Gunnar Eigil Jonsson

Characterization of microporous membranes for use in membrane contactors

Abstract Methods of selecting applicable membranes for use in membrane contactors for flue gas desulfurization are proposed in this paper. The mass transfer mechanism for SO2 diffusion through gas filled pores is explored by simple measurements in order to identify suitable membrane structures for use in contactors for flue gas cleaning. It is attempted to correlate the experimentally determined membrane mass transfer coefficient to intrinsic physical properties of the membrane by applying theoretical and empirical correlations for the porosity-tortuosity relationship of the porous structure. Thereby limiting fluxes can be predicted with good accuracy from data quoted in the manufactures catalogue.

Boundary layer phenomena during ultrafiltration of dextran and whey protein solutions

Abstract The osmotic pressure and viscosity of dextran and whey protein solutions were measured at concentrations up to 50 wt%. It is shown that the flux-pressure data for ultrafiltration at varying concentrations and circulation velocities can be explained by the increased osmotic pressure of the concentrated boundary layer. Experiments with colored macromolecules in a transparent thin channel system, show that dissipative structures are formed on the membrane surface. In certain areas of the flow channel, the concentrated boundary layer is moving in thin stripes along the membrane surface in the flow direction. In these areas the concentrated boundary solution can be skimmed off through small holes in the skin layer of the membrane.

Lactic acid production from wheat straw hemicellulose hydrolysate by Lactobacillus pentosus and Lactobacillus brevis.

Lactic acid production by Lactobacillus brevis and Lactobacillus pentosus on a hemicellulose hydrolysate (HH) of wet-oxidized wheat straw was evaluated. The potential of 11-12 g/l fermentable sugars was released from the HH through either enzymatic or acidic pretreatment. Fermentation of added xylose in untreated HH after wet-oxidation, showed no inhibition on the lactic acid production by either Lb. pentosus or Lb. brevis. Lb. pentosus produced lactate corresponding to 88% of the theoretical maximum yield regardless of the hydrolysis method, whereas Lb. brevis produced 51% and 61% of the theoretical maximum yield after enzymatic, or acid treatment of HH, respectively. Individually, neither of the two strains were able to fully utilize the relatively broad spectra of sugars released by the acid and enzyme treatments; however, lactic acid production increased to 95% of the theoretical maximum yield by co-inoculation of both strains. Xylulose was the main sugar released after enzymatic treatment of HH with Celluclast. Lb. brevis was able to degrade xylobiose, but was unable to assimilate xylulose, whereas Lb. pentosus was able to assimilate xylulose but unable to degrade xylobiose.

Factors affecting flux and ethanol separation performance in vacuum membrane distillation (VMD)

Abstract Membrane distillation (MD) has a great potential as a concentration process for temperature labile liquids such as fruit juices, etc. Besides water, also aroma compounds will permeate through the membrane depending on their volatility and how the MD process is operated on the permeate side. In this paper, an experimental and theoretical investigation of the influence of concentration polarisation and temperature polarisation on the flux and selectivity of binary aqueous mixtures of ethanol is presented for vacuum membrane distillation (VMD) processes. Experimental results include changes of the following parameters: nature of solutions, membrane material and pore size, feed temperature, recirculation flow rate. One method was proposed in order to evaluate the concentration polarisation effects from the fit of the experimental data. General models taking into account Knudsen and viscous flows were proposed, but viscous contribution resulted to be negligible under our operating conditions. Therefore, theoretical fluxes were estimated using Knudsen model and a good agreement between them and the experimental ones was found.

Water and solute transport through cellulose acetate reverse osmosis membranes

Abstract Reverse osmosis data on two different cellulose acetate membranes using seven organic solutes of varying molecular weight have been obtained. A combined viscous-flow and frictional model is presented and used to estimate the maximum retention, the friction between solute and membrane, the distribution coefficient for solute and the pore radius. The calculated values of the maximum retention and distribution coefficient have been compared with the Ferry-Faxen equation. For the more open membrane these are in good agreement. The tighter one, however, shows a greater interaction between solute and membrane than predicted by the Faxen equation. Some data on two-solute systems are presented and shown to give variation in the retention, which can be explained from the convection term. Furthermore, for experiments with dextran the permeate shows a significant reduction in both M n and M t

