Pervaporation Transport Research Articles

Mass transfer fundamentals in pervaporation, perstraction and sorption: A unified approach

The high affinity between an organic solute and a polymeric sorbent or membrane is the basis for separating organic contaminants from water via sorption, pervaporation and perstraction. These processes all share the same features of dissolution and diffusion, and thus their mass transfer characteristics can be correlated. However, they are often dealt with independently, and the research findings from one process are rarely applied to the other. The permeability coefficient in perstraction (for liquid permeation) based on concentration gradient as the driving force for mass transfer and the permeability coefficient for pervaporative transport expressed customarily in analog to vapor permeation can hardly be compared directly, and sometimes seemingly contradictive trends in temperature dependencies of the permeability coefficients for the two process modes may result. Looking into the mass transfer fundamentals pertaining to sorption, pervaporation and perstraction, this work attempted to provide a unified approach to the mass transfer in all these processes. While the permeability coefficient in pervaporation uses equivalent vapor pressure gradient across the membrane as the driving force, this work provided a generalized treatment by taking into account the sorption constants embedded in the permeability coefficients of the different processes so that the performance parameter for one process could be applied to another, which would be especially useful for screening and developing appropriate membrane/sorbent materials. In addition, the relationships among the selectivities of these separation processes were elucidated, which allowed for an intuitive illustration of how the process selectivity changed due to the process mode. Aniline removal from water via pervaporation, perstraction and sorption using a poly(ether-b-amide) membrane/sorbent was used as an example to illustrate and validate the approach.

Clustering analysis for pervaporation performance assessment of alginate hybrid membranes in dehydration of ethanol

Pervaporative dehydration of 96 vol% ethanol solutionwas used as an experimental model for the determination of hybrid alginate membranes separation performance. Series of membranes filled with different metal oxides particles (magnetite, zinc, silver, titanium(IV) and chromium(III) oxide) were prepared and the effect of the oxide’s nature and filling ratio on pervaporative transport and separation effectiveness was discussed. Hierarchical agglomerative cluster analysis of the evaluated parameters was applied to divide all membranes into four clusters, with the most effective comprising of 10 and 15 wt% magnetite loaded for which pervaporative separation index reached the value of 56.79 and 76.59, respectively.

Pervaporative transport mechanisms in phosphonium ionic liquid-based supported liquid membrane and its stability with actual fermentation broth feed

ABSTRACTThe pervaporative transport interactions between trihexyl(tetradecyl)phosphonium bis(trifluoromethylsulfonyl)imide ionic liquid-based membrane and the components of ABE fermentation broth were elucidated in this study. Acetone has a suggested viscosity decreasing effect on the ionic liquid and exhibited coupling effect with 1-butanol. Temperature increase consistently caused increase on 1-butanol permeance. The presence of acetic and butyric acids has no observed effect on 1-butanol and water permeation. No permeation of ethanol and acetic acid was noted, while butyric acid permeation can be suppressed by pH control. The membrane was proven stable in separating 1-butanol from actual fermentation broth for long period of time.

A novel simple and efficient procedure for the pervaporation transport study of binary mixtures through polymeric membranes: tested systems propanol isomers—water–polyethylene membrane

A novel simple and efficient procedure for the study of binary mixtures pervaporation transport through polymer membranes is reported. The basic idea is a gradual increase of one component concentration in the feed mixture instead of time-consuming repetitive measurements of several solutions of different compositions. Presented procedure called "component addition method" allows obtaining the required experimental data much faster and easier. The study of the transport behavior of propanol isomers (propan-1-ol and propan-2-ol)/water mixtures during the pervaporation through a polyethylene membrane was performed using the proposed novel method. The effects of temperature, feed composition and the shape of the isomer molecules were investigated. Higher temperature effect on pervaporation transport of propan-2-ol was registered. Lower mass fluxes and diffusion coefficients of propan-2-ol for all experimental temperatures were found; the reason may be the more branched molecular structure. With respect to the feed composition, an interesting trend of propanols transport parameters were obtained; increasing water content in the feed caused the slight diffusion coefficient increase and the mass flux decrease for both isomers; however, in the concentration range 30–70 wt% the flux decrease was slower than in other regions.

