Editing Haber process (section)

=== Ammonia production ===
The hydrogen is catalytically reacted with nitrogen (derived from [[air separation]]{{clarify|date=September 2024}}) to form anhydrous [[liquid ammonia]]. It is difficult and expensive, as lower temperatures result in slower [[reaction kinetics]] (hence a slower [[reaction rate]]){{sfn|Clark|2013|loc=However, 400–450&nbsp;°C isn't a low temperature! Rate considerations: The lower the temperature you use, the slower the reaction becomes. A manufacturer is trying to produce as much ammonia as possible per day. It makes no sense to try to achieve an equilibrium mixture which contains a very high proportion of ammonia if it takes several years for the reaction to reach that equilibrium".}} and high pressure requires high-strength pressure vessels{{sfn|Clark|2013|loc="Rate considerations: Increasing the pressure brings the molecules closer together. In this particular instance, it will increase their chances of hitting and sticking to the surface of the catalyst where they can react. The higher the pressure the better in terms of the rate of a gas reaction. Economic considerations: Very high pressures are expensive to produce on two counts. Extremely strong pipes and containment vessels are needed to withstand the very high pressure. That increases capital costs when the plant is built"}} that resist [[hydrogen embrittlement]]. [[Diatomic]] nitrogen is bound together by a [[triple bond]], which makes it relatively inert.<ref name=XiaopingZhang_2024>{{cite journal |journal=[[Nature Communications]] |volume = 15 |issue = 1 |article-number = 1535 |doi=10.1038/s41467-024-45832-9 |title=Efficient catalyst-free N<sub>2</sub> fixation by water radical cations under ambient conditions |year=2024 |first1=Xiaoping|last1=Zhang|first2=Rui|last2=Su|first3=Jingling|last3=Li|first4=Liping|last4=Huang|first5=Wenwen|last5=Yang|first6=Konstantin|last6=Chingin|first7=Roman|last7=Balabin|first8=Jingjing|last8=Wang|first9=Xinglei|last9=Zhang|first10=Weifeng|last10=Zhu|first11=Keke|last11=Huang|first12=Shouhua|last12=Feng|first13=Huanwen|last13=Chen|page = 1535 |pmid=38378822|pmc=10879522|bibcode = 2024NatCo..15.1535Z }}</ref><ref>{{cite web |date=2019-06-05 |title=Chemistry of Nitrogen |url=https://chem.libretexts.org/Bookshelves/Inorganic_Chemistry/Supplemental_Modules_(Inorganic_Chemistry)/Descriptive_Chemistry/Elements_Organized_by_Block/2_p-Block_Elements/Group_15%3A_The_Nitrogen_Family/Z%3D007_Chemistry_of_Nitrogen_(Z%3D7) |access-date=2019-07-07 |website=Chem.LibreTexts.org |department=Compounds}}</ref> Yield and efficiency are low, meaning that the ammonia must be extracted and the gases reprocessed for the reaction to proceed at an acceptable pace.{{sfn|Clark|2013|loc="At each pass of the gases through the reactor, only about 15% of the nitrogen and hydrogen converts to ammonia. (This figure also varies from plant to plant.) By continual recycling of the unreacted nitrogen and hydrogen, the overall conversion is about 98%"}}

This step is known as the ammonia synthesis loop:

:<chem>3 H2 + N2 -> 2 NH3</chem>

The gases (nitrogen and hydrogen) are passed 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 unreacted gases are recycled, and eventually conversion of 97% is achieved.<ref name="Appl" />

Due to the nature of the (typically multi-promoted [[magnetite]]) catalyst used in the ammonia synthesis reaction, only low levels of oxygen-containing (especially CO, CO<sub>2</sub> and H<sub>2</sub>O) compounds can be tolerated in the hydrogen/nitrogen mixture. Relatively pure nitrogen can be obtained by [[air separation]], but additional [[oxygen]] removal may be required.

Because of relatively low single pass conversion rates (typically less than 20%), a large recycle stream is required. This can lead to the accumulation of inerts in the gas.

Nitrogen gas (N<sub>2</sub>) is unreactive because the [[atoms]] are held together by [[Chemical bond|triple bonds]]. The Haber process relies on catalysts that accelerate the [[scission (chemistry)|scission]] of these bonds.

