Energy-saving natural gas liquefaction process system based on waste heat recovery

Abstract

The invention provides an energy-saving natural gas liquefaction process system based on waste heat recovery, and belongs to the technical field of natural gas liquefaction, and the energy-saving natural gas liquefaction process system based on waste heat recovery comprises a cooling circulation system and a waste heat recovery system. And an efficient and closed-loop energy gradient utilization network is constructed. The system not only fully utilizes high-temperature waste gas discharged by the gas turbine to drive the steam turbine to generate power, but also further recovers medium-low-grade waste heat in exhaust gas of the steam turbine and heat generated in the compression process of a mixed refrigerant for pre-cooling raw material natural gas. According to the multi-stage and multi-source waste heat recovery mode, the energy consumption of a traditional pre-cooling link is remarkably reduced, the overall thermodynamic perfection degree of the system is improved, and efficient and low-carbon utilization of energy is achieved.

Description

Technical Field

[0001] This invention belongs to the field of natural gas liquefaction technology, specifically relating to an energy-saving natural gas liquefaction process system based on waste heat recovery. Background Technology

[0002] Natural gas, as a clean and efficient fossil fuel, has seen continuous growth in global consumption. To facilitate long-distance transportation and storage, natural gas is often cooled to approximately -162°C at atmospheric pressure, converting it into liquefied natural gas (LNG). However, the LNG liquefaction process is a high-energy-consuming industrial process, with the refrigerant compression and cooling stage being the primary energy source. Therefore, reducing energy consumption in the liquefaction process is crucial for improving project economics and promoting green and low-carbon development.

[0003] In existing technologies, to improve energy efficiency, some liquefaction plants have attempted to recover high-temperature exhaust gases (typically 400℃-600℃) emitted by gas turbines, for example, by using waste heat boilers to generate steam for power generation. However, this recovery method has significant limitations and incompleteness, as detailed below;

[0004] First, the high-temperature exhaust gas recovery is incomplete. The low- and medium-grade gas-liquid mixture emitted after the steam turbine generates electricity still carries a considerable amount of heat. In the existing system, this heat is often directly discharged into the environment through the condenser, resulting in a secondary waste of energy.

[0005] Secondly, the low- and medium-grade waste heat in the core refrigeration cycle is neglected. Inside the cooling cycle system, after the mixed refrigerant is discharged by the compressor, its temperature and pressure increase significantly. Before entering the expansion device (such as a throttle valve or expander) for cooling and liquefaction, this high-temperature and high-pressure refrigerant working fluid must be cooled in the heat exchanger. Existing processes generally use external cold sources such as ambient air or cooling water to directly cool it. In essence, the heat converted from the compression work, which has a grade significantly higher than that of the environment, is simply and unidirectionally dissipated. This process wastes a huge amount of low- and medium-temperature heat energy, resulting in the energy contained in the compression work not being fully utilized. This is an important reason for the low thermodynamic perfection of the entire liquefaction system.

[0006] Finally, the precooling process is disconnected from the internal waste heat. At the front end of the liquefaction process, the raw natural gas usually needs to be precooled before entering the main liquefaction heat exchanger. Currently, the precooling process relies on independent precooling units that burn more fuel or consume additional electricity, which further increases the system's energy consumption and carbon emissions.

[0007] Therefore, to solve the above problems, an energy-saving natural gas liquefaction process system based on waste heat recovery is needed. Summary of the Invention

[0008] The purpose of this invention is to provide an energy-saving natural gas liquefaction process system based on waste heat recovery, which aims to solve the problem of low-grade waste heat being wasted in the natural gas liquefaction process in the prior art.

[0009] To achieve the above objectives, the present invention provides the following technical solution:

[0010] An energy-saving natural gas liquefaction process system based on waste heat recovery includes a cooling circulation system and a waste heat recovery system;

[0011] The cooling cycle system includes a first heat exchange device, a compressor, an expansion device, a gas turbine, a refrigerant supply module, and a second heat exchange device. The hot flow channel of the first heat exchange device is for the liquefied natural gas to pass through. The cold flow channel of the first heat exchange device, the compressor, the hot flow channel of the second heat exchange device, and the expansion device are connected by pipes to form a cooling cycle pipeline. The refrigerant supply module is connected to the cooling cycle pipeline to replenish the mixed refrigerant. The output shaft of the gas turbine is connected to the compressor to drive its operation.

