A Study on the Relationship Between the Boundary Flux Parameters and Membrane Process Requirements

Abstract

The boundary flux concept permits to describe the fouling behaviour of membrane systems as a function of the operating time. The method relies on a set of equations that is possible to integrate in time, thus permitting to evaluate the separation process outcome and performances. This study focuses on the relationship between the membrane area requirements and specific parameters of the boundary flux concept on different membrane systems characterized by different waste feed streams and operating conditions. The target of the analysis was to identify which parameters of the boundary flux equations are the most influent one on the membrane area requirements. The knowledge of the relevant parameters can strongly assist membrane process designers to minimize the capex of the developed plants. The study was performed on many different systems, available in literature, and at different operating conditions. In order to avoid the triggering of irreversible fouling, the operation must be performed in sub-boundary operating conditions, and as a consequence, the condition of the permeate flux J_p equal to the boundary flux J_b must be met at the end of operation. Summarizing briefly the findings, it was possible to observe that the membrane area requirements are minimized for higher pure water membrane permeability values (w) for recoveries up to 75% Y_max, but then, in the range up to 90% Y_max, to achieve the same target, it appears to be more important to have higher J_b values. In other words, in those systems characterized by VRF (volume recovery factor) less than 4, high membrane permeabilities appears advantageous for the choice, even in presence of some extent of fouling. This appears not to be the case for those systems targeting higher VRF values: in this case, high J_b values must be achieved. Since the value of J_b depends on many parameters, such as T, Re and other physical-chemical characteristics, the minimization of the membrane area requirements requires the proper design of the membrane process, pre-treatment steps and operating conditions choice.