Using Dynamic Simulation to Improve a Prico LNG Liquefaction Plant Operation

The CHALLENGE

Meeting product specifications while optimizing power and fuel consumption can be a significant challenge as a plant operates at points away from the design conditions. Understanding how variables like feed flow, composition, weather, mechanical limitations, and others affect each other and the total system performance can be complex. Steady state process modeling struggles to provide meaningful insight as to what site operations should do or what is currently happening for a given set of conditions. Dynamic simulation is a more useful tool for optimizing plant operations and training plant operators. The benefits of dynamic simulation are explored through the lens of a PRICO LNG liquefaction plant. The system consists of a main cryogenic heat exchanger (MCHE), three (3) drums, one (1) compressor with two sections, two (2) air coolers and two (2) pumps each with operating variables that can be manipulated.

The SOLUTION

Dynamic simulation first validates the control strategy of the PRICO LNG plant. There are two major control loops for the mixed refrigerant: one is the mixed refrigerant flow and the other one is the compressor suction pressure. It is very intuitive to use the VFD on the compressor to control the mixed refrigerant flow and ultimately the LNG temperature. From a steady state simulation perspective, the mixed refrigerant control valve will be used to control the compressor suction pressure. However, dynamic simulation proves this is a bad strategy. From time to time, due to the wrong set point of the suction pressure, the compressor is in the risk of liquid at the suction. The correct way is to use the control valve to control the suction superheat, instead of the pressure. With this new strategy, the control is stable and there is very little chance the compressor is flooded with liquid.

Then dynamic simulation is used to optimize the plant operation. Since LNG temperature is specified, the compressor speed will be adjusted to accommodate the requirement. However, the superheat set point is not directly related to any hard product specification, thus it can be optimized to reduce the total power consumption of the compressor. Thanks to the dynamic simulation, users can optimize the virtual plant just like operators sitting in front of the HMI of the real plant. The full optimization results will be presented during the conference.