[0012] The waste heat recovery system includes a waste heat boiler, a steam turbine, a generator, a third heat exchange device, a liquid storage tank, and a drive pump. The hot flow channels of the waste heat boiler, the steam turbine, the third heat exchange device, the liquid storage tank, the drive pump, and the cold flow channel of the second heat exchange device are connected by pipelines to form a recovery circulation pipeline for recovering the waste heat generated by the steam turbine and the compressor.

[0013] Preferably, the gas turbine includes an air inlet, a feed inlet, and a discharge outlet. The feed inlet of the gas turbine is connected to a fuel supply device for supplying natural gas fuel to the gas turbine. The waste heat boiler includes an air inlet, an air outlet, a liquid inlet, and a liquid outlet. The discharge outlet of the gas turbine is connected to the air inlet of the waste heat boiler for supplying high-temperature waste gas to the waste heat boiler. The high-temperature waste gas entering the waste heat boiler can heat the liquid inside the boiler body to generate steam. The high-temperature waste gas after use is discharged through the air outlet of the waste heat boiler.

[0014] Preferably, the steam turbine includes a steam inlet and a steam outlet. The liquid outlet of the waste heat boiler is connected to the steam inlet of the steam turbine to provide steam to the steam turbine and drive it to work. The output shaft of the steam turbine is connected to a generator, which drives the generator to work through the steam turbine, thereby achieving energy saving in power generation. The heat flow channel inlet of the third heat exchange device is connected to the steam outlet of the steam turbine.

[0015] Preferably, the waste heat recovery system further includes a raw material supply module and a transfer valve. The transfer valve includes an inlet, a first feed port, and a second feed port. The outlet of the raw material supply module is connected to the inlet of the transfer valve. The first feed port of the transfer valve is connected to the inlet of the cold flow channel of the third heat exchange device. The second feed port of the transfer valve and the outlet of the cold flow channel of the third heat exchange device are both connected to the inlet of the hot flow channel of the first heat exchange device.

[0016] Preferably, a first temperature detection device is provided at the steam outlet of the steam turbine to detect the temperature W1 of the gas-liquid mixture, and a second temperature detection device is provided at the feed inlet of the transfer valve to detect the temperature W2 of the natural gas entering the transfer valve.

[0017] Preferably, the temperature W1 of the gas-liquid mixture and the temperature W2 of the natural gas are obtained. If the temperature W2 of the natural gas is higher than the temperature W1 of the gas-liquid mixture, the first feed port is opened and the second feed port is closed. The gas-liquid mixture discharged from the steam outlet of the steam turbine can exchange heat with the natural gas to be liquefied in the cold flow channel of the third heat exchange equipment when it flows through the hot flow channel of the third heat exchange equipment. This can raise the temperature of the gas-liquid mixture and lower the temperature of the natural gas to be liquefied, thus achieving the initial pre-cooling of the natural gas.

[0018] If the temperature W2 of the natural gas is lower than or equal to the temperature W1 of the gas-liquid mixture, the first feed port is closed and the second feed port is opened, allowing the natural gas inside the transfer valve to enter the heat flow channel of the first heat exchanger.

[0019] Preferably, the drive pump is used to control the circulation of liquid inside the storage tank in the recovery circulation pipeline.

[0020] Preferably, a third temperature detection device is provided at the inlet of the heat flow channel of the second heat exchange device to detect the temperature W3 when the mixed refrigerant enters, and a fourth temperature detection device is provided at the outlet of the heat flow channel of the second heat exchange device to detect the temperature W4 when the mixed refrigerant is discharged.

[0021] Preferably, a target temperature T is set, temperatures W3 and W4 are obtained, and the pump speed S of the drive pump is calculated. The heat recovery coefficient K is then obtained, and the speed of the drive pump is adjusted according to the obtained heat recovery coefficient K to ensure that the temperature W4 is equal to the target temperature T.

