Design, simulation and control of a flexible LNG reception terminal
This research is a preliminary design study relating to the importation of liquefied natural gas (LNG) in the United Kingdom, focusing on the process design, simulation and control of a flexible LNG reception terminal. The primary operating objective of the proposed LNG reception terminal is to accept a wide variety of LNG feedstocks, and to continuously produce output with consistent specifications. This requirement to provide "feedstock flexibility" was the major influence in the development of this research. The process design was derived from the application of selection criteria to assess the impact of feedstock flexibility on process unit performance. The subsequent dynamic simulation had to represent multiple operating conditions and feedstock changeovers for a multicomponent system. This required the incorporation of an interactive physical properties database and the adoption of a pseudo-binary distillation column model in the simulation. Regulatory control loops were developed and tuned to provide stable operation for all the plant's operating conditions. In order to assess the impact of multiple feedstock conditions, control system performance was evaluated for disturbance rejection and control valve action. A changeover mechanism was developed which enabled successful dynamic simulation of feedstock changeovers. The simulation also identified significant interactions between certain process units in the flowsheet. In conclusion, the primary objective of the supervisory control system would be management of these interactions as opposed to maintenance of product specifications. This work has provided a preliminary flowsheet and a design tool for process and control studies, for a flexible LNG reception terminal. These will enable the design engineer to investigate a range of changes to the process design and to assess their implications for the plant's overall dynamic behaviour.