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Influence of Reaction Temperature

In RD processes isothermal operation of the catalyst bed is not possible. At the top of the column temperatures in the range of 60 °C are common, with higher temperature towards the bottom. Higher temperatures enhance the reaction more than mass transfer. Therefore mass-transport phenomena will have more pronounced effects at higher temperatures. We tested the catalysts at temperatures of 60, 75, and 90 °C. The results are shown in Fig. 8.13 and Fig. 8.15 for polymer/carrier catalysts with different polymer content. GFP-15 is a catalyst with low polymer content; GFP-12 has twice the polymer content of GFP-15. GFP-15 was chosen because it shows the classical MTBE kinetic pattern. GFP-12 was tested in an RD column.

As expected the maxima for GFP-15 are shifted towards higher methanol concentrations with increasing temperature. GFP-12 shows no maximum: rising temperature and increasing methanol concentration lead to higher reaction rates. At low methanol concentration both catalysts give a lower reaction rate.

To estimate the operating region of the catalysts, activation energies were measured. For GFP-12 the effective activation energy is 42 kj/mol, for GFP-15 the value is 74 kj/mol. This indicates that internal mass-transport phenomena influence the observed reaction rates. The activation energy for MTBE synthesis without internal mass-transport limitations should have a value of 92 kj/mol [11].

At 90 °C the GFP-15 has observed rates more than twice the rate of GFP-12. Despite having only half the polymer content per volume fraction of GFP-12, GFP-15 has the same catalytic performance in MTBE synthesis. Regarding selectivity, it is more advantageous to use GFP-12 in the region of low methanol concentration and high temperature. This can be seen in Fig. 8.13, Fig. 8.14, and Fig. 8.15. So we recommend the use of a low polymer content polymer/carrier catalyst at the top of the tower and a high polymer content polymer/carrier catalyst at the bottom.

Polymer/carrier Raschig rings have shown high activity and selectivity during tests in a laboratory RD column. Detailed investigations on this new catalyst have been reported [24].

Fig. 8.13 Influence of temperature on MTBE synthesis with polymer/carrier catalysts

Fig. 8.14 Influence of temperature on MTBE synthesis with polymer/carrier catalysts

Methanol concentration, c^^n [mol/l]

Fig. 8.14 Influence of temperature on MTBE synthesis with polymer/carrier catalysts

Fig. 8.15 Influence of temperature on the formation of diisobutene by using polymer/carrier catalysts with different polymer load
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