Rough decarburization system and process for natural gas with high CO2 content

Abstract

The invention discloses a high-CO2-content natural gas coarse decarbonization system and process, and the coarse decarbonization process comprises the following steps: raw material gas is subjected to primary flash separation after primary heat exchange cooling, a gas phase product obtained through separation is subjected to secondary flash separation after secondary heat exchange cooling, and coarse decarbonized natural gas is obtained; liquid-phase products obtained through two times of flash separation are subjected to throttling refrigeration and then serve as a heat exchange cooling medium to be reheated, and finally CO2 is recycled through a compression and purification technology. Throttling refrigeration is achieved through the high pressure of the feed gas, meanwhile, the two-stage flash separation device is arranged to make full use of cold energy, and therefore low-energy-consumption large-proportion coarse decarburization is achieved, natural gas purification can be further conducted in cooperation with other processes, and then the overall process energy consumption is greatly reduced. In addition, CO2 is efficiently recovered by adopting a compression purification technology, so that high-value utilization is facilitated, and the method is green and environment-friendly.

Description

Technical Field

[0001] The present invention belongs to the technical field of natural gas decarbonization and purification, and in particular relates to a system and process for crude decarbonization of natural gas with high CO2 content. Background Art

[0002] In the context of responding to global climate change, natural gas, as an important low-carbon fossil energy, plays a particularly key role in the transition period of energy transformation. As CO2 injection and natural gas production projects deepen, the CO2 content in natural gas wells is gradually enriched, with a volume fraction of up to 70%. The recovered gas is difficult to be used as a resource, and a large number of high-CO2 natural gas wells are waiting to be developed.

[0003] Natural gas contains acidic impurity gas CO2 during the extraction process, which not only reduces the calorific value of natural gas, but also causes corrosion to the transmission pipeline and increases transportation costs. In order to meet the needs of natural gas use and pipeline transportation, carbon capture technology must be used to strictly remove CO2. Conventional decarbonization technologies include membrane separation, pressure swing adsorption, alcohol amine absorption, and cryogenic methods. However, existing natural gas decarbonization technologies are not suitable for working conditions with such high CO2 content, and face huge challenges in terms of decarbonization energy consumption, CH4 purity, and recovery rate.

[0004] The decarbonization of high-CO2 natural gas is a process of gradually changing from high-concentration CO2 decarbonization to low-concentration CO2 decarbonization and gradually purifying CH4. A single natural gas decarbonization technology is difficult to take into account both the process energy consumption and the requirements of CH4 product purity. The combined process of different CO2 capture technologies is a reasonable choice to overcome these challenges. It can integrate the advantages of a single technology and make up for the shortcomings of each technology. Therefore, it is urgent to design a system and process for crude decarbonization of high-CO2 natural gas that can achieve low energy consumption and large-proportion crude decarbonization. Summary of the invention

[0005] Purpose of the invention: In order to overcome the technical problems existing in the prior art, the present invention provides a system and process for crude decarbonization of natural gas with a high CO2 content, which utilizes the high-pressure throttling of the raw gas to convert it into cold energy, and then utilizes the cold energy to achieve gas-liquid separation, greatly reducing the energy consumption of decarbonization, and at the same time adopts compression and purification technology to efficiently recover CO2, which is conducive to high-value utilization.

[0006] Invention content: To achieve the above objectives, the present invention provides a system for crude decarbonization of natural gas with high CO2 content, comprising a primary heat exchanger, a primary flash separator, a primary throttle valve, a secondary heat exchanger, a secondary flash separator and a secondary throttle valve;

[0007] The raw gas is cooled by heat exchange in the primary heat exchanger and then enters the primary flash separator for primary gas-liquid separation. The separated gas phase product is further cooled by heat exchange in the secondary heat exchanger and then enters the secondary flash separator for secondary gas-liquid separation, thereby obtaining crude decarbonized natural gas.

