Robert Clément

Industrial state-of-the-art of pervaporation and vapour permeation in the western countries

This work reviews the current trends of industrial pervaporation (PV) and vapour permeation (VP) in Europe and the USA after the great changes that have been occurring since 1995. More than simply being an up-to-date state-of-the-art, the paper provides key data and information about patented applications and current suppliers for PV and VP membranes and membrane separation systems. It also discusses several examples for PV and VP industrial applications and points out many inherent advantages of these separation processes as compared to other more conventional approaches, in particular in terms of very significant savings in energy and raw materials. On the basis of information provided by the leading industrial companies in the field, the past trends and future prospects of PV and VP are eventually analysed to help draw new guidelines for the promotion of these technologies in the near future.

Sorption, diffusion and vapor permeation of various penetrants through dense poly(dimethylsiloxane) membranes: a transport analysis

Abstract A detailed analysis of the transport of chloroform, isomeric butanols, methanol and water vapors at 40°C through dense silicone rubber membranes has been undertaken by the vapor permeation technique. The variation of fluxes versus upstream activity, coupled to sorption isotherms of the investigated compounds in silicone rubber at 40°C (determined by the swelling technique), offer the possibility to determine the sorption and diffusion steps in the overall permeant flux. It is shown that the chloroform isotherm follows the Flory-Huggins theory, while alcohols require a more sophisticated approach (Koningsveld and Kleinjtens type) in order to fit the isotherms. The water sorption isotherm shows a typical sigmoid shape which could not be fitted correctly by classical sorption equations. The diffusion coefficient of chloroform in PDMS appears to be almost unaffected by solvent concentration in the membrane, similarly to the results obtained with other good solvents in PDMS. Diffusion coefficients of alcohols and water show, however, a common tendency to decrease with their local concentration. An interpretation of the sorption isotherms based on the cluster integral of Zimm and Lundberg suggests some clustering behavior, which seems consistent with the diffusion coefficient variation. The clustering tendency of the solvent molecules in the PDMS matrix was related to their proton donating power, which follows the sequence: water > methanol > 1-butanol > 2-butanol > 2-methyl-2-propanol > chloroform.

Permeability of block copolymers to vapors and liquids

Abstract Permeability of block copolymers is an important feature for a broad range of applications including packaging, bio-materials (e.g. for controlled release or encapsulating membranes), barrier materials, high performance impermeable breathable clothing and membrane separation processes. This paper reviews the literature on permeability of block copolymers to pure or mixed vapors and liquids for the past 40 years. Different types of block copolymers are considered, starting with the most investigated polyurethanes and polyurethaneureas followed by polyimides, polyamides and miscellaneous types of block copolymers (e.g. block copolymers containing siloxane segments, hydrocarbon block copolymers and related materials). Block copolymers offer a great structure versatility which is highly interesting for a fundamental analysis of permeation through polymeric materials. A special attention has been paid to the key permeability properties and their correlation to the chemical structure of block copolymers. Systematic structure/property relationships have been emphasized to promote the future design of block copolymers with improved permeability properties. Far from being limited to fundamental investigations, this literature review points out the originality of block copolymers, which combine a great structure versatility with several high potential industrial prospects. Many patents on block copolymers indeed reflect a strong industrial interest in applications based on their (selective) permeability. A sharp increase in the related patents for the past 10 years certainly shows the strong potential of block copolymers to take up several current challenges in the medical, chemical and petrochemical fields.

