Effect of process parameters on transmembrane flux during direct osmosis

Direct osmosis is a non-thermal membrane process employed for the concentration of fruit juices at ambient temperature and atmospheric pressure, thereby maintaining the organoleptic and nutritional properties of fruit juices. In the present study, concentration of pineapple juice by direct osmosis was explored. Aqueous solution of sucrose (0–40%, w/w)–sodium chloride (0–26%, w/w) combination was investigated as an alternative osmotic agent. The sucrose–sodium chloride combination can overcome the drawback of sucrose (low flux) and sodium chloride (salt migration) as osmotic agents during direct osmosis process. The effect of the hydrodynamic conditions in the module and feed temperature (25–45 °C) on transmembrane flux was evaluated. For a range of hydrodynamic conditions studied, it was observed that transmembrane flux increases with Reynolds number. The increase in feed temperature resulted in an increase in transmembrane flux. The pineapple juice was concentrated upto a total soluble solids content of 60 °Brix at ambient temperature. The effect of direct osmosis process on physico-chemical characteristics of pineapple juice was also studied. The ascorbic acid content was well preserved in the pineapple juice concentrate by direct osmosis process.     

The effect of imposed flux on biofouling in reverse osmosis: Role of concentration polarisation and biofilm enhanced osmotic pressure phenomena

This paper describes a systematic study of biofouling in reverse osmosis process using model bacteria of Pseudomonas fluorescens and employing a sodium chloride tracer response technique for fouling characterization. It was found that the growth of biofilm at constant flux following initial bacteria colonization of the membrane surface increased with imposed flux. The rationale was that biofilm growth was nutrient dependent, where the nutrient availability at the membrane wall was controlled by the magnitude of concentration polarization, which is driven by flux. The salt tracer response showed that the biofouling comprised a hydraulic resistance and induced an enhanced osmotic pressure phenomenon; known as the biofilm enhanced osmotic pressure (BEOP) effect [M. Herzberg, M. Elimelech, Biofouling of reverse osmosis membranes: role of biofilm-enhanced osmotic pressure, Journal of Membrane Science 295 (2007) 11–20], due to hindered back diffusion of solutes through the tortuous path of the heterogeneous structure of the biofilm. For the conditions studied, the contribution of BEOP to transmembrane pressure increase was greater than the hydraulic resistance.     

Flux decline during nanofiltration of naturally-occurring dissolved organic matter: effects of osmotic pressure,membrane permeability,and cake formation

Nanofiltration of naturally-occurring dissolved organic matter (NOM) by an aromatic polyamide membrane was measured in a crossflow bench-scale test cell and modeled using a semi-empirical osmotic pressure/cake formation model. Our objective was to examine flux decline due to NOM fouling while explicitly accounting for flux decline due to osmotic effects and changes in membrane permeability. This approach allowed quantification of the effect of ionic composition on specific NOM cake resistance, and yielded insight into flux decline due to enhanced NaCl rejection by the NOM deposit. In the absence of NOM, increasing NaCl concentration reduced salt rejection and decreased membrane permeability. Flux decline was modeled by accounting for changes in osmotic pressure with time, and by employing an effective permeability. The addition of calcium significantly reduced rejection of sodium and feed conductivity, and thus mitigated flux decline. Increasing pH from 4 (near membrane pI) to 10 increased the effective permeability but also increased NaCl rejection, which resulted in greater flux decline. The presence of NOM caused greater flux decline resulting from a combination of NOM cake resistance and increased rejection of NaCl by negatively charged NOM functional groups. Increasing NaCl concentration had little effect on the mass of NOM deposited, but significantly increased the specific resistance of the NOM cake. The effect of ionic strength on specific resistance correlated with a reduction in NOM size, estimated by separate UF permeation experiments and size exclusion chromatography analysis of UF permeate. Therefore, increased specific cake resistance is consistent with a more compact, less porous cake. Flux decline by NOM solutions showed a maximum at pH 7, where salt rejection was also a maximum. Binding of calcium reduced the ability of NOM to enhance NaCl rejection, and likely increased NOM cake resistance. Flux decline caused by NOM fouling in the presence of calcium was only significantly different than that caused by NOM in a solution of NaCl at the same ionic strength when the calcium concentration corresponded to saturation of NOM binding sites.     

