HARBER BOSCH PROCESS
1. Context
A hundred million tonnes of nitrogen are now removed from the atmosphere and converted into fertilizer via the Haber-Bosch process, adding 165 million tonnes of reactive nitrogen to the soil. To compare, biological processes replenish an estimated 100-140 million tonnes of reactive nitrogen every year. Without the industrial synthesis of ammonia from nitrogen and hydrogen, we would have had no way to meet the world’s expanding demand for food.
2. Nitrogen Molecule
- Nitrates are molecules composed of oxygen and nitrogen, which are abundant in Earth's atmosphere. Despite nearly eight metric tonnes of nitrogen resting on every square meter of the planet's surface, it cannot nourish even a single blade of grass in its atmospheric form.
- Most nitrogen in the air exists as N2, where two nitrogen atoms are held together by a triple bond, sharing three pairs of electrons, making the molecule extremely stable.
- The energy needed to break this triple bond is very high (946 kJ/mol), making molecular nitrogen almost inert.
- However, once the bond is broken, nitrogen atoms can combine with other elements to form reactive compounds like ammonia (NH3), ammonium (NH4+), or nitrates (NO3–).
- These reactive nitrogen forms are essential for plants, which use them to synthesize enzymes, proteins, and amino acids. Healthy plants typically contain 3-4% nitrogen in their above-ground tissues, much more than other nutrients
3. Natural Nitrogen in Nature
- Among natural phenomena, only lightning possesses sufficient energy to break the N2 triple bond. During a lightning strike, nitrogen in the atmosphere reacts with oxygen, producing nitrogen oxides such as NO and NO2.
- These oxides can further react with water vapor to form nitric and nitrous acids (HNO3 and HNO2).
- When it rains, these reactive nitrogen-rich droplets act as fertilizers for agricultural lands, forests, and grasslands, replenishing the soil with approximately 10 kg of nitrogen per acre annually.
- In addition to lightning, a slow metabolic process conducted by Azotobacter bacteria can also generate reactive nitrogen.
- Certain microorganisms, like Rhizobia, have formed symbiotic relationships with legume plants (such as clover, peas, beans, alfalfa, and acacia) to supply reactive nitrogen in exchange for nutrients.
- Azolla, an aquatic fern that has a symbiotic relationship with the cyanobacterium Anabaena azollae, can absorb and convert atmospheric nitrogen into reactive nitrogen, making dried and decayed Azolla an effective fertilizer for farmland
4. Nitrogen Cycle
- Plants primarily obtain their reactive nitrogen from the soil, where they absorb minerals dissolved in water, including ammonium (NH4+) and nitrate (NO3-).
- Humans and animals require nine essential nitrogen-rich amino acids, which they get from plants. Nitrogen constitutes about 2.6% of the human body. The nitrogen consumed by both plants and animals returns to the soil through waste products and the decomposition of organic matter.
- However, this cycle is not fully closed, as some nitrogen is released back into the environment in its molecular form, and nitrogen from human waste is seldom returned to agricultural fields.
- While legumes have the ability to produce nitrogen on their own, key food crops such as rice, wheat, corn, and potatoes, as well as less common crops like cassava and bananas, rely on soil nitrogen.
- As the global population continues to grow, the nitrogen levels in agricultural soils are depleting more rapidly, necessitating the use of fertilizers to replenish them.
- Farmers have long recognized this issue. They cultivated legumes or applied ammonia-based fertilizers to boost crop yields when possible. Additionally, they utilized ammonium-rich minerals from volcanic activity and naturally occurring nitrates found in caves, walls, and rocks as sources of fertilizer
5. Generation of Ammonia
- Ammonia (NH4) consists of nitrogen and hydrogen, both of which naturally occur as diatomic molecules. When subjected to extreme heat, these molecules dissociate to form a compound, but this compound is only temporary due to the high temperature.
- To produce significant quantities of ammonia, the reversible reaction N2 + 3H2 = 2NH3 (with the '=' symbol representing bidirectional arrows) must occur under specific conditions.
- The German chemist Fritz Haber experimented by heating a mixture of N2 and H2 in a platinum cylinder at various temperatures while measuring the amount of ammonia produced.
- He also decomposed hot ammonia back into nitrogen and hydrogen in an effort to approach the equilibrium point from the reverse direction. At a temperature of 1,000 degrees Celsius, he discovered that the yield of usable ammonia was merely 0.01% of the mixture—insufficient for commercial production.
