Effect of recycle on control of a purely integrating process

Chemical plant designs with heat or material recycle are becoming more common due to economic and environmental reasons. Underperforming control loops in chemical plants may in some cases be attributed to the presence of recycle streams. This is because recycle alters the dynamics of neighboring processes. During commissioning work, when control engineers are tuning control loops, they are often under time pressure and it is therefore not unusual to try to perform as short step response experiments as possible. Recycle dynamics could then be missed altogether. Recycle could depend on a controlled variable, an uncontrolled variable or a manipulated variable. This paper will shed some light on how control of a purely integrating process is affected by the addition of a recycle stream that is a function of the manipulated variable.