Economic and Dynamic Impact of the Use of Excess Reactant in Reactive Distillation Systems

Abstract

Reactive distillation columns are sometimes operated with an excess of one of the reactants, but this mode of operation may have the disadvantage of requiring the recovery and recycle of the reactant that is in excess. The flowsheet typically consists of a two-column system: a reactive column and a recovery column. An alternative is to use a single reactive column that is operated “neat” (exact stoichiometric amounts of the reactants fed and consumed). The one-column process has lower capital investment and energy costs than the two-column system. However, Al-Arfaj and Luyben1 recently explored the control of a one-column “neat” system and demonstrated that a composition analyzer may be required to detect the inventory of one of the reactants so that the fresh feed can be adjusted. Because composition analyzers are expensive and sometimes unreliable, the economic advantages of the single-column system need to be assessed. The purpose of this paper is to quantitatively compare the single-column (neat operation) and the two-column system (excess of one of the reactants fed to the reactive column). The comparison includes both steady-state economics and dynamic controllability. The reversible reaction is A + B ⇌ C + D. 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. In the two-column system, an excess of B is fed to the reactive column. The distillate from the reactive column is product C. The bottoms is a mixture of components D and B, which are separated in the second column. The distillate of the second column is mostly B and is recycled back to the first column. Steady-state economics show a very significant incentive for the single-column system. Total annual cost of the two-column system is 50% more than that of the single-column process if a 10% reactant excess is needed to compensate for flow-measurement uncertainties and fresh-feed composition changes. If a 20% excess is needed, the total annual cost is 100% more. Both systems are dynamically controllable, but the dynamic response of the two-column process is slower because of the presence of the recycle and is more complex. Only tray temperature control is required in the two-column process.