About once a month, a Google News alert that I set up is populated with a report of an accident in a chemistry lab. The cause is usually the same: nitric acid in a garbage can where someone inadvertently adds some acetone rinse waste or some other organic solvent, creating an explosion that destroys a hood. It’s a demonstration of the power of concentrated nitric acid as a reactant – one that chemists routinely use to add nitrogen to their molecule and one that industry uses in chemical processes.
It is difficult to overestimate the history and importance of nitration in both modern life and chemistry itself. The Nobel Prizes came from the fortune of Alfred Nobel, who stabilized nitroglycerin in diatomite to make dynamite; Nitroglycerin is made from the reaction of nitric acid and glycerin. You can’t just use any nitric acid like the concentrated nitric acid that you have in your laboratory in the bottle with the red cap. Instead, you must use “white fuming nitric acid” which contains 95% nitric acid and does not contain yellow or brown oxides of nitrogen.
While nitrate esters have made history as explosives, other forms of nitrates are ubiquitous. Ammonium nitrate is a common fertilizer and contains the nitrogen that plants need to grow. It is made by the simple reaction of ammonia and nitric acid. Again, there is an unfortunate history of large-scale industrial explosions, including the 1921 Oppau explosion in Germany and the Texas City disaster in the US in 1947. In 2013, a fire broke out near an ammonium nitrate warehouse in West City, Texas, and the subsequent explosion killed 15 people and destroyed much of the city.
No wonder then that Bretherick’s informative reference Handbook of the Hazards of Reactive Chemicals has this dire statement about the risk associated with nitriding: “The most widespread and most destructive industrial process operation. Because nitric acid can under certain circumstances cause a complete and strongly exothermic conversion into gases. ‘
However, millions of tons of products are nitrided worldwide every year. Nitric acid and other nitrating agents are used to make intermediates for active pharmaceutical ingredients (since nitro groups are still a good way to incorporate a nitrogen into a molecule). The extremely popular polyurethanes found in furniture and synthetic fibers use two monomers that require aromatic nitration; in the case of methylenediphenyl diisocyanate, benzene is mononitrated to produce nitrobenzene which is then reduced to aniline. In the case of toluene diisocyanate, toluene is dinitrated and then reduced to the amine functionality.
Industrial scale nitrations use a mixture of nitric and sulfuric acids. The sulfuric acid acts both as a catalyst and as a dehydrating agent for the water produced during the reaction. When you are dealing with millions of tons of nitric and sulfuric acid, disposal is a matter of concern. Dealing with just a few thousand liters of acid is not too difficult – you can simply neutralize the acid with an inexpensive base such as caustic soda and send it for disposal. However, if you’re dealing with nitric and sulfuric acids in the cars, you’d better regenerate the acid and return it to the process. It requires a lot of specialized equipment, but it’s better than spending your time neutralizing it.
Waste isn’t the only difficulty nitriding plants encounter – you also need to think about corrosion in your reactors. While choosing the right building materials is crucial, you still need to remember to check your equipment to see if the nitric acid has corroded the surfaces. Surprisingly, coating the surfaces with PTFE may not be sufficient, as the nitrogen oxide species, which are often dissolved in nitric acid, can penetrate this layer. Checking for early signs of corrosion can be tedious and certainly requires downtime and maintenance. However, it is much better than the different display options for leaks – sometimes discreetly with a slight pressure drop or sometimes with a stream of acid flowing from the reactor onto the system floor.
Unless you’re making millions of tons of nitrided material, there are many newer technologies to consider. Flow units for nitration are a perfect setup for safely performing kilogram-scale nitration as they control temperature while preventing nitration of a large amount of reactants. Remember to separate the nitric acid waste at the end of the process!