G.H. Koops

Characterization of morphology controlled polyethersulfone hollow fiber membranes by the addition of polyethylene glycol to the dope and bore liquid solution

The preparation of polyethersulfone (PES) hollow fiber membranes has been studied using N-methylpyrrolidone (NMP) as solvent, polyethylene glycol 400 (PEG 400) as weak nonsolvent and water as strong nonsolvent. When PEG 400 is used as polymeric additive to the spinning dope the viscosity of the PES solution is strongly enhanced. Furthermore, it was observed that PEG 400 could be added to the solution in large amounts without causing phase separation (NMP/PEG ratio 1:9, PES concentration approximately 11 wt.%). Membranes prepared from a solution containing a NMP/PEG ratio of 1:1 results in higher fluxes than when a ratio of 1:4 is used. Similar fluxes were obtained for PES concentrations of 16 and 20 wt.%. Looking at the fiber cross-section it became clear that macrovoid formation could not be suppressed by the addition of PEG 400 alone, not even at concentrations as high as 38 wt.%. Only when relatively large amounts of water were added to the dope solution macrovoids disappeared and nice spongy structures were obtained. Variation of the bore liquid composition using the components NMP, PEG 400 and water showed to be a powerful method to control the pore size of the bore surface. Pores of 5–28 nm were obtained in combination with high pure water fluxes; e.g. a membrane with pores of 7 nm had a pure water flux of 940 l/(m2 h bar) and showed 100% BSA retention. When an air gap larger than 10 mm was applied the shell surface contained relatively large pores. Spinning directly in water (airgap=0) resulted in shell side pores of 8–10 nm, while an air gap of 10 mm resulted in pore sizes of 40–54 nm.

Polyimide hollow fiber gas separation membranes: preparation and the suppression of plasticization in propane/propylene environments

Asymmetric hollow fiber membranes were prepared using the polyimide Matrimid® 5218. The fibers had an effective top layer thickness of 0.3–0.4 μm. The fibers were used in propane and propylene permeation experiments. Whereas the propane permeance remained more or less constant, the propylene permeance increased with feed pressure greater than 1 bar. This indicated that propylene plasticized the membrane material. The fibers were given different heat-treatments in order to investigate the possibilities to suppress the propylene plasticization. This treatment also reduced the permeance considerably, the effect being more pronounced the more intense the heat-treatment was. This was in agreement with scanning electron microscopy studies, which revealed that densification of the fibers occurred due to the heat-treatments. Most important, relatively mild heat-treatments already appeared to be effective in suppressing the propylene plasticization. Since these heat-treated fibers still readily dissolved it is concluded that the plasticization suppression was not due to crosslinking, but to an annealing effect. Due to thermal curing (annealing) at temperatures below the Tg aromatic polyimides tend to form charge transfer complexes, which restrict the polymer chain mobility. Presence of these complexes seems to be responsible for suppression of propylene plasticization.

High flux polyethersulfone-polyimide blend hollow fiber membranes for gas separation

In this work, the preparation of gas separation hollow fibers based on polyethersulfone Sumikaexcel (PES) and polyimide Matrimid 5218 (PI) blends, for three different compositions (i.e. PES/PI: 80/20, 50/50 and 20/80 wt.%), is reported. The dry/wet spinning process has been applied to prepare asymmetric hollow fibers by using blends of two different polymers with a common solvent. Dope viscosity measurements were performed to locate the blend concentrations where significant chain entanglement occurs. Cloud point measurements were carried out to estimate the tolerance of both pure components and blends in water. Scanning electron microscopy (SEM) was used to investigate the morphological characteristics and the structure of asymmetric hollow fibers. The permeation rates of CO2 and N2 were measured by the variable pressure method. In all cases, hollow fibers exhibit a typical asymmetric structure with a dense skin layer and a finely porous substructure. Macrovoids in the membrane substructure were observed only for the fibers spun at high PES concentration (80 wt.%). After coating with a silicone rubber solution, the developed hollow fibers exhibit a CO2 permeance varying from 31 to 60 gas permeation units (GPU) and a CO2/N2 selectivity varying from 40 to 35, at room temperature. The thickness of the skin layer, which corresponds to these permeation rates, varies from 0.1 to 0.15 μm. The effect of air-gap distance on hollow fibers structure and permeation performance is examined. The aforementioned permeation properties, establish PES/PI hollow fibers as excellent candidates membranes for the separation of gaseous mixtures in industrial level.

