Process for preparing liquefied natural gas (LNG) by low concentration coal bed methane oxygen bearing copious cooling liquefaction

Abstract

The invention discloses a process for preparing liquefied natural gas (LNG) by low concentration coal bed methane oxygen bearing copious cooling liquefaction. The process for preparing LNG by low concentration coal bed methane oxygen bearing copious cooling liquefaction comprises the following steps: (1), compression and purification process; (2), liquefaction and separation process which comprises a, main procedure process; b, refrigeratory process, wherein the refrigeratory process comprises b1, mixing cryogen process; and b2, nitrogen cryogen process. Nitrogen cryogen sequentially passes through a first level heat exchanger, a second level heat exchanger, a third level heat exchanger and a subcooler for cooling after being compressed and cooled, the nitrogen cryogen after throttling enters an overhead condenser to output cold quantity, and then the gaseous nitrogen cryogen sequentially passes through the subcooler, the third level heat exchanger, the second level heat exchanger, and the first level heat exchanger, the gaseous nitrogen cryogen is warmed gradually to normal temperature, and then flows back to a nitrogen compressor for being circularly used. The mixing cryogen process is used for controlling evaporation capacity at the bottom of a tower, the nitrogen cryogen process is used for controlling condensation capacity at the top of the tower, and independently adjusting purity coefficient and yield coefficient of natural gas products is achieved. The cryogen in a low temperature region only possesses a nitrogen component, and heavy components of isobutane, isopentane and the like do not exist in the low temperature region, and therefore the problems of being free of refrigeration in the process of throttling, or congestion of a cryogen channel are solved.

Description

technical field [0001] The invention belongs to the technical field of liquefaction, solidification or separation of gas or gas mixture through pressurization and cooling treatment, and specifically relates to a process for producing LNG by cryogenic liquefaction of low-concentration coal bed gas containing oxygen. Background technique [0002] Oxygen-containing coalbed methane is the primary by-product pumped out during coal mining to prevent gas explosions and outbursts and ensure safe production in coal mines. Its main component is methane. From its composition content, it can be seen that coalbed methane is an important energy source. and chemical raw materials. However, due to its complex composition, especially the oxygen contained in coalbed methane, it is a very dangerous combustion and explosion-supporting agent, which restricts the comprehensive utilization of oxygen-containing coalbed methane. In practice, in order to save costs, coalbed methane is generally used ...

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Problems solved by technology

[0004] (1) The raw material gas (i.e. low-concentration coalbed methane) comes from the underground extraction of coal mines, and the methane content fluctuates greatly, which will inevitably affect the stable operation of the liquefaction process, and the device needs to be adjusted; and the purity of the natural gas product in the method and device And the yield is controlled by the evaporation at the bottom of the tower and the condensation at the top of the tower, but the refrigerant fluid for heating the bottom of the tower and cooling for the top of the tower is the same, so when adjusting the product purity, it will affect the condensation at the top of the tower (yield) , similarly, when adjusting the product yield, it will affect the bottom evaporation (purity);

[0005] (2) If the working condition of the refrigerant compressor is unstable, a large amount of heavy components (such as isobutane and isopentane) in the mixed refrigerant will enter the low temperature area of ​​the liquefaction device (the temperature is lower than -165°C), causing the The temperature of the refrigerant in the first stage is lower than its lower conversion temperature, so that the temperature rises after throttling (that is, no cooling), and in severe cases, it will also cause solidification and block the refrigerant channel;

[0006] (3) After the methane in the coalbed methane is extracted, the nitrogen and oxygen tail gas is directly reheated and emptied under the operating pressure (0.3MPa) of the rectification tower, and its energy (pressure energy) is not fully recovered, which is a waste of energy

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