Overall Summary of Findings on Design Issues

The discussions and investigations presented in Chapters 3-6 were motivated by a lack of understanding of many practical aspects of hybrid reactive distillation design. It was shown that a reactive distillation simulation model could be built using the equilibrium stage model with appropriate modifications to reflect the reaction(s), and that this model accurately predicted the operating conditions and product compositions of an actual MTBE column that was used to validate the modelling process. The model can be modified to simulate both equilibrium reactions and kinetically controlled reactions so that it provides a satisfactory basis for the analysis of the operating characteristics of hybrid reactive distillation and for design studies.

The full hybrid reactive distillation model was used to simulate several ETBE and MTBE columns to determine the effect of key process variables. These effects have not previously been documented and were shown to vary significantly from what would be expected in a conventional distillation column. For example, the operating pressure of a reactive column should be optimised to balance the reaction and separation functions of the system. In addition to the readily apparent effects of pressure on the reaction equilibrium and rate constants (via changes in the phase equilibrium temperature) and on the relative volatility, the operating pressure affects the stage-to-stage compositions in a manner which can influence the direction and extent of the reaction.

it was also found that increasing fractionation is sometimes detrimental for the reaction and that the highest product purity and reactant conversion in a hybrid column are obtained with an intermediate number of stages and high reflux ratio. It is often not possible to match the effects of reflux with additional stages as can be done with non-reactive distillation.

There is an absence of suitable design methods for hybrid reactive distillation in the literature. However, residue curves diagrams and reactive residue curves diagrams can be used to indicate the feasibility of a reaction-separation and provide useful a priori information regarding the internal column compositions. Rigorous simulations can then be used to optimise the operating conditions and column topography and to perform the detailed design. A design strategy was proposed to assist this process.

The integration of hybrid reactive distillation technology within a process plant was also considered. It was found that ETBE could be a more attractive synthesis than MTBE (despite less favourable reaction thermodynamics) where the hydrocarbon feed was lean in isobutene. It was also found that the choice of ETBE production (rather than MTBE production) could sometimes result in a significantly lower capital outlay as some recovery equipment might be made redundant. The optimisation of an integrated ETBE process was then considered and the target operating conditions (i.e. control system set-points) were determined from a set of assumed economic constraints.

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