Separation of liquid mixtures by using polymer membranes. I. Permeation of binary organic liquid mixtures through polyethylene

The permeation characteristics and the separation behavior of 25 combinations of binary liquid mixtures through low-density polyethylene membrane have been investigated. The organic compounds studied were members of the homologous series of liquid aliphatic hydrocarbons between n-pentane and n-nonane as well as some aromatic and cyclic compounds. A special permeation cell was designed in order to study permeation rates at different temperatures ranging from 25 to 45°C. The rate of permeation increased with temperature, and it was found that the temperature dependence of the permeation rate for both pure compounds and mixtures could be expressed by Arrheniustype relationships. The efficiency of separation, however, decreased with increasing temperature. Activation energies of permeation rànged from 16–22 kcal/mol for pure compounds and binary mixtures of benzene, n-hexane, cyclohexane, and 2–2-dimethylbutane. The effects of chemical nature, molecular size, and molecular shape of the diffusing species on the permeation and separation were studied and qualitative guidelines were suggested. The effect of the composition of the binary mixtures on the permeation rate has been investigated for several systems. Permeation enhancement effects were observed in which the mixtures permeate considerably faster than either of the pure components. Maximum permeation rates occurred at about 50 wt-% mixtures for the systems benzene–n-hexane and benzene–cyclohexane. This phenomenon is explained in terms of a combined internal plasticizing and solubility effect.     

Permeation of binary–liquid mixtures through elastomers

Permeation of binary liquid mixtures through two commercially available elastomers were examined. The four systems studied are benzene–cyclohexane/neoprene, toluene–n-hexane/neoprene, toluene–methanol/nitrile, and toluene–o-xylene/nitrile. Permeation rate–time curves for several mixture compositions were prepared for each system. In the first three systems, the difference in the plasticizing effect of the two liquids on the membrane was very large. The liquid with the smaller plasticizing effect showed a permeation enhancement and the liquid with the larger plasticizing effect experienced a permeation depression. Toluene–o-xylene/nitrile represented a system in which the two liquids plasticized the membrane at almost the same rate. This system showed a permeation depression for both liquids.     

Permeation of organic vapors through polymer in the condensation region

Permeability coefficients have been measured for normal butane, isobutane, isobutylene, and butene-1 in Zendel Copolymer-I at temperatures between 30° and 70°C and at penetrant pressures p₁ up to 5.5 atm. The permeability coefficients of these organic vapors show behavior analogous to that observed for C₃ and C₂ hydrocarbons as detailed in our earlier work; i.e., near the condensation point of the penetrant, P increases as the temperature is decreased. Isothermal plots of log P versus p₁ in that region are generally linear and can be represented by empirical relations of the form where P₀ is a constant. The slope a is a function of temperature: where a₀ is a constant and b has the same value for the four hydrocarbons investigated.     

Permeation of mixtures of organic liquids through polymeric membranes: Role of liquid–liquid interactions

The permeation of pure organic liquids and mixtures of organic liquids through commercial butyl, neoprene, and nitrile membranes was studied using dynamic material deformation (swelling) and permeation techniques. The derived parameters, the breakthrough time (t_BT), steady‐state permeation rate (SSPR), and initial swelling rate (SR), show deviations from additivity for the mixtures, based on the parameters of the pure liquids on a mol fraction basis. In the majority of cases for the three membranes examined, the deviations are independent of the nature of the membranes, and the signs of the deviations for t_BT are opposite to those for SSPR or SR, provided that the membranes are not degraded by one of the solvents. An approach that considers only solvent–solvent interactions based on the enthalpy of mixing was used to predict deviations for mixtures. For mixtures where the enthalpy of mixing is large and exothermic, the permeation of the mixture is less than expected, while for systems where the enthalpy of mixing is large and endothermic, the permeation is larger than expected. A simple semiempirical model predicts the sign and magnitude of the permeation of 73% of the system–permeation property combinations investigated, which show significant deviations from ideality. It is interesting to note that the wrong predictions are for systems where the predictions are positive, that is, for SSPR and SR rates with endothermic systems and for t_BT with exothermic systems. The exceptions also seem to be for systems that correspond to materials having a high resistance to one of the solvents and a very low resistance to the other solvent. Examples of ternary–mixture permeation data are also given and show that, even if two of the pure components do not permeate through a membrane, the membrane will offer little protection if the third component shows a high affinity for the membrane and if the enthalpies of mixing of this component with the other liquids are endothermic. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 195–215, 2002     

Permeation of oxygen through polyurethane–polyepoxide interpenetrating polymer networks

Oxygen permeation studies on polyurethane (PU)/polyepoxide (EP) interpenetrating polymer networks show that the increased crosslinking density owing to additional permanent chain entanglement (resulting from interpenetration) can decrease the coefficients of permeation, diffusion, and oxygen solubility. At 20% PU, at which the crosslinking density is maximum, these coefficients retain minimum values, while the tensile strength retains a maximum value.     

