Optimization of Propane Reliquefaction Cycle in LPG Plant

Reliquefaction is one of the most important systems in the cryogenic process of a propane plant which functions to minimize BOG (Boil Off Gas) on propane tanks. The reliquefaction plant in a propane plant liquefies BOG back to liquid phase. Cold liquids of propane and butane from a butane plant are mixed to form LPG. BOG causes large propane loss if not handled properly. BOG occurs due to a natural increase in propane temperature, which is normally at -42°C, in the tank and causes a phase change from liquid to gas accompanied by an increase in pressure in the propane tank. At present, there is a decrease in coefficient of performance (CoP) of the propane reliquefaction cycle from 2.16 to 1.90 suspected to occur. The present work attempts to reduce the compressor work by reducing the refrigerant temperature in the compressor cooler exit by utilizing cold seawater of 17°C exiting an heat exchanger between cold liquid propane and seawater streams to replace seawater stream of 30°C. optimization work has been conducted on the compressor cooler exit unit involving varying heat flux and heat transfer surface area affecting the investment cost. The optimization results in decrease in the compressor work from 213.04 to 179.92 kJ/kg and recover CoP of the reliquefaction cycle to 2.16.