Graduation Year

2009

Document Type

Dissertation

Degree

Ph.D.

Degree Granting Department

Civil Engineering

Major Professor

Mahmoud Nachabe, Ph.D.

Keywords

Reverse osmosis, Concentration polarization, Desalination, Diffusion, Peclet number

Abstract

Fouling in a nanofitration membrane module is usually a result of concentration polarization. The effect of permeate suction on the slightly negatively charged spiral wound nanofiltration membrane is investigated. According to the film theory, the mass transfer coefficient is inversely proportional to concentration polarization. The effect of permeate suction destabilizes the boundary layer. This will decrease the concentration polarization layer, and consequently will increase mass transfer through the membrane's surface. To validate the hypothesis, experiments were carried out on a NF membrane that can be described by the solution-diffusion model. This model has coefficients that can be measured experimentally. Using the membrane wall concentration in this model instead of the bulk feed concentration can help estimating the mass transfer coefficient more appropriately.

Two experimental studies were carried out, one with a standard high pressure pump, and another one with the added effect of suction pressure applied to the permeate collector tube. Three different concentrations of binary dilute solutions of NaCl,MgSO4, and MgCl2, at three different pressures (low, medium, and high) were tested. For all tested solutions, permeate suction increased the diffusive Peclet number as a function of the feed concentration (x) according to the equation Pe = a1x²+b1x+c1, with R²>0.99, where x is the feed concentration in Mol/l, and a1, b1, and c1 are coefficients dependent on feed pressure for every salt solution. With the increase of the Peclet number, it was observed that the concentration polarization decreased, and both the product flow and the product quality were improved.

Suction had the greatest impact at the range of 100 to 110 psi feed pressure, where the concentration polarization reduced approximately 14 to 20 %. ANOVA for the concentration polarization showed that suction was significant in reducing the calculated concentration polarization layer for all tested solutions. It was concluded that permeate suction reduced concentration polarization, increased product flow rate, and improved product quality. Thus, adding permeate suction has beneficial consequences because it reduces membrane fouling and extends its useful service life.

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