Separation of aqueous organic liquid solutions through polymer membranes

Characteristics of permeation and separation for aqueous dimethyl sulfoxide, acetic acid and ethanol solutions through some polymer membranes such as chitosan, poly(vinyl chloride) and poly(dimethyl siloxane) membrane were studied by evapomeation. In temperature difference controlling evapomeation (TDEV), when the temperature of the feed solution was kept constant and the temperature of the membrane surroundings was changed, permselectivity for water increased in the chitosan and poly(vinyl chloride) membranes; in the poly(dimethyl siloxane) membrane the perm-selectivity for ethanol increased while decreasing the temperature of the membrane surroundings. Permeation and separation mechanisms for aqueous organic liquid solutions through the above polymer membranes by the TDEV method are discussed.     

Oriented polymer networks obtained by photopolymerization of liquid-crystalline monomers

In this paper a new class of oriented polymers is introduced: highly crosslinked polymer networks obtained by photopolymerization of oriented liquid crystalline (LC) monomers. Orientation of these monomers can be induced in the same manner as usual for conventional LCs applied in LCDs: with the aid of an electric or a magnetic field, or using surface orientation. Photopolymerization offers the advantage that a temperature can be chosen right within the LC temperature region. The resulting polymer networks show a high degree of ordering which is somewhat dependent on the polymerization temperature, as demonstrated by the amount of birefringence. This ordering is maintained over a wide temperature region, contrary to that in LC side-chain polymers. Apart from their optical properties, uniaxially ordered polymer networks show anisotropy also in other physical properties, such as the thermal expansion coefficient and modulus.     

Studies on syntheses and permeabilities of special polymer membranes. III. Permeation of aqueous alcohol solutions and binary organic liquid mixtures through alkali bridged poly(vinyl alcohol) membranes

The preparation conditions of water insoluble poly(vinyl alcohol) membranes were investigated using three kinds of monovalent alkali metal compounds, namely, lithium hydroxide, sodium hydroxide, and potassium hydroxide. The mechanism for the formation of water insoluble membranes is discussed. The permeabilities of pure alcohols were improved by adding ethylene glycol to the casting solutions and they depended upon the radii of alkali metal ions and the density of locations bridged with these ions. The permeation and separation characteristics of three different alcohol/water systems and three different alcohol/benzene systems through alkali bridged poly(vinyl alcohol) membranes were studied by changing the feed composition of the binary mixtures. In alcohol/water solutions, the depression of permeation depended upon the compaction of membrane under pressure. The permeation rate of water/lower alcohol solution was faster than that of water/higher alcohol solution in all feed compositions. Separation of water/alcohol at given water contents increased with an increase in the molecular length of the alcohols. In the three different alcohol/benzene systems, the permeation rates increased with an increase in the alcohol fraction in feed, and the permeation rate of benzene/lower alcohol mixture was faster than that of benzene/higher alcohol mixture in all feed compositions. Separation of benzene/alcohol mixtures at given benzene contents decreased with an increase in the molecular length of alcohols. These phenomena are discussed from the standpoint of the molecular size, the molecular shape, and the physicochemical natures of the permeating species and the polymer membrane.     

Permeation of hydrocarbon gases through polymer membranes

Experimental results are presented for the permeation of propane and ethylene through polyethylene membranes and for the permeation of propane through “Saran” in the temperature range from —10 to 20°C and the pressure range from 20.0 to 50.0 psig. The flux of propane through polyethylene exhibits a minimum as the temperature approaches the condensation temperature. No such minimum was observed either for the flux of ethylene through polyethylene or for propane through “Saran”. The permeability coefficients for propane through polyethylene are compared with those predicted on the assumption of a diffusion coefficient exponentially dependent on the concentration of permeability.     

