(176c) Optimal Operating Policies for Reactive Chemical Systems and Application to Salicylic Acid Nitration by a Nitric Acid/ Acetic Acid Mixture

Salicylic acid can be nitrated to produce 5-nitrosalicylic acid which is an important intermediate in the pharmaceutical industry. Many industrial nitration processes use a mixed acid system of sulfuric and nitric acids; Andreozzi et al. propose an acetic acid/nitric acid system for the nitration of salicylic acid. This system results in higher yields, easier separation of the desired solid product, and improved process safety and can be considered a possible alternative to the current industrial nitration processes.

The nitration of salicylic acid by a nitric acid/acetic acid mixture results in several solid products that have low solubility in the reacting medium compared to the reactant species. Thus, the solid products can precipitate out of the reacting solution allowing for easy separation by filtration leaving the remaining liquid phase species to be separated. We propose a flowsheet for this process, shown below, where the reactant species, nitric acid, and the solvent, acetic acid, are separated by distillation and recycled.

When a new chemical plant is at the design and start-up stage there is the issue of optimal usage of excess equipment capacity; this question also arises during nominal operating conditions or times of increased/decreased production rate. Ward et al. and subsequently Griffin et al. have developed a simple decision hierarchy to determine optimal operating policies for different classes of process chemistries. Based on the kinetics of a reaction system, it can be determined quickly if it is economically optimal to operate the reactor completely full at all times (?bounded?) or not (?non-bounded). For ?bounded? chemistries there can be economic losses corresponding to selectivity losses and/or separation costs if the reactor is not operated at its maximum volume constraint under all operating conditions. The nitration system of salicylic acid is used here to demonstrate the usefulness and simplicity of this methodology and its possible applications to multiphase industrial processes. It is shown that the economic optimal operating point for this process is on the reactor volume constraint for all production rates corresponding to a bounded chemistry, as predicted by the decision tree.

Andreozzi, R., M. Canterino, V. Caprio,
I. Di Somma, and R. Sanchirico. ?Salicyclic Acid Nitration by Means of Nitric Acid/Acetic Acid System: Chemical and Kinetic Characterization,? Organic Process Research & Development, 10, 1199 (2006).

Griffin, D. W., J. D. Ward, D. A. Mellichamp, and M. F. Doherty. ?Steady-State Operating Policies for Plants with Multiple Reactions of Equal Overall Order,? Industrial & Engineering Chemical Research, 45, 8056 (2006).
Griffin, D. W., J. D. Ward, D. A. Mellichamp, and M. F. Doherty. ?Selectivity vs. Conversion and Optimal Operating Policies for Plants with Recycle,? Submitted 2007.

Ward, J. D., D. A. Mellichamp, and M. F. Doherty. ?The Importance of Process Chemistry in Selecting the Operating Policy for Plants with Recycle,? Industrial & Engineering Chemical Research, 43, 3957 (2004).