"},"pronounce":{"wt":"urea {{IPAc-en|j|ʊəˈr|iː|ə}}, carbamide {{IPAc-en|ˈ|k|ɑːr|b|ə|m|aɪ|d}}"},"OtherNames":{"wt":"{{ubl|Carbamide|Carbonyldiamide|Carbonyldiamine|Diaminomethanal|Diaminomethanone}}"},"IUPACName":{"wt":""},"Section1":{"wt":"{{Chembox Identifiers\n| IUPHAR_ligand = 4539\n| ChEBI_Ref = {{ebicite|correct|EBI}}\n| ChEBI = 16199\n| DrugBank_Ref = {{drugbankcite|correct|drugbank}}\n| DrugBank = DB03904\n| SMILES = C(=O)(N)N\n| UNII_Ref = {{fdacite|correct|FDA}}\n| UNII = 8W8T17847W\n| KEGG_Ref = {{keggcite|correct|kegg}}\n| KEGG = D00023\n| InChI = 1/CH4N2O/c2-1(3)4/h(H4,2,3,4)\n| InChIKey = XSQUKJJJFZCRTK-UHFFFAOYAF\n| ChEMBL_Ref = {{ebicite|correct|EBI}}\n| ChEMBL = 985\n| StdInChI_Ref = {{stdinchicite|correct|chemspider}}\n| StdInChI = 1S/CH4N2O/c2-1(3)4/h(H4,2,3,4)\n| StdInChIKey_Ref = {{stdinchicite|correct|chemspider}}\n| StdInChIKey = XSQUKJJJFZCRTK-UHFFFAOYSA-N\n| CASNo = 57-13-6\n| CASNo_Ref = {{cascite|correct|CAS}}\n| PubChem = 1176\n| ChemSpiderID_Ref = {{chemspidercite|correct|chemspider}}\n| ChemSpiderID = 1143\n| RTECS = YR6250000\n| Beilstein = 635724\n| Gmelin = 1378\n }}"},"Section2":{"wt":"{{Chembox Properties\n| Formula = {{chem2|CO(NH2)2}}\n| MolarMass = 60.06 g/mol\n| Appearance = White solid\n| Density = 1.32 g/cm3\n| Solubility = 545 g/L (at 25 °C){{cite book|url=https://books.google.com/books?id=cfFzJFthLCIC&pg=PP1|title = Handbook of Aqueous Solubility Data|isbn = 9781439802465|last1 = Yalkowsky|first1 = Samuel H.|last2 = He|first2 = Yan|last3 = Jain|first3 = Parijat|date = 19 April 2016| publisher=CRC Press }}\n| SolubleOther = 500 g/L glycerol{{cite web|url=http://msdssearch.dow.com/PublishedLiteratureDOWCOM/dh_0032/0901b8038003228b.pdf?filepath=glycerine/pdfs/noreg/115-00668.pdf|title=Solubility of Various Compounds in Glycerine|work=msdssearch.dow.com|access-date=2014-04-12|archive-url=https://web.archive.org/web/20140413155224/http://msdssearch.dow.com/PublishedLiteratureDOWCOM/dh_0032/0901b8038003228b.pdf?filepath=glycerine%2Fpdfs%2Fnoreg%2F115-00668.pdf|archive-date=2014-04-13|url-status=dead}}\n 50 g/L ethanol
\n ~4 g/L [[acetonitrile]]{{cite journal|last1=Loeser|first1=Eric|last2=DelaCruz|first2=Marilyn|last3=Madappalli|first3=Vinay | name-list-style = vanc |title=Solubility of Urea in Acetonitrile–Water Mixtures and Liquid–Liquid Phase Separation of Urea-Saturated Acetonitrile–Water Mixtures|journal=Journal of Chemical & Engineering Data|date=9 June 2011|volume=56|issue=6|pages=2909–2913|doi=10.1021/je200122b}}\n| MeltingPtC = 133 to 135\n| BoilingPt = decomposes\n| MeltingPt_notes =\n| pKa =\n| pKb = 13.9Calculated from 14−pKa. The value of pKa is given as 0.10 by the ''CRC Handbook of Chemistry and Physics'', 49th edition (1968–1969). A value of 0.18 is given by {{cite web|url=http://research.chem.psu.edu/brpgroup/pKa_compilation.pdf|title=pKa Data|last1=Williams|first1=R.