Study on polybenzoxazinone membrane in pervaporation processes

ABSTRACTPoly(benz‐3,1‐oxazinone‐4) was prepared by thermal cyclization of its hydrolytically stable precursor polyamic acid. Both polymer and its precursor were investigated as membrane materials. Thermogravimetric analysis and contact angle measurements were carried out for characterization of peculiarity of membrane compositions and analysis of membrane surface. Pervaporation of water–isopropanol mixture was studied in the wide range of feed composition. To interpret the pervaporation transport properties of the membranes, swelling experiments were performed, kinetic curves of sorption and desorption were plotted, and basic sorption and diffusion parameters were analyzed. It was established that poly(benz‐3,1‐oxazinone‐4) membrane is extremely effective in dehydration of water–isopropanol mixture and shows high separation factor. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 4024–4031, 2013

The influence of water on butanol isomers pervaporation transport through polyethylene membrane

Pervaporation transport of binary mixtures of four butanol isomers (n-butanol, sec-butanol, iso-butanol and tert-butanol) with water through polyethylene membrane was studied. The pervaporation experiments were performed with butanol–water binary mixtures of wide concentration range and at different temperatures.The effects of temperature, feed composition and molecular shape of the permeating species on pervaporation characteristics were investigated. The obtained results shown that in comparison with pure butanols, the butanol permeation fluxes initially steeply decreases with increase of water concentration in the feed but then at higher water concentration levels. In the case of higher content of water in mixtures (15–95wt.%) they remained practically constant. It was found that the butanol fluxes decrease in the following order: sec-butanol, n-butanol, tert-butanol to iso-butanol.Butanol diffusivities with small increase of water concentration in the feed increase only slightly, for higher water concentration they appear approximately concentration independent. From the comparison of butanol isomers flux and diffusivity it follows that diffusivity decreases as expected with decreasing of “linearity” of butanol molecules in the sequence –n-butanol, sec-butanol, iso-butanol and tert-butanol.The temperature dependencies of butanol isomers sorption in polyethylene membrane were measured and compared. The sorption of isomers corresponds to their vapor pressure – the sorption amount decreases from tert-butanol, sec-butanol, iso-butanol to n-butanol.Calculated butanol fluxes using the first Fick’s law under the assumption constant diffusion coefficient and equilibrium concentration of permeant at the feed side of the membrane are in very good agreement with experimental values.

PVDF/Nanosilica Dual-Layer Hollow Fibers with Enhanced Selectivity and Flux as Novel Membranes for Ethanol Recovery

In this work, we have demonstrated the design and engineering of poly(vinylidene fluoride) (PVDF)/nanosilica dual-layer hollow fibers as novel pervaporation membranes for ethanol recovery. The newly developed dual-layer hollow fiber membrane can exhibit a high separation factor of up to 29 with a sustainable high flux of 1.1 kg m–2 h–1, which is equivalent to the separation performance regime of inorganic membranes. Central to this performance achievement is the synergy of (1) desirable membrane morphology, nanopore size, and high surface porosity of a thin-PVDF/nanosilica composite on a fully porous substrate accomplished by the dual-layer coextrusion technology, and (2) optimal operating downstream pressure with the aid of controlled pervaporation transport. The membrane selectivity-downstream pressure dependence of PVDF/nanosilica hybrid membranes is comprehensible via a modified pore-flow model. This study may represent a new class of membranes for ethanol–water separation.

Experimental data based modelling and simulation of isopropanol dehydration by pervaporation

A simulation tool is developed in order to support the operation of an industrial scale hybrid distillation–pervaporation system that is used to regenerate isopropanol. Laboratory-scale experiments were carried out to determine the parameters of a semi-empirical solution–diffusion membrane transport model that is used for the modelling of the pervaporation transport. Isotherm experiments were made at three different temperatures (60, 80, 90°C) using a hydrophilic flat sheet membrane (PERVAP 2210, Sulzer Chemtech). The permeate fluxes and compositions were measured at different constant feed compositions. The parameters of the applied transport model were fitted to the experimental data. For simulating the pervaporation apparatus, an earlier developed user added pervaporation module within the ChemCad software package was applied. The user added module and the fitted transport model parameters were used for the simulation of our measured laboratory-scale data. Simulation of the industrial scale distillation pervaporation hybrid system was also carried out. The simulated and measured data were compared.

