Inherent Safety Analysis of a Propane Precooled Gas-Phase Liquified Natural Gas Process

Refrigeration is widely used in chemical and petrochemical industries and in the liquefaction of gases including natural gas (LNG). There are many commercial processes being used in the LNG industries. These processes are energy intensive and require large capital investment. Conventional refrigeration processes such as the single mixed refrigerant process and the cascade refrigerant process operate by evaporation of the refrigerant. Gas-phase refrigeration processes have one important advantage over these processes and this is their very low hydrocarbon inventory. However, precooling the natural gas feed stream with a conventional propane refrigeration system significantly improves the efficiency of the gas phase refrigeration process. However, due to the propane precooling cycle, the liquid hydrocarbon inventory in the process increases, and hence, the hazard potential of the overall process increases. Therefore, there is a tradeoff between the energy efficiency and the inherent safety of the process. To analyze this tradeoff, a multiobjective optimization study is performed on the propane precooled dual independent expander process. The study includes four case studies with different objectives such as the minimization of the total shaftwork requirement, the capital cost, the total annual cost, and the total hydrocarbon inventory. The cost of hydrocarbon inventory reduction is calculated from the Pareto-optimal solutions.