Process for preparing CNG (compressed natural gas) and co-producing carbon dioxide from biogas and system for preparing CNG and

Abstract

The invention discloses a process for preparing CNG (compressed natural gas) and co-producing carbon dioxide from biogas. The process comprises the following steps: 1, obtaining tower bottom gas after a PSA (pressure swing adsorption) process; and 2, only carrying out gas liquefaction on the tower bottom gas to obtain liquid carbon dioxide gas. The invention also provides a system for preparing CNG and co-producing carbon dioxide from biogas, which comprises an induced draft system, a purification system, a compression system, a separation and purification system and a pressurized liquefaction rectification system, and the pressurized liquefaction rectification system is connected with the PSA adsorption tower and used for receiving tower bottom gas. And the pressurized liquefaction rectification system can be used for carrying out pressurized liquefaction and liquefaction rectification on the tower bottom gas to obtain liquid carbon dioxide with the purity not lower than 99.9%. According to the invention, CNG and liquid carbon dioxide products can be simultaneously produced, only the liquefied tail gas part is cooled in the whole process, the energy consumption for liquefying all biogas is saved, the manufacturing cost of CNG is reduced, and meanwhile, the low-energy-consumption and high-efficiency recovery of carbon dioxide is also realized.

Description

Technical Field

[0001] The present invention belongs to the technical field related to biogas to CNG, and more specifically, particularly relates to a biogas to CNG co-producing carbon dioxide process and a biogas to CNG co-producing carbon dioxide system. Background Art

[0002] At present, in the production process of physically adsorbing biogas to prepare CNG, the tail gas after purifying methane will directly enter the chimney for emission. A large amount of carbon dioxide is contained in the emitted tail gas, and its concentration is about more than 90%. This tail gas containing high-concentration carbon dioxide not only pollutes the environment but also causes waste of resources. In the prior art, methods such as the PSA method, VPSA / QPSA, etc. only consider the methane recovery rate. The higher the methane recovery rate, the higher the carbon dioxide concentration in the tail gas, which can reach up to 98% at most. No matter which method is used, the tail gas after methane recovery is directly discharged.

[0003] In order to reduce the environmental pollution caused by carbon dioxide and effectively recover carbon dioxide for the reuse of available resources, the following technology is proposed in the article "Method for Extracting Pure Methane and Carbon Dioxide from Biogas Impurities and Its Special Device": The gas is liquefied by cooling, and then gaseous methane and gaseous carbon dioxide are separated by gasification. If this technology is applied to industrial CNG production, because it increases the liquefaction and condensation energy consumption and gasification energy consumption, it will lead to the problem of too high energy consumption in the entire process flow. Summary of the Invention

[0004] (I) Technical Problem

[0005] In summary, how to achieve low-energy consumption recovery of carbon dioxide in the production process of physically adsorbing biogas to prepare CNG has become an urgent problem to be solved by those skilled in the art.

[0006] (II) Technical Solution

[0007] The present invention provides a biogas to CNG co-producing carbon dioxide process. In the present invention, the biogas to CNG co-producing carbon dioxide process includes the following steps:

[0008] Step 1: Obtain the bottom gas after the PSA process;

[0009] Step 2: Only liquefy the bottom gas to obtain liquid carbon dioxide gas.

[0010] Preferably, in the process of producing CNG and co-producing carbon dioxide from biogas provided by the present invention, in the first step, CNG top gas with a concentration of not less than 96% is obtained; in the first step, the bottom gas of the tower contains methane gas, oxygen and carbon dioxide gas, wherein the concentration range of carbon dioxide gas is 95%-98%; in the second step, the purity of the liquid carbon dioxide gas is not less than 99.9%.

[0011] Preferably, in the process of producing CNG and co-producing carbon dioxide from biogas provided by the present invention, in the first step, biogas is used as the raw material gas to obtain the top gas for preparing CNG gas.

