A Silicon Microseparator based Pervaporation Process for Separation of Ethanol/Water Mixtures using a Polymer Membrane

Abstract

The selective removal of water from ethanol through pervaporation was demonstrated in a microchannel device using a commercial membrane. Photolithography and dry etching techniques were employed for fabrication of the microseparator with hydraulic diameters of 30 µm to 80 µm. Experiments conducted at 90°C and 2–3 Torr, with Reynolds Numbers ranging from 8 to 91, resulted in an average water and ethanol permeance of 1.2×10^?3 and 8×10^?5 cm³/cm² · s · cmHg respectively. A mass transfer analysis involving Sherwood correlations was used to calculate the theoretical boundary layer resistance. The comparison of overall mass transfer coefficient with the boundary layer coefficients suggests that the membrane was the dominant resistance for this system.     

Pervaporative Dehydration of Binary Ethanol/Water and Ternary Ethanol/Water/Methanol Mixtures Using a Methylated Silica Membrane: A Mechanistic Approach

The pervaporation properties of a methylated-silica membrane were studied on binary ethanol/water and ternary ethanol/water/methanol mixtures. The aim was to acquire a better understanding of the pervaporation mechanisms by studying the effects of feed temperature, permeate pressure, and feed composition on molecular transport. Emphasis was placed on the role of competitive adsorption and dragging and blocking effects between the components in the context of the adsorption-diffusion model. The results show the potential of the membrane for the coupled removal of water and methanol from bioethanol. This attractive application for process intensification was suggested for the first time in this paper.     

Stabilization of bioethanol recovery with silicone rubber‐coated ethanol‐permselective silicalite membranes by controlling the pH of acidic feed solution

In order to produce highly concentrated bioethanol by pervaporation using an ethanol‐permselective silicalite membrane, techniques to suppress adsorption of succinic acid, which is a chief by‐product of ethanol fermentation and causes the deterioration in pervaporation performance, onto the silicalite crystals was investigated. The amount adsorbed increased as the pH of the aqueous succinic acid solution decreased. The pervaporation performance also decreased with decreasing pH when the ternary mixtures of ethanol/water/succinic acid were separated. Using silicalite membranes individually coated with two types of silicone rubber, pervaporation performance was significantly improved in the pH range of 5 to 7, when compared with that of non‐coated silicalite membranes in ternary mixtures of ethanol/water/succinic acid. Moreover, when using a silicalite membrane double‐coated with the two types of silicone rubber, pervaporation performance was stabilized at lower pH values. In the separation of bioethanol by pervaporation using the double‐coated silicalite membrane, removal of accumulated substances having an ultraviolet absorption maximum at approximately 260 nm from the fermentation broth proved to be vital for efficient pervaporation. Copyright © 2005 Society of Chemical Industry     

Die Entwicklung von Iösungsmittelselektiven Membranen und ihre Anwendung in der Gastrennung und Pervaporation

Development of solvent-selective membranes and their use in gas separation and pervaporation. Pervaporation and gas separation are two membrane processes whose industrial and economic importance has increased considerably in recent years because they are particularly suitable for selective removal of organic solvents from aqueous and gaseous mixtures. The problem of solvent removal is encountered, inter alia, in the purification of certain industrial gaseous and liquid effluents. However, the removal of solvents is also significant in petrochemistry and biotechnology and both pervaporation and gas separation can again be efficiently used here. On the basis of the equations relating to mass transfer in homogeneous polymers, criteria for the choice of polymers for high-selectivity membrane production are discussed. The development of suitable membranes and membrane modules is described. Two industrially and economically relevant examples, viz. recovery of organic solvents from gaseous effluent and the continuous removal of ethanol from bioreactors, illustrate the scope and limitations of the procedures.     

