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.     

Effect of operating conditions on water transport during the concentration of sucrose solutions by osmotic distillation

A recent membrane technique, osmotic distillation (OD), is used to concentrate binary water–sucrose solutions at ambient temperature under atmospheric pressure. The principle is based on the extraction of water vapour from a dilute aqueous solution, which is put in contact with a hypertonic salt solution by means of a macroporous hydrophobic membrane. The concentration difference between both solutions translates into a transmembrane vapour pressure drop, that constitutes the driving force for mass transfer. An experimental device is designed at laboratory scale for this study, allowing achievement of vapour fluxes of 10 kg m⁻² h⁻¹ under standard conditions. The effect of various operating parameters on vapour flux is studied. The solute content results in the most influencing variable via water activity in brine and via viscosity in sugar solutions. The effect of concentration polarisation on the brine side is not negligible and would have to be taken into account for process optimisation. This phenomenon could not be quantified on the sugar solution side due to pressure drop limits of the pilot rig. Eventually, the vapour flux can be significantly increased by adding a temperature difference to the transmembrane concentration difference, when pure water is evaporated.     

Air gap membrane distillation of sucrose aqueous solutions

In this paper results obtained with air gap membrane distillation (AGMD) using sucrose aqueous solutions are shown. The role of the relevant process parameters has been investigated experimentally (the flow rate through the cell, the feed initial concentration, the type of membrane, the air gap thickness, etc.). Equations have been proposed to estimate the intermediate temperatures for the air gap configuration. The fluxes given by different gas stagnant film diffusion models showed good agreement with the experimental results over the entire range of temperatures studied. Also a model which accounts for the thermal diffusion phenomenon was used. From the fits of the experimental flux data to the theoretical equations, the diffusion coefficient of the water vapour–air mixture, D_AB, and the thermal diffusion coefficient, K_T (only in the last case), were obtained and the results were analysed. For the D_AB coefficient higher values than the tabulated ones have been obtained, although of the same order of magnitude, and still higher when the thermal diffusion is considered.     

Response surface modelling and optimization in pervaporation

Both the conventional method of experimentation, in which one of factors is varied maintaining the other factors fixed at constant levels and the statistically designed experimental method, in which all factors are varied simultaneously are carried out for organic removal from water by pervaporation. Binary acetonitrile–water mixtures are considered. The effects of the operating parameters on the pervaporation performance of the membrane system have been investigated. The overall mass transfer coefficients have been determined for different conditions of feed temperature and initial organic concentration. In addition, the activation energy associated to the permeation process has been determined and discussed for each feed organic mixture. Statistical experimental design and response surface methodology, RSM, have been applied to optimize the operational conditions of pervaporation process in order to maximize the output responses, which are permeate flux ratio and concentration of organic in permeate. The input variables employed for experimental design were the feed temperature, initial concentration of organic in feed and operational downstream pressure. Based on the design of experiment the quadratic response surface models have been developed to link the output responses with the input variables via mathematical relationships. The constructed response models have been tested using the analysis of variance and the canonical analysis. The obtained optimal point by means of Monte Carlo simulation method and desirability function corresponds to a feed temperature of 57.69 °C, a feed acetonitrile concentration of 6.96 wt% and a downstream pressure of 28.95 kPa. The maximal values of the permeate flux ratio and the concentration of organic in permeate obtained under optimal process conditions have been confirmed experimentally.     

Effective arsenic removal using polyacrylonitrile-based ultrafiltration (UF) membrane

Applicability of polyacrylonitrile (PAN)-based negatively charged ultrafiltration (UF) membrane for effective arsenic removal has been demonstrated, to our knowledge, for the first time. The hydrolysis of PAN-based UF membrane surface by NaOH leading to the formation of carboxylate (COO⁻) groups and reduction in initial pore size rendered As-V rejection capability by Donnan exclusion principle. A lowering in pore size was indicated by the reduction in water flux and elevation in rejection of protein and polyethylene glycol (PEG). NaOH treatment leading to formation of carboxylate group on the membrane surface was indicated by FTIR-ATR, while contact angle measurement indicated increased hydrophilicity. This treatment rendered membrane surface smoothening as confirmed by SEM and AFM analyses. The molecular weight cut off after the NaOH treatment was found to be 6 kDa. The rejection of pentavalent arsenic (As-V) by these surface modified membranes was studied with different feed concentration, cross-flow velocity, pressure, temperature and pH. Experiments with 50 ppb As-V in feed showed that arsenic rejection was close to 100% and remained constant up to 6 h. Feed sample concentration of 1000 ppb and 50 ppm of As-V showed >95% rejection at pH 7 and room temperature, but for 1000 ppm feed concentration, the rejection was 40–65%. For concentrations ≤50 ppm of arsenic in the feed, the rejection coefficient was not dependent on cross-flow velocity or transmembrane pressure. The rejection for 1000 ppm concentration of As-V varied from 40 to 65% with variation in the cross-flow velocity and transmembrane pressure as the concentration polarization was important.     