|date=2001-10-24|archive-url=https://web.archive.org/web/20030824213333/http://research.chem.psu.edu/brpgroup/pKa_compilation.pdf|archive-date=August 24, 2003}}\n| ConjugateAcid = Uronium\n| MagSus = −33.4·10−6 cm3/mol\n}}"},"Section3":{"wt":"{{Chembox Structure\n| Dipole = 4.56 [[Debye|D]]\n }}"},"Section4":{"wt":"{{Chembox Thermochemistry\n| Thermochemistry_ref = CRC Handbook\n| HeatCapacity =\n| Entropy =\n| DeltaHf = −333.19 kJ/mol\n| DeltaGf = −197.15 kJ/mol\n| DeltaHc =\n }}"},"Section5":{"wt":""},"Section6":{"wt":"{{Chembox Pharmacology\n| ATCCode_prefix = B05\n| ATCCode_suffix = BC02\n| ATC_Supplemental = {{ATC|D02|AE01}}\n}}"},"Section7":{"wt":"{{Chembox Hazards\n| ExternalSDS = [https://www.inchem.org/documents/icsc/icsc/eics0595.htm ICSC 0595]\n| FlashPt = Non-flammable\n| LD50 = 8500 mg/kg (oral, rat)\n| NFPA-H = 1\n| NFPA-F = 1\n| NFPA-R = 0\n}}"},"Section8":{"wt":"{{Chembox Related\n| OtherFunction = [[Thiourea]]
[[Hydroxycarbamide]]\n| OtherFunction_label = ureas\n| OtherCompounds = {{ubl|[[Carbamide peroxide]]|[[Urea phosphate]]|[[Acetone]]|[[Carbonic acid]]|[[Carbonyl fluoride]]}}\n}}"}},"i":1}}]}" id="mwCA">
Names | |||
---|---|---|---|
Pronunciation | urea /jʊəˈriːə/ , carbamide /ˈkɑːrbəmaɪd/ | ||
Preferred IUPAC name Urea [1] | |||
Systematic IUPAC name Carbonic diamide [1] | |||
Other names
| |||
Identifiers | |||
3D model (JSmol) | |||
635724 | |||
ChEBI | |||
ChEMBL | |||
ChemSpider | |||
DrugBank | |||
ECHA InfoCard | 100.000.286 | ||
E number | E927b (glazing agents, ...) | ||
1378 | |||
KEGG | |||
PubChem CID | |||
RTECS number |
| ||
UNII | |||
CompTox Dashboard (EPA) | |||
| |||
| |||
Properties | |||
CO(NH2)2 | |||
Molar mass | 60.06 g/mol | ||
Appearance | White solid | ||
Density | 1.32 g/cm3 | ||
Melting point | 133 to 135 °C (271 to 275 °F; 406 to 408 K) | ||
Boiling point | decomposes | ||
545 g/L (at 25 °C) [2] | |||
Solubility | 500 g/L glycerol [3] 50 g/L ethanol | ||
Basicity (pKb) | 13.9 [5] | ||
Conjugate acid | Uronium | ||
−33.4·10−6 cm3/mol | |||
Structure | |||
4.56 D | |||
ThermochemistryCRC Handbook | |||
Std enthalpy of formation (ΔfH⦵298) | −333.19 kJ/mol | ||
Gibbs free energy (ΔfG⦵) | −197.15 kJ/mol | ||
Pharmacology | |||
B05BC02 ( WHO ) D02AE01 ( WHO ) | |||
Hazards | |||
NFPA 704 (fire diamond) | |||
Flash point | Non-flammable | ||
Lethal dose or concentration (LD, LC): | |||
LD50 (median dose) | 8500 mg/kg (oral, rat) | ||
Safety data sheet (SDS) | ICSC 0595 | ||
Related compounds | |||
Related ureas | Thiourea Hydroxycarbamide | ||
Related compounds | |||
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). |
Urea, also called carbamide (because it is a diamide of carbonic acid), is an organic compound with chemical formula CO(NH2)2. This amide has two amino groups (–NH2) joined by a carbonyl functional group (–C(=O)–). It is thus the simplest amide of carbamic acid. [6]
Urea serves an important role in the cellular metabolism of nitrogen-containing compounds by animals and is the main nitrogen-containing substance in the urine of mammals. Urea is Neo-Latin, from French urée , from Ancient Greek οὖρον (oûron) 'urine', itself from Proto-Indo-European *h₂worsom.
