Haber process

The Haber process, also called the Haber–Bosch process, is the nitrogen fixation reaction of nitrogen gas and hydrogen gas, over an enriched iron or ruthenium catalyst, which is used to produce ammonia. The Haber process is important because ammonia is difficult to produce on an industrial scale, and the fertilizer generated from the ammonia is responsible for sustaining one-third of the Earth's population. Despite the fact that 78.1% of the air we breathe is nitrogen, the gas is relatively unreactive because nitrogen molecules are held together by strong triple bonds. It was not until the early 20th century that this method was developed to harness the atmospheric abundance of nitrogen to create ammonia, which can then be oxidized to make the nitrates and nitrites essential for the production of nitrate fertilizer and explosives.

History

File:Fritz Haber.png

Fritz Haber, 1918

Early in the twentieth century several chemists tried and failed to produce ammonia from atmospheric nitrogen. The enormous technical problems associated with the process were first solved by German chemist Fritz Haber (with the invaluable help of Robert Le Rossignol, who developed and built the necessary high-pressure devices). They first demonstrated their success in the summer of 1909, producing ammonia from air drop by drop, at the rate of about a cup every two hours. The process was purchased by the German chemical company BASF, which assigned Carl Bosch the difficult task of scaling up Haber's tabletop machine to industrial-level production. Haber and Bosch were later awarded Nobel prizes, in 1918 and 1931 respectively, for their work in overcoming the chemical and engineering problems posed by the use of large-scale, continuous-flow, high-pressure technology. Ammonia was first manufactured using the Haber process on an industrial scale in 1913 in BASF's Oppau plant in Germany. During World War I, production was shifted from fertilizer to explosives, particularly through the conversion of ammonia into a synthetic form of Chile saltpeter, which could then be changed into other substances for the production of gunpowder and high explosives (the Allies had access to large amounts of saltpeter from natural nitrate deposits in Chile that belonged almost totally to British industrials; Germany had to produce its own). It has been suggested that without this process, Germany would not have fought in the war, or would have had to surrender years earlier.

The process

By far the major source of the hydrogen required for the Haber-Bosch process is methane from natural gas, obtained through a heterogeneous catalytic process, which requires far less external energy than the process used initially by Bosch at BASF, the electrolysis of water. Far less commonly, in some countries like China, coal is used as source of hydrogen through a process called coal gasification. However, the source of the hydrogen makes no difference to the Haber-Bosch process, which is only concerned with synthesizing ammonia from nitrogen and hydrogen.

Synthesis gas preparation

Main article: Hydrogen production

First, the methane is cleaned, mainly to remove sulfur impurities that would poison the catalysts.

The clean methane is then reacted with steam over a catalyst of nickel oxide. This is called steam reforming:

CH₄ + H₂O → CO + 3 H₂

Secondary reforming then takes place with the addition of air to convert the methane that did not react during steam reforming.

2 CH₄ + O₂ → 2 CO + 4 H₂

CH₄ + 2 O₂ → CO₂ + 2 H₂O

Then the water gas shift reaction yields more hydrogen from CO and steam.

CO + H₂O → CO₂ + H₂

The gas mixture is now passed into a methanator which converts most of the remaining CO into methane for recycling:

CO + 3 H₂ → CH₄ + H₂O

This last step is necessary as carbon monoxide poisons the catalyst. (Note, this reaction is the reverse of steam reforming). The overall reaction so far turns methane and steam into carbon dioxide, steam, and hydrogen.

Ammonia synthesis – Haber process

The final stage, which is the actual Haber process, is the synthesis of ammonia using a form of magnetite, iron oxide, as the catalyst:

N₂ (g) + 3 H₂ (g) 15px 2 NH₃ (g) (ΔH = -92.4 kJ·mol⁻¹)

This is done at 15–25 MPa (150–250 bar) and between 300 and 550 °C, passing the gases over four beds of catalyst, with cooling between each pass to maintain a reasonable equilibrium constant. On each pass only about 15% conversion occurs, but any unreacted gases are recycled, so that eventually an overall conversion of 98% can be achieved.

The steam reforming, shift conversion, carbon dioxide removal, and methanation steps each operate at absolute pressures of about 2.5–3.5 MPa (25–35 bar), and the ammonia synthesis loop operates at absolute pressures ranging from 6–18 MPa (60–180 bar), depending upon which proprietary design is used.

There are many engineering and construction companies that offer proprietary designs for ammonia synthesis plants. Haldor Topsoe of Denmark, Lurgi AG of Germany, Uhde of Germany, Saipem/Snamprogetti of Italy and Kellogg, Brown and Root of the United States are among the most experienced companies in that field.