[0022] Preferably, the first heat exchange device, the second heat exchange device, and the third heat exchange device all employ coiled tube heat exchangers.

[0023] Compared with the prior art, the beneficial effects of the present invention are:

[0024] 1. This invention constructs a highly efficient, closed-loop energy cascade utilization network by combining a cooling circulation system with a waste heat recovery system. The system not only fully utilizes the high-temperature exhaust gas from the gas turbine to drive the steam turbine for power generation, but also further recovers medium- and low-grade waste heat from the steam turbine exhaust and the heat generated during the compression of the mixed refrigerant for pre-cooling the feedstock natural gas. This multi-stage, multi-source waste heat recovery method significantly reduces the energy consumption of the traditional pre-cooling process, improves the overall thermodynamic perfection of the system, and achieves efficient and low-carbon energy utilization.

[0025] 2. This invention further introduces an intelligent temperature control and flow regulation mechanism. By real-time detection of the temperature difference between the gas-liquid mixture and the raw material natural gas, the flow direction of the transfer valve is intelligently controlled to ensure that the pre-cooling stage only starts when there is actual heat exchange potential, avoiding energy loss caused by ineffective heat exchange. Simultaneously, the system dynamically adjusts the speed of the drive pump based on the deviation between the inlet and outlet temperatures of the mixed refrigerant and the target temperature, precisely controlling the intensity of waste heat recovery. This prevents a decrease in throttling effect due to over-cooling of the mixed refrigerant and avoids flash evaporation caused by insufficient cooling, thereby ensuring the stable and efficient operation of the refrigeration cycle and further improving the system's energy-saving effect and operational reliability. Attached Figure Description

[0026] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used in conjunction with embodiments of the invention to explain the invention and do not constitute a limitation thereof. In the drawings:

[0027] Figure 1 This is a schematic diagram of the overall process of the present invention;

[0028] Figure 2 This is a schematic diagram of the control flow of the transfer valve of the present invention;

[0029] Figure 3 This is a schematic diagram of the control process for the pump driven by the present invention.

[0030] In the diagram: 1. First heat exchange device; 2. Compressor; 3. Expansion device; 4. Gas turbine; 5. Refrigerant supply module; 6. Second heat exchange device; 7. Waste heat boiler; 8. Steam turbine; 9. Generator; 10. Storage tank; 11. Third heat exchange device; 12. Liquid storage tank; 13. Drive pump; 14. Raw material supply module; 15. Transfer valve; 16. First temperature detection device; 17. Second temperature detection device; 18. Third temperature detection device; 19. Fourth temperature detection device; 20. Fuel supply device. Detailed Implementation

[0031] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0032] Example 1

[0033] In existing technologies, in order to improve energy efficiency, some liquefaction plants have attempted to recover high-temperature exhaust gases (usually 400-600°C) emitted by gas turbines. However, this recovery method has significant limitations and is incomplete.

[0034] First, the high-temperature exhaust gas recovery is incomplete. The low- and medium-grade gas-liquid mixture emitted after the steam turbine generates electricity still carries a considerable amount of heat. In the existing system, this heat is often directly discharged into the environment through the condenser, resulting in a secondary waste of energy.

[0035] Secondly, the low- and medium-grade waste heat in the core refrigeration cycle is neglected. Inside the cooling cycle system, after the mixed refrigerant is discharged by the compressor, its temperature and pressure increase significantly. Before entering the expansion device (such as a throttle valve or expander) for cooling and liquefaction, this high-temperature and high-pressure refrigerant working fluid must be cooled in the heat exchanger. Existing processes generally use external cold sources such as ambient air or cooling water to directly cool it. In essence, the heat converted from the compression work, which has a grade significantly higher than that of the environment, is simply and unidirectionally dissipated. This process wastes a huge amount of low- and medium-temperature heat energy, resulting in the energy contained in the compression work not being fully utilized. This is an important reason for the low thermodynamic perfection of the entire liquefaction system.

