Activity 1: Ammonia synthesis

Activity summary

What you must remember :

Skills linked to the curriculum :

Compétences Capacités à maîtriser

APP

Analyser un ou plusieurs procédés industriels de synthèse d'une même espèce chimique en s'appuyant sur les principes de la chimie verte : matières premières, sous-produits, énergie, catalyseur, sécurité.

COM

Formuler et argumenter des réponses structurées.
Formuler et présenter une conclusion.

Document 1: History of the 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 catalyst, to produce ammonia.

$$N_{2~(g)}+3~H_{2~(g)}=2~NH_{3~(g)}~~~~~\Delta_rH°=-92.4~kJ.mol^{-1}$$

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.

Source: https://en.wikipedia.org/

Document 2: Synthesis gas preparation (syngas)

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:

$$CH_4+H_2O=CO+3~H_2$$

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

$$2~CH_4+O_2=2~CO+4~H_2$$
$$CH_4+2~O_2=CO_2+2~H_2O$$

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

$$CO+H_2O=CO_2+H_2$$

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

$$CO+3~H_2=CH_4+H_2O$$

This last step is necessary as carbon monoxide poisons the catalyst. The overall reaction so far turns methane and steam into carbon dioxide, steam, and hydrogen.

Source: https://en.wikipedia.org/

Document 3: Ammonia synthesis

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

$$N_{2~(g)}+3~H_{2~(g)}=2~NH_{3~(g)}~~~~~\Delta_rH°=-92.4~kJ.mol^{-1}$$

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.

Source: https://en.wikipedia.org/

Document 4: Pressure and temperature compromise

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.

As the temperature increases, the equilibrium is shifted and hence, the amount of product drops dramatically according to Le Chatelier’s principle. 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 and the pressure used (around 200 atm) alters the equilibrium concentrations to give a profitable yield.

Conditions: T = 450°C, P = 200 atm, yield = 30%

Source: https://en.wikipedia.org/

Acquiring vocabulary.

French English
réaction réversible
équilibre thermodynamique
réaction exothermique
réactif
produit
réaction dans le sens direct
réaction dans le sens indirect
libérer de l’énergie
déplacer un équilibre
favoriser le sens direct
influencer/affecter une réaction
le rendement
cinétique (d’une réaction)
un catalyseur
enlever (un produit)
réacteur à lit fixe

Pressure and temperature comprimise.
What is the objective of the syngas step?

Write the reaction for the synthesis of ammonia:

Does this reaction give off or require heat?

In the dynamic equilibrium, do we want the forward or the reverse reaction?

How can we shift the dynamic equilibrium?

What is the effect of pressure on the yield?

What is the effect of temperature on the yield?

What is the effect of temperature on kinetics?

What is the best compromise in terms of temperature?

Why do we use a reactor packed with iron oxide?
Modifié le: dimanche 7 janvier 2018, 15:05