Optimization of small-scale skid-mounted liquefaction device for coalbed methane

Coalbed methane (CBM) fields are characterized by multiple wells, low pressure and low production. It is not economical to build an independent pipeline system for small pilot wells or remote wells, so the produced gas can not be transported efficiently via gas pipelines. In this paper, a new type of mixed refrigerant liquefaction process suitable for CBM skid-mounted liquefaction device was put forward. First, the feed gas is pre-cooled so that the impurities in the feed gas can be precipitated under the effect of low temperature. Second, the impurities are adsorbed into the first/second sensible heat exchanger for removal. And third, the original fuel gas is used for polythermal flush and the removed impurities flow into electric generators together with the fuel gas for combustion. The first and the second sensible heat exchangers run alternatively. Therefore, this pretreatment process becomes simpler. For CBM skid-mounted liquefaction devices which are driven by oil lubricated screw compressors, a J-T refrigerator structure with mixed refrigerant was proposed to separate lubricant from refrigerant deeply. The cold box structure was optimized by combining a plate-fin heat exchanger with a spiral tube heat exchanger with micro tube structure. Finally, a CBM skid-mounted liquefaction prototype was developed and then trial run tests were carried out. It is shown that its minimum unit power consumption is 0.612 (kW·h)/m³ and its liquefaction performance index is equivalent to that of centralized device with treatment rate of 10×10⁴ m³.