Ronaldo Nobrega

Transfer of dextran through ultrafiltration membranes: a study of rejection data analysed by gel permeation chromatography

Abstract A method is proposed for obtaining a measure of the flux distribution through an ultrafiltration membrane. In a previous paper [9], the mechanisms of mass transfer of polydisperse polymer solutes through a membrane were studied using the molecular weight distributions of the feed and of the permeates corresponding to different operating conditions. As a result, partial rejection coefficients were defined, each one characteristic of a given molecular weight fraction. For the ultrafiltration of a dextran solution having a widely spread molecular weight distribution (mixture of various commercial fractions) through a polysulfone outer-skinned hollow fiber, we show that these partial rejection coefficients give information on the characteristics of the membrane. More precisely, for operating conditions chosen in order to minimize the phenomena that cause deformation of the macromolecules, it is possible to use the variations in the partial rejection coefficients extrapolated to zero applied pressure to obtain a measure of the flux distribution through the membrane, related to dextran molecular weight. To determine to what extent the method developed could give information about fouling, it has been applied to an inorganic membrane before and after ultrafiltration of a protein solution.

Removal of aromatics from multicomponent organic mixtures by pervaporation using polyurethane membranes: experimental and modeling

Polyurethane (PU) dense membranes were used in the separation of binary and multicomponent aromatic/aliphatic mixtures by pervaporation. The PU membranes found to be selective towards aromatics in all systems studied. Both, swelling of polymer matrix and the permeate flux increase with aromatic weight fraction in the feed. The highest selectivity was achieved with benzene/n-hexane mixture in the whole feed composition range. The permeation of the organics through the PU membrane was modeled based on the sorption–diffusion mechanism. The sorption equilibrium was calculated using Flory–Huggins (FH) or UNIQUAC equations and Stefan–Maxwell (SM) equations were used to describe the transport inside the membrane matrix. A correction was introduced in the diffusion coefficients of the permeating species to take into account the polymer matrix plasticization. The experimental and predict data showed a good agreement, indicating that the PU membrane are useful to reduce the aromatic content of industrial solvent by pervaporation process.

New insights in the removal of diluted volatile organic compounds from dilute aqueous solution by pervaporation process

The present work aimed the mass transfer investigation in the removal of organic contaminants from water by the pervaporation process. The terpolymer ethene-propene-diene (EPDM) was used as the selective elastomer. Two classes of model organic solutes were chosen: chlorinated hydrocarbons (trichloroethylene, dichloromethane and trichloromethane) and aromatic ones (toluene, phenol and aniline). Pervaporation tests were carried out using dense and composite membranes with different thickness, solute concentrations and feed flow velocities at room temperature. The liquid boundary layer resistance (i.e., concentration polarization phenomenon) was observed for all solutes. The resistance-in-series model was used to determine liquid and polymer phase resistances. The results obtained indicate that the model would be better written considering the chemical potential gradient as driving force, in order to take into account affinity between water and the organic solutes, as well as their interactions with the polymer selective layer. The rational activity coefficients of the solutes in the polymer phase were determined by inverse gas chromatography (IGC) and related to the mass transfer coefficient in the polymer phase.

The deformation of dextran molecules. Causes and consequences in ultrafiltration

Abstract The transfer of dextran T70 solutions through a skinned polysulfone hollow fiber membrane was studied with and without applied pressure. The molecular weight distributions of dextran in the feed and in the permeate were obtained by high pressure liquid chromatography. Two different phenomena appear to play important roles with regard to solute transfer. One is related to the shear stress imposed by the flow at the pore entrances, i.e. to permeate flux, and the other is related to the influence of solute concentration on the expansion of the macromolecular chains. These phenomena explain the observed variations with operating conditions of the overall rejection coefficient.

Membranes obtained by simultaneous casting of two polymer solutions

Abstract Simultaneous casting of two polymer solutions allows formation of each membrane layer by different concepts. However, mass transfer between the polymer solutions increases the complexity of the involved phenomena. In the present work, it was investigated the simultaneous casting of two different polymer solutions to form the top and support layers of the membrane, respectively. Polyetherimide and polyethersulfone were used as base polymers. In order to form the support layer, it was used as a polymer solution composed of Lewis acid:base complex (adipic acid — Lewis acid, and n-methyl-2-pyrrolidone — Lewis base), and another composed of a macromolecular additive (polyvinylpyrrolidone). In order to form the top layer, different polymer solutions composed of a volatile component tetrahydrofuran (THF) were used. The velocity of precipitation of the polymer solutions and the morphology of the membranes were characterized by light transmission measurements and scanning electron microscopy (SEM), respectively. The morphology of the resulting membranes were also compared with the membranes obtained by single casting of each polymer solution. The results mainly showed that adhesion of the membrane layers seemed to occur when there was sufficient time for the solutions to interpenetrate through each other. This was probably favored when the region close to the interface of the solutions remains stable for a longer period. The presence of polyvinylpyrrolidone seemed also to favor adhesion between the layers.

