Editing Haber process (section)

== Industrial production ==

=== Synthesis parameters ===
{| class="wikitable floatright"
|+Change of the equilibrium constant K<sub>eq</sub> as a function of temperature<ref name="brown">{{Cite book |last1=Brown |first1=Theodore L. |title=Chemistry the Central Science |last2=LeMay |first2=H. Eugene |last3=Bursten |first3=Bruce Edward |date=2003 |publisher=Prentice Hall |isbn=978-0-13-038168-2 |editor-last=Brunauer |editor-first=Linda Sue |edition=9th |location=Upper Saddle River, New Jersey, Pakistan Punjab|language=en}}</ref>
|-
! temperature (°C)
! K<sub>eq</sub>
|-
|align="center" | 300
| 4.34 × 10<sup>−3</sup>
|-
|align="center" | 400
| 1.64 × 10<sup>−4</sup>
|-
|align="center" | 450
| 4.51 × 10<sup>−5</sup>
|-
|align="center" | 500
| 1.45 × 10<sup>−5</sup>
|-
|align="center" | 550
| 5.38 × 10<sup>−6</sup>
|-
|align="center" | 600
| 2.25 × 10<sup>−6</sup>
|}

The reaction is:

:<math chem>\ce{N2 + 3H2 <=> 2NH3} \qquad {\Delta H^\circ_{\mathrm{298~K}} = -92.28 \; \ce{kJ/mol}}) </math><ref name="Holleman">{{Holleman&Wiberg|edition=102|pages=662–665}}</ref>

The reaction is an exothermic equilibrium reaction in which the gas volume is reduced. The equilibrium constant K<sub>eq</sub> of the reaction (see table) and obtained from:

: <math chem>K_{eq} = \frac{p^2 \ce{(NH3)}}{p \ce{(N2)}\cdot p^3 \ce{(H2)}}</math>

Since the reaction is [[Exothermic reaction|exothermic]], the equilibrium of the reaction shifts at lower temperatures to the ammonia side. Furthermore, four volumetric units of the raw materials produce two volumetric units of ammonia. According to [[Le Chatelier's principle]], higher pressure favours ammonia. High pressure is necessary to ensure sufficient surface coverage of the catalyst with nitrogen.<ref name="cornils">{{Cite book |last1=Cornils |first1=Boy |title=Catalysis from A to Z: A Concise Encyclopedia |last2=Herrmann |first2=Wolfgang A. |last3=Muhler |first3=M. |last4=Wong |first4=C. |date=2007 |publisher=Verlag Wiley-VCH |isbn=978-3-527-31438-6 |page=31}}</ref> For this reason, a ratio of nitrogen to hydrogen of 1 to 3, a [[pressure]] of 250 to 350 bar, a temperature of 450 to 550&nbsp;°C and α iron are optimal.

The catalyst [[Ferrite (iron)|ferrite]] (α-Fe) is produced in the reactor by the reduction of magnetite with hydrogen. The catalyst has its highest efficiency at temperatures of about 400 to 500&nbsp;°C. Even though the catalyst greatly lowers the [[activation energy]] for the cleavage of the [[triple bond]] of the nitrogen molecule, high temperatures are still required for an appropriate reaction rate. At the industrially used reaction temperature of 450 to 550&nbsp;°C an optimum between the decomposition of ammonia into the starting materials and the effectiveness of the catalyst is achieved.<ref name="Oberstufe">{{Cite book |title=Fokus Chemie Oberstufe Einführungsphase |date=2010 |publisher=Cornelsen-Verlag |isbn=978-3-06-013953-8 |location=Berlin |page=79 |language=de}}</ref> The formed ammonia is continuously removed from the system. The volume fraction of ammonia in the gas mixture is about 20%.

