Inherent SAFETY: WHEN "SAFE" CAN BECOME "Safer”. A CASE STUDY USING 2- and 3-Methylpyridine N-Oxidation In A Closed Reactor

A. Pineda-Solano¹, L. Saenz-Noval¹_,M. Papadaki², V. H. Carreto-Vazquez¹, Simon Waldram³, Subramanya Nayak³, and M.S. Mannan¹

¹Mary Kay O’Connor Process Safety Center, Artie McFerrin Department of Chemical Engineering, Texas A&M University, College Station, Texas 77843-3122, USA

²Department of Environmental and Natural Resources Management, University of Ioannina, Seferi 2, Agrinio, GR 30100, Greece

³Chemical Engineering Program, Texas A&M University at Qatar, PO Box 23974, Education City, Doha, Qatar

 ABSTRACT

Alkylpyridines and their N-oxides are extensively used in the pharmaceutical industry. Alkylpyridine-N-oxides are typically produced isothermally, in an open semi-batch reactor, near the mixture normal bubble-point, approximately 373 K, following a protocol described in more detail elsewhere [1]. Phosphotungstic acid, a highly selective complex metal compound, acts as a homogeneous catalyst and an aqueous solution of hydrogen peroxide as the oxidant. Approximately 30% of the added H₂O₂ decomposes via a parallel to the N-oxidation unwanted reaction, creating an oxygen rich environment in the flammable atmosphere formed by the alkylpyridine, thus compromising the safety of the process.

Previous calorimetric studies [2] performed in an open system, suggested that increased temperatures and/or catalyst concentrations can dramatically increase the selectivity towards N-oxidation thus reducing the use of H₂O₂ and practically eliminating its decomposition. Owing to the complexity of the measurements, their prolonged duration and physical restrictions set by the mixture thermal and thermodynamic properties, previous work was performed in an open system and a narrow temperature and catalyst concentration range. The industrially employed open system ensures the escape of the produced oxygen, but it simultaneously sets an upper limit on the permitted temperature of operation. Given that previous studies indicate practically 100% selectivity towards N-oxidation, the current research presents results obtained during 2- and 3-methylpyridine N-oxidation in a closed system. Based on those results, the paper compares, from a qualitative point of view, the inherent safety choices made for the open and the closed reactor operation and how they can affect the process efficiency. Potential causes of runaways and their consequences are also discussed.

   REFERENCES

1.         Pineda-Solano, A., et al. Inherently Safer Reactor Design for Complex Reactions Based on Calorimetry Studies. in 7th Global Congress on Process Safety. 2011. Chicago, Illinois.

2.         Papadaki, M. and J. Gao, Kinetic models of complex reaction systems. Computers and Chemical Engineering, 2005. 29: p. 2449-2460.

  ACKNOWLEDGMENTS

The financial support of the Mary Kay O'Connor Process Safety Center is greatly acknowledged.