Synthetic ammonia

Ammonia synthesis refers to the direct synthesis of ammonia from nitrogen and hydrogen under high temperature and pressure in the presence of a catalyst; it is a fundamental inorganic chemical process. In the modern chemical industry, ammonia serves as a key raw material for the fertiliser industry and basic organic chemical production.

Ammonia is a vital basic raw material in the chemical industry. The feedstocks used in ammonia synthesis include natural gas, coal, naphtha and heavy oil. Of these, ammonia produced from natural gas accounts for 80 per cent of global production.

Natural gas is first desulphurised, then undergoes secondary conversion, followed by processes such as the carbon monoxide shift and carbon dioxide removal. The resulting nitrogen-hydrogen mixture still contains approximately 0.1 per cent to 0.3 per cent (by volume) of carbon monoxide and carbon dioxide; which is subsequently removed via methanation to produce a pure gas with a hydrogen-to-nitrogen molar ratio of 3. This gas is then compressed by a compressor and fed into the ammonia synthesis loop to produce ammonia.

To improve efficiency, catalysts are required during the process. However, whilst the catalyst remains active, even minimal contact with impurities can cause a significant decline in activity or even complete deactivation, leading to catalyst poisoning. It is generally believed that poisoning occurs when active sites on the catalyst surface are occupied by impurities. Poisoning is classified into two types: temporary and permanent. For example, in the case of iron catalysts used in the ammonia synthesis reaction, O_₂, CO, CO_₂ and water vapour can all cause catalyst poisoning.

In the ammonia synthesis process, it is necessary to monitor the concentrations of gas components in the fresh gas and the outlet gas from the recirculated gas stream for the purposes of process feedback control and optimisation of the production process.

(1) Feed gas preparation involves converting raw materials such as coal and natural gas into crude feed gas containing hydrogen and nitrogen. For solid feedstocks such as coal and coke, gasification is typically used to produce synthesis gas; for residual oil, synthesis gas can be obtained via non-catalytic partial oxidation; for gaseous hydrocarbons and naphtha, the two-stage steam reforming process is utilised in industry to produce synthesis gas.

(2) Purification: The crude feed gas undergoes purification to remove impurities other than hydrogen and nitrogen; this primarily comprises the shift reaction, desulphurisation and decarbonisation processes, and gas refining.

① Carbon monoxide shift process: In ammonia synthesis, feed gases produced by various methods all contain CO, typically at a volume fraction of 120. As the two components required for ammonia synthesis are H₂ and N₂, the CO must be removed from the synthesis gas.

The shift reaction is as follows: CO + H₂O → H₂ + CO₂ ΔH = −41.2 kJ/mol

As the CO shift process is a strongly exothermic reaction, it must be carried out in stages to facilitate the recovery of reaction heat and to control the residual CO content at the outlet of the shift section.

The first step is high-temperature shift, which converts the majority of CO into CO₂ and H₂;

The second step is low-temperature shift, which reduces the CO content to approximately 0.3. Consequently, the CO shift reaction serves both as a continuation of feed gas production and as a purification process, thereby creating the conditions for the subsequent decarbonisation process.

② Desulphurisation and decarbonisation process: Crude feed gas produced from various raw materials contains some sulphur and carbon oxides. To prevent catalyst poisoning in the ammonia synthesis process, these must be removed prior to the ammonia synthesis stage. In the steam reforming process using natural gas as feedstock, the first stage is desulphurisation, which serves to protect the conversion catalyst. In the partial oxidation process using heavy oil and coal as feedstocks, the location of desulphurisation is determined by whether a sulphur-tolerant catalyst is used in the carbon monoxide shift reaction. There are many types of industrial desulphurisation methods; typically, physical or chemical absorption methods are employed, with common examples including the low-temperature methanol scrubbing method (Rectisol) and the polyethylene glycol dimethyl ether method (Selexol).

After the CO shift reaction of the crude feed gas, the shift gas contains not only H₂ but also CO₂, CO and CH₄, amongst other components, with CO₂ being the most abundant. CO₂ acts both as a poison for ammonia synthesis catalysts and as a key raw material for the production of nitrogen fertilisers such as urea and ammonium bicarbonate. Consequently, the removal of CO₂ from the shift gas must take both of these requirements into account.

Solution absorption is generally employed to remove CO₂. Depending on the properties of the absorbent, this method can be divided into two main categories.

One category comprises physical absorption methods, such as the low-temperature methanol scrubbing process (Rectisol), the polyethylene glycol dimethyl ether process (Selexol) and the propylene carbonate process.

The other category comprises chemical absorption methods, such as the hot potassium alkali process, the low-heat-consumption Benfield process, the activated MDEA process and the MEA process.

③ Gas Purification Process Following CO conversion and the removal of CO₂, the feed gas still contains small amounts of residual CO and CO₂. To prevent poisoning of the ammonia synthesis catalyst, the total content of CO and CO₂ must not exceed 10 cm³/m³ (by volume). Consequently, prior to entering the synthesis stage, the feed gas must undergo final purification, i.e. the refining process.

In industrial production, the final purification methods are categorised as cryogenic separation and methanation. The cryogenic separation method primarily involves liquid nitrogen scrubbing, which utilises liquid nitrogen to absorb and separate small amounts of CO under deep-freezing conditions (<−100>³/m³ of inert gases. The cryogenic purification method is typically combined with air separation and low-temperature methanol washing. The methanation process is a purification method in which, in the presence of a catalyst, small amounts of CO, CO₂ and H₂ react to produce CH₄ and H₂O. It requires that the carbon oxide content (volume fraction) in the feed gas to generally be less than 0.7. The methanation process can remove carbon oxides (CO + CO₂) from the gas to levels below 10 cm³/m³, but it consumes the active component H₂ and increases the content of the inert gas CH₄. The methanation reaction is as follows:

CO + 3H₂ → CH₄ + H₂O -206.2 kJ/mol 0298HΔ

CO₂ + 4H₂ → CH₄ + 2H₂O – 206.2 kJ/mol 0298HΔ

(3) Ammonia synthesis involves compressing a mixture of pure hydrogen and nitrogen to high pressure and synthesising ammonia under the action of a catalyst. Ammonia synthesis is the process that yields liquid ammonia and forms the core of the entire ammonia production process.

The ammonia synthesis reaction takes place under high pressure and in the presence of a catalyst. As the ammonia content in the post-reaction gas is relatively low—typically only 100—a process utilising the recirculation of unreacted hydrogen and nitrogen is employed. The chemical equation for ammonia synthesis is as follows:

N₂ + 3H₂ → 2NH₃ -92.4 kJ/mol

In this process, trace amounts of N₂, H₂, CO, CO₂, Ar, CH₄ and other gases at each stage can be monitored using an online mass spectrometer, thereby saving energy, reducing consumption and improving production efficiency.