EFFECTS OF THERMAL AND CONCENTRATION BOUNDARY-LAYERS ON VAPOR PERMEATION IN MEMBRANE DISTILLATION OF AQUEOUS LITHIUM BROMIDE SOLUTION

The permeation flux of water vapor in membrane distillation is affecte d by membrane properties, the difference in the vapor pressure between the opposite sides of the membrane, and the operational conditions. S ince the vapor pressure is a function of the concentration and the tem perature, the vapor flux through a membrane is decreased with increasi ng concentration and decreasing temperature of the salt solution. Perm eation experiments with a PTFE membrane (80 mu thick, 0.2 mu m pore di ameter, 0.75 porosity, 2.38 x 10(-3) m(2) effective surface area) were conducted by using a batch cell, which had two reservoirs stirred by magnetic bars, separated by a membrane, and thermostated by hot water and cool water circulated in respective jackets. The temperature of th e hot water was ranged from 308 to 373 K, and the temperature of the c ool side was kept at 288 K. The stirring rate was varied from 200 to 8 00 rpm. The concentration of aqueous lithium bromide solution was rang ed within 0 to 55 wt%. The water flux was obtained by the moving rate of the water meniscus in the capillary connected to the sealed cell. T he permeate flux was affected by the thermal and concentration boundar y layers. The analogy analysis for the thermal and concentration bound ary layers was applied to the permeation mechanism, and well explained the effects of the stirring rate and the difference in the temperatur e on the permeation flux. The thickness of the thermal boundary layer was found to be larger than that of the concentration boundary layer, and both layers were hardly negligible under the conditions of this wo rk.