Natural gas liquefaction process selection for the LNG plant
So far, there are mainly three mature liquefaction processes in the field of natural gas liquefaction: cascade refrigeration cycle process, mixed refrigeration cycle process and expander refrigeration cycle process.
A) Cascade refrigeration cycle process
The cascade refrigeration cycle is a three-stage refrigeration cycle with refrigerants such as propane (or propylene), ethane (or ethylene), methane (or nitrogen) (evaporating temperatures are - 100 °F, - 121 °F , - 256 °F , respectively ). Stage refrigeration enables natural gas to exchange heat with corresponding refrigerants in multiple temperature grades, thereby cooling and liquefying it.
B) Mixed refrigeration cycle process
The mixed refrigerant refrigeration cycle is a process that uses a mixture of N 2 and C 1 to C 5 hydrocarbons as a circulating refrigerant.
C) Expansion refrigeration cycle process
The expander refrigeration cycle refers to the Crowder cycle refrigeration in which high-pressure refrigerant expands adiabatically through a turbo expander to realize the liquefaction of natural gas. While the gas expands and cools down in the expander, it can output work, which can be used to drive the compressor in the process.
According to different refrigerants, the expander refrigeration cycle can be divided into: nitrogen expander refrigeration cycle, nitrogen-methane expander refrigeration cycle, natural gas expansion refrigeration cycle.
D) Techno-economic comparison of the three processes
The energy consumption of the cascade refrigeration cycle is set as 1, the comparison of various refrigeration cycles is listed in Table 5.1-2, and the characteristics of various refrigeration cycles are compared in Table 5.1-3.
Table 5.1-2 Comparison of Efficiency of Various Refrigeration Cycles
|
Refrigeration process |
Relative Energy Consumption to Cascade Cooling |
|
cascade refrigeration cycle |
1.00 |
|
mixed refrigerant refrigeration cycle |
1.15 |
|
R22 Precooling N 2 -CH 4 Expansion Refrigeration Cycle |
1.35 |
Table 5.1-3 Comparison of various refrigeration cycle characteristics
|
index |
cascade cooling |
mixed refrigerant |
Expansion refrigeration |
|
efficiency |
high |
middle |
Low |
|
Complexity |
high |
middle |
middle |
|
Main moving equipment |
3 compressors |
1 compressor |
compressor + expander |
|
heat exchanger type |
plate fin |
Plate fin or wound tube |
plate fin |
|
heat exchanger area |
big |
big |
Small |
|
adaptability |
high |
middle |
middle |
|
refrigerant source |
3 refrigerants are required |
3 to 6 refrigerants are required, and the refrigerant can be directly extracted from the Feed gas |
Only 1 refrigerant is needed: Nitrogen, which can be extracted from the air |
|
invest |
high |
middle |
Moderately high |
Comprehensively weighing factors such as investment, energy consumption and operation and maintenance, the liquefaction process of this device adopts the mixed refrigerant refrigeration process.
Rongteng specializes in gas field wellhead treatment, natural gas purification, LPG recovery, LNG plant and natural gas generator set.
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