August 12, 2017 · 7 min read
I recently finished The Alchemy of Air, by Thomas Hager. It’s the story of the Haber-Bosch process, the lives of the men who created it, and its consequences for world agriculture and for Germany during the World Wars.
What is the Haber-Bosch process? It’s what keeps billions of people in the modern world from starving to death. In Hager’s phrase: it turns air into bread.
Some background. Plants, like all living organisms, need to take in nutrients for metabolism. For animals, the macronutrients needed are large, complex molecules: proteins, carbohydrates, fats. But for plants they are elements: nitrogen, phosphorus and potassium (NPK). Nitrogen is needed in the largest quantities.
Nitrogen is all around us: it constitutes about four-fifths of the atmosphere. But plants can’t use atmospheric nitrogen. Nitrogen gas, N2, consists of two atoms held together by a triple covalent bond. The strength of this bond renders nitrogen mostly inert: it doesn’t react with much. To use it in chemical processes, plants need other nitrogen-containing molecules. These substances are known as “fixed” nitrogen; the process of turning nitrogen gas into usable form is called fixation.
In nature, nitrogen fixation is performed by bacteria. Some of these bacteria live in the soil; some live in a symbiotic relationship on the roots of certain plants, such as peas and other legumes.
Nitrogen availability is one of the top factors in plant growth and therefore in agriculture. The more fixed nitrogen is in the soil, the more crops can grow. Unfortunately, when you farm a plot of land, natural processes don’t replace the nitrogen as fast as it is depleted.
Pre-industrial farmers had no chemistry or advanced biology to guide them, but they knew that soil would lose its fertility over the years, and they had learned a few tricks. One was fertilization with natural substances, particularly animal waste, which contains nitrogen. Another was crop rotation: planting peas, for instance, would replace some of the nitrogen in the soil, thanks to those nitrogen-fixing bacteria on their roots.
But these techniques could only go so far. As the world population increased in the 19th century, more and more farmland was needed. Famine was staved off, for a time, by the opening of the prairies of the New World, but those resources were finite. The world needed fertilizer.
An island off the coast of Peru where it almost never rains had accumulated untold centuries of—don’t laugh—seagull droppings, some of the world’s best known natural fertilizer. An industry was made out of mining guano on these islands, where it was piled several stories high, and shipping it all over the world. When that ran out after a couple decades, attention turned inland to the Atacama Desert, where, with no rainfall and no life, unusual minerals grew in crystals on the rocks. The crystals included salitre, or Chilean saltpeter, a nitrogen salt that could be made into fertilizer.
It could be made into something else important, too: gunpowder. It turns out that nitrogen is a crucial component not only of fertilizer, but also of explosives. Needing it both to feed and to arm their people, every country considered saltpeter a strategic commodity. Peru, Chile and Bolivia went to war over the saltpeter resources of the Atacama in the late 1800s (Bolivia, at the time, had a small strip of land in the desert, running to the ocean; it lost that strip in the war and has remained landlocked ever since).
By the end of the 19th century, as population continued to soar, it was clear that the Chilean saltpeter would run out within decades, just as the guano had. Sir William Crookes, head of the British Academy of Sciences, warned that the world was heading for mass famine, a true Malthusian catastrophe, unless we discovered a way to synthesize fertilizer. And he called on the chemists of the world to do it.
Nearby, in Germany, other scientists were thinking the same thing. Germany was highly dependent on salt shipped halfway around the world from Chile. But Germany did not have the world’s best navy. If—God forbid—Germany were ever to be at war with England (!), they would quickly blockade Germany and deprive it of nitrogen. Germany would have no food and no bombs—not a good look, in wartime.
The prospect of synthesizing fixed nitrogen was tantalizing. After all, the nitrogen itself is abundant in the atmosphere. A product such as ammonia, NH3, could be made from that and hydrogen, which of course is present in water. All you need is a way to put them together in the right combination.
The problem, again, is that triple covalent bond. Owing to the strength of that bond, it takes very high temperatures to rip N2 apart. More troublesome is that ammonia is by comparison a weak molecule. So at temperatures high enough to separate the nitrogen atoms, the ammonia basically burns up.
Fritz Haber was the chemist who solved the fundamental problem. He found that increasing the pressure of the gases allowed him to decrease the temperature. At very high pressures, he could start to get an appreciable amount of ammonia. By introducing the right catalyst, he could increase the production to levels that were within reach of a viable industrial process.
