Optimization of Multiple-Fan Air-Cooler Control in a Distillation Column

Abstract

Air coolers are often used in refineries and petrochemical plants to cool process fluid or circulating cooling water. In general, an overhead air-cooler of atmospheric distillation columns consists of multiple fans including fixed fans and variable flowrate fans that can change flowrate by controlling the pitch angle or the rotation speed. Even when the air-coolers are affected by disturbances on the process side or cooling side, the influence of the disturbances can usually be suppressed by variable fans which control the column overhead pressure or the outlet temperature. However, when the controller output exceeds the operating range of the variable fans, it is necessary to return the controller output into the operating range by starting or stopping the fixed fan in order to restart control. There are two issues to consider in the operation of an air-cooler with multiple fans. One is that the air-flow rate fluctuates when the fixed fan is started or stopped, and the other is that the power consumption of the fans cannot be minimized. In this paper, first, in order to suppress the start/stop disturbance of the fixed fan, a cascade control method which estimates total air flowrate and defines this as an intermediate variable is proposed. On the process dynamic simulator Aspen HYSYS^® Dynamics, it was shown that when starting and stopping the fixed fan, the proposed control system immediately manipulates the variable fans to stabilize the air flowrate and the tower overhead pressure. Next, we propose a method for optimizing the number of fixed fans in operation so as to minimize the power consumption while reducing the frequency of starting and stopping of fixed fans. As a result of applying the proposed method to the simulation data, it was elucidated that the power consumption can be expected to be reduced without changing the frequency of changing the number of fixed fans compared to the conventional method.