Reaction rate and equilibrium

There are two opposing considerations in this synthesis: the position of the equilibrium and the rate of reaction. At room temperature, the reaction is slow and the obvious solution is to raise the temperature. This may increase the rate of the reaction but, since the reaction is exothermic, it also has the effect, according to Le Chatelier's principle, of favouring the reverse reaction and thus reducing the amount of product, given by:

<math>K_\mathrm{eq} = \mathrm{\frac{p_{NH_3}^2}{p_{N_2}.p_{H_2}^3}}</math>

Variation in K_eq for the Equilibrium
N₂ (g) + 3H₂ (g) 15px 2NH₃ (g)
as a Function of Temperature

Temperature (°C)	Keq
300	4.34 x 10−3
400	1.64 x 10−4
450	4.51 x 10−5
500	1.45 x 10−5
550	5.38 x 10−6
600	2.25 x 10−6

As the temperature increases, the equilibrium is shifted and hence, the amount of product drops dramatically according to the Van't Hoff equation. Thus one might suppose that a low temperature is to be used and some other means to increase rate. However, the catalyst itself requires a temperature of at least 400 °C to be efficient.

Pressure is the obvious choice to favour the forward reaction because there are 4 moles of reactant for every 2 moles of product (see entropy), and the pressure used (around 200 atm) alters the equilibrium concentrations to give a profitable yield.

Economically, though, pressure is an expensive commodity. Pipes and reaction vessels need to be strengthened, valves more rigorous, and there are safety considerations of working at 200 atm. In addition, running pumps and compressors takes considerable energy. Thus the compromise used gives a single pass yield of around 15%.

Another way to increase the yield of the reaction would be to remove the product (i.e. ammonia gas) from the system. In practice, gaseous ammonia is not removed from the reactor itself, since the temperature is too high; but it is removed from the equilibrium mixture of gases leaving the reaction vessel. The hot gases are cooled enough, whilst maintaining a high pressure, for the ammonia to condense and be removed as liquid. Unreacted hydrogen and nitrogen gases are then returned to the reaction vessel to undergo further reaction.

Catalysts

The catalyst has no effect on the position of chemical equilibrium; rather, it provides an alternative pathway with lower activation energy and hence increases the reaction rate, while remaining chemically unchanged at the end of the reaction. The first Haber–Bosch reaction chambers used osmium and ruthenium as catalysts. However, under Bosch's direction in 1909, the BASF researcher Alwin Mittasch discovered a much less expensive iron-based catalyst that is still used today. Part of the industrial production now takes place with a ruthenium rather than an iron catalyst in the KAAP-process.

In industrial practice, the iron catalyst is prepared by exposing a mass of magnetite, an iron oxide, to the hot hydrogen feedstock. This reduces some of the magnetite to metallic iron, removing oxygen in the process. However, the catalyst maintains most of its bulk volume during the reduction, and so the result is a highly porous material whose large surface area aids its effectiveness as a catalyst. Other minor components of the catalyst include calcium and aluminium oxides, which support the porous iron catalyst and help it maintain its surface area over time, and potassium, which increases the electron density of the catalyst and so improves its activity.

The reaction mechanism, involving the heterogeneous catalyst, is believed to be as follows:

1. N₂ (g) → N₂ (adsorbed)
2. N₂ (adsorbed) → 2 N (adsorbed)
3. H₂(g) → H₂ (adsorbed)
4. H₂ (adsorbed) → 2 H (adsorbed)
5. N (adsorbed) + 3 H(adsorbed)→ NH₃ (adsorbed)
6. NH₃ (adsorbed) → NH₃ (g)

Reaction 5 occurs in three steps, forming NH, NH₂, and then NH₃. Experimental evidence points to reaction 2 as being the slow, rate-determining step.

A major contributor to the elucidation of this mechanism is Gerhard Ertl.

Economic and environmental aspects

The Haber process now produces 100 million tons of nitrogen fertilizer per year, mostly in the form of anhydrous ammonia, ammonium nitrate, and urea. 3–5% of world natural gas production is consumed in the Haber process (~1–2% of the world's annual energy supply). That fertilizer is responsible for sustaining one-third of the Earth's population, as well as various deleterious environmental consequences. Hydrogen production using electrolysis of water powered by renewable energy is not yet competitive cost-wise with hydrogen from fossil fuels, such as natural gas, and so has been responsible for only 4% of current hydrogen production (almost all as a byproduct of the chloralkali process). Notably, the rise of the Haber industrial process led to the "Nitrate Crisis" in Chile when the natural nitrate mines were no longer profitable and were closed, leaving a large unemployed Chilean population behind.

See also

• Chemical kinetics
• Reaction rate
• Rate equation
• Timeline of hydrogen technologies

References

• "The Haber Process". Chemguide.co.uk. 

External links

• What is the Haber-Bosch Process?
• Haber-Bosch process, most important invention of the 20th century, according to V. Smil, Nature, July 29 1999, p 415 (by Jürgen Schmidhuber)
• Britannica guide to Nobel Prizes: Fritz Haber
• Nobel e-Museum - Biography of Fritz Haber
• Uses and Production of Ammonia
• Haber Process for Ammonia Synthesisar:عملية هابر

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