Pervaporation of aqueous ester solutions through hydrophobic poly(ether-block-amide) copolymer membranes

The pervaporation of three ethyl esters (aroma model compounds) in aqueous solutions was studied with poly(ether-block-amide) copolymer (PEBA) membranes of different compositions. The ester flux increases linearly with the ether-unit (tetramethylene oxide) content. The slope of the increase changes from a low to a higher value at 50 wt% polyether content. This behavior is explained by a change in the membrane structure from a continuous polyamide matrix structure to a polyether matrix one. The ester flux from saturated aqueous solutions increases from butyrate to propionate, then to acetate. This order differs from that of the poly(dimethylsiloxane) (PDMS) membrane whose flux was highest for butyrate and lowest for propionate ester. The PEBA membrane permeability and selectivity to the esters increase significantly with the polyether content. However, the best PEBA membrane had lower flux and selectivity to esters in aqueous solutions than the PDMS membrane. The activation energy for ester permeation was always lower than that for water permeation; its value was the highest for the PEBA membrane with the highest polyamide content.

Sorption of organic solvents into dense silicone membranes. Part 1.—Validity and limitations of Flory–Huggins and related theories

Sorption isotherms of various organic solvents and aqueous mixtures into dense polydimethylsiloxane (PDMS) membranes have been analysed and the results are compared with Flory–Huggins theory, already reported to hold for solvent sorption into elastomeric materials. The original Flory–Huggins equation, as well as more complex versions derived later by different authors (variable interaction parameter, elastic contribution, modified entropic contribution) have been screened; it appears that although excellent agreement is offered by the Flory–Huggins simplest expression for good PDMS solvents (e.g. hydrocarbons and halogenated hydrocarbons), none of the modified Flory–Huggins theories leads to satisfactory data interpretation for poor solvents (ketones, alcohols), unless at least three adjustable parameters are used for the enthalpic contribution term. Cluster formation at high solvent activity, considered on the basis of Zimm–Lundberg clustering analysis, is supposed to explain the observed deviations for poor solvents. A semi-empirical power law (Freundlich type) gives the best fit in the latter case.

The engaged species induced clustering (ENSIC) model: a unified mechanistic approach of sorption phenomena in polymers

Abstract The description of sorption equilibria of solvents in polymers is a key problem which has to be solved in order to achieve a correct understanding of transport processes through dense membranes. Up to now, this purpose is limited essentially to solvents in elastomers, which are described by Flory-Huggins thermodynamics theory, while sorption of gases in glassy polymers is usually based on a mechanistic one, the so-called dual mode model. Given the difficulties encountered as soon as polymer solvent peculiarities (non-regular mixing enthalpy, network elastic contribution) are taken into account, other types of mixtures can hardly be described by purely thermodynamic models unless complicated expressions are used. A simple mechanistic approach has been developed in order to circumvent these limitations; it is based on the assumption that insertion of a solvent molecule into the polymer solvent matrix will be governed by the intrinsic affinity of the solvent for either a polymer segment or an already sorbed solvent molecule. The model enabling cluster formation description has been named engaged species induced clustering (ENSIC). A series of data related to the most frequently used polymer dense membrane materials have been used in order to check the fitting efficiency of the newly developed expression; it is shown to give a very good description of sorption isotherms of many binary polymer solvent systems, including polar and apolar compounds, in either elastomeric, glassy or thermoplastic membrane materials. The physico-chemical interpretation of the two probabilistic insertion parameters used in the ENSIC model, as well as prospects concerning the extension to multicomponent systems or transport mechanisms simulations are discussed.

Application of Flory-Huggins theory to ternary polymer-solvents equilibria: A case study

Abstract The ternary equilibrium data of the following systems: benzene/heptane/NBR at 60 °C (system 1), heptane/isooctane/PE at 25 °C (system 2), ethanol/water/CA at 20 °C (system 3) and ethanol/water/ P(E-co-VAc) at 32 °C (system 4), already reported in the literature, have been used and experimental results compared to predictions offered by the Flory-Huggins theory applied to ternary mixtures (one polymer and two liquids), with constant interaction parameters and negligible elastic contribution. Polymer-solvent interaction parameters have been determined from swelling in pure liquid, while liquid-liquid interaction parameters have been estimated from liquid-vapour equilibrium data curve fitting. It is shown that the Flory-Huggins theory offers reasonable prediction in the case of apolar liquids in an elastomeric matrix (system 1), while approximate isotherm patterns with significant discrepancies are obtained with apolar liquids in a thermoplastic (system 2). The equilibrium data of polar liquids in either homopolymer or copolymer (systems 3 and 4) can hardly be achieved by Flory-Huggins theory, even when a variable liquid-liquid interaction parameter is used. Implications in ternary diagram simulations and possible prediction improvements are discussed.

