Design of novel BOG re-liquefaction system using BOG spray recondenser and experimental investigation on condensation performance on liquid surface in thermally stratified state pressurized tank

Abstract

This study presents the design of a novel boil-off gas (BOG) re-liquefaction technology using a BOG recondenser system. The BOG recondenser system targets the liquefied natural gas (LNG) bunkering operation, in which the BOG phase transition occurs in a pressure vessel instead of a heat exchanger. The BOG that is generated during LNG bunkering operation is characterized as an intermittent flow with various peak loads. The system was designed to temporarily store the transient BOG inflow, condense it with subcooled LNG and store the condensed liquid. The superiority of the system was verified by comparing it with the most extensively employed conventional re-liquefaction system in terms of consumption energy and via an exergy analysis. The gas-liquid condensation phenomenon in a pressure tank in a thermally stratified state was experimentally investigated. Gas under designated conditions was injected into the pressure tank, which contained low-temperature test fluid in a saturated liquid state (R290), for identification of the pressurisation phenomenon in the thermal stratification state. The gas-liquid condensation phenomenon was qualitatively and quantitatively analysed by investigating the heat and mass transfer at this time. The gas injected from the exterior was partially condensed, yielding a non-equilibrium state between the gas and liquid inside the tank. Further, a temperature stratification phenomenon occurred between the upper and lower regions of the liquid. This non-equilibrium state constitutes a different result to that for gas-liquid condensation obtained through thermodynamic modelling assuming an equilibrium state. The factors affecting the gas-liquid condensation phenomenon include the mass and initial pressure of the liquefied inventory in the test cell, along with the temperature, pressure, and flow rate of the inlet vapour flow. Thus, the gas-liquid condensation phenomenon was experimentally clarified in this study by setting those factors as variables.