Editing Urea (section)

==Industrial production==
In 2020, worldwide production capacity was approximately 180 million tonnes.<ref>{{cite web |title=Urea production statistics |url=https://www.ifastat.org/supply/Nitrogen%20Products/Urea |website=www.ifastat.org |publisher=International Fertilizer Association |access-date=19 April 2023}}</ref>

For use in industry, urea is produced from synthetic [[ammonia]] and [[carbon dioxide]]. As large quantities of carbon dioxide are produced during the ammonia manufacturing process as a byproduct of burning [[hydrocarbons]] to generate heat (predominantly natural gas, and less often petroleum derivatives or coal), urea production plants are almost always located adjacent to the site where the ammonia is manufactured.

===Synthesis===
[[File:THC 2003.902.071 Urea Plant.tif|thumb|right | Urea plant using ammonium carbamate briquettes, Fixed Nitrogen Research Laboratory, ca. 1930]]
The basic process, patented in 1922, is called the ''[[Bosch–Meiser urea process]]'' after its discoverers [[Carl Bosch]] and Wilhelm Meiser.<ref>{{cite patent |country=US |number=1429483 |title=Process of Manufacturing Urea |gdate=1922-09-19 |fdate=1920-07-09 |invent1=Carl Bosch |invent2=Wilhelm Meiser |assign1=[[BASF]] |url=https://patentimages.storage.googleapis.com/36/5a/32/ccce45be728586/US1429483.pdf}}</ref> The process consists of two main [[equilibrium reaction]]s, with incomplete conversion of the reactants. The first is '''carbamate formation''': the fast [[exothermic]] reaction of liquid ammonia with gaseous carbon dioxide ({{chem2|CO2}}) at high temperature and pressure to form [[ammonium carbamate]] ({{chem2|[NH4]+[NH2COO]-}}):<ref name="Meessen 2012"/>

{{block indent |left=1.5 |text={{chem2|2 NH3 + CO2 ⇌ NH4CO2NH2}}{{space|5}}(Δ''H'' = −117&nbsp;kJ/mol at 110&nbsp;atm and 160&nbsp;°C)<ref name="Meessen 2012"/><ref name="Brouwer 2009">{{cite web |last1=Brouwer |first1=Mark |title=Thermodynamics of the Urea Process |url=https://ureaknowhow.com/pdflib/391_2009%2006%20Brouwer%20UreaKnowHow.com%20Thermodynamics%20of%20the%20%20Urea%20Process.pdf |website=ureaknowhow.com |access-date=26 February 2023}}</ref>}}

The second is '''urea conversion''': the slower [[endothermic]] decomposition of ammonium carbamate into urea and water:

{{block indent |left=1.5 |text={{chem2|NH4CO2NH2 ⇌ CO(NH2)2 + H2O}}{{space|5}}(Δ''H'' = +15.5&nbsp;kJ/mol at 160–180&nbsp;°C)<ref name="Meessen 2012"/><ref name="Brouwer 2009"/>}}

The overall conversion of {{chem2|NH3}} and {{chem2|CO2}} to urea is exothermic, with the reaction heat from the first reaction driving the second. The conditions that favor urea formation (high temperature) have an unfavorable effect on the carbamate formation equilibrium. The process conditions are a compromise: the ill-effect on the first reaction of the high temperature (around 190&nbsp;°C) needed for the second is compensated for by conducting the process under high pressure (140–175&nbsp;bar), which favors the first reaction. Although it is necessary to compress gaseous carbon dioxide to this pressure, the ammonia is available from the ammonia production plant in liquid form, which can be pumped into the system much more economically. To allow the slow urea formation reaction time to reach equilibrium, a large reaction space is needed, so the synthesis reactor in a large urea plant tends to be a massive pressure vessel.

===Reactant recycling===
Because the urea conversion is incomplete, the urea must be separated from the unconverted reactants, including the ammonium carbamate. Various commercial urea processes are characterized by the conditions under which urea forms and the way that unconverted reactants are further processed.

