The operation limitation of the two-stage reliquefaction cycle employed on board the ethylene carrier

Abstract

Ethylene, propane, commercial propane or HD-5 is cargoes very often carried by sea. In order to reduce the time of loading and later cooling down to required parameters of temperature and pressure in the cargo tanks two-stage reliquefaction cycle is used. For above-mentioned cargoes, two-stage cycle with liquid subcooling and interstage cooler is one of the most often utilized. In this paper operation of this cycle is discussed, based on parameters of the reciprocating cargo compressor and its cycle used on board the one of the biggest – 21 500 m3 – ethylene carrier in the world. It is explained how some vapour mixtures decrease cooling rate during reliquefaction of the cargo and some parameters of pressure and temperature of these mixtures are calculated with help of ProSimPlus – thermodynamic simulator. In this way, changes of the mixtures composition across reliquefaction plant are discussed. Increased pressure of the cargoes condensing as a way of solving of explained problem is analysed. Three stage – cycle of reliquefaction plant used for pure propane or ethylene is shown and some of its disadvantages are explained. Based on this discussion, three-stage cycle with liquid injection is proposed to fulfil operation requirements of the cargoes and improve their loading and cooling processes on board the ethylene carrier. Analysis of refrigeration capacity for both cycles is carried out.