Pervaporation of water-ethanol mixtures through a poly(acrylic acid) grafted polyethylene membrane. Influence of temperature and nature of counter-ions

Both the permeability and the selectivity to water of poly (acrylic acid) grafted polyethylene films increase when the counter-ion is changed from Li+ to Na+ or K+. The acid membrane behaves in a singular manner: high permeability at low water contents, slight selectivity to water for ethanol-rich mixtures, reversal in selectivity at 60 wt.% of water (at 60°C) and slight selectivity to ethanol in water-rich mixtures. Both partial water and ethanol fluxes increase with pervaporation temperature, according to an Arrhenius-like relationship, but with different activation energies. The activation energy for ethanol permeation is always higher than that for water permeation, but both of them decrease with increasing water content and when the counter-ion is changed from Li+ to Na+ or K+. As a consequence, selectivity to water is strongly reduced by an increase in pervaporation temperature, especially for the K+ membrane. Most of the results can be interpreted on the basis of the extent of ionization of the carboxylate groups, and the effects of swelling on the activation energy.

Sorption of organic solvents into dense silicone membranes. Part 2.—Development of a new approach based on a clustering hypothesis for associated solvents

The difficulties encountered in fitting sorption isotherms of associated liquids (alcohols, ketones) in polydimethylsiloxane (PDMS) by Flory–Huggins and related theories have been reported in a previous work; cluster formation, expected to explain these deviations, has been examined according to various approaches already reported to take into account such a phenomenon (polycondensation in an inert matrix, dimer formation). Given the poor results obtained by these approaches, a new equation, based on elementary affinity constants between polymer and sorbed solvent species in a lattice graph has been tested. Results obtained by fitting the resulting two-parameter equation to alcohol–PDMS results show the ability of the proposed equation to fit precisely the isotherms. Furthermore, pictures of the polymer–solvent lattice can be easily obtained by this approach, by computer simulation. Generalizations of the novel approach to other polymer solvent systems as well as to multicomponent systems are discussed. Diffusion studies on the basis of this heterogeneous matrix picture could possibly be undertaken later.

Sorption and diffusion of solvent vapours in poly(vinylalcohol) membranes of different crystallinity degrees

Solvent sorption and diffusion are the key processes that control membrane performances in membrane processes. The sorption characteristic of water and ethanol vapours in poly(vinylalcohol) (PVA) membranes of different crystallinity degrees was measured by microgravimetry and the diffusion characteristic was calculated from the sorption kinetics at different water activities by curve fitting. The sorption isotherms for water vapour in membranes of 28, 37, 44 and 56% crystallinity degrees at 40°C obey the Flory equation based on the polymer lattice model. When the sorption extent was corrected by assuming that only the polymer amorphous phase is accessible to the penetrant, a unique Flory χ interaction parameter, 0.3, was obtained for all samples except for the 28% crystallinity sample. For the latter sample, the lower χ value (0.18) obtained can be explained by a change in the sorption behaviour of the original crystalline domains which may undergo partial destruction. The diffusion coefficient increases with the average water content in the membrane according to an exponential relationship characterized by a limit diffusion coefficient and a plasticization coefficient. The higher the crystallinity of the membrane, the lower the values of the limit diffusion coefficient and the plasticization coefficient. The ethanol sorption was also well described by the Flory-Huggins equation. The limit diffusion coefficient for water was two orders of magnitude larger than that for ethanol.