Concentration and temperature polarization effects during osmotic membrane distillation

Pineapple juice is one of the popular fruit juice due to its pleasant aroma and flavor. Concentration of clarified pineapple juice was carried out by osmotic membrane distillation in a plate and frame membrane module. Concentration and temperature polarization effects are found to have significant role on flux reduction during osmotic membrane distillation process. The contribution of these polarization effects on reduction of the driving force (in turn the flux) at various process conditions such as osmotic agent concentration (2–10 mol/kg (1 molality = 1 mol/kg)), flow rate (25–100 ml/min) of feed and osmotic agent are studied. Concentration polarization has more significant effect on flux reduction when compared to temperature polarization. The experimental fluxes were in good agreement with theoretical fluxes when calculated by considering both concentration and temperature polarization effects. The pineapple juice was concentrated up to a total soluble solids content of 62°Brix at ambient temperature.     

Electrodialysis of uranium(VI) through cation exchange membranes and modeling of electrodialysis processes

The transport of ionic species through ion exchange membranes found several applications for water effluents purification and metal ion separation. To enhance the transport performance, the effect of electric fields was suggested in this work. The transport of U(VI) species in nitric acid solutions across cation exchange membranes was investigated. Different parameters affecting the transport of U(VI) were studied. These parameters include: nitric acid concentration in the feed solution, stripping solution concentration and applied electric field. From the results obtained, the cationic flux of U(VI) was 6.5^.10^–8 geq^.cm^–2.s^–1 at the optimal conditions of 10^–3M HNO₃ in the feed solution, 0.5M Na₂CO₃ in the stripping solution and 30 V. The modeling of the electrodialysis process was also made. The model correlates the mass transfer as a function of current density and voltage as variables and takes into account the electro-osmotic effect. The model is applied to the experimental data.     

Nitric acid transport across TBP—Kerosene oil supported liquid membranes

HNO₃ transport across tri-n-butyl phosphate kerosene oil supported liquid membrane with or without uranyl ion transport has been studied. Parameters studied are the effect of TBP in the membrane, nitric acid in the feed solution and nitrate ion concentration in the feed solution. The flux of protons for 1 to 10 mol·dm^–3 HNO₃ solution is in the range of (0–25)·10^–4 mol·m^–2·s^–1 and for the TBP concentration range of 0.359 to 3.59 mol·dm^–3, the flux determined is (8.9 to 22)·10^–4 mol·m^–2·s^–1. From the experimental data and using theoretical equations the complex under transport through the membrane appears to be 2TBP·HNO₃ both in the presence and absence of uranyl ions. The diffusion coefficient for H⁺ ions through the membrane as a function of TBP concentration varies from (53 to 6)·10^–12 m²·s^–1, based on experimental flux and permeability data. The values of this coefficient supposing 2TBP·HNO₃ as diffusing species, based on viscosity data and theoretical estimation varies from (82.50 to 3.30)·10^–12 m²·s^–1. The value of distribution coefficient varies in the reverse direction from 0.06 to 1.46 at the same TBP concentration.     

Free-solvent model shows osmotic pressure is the dominant factor in limiting flux during protein ultrafiltration

In protein ultrafiltration (UF), the limiting flux phenomenon has been generally considered a consequence of the presence of membrane fouling or the perceived formation of a cake/gel layer that develops at high operating pressures. Subsequently, numerous theoretical models on gel/cake physics have been made to address how these factors can result in limiting flux. In a paradigm shift, the present article reestablishes the significance of osmotic pressure by examining its contribution to limiting flux in the framework of the recently developed free solvent osmotic pressure model. The resulting free-solvent-based flux model (FSB) uses the Kedem–Katchalsky model, film theory and the free solvent representation for osmotic pressure in its development. Single protein tangential-flow diafiltration experiments (30 kDa MWCO CRC membranes) were also conducted using ovalbumin (OVA, 45 kDa), bovine serum albumin (BSA, 69 kDa), and immuno-gamma globulin (IgG, 155 kDa) in moderate NaCl buffered solutions at pH 4.5, 5.4, 7 and 7.4. The membrane was preconditioned to minimize membrane fouling development during the experimental procedure. The pressure was randomly selected and flux and sieving were determined. The experimental results clearly demonstrated that the limiting flux phenomenon is not dominated by membrane fouling and the FSB model theoretically illustrates that osmotic pressure is the primary factor in limiting flux during UF. The FSB model provides excellent agreement with the experimental results while producing realistic protein wall concentrations. In addition, the pH dependence of the limiting flux is shown to correlate to the pH dependency of the specific protein diffusion coefficient.     