[0008] The liquid products of the primary flash separator and the secondary flash separator are throttled and refrigerated by the primary throttling valve and the secondary throttling valve respectively, and then reheated as a heat exchange cooling medium, and finally CO2 is recovered through compression purification technology.

[0009] Specifically, the liquid phase product of the first-stage flash separator is throttled and cooled in a first-stage throttle valve, and then reheated as a cooling medium in a first-stage heat exchanger;

[0010] The liquid phase product of the secondary flash separator is firstly reheated as the cooling medium of the secondary heat exchanger, then throttled and cooled by the secondary throttle valve, and finally reheated by the secondary heat exchanger and the primary heat exchanger in sequence.

[0011] The reason why the liquid product of the secondary flash separator is first reheated as the cooling medium of the secondary heat exchanger and then throttled and cooled by the secondary throttling valve is, on the one hand, to make full use of the cold capacity. The temperature of the liquid product obtained by the secondary flash separation can reach about -54°C. The cold capacity is first recovered through the secondary heat exchanger to increase the stream temperature, and then the stream temperature is reduced through the secondary throttling valve to further recover the cold capacity. On the other hand, it is to ensure that the stream temperature after throttling is higher than the triple point temperature of CO2, that is, -56.6°C, to avoid blockage caused by condensation of CO2 in the stream pipeline.

[0012] Furthermore, the system also includes a compressor, a refrigeration unit and a distillation tower, wherein the two-stage liquid products after throttling and reheating are pressurized by the compressor, and then pass through a primary heat exchanger and a refrigeration unit in sequence for cooling and liquefaction, and then enter the distillation tower for distillation and purification, thereby obtaining high-purity CO2 liquid, whose CO2 volume fraction can reach 99.9%, and can be directly used for high value.

[0013] Furthermore, the gas phase product at the top of the distillation tower is mixed with the feed gas and then circulated for decarbonization, so that the natural gas component in the two-stage liquid phase products can be further recovered.

[0014] Furthermore, the gas phase product of the secondary flash separator is successively passed through a secondary heat exchanger and a primary heat exchanger to recover cold energy, thereby obtaining a crude decarbonized natural gas.

[0015] Specifically, the raw gas is natural gas with a high CO2 content after dehydration and drying, wherein the volume fraction of CO2 is higher than 50%.

[0016] Specifically, the volume fraction of CO2 in the crude decarbonized natural gas is lower than 28%, thereby achieving the goal of large-scale crude decarbonization and effectively reducing the power consumption of the subsequent CH4 purification process.

[0017] In addition, the present invention also provides a crude decarbonization process for high-CO2-containing natural gas, comprising the following steps:

[0018] The raw gas is subjected to a primary heat exchange cooling and then a primary flash separation, and the separated gas phase product is subjected to a secondary heat exchange cooling and then a secondary flash separation, thereby obtaining a crude decarbonized natural gas;

[0019] The liquid products obtained from the two flash separations are throttling refrigerated respectively, and then reheated as a heat exchange cooling medium, and finally the CO2 is recovered through compression purification technology.

[0020] Specifically, the liquid product obtained from the primary flash separation is reheated as a primary heat exchange cooling medium after throttling refrigeration;

[0021] The liquid product obtained from the secondary flash separation is firstly reheated as a secondary heat exchange cooling medium, then throttling cooled, and finally reheated as a secondary heat exchange cooling medium and a primary heat exchange cooling medium in sequence.

[0022] Specifically, the liquid phase product obtained by the two flash separations is first compressed and cooled for liquefaction after throttling and reheating, and then distilled and purified to obtain high-purity CO2 liquid, wherein the gas phase product obtained by distillation and purification is mixed with the raw gas and then circulated for decarbonization.