Spinning process variables and polymer solution effects in the die-swell phenomenon during hollow fiber membranes formation

During hollow fiber spinning many variables are involved whose effects are still not completely clear. However, its understanding is of great interest because the control of these variables may originate membranes with the desired morphologies and physical properties. In this work, the phase inversion process induced by the immersion precipitation technique was applied to prepare hollow fibers membranes. It was verified that some of the variables involved, can promote a visco-elastic polymer solution expansion, called die-swell phenomenon, which is undesired since it may lead to low reproducibility of the permeation properties. The effects of the distance between spinneret and precipitation bath, the bore liquid composition, and the polymer solution composition were analyzed and discussed in order to avoid this phenomenon. According to the results, it was verified that the parameters investigated might promote a delay precipitation, which restrained the visco-elastic expansion.

Membrane for processing tropical fruit juice

In the present work, membrane processing was investigated in order to clarify and to recover aroma from tropical fruit juices. Pineapple and passion fruit juices were selected. Pervaporation experiments were also carried out with binary synthetic aqueous solutions of typical aroma components. Experimental and simulation results indicated that olefin pervaporation membranes presented a good performance due to its low water permeability and high enrichment factor to the components of aroma from the investigated synthetic solutions, as well as the single strength and the clarified fruit juices.

Microporous anisotropic phase inversion membranes from bisphenol-A polycarbonate: study of a ternary system

The aim of this work was to investigate anisotropic microporous membrane formation by phase inversion; using immersion precipitation technique and a ternary polymer system consisting of bisphenol-A polycarbonate, N-methyl-2-pyrrolidone as solvent and water as non-solvent to the polymer. Membrane morphology was observed by scanning electron microscopy (SEM) and related to some variables involved in membrane synthesis with the help of cloud-point measurements and precipitation kinetics, determined by means of light transmission experiments. The results show that the studied system presents a very small miscibility gap, which favors instantaneous demixing and the mechanism of nucleation and growth of the polymer lean phase, as well as macrovoid formation, yielding membranes with big macrovoids and low-pore interconnectivity, when directly immersed into a non-solvent bath.

Influence of oxygen availability on cell growth and xylitol production by Candida guilliermondii.

Oxygen availability is the most important environmental parameter in the production of xylitol by yeasts, directly affecting yields and volumetric productivity. This work evaluated the cell behavior in fermentations carried out with different dissolved oxygen concentrations (0.5-30.0% of saturation), as well as a limited oxygen restriction (0% of saturation), at several oxygen volumetric transfer coefficients (12 < or = kLa < or = 70 h(-1)). These experiments allowed us to establish the specific oxygen uptake rate limits to ensure high yields and volumetric productivity. When oxygen availability was limited, the specific oxygen uptake rate values were between 12 and 26 mg of O2/of g cell x h, resulting in a yield of 0.71 g of xylitol/xylose consumed, and 0.85 g/[L x h] for the volumetric productivity. According to the results, the effective control of the specific oxygen uptake rate makes it possible to establish complete control over this fermentative process, for both cell growth and xylitol production.

Xylitol production from D-xylose in a membrane bioreactor

Abstract Economical feasibility of xylitol production by microorganisms depends directly on the efficiency of conversion and productivity. A potential alternative to ensure this objective is continuous bioconversion. Xylitol production and separation methodology by a biotechnological route using the benefits of membrane technology were investigated. Xylitol produced by a strain of Candida guilliermondii was obtained using a synthetic medium containing D-xylose in a continuous membrane bioreactor. Moreover, hollow-fiber membranes were prepared and modules suitable for continuous fermentation medium sterilization were designed. The membrane that presented the best performance had maximum pore diameter of 0.2 μm and permeability of 42.9 L/(m 2 .h.bar) for a simulated medium. The best results obtained with this system for the continuous production of xylitol in a membrane bioreactor was attained with a dilution rate of 0.03 h −1 , providing an efficiency of D-xylose conversion of 86% and productivity values up to 1.14 g of xylitol/L.h.