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

•Polarization layer in crossflow RO has been visualized by Holographic Interferometry.•Evolution of the polarization layer (PL) along the channel has been directly observed.•PL is mostly affected by the CF velocity, but it hardly influences permeate flux.•Feed concentration reduces considerably the permeate flux due to the osmotic pressure.•Experimental results have been compared with calculated data from CFD.

The evolution of the concentration polarization layer during crossflow reverse osmosis in a slit channel has been studied. Digital Holographic Interferometry allows visualizing the polarization layer as an interference fringe pattern. An especial module with four windows has been designed to see the development of the polarization layer along the membrane channel. Several experiments with a constant transmembrane pressure of 6 bar, three different feed concentrations (3, 6 and 9 kg/m3) and three different Re (13, 38 and 111) had been carried out. The process has been modeled simulating the experimental conditions. The computed results were found to be consistent with the experimental ones. All the experiments show a continuous increase of the polarization layer along the channel, regardless of the crossflow velocity, except near the outlet of the cell due to an edge effect. This increase in the polarization layer is greater at lower Re, although it does not influence very much the permeate flux. In contrast, what substantially affects the permeate flux, much more than Re number, is the feed concentration due to its osmotic pressure.