Mixing in Spacer–Filled Channels

Abstract

Coated flat–sheet contacting devices are currently under development for a variety of membrane applications, e.g. in air conditioning [1], in the removal of CO₂ from fuel gases of biological origin [2] or in the elimination of trace organics from wastewater [3]. Generally, the relative take-up capacity of the stripping fluids is very large leading to low cross–flow velocities in the permeate channel. This undesirable consequence broadens the residence time distribution so that the stripping fluid is loaded unevenly. However, its effect can be reduced by enhancing lateral mixing. To quantify the mixing effects of spacers at different hydraulic conditions, this paper presents a suitable measurement method for lateral dispersion in spacer–filled channels. The measuring concept is based on mixing an acid and an alkaline stream, both marked with a pH indicator. The points in space where the pH indicator shifts its color are directly observed by photometric analysis. The resulting image is interpreted by transforming the axial space coordinate to a time coordinate, so that an analytical solution of Fick’s Second Law can be fitted to the color–change front. The dispersion coefficient, which is a measure for lateral mixing in the spacer–filled channel, can be therewith obtained directly.