[0012] Preferably, in the process of producing CNG and co-producing carbon dioxide from biogas provided by the present invention, the first step includes: step a, biogas storage; step b, gas purification; step c, gas pressurization; step d, gas separation using the PSA process.

[0013] Preferably, in the process of producing CNG and co-producing carbon dioxide from biogas provided by the present invention, in step a, the induced draft system provides power for the biogas and transports it to the gas storage tank for biogas storage.

[0014] Preferably, in the process of producing CNG and co-producing carbon dioxide from biogas provided by the present invention, in step b, the gas is separated from impurities through a desulfurization device, a gas-liquid separator and a drying device to obtain clean and dry biogas.

[0015] Preferably, in the process of producing CNG and co-producing carbon dioxide from biogas provided by the present invention, in step c, the purified biogas is pressurized to 0.6-1 MPa by a compression system, wherein the compression system is composed of an industrial gas compressor to provide kinetic energy for gas separation.

[0016] Preferably, in the process of producing CNG and co-producing carbon dioxide from biogas provided by the present invention, in step d, a separation and purification system is used to separate the target gas from the pressurized biogas. The separation and purification system includes an adsorption tower, in which a target gas adsorbent is provided. After the target gas is adsorbed by the target gas adsorbent, it is released as the top gas. A buffer tank is connected to the adsorption tower for buffering and storing the top gas; wherein the target gas is methane gas, and the bottom gas discharged from the adsorption tower contains carbon dioxide gas with a concentration of 95%-98%, methane gas and oxygen.

[0017] Preferably, in the process of producing CNG and co-producing carbon dioxide from biogas provided by the present invention, in the second step, the bottom gas of the tower is pressurized to obtain liquefied bottom gas of the tower, the liquefied bottom gas of the tower is liquefied and rectified to obtain liquid carbon dioxide with a purity of not less than 99.9%, and the tail gas after liquefaction and rectification is directly discharged.

[0018] The present invention also provides a biogas-to-CNG co-production carbon dioxide system. In the present invention, the biogas-to-CNG co-production carbon dioxide system includes:

[0019] An induced draft system, the induced draft system includes an induced draft fan, which provides power for biogas through the induced draft fan and can store biogas in a gas storage tank;

[0020] A purification system, the purification system includes a desulfurization device for desulfurizing biogas, a gas-liquid separator for gas-liquid separation of biogas, and a drying device for drying the gas. The desulfurization device, the gas-liquid separator, and the drying device are connected in sequence, and the purification system is connected to the gas storage tank;

[0021] A compression system, the compression system is used to pressurize the gas, and the pressurization pressure range is 0.6 - 1 MPa. The compression system is used to provide kinetic energy for gas separation, and the compression system is connected to the purification system;

[0022] A separation and purification system, the separation and purification system includes a PSA adsorption tower and a buffer tank connected to the PSA adsorption tower for receiving the top gas. The separation and purification system is connected to the compression system;

[0023] A pressurized liquefaction and rectification system, the pressurized liquefaction and rectification system is connected to the PSA adsorption tower for receiving the bottom gas of the tower. The pressurized liquefaction and rectification system can pressurize and liquefy the bottom gas of the tower and perform liquefaction rectification to obtain liquid carbon dioxide with a purity of not less than 99.9%.

[0024] (III) Beneficial effects

[0025] The present invention provides a process for co-producing CNG from biogas and carbon dioxide, comprising: Step 1, obtaining the bottom gas after the PSA process; Step 2, only liquefying the bottom gas to obtain liquid carbon dioxide gas. In addition, the present invention also provides a system for co-producing CNG from biogas and carbon dioxide. The system for co-producing CNG from biogas and carbon dioxide includes: an induced draft system, which includes an induced draft fan, provides power for the biogas through the induced draft fan and can store the biogas in a gas storage tank; a purification system, which includes a desulfurization device for desulfurizing the biogas, a gas-liquid separator for separating gas and liquid from the biogas, and a drying device for drying the gas. The desulfurization device, the gas-liquid separator, and the drying device are connected in sequence, and the purification system is connected to the gas storage tank; a compression system, which is used to pressurize the gas, the pressurization pressure range is 0.6 - 1 MPa, the compression system provides kinetic energy for gas separation, and the compression system is connected to the purification system; a separation and purification system, which includes a PSA adsorption tower and a buffer tank connected to the PSA adsorption tower for receiving the top gas, and the separation and purification system is connected to the compression system; a pressurized liquefaction and rectification system, which is connected to the PSA adsorption tower and is used to receive the bottom gas. The pressurized liquefaction and rectification system can pressurize and liquefy the bottom gas and perform liquefaction rectification to obtain liquid carbon dioxide with a purity of not less than 99.9%.

