Abstract:
The control of the distillation unit in Khartoum refinery, by keeping the operating conditions in certain limits is essential to adjust disturbances which lead to deviation from the desired values. The aim of this work is to select the suitable controller and the appropriate tuning method. The control loops are designed depending upon controllability and performance. These loops were Control of furnace temperature, Control of interface level at reflux drum, Control of level at stripper one, Control of level at bottoms of the column and Control of pressure at the top of the column. The mathematical model for each loop was determined using MATLAB toolbox system identification by designing Graphical User Interface (GUI). The same was used to identify the transfer function for each loop. Having identified the transfer functions, each loop was closed and the characteristic equation was obtained from the overall transfer function. Using Z-N tuning method to get the ultimate gain and ultimate period, from which the adjustable parameters were determined using Z-N criterion. These were used to investigate the offset upon a unit step change in the set point using proportional only (P), proportional integral (PI) and proportional integral derivative controller (PID).The controller that gave the minimum offset is found to be proportional integral derivative controller (PID) and has been selected. The procedure is repeated using Root-Locus and decay ratio criterions. In each case the controller that gave the lowest offset was selected. The three methods of Z-N, Roots Locus and decay ratio were compared according to their offset and found to be within good accuracy. Stability performance was examined using Bode, Nyquist and Root locus Criteria. Having closed the loops in conventional system, the same was transformed to digital computer control. The analysis of the offset investigation and system stability were performed on (Z-domains). The results are in agreement with conventional analysis. In conclusion a complete control system of a distillation unit was designed, drawn in block diagram and interfaced to a digital computer control.
