Selection of elastomeric membranes for the removal of volatile organics from water

A wide range of homogeneous elastomeric membranes has been prepared using dicumylperoxide as a general cross-linking agent. The membranes have been used for both equilibrium sorption measurements and steady-state pervaporation experiments to study solution-diffusion phenomena in the removal of volatile organic components from aqueous solutions. Pervaporation experiments have been performed under identical hydrodynamic conditions in order to fix the boundary layer mass transfer coefficient at a constant and known value. For comparison of the permeabilities of different pervaporation membrane materials, this is of utmost importance. A wide range of selectivity factors up to a value of 100,000 are obtained, whereas usually the permeabilities for the organic component are in the range of 10-10-10-9m2/s and 10-14-10-12m2/s for water. The permeation and sorption data obtained for the various elastomers have been related to the chemical and physical nature of the elastomers through the solubility parameter and the glass transition temperature, respectively. Both diffusional and sorption effects seem to be important, determining the water-transport behavior in the elastomeric membranes. The solubility of the organic component appears to be independent of this combined solubility parameter. Differences in the permeabilities of the organic component can primarily be ascribed to structural parameters in the membrane material, like degree of unsaturation and presence of steric side groups

Diffusional phenomena in membrane separation processes

Nowadays membrane filtration processes are used industrially as an alternative to conventional separation methods. Membrane separation methods can be divided into classes according to their separation characteristics: (i) separation by sieving action; (ii) separation due to a difference in affinity and diffusivity; (iii) separation due to a difference in charge of molecules; (iv) carrier-facilitated transport, and (v) the process of (time-) controlled released by diffusion. In all these cases diffusion processes play an important role in the transport mechanism of the solutes. Various mechanisms have been distinguished to describe the transport in membranes: transport through bulk material (dense membranes), Knudsen diffusion in narrow pores, viscous flow in wide pores or surface diffusion along pore walls. In practice, the transport can be a result of more than only one of these mechanisms. For all of these mechanisms models have been derived. The characteristics of a membrane, e.g. its crystallinity or its charge, can also have major consequences for the rate of diffusion in the membrane, and hence for the flux obtained. Apart from the diffusion transport processes in membranes mentioned above, other important diffusion processes are related to membrane processes, viz. diffusion in the boundary layer near the membrane (concentration polarization phenomena) and diffusion during membrane formation. The degree of concentration polarization is related to the magnitude of the mass transfer coefficient which, in turn, is influenced by the diffusion coefficient. The effect of concentration polarization can be rather different for the various membrane processes. The phase inversion membrane formation mechanism is determined to a large extent by the kinetic aspects during membrane formation, which are diffusion of solvent and of non-solvent and the kinetics of the phase separation itself

The stability of supported liquid membranes

In this paper a new hypothesis about the instability mechanism of SLMs will be discussed: emulsion formation induced by lateral shear forces. Experimental results show that a water phase with a low salt concentration which flows along the membrane interface causes the removal of both solvent and carrier from the membrane. There is a significant correlation between the instability of the liquid membrane and the stability of emulsions formed with the same system. Therefore, the development of stable SLMs needs conditions in which formation of relatively stable emulsions is prevented. This can be realized by gelation of the liquid membrane. A gel network was created in the pores of the membrane in such a way that the permeability is not decreased while the stability increases to values which are very promising

Supported liquid membranes: stabilization by gelation

A new method has been developed to increase the stability of supported liquid membranes. By applying a homogeneous gel network in the pores of the support both the mechanical stability (against liquid displacement) and the long term permeability increase substantially. The flux decreases only slightly because of the open structure of the gel network. A second technique, by which a thin dense gel layer is applied to the feed side of the membrane, results in a specific suppression of the formation of emulsion droplets. The stability of the membrane increases by this treatment to values which are very promising

The formation of nodular structures in the top layer of ultrafiltration membranes

The formation of nodular structures in the top layer of ultrafiltration membranes is considered. A critical review of mechanisms described in the literature is given. Flat-sheet poly(ether sulfone) membranes and hollow-fiber poly(ether sulfone)/polyvinylpyrrolidone membranes were made by coagulation of a polymer solution in a nonsolvent medium under different circumstances. From these experiments, a number of empirical rules are found to describe the resulting morphology of the top layer. A new mechanism for the formation of a nodular structure is proposed. It is based on the small diffusion coefficient of the polymer molecules compared to the diffusion coefficient of solvent and nonsolvent combined with a high degree of entanglement of the polymer network. For unstable compositions, phase separation will proceed by growth in amplitude of concentration fluctuations. The rapid diffusional exchange of solvent for nonsolvent in the top layer leads to vitrification of the maxima of the concentration fluctuations that form the nodules. Complete disentanglement of the polymer chains between the nodules is not reached, which explains the small pores and the low porosity of ultrafiltration membranes

