention can be combined with each other to form new technical solutions.

[0066] Unless otherwise specified, all technical features and optional technical features of this invention can be combined to form new technical solutions.

[0067] Unless otherwise specified, the terms "comprising" and "including" as used in this invention can be open-ended or closed-ended. For example, "comprising" and "including" can mean that other components not listed may also be included, or that only the listed components may be included.

[0068] Unless otherwise specified, the term "or" is inclusive in this invention. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, the condition "A or B" is satisfied by any of the following conditions: A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0069] In this invention, lean methanol refers to methanol that has been fully regenerated and contains virtually no other impurities. The concentration of methanol in the lean methanol is not less than 99.5 mol / L.

[0070] In this invention, semi-lean methanol refers to methanol with low carbon content and virtually no H2S. Preferably, the molar content of CO2 in the semi-lean methanol is 5-20%.

[0071] In this invention, carbon-rich methanol refers to methanol with a high carbon content and virtually no H2S. Preferably, the carbon-rich methanol contains 1-30% CO2 molar content.

[0072] In this invention, sulfur-rich methanol refers to methanol that contains both sulfur and carbon. Preferably, the sulfur-rich methanol contains 0.1-5% H2S and 1-40% CO2.

[0073] In this invention, it should be noted that the levels of carbon and sulfur content in lean methanol, semi-lean methanol, carbon-rich methanol, and sulfur-rich methanol are relative to each other.

[0074] The first aspect of the present invention provides a food-grade carbon dioxide acid gas purification system, wherein the system includes a primary cooling and liquid separation unit, an absorption unit, a secondary cooling and liquid separation unit, a pressure recovery unit, and a distillation unit that are interconnected by pipelines.

[0075] The primary cooling and liquid separation unit is used to sequentially perform primary cooling and primary liquid separation on the raw material gas 1 to obtain primary cooled raw material gas 3 and primary condensate 21. The raw material gas 1 contains carbon dioxide and hydrogen sulfide.

[0076] The absorption unit includes a desulfurization section and a decarbonization section;

[0077] The desulfurization section is used to desulfurize the primary cooling raw gas 3 to obtain desulfurized raw gas 4;

[0078] The secondary cooling and liquid separation unit is used to sequentially perform secondary cooling and secondary liquid separation on the desulfurization raw material gas 4 to obtain secondary cooled raw material gas 7 and carbon dioxide recovery liquid 6.

[0079] The decarbonization section is used to decarbonize the secondary cooling raw material gas 7 to obtain purified raw material gas 8;

[0080] The pressure recovery unit includes a pressure reducing valve and a pressure recovery device S4;

[0081] The pressure reducing valve is used to reduce the pressure of the carbon dioxide recovery liquid to obtain a reduced-pressure carbon dioxide recovery liquid;

[0082] The pressure recovery device S4 is used to recover the pressure generated during decompression.

[0083] The distillation unit is used to distill the reduced-pressure carbon dioxide recovery liquid to obtain distilled carbon dioxide liquid 10;

[0084] The adsorption unit S3 is used to perform temperature-switched adsorption on the distilled carbon dioxide liquid 10 to obtain food-grade carbon dioxide product gas.

[0085] In existing technologies, the purity of the carbon dioxide product gas is not high (concentration of about 98.5 mol%), the carbon dioxide tail gas contains some nitrogen, and the pressure of these two gases is extremely low. It is difficult to directly recover the carbon dioxide in the gas, so it is directly released into the air, resulting in a large amount of carbon emissions and a waste of a large amount of CO2.

[0086] The system provided by this invention uses a two-stage cooling and liquid separation unit coupled with a distillation unit to obtain distilled carbon dioxide liquid. The distilled carbon dioxide liquid has a high concentration and high pressure. After further coupling with an adsorption unit to remove methanol through temperature-switched adsorption, food-grade carbon dioxide product gas can be obtained. This food-grade carbon dioxide product gas can be sold directly, reducing the carbon emissions of the device and has high promotional value.

[0087] The system provided by this invention has a pressure recovery unit that can directly recover the pressure generated during the decompression process of carbon dioxide recovery liquid, thereby achieving further energy recovery and further reducing the energy consumption of the device.

[0088] In the preferred embodiment of the system provided by the present invention, the heat source required by the reboiler in the distillation unit comes from the heat regeneration unit provided by the system itself. The reboiler does not need to introduce an additional heating medium and can directly extract heat from the system, thereby reducing the energy consumption of the device.

[0089] The system provided by this invention has a pressure recovery unit that can directly recover the pressure generated during the decompression process of carbon dioxide recovery liquid, thereby achieving further energy recovery and further reducing the energy consumption of the device.

[0090] In this invention, the specific configuration of the primary cooling unit is not particularly limited, as long as it can achieve primary cooling and primary liquid separation of the raw gas. Preferably, the primary cooling unit includes a primary cooler E1 and a primary liquid separator V1 connected in sequence.

[0091] In this invention, cooling the raw gas using a primary cooler facilitates the effective removal of water from the raw gas, preventing water in the raw gas from freezing and clogging the equipment during primary cooling; it also removes methanol carried in the raw gas from the methanol spraying unit described below. Preferably, the primary cooler E1 is used to perform primary cooling on the raw gas 1, obtaining a mixture 2 containing primary cooled raw gas and primary coolant.

[0092] In this invention, there is no particular limitation on the specific type of primary cooler, as long as it can achieve the cooling and removal of water from the raw gas and the heat exchange function. Preferably, the primary cooler E1 is provided with multiple tube passes; more preferably, the primary cooler E1 is a coiled tube heat exchanger cooler.

[0093] In this invention, there is no particular limitation on the specific type of primary liquid separator, as long as it can achieve gas-liquid separation. Preferably, the primary liquid separator V1 is used to perform primary liquid separation on the mixture 2 containing primary cooling feed gas and primary cooling liquid to obtain primary cooling feed gas 3 and primary condensate 21.

[0094] In this invention, methanol is selected as the absorbent solvent to remove acidic gases. Preferably, the system further includes a methanol spray unit (not shown in the figure) located at the feed gas inlet of the primary cooler E1, which is used to spray the feed gas 1. This preferred embodiment helps to prevent water in the feed gas from freezing during the primary cooling process.

[0095] This invention does not specifically limit the type of methanol spraying unit; any device conventionally defined in the art that can perform methanol spraying is applicable to this invention. Furthermore, this invention does not specifically limit the source of the methanol required in the methanol spraying unit; for example, it can be provided internally by the system.

[0096] In this invention, preferably, the absorption unit includes an absorption tower C1.