Modeling and analysis of vacuum membrane distillation for the recovery of volatile aroma compounds from black currant juice

A vacuum membrane distillation (VMD) model has been developed and validated with experimental data. The model consists of an extended transport model for the VMD process and is able to predict the effects of concentration and temperature polarization on the overall process performance. To validate the model, first it was tested with few experimental case studies from literature [S. Bandini, G.G. Sarti, Heat and mass transport resistances in vacuum membrane distillation per drop, AIChE J. 45 (7) (1999) 1422–1433; K.W. Lawson, D.R. Lloyd, Membrane distillation. I. Module design and performance evaluation using vacuum membrane distillation, J. Membr. Sci. 120 (1996) 111–121; A.M. Urtiaga, G. Ruiz, I. Ortiz, Kinetic analysis of the vacuum membrane distillation of chloroform from aqueous solutions, J. Membr. Sci. 165 (2000) 99–110]. Then the VMD model has been validated with experimental data collected from the recovery of aroma compounds from black currant [R.B. Jørgensen, A.S. Meyer, C. Varming, G. Jonsson, Recovery of volatile aroma compounds from black currant juice by vacuum membrane distillation, J. Food Eng. 64 (2004) 23–31]. In this work, recovery of 12 characteristic volatile aroma compounds from black currant juice has been studied. The simulated results from the VMD model, in terms of aroma concentration in the permeate have been compared with those obtained from laboratory experiments. The validated model has been used to study the effects of various process and membrane parameters on the concentration factor. The physical properties of various aroma compounds have been predicted using group contribution method as a function of temperature.     

Extraction of Pd(II) ions using tri-n-octylamine xylene base supported liquid membranes

Membranes, based on tri-n-octylamine (TOA) xylene liquid, supported in hydrophobic microporous films have been used to study the transport of Pd(II) ions, after extraction into the membrane. Various parameters, such as the effect of hydrochloric acid concentration in the feed solution, TOA concentration in the membrane phase, effect of stripping agent like nitric acid concentration, and temperature on the flux of Pd(II) ions across the liquid membranes have been investigated. The optimum conditions of transport for these metal ions determined are, TOA concentration, 1.25 mol·dm^–3, HCl concentration in the feed solution, 5 mol·dm^–3, and concentration of nitric acid used as a stripping, agent 5 mol·dm^–3. The maximum values of the flux and permeability determined under the optimum condition are 23·10^–6 mol·m^–2·s^–1 and 2.40·10³ m²·s^–1 at 25°C. The results obtained have been used to elucidate the mechanism of palladium transport.     

Thermodynamic Properties of NaCl Solutions at Subzero Temperatures

Heat capacities at infinite dilution of NaCl (aq) for the temperature range 0 to –25°C and apparent molar volumes at infinite dilution for 0 to –15°C have been estimated from a synthesis of experimental data collected at subzero temperatures. The parameters of the Helgeson–Kirkham–Flowers (HKF) equation for Na⁺ (aq) have been obtained, from which the Gibbs energies of Na⁺ and Cl^– have been calculated. The estimated values of Pitzer-equation parameters for thermal and activity-coefficient properties have been adjusted for subzero temperatures. The experimental phase diagram for the NaCl–H₂O system could be reproduced with these data, demonstrating the low-temperature applicability of the HKF model to extrapolate thermodynamic properties of aqueous-solution species at infinite dilution.     

