Optimal Design of Membrane Processes: a Problem of Choices Between Process Layout, Operating Conditions and Adopted Control System

Abstract

The development of membrane processes as a technology for environmental treatment applications and in particular for the purification of wastewater streams has significantly increased in the last decades. Fouling on membranes appears to be one of the main technical limit of this technology. This phenomenon causes the unavoidable deposition of particles on the membrane surface, building a resistive growing layer to permeability. Sensible fouling of the membrane leads to a significant reduction of the performances, a decrease of the operating life and, as a consequence, the increase of the operational costs due to the replacement or cleaning of the exhausted membrane modules. The presence of the fouling phenomena makes the proper design and control of membrane systems a difficult task. Optimal design of the membrane processes will be here discussed. The procedure requires to determine the optimal process layout given the input data and target requirements. At the end, the required membrane area is calculated. This latter property is strictly dependant of the adopted operating conditions, most importantly by the adopted value of transmembrane pressure (TMP). Moreover, it depends if the value of TMP remain fixed as a function of time or is variable (as in case of fixed permeate flow rates). Therefore, the optimal design of the system may occur only if the adopted control strategy is defined a priori. As a consequence, design choices of the membrane process layout, operating condition and adopted control system are strictly dependant, and connections between these different aspects should not be neglected during the engineering and P&I development stage of membrane systems. This paper will start from the theory of the boundary flux, in order to describe a novel design approach to membrane systems. Parallel to this, the development of an advanced control system, that allows to limit fouling formation during operation, is presented. The advanced control system relies on a suitable simulation software capable to predict the boundary flux, that changes the controller's set-points accordingly. Finally, the paper will merge all elements together, and report about the optimal design of membrane processes equipped with the advanced membrane process control system; validation of the proposed approach will be based on the use of a custom simulation model in ASPEN HYSYS and by experiments on lab scale.