When we talk about safety, we are typically concerned with protecting people, property, and the environment. Historically, safety practices have generally involved features we need to add on to our processes or procedures to prevent some type of accident or harm to people,
Green Chemistry and Engineering: A Practical Design Approach, By Concepcio´n Jimenez-Gonzalez and David J. C. Constable
Copyright2011 John Wiley & Sons, Inc.
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equipment, or both, and these have been known as passive, active, and procedural actions.
A brief description of these traditional safety program elements follows:
. Passive: includes things like designing equipment to withstand a certain pressure, or putting a dike around a storage tank.
. Active: includes systems such as safety interlocks or having alarm systems that would have multiple active elements, such as a sensor to detect a hazardous condition, a logic device to decide what to do, and a control element that would implement a corrective action. All of these things are designed to prevent incidents or to mitigate the consequences of incidents. Everyday examples of active safety systems include a child lock in a car, a lock on a front-loading washer that prevents the door from opening when the washer is in a middle of a wash cycle, and in a chemical plant, a low-level alarm that triggers a refill or shuts down the reactor’s heating elements.
. Procedural: standard operating procedures, safety rules, emergency response proce- dures, and, of course, a lot of training.
Traditional safety activities have also focused onmitigation measures, things we might do in the event of an accident. Such things as sprinkler or deluge systems, water curtains, emergency response systems, and catastrophic event planning would fall into the category of mitigation measures. All these mitigation measures are reactive or responsive activities; that is, they occur following a process upset or in response to an accident or unexpected event and are designed to minimize the overall risk of a catastrophic loss. The combining of elements of these loss-prevention strategies, generally known aslayers of protection, is illustrated in Figure 14.1.
Over a period of years and in response to some major accidents, such as the 1974 accident at the Nypro plant in Flixborough, UK,1the 1989 explosion at a petrochemical facility in Pasadena, Texas,2or the 1995 Lodi, New Jersey explosion during a blending operation that
Process Design
Community Emergency Response Plant Emergency Response
Physical Protection (dikes) Physical Protection (relief devices)
Automatic Action SIS or ESD Critical Alarms, Operator Supervision,
and Manual Intervention Basic Controls, Process Alarms,
and Operator Supervision
FIGURE 14.1 Layers of protection in classic chemical plant safety.
resulted in five fatalities and destruction of much of the manufacturing facility,3people began to think about safety in a different way. Perhaps the earliest pioneer in inherent safety was Trevor Kletz, who worked for ICI in the UK and following the Flixborough disaster began to work on specific design principles for the chemical industry. Additional notable work has been carried out by Preston and Turney, also of ICI, Hendershot at Rohm & Haas, Englund and Rogers at Dow, and many others. The point here is that inherent safety as a concept in many respects preceded green chemistry and engineering and to a certain extent, pollution prevention, and industry has always provided the intellectual and practical development of the discipline.
Clearly, all of the mitigation practices cited above generally work, work well, and continue to be used to this day. We also know that we can safely handle a variety of extremely hazardous materials on a daily basis; indeed, society would not have as many of its modern conveniences if this were not the case. But why not eliminate the hazard from the manufacturing process altogether rather than managing or controlling the risk? As we have noted throughout this book, avoiding a hazard and its attendant risk is always cheaper, safer, and provides less of an impact. Inherent safety is all about designing-out potential issues at the earliest possible moment in the development of a chemical synthetic route and its associated process.
Inherent safety fits very well into any discussion about green chemistry and green engineering. In each case, we are interested in avoiding the short- and long-term consequences of fires, explosions, toxic material releases, and so on (see Figure 14.2).
We can accomplish this in ways that are a natural fit with what we already do; evaluate reaction pathways, evaluate the chemistries and the materials we use, carefully consider our reaction kinetics and thermodynamics, use technology that ideally eliminates the need for hazardous substances and/or activities, and so on. We can also think about where we site our plants, the ways in which we transport our raw materials to and from those facilities, and how we store things. Finally, we can think about the details of equipment design and how our processing approach can eliminate risk even if there is a need to use a hazardous material.
Example 14.1 As part of a chemical synthesis you are designing, there are several stages where you are isolating powders and drying them. What are classic and inherent safety approaches that you might take to ensure safe handling of these powders?
Solution First, you need to think about the potential issues. The powder is going to be organic, and most powders will support combustion. Are they explosible? When they explode, what would the force of the explosion be? How quickly would the
design S, H & E safeguards
-passive -active -procedural Inherent
approach
-avoid/eliminate -minimize at source
FIGURE 14.2 Traditional vs. inherent safety approach.
explosion propagate? How much energy would be required to ignite the powder: a low- or a high-energy spark? What are some of the approaches I might take to reduce the risk?
. Passive
Build stronger equipment: greater than 120 psig pressure rating
. Active
Inerting (assuming an active system to maintain an inert atmosphere) Explosion venting
Explosion containment Explosion suppression systems
. Procedural
Procedures to maintain electrical grounding and bonding of equipment Procedures to avoid getting metal objects into the system (spark hazard) Procedures to avoid the generation of dust clouds
. Inherent
Use a different (noncombustible) material
Larger particles: granules or pellets (eliminate, or more likely, reduce the dust explosion hazard)
Eliminate the drying step: pass to the next stage as a wet cake
Additional Points to Ponder What other hazards and risks might there be in this example? Will any of the solutions introduce or increase the magnitude of other hazards and risks?