Energy and Exergy Analysis of Liquid Piston Compression for Small-Scale Methane Liquefaction

Abstract

This study evaluates a small-scale methane liquefaction system using liquid piston compression. While methane capture offers environmental benefits and energy potential, its low volumetric energy density challenges its practical viability. The proposed system uses 6mm inserts to enhance heat transfer in the liquid piston, and its performance is compared to traditional compressors through a validated lumped parameter model and Aspen HYSYS thermodynamic modelling. For a compression ratio of 10 and 15 seconds stroke time, 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 (polytropic index of 1.1). Additionally, results showed enhancing surface area was significantly better at lowering the polytropic index of compression than increasing stroke time. Using the liquid piston with inserts in the proposed reverse Brayton cycle reduced the specific energy by 39% compared to the reciprocating compressor. The 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 given by the liquid piston with inserts brought an 83% reduction in total exergy destruction in the compressor train.

Keywords: Liquid Piston, Methane, Exergy, Specifc Energy, Small Scale Liquefaction