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This section covering ammonia production comprises these pages:
Introduction
Energy efficient ammonia production
Introduction
Short history
Sir William Crookes was perhaps the first person to focus on the impact of diminishing fertiliser imports (guano) from South America on feeding the world’s population. He opened his speech on becoming president of the British Academy of Science (1898) with the statement: “England and all civilised nations stand in deadly peril of not having enough to eat. As mouths multiply, food resources dwindle.”
Crookes outlined that based on his research there was no additional farmland to be gained as the Great Plains in USA and Canada and the Steppes of Russia were already discovered; no new farmland could be cultivated in order to use the benefits of virgin soil. With crop yields declining on depleted soils, there was only one answer; to bring in vast amounts of fertiliser as bird manure. Extensive deposits of guano found in Chile had provided a significant supply of fertiliser for some time, but calculations were able to show with reasonable accuracy when these supplies would run out. As supplies from South America were limited, new sources had to be established.
The only solution Crookes could come up with was that chemists had to devise a new way of producing nitrogen containing compounds, otherwise, according to his research the World’s population would start starving around 1930. The chemists had to save the day.
Looking back to these old days, BASF had already started working on the challenge of how to use the nitrogen in the air to produce nitrogen compounds.
In 1897, Schönherr and Hessberger started work on improving the arc process for nitric acid production. They actually designed the process up to a full industrial attempt, in Norway, in co-operation with Norsk Hydro. But, owing to the high cost of electricity, the downstream products from the arc process were still more expensive in comparison to imported Chilean nitrates.
In 1900, Willhelm Oswald applied for a patent for the synthesis of ammonia from nitrogen and hydrogen using an iron catalyst. Oswald tried to sell his patent to BASF. Carl Bosch, as a new chemist at BASF, was assigned the task of verifying Oswald’s findings. In doing so, he discovered that the ammonia apparently created on the iron catalyst actually resulted from a previous experiment. There was no continuous ammonia production under the given test conditions and in the end, Oswald withdrew his patent application.
In 1906, Fritz Haber obtained his first results regarding ammonia synthesis from the elements, which he presented in May 1907 at the Bunsen Society meeting. There a heated discussion started with Walther Nernst, who doubted Haber’s results and declared the yields as far too high. Annoyed by this discussion, Haber increased his efforts as well as the pressure in his experiments and obtained even higher yields. Fritz Haber approached BASF in 1908 and signed a contract which gave him the financial means to increase his workload even further. By the end of 1908 the first BASF patent for ammonia synthesis from the elements was granted.
With a process established and a suitable catalyst recipe identified, the next challenge was to scale the process up to a production scale; in this the key innovator was Carl Bosch. After many experiments, Bosch had the idea of separating the task of getting a tightly sealed reactor from the task of bearing the pressure. A liner tube of soft carbon-free iron was fitted on the inside of the reactor, which effectively sealed the reactor. He used a carbon steel shell capable of bearing the pressure, into which holes were drilled to release the hydrogen that still drifted into the outer shell; in this way Bosch created a reactor which lasted practically indefinitely. The workers called the holes ‘Bosch holes’. In this way hydrogen diffusing through the soft iron liner could be released before it could accumulate and create a high pressure and so the pressure bearing reactor wall was protected from hydrogen attack. The loss of hydrogen that occurred in this construction was so limited, that it was neither a safety risk nor of commercial relevance.
The contribution of Bosch in taking the ammonia process from a laboratory scale to an industrial activity led to Bosch and Bergius being jointly awarded the Nobel Prize in Chemistry (1931), in recognition of their contributions to the invention and development of chemical high pressure methods, much later than the award to Haber for his work on the ammonia process (1918).
Since these early days, the so-called Haber-Bosch process for manufacturing ammonia has been much studied, developed, and improved (Reuvers, Brightling, Seldon, 2013; Reuvers, 2017). Early processes used coal as a source of hydrogen, more modern processes use natural gas. When natural gas is used as a material source for hydrogen, the gas is steam reformed to produce hydrogen and carbon dioxide.
Even today, 2021, the most modern commercial units use methane steam reforming for the production of hydrogen, which is then reacted with purified nitrogen from air to produce ammonia (NH3). All commercial present-day processes are still based on the Haber-Bosch technology.
This ammonia history is nicely explained by T. Hager in his book “The Alchemy of Air” (2008).
Properties and reactivity of ammonia
Ammonia, NH3, is a colourless gas with a characteristically pungent smell. It is lighter than air, its density (100 wt-%) being 0.618 g/cm3. It is easily liquefied (owing to the strong hydrogen bonding between individual molecules); the liquid boils at -33.3 °C, and freezes at -77.7 °C.
Ammonia does not burn readily or sustain combustion, except under narrow fuel-to-air mixtures of 15–25 % air. When mixed with oxygen, it burns with a pale yellowish-green flame.
Ammonia readily dissolves in water. The aqueous solution of ammonia is basic.
Ammonia will react with acids to form ammonium salts, such as ammonium chloride (salmiak salt), ammonium nitrate and ammonium sulphate. Ammonia reacts strongly with chlorine forming nitrogen and hydrogen chloride; if chlorine is present in excess, the highly explosive nitrogen trichloride (NCl3) is also formed.
At high temperature, and in the presence of a suitable catalyst, ammonia is decomposed into its constituent elements. Decomposition of ammonia is a slightly endothermic process requiring 23 kJ/mol of ammonia, and yields hydrogen and nitrogen gas.
There are many uses for ammonia although the majority of applications is as a direct fertiliser in its own right or as a starting material for various fertiliser formulations, including urea, ammonium nitrate, and ammonium sulphate. Worldwide, about 80-85 % of all ammonia produced is used in fertiliser applications. Mainly in the United States, ammonia is applied to fields in the form of anhydrous ammonia, here the share of directly applied / injected ammonia amounts to about 30 % of all agricultural nitrogen.
Other uses involve the synthesis of many industrial nitrogen compounds, in cleaning agents, as a refrigerant, or as a fuel (see below).
Safety and health aspects related to ammonia are well described in Proceedings of the International Fertiliser Society.
Further information on the properties and reactivity of ammonia can be found in literature (Ullmann 1985). The International Fertiliser Society offers a wealth of data and information (Shah, 1997 and 2007) and a dedicated USB is available for past ammonia proceedings.
References
Appl, M. (1999). Ammonia: Principles and Industrial Practice, Wiley ISBN 3-527-29593-3
Hager, T. (2008). The Alchemy of Air. Harmony Books, New York, United States of America. ISBN 978-0-307-35178-4. OCLC 191318130.
Korkhaus, K. and Bachtler, M. (2013). The Ammonia Process – A Challenge for Materials, Fabrication and Design of the Components. AIChE Ammonia Technical Manual Vol 54.
Reuvers, J. G., Brightling, J. R. and Seldon, D. T. (2013). Ammonia technology development from Haber-Bosch to current times, Proceedings International Fertiliser Society, 747.
Reuvers, J. G. (2017), Changes, Challenges and Opportunities in Fertilizer-Manufacturing Processes: A personal Review and Outlook , Proceedings International Fertiliser Society, 803.
Shah, K. D. (2007). Safety Issues in Ammonia Handling and Distribution. Proceedings International Fertiliser Society, 604
Shah, K. D. (1997). Ammonia: Safety, Health and Environmental Aspects . Proceedings International Fertiliser Society, 401.
Ullmann (1985). Ullmann’s Encyclopedia of Industrial Chemistry “Ammonia”, VCH-Verlagsgesellschaft, Weinheim, Germany.
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