Use of pervaporation to separate butanol from dilute aqueous solutions: Effects of operating conditions and concentration polarization

This study deals with the separation of n-butanol from aqueous solutions by pervaporation. The effects of feed concentration, temperature, and membrane thickness on the separation performance were investigated. Over the low feed butanol concentration range (0.03–0.4 wt%) studied, the butanol flux was shown to increase proportionally with an increase in the feed butanol concentration, whereas the water flux was relatively constant. An increase in temperature increased both the butanol and water fluxes, and the increase in butanol flux was more pronounced than water flux, resulting in an increase in separation factor. While the permeation flux could be enhanced by reducing the membrane thickness as expected for all rate-controlled processes, the separation factor was compromised when the membrane became thinner. The effect of membrane thickness on the separation performance was analyzed taking into account the boundary layer effect. This could not be fully attributed to the concentration polarization, which was found not significant enough to dominate the mass transport. A variation in the membrane thickness would vary the local concentration of permeant inside the membrane, thereby affecting the permeation of butanol and water differently. Thus, caution should be exercised in using permeation flux normalized by a given thickness to predict the separation performance of a membrane with a different thickness because the membrane selectivity can be affected by the membrane thickness even in the absence of boundary layer effect.     

Membrane potentials and electrolyte permeation in a cation-exchange membrane

A method has been developed which permits one to determine the electrolyte permeation velocities and the membrane system permeability from measurements of the membrane potential as a function of time. The method has been applied to a cation-exchange membrane separating two aqueous KCl solutions, at the same pressure and temperature, but of different concentrations. The experiments have been carried out in two concentration ranges and at different solution stirring rates. The obtained results showed that the membrane system permeability depends notably on the stirring rate, this dependence being greater for the highest concentration range. The intrinsic permeability of the membrane was determined from considerations about the concentration polarization effect, its value being greater in the lower concentration range.     

Effect of operating variables on the flux and selectivity in sweep gas membrane distillation for dilute aqueous isopropanol

Sweep gas membrane distillation was examined as a possible technique for isopropanol (IPA)–water separation using PTFE hollow fiber membrane module. The composition and flux of the permeate were monitored when feed concentration, operating temperature and flow rate were varied. The upper feed concentration tested was 10 wt.% IPA. Within the feed temperature range of about 20–50°C, IPA selectivity of 10–25 was achieved. Since the concentration near the surface on the membrane increased by the selective adsorption of IPA on the hydrophobic membrane, the selectivity increases. The permeate flux and IPA selectivity increase as feed temperature increase. The flux and selectivity increase at higher flow rates is mainly due to the reduced effects of concentration and temperature polarization. The effect of salt addition to the feed mixture was also examined.     

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.     

藻朊酸钠/壳聚糖聚电解质复合物膜 Ⅰ.对水/有机液体系的渗透汽化特性

制备了藻朊酸钠／壳聚糖聚电解质复合物复合膜，研究了进料液浓度、温度等对水／乙醇体系渗透汽化特性的影响；发现膜的不同表面接触进料液时膜的分离性能不同。同时，对其它水／有机液体系，该膜也具有优异的分离性能。     

Establishing SI traceability for measurements of mercury vapour

The majority of measurements of mercury vapour, for example those to determine mass concentration in air, are currently ultimately traceable to the vapour pressure of mercury, usually via a bell-jar calibration apparatus. This allows a saturated concentration of mercury vapour in air to develop in a confined space in equilibrium with ambient conditions, from which a known mass of mercury can be removed for calibration purposes. Several empirical equations are available to describe the vapour pressure of mercury at a given temperature, but the agreement between them is not good, with data from different equations sometimes differing by 5% or more. In order to remove the dependence of mercury vapour measurement on these empirical equations, and to provide stability, comparability and coherence for mercury vapour measurements, this paper describes work undertaken to link directly mercury vapour measurements to standards of mass, and therefore to establish traceability for these measurements to the SI system of units. This has been achieved by measuring the mass output rate of a dynamic mercury vapour generator gravimetrically, and linking this to the expected mass concentration in the bell-jar apparatus. The SI traceable mercury vapour measurements have been shown to agree with the predicted output from the bell-jar, as defined by the most commonly used empirical mercury vapour pressure equation, within the uncertainty of the measurement.     