[0097] This invention does not impose particular limitations on the specific structure of the absorption unit. It can be a single absorption tower with separate desulfurization and decarbonization sections, or multiple absorption towers connected in series to form an absorption unit, as long as the purpose of desulfurization followed by decarbonization is achieved. Preferably, the decarbonization section is located in the upper section of absorption tower C1, and the desulfurization section is located in the lower section of absorption tower C1. The advantage of this preferred embodiment is that the primary cooling gas undergoes a desulfurization reaction first to remove most of the sulfur, followed by a decarbonization reaction, thus achieving maximum removal of impurity gases.

[0098] In this invention, preferably, the decarbonization section is provided with a first decarbonizing agent inlet and a second decarbonizing agent inlet.

[0099] In this invention, preferably, the first decarbonizing agent inlet is higher than the second decarbonizing agent inlet along the gas flow direction. In this preferred embodiment, semi-lean methanol is introduced as a decarbonizing agent at the second decarbonizing agent inlet for coarse gas washing. The gas after coarse washing is then introduced with clean lean methanol at the first decarbonizing agent inlet for fine washing, ultimately ensuring that the final purified gas meets the required standards. It should be noted that the gas flow direction here refers to the flow direction of the gas to be treated.

[0100] In this invention, preferably, the secondary cooling and liquid distribution unit includes a cryocooler E2 and a secondary liquid distribution tank V2.

[0101] In this invention, a cryocooler is used to cool the desulfurization feed gas, thereby liquefying the carbon dioxide in the feed gas and reducing the carbon dioxide content in the subsequent secondary cooling feed gas. This reduces the required absorption agent circulation volume and minimizes the overall system's steam, nitrogen stripping, and electricity consumption. Simultaneously, the overall system equipment and piping dimensions are reduced, significantly lowering investment costs. Preferably, the cryocooler E2 is used for secondary cooling of the desulfurization feed gas 4, resulting in a mixture 5 containing the secondary cooling feed gas and recovered carbon dioxide liquid. This invention does not specifically limit the type of cryocooler, as long as it can achieve partial liquefaction of the desulfurization feed gas.

[0102] In this invention, preferably, the cryogenic device E2 is provided with a refrigerant inlet 22 and a refrigerant outlet 23, the refrigerant inlet 22 is connected to an external refrigerant supply unit, and the refrigerant outlet 23 is connected to an external refrigerant recovery unit.

[0103] In this invention, there is no particular limitation on the specific type of secondary separator, as long as it can achieve gas-liquid separation. Preferably, the secondary separator V2 is used to perform secondary separation on the mixture 5 containing secondary cooling feed gas and carbon dioxide recovery liquid to obtain secondary cooling feed gas 7 and carbon dioxide recovery liquid 6.

[0104] In this invention, the specific structure of the pressure recovery device is not particularly limited. Any pressure recovery device conventionally defined in the art that can achieve pressure recovery is applicable to this invention, and those skilled in the art will not make any particular limitations on it.

[0105] In this invention, preferably, the distillation unit includes a distillation column C2 and a reboiler E3. Distillation columns are typically equipped with reboilers, as is well known to those skilled in the art, and this invention does not impose any particular limitation on this.

[0106] In this invention, high-purity carbon dioxide can be obtained by distilling the vacuum carbon dioxide recovery liquid using a distillation column. Preferably, the distillation column C2 is used to distill the vacuum carbon dioxide recovery liquid to obtain distilled carbon dioxide liquid 10 and distilled impurity gas 12.

[0107] In this invention, there is no particular limitation on the specific type of distillation column. Any distillation column conventionally defined in the art that can achieve the purpose of distillation can be applied to this invention.

[0108] In this invention, preferably, the number of theoretical plates of the distillation column C2 is 5-15, and more preferably 8-12.

[0109] In this invention, preferably, the height-to-diameter ratio of the distillation column C2 is 3-12:1, and more preferably 6-10:1.

[0110] In this invention, preferably, the diameter of the distillation column is 2000-4000 mm. In this invention, the diameter of the distillation column refers to the diameter of the bottom surface of the column.

[0111] In this invention, preferably, the system further includes a flash evaporation unit S1. The specific type of flash evaporation unit is not particularly limited in this invention; flash evaporation towers conventionally defined in the art are all applicable to this invention.

[0112] In this invention, preferably, the gas phase inlet of the flash evaporation unit S1 is connected to the gas phase outlet of the distillation column C2. By connecting the flash evaporation unit to the distillation column, this invention enables the transport of distillation impurity gases obtained during the distillation process to the flash evaporation unit for recovery of effective gases (CO, H2).

[0113] In this invention, preferably, the gas phase outlet of the flash evaporation unit S1 is connected to the product gas phase inlet of the primary cooler E1. This invention achieves the recovery of low-purity carbon dioxide and exhaust gas by connecting the flash evaporation unit to the primary cooler.

[0114] In this invention, preferably, the first liquid phase inlet of the flash evaporation unit S1 is connected to the first liquid phase outlet of the desulfurization section. This invention connects the flash evaporation unit to the desulfurization section; after the first-stage cooling gas desulfurization is completed, sulfur-rich methanol is transported to the flash evaporation unit to desorb some of the dissolved CO2 from the methanol.

[0115] In this invention, preferably, the first liquid phase outlet of the flash evaporation unit S1 is connected to the second decarbonizing agent inlet of the decarbonization section. This invention, by connecting the flash evaporation unit to the decarbonization section, provides the required semi-lean methanol for the decarbonization of the secondary cooling feed gas.

[0116] In this invention, preferably, the liquid phase outlet of the decarbonization section is connected to the desulfurizing agent inlet of the desulfurization section and / or the second liquid phase inlet of the flash evaporation unit S1. This invention connects the decarbonization section to the desulfurization section to transport the carbon-rich methanol produced in the decarbonization section to the desulfurization section for cyclic desulfurization; the decarbonization section is also connected to the flash evaporation unit to transport the carbon-rich methanol produced in the decarbonization section to the flash evaporation unit to desorb some of the dissolved CO2 from the methanol.

[0117] In this invention, preferably, the system further includes a methanol thermal regeneration unit S2, which is used for methanol regeneration. The specific type of methanol thermal regeneration unit is not particularly limited in this invention, as long as it can achieve methanol regeneration.

