Control of Delayed Recycling Systems with Unstable First Order Forward Loop
This paper considers the plantwide control structure design for improved disturbance rejection. The balanced control scheme of ‘Comput. Chem. Eng. 20 (1996) 1291’ has the advantage of alleviating the snowball effect for plants with recycles by changing condition in several units in the process so as to distribute the work evenly among process units as production rate changes. However, the balanced scheme may lead to rather complex control configurations, especially for composition control. Based on steady-state disturbance sensitivity analyses, we are able to simplify the control structure by eliminating some of the compositions loops and still maintain a balanced partial composition control structure. More importantly, this is achieved with minimal process information, i.e., material balances and characteristics of the balanced scheme. For a simple recycle plant, results show that only one composition loop is sufficient to keep all three compositions near set points. A number of single composition control alternatives are explored. Two attractive alternatives are: (1) letting the reactor level uncontrolled or (2) using a proportional only reactor level controller. Nonlinear simulations show that effective composition control can be obtained with rather simple control structures. # 2002 Elsevier Science Ltd. All rights reserved.