Distillation Operating Window

Fig. 20. Example of constraint control operating window for a distillation column, where (-----) is the vessel pressure limit, and ( ) represents constant...

Table 1 gives an overview of the possible applications of reactive distillation reported in the literature. Very few of them have been realized so far on the commercial scale. One of the common factors that hinders a broader application of reactive distillation is a small feasible operation window. The overlap region in the pressure-temperature domain, in which chemical reaction and separation and apparatus design are feasible, is usually quite narrow (see Figure 2 in Chapter 9). A possible remedy for this limitation is sought in the development of new types of catalysts that would allow one to significantly broaden the feasible operation window for chemical reaction. 

A further opportunity for process optimisation involves the judicious selection of reaction conditions (temperature, pressure and hydrogen / hydrocarbon ratio) so as to promote the transfer of primary middle-distillate product into the vapour phase with a view to reducing its residence time in the catalyst bed and, consequently, preventing distillate loss via secondary cracking. This approach to improved overall distillate selectivity has previously been referred to by Filers et al. [2] and provides the incentive to optimise catalyst performance within the constraints of the operating window so defined. 

Can move within a range (operating window) while both distillate and bottoms compositions stay within spec. 

The best-known and (commercially) most successful example of combining reaction and separation is the reactive distillation. Reactive distillation has been investigated widely [3, 4 see also Chapters 3, 4, and 5] and is applied to many processes [5], However, the integration of more than one functionality in an apparatus leads to a loss in degrees of freedom. For a successful integration, the feasible window of operation concerning process conditions such as pressure and temperature must coincide for the reaction, the separation and the apparatus (Fig. 8.1). 

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