- Haber then speculated that increasing pressure might enhance the reaction. He determined that hydrogen and nitrogen would only remain bonded under extreme conditions, specifically at temperatures of 200 degrees Celsius and pressures of 200 atmospheres (which is 200 times the average air pressure at sea level).
- However, the rate of ammonia production remained too slow, leading Haber to search for a suitable catalyst. He also realized that by cooling the ammonia into a liquid state, he could efficiently collect a larger portion of it
6. Haber-Bosch process
- A young Englishman named Robert Le Rossignol, skilled in practical engineering, had recently joined Haber’s laboratory. He designed the seals necessary to maintain high pressure within an experimental chamber.
- Friedrich Kirchenbauer, an adept mechanic, constructed the corresponding equipment. Haber later acknowledged both Le Rossignol and Kirchenbauer during his Nobel Prize acceptance speech, sharing the patents and prize money with them.
- In the experiment, the heated mixture of hydrogen and nitrogen would flow through a steel chamber maintained at a pressure of 200 atm. This chamber featured a valve capable of withstanding high pressure while allowing the N2-H2 mixture to move through.
- Additionally, Haber developed a device to transfer heat from the hot gases exiting the reaction chamber to the cooler incoming gases. This design enabled the outgoing mixture to cool rapidly while simultaneously heating the incoming gas, effectively serving two purposes.
- Serendipity played a role in finding the catalyst. At that time, innovators were searching for a suitable material for lightbulb filaments (with Thomas Edison yet to discover tungsten).
- Auergesellschaft of Berlin, a manufacturer of gas lamps and electric lights, had approached Haber for recommendations on a filament and provided his lab with various rare materials. Haber began experimenting with these materials to see if any could serve as effective ammonia catalysts.
- One such material was osmium, a rare and dense metal found in trace amounts in the Earth. When Haber introduced an osmium sheet into the pressure chamber, filled it with the N2-H2 mixture, and heated it, the nitrogen triple bond broke, allowing reactive nitrogen to combine with hydrogen and produce a significant quantity of ammonia. German propagandists soon celebrated this achievement with the phrase “brot aus luft!” — likening it to the biblical miracle of producing bread from air.
- Haber continued testing various catalysts and found that uranium was also effective. However, both osmium and uranium were too costly for large-scale applications. When Badische Anilin- und Soda-Fabrik (BASF), a major German chemical company, decided to scale up Haber’s experiment for industrial production, it tasked its chemist Alwin Mittasch with finding a more affordable and readily available catalyst.
- Mittasch conducted thousands of experiments with 4,000 different materials and discovered that certain iron oxides worked well as catalysts. Eventually, innovative engineering by BASF’s Carl Bosch transformed Haber’s small-scale setup into a full industrial process for producing fertilizer.
- Five years after Haber and his team's breakthrough, BASF inaugurated its first ammonia factory in 1913
7. Challenges with fertilisers
- A century ago, the average life expectancy at birth for an Indian was just 19 years, whereas today it exceeds 67 years. The Haber-Bosch method enabled industries to produce inexpensive synthetic fertilizers, playing a crucial role in the sevenfold increase in global food production during the 20th century.
- One estimate suggests that without the Haber process for nitrogen fixation, approximately one-third of the world’s population—around two billion people—would face food scarcity.
- However, environmentalists caution that the benefits of fertilizers should not be taken for granted. For instance, Haber’s nitrogen fertilizers are not without their drawbacks. An average adult contains about 1-2 kg of nitrogen in their body, yet in many regions, the annual application of fertilizers now exceeds 50 kg of nitrogen per person, with a global average of around 13 kg.
- This "extra" nitrogen is absorbed by plants along with larger amounts of other minerals. It also promotes bacterial growth and accelerates biochemical processes that release reactive nitrogen into the atmosphere, resulting in acid rain that degrades and damages land.
- Additionally, nitrogen fertilizers contribute to surface runoff into freshwater and coastal ecosystems, unintentionally fertilizing these bodies of water and causing deoxygenation. Even a small amount of moisture in these areas can lead to rampant weed growth.
- In conclusion, while Haber’s contributions were vital, they alone are insufficient to eliminate hunger and malnutrition.
- These issues persist alongside warehouses overflowing with grain surpluses, even within the same nation.
- The story of nitrogen fixation imparts a broader lesson: technological solutions alone cannot resolve societal challenges; political action and social mobilization are also essential
For Prelims: Current events of national and international importance
For Mains: General Studies III: Science and Technology - developments and their applications and effects in everyday life
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Source:The Hindu