Consider Inherent Safety at Your Plant

Many sites can benefit -- but confusion about how to identify options stymies efforts.

By Dennis C. Hendershot, process safety consultant

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1. Large scale mixing. The nitric and sulfuric acids were fed through a dip pipe into the batch reactor and had to be mixed throughout the several thousand gallons of vessel volume to contact the organic substrate. Poor mixing would result in large concentration and temperature gradients, prompting more side reactions, reduced purity product and lower yield.

2. Micromixing. Nitric acid and organic substrate reacted quickly once they came into contact. However, the nitric acid was in an aqueous phase and the organic substrate in an organic solvent phase. What really controlled the rate of reaction was mass transfer from the aqueous to the organic phase. One factor that controls mass transfer is surface area between the phases — so designing a mixing system to maximize surface area (by providing many very small droplets of the aqueous phase) will maximize reaction rate.

3. Heat removal. Because the reaction is extremely exothermic, rapid removal of the heat of reaction is required to maximize reaction rate and minimize reactor size.

By knowing which process parameters are important, it's possible to design a reactor that optimizes them. A continuous stirred tank reactor with a few-hundred-gallon volume, an extremely high intensity mixing system and a large heat transfer area (from the reactor jacket and internal coils) was designed. The system was safer because the reactor was much smaller, product quality was better and raw material yield was higher. It probably would have been possible to reduce the size further with a plug-flow pipe reactor containing mixing elements. Similar technology, using an eductor as a reactor, has been used to make explosives.

The Crucial Element
The key to implementing ISD in any plant, new or existing, is a basic and thorough understanding of the process. What are the hazards? What physical and chemical parameters control the process? Such knowledge should underpin your efforts to eliminate or reduce hazards. Tools and checklists are available to help you ask the right questions, so you can use your process knowledge to identify inherently safer process options. But, without that process understanding, these tools won't do the job on their own. Ultimately, implementation of ISD depends on process understanding — this is exactly what you need to design and operate the most efficient and profitable plant. 


Dennis C. Hendershot is a process safety consultant based in Bethlehem, Pa., after having retired as Senior Technical Fellow at Rohm and Haas and principal process safety specialist at Chilworth Technology. E-mail him at d.c.hendershot@att.net.

References
1. Hendershot, D. C., "A New Spin on Safety," p. 16, Chemical Processing, May 2004, www.ChemicalProcessing.com/articles/2004/33.html.
2. Hendershot, D. C., "Rethink Your Approach to Process Safety," p. 36, Chemical Processing, September 2007, www.ChemicalProcessing.com/articles/2007/158.html.
3. "Inherently Safer Chemical Processes: A Life Cycle Approach," 2nd ed., Center for Chemical Process Safety, John Wiley & Sons, Hoboken, N.J. (2009).
4. "Guidelines for Design Solutions for Process Equipment Failures," Center for Chemical Process Safety, American Institute of Chemical Engineers. New York City (1998) (now marketed by John Wiley & Sons, Inc., Hoboken, N.J.).

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