Incorporating ISD considerations into the plant PHA follows a procedure similar to that used in an ISD-specific PHA. However, the team doesn't restrict its recommendations to ISD but considers ISD solutions as one of many options available for managing hazards and risks. (See the sidebar for some tips.) When the team identifies a danger, it first seeks an ISD solution, trying to eliminate or reduce the hazard. It also considers other alternatives, including active, passive and procedural risk-management strategies. If the facility is located in a jurisdiction that requires consideration of ISD, it's important to clearly document evaluation of ISD.
Understand Your Process!
Identifying and implementing ISD demands a thorough grasp of the manufacturing process. Obviously you must appreciate all the hazards of your current route and potential alternatives to eliminate or minimize them. But to identify inherently safer alternatives, you must have a fundamental understanding of how your process works and what physical and chemical factors are most important in controlling its behavior. Then you're in a position to properly determine process and equipment alternatives that optimize these important factors, minimizing the required size of equipment while improving control of the process and reducing or eliminating hazards. I can't overemphasize the importance of understanding what's important in controlling the process — in general a plant that's under control is safe and will produce the desired product quality and quantity, maximizing profitability.
As an example, consider a nitration process. Nitration chemistry can be very hazardous. The reaction usually is highly exothermic; loss of control can result in a runaway reaction and explosion. Products can be unstable and it's possible to get unstable byproducts if reactions are improperly controlled. For one particular product, a company developed a semi-batch process in which an organic substrate was mixed with an organic solvent and then a mixture of nitric acid and sulfuric acid catalyst was fed at a rate to maintain a specified batch temperature. Initial design called for a several-thousand-gallon reactor; reactant feed would take many hours. Because of the large reactor size, any runaway reaction posed major consequences. To consider ISD options, it was essential to fully understand what physical and chemical factors dominated this process. The actual chemical reaction was of little importance — the nitric acid and organic substrate reacted extremely rapidly once they contacted each other. Three things were really important in optimizing this process from both an inherent safety and economic viewpoint:
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 firstname.lastname@example.org.
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.).