[0118] In this invention, preferably, the first liquid phase inlet of the methanol thermal regeneration unit S2 is connected to the outlet of the reboiler, and the first liquid phase outlet of the methanol thermal regeneration unit S2 is connected to the inlet of the reboiler E3. This invention provides a high-temperature heat source to the reboiler by connecting the methanol thermal regeneration unit to the reboiler; simultaneously, the low-temperature heat source after heat exchange in the reboiler is returned to the thermal regeneration unit for repeated regeneration and recycling. The reboiler does not require an additional heating medium and directly extracts heat from within the system, thereby reducing the energy consumption of the device.

[0119] In this invention, preferably, the second liquid phase inlet of the methanol thermal regeneration unit S2 is connected to the second liquid phase outlet of the flash evaporation unit. This invention connects the flash evaporation unit to the methanol thermal regeneration unit, allowing the methanol-containing solution generated during the flash evaporation process to be transported to the thermal regeneration unit for methanol regeneration.

[0120] In this invention, preferably, the second liquid phase outlet of the methanol thermal regeneration unit S2 is connected to the first decarbonizing agent inlet of the decarbonization section. This invention, by connecting the methanol thermal regeneration unit to the decarbonization section, provides the required lean methanol for the secondary cooling feed gas decarbonization.

[0121] In this invention, preferably, the third liquid phase inlet of the methanol thermal regeneration unit S2 is connected to the liquid phase outlet of the primary separator V1. This invention connects the methanol thermal regeneration unit to the primary separator, allowing the primary condensate in the primary separator to be transported to the thermal regeneration unit for methanol recovery.

[0122] In this invention, the specific type of adsorption unit is particularly limited. In a preferred case, the adsorption unit S3 is a temperature-switching adsorption unit.

[0123] A second aspect of the present invention provides a method for purifying food-grade carbon dioxide acid gas, wherein the method includes the following steps:

[0124] (1) The raw material gas 1 is subjected to primary cooling and primary liquid separation in sequence to obtain primary cooled raw material gas 3 and primary condensate 21. The raw material gas 1 contains carbon dioxide and hydrogen sulfide.

[0125] (2) Desulfurize the primary cooling raw gas to obtain desulfurized raw gas 4;

[0126] (3) The desulfurization raw material gas 4 is subjected to secondary cooling and secondary liquid separation in sequence to obtain secondary cooled raw material gas 7 and carbon dioxide recovery liquid 6;

[0127] (4) The secondary cooling raw material gas 7 is decarbonized to obtain purified raw material gas 8;

[0128] (5) The carbon dioxide recovery liquid 6 is subjected to reduced pressure to obtain reduced-pressure carbon dioxide recovery liquid;

[0129] (6) The reduced pressure carbon dioxide recovery liquid is distilled to obtain distilled carbon dioxide liquid 10;

[0130] (7) The distilled carbon dioxide liquid 10 is subjected to temperature-switched adsorption to obtain food-grade carbon dioxide product gas 26.

[0131] The method provided by this invention employs a direct coupling of two-stage cooling, two-stage liquid separation, and distillation to achieve the liquefaction and separation of carbon dioxide in the desulfurization feed gas, reducing the content of gases requiring further impurity removal, decreasing the circulation volume of decarbonizing and desulfurizing agents, and reducing energy consumption. Furthermore, by combining the temperature-switching adsorption of carbon dioxide liquid, the adsorbed and removed methanol can be sold as food-grade carbon dioxide product gas, reducing the carbon emissions of the equipment and possessing high value for promotion and application.

[0132] The method provided by this invention can directly recover the pressure generated during the decompression process of carbon dioxide recovery liquid, thereby achieving further energy recovery and reducing energy consumption.

[0133] In this invention, there is no particular limitation on the content of each component in the raw gas. Preferably, in step (1), based on the total amount of raw gas 1, the content of carbon dioxide in raw gas 1 is 15-85 mol%, preferably 30-60 mol%; the content of hydrogen sulfide is 0.1-5 mol%, preferably 0.2-2.5 mol%.

[0134] In this invention, under certain conditions, the raw material gas 1 also contains hydrogen, carbon monoxide, water, nitrogen, and an acidic gas containing carbon dioxide and hydrogen sulfide.

[0135] In this invention, preferably, the temperature of the raw material gas 1 is 15-45°C and the pressure is 3-7 MPaG.

[0136] In this invention, there is no particular limitation on the source of the raw gas, and it can be a raw gas obtained by various means in the art. Acidic gases conventionally defined in the art can all be applied to this invention.

[0137] In this invention, it is understood that hydrogen and carbon monoxide are used as effective gas components in the raw material gas 1 and are sent to the downstream device.

[0138] In this invention, there are no particular limitations on the conditions for primary cooling, as long as it enables partial liquefaction of carbon dioxide in the raw material gas. Preferably, in step (1), the conditions for primary cooling include: a final primary cooling temperature of -30°C to -10°C.

[0139] In this invention, preferably, the primary cooling is carried out in a primary cooler. This invention does not particularly limit the type of primary cooler, and it can be the same as the primary cooler described in the first aspect.

[0140] In this invention, preferably, the primary separation is carried out in a primary separation tank. This invention does not particularly limit the type of primary separation tank, which can be the same as the primary separation tank described in the first aspect.

[0141] In this invention, methanol is used as the absorption solvent to purify acidic gases. Preferably, the method further includes spraying the raw gas 1 with methanol before the primary cooling in step (1). This preferred embodiment helps prevent water in the raw gas from freezing during the primary cooling process. In this invention, there are no particular limitations on the specific conditions of the spraying, as long as the purpose is to prevent water in the raw gas from freezing and clogging the heat exchanger after cooling.

[0142] In this invention, preferably, the primary condensate 21 mainly consists of methanol and water. Preferably, the method further includes: thermally regenerating the primary condensate 21 in step (1) to recover methanol from the primary condensate 21. By recovering methanol from the primary condensate, the methanol-absorbing solvent is recycled, saving resources and possessing significant industrial value.

[0143] In this invention, preferably, the temperature of the primary cooling raw material gas 3 is -30°C to -10°C, and the pressure is 3-7 MPaG.

[0144] In this invention, the desulfurization conditions are not particularly limited, and those skilled in the art can adjust them according to actual needs. Preferably, in step (2), the desulfurization conditions are such that the hydrogen sulfide content in the desulfurization feed gas 4 is not higher than 5 ppm.

[0145] In this invention, preferably, in step (2), the temperature of the desulfurization feed gas is -25°C to -5°C, and the pressure is 2.5-6 MPaG.

[0146] In this invention, the specific desulfurization process is not particularly limited, as long as hydrogen sulfide is removed from the primary cooling feed gas. Preferably, the desulfurization in step (2) includes: counter-contacting the primary cooling feed gas 3 with carbon-rich methanol 24 to obtain desulfurized feed gas 4 and sulfur-rich methanol 25.