On the prediction of permeate flux for nanofiltration of concentrated aqueous solutions with thin-film composite polyamide membranes

The nanofiltration of binary aqueous solutions of glucose, sucrose and sodium sulfate was investigated using thin-film composite polyamide membranes with different molecular weight cut-off's. The NF experiments, in total recycle mode, were performed in a plate-and-frame module Lab 20 (AlfaLaval), at 22 °C and with a flowrate of 8.2 L/min, using the membranes NF90, NF200 and NF270 from FilmTec (Dow Chemical), for transmembrane pressures between 1 and 6 MPa and with aqueous solutions with osmotic pressures of between 0.5 and 3.0 MPa. The permeate flux was predicted by the osmotic pressure model, using the membrane hydraulic resistance and the solution viscosity inside the membrane pores, and computing the concentration polarization with recourse to a mass-transfer correlation specific for the plate-and-frame module used. The flux predictions, using the pure water viscosity, agree reasonably with the experimental data only for low transmembrane pressures and with the most diluted solutions. For higher transmembrane pressures and for higher solute concentration the predicted fluxes can be as far as 2.5, 4.1 and 9.6 times higher than the experimental one, for the aqueous solutions of Na₂SO₄, glucose and sucrose, respectively. These deviations are strongly reduced when the pure water viscosity is replaced by the solution viscosity adjacent to the membrane. In this case, the maximum deviation between predictions and experiments occurs also for higher transmembrane pressures and for higher solute concentration, but the maximum ratio between predicted values and the experiments were reduced now to 1.8, 2.1 and 2.9, for the aqueous solutions of Na₂SO₄, glucose and sucrose, respectively. Even using the solution viscosity adjacent to the membrane, and for the systems investigated, the osmotic pressure model must used with caution for design purposes because it may over predict the permeate flux by a factor of about 2 when the solute concentration is high.     

Freezing points and related properties of electrolyte solutions. III. The systems NaCl−CaCl2−H2O and NaCl−BaCl2−H2O

The freezing temperatures of aqueous calcium chloride and barium chloride and their mixtures with sodium chloride were measured at equivalent molalities of 0.1 to 1.5 mole-kg^?1. Osmotic and activity coefficients of the mixtures were calculated at the freezing points of the mixtures. From the freezing points and calorimetric enthalpies of mixing of sodium chloride-magnesium chloride solutions, osmotic and activity coefficients were calculated at 298°K. Agreement of the calculated properties with isopiestic and electrochemical measurements at 298°K is excellent.     

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.     

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.     

A controlled porosity osmotic pump system with biphasic release of theophylline

A controlled porosity osmotic pump system with biphasic release of theophylline was developed for the nocturnal therapy of asthma. The developed system was composed of a tablet-in-tablet (TNT) core and a controlled porosity coating membrane. Release pattern of the developed system was influenced by amount of pore former (18.2-45.5%, w/w of polymer), weight gain (16-26 mg per tablet) of the coating membrane and osmotic agents used in inner layer of the TNT core. When sodium phosphate and sodium chloride were selected as the osmotic agents in inner and outer layer of the TNT core respectively, target release profile was obtained with coating solution cellulose acetate-polyethylene glycol 400-diethyl phthalate (54.5-36.4-9.1%, w/w) at a weight gain of 16-22 mg per tablet. To examine the mechanism of drug release, release profiles of osmotic agents, micro-environmental osmotic pressure and micro-environmental pH of the formulation during dissolution were studied. Micro-environmental osmotic pressure decreased and micro-environmental pH increased continuously during the whole dissolution process, theophylline release was dominated by the successive dissolution of sodium chloride and sodium phosphate. Theophylline solubility increased as environmental pH exceeded 10.8. At the last stage of the biphasic release, micro-environmental pH in the developed formulation reached 10.9, and theophylline release was promoted by its elevated solubility despite of the decrease of micro-environmental osmotic pressure in the developed formulaiton.     

