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.     

Pervaporation performance analysis and prediction—using a hybrid solution-diffusion and pore-flow model

A hybrid mathematic model for pervaporation is proposed which incorporates the concepts of solution-diffusion model and pore model. The model allows performance prediction as well as the establishment of the internal concentration and pressure profiles within the membrane. The model parameters specific to the particular membrane and mixture system are determined using liquid sorption and pervaporation experimental data. The model is experimentally examined using ethanol–water mixtures and poly(dimethyl siloxane)–poly(vinyldiene fluoride) (PDMS–PVDF) composite membranes. The characteristics of flux and separation factor predicted using the model are in fair agreement with the experimental data under various feed concentrations and downstream pressures for different membrane arrangements, including single-layer, reverse single-layer and double-layer PDMS–PVDF composite membranes. Internal profiles of pressure, concentration and component mole fraction can be established using the model. Concentration polarization phenomena for ethanol and water are located at membrane interfaces and vapor–liquid interfaces, respectively. Performances of several different membrane designs are compared using the model.     

Effect of transfer in the vapour phase on the extraction by pervaporation through organophilic membranes: experimental analysis on model solutions and theoretical extrapolation

Mass transfer in pervaporation is usually regarded as limited by the solution-diffusion step inside the dense selective polymer layer. In the case of pervaporation for the extraction of volatile organic compounds through organophilic membranes, especially at low feed temperature (about 300 K), the influence of the downstream pressure cannot be neglected. A contribution to the study of the operating parameters on the vapour side in a pilot plant — from the membrane to the condenser — to the overall mass transfer is presented.

A “convection-diffusion” model has been established to calculate the partial pressure gradients in the vapour phase up to the downstream face of the membrane. This equation has been combined with a relation for the mass transfer inside the membrane with a driving force expressed as a difference in fugacities.

The partial permeate pressures and the pervaporate fluxes obtained first with a pure compound (water) and secondly with binary mixtures (water-ethanol) pervaporated through membranes of polydimethylsiloxane (PDMS) on a pilot plant scale are well predicted by the model. Moreover, on the permeate side, the effects of unavoidable non-condensable gases, of the condenser temperature and of the distance between the module and the condenser on the flux and on the selectivity have been established for different total permeate pressures (300–3000 Pa). At high pressure, the pervaporation selectivity towards ethanol exhibits a minimum value as a function of the permeate circuit design.     

Pervaporation of methanol–water mixture by poly(γ-methyl L-glutamate) membrane and synergetic effect of their mixture on diffusion rate

The pervaporation behaviors of methanol–water by poly(γ-methyl L -glutamate) (PMLG) membrane at non-steady- and steady-state permeation were investigated. The values of t_1/2 (time required to reach a half value of steady-state permeation flux) for methanol and water changed and did not change with the component in feed, respectively. Both of the average diffusion coefficients for methanol–and water–PMLG from the mixture changed exponentially with the sorption amount of methanol by the synergetic effect on diffusion. The difference in behavior of non-steady and steady state diffusion was explained by whether D_o (diffusion coefficient at zero penetrant concentration) was influenced by the concentration distribution of penetrant in PMLG membrane.     

Pervaporation of benzene–cyclohexane mixture by poly(γ-methyl L-glutamate) membrane and synergetic effect of their mixture on diffusion rate

The pervaporation of binary liquids mixture of benzene and cyclohexane was examined by use of poly(γ-methyl L -glutamate) (PMLG) membrane. The permeation rate–time curve for each of benzene and cyclohexane from their mixtures changed to the longer times side by increasing the cyclohexane in the mixtures. t_1/2 (the time required to reach a half-value of the steady state permeation rate) for the each component increased exponentially with increasing of cyclohexane, which has a smaller plasticizing effect on PMLG membrane than benzene, in the mixtures. The apparent diffusion coefficients, obtained from the steady state permeation and the sorption experiments, for benzene–PMLG and cyclohexane–PMLG are dependent exponentially on the sorbed amounts of benzene. This result was explained on the bases that the diffusion of cyclohexane was enhanced synergetically with benzene coexisting in the system. This effect influenced negatively the separation of the liquids mixture by pervaporation.     

