Thermodynamic Analysis on of Skid-Mounted Coal-bed Methane Liquefaction Device using Cryogenic Turbo-Expander

Coal-bed methane (CBM) reserves are rich in Sinkiang of China, and liquefaction is a critical step for the CBM exploration and utilization. Different from other CBM gas fields in China, CBM distribution in Sinkiang is widespread but scattered, and the pressure, flow-rate and nitrogen content of CBM feed vary significantly. The skid-mounted liquefaction device is suggested as an efficient and economical way to recover methane. Turbo-expander is one of the most important parts which generates the cooling capacity for the cryogenic liquefaction system. Using turbo-expander, more cooling capacity and higher liquefied fraction can be achieved. In this study, skid-mounted CBM liquefaction processes based on Claude cycle are established. Cryogenic turbo-expander with high expansion ratio is employed to improve the efficiency of CBM liquefaction process. The unit power consumption per liquefaction mole flow-rate for CBM feed gas is used as the object function for process optimization, compressor discharge pressure, flow ratio of feed gas to turbo-expander and nitrogen friction are analyzed, and optimum operation range of the liquefaction processes are obtained.