Modulating the biocompatibility of polymer surfaces with poly(ethylene glycol): Effect of fibronectin

A novel approach described earlier for improving polymer substratum biocompatibility(1) is further elucidated. Polysulfone (PSf) spin-coating films were modified by covalent end-on grafting of hydrophilic and sterically demanding photo-reactive poly(ethylene glycol) (PEG) conjugates (ABMPEG; 10 kDa). The degree of grafting density was varied systematically, yielding a wide spectrum of attained surface characteristics monitored by air-water contact angles (captive bubble method). Fibronectin (FN) adsorption was studied by in situ ellipsometry and found to decrease monotonically as ABMPEG grafting density increased. The adhesive interaction of human skin fibroblasts with these substrata and, in particular, the effect of FN precoating were investigated in detail. A clear optimum of cell-substratum interactions was found for mildly modified substrata, employing well established microscopic and immunofluorescence techniques, namely the monitoring of cell adhesion and spreading, overall cell morphology, organization of FN receptors, and focal adhesions as well as FN matrix formation. The results suggest that cell interactions with hydrophobic polymer substrata are enhanced considerably when modified with hydrophilic and sterically demanding PEG moieties at a low surface coverage due to enhanced biologic activity of adsorbed and intercalated adhesive proteins such as FN.

A comparative study of techniques used for porous membrane characterization: pore characterization

A range of commerical UF membranes have been characterized by thermoporometry, biliquid permporometry and molecular weight cut-off experiments. A comparison of results from these three independent techniques for the same types of membrane shows an indication of the strength and weakness of the methods. MWCO values determined from actual rejection values using PEG and dextran were significantly lower than the manufacturer supplied data. The data obtained using the biliquid permporometry and solute rejection tests produced contrasting results for Amicon polysulfone (PM30) and regenerated cellulose (YM30) membranes. While MWCO determination resulted in sharper cut-off curves, the biliquid permporometry offered a broader size distribution with the PM30 and vice versa with the YM30. The pore sizes obtained by thermoporometry were significantly larger than those by the biliquid permporometry. The biliquid permporometry and thermoporometry give significantly higher values than the MWCO method. The closest comparison is obtained between the EM values and the MWCO method. This suggests that the controlling pore dimension for separation is the surface skin dimension.

Fouling phenomena in microporous membranes. Flux decline kinetics and structural modifications

Abstract The fouling mechanisms in microfiltration are analyzed with two symmetric and one asymmetric membranes. Fouling kinetics when microfiltering bovine serum albumin solutions (BSA) are studied for both the consecutive processes of surface blocking and cake formation. Influence of shear stresses and aggregates is analyzed. Permeate flux is recovered by cleaning with different agents and temperatures, thus with different removal and cake coagulation performances. Finally, modifications of the structure of the porous matrix due to the appearance of a cake on the retentate layer of the membrane are studied by air permeation porometry. The flux decay and recovery characteristics as well as the structural modifications obtained agree with the two consecutive steps in the fouling mechanism leading to a solute accumulation on the pore entrances whose consequences are analyzed here. This model predicts asymmetries in the recovered water flux after cleaning which are effectively found.

Synthesis of photoreactive α‐4‐azidobenzoyl‐ω‐methoxy‐poly(ethylene glycol)s and their end‐on photo‐grafting onto polysulfone ultrafiltration membranes

Covalent end-on grafting of poly(ethylene glycol) (PEG) onto a polysulfone (PSf) surface using α-4-azidobenzoyl-ω-methoxy-PEG (ABMPEG) is described. Photoreactive ABMPEG was synthesized by reacting monomethoxy-PEG (MPEG) with 4-azidobenzoyl chloride, yielding complete substitution of the hydroxyl groups. After adsorption from aqueous solutions, ABMPEG was photo-grafted under wet conditions onto PSf ultrafiltration (UF) membranes. Contact angle (CA) measurements showed the increased hydrophilicity of modified membranes and the irreversibility of the modification. Bovine serum albumin (BSA) adsorption decreased by 70% and the permeability decay after protein adsorption became less severe for the modified membranes compared to unmodified reference membranes.

Overview of theories for water and solute transport in9 UF/RO membranes

Abstract The applicability of various models for the transport of water and solute in UF/RO membranes is briefly outlined. The selectivity of various solutes in ultrafiltration and reverse osmosis is further discussed.