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 placed, an intermediate situation occurred. The combination of the diffusive movement of the solute (horizontal) and the natural convection currents (vertical) caused by the density gradient established in the module produced deformations on the interference fringes. The main consequence of the buoyancy effect is a great enhancement of the membrane performance and, therefore, a higher effectiveness of the reverse osmosis process.

Buoyancy effect on the ultrafiltration of PEG 10,000. Visualization by digital holographic interferometry

“…In dead‐end UF, only part of the macromolecules that reach the vicinity of the membrane remain dissolved in the polarization layer, whereas another important part of them is retained on the membrane surface. Then, the behavior of the instabilities during UF processes is expected to be different from that observed in the research about buoyancy in RO processes 5…”

“…In a previous experimental work,5 real‐time holographic interferometry (IH) was used to visualize the buoyancy effects on dead‐end RO of salts by rotating the cell 90 and 180° from its original position (0°) (see Figure 1). When the module was in 180° position, great instabilities were observed in the polarization layer due the natural convection currents caused by the density gradient.…”

“…The aim of this work, as a continuation and a complement to the previous RO research,5 is to visualize and study the buoyancy effects on an UF process. Thus, besides studying in UF processes, the buoyancy effect on the permeate flux in Ref.…”

“…4, it is intended to visualize the polarization layer while the buoyancy phenomenon occurs. The experimental technique used is digital holographic interferometry (DHI), a variation of the conventional HI technique previously used to study diffusion in transparent media,6, 7 the concentration polarization phenomenon when ultrafiltering BSA8 and buoyancy effects in a RO process,5 the main difference of which is the recording element. In classical HI, a holographic plate, photographically developed, is used.…”

Diffusion studies in polarized reverse osmosis processes by holographic interferometry

“…Classic interferometry [11][12][13][14][15][16] and holographic interferometry [17][18][19][20] have been successfully used to measure concentration profiles near the membrane during mass transfer processes. The knowledge of these profiles allows the evaluation of which are the mechanisms implied in the process.…”