Conceptual Process Design and Optimization of Refrigeration Cycles for the Liquefaction of Boil-off Gas

Abstract

Improving thermodynamic efficiency for refrigeration cycles is one of the key elements in conceptual process design activities for achieving cost-effective and sustainable production of liquefied natural gas. Various structural and heat recovery options are available for the design of refrigeration cycles, with which shaft power required to drive compressors is attempted to be minimized. Choice of refrigerants to be employed is another important degree of freedoms in a process design of refrigeration cycles. Hence, systematic investigation in a holistic manner is required to screen different liquefaction technologies and evaluate their techno-economic impacts. In this study, the focus is made to gain conceptual understanding in the design of liquefaction processes for boil-off gas. The current study aims to provide guidelines for the selection of the most appropriate technologies for the application of boil-off gas liquefaction as well as for determining optimal design and operating conditions of refrigeration cycles. A heat-integrated design framework is adopted in this work to evaluate energy efficiency in a systematic manner, while optimization methods are applied to systematically determine the most appropriate operating conditions for the liquefaction cycles. From the case study with 1 ton/day of liquefaction capacity, it was found that the power consumption for a single mixed refrigerant cycle is about 12 % less than that of an N2 expander cycle.