Temperature Polarization Effects Research Articles

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.

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.

Concentration of sucrose solutions via vacuum membrane distillation

The potential use of vacuum membrane distillation (VMD) process for the concentration of sucrose solution was examined. The effect of several parameters, including feed temperature, flow rate, and initial sucrose concentration on the flux quality and quantity was studied. VMD process was found effective in concentrating aqueous sucrose solutions as the permeate was absolutely pure water. The feed temperature significantly affected the flux. The increase in feed flow rate increased the permeate flux and this increase was more pronounced in the laminar than in the turbulent region. The increase in the initial sucrose concentration resulted in a slight decrease in the flux without affecting the quality of the permeate. A mathematical model describing the variation of sucrose concentration with time was developed and validated with the experimental results. Concentration and temperature polarization effects were also considered in this work.

Using Correlations to Understand Changes in Actinide Bonding

AbstractAn important issue in actinide science is the changing role of the 5f electrons, both when progressing across the series, as well as how experimental variables affect these roles in a particular element's chemistries and physics. The function of these 5f electrons can be changed by experimental conditions: temperature and pressure being two of many such variables. The 5f electrons of several actinide metals, their alloys and compounds are affected greatly by pressure, due to the very large decreases in interatomic distances encountered under pressure. The latter bring about significant changes in the total energy of the system and in the electronic energy levels, which in turn affect the potential for overlap/hybridization) of their orbitals, promotion of electrons to other orbitals, etc. The physical state, temperature, pressure, specific structures, magnetic interactions and spin polarization effects are all critical parameters for bonding. Often correlations of behavior with these parameters can provide unique insights and understanding into the bonding and the changes that occur in it. With the advancement of modern computation approaches using FPMTO, or other approaches, theory has enlightened greatly the understanding of not only the bonding behavior of these elements but also the understanding of changes observed experimentally. But these computational efforts have some complications and limitations, and at times experimental findings and theory are not always in full agreement. In contrast to the behaviors of the elements, changes observed with compounds often are not be linked directly to the involvement of 5f electrons, due in part to the presence and bonding role of non-actinide atoms. The latter affect both the actinide interatomic distances and the type of electronic orbtals that interact. Presented here is an overview of the pressure behavior several actinide elements, some insights into the bonding behavior of compounds under pressure and selected correlations that help explain changes occurring in electronic configurations and bonding.

Orange juice concentration by osmotic evaporation and membrane distillation: A comparative study

Abstract In this work, the osmotic evaporation (OE) and membrane distillation (MD) processes were compared in terms of water flux and aroma retention. The concentration of a sucrose solution, used as a model fruit juice, was carried out in a hollow fibre membrane contactor. Although a similar overall driving force was used, the flux obtained using the MD process was less than half of that observed using the OE process, due to temperature polarization effects. The transport of citral and ethyl butyrate, two aroma compounds relevant in the orange juice aroma, was also compared for both processes. A higher retention of these compounds was observed with the OE process. From these studies, it could be concluded that, the OE process has advantages over the MD process, not only in terms of water flux, but also regarding the retention of aroma compounds. Finally, the concentration of a commercial orange juice by osmotic evaporation, was carried out. Due to the presence of suspended solids and macromolecules, a higher mass transfer resistance was observed.

Treatment of waters colored with methylene blue dye by vacuum membrane distillation

Textile industries consume large amounts of water and generate highly dye-contaminated effluents. Textile wastewaters have to be treated in order to be recycled in the process or to meet legislative requirements before being discharged. The objective of the present study was to examine the potential use of the vacuum membrane distillation process for the treatment of dyed solutions. Methylene blue (MB) was used as a model dye. Batch experiments were conducted on dilute MB-water mixtures using a tubular polypropylene membrane module. The concentration of MB dye within the feed reservoir was monitored over time. The impact of operating variables such as feed temperature, flow rate and initial dye concentration was investigated. A mathematical model incorporating temperature and concentration polarization effects was developed and validated on the experimental data.

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.