[0036] Finally, the precooling process is disconnected from the internal waste heat. At the front end of the liquefaction process, the raw natural gas usually needs to be precooled before entering the main liquefaction heat exchanger. Currently, the precooling process relies on independent precooling units that burn more fuel or consume additional electricity, which further increases the system's energy consumption and carbon emissions.

[0037] Please see Figure 1 The present invention provides the following technical solution: an energy-saving natural gas liquefaction process system based on waste heat recovery, including a cooling circulation system and a waste heat recovery system;

[0038] The cooling cycle system includes a first heat exchange device 1, a compressor 2, an expansion device 3, a gas turbine 4, a refrigerant supply module 5, and a second heat exchange device 6. The hot flow channel of the first heat exchange device 1 is for the liquefied natural gas to pass through. The cold flow channel of the first heat exchange device 1, the compressor 2, the hot flow channel of the second heat exchange device 6, and the expansion device 3 are connected by pipes to form a cooling cycle pipeline. The refrigerant supply module 5 is connected to the cooling cycle pipeline to replenish the mixed refrigerant. The output shaft of the gas turbine 4 is connected to the compressor 2 to drive its operation.

[0039] The waste heat recovery system includes a waste heat boiler 7, a steam turbine 8, a generator 9, a third heat exchange device 11, a liquid storage tank 12, and a drive pump 13. The hot flow channel of the waste heat boiler 7, the steam turbine 8, the third heat exchange device 11, the liquid storage tank 12, the drive pump 13, and the cold flow channel of the second heat exchange device 6 are connected by pipelines to form a recovery circulation pipeline for recovering the waste heat generated by the operation of the steam turbine 8 and the compressor 2.

[0040] Compressor 2, expansion device 3, gas turbine 4, refrigerant supply module 5, waste heat boiler 7, and steam turbine 8 are all existing technologies and will not be described in detail. The expansion device can use a throttle valve.

[0041] The liquefied natural gas, which passes through the heat flow channel of the first heat exchange device 1, enters the storage tank 10 for storage.

[0042] Mixed refrigerants are refrigerants made by mixing two or more pure refrigerants, such as nitrogen, methane, ethane, propane, and butane, in a certain proportion. This is a current technology and will not be described in detail here.

[0043] The gas turbine 4 includes an air inlet, a feed inlet, and a discharge outlet. The feed inlet of the gas turbine 4 is connected to a fuel supply device 20 for supplying natural gas fuel to the gas turbine 4. The waste heat boiler 7 includes an air inlet, an air outlet, a liquid inlet, and a liquid outlet. The discharge outlet of the gas turbine 4 is connected to the air inlet of the waste heat boiler 7 for supplying high-temperature exhaust gas to the waste heat boiler 7. The high-temperature exhaust gas entering the waste heat boiler 7 can heat the liquid inside the furnace body to generate steam. After use, the high-temperature exhaust gas is discharged through the air outlet of the waste heat boiler 7.

[0044] The cold flow channel outlet of the second heat exchange device 6 is connected to the liquid inlet of the waste heat boiler 7, forming a loop.

[0045] The steam turbine 8 includes a steam inlet and a steam outlet. The outlet of the waste heat boiler 7 is connected to the steam inlet of the steam turbine 8 to provide steam to the steam turbine 8 and drive the steam turbine 8 to work. The output shaft of the steam turbine 8 is connected to the generator 9, and the generator 9 is driven by the steam turbine 8 to achieve power generation and energy saving. The inlet of the heat flow channel of the third heat exchange device 11 is connected to the steam outlet of the steam turbine 8.

[0046] The drive pump 13 pumps the liquid in the storage tank 12 into the cold flow channel of the second heat exchange device 6, and then into the waste heat boiler 7 to be heated to generate steam. After the steam drives the steam turbine 8, the exhaust gas enters the hot flow channel of the third heat exchange device 11 for condensation, and finally returns to the storage tank 12.

