The impact of process design decisions on operability and control of an LNG process

This paper describes a framework for integrated process and control structure design, and applies this framework to a Liquefied Natural Gas (LNG) process design. The overall aim of the work is to contribute to the methodological basis for improved design and operability of gas processing plants. Good operability means essentially that a plant can be operated easily, i.e. it can cope with unknown disturbances, offsets and other uncertainties with the smallest possible profit loss and without frequent shutdowns. This is obtained both through the design of the process itself and the design of the control system. There is a potential for improved operability of process plants, and thereby reduced profit loss, by considering these two aspects together. The main message is that this is handled by considering control structure design when process design changes are made. in steps for developing an improved procedure for integrated process and control design are suggested. Such a procedure includes analyses of how altering key parameters in the process design affects the best possible control structure in the presence of defined disturbance scenarios. An important ingredient is the use of a dynamic, control relevant simulation model. The paper presents an analysis of how altering compressor size affects the choice of control structure for the Tealarc LNG process. This analysis has been carried out using a self-optimizing control methodology.