Applicability of Reactive Distillation

The reactive distillation process for MTBE quickly gained acceptance in the market place after the first commercial application was installed in 1981 (Smith and Huddleston, 1982). Today, most of the new MTBE plants which are currently in either the planning or construction phases will use processes based around reactive distillation (Riddle, 1996). Although the advantages which reactive distillation brings to MTBE production are well recognised, it is not clear whether the same benefits can be realised for ETBE synthesis.

The most obvious difference between the MTBE and ETBE reaction systems is the more restrictive ETBE thermodynamics. This implies a lower isobutene conversion at the same temperature but it does not preclude the use of reactive distillation. Indeed, the improvement in the stoichiometric ratio of reactants is potentially even more advantageous since there is more to gain. The ETBE reaction rate is similar to MTBE so that the required catalyst volume should not be prohibitive. The reaction is also equally selective (up to 99%) so that the side-reactions (predominantly dimerisation and hydration) are not likely to be any more significant.

Ethanol and ETBE are less volatile than methanol and MTBE, leading to higher distillation temperatures at the same pressure. This would result in a faster reaction rate but a lower equilibrium constant, although the operating pressure could be adjusted to mostly compensate for the VLE differences. The most important distinction in the phase behaviour of the two systems is the absence of azeotropes between ethanol and butenes at pressures below approximately 1400 kPag. Comparable azeotropes in the MTBE system are important in recycling methanol to the reaction zone and this variance suggests that a slightly different operating strategy might be required, notably that excess ethanol should be recovered with the bottoms rather than the distillate. The absence of these azeotropes will also affect the reaction via changes in the internal distribution of components although the exact effect is hard to predict without detailed simulations.

Overall, there appear to be sufficient similarities between the MTBE and ETBE systems to suggest that reactive distillation could deliver productivity gains to ETBE manufacture that are similar to those already demonstrated for MTBE. However, there are also enough differences between the two systems to indicate that specialised designs might be required to maximise the process benefits that could be obtained from reactive distillation.

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