Opposing-reactants geometry

Shown in Figure 8.3 is the opposing reactants geometry in which the two reactants enter the membrane from its opposite sides. A reaction plane is formed inside the catalytically active membrane. This implies that the flow front of either reactant is fairly uniform due to the well-engineered microstructure of inorganic membranes. The reactants arrive at the reaction plane in a stoichiometric ratio. Thus undesirable side reactions are reduced. It is noted that as any of the reactant flow rate or concentration changes, the reaction plane will migrate to a new position inside the membrane so that mass balance is maintained. 

The opposing-reactant geometry has been studied recently by several investigators for a number of catalytic reactions including destruction of NOx [Zaspalis et al., 1991d], conversion of hydrogen sulfide to sulfur [Sloot et al, 1990 and 1992] and oxidation of carbon monoxide [Veldsink et al, 1992]. 

Similarly the fast oxidation reaction of carbon monoxide proves to be amenable to the concept of opposing-reactant geometry [Veldsink et al., 1992]. In this case, alpha-alumina membrane pores are deposited with platinum as the catalyst for the reaction to proceed at about 250°C. 

CATALYTIC NON-PERMSELECTIVE MEMBRANE REACTORS WITH AN OPPOSING REACTANTS GEOMETRY (CNMR/ORG)... 

Concept of a Catalytic Non-Permselective Membrane Reactor with an Opposing Reactants Geometry... 

Figure 10.15 Schematic of a catalytic non-pcrmselcctive membrane reactor with an opposing reactants geometry [Sloot et al., 1990]...

As a building block for simulating more complex and practical membrane reactors, various membrane reactor models with simple geometries available from the literature have been reviewed. Four types of shell-and-tube membrane reactor models are presented packed-bed catalytic membrane reactors (a special case of which is catalytic membrane reactors), fluidized-bed catalytic membrane reactors, catalytic non-permselecdve membrane reactors with an opposing reactants geometry and catalytic non-permselective membrane multiphase reactors. Both dense and porous inorganic membranes have been considered. 

Nakao, S.-I., Suzuki, T., Sugawara, T., Tsuru, T., Kimura, S. (2000). Preparation of micro-porous membranes by TEOS/03 CVD in the opposing reactants geometry. Microporous and Mesoporous Materials, 37, 145—152. 

Considerations of this kind, that were not emphasized in connection with the unimolecular reactions dealt with in the preceding chapter, attain crucial importance when the geometric requirements of cycloadditions and cycloreversions are compared. Like the isomerizations previously discussed, cycloreversions are unimolecular a non-totally symmetric vibrational motion that may be called for by the correspondence diagram will ordinarily be opposed by a restoring force. Cycloadditions, at least the prototypical ones, are bimolecular the two reactants can approach each other in a variety of ways, their reorientation in space costing no energy at all. It then becomes reasonable to ask how the conclusions which may be reached by the orbital symmetry analysis depend on the initial geometry assumed for the approach of the reactants towards one another. 

Transition state geometries for reactions of open-shell molecules, particularly radical ions, pose special problems for DFT methods. In contrast to closed-shell systems whose ground state wavefunction is always totally symmetric, rearrangements of radicals and radical ions frequently involve crossings of states of different symmetry. In this situation, the molecule must lose symmetry to effect an adiabatic passage from reactants to products. In radical ions this loss often involves a localization of spin and charge, and it seems that DFT methods tend to oppose this localization. 

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