Two opposing considerations are relevant: the equilibrium position and the [[reaction rate]]. At room temperature, the equilibrium is in favor of ammonia, but the reaction does not proceed at a detectable rate due to its high activation energy. Because the reaction is [[exothermic reaction|exothermic]], the equilibrium constant decreases with increasing temperature following [[Le Châtelier's principle]]. It becomes unity at around {{convert|150|–|200|°C|F|abbr=on}}.<ref name="Appl" />

{| class="wikitable" style="clear:right; float:right; margin-left: 1em;" border="3"
|+ ''K''(''T'') for {{chem|N|2}} + 3 {{chem|H|2}} ⇌ 2 {{chem|N|H|3}}<ref>{{Cite book |last1=Brown |first1=Theodore L. |title=Chemistry: The Central Science |last2=LeMay |first2=H. Eugene Jr. |last3=Bursten |first3=Bruce E. |date=2006 |publisher=Pearson |isbn=978-0-13-109686-8 |edition=10th |location=Upper Saddle River, NJ |chapter=Table 15.2 |chapter-url=https://archive.org/details/chemistry00theo_0 |chapter-url-access=registration}}</ref>
! Temperature<br />(°C)
! ''K''<sub>p</sub><br />{{clarify|date=September 2024}}
|-
| 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>
|}
Above this temperature, the equilibrium quickly becomes unfavorable at atmospheric pressure, according to the [[Van 't Hoff equation]]. Lowering the temperature is unhelpful because the catalyst requires a temperature of at least 400&nbsp;°C to be efficient.<ref name="Appl" />

Increased [[pressure]] favors the forward reaction because 4&nbsp;moles of reactant produce 2&nbsp;moles of product, and the pressure used ({{convert|15|–|25|MPa|abbr=on|bar psi}}) alters the equilibrium concentrations to give a substantial ammonia yield. The reason for this is evident in the equilibrium relationship:

<math chem="" display="block">K = \frac{y_\ce{NH3}^2}{y_\ce{H2}^3 y_\ce{N2}} \frac{\hat\phi_\ce{NH3}^2}{\hat\phi_\ce{H2}^3 \hat\phi_\ce{N2}} \left(\frac{P^\circ}{P}\right)^2,</math>

where <math>\hat\phi_i</math> is the [[fugacity coefficient]] of species <math>i</math>, <math>y_i</math> is the [[mole fraction]] of the same species, <math>P</math> is the reactor pressure, and <math>P^\circ</math> is standard pressure, typically {{convert|1|bar|MPa}}.

Economically, reactor pressurization is expensive: pipes, valves, and reaction vessels need to be strong enough, and safety considerations affect operating at 20&nbsp;MPa. Compressors take considerable energy, as work must be done on the (compressible) gas. Thus, the compromise used gives a single-pass yield of around 15%.<ref name="Appl" />

While removing the ammonia from the system increases the reaction yield, this step is not used in practice, since the temperature is too high; instead it is removed from the gases leaving the reaction vessel. The hot gases are cooled under high pressure, allowing the ammonia to condense and be removed as a liquid. Unreacted hydrogen and nitrogen gases are returned to the reaction vessel for another round.<ref name="Appl" /> While most ammonia is removed (typically down to 2–5&nbsp;mol.%), some ammonia remains in the recycle stream. In academic literature, a more complete separation of ammonia has been proposed by absorption in [[metal halides]], [[metal-organic framework]]s or [[zeolites]].<ref>{{Cite journal |last1=De Alwis Jayasinghe |first1=Dukula |last2=Chen |first2=Yinlin |last3=Li |first3=Jiangnan |last4=Rogacka |first4=Justyna M. |last5=Kippax-Jones |first5=Meredydd |last6=Lu |first6=Wanpeng |last7=Sapchenko |first7=Sergei |last8=Yang |first8=Jinyue |last9=Chansai |first9=Sarayute |last10=Zhou |first10=Tianze |last11=Guo |first11=Lixia |last12=Ma |first12=Yujie |last13=Dong |first13=Longzhang |last14=Polyukhov |first14=Daniil |last15=Shan |first15=Lutong |last16=Han |first16=Yu |last17=Crawshaw |first17=Danielle |last18=Zeng |first18=Xiangdi |last19=Zhu |first19=Zhaodong |last20=Hughes |first20=Lewis |last21=Frogley |first21=Mark D. |last22=Manuel |first22=Pascal |last23=Rudić |first23=Svemir |last24=Cheng |first24=Yongqiang |last25=Hardacre |first25=Christopher |last26=Schröder |first26=Martin |last27=Yang |first27=Sihai|date=8 November 2024 |title=A Flexible Phosphonate Metal–Organic Framework for Enhanced Cooperative Ammonia Capture |journal=Journal of the American Chemical Society |volume=146 |issue=46 |pages=32040–32048  |doi=10.1021/jacs.4c12430|pmid=39513623 |pmc=11583364 |bibcode=2024JAChS.14632040D }}</ref> Such a process is called an ''absorbent-enhanced Haber process'' or ''adsorbent-enhanced Haber–Bosch process''.<ref>{{Cite journal |last1=Abild-pedersen |first1=Frank |last2=Bligaard |first2=Thomas |date=1 January 2014 |title=Exploring the limits: A low-pressure, low-temperature Haber–Bosch process |url=https://www.academia.edu/25092433 |journal=Chemical Physics Letters |volume=598 |page=108 |bibcode=2014CPL...598..108V |doi=10.1016/j.cplett.2014.03.003 |via=academia.edu}}</ref>