Zeolite membrane case study

Recent results on isobutane dehydrogenation have been reported, and a conventional reactor has been compared with membrane reactors consisting of a fixed-bed Pt-based catalyst and different types of membrane [51]. In the case of a mesoporous y-AKOi membrane (similar to those used in several studies reported in the literature), the observed increase in conversion could be fully accounted for simply by the decrease in the partial pressures due to the complete mixing of reactants, products and sweep gas. When a permselective ultramicroporous zeolite membrane is used, this mixing is prevented the increase in conversion (% 70%) can be attributed to the selective permeation of hydrogen shifting the equilibrium. 

Tetrahydrofuran, THF, is an important industrial solvent and forms an azeotropic mixture at 5.3 wt% with water (see Table 10.3). To separate water/THF, Li et al. [148] tested the pervaporation performance of different hydrophihc zeolite membranes, zeolite A, zeohte Y, MOR, and ZSM-5. The preliminary test showed that the separation factor increased as the Si/Al ratio of the zeohte decreased, except for the case of zeolite A. This fact is probably due to the lower quality of this membrane with respect to the others since in the permeation of triisopropylbenzene (TIPB), showed the highest flux, 3.1 g/m h, indicating the presence of nonselective defects. Therefore, the best results were obtained with zeolite Y, rendering a separation factor of 300 with a water flux of 2.24 kg/m h at 60°C. The water flux increased with water concentration in the feed, up to a value of 15 wt%, indicating that the zeolite was saturated, as was the same for the case of water/ethanol mixtures in zeolite A, previously described. At the same time, the separation factor decreases as water concentration decreased. The stabihty of the membrane was also studied, showing a stable performance after 35 h of operation. 

Various chapters on membrane reactors (MR) consider different aspects of the integration of membranes with other conventional systems pervapo-ration, zeolite, bioreactors, fuel cells, wastewater treatment, systems for electrical energy, and so on. However, among the various possible examples not cited in these chapters, in the following, due to the lack of space, only seven, but very interesting, case studies are taken in consideration. 

Case studies of heat and mass transport across the zeolite membrane... 

In this chapter, we have presented results obtained for transport under non-equilibrium conditions, and shown how this method models transport across a zeolite membrane in agreement with the second law of thermodynamics. We clearly see changes in temperature at the surface of the zeolite, and this effect needs further study. In this case we have only presented transport of single components across the zeolite. In future work, the selectivity and separation purposes should be studied further. 

In many studies the separation factor, which is indicative of the membrane s ability to separate two gases in a mixture, is predominantly governed by Knudsen diffusion. Knudsen diffusion is useful in gas separation mostly when two gases are significantly different in their molecular weights. In other cases, more effective uansport mechanisms are required. The pore size of the membrane needs to be smaller so that molecular sieving effects become operative. Some new membrane materials such as zeolites and other molecular sieve materials and membrane modifications by the sol-gel and chemical vapor deposition techniques are all in the horizon. Alternatively, it is desirable to tailor the gas-membrane interaction for promoting such transport mechanisms as surface diffusion or capillary condensation. 

A method for determining the effect of particle size on the effective permeability values of zeolite-polymer mixed matrix membranes has been developed in this study. The model presented is a modified form of the effective medium theory, including the permeability and thickness of an additional phase, the interphase, which is assumed to surround the zeolite particles in the polymer environment. The interphase thickness and permeability values were determined by taking into consideration the assumptions that in case the size of the zeolite particles is held constant, the interphase thickness should be equal for different gases and in case the zeolite particle size is varied, the interphase permeability should remain constant for the same gas. The model seems to fit the experimental permeability data for O2, N2 and CO2 in the silicalite-PDMS mixed matrix membranes well. 

Okamotoetal. [193] studied several water/organic systems that are Usted in Table 11.5, and the performance of the zeolite A membrane was excellent for all the separations. These results could be also compared with the ones obtained using microporous silica manbranes [213]. Silica membranes, fora water/dioxane (10/90 wL%) mixture at 60°C, showed a separation factor of 125 and a water flux of 2.2 kg/(m h). In the case of DMF, the results obtained for a water/DMF mixture (13.2/86.8 wt.%) were 30 and 0.225 kg/(m h) for the separation factor and water flnx, respectively. In both separations, zeolite A ontperforms the microporous silica manbrane. 

The kinetics of polymer adsorption on porous substrates is much more difficult to tackle. Besides adsorption, desorption, and exchange, size exelusion has to be taken into account. Also, most in situ methods are not applicable to porous substrates. A major difficulty is that with all available methods smeared-out properties are measured while it is likely that strong gradients in the axial direetion of the cylindrical pore are present. The process of axial equilibration is poorly understood and in many cases extremely slow. Most studies were performed with porous substrates with broad pore size and shape distributions. Controlled-pore glasses, zeolites, or porous membranes could be used as model systems with pores of molecular size. Application of glass capillaries is interesting for controlling the hydrodynamics in a curved system. 

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