====Conventional recycle processes====

In early "straight-through" urea plants, reactant recovery (the first step in "recycling") was done by letting down the system pressure to atmospheric to let the carbamate decompose back to ammonia and carbon dioxide. Originally, because it was not economic to recompress the ammonia and carbon dioxide for recycle, the ammonia at least would be used for the manufacture of other products such as [[ammonium nitrate]] or [[ammonium sulfate]], and the carbon dioxide was usually wasted. Later process schemes made recycling unused ammonia and carbon dioxide practical. This was accomplished by the "total recycle process", developed in the 1940s to 1960s and now called the "conventional recycle process". It proceeds by depressurizing the reaction solution in stages (first to 18–25 bar and then to 2–5 bar) and passing it at each stage through a steam-heated ''carbamate decomposer'', then recombining the resulting carbon dioxide and ammonia in a falling-film ''carbamate condenser'' and pumping the carbamate solution back into the urea reaction vessel.<ref name="Meessen 2012" />

==== Stripping recycle process ====

The "conventional recycle process" for recovering and reusing the reactants has largely been supplanted by a [[Stripping (chemistry)|stripping]] process, developed in the early 1960s by [[Stamicarbon]] in The Netherlands, that operates at or near the full pressure of the reaction vessel. It reduces the complexity of the multi-stage recycle scheme, and it reduces the amount of water recycled in the carbamate solution, which has an adverse effect on the equilibrium in the urea conversion reaction and thus on overall plant efficiency. Effectively all new urea plants use the stripper, and many total recycle urea plants have converted to a stripping process.<ref name="Meessen 2012"/><ref name="Meessen 2014">{{cite journal |last=Meessen |first=Jozef |title=Urea synthesis |journal=Chemie Ingenieur Technik |publisher=Wiley |volume=86 | issue=12| date=2014 |issn=0009-286X |doi=10.1002/cite.201400064 |pages=2180–2189|doi-access=free }}</ref>

In the conventional recycle processes, carbamate decomposition is promoted by reducing the overall pressure, which reduces the partial pressure of both ammonia and carbon dioxide, allowing these gasses to be separated from the urea product solution. The stripping process achieves a similar effect without lowering the overall pressure, by suppressing the partial pressure of just one of the reactants in order to promote carbamate decomposition. Instead of feeding carbon dioxide gas directly to the urea synthesis reactor with the ammonia, as in the conventional process, the stripping process first routes the carbon dioxide through the stripper. The stripper is a carbamate decomposer that provides a large amount of gas-liquid contact. This flushes out free ammonia, reducing its partial pressure over the liquid surface and carrying it directly to a carbamate condenser (also under full system pressure). From there, reconstituted ammonium carbamate liquor is passed to the urea production reactor. That eliminates the medium-pressure stage of the conventional recycle process.<ref name="Meessen 2012"/><ref name="Meessen 2014"/>

=== Side reactions ===
The three main side reactions that produce impurities have in common that they decompose urea.

Urea hydrolyzes back to ammonium carbamate in the hottest stages of the synthesis plant, especially in the stripper, so residence times in these stages are designed to be short.<ref name="Meessen 2012"/>

[[Biuret]] is formed when two molecules of urea combine with the loss of a molecule of ammonia.

{{block indent |left=1.5 |text={{chem2|2 NH2CONH2 → NH2CONHCONH2 + NH3}}}}

Normally this reaction is suppressed in the synthesis reactor by maintaining an excess of ammonia, but after the stripper, it occurs until the temperature is reduced.<ref name="Meessen 2012"/> Biuret is undesirable in urea fertilizer because it is toxic to crop plants to varying degrees,<ref name="Mikkelsen 1990">{{cite journal | last=Mikkelsen | first=R. L | title=Biuret in urea fertilizer | journal=Fertilizer Research | volume=26 | issue=1–3 | year=1990 | issn=0167-1731 | doi=10.1007/bf01048769 | pages=311–318| s2cid=5970745 }}</ref> but it is sometimes desirable as a nitrogen source when used in animal feed.<ref>{{cite journal | last1=Fonnesbeck | first1=Paul V. | last2=Kearl | first2=Leonard C. | last3=Harris | first3=Lorin E. | title=Feed Grade Biuret as a Protein Replacement for Ruminants. A Review | journal=Journal of Animal Science | publisher=Oxford University Press (OUP) | volume=40 | issue=6 | date=1975 | issn=0021-8812 | doi=10.2527/jas1975.4061150x | pages=1150–1184 | url=https://academic.oup.com/jas/article-abstract/40/6/1150/4699012}}</ref>

