C.A. Smolders

Microstructures in phase-inversion membranes. Part I. Formation of macrovoids

A new mechanism for the formation of macrovoids in phase-inversion membranes is proposed. It is based on the observed difference in type of demixing of a thin film of a polymer solution when immersed in a nonsolvent bath: delayed or instantaneous demixing. The explanation for macrovoid formation assumes local conditions of delayed demixing in front of a certain layer of nuclei already formed, due to a change in the interfacial compositions at the advancing coagulation front, as compared to the original composition at the interface film-bath. Effects of variations in membrane formation conditions which can be calculated using the model of diffusive mass transport in thin films of polymeric solutions in combination with phase separation in phase-inversion membranes, completely support the mechanism as proposed.

Microstructures in phase inversion membranes. Part II. The role of a polymeric additive

Membranes were prepared from a casting solution of a water-soluble polymer, poly (vinyl pyrrolidone) (PVP), and a membrane forming polymer, poly (ether sulfone), in 1-methyl-2-pyrrolidone (NMP) as solvent by immersing them in mixtures of water and NMP. It was found that the addition of PVP to the ternary system suppresses the formation of macrovoids in the sub-layer, while the ultrafiltration-type top-layer consists of a closely packed layer of nodules. Using a model for pass transfer in this quaternary system, it is possible to explain the effects of the additive on macrovoid formation. Strong indications are found that the appearance of a nodular structure in the top-layer follows a mechanism of spinodal decomposition during the very early stages of the immersion step.

Formation of membranes by means of immersion precipitation : Part I. A model to describe mass transfer during immersion precipitation

Equations and boundary conditions are derived for the isothermal diffusion processes in the coagulation bath and in the polymer solution after immersion of a cast (ternary) polymer solution into a (binary) coagulation bath. The mass transfer is expressed in terms of thermodynamic driving forces and frictional coefficients. The frictional coefficients in the ternary system are assumed to be interrelated through the Onsager reciprocal relations and to be related to the measurable frictional coefficients defined in the three limiting binary composition ranges. In combination with knowledge about the demixing processes which can take place in the polymer solution (liquid-liquid phase separation or solid-like aggregate formation), this model makes it possible to calculate the polymer concentration profile in the immersed film at the moment of demixing of the polymer solution as a function of several process parameters. The calculated concentration profile and its relation to the asymmetric structure of the ultimate membrane are presented in Part II1.

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.

Hydrodynamic resistance of concentration polarization boundary layers in ultrafiltration

The influence of concentration polarization on the permeate flux in the ultrafiltration of aqueous Dextran T70 solutions can be described by (i) the osmotic pressure model and (ii) the boundary layer resistance model. In the latter model the hydrodynamic resistance of the non-gelled boundary layer is computed using permeability data of the Dextran molecules obtained by sedimentation experiments. It is shown both in theory and experiment that the two models are equivalent.

Adsorbent filled membranes for gas separation. Part 1. Improvement of the gas separation properties of polymeric membranes by incorporation of microporous adsorbents

The effect of the introduction of specific adsorbents on the gas separation properties of polymeric membranes has been studied. For this purpose both carbon molecular sieves and zeolites are considered. The results show that zeolites such as silicate-1, 13X and KY improve to a large extent the separation properties of poorly selective rubbery polymers towards a mixture of carbon dioxide/methane. Some of the filled rubbery polymers achieve intrinsic separation properties comparable to cellulose acetate, polysulfone or polyethersulfone. However, zeolite 5A leads to a decrease in permeability and an unchanged selectivity. This is due to the impermeable character of these particles, i.e. carbon dioxide molecules cannot diffuse through the porous structure under the conditions applied. Using silicate-1 also results in an improvement of the oxygen/nitrogen separation properties which is mainly due to a kinetic effect. Carbon molecular sieves do not improve the separation performances or only to a very small extent. This is caused by a mainly dead-end (not interconnected) porous structure which is inherent to their manufacturing process.

Microporous hollow fibre membrane modules as gas-liquid contactors. Part 1: Physical mass transfer processes. A specific application: mass transfer in highly viscous liquids

Gas-liquid mass transfer has been studied in a membrane module with non-wetted microporous fibres in the laminar flow regime. This new type of gas/liquid contactor can be operated stabily over a large range of gas and liquid flows because gas and liquid phase do not influence each other directly. Therefore foam is not formed in the module, gas bubbles are not entrained in the liquid flowing out of the reactor and the separation of both phases can be achieved very easily. These phenomena often limit the applicability of conventional contactors, e.g. a bubble column which was also studied in the present work. The large mass transfer area of a bundle of small fibres offers the possibility of creating a compact gas/liquid mass exchanger. However, owing to the small channels in and around the fibres the flow of either gas or liquid becomes laminar which reduces the mass transfer capacity of the module. Therefore the mass transfer coefficients in the laminar flow regime were determined experimentally. For mass transfer determined by the transport in the liquid phase it was found that the active mass transfer area is equal to the total membrane area, regardless the porosity of the fibre. For processes with liquid flowing through the fibres, the influence of fibre diameter, diffusivity in the liquid, liquid viscosity and liquid velocity on mass transfer can be correlated extremely well with the Graetz-Leveque solution derived for the analogous case of heat transfer. For liquid flowing around regularly packed fibres mass transfer was described satisfactory with a correlation derived from a numerical solution for the similar heat transfer problem [Miyatake and Iwashita, Int. J. Heat Mass Transf., 33 (1990) 416]. Correlating mass transfer in liquid flowing around irregularly packed fibres was not possible because of the undefined dimensions of the different channels between the fibres.

Microporous hollow fibre membrane modules as gas-liquid contactors. Part 2. Mass transfer with chemical reaction

Absorption determined by mass transfer in the liquid is described well with the Graetz-Leveque equation adapted from heat transfer. The influence of a chemical reaction on the mass transfer was simulated with a numerical model and tested on the absorption of CO2 in a hydroxide solution. Absorption determined by mass transfer in the gas phase and the pores of the membrane was also analysed experimentally and numerically. It was found that the gas phase concentration profile is established at a very short distance from the entrance of the fibre. This results in a constant Sherwood number along the fibre. A module coated with a very thin silicone rubber layer showed absorption rates comparable to the uncoated membranes. If absorption liquids are used which wet the membranes, resulting in leaky membranes, such a coating can enlarge the application of microporous hollow fibre membrane module.

Phase separation phenomena in solutions of polysulfone in mixtures of a solvent and a nonsolvent: relationship with membrane formation☆

The phase separation phenomena in ternary solutions of polysulfone (PSf) in mixtures of a solvent and a nonsolvent (N,N-dimethylacetamide (DMAc) and water, in most cases) are investigated. The liquid-liquid demixing gap is determined and it is shown that its location in the ternary phase diagram is mainly determined by the PSf-nonsolvent interaction parameter. The critical point in the PSf/DMAc/water system lies at a high polymer concentration of about 8% by weight. Calorimetric measurements with very concentrated PSf/DMAc/water solutions (prepared through liquid-liquid demixing, polymer concentration of the polymer-rich phase up to 60%) showed no heat effects in the temperature range of −20°C to 50°C. It is suggested that gelation in PSf systems is completely amorphous. The results are incorporated into a discussion of the formation of polysulfone membranes.