Natural gas liquefaction system and method and process

Technical realization elements:

4. The object of the present invention is to provide a natural gas liquefaction system and method, which can use propane and mixed refrigerant to realize the liquefaction of natural gas, and can use the opening and closing of the valve to adjust the number of stages of the three-stage separation tank according to local conditions. Free switching, simplifying the equipment while reducing the investment in the production cost of the enterprise.

5. The natural gas liquefaction system provided by the present invention includes a first liquefaction cold box and a second liquefaction cold box;

6. A three-stage pre-cooling section is formed in the first liquefaction cold box through a propane pre-cooling cycle system, that is, the first liquefaction cold box includes three heat exchange sections, so as to realize the early pre-cooling of natural gas and mixed refrigerant ；

7. In the second liquefaction cold box, a two-stage cryogenic section is formed through a mixed refrigerant cycle system, that is, the second liquefaction cold box includes two heat exchange sections, so as to realize the deep cooling of natural gas and the mixed refrigerant cycle use;

8. The inlet end of the three-stage pre-cooling section is connected to the natural gas intake pipeline, the outlet end of the three-stage pre-cooling section is connected with the inlet end of the two-stage cryogenic section, and the outlet of the two-stage cryogenic section The end is connected with the LNG outlet pipeline, and the LNG outlet pipeline is provided with a sixth throttle valve.

9. Specifically, the structure of the propane pre-cooling cycle system is as follows:

10. The outlet end of the propane compressor is connected to a propane cooler, and the outlet end of the propane cooler is connected to the upper inlet end of the three-stage separator tank through a pipe provided with a first throttle valve;

11. The liquid phase at the upper outlet end of the three-stage separator tank is divided into two paths, one path is connected to the inlet end of the first-level reheating section in the first liquefaction cold box, and the other path is provided with a second throttle valve The pipeline is connected to the middle inlet end of the three-stage separation tank;

12. The liquid phase at the middle outlet end of the three-stage separation tank is divided into two paths, one is connected to the inlet end of the second reheat section in the first liquefaction cold box, and the other is through the third throttle valve. The pipeline is connected with the inlet end of the lower part of the three-stage separation tank;

13. The liquid phase outlet at the bottom of the three-stage separation tank is connected to the inlet of the three-stage reheat section in the first liquefaction cold box

catch;

14. The outlet ends of the first-stage reheating section, the second-stage reheating section, and the third-stage reheating section and the gas-phase outlet end of the top of the three-stage separation tank are combined with the inlet end of the propane compressor. Connect, thus forming a propane cycle.

15. Specifically, the structure of the mixed refrigerant cycle system is as follows:

16. The outlet end of the three-stage pre-cooling section in the first liquefaction cold box is connected with the inlet end of a separator, and the gas-phase outlet end of the top of the separator is connected with the inlet end of the first-level cryogenic section in the second liquefaction cold box Section connection, the outlet end of the secondary cryogenic section in the second liquefaction cold box is connected to the inlet end of the secondary reheat section in the second liquefaction cold box through a fourth throttle valve;

17. The liquid phase outlet end of the bottom of the separation tank is connected to the inlet end of the first-level cryogenic section in the second liquefaction cold box, and the outlet end of the separation tank is connected to the second stage through a pipeline provided with a fifth throttle valve. The outlet end of the stage reheating section is collected and connected to the inlet end of the first stage reheating section in the second liquefaction cold box, and the outlet end of the first stage reheating section is connected to the inlet end of the mixed refrigerant compressor. The outlet end of the mixed refrigerant compressor is connected with the mixed refrigerant cooler through a pipeline, and returns to the primary pre-cooling section in the first liquefied cold box to form a mixed refrigerant circulation circuit.

18. In the natural gas liquefaction system of the present invention, the three-stage separation tank is provided with a cofferdam baffle and a cap, and the lower part of the cofferdam baffle with a certain thickness is fixed on the tank wall of the three-stage separation tank The cap is fixed above the cofferdam baffle at an equal angle by a bracket, the cofferdam baffle realizes the collection of liquid, and the cap serves as a rising channel for gas and prevents the inflow of liquid.

19. The present invention also provides a method for liquefying natural gas by using the natural gas liquefaction system, which includes the following steps:

20. Natural gas enters the first liquefaction cold box through the natural gas intake pipeline, and is gradually cooled to

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