Mixing in Spacerâ€“Filled Channels
AbstractCoated flatâ€“sheet contacting devices are currently under development for a variety of membrane applications, e.g. in air conditioning , in the removal of CO2Â from fuel gases of biological origin  or in the elimination of trace organics from wastewater . Generally, the relative take-up capacity of the stripping fluids is very large leading to low crossâ€“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 spacerâ€“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 Fickâ€™s Second Law can be fitted to the colorâ€“change front. The dispersion coefficient, which is a measure for lateral mixing in the spacerâ€“filled channel, can be therewith obtained directly.
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