by Ronan K. McGovern
Inspired by the thinking of Randy Truby and Jim Birkett to find a single figure of merit for seawater RO membranes, I thought I’d see if I could compare the performance of a selection of membranes currently on the market.
To begin I plotted the nominal flux and rejection of 8” membranes from Toray, Filmtec, Hydranautics and NanoH2O – note the membranes that compete directly (e.g. at 99.8% rejection and a flux of about 38 lmhb). The problem I have with data in this form is that neither flux nor rejection are properties of the membranes. Furthermore, rejection depends upon flux – with higher fluxes tending to increase the relative amount of water passing compared to salts and thus increasing rejection. Therefore, I took the data from Fig. 1 and computed the water permeability and salt permeation coefficient for each membrane*. I then took this data and generated Fig. 2.
Fig. 2 has a number of remarkable points:
We typically regard both membrane permeability and the salt permeation coefficient to scale linearly with the thickness of the active layer (the solution diffusion model). For this reason, any straight line drawn from the origin represents a membrane with the same active layer chemical composition but of decreasing thickness as one moves away from the origin.
Following the above logic, the three Hydranautics membranes appear to have similar active layer chemistry but different thicknesses. The Toray membranes seem to include membranes of two different chemical compositions, as do the NanoH2O membranes.
Hydranautics, Toray and NanoH2O membranes show the expected trade-off between water permeability and salt permeation coefficient. An improvement in one leads to a dis-improvement in the other. By contrast, the Filmtec membranes do not illustrate this trend. This is surprising and begs the […]