Phase-separation phenomena in solutions of poly(2,6-dimethyl-1,4-phenylene oxide). I. Thermodynamic parameters of solutions in toluene

New experimental data have been collected on thermodynamic properties of solutions of poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) in toluene. The Flory-Huggins interaction parameters g have been determined from light scattering measurements. These values are in agreement with values obtained by osmotic measurements at low concentrations and they allow the calculation of a melting point curve which fits the experimental melting points. No liquid-liquid phase separation can be calculated, as was concluded in a preceding paper. Spinodals could not be detected by light scattering or DSC-measurements. This also indicates that liquid-liquid phase separation does not occur. The phase separation on cooling of a PPO-toluene solution is thus believed to be a crystallization phenomenon

Dilation kinetics of glassy, aromatic polyimides induced by carbon dioxide sorption

Over the past years, the equilibrium sorption of gases in polymers has been intensively studied. Mostly, glassy polymers were investigated because of their excellent selective mass transport properties. This work does not focus on the equilibrium sorption but on the kinetics to reach the equilibrium. We developed a new experimental method measuring the sorption-induced dilation kinetics of a polymer film. Carbon dioxide and glassy, aromatic polyimides were chosen as model systems. Low-pressure experiments demonstrate that the measured dilation kinetics represent the sorption kinetics. A significant delay between the sorption and dilation kinetics is based on the fact that dilation kinetics occurs simultaneously with the concentration increase in the center of the polymer film. High-pressure experiments reveal significant differences in dilation kinetics compared to low-pressure experiments. Generally, three regimes can be distinguished in the dilation kinetics: a first, fast volume increase followed by two much slower regimes of volume increase. The magnitude of fast and slow dilation kinetics strongly depends on the swelling history of the polymer sample. The results of the experiments are analyzed in the light of a model relating the fast dilation kinetics to a reversible Fickian dilation and the slower dilation kinetics to an irreversible, relaxational dilation

Nitrate removal using supported liquid membranes: transport mechanism

A new method is developed for the removal of nitrate ions from water. Nitrate ions can be removed from water almost completely, with a mobile carrier, by counter-transport of chloride ions through a supported liquid membrane. The transport characteristics of this process, in which the water phases are flowing parallel to flat membranes, are described. The results show that depending on the experimental conditions the flux is determined by the diffusion of the carrier through the membrane or by the diffusion of the nitrate ions through a laminar water layer at the feed side. The selectivity of the membrane, which depends on the type of the organic solvent, determines the influence of the chloride concentration in the stripping phase on the membrane flux. Furthermore the effect of carrier concentration is investigated

Membranes of semicrystalline aliphatic polyamide nylon 4,6: Formation by diffusion-induced phase separation

The preparation of membranes of nylon 4,6 by diffusion-induced phase separation (DIPS) using formic acid as a solvent and water as a nonsolvent was studied. Nylon 4,6 is a semi-crystalline polymer; phase separation from a solution can occur by solid-liquid (s-l) de-mixing as well as by liquid-liquid (l-l) demixing. Upon quenching films of solutions with low polymer concentration (< 17 wt %) in a nonsolvent bath containing water, the morphology of the membranes show a foam-like structure typical for l-l demixing. When phase separation is induced by water vapor a transition in structure occurs from the cellular type to a morphology typical for s-l phase separated films. At higher polymer concentrations membranes exhibit structures consisting of spheres or smaller crystal-like units resulting from an s-l phase separation process. The addition of 2 wt % or more of water to polymer solutions with low concentration (up to 15 wt %) resulted in s-l demixing as well. In a DIPS process s-l demixing is kinetically competitive with l-l demixing if nuclei are already present in the starting solutions (heterogeneous nucleation), or if a relatively long time is available for crystal nuclei to be formed. The morphology resulting from s-l demixing is a result of spherulitic crystallization. A certain concentration of nuclei or of precursor particles already present results in a small nucleation density during precipitation and thus large spherulites can be grown; at higher polymer and/or water concentrations the nucleation density increases resulting in an axialitic morphology of the membranes