Figure 2. The small size of this reactor cuts inventory but makes the process more susceptible to an unstable reaction mixture.
A designer should ask four key questions when he identifies a hazard.
1. Can I redesign the system to completely eliminate the hazard? This is the inherently safer design approach. The extensive literature available provides guidance and checklists to help determine specific approaches for a particular system.
2. Can I modify the system to reduce the potential damage from the hazard? This is the second phase of a search for inherently safer alternatives. If the hazard cannot be eliminated, can it be significantly decreased in magnitude?
3. Do the modifications to the system identified in Questions 1 and 2 introduce new hazards or increase the potential damage from existing hazards? This is an important question that the designer must always ask. After focusing on a particular hazard or set of hazards and identifying potential improvements, the designer must step back and evaluate the entire system, considering all hazards, using the appropriate system hazard identification tools, such as process safety checklists or HAZOP. Any new or increased hazards must be evaluated against the overall benefits of the proposed changes, using engineering tools for process simulation, accident consequence analysis and accident likelihood. The designer also can evaluate the relative difficulty, cost and effectiveness of other risk-management strategies (passive, active and procedural). It also may be appropriate to apply decision-making tools that consider the relative importance of various kinds of hazards or different impacted populations.
4. What passive, active and procedural design features are required to adequately manage risk from the remaining hazards? It is almost impossible to develop a design that eliminates all hazards. So, there always will be a need to incorporate passive, active and procedural layers to reach safety goals. However, engineers too often accept the hazards in a system and jump directly to identifying systems and procedures to control and manage those hazards. A better approach is to first ask if the hazards can be eliminated or significantly reduced. But, it also is important to avoid focusing on only one or a few of the many hazards. Good decisions about inherently safer design require full knowledge and consideration of all hazards.
The next steps
Inherently safer design is an important tool for improving safety in the chemical industry. Properly applying it requires recognition that all systems have multiple hazards and that decisions should be based on the best available information about all of those hazards. Unfortunately, the concept does not get the attention it deserves. There needs to be continuous publicity, further development of methods and, perhaps most importantly, increased attention to the concept in the education of chemical engineers and chemists.
Figure 3. After loss of organic feed to the continuous reactor, it only takes about three minutes to an unstable reaction mixture to form.
Engineers should consider inherent safety in all of their activities. This extends from product and process research and development through plant design and operation -- and even to decommissioning and shutdown.
Regulations mandating consideration of inherently safer design will be difficult to implement and enforce. The language in proposed legislation is very general, requiring consideration of inherently safer technology. What does that mean? Inherently safer with respect to which of the many hazards in a process? Inherently safer to whom? Different organizations may evaluate the same technology options and make different choices because of local circumstances and considerations. If there are conflicts, how are choices to be made? Furthermore, these questions only consider the safety aspects of the choices. Other factors, such as economic and technical feasibility and the state of knowledge on the proposed alternatives, also are important.
Dennis Hendershot is senior technical fellow for the Engineering Division of Rohm and Haas Co., Croydon, Pa.