Preparation and characterization of highly selective dense and hollow fiber asymmetric membranes based on BTDA-TDI/MDI co-polyimide

In this work, the preparation, the characterization, and the permeation properties of dense flat sheet and asymmetric hollow fiber membranes, based on BTDA-TDI/MDI co-polyimide (P84), are reported. Results are shown for pure gases and for the separation of a CO2/N2 (80/20) mixture. Dope viscosity measurements were performed to locate the polymer concentrations where significant chain entanglement occurs. Asymmetric hollow fibers were spun, using the dry/wet phase inversion process. Scanning Electron Microscopy (SEM) was used to investigate the morphological characteristics and the structure of the developed fibers. The permeation rates of He, CO2, O2, and N2 were measured by the variable pressure method at different feed pressures and temperatures. P84 co-polyimide proved to be one of the most selective glassy polymers. The achieved ideal selectivity coefficients are: 285–300 for He/N2, 45–50 for CO2/N2, and 8.3–10 for O2/N2, which are in the range of the highest values reported ever for polymeric membranes. The permeability of CO2 is relatively low (1 barrer, 25 °C), however it is independent of feed pressure indicating that the P84 dense membranes are not plasticized at CO2 feed pressures up to 30 bar. To the contrary, the permeance of CO2 through the asymmetric hollow fiber membranes increases with pressure, indicating that the plasticization behavior of asymmetric membranes differs from the respective dense ones. However, no evidence of plasticization was observed when a CO2/N2 (80/20) mixture was fed to the hollow fiber membranes for pressures up to 30 bar. In all cases, CO2 permeance decreased with pressure while that of N2 remained constant.

Carbon molecular sieve membranes prepared from porous fiber precursor

Carbon molecular sieve (CMS) membranes are usually prepared from dense polymeric precursors that already show intrinsic gas separation properties. The rationale behind this approach is that the occurrence of any kind of initial porosity will deteriorate the final CMS performance. We will show that it is not necessary to produce a non-porous precursor in order to obtain a selective CMS membrane. We used tight ultra-filtration (UF) fiber membranes as a precursor. These fibers did not have any gas separation properties before the pyrolysis treatment, nor were coatings applied to these fibers before or subsequent to the pyrolysis. After a heat treatment in air followed by a pyrolysis in a nitrogen atmosphere CMS fiber membranes were obtained. The CMS fibers were analyzed using scanning electron microscopy, thermo gravimetrical analysis, and gas permeation. From the permeation rates and permselectivity values measured for He, H2, CO2, Ar, O2, N2, CH4, C2H4, C2H6, C3H6, C3H8 and SF6 the evolution of the mean pore diameter was investigated. It was found that the pore diameter increases with pyrolysis temperature up to 800 °C, but decreases as the temperature is raised to 900 °C. The overall porosity reaches its highest value at 900 °C.

The development of electro-membrane filtration for the isolation of bioactive peptides: the effect of membrane selection and operating parameters on the transport rate

The ability to produce functional food ingredients from natural sources becomes increasingly attractive to the food industry. Antimicrobial (bioactive) ingredients, like peptides and proteins, can be isolated from hydrolysates with membrane filtration and/or chromatography. Electro-membrane filtration (EMF) is an alternative for the isolation of these usually strongly charged components. It is believed to be more selective than membrane filtration and less costly than chromatography. The isolation of bioactive peptides from a hydrolysate of αs2-casein, a protein originating from milk, was studied as a model separation for the development of EMF. This separation can be used as an example application for the isolation of other charged components from complex feedstocks in sseveral industries. After 4 h EMF the product consisted for 100% of proven or anticipated charged bioactive components. Diffusion and convection were negligible in relation to electrophoretic transport, since only charged components were recovered in the permeate product. The most important peptide (26% on total protein, starting from 7.5% in the feed) was αs2-casein ƒ(183–207), a very potent peptide against Gram positive and Gram negative microorganisms. The transport rate of αs2-casein ƒ(183–207) was reduced strongly when a polysulphone membrane with a molecular weight cut-off below 20 kDa was used. The amount of αs2-casein ƒ(183–207) transported increased practically linearly with the concentration and the applied potential difference. The use of desalinated feeds to further increase the electrical field strength in the feed compartment resulted in higher transport rates, but this increase was lower than expected probably due to the lower electrophoretic mobility. An average transport rate of 2.5 and 4 g/m2.h at maximum was achieved during 4 h EMF using GR60PP (25 kDa) and GR41PP (100 kDa) membranes respectively.

