ReviewPolymeric membrane pervaporation
Introduction
Pervaporation is a membrane process for liquid separation [1], [2], a polymeric or zeolite membrane [3], [4], [5] usually serves the separating barrier for the process. When a membrane is in contact with a liquid mixture, one of the components can be preferentially removed from the mixture due to its higher affinity with, and/or quicker diffusivity in the membrane. As a result, both the more permeable species in the permeate, and the less permeable species in the feed, can be concentrated. In order to ensure the continuous mass transport, very low absolute pressures (e.g., 133.3–400.0 Pa (1–3 mmHg)) are usually maintained at the downstream side of the membrane, removing all the molecules migrating to the face, and thus rendering a concentration difference across the membrane. As a variant, the use of a sweeping gas [6], [7] in the downstream side of the membrane is also a feasible alternative for the generally used vacuum operation.
It is well known that the phase change from liquid to vapor takes place in pervaporation. Processes involving phase changes are generally energy-intensive, and distillation is a notorious example of them. Pervaporation cleverly survives the challenge of phase change by two features. (1) Pervaporation deals only with the minor components (usually less than 10 wt.%) of the liquid mixtures, and (2) pervaporation uses the most selective membranes. The first feature effectively reduces the energy consumption of the pervaporation process. Compared with the distillation, because of the characteristics of pervaporation operation, it is essentially true that only the minor component in the feed consumes the latent heat. The second feature generally allows pervaporation the most efficient liquid-separating technology. Take the separation of isopropanol/water mixtures for example, if the water content in the feed is 10 wt.%, the maximum single plate separation factor (isopropanol to water) in distillation is about 2, however, a pervaporation membrane can normally offer an one-through separation factor (water over isopropanol) of 2000–10,000 [8], [9], [10]. Furthermore, combination of these two features ranks pervaporation the most cost-effective liquid separation technology [11], [12]. In addition, pervaporation also demonstrates incomparable advantages in separating heat-sensitive, close-boiling, and azeotropic mixtures [13], [14], [15], [16] due to its mild operating conditions, no emission to the environment, and no involvement of additional species into the feed stream. More recently, the hybrid processes [12], [17], [18], [19] integrating pervaporation with other viable liquid-separating technologies, and processes are gaining momentum. With these developments, we have more reasons to believe that pervaporation will play even more important roles in the future.
To date, pervaporation has found viable applications [20] in the following three areas: (i) dehydration of organic solvents (e.g., alcohols, ethers, esters, acids); (ii) removal of dilute organic compounds from aqueous streams (e.g., removal of volatile organic compounds, recovery of aroma, and biofuels from fermentation broth); (iii) organic–organic mixtures separation (e.g., methyl tert-butyl ether (MTBE)/methanol, dimethyl carbonate (DMC)/methanol). Among them, dehydration of organic solvents is best developed. This resulted from the so-called synergic effect [21]: water is both preferentially dissolved and transported in the hydrophilic membranes due to its much smaller molecular size. When pervaporation is used for removing organic compounds from water, the preferential transport of the organic species cannot be achieved in the organophilic membrane. As a result, the permselectivity of the pervaporation process is reduced, and less than the sorption selectivity. Theoretically speaking, pervaporation in these cases demonstrates no advantage over the adsorption technique. However, when the concentration of organic compounds in water is relatively high, pervaporation tends to be superior to the adsorption technology since pervaporation is a continuous process, it therefore suffers no limitation of the saturated adsorption capacity, which is however an intrinsic weakness of the adsorption process.
Separation of organic–organic mixtures represents the most challenging application for pervaporation [22]. Most liquid pairs in this category are of industrial importance [19], such as polar/non-polar, e.g., methanol/MTBE [23], [24], aromatic/aliphatic, e.g., benzene/n-hexane [25], [26], aromatic/alicyclic, e.g., benzene/cyclohexane [27], [28], and isomers, e.g., p-xylene, m-xylene, and o-xylene [29], [30], [31]. Research in this category gained some successes in the separation of polar/non-polar liquid mixtures as shown previously, but has not yet seen much significant progress in the other liquid pairs.