Time lag and permeation rate in liquid/liquid dialysis

Time lag and permeation rate in liquid/liquid dialysis in general are found different from those in pervaporation. Theoretical treatments of time lag in liquid/liquid dialysis indicate that time lag, in certain cases, may be correlated to the “relaxation time” of the membrane, defined as the time required for the membrane to relax, or as the change of the permeation flux of diffusable species from initial flux to steady state due to the long time required for the dialysis solvent to reach steady state in the membrane. The permeation of dichlorobenzene in a Teflon-FEP membrane for both the operations of liquid/liquid dialysis and pervaporation was measured as an example. It was found that the time lag in liquid/liquid dialysis is much longer than that calculated from a diffusion constant calculated from pervaporation. Permeation rate and activation energy are also different in both operations.     

乙醇－水－甲醇精馏过程中温度和组分浓度沿塔高的分布

本文用全回流筛板精馏塔测定了四个压力下的乙醇－水和乙醇－水－甲醇两个物系沿塔高温度与浓度的分布。用两元实测结果计算了不同压力下各板的默弗里板效，并以ＵＮＩＦＡＣ活度系数方程和维里状态方程为基础的热力学模型建立了精馏计算模型。用该模型对三元体系进行了计算，与实测结果符合良好。     

Pervaporation of high boilers using heated membranes

Hydrophobic polymer membranes for organic pervaporation from aqueous-organic feed solutions are stabilized against swelling distortion by incorporating a woven fabric near the non-swelling downstream interface. When using electrically conducting metal wire fabric, the membrane may be heated, thereby reversing the temperature gradient across the membrane. An apparatus for pervaporation using electrically heated membranes is described, and test results on pervaporation of high boilers (phenol, benzoic acid, γ-decalactone) as a function of power supplied are presented.     

硅橡胶膜生物反应器中乙醇发酵与渗透汽化的耦合

用硅橡胶膜生物反应器(SMBR)实验研究连续发酵-渗透汽化的耦合性能。发酵微生物采用酿酒干酵母,所用碳源为工业级葡萄糖。发酵过程由于产物抑制作用,在乙醇质量浓度达到73 g/L时趋于停滞,而耦合渗透汽化膜后,发酵罐内的乙醇质量浓度降低并维持在40 g/L,使发酵可以连续稳定地进行。在SMBR运行达到稳态后,乙醇的体积产率为4.02 g/(L.h)。发酵液中乙醇质量浓度维持在20~63 g/L,聚二甲基硅氧烷(PDMS)膜的总渗透通量为1 220~800 g/(m2.h),分离因子为5~9.2。与传统发酵和分离相同进料质量分数的乙醇溶液相比,乙醇发酵和渗透汽化在硅橡胶膜生物反应器中能相互耦合并得到强化。与较小规模耦合系统(发酵体积1 L和2 L)比较,性能稳定良好。     

丙酮、乙醇对丁醇渗透汽化性能的影响

考察了全硅沸石silicalite-1对丁醇-水、丙酮-水、乙醇-水、丙酮-丁醇-水、乙醇-丁醇-水5种体系中各溶剂的吸附作用。采用自制的silicalite-1/硅橡胶杂化渗透汽化透醇膜,研究了温度对丙酮、丁醇、乙醇分离性能的影响以及不同分离温度下丙酮、乙醇的浓度对丁醇、水渗透汽化性能的影响,结果表明丙酮和乙醇的存在会促进丁醇的透膜性。     

Characterization of permeation behaviors of ethanol–water mixtures through sodium alginate membrane with crosslinking gradient during pervaporation separation

