Structural Modifications on Single Mixed Refrigerant Cycles for Performance Improvements in Small-Scale LNG Processes

Abstract

Production of liquefied natural gas (LNG) at a small scale typically relies on single mixed refrigerant (SMR) cycles to provide the necessary refrigeration. The shaft power required for refrigerant compression is by far the largest contributor to the operating costs of the entire LNG plant. The highly nonlinear interactions between operating variables mean that the optimization of these SMR cycles is very challenging. Current design methodologies for SMR cycles mostly rely on stochastic search algorithms to search the solution space. However, the quality of the results obtained from these methodologies is difficult to assess. Exergy analysis can be performed, but it provides limited information. This work provides a thermodynamic-based analysis to minimize the shaft power demand. Four different SMR cycle configurations are studied in one case study. The analysis brings insights regarding their optimized conditions, including the refrigerant composition. These insights are demonstrated to be useful with a second case study, which is used to develop effective design methodologies for SMR cycles. Overall, the analysis of structural modifications to SMR cycles in this work enables a more fundamental understanding of their optimal design for high energy efficiency.

Liquefied Natural Gas Value Chain: A Comprehensive Review and Analysis

“…The necessary cold energy is supplied by mixed refrigerants, which comprise nitrogen, methane, propane, ethane, and butane. The method is renowned for its great flexibility, high dependability, and small capital and operating cost 30. Propane‐precooled mixed refrigerant: This process uses two refrigeration cycles: a pre‐cooling cycle that uses propane as refrigerant and a liquefaction subcooling cycle that operates by using a mixed refrigerant. In the pre‐cooling cycle, a multistage refrigeration cycle with three to four different pressure levels is used to exchange heat with the gas streams and warm mixed refrigerant so that both streams reach a temperature of about –35 °C.…”

“…The necessary cold energy is supplied by mixed refrigerants, which comprise nitrogen, methane, propane, ethane, and butane. The method is renowned for its great flexibility, high dependability, and small capital and operating cost 30.…”

“…The necessary cold energy is supplied by mixed refrigerants, which comprise nitrogen, methane, propane, ethane, and butane. The method is renowned for its great flexibility, high dependability, and small capital and operating cost [30]. (ii) Propane-precooled mixed refrigerant: This process uses two refrigeration cycles: a pre-cooling cycle that uses propane as refrigerant and a liquefaction subcooling cycle that operates by using a mixed refrigerant.…”