OPTIMIZATION OF CBM LIQUEFACTION PROCESS ADOPTING MIXED REFRIGERANT CYCLE

摘要

Coalbed methane (CBM) is an important energy resource in the world. Liquefaction is a good option for recovery of CBM. Different from ordinary natural gas, CBM usually consists of a lot of nitrogen and oxygen, which cannot be removed by the ordinary purification technology of LNG. Nitrogen can be separated from CBM by adsorption before liquefaction or by distillation after liquefaction. For the liquefaction-distillation process, nitrogen is liquefied together with methane, and it makes the liquefaction system performance change along with the nitrogen content of CBM feed gas. The liquefaction process using a mixed refrigerant cycle is discussed in this paper, which has the advantages of high efficiency and relative simplicity, and is suitable for small-scale liquefaction plants. The mixed refrigerant usually consists of nitrogen and hydrocarbons from methane to pentane, which are all natural refrigerants. To realize optimal system performance, the composition of the mixed refrigerant may vary significantly with different CBM conditions. Taking the unit product liquefaction power consumption as the major index for analysis, the optimal composition of mixed refrigerant is worked out and corresponding system performance is obtained at different nitrogen content of CBM feed gas when liquefaction rate and methane recovery rate are fixed respectively.