Multivariable Model Predictive Controller Improves Turbo Expander Demethanizer Column Performance

Since its application to the NGL process in 1964, the turbo-expander based demethanizer process has been the workhorse of many gas plants. The turbo-expander process isentropically expands the gas through a high speed turbine to create cryogenic temperatures facilitating the separation of ethane and heavier components from the methane. This process also uses significant heat integration to reduce energy consumption. These aspects combined with the need to match the demethanizer capacity with the incoming feed rate, create a complex control problem. The use of traditional independent PID feedback control loops on this process often results in poor performance and excessive operator intervention. The application of an advanced multivariable model predictive controller (MPC) to this complex process provides significant improvement in the process performance and greatly reduces the need for manual operator intervention. We will describe how to apply MPC to the turbo-expander based demethanizer process and show the benefits of its application in actual projects.