Theoretical and experimental studies on desalination using the sweeping gas membrane distillation method

Sweeping gas membrane distillation was investigated as a possible technique for desalination by using a shell-and tube polypropylene membrane module. Humid air was employed as the vapor carrier gas and the feed solution was circulated in counter-current through the lumen side of the membrane module. The effects of the process parameters, liquid feed flow rate, feed temperature, air flow rate and salt concentration on the distillate flux have been investigated. A theoretical model that considers the heat and mass transfer through microporous and hydrophobic membranes as well as the temperature and concentration polarization effects was developed and validated with the experimental data of distilled water and sat aqueous feed solutions. The distillate flux decreases with the salt concentration, but no drastic flux decline was observed with the increase of the feed electrical conductivity up to 182 mS/cm. High-purity water was obtained.

Temperature polarization in pervaporation

The incidence of temperature polarization has received considerable attention in membrane processes such as membrane distillation and thermoosmosis but has been much less investigated in pervaporation. Based on a theoretical computation on a case study (pervaporation of water through a dense cellulose acetate membrane), it has been confirmed that this phenomenon is likely to occur frequently, as soon as a thin and permeable membrane is used, with the temperature drop being located essentially in the liquid boundary layer. Experimental investigations on the importance of temperature polarization and its influence on pervaporation flux have been further performed with pure liquids (water and organic compounds) by changing the impeller agitation rate in a batch stirred cell. Different membrane materials (cellulose acetate, Cuprophan, PDMS, PEBA) and various liquids (water, ethanol, butanol, pentanol, ethyl propionate) were investigated in order to point out the influence of either liquid or membrane material physicochemical properties on the temperature polarization effect. A simplified data treatment of the observed results, derived from theoretical computations, is proposed and discussed.

Sensitivity Analysis of Air Gap Membrane Distillation

In this study, parametric sensitivity analysis using dimesionless sensitivity analysis and temperature polarization was used to investigate the sensitivity of the mass flux to the different parameters associated with the air gap membrane distillation (AGMD) process for pure water production. The model of AGMD used in this study is the approximate model proposed by Jonsson et al., which neglects the temperature polarization effect. The effect of temperature polarization is studied using another model developed by Banat and Simandl. The results obtained show that the mass flux of pure water production is highly sensitive to the feed bulk temperature, membrane porisity at low porosity values, and air gap width. Results also show that increasing the membrane thickness decreases the mass flux of pure water and decreases the temperature polarization effect. In addition, results show that the temperature polarization effect becomes significant as feed bulk temperature increases. Increasing the film heat‐transfer coefficients, increasing the diffusion path, or decreasing the membrane porosity can reduce the temperature polarization effect significantly.

Measurement of 2D-temperature distributions in a pervaporation membrane module using ultrasonic computer tomography and comparison with computational fluid dynamics calculations

Temperature polarization effects can considerably limit the overall performance of pervaporation modules. This study shows the measurement of temperatures in a pervaporation membrane module using ultrasonic computer tomography (U-CT). With this technique complete 2D-temperature distributions that occur in the module can be measured within seconds. Temperature distributions were also determined using computational fluid dynamics (CFD). Both temperature distributions are comparable and show that in the tubular pervaporation system studied, temperature segregation occurs. This segregation is a result of the occurring hydrodynamics in the system that is determined by mixed convection. Also, the measured water fluxes through the membrane show to be dependent of forced and natural convection.

PARAMETRIC SENSITIVITY ANALYSIS OF DIRECT CONTACT MEMBRANE DISTILLATION

Membrane distillation is a thermally driven mass transfer in which a porous hydrophobic membrane separates two liquid phases and a temperature difference is maintained as the driving force. In this study, the technique of direct contact membrane distillation (DCMD) process, where the liquid phases are in direct contact with both sides of the membrane, has been investigated for the case of pure water production. Parametric sensitivity analysis and temperature polarization factor are used to study the sensitivity of the mass flux to the different parameters associated with DCMD for water production, and the effect of these parameters on the thermal efficiency of the DCMD process has been investigated. The results obtained show that the mass flux of pure water production is highly sensitive to the feed bulk temperature, and at low flow rates of hot and cold fluids, to the heat transfer coefficients. Results also show that increasing membrane thickness decreases the mass flux of pure water and decreases temperature polarization effect. In addition, results show that temperature polarization effect becomes significant as feed bulk temperature increases.