Tailoring A Poly(ether sulfone) Bipolar Membrane: Osmotic-Energy Generator with High Power Density

Osmotic energy, obtained through different concentrations of salt solutions, is recognized as a form of a sustainable energy source. In the past years, membranes derived from asymmetric aromatic compounds have attracted attention because of their low cost and high performance in osmotic energy conversion. The membrane formation process, charging state, functional groups, membrane thickness, and the ion-exchange capacity of the membrane could affect the power generation performance. Among asymmetric membranes, a bipolar membrane could largely promote the ion transport. Here, two polymers with the same poly(ether sulfone) main chain but opposite charges were synthesized to prepare bipolar membranes by a nonsolvent-induced phase separation (NIPS) and spin-coating (SC) method. The maximum power density of the bipolar membrane reaches about 6.2 W m⁻² under a 50-fold salinity gradient, and this result can serve as a reference for the design of bipolar membranes for osmotic energy conversion systems.     

Direct osmotic concentration of tomato juice in tubular membrane – module configuration. II. The effect of using clarified tomato juice on the process performance

A study was carried out by using a new tubular direct osmosis module, constructed by PCI UK and equipped with a novel AFC99 membrane 400 μm thick, to investigate the effect of clarifying tomato juice on the rate of direct osmotic concentration. Under virtually the same operating conditions five respective clarifications of juice were tried including full, unclarified, tomato juice. The process performance was calculated in terms of permeation flux. In all the experimental runs the osmotic medium was sodium chloride brine (initial concentration approx. 23% NaCl). A remarkable increase of permeation flux (over 100%) was observed shifting from unclarified to the clarified tomato juice. Clarification was carried out by passing the juice through 35 μm mesh and also by using membranes of molecular weight cut-off 200 000, 100 000 and 25 000 Daltons (Da), respectively, in order to obtain the juice ultrafiltrates. It is also worth mentioning that the flux value obtained with 25 000 Daltons (Da) ultrafiltrate appeared to be considerably higher than values reported in experiments carried out by other researchers, where unclarified juice was used, despite the disadvantage of a far thicker membrane being used in the present investigation. This specific finding discloses great potential in using a combined low temperature and pressure ultrafiltration and direct osmosis process to produce tomato concentrates.     

Thermodynamics of mixed electrolyte solutions: The systems H2O−NaCl−CoCl2 and H2O−CaCl2−CoCl2 at 25°C

Osmotic coefficients for aqueous solutions of sodium chloride + cobalt chloride and calcium chloride + cobalt chloride obtained by the isopiestic method are interpreted in terms of the Scatchard neutral-electrolyte treatment to derive activity coefficients of the solutes. Excess Gibbs energy of mixing are calculated, and difficulties in the evaluation of this quantity are discussed. The mixture parameters of Pitzer's method are evaluated for the two systems.     

Tailoring A Poly(ether sulfone) Bipolar Membrane: Osmotic‐Energy Generator with High Power Density

Osmotic energy, obtained through different concentrations of salt solutions, is recognized as a form of a sustainable energy source. In the past years, membranes derived from asymmetric aromatic compounds have attracted attention because of their low cost and high performance in osmotic energy conversion. The membrane formation process, charging state, functional groups, membrane thickness, and the ion‐exchange capacity of the membrane could affect the power generation performance. Among asymmetric membranes, a bipolar membrane could largely promote the ion transport. Here, two polymers with the same poly(ether sulfone) main chain but opposite charges were synthesized to prepare bipolar membranes by a nonsolvent‐induced phase separation (NIPS) and spin‐coating (SC) method. The maximum power density of the bipolar membrane reaches about 6.2 W m^?2 under a 50‐fold salinity gradient, and this result can serve as a reference for the design of bipolar membranes for osmotic energy conversion systems.     

