Optimal design and operating condition of boil-off CO2 re-liquefaction process, considering seawater temperature variation and compressor discharge temperature limit

Abstract Low-temperature liquid CO 2 could boil off during ship transportation because of the heat ingress from the surroundings to inside the tank, which causes the tank pressure to increase. To maintain the operating pressure range of the tank, the re-liquefaction process is indispensable. Three design alternatives to the re-liquefaction process using boil-off CO 2 as a refrigerant are proposed and compared. A systematic procedure to find the optimal design of CO 2 re-liquefaction is provided considering operational constraints such as the cooling water temperature and compressor discharge temperature. The optimal operating conditions of the proposed processes are determined by solving nonlinear programming. The compressor power consumption as the operation energy for the CO 2 re-liquefaction ranges from 60 to 120 kW/t CO 2 given the operational constraints. As the seawater temperature is lower and the discharge temperature limit is higher, the proposed Alternative 2 design consumes less power than the other designs.