1,721,135 research outputs found
Oligosaccharide nanofiltration: an experimental study about electrolyte effect on the separation
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Effect of temperature, pH and composition on nanofiltration of mono/disaccharides: experiments and modeling assessment
No full textThe 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
Mass transfer in 1812 spiral wound modules: Experimental study in dextrose-water nanofiltration
No full textBiotech 1812 spiral-wound elements manufactured by GE Powe&Water have been characterized, operating in NF with aqueous solutions containing 50 g/dm3dextrose at 50 °C and pH = 4. Operative conditions were selected in order to get experimental results of flux and observed rejections highly dependent on feed flow rate, so that a confident mass transfer correlation in the feed side has been obtained. The mass transfer correlation accounts of the feed spacer geometrical characteristics included in the description of the hydraulic diameter. It matches in a surprising manner with the well-known correlation derived from heat and mass transfer analogies in turbulent flow regime, it is in a good agreement with a recently published correlation derived from OSN in 1812 modules, whereas it is heavily in contrast with the widely used Shock and Miquel equation. In addition, the elaboration of the experimental data according to the velocity variation method does not lead to confident results. The correlation here presented can be extended to the simulation of industrial modules operating at feed flow conditions corresponding to Reynolds number in the range from 100 to 700, since it is rather independent of the way in which it was calculated. A critical discussion is also presented about the differences between the values of membrane permeability and of the module permeability and about the role of the âmodule length to membrane widthâ ratio in data elaboration. A sensitivity analysis concludes the work, in which authors discuss how the results of module characterization depend on the quality of the mass transfer correlation in the feed side and give some recommendations for a proper elaboration of experimental results
Transport phenomena in reverse osmosis/nanofiltration membranes
No full textA general unique structural vision of transport phenomena in Reverse Osmosis (RO) and Nanofiltration (NF) membranes is discussed. The most common models for RO (such as the solution-diffusion and/or the Spiegler-Kedem model) and for NF membranes (the extended Nernst-Planck equation) are introduced as particular cases of the general “statistical-mechanical theory” of membrane transport developed by Mason and Lonsdale in 1990. The use of that approach is recommended to develop a structural model when the physical meaning of the parameters is desired.
The typical trends of solute rejection and of the total volume flux in RO processes are discussed and the meaning of the model parameters is explained.
NF modelling is presented according to the conditions of the porous vision of the Donnan-Steric-Pore-and-Dielectric-Exclusion model (DSPM-DE). The complexity of the physical phenomena involved in the partitioning mechanisms is widely discussed: mechanisms of charge formation and of dielectric exclusion (image forces and Born partitioning) are described in detail. The general DSPM-DE model is adapted for the case of neutral solutes and for electrolyte mixtures: for each case, the basic equations are developed and the typical approximations are presented. The procedures for membrane parameters calculations are introduced and a detailed discussion about the recommended correct method for data elaboration is presented.
The final discussion is focused on the problems not yet completely solved as well as on the possible future trends
On the Morphological Characterization Procedures of Multilayer Hydrophobic Ceramic Membranes for Membrane Distillation Operations
Full text linkThe paper introduces some aspects of the characterization of hydrophobized multilayer ceramic membranes intended for use in membrane distillation (MD) operations. Four-layer hydrophobic carbon-based titania membranes, manufactured by the Fraunhofer Institute for Ceramic Technologies and Systems (IKTS, Hermsdorf, Germany), were tested according to the gas permeation technique. Gas permeance data were elaborated following the premises of the dusty gas model, to calculate the average pore size and the porosity-tortuosity ratio of each layer. Membrane testing was the opportunity to discuss which characterization method is more appropriate to obtain the membrane parameters necessary for the simulation of membranes in MD processes. In the case of multilayer membranes, the calculation of the morphological parameters should be performed for each layer. The "layer-by-layer gas permeation" method, previously introduced by other authors and completed in this work, is more appropriate for obtaining representative parameters of the membrane. Conversely, the calculation of morphological parameters, averaged over the entire membrane, might lead to heavy underestimations of the total membrane resistance and then to a heavy error on the transmembrane flux simulation
Intrinsic Performances of Reverse Osmosis and Nanofiltration Membranes for the Recovery and Concentration of Multicomponent Mixtures of Volatile Fatty Acids: A Semi-Pilot Study
Full text linkThis study presents data from Reverse Osmosis (RO) and Nanofiltration (NF) spi-ral-wound polyamide modules tested in a semi-pilot plant with multicomponent mix-tures of Volatile Fatty Acids (VFAs) comprising acetic, propionic, butyric, valeric, and hexanoic acids. A robust method combining film theory and dissociation equilibria was developed to estimate interfacial concentrations, enabling accurate analysis of concentra-tion polarization, real rejection, and effective transmembrane driving force. Concentration polarization strongly affects NF membranes, resulting in real rejections up to 20% higher than apparent values, while its effect is negligible for RO membranes. NF rejections show marked sensitivity to pH and VFAs feed concentration: at 20 g/L and highest flux, acetic acid real rejection increases from 80% to 91% as pH rises from 6 to 9. At pH 7, rejections decline with feed concentration, with acetic acid dropping from 55% at 20 g/L to 32% at 63 g/L, at the same flux. These changes correlate with the molecular weight of the acids. Conversely, RO rejections are marginally affected by pH and not influenced by concentra-tion due to dominant steric exclusion. Membrane permeabilities remain unaffected by VFAs and align with pure water values. The data analysis framework is effective and ap-plicable across a wide range of conditions and membranes
The Role of the Morphological Characterization of Multilayer Hydrophobized Ceramic Membranes on the Prediction of Sweeping Gas Membrane Distillation Performances
Full text linkThis paper shows which morphological characterization method is most appropriate to simulating membrane performance in sweeping gas membrane distillation in the case of multilayer hydrophobized ceramic membranes. As a case study, capillary four-layer hydrophobic carbon-based titania membranes arranged in bundles in a shell-and-tube configuration were tested with NaCl-water solutions using air as sweeping gas, operating at temperatures from 40 to 110 °C and at pressures up to 5.3 bar. Contrary to what is generally performed for polymeric membranes and also suggested by other authors for ceramic membranes, the mass transfer across the membrane should be simulated using the corresponding values of the mean pore diameter and the porosity-tortuosity ratio of each layer and measured by the layer-by-layer (LBL) method. Comparison of the modeling results with experimental data highlights that the use of parameters averaged over the entire membrane leads to an overestimation by a factor of two to eight of the modeled fluxes, with respect to the experimental values. In contrast, the agreement between the modeled fluxes and the experimental values is very interesting when the LBL parameters are used, with a discrepancy on the order of +/−30%. Finally, the model has been used to investigate the role of operative parameters on process performances. Process efficiency should be the optimal balance between the concomitant effects of temperature and velocity of the liquid phase and pressure and velocity of the gas phase
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