Productivity and energy consumption as performance criteria to overcome bias caused by membrane resistance

Abstract

Membrane scientists and engineers routinely compare the performance of membranes for different applications. Such comparisons are required to optimize surface chemistries and to evaluate and select the best membrane to scale-up industrial applications. In most cases, the potential of the feed to foul membrane surfaces is also critical. When different membranes under consideration have different resistances Rm, such comparisons must be done with care. Here we elucidate the impact of Rm on fouling kinetics for a range of fouling mechanisms and experimental protocols. We show that traditional plots of flux versus time for constant pressure operation, or pressure versus volume in constant flux operation, obscure two important performance criteria: accumulated volume over a defined time period (i.e., productivity) and energy consumption; practical implications include the risk that higher performance may be attributed to membranes that actually have lower productivity and/or that consume more energy, and that lower performing membranes may be inadvertently chosen from screening studies as offering superior performance. To facilitate membrane comparison with different Rm, we have developed a graphical approach using normalized coordinates. This approach yields linear plots having slopes that are a function of fouling parameters only, and are independent of membrane resistance Rm. Therefore, such normalization coordinates, presented for each fouling mechanism and each operation mode, can be employed to isolate fouling potential. In addition, we developed a new graphical approach employing contour plots of iso-volume or iso-energy lines to visualize the potential trade-offs between antifouling performance, membrane permeability, and either productivity in the case of constant pressure operation, or specific energy in the case of constant flux operation. This study provides a framework for judging the performance of different membranes under fouling conditions, considering fouling, energy consumption and productivity as metrics, independent of membrane resistance. This framework will help guide process design and the design and preparation of antifouling membranes.