Reliable production of highly concentrated bioethanol by a conjunction of pervaporation using a silicone rubber sheet‐covered silicalite membrane with adsorption process

For the production of highly concentrated bioethanol by pervaporation using an ethanol‐permselective silicalite membrane, pervaporation performance was investigated using a silicalite membrane entirely covered with a silicone rubber sheet to prevent direct contact with acidic compounds. By using a resistance model for membrane permeation, the separation factor of the covered silicalite membrane towards ethanol can be estimated from the individual pervaporation performances of the silicalite membrane and the silicone rubber sheet. No decrease in the ethanol concentration through the silicone rubber sheet‐covered membrane was caused when ethanol solutions containing succinic acid were supplied. By directly passing the permeate‐enriched ethanol vapor mixed with water vapor through a dehydration column packed with a molecular sieve of pore size 0.3 nm, highly concentrated bioethanol up to 97% (w/w), greater than the azeotropic point in the ethanol/water binary systems, can be obtained from 9% (w/w) fermentation broth. Copyright © 2004 Society of Chemical Industry     

On a rapid method to characterize intercrystalline defects in zeolite membranes using pervaporation data

This work presents a mass-balance-based model devoted to the characterization of intercrystalline defects active to permeation in film-like zeolite membranes using pervaporation data. Relevant structural information concerning such domains can be obtained from the slope and intercept of the line that describes the increase of the flux pervaporated through a membrane with the absolute pressure in the liquid feed. Capillary forces appear to govern mass transfer within intercrystalline pores. The suitability of the model is assessed by using a collection of zeolite NaA membranes prepared on our premises for the separation of ethanol/water mixtures. The results show that, although a membrane displays high selectivity, a reduced number of defects in the zeolite layers cannot be ruled out. The model offers the advantage of enabling the characterization of large defects in pervaporation membranes at experimental conditions similar to those in which they are likely to be used.     

Application and development of synthetic polymer membranes. II. Separation of aqueous ethanol solution through poly(tert-butyl methacrylate-co-styrene) membranes

Separation of aqueous ethanol solution was carried out by pervaporation using a membrane which consisted of common polymer membranes. A membrane obtained from poly(tert-butyl methacrylate-co-styrene) was effective for a selective separation of ethanol from aqueous ethanol solution by pervaporation technique. The pervaporation ethanol–water mixture through the present membranes was analyzed as a solution–diffusion process, on the assumption that the both diffusion coefficients of each component are an exponential function of ethanol concentration.     

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.     

乙醇/水及乙酸/水体系的渗透汽化分离

以乙醇/水及乙酸/水体系为研究对象,研究了渗透汽化过程中料液浓度、温度因素对分离效果的影响;结合乙醇、乙酸对聚二甲基硅氧烷(PDMS)膜的溶胀特性差别,分析并讨论了两者在渗透汽化过程中可能的分离机理. 研究表明,PDMS膜能够优先透醇,但乙酸分子的缔合物以及羧基与疏水PDMS膜高分子链的强相互作用降低了其在膜中的扩散速率,使低温时乙酸/水体系优先透水,只有当温度在60℃以上时才表现出优先透酸,且分离效果较差.     

TMCS修饰MFI分子筛膜的制备及乙醇/水分离稳定性的研究

采用三甲基氯硅烷（TMCS）作为修饰源对MFI分子筛膜进行表面改性,系统考察了TMCS浓度以及修饰时间对于MFI分子筛膜在分离乙醇/水混合物时的性能影响。SEM、XRD、²⁹Si NMR、FT-IR、接触角实验及分离实验结果表明,TMCS可以与硅羟基反应,嫁接分子筛膜表面,在消除膜表面硅缺陷的同时提高膜的疏水性及膜分离性能的稳定性。随着TMCS浓度以及反应时间的增加,修饰后MFI分子筛膜的通量及分离因子略有下降,但稳定性增强。在TMCS的浓度为0.4%（质量）,修饰时间为2 h时,所得到的膜具有最佳渗透汽化分离性能,并可在60℃下分离5%（质量）乙醇/水混合物时保持良好的稳定性。在连续90 h渗透汽化分离过程中,其渗透通量稳定在1.61 kg·m^-2·h^-1 左右,分离因子保持在20以上。     

生物乙醇提浓工艺中渗透蒸发膜材料的研究进展

渗透蒸发膜分离技术具有分离效率高、低能耗、易于和发酵装置耦合等优势,在生物乙醇的分离、提浓工艺中得到广泛应用。结合国内外生物乙醇的研究现状,综述了渗透蒸发膜分离技术的研究进展,并对渗透蒸发膜分离技术的核心材料--膜材料的制备与应用进行详细介绍,展望了生物乙醇的渗透蒸发膜分离技术的发展前景。     

