Use of nanofiltration for concentration and demineralization in the dairy industry: Model for mass transport

Abstract

For whey products intended for human or animal consumption, demineralization enhances the nutritional value of the product. In industrial processes, whey is concentrated by evaporation (EV) and subsequently demineralized by electrodialysis (ED) and/or ion-exchange. Nanofiltration (NF) is an alternative for partial demineralization of whey. NF-membranes which are suitable for dairy applications have a high permeability for (monovalent) salts (NaCl, KCl) and a very low permeability for organic compounds (lactose, proteins, urea). The use of NF instead of EV followed by ED has the advantage of simultaneous concentration and demineralization of whey. This eventually reduces processing costs. Models were developed, based on the extended Nernst-Planck equation, which describe the salt rejection as a function of the flux for binary and ternary salt solutions. Effects of concentration polarization, composition of feed and concentration are incorporated in the model. In laboratory-scale experiments, rejection-flux curves of four different commercial membranes were established for three different model solutions (NaCl, CaCl 2 and (NaCl+CaCl 2)) and for ultrafiltration (UF) whey-permeate (pH 4.6, 5.8 and 6.6). The results indicated that the salt transport through all the NF membranes investigated depends on the flux. At low flux, when the contribution of diffusive transport is the most important, permeation of (especially monovalent) cations is high. At high flux, when transport by convection is the most important, rejection reaches a maximum (constant) value. From this it follows that the salt transport can be controlled by the flux. For binary salt solutions (NaCl or CaCl 2), rejection data could be described by the (two-parameter) model for binary systems. For ternary systems (NaCl and CaCl 2) the model was simplified from a model with four transport parameters to a model with three transport parameters. Rejection data for a ternary system could also be described adequately. Decoupling of transport parameters allowed that the model for the ternary system could be reduced from a four-parameter model to a three-parameter model without losing accuracy. For ultrafiltration (UF)-whey-permeate, a multicomponent mixture, it is shown that an approach in which monovalent cations, divalent cations and anions were grouped separately and lumped into one concentration can be used to describe the rejection-flux data adequately. The experimental data for the (cumulative) anion equivalent charges were predicted accurately only at pH 4.6 and 5.8. At pH 6.6, the rejection calculated for the anions based on equivalent charges was somewhat lower than the rejection actually measured. About halve of the difference could be ascribed to lactate and carbonate. which were not determined separately. As a result there was also a non-matching charge balance. The transport parameters derived from the results with UF-whey-permeate can be used to predict the salt rejection for similar multi-component systems like whey and UF-permeate in indutrial systems.