Refrigeration cycles in low-temperature distillation processes for the purification of natural gas

The increasing energy demand has made low-quality natural gas reserves worthy of consideration for exploitation. As a consequence, industries have developed new process solutions in order to exploit these gas reservoirs in a profitable way. Most of these solutions are natural gas purification processes by distillation at low-temperature, involving or not solid CO2 formation. Due to the low-temperatures reached in this type of processes, the choice of the appropriate refrigeration cycle becomes of paramount importance for limiting their energy consumptions and, thus, their operating costs. The aim of this work is to compare the performances of different types of refrigeration cycles using the coefficient of performance (COP) as discriminating factor. Several compounds (such as nitrogen, light hydrocarbons and ethylene) and their mixtures have been considered as working fluids and both non-cascade and cascade systems have been taken into account. Simulations by means of Aspen Hysys® V7.3 have led to conclude that the propane-ethylene cascade refrigeration cycle allows to attain the best performances.