Propane and iso-butane pre-cooled mixed refrigerant liquefaction process for small-scale skid-mounted natural gas liquefaction

•A propane and iso-butane pre-cooled mixed refrigerant liquefaction process.•C3&C4-MR process has advantages over the widely used C3-MR process in terms of operating costs.•Energy consumption and exergy analysis are performed.•The guidance and recommendations for mixed refrigerant ratio are summarized.•Compare C3&C4 and R410a as pre-coolants respectively and obtain the best ratio of propane and iso-butane. The development of a small skid-mounted natural gas liquefaction plant can economically, environmentally-friendly, and efficiently collect natural gas in mountainous areas where are not easy to lay pipelines. A propane and iso-butane pre-cooled mixed refrigerant liquefaction process (C3&C4-MR) is proposed in this paper. Compared with the propane pre-cooled mixed refrigerant liquefaction process, the improved liquefaction process can reduce system energy consumption and increase exergy efficiency greatly. Genetic algorithm is applied to optimize energy consumption of C3&C4-MR process. The optimized specific energy consumption STotal is 0.301 kW·h/kg, which is 18.0% lower than the optimized C3-MR process. And compared with the optimized R410a pre-cooled mixed refrigerant liquefaction process, STotal of C3&C4-MR process is 5.64% lower than that of R410a-MR process. Besides, mixed propane and isobutane as pre-coolant is easier to obtain from the feed gas. By analyzing the temperature sensitivity of mixed refrigerant and calculating the unit compression power consumption of a single refrigerant, the guidance and recommendations for mixed refrigerant ratio are summarized. Due to the property of iso-butane, the optimal percentage of iso-butane in the pre-coolant was analyzed. In conclusion, C3&C4-MR process is energy-saving and advanced in terms of system operating costs to recycle scattered natural gas in remote areas.