Multistage and Multicomponent Separation in Pervaporation: The Non-isothermal Model

This paper presents the design of a pervaporator in order to separate water from an azeotropic mixture, light oil. This mixture is composed of five alcohols ranging from C₂–C₅. From the mass transfer equations obtained from our experimental results, we develop three basic equations governing the pervaporation process formulating mass and energy balance. A very simple heat balance is used considering the vaporization enthalpy of the permeate supplied by the feed mixture under adiabatic conditions. It was clearly seen that the membrane area required to obtain a specific final concentration neglecting the feed temperature drop was underestimated. The membrane surface area is important because the pervaporation modules called stages have higher surface area.     

硅橡胶复合膜用于渗透蒸发的膜传质动力学(Ⅰ)膜面上的对流传质

利用自制的硅橡胶平板复合膜对低浓度乙醇水溶液进行渗透蒸发分离乙醇实验 ,研究了过程的传质动力学。基于液 -膜的串联传质阻力模型 ,通过实验测定了膜的总传质系数 ,采用对比差值方法将总传质系数拆分为膜面上的液膜传质系数和膜内的扩散传质系数两部分 ,分析了液相边界层阻力和膜扩散阻力对总传质系数的影响。特别针对膜面上液体流动状况对膜传质的影响进行了探讨 ,得出了液膜传质系数与Reynolds数及温度的关联式。     

CONCENTRATION POLARIZATION ANALYSIS AT THE ENTRANCE REGION OF A FLAT SHEET PERVAPORATION MODULE FOR VOC REMOVAL

Concentration polarization is a phenomenon that is inherent in all membrane separation processes, which is difficult if not impossible to measure experimentally. Concentration polarization in a pervaporation module causes flux decline and is therefore an important issue in predicting the performance of the membrane unit for evaluation and optimization. Short-form (small L/D ratio) membrane configurations, commonly used for membrane evaluations or certain material separations, compound the complexity of process modeling that addresses concentration polarization since a substantial portion of the membrane flow channel would be considered as an “entrance region” based on the flow profile that is not fully developed. This article employed the classic boundary layer theory, combined with mass transfer phenomena in a pervaporation process that is used in volatile organic compound (VOC) removal from contaminated water sources, to theoretically analyze the concentration polarization severity in the entrance region of a flat sheet membrane module.     

Energy optimization of bioethanol production via gasification of switchgrass

In this article, we address the conceptual design of the bioethanol process from switchgrass via gasification. A superstructure is postulated for optimizing energy use that embeds direct or indirect gasification, followed by steam reforming or partial oxidation. Next, the gas composition is adjusted with membrane‐PSA or water gas shift. Membrane separation, absorption with ethanol‐amines and PSA are considered for the removal of sour gases. Finally, two synthetic paths are considered, high alcohols catalytic process with two possible distillation sequences, and syngas fermentation with distillation, corn grits, molecular sieves and pervaporation as alternative dehydration processes. The optimization of the superstructure is formulated as an mixed‐integer nonlinear programming problem using short‐cut models, and solved through a special decomposition scheme that is followed by heat integration. The optimal process consists of direct gasification followed by steam reforming, removal of the excess of hydrogen and catalytic synthesis, yielding a potential operating cost of $0.41/gal. © 2011 American Institute of Chemical Engineers AIChE J, 2011     

浅析渗透汽化膜脱水及其应用

渗透汽化作为一种节能、低能耗、绿色环保的新型膜分离技术,正受到世界范围内越来越广泛的关注和研究。文中简述了渗透汽化膜技术的基本原理、工艺流程和传质模型,介绍了影响分离效果的因素及特点;分析渗透汽化技术在有机溶剂脱水、水中脱除有机物以、有机物/有机物的分离以及化学反应中的工业应用情况,并进行经济性分析,最后展望了该技术的应用前景。     