[0047] The waste heat recovery system also includes a raw material supply module 14 and a transfer valve 15. The transfer valve 15 includes an inlet, a first feed port, and a second feed port. The outlet of the raw material supply module 14 is connected to the inlet of the transfer valve 15. The first feed port of the transfer valve 15 is connected to the inlet of the cold flow channel of the third heat exchange device 11. The second feed port of the transfer valve 15 and the outlet of the cold flow channel of the third heat exchange device 11 are both connected to the inlet of the hot flow channel of the first heat exchange device 1.

[0048] The adapter valve 15 uses a three-way solenoid valve, which is existing technology and will not be described in detail here.

[0049] The drive pump 13 is used to control the circulation of liquid inside the storage tank 12 in the recovery circulation pipeline.

[0050] The first heat exchange device 1, the second heat exchange device 6, and the third heat exchange device 11 all use coiled tube heat exchangers.

[0051] In operation, the mixed refrigerant cools the natural gas in the first heat exchange device 1, is then compressed and heated by the compressor 2, and subsequently cooled by the working fluid of the waste heat recovery system in the second heat exchange device 6. After being cooled by the expansion device 3, it returns to the first heat exchange device 1, completing the refrigeration cycle. At the same time, the exhaust gas of the gas turbine 4 drives the waste heat recovery system to generate steam for power generation. Its gas-liquid mixture and other medium and low grade waste heat in the system are used to pre-cool the raw material natural gas, forming an efficient, closed-loop energy cascade utilization network.

[0052] It should be noted that the high-temperature exhaust gas emitted from the discharge port of gas turbine 4 is typically between 400℃ and 600℃. This gas can be used to heat waste heat boiler 7 to generate steam, which drives steam turbine 8, achieving power generation and energy saving. Since the principle of steam doing work in steam turbine 8 is expansion, the steam flows from a high-pressure state into the low-pressure outlet of steam turbine 8. This huge pressure difference is the fundamental driving force for the steam to drive the impeller rotation, meaning that the steam can expand from the initial high pressure to a lower pressure, resulting in a greater pressure drop, more work done, and more complete energy conversion. The absolute pressure at the exhaust port of the steam turbine 8 is typically designed to be between 5 kPa and 10 kPa. The temperature of the gas-liquid mixture discharged from the steam turbine 8 is very low, approximately 30℃-45℃. However, the temperature of the mixed refrigerant after flowing through the compressor 2 is between 90℃ and 130℃, which is higher than that of the gas-liquid mixture. The waste heat generated by the mixed refrigerant as it flows through the compressor 2 can be recovered through the gas-liquid mixture, which can raise the temperature of the gas-liquid mixture so that it can subsequently enter the waste heat boiler 7, reducing the heating and steaming time and effectively improving efficiency.

[0053] In summary, this invention constructs a highly efficient, closed-loop energy cascade utilization network by combining a cooling circulation system with a waste heat recovery system. The system not only fully utilizes the high-temperature exhaust gas from gas turbine 4 to drive steam turbine 8 for power generation, but also further recovers the low- and medium-grade waste heat from the exhaust gas of steam turbine 8, as well as the heat generated during the compression of the mixed refrigerant, for pre-cooling the feedstock natural gas. This multi-stage, multi-source waste heat recovery method significantly reduces the energy consumption of the traditional pre-cooling process, improves the overall thermodynamic perfection of the system, and achieves efficient and low-carbon energy utilization.

[0054] Example 2

[0055] Based on the above embodiments, considering that the liquefied natural gas (LNG) is affected by the ambient temperature during its passage through the pipeline from the feedstock module to the transfer valve, if the LNG temperature is lower than that of the gas-liquid mixture, its temperature will rise upon entering the third heat exchange device. This increases the cooling load on the LNG liquefaction process, leading to a significant increase in the circulation volume and compression power consumption of the mixed refrigerant, thus pushing up overall energy consumption. Simultaneously, the recovery of waste heat from the compressed mixed refrigerant must be precisely controlled. Excessive waste heat recovery will lower the temperature and pressure of the mixed refrigerant, weakening the refrigeration effect of the throttling expansion. Conversely, insufficient waste heat recovery will result in a higher temperature of the mixed refrigerant, causing significant flash evaporation as it flows through the expansion device. Both scenarios disrupt the stability and efficiency of the refrigeration cycle, ultimately affecting the liquefaction effect of the natural gas and the system's energy efficiency.

