Concentration Polarization in Hyperfiltration Systems
When using hyperfiltration membranes for brackish or seawater desalination, an enrichment of the solute (concentration polarization) always occurs at the membrane surface juxtaposed with the brine solution. This is due to the formation of a boundary layer separating the membrane surface from the bulk flow. The thickness of this boundary layer is dependent upon the stirring or streaming velocity. Because the salt transport within the boundary layer is not affected by the bulk flow, the rejected salt at the membrane surface must return to the bulk flow by means of diffusion only. This diffusion of salt into the well stirred or streaming solution is counterbalanced by the convective salt flux (associated with the permeation of water through the membrane), thereby forming a concentration gradient within the boundary layer. The enhanced salt concentration at the membrane surface results in a lower water flux and salt rejection. Thus, the “intrinsic water flux” and the “intrinsic salt rejection”, which would be observed without concentration polarization, are always greater than the observed water flux and salt rejection. The effect of concentration polarization upon volume flow, q, and salt rejection, robs, of a membrane was first investigated by Merten. He showed that the volume flow is limited by concentration polarization because of the effect on local osmotic pressure.
KeywordsMembrane Surface Mass Transfer Coefficient Boundary Layer Thickness Concentration Polarization Bulk Flow
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- 3.W. Banks and A. Sharpies, “The Mechanism of Desalination by Reserve Osmosis and its Relation to Membrane Structure”, Office of Saline Water Res. Develop. Progr. Rept. No. 143, June 1965.Google Scholar
- 6.P.L.T. Brian in Desalination by Reverse Osmosis, U. Merten (Ed.), M.I.T. Press, Cambridge, Mass. 1966, pp. 161–202.Google Scholar
- 8.T. K. Sherwood, Chem. Eng. Progr. Symp. Ser., 55, 71 (1959).Google Scholar
- 9.Chandrasekhar, Hydrodynamic and Hydromagnetic Stability, Academic Press, New York, 1954.Google Scholar
- 17.H. Strathmann, “Control of Concentration Polarisation in Reverse Osmosis Desalination of Water”, Office of Saline. Water Res. Develop. Progr. Rept. No. 336, April 1968;Google Scholar
- 18.H. J. Bixler and R. A. Cross, “Final Report on Control of Concentration Polarization in Reverse Osmosis Desalination of Water”, Office of Saline Water Res. Develop. Progr. Rent. No. 469, October 1969.Google Scholar
- 20.T. K. Sherwood, P.L.T. Brian, and R. E. Fisher, MIT Desalination Research Laboratory, Rept. 295–1, August 1963.Google Scholar
- 21.J. S. Johnson, Jr., L. Dresner, and K. A. Kraus in Principles of Desalination, by K. S. Spiegler (Ed.), Academic Press, New York, 1966, pp. 345–439.Google Scholar
- 22.L. Dresner, Oak Ridge Natl. Lab. Rept. No. 3621, May 1964.Google Scholar
- 23.R. E. Fisher, T. K. Sherwood, and P.L.T. Brian, MIT Desalination Research Laboratory, Rept. 295–5, 1964.Google Scholar
- 26.H. Goldsmith and H. Lolachi, Office of Saline Water Res. Develop. Progr. Rept. No. 527, May 1970.Google Scholar