Power generation with pressure retarded osmosis: An experimental and theoretical investigation

Pressure retarded osmosis (PRO) was investigated as a viable source of renewable energy. In PRO, water from a low salinity feed solution permeates through a membrane into a pressurized, high salinity draw solution; power is obtained by depressurizing the permeate through a hydroturbine. A PRO model was developed to predict water flux and power density under specific experimental conditions. The model relies on experimental determination of the membrane water permeability coefficient (A), the membrane salt permeability coefficient (B), and the solute resistivity (K). A and B were determined under reverse osmosis conditions, while K was determined under forward osmosis (FO) conditions. The model was tested using experimental results from a bench-scale PRO system. Previous investigations of PRO were unable to verify model predictions due to the lack of suitable membranes and membrane modules. In this investigation, the use of a custom-made laboratory-scale membrane module enabled the collection of experimental PRO data. Results obtained with a flat-sheet cellulose triacetate (CTA) FO membrane and NaCl feed and draw solutions closely matched model predictions. Maximum power densities of 2.7 and 5.1 W/m² were observed for 35 and 60 g/L NaCl draw solutions, respectively, at 970 kPa of hydraulic pressure. Power density was substantially reduced due to internal concentration polarization in the asymmetric CTA membranes and, to a lesser degree, to salt passage. External concentration polarization was found to exhibit a relatively small effect on reducing the osmotic pressure driving force. Using the predictive PRO model, optimal membrane characteristics and module configuration can be determined in order to design a system specifically tailored for PRO processes.     

Thermal effects in osmotic distillation

Osmotic distillation (OD) is a concentration technique for aqueous mixtures based on porous hydrophobic membranes in contact on both sides with liquid phases at pressure lower than the pressure needed to displace the gas phase in the pores. The driving force for the water vapour diffusion through the gas phase immobilised within the membrane pores is sustained by an activity difference by using a hypertonic solution, typically concentrated brines, downstream the membrane. The mass transfer causes a cooling down of the feed and a warm up of the brine, as a consequence a temperature difference is created which reduces the effective driving force for mass transfer. This ‘thermal effect’ is investigated both theoretically and experimentally, it is shown that the effect on the flux is substantial.     

Experimental study of desalination using direct contact membrane distillation: a new approach to flux enhancement

New membrane distillation configurations and a new membrane module were investigated to improve water desalination. The performances of three hydrophobic microporous membranes were evaluated under vacuum enhanced direct contact membrane distillation (DCMD) with a turbulent flow regime and with a feed water temperature of only 40 °C. The new configurations provide reduced temperature polarization effects due to better mixing and increased mass transport of water due to higher permeability through the membrane and due to a total pressure gradient across the membrane. Comparison with previously reported results in the literature reveals that mass transport of water vapors is substantially improved with the new approach. The performance of the new configuration was investigated with both NaCl and synthetic sea salt feed solutions. Salt rejection was greater than 99.9% in almost all cases. Salt concentrations in the feed stream had only a minor effect on water flux. The economic aspects of the enhanced DCMD process are briefly discussed and comparisons are made with the reverse osmosis (RO) process for desalination.     

Solute rejection by porous thin film composite nanofiltration membranes at high feed water recoveries

The authors developed a rigorous framework to model nanofiltration (NF) membrane selectivity at high feed water recoveries and verify it experimentally. The phenomenological model and the Donnan steric partitioning pore model (DSPM) were incorporated into a differential element approach for predicting removal of a variety of solutes from single salt solutions and natural water by NF membranes up to 90% feed water recovery in the temperature range 5-41 degrees C. In this approach, the entire membrane ensemble was divided into numerous sub-elements analogous to real-world full-scale NF installations, where concentrate (or reject) from one element feeds into the next element. Fundamental membrane properties (average pore radius, surface charge density, and ratio of thickness to porosity) and the reflection coefficient and permeability coefficient were first independently obtained for each solute-membrane-temperature combination using separate low recovery experiments with negligible concentration polarization and later used as model inputs to calculate solute removal in a purely predictive fashion for 5-90% recovery. This modeling approach accurately predicted removals from single salt solutions of NaCl and MgSO(4) as well as natural organic matter, disinfection by-product precursors, and several ions from pretreated Lake Houston water in a wide range of operating conditions demonstrating its use to simulate NF permeate water quality under real-world conditions of high feed water recovery.     

Nature of flow on sweeping gas membrane distillation

The process of sweeping gas membrane distillation (SGMD), with the liquid feed and the sweeping gas counterflowing in a plate and frame membrane module, has been studied. A theoretical model, which was presented in a previous paper and permitted to obtain the temperature profiles inside the fluid phases, has been developed in order to analyse the physical nature of the transmembrane water flux. Two porous hydrophobic membranes have been studied in different experimental conditions. The influence of some relevant parameters, such as the inlet and outlet temperatures or the circulation velocities of the fluids, has been studied. The experimental results have been analysed according to the model and the conclusion is that the water transport takes place, apparently, via a combined Knudsen and molecular diffusive flow mechanism. From the temperature profiles, a local temperature polarisation coefficient may be defined. From this local value, an overall one for the whole system is then defined. The new theoretical predictions have been applied to the obtained results and the accordance may be considered good.     