Dehydration of caprolactam–water mixtures through cross-linked PVA composite pervaporation membranes

ɛ-Caprolactam (CPL) is the monomer of nylon-6 used extensively in the manufacture of high quality nylon-6 fibers and resins. Since CPL is a very heat-sensitive substance and water is the most important impurity in the final CPL purification, reduced pressure distillation through triple-effect evaporation sets is widely adopted by chemical industry. But high energy consumption, high wastage and condensate pollution have restricted the wide-scale production of CPL and the commercial profit of the manufactures. To improve or develop a new CPL dehydration process, pervaporation separation of CPL–water mixtures was investigated using composite membranes consisting of a selective poly(vinyl alcohol) (PVA) membrane as top layer and a porous polyacrylonitrile (PAN) substrate. The selective layer was formed by PVA matrix followed by the cross-linking reaction of PVA with glutaraldehyde. The membranes were characterized by SEM, FTIR and TGA. Due to the formation of more compact cross-linked structure, cross-linked PVA composite membranes exhibited enhanced thermal stability. Through evaluating the pervaporation performance, we introduced a pervaporation transport equation to normalized permeation fluxes in terms of water permeance, CPL permeance and selectivity. The evaluated results have revealed that the separation performances of PVA composite membranes are strongly related to its hydrophilic/hydrophobic nature as well as the operating parameters, such as feed concentration and operating temperature. Experimental data showed that the PVA composite membranes had superior pervaporation separation performances for dehydration of caprolactam–water mixtures, which provided a new way for CPL dehydration.

On the Driving Force of Methanol Pervaporation through a Microporous Methylated Silica Membrane

The pervaporation transport of methanol through an amorphous microporous methylated silica membrane was studied experimentally and analyzed through modeling within a Maxwell−Stefan (MS) framework. The experimental conditions cover a temperature range of 60−155 °C and absolute pressures up to 16 bar. Exerting higher absolute pressures on the liquid feed-side of the membrane did not lead to enhanced fluxes, confirming that (i) the selective layer of the 56.8 cm2 membrane was a closed layer and defect-free, and (ii) the chemical potential gradient of the permeating component is an appropriate driving force to describe the transport through the microporous membrane. Both the adsorption isotherm and the heat of adsorption (−ΔHi,ads) were determined, and a two-site Langmuir isotherm adequately correlated the heterogeneous adsorption behavior of methanol in amorphous silica. Different levels of detail were adopted for modeling the diffusion transport through the selective layer, after having accounted for the re...

A simple model for pervaporative transport of binary mixtures through rubbery polymeric membranes

A new model for pervaporation of binary mixtures through rubbery polymeric membranes is proposed. The model deals with the inherent problem of plasticization in a very simple, but physically realistic manner, and consequently contains fewer parameters than prior models; thus, concentration profiles can be computed from a minimum of experimental results for pervaporation and sorption. Results from a series of ionically crosslinked poly( n-alkyl acrylate) membranes and toluene/ i-octane binary mixtures are used to test the model. The model reasonably represents experimental results for pervaporative transport of toluene/ i-octane mixtures through these membranes. Concentration profiles for the penetrants across the membranes are calculated from the model using experimental results and parameters obtained in the fitting process.

Separation of 1-hexene/n-hexane mixtures using a hybrid membrane/extraction system

Abstract Previously reported techniques for the separation of gaseous olefins from paraffins by means of membrane contactors, and fluid olefins from paraffins by means of liquid extraction, have been extended to the separation of liquid olefin/paraffin mixtures by means of a hybrid membrane/extraction system. In this paper, emphasis is placed on the development of a hybrid membrane contacting system for the separation of liquid olefin/paraffin mixtures in the range up to C6. The membrane contacting system described comprises non-porous polymeric membranes and a high-boiling-point selective liquid extractant flowing along polymeric membranes between the extractor and the membrane stripper at various temperatures and flow rates. The olefin components permeated through the polymeric films at elevated temperatures in a membrane stripper, making it possible to effect pervaporation transport. Selectivities of 1-hexene/n-hexane separations ranged from 40 to 70. The 1-hexene permeation rate achieved at 62°C was up to 1.4×10−6 g/s cm2.

