Design of boil-off CO2 re-liquefaction processes for a large-scale liquid CO2 transport ship

This paper studies re-liquefaction properties of CO2 and introduces feasible designs of boil-off CO2 re-liquefaction processes suited for a large-scale transport ship. There are additional design considerations besides re-liquefaction efficiency due to the ship. Firstly, re-liquefaction facilities should be located within a limited space of a transport ship, which induces the number of main equipment as a constraint. Secondly, if an external closed-loop refrigeration is necessary, refrigerants should be re-chargeable and safe. From this perspective, CO2 would be the most appropriate one due to the availability in a liquefied CO2 ship and its safety if it can have similar performance to conventional refrigerants. According to CO2 liquefaction simulation, CO2 as a refrigerant shows similar performance to C3H8 and NH3 with additional 5-7% power consumptions. With this preliminary investigation, four boil-off CO2 re-liquefaction processes for a large-scale transport ship are designed. A re-liquefaction process which employs a distillation column shows 99.7% of re-liquefaction fraction but it has disadvantages of power consumption and the number of main equipment. On the contrary, another re-liquefaction process which has an external closed-loop refrigeration cycle with a CO2 refrigerant meets the re-liquefaction target (> 70.0%) with the lowest power consumption. This process also requires nominal level of the number of main equipment and the capital expenditure. Given these results, an external closed-loop CO2 refrigeration cycle based re-liquefaction process is expected to be appropriate for a large-scale liquid CO2 transport ship. Hopefully, the results addressed in this study will contribute to further development of liquefied CO2 transport ships in near future.