Dense sodium alginate (SA) membranes crosslinked with glutaraldehyde were prepared by a new solution technique, which had different extents of crosslinking gradient structures. The SA membranes having a crosslinking gradient structure were fabricated by exposing one side of the membrane to the reaction solution while blocking the other side by a polyester film to prevent the reaction solution from contacting it. The extent of the crosslinking gradient was controlled by the exposing time. When the swelling measurements were performed with uniformly crosslinked membranes in aqueous solutions of 70–90 wt % ethanol contents, it was observed that the crosslinking could reduce both the solubility of water in the membrane and the permselectivity of the membrane toward water. The pervaporation separation of the ethanol–water mixture of 90 wt % ethanol content was carried out with membranes with different extents of crosslinking gradients. As the crosslinking gradient was developed more across the membrane, the resulting flux as well as the separation factor to water was found to decrease while the membrane became stable against water. The pervaporation performances of the membranes with different membrane loadings in a membrane cell were also discussed using the schematic concentration and activity profiles of the permeant developed in them. The pervaporation separations of the ethanol/water mixtures through the membrane with an optimal crosslinking gradient were performed at different feed compositions and temperatures ranging from 40 to 80°C. The change in the membrane performance due to the relaxation process during pervaporation was observed with the operating temperature and feed composition. The relaxational phenomena were also elucidated through an analysis of the experimental data of the membrane performance measured by repeating the operation in a given temperature range. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 1607–1619, 1998     

中空纤维渗透汽化复合膜及组件研究进展

中空纤维渗透汽化膜组件具有器件小型化及成本较低等方面的优势,其工业应用潜力巨大。本文介绍了中空纤维渗透汽化复合膜及组件的研究进展,阐述了膜材料、成膜方法以及组件结构参数等对组件渗透汽化性能的影响,并对中空纤维渗透汽化膜组件的中试研究进行了总结。通过组件放大及中试研究发现,中空纤维渗透汽化膜组件的装填密度、长度以及抽吸方式均会影响其下游侧的真空度,从而影响其渗透汽化性能。膜材料的分子设计、组件的结构参数优化以及耐溶剂耐高温封装将是中空纤维渗透汽化膜组件未来工业放大过程中的关键环节。     

Modelling of the pervaporative flux through hydrophilic membranes

This work presents a simplified model of wide applicability for the determination of the pervaporation flux through hydrophilic membranes, provided that the adsorption isotherm of the permeating species onto the pervaporation membrane has a linear shape. The model predicts the pervaporation flux as a function of the activity of the penetrant in the liquid phase and the operation temperature. Experimental results obtained working with two polymeric membranes (CMC‐CF‐23 and Pervap 2256) applied to the dehydration of tetrahydrofuran (THF) and to the separation of methanol from alcohol–ether mixtures, respectively, have been satisfactorily correlated and the characterising parameters have been obtained. Furthermore, the model has been also tested against results obtained with two ceramic membranes, Pervap SMS and zeolite NaA, applied to the dehydration of ketonic mixtures and of tetrahydrofuran respectively. Copyright © 2005 Society of Chemical Industry     

Separation of binary mixtures by dense membrane processes: influence of inert gas entrance under variable downstream pressure conditions

The influence of an inert gas on the separation performances of a dense polymeric membrane module working under partial vacuum on the downstream side, such as possibly encountered in gas permeation, vapor permeation or pervaporation, has been investigated through an experimental and theoretical study. A whole range of situations on the downstream side, covering ideal vacuum pumping (i.e. zero downstream pressure under leak free conditions) to inert gas sweeping under atmospheric pressure has been tested. A theoretical framework, previously developed for single permeant situation has been extended to the multicomponent case. The separation of methanol and 2-propanol by a dense silicone rubber membrane confirms the ability of this simple modelling strategy to offer quantitative predictions of the permeate composition under variable downstream pressure and inert gas flowrate conditions. Based on this observation, the implications of an inert gas contribution on pervaporation or gas separation operation are discussed, particularly in relationship to the global energy consumption of the system or to analytical devices making use of a gas sweep.     