Effect of temperature on the intrinsic viscosity of poly(ethylene glycol)/poly(vinyl pyrrolidone) blends in aqueous solutions

In this work the intrinsic viscosity of poly(ethylene glycol)/poly(vinyl pyrrolidone) blends in aqueous solutions were measured at 283.1–313.1 K. The expansion factor of polymer chain was calculated by use of the intrinsic viscosities data. The thermodynamic parameters of polymer solution (the entropy of dilution parameter, the heat of dilution parameter, theta temperature, polymer–solvent interaction parameter and second osmotic virial coefficient) were evaluated by temperature dependence of polymer chain expansion factor. The obtained thermodynamic parameters indicate that quality of water was decreased for solutions of poly(ethylene oxide), poly(vinyl pyrrolidone) and poly(ethylene oxide)/poly(vinyl pyrrolidone) blends by increasing temperature. Compatibility of poly(ethylene oxide)/poly(vinyl pyrrolidone) blends were explained in terms of difference between experimental and ideal intrinsic viscosity and solvent–polymer interaction parameter. The results indicate that the poly(ethylene glycol)/poly(vinyl pyrrolidone) blends were incompatible.     

Concentration of noni juice by means of osmotic distillation

Osmotic distillation (OD) or osmotic evaporation (OE) is a promising membrane process generally applied to concentrate solutions under isothermal conditions. In this work, this process was applied to concentrate commercial noni juice (Morinda citrifolia). Several nutraceutical properties have been reported for noni-derived products, mainly associated to the phenolic content of the fruit.The analyzed system is an osmotic distillation system where the solutions are circulated through a hollow fiber membrane contactor operating in transient configuration with circulation rates between 0.1 and 1.0 L min⁻¹ and concentrated solutions of CaCl₂ were used as extraction brine. At isothermal conditions (30 °C), transmembrane vapor water flux was experimentally determined from 0.090 up to 0.413 kg h⁻¹ m⁻². Noni juice was concentrated from 8 to 32 °Brix after 60 min of treatment. The content of phenolic compounds was preserved after this processing.Simulation algorithms based on phenomenological equations of heat and mass transfer were developed considering a resistances-in-series model to predict the performance of the process from theoretical information. The values of transmembrane water flux obtained by simulations showed deviations between 2.35 and 16.19% with the experimental ones for the operating conditions applied in this work.     

Separation of Bulk Carbon Dioxide-Hydrogen Mixtures by Selective Surface Flow Membrane

The separation performance of carbon dioxide-hydrogen mixtures by a nanoporous carbon membrane called selective surface flow membrane is described. The membrane selectively permeates CO2 from H2 and a H2 enriched gas is produced at the feed gas pressure. Extensive experimental data for the separation using feed gas pressures from 0.24 to 1.13 MPa and CO2 compositions from 5 to 75 (mol%) in H2 are reported. The data can be empirically correlated using a simple equation with a single adjustable-parameter. The adjustable parameter is found to be a linear function of the feed gas CO2 partial pressure.The membrane separates CO2-H2 mixture very efficiently even at a low total feed gas pressure (0.4 MPa). The membrane area required for a given separation decreases drastically with increasing feed gas pressure in the range of 0.24–0.92 MPa and then it becomes insensitive to the feed gas pressure.     

Membrane potential across reverse osmosis membranes under pressure gradient

Membrane potential measurement has been widely used for the characterization of ionic membranes such as ion-exchange membranes without solvent permeability. However, there have been few studies on membrane potentials across pressure-driven processes such as reverse osmosis (RO) membranes with solvent permeability. In the present study, the membrane potential across RO membranes in NaCl and MgCl2 under the pressure gradient, DeltaP=0-0.3 MPa, was measured. The experimental results were analyzed by the theoretical model based on the Donnan equilibrium and the extended Nernst-Planck flux equation considering the pressure effect. The theoretical values agreed well with the experimental ones. This indicates that membrane potential is useful for characterizing the effective charge density of the active layer of RO membranes under pressure gradient.     

A facile route for fabricating novel polyelectrolyte complex membrane with high pervaporation performance in isopropanol dehydration

Polyelectrolyte complexes (PECs) of sodium carboxymethyl cellulose (CMCNa) and poly(diallyldimethylammonium chloride) (PDDA) were prepared in dilute hydrochloric acid (HCl) aqueous solution and obtained in its solid form. Element analysis and FT-IR showed that the composition of PECs could effectively be tuned by the concentration of HCl in parent polyelectrolyte solution. The PECs were then dissolved in 0.1 mol/L aqueous NaOH and subsequently cast onto polysulfone ultra-filtration membrane. This composite membrane, which has a unique homogeneous PECs separation layer, was subjected to pervaporation test for the first time and gave a performance of J = 3.0 kg/m² h, α = 960 for 10 wt% water–isopropanol feed at 75 °C. Meanwhile, performance of the PECs membrane displays good stability and unique dependence on feed temperature. These findings, together with its ultra-high performance, are primarily explained by the structure characteristic of PECs.     

