Simple technique for measuring the concentration polarization level in a reverse osmosis system

This paper describes the application of a simple technique for determining the mass transfer coefficient and the concentration polarization level in a reverse osmosis (RO) system. The technique is based on evaluation of the permeate flux decline induced by the addition of a salt solution to an initially salt-free water feed. Since the net pressure driving force is influenced by the level of the osmotic pressure prevailing on the membrane surface, the magnitude of flux decline enables the evaluation of membrane surface concentration, and hence the determination of the mass transfer coefficient k. The mass transfer coefficient is given by: k = (J(v))(salt)/In {DeltaP/pi (b)-pi (p).(1=(J(v))(salt)/(J(v))M2O)} Hence, the value of k can be simply determined from the osmotic pressures pi (b), and pi (p), of the saline feed and of the permeate respectively and by measuring (J(v))H2O, the permeate flux of the salt-free water, and (J(v))H2O, the permeate flux of the saline solution. The proposed technique was verified by experiments performed in a tubular RO system under turbulent flow conditions. Experiments covering the Reynolds number range of 2,600-10,000 yielded the following mass transfer correlation: Sh = k . d/D = 0.020 . Re (0.91) . Sc (0.25) This expression is practically identical with the theoretically anticipated Deissler correlation, thus lending strong support to the proposed mass transfer measurement technique.