Fundamentals of Crossflow Filtration

Abstract

Traditional Crossflow Filtration and its Bottlenecks

Crossflow filtration is a method applied mainly to hyper-and ultrafiltration as well as, to some extent, microfiltration. In crossflow filtration the solution or suspension fed to the filter flows parallel to the filter medium or membrane. Perhaps the name “parallel” or “tangential” filtration would be more illustrative than “crossflow.” The filtration product, i.e., the filtrate or permeate (depending on whether a suspension or solution is separated) leaves the filter channel at right angles to the medium. The crossflow method is just the opposite of the so called “dead-end” filtration used in almost all traditional filtering processes. In “dead-end” filtration the flow of suspension is directed at right angles to the medium and the filtrate leaves the medium in the same direction (Fig. 1.) This difference between parallel and perpendicular flow is significant because perpendicular flow entails cake formation, whereas crossflow is intended to prevent such cake build-up. Dead-end filtration is thus applied when the cake is to be collected, when the main purpose is the recuperation of suspended solids, or when relatively rapid cake formation cannot be avoided. The crossflow method is applied, on the other hand, when formation of the deposit on the medium is to be prevented as far as possible. This is the case for solutions and suspensions with very low solids content, such as liquids treated by reverse osmosis, ultra- and microfiltration. For these three kinds of processes there is the same flow pattern as well as very similar hardware.