Alginate-coated microporous PTFE membranes for use in the osmotic distillation of oily feeds

Osmotic distillation (OD) has two main advantages over thermally driven concentration processes attributable to its ambient temperature operation. These are maintenance of the integrity of thermally labile components and minimisation of the loss of volatile flavour/fragrance components. However, a major disadvantage of osmotic distillation is the potential for wet-out of the hydrophobic membrane when fouled by surface-active agents such as citrus oils. In this work, sodium alginate hydrogel coatings were applied to PTFE membranes for protection against wet-out. The coating technique developed for this purpose resulted in a 10-fold increase in adhesion strength over that achievable by simple casting. This was effected by increased intrusion of the coating solution meniscus into the porous PTFE structure by surface tension adjustment with ethanol, precipitation of sodium alginate by the selective removal of water, and finally alginate crosslinking. Precipitation occurred both on the surface and in the void spaces between the PTFE fibres, thereby providing better anchorage for the coating. The coating decreased the overall OD mass transfer coefficient by less than 5%. OD flux measurements using coated membranes with 0.2, 0.4 and 0.8 wt.% orange oil–water mixtures over a period of 300 min indicated that the coating was successful in protecting the membrane against wet-out. An uncoated membrane was immediately wet out by a 0.2 wt.% orange oil–water OD feed. In a separate trial, a coated membrane retained its integrity after contact with a 1.2 wt.% oil–water mixture for 72 h.     

Calcium-induced volume transition in polyacrylate hydrogels swollen in physiological salt solutions

Osmotic and small-angle neutron-scattering measurements are performed to study the volume transition that occurs in sodium polyacrylate gels swollen in sodium chloride solutions when calcium ions are introduced. In the presence of calcium ions, the osmotic pressure depends sensitively on the sodium chloride concentration, indicating that calcium preferentially replaces condensed sodium ions. This substitution modifies the effective attractive interaction between polymer chains. Analysis of the osmotic data in terms of the Flory–Huggins theory reveals a sharp increase in the third-order ternary thermodynamic interaction parameter upon introduction of calcium ions. The neutron-scattering response at low scattering vectors q displays power-law behavior with a slope of approximately −3.6, consistent with scattering from surfaces of large objects. These results are in agreement with the development of dense polymer-rich regions dispersed in a soft polymer matrix. At larger q, a region with slope −1 is observed, characteristic of rigid linear structures.

Small-angle neutron-scattering spectra of polyacrylate hydrogels swollen by 40 mM sodium chloride solutions containing different amounts of CaCl₂ (+: 0.5 mM , ○: 0.85 mM , ×: 1.7 mM ). The dashed curve shows the least squares fit of the 0.85 mM CaCl₂ data to Equation ( 5 ) in which the first term is replaced by Equation ( 8 ), and the second term is approximated by a simple power law.     

Unsteady state flux response: a method to determine the nature of the solute and gel layer in membrane filtration

The unsteady-state permeate flux response to a step change in transmembrane pressure is shown to result in unique flux–pressure profiles for the three types of solutes common in membrane ultrafiltration (UF): (a) solutes which exert an osmotic pressure but do not form a ‘gel’; (b) solutes which do not exert an osmotic pressure but form a ‘gel’ and (c) solutes which exert an osmotic pressure and also form a ‘gel’. It is also shown that for stirred cell UF, changes in the bulk feed solution properties (concentration, volume) are negligible on the time scale needed to attain a stable permeate flux. Unsteady-state permeate flux measurements could therefore be made at short filtration times so that the results would not be masked by changes in bulk properties.     

Chemical Cleaning of Microfiltration Membranes Fouled by Whey

Millipore hydrophobic polyvinylidene fluoride (PVDF) microfiltration membranes were used for whey processing. Fouled membranes were cleaned with acid (HCl), alkaline (NaOH) and surfactant (Triton‐X100). The latter resulted in maximum flux recovery and resistance removal. Hydrochloric acid had a moderate effect and sodium hydroxide was the weakest cleaning agent. This is due to the cleaning strength of emulsifiers compared to acid or alkali. However acids are more efficient than alkaline solutions for removal of mineral compounds which remain on the membrane surface. Cleaning efficiency depends on the concentration of cleaning agent being higher for higher surfactant concentration. For acids and alkali, the efficiency increases with increasing the concentration of the reagent reaches a maximum (optimum concentration) and then decreases. This can be explained by changes in permeability of the deposit layer with the concentration of the cleaning agent. Another explanation is the breakage of proteins by acid or alkali which produces more fouling materials and causes less cleaning efficiency. Operating conditions affect the cleaning process. At higher stirring speeds (turbulent flow) or longer cleaning time better removal of deposits and higher cleaning efficiency were observed. The sequential cleaning process may or may not improve the cleaning efficiency. When acidic cleaning was followed by washing with a surfactant an improvement was achieved. This can be attributed to the incomplete removal of deposits by acid. However further cleaning with acid can not improve the cleaning efficiency. During whey processing fouling occurs by deposition of foulants of mostly proteins and macromolecules on the membrane surface or in the membrane matrix. Large substances (compared to the membrane pores) settle on the membrane surface and the small species penetrate and are adsorbed in the membrane pores. Cleaning dissolves and removes the adsorbed foulants from the membrane.     

