In a propane pre-cooled mixed refrigerant (C3MR) process, what is the primary benefit of using propane for pre-cooling?

The primary benefit of using propane for pre-cooling in a propane pre-cooled mixed refrigerant (C3MR) process is to reduce the overall energy consumption of the liquefaction plant by enabling the mixed refrigerant (MR) cycle to operate more efficiently. In this process, the feed gas is cooled in stages. First, it is cooled by propane refrigerant in the pre-cooling stage. This propane refrigerant is typically in a closed-loop cycle, where it evaporates at a low temperature to absorb heat from the feed gas and is then compressed and condensed to release heat, before returning to the evaporation stage. By using propane to pre-cool the feed gas to a moderately low temperature (typically around -35°C), the temperature difference that the mixed refrigerant cycle needs to bridge is reduced. The mixed refrigerant cycle is designed to handle the cryogenic cooling required for actual liquefaction (around -160°C). Because the propane pre-cooling stage removes a significant portion of the heat load at a higher temperature, the mixed refrigerant cycle can operate with a smaller temperature lift, resulting in a lower compression ratio and reduced power consumption in the MR compressors. This staging effect enhances the thermodynamic efficiency of the overall liquefaction process. Moreover, propane is readily available and relatively inexpensive, making it an economically attractive option for pre-cooling. The pre-cooling stage also allows for the efficient removal of heavier hydrocarbons from the feed gas, further improving the performance of the MR cycle and the quality of the LNG product.