Flocculation in laminar tube flow

Abstract

The extent of orthokinetic flocculation of suspensions as a result of laminar tube flow has been calculated using Smoluchowski theory and shown to depend primarily on the tube dimensions and not on the flow rate. When a correction factor allowing for hydrodynamic resistance between approaching particles is introduced, the calculated flocculation rate can be much lower than the Smoluchowski value and depends inversely on flow rate and particle size. A further effect which has to be included is the non-uniformity of shear rate in the tube, which leads to less flocculation than expected on the basis of a uniform shear rate throughout the tube. Coiling of the tube should given an extra degree of mixing to the flowing suspension and hence a more uniform shear rate and an enhanced flocculation. Preliminary experiments using model suspensions flowing through straight and coiled tubes have shown fair agreement with these predictions. It is suggested that laminar flow through coiled tubes could provide the basis of a practical flocculation test method.