[0026] In the present invention, the separation and purification system consists of an adsorption tower, an adsorbent, a buffer tank, etc., and can separate other gases such as methane and carbon dioxide. The main component of the top gas is methane, which is directly output as CNG. The discharged concentration at the bottom of the tower is about 95% - 98% carbon dioxide, methane, oxygen, etc. After liquefaction rectification of this part of the gas, liquid carbon dioxide with a purity of more than 99.9% is obtained, and the other part is pre-cooled and heat-exchanged with the bottom gas and then discharged. The control system includes valves, PLCs, solenoid valves, etc., and controls the opening, closing, and operation of the recovery process. Through the optimization of the process, the present invention can simultaneously produce CNG and liquid carbon dioxide products. Only the tail gas of the cooling liquefaction part is involved in the whole process, saving the energy consumption of liquefying all the biogas, reducing the manufacturing cost of CNG, and at the same time realizing the low-energy and high-efficiency recovery of carbon dioxide. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is a flow block diagram of the process for co-producing CNG from biogas and carbon dioxide in an embodiment of the present invention.

[0028] In Figure 1 the corresponding relationship between the component names and the reference numerals of the drawings is as follows:

[0029] Induced draft fan 1, gas storage tank 2, purification system 3, compression system 4, PSA adsorption tower 5,

[0030] Pressurized liquefaction and rectification system 6. DETAILED DESCRIPTION OF THE INVENTION

[0031] To make the objectives, technical solutions and advantages of the present invention more clear, the embodiments of the present invention will be further described in detail below with reference to the accompanying drawings.

[0032] In addition, in the description of the present invention, the orientation or positional relationship indicated by the terms "longitudinal", "transverse", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", etc. is based on the orientation or positional relationship shown in the accompanying drawings. It is only for the convenience of describing the present invention rather than requiring the present invention to be constructed and operated in a specific orientation. Therefore, it should not be construed as a limitation to the present invention. The terms "connected" and "coupled" used in the present invention should be understood in a broad sense. For example, it can be a fixed connection or a detachable connection; it can be directly connected or indirectly connected through an intermediate component. For those of ordinary skill in the art, the specific meanings of the above terms can be understood according to specific circumstances.

[0033] Please refer to Figure 1 , Figure 1 which is a process flow block diagram of a biogas-to-CNG combined production of carbon dioxide process in an embodiment of the present invention.

[0034] The present invention provides a biogas-to-CNG combined production of carbon dioxide process. In this process, the present invention includes the following two steps, namely: Step 1, obtaining the bottom gas after the PSA process; Step 2, only liquefying the bottom gas to obtain liquid carbon dioxide gas. In the PSA process, the adsorption tower adsorbs the target gas (methane). After a large amount of methane gas in the biogas is adsorbed, the remaining gas (tail gas, or bottom gas) mainly contains carbon dioxide gas, a small amount of methane gas, a small amount of oxygen, and other impurity gases. The target gas is absorbed by the adsorbent in the adsorption tower and then released, and discharged as the top gas to prepare CNG, and the tail gas enters the next process as the bottom gas. In the present invention, in Step 1, CNG top gas with a concentration of not less than 96% is obtained (after the top gas is output, it enters the buffer tank and then CNG is prepared); in Step 1, the bottom gas contains methane gas, oxygen, and carbon dioxide gas, wherein the concentration range of carbon dioxide gas is 95%-98%; in Step 2, the purity of the liquid carbon dioxide gas is not less than 99.9%.

