Design and analysis of CO2 cryogenic separation process for the new LNG purification cold box

A CO2 cryogenic separation process is proposed and designed for the new liquefied natural gas (LNG) purification cold box. This process is based on the liquefaction process using brazed plate heat exchanger (BPHE) and two separators are embedded between the liquefaction and subcooling heat exchangers to remove frozen CO2. The separator adopts one-use one-standby mode to ensure uninterrupted operation. The main parameters are first obtained through the phase diagram and solubility curve, and then verified by HYSYS simulation. The simulation results show that the preset values are in good agreement with the simulation. LNG enters the separator at 163 K and then is cooled down to 132.5 K to freeze and separate the CO2, so that the CO2 solids will not block the heat exchanger when they are further subcooled. When the mole fraction of CO2 inside the separator accumulates to 10%, the process will switch to another separator to work. The mixed refrigerants (MRs) heat the frozen separator to above 189 K, and the mixtures (solid CO2 and LNG) become liquid and flow out. The change of the process does not cause the temperature crossover inside the heat exchanger. Furthermore, the exergy losses of the proposed process are slightly increased by 1.3% compared to the original process.