[0147] In this invention, preferably, the desulfurization in step (2) is carried out in the desulfurization section of the absorption unit described in the first aspect.

[0148] In this invention, preferably, step (2) further includes: flash evaporating sulfur-rich methanol 25 to recover methanol from it. This invention achieves the recycling and utilization of methanol, thereby increasing its industrial production value.

[0149] In this invention, preferably, the flash evaporation of sulfur-rich methanol 25 is carried out in the flash evaporation unit described in the first aspect.

[0150] In this invention, there are no particular limitations on the conditions for secondary cooling in step (3). The conditions for secondary cooling in step (3) include: a final secondary cooling temperature of -45°C to -30°C. By controlling the secondary cooling temperature, most of the carbon dioxide is liquefied, reducing the content of substances to be processed subsequently, thereby reducing energy consumption.

[0151] In this invention, preferably, in step (3), the secondary cooling is carried out in the presence of a refrigerant. Through secondary cooling, most of the carbon dioxide in the desulfurization feed gas 4 is condensed in the presence of a refrigerant to obtain carbon dioxide recovery liquid 6.

[0152] In this invention, there is no particular limitation on the source of the refrigerant. Preferably, the refrigerant is provided by an external refrigerant supply unit.

[0153] In this invention, there is no particular limitation on the specific type of refrigerant, as long as it can provide sufficient cooling capacity. Preferably, in step (3), the refrigerant is selected from at least one of propylene, ammonia, and lithium bromide.

[0154] In this invention, preferably, in step (3), the temperature of the refrigerant is -50°C to -30°C.

[0155] In this invention, preferably, the purity of the carbon dioxide recovery liquid 6 in step (3) is not less than 95 mol%.

[0156] In this invention, preferably, the secondary cooling in step (3) is carried out in the cryogenic cooler E2 described in the first aspect.

[0157] In this invention, there is no particular limitation on the specific method of decarbonization. Preferably, the decarbonization in step (4) includes: counter-current contact between the secondary cooling raw material gas 7 and lean methanol 16 and semi-lean methanol 15 to obtain purified raw material gas 8.

[0158] In this invention, there are no particular limitations on the decarbonization conditions, as long as the decarbonization requirements are met. Preferably, in step (4), the decarbonization conditions ensure that the carbon dioxide content in the purified raw material gas 8 is not higher than 20 ppm.

[0159] In this invention, preferably, step (4) further includes: recovering the cooling capacity of the purified raw material gas 8 to obtain purified product gas 9. The purified gas of this invention can be directly transported as product gas to downstream devices.

[0160] In this invention, preferably, the decarbonization is carried out in the decarbonization section of the absorption unit described in the first aspect.

[0161] In this invention, there are no particular limitations on the conditions for decompression. In step (5), the conditions for decompression include: decompression of 1-3.5 MPaG, preferably 1.5-3 MPaG. The pressure energy obtained through decompression can be further recovered through pressure recovery devices such as hydraulic turbines, thereby reducing the energy consumption of the device.

[0162] In this invention, preferably, in step (6), the purity of the distilled carbon dioxide liquid 10 is not less than 99 mol%.

[0163] In this invention, there are no particular limitations on the distillation conditions. Preferably, in step (6), the distillation conditions include: a pressure of 2-3 MPaG, a top temperature of -45°C to -30°C, and a bottom temperature of -20°C to -5°C.

[0164] In this invention, preferably, in step (6), the distillation is carried out in a distillation column. Through distillation in the column, liquid carbon dioxide 10 is obtained at the bottom of the column.

[0165] In this invention, preferably, in step (6), the number of theoretical plates of the distillation column is 5-15, more preferably 8-12.

[0166] In this invention, preferably, in step (6), the height-to-diameter ratio of the distillation column is 3-12:1, more preferably 6-10:1.

[0167] In this invention, preferably, step (6) further includes: recovering the cold energy from the distilled carbon dioxide liquid to obtain carbon dioxide product gas 11. The carbon dioxide product gas obtained by the method of this invention not only has high purity but also high pressure.

[0168] In this invention, preferably, the distillation in step (6) also yields distilled impurity gas 12.

[0169] In this invention, preferably, step (6) further includes: flash evaporating the distillation impurity gas 12 to recover the effective gas (H2 and CO) in the distillation impurity gas 12, to obtain low-purity carbon dioxide 17.

[0170] In this invention, preferably, the flash evaporation is carried out in the flash evaporation unit described in the first aspect.

[0171] In this invention, preferably, the method further includes: recovering the cold energy of low-purity carbon dioxide 17 to obtain low-purity carbon dioxide product gas 19. The advantage of this preferred embodiment is that the low-purity carbon dioxide product gas can be directly sent to downstream devices. If there is no user for this product gas, it will be directly vented after methanol removal along with the tail gas.

[0172] In this invention, preferably, the purity of the low-purity carbon dioxide product gas 19 is 98.5-99 mol%.

[0173] In this invention, preferably, tail gas 18 is also generated during the flash evaporation process; more preferably, the method further includes: recovering the cold energy of the tail gas 18 through a cold cycle to obtain waste gas 20, and then further removing methanol before directly venting it.

[0174] In this invention, preferably, the temperature-switching adsorption in step (7) includes: subjecting the carbon dioxide product gas 11 to temperature-switching adsorption to obtain food-grade carbon dioxide product gas 26.

[0175] In this invention, preferably, in step (7), the conditions for temperature-switching adsorption include: a temperature of 20-40°C and a pressure of 1-3.5 MPaG.

[0176] In this invention, preferably, the temperature-switching adsorption is carried out in the presence of an adsorbent. The type of adsorbent is not particularly limited in this invention; for example, it can be a molecular sieve, activated carbon, etc. The amount of adsorbent is not particularly limited in this invention, and those skilled in the art can adjust it according to actual needs.

[0177] In this invention, preferably, the cooling energy recovered from the distilled carbon dioxide liquid 10, the purified raw material gas 8, the tail gas 18, and the low-purity carbon dioxide 17 is used for the first cooling in step (1).

[0178] In this invention, preferably, the method is performed in the system described in the first aspect.

[0179] In this invention, preferably, the method is performed continuously.

