Conceptual Design, Optimization and Control on Chemical Processes for the Recovery of Monoethylene Glycol and Boil-off Gas

Abstract

As energy consumption is expected to increase continuously which is highly dependent on fossil fuel, searching for a new source of energy is crucial against limited reserves of fossil fuel. In addition, energy’s high dependency on fossil fuel causes pollution emission such as NOx and SOx, especially in the ship industry. To solve these problems, offshore resources can be considered as a new source of energy, where repeatedly-used hydrate inhibitor, namely, monoethylene glycol (MEG), is required to possess high recovery to drive cost-effectiveness in the offshore production. On the other hand, the introduction of LNG-fueled ship which substitutes conventional ship fuel with liquefied natural gas (LNG) can reduce the pollution. In response to the tightened regulations on environmental contamination and minimizing fuel loss, boil-off gas (BOG) from LNG tank is recovered with re-liquefaction process, and its design and operation should remain efficient to save operation cost. Hence, in this thesis, MEG regeneration process is conceptually designed to examine MEG recovery. For BOG re-liquefaction process, optimization is covered to improve energy efficiency in the LNG-fueled ship, which is relatively lower than other ship applications; process control is covered to achieve stable operation under fluctuating ocean environment. First, a process model for MEG regeneration is developed based on the conceptual design. It is composed of industrial data and stepwise design considering process configuration, salt precipitation, and operating conditions, whose simulation results confirm the model validity by identifying MEG recovery close to commercial industrial processes. In addition, critical parameters, which have an impact on MEG loss and energy consumption, are examined with sensitivity analysis. Application of life-cycle reservoir conditions to the MEG regeneration process proves the impacts of feed conditions on energy consumption and MEG loss, which contributes to the understanding of offshore production, paving a way for the improved economics. Subsequently, optimization of structure and operating condition is carried out based on dual expander cycle to improve energy efficiency of the BOG re-liquefaction process in the LNG-fueled ship. Dual expander cycle is made by introducing additional expander to the single expander cycle, which leads to three configurational options. In overall, 11 process schemes are proposed including alternative cascaded process, followed by optimization of operating conditions to minimize energy consumption. As a result, largest reduction in energy consumption is made by 23%, and its effects are analyzed in qualitative and quantitative manner. Application of different feed conditions to the optimization framework enables to demonstrate a preference to specific configurational options to improve energy efficiency. Finally, enhancing operability of the BOG re-liquefaction process directs to develop integrated operation guidelines which include design of control algorithms for steady-state and operation logics for non-steady state. With controllability analysis (degree of freedom analysis, open loop gain, open loop disturbance gain, and dynamic simulation), optimum control algorithm is proposed to reach a setpoint in a short time against disturbances. In addition, mechanical limitations for small-scale process and industrial practice are considered for the operation of start-up and shut-down, with which simple operation appropriate for the ocean environment is achieved without any malfunction.