Inhibiting runaway reaction

The basic requirements of a reactor are 1) fissionable material in a geometry that inhibits the escape of neutrons, 2) a high likelihood that neutron capture causes fission, 3) control of the neutron production to prevent a runaway reaction, and 4) removal of the heat generated in operation and after shutdown. The inability to completely turnoff the heat evolution when the chain reaction stops is a safety problem that distinguishes a nuclear reactor from a fossil-fuel burning power plant. 

However, the initial absence of unstable groups is no guarantee for long-term stability of the compound. For example, some aldehydes and ethers are easily converted to peroxides by reaction with oxygen from air [35,37,38]. Organic peroxides represent a class of unstable materials while monomers represent a class of substances that can self-react by polymerization if not properly inhibited and if the temperature is not properly maintained. Runaway reactions can result in both of these examples. 

Safety based on inhibiting a runaway reaction can take three forms, all of which require a knowledge of the mechanism of the decomposition reaction (see also case histories Al 66-68, page 186). They are ... 

Quenching and dumping are more usual methods of inhibiting a runaway reaction. In both cases a quantity of cold inert diluent is added to the reaction mixture and stops the reaction by cooling it. The ideal substance is water, when applicable, which is cheap, readily available and has a high specific heat. 

It is perhaps not generally realised that a switch from batch to continuous reactor design has intrinsic beneficial PI and safety implications when exothermic reactions and their associated runaway risks are involved. For batch operation, the time during which the reaction exotherm is generated is only a fraction of the batch cycle time. In order to control the reactions, it is imperative that provision is made to cope with the maximum likely heat evolution load so as to inhibit runaway. On the other hand, the heat exchanger provision for a continuous process operating at the same production rate needs to be considerably less than that for the batch equivalent, because the heat load is uniformly time distributed rather than being concentrated in a fraction of the batch residence time. Hence, continuous versions of batch processes have both safety and intensification benefits. 

A calorimetric study of reaction with sodium or potassium hydroxides in ethanol or 2-propanol is given. At starting temperatures below 70°C the product is the appropriate nitrophenyl ether above that temperature, reduction of the nitro groups may come into play, to give much more energy and a variety of other products. This reaction is inhibited by oxygen. There is potential for runaway if such reactions are operated industrially with poor temperature control. The editor suspects that the stimulus for this study was an accident which sprayed the German environment with 2-nitroanisole. 

In the production of formic acid, a slimy of calcium formate in 50% aqueous formic acid containing urea is acidified with strong nitric acid to convert the calcium salt to free acid, and interaction of formic acid (reducant) with nitric acid (oxidant) is inhibited by the urea. When only 10% of the required amount of urea had been added (unwittingly, because of a blocked hopper), addition of the nitric acid caused a thermal runaway (redox) reaction to occur which burst the (vented) vessel. A small-scale repeat indicated that a pressure of 150-200 bar may have been attained. A mathematical model was developed which closely matched experimental data. 

Kammel, U., S. Schluter, A. Steiff, and P.-M. Weinspach (1996). "Control of Runaway Polymerization Reactions by Injection of Inhibiting Agents - A Contribution to the Safety of Chemical Reactors." Chemical Engineering Science 51, 10, 2253-59. 

Bilous and Amundson [1] were the first to describe the phenomenon of parametric sensitivity in cooled tubular reactors. This parametric sensitivity was used by Barkelew [2] to develop design criteria for cooled tubular reactors in which first order, second order and product- inhibited reactions take place. He presented diagrams from which for a certain tube diameter dt the required combination of CAO and Tc can be derived to avoid runaway or vice versa. Later van Welsenaere and Froment [3] did the same for first order reactions, but they also used the inflexion points in the reactor temperature T versus relative conversion XA trajectories, which describe the course of the reaction in the tubular reactor. With these trajectories they derived a less conservative criterion. Morbidelli and Varma [4] recently devised a method for single order reactions based on the isoclines in a temperature conversion plot as proposed by Oroskar and Stern [5]. 

True inhibition is usually applicable only to intermolecular reactions rather than decompositions. The polymerization of vinyl compounds is an example when a runaway is detected, an inhibitor such as tertiary butyl catechol can be added to stop the reaction by removing the free radicals which propagate it. This method can be applied to other types of reaction, but a laboratory study is always necessary to select the inhibitor and conditions such as inhibitor concentration, mixture temperature and speed of addition. The reliability of the inhibition system should be quantified and demonstrated to be acceptable if inhibition is to be used as the ultimate basis of safety. 

Flame retardant additives are added to the electrolyte to inhibit or delay the onset of radical propagation reaction and prevent the spread of fire when a thermal runaway occurs in a battery. Flame retardant additives have been widely used in other areas such as the plastics and coating industry, and battery flame retardant additives are designed with the same principle. However, more stringent requirements are applied to the battery additives, as the additives must be electrochemically inert, and preferably would have a positive effect on the battery performance. 

---