[0180] According to a specific embodiment of the present invention, the method is in Figure 1 The method, performed in the system shown, includes the following steps:

[0181] (1) The raw gas 1 from the upstream unit (containing carbon dioxide and hydrogen sulfide) enters the primary cooler E1 after being sprayed with methanol for primary cooling, resulting in a mixture 2 containing primary cooling raw gas and primary condensate. The mixture 2 containing primary cooling raw gas and primary condensate enters the primary separator V1 for primary separation, resulting in primary cooling raw gas 3 and primary condensate 21. The primary condensate 21 is transported to the methanol thermal regeneration unit S2 for methanol thermal regeneration to recover the methanol in the primary condensate 21.

[0182] (2) The primary cooling raw material gas 3 and carbon-rich methanol 24 are in counter-current contact and desulfurization is carried out in the desulfurization section of the absorption tower C1 of the absorption unit to obtain desulfurized raw material gas 4 and sulfur-rich methanol 25. The sulfur-rich methanol 25 is transported to the flash evaporation unit S1 for flash evaporation to recover the methanol in the sulfur-rich methanol 25.

[0183] (3) The desulfurization raw material gas 4 enters the cryogenic cooler E2 and undergoes secondary cooling in the presence of refrigerant (the refrigerant is supplied by an external refrigerant supply unit and enters the cryogenic cooler E2 through the refrigerant inlet 22, and flows out through the refrigerant outlet 23 after the secondary cooling is completed) to obtain a mixture 5 containing the secondary cooling raw material gas and carbon dioxide recovery liquid. The mixture 5 containing the secondary cooling raw material gas and carbon dioxide recovery liquid enters the secondary liquid separator V2 for secondary liquid separation to obtain the secondary cooling raw material gas 7 and carbon dioxide recovery liquid 6.

[0184] (4) The secondary cooling raw material gas 7 is in counter-current contact with the semi-lean methanol 15 provided by the flash unit S1 and the lean methanol 16 provided by the thermal regeneration unit. It is decarbonized in the decarbonization section of the absorption tower C1 of the absorption unit to obtain purified raw material gas 8 and carbon-rich methanol 24. The purified raw material gas 8 is sent to the primary cooler E1 to recover the cold energy and obtain purified product gas 9. At least part of the carbon-rich methanol 24 is sent to the flash unit S1 for flash evaporation to recover methanol for reuse. At least part of the carbon-rich methanol 24 is sent to the desulfurization section of the absorption tower C1 for desulfurization and reuse.

[0185] (5) The carbon dioxide recovery liquid 6 is depressurized by the pressure reducing valve to obtain depressurized carbon dioxide recovery liquid. The pressure obtained during the depressurization process is recovered by the pressure recovery device S4.

[0186] (6) The depressurized carbon dioxide recovery liquid enters the distillation column C2 for distillation to remove impurities, resulting in distilled carbon dioxide liquid 10 and distilled impurity gas 12. Distilled carbon dioxide liquid 10 is sent to the primary cooler E1 to recover the cold energy and obtain carbon dioxide product gas 11. Distilled impurity gas 12 is sent to the flash evaporation unit S1 for flash evaporation to recover the effective gases (H2 and CO) in distilled impurity gas 12, resulting in low-purity carbon dioxide 17. Low-purity carbon dioxide 17 is then used to recover the cold energy and obtain low-purity carbon dioxide product gas 19, which is directly sent to the downstream unit. During the distillation process, the heat source of reboiler E3 is provided by the high-temperature heat source of methanol thermal regeneration unit S2. The low-temperature heat source after heat exchange in reboiler E3 is returned to methanol thermal regeneration unit S2 for repeated regeneration. Throughout the process, the tail gas 18 generated in flash evaporation unit S1 can be directly sent to the primary cooler E1 to recover the cold energy and then discharged as waste gas 20.

[0187] (7) The carbon dioxide product gas 11 is transported to the adsorption unit S3 for temperature-switched adsorption to obtain food-grade carbon dioxide product gas 26.

[0188] The present invention will be described in detail below through embodiments.

[0189] In the following examples, the composition of each gas can be determined by a gas chromatograph.

[0190] In the following examples, the composition of each gas is expressed in mol%;

[0191] In the following examples, the recovery rate of distilled carbon dioxide liquid was calculated as follows:

[0192] Recovery rate (%) = Molar content of carbon dioxide in distilled carbon dioxide liquid / Molar content of carbon dioxide in desulfurization feed gas × 100%.

[0193] Example 1

[0194] according to Figure 1 The system shown uses the method provided by this invention to purify acidic gases containing carbon dioxide. Specifically:

[0195] (1) Raw material gas 1 from the upstream unit (pressure 5.5 MPaG, temperature 40℃, flow rate 246281 Nm³) 3 The feed gas 1, with main components of CO2: 43.39%, H2: 55.13%, CO: 0.46%, H2S: 0.42%, H2O: 0.18%, N2: 0.28%, is sprayed with methanol (flow rate 850 kg / h) and then enters the primary cooler E1 (a coiled tube heat exchanger) for primary cooling (the primary cooling condition is: final cooling temperature to -12℃), resulting in a mixture 2 containing primary cooled feed gas and primary coolant. This mixture 2 then enters the primary separator V1 for primary separation, yielding primary cooled feed gas 3 (pressure 5.5 MPaG, temperature -6.1℃, flow rate 233381.2 Nm³). 3 The first-stage condensate 21 (mainly containing methanol and water, pressure 5.5 MPaG, temperature -6.1℃, flow rate 802.1 kg / h) is transported to the methanol thermal regeneration unit S2 for methanol thermal regeneration to recover the methanol in the first-stage condensate 21.

[0196] (2) Primary cooling feed gas 3 and carbon-rich methanol 24 (temperature -25.2℃, pressure 5.6MPaG, flow rate 161295kg / h) undergo counter-current desulfurization in the desulfurization section of absorber C1 to obtain desulfurization feed gas 4 (pressure 5.5MPaG, temperature -15.4℃, flow rate 206429Nm³). 3 The desulfurization feed gas 4 mainly consists of CO2: 33.13%, H2: 65.78%, CO: 0.55%, CH3OH: 0.049%, N2: 0.33%, H2S 4ppm) and sulfur-rich methanol 25 (temperature: -10℃, pressure: 5.6MPaG, flow rate: 175020kg / h). The sulfur-rich methanol 25 is transported to the flash evaporation unit S1 for flash evaporation to recover methanol from the sulfur-rich methanol 25.

