The influence of incondensible gases on the refrigeration capacity of the reliquefaction plant during ethylene carriage by sea

This paper presents a performance of the reliquefaction cascade plant during ethylene carriage by sea. Parameters of this plant are discussed because of dramatic low cooling rate of the cargo. Ability to decrease ethylene temperature from approx. minus 96°C to minus 103°C on board the ship is essential. Modern ethylene carriers are able to keep cooling rate about 2K per day, on the other hand old ones do it with rate approx. 0,2 K per day and require weeks to achieve proper temperature of ethylene for discharging. Of course, economic issue of such journey is not very satisfying but many factors influence on this. In this paper almost new one – nine months – old ship is considered, with capacity 21 000m3. Main reason of problem is analysed, including heat ingress through the cargo tanks insulation. Gassing-up of the ship is explained and influence of presence incondensable gases in the cargo tanks described. Operation of the reliquefaction plant is shown, especially with the vent valve of LPG condenser. Analysis of pressures in the cargo tanks is carried out – on board the ship and with help of ProSimPlus – thermodynamic simulator. Conclusions contain discussion of the reliquefaction plant problems and remarks what should be improved on board the ethylene carrier to avoid such problems.