Carl Bosch was the industrialist at the German chemical company BASF who led the team that figured out how to turn this into a profitable process, at scale. The challenges were enormous. To start with, the pressures required by the process were immense, around 200 atmospheres. The required temperatures, too, were very high. No one had ever done industrial chemistry in that regime before, and Bosch’s team had to invent almost everything from scratch, pioneering an entirely new subfield of high-pressure industrial chemistry. Their furnaces kept exploding—not only from the pressure itself, but because hydrogen was eating away at the steel walls of the container, as it forced into them. No material was strong enough and inexpensive enough to serve as the container wall. Finally Bosch came up with an ingenious system in which the furnaces had an inner lining of material to protect the steel, which would be replaced on a regular basis.
A further challenge was the catalyst: Haber had used osmium, an extremely rare metal. BASF bought up the entire world’s supply, but it wasn’t enough to produce the quantities they needed. They experimented with thousands of other materials, finally settling on a catalyst with an iron base combined with other elements.
This is the Haber-Bosch process: it turns pure nitrogen and hydrogen gas into ammonia. The nitrogen can be isolated from the atmosphere (by cooling air until it condenses into liquid, then carefully increasing the temperature: different substances boil at different temperatures, so this process separates them). Hydrogen can be produced from water by electrolysis, or, these days, found in natural gas deposits. The output of the process, ammonia, is the precursor of many important products, including fertilizers and explosives.
The new BASF plant that Bosch built began turning out tons of ammonia a day. It beat out all competing processes (including one that used electric arcs through the air), and provided the world with fertilizer—cheaper and of more consistent quality than could be obtained from the salts of Chile, which were abandoned before they ran out.
Haber-Bosch fed the world—but it also prolonged World War I, and later helped fuel the rise of Hitler.
The Alchemy of Air is as much about the lives of Haber and Bosch, and what happened after their process became a reality, as it is about the science and technology of the process itself. Even though the technology was my main interest this time, I found the history captivating.
Haber was a Jew, at a time when Jews were second-class citizens in Germany. Rather than denouncing the society he lived in, this seemed to cause Haber to seek its approval. After his scientific achievement with ammonia, he got a high-status job at the Kaiser Wilhelm Institutes in Berlin, and sought to be an adviser to the Kaiser himself. Jews were barred from military service, but Haber was able to become a science adviser to the military—even pioneering the use of poison gas in WW1, a role that left him with a reputation as a war criminal.
Haber believed that if Jews showed what good, patriotic German citizens they could be, they could eventually be accepted as equals. Decades later, when the Nazis came to power and began “cleansing” Jews first out of the German government, then out of all of society, Haber saw his dream of acceptance fall completely to pieces. He died, shortly before WW2, in great distress.
Bosch, on the other hand, held liberal political views and was against the Nazis. He even tried to speak out against them, and in a personal meeting with Hilter made a futile argument for freedom of inquiry and better treatment of the Jews. But at the same time he made deals with the Nazis to secure funding for his chemical company—by then he was the head, not only of BASF, but of a broader industry association called IG Farben. He was building a massive chemical plant in the heart of Germany, at Leuna, to produce not only ammonia but also what he saw as his magnum opus: synthetic gasoline, made from coal. In the end Farben became virtually a state company and provided much of the material Germany needed for WW2, including ammonia, gasoline, and rubber.
Bosch died shortly after the war began. On his deathbed, he predicted that the war would be a disaster for Germany. It would go well at first, he said, and Germany would occupy France and maybe even Britain. But then Hitler would make the fatal mistake of invading Russia. In the end, the skies would darken with Allied planes, and much of Germany would be destroyed. It happened as he predicted, and Bosch’s beloved Leuna was a major target, ultimately crippled by wave after wave of Allied bombing raids.
Synthetic ammonia is one of the most important industrial products of the modern world, and so Haber-Bosch is one of the most important industrial processes. Around 1% of the total energy of the economy is devoted to it, and Hager estimates that half the nitrogen atoms in your body came from it. It’s a crucial part of the story of industrial agriculture, and so a crucial part of the story of how we became smart, rich and free.
Social media link image credit: Stacy Spensley, CC BY 2.0
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