USE OF NANOFILTRATION FOR CONCENTRATION AND DEMINERALIZATION IN THE DAIRY-INDUSTRY - MODEL FOR MASS-TRANSPORT

For whey products intended for human or animal consumption, deminerali zation enhances the nutritional value of the product. In industrial pr ocesses, whey is concentrated by evaporation (EV) and subsequently dem ineralized by electrodialysis (ED) and/or ion-exchange. Nanofiltration (NF) is an alternative for partial demineralization of whey. NF-membr anes which are suitable for dairy applications have a high permeabilit y for (monovalent) salts (NaCl, KCI) and a very low permeability for o rganic compounds (lactose, proteins, urea). The use of NF instead of E V followed by ED has the advantage of simultaneous concentration and d emineralization of whey. This eventually reduces processing costs. Mod els were developed, based on the extended Nernst-Planck equation, whic h describe the salt rejection as a function of the flux for binary and ternary salt solutions. Effects of concentration polarization, compos ition of feed and concentration are incorporated in the model. In labo ratory-scale experiments, rejection-flux curves of four different comm ercial membranes were established for three different model solutions (NaCl, CaCl2 and (NaCl + CaCl2)) and for ultrafiltration (UF) whey-per meate (pH 4.6, 5.8 and 6.6). The results indicated that the salt trans port through all the NF membranes investigated depends on the flux. At low flux, when the contribution of diffusive transport is the most im portant, permeation of (especially monovalent) cations is high. At hig h 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 (N aCl or CaCl2), rejection data could be described by the (two-parameter ) model for binary systems. For ternary systems (NaCl and CaCl2) the m odel was simplified from a model with four transport parameters to a m odel with three transport parameters. Rejection data for a ternary sys tem could also be described adequately. Decoupling of transport parame ters allowed that the model for the ternary system could be reduced fr om a four-parameter model to a three-parameter model without losing ac curacy. For ultrafiltration (UF)-whey-permeate, a multicomponent mixtu re, it is shown that an approach in which monovalent cations, divalent cations and anions were grouped separately and lumped into one concen tration can be used to describe the rejection-flux data adequately. Th e 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 lo wer than the rejection actually measured. About halve of the differenc e could be ascribed to lactate and carbonate, which were not determine d separately. As a result there was also a non-matching charge balance . The transport parameters derived from the results with UF-whey-perme ate can be used to predict the salt rejection for similar multi-compon ent systems like whey and UF-permeate in industrial systems.