[0023] Beneficial effects: The present invention utilizes the high pressure of the raw gas itself to achieve throttling refrigeration, and at the same time sets up a two-stage flash separation device to fully utilize the cooling capacity, thereby achieving low-energy consumption and large-proportion crude decarbonization. The present invention is based on the concept of graded decarbonization. On the basis of achieving low-energy consumption and large-proportion crude decarbonization, it further combines other processes for natural gas purification, which can greatly reduce the overall process energy consumption. In addition, the present invention uses compression purification technology to efficiently recover CO2, which is conducive to high-value utilization, green and environmental protection, energy saving and high efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a schematic diagram of the process of a crude decarbonization system for high-CO2-containing natural gas in an embodiment of the present invention;

[0025] The figure includes: 1. a primary flash separator, 2. a primary throttle valve, 3. a primary heat exchanger, 4. a secondary heat exchanger, 5. a secondary flash separator, 6. a secondary throttle valve, 7. a compressor, 8. a refrigeration unit, and 9. a distillation tower. DETAILED DESCRIPTION

[0026] To make the technical solution of the present invention clearer, the technical solution in the embodiment of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiment of the present invention. Obviously, the described embodiment is a part of the embodiment of the present invention, not all the embodiments. Based on the embodiment of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0027] Reference Figure 1 , this embodiment provides a system for crude decarbonization of natural gas with high CO2 content, comprising a primary heat exchanger 3, a primary flash separator 1, a primary throttle valve 2, a secondary heat exchanger 4, a secondary flash separator 5, a secondary throttle valve 6, a compressor 7, a refrigeration unit 8 and a distillation tower 9;

[0028] The raw gas is mixed with the gas phase reflux product at the top of the distillation tower 9, and after heat exchange and cooling in the primary heat exchanger 3, enters the primary flash separator 1 for primary gas-liquid separation. The top gas phase product obtained by separation is further heat exchanged and cooled in the secondary heat exchanger 4, and enters the secondary flash separator 5 for secondary gas-liquid separation, thereby obtaining crude decarbonized natural gas;

[0029] The bottom liquid product of the primary flash separator 1 is throttled and cooled in the primary throttle valve 2, and then reheated as the cooling medium of the primary heat exchanger 3; the bottom liquid product of the secondary flash separator 5 is first reheated as the cooling medium of the secondary heat exchanger 4, then throttled and cooled through the secondary throttle valve 6, and finally reheated through the secondary heat exchanger 4 and the primary heat exchanger 3 in sequence;

[0030] The two-stage liquid products after throttling and reheating are pressurized by the compressor 7, and then pass through the primary heat exchanger 3 and the refrigeration unit 8 for cooling and liquefaction, and then enter the distillation tower 9 for distillation and purification. A high-purity CO2 liquid product is obtained from the bottom outlet of the distillation tower 9, and the gaseous product at the top of the distillation tower 9 is mixed with the raw gas for circulating decarbonization.

[0031] Specifically, the raw gas is a high-CO2 natural gas after dehydration and drying, in which the volume fraction of CO2 is >50%. At the same time, based on the original properties of the gas well, the raw gas itself has high pressure (above 3MPa, up to tens of MPa). Based on this, the present invention creatively uses the high pressure of the raw gas itself to achieve throttling refrigeration, and adopts a two-stage throttle valve to produce cold capacity, thereby meeting the heat exchange requirements. In order to avoid blockage caused by condensation of CO2 in the stream pipeline, the stream temperature after throttling by the two-stage throttle valve should be higher than the triple point temperature of CO2, that is, -56.6°C, and on this basis, the primary and secondary flash evaporation temperatures are reasonably set. Generally speaking, the primary flash evaporation temperature can reach about -25°C, and the secondary flash evaporation temperature can reach about -54°C, thereby realizing the liquefaction separation of CO2 through two heat exchange cooling.

