It is important to remember that as much as we might like to, we can’t get something for nothing. So, as in the case of pollution prevention, it is generally impossible to eliminate all waste and it is equally impossible to make everything perfectly safe. Consequently, we should talk about something being inherently safer rather than inherently safe; a certain relativity needs to be understood as we move forward. But what do we really mean when we say that something is inherently safer? If we dissect the term,inherentmeans that there is a feature that exists in something, or there is an attribute of something which is a permanent or inseparable part of that thing. So the phrase “an inherently safer process” means that safety is an inseparable or permanent part of a process from first principles or by design; it is not added or “bolted” on, but built in. Ideally, this means that hazard is eliminated in a process so that the process is risk-free. This is clearly something that exists only as an ideal; it is very difficult to attain in practice.
Having said that, there is much that we can do to make our chemical synthetic routes and their associated processes inherently safer, and there are a great many resources available to assist us.4–9As with pollution prevention, we speak of a hierarchy of controls or approach, as illustrated in Figure 14.3. Let’s look a bit more closely at each of the steps of the inherent safety hierarchy.
1. Intensification or minimization. Intensification or minimization can be understood in several ways: from a materials perspective, from a combined equipment and materials perspective, and from an energy perspective. From a materials perspective, the objective is to reduce the amount of a hazardous material or materials in a process or a plant. A useful way of looking at this is: “What you don’t have can’t leak.” This objective can be met through such strategies as in situ generation, or using less of a hazardous material at a given time through logistical planning and just-in-time delivery. It also has to do with simple things such as storage, intermediate storage, piping, and the types of process equipment in use. Alternatively, process intensification strategies such as those described in Chapter 15 can be used to probe what can be done to run reactions at a higher concentration or by using less material through a change in reactor design. For process intensification strategies that rely on increasing reaction concentrations, we must ensure that doing so will not result in an uncontrolled event. This is clearly where alternative reaction technology can and should play a roll.
As with many things in green chemistry, it is imperative that you understand what controls any chemical reaction so that process equipment can be designed to optimize the reaction. In general, you will want to develop reactions that occur very rapidly but with the proviso that they are not highly exothermic. One also needs to pay attention to energy and mass transfer or transport phenomena to optimize mixing, chemical equilibrium, and other molecular phenomena across phases and surfaces. Reactions that occur at room temperature and pressure, reactions that use single-phase systems of low viscosity, and reactions that are not sensitive to variations in reaction conditions are all desirable. Ideally, changes in temperature, pressure, concentration, or the presence of trace quantities of contaminants such as water, oil, or particulate matter such as rust would not create problems for the system. There is also the possibility of employing phase-transfer catalysts as a means of overcoming phase separation issues, or permitting the use of biphasic solvent systems that enhance the separability and isolation of the desired product or that can be used to control reaction kinetics.
2. Substitution. As with pollution prevention, the strategy here is to use a safer material or a set of reactions to replace more hazardous materials: replacing highly flammable substances with those that are less flammable, or toxic substances with those that are less toxic. If possible, complete elimination of a hazardous substance or set of conditions is ideal but is often not possible. It is also important to evaluate a substance and the volume required.
Source Reduction
(Elimination, Substitution)
Control
(Recycle, Integration)
Treatment
Cost(treatment & legislation)
Time to resolve
Hazard and Risk
Intensify or Minimize Substitute
Moderate Limitation of Effects
Intensify or Minimize Substitute
Moderate Limitation of Effects
Cost(treatment & legislation)
Time to resolve
Hazard and Risk
(a) (b)
FIGURE 14.3 (a) Pollution prevention hierarchy; (b) inherent safety hierarchy.
In some instances, replacing a small volume of a very hazardous material with a large volume of a less hazardous material may create greater risk of another type (e.g., environmental, occupational exposure, or other safety risk). Each situation needs to be evaluated carefully and holistically to arrive at the best solution.
3. Attenuation or moderation. In the attenuation or moderation step of the hierarchy, the desire is to use a chemical, hazardous or nonhazardous, under conditions that are less severe. The goal here would be to lower the reaction pressure or temperature. A good example of this would be storing chlorine or ammonia as refrigerated liquids at atmospheric pressure rather than at high pressure and at ambient temperature. The lower pressure means that if there were a leak, the leak rate would be lower while the decreased temperature reduces the rate of vaporization. Another possible approach would be to employ catalysts.
