Haber process

• 1. Introduction

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  The Haber process makes ammonia out of ordinary air. In the summer of 1909, the German chemist Fritz Haber and his assistant Robert Le Rossignol showed it working, and the result arrived drop by drop. Their apparatus produced about 125 mL of ammonia per hour. That trickle was enough to prove a point that chemists had doubted. Within a few years the same chemistry, also called the Haber-Bosch process, was running as a factory operation. It has since touched farming, warfare and the balance of nitrogen on the planet. This documentary asks three questions. Why did anyone need to pull fertilizer out of thin air? How do you force the most stubborn gas in the atmosphere to react? And what has the world paid for the answer?

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• 2. Nitrate Shortage

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  Through the 19th century, demand for nitrates and ammonia climbed rapidly. Fertilizers use them to supply plants with the nutrients they need to grow, and factories used them as industrial feedstocks. The main supply came from mining niter deposits and guano from tropical islands. By the start of the 20th century, these reserves were thought too small to meet future demand. Research into new sources of ammonia grew more urgent. The frustrating part was that nitrogen was everywhere. It makes up roughly 78% of the air. But atmospheric nitrogen, written N2, is exceptionally stable and does not readily react with other chemicals. Some answers already existed, including the Birkeland-Eyde and Frank-Caro processes. Haber and Bosch improved on their efficiency, which counted as a major advance. The cyanamide process was another rival. It consumed large amounts of electrical power and needed more labor than Haber's method.

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• 3. Bench To Factory

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  Carl Bosch succeeded in 1910 at a task handed to him by BASF, the German chemical company that had purchased the process. His job was to scale Haber's tabletop machine up to industrial size. The first full-scale manufacture followed in 1913 at BASF's Oppau plant in Germany. By 1914 the plant was reaching 20 tonnes a day. The early reactors had a nasty flaw. They burst under high pressure, because atomic hydrogen inside the carbon-bearing steel partly recombined into methane and cracked the metal. Bosch answered with tube reactors. A pressure-bearing steel tube held an inserted lining tube of low-carbon iron, which was filled with catalyst. Hydrogen that seeped through the lining escaped through thin holes in the outer jacket, known as Bosch holes. Later, hydrogen-resistant chromium-molybdenum steels made single-walled pipes possible. Recognition followed for both men. Haber received the Nobel Prize in Chemistry in 1918 for ammonia synthesis. Bosch received his in 1931 for related work in high-pressure chemistry. Modern single-line plants exceed 3000 tons a day, a long way from that first 20 tonnes.

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• 4. Wartime Necessity

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  When World War I demanded huge quantities of nitrate for munitions, Germany faced a problem of geography. Large deposits of sodium nitrate, called Chile saltpetre, sat in Chile under the control of British companies. India held large supplies too, also under British control. Even where German commercial interests held nominal legal rights, the Allies commanded the sea lanes. Their effective blockade would have stopped such cargoes from reaching Germany. Synthetic ammonia from the Haber process solved the shortage. It was used to make nitric acid, a precursor to the nitrates in explosives. The process proved so essential that it is considered virtually certain Germany would have been defeated within months without it. After the war, the same ammonia would find its largest market in fertilizer.

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• 5. Heat Versus Pressure

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  At room temperature, the chemistry actually favors ammonia, yet nothing measurable happens. The nitrogen molecule is held together by a triple bond, which makes it relatively inert. The reaction also releases heat, so raising the temperature pushes the balance back toward the starting gases. Equilibrium becomes unity at around 150-200 degrees Celsius. At 300 degrees the equilibrium constant is 434 × 10-5, and by 600 degrees it has dropped to 0.225 × 10-5. Cooling the reactor does not help, because the catalyst needs at least 400 degrees to work well. Pressure offers the other lever. Four volumes of reactant gas become two of product, so higher pressure favors ammonia. But pressure is expensive. Pipes, valves and vessels must be strong, safety becomes a concern near 20 MPa, and compressors burn considerable energy. The compromise yields only about 15% conversion on each pass. Operators therefore cool the hot gases under pressure until the ammonia condenses as a liquid. They return the unreacted hydrogen and nitrogen to the reactor. After enough recycling, conversion reaches 97%. During the interwar years, Luigi Casale and Georges Claude proposed far higher pressures of 80-100 MPa. Most plants still run something close to the original 20 MPa and 500 degrees Celsius, with better efficiency. Recycling brings a side effect. Inert gases such as argon build up, so some gas is bled off and the argon is separated using the Linde process.