Experimental Determination of Mass Transfer at the Active Surface of a Membrane Pervaporation Cell

This work is dedicated to an experimental study of overall mass transfer at the bottom of a rectangular cavity subjected to forced convective flow at its mouth. Mass transfer coefficients have been measured using an electrochemical method for different aspect ratios, H/h, between the depth of the cavity to the height of the upstream channel, range between 0.5 and 1.67. Reynolds number, based on the hydraulic diameter of the upstream duct, has been varied from 1 to 3000 and Schmidt number between 1010 and 8400. Overall mass transfer at the bottom edge of the cavity is found to increase with both Reynolds and Schmidt numbers and to decrease with the aspect ratio, H/h. General correlations taking into account these different parameters have been established in order to predict mass transfer at the bottom of the cavity. This work is the first part of a global investigation of a pervaporation membrane separation cell involving mass transfer in liquid phase at one side of the membrane surface, followed by a transfer of the species across the membrane and finally vaporization on the downstream side of the membrane occurs.     

Permeation behaviour in cellulose triacetate dense membrane during pervaporation separation of methanol/methyl tert-butyl ether mixture

The pervaporation separation of methanol/methyl tert-butyl ether through cellulose triacetate dense membranes has been carried out under different feed compositions, permeation temperatures and temperature cycles to investigate the permeation behaviour of the membrane during pervaporation process. The experimental data indicate that the plasticization effect has a decisive influence on pervaporation flux, permselectivity and permeation activation energy. The results show that the penetrants of different feeds are transported through the membrane by different pathways with the influence of plasticization effect at different temperatures. It has also been observed that plasticization had great effect on the membrane swelling and sorption selectivity. Furthermore, DSC results show that the membranes retain the influence of the plasticization effect after pervaporation separations are tested. © 2000 Society of Chemical Industry     

Liquid transport through membranes prepared by grafting of polar monomers onto poly(tetrafluoroethylene) films. II. Some factors determining pervaporation rate and selectivity

The effect of some factors including temperature, pressure, film thickness, and grafting ratio on the fractionation of binary liquid mixtures has been investigated by pervaporation through poly(tetrafluoroethylene) films grafted with N-vinylpyrrolidone or 4-vinylpyridine. The purpose was to estimate the best conditions in which the pervaporation process must be carried out. It was concluded that the pervaporation rate is increased at roughly constant selectivity when the temperature of the liquid charge is higher or when the downstream vapor pressure or the film thickness are decreased. A pressure higher than 1 atmosphere above the liquid does not increase the rate. When the grafting ratio is increased, the rate shows a maximum, and for particular conditions, the selectivity becomes much higher.     

水性聚氨酯膜渗透蒸发分离苯/环己烷的条件及过程

以聚己二酸1,4-丁二醇酯二醇（PBA-2000）和甲苯二异氰酸酯（TDI）为主要原料合成水性聚氨酯膜,并对苯/环己烷混合液渗透蒸发性能进行测试,讨论了苯/环己烷混合体系的渗透蒸发分离过程特点。结果表明,当膜厚度增大时,分离因子提高而渗透通量随之下降,这个变化趋势在膜较薄时很明显,达到一定厚度后则变化比较平缓;膜下游侧真空度提高会同时提高膜的分离因子和通量;增大料液中苯的浓度,提高料液温度会提高通量并降低分离因子。液体被分离组分在渗透蒸发膜中经历了吸附溶胀-膜内汽化-气体扩散的质量传递过程,“干区”对分离的影响作用更加显著。     

Intermittent drying of porous materials containing binary mixtures

The selectivity during the drying of porous materials containing binary mixtures depends on the thermodynamic equilibrium, the gas-side mass transfer and the liquid-side mass transfer. Very often the drying process is nonselective owing to the controlling liquid-side mass transfer. However, in this case at the early stages a selectivity might be expected, because the liquid-side mass transfer resistance is negligible at the beginning. This so-called initial selectivity can be extended to the whole drying process by intermittent drying. For this purpose the time required for the development of the steady state concentration profile at the beginning (transient time t_T) and the time required for the degraation of the steady state concentration profile during recovery (relaxation time t_R) had been estimated theoretically and experimentally in this work.