Separation of lignosulfonate from its aqueous solution using supported liquid membrane

This paper presents an experimental and theoretical study on facilitated transport of lignosulfonate (LS) through a flat sheet supported liquid membrane using trioctylamine (TOA) as carrier and dichloroethane as diluent. The studies were carried out with various support materials and operating conditions (viz. carrier concentration, strip phase concentration, salt concentration, etc.) and their effects on the transport of LS. The results were analyzed to identify a suitable combination of support and operating condition that would yield best performance of the supported liquid membrane (SLM) in terms of fast and efficient transport of LS. The stability of the SLM was assessed in terms of loss of liquid from the pores of membrane support. The SLM is found to be stable till 10 h. Co-transport mechanism has been adopted in this work by using NaOH as the strip phase. It was observed that extraction of LS is increased with increase in concentration of NaOH up to a limiting value of 0.5 M NaOH. Difference of salt concentration between feed and strip phase considerably affect the separation process. The diffusional resistances of organic membrane (Δorg) and aqueous solution (Δaq) calculated from the permeation model, which is again a combination of three unique mechanisms viz., diffusion through a feed aqueous layer, a fast interfacial chemical reaction, and diffusion of carrier–complex through the organic membrane, are found to be 609.9 and 176.6 s cm⁻¹, respectively. The values of the diffusion coefficient in the membrane (D_org) and in the bulk organic phase (D_complex) are 1.67×10⁻⁹ and 6.68 × 10⁻⁸ m²s⁻¹, respectively. The extraction of LS is about 90%. Nearly 43% of LS can be recovered at optimum condition.     

Fabrication and characterization of γ-Al2O3–clay composite ultrafiltration membrane for the separation of electrolytes from its aqueous solution

In this paper, we have reported the preparation of low cost γ-Al₂O₃ membrane on a macroporous clay support by dip-coating method. For the synthesis of γ-Al₂O₃ top layer on the support, a stable boehmite sol is prepared using aluminium chloride salt as a starting material by sol–gel route. The structural properties of the composite membrane as well as γ-Al₂O₃ powder is carried out using scanning electron microscopy (SEM), X-ray diffraction (XRD), nitrogen adsorption–desorption isotherm data, Fourier transform infrared analysis (FTIR) and dynamic light scattering (DLS) analysis. The mean particle size of the boehmite sol used for coating is found to be 30.9 nm. The pore size distribution of the γ-Al₂O₃–clay composite membrane is found to be in the range of 5.4–13.6 nm. Separation performance of the membrane in terms of flux and rejection of single salts solution such as MgCl₂ and AlCl₃ as a function of pH, salt concentration and applied pressure is also studied. The rejection and flux behavior are found to be strongly dependent on electrostatic interaction between the charged molecules and γ-Al₂O₃–clay composite membrane. The intrinsic rejection has been determined by calculating the concentration at membrane surface (C_m) using Speigler–Kedem model. It is found that the observed rejection shows anomalous trend with increase in applied pressure and the intrinsic rejection increases with increase in applied pressure, a trend typical of the separation of electrolyte through charged membranes. At acidic pH, both the salt solution shows higher rejection. With increase in the salt concentration, observed rejection of salt decreases due to the enhanced concentration polarization. The maximum rejection of MgCl₂ and AlCl₃ is found to be 72% and 88%, respectively for salt concentration of 3000 ppm.     

Extraction and stripping study of strontium ions across D2EHPA-TBP-kerosene oil based supported liquid membranes

A Sr ion transport study across D2EHPA-TBP kerosene oil based liquid membranes supported on microporous polypropylene film has been performed. The parameters studied were the effect of di(2-ethylhexyl)phosphoric acid (D2EHPA) and TBP concentration variation in the membrane liquid, HNO₃ concentration variation in the stripping phase and citric acid concentration variation in the feed solution. The optimum conditions of transport are 0.3 mol/dm³ D2EHPA, 0.1 mol/dm³ TBP, 0.01 mol/dm³ citric acid in feed and 2 mol/dm³ HNO₃ in the stripping phase. The mechanism of transport observed is counter-ion coupled transport. The coupling ions are protons. The maximum flux for Sr ion transport observed is 5.33·10^–5 mol·m^–2·s^–1 and maximum permeability under optimum conditions observed is 8.08·10^–11 m^–2·s^–1.     

