Investigating the Effects of Isentropic Efficiency on a Turbo-Expander for Optimal Extraction of Natural Gas Liquid Using Model Predictive Controller

Optimal control of a turbo-expander is paramount for high isentropic efficiency such that the desired higher output in NGL extraction is achieved. In this work, the process flow was implemented in two instances using MPC and PID control systems. In both instances, the operation process variables’ data of the turbo-expander from the Mobil Producing Nigeria NGL extraction plant known as GX-facility were used. Mathematical functions for computing the cost function, defining the constraints, input, and state estimation were used as subsystem blocks in Simulink and wired with other process components such as the heat exchangers, sensors, transfer-function blocks and compressor sub-systems to form the NGL extraction plant. A simulation was performed in five locations at intervals of 5sec using the MPC and the PID control systems at 650psi and 1015psi for the lower and upper limits pressures respectively. A baseline of 300mscfd steady-state flow rate was used. The performances of the controllers were evaluated based on the isentropic efficiency achieved for the extraction process. The simulation result showed that average isentropic efficiencies of 43% and 76% were achieved for the PID and the MPC control systems respectively, as the pressure ratio increases. The result indicates that the PID controller greatly degraded with an increase in pressure ratio, while the MPC system experienced much slight decrease.