Energy and exergy analysis of liquid piston compression for small-scale methane liquefaction

This study evaluates a proposed small-scale methane liquefaction system with two compressor configurations: a reciprocating compressor and a liquid piston. While capturing gaseous methane offers environmental benefits and energy potential, its low volumetric energy density challenges its practical and economic viability. The proposed configuration with a liquid piston uses 6 mm diameter inserts to enhance heat transfer. Its performance is compared to that of the reciprocating compressor through a validated lumped parameter model and Aspen HYSYS thermodynamic simulations. For a compression ratio of 10 and a stroke time of 15 s, the final gas compression temperatures were 550 K, 492 K, and 319 K for the adiabatic process, liquid piston, and liquid piston with pipe inserts, respectively. The liquid piston with pipe inserts achieved near-isothermal compression (with a polytropic index of 1.1). Additionally, results indicated that enhancing surface area was significantly more effective at lowering the polytropic index of compression than increasing stroke time. The reverse Brayton cycle using the liquid piston configuration reduced specific energy by 39 % compared to the reciprocating compressor. This decrease in specific energy made the cycle competitive with other methane liquefaction cycles, producing 3.7 tonnes of liquid methane daily. The lower cooling requirements offered by the liquid piston with inserts resulted in an 83 % reduction in total exergy destruction within the compressor train.