Every strategy doesn’t have to result in the complete elimination of the hazard or risk scenario. When we can make an incorrect action or assembly impossible (or at least very difficult) or design to accommodate the error without harm, we use the term “mistake proofing.” Where doable at a reasonable cost, this may be an attractive strategy because it rarely introduces alternative scenarios. For our chlorine cylinder example, mistake proofing might include using unique connections for the hoses.
In contrast, mistake tolerant systems provide timely feedback when a mistake happens, the means (either before or after loss of containment) to correct the error before an undesirable outcome occurs, or, if not corrected, reduced consequences from the mistake. For the chlorine cylinder, a mistake tolerant strategy might involve isolating chlorine inside buildings that have a chlorine vapor recovery system.
Putting The Strategies To Use
To illustrate the application of inherent safety strategies, let’s look at several real-world situations: sulfonic acid plant design, aluminum chloride (AlCl3) handling, a utility station and an electrical switchgear.
Sulfonic acid plant design. Reacting sulfur trioxide (SO3) dissolved in sulfur dioxide (SO2) with an alkylate feed produces sulfonic acid. This is an exothermic reaction that boils off SO2 as its primary means of heat removal. The SO2 performs the role of mutual solvent to allow intimate contacting between alkylate and SO3, which otherwise would only react at their mutual surface. All of the materials are flammable. The SO2 and SO3 are both inhalation toxics.
The heat of reaction boils the SO2 and SO3 from the reactor. In the traditional plant design (Figure 1), two drums collect the boiled-off vapor and allow the return of SO3 and any knocked-out liquid to the reactor. A compressor and cooling water exchanger provide cooled, liquefied SO2 for recycling to the reactor.
Following inherently safer design principles, the process was modified to eliminate the compressor and collector drums and replace the standard pumps with seal-less ones (Figure 2). This very significantly reduced the inventory of SO2 required to operate the process and removed two pieces of rotating equipment, each of which had the potential to leak toxic material to the air. In addition, because a Freon refrigerant is used, the bulk of the SO2 now is at a temperature not far from its boiling point, which minimizes vaporization in the event of a leak. However, these process safety improvements were achieved by using an ozone reactive material rather than cooling water.
The minimization and moderation strategies enhanced process safety — but opportunities exist to make the process even more inherently safe:
• Use the cooling exchanger as knockout pot and provide for gravity drain of cooled SO2 back to the reactor, eliminating the pump. (This requires relocation of the SO3 injection point.)
• Find a safer solvent than SO2.
In addition, even greater inherent safety may be possible by avoiding the process altogether, such as by switching to sulfonic acid alternatives that are made via inherently safer processes.
Aluminum chloride handling, part 1. Figure 3 depicts part of a process that uses AlCl3 as an ionic polymerization catalyst. AlCl3 is a powder that reacts violently with water to form toxic hydrogen chloride (HCl) gas and aluminum hydroxide (Al(OH) 3). Its contact with skin results in burns. Low-pressure nitrogen is used to unload AlCl3 from delivery trucks and transport the material to smaller vessels from which it is conveyed into the reactor. The AlCl3 is a very fine powder, some of which will travel with the nitrogen. All conveying nitrogen is returned to a silo that can contain as much as 80,000 lb of AlCl3. It then passes through a filter that returns most of the AlCl3 to the silo. What passes through the filter is scrubbed from the nitrogen in a packed tower where water is sprinkled down through the bed as the nitrogen rises and is released from an elevated vent stack. The slightly acidic water drops through a “p-trap” and then goes to the wastewater sewer.