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• 6. The Iron Catalyst

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  Haber's first catalysts were built on osmium and uranium, and each had a drawback. Osmium was available only in extremely small quantities. Uranium reacted to form a nitride during use. In 1909, the BASF researcher Alwin Mittasch found a much cheaper iron-based catalyst, and it is still used today. Gerhard Ertl is named as a major contributor to the catalytic work. The finished catalyst is a layered particle. A magnetite core is wrapped in a shell of wüstite, which is surrounded by an outer shell of metallic iron. Promoters including potassium, calcium and aluminium oxides help it keep its porous, high surface area. Reducing a fresh catalyst to full capacity takes four to ten days. Pre-reduced versions need only 30 to 40 hours. The surface work is slow because splitting the nitrogen molecule is the rate-determining step. Certain iron crystal faces, Fe(111) and Fe(211), are by far the most active. Only they have iron atoms with seven closest neighbours. Impurities can ruin the whole effort. Sulfur, phosphorus, arsenic and chlorine compounds are permanent poisons, while water, carbon monoxide, carbon dioxide and oxygen are temporary ones. Ruthenium offers a second generation of catalyst that allows milder pressures and temperatures. The KBR Advanced Ammonia Process has used it industrially since 1992. In 2012, Hideo Hosono's group reported a ruthenium-loaded electride catalyst. In 2019 the same group found a perovskite oxynitride-hydride that works at lower temperature and without costly ruthenium.

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• 7. Making The Hydrogen

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  Making ammonia takes 1-2% of global energy consumption, and the process also accounts for 3% of global carbon emissions. It uses 3-5% of natural gas consumption. Much of that appetite comes from the hydrogen. The major source is methane. As of 2012, steam reforming of natural gas supplied 72% of global ammonia production. In China in 2022, natural gas supplied 20% and coal supplied 75%. Steam reforming begins by stripping sulfur from the feedstock, since sulfur deactivates the catalysts used later. Zinc oxide beds capture the hydrogen sulfide as solid zinc sulfide. A nickel catalyst then reforms the methane with steam into hydrogen and carbon monoxide. A shift conversion step turns that carbon monoxide into carbon dioxide and more hydrogen. The carbon dioxide is removed, and a final methanation step clears residual traces. In large plants, a secondary reformer is supplied with air, which also adds the nitrogen the synthesis needs. Electrolysis is an alternative. At one time, most of Europe's ammonia came from the hydro plant at Vemork. Green hydrogen, made without fossil fuels, can come from biomass, water electrolysis or solar water splitting. One newer approach, reverse fuel cell technology, turns electricity, water and nitrogen into ammonia without a separate electrolysis step.

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• 8. Fertilizer And Fallout

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  As of 2018, the Haber process was producing 230 million tonnes of anhydrous ammonia per year. Most of it becomes nitrogen fertilizer, applied as ammonia itself, as ammonium nitrate or as urea. Together with advances in breeding, herbicides and pesticides, these fertilizers raised the productivity of farmland. If average crop yields had stayed at their 1900 level, the harvest of the year 2000 would have needed nearly four times more land. Cultivation would then have claimed nearly half of all ice-free continents, rather than under 15% of total land area. Nearly 50% of the nitrogen in human tissues originated from the Haber-Bosch process. The process has been called the "detonator of the population explosion", as world population grew from 1.6 billion in 1900 to 7.7 billion by November 2018. The costs are real. Nitrates leach into groundwater, rivers, ponds and lakes, and recurring eutrophication expands dead zones in coastal oceans. Emissions of nitrous oxide, now the third most important greenhouse gas, are also higher. Because nitrogen use efficiency is typically below 50%, farm runoff disrupts habitats. The process is among the largest contributors to a buildup of reactive nitrogen in the biosphere. Cheap synthetic nitrogen also weakens the incentive to rotate crops with legumes, plants that fix nitrogen naturally.

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