Exergy analysis, optimization approach and transient mode operation study of non-flammable biomethane liquefaction process

Abstract

Since the advent of the industrial age, the planetary energy demand has substantially increased to improve daily comfort. In a view to counterbalance the growing energy consumption of fossil fuels, such as natural gas, various strategies have been deployed for increasing the share of renewable energy (i.e., solar, wind, biogas...) in the global energy mix. A particular interest is paid to the use of biogas as natural gas substitute. For producers geographically located far from natural gas grid infrastructures, biogas shall be liquefied to facilitate its transport. Existing mixed refrigerant liquefaction units, operate with flammable hydrocarbons. However, increasingly stringent safety requirements impel to predict the inherent risks of handling hydrocarbons, in particular in biogas units close to farms. The use of non flammable alternative refrigerants is a potential solution currently barely considered. In this paper, a novel mixed refrigerant cycle for biogas micro-liquefaction process using a non flammable mixed refrigerant is proposed. The proposed cycle is first simulated in Aspen HYSYS® and further assessed by the exergy analysis method in order to evaluate process performance, to locate exergy destruction and to define key parameters for optimization approach. In attempt to improve the energy efficiency of the cycle, a link between Aspen HYSYS®, PIKAIA genetic algorithm tools (Charbonneau, 2002) and Microsoft Visual Basic was developed. In the proposed methodology the objective function consists on the maximization of the amount of the liquefied biomethane per unit of the required compressor work. The combination of exergy analysis and genetic algorithm optimization improved the proposed architecture performance leading to higher energy efficiency. Actually the operating parameters optimization contributed, in comparison to the reference case, to a 27 % reduction in electric power consumption and 26 % in exergy destruction. As a second step and with the aim of studying the transient-mode operation of liquefaction units, results obtained for a steady state operation are considered in order to assess the suitability of optimized solutions and robustness of systems against quick start-and-stop or partial load operating mode.