Application of europium(III) extraction with organophosphinic acid to liquid membrane transport

The extraction behavior of Eu(III) has been studied using di(2,4,4-trimethylpentyl)phosphinic acid (DTMPPA, HA) in kerosene. Europium was extracted as Eu(HA₂)₃ with the extraction constant of 2.0·10^–3. This extraction system was applied to the transport of Eu(III) across a DTMPPA liquid membrane supported on porous polytetrafluoroethylene. Europium was quantitatively moved through the liquid membrane containing 0.1M (HA)₂ as a mobile carrier from the feed solution of pH above 3 into the product solution of 0.1M HNO₃, yielding a concentration factor of ten. The transport rate increased with increasing pH and DTMPPA concentration.     

Effects of polyether–polyamide block copolymer coating on performance and fouling of reverse osmosis membranes

The effects of a water-permeable polymer coating on the performance and fouling of high-flux (ESPA1 and ESPA3) and low-flux (SWC4) polyamide reverse osmosis (RO) membranes were investigated. It was anticipated that the coating would create a smoother hydrophilic surface that would be less susceptible to fouling when challenged with a motor-oil/surfactant/water feed emulsion (used as a model foulant). AFM and FT-IR analyses confirm that a 1 wt.% polyether–polyamide (PEBAX^® 1657) solution applied to ESPA and SWC4 membranes produces a continuous polymer coating layer and, thereby, provides smoother membrane surfaces. However, pure-water permeation data combined with a series-resistance model analysis reveal that the coating does not only cover the surface of the polyamide membrane, but also penetrates into its porous ridge-and-valley structure. During a long-term (106-day) fouling test with an oil/surfactant/water emulsion, the rate of flux decline was slower for coated than for uncoated membranes. This improvement in fouling resistance compensated for the decrease in permeate flux for SWC4 over a period of approximately 40 days. However, the coating material is believed to penetrate more deeply into the polyamide surface layer of the high flux, high surface area ESPA membranes relative to the low-flux SWC4, resulting in significant water flux reduction.     

Concentration polarization in gas permeation

The aim of this article is to show the importance of concentration polarization effects in the separation of gas–vapor mixtures using membranes. In the experimental part of this work, gas mixture measurements are conducted with a specially designed test cell. The experimental data are analyzed using a two-resistance model for the transport through the membrane, which is derived in the theoretical part of this work. The two resistances considered are the transport through the boundary layer on the feed side of the membrane and through the separation layer. For the transport through the separation layer an extended free volume model is derived. This model considers not only the feed side but also the influence of the permeate side on the separation properties of the membrane. The results of the measurements show the influence of concentration polarization effects and their dependencies on feed pressure, membrane thickness, and feed flow rate.     

Nitrate ion transport across TBP-kerosene oil supported liquid membranes coupled with uranyl ions

The role of nitrate ions in uranyl ions transport across TBP-kerosene oil supported liquid membranes (SLM) at varied concentrations of HNO₃ and NaNO₃ has been studied. It has been found that nitrate ions move faster compared to uranyl ions at the uranium feed solution concentrations studied. The nitrate to uranyl ions flux ratio vary from 355 to 2636 under different chemical conditions. At low uranium concentration the nitrate ions transport as HNO₃ · TBP, in addition to as UO₂(NO₃)₂ · 2TBP type complex species. The flux of nitrate ions is of the order of 12.10 · 10^–3 mol · m^–2 · s^–1 compared to that of uranium ions (4.56 · 10^–6 mol · m^–2 · s^–1). The permeability coefficient of the membrane for nitrate ions varies with chemical composition of the feed solution and is in the order of 2.5 · 10^–10 m^–2 · s^–1. The data is useful to estimate the nitrate ions required to move a given amount of uranyl ions across such an SLM and in simple solvent extraction.