Separation of liquid mixtures by using polymer membranes. II. Water-alcohol separation by pervaporation through porous PVA membranes

The permeation and separation characteristics of alcohol/water systems through porous PVA membranes were investigated. Porous PVA membranes with different pore size and number were prepared by solution blending of PVA with several synthesized polymers or copolymers, such as polyacrylic acid, polyacrylamide, polyacrylonitrile, and methylmethacrylate-co-maleic anhydride, etc. Then casting, and finally extracting the blended polymers or copolymers by solvent from the membranes. The dependency of both permeation and separation on the molecular size and shape of the permeating species was dicussed qualitatively. Moreover, the permselectivity was investigated with attention to the feed composition of alcohol/water mixture and the effect of pore size and number. The selectivity was found to depend on the weight ratio and the molecular weight of polymer introduced to the membrane. When the weight ratio of polymer introduced into the membrane was larger than 0.1, methanol was permeated through membrane preferentially in methanol/water system, and the separation factor increased with increasing the methanol feed concentration. On the other hand, membrane had a selective permeability for water in the other alcohol/water systems. The influence of operating conditions was also studied.     

Separation of liquid mixtures by using polymer membranes. II. Permeation of aqueous alcohol solutions through cellophane and poly(vinyl alcohol)

The permeation and separation characteristics of four different alcohol—water systems through cellophane and poly(vinyl alcohol) membranes were investigated. The homologous series of linear alcohols n-propanol, ethanol, and methanol as well as isopropanol were studied. A specially designed permeation cell was used to study permeation rates at temperatures ranging from 30° to 50°C. The dependency of both permeation and separation on the molecular size and shape of the permeating species was discussed qualitatively. The temperature dependence of the permeation rate for both pure compounds and binary mixtures was expressed by Arrhenius-type relationships. The rate was found to increase with increasing temperature while the separation decreased. Activation energies of 4–9 kcal/mole were calculated for alcohol—water solutions through cellophane, and of 8–15 kcal/through poly(vinyl alcohol). Departure of permeation rates from the ideal rates were discussed in terms of permeation “enhancement” or “depression.” These phenomena were explained in terms of both the plasticizing action of water and the “clustering” of water molecules within the polymer network.     

Nanoporous organic polymer networks

Nanoporous organic polymer networks are a class of materials consisting solely of the lighter elements in the periodic table. These materials have potential uses in areas such as storage, separation, and catalysis. Here, we review the different classes of nanoporous polymer networks including covalent organic frameworks, hypercrosslinked polymers, conjugated microporous polymers, and polymers of intrinsic microporosity. The growing variety in synthetic routes to these materials allows a range of different polymer networks to be formed, including crystalline and amorphous structures. It is also possible to incorporate many different kinds of functional groups in a modular fashion. So far, most networks have been examined from the perspective of gas sorption, and this area is discussed critically and in depth in this review. The use of nanoporous organic polymers for applications such as catalysis and separations is an important developing area, and we discuss recent developments as well as highlighting potential future opportunities.     

Permeation of benzene through liquid surfactant membranes

The effect of time on the permeation of benzene through aqueous surfactant membranes formed with two non-ionic surfactants has been studied on a batch scale under two different conditions of surfactant concentration and hydrocarbon: water ratio in the emulsions. The data have been correlated by an equation based on Casamatta's model of hydrocarbon permeation from an emulsion drop. The results indicate that the correlation can be used to treat the data until the onset of membrane rupture. The membrane thickness depends on the hydrocarbon: water ratio in the emulsion, and varies from about 1.57 to 5.10 μm.     

Permeation of ethane–butane mixtures through polyethylene

Permeation of ethane–butane mixtures a t atmospheric pressure through polyethylene was investigated in the temperature interval of 30–60°C. The integral permeation constant P? and the integral diffusion coefficient D? of both ethane and butane were satisfactorily correlated by using an exponential and also a linear function of butane concentration. This was attributed to the plasticizing of the film by butane, whose solubility constant is approximately ten times that of ethane in the temperature interval investigated. The separation factor G_{i, j} defined as the ratio of the permeation constant of butane to the permeation constant of ethane over the range of mixture compositions and temperatures investigated, remained nearly constant at values in the range of 2.8–3.2. The insignificant change in G_{i, j} is due to the proximity of the activation energies of permeation of ethane and butane (10.95 and 10.75 kcal/mol, respectively) and the similar magnitude of change in the permeation constants with increasing butane concentration. Experiments were run in which the film was initially in equilibrium with the same partial pressure of butane as in the mixture to be studied. It can be shown that the diffusion coefficient of ethane in this case is related to the following function of film position: D = D(o)(β₀ + β₁x)^δ. The time lag of diffusion of ethane in these experiments decreased with inrreasing butane concentration.     

Separation of liquid mixtures by using polymer membranes. III. Water–alcohol separation by pervaporation through modified PAN membrane

The modification of polyacrylonitrile membrane with ethanolamine was carried out, and the permeation characteristics in pervaporation were examined using the aqueous alcohol solutions. In pervaporation of a water/alcohol solution, preferential permeation of water was observed for all these membranes because of the hydrogen-bonding interaction. The selectivity of the modified polyacrylonitrile (PAN) membrane depended on operating temperature, but was independent on the thickness of the membrane. Furthermore, it was found that the membrane with more ethanolamine content had a higher affinity to water. The effect of feed concentration and the molecular size of the permeating species on the separation factor and permeation flux was also investigated.     

