Cd N

Fig. 5.29. Simulated reaction rates and liquid-phase component activity profiles inside a catalyst particle for MTBE synthesis

For illustration, here we will briefly summarize some phenomena that will originate from the diffusion-reaction interaction at acid ion-exchange catalysts during the MTBE synthesis. In Fig. 5.29 simulated profiles of reaction rates and component activities within a spherical catalyst particle are given, assuming an excess of isobutene in the surrounding bulk phase. As can be seen from this figure, due to internal diffusion limitations the methanol concentration decreases towards the particle center. In contrast, the MTBE-content increases towards the catalyst center. As a result of the methanol decrease, the MTBE-formation rate is increasing, reaches a maximum, and then is sharply decreasing until it drops nearly to zero. The reason for this peculiar phenomenon, is the negative reaction order of the reaction rate (5.63), with respect to the reactant methanol. Consequently, there is a particle core that is nearly free of methanol. This gives rise to an unde-sired side reaction, that is the dimerization of isobutene and its further reaction to higher oligomers that can deactivate the catalyst [51].

Dimer formation leads to a significant increase of the boiling temperatures along the stripping section of a RD column as can be seen from the experimental data shown in Fig. 5.30. The comparison with two alternative process models reveals that only a heterogeneous column model, that is a model that includes the mass transport phenomena within the catalyst rings, is able to predict the observed steady-state process behavior with respect to axial column temperatures (Fig. 5.30a) as well as with respect to the axial liquid-phase compositions (Fig. 5.30b).

j C4 = IB + 1B

Fig. 5.30. a) Experimental and simulated temperature profiles for packed MTBE RD column. b) Experimental and simulated liquid-phase compositions for MTBE synthesis in a packed RD column

DIB Me0H

Fig. 5.30. a) Experimental and simulated temperature profiles for packed MTBE RD column. b) Experimental and simulated liquid-phase compositions for MTBE synthesis in a packed RD column

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