pH control structure design for a periodically operated membrane separation process

A bioreactor integrated with an electrically driven membrane separation process (Reverse Electro-Enhanced Dialysis – REED) is under investigation as potential technology for intensifying lactic acid bioproduction. In this contribution the pH regulation issue in the periodically operated REED module is studied. A methodology for control structure design is proposed to handle the dynamic system. A sensitivity analysis is used for the conceptual design of the control structure. Dynamic simulations are employed to evaluate the sensitivity index. From the analysis a periodic input-resetting control structure is selected. The system controls pH using the imposed current density and resets the current density manipulating the hydroxide inlet concentration to the dialysate channel. The control structure is satisfactorily achieving a desired pH at the outlet of the feed channel in REED from period to period and resetting the current density. Thus suitable performance is achieved within a large part of the operating window.

► The proposed dynamic sensitivity index assists the controlled/manipulated variables pairing. ► pH is more sensitive to current density than to base concentration at high frequencies. ► At low frequencies both manipulated variables have relevant influence on the pH. ► An input-resetting control structure can exploit the sensitivities and manage input constraints. ► The proposed control structure is able to control the system and avoid constraints.