The inert components, especially the noble gases such as [[argon]], should not exceed a certain content in order not to reduce the [[partial pressure]] of the reactants too much. To remove the inert gas components, part of the gas is removed and the argon is separated in a [[gas separation plant]]. The extraction of pure argon from the circulating gas is carried out using the [[Linde process]].<ref name="ullmann">P. Häussinger u. a.: ''Noble Gases.'' In: ''Ullmann's Encyclopedia of Industrial Chemistry.'' Wiley-VCH, Weinheim 2006. {{doi|10.1002/14356007.a17_485}}</ref>

=== Large-scale implementation ===
Modern ammonia plants produce more than 3000 tons per day in one production line. The following diagram shows the set-up of a modern (designed in the early 1960s by [[KBR, Inc.|Kellogg]]<ref>https://acshist.scs.illinois.edu/bulletin_open_access/v47-1/v47-1%20p50-61.pdf</ref>) "single-train" Haber–Bosch plant:

[[File:Haber-Bosch-En.svg|upright=3.6|thumb|center|{{Farbindex|ecbae7|primary reformer}} {{Farbindex|4d94e1|air feed}} {{Farbindex|f2c500|secondary reformer}} {{Farbindex|cadaeb|CO conversion}} {{Farbindex|cadaeb|washing tower}} {{Farbindex|f2c500|ammonia reactor}} {{Farbindex|4d94e1|heat exchanger}} {{Farbindex|fffc51|ammonia condenser}}]]

Depending on its origin, the synthesis gas must first be freed from impurities such as [[hydrogen sulfide]] or organic sulfur compounds, which act as a [[catalyst poisoning|catalyst poison]]. High concentrations of hydrogen sulfide, which occur in synthesis gas from [[carbonization]] coke, are removed in a wet cleaning stage such as the [[sulfosolvan process]], while low concentrations are removed by adsorption on [[activated carbon]].<ref name="leibnitz">{{Cite journal |last1=Leibnitz |first1=E. |last2=Koch |first2=H. |last3=Götze |first3=A. |year=1961 |title=Über die drucklose Aufbereitung von Braunkohlenkokereigas auf Starkgas nach dem Girbotol-Verfahren |journal=Journal für Praktische Chemie |language=de |volume=13 |issue=3–4 |pages=215–236 |doi=10.1002/prac.19610130315}}</ref> Organosulfur compounds are separated by [[pressure swing adsorption]] together with carbon dioxide after CO conversion.

To produce [[hydrogen]] by steam reforming, methane reacts with water vapor using a nickel oxide-alumina catalyst in the primary reformer to form [[carbon monoxide]] and hydrogen. The energy required for this, the [[enthalpy]] ΔH, is 206 kJ/mol.<ref name="steinborn1">{{Cite book |last=Steinborn |first=Dirk |title=Grundlagen der metallorganischen Komplexkatalyse |date=2007 |publisher=Teubner |isbn=978-3-8351-0088-6 |location=Wiesbaden |pages=319–321 |language=de}}</ref>

:<math chem>\ce{ {CH4_{(g)} } + H2O_{(g)} -> {CO_{(g)} } + 3H2_{(g)} } \qquad {\Delta H^\circ = +206\ \ce{kJ/mol} }</math>

The methane gas reacts in the primary reformer only partially. To increase the hydrogen yield and keep the content of inert components (i. e. methane) as low as possible, the remaining methane gas is converted in a second step with oxygen to hydrogen and carbon monoxide in the secondary reformer. The secondary reformer is supplied with air as the oxygen source. Also, the required nitrogen for the subsequent ammonia synthesis is added to the gas mixture.

:<math chem>\ce{ {2CH4_{(g)} } + O2_{(g)} -> {2CO_{(g)} } + 4H2_{(g)} } \qquad {\Delta H^\circ = -71\ \ce{kJ/mol} }</math>

In the third step, the carbon monoxide is oxidized to [[carbon dioxide]], which is called CO conversion or [[water-gas shift reaction]].