基于多场协同理论的渗透蒸发传质模型

Toprovide a theoretical basis for optimizing the pervaporation procedure, a mass transfer model for pervaporation for binary mixtures was developed basedon the multi-fields synergy theory. This model used the mechanism of sorption-diffusion-desorption and introduced a diffusion coefficient, which was dependent on the feed concentration and temperature. Regarding the strong coupling effect in the mass transfer, the concentration distribution in membrane was predicted using the Flory-Huggins thermodynamic theory. The batch experiments and other experiments with constant composition-were conducted-using a modified chitosan pervaporatioffmembrane to separate tert-butyl alcohol （TBA）-water mixtures. The parameters of the mass transfer model were obtained from the flux of the experiments with a constant composition and the activity coefficients available through phase equilibrium equation, using the Willson equation in the feed side and the Flory-Huggins thermodynamic theory within the membrane The simulation results of the experiments .are in good agreement with the results, of the experiments.     

Simulation of pervaporation process for ethanol dehydration by using pilot test results

This paper focuses on providing a pervaporation simulation method for ethanol dehydration from a practical point of view. The simulation procedure is performed by setting up simulation equations which describe the pervaporation process, obtaining the necessary data from pervaporation batch mode pilot tests, verifying the simulation tool through simulations of continuous mode pilot tests, and comparing the simulation results with the real pilot test results. We considered the mass and energy balances that describe separating an ethanol/water mixture by a pervaporation membrane. The simulation equations were mathematically expressed into simultaneous non-linear differential equations based on these balances. The necessary data for simulation consist of the thermophysical properties for the ethanol-water mixture and the characteristic data of a PVA composite membrane. The membrane characteristic data are permeation flux and membrane selectivity, which are functions of feed composition and operating conditions. These data were experimentally determined by a batch mode pilot test. The continuous mode pilot tests were simulated and the simulation results were compared to the real test results. The resulte were fairly good.     

Separation and concentration of bilberry impact aroma compound from dilute model solution by pervaporation

BACKGROUND: In the present work, the flavor compounds of natural juice of bilberries were qualitatively analyzed by GC‐MS, leading to the identification of trans‐Hex‐2‐en‐1‐ol as one of the major ‘impact aroma’ compounds of this fruit. The pervaporation of trans‐Hex‐2‐en‐1‐ol from diluted aqueous solutions was studied using commercial polydimethylsiloxane (PDMS) capillary membranes. The influences of solvent composition (water/ethanol mixtures), initial concentration of the aroma compound, flow rate and temperature of the feed were studied. RESULTS: High selectivity of the PDMS membrane towards the aroma compound was obtained, leading to enrichment factors in the range 100 < β < 200. Mass transfer resistance was found to be located in the pervaporation membrane. Experimental data showed a linear dependency of the permeation flux of trans‐Hex‐2‐en‐1‐ol on the differences in partial pressures of the compound across the pervaporation membrane. The permeability coefficient of the PDMS membrane to the transport of trans‐Hex‐2‐en‐1‐ol was calculated as P_{m, Hex}(50 °C) = 7.62 × 10^?11 mol m^?1 s^?1 Pa^?1. CONCLUSION: The membrane used was found to be very selective toward trans‐Hex‐2‐en‐1‐ol. A model based on the solution‐diffusion mechanism was applied. The mass transfer parameters needed for the design of a pervaporation process for aroma compound recovery were obtained. Copyright © 2008 Society of Chemical Industry     

CONCENTRATION POLARIZATION ANALYSIS AT THE ENTRANCE REGION OF A FLAT SHEET PERVAPORATION MODULE FOR VOC REMOVAL

Concentration polarization is a phenomenon that is inherent in all membrane separation processes, which is difficult if not impossible to measure experimentally. Concentration polarization in a pervaporation module causes flux decline and is therefore an important issue in predicting the performance of the membrane unit for evaluation and optimization. Short-form (small L/D ratio) membrane configurations, commonly used for membrane evaluations or certain material separations, compound the complexity of process modeling that addresses concentration polarization since a substantial portion of the membrane flow channel would be considered as an “entrance region” based on the flow profile that is not fully developed. This article employed the classic boundary layer theory, combined with mass transfer phenomena in a pervaporation process that is used in volatile organic compound (VOC) removal from contaminated water sources, to theoretically analyze the concentration polarization severity in the entrance region of a flat sheet membrane module.