Highlighting Thermodynamic Coupling Effects in Alcohol/Water Pervaporation across Polymeric Membranes

Abstract

The pervaporation of binary alcohol/water mixtures across polymeric membranes is modeled by combining the Maxwell–Stefan (M-S) diffusion formulation with the Flory–Huggins (F-H) description of sorption equilibrium. The combined M-S/F-H model shows that the flux of each penetrant species is coupled to the driving force of its partner penetrant. Two types of coupling contributions can be distinguished: (i) coupling arising out of correlated motions of penetrants in the polymer matrix and (ii) thermodynamic coupling. The focus of this article is on the contribution of thermodynamic coupling, which is quantified by the set of coefficients , where ai, the activity of species i, is dependent on the volume fractions ϕi,ϕj, of both penetrants in the polymeric membrane. Detailed analyses of published experimental data for pervaporation of ethanol/water feed mixtures of varying compositions in both hydrophobic (poly(dimethylsiloxane)) and hydrophilic (cellulose acetate, polyimide, and polyvinyl alcohol/polyacrylonitrile composite) membranes show that in all cases, the cross-coefficients Γij (i ≠ j) are negative and may attain large magnitudes in relation to the diagonal elements Γii. The net result is that the permeation fluxes of each penetrant are suppressed by its partner, resulting in mutual slowing down of permeation fluxes. If thermodynamic coupling effects are ignored, significantly higher fluxes are anticipated than those that are experimentally observed.

Water/Alcohol Mixture Adsorption in Hydrophobic Materials: Enhanced Water Ingress Caused by Hydrogen Bonding

“…Due to the formation of azeotropes, distillation of a water/alcohol mixture is energy-intensive. The use of membrane pervaporation devices in hybrid distillation-membrane processing schemes offers energy-efficient alternatives in production of purified alcohols. − The membranes may be constructed as thin films of zeolites (e.g., CHA, , DDR, , LTA, MFI , ) or zeolitic imidazolate frameworks (ZIFs) . The permeation selectivity of membrane constructs, S perm , is dictated by a combination of the adsorption selectivity, S ads , and the diffusion selectivity, S diff . − The alcohol/water adsorption selectivity, S ads , is defined by where q 1 and q 2 are the molar loadings of the water and alcohol, respectively, in the adsorbed phase in equilibrium with a bulk fluid phase mixture with partial fugacities f 1 and f 2 and composition y 1 = 1 – y 2 .…”

Highlighting Thermodynamic Coupling Effects in the Immersion Precipitation Process for Formation of Polymeric Membranes

“…At any time t , during the immersion precipitation process, we have thermodynamic equilibrium at the interface between the two immiscible phases, at compositions A* and CB*. The volume fractions ϕ i I and ϕ 1bI are determined by the thermodynamic equilibrium constraints The Flory–Huggins (F–H) description of phase equilibrium thermodynamics − ,− is used to solve the set of eq ; calculation details and F–H input parameters for all investigated systems are provided in the SI.…”

“…The primary objective of this article is to trace the theoretical origins of the diffusional foray into the metastable region as witnessed for the trajectory C–C*. For this purpose, we set up a model to describe the equilibration trajectories by combining the Flory–Huggins description of phase equilibrium with the Maxwell–Stefan formulations for diffusion. − By detailed analysis of four different systems: water/acetone/CA, water/DMF/PVDF, water/NMP/PSF, and water/NMP/PEI (CA = cellulose acetate; PVDF = poly­(vinylidene fluoride); PSF = polysulfone; PEI = polyetherimide; DMF = dimethyl formamide; NMP = N -methyl-2-pyrrolidone), we aim to show that the diffusional forays into metastable regions are primarily engendered by thermodynamic coupling effects that are quantified by the off-diagonal elements Γ ij ( i ≠ j ). The secondary objective of this article is to demonstrate that the same concepts and modeling approaches are applicable to describe diffusion-induced ingress into metastable regions, resulting in emulsification − and production of nanospheres and nanoparticles by the exploitation of the “Ouzo effect”. − …”

Merging Proline:Xylitol Eutectic Solvent in Crosslinked Chitosan Pervaporation Membranes for Enhanced Water Permeation in Dehydrating Ethanol

“…Here, the solvent's polarity becomes important for its own transport and separation [32]. Apart from that, the diffusion of water across the membranes can increase with an increase in feed water activity, which is ascribed to the plasticization effect on the membrane due to the increase in the free volume between the polymer chains, facilitating the diffusion of the molecules through the membrane matrix [41,42]. Fundamentally, the first transport resistance in a PV system is generally assumed to be in the membrane itself, according to its non-porous structure.…”