Cristiano P. Borges

Membrane formation mechanism based on precipitation kinetics and membrane morphology : flat and hollow fiber polysulfone membranes

Abstract Membrane formation from poly(bisphenol-A sulfone)/poly(vinyl pyrrolidone)/dimethylacetamide/water systems by phase inversion process using immersion–precipitation technique was investigated. The initial precipitation rate was determined from light transmission experiments and the membranes morphologies were observed by scanning electron microscopy (SEM). These results were used to explain an observed oscillatory behavior in macrovoid occurrence, as well as to identify the region where spinodal demixing dominates the early stages of the phase inversion process. It is proposed as a qualitative model dividing the solution in three different layers during the polymer solution mass exchange with the coagulation bath. Each layer is associated with different precipitation kinetics leading to distinct morphologies. The model assumes that the macrovoids development is a function of the resistances created by precipitation kinetics of former layers.

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.

Separation of fructose from a mixture of sugars using supported liquid membranes

The main purpose of this study was to investigate the feasibility of using supported liquid membranes to extract fructose from a mixture of sugars contained in a fermentation broth. The membrane consisted of a microporous polypropylene support impregnated with a solution of a phenylboronic acid derivative in 2-nitrophenyl octyl ether. Transport through a flat sheet membrane was studied as a function of carrier and feed concentration. A hollow fiber system was also examined, and the effects of the carrier and feed concentrations, as well as the flow rate through the fiber lumen, on the glucose and fructose fluxes and fructose selectivity were studied. The hollow fiber system is more stable than the flat sheet supported liquid membrane, and produces higher fructose selectivities using a lower carrier concentration. The hollow fiber supported liquid membrane is able to remove fructose from a fermentation broth, although the membrane flux and long term stability need further improvement.

Economic analysis of ethanol and fructose production by selective fermentation coupled to pervaporation: effect of membrane costs on process economics☆

This work presents a preliminary economic analysis of the production of ethanol using different fermentation processes. The installation of a new plant and the adaptation of an existing ethanol plant to the selective fermentation of glucose from sugar cane hydrolysate were considered. In the latter process, glucose is converted to ethanol and fructose accumulates in the fermentation broth. The use of pervaporation for continuous removal of ethanol from the fermentation broth is also considered, in order to minimize inhibition of the microorganism and to facilitate product recovery. The results show that the selective fermentation coupled to membrane processes to removal of ethanol is an attractive process to increase ethanol production economics, although membrane performance and costs should still be improved.

Evaluation of flat sheet and hollow fiber supported liquid membranes for fructose pertraction from a mixture of sugars

This work investigates the use of supported liquid membranes (SLM) in the pertraction of fructose from a mixture of sugars contained in a fermentation broth. Membranes consisted in a porous polypropylene support impregnated with different kinds of carriers using 2-nitrophenyl octyl ether as solvent. Transport through flat sheet and hollow-fiber membranes was studied as a function of carrier and feed concentration. The results show that a boronic acid derivative yields the highest fructose selectivity and the hollow fiber supported liquid membrane (HFSLM) is more stable than the flat sheet system, also yielding higher fructose selectivities using lower carrier concentration. The HFSLM using a boronic acid as carrier was able to remove fructose selectively from a fermentation broth. Simulations of fructose removal from a fermentation broth were carried out using the experimental results obtained in this work. The results show that fructose removal from the fermentation broth can reduce microorganism inhibition and increase the system performance, although, further improvement in membrane stability and fluxes are still necessary.

Study of the synthesis of poly(4,4′-diphenylether-1,3,4-oxadiazole) in solutions of poly(phosphoric acid)

Abstract Using statistical design of experiments, samples of poly(4,4′-diphenylether-1,3,4-oxadiazole)s, POD–DPE, were synthesized through solution polycondensation of 4,4′-diphenylether dicarboxylic acid, DPE, and hydrazine sulphate, HS, in poly(phosphoric acid), PPA, under different reaction conditions. Final polymer molecular structure (average molecular weights, polymer composition, amount of hydrazide groups) is shown to depend strongly upon the polymerization conditions and that adequate properties for membrane synthesis (high average molecular weight, low amounts of hydrazide groups and low gel contents) can only be obtained if high synthesis temperatures (160°C) are used. Empirical and semi-theoretical models are developed to describe the kinetics of POD–DPE production in PPA solutions, showing that the likely existence of cross-linking reactions may explain both the relatively high values and the large fluctuations of the average molecular weight obtained experimentally.

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.

Hollow fibre modules for orange juice aroma recovery using pervaporation

The use of poly (dimethyl siloxane) PDMS hollow fibres was investigated in orange juice aroma recovery. Modules for pervaporation were designed and built for this purpose, scrutinized with binary ester-water feeds and compared to other module geometries using experimental enrichment factors and mass transfer coefficient values. Operational variables tested for the aroma separation included feed flow rate and aroma feed concentration. Further experimental runs were carried out with an industrial multicomponent feed consisting of an aqueous orange juice by-product. The results show the good potential of the modules for dilute aroma recovery.

Synthesis and characterization of flexible polyoxadiazole films through cyclodehydration of polyhydrazides

Abstract Aiming to produce flexible films of poly(4,4′-diphenylether-1,3,4-oxadiazole), POD-DPE, the synthesis of POD-DPE through thermal cyclodehydration of polyhydrazide in solutions of poly(phosphoric acid), PPA, and in film form is studied here. It was observed that POD-DPE polymer samples obtained when the cyclodehydration of polyhydrazide is performed in solutions of PPA are insoluble in the solvents normally used for film preparation. On the other hand, the cyclodehydration of polyhydrazide in film form can lead to brittle POD-DPE films due to the high temperatures required for reaction to occur. In order to obtain flexible POD-DPE films with high conversion through cyclodehydration of polyhydrazide films, it is shown here that it is very important to adjust the cyclodehydration temperature ( T c ) as a function of the glass transition temperature ( T g ) and the weight average molecular weight ( M w ) of the polyhydrazide. In all cases analyzed, polyhydrazides were synthesized by low temperature solution polycondensation reactions and were characterized by intrinsic viscosity measurements, size exclusion chromatograpy, nuclear magnetic resonance, infrared spectroscopy and thermal analysis. Polyhydrazides with weight average molecular weights up to 74,100 g/mol were obtained. The POD-DPE films obtained were characterized by infrared spectroscopy and nuclear magnetic resonance, showing that the production of POD-DPE films is indeed possible from cyclodehydration of polyhydrazide films.