On the mechanism of separation of ethanol/water mixtures by pervaporation I. Calculations of concentration profiles

A solution—diffusion model for the permeation of liquid mixtures through polymeric membranes taking into account coupling of fluxes has been developed. The model is applied to the separation by pervaporation of ethanol—water mixtures through cellulose acetate. In order to determine the activities of the permeating components in the polymeric membrane, values of polymer—liquid and liquid—liquid interaction parameters are needed; polymer—liquid interaction parameters have been determined from swelling experiments and liquid—liquid interaction parameters have been calculated from excess free energy of mixing data taken from the literature.\ud \ud Concentration profiles of water and ethanol in cellulose acetate membranes have been calculated using (a) apparent concentration independent diffusion coefficients, and (b) diffusion coefficients with exponential concentration dependence and two adjustable parameters. It is discussed that the transport of ethanol—water mixtures by pervaporation cannot be explained by using concentration independent diffusion coefficient

Gas separation properties of a thermally stable and chemically resistant polytriazole membrane

The polymer poly (1,3-phenyl-1,4-phenyl)-4-phenyl-1,3,4-triazole has been investigated for its gas separation properties. This thermally stable and chemically resistant polymer can be processed into membranes by the phase-inversion technique because of its unexpectedly good solubility in formic acid. Homogeneous membranes have been tested with respect to their permeability for several gases, and the influence of time and temperature upon permeation has been investigated. The polymer shows reasonable permeabilities for several gases and excellent selectivities. After a conditioning time of several days in which the permeability of the faster-moving gases increases by a factor of about 2, the permeation properties of the polymer remain constant for at least two months. A thermal treatment at 295°C, just above the glass transition temperature, can reduce the conditioning time and can prevent the film from shrinkage at high permeation temperatures without affecting the permeation properties

Cyclo dehydration reaction of polyhydrazides. II. Kinetic parameters obtained from isothermal thermogravimetry

The kinetics of the thermal conversion reaction of poly-(1,3-phenyl-1,4-phenyl)-hydrazide into poly-(1,3-phenyl-1,4-phenyl)-1,3,4-oxadiazole have been studied with isothermal thermogravimetry in continuation of a study with nonisothermal thermogravimetry described in a previous paper. Although the isothermal measurements are much more time-consuming, they provide some new information and insight about the cyclo dehydration reaction of the polyhydrazide. The physical state of the sample, rubbery or glassy, seems to influence the kinetics considerably. The kinetic parameters determined with the isothermal method for the polymer in its glassy state agree well with the parameters derived from the previously reported nonisothermal measurements, while the kinetic parameters for the expected rubbery state differ considerably. The morphological state or the history of the polymer has also a considerable influence on the kinetics of the isothermal conversion process. The powder form of the polymer has a much lower isothermal conversion rate than the film form

Thermal behavior of polytriazole films: a thermal analysis study

The thermal behavior of poly(1,3-phenyl-1,4-phenyl)-4-phenyl-1,2,4-triazole has been investigated using different scanning calorimetry (DSC) and thermogravimetry (TG). Processes are studied for this thermally stable polymer that take place between 200 and 500°C. While the polycondensation reaction product in powder from appeared to be partially crystalline, films prepared by casting from a formic acid solution appeared to be completely amorphous. A thermal treatment between Tg(~ 270°C) and Tm(~430°C) can introduce crystallinity in the films because of the polymer's ability to cold crystallize. The cold crystallization temperature Tc seems to be dependent on the preparation history of the solid polymer phase. Thermal annealing of the films just below Tg does not introduce crystallinity but inhibits subsequent cold crystallization at higher temperatures. Crystallization upon cooling from the crystalline melt has not been observed either. At temperatures just above the crystalline melting point the polymer starts to decompose in an exothermic reaction

Phase separation phenomena during the formation of asymmetric membranes

The formation of membranes from two systems has been studied. In the system polyurethane-dimethylformamide-water, the mechanism for the formation of the sponge-like structure proves to be a liquid-liquid phase separation with nucleation and growth of the diluted phase. This mechanism has been confirmed for the system modified polystyrene-polyisoprene-polystyrene/o-dichlorobenzene/(methanol-water). Crystallization and gelation is discussed. The membranes prepared showed hyperfiltration activity. The mechanism proposed here is believed to be valid for other systems, too

