Summary: Comparison of Alternative Control Structures for an Ideal
Two-Product Reactive Distillation Column
Muhammad Al-Arfaj and William L. Luyben*
Department of Chemical Engineering, Lehigh University, Iacocca Hall, Bethlehem, Pennsylvania 18015
Although steady-state design and open-loop dynamics of reactive distillation columns have been
explored in many papers, very few papers have dealt with closed-loop control. Most of these
control papers consider reactive distillation columns in which there is only one product, and an
excess of one of the reactants is sometimes assumed. This paper explores the closed-loop control
of a reactive distillation column in which two products are produced in a single column and
stoichiometric amounts of fresh feeds are desired. The reversible reaction is A + B h C + B.
The relative volatilities are favorable for reactive distillation; i.e., the reactants are intermediate
boilers between the light product C and the heavy product D. Simple ideal physical properties,
kinetics, and vapor-liquid equilibrium are assumed so that the basic control issues of reactive
distillation can be explored without being clouded by complexities of a specific chemical system.
Six alternative control structures are evaluated via rigorous dynamic simulation. All of the
schemes use a composition analyzer in the reactive zone of the column to detect the inventory
of one of the reactants so that fresh feed can be manipulated to satisfy the reaction stoichiometry.
Therefore, the use of excess reactant is not required. The interaction between design and control
is illustrated by the impact of holdup in the reactive zone: increasing holdup (catalyst) improves
the dynamic controllability of the process. Single-end temperature control can keep both products