To date, several reviews [2], [11], [12], [17], [20], [22], [32], [33] on pervaporation have been available. A detailed review on the zeolite membrane pervaporation had also been conducted by Bowen et al. [11], this review will thus focus on the polymeric membrane pervaporation, with the emphasis given to the fundamental understanding of the membranes, where an analytical overview on the potential of the pervaporation for separating various liquid pairs was presented, the challenges and opportunities, and the prospect of pervaporation was also tentatively discussed.
Section snippets
Pervaporation in terms of the Hansen solubility parameter and the kinetic diameter
If reviewing the evolution of pervaporation, one may recognize the fact that the Hansen solubility parameter exerted incomparable influence on the development of pervaporation. The Hansen solubility parameter refers to the density of cohesive energy [34], which consists of three components: δh: the contribution of the hydrogen bonding interaction; δp: the contribution of the polar interaction; δd: the contribution of the dispersion interaction. The solubility parameter can thus be represented
Solution-Diffusion theory
Solution-diffusion is the generally accepted mechanism of mass transport through non-porous membranes, which was first proposed by Graham [38] based on his extensive research on gas permeation through homogeneous membranes. It is held that gas permeation through a homogeneous membrane consists of three fundamental processes: (1) solution of gas molecules in the upstream surface of the membrane. (2) Diffusion of the dissolved species across the membrane matrix. (3) Desorption of the dissolved
A closer view of the pervaporation membranes
Generally speaking, because of the presence of strong membrane–species interactions, the pervaporation membrane can no longer be treated as a uniform medium for permeation. Shimidzu and co-workers [62], [63], [64] believed that the polar groups in the membrane matrix, responsible for the membrane hydrophilicity, act as the fixed carriers for mass transport in the membrane. In the case of dehydration of organic solvents, it is believed that water transport in the membrane proceeds in a special
The coupled transport in pervaporation membranes
The coupled transport is a frequently observed mass transport phenomenon in pervaporation membranes [68], [69], [70], [71]. According to Mulder et al. [72], there are generally two types of coupled transport, i.e., the thermodynamic, and kinetic coupling. The thermodynamic coupling results from the interaction between the dissolved species in the membrane. As is well known that the Gibbs free energy of one species can be changed by the presence of other species, and the changes in the free
Structural stability of composite pervaporation membranes
Pervaporation membranes fall into two categories, the homogeneous membrane and the composite membrane. The composite membrane can offer a higher permeation flux than the homogeneous one due to the much thinner thickness of the homogeneous membrane supported on a porous substrate. The composite membrane is thus suitable for industrial use. Ideally, the porous substrate of a composite membrane presents negligible resistance to mass transport [73]. Otherwise, the substrate resistance leads to
Organic solvent dehydration
Dehydration of organic solvents (e.g., alcohols, ethers, acids, and ketones) largely represents the applications of the pervaporation [90], [91], [92], [93], [94]. The materials used in earlier dehydration research were the naturally occurring polymers, e.g., cellulose and cellulose derivatives [1]. Synthetic polymers [95], [96], e.g., poly(acrylic acid) (PAA), poly(vinyl alcohol) (PVA), polyacrylonitrile (PAN), and nylon 6, were subsequently investigated, with a focus mainly on ethanol
Commercial and engineering aspects of pervaporation
Since the commercialization of pervaporation for ethanol dehydration launched by GFT in 1980s based on the cross-linked PVA/PAN composite membrane, both the scope of application and the types of the pervaporation membranes were extensively enlarged [166]. According to the website of Sulzer Chemtech, a wide array of solvents have been covered in its dehydration market, which includes various alcohols, ethers, ketones, acids, and some polymer solvents like THF, dioxane, etc. The SULTZER PERVAP®
Concluding remarks
Pervaporation has played an important role in solvent dehydration, and this application can be further extended by integrating pervaporation with other viable liquid-separating technologies, and by finding right materials for dehydrating some caustic solvents (e.g., nitric acid). Higher membrane productivity and selectivity is always a concern for application, and this was conventionally accomplished by operating the pervaporation membranes at higher temperatures. The improved membrane flux can
Acknowledgement
The financial support from Natural Sciences and Engineering Research Council (NSERC) of Canada is gratefully acknowledged.
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