[0056] Flash evaporation refers to the instantaneous and rapid partial vaporization of a high-pressure saturated liquid when its pressure suddenly drops, because its temperature is higher than the saturation temperature under the new pressure. Excessive flash evaporation will produce a large amount of gaseous refrigerant, significantly reducing the proportion of liquid refrigerant and thus severely weakening the ability of the mixed refrigerant to cool and liquefy natural gas.

[0057] Please see Figure 2 and Figure 3 The steam turbine 8 is equipped with a first temperature detection device 16 at the steam outlet to detect the temperature W1 of the gas-liquid mixture, and a second temperature detection device 17 is equipped at the feed inlet of the transfer valve 15 to detect the temperature W2 of the natural gas entering the transfer valve 15.

[0058] The temperature W1 of the gas-liquid mixture and the temperature W2 of the natural gas are obtained. If the temperature W2 of the natural gas is higher than the temperature W1 of the gas-liquid mixture, the first feed port is opened and the second feed port is closed. The gas-liquid mixture discharged from the steam outlet of the steam turbine 8 can exchange heat with the natural gas to be liquefied in the cold flow channel of the third heat exchange device 11 as it flows through the hot flow channel of the third heat exchange device 11. This can raise the temperature of the gas-liquid mixture and lower the temperature of the natural gas to be liquefied, thus achieving the initial pre-cooling of the natural gas.

[0059] If the temperature W2 of the natural gas is lower than or equal to the temperature W1 of the gas-liquid mixture, the first feed port is closed and the second feed port is opened, allowing the natural gas inside the transfer valve 15 to enter the heat flow channel of the first heat exchange device 1.

[0060] A third temperature detection device 18 is provided at the inlet of the hot flow channel of the second heat exchange device 6 to detect the temperature W3 when the mixed refrigerant enters, and a fourth temperature detection device 19 is provided at the outlet of the hot flow channel of the second heat exchange device 6 to detect the temperature W4 when the mixed refrigerant is discharged.

[0061] Set the target temperature T, obtain the temperatures W3 and W4 and the pump speed S of the drive pump 13, calculate the heat recovery coefficient K, and adjust the speed of the drive pump 13 according to the obtained heat recovery coefficient K to ensure that the temperature W4 is equal to the target temperature T, thereby improving waste heat recovery. When the temperature W4 is higher than the target temperature T, control the drive pump 13 to accelerate the circulation of the liquid inside the storage tank 12 in the recovery circulation pipeline. When the temperature W4 is lower than the target temperature T, control the drive pump 13 to reduce the circulation of the liquid inside the storage tank 12 in the recovery circulation pipeline.

[0062] The system is equipped with a first temperature detection device 16 at the steam outlet of the steam turbine 8 to detect the temperature W1 of the gas-liquid mixture in real time. A second temperature detection device 17 is installed at the feed inlet of the transfer valve 15 to detect the temperature W2 of the raw material natural gas in real time.

[0063] Compare W1 and W2. If W2 > W1, it indicates that the temperature of the raw natural gas is higher than that of the gas-liquid mixture, and there is pre-cooling potential. Control the transfer valve 15 to open its first feeding port and close the second feeding port. Let the raw natural gas flow through the cold flow channel of the third heat exchange device 11 and utilize the cold energy of the gas-liquid mixture for effective pre-cooling. If W2 ≤ W1, it indicates that the gas-liquid mixture has no pre-cooling capacity or the pre-cooling effect is poor at this time. Control the transfer valve 15 to close the first feeding port and open the second feeding port, so that the raw natural gas bypasses the third heat exchange device 11 and directly enters the first heat exchange device 1 to avoid ineffective heat exchange.