[[Isocyanic acid]] HNCO and [[ammonia]] {{chem2|NH3}} results from the thermal decomposition of [[ammonium cyanate]] {{chem2|[NH4]+[OCN]-}}, which is in [[chemical equilibrium]] with urea:
{{block indent |left=1.5 |text={{chem2|CO(NH2)2 → [NH4]+[OCN]- → HNCO + NH3}}}}
This decomposition is at its worst when the urea solution is heated at low pressure, which happens when the solution is concentrated for prilling or granulation (see below). The reaction products mostly volatilize into the overhead vapours, and recombine when these condense to form urea again, which contaminates the process condensate.<ref name="Meessen 2012"/>

=== Corrosion ===

[[Ammonium carbamate]] solutions are highly corrosive to metallic construction materials – even to resistant forms of [[stainless steel]] – especially in the hottest parts of the synthesis plant such as the stripper. Historically [[corrosion]] has been minimized (although not eliminated) by continuous injection of a small amount of [[oxygen]] (as air) into the plant to establish and maintain a [[passivation (chemistry)|passive]] oxide layer on exposed stainless steel surfaces. Highly corrosion resistant materials have been introduced to reduce the need for passivation oxygen, such as specialized [[duplex stainless steel]]s in the 1990s, and [[zirconium]] or zirconium-clad titanium tubing in the 2000s.<ref name="Meessen 2012"/>

=== Finishing ===

Urea can be produced in solid forms ([[prill]]s, [[:wikt:granule|granules]], pellets or crystals) or as solutions.

==== Solid forms ====

For its main use as a fertilizer urea is mostly marketed in solid form, either as prills or granules. Prills are solidified droplets, whose production predates satisfactory urea granulation processes. Prills can be produced more cheaply than granules, but the limited size of prills (up to about 2.1&nbsp;mm in diameter), their low crushing strength, and the caking or crushing of prills during bulk storage and handling make them inferior to granules. Granules are produced by acretion onto urea seed particles by spraying liquid urea in a succession of layers. [[Formaldehyde]] is added during the production of both prills and granules in order to increase crushing strength and suppress caking. Other shaping techniques such as pastillization (depositing uniform-sized liquid droplets onto a cooling conveyor belt) are also used.<ref name="Meessen 2012"/>

==== Liquid forms ====

Solutions of [[UAN|urea and ammonium nitrate]] in water (UAN) are commonly used as a liquid fertilizer. In admixture, the combined solubility of ammonium nitrate and urea is so much higher than that of either component alone that it gives a stable solution with a total nitrogen content (32%) approaching that of solid ammonium nitrate (33.5%), though not, of course, that of urea itself (46%). UAN allows use of ammonium nitrate without the explosion hazard.<ref name="Meessen 2012"/> UAN accounts for 80% of the liquid fertilizers in the US.<ref name="Ren 2021">{{cite journal | last1=Ren | first1=Baizhao | last2=Guo | first2=Yanqing | last3=Liu | first3=Peng | last4=Zhao | first4=Bin | last5=Zhang | first5=Jiwang | title=Effects of Urea-Ammonium Nitrate Solution on Yield, N2O Emission, and Nitrogen Efficiency of Summer Maize Under Integration of Water and Fertilizer | journal=Frontiers in Plant Science | volume=12 | date=2021-08-03 | page=700331 | issn=1664-462X | doi=10.3389/fpls.2021.700331 | pmid=34413867 | pmc=8369924 | doi-access=free }}</ref>