Critical points in the analysis of membrane pore structures by thermoporometry

Several ultrafiltration membranes of the anisotropic and isotropic type were characterized by means of the thermoporometry. Successive cooling and heating runs were performed in order to investigate the effects of the water-ice and ice-water phase transitions on the structure of the membranes. The results found for membranes having different casting thicknesses indicate that, in some cases, pores in the sublayer of anisotropic UF membranes frustrate the measurement of the, for the separation, relevant porse present in the top layer

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

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

Permporometry: the determination of the size distribution of active pores in UF membranes

Permporometry is a method by which the characteristics of the interconnecting 'active' pores of an ultrafiltration membrane can be measured. It is these active' pores that are responsible for the actual membrane performence. Application of permporometry on different membrane types, including ceramic as well as polymeric membranes, shows that the method can provide objective information on the `active¿ pore size present

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

Zeolite-filled silicone rubber membranes. Experimental determination of concentration profiles

Permeant concentrations in silicalite-filled silicone rubber membranes during pervaporation of propanol/water mixtures were measured using multi-layered membranes. Experimentally determined concentration profiles show that the propanol concentration in the silicalite-filled membrane increases with increasing silicalite content. The water concentration in the membrane is low and no water is present in the silicalite particles during pervaporation.\ud The concentration profiles measured here support the observations from the resistance model that the diffusion through the membrane determines the transport rate, i.e., adsorption is a fast process

Supported liquid membranes: instability effects

The instability behavior of several supported liquid membranes (SLMs) has been studied for a system in which nitrate ions are removed from an aqueous feed phase and concentrated in a stripping phase. The composition of the aqueous phases and of the membrane liquid has been determined after the aqueous phases had flowed parallel to the membranes for a period of six days. From the experimental data it can be concluded that SLM-failure results from the removal of LM-phase from the support. Contrary to literature data this is not caused by an osmotic pressure difference. It is shown that the membrane stability depends largely on the type of solvent and the molecular structure of the carrier. Furthermore the membrane stability increases with an increasing salt content in the stripping phase (at constant composition of the feed solution)

Zeolite-filled silicone rubber membranes : Part 1. Membrane preparation and pervaporation results

Amongst the alternative fuels obtained from renewable resources alcohol from fermentation may become one of the most important. The combination of fermentation with pervaporation in a membrane bioreactor offers the advantage of continuous processing. In this membrane bioreactor alcohol-selective membranes are needed. The performance of the membranes available at present is poor. Much research is being carried out on silicone rubber but the selectivity of this material for alcohol is too low. Addition to the membrane of a sorptive filler with a high selectivity towards alcohol appears to improve both selectivity and flux. Silicalite, a novel type of hydrophobic zeolite, has been used for that purpose. Results presented in this paper indicate that transport through the zeolite pores contributes to a major extent to the total transport through the membrane

Wetting criteria for the applicability of membrane distillation

Membrane distillation can only be applied on liquid mixtures which do not wet a microporous hydrophobic membrane. Solutions of inorganic material in water have such high values of surface tension (γLgreater-or-equal, slanted72x10−3 N/m) that the non-wetting condition is fulfilled for a number of hydrophobic membranes. As soon as organic solutes are present in the solution, the surface tensionγL will be lowered, and if the concentration of organic material becomes too high, wetting of the membrane will occur. By means of theoretical considerations a critical solute concentration or surface tension at which a homogeneous smooth material will be wetted (gq < 90/deg) can be calculated. For a (micro)porous membranes no such theoretical relation can be derived. Therefore, a simple experimental method is described to measure the maximum allowable concentration for a (micro)porous membrane. On the basis of these measurements, the maximum allowable concentration under process conditions can be determined

Characterization of anisotropic UF-membranes:top layer thickness and pore structure

Anisotropic poly(2,6-dimethyl-, 1,4-phenylene oxide) (PPO) ultrafiltration membranes are characterized by means of two techniques. A new method for the determination of skin thicknesses, the gold sol method, is introduced and applied to these membranes. The membranes appeared to have a well-defined skin with a thickness of 0.2 μm. The results were combined with permporometry, a method that measures the size of the active pores that are actually responsible for the membrane performance. By following this approach it is possible to calculate the porosity of the skin layer and to obtain a suitable prediction of the pure water flux