The Stability Analysis of a Modified Cascade Control Strategy for Khartoum Refinery Delayed Coking Unit's Drums

Abstract

The delayed coking unit is a process that converts heavy oil residues into more valuable products. The pressure control of the coking drums is important for process safety, yield, and stability. However, it has received limited study compared to temperature control. Cascade control is suitable for many industrial processes, especially when enhanced by combining proportional (P), integral (I), and derivative (D) actions in the control loops. This study focused on modifying a previous cascade control strategy developed to control the pressure inside the coking drums at Khartoum Refinery by controlling the heating fuel flow rate. The modification included replacing the Proportional (P) controller in the secondary loop with Proportional-Integral (PI) and Proportional-Integral-Derivative (PID) controllers to evaluate their impact on the overall control system stability. MATLAB simulations were used to calculate system transfer functions and test stability through direct substitution, root locus, Bode plots, and step response analysis. The results showed instability with both controllers due to reduced phase margins and increased high-frequency noise. Several control strategies were recommended to enhance the system performance, such as Lead/Lag compensators, Model Predictive Control (MPC), and State Feedback control.