[0197] (3) The desulfurization feed gas 4 enters the cryogenic cooler E2 and undergoes secondary cooling in the presence of a refrigerant (a gas-liquid mixture of propylene at -40°C, pressure 0.039 MPaG, and flow rate 32694 kg / h supplied by an external refrigerant supply unit). The outlet operating temperature of the cryogenic cooler E2 is -36°C, the pressure is 5.4 MPaG, and the flow rate is 149433 kg / h, resulting in a mixture 5 containing the secondary cooling feed gas and carbon dioxide recovery liquid. This mixture 5 enters the secondary separator V2 for secondary separation, resulting in secondary cooling feed gas 7 (pressure 5.4 MPaG, temperature -36°C, and flow rate 26741 Nm). 3 The secondary cooling feed gas 7 has the following main components: CO2: 28.54%, H2: 70.34%, CO: 0.58%, CH4: 0.09%, N2: 0.35%; and the carbon dioxide recovery liquid 6 has the following main components: CO2: 95.95%, H2: 3.22%, CO: 0.05%, CH3OH: 0.71%, N2: 0.03%.

[0198] (4) The secondary cooling feed gas 7, semi-lean methanol 15 (temperature -60.8℃, pressure 5.4MPaG, flow rate 79325kg / h) provided by flash evaporation unit S1, and lean methanol 16 (temperature -53℃, pressure 5.3MPaG, flow rate 219182kg / h) provided by thermal regeneration unit S2 are decarbonized in counter-current contact in the decarbonization section of absorption tower C1 to obtain purified feed gas 8 (temperature -52℃, pressure 5.1MPaG, flow rate 137933Nm). 3 The purified feed gas 8 consists of CO: 0.82%, H2: 98.44%, N2: 0.5%, with the balance being CH4 and AR (carbon dioxide content 18ppm) and carbon-rich methanol 24. After the purified feed gas 8 is sent to the primary cooler E1 to recover the cooling capacity, purified product gas 9 is obtained. Purified product gas 9 (temperature 30℃) is directly sent to downstream units for use. At least a portion of the carbon-rich methanol 24 is sent to the flash evaporation unit S1 for flash evaporation and reuse, and at least a portion of the carbon-rich methanol 24 is sent to the desulfurization section of the absorption unit for desulfurization and reuse.

[0199] (5) The carbon dioxide recovery liquid 6 is depressurized to 2.1 MPaG through a pressure reducing valve, the temperature is -38.6℃, and the flow rate is 26741 kg / h to obtain depressurized carbon dioxide recovery liquid. The pressure obtained during the depressurization process is used to recover energy through the pressure recovery device S4.

[0200] (6) The reduced-pressure carbon dioxide recovery liquid enters the distillation column C2 for distillation under the following conditions: temperature -38.6℃, pressure 2.1MPaG, number of distillation trays 11, height-to-diameter ratio 7:1, and column diameter 3000mm. Impurities CO, H2, and methanol are removed to obtain distilled carbon dioxide liquid 10 (pressure 2.1MPaG, temperature -16.2℃, flow rate 25601.1kg / h, mainly composed of CO: 0.01%; CO2: 99.2%; CH4: 0.01%; CH3OH: 0.76%; H2: 0.01%) and distilled impurity gas 12 (pressure 2.1MPaG, temperature -38.6℃, flow rate 1088.4kg / h). The distillation impurity gas 12 has the following composition: CO: 0.58%, H2: 45.68%, CO2: 53.1%, N2: 0.38%, with the balance being CH4 and AR. The distilled carbon dioxide liquid 10 is sent to the primary cooler E1 to recover its cooling capacity, yielding carbon dioxide product gas 11. This carbon dioxide product gas 11 enters a temperature-switched adsorption unit to remove methanol entrained in the product gas. The adsorption conditions are: adsorption temperature 30℃, adsorption pressure 2.1 MPaG. After adsorption, food-grade carbon dioxide product gas 26, which can be directly sold, is obtained. Food-grade carbon dioxide product gas 26 contains CO2 ≥ 99.9%, methanol < 10 ppm, pressure 2.0 MPaG, temperature 30℃, and flow rate 129684 Nm³. 3 During the / h distillation process, the heat source for reboiler E3 is provided by the high-temperature heat source 13 of methanol thermal regeneration unit S2. The low-temperature heat source 14, after heat exchange in reboiler E3, returns to methanol thermal regeneration unit S2 for repeated regeneration. In this process, the recovered heat is mainly used to heat the CO2 liquid produced by distillation from -16.2℃ to 30℃; the recovered cold energy has a pressure of 2.1MPag and a flow rate of 25601.1kg / h.

[0201] Throughout the process, low-purity carbon dioxide 17 (CO2: 98.5%, pressure 0.1 MPaG, temperature 30℃, flow rate 4200 Nm³) is generated in flash unit S1. 3 The exhaust gas 18 and the tail gas 18 are respectively sent to the primary coolant E1 to recover the cooling capacity, resulting in low-purity carbon dioxide product gas 19 and waste gas 20. Low-purity CO2 product gas 19 can be sent to downstream units. If there is no user for low-purity carbon dioxide product gas 19, it will be directly vented after methanol removal together with waste gas 20.

[0202] Comparative Example 1

[0203] according to Figure 2 The system shown employs existing technologies for purifying acidic gases containing carbon dioxide. Specifically:

[0204] (1) Raw material gas 1 from the upstream unit (pressure 5.5 MPaG, temperature 40℃, flow rate 246281 Nm³) 3 The feed gas 1, with main components of CO2: 43.39%, H2: 55.13%, CO: 0.46%, H2S: 0.42%, H2O: 0.18%, N2: 0.28%, is sprayed with methanol (flow rate 850 kg / h) and then enters the primary cooler E1 (a coiled tube heat exchanger) for primary cooling (the primary cooling condition is cooling to -10℃), resulting in a mixture 2 containing primary cooling feed gas and primary cooling liquid. This mixture 2 then enters the primary separator V1 for primary separation, yielding primary cooling feed gas 3 (pressure 55 MPaG, temperature -10℃, flow rate 245830 Nm³). 3 The first-stage condensate 4 (mainly containing methanol and water, pressure 5.5 MPaG, temperature -10℃, flow rate 853.4 kg / h) is transported to the thermal regeneration unit S2 for methanol thermal regeneration to recover methanol from the first-stage condensate 4.