[0035] Specifically, in Step 1, biogas is used as the raw material gas to obtain the top gas for preparing CNG gas.

[0036] Regarding Step 1, it specifically includes:

[0037] Step 1a, biogas storage; in Step 1a, the induced draft system provides power for the biogas and transports it to the gas storage tank 2 (in the present invention, a drain pipe is provided at the bottom of the gas storage tank 2 for discharging impurities at the bottom of the gas storage tank 2) for biogas storage.

[0038] Step 1b, gas purification; in Step 1b, the gas is separated from impurities and clean and dry biogas is obtained through a desulfurization device (in the present invention, water washing desulfurization is adopted, and a spray pipe is arranged at a position near the top of the desulfurization device for spraying desulfurization liquid, preferably water, and the gas enters the inside from the bottom of the desulfurization device and flows in a countercurrent manner with the desulfurization liquid), a gas-liquid separator, and a drying device.

[0039] Step 1c, gas pressurization; in Step 1c, the purified biogas is pressurized to 0.6 - 1 MPa through a compression system 4, where the compression system 4 is composed of an industrial gas compressor to provide kinetic energy for gas separation.

[0040] Step 1d, gas separation using the PSA process; in Step 1d, a separation and purification system is used to separate the target gas from the pressurized biogas. The separation and purification system includes an adsorption tower, in which a target gas adsorbent is arranged. After the target gas is adsorbed by the target gas adsorbent, it is released as the top gas of the tower. A buffer tank is connected to the adsorption tower for buffering and storing the top gas.

[0041] In the present invention, the target gas refers to methane gas, and the bottom gas discharged from the adsorption tower contains carbon dioxide gas, methane gas, and oxygen with a concentration of 95% - 98%.

[0042] In Step 2, the bottom gas of the tower is pressurized to obtain liquefied bottom gas of the tower, and the liquefied bottom gas of the tower is subjected to liquefaction rectification to obtain liquid carbon dioxide with a purity of not less than 99.9%. The tail gas after liquefaction rectification is directly discharged to the atmosphere.

[0043] The present invention also provides a biogas-to-CNG co-production carbon dioxide system, which includes: an air induction system, the air induction system includes an air induction fan 1, which provides power for the biogas through the air induction fan 1 and can store the biogas in a gas storage tank 2; a purification system 3, the purification system 3 includes a desulfurization device for desulfurizing the biogas, a gas-liquid separator for gas-liquid separation of the biogas, and a drying device for drying the gas. The desulfurization device, the gas-liquid separator, and the drying device are connected in sequence, and the purification system 3 is connected to the gas storage tank 2; a compression system 4, the compression system 4 is used to pressurize the gas, and the pressurization pressure range is 0.6-1 MPa. The compression system 4 provides kinetic energy for gas separation, and the compression system 4 is connected to the purification system 3; a separation and purification system, the separation and purification system includes a PSA adsorption tower 5 and a buffer tank connected to the PSA adsorption tower 5 for receiving the top gas. The separation and purification system is connected to the compression system 4; a pressurized liquefaction and rectification system 6, the pressurized liquefaction and rectification system 6 is connected to the PSA adsorption tower 5 and is used to receive the bottom gas of the tower. The pressurized liquefaction and rectification system 6 can pressurize and liquefy the bottom gas of the tower and perform liquefaction rectification to obtain liquid carbon dioxide with a purity of not less than 99.9%.

[0044] In the production process of preparing CNG by physical adsorption of biogas, in order to achieve low-energy consumption recovery of carbon dioxide, the present invention proposes the following technical solution: the biogas is subjected to air induction - storage - desulfurization and impurity removal - compression - PSA purification, so as to obtain CNG top gas with a purity of more than 96%. The bottom gas of the tower is a mixture of carbon dioxide and methane. The mixture is pressurized and liquefied after entering the compression unit to obtain a liquid carbon dioxide product with a purity of more than 99.9%.

