27.2.

Two processes enabled the twentieth century to be one of both unprecedented violence and unprecedented prosperity. The Haber-Bosch process for the synthesis of ammonia made it possible to produce nitrogen fertilizers from air. The Ostwald process made it possible to produce nitric acid from ammonia. Together, these processes freed both farmers and soldiers from their historical dependence on saltpeter, either extracted from animal manure or imported from South America. In the twenty-first century, the production of food and munitions remains subject to political and economic, but not chemical limitations.

Figure 27-1. The Haber-Bosch and Ostwald Processes

The schematic for these processes is deceptively simple. The starting materials, oxygen, hydrogen, nitrogen, and water enter from the left of Figure 27-1, as usual. Oxygen and nitrogen come from the air. Hydrogen might well be supplied by a chloralkali plant, as discussed in Chapter 25, but is more economically generated from natural gas. Water may be obtained from any convenient source. Some engineering is required to separate and purify the starting materials, but the details have been left out of the process schematic for simplicity. The purified starting materials pass through three reactors in turn, a furnace, a burner, and an absorber.

The first reactor in the schematic, the Haber-Bosch furnace, reacts nitrogen and hydrogen to produce ammonia. This reaction is so simple that you might well wonder why it wasn't developed in the nineteenth century. The reason is that the reaction requires both high temperature (500C) and high pressure. These extreme conditions were a challenge to engineers trained on the low-pressure reactors of Solvay soda plants, but even with a suitable high-pressure furnace, the reaction goes only at a snail's pace. To speed up the process, hydrogen and nitrogen are passed over pellets of iron oxide and aluminum which serve as a catalyst, a material which makes a slow reaction occur faster but which is neither produced nor consumed in the reaction.

The automotive catalytic converter is, perhaps, the most familiar example of a catalyst. Hot exhaust gases pass over the catalyst, often an alloy of platinum and rhodium. Nitrogen oxides are converted to nitrogen and carbon monoxide is converted to carbon dioxide, reducing the emission of these pollutants from the automobile tailpipe. Unlike gasoline, which is consumed by the car; unlike exhaust, which is produced by the car; the catalyst is unchanged by the reaction. You don't have to refill the converter with catalyst. It is possible, however, for a catalyst to be poisoned, destroying its catalytic properties. The automotive catalytic converter, for example, is poisoned by tetraethyl lead, formerly used as an anti-knock additive in gasoline. Coincidentally, the automotive platinum/rhodium catalyst is also employed in the second reactor of Figure 27-1.

The ammonia produced by the Haber-Bosch furnace is a commodity in its own right, but a large fraction of the ammonia is used to produce nitric acid via the Ostwald process. Except for reactants and products, the Ostwald process is schematically identical to the Lead Chamber process of Figure 18-1. The diverted ammonia enters a burner where it combines with atmospheric oxygen over a platinum/rhodium or gold/palladium catalyst, producing nitrogen monoxide (NO) and water. It should be noted that while a furnace sits over a burner, whose reactants and products are irrelevant to the process, the Ostwald burner is actually fueled with ammonia and it is the exhaust gases themselves which pass to the third reactor in the schematic, an absorber. Atmospheric oxygen is supplied to the absorber to convert these exhaust gases into nitric acid, HNO3. The dilute nitric acid from the absorber can be distilled to produce concentrated nitric acid, which is approximately 70% acid and 30% water. Concentrated nitric acid, like ammonia, is a commodity in its own right, used primarily in the manufacture of fertilizers and explosives.

Black powder is a mixture of charcoal, sulfur, and potassium nitrate, an oxidant which takes the place of atmospheric oxygen. The charcoal fuel can burn much faster than usual, since it doesn't have to wait for fresh air to replace its exhaust gases. But black powder is still just a mixture, separate particles of fuel and oxidant. An even more powerful explosive results if the fuel and the oxidant are present in the same compound. For example, trinitrotoluene (TNT), nitrocellulose (smokeless powder), and nitroglycerin (dynamite) each incorporate fuel and oxidant in a single molecule. Each is produced by the reaction of its base fuel with nitric acid in the presence of sulfuric acid. Nitroglycerin, for example, is produced by the following reaction:

C3H8O3(l) + 3 HNO3(l) = C3H5(NO3)3(l) + 3 H2O(l)

When nitroglycerin explodes, it does so without requiring a separate oxidizer:

4 C3H5(NO3)3(l) = 12 CO2(g) + 6 N2(g) + 10 H2O(g) + O2(g)

Similarly, nitrocellulose produces no solid combustion products, hence the term, smokeless powder. If nitric acid's only use were the manufacture of high explosives it would be an important chemical, but an even bigger market for nitric acid exists in the manufacture of fertilizers.

Neither nitric acid nor ammonia can be used directly as a fertilizer; nitric acid is a corrosive liquid and ammonia is an alkaline gas. For use as fertilizers both must be neutralized, as shown in the optional reaction of Figure 27-1. Ammonia is often reacted with sulfuric acid to produce ammonium sulfate fertilizer, but since ammonia and nitric acid are produced at the same plant, it's natural to react the one with the other to produce ammonium nitrate. Both the sulfate and the nitrate have neutral pH and make excellent fertilizers, but ammonium nitrate has the advantage of providing a double dose of nitrogen, one in the ammonium ion, the other in the nitrate ion. In fact, ammonium nitrate is just about ideal as a fertilizer except that it's also a powerful oxidizing agent; when combined with a fuel, either accidentally or intentionally, it makes a powerful explosive. An accident aboard the ship, S. S. Grande Camp, caused its cargo of fertilizer to explode in 1947, destroying much of Texas City. Ammonium nitrate is often combined with fuel oil to make a blasting compound for construction, mining, or terrorism. Is ammonium nitrate good, or bad? As with many of the materials we've encountered in this book, ugly is in the eye of the beholder.

WarningMaterial Safety
 

Locate MSDS's for cellulose (CAS 9004-34-6), sodium nitrate (CAS 7631-99-4), sulfuric acid (CAS 7664-93-9), nitric acid (400OxCAS 7697-37-2), nitrocellulose (CAS 9004-70-0), ethanol (CAS 64-17-5), and ether (CAS 60-29-7). Summarize the hazardous properties in your notebook, including the identity of the company which produced each MSDS and the NFPA diamond for each material.[1]

Your most likely exposure is to sulfuric acid. Wash any affected areas immediately with plenty of running water. If eye contact occurs, flush with water and call an ambulance.

You should wear safety glasses and rubber gloves while working on this project. Spent acid solution may be poured down the drain with plenty of water. Nitrocellulose should be burned in a safe place as soon as it is dry. Storage of nitrocellulose is a fire hazard.

NoteResearch and Development
 

So there you are, studying for a test, and you wonder what will be on it.

  • Study the meanings of all of the words that are important enough to be included in the index or glossary.

  • You should know all of the Research and Development points from Chapter 23 and Chapter 24.

  • You should understand the process schematic shown in Figure 27-1 and know the equations given there.

  • You should know the major industries which depend on ammonia and nitric acid.

  • You should know the hazardous properties of sulfuric acid, nitric acid, and nitrocellulose, ethanol, and ether.

  • You should be able to articulate the concerns raised by Crookes and describe the contributions of Ostwald, Haber, Bosch, and BASF, to the fixation of nitrogen.

Notes

[1]

The NFPA diamond was introduced in Section 15.2. You may substitute HMIS or Saf-T-Data ratings at your convenience.