Preferential sorption versus preferential permeability in pervaporation

Transport of liquids by pervaporation takes place by a solution—diffusion mechanism. In order to investigate the “solution part” of this transport model, preferential sorption has been compared with preferential permeability. Sorption equilibria and pervaporation experiments for the systems water—ethanol—cellulose acetate, water—ethanol—polyacrylonitrile and water—ethanol—polysulfone have been investigated. Theoretical values of preferential sorption have been derived from Flory—Huggins thermodynamics, extended with concentration dependent interaction parameters. These calculated sorption values show a reasonable agreement with experimental values. The large difference in molar volumes between water and ethanol determines the preferential sorption of water in these systems to a great extent, and this effect increases with decreasing swelling value. Comparison of preferential sorption experiments with pervaporation experiments indicates that, apart from the effect of differences in diffusivity for the permeating components, preferential sorption contributes to a major extent to selective transport

On the mechanism of separation of ethanol/water mixtures by pervaporation II. Experimental concentration profiles

Ethanol—water concentration profiles in cellulose acetate membranes were measured under steady-state pervaporation conditions. Knowledge of these profiles leads to a better understanding of the diffusion process during pervaporation. The concentration profiles were determined by a film-stack method, using three to six layers. It is shown that permeation of ethanol—water mixtures proceeds in a coupled way and that crossterm diffusion coefficients need to be considered. Furthermore, the occurrence of sorption resistances at the feed/membrane interface can be established from these experiment

Cyclo dehydration reaction of polyhydrazides. I. Kinetic parameters obtained with nonisothermal thermogravimetry

The thermal conversion reaction of poly-(1,3-phenyl-1,4-phenyl)-hydrazide into poly-(1,3-phenyl-1,4-phenyl)-1,3,4-oxadiazole has been studied using thermogravimetry (TG). For the evaluation of the energie of activation and other kinetic parameters of this cyclo dehydration reaction a method developed by Ozawa was used, where polymer samples are heated with different constant heating rates. With this method the energy of activation can be determined accurately as a function of the degree of conversion. In this way a parallel reaction could be observed starting at the end of the nonisothermal conversion process. The polymer was used in two different morphological states, a powder and a film. A slightly higher energy of activation and a considerably higher pre-exponential factor were observed for the film indicating a dependency of the kinetics on the morphological state or on the history of the polymer sample

Separation of isomeric xylenes by pervaporation through cellulose ester membranes

The interaction between the isomeric xylenes and different cellulose esters was investigated using solubility parameter considerations and through measurements of swelling values. p]Hansen's three-dimensional solubility parameters δd, δp, δh of all the components have been calculated. These values have been used to predict the interaction between polymer and penetrant. A measure for this interaction is given by Δ, which is the distance between polymer and penetrant in the δd, δr, δh space. As expected, the experimental swelling values varied in inverse proportion to the calculated Δ values. p]Pervaporation characteristics of different cellulose ester membranes were determined by measuring product rates and selectivity. The differences in membrane characteristics have been explained qualitatively in terms of the solubility parameter concept

A survey of structure characterization methods for ultrafiltration and reverse osmosis membranes

Asymmetric membranes consist of a thin skin, which is permselective to certain molecules in solution, and a porous support, serving as a mechanical support layer and also as a transport layer for the permeate. Both in ultrafiltration and in hyperfiltration (reverse osmosis) asymmetric membranes are in use. Two different types of methods can be distinguished for the characterization of porous properties of membranes, aiming resp. at morphological structure and permeability of membranes.\ud \ud The methods at hand to characterize the morphological structure of membranes are akin to those used for wet spun polymer fibers. Thus, scanning electron microscopy, thanks to its large depth of focus has given insight as to the type of pores existing in support layers (closed or open cells). It is much more difficult to get relevant information on morphological structures existing in the skin. Transmission electron microscopy, which is able to show domain structures (≈ 500 P.) in thin films of block-copolymers, should give some of the answers here.\ud \ud Further ways to characterize porous membrane structures are to be explored in mercury porosimetry (pressing mercury in dried and evacuated membrane materials) and through gas adsorption measurements (BET-method). In both cases membrane treatment must be such that the pores are kept in their original size and shape (liquid exchange to non-swelling liquids before drying).\ud \ud The second and more direct way in defining membrane performance concerns selective permeability and is characterized by the so-called cut-off of membranes. A basic assumption made very often here is that permeability decreases with molecular size, because of a given pore size distribution in the skin. It should be realized that except for the pore sizes in the membrane and the size of the permeating substance, also the chemical nature (charge, extent of hydration) of the substances to be separated and that of the membrane material are important. Some relevant criteria for the choice of testing compounds in cut-off studies will be given, and typical results for all the foregoing topics will be presented