[0045] The present invention consists of systems such as an air induction system, a purification system 3, a compression system 4, a separation and purification system, and a control system. The air induction system provides power for the biogas, so that it is transported to the gas storage tank 2 through the air induction fan 1. The purification system 3 provides a clean gas source for the downstream separation and purification system, including a desulfurization device, a gas-liquid separator, and a drying device. The compression system 4 pressurizes the biogas to 0.6-1 MPa. The compression system 4 is composed of an industrial gas compressor and provides kinetic energy for gas separation. The separation and purification system is the core of this process, consisting of an adsorption tower, adsorbent, buffer tank, etc., and can separate other gases such as methane and carbon dioxide. The main component of the top gas is methane, which is directly output as CNG. The discharged concentration at the bottom of the tower is about 95%-98% carbon dioxide, methane, oxygen, etc. This part of the gas is liquefied and rectified to obtain liquid carbon dioxide with a purity of more than 99.9%. The others are pre-cooled and heat-exchanged with the bottom gas of the tower and then discharged. The control system includes valves, PLCs, solenoid valves, etc., and controls the opening and closing and operation of the recovery process. In the present invention, CNG and liquid carbon dioxide products can be produced simultaneously. Only the cooled and liquefied tail gas part is cooled in the whole process, saving the energy consumption of liquefying all biogas and reducing the manufacturing cost of CNG.

[0046] In the present invention, the gas passing through the separation and purification system is split into overhead gas and bottom gas. The overhead gas is used to prepare CNG, and the bottom gas is a clean gas containing a large amount of carbon dioxide gas. The bottom gas will enter the pressurized liquefaction and rectification system 6. The specific structure of the pressurized liquefaction and rectification system 6 is as follows: It includes a rectification tower. A plurality of rectification towers are provided for multi-stage rectification of the bottom gas. A precooler, a liquefier, and a reboiler provided at the bottom of the tower are arranged in the rectification tower.

[0047] The precooler can precool the gas (bottom gas). The liquefier is arranged downstream of the precooler process and can liquefy the precooled gas. The low-temperature gas led from the top of the rectification tower is precooled and then enters the liquefier. The gas is further cooled and liquefied. The liquid raw material (all the bottom gas is liquefied, and this part of the liquid contains a large amount of liquid carbon dioxide and a very small amount of liquid nitrogen, liquid oxygen, liquid methane, etc.) enters the food-grade rectification tower (secondary rectification tower) for further rectification and purification. The non-condensable gases nitrogen, oxygen, and methane are discharged from the top of the rectification tower. The reboiler provided at the bottom of the tower can obtain the food-grade product (food-grade carbon dioxide). The product is led out from the bottom of the reboiler (output in the form of liquid carbon dioxide), and after throttling, depressurizing, and subcooling, it is directly sent to the food-grade finished product tank for storage.

[0048] The cooling and liquefaction of carbon dioxide gas are realized by the refrigeration system. The gas Freon of the refrigeration system enters the screw chiller for compression and then enters the evaporative condenser. The gaseous Freon is cooled by air and cooled down to become liquid and is stored in the liquid receiver. The liquid Freon led out from the liquid receiver is respectively sent to the liquefier and the top total condenser. After throttling, the gas carbon dioxide is cooled and liquefied, and the liquid Freon itself is vaporized and returns to the chiller again. The refrigeration unit consists of a screw compressor, an evaporative condenser, a liquid Freon receiver, a gas-liquid separator, etc.

[0049] As is known by common technical knowledge, the present invention can be implemented by other embodiments without departing from its spiritual essence or essential features. Therefore, the above-disclosed embodiments are illustrative in all aspects and are not the only ones. All changes within the scope of the present invention or within the scope equivalent to the present invention are encompassed by the present invention.