As catalysts lower the activation energy of a reaction, reactions can often be run at lower temperatures and pressures. As with intensification and minimization, phase-transfer catalysts may be used to good effect to moderate reactions or control reaction kinetics.
4. Limitation of effects. Simple changes in the way that chemicals are introduced into a reaction vessel can have profound effects on the overall safety of a process. So changing reactor designs, changing process conditions, or relying on protective equipment is desirable. For example, we can lower the final temperature of reaction liquors if we introduce a reactive chemical at a reduced temperature, or we might change the order or rate of addition of a very reactive substance rather than relying on a control system to sense a problem and attempt to control it.
5. Simplicity. The acronym KISS, “keep it simple, stupid,” is in view here. While we may need to build complex molecules, complexity in synthetic and process design is generally not a good thing. Simpler synthetic designs and simpler plants are generally safer than complex syntheses, processes, and plants because they generally provide fewer opportunities for human error and equipment failure. They are also easier to control if there are deviations from normal conditions before the situation turns into a catastrophic accident.
Now that we have the broad outline of the inherent safety hierarchy, you might ask: How do we know if a system or process is inherently safer? A reasonably simple way to look at this would be to say that if a system or process is perturbed yet remains within or returns to a safe and stable condition in the absence of human intervention or automatic controls, it is inherently safe. Getting to the point where we can use the hierarchy effectively will, of course, require a strong fundamental knowledge of the physical and chemical processes that are underlying and governing our reactions and manufacturing processes. It is important to remember that inherently safer design, like green chemistry, is first and foremost a way of thinking about the problems facing us and the tools and methods we use to solve those problems. It is how we think about things: from the conception of a route during retro- synthetic analysis to the final aspects of how we design our processes.
Example 14.2: Carbaryl Process Many of you may be familiar with the first process shown below as the process used by the former Union Carbide in Bhopal, India to make the pesticide carbaryl. Contamination of the methyl isocyanate with water resulted in the overpressurization of a tank holding methyl isocyanate and the rupture of a pressure relief valve. Because over 100 tons of the intermediate was stored on site, and the emergency safety systems were either inactivated or ineffective, over 2000 people were killed, many
then proposed and is shown below. Which process is inherently safer, and why?
Old process:
C H3 NH2
O Cl
Cl H3CN O
N O
C H3
OH O
O N CH3
+ + 2 HCl
methylamine phosgene methyl
isocyanate
hydrogen chloride
methyl isocyanate
+
(1)
(2)
α-naphthol carbaryl
New process:
O Cl Cl
OH O
O Cl
C H3 NH2
O O
Cl O
O N CH3 phosgene
+
α-naphthol
+
α-naphthol chloroformate
+ 2HCl (1)
methylamine
α-naphthol chloroformate
carbaryl
+ 2HCl (2)
Solution First, it should be noted that phosgene and methylamine are both toxic, hazardous compounds and have a certain degree of associated risk in their manufacture, handling, and storage. More recent synthetic strategies for making carbaryl have eliminated the use of both compounds, and these satisfy the second step in the IS hierarchy. In the Bhopal case, the methyl isocyanate (MIC) intermediate was a denser-than-air but still very volatile gas that is highly reactive with water. Storing 100 tons of MIC was not necessary, as the process could
have been run close to continuously with only a few kilograms of MIC needed at any time.
Had the plant operators made and used only the precise amount of MIC that was needed, they would have fulfilled the first step of the IS hierarchy and intensified or minimized the hazard and resulting risk of an adverse event.
In the second synthetic strategy, in situ generation of phosgene could be reacted with the a-naphthol to form the nonvolatilea-naphthol chloroformate. Thea-naphthol chlorofor- mate could then be reacted with methyl amine to form the desired carbaryl. The in situ generation of phosgene fulfills the first step of the IS hierarchy by minimizing the amount of phosgene on hand at any given time, and the fourth step, limitation of effects, as only the exact amount of phosgene needed is generated. Going through thea-naphthol chloroformate intermediate fulfills the second step of the IS hierarchy by substituting a less hazardous intermediate for a more hazardous one. Overall, the second process, although still exhibiting safety hazards and risks, is undoubtedly an inherently safer process.
Additional Points to Ponder What other changes would you propose to the newer process to make it inherently safer? What are other less tangible benefits of applying inherent safety principles?