[0064] At the inlet and outlet of the heat flow channel of the second heat exchange device 6, the system is respectively equipped with a third temperature detection device 18 and a fourth temperature detection device 19, which are used to detect the temperature W3 when the mixed refrigerant enters and the temperature W4 when it is discharged in real time. The system presets a target temperature T, which is the ideal temperature that the mixed refrigerant hopes to reach after being cooled in the second heat exchange device 6. The purpose is to avoid the mixed refrigerant having too high or too low temperature. If the temperature is too high, a large amount of flashing will occur when flowing through the expansion device 3. If the temperature is too low, its pressure will also become low, which will affect the subsequent cooling effect of the mixed refrigerant when passing through the expansion device 3 and the subsequent effect of cooling and liquefying natural gas. Obtain W3, W4 and the current pump speed S of the driving pump 13. The system calculates a heat recovery coefficient K, and its calculation formula is: K = (W4 - W3) / S. This coefficient reflects the heat recovered by the flow of the gas-liquid mixture per unit pump speed.

[0065] The controller compares W4 with the target temperature T. When W4 > T, it indicates that the mixed refrigerant is not sufficiently cooled and the waste heat it carries is not fully taken away. The controller will increase the rotational speed S of the driving pump 13 to accelerate the circulation flow rate of the cooling working medium in the waste heat recovery system, thereby enhancing the cooling capacity of the second heat exchange device 6 and making W4 approach T. When W4 < T, it indicates that the cooling is excessive and the pump work may be wasted. The controller will reduce the rotational speed S of the driving pump 13 to slow down the circulation flow rate and make the system operate in a more energy-saving manner on the premise of meeting the cooling requirements.

[0066] To sum up, the present invention further introduces an intelligent temperature control and flow regulation mechanism. By detecting the temperature difference between the gas-liquid mixture and the raw natural gas in real time, it intelligently controls the flow direction of the transfer valve 15 to ensure that the pre-cooling link is only started when there is actual heat exchange potential, avoiding energy loss caused by ineffective heat exchange. At the same time, the system dynamically adjusts the rotational speed of the driving pump 13 according to the deviation between the inlet and outlet temperatures of the mixed refrigerant and the target temperature, precisely controlling the waste heat recovery intensity, preventing both the throttling effect decline caused by excessive cooling of the mixed refrigerant and the flashing phenomenon caused by insufficient cooling, thereby ensuring the stable and efficient operation of the refrigeration cycle and further improving the energy-saving effect and operation reliability of the system.

[0067] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. An energy-saving natural gas liquefaction process system based on waste heat recovery, characterized in that: This includes a cooling circulation system and a waste heat recovery system; The cooling cycle system includes a first heat exchange device (1), a compressor (2), an expansion device (3), a gas turbine (4), a refrigerant supply module (5), and a second heat exchange device (6). The hot flow channel of the first heat exchange device (1) is for the liquefied natural gas to pass through. The cold flow channel of the first heat exchange device (1), the compressor (2), the hot flow channel of the second heat exchange device (6), and the expansion device (3) are connected by pipes to form a cooling cycle pipeline. The refrigerant supply module (5) is connected to the cooling cycle pipeline to replenish the mixed refrigerant. The output shaft of the gas turbine (4) is connected to the compressor (2) to drive its operation. The waste heat recovery system includes a waste heat boiler (7), a steam turbine (8), a generator (9), a third heat exchange device (11), a liquid storage tank (12), and a drive pump (13). The hot flow channels of the waste heat boiler (7), the steam turbine (8), the third heat exchange device (11), the liquid storage tank (12), the drive pump (13), and the cold flow channel of the second heat exchange device (6) are connected by pipelines to form a recovery circulation pipeline for recovering the waste heat generated by the steam turbine (8) and the compressor (2).

2. The energy-saving natural gas liquefaction process system based on waste heat recovery according to claim 1, characterized in that: The gas turbine (4) includes an air inlet, a feed inlet, and a discharge outlet. The feed inlet of the gas turbine (4) is connected to a fuel supply device (20) for supplying natural gas fuel to the gas turbine (4). The waste heat boiler (7) includes an air inlet, an air outlet, a liquid inlet, and a liquid outlet. The discharge outlet of the gas turbine (4) is connected to the air inlet of the waste heat boiler (7) for supplying high-temperature waste gas to the waste heat boiler (7). The high-temperature waste gas entering the waste heat boiler (7) can heat the liquid inside the furnace body to generate steam. The high-temperature waste gas after use is discharged through the air outlet of the waste heat boiler (7).