Transport Phenomena of Chitosan Membrane in Pervaporation of Water-Ethanol Mixture

ABSTRACT The pervaporation transport process of H2O-EtOH solution was studied on a chitosan membrane and on a H2SO4 crosslinked chitosan membrane. The influence of concentration, temperature, and crosslinking was also studied. The dependence of permeation fluxes on feed concentration showed strong coupling effects existed in the permeation process. That the thermodynamic swelling—distribution relationship changed with the feed concentration also showed that a strong coupling effect existed in the thermodynamic swelling process. The permeation fluxes and thermodynamic swelling processes showed analogous relationships versus the concentration in the feed. The high swelling ratio and the high selectivity of the membrane in the thermodynamic swelling distribution process was the basis ofhigh flux and high permselectivity of pervaporation. With an increase of temperature, the permeation fluxes increased quickly, but the swelling ratio of water and EtOH in the membrane scarcely changed. This showed that an increase of temperature promoted the diffusion process but had little influence on permselectivity. The permselectivity of pervaporation depended strongly on the thermodynamic swelling process.

POSSIBILITY OF CONCENTRATION POLARIZATION OCCURRING INSIDE THE MEMBRANE DURING STEADY STATE PERVAPORATION

Mathematical equations are derived for describing the pervaporation transport of pure penetrant through polymeric membranes by assuming the chemical potential gradient as the driving force for the flow of penetrant. An imaginary phase (liquid or vapor) is assumed to be present and is in thermodynamic equilibrium with the membrane phase for deriving these equations. The mathematical equations obtained are similar to those derived by Okada and Matsuura (1991) using the pore-flow model. The possibility of concentration polarization occurring inside the membrane is predicted based on the analysis of binary mixture pervaporation. Experimental profiles of binary penetrant concentration in the membrane are established and these profiles substantiate the prediction of concentration polarization occurring inside the membrane. Pervaporation and sorption data from liquid and vapor phase at 25° C are reported for the acetic acid-water-polyamide system.

Pervaporative transport modelling in a ternary system: ethyltertiarybutylether/ethanol/polyurethaneimide

The study deals with the analysis of diffusion and mass transfer modelling during pervaporation in a true ternary system involving a polar liquid mixture (ETBE/EtOH) and a polar block copolymer (polyurethaneimide or PUI). A survey of methods of pervaporative transfer modelling in ternary systems is first developed. From differential permeation experiments carried out with both pure liquids, it appears that both permeants obey a Fickian law. Moreover, the diffusional behaviour is consistent with Long's model, which has thus been assumed for the related ternary system. An extension of the Brun's model is then derived, which takes into account the diffusion coupling as well the significant deviation from sorption ideality. From a practical point of view, the calculated values of fluxes show generally good agreement with the experimental results, although a small deviation occurs for mixtures of low ethanol content. Diffusion coefficients of both pure solvents corresponding to transient or steady state are compared. A very good agreement is found for the aprotic permeant (ETBE). whereas the diffusion coefficient of ethanol in transient state is only the quarter of the value corresponding to steady state. The results are discussed in comparison with related investigations in the literature, involving specific liquid-polymer interactions.

Pervaporative transport of aqueous ethanol: Dependence of permeation rates on ethanol concentration and permeate side pressures

The pervaporative transport of aqueous solutions of high ethanol content was studied on polyion complex membranes. It was found that the permeation rate of water became smaller with the decrease in the water content of the feed solution and/or with the increase in the permeate side pressure. This fact suggested that the permeation rate was dependent on the activity of water in the membrane. Therefore, on the basis of the solution-diffusion theory, an equation of the permeation rate of water was derived on the assumption that both the activity and the diffusion constant of water depended on concentration. The permeation rate was calculated for the membranes of different thickness and under various permeate side pressures. It was confirmed that the derived equation accurately described the experimental data for permeation rate of water. On the other hand, the permeation of ethanol resulted mostly from the volume flow through incomplete parts of the membrane but was negligible compared with the permeating water flow.

