Buoyancy Effects in Dead-End Reverse Osmosis: Visualization by Holographic Interferometry

Abstract

Real-time holographic interferometry, previously used to measure concentration profiles in the polarized layer during membrane processes, has been applied to visualize the buoyancy effects on dead-end reverse osmosis of salts by rotating the cell 90° and 180° from its original position (0°). Sets of experiments have been carried out, each one with the membrane in a different gravitational orientation, using NaCl and Na2SO4 with a feed concentration ranging from 1 to 7 kg/m3 at a constant pressure of 600 kPa. The interferometric fringe patterns obtained in each membrane position were very different. At the 0° position, the evolution of the polarization layer was observed by means of several interferometric fringes parallel to the membrane surface. At 180° position, the interferometric fringe pattern obtained pointed out the existence of a natural convection or buoyancy flow in the vicinity of the membrane surface which prevented the growth of the polarization layer. Finally, with the membrane vertically pl...