3. The energy-saving natural gas liquefaction process system based on waste heat recovery according to claim 2, characterized in that: The steam turbine (8) includes a steam inlet and a steam outlet. The outlet of the waste heat boiler (7) is connected to the steam inlet of the steam turbine (8) to provide steam to the steam turbine (8) and drive the steam turbine (8) to work. The output shaft of the steam turbine (8) is connected to the generator (9). The generator (9) is driven by the steam turbine (8) to achieve power generation and energy saving. The heat flow channel inlet of the third heat exchange device (11) is connected to the steam outlet of the steam turbine (8).

4. The energy-saving natural gas liquefaction process system based on waste heat recovery according to claim 3, characterized in that: The waste heat recovery system also includes a raw material supply module (14) and a transfer valve (15). The transfer valve (15) includes an inlet, a first feed port and a second feed port. The outlet of the raw material supply module (14) is connected to the inlet of the transfer valve (15). The first feed port of the transfer valve (15) is connected to the inlet of the cold flow channel of the third heat exchange device (11). The second feed port of the transfer valve (15) and the outlet of the cold flow channel of the third heat exchange device (11) are both connected to the inlet of the hot flow channel of the first heat exchange device (1).

5. The energy-saving natural gas liquefaction process system based on waste heat recovery according to claim 4, characterized in that: The steam turbine (8) is equipped with a first temperature detection device (16) at the steam outlet to detect the temperature W1 of the gas-liquid mixture, and the transfer valve (15) is equipped with a second temperature detection device (17) at the feed inlet to detect the temperature W2 of the natural gas entering the transfer valve (15).

6. The energy-saving natural gas liquefaction process system based on waste heat recovery according to claim 5, characterized in that: The temperature W1 of the gas-liquid mixture and the temperature W2 of the natural gas are obtained. If the temperature W2 of the natural gas is higher than the temperature W1 of the gas-liquid mixture, the first feed port is opened and the second feed port is closed. The gas-liquid mixture discharged from the steam outlet of the steam turbine (8) can exchange heat with the natural gas to be liquefied in the cold flow channel of the third heat exchange device (11) as it flows through the hot flow channel of the third heat exchange device (11). This can raise the temperature of the gas-liquid mixture and lower the temperature of the natural gas to be liquefied, thus achieving the initial pre-cooling of the natural gas. If the temperature W2 of the natural gas is lower than or equal to the temperature W1 of the gas-liquid mixture, the first feed port is closed and the second feed port is opened, so that the natural gas inside the transfer valve (15) can enter the heat flow channel of the first heat exchange device (1).

7. The energy-saving natural gas liquefaction process system based on waste heat recovery according to claim 6, characterized in that: The drive pump (13) is used to control the circulation of liquid inside the storage tank (12) in the recovery circulation pipeline.

8. The energy-saving natural gas liquefaction process system based on waste heat recovery according to claim 7, characterized in that: The second heat exchange device (6) is provided with a third temperature detection device (18) at the inlet of the heat flow channel to detect the temperature W3 when the mixed refrigerant enters, and a fourth temperature detection device (19) is provided at the outlet of the heat flow channel to detect the temperature W4 when the mixed refrigerant is discharged.

9. The energy-saving natural gas liquefaction process system based on waste heat recovery according to claim 8, characterized in that: Set the target temperature T, obtain the temperatures W3 and W4 and the pump speed S of the drive pump (13), calculate the heat recovery coefficient K, and adjust the speed of the drive pump (13) according to the obtained heat recovery coefficient K to ensure that the temperature W4 is equal to the target temperature T.

10. The energy-saving natural gas liquefaction process system based on waste heat recovery according to claim 9, characterized in that: The first heat exchange device (1), the second heat exchange device (6) and the third heat exchange device (11) all adopt a coiled tube heat exchanger.

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