:<math chem>\ce{ {CO_{(g)} } + H2O(g) -> {CO2_{(g)} } + H2_{(g)}} \qquad {\Delta H^\circ = -41\ \ce{kJ/mol} }</math>

Carbon monoxide and carbon dioxide would form [[carbamate]]s with ammonia, which would clog (as solids) pipelines and apparatus within a short time. In the following process step, the carbon dioxide must therefore be removed from the gas mixture. In contrast to carbon monoxide, carbon dioxide can easily be removed from the gas mixture by [[Scrubber|gas scrubbing]] with [[triethanolamine]]. The gas mixture then still contains methane and noble gases such as argon, which, however, behave inertly.<ref name="roempp" />

The gas mixture is then compressed to operating pressure by [[Turbo-compressor|turbo compressors]]. The resulting compression heat is dissipated by [[heat exchanger]]s; it is used to preheat raw gases.

The actual production of ammonia takes place in the ammonia reactor. The first reactors were bursting under high pressure because the atomic hydrogen in the carbonaceous steel [[High temperature hydrogen attack|partially recombined into methane]] and produced cracks in the steel. Bosch, therefore, developed tube reactors consisting of a pressure-bearing steel tube in which a low-carbon iron lining tube was inserted and filled with the catalyst. Hydrogen that diffused through the inner steel pipe escaped to the outside via thin holes in the outer steel jacket, the so-called Bosch holes.<ref name="Holleman" /> A disadvantage of the tubular reactors was the relatively high-pressure loss, which had to be applied again by compression. The development of hydrogen-resistant chromium-molybdenum steels made it possible to construct single-walled pipes.<ref name="forst" />

[[File:Ammoniakreaktor MS.svg|thumb|upright=1.2|Modern ammonia reactor with heat exchanger modules: The cold gas mixture is preheated to reaction temperature in heat exchangers by the reaction heat and cools in turn the produced ammonia.]]

Modern ammonia reactors are designed as multi-storey reactors with a low-pressure drop, in which the catalysts are distributed as fills over about ten storeys one above the other. The gas mixture flows through them one after the other from top to bottom. Cold gas is injected from the side for cooling. A disadvantage of this reactor type is the incomplete conversion of the cold gas mixture in the last catalyst bed.<ref name="forst">{{Cite book |last1=Forst |first1=Detlef |title=Chemie für Ingenieure |last2=Kolb |first2=Maximillian |last3=Roßwag |first3=Helmut |date=1993 |publisher=Springer Verlag |isbn=978-3-662-00655-9 |pages=234–238 |language=de}}</ref>

Alternatively, the reaction mixture between the catalyst layers is cooled using heat exchangers, whereby the hydrogen-nitrogen mixture is preheated to the reaction temperature. Reactors of this type have three catalyst beds. In addition to good temperature control, this reactor type has the advantage of better conversion of the raw material gases compared to reactors with cold gas injection.

Uhde has developed and is using an ammonia converter with three radial flow catalyst beds and two internal heat exchangers instead of axial flow catalyst beds. This further reduces the pressure drop in the converter.<ref>{{Cite web |title=Ammoniakkonverter – Düngemittelanlagen |url=https://www.thyssenkrupp-industrial-solutions.com/de/produkte-und-services/duengemittelanlagen/ammonia-plants-by-uhde/ammonia-plants-500mtpd/ammoniakkonverter |access-date=8 December 2021 |website=Industrial Solutions |language=de}}</ref>

The reaction product is continuously removed for maximum yield. The gas mixture is cooled to 450&nbsp;°C in a heat exchanger using water, freshly supplied gases, and other process streams. The ammonia also condenses and is separated in a pressure separator. Unreacted nitrogen and hydrogen are then compressed back to the process by a [[Gas compressor|circulating gas compressor]], supplemented with fresh gas, and fed to the reactor.<ref name="forst" /> In a subsequent distillation, the product ammonia is purified.