Influence of Catalyst Holdup on Process Intensification in a Reactive Distillation Column

Abstract

Reactive holdup is one of the crucial design considerations in a reactive distillation (RD) column. Column sizing in RD is an iterative procedure because the diameter governed by vapor rates varies with tray holdup due to the interplay between reaction and separation. In other words, the reactive holdup depends on column geometry, and it cannot be set arbitrarily. The fact is that inappropriate reactive holdup reduces reaction residence time, which eventually leads to poor conversion. Larger holdups are also essential for reactions with slow kinetics. However, the diameter established by vapor loading restricts the reactive holdup on individual trays due to hydraulic limitations. Larger holdup would either require large liquid heights (excessive pressure drop) or a larger column diameter. An alternative design is proposed to accommodate larger holdup by making the column diameter larger than that required by vapor loading. This design alternative may be expensive in terms of capital investment, but it can accommodate large holdup and overcome hydraulic limitations. The larger holdup also reduces the required energy input in the reboiler due to better internal heat integration. These striking facts in the proposed design demonstrate that larger reactive holdup requires a wider diameter, which increases capital cost but reduces energy cost, so an economic optimum exists. In this study, the competition of these parameters in the proposed alternative design of an RD column is investigated. Simulations are carried out using Aspen Plus for three industrial chemical processes, and their optimal design configurations are detailed by minimizing the total annual cost as the objective function. The results affirm the importance of this trade-off in these alternative designs and their quantitative analysis to obtain the optimal RD design.