[0205] (2) Primary cooling feed gas 3 and carbon-rich methanol 7 (temperature -25.2℃, pressure 5.6MPaG, flow rate 185162kg / h) undergo counter-current desulfurization in the absorption unit to obtain desulfurization products (pressure 5.4MPaG, temperature -15.4℃, flow rate 206429Nm³). 3 The desulfurization products mainly consist of CO2: 33.13%, H2: 65.78%, CO: 0.55%, CH3OH: 0.049%, N2: 0.33%) and sulfur-rich methanol 10 (temperature -10℃, pressure 5.6MPaG, flow rate 198887kg / h, sulfur-rich methanol 10 is transported to flash evaporation unit S1 for flash evaporation to recover methanol from sulfur-rich methanol 10);

[0206] (3) The desulfurization products are directly reacted with semi-lean methanol 5 (temperature -60.8℃, pressure 5.4MPaG, flow rate 176272kg / h) provided by flash evaporation unit S1 and lean methanol 6 (temperature -53℃, pressure 5.3MPaG, flow rate 230718kg / h) provided by thermal regeneration unit in counter-current contact in absorption tower C1 for decarbonization, resulting in purified feed gas 8 (pressure 5.1MPaG, flow rate 137933Nm). 3The purified feed gas 8 consists of CO: 0.82%, H2: 98.44%, N2: 0.5%, with the balance being CH4 and AR (carbon dioxide content 18ppm) and carbon-rich methanol 7. After the purified feed gas 8 is sent to the primary cooler E1 to recover the cooling capacity, purified product gas 9 (temperature 30℃) is obtained. Purified product gas 9 is directly sent to downstream units for use. At least a portion of the carbon-rich methanol 7 is sent to the flash evaporation unit S1 for flash evaporation and reuse, and at least a portion of the carbon-rich methanol 7 is sent to the absorption tower C1 for desulfurization and reuse.

[0207] (4) Low-purity carbon dioxide 13 (CO2: 98.5%) is generated in flash evaporation unit S1 at a temperature of 30°C, a pressure of 0.1 MPaG, and a flow rate of 11000 Nm³. 3 The exhaust gas 11 and the tail gas 11 are respectively sent to the primary cooler E1 to recover the cooling capacity, resulting in low-purity carbon dioxide product gas 14 and waste gas 12. Low-purity carbon dioxide product gas 14 can be sent to downstream units. If there is no user for low-purity carbon dioxide product gas 14, it will be directly vented after methanol removal together with waste gas 12.

[0208] The parameters involved in the above embodiments and comparative examples are listed in Table 1.

[0209] Table 1

[0210]

[0211]

[0212] Note: Carbon emissions refer to the amount of CO2 in exhaust gas emitted at normal temperature and pressure.

[0213] Low-pressure steam and low-pressure steam are used to ensure the quality of lean methanol and semi-lean methanol.

[0214] The circulating cooling water and cooling capacity provide cooling for the system.

[0215] As can be seen from the results in Table 1, the system provided by this invention is particularly suitable for the purification of acidic gases containing carbon dioxide. By reducing the amount of secondary cooling feed gas requiring further decarbonization, the circulating amount of decarbonizing agent used for further decarbonization is reduced, thereby significantly reducing the system's energy consumption. Simultaneously, the overall system equipment and piping dimensions are reduced, thus significantly lowering equipment investment. Furthermore, by recovering the pressure generated during the depressurization process, recoverable energy (up to 40 kW) can be obtained for use in other units within the system. Even further, this invention can ultimately produce food-grade carbon dioxide product gas with high purity (up to 99.9 mol%) and high pressure (up to 2.0 MPaG), demonstrating high potential for widespread application.

[0216] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A food-grade carbon dioxide acid gas purification system, characterized in that, The system includes a primary cooling and liquid separation unit, an absorption unit, a secondary cooling and liquid separation unit, a pressure recovery unit, a distillation unit, and an adsorption unit, all interconnected by pipelines. The primary cooling and liquid separation unit is used to sequentially perform primary cooling and primary liquid separation on the raw gas to obtain primary cooled raw gas and primary condensate. The raw gas contains carbon dioxide and hydrogen sulfide. The absorption unit includes a desulfurization section and a decarbonization section; The desulfurization section is used to desulfurize the primary cooling raw gas to obtain desulfurized raw gas; The secondary cooling and liquid separation unit is used to sequentially perform secondary cooling and secondary liquid separation on the desulfurization feed gas to obtain secondary cooled feed gas and carbon dioxide recovery liquid. The decarbonization section is used to decarbonize the secondary cooling raw gas to obtain purified raw gas; The pressure recovery unit includes a pressure reducing valve and a pressure recovery device; The pressure reducing valve is used to reduce the pressure of the carbon dioxide recovery liquid to obtain a reduced-pressure carbon dioxide recovery liquid; The pressure recovery device is used to recover the pressure generated during decompression. The distillation unit is used to distill the reduced-pressure carbon dioxide recovery liquid to obtain distilled carbon dioxide liquid; The adsorption unit is used to perform temperature-switched adsorption on the distilled carbon dioxide liquid to obtain food-grade carbon dioxide product gas.

2. The system according to claim 1, wherein, The primary cooling unit includes a primary cooler and a primary liquid separator connected in sequence. Preferably, the primary cooler is used to perform primary cooling on the raw material gas to obtain a mixture containing primary cooled raw material gas and primary coolant; Preferably, the primary separator is used to perform primary separation of the mixture containing primary cooling feed gas and primary cooling liquid to obtain primary cooling feed gas and primary condensate. Preferably, the system further includes a methanol spraying unit installed at the feed gas inlet of the primary cooler, the methanol spraying unit being used to spray the feed gas.

3. The system according to claim 1 or 2, wherein, The absorption unit includes an absorption tower; Preferably, the decarbonization section is located in the upper section of the absorption tower, and the desulfurization section is located in the lower section of the absorption tower; Preferably, the decarbonization section is provided with a first decarbonizing agent inlet and a second decarbonizing agent inlet; more preferably, along the gas flow direction, the first decarbonizing agent inlet is higher than the second decarbonizing agent inlet. Preferably, the secondary cooling and liquid distribution unit includes a cryocooler and a secondary liquid distribution tank; Preferably, the cryocooler is used to perform secondary cooling on the desulfurization feed gas to obtain a mixture containing secondary cooled feed gas and carbon dioxide recovery liquid; Preferably, the cryogenic device is provided with a refrigerant inlet and a refrigerant outlet, the refrigerant inlet being connected to an external refrigerant supply unit and the refrigerant outlet being connected to an external refrigerant recovery unit; Preferably, the secondary separator is used to perform secondary separation of the mixture containing secondary cooling feed gas and carbon dioxide recovery liquid to obtain secondary cooling feed gas and carbon dioxide recovery liquid.

4. The system according to claim 3, wherein, The distillation unit includes a distillation column and a reboiler; Preferably, the distillation column is used to distill the reduced-pressure carbon dioxide recovery liquid to obtain distilled carbon dioxide liquid and distilled impurity gas; Preferably, the number of theoretical plates in the distillation column is 5-15, more preferably 8-12; Preferably, the height-to-diameter ratio of the distillation column is 3-12:1, more preferably 6-10:1; Preferably, the diameter of the distillation column is 2000-4000 mm.

