(184g) Countercurrent Cascaded Reactors: Their Role in the Design of Reactive Distillation Columns

Multifunctional process units are an integral part of process intensification and reactive distillation (RD) is one of the most common examples of a multifunctional processes. Despite recent progress in the feasibility analysis, design, and, in some cases, control of reactive distillation, shortcut design of reactive distillation is not well understood, especially for the design of the reaction zone inside a column. Several authors proposed methods for the feasibility analysis and also for the calculation of minimum?reflux ratio.1-3 Lee et al. (2003) describes a method to estimate the minimum catalyst loading based on the nonreactive rectification body method, the critical composition profiles, the analysis of eigenvectors of pinch points, and geometrical design insights. Other than this, however, the quantitative description of catalyst loading and distribution is rarely discussed.

In this work, we use the concept of countercurrent cascaded reactors (CCRs) to describe the reactive zone. An analytical expression for the minimum theoretical catalyst loading can be derived as the number of CCRs approaches infinity. Next, McCabe-Thiele type column stepping can be employed to complete the reactive distillation system description.4 The tradeoff between the catalyst amount and the energy consumption can be found using the proposed methodology and subsequently, a shortcut design procedure can be derived. This shortcut method provides a minimum reflux ratio and the corresponding theoretical catalyst amount so that the number of separation stages and the number of reaction stages can be obtained.

An ideal binary system with the elementary reaction A↔B and ideal vapor liquid equilibrium is used to illustrate the design. Different parameters of the system, including relative volatility, specific reaction constant, equilibrium constant and product purity were evaluated and optimized designs with different parameters were compared to the shortcut design. The sensitivities of different parameters were calculated, and the specific reaction rate was found to have a significant effect on the catalyst amount which affects the calculation of the number of stages in the column. At moderate specific reaction rate, the design calculated by the shortcut method has a good agreement with the optimal design. Although this shortcut method does not always work well, it can be used for a preliminary process design or as the initial case for a rigorous optimization.

References:

1. Barbosa, D.; Doherty, M. F., Design and Minimum-Reflux Calculations for Single-Feed Multicomponent Reactive Distillation-Columns. Chemical Engineering Science 1988, 43 (7), 1523-1537.

2. Hoffmaster, W. R.; Hauan, S., Difference points in reactive and extractive cascades. III - Properties of column section profiles with arbitrary reaction distribution. Chemical Engineering Science 2004, 59 (17), 3671-3693.

3. Lee, J. W.; Bruggemann, S.; Marquardt, W., Shortcut method for kinetically controlled reactive distillation systems. AIChE Journal 2003, 49 (6), 1471-1487.

4. Lee, J. W.; Hauan, S.; Westerberg, A. W., Graphical methods for reaction distribution in a reactive distillation column. AIChE Journal 2000, 46 (6), 1218-1233.