Effect of evaporation time on the pervaporation characteristics through homogeneous aromatic polyamide membranes. I. Pure water permeation and membrane characterization by sorption measurements

AbstractThe pervaporation of pure water through homogeneous aromatic polyamide membranes was investigated. The structure of the prepared membranes was controlled by varying the solvent evaporation times before the gelation step from 5 min to 240 min. The permeation flux of pure water decreased rapidly when the solvent evaporation time increased from 5 min to 30 min, and the decrease was rather moderate at higher evaporation times. Vapor and liquid sorption measurements were used to characterize the membranes. The amount of vapor sorption at a given relative pressure increased with an increase in the solvent evaporation time. The specific surface area of the membranes, calculated from the vapor sorption isotherm, increased with evaporation time up to 30 min, and remained constant thereafter. Liquid sorption volume, on the other hand, decreased monotonically with an increase in evaporation time. The formation of channels in the membrane is used to explain the morphology change during the solvent evaporation. The increase in the solvent evaporation time gradually increases the number of the channels in the membrane and decreases their average size. Good correlation was observed between the average size of the channels (represented by the ratio of specific volume to the specific surface area) and the parameters associated with pervaporation transport. © 1995 John Wiley & Sons, Inc.

A simple predictive treatment of the permeation process in pervaporation

In pervaporation, a liquid feed contacts one side of a membrane and a vacuum is drawn on the other side of the membrane, producing a permeate vapor. Conventional transport models describe pervaporation in terms of sorption equilibrium between the liquid feed and the membrane material, diffusion through the membrane driven by a chemical potential gradient, and desorption into a vapor phase on the permeate side of the membrane. This is called the solution-diffusion model. The treatment presented in this paper is based on the same model but the driving force for permeation is expressed as a vapor pressure difference rather than a concentration difference. To simplify the mathematical treatment of pervaporation, the initial sorption step between the liquid feed solution and the membrane is separated into two sequential steps that together are thermodynamically equivalent to sorption from the liquid. These steps are: (1) evaporation of the feed liquid to produce a saturated vapor phase, and (2) permeation of the vapor through the membrane, driven by a partial pressure gradient. It is important to note that step (1) is conceptual only; in pervaporation, no vapor is actually present on the feed side of the membrane. This simple modification to the model leads to an equation for the pervaporation separation factor, β pervap, which contains a term β pervap, defined by the vapor-liquid equilibrium of the feed mixture, and a term α mem, the membrane selectivity, which is identical to the parameter used in gas and vapor separation. The resulting pervaporation transport equation contains the feed-side and permeate-side partial vapor pressures and the membrane normalized permeation flux, as commonly defined in gas and vapor separation. An advantage of our approach is that the role of the operating conditions of pervaporation (feed temperature and permeate pressure) becomes clear. Membrane performance can be separated from the operating conditions, so that comparison of pervaporation separation data from various sources can be made, even if the operating conditions are not identical. Another advantage is that the contributions of the vapor-liquid equlibirium and the membrane selectivity to the overall pervaporation separation factor are recognized. Experimental data taken from our own work and from the literature are analyzed using the proposed model. The analysis confirms the usefulness of our approach.

Driving force for hydraulic and pervaporative transport in homogeneous membranes

AbstractExperiments were designed to demonstrate that the chemical potential gradient required for liquid transport through swollen network polymer membranes manifests itself as a concentration gradient and that the rate of transport is independent of how this gradient is established. The fluxes of various liquids through a crosslinked rubber membrane were measured in hydraulic and pervaporation modes of permeation. The pressure applied downstream in the latter act simply to fix the activity of the liquid in the downstream membrane surface. The experiments show the flux is a unique function of this activity, and it does not matter how it is established. Sorption data were used to convert these results into a plot of flux versus concentration differential across the membrane which was analyzed by Fick's law using a model for the concentration dependence of the diffusion coefficient. Measured ceiling fluxes for pervaporation for a number of liquids were found to be the same as those estimated from hydraulic permeation data. A simple mathematical representation for an ideal system is used as a pedagogical device to demonstrate the conclusions.