[0032] Specifically, the primary heat exchanger 3 and the secondary heat exchanger 4 are both multi-stream heat exchangers to fully utilize the cooling capacity generated by high-pressure throttling; if the inlet stream pressures of the compressor 7 are different, two compressors 7 can be provided for the compressor 7, and the streams are respectively compressed at high and low pressures and then merged, or an intermediate gas-filling compressor 7 can be provided; the process combines throttling refrigeration and ammonia refrigeration mechanism refrigeration, and the raw gas is liquefied and then subjected to preliminary flash separation, and then distillation and purification to ensure the purity of the CO2 liquid product; no condenser is provided at the top of the distillation tower 9, and a reboiler is provided at the bottom to achieve the discharge of high-purity CO2 liquid products from the bottom outlet of the distillation tower 9, wherein the volume fraction of CO2 can reach 99.9%, which can be directly used for high value.

[0033] Furthermore, the top gas phase product of the secondary flash separator 5 passes through the secondary heat exchanger 4 and the primary heat exchanger 3 in sequence to further recover cold energy, thereby obtaining a crude decarbonized natural gas product, in which the volume fraction of CO2 is 23% to 26%, so as to achieve a stable CO2 concentration in the crude decarbonized natural gas product and reduce the impact on the subsequent CH4 purification process.

[0034] In addition, this embodiment also provides a crude decarbonization process for high CO2-containing natural gas, which specifically includes:

[0035] The raw gas is subjected to a primary heat exchange cooling and then a primary flash separation, and the separated gas phase product is subjected to a secondary heat exchange cooling and then a secondary flash separation, thereby obtaining a crude decarbonized natural gas;

[0036] The liquid product obtained from the primary flash separation is reheated as the primary heat exchange cooling medium after throttling refrigeration; the liquid product obtained from the secondary flash separation is first reheated as the secondary heat exchange cooling medium, then throttling refrigeration is performed, and finally reheated as the secondary heat exchange cooling medium and the primary heat exchange cooling medium in sequence;

[0037] The liquid product obtained from the two flash separations is first compressed and cooled for liquefaction after throttling and reheating, and then distilled and purified to obtain high-purity CO2 liquid, wherein the gas product obtained by distillation and purification is mixed with the raw gas and then circulated for decarbonization.

[0038] After the raw gas is treated by the crude decarbonization process of high-CO2 natural gas, it can be combined with other CH4 purification processes, such as amine absorption, pressure swing adsorption and membrane separation, to further achieve low-concentration CO2 natural gas decarbonization. Taking this process combined with the amine absorption decarbonization process as an example, when the CO2 concentration of the raw gas is 70%, the decarbonization amount of the crude decarbonization process can account for 87% of the total decarbonization amount, while the decarbonization energy consumption of the traditional amine absorption method can reach 300kWh / t CO2. Since the decarbonization amount of the purification process only accounts for 13% of the total, the overall energy consumption of the combined decarbonization process can be reduced to 120kWh / t CO2. Compared with the single amine absorption process, the combined decarbonization process can significantly reduce the overall energy consumption.

[0039] The following examples use three different concentrations of high CO2 (>50%) natural gas for experimental simulation. The simulated raw gas components and contents are shown in Table 1. In the experiment, the mechanical loss of the device, pipeline pressure loss, etc. are negligible, the insulation efficiency of equipment such as the compressor is 0.85, the pressure drop in the distillation tower is calculated as 2kPa, and the heat exchanger channel resistance is calculated as 1kPa. Referring to the actual project, the temperature difference at the heat exchanger end of the refrigerator is 10°C, the minimum temperature difference at the heat exchanger end is 1.5°C, the temperature difference at the interstage cooler end of the compressor is 5°C, and the cooling circulating water temperature is 30-40°C.