Claims

1. A biogas-to-CNG combined production of carbon dioxide process, characterized in that, Comprising: Step 1: Obtain the bottom gas of the tower after the PSA process; Step 2: Only liquefy the bottom gas of the tower to obtain liquid carbon dioxide gas.

2. The biogas-to-CNG co-production of carbon dioxide process according to claim 1, wherein in the said Step 1, obtain CNG top gas with a concentration of not less than 96%; in the said Step 1, the bottom gas of the tower contains methane gas, oxygen and carbon dioxide gas, wherein the concentration range of carbon dioxide gas is 95% - 98%; in the said Step 2, the purity of the liquid carbon dioxide gas is not less than 99.9%.

3. The biogas-to-CNG co-production of carbon dioxide process according to claim 1, wherein in the said Step 1, use biogas as the raw material gas to obtain the top gas for preparing CNG gas.

4. The biogas-to-CNG co-production of carbon dioxide process according to claim 1, wherein the said Step 1 includes: Step 1a: Biogas storage; Step 1b: Gas purification; Step 1c: Gas pressurization; Step 1d: Use the PSA process for gas separation.

5. The biogas-to-CNG co-production of carbon dioxide process according to claim 4, wherein in the said Step 1a, the induced draft system provides power for the biogas and transports it to the gas storage tank for biogas storage.

6. The biogas-to-CNG co-production of carbon dioxide process according to claim 4, wherein in the said Step 1b, the gas is separated from impurities through a desulfurization device, a gas-liquid separator and a drying device to obtain clean and dry biogas.

7. The biogas-to-CNG co-production of carbon dioxide process according to claim 4, wherein in the said Step 1c, the purified biogas is pressurized to 0.6 - 1 MPa through a compression system, wherein the compression system consists of an industrial gas compressor to provide kinetic energy for gas separation.

8. The biogas-to-CNG co-production of carbon dioxide process according to claim 4, wherein in the said Step 1d, a separation and purification system is used to separate the target gas from the pressurized biogas. The separation and purification system includes an adsorption tower, in which a target gas adsorbent is arranged. After the target gas is adsorbed by the target gas adsorbent, it is released as the top gas. A buffer tank is connected to the adsorption tower for buffering and storing the top gas; wherein the target gas is methane gas, and the bottom gas discharged from the adsorption tower contains carbon dioxide gas, methane gas and oxygen with a concentration of 95% - 98%.

9. The biogas-to-CNG co-production of carbon dioxide process according to any one of claims 1 to 8, wherein in the said Step 2, the bottom gas of the tower is pressurized to obtain liquefied bottom gas of the tower, the liquefied bottom gas of the tower is liquefied and rectified to obtain liquid carbon dioxide with a purity of not less than 99.9%, and the tail gas after liquefied rectification is directly discharged.

10. A biogas-based CNG production system with carbon dioxide co-production, characterized in that, Comprising: An induced draft system, the induced draft system includes an induced draft fan (1), and the induced draft fan provides power for the biogas and can store the biogas in the gas storage tank (2); Purification system (3), the purification system includes a desulfurization device for desulfurizing biogas, a gas-liquid separator for gas-liquid separation of biogas, and a drying device for drying the gas. The desulfurization device, the gas-liquid separator, and the drying device are connected in sequence, and the purification system is connected to the gas storage tank; Compression system (4), the compression system is used to pressurize the gas, and the pressurization pressure range is 0.6 - 1 MPa. The compression system is used to provide kinetic energy for gas separation, and the compression system is connected to the purification system; Separation and purification system, the separation and purification system includes a PSA adsorption tower (5) and a buffer tank connected to the PSA adsorption tower for receiving the top gas. The separation and purification system is connected to the compression system; Pressurized liquefaction and rectification system (6), the pressurized liquefaction and rectification system is connected to the PSA adsorption tower and is used to receive the bottom gas of the tower. The pressurized liquefaction and rectification system can pressurize and liquefy the bottom gas of the tower and perform liquefaction rectification to obtain liquid carbon dioxide with a purity of not less than 99.9%.

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