Effect of colloids on salt transport in crossflow nanofiltration

The role of colloid deposition on the performance of a salt-rejecting NF membrane was evaluated by modeling salt transport using a two-layer transport model, which quantified the relative contributions of advection and diffusion in the cake and the membrane layers, and the effects of flux on the membrane sieving coefficient. The model was able to accurately describe how the measured permeate concentration, rejection, osmotic pressure, and flux decline varied with time. The two-layer model confirmed that the Peclet number in the cake layer was about an order of magnitude higher than that in the membrane layer, leading to significant concentration polarization at the membrane surface, as shown by others. However, the cake layer also increased overall resistance, which resulted in flux decline during constant pressure operation. Flux decline caused an increase in the actual sieving coefficient, leading to higher solute flux, lower observed rejection, and thus lower the bulk concentration. These coupled phenomena tended to mitigate the increase in concentration polarization caused by the cake. Therefore, as predicted by the model and verified by experiment, the osmotic pressure does not increase monotonically as the cake grows, and in fact can decrease when the cake layer is thick and the flux decline is significant. In our experimental system, the pressure drop across the cake layer, which was proportional to the cake thickness, was significant under the conditions studied. The effects of cake-enhanced osmotic pressure analyzed here are lower than those observed in previous studies, possibly because the transport model employed explicitly accounts for the effect of flux decline due to cake growth on the membrane sieving coefficient, and possibly because we used a somewhat different methodology to estimate cake porosity.     

Accurate calibration of mercury vapour measurements

Almost all measurements of mercury vapour, for example those to determine mass concentration in air, are currently ultimately traceable to the vapour pressure of mercury, usually via a bell-jar calibration apparatus. This allows a saturated concentration of mercury vapour in air to develop in a confined space in equilibrium with ambient conditions, from which a known mass of mercury can be removed for calibration purposes. Setting aside the uncertainty in the vapour pressure of mercury at a given temperature, the accuracy of vapour phase mercury determinations depends critically on fully understanding the operation and sensitivities of the mercury bell-jar apparatus. This paper discusses the thermodynamic and kinetic considerations that must be taken into account when using the bell-jar apparatus, provides the theoretical basis for understanding the operation of the bell-jar, and presents experimental data demonstrating the systematic biases which may be obtained if the bell-jar is used incorrectly. These biases depend on the temperature difference between the mercury vapour in the bell-jar and the syringe used to remove the mercury vapour from the bell-jar, but they may be well in excess of 10% under some operating conditions. The results from this study have been used to propose best practice solutions for mercury vapour calibrations using the bell-jar.     

Membrane distillation for dilute ethanol: Separation from aqueous streams

Air-gap membrane distillation was examined as a possible technique for ethanol–water separation using PVDF membranes. The composition and flux of the permeate were monitored as feed concentration, feed temperature, feed flow rate, cooling temperature and cooling flow rate were varied. The effect of salt addition to the feed mixture was also examined. The upper feed concentration tested was 10 wt.% ethanol. Within the feed temperature range of 40–70°C, ethanol selectivity of 2–3.5 was achieved. Two versions of a general mathematical model were solved numerically for the ethanol–water system; one did not include temperature and concentration polarization effects while the other did. Good agreement between experimental and predicted values was obtained with the latter version of the model.     

Concentration polarization,separation factor,and Peclet number in membrane processes

Concentration polarization affects almost all the membrane separation processes and can be the cause of a substantial reduction in the separation factor and flux. A generalized equation relating the modified Peclet number to the concentration polarization occurring in the boundary layer is proposed and shown applicable to the majority of membrane separation processes like gas separations, reverse osmosis, ultrafiltration, pervaporation, and dissolved gas permeation in liquid. The membrane permeability, separation factor (or solute rejection), membrane thickness, boundary layer mass transfer coefficient, and Henry's law coefficient are the factors that determine the extent of polarization. An analysis is presented to offer a clean division of the hydrodynamic effect from the pure membrane property for membrane separation processes of liquid phases. Also the effect of membrane thickness on polarization is discussed. An attempt has been made to reconcile the different approaches taken for different membrane processes in the literature. Experimental data from widely different sources illustrate and confirm the present theory for pervaporative separation of dilute solutions of volatile organic compounds, dissolved gas permeation, and ultrafiltration of proteins and carbowax. Specific suggestions are made to obtain independent experimental measurements of the Peclet number and polarization index in terms of measurable quantities like the actual and intrinsic separation factors.