Control and operability of recycle processes

Abstract

<p>This thesis presents an investigation into the effect of recycle on the dynamic controllability and operability of chemical processing systems. In particular, this investigation focuses on the effect that recycle has on the behaviour of individual chemical units when they are configured such that the product of the last unit is returned as a partial feed to the first unit. An experimental extractive distillation unit, which separates a mixture of methanol and acetone using a water solvent, was chosen for this study. Two aspects of this system were highlighted in this thesis. The first is that the system is subject to, and is to be controlled against, feed flow and compositional disturbances. The second aspect is that the system contains deadtime, and the analysis techniques must take this into account. There are currently several techniques for analyzing the dynamic behaviour of chemical systems (without resorting to exhaustive numerical simulations). It is demonstrated that these techniques contain severe limitations which preclude their use for a recycle system. Modifications presented in this thesis to these techniques allow for their successful application on a recycle system; furthermore, these modifications are not limited to recycle systems exclusively, but should be utilized on chemical engineering systems in general. In particular, these modifications are: (1) A methodology was formulated for determining interaction for regulatory control systems. To date, dynamic interaction techniques were constrained to servo systems; the technique presented in this thesis is the first to consider the regulatory interactions of the system. (2) Interaction techniques were modified to take into account deadtime, as all present techniques have the shortcoming of being invariant to deadtime. (3) A technique for simplifying recycle transfer functions was formulated. This was necessary because recycle transfer functions formed from their constituent subsystems cannot be inverted to the time domain or used to design controllers. These techniques were applied to the extraction unit and compared against simulations to demonstrate their effectiveness. The large effect that recycle has on the open and closed loop response of this system was also demonstrated, illustrating the importance of considering the response of the entire system, and not just its component parts.</p>