新型有机分离体系渗透汽化膜材料

综述了固载活性基团促进传递膜、新型共聚物和树枝型聚合物材料在提高膜的选择性与渗透通量中的作用.介绍了从分子结构角度设计的材料在提高分离效果与成膜稳定性中的应用,指出了新型高聚物膜材料在渗透汽化分离有机混合物领域具有较好的工业化应用前景.     

Permeation and separation characteristics of alcohol–water mixtures through poly(dimethyl siloxane) membrane by pervaporation and evapomeation

Permeation and separation characteristics for aqueous alcoholic solutions such as methanol/water, ethanol/water and 1-propanol/water were studied using a poly(dimethyl siloxane) membrane by pervaporation and evapomeation. Poly(dimethyl siloxane) membrane preferentially permeated alcohol from aqueous alcoholic solutions in both methods. The concentration of alcohol in the permeate by evapomeation was higher than that by pervaporation. However, the permeation rate for the former method was smaller than that for the latter method. In evapomeation with a temperature difference between the feed solution and the membrane surroundings, when the temperature of the membrane surroundings was kept constant and the temperature of the feed solution was raised, both the permeation rate and the permselectivity for ethanol increased with increasing temperature of the feed solution. On the other hand, as the temperature of the feed solution was kept constant and the temperature of the membrane surroundings was changed, the permeation rate decreased, but the permselectivity for ethanol increased remarkably with dropping temperature in the membrane surrounding. Under permeation conditions of a feed solution of 40°C and a membrane surrounding temperature of ?30°C in evapomeation, an aqueous solution of 10 wt % ethanol in the feed was concentrated to about 90 wt % in the permeate. The permselectivity for alcohol was in the order of methanol <ethanol <1-propanol. The above permeation and separation characteristics are discussed from the viewpoint of the physicochemical properties of the poly(dimethyl siloxane) membrane and the permeating molecules.     

Separation of liquid mixtures by using polymer membranes. I. Water–alcohol separation by pervaporation through PVA-g-MMA/MA membrane

A membrane was designed for the separation of a water–alcohol mixture by pervaporation on the basis of the difference in hydrogen-bonding interaction between two components of the membrane. Two kinds of poly(vinyl alcohol)-g-maleic anhydride/methyl methacrylate membrane were prepared by different methods: (1) A homogeneous membrane was formed by casting from dimethyl sulfoxide solution after purification. (2) A porous membrane was obtained directly by casting from the reaction solution, and then was purified. It is found that water was permeated through a homogeneous membrane preferentially in all ranges of feed compositions. Moreover, the flux was found to decrease with increasing PVA content in the membrane. The porous membrane after heat treatment has selective permeability for methanol. Pervaporation of water was investigated with respect to the feed concentration and also to the operating conditions. The effect of the molecular size of the permeating species on both permeation and separation is also discussed.     

Computer simulation of phase separation in binary polymer mixtures

Phase separation of binary polymer mixtures was numerically simulated by solving the time-dependent Langevin equation with Flory–Huggins free energy in two dimensions by using a finite difference method. Spinodal decomposition following structure coarsening was calculated. Simulation results were verified by evaluating the evolution of the wave number obtained from the calculated phase structure by Fourier transformation. Then, computer experiments were carried out to investigate effects of volume fraction and polymer characters, the number of segments, and solubility parameter on morphology. The phase separation time, when the phase began to separate, decreased with deviating volume fraction from 0.5 and with decreasing number of segments and difference between solubility parameters. The difference between solubility parameters had the largest influence on the phase separation time among them and had two effects, the acceleration of phase separation and the restriction of structure coarsening. © 1995 John Wiley & Sons, Inc.     

渗透汽化技术在液体分离中的研究新进展

渗透汽化是近年膜科学研究中最活跃的领域之一,在分离液体混合物,尤其是痕量、微量物质的移除,近、共沸物质的分离等方面有独特优势。简要介绍了渗透汽化技术的历史及发展过程,着重介绍了近几年渗透汽化技术在液体分离方面的研究进展。从研究结果来看,从水溶液中分离有机物研究主要集中在醇、酯的分离。在有机物脱水方面,开发出了选择性更好、渗透量更高的膜,乙醇脱水也有不少新的研究成果。特别是在有机混合物分离方面,取得了长足的进展,可以分离更多的有机物。