To illustrate how ISD can be applied at various levels of process development and design, let’s consider production of a generic chlorinated organic chemical.
Selection of basic technology. There may be a variety of chemistry options for producing the molecule of interest. Some may use elemental chlorine, while others may rely on other chlorinating agents or be based on other readily available chlorinated organic chemicals that eliminate the need for a chlorination step. The research chemists should search for alternative synthesis routes, consider the hazards associated with the available chemistries and look for options that reduce the inherent hazards of the process.
Implementation of the technology. There may be many options available for implementing the technology chosen. For instance, if the chemistry requires elemental chlorine, the process engineers and chemists should consider whether to ship in or generate the element at the site. Each option has specific ISD characteristics relative to various hazards of concern. Other factors such as economics and availability of technology also come into play, of course.
Plant design. At this point in the process life cycle, the designer must consider ISD for a variety of factors, including:
- location of the plant relative to surrounding population and sensitive environmental areas;
- general layout of the equipment on the plant site;
- number of parallel systems and size of those systems (one big unit, or two or more smaller trains, for example); and
- size of storage facilities for hazardous materials.
Detailed equipment design. There are many options in the design of equipment such as heat exchangers, chlorine vaporizers and other devices that might be included in the plant. Different equipment designs will have different ISD characteristics — for example, the inventory of material in the equipment or the operating temperature and pressure. Also, the detailed layout of the equipment will impact plant safety characteristics such as the length and diameter of piping containing hazardous materials. In addition, ISD demands consideration of human factors for equipment, to minimize the potential for mis-operation and errors by personnel.
Operation. ISD should be considered in the development of operating and maintenance procedures. These must be clear, logical and consistent with actual human behavior. Also, the plant should keep ISD options in mind throughout the operational lifetime, particularly when modifications are made or if new technology becomes available.
ISD is not a magic bullet that will eliminate all potential risks associated with chemical processing. After all, in many cases the characteristic of a material or technology that makes it hazardous is the same that makes it useful. For example:
- Jet airplanes travel several hundred miles per hour. So, they can transport people long distances in a short time. But the speed also makes an airplane hazardous because its kinetic energy can cause major damage if the plane hits something.
- Vinyl monomers contain a double bond that can be highly reactive. When properly controlled, this reactivity allows the manufacture of a wide variety of polymers with useful properties. But, if the reactivity isn’t properly controlled, a runaway polymerization can prompt an explosion and fire, with potential for injury or fatality and significant property damage.
In some cases alternative technologies may be less hazardous or easier to control. But, for many technologies, inherently safer technologies don’t exist or aren’t economically feasible; so, we must rely on passive, active and procedural safety strategies to manage the risk. These strategies can be highly effective — travel by airplane, despite the significant inherent risks of flying, is extremely safe because of the highly effective safety management systems in place in the air transport system.
As discussed previously, any change to a technology designed to reduce one or more hazards may perhaps increase or introduce others. Chlorofluorocarbon (CFC) refrigerants provide an example. When first developed in the 1930s, CFCs were considered to be safer alternatives to existing refrigerants such as ammonia and light hydrocarbons. CFCs have low acute toxicity and are not flammable.
Toward the end of the 20th century, their adverse environmental impacts were recognized and many CFCs have since been phased out. While CFCs are still inherently safer than many alternatives with respect to flammability and acute toxicity hazards, society has decided that the previously unknown hazard of adverse environmental impact is unacceptable and is willing to apply passive, active and procedural strategies to manage the hazards associated with replacement refrigerants.
Such switches aren’t necessarily easy or straightforward. For a home refrigerator, it may not be a good idea to simply replace the CFC with another refrigerant — say, a light hydrocarbon. The quantity of light hydrocarbon (perhaps several kilograms), if it leaked, would be sufficient to create an explosive atmosphere in a room the size of a kitchen. Many “green” refrigerators feature a complete redesign to significantly reduce the amount of refrigerant to as little as a couple of hundred grams to minimize the fire and explosion hazard in case of a leak. This illustrates the importance of considering the design of a complete system when implementing ISD to ensure that all known hazards are adequately managed.
Different groups may perceive the inherent safety of technology options differently. A plant using chlorine has a choice between getting it in 1-ton cylinders or 90-ton railroad tank cars. Neighbors several miles away from the plant would consider the 1-ton cylinders to be inherently safer because a leak from one of these containers probably wouldn’t affect them. But plant operators would have to connect and disconnect 90 cylinders instead of one tank car, and each time they are at risk from chlorine exposure. So, the operators would consider the railroad car to be inherently safer because it requires less handling.