[0040] Table 1. Composition and content of simulated raw gas

[0041]

[0042] Embodiment 1:

[0043] Using Aspen Plus system for simulation calculation, the CO2 volume fraction in the raw gas is 50%, and the raw gas pressure after dehydration and drying is 3MPa. Its main production indicators are as follows:

[0044] Crude degassing gas stream:

[0045] Ingredients: CO2: 23.89%, CH4: 75.73%, C2H4: 0.227%, N2: 0.154%;

[0046] Temperature: 10℃;

[0047] CH4 recovery rate: 99.99%;

[0048] Product CO2 stream:

[0049] Ingredients: CO2: 99.9%, CH4: 0.0008%, C2H4: 0.149%, N2: 0%;

[0050] Pressure: 30bar;

[0051] Temperature: -5.4℃;

[0052] Liquid CO2 product mass flow rate: 12.16t / h;

[0053] Liquid CO2 product purity: 99.9%;

[0054] Rough CO2 removal ratio: 70%;

[0055] Energy consumption of refrigeration unit: 520.2kW;

[0056] Compressor energy consumption: 646.6kW;

[0057] Cooling water consumption: 2111.9t / h;

[0058] System unit energy consumption (calculated based on CO2 in the product): 103.9kWh / t CO2.

[0059] Embodiment 2:

[0060] Using Aspen Plus system for simulation calculation, the CO2 volume fraction in the raw gas is 70%, and the raw gas pressure after dehydration and drying is 3MPa. Its main production indicators are as follows:

[0061] Crude degassing gas stream:

[0062] Ingredients: CO2: 23.88%, CH4: 75.58%, C2H4: 0.284%, N2: 0.259%;

[0063] Temperature: 10℃;

[0064] CH4 recovery rate: 99.99%;

[0065] Product CO2 stream:

[0066] Ingredients: CO2: 99.9%, CH4: 0.003%, C2H4: 0.147%, N2: 0%;

[0067] Pressure: 30bar;

[0068] Temperature: -5.3℃;

[0069] Liquid CO2 product mass flow rate: 21.30t / h;

[0070] Liquid CO2 product purity: 99.9%;

[0071] Rough CO2 removal ratio: 87%;

[0072] Energy consumption of refrigeration unit: 910.3kW;

[0073] Compressor energy consumption: 947.9kW;

[0074] Cooling water consumption: 352.8t / h;

[0075] System unit energy consumption (calculated based on CO2 in the product): 96.1kWh / t CO2.

[0076] Embodiment 3:

[0077] Using Aspen Plus system for simulation calculation, the CO2 volume fraction in the raw gas is 80%, and the raw gas pressure after dehydration and drying is 3MPa. Its main production indicators are as follows:

[0078] Crude degassing gas stream:

[0079] Ingredients: CO2: 23.87%, CH4: 75.40%, C2H4: 0.347%, N2: 0.393%;

[0080] Temperature: 10℃;

[0081] CH4 recovery rate: 99.99%;

[0082] Product CO2 stream:

[0083] Ingredients: CO2: 99.9%, CH4: 0.003%, C2H4: 0.150%, N2: 0%;

[0084] Pressure: 30bar;

[0085] Temperature: -5.3℃;

[0086] Liquid CO2 product mass flow rate: 25.86t / h;

[0087] Liquid CO2 product purity: 99.9%;

[0088] Rough CO2 removal ratio: 93%;

[0089] Energy consumption of refrigeration unit: 1164.2kW;

[0090] Compressor energy consumption: 1178.7kW;

[0091] Cooling water consumption: 450.3t / h;

[0092] System unit energy consumption (calculated based on CO2 in the product): 101.9kWh / t CO2.

[0093] It can be seen from the above examples that for raw gas with different CO2 concentrations, this process can achieve low energy consumption and large-scale decarbonization. The high-pressure throttling refrigeration of the raw gas can effectively reduce the decarbonization energy consumption. At the same time, the CO2 removal ratio of the raw gas can reach up to 90%, and a CO2 liquid product with a purity of 99.9% can be obtained, which can be directly used for high value. In summary, the combined decarbonization process based on this rough decarbonization process can significantly reduce the overall process decarbonization energy consumption compared to a single process.

