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Physicochemical Transport Phenomena

It is preferable if the reactions in RD columns are catalyzed by heterogeneous catalysts because they are easily kept in the reactor and they allow hybrid RD columns combining reactive and non-reactive sections. Fig. 5.25 illustrates a column randomly packed with supported ion exchangers in rings. Such a heterogeneous cat-

Catalytically Active Rings (8x8 mm)

Supported Ion Exchange Resin

Feed

Fig. 5.25. Installation of ring-shaped ion-exchange catalyst in distillation column

Catalytically Active Rings (8x8 mm)

Supported Ion Exchange Resin

Feed

Fig. 5.25. Installation of ring-shaped ion-exchange catalyst in distillation column alyst was successfully used for the synthesis of the fuel ether MTBE [25], the fuel ether TAME [26, 27], and the ester methyl acetate [28],

Fig. 5,26 gives a schematic representation of the mass and energy transport phenomena occurring in the RD process with a heterogeneous catalyst. In this process, the chemical reaction takes place in the solid catalyst phase. Along the external catalyst surface, the liquid phase trickles down the column. The liquid phase and the rising vapor phase are in counter-current flow. The sketch in Fig. 5.26 assumes the catalyst to be totally wetted at its external surface. Internally the porous catalyst bodies very often are completely wetted due to capillary forces but in some cases this situation can be changed by internal boiling processes [29].

The directions of the mass and energy fluxes between the phases cannot be predicted because of the influence of convection in axial column direction, and due to the fact that multicomponent mass-transport phenomena may result in uphill transport [30]. There are three types of transport resistances present in the considered three-phase (vapor/liquid/solid) process

- transport between the vapor and the liquid phase,

- transport between the liquid and the solid phase,

- intraparticle transport within the solid phase.

If supported macroreticular ion exchangers are used as catalysts, the intraparticle transport processes can be decomposed into gel phase diffusion and macropore diffusion [31]. For a combination of the two mechanisms special catalyst models were proposed [32].

Furthermore, it should be mentioned that for structured packing additional mass-transport resistances may exist between the catalyst particles in the structured wire gauzes of the packing. So far, relatively little is known about these inter-particle resistances. However, they may play a major role in RD operations since a v-i v-i

Porous Catalyst

Porous Catalyst

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