Effect of calcium and carbonate concentrations on anionic membrane fouling during electrodialysis

A previous study on electrodialysis of calcium and carbonate high concentration solutions demonstrated that calcium migrated through the cation-exchange membrane (CEM) was blocked by the anion-exchange membrane (AEM) where it formed another fouling. The aim of the present work was to complete the identification of the deposit formed on AEM during electrodialysis and to characterize its physical structure at the interface of the membrane. No fouling was found on the anionic membranes treated without calcium chloride in presence of sodium carbonate, while membranes used during ED process of solutions containing calcium chloride and sodium carbonate were slightly fouled. A thin layer of precipitates was observed on the anionic membrane surface. The appearance of precipitates was typical of a crystalline substance. The size and form of crystal increased in proportion to the concentration of calcium chloride in solution. Large and cubic crystals were the best defined on the membrane treated at 1600 mg/L of CaCl2. The precipitate was identified as calcium hydroxide. However, this fouling was not found to affect significantly the electrical conductivity and the thickness of the membranes. Furthermore, the fouling formed was reversible.     

The Pharmaceutical Use of Captisol®: Some Surprising Observations

The purpose of this paper is to share some recent observations on the pharmaceuticaluses and properties of Captisol^® or SBE_7M--CD in controlled porosity osmotic pump tablets (CP-OPT) and the underlying mechanism/sthat lead to apparent zero-order drug release pattern. It would have been simple toattribute the apparent zero-order release mechanism/s of poorly water-soluble drugsfrom CP-OPTs and pellets utilizing Captisol^®as both a solubilizing andosmotic agent, to purely osmotic and diffusional components. However, the mechanismmay be more related to a counterbalancing of physical properties as the concentration of Captisol^®changes within the matrix. Specifically, the initial concentration of Captisol^®within a core is 0.3–0.4M. When this drops to lower values an osmotic pressure drop occurs across the membrane. Therefore, drug release should not follow apparent zero-order kinetics if all the drug is solubilized. However, as the viscosity within the tablet also drops, the apparent diffusion coefficient of both Captisol^® and drug increases. Therefore, it appears that there is an initial resistance (hydraulic pressure) to fluid flow from the tablet through the rate-limiting microporous membrane. This resistance decreases so that even as osmotic pressure and concentration differences drop with time, counterbalancing faster release occurs. Osmotic driving force appears to be the most important initial driving force but a diffusional component becomes more significant with time.     

Ideal limiting fluxes in ultrafiltration: comparison of various theoretical relationships

The ultrafiltration of macromolecules is characterised by a limiting flux at high transmembrane pressures. There is also some evidence that at high pressures and low crossflow velocities the flux decreases slightly with increasing pressure. It is confirmed from a theoretical viewpoint that this can only be caused by a decrease in the average mass-transfer coefficient due to concentration increases in the boundary layer. At the practical level, we propose an expression which, for a given system, enables the ideal flux to be estimated a priori as a function of the transmembrane pressure. The ideal flux is defined as that flux which would occur in the absence of fouling and gelation. The model includes the influence of both osmotic pressure and the variation in viscosity due to concentration polarisation. Thus for predictive purposes knowledge of osmotic pressure and viscosity as a function of concentration is required. The only membrane parameter that has to be experimentally determined is the membrane permeability. In the absence of adsorption (which is the ideal case) this is the permeability to the pure solvent. The model has been tested against Jonsson's data for the ultrafiltration of dextran solutions. The results are most encouraging.