Operability study on small-scale BOG (boil-off gas) re-liquefaction processes

Abstract

This study aims at analyzing dynamic behavior for the full operation cycle considering various alternatives and constraints related to small-scale and seaborne environment for a BOG re-liquefaction process in LNG-fueled ships, leading to significant enhancement of its operability. The design framework includes steady-state modeling, dynamic modeling and control system, with which system-wide dynamic characteristics are readily investigated and operating guidelines for achieving optimal process operations are obtained. Equipment sizing with the appropriate choice of design parameters is made such that the dynamic model developed in this study simulates non-steady-state characteristics in a realistic manner. 8 different process control schemes are proposed, and their technical gains as well as dynamic characteristics are investigated in details. The control algorithms are assessed through controllability analysis, including open loop test, open loop disturbance test and dynamic response against disturbances. For improving process efficiency, operation procedure for shut-down and start-up is designed with constraints imposed such as rotating speed variation and inert material filling for the effective and practically-adequate control for a refrigeration cycle, with which feasible heat transfer can be readily maintained. The case study presented fully demonstrates the validity and applicability of the control system proposed in this study.