Wet spinning of integrally skinned hollow fiber membranes by a modified dual-bath coagulation method using a triple orifice spinneret

Three main routes are known to prepare hollow fiber membranes; melt spinning, dry spinning and wet spinning (or dry/wet spinning). The latter is the most important technique for the preparation of industrial hollow fiber membranes. In this process the extruded polymer solution is immersed in a nonsolvent bath where phase separation occurs. Generally there is an air gap between the spinneret and non-solvent and this air gap has a large impact on the ultimate membrane properties. In order to control the processes in this air gap a new type of spinneret has been developed where the extrusion mouth has three openings. The membrane formation process can be controlled by the choice of the nonsolvent which is extruded through the outer opening as a thin film adhering to the polymer solution. In this way both asymmetric microfiltration and ultrafiltration hollow fibers can be prepared as well as integrally skinned hollow fibers with a defect-free top layer suitable for gas separation and pervaporation. The advantage of this spinneret in relation to the phase inversion process will be described and some examples will be given in the field of gas separation and pervaporation.

Preparation and characterization of gas separation hollow fiber membranes based on polyethersulfone-polyimide miscible blends

In this work the preparation and characterization of gas separation hollow fibers based on polyethersulfone Sumikaexcel (PES) and polyimide Matrimid 5218 (PI) blends are reported. Scanning Electron Microscopy (SEM) was used to investigate the morphological characteristics and structure of the asymmetric hollow fibers. Differential Scanning Calorimetry (DSC) was used to determine the glass transition temperatures of the blends. The permeation rates of CO2 and N2, for both uncoated and PDMS coated fibers, were measured by the variable pressure method. Experimental results were used in conjunction with the resistance model in order to determine the surface porosity of uncoated fibers. PES/PI fibers exhibit an asymmetric structure and one glass transition temperature. Increase of the air gap distance during spinning results in higher surface porosity and gas permeation rates. PDMS coated fibers exhibit a CO2 permeance varying from 31 to 60 GPU and a CO2/N2 selectivity varying from 35 to 40, at room temperature. The thickness of the skin layer, which corresponds to these permeation rates, varies from 0.1 to 0.15 ?m. After plasticization with CO2, ultrathin hollow fibers especially rich in PES, exhibit reversible reduction of CO2/N2 selectivity. These properties establish PES/PI hollow fibers as excellent candidate membranes for the separation of gaseous mixtures in industrial level.

Native protein recovery from potato fruit juice by ultrafiltration

Potato fruit juice, i.e. the stream resulting after the extraction of the starch from the potato, contains up to 2.5% [w/w] of proteins that are potentially valuable for the food market. However, today the recovery of protein from the potato fruit juice with reverse osmosis membranes results in a protein concentrate that is not suitable for human consumption. The described research shows that the use of ultrafiltration with additional diafiltration is able to produce a higher quality protein. Tests with the produced protein show that the quality depends on the rate of diafiltration used and that the product has functional properties that are equal or better than the compared commercial food product that are currently used.

Effect of spinning conditions on the structure and the gas permeation properties of high flux polyethersulfone-polyimide blend hollow fibers

In this work, the effects of major spinning parameters, such as: polymer concentration, air gap distance, bore fluid composition, and take-up velocity on the structure and the permeation properties of polyethersulfone-polyimide gas separation hollow fibers are discussed in detail. It is shown that a spinning dope starts to exhibit significant chain entanglement at a critical polymer concentration. Fibers spun from this critical concentration exhibit theoretically the thinnest skin layer and minimum surface porosity. The longer the nascent hollow fiber membrane is exposed to a humid air-gap, the higher the water content in the top layer before demixing occurs. This results in higher surface porosity and gas permeance. Better mixing between the polymer solution and the bore liquid is achieved by adjusting the composition of the bore fluid (NMP/H20). Finally, by increasing the velocity of the take-up drum, the permeance of both CO2 and N2 decrease while their permselectivity remains constant. Suitable selection of the spinning conditions results in gas separation hollow fibers with thin skin layers (0.1 ?m), macrovoid-free substructure, high permeation rates (CO2: 40?60 GPU) and selectivity coefficients (a CO2/N2: 40). These results compete directly with the performance of commercial gas separation membranes.