| Summary: | Pervaporation has gained increasing attention as an energy saving process for
separating azeotropes such as ethanol and water mixtures. Pervaporation
distinguishes itself from other membrane processes in that it entails a phase
transition step that occurs during the diffusion through the membrane, from the
liquid phase in the feed to a vapor phase in the permeate. Pervaporation
performance is mainly regulated by the physicochemical structure of the
membrane rather than the vapor-liquid equilibrium of the system. A significant
amount of literature is available to show the successful developments in terms of
membranes and their use for pervaporation applications.
In spite of the substantial progress in pervaporation using polymeric membranes,
as has been reviewed in several articles, zeolite membranes have various
advantages over polymeric membranes, most notably their chemical and thermal
stability. Due to their uniform molecular-sized pores, zeolites are highly suitable for
the separation of molecules in mixtures both through their adsorption capacity and
molecular sieving effects. It was the aim of this study to evaluate the suitability of
our in-house manufactured centrifugally casted ceramic support for coating with a
thin defect free NaA zeolite layer. The composite membrane was used to optimise
some of the variables pertaining to water ethanol pervaporation.
Both single and double coated NaA ceramic composite membranes were
manufactured. The integrity of the zeolite layer was confirmed by SEM. XRD was
used to show that the coated layer consisted of the zeolite NaA. According to the
XRD no impurities were present.
Both the single and double coated membranes were used for pervaporation.
During pervaporation, the influence of the feed temperature and composition on
both the single components and binary mixtures was determined. The binary
mixtures were evaluated by varying the feed composition from 5 to 95% water and
the feed temperature from 308K to 328K.
The single coated membrane performed better than the double coated membrane
both in terms of flux and selectivity. The single coated membrane yielded a
maximum flux of 4.50 kg.m-2h-1at a selectivity of nearly 20 000, compared to the
highest flux for the double coated membrane of 0.70 kg.m-2.h-1and a selectivity of
4000. While the fluxes for the single components were higher than for the binary
mixtures, the real selectivity for the binary mixture increased substantially from the
ideal selectivity obtained with the single components. This was explained in terms
of the preferential adsorption and condensation of water in the hydrophilic zeolite
pores (both intra- and intercrystalline). Due to the condensation of water in the
pores, the permeation of the ethanol is restricted, resulting in the significant
separation factor obtained.
For the binary mixture, it was found that both the total flux and the water flux
increase with increasing temperature and water content in the feed. The best
compromise in terms of flux and selectivity, i.e. an average flux and maximum
selectivity was obtained between 55 and 75% water in the feed at 328K. The high
selectivity obtained throughout this study confirmed that a defect free zeolite NaA
had been grown onto the smooth inside surface of the tubular ceramic support.
The zeolite layer was furthermore very thin, confirmed by the high fluxes obtained
compared to literature. === Thesis (M.Sc. (Chemistry))--North-West University, Potchefstroom Campus, 2006.
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