[0094] The foregoing is merely a specific embodiment of the present invention, which enables those skilled in the art to understand or implement the present invention. Various modifications to these embodiments will be apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention will not be limited to the embodiments shown herein, but rather to the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A system for raw decarbonization of high-CO2 natural gas, characterized in that: It includes a primary heat exchanger, a primary flash separator, a primary throttle valve, a secondary heat exchanger, a secondary flash separator and a secondary throttle valve; The raw gas is cooled by heat exchange in the primary heat exchanger and then enters the primary flash separator for primary gas-liquid separation. The separated gas phase product is further cooled by heat exchange in the secondary heat exchanger and then enters the secondary flash separator for secondary gas-liquid separation, thereby obtaining crude decarbonized natural gas. The liquid products of the primary flash separator and the secondary flash separator are throttled and refrigerated by the primary throttling valve and the secondary throttling valve respectively, and then reheated as a heat exchange cooling medium, and finally CO2 is recovered through compression purification technology.

2. The system for raw decarbonization of high CO2-containing natural gas according to claim 1, characterized in that: The liquid phase product of the first-stage flash separator is throttled and refrigerated in the first-stage throttle valve, and then reheated as the cooling medium of the first-stage heat exchanger; The liquid phase product of the secondary flash separator is firstly reheated as the cooling medium of the secondary heat exchanger, then throttled and cooled by the secondary throttle valve, and finally reheated by the secondary heat exchanger and the primary heat exchanger in sequence.

3. The system for raw decarbonization of natural gas with high CO2 content according to claim 1, characterized in that: It also includes a compressor, a refrigeration unit and a distillation tower, wherein the two-stage liquid products after throttling and reheating are pressurized by the compressor, and then pass through a primary heat exchanger and a refrigeration unit in sequence for cooling and liquefaction, and then enter the distillation tower for distillation and purification, thereby obtaining high-purity CO2 liquid.

4. The system for raw decarbonization of high CO2-containing natural gas according to claim 3 is characterized in that: The gas phase product at the top of the distillation tower is mixed with the feed gas and then circulated for decarbonization.

5. The system for raw decarbonization of natural gas with high CO2 content according to claim 1, characterized in that: The gas phase product of the secondary flash separator is passed through the secondary heat exchanger and the primary heat exchanger in sequence to recover cold energy, thereby obtaining a crude decarbonized natural gas.

6. The system for raw decarbonization of natural gas with high CO2 content according to claim 1, characterized in that: The raw gas is natural gas with a high CO2 content after dehydration and drying, wherein the volume fraction of CO2 is higher than 50%.

7. The system for raw decarbonization of natural gas with high CO2 content according to claim 1, characterized in that: The volume fraction of CO2 in the raw decarbonized natural gas is lower than 28%.

8. A process for crude decarbonization of high CO2-containing natural gas, characterized in that: The following steps are involved: The raw gas is subjected to a primary heat exchange cooling and then a primary flash separation, and the separated gas phase product is subjected to a secondary heat exchange cooling and then a secondary flash separation, thereby obtaining a crude decarbonized natural gas; The liquid products obtained from the two flash separations are throttling refrigerated respectively, and then reheated as a heat exchange cooling medium, and finally the CO2 is recovered through compression purification technology.

9. The process for crude decarbonization of high CO2-containing natural gas according to claim 8, characterized in that: The liquid product obtained from the primary flash separation is reheated as a primary heat exchange cooling medium after throttling refrigeration; The liquid product obtained from the secondary flash separation is firstly reheated as a secondary heat exchange cooling medium, then throttling cooled, and finally reheated as a secondary heat exchange cooling medium and a primary heat exchange cooling medium in sequence.

10. The process for crude decarbonization of high CO2-containing natural gas according to claim 8, characterized in that: The liquid product obtained from the two flash separations is first compressed and cooled for liquefaction after throttling and reheating, and then distilled and purified to obtain high-purity CO2 liquid, wherein the gas product obtained by distillation and purification is mixed with the raw gas and then circulated for decarbonization.

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