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graph in Figure 3.18, the average temperature in the reactive zone TRx;av for the a390 = 0.95 case is 354 K. This gives relative volatilities of 1.34. Remember that, at any temperature, all of the relative volatilities are the same among the adjacent components.

Results for different a390 cases are displayed in Table 3.8. These are the optimum designs in terms of the four design optimization variables: column pressure and the number of stripping, rectifying, and reactive trays. Reducing the relative volatility increases both capital and energy costs.

Comparison. The top graph in Figure 3.20 gives a direct comparison of the TACs of both processes for the temperature-dependent cases. There is a small increase in TAC for the conventional multiunit process as the relative volatilities decrease, but there is a very rapid increase for the reactive distillation process.

The lower graph in Figure 3.20 shows how the reactor temperature in the conventional process and the average temperature in the reactive section of the reactive distillation column change as temperature-dependence changes. At base case conditions with a390 = 2 (no temperature dependence), the optimum reactor temperature in the conventional process is 367 K, and we assumed this did not change for other values of a390. At base case conditions with a390 = 2 (no temperature dependence), the optimum average reaction zone temperature in the reactive distillation column is 394 K. This is higher than the conventional reactor temperature. Because the reactive distillation column is removing products from the reaction zone, a smaller chemical equilibrium constant can be tolerated (higher temperature). The conventional process, in which no products are removed from the reaction zone, is favored by a lower reactor temperature because it gives a higher chemical equilibrium constant.

However, as the value of a390 decreases, the optimum average reaction zone temperature in the reactive distillation decreases because the separation is becoming more difficult as temperature increases.

These results clearly illustrate the fundamental difference between a conventional process and a reactive distillation process. In a conventional process, the temperature for reaction and the temperatures for separation can be independently set. This is not true for reactive distillation. Therefore, reactive distillation is not economical for systems in which the temperatures for reaction and for separation are not similar.

3.4.3 Real Chemical Systems

Some confirmation of the results presented above for the ideal quaternary chemical system can be seen in several articles that compare conventional processes with reactive distillation for real chemical systems. Two articles were found that make such a comparison and provide sufficient detail about process conditions.

ETBE Process. An article by Sneesby et al.1 provides a description of the conventional ETBE process. There are two reactors in series, the first operating at 90 °C and the second at 50-60 °C. The lower temperature in the second reactor gives a higher equilibrium constant because the reaction of ethanol and isobutene to produce ETBE is exothermic.

The temperature in the reactive zone of the ETBE reactive distillation process is about 70 °C. Thus, the conventional and reactive distillation processes have similar temperatures. Therefore, we would expect the reactive distillation process to be more economical, which is indeed the case.

Toluene Disproportionation Process. A article by Stitt2 compares a conventional process to produce benzene from toluene with a reactive distillation process. Several steady-state economic indicators are used to show that the reactive distillation process "does not prove to be a fruitful development opportunity ... due to economic considerations."

The reactor temperature in the conventional process is 400 °C (vapor-phase reaction). The separation section consists of several distillation columns operating at normal temperature levels for benzene/toluene separation.

The reactive distillation column has a temperature of about 280 °C in the reactive zone. To achieve this temperature, the column must operate at 30 bar.

The difference between the conventional reactor temperature and the reactive distillation temperature indicates that the optimum temperature for reaction is different than the optimum temperature for separation. Therefore, we would expect that the reactive distillation column would not be superior to the conventional process, which is precisely what Stitt found.

'M. G. Sneesby, M. O. Tade, R. Datta, and T. N. Smith, ETBE synthesis via reactive distillation. 1. Steady-state simulation and design aspects, Ind. Eng. Chem. Res. 36, 1855-1869 (1997).

2E. H. Stitt, Reactive distillation for toluene disproportionation: A technical and economical evaluation, Chem. Eng. Sci. 57, 1537-1543 (2002).

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