EFFECTS OF RECYCLE ON CONTROL OF CHEMICAL PROCESSES

The effects of the recycle of mass or energy on the control of chemica l processes are studied. The recycle can be considered analogous to a feedback and significantly changes the characteristics of the process. The study on simple transfer function models indicates that the recyc le loop gain strongly and time constant of the recycle unit somewhat l ess strongly influence the behaviour and the controllability of the pr ocess. Higher time constants of the recycle, the use of large surge ta nks, and the control of the recycle unit of smaller time constants imp roves the range of stability. Two case studies, the distillation train of the manufacture of styrene and a simple reactor-distillation colum n system, demonstrate that chemical processes with internal recycle ca n usually be controlled with decentralised control structure with indi vidually tuned controllers. The case studies also show that in the cas e of the control structure design the connections of the different uni ts should be considered and the control of the recycle unit is also im portant.