Efficiency evaluation of closed and open cycle pure refrigerant cascade natural gas liquefaction process through exergy analysis

Abstract

Natural gas liquefaction allows to transport this energy vector over long distances from stranded reserves to the market. Pure Refrigerant Cascade processes will be responsible for more than 105 mtpa of the LNG base-load production by 2021. Classical Cascade process developed by ConocoPhillips consists of three pure component closed refrigeration loops at several pressure levels. Alternatively, the open cycle Optimized Cascade process operates the methane loop in open cycle, where the flashed vapours are recompressed and recycled to the natural gas feed, after cold recovery. Literature on Pure Refrigerant Cascade natural gas liquefaction processes is scarce, therefore a comprehensive evaluation of the two configurations is presented in this study. Specific power consumption resulted in 357,2 and 323,7 kWh/ton LNG for the closed and open cycle base cases respectively, i.e. approximately 10% higher LNG throughput for the latter for equivalent available horsepower. Sensitivity to the presence of N2 in the feed stream reveals a larger detrimental effect on the performance for the open cycle process, resulting in a reduction to 5% additional LNG throughput relative to the closed configuration, when the feed stream contains up to 0,5% mol of N2. The sensitivity to the cooling media resulted in a similar increase of 0,8% LNG production for every °C decrease in the process stream aftercooling temperature for both models. The subsequent exergy analysis showed that the irreversible losses for valves, cryogenic exchangers, compressors and mixers were lower for the open cycle topology, while the exergy destruction taking place in the heat rejection units notably increased. Overall, the exergy efficiency of the open cycle was around 4%-points higher than for the closed cycle. The calculation of the coefficients of structural bonds (CSB), revealed that investment and research efforts should preferentially focus on minimizing temperature approaches in the cryogenic exchangers.