Development of exopolysaccharide membranes for dehydration processes

This PhD work proposes the use of two microbial exopolysaccharides, GalactoPol and FucoPol, for the development of membranes to be applied in relevant dehydration processes in industry. The starting material is obtained from a low cost, abundant carbon source, the glycerol produced as a by-product from the biodiesel industry, which represents a key economic advantage. A special attention was given to the development of membranes with enhanced mechanical and transport properties, and superior water selectivity for application in dehydration processes by pervaporation and vapour permeation. The approaches selected for designing target membrane properties included polymer cross-linking and the development of hybrid membranes using the sol-gel method. Firstly, the membranes were modified using different crosslinking agents: trichloroacetic acid for GalactoPol and genipin for Fucopol. Different crosslinking protocols were studied in order to evaluate their potential use for ethanol dehydration by pervaporation. These membranes were characterised in terms of their morphological structure, resistance to solvents and mechanical properties. They showed high water affinity, good chemical resistance towards organic solvents and adequate mechanical properties. Pervaporation experiments were performed using both types of membranes for different water concentration in the feed stream (5.0 to 10.0 wt.%) at a constant temperature of 30 ºC. In addition, the impact of the exopolysaccharide purification protocol – by dialysis and by-dia-ultrafiltration – on the properties and transport performance of composite FucoPol membranes for pervaporation was also studied. Significant differences were found in the swelling behaviour and transport selectivity depending on the purification method. The shear stress imposed during purification by the dia-ultrafiltration method led to a disintegration of polysaccharide aggregates, and, as consequence, denser membranes were obtained, affecting the transport selectivity. The membranes developed, in particular, the composite GalactoPol-PES and dia-ultrafiltrated FucoPol-PES membrane, exhibited a high potential for ethanol dehydration, since a water/ethanol selectivity of 134 and 143, respectively, at 10.0 wt.% water in ethanol was achieved. Although they showed excellent affinity for water, they become progressively unstable in aqueous solutions. Thus, novel hybrid FucoPol membranes were developed combining the best properties of the inorganic network with the selectivity of the microbial polysaccharides, to be applied in pervaporation processes for ethanol dehydration, as well as, in gas dehydration. The hybrid membranes were prepared by incorporation of a SiO2 network homogeneously dispersed by a sol-gel method using (3-Glycidyloxypropyl) trimethoxysilane (GPTMS) as a crosslinker silica precursor. These membranes were structurally, mechanically and thermally characterised. They presented a dense and homogeneous structure, resistant to deformation, a glass transition temperature (Tg) of 43 ºC and a thermal decomposition between 240-251 ºC. Hybrid FucoPol membranes were successfully applied in ethanol dehydration, with higher selectivity values than commercial membranes PERVAP® 4101. However, they lost their stability in contact with solutions of 10.0 wt.% water in ethanol after three days of operation. In contrast, when these membranes were applied for N2 dehydration, they were stable, showed reproducible results and extremely high water selectivities. Permeation of water vapour and pure gases (CO2, CH4 and N2), at different conditions of gas humidity content was monitored by mass spectrometry. Gas mixtures containing 20 vol.% CO2 + 80 vol.% N2 and 70 vol.% CH4 + 30 vol.% CO2 were also studied to mimic industrial applications, namely flue gas and biogas dehydration. The hybrid membranes developed showed barrier properties to all gases studied, with a gas permeability below 1 barrer. On the other hand, they exhibited high water permeabilities and selectivities. When processing the biogas mixture, the water permeability was found to be three times higher than water permeability in the flue gas mixture, leading to a H2O/CH4 selectivity much higher than H2O/N2 selectivity (4042 and 294, respectively). The hybrid FucoPol membranes showed that, in real situations, have the capacity to dehydrate mixtures, with the advantage of not losing N2 or CH4, due to the low permeability values of these gases. This work shows that microbial polysaccharides obtained from a renewable source and purified without using solvents can be a sustainable alternative to other materials used in industrial dehydration processes. Strategies for further improvement should include optimization of polymer cross-linking conditions and optimization of the thickness of the active layer of the composite membranes in order to improve the long term stability of the membranes and promote higher fluxes, without compromising selectivity.