Measurement of Concentration Profiles by Holographic Interferometry and Modelling in Unstirred Batch Reverse Osmosis

Abstract

An optical method (real-time holographic interferometry) has been used to visualize concentration changes in the vicinity of the membrane surface during the dead-end reverse osmosis of salt solutions. This interferometric technique is based on the fact that changes in refractive index, which are associated with changes in concentration, can be visualized as interference fringes. Reverse osmosis experiments with NaCl and Na 2 SO 4 solutions with feed concentration in the range of 1-7 kg/m 3 at a constant pressure of 600 kPa have been conducted. Interferograms obtained under different experimental conditions, as well as permeate flux and membrane rejection, are presented. Concentration profiles in the concentration polarization layer have been determined from these interferograms and compared with those calculated using the mixed convection-diffusion and osmotic pressure theory or Fick's second law of diffusion, depending on whether the profiles correspond to the development or the disappearance of the layer. The reasonable agreement obtained between experimental and calculated results seems to support the validity of the mathematical models proposed in the range of the experimental conditions studied.

Visualization and modeling of the polarization layer in crossflow reverse osmosis in a slit-type channel

“…Digital Holographic Interferometry (DHI) has been used in this research and was previously applied to the study of the CP layer [22,23]. It is an interferometric technique very similar to classical Holographic Interferometry previously used [24][25][26][27] but replacing the holographic plate, which needs to be photographically developed, by the CCD chip of a video camera [28]. The interferometric techniques are based in the appearance of interference fringes related to changes in the refractive index of an object where a laser beam has passed through.…”

Pore size analysis from retention of neutral solutes through nanofiltration membranes. The contribution of concentration–polarization

“…Knowing that the flow is "a priori" trivially measured, and retention values could be calculated from concentration at both sides. However, due to the concentration polarization effect, to measure real concentrations on the membrane surfaces, although possible, would require complex experimental techniques [5][6][7][8][9]. Some of these techniques are, for example: interferometric measurements of the concentration polarization profile in an unstirred batch cell [10,11], light deflection techniques (shadowgraphy, refractometry), magnetic 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 -3-resonance imaging, radio isotope labeling, electron diode array microscope or direct pressure measurements as is reviewed in [12].…”

Experimental study of concentration polarization in a crossflow reverse osmosis system using Digital Holographic Interferometry

“…Cellulose acetate membranes were employed with solutions of NH 4 NO 3 , NaNO 3 , NaCl, NaSO 4 and MgSO 4 . In a relatively recent paper Since the review made by Sablani et al [2], only two papers [16,17] presenting experimentally determined profiles of the CPL have been found. In both cases, experiments were developed on unstirred batch conditions.…”

“…Twelve small-diameter stainless steel tubes were fitted to run parallel to the length of the experimental column, each tube terminating at a different position within the concentration polarization layer. In the other paper [17], holographic interferometry was used to visualize the appearance, evolution with time and disappearance of the concentration polarization layer in unstirred batch reverse osmosis.…”

“…In previous papers [17,[19][20][21][22][23][24], the optical holographic interferometry technique was used to visualize the evolution of the concentration polarization layer during UF of BSA and PEG solutions, as well as RO of salts. This technique, which has also been used to study diffusion processes [25][26], allows interferometric fringe patterns to be obtained, that are indicative of changes in the optical path followed by the light and are related to changes in the refractive index.…”