5. The system according to claim 4, wherein, The system also includes a flash evaporation unit; Preferably, the vapor inlet of the flash evaporation unit is connected to the vapor outlet of the distillation column; Preferably, the vapor phase outlet of the flash evaporation unit is connected to the product vapor phase inlet of the primary cooler; Preferably, the first liquid phase inlet of the flash evaporation unit is connected to the first liquid phase outlet of the desulfurization section; Preferably, the first liquid phase outlet of the flash evaporation unit is connected to the second decarbonizing agent inlet of the decarbonization section; Preferably, the liquid phase outlet of the decarbonization section is connected to the desulfurizing agent inlet of the desulfurization section and / or the second liquid phase inlet of the flash evaporation unit.

6. The system according to claim 5, wherein, The system also includes a methanol thermal regeneration unit, which is used for methanol regeneration; Preferably, the first liquid phase inlet of the thermal regeneration unit is connected to the outlet of the reboiler, and the first liquid phase outlet of the thermal regeneration unit is connected to the inlet of the reboiler; Preferably, the second liquid phase inlet of the thermal regeneration unit is connected to the second liquid phase outlet of the flash evaporation unit; Preferably, the second liquid phase outlet of the thermal regeneration unit is connected to the first decarbonizing agent inlet of the decarbonization section; Preferably, the third liquid phase inlet of the thermal regeneration unit is connected to the liquid phase outlet of the primary separator.

7. A method for purifying food-grade carbon dioxide acid gas, characterized in that, The method includes the following steps: (1) The raw gas is subjected to primary cooling and primary liquid separation in sequence to obtain primary cooled raw gas and primary condensate. The raw gas contains carbon dioxide and hydrogen sulfide. (2) Desulfurize the primary cooling raw gas to obtain desulfurized raw gas; (3) The desulfurization raw gas is subjected to secondary cooling and secondary liquid separation in sequence to obtain secondary cooled raw gas and carbon dioxide recovery liquid; (4) The secondary cooling raw gas is decarbonized to obtain purified raw gas; (5) The carbon dioxide recovery liquid is subjected to reduced pressure to obtain reduced-pressure carbon dioxide recovery liquid; (6) The reduced pressure carbon dioxide recovery liquid is distilled to obtain distilled carbon dioxide liquid; (7) The distilled carbon dioxide liquid is subjected to temperature-switched adsorption to obtain food-grade carbon dioxide product gas.

8. The method according to claim 7, wherein, In step (1), based on the total amount of raw gas, the content of carbon dioxide in the raw gas is 15-85 mol%, and the content of hydrogen sulfide is 0.1-5 mol%. Preferably, the temperature of the raw material gas is 15-45°C and the pressure is 3-7 MPaG; Preferably, in step (1), the conditions for the first-stage cooling include: the final temperature of the first-stage cooling is -30°C to -10°C; Preferably, the method further includes: spraying the raw gas with methanol before the raw gas undergoes primary cooling in step (1); Preferably, the method further includes: performing methanol thermal regeneration on the primary condensate from step (1) to recover methanol from the primary condensate; Preferably, the temperature of the primary cooling raw material gas is -30°C to -10°C, and the pressure is 3-7 MPaG.

9. The method according to claim 7 or 8, wherein, In step (2), the desulfurization conditions are such that the hydrogen sulfide content in the desulfurization feed gas is no higher than 5 ppm; Preferably, in step (2), the temperature of the desulfurization feed gas is -25°C to -5°C, and the pressure is 2.5-6 MPaG; Preferably, the desulfurization in step (2) includes: counter-current contact between the primary cooling feed gas and carbon-rich methanol to obtain desulfurized feed gas and sulfur-rich methanol; Preferably, step (2) further includes: flash evaporating the sulfur-rich methanol to recover the methanol from the sulfur-rich methanol.

10. The method according to claim 7 or 8, wherein, In step (3), the conditions for the secondary cooling include: the final temperature of the secondary cooling is -45℃ to -30℃; Preferably, in step (3), the secondary cooling is carried out in the presence of refrigerant, and preferably the refrigerant is provided by an external refrigerant supply unit; Preferably, in step (3), the refrigerant is selected from at least one of propylene, ammonia, and lithium bromide; Preferably, in step (3), the temperature of the refrigerant is -50°C to -30°C; Preferably, the purity of the carbon dioxide recovery liquid in step (3) is not less than 95 mol%.

11. The method according to claim 7 or 8, wherein, The decarbonization in step (4) includes: counter-current contact between the secondary cooling raw gas and lean methanol and semi-lean methanol to obtain purified raw gas; Preferably, in step (4), the decarbonization conditions are such that the carbon dioxide content in the purified raw gas is not higher than 20 ppm. Preferably, step (4) further includes: recovering the cold energy of the purified raw material gas to obtain purified product gas.

12. The method according to claim 7 or 8, wherein, In step (5), the decompression conditions include: decompression to 1-3.5 MPaG, preferably 1.5-3 MPaG; Preferably, in step (6), the purity of the distilled carbon dioxide liquid is not less than 99 mol%. Preferably, in step (6), the distillation conditions include: a pressure of 2-3 MPaG, a top temperature of -45°C to -30°C, and a bottom temperature of -20°C to -5°C. Preferably, in step (6), the distillation is carried out in a distillation column; Preferably, in step (6), the number of theoretical plates of the distillation column is 5-15, more preferably 8-12; Preferably, in step (6), the height-to-diameter ratio of the distillation column is 3-12:1, and more preferably 6-10:1; Preferably, step (6) further includes: recovering the cold energy from the distilled carbon dioxide liquid to obtain carbon dioxide product gas; Preferably, the distillation in step (6) also yields a distillation impurity gas. More preferably, step (6) further includes: flash evaporating the distillation impurity gas to recover the effective gas in the distillation impurity gas, thereby obtaining low-purity carbon dioxide. Preferably, the method further includes: recovering the cold energy of low-purity carbon dioxide to obtain low-purity carbon dioxide product gas; Preferably, the purity of the low-purity carbon dioxide product gas is 98.5-99 mol%.

13. The method according to claim 7 or 8, wherein, In step (7), the conditions for temperature-switching adsorption include: a temperature of 20-40℃ and a pressure of 1-3.5 MPaG.

14. The method according to any one of claims 7-13, wherein, The method is performed in the system described in any one of claims 1-6; Preferably, the method is performed continuously.

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