Effect of temperature, pH and composition on nanofiltration of mono/disaccharides: experiments and modeling assessment

The paper introduces a systematic experimental investigation on the role of temperature and composition on the membrane permeability and on the solute rejection, in association with suitable modeling. The case of Nanofiltration of aqueous solutions containing single sugars (dextrose or fructose or maltose) is considered with polyamide membranes. Real rejections are reported as a function of total volume fluxes in the complete rage from 0% to 100%, at 30 °C and 50 °C, at pH=4 and 6, with sugar composition from 1 to 300 g/dm3, in the pressure range from 3 to 30 bar. Hydraulic permeabilities are also measured at 30,40,50 °C, at pH=4, for NF membranes (GE-DK and GE-DL) and for brackish water RO membranes (GE-AG and GE-AK). The wide experimentation allows to obtain general trends and to define a criterion for the experimental protocol required for a complete characterization of membrane performances. A revision of the Steric Pore Model is also introduced, in which a porous vision of the membrane is applied both to the description of the solute flux and of the total flux. The model allows to understand and to explain the nature of the solute/membrane and/or of the solvent/membrane interactions, and to develop a criterion for the primary elaboration of experimental data. Hydrodynamic coefficient of the solute becomes the main adjustable parameter of the model related to the solute type, assuming the meaning of a binary interaction parameter accounting of solute/membrane interactions, including membrane geometry and molecular dimensions. For the GE-DK and GE-DL membranes no swelling effect of temperature nor of the solute type is observed on membrane permeability, whereas solute rejection is greatly affected by temperature. Experimental results are useful to test the model and the corresponding procedure for the parameters calculation, developed in this work. A critical discussion about the validity of the procedure is presented and compared with other elaboration techniques reported in literature.