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.

A method to evaluate coefficients affecting flux in membrane distillation

A method is proposed that permits to evaluate the membrane mass transfer coefficient, the membrane heat transfer coefficient and the boundary layer heat transfer coefficient in a membrane distillation system. The method completes a method previously proposed by R.W. Schofield, A.G. Fane, C.J.D. Fell, (1987). The coefficients sought after are obtained from a set of measurements of mass fluxes and evaporation efficiencies. The method has been applied to four porous hydrophobic membranes. The method also permits to quantify the temperature polarization effects.

Study of Polarization Phenomena in Membrane Distillation of Aqueous Salt Solutions

In this paper we study water transport in membrane distillation using a flat PTFE membrane. Experiments have been carried out with water and aqueous solutions of NaCl as feed. The effects of temperature and concentration polarization on the reduction of vapor pressure differences across the membrane with regard to the vapor pressure differences corresponding to the bulk phases which are separated by the membrane have been evaluated. A coefficient which measures this reduction has been introduced. This coefficient and the known temperature polarization coefficient coincide when water is used as feed, but they are more and more different when the salt concentration of feed increases. In this last case it is the polarization coefficient introduced here that measures the reduction of flux through the effect of the polarization phenomena. The measured flux results and the calculated polarization results are discussed for different temperatures, recirculation rates, and feed concentrations.

Simultaneous removal of acetone and ethanol from aqueous solutions by membrane distillation: prediction using the Fick's and the exact and approximate Stefan-Maxwell relations

The simultaneous removal of dilute acetone and ethanol from aqueous solutions by air gap membrane distillation is theoretically investigated. A combined heat and mass transfer model that includes temperature and concentration polarization effects as well as temperature and concentration variation along the module length is employed to predict the flux and selectivity of acetone and ethanol under the relevant process operating conditions. Three mass transfer solutions are considered in the model, namely; the exact Stefan-Maxwell (S-M), the approximate Stefan-Maxwell and the Fickian binary solution. Although, qualitatively, the three solutions exhibit the same trends, quantitatively some differences exist between the Fickian-based solution on one hand and the Stefan-Maxwell solutions on the other hand. The exact and approximate solutions of the Stefan-Maxwell equation showed similar capability in predicting the process performance under all process conditions. Predictions showed that acetone selectivity and flux were strongly dependent on feed conditions and air gap width.

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.

Theoretical investigation of membrane distillation role in breaking the formic acid-water azeotropic point: Comparison between Fickian and Stefan-Maxwell-based models

A comparison between the Stefan-Maxwell and the Fickian mass transfer-based mathematical models for predicting the air-gap membrane distillation process performance in breaking azeotropic mixtures such as formic acid/ water system is carried out in this paper. The predictions of Stefan-Maxwell model which incorporated temperature and concentration polarization effects were closer to the reported literature experimental data than the Fickian-based model that was developed by Udriot et al. The model was also used to predict the process behavior under other experimentally unstudied process parameters.

Separation of water and glycols by direct contact membrane distillation

In the present paper the feasibility of using direct contact membrane distillation for the concentration of glycol from used coolant liquids is analyzed. Membrane distillation experiments were performed with water–ethylene glycol mixtures, using a tangential flow cell, various membranes and different temperatures. The experiments show that the membranes reject almost completely the glycol and effective concentration is achieved. This result is expected, since the vapor pressure of glycol is negligible, compared to that of water, at the temperatures employed in the experiments. On the other hand, the effects of both temperature and concentration polarization were very important, and a specific computer code was developed in order to take them into account. From the computer code, values are obtained for the permeability of the membrane, and for the heat and mass transfer coefficients of the boundary layers. The values obtained agree with those of other literature sources and with the theoretical expectations.

Application of Stefan–Maxwell approach to azeotropic separation by membrane distillation

A multicomponent mass transfer model based on the Stefan–Maxwell formalism is developed to predict membrane distillation performance in separating azeotropic mixtures. The developed model accounts for all coupling interactions between the diffusing species as well as for temperature and concentration polarization effects. The model is validated with previously published experimental data of propionic acid/water azeotropic mixture. The model predicts the effect of the process relevant parameters very well.