It is a colorless, odorless solid, highly soluble in water, and practically non-toxic (LD50 is 15 g/kg for rats). [7] Dissolved in water, it is neither acidic nor alkaline. The body uses it in many processes, most notably nitrogen excretion. The liver forms it by combining two ammonia molecules (NH3) with a carbon dioxide (CO2) molecule in the urea cycle. Urea is widely used in fertilizers as a source of nitrogen (N) and is an important raw material for the chemical industry.
In 1828, Friedrich Wöhler discovered that urea can be produced from inorganic starting materials, which was an important conceptual milestone in chemistry. This showed for the first time that a substance previously known only as a byproduct of life could be synthesized in the laboratory without biological starting materials, thereby contradicting the widely held doctrine of vitalism, which stated that only living organisms could produce the chemicals of life.
The structure of the molecule of urea is O=C(−NH2)2. The urea molecule is planar when in a solid crystal because of sp2 hybridization of the N orbitals. [8] [9] It is non-planar with C2 symmetry when in the gas phase [10] or in aqueous solution, [9] with C–N–H and H–N–H bond angles that are intermediate between the trigonal planar angle of 120° and the tetrahedral angle of 109.5°. In solid urea, the oxygen center is engaged in two N–H–O hydrogen bonds. The resulting hydrogen-bond network is probably established at the cost of efficient molecular packing: The structure is quite open, the ribbons forming tunnels with square cross-section. The carbon in urea is described as sp2 hybridized, the C-N bonds have significant double bond character, and the carbonyl oxygen is relatively basic. Urea's high aqueous solubility reflects its ability to engage in extensive hydrogen bonding with water.
By virtue of its tendency to form porous frameworks, urea has the ability to trap many organic compounds. In these so-called clathrates, the organic "guest" molecules are held in channels formed by interpenetrating helices composed of hydrogen-bonded urea molecules. In this way, urea-clathrates have been well investigated for separations. [11]
Urea is a weak base, with a pKb of 13.9. [5] When combined with strong acids, it undergoes protonation at oxygen to form uronium salts. [13] [14] It is also a Lewis base, forming metal complexes of the type [M(urea)6]n+. [15]
Urea reacts with malonic esters to make barbituric acids.
Molten urea decomposes into ammonium cyanate at about 152 °C, and into ammonia and isocyanic acid above 160 °C: [16]
Heating above 160 °C yields biuret NH2CONHCONH2 and triuret NH2CONHCONHCONH2 via reaction with isocyanic acid: [17] [16]
At higher temperatures it converts to a range of condensation products, including cyanuric acid (CNOH)3, guanidine HNC(NH2)2, and melamine. [17] [16]
In aqueous solution, urea slowly equilibrates with ammonium cyanate. This elimination reaction [18] cogenerates isocyanic acid, which can carbamylate proteins, in particular the N-terminal amino group, the side chain amino of lysine, and to a lesser extent the side chains of arginine and cysteine. [19] [20] Each carbamylation event adds 43 daltons to the mass of the protein, which can be observed in protein mass spectrometry. [20] For this reason, pure urea solutions should be freshly prepared and used, as aged solutions may develop a significant concentration of cyanate (20 mM in 8 M urea). [20] Dissolving urea in ultrapure water followed by removing ions (i.e. cyanate) with a mixed-bed ion-exchange resin and storing that solution at 4 °C is a recommended preparation procedure. [21] However, cyanate will build back up to significant levels within a few days. [20] Alternatively, adding 25–50 mM ammonium chloride to a concentrated urea solution decreases formation of cyanate because of the common ion effect. [20] [22]
Urea is readily quantified by a number of different methods, such as the diacetyl monoxime colorimetric method, and the Berthelot reaction (after initial conversion of urea to ammonia via urease). These methods are amenable to high throughput instrumentation, such as automated flow injection analyzers [23] and 96-well micro-plate spectrophotometers. [24]
Ureas describes a class of chemical compounds that share the same functional group, a carbonyl group attached to two organic amine residues: R1R2N−C(=O)−NR3R4, where R1, R2, R3 and R4 groups are hydrogen (–H), organyl or other groups. Examples include carbamide peroxide, allantoin, and hydantoin. Ureas are closely related to biurets and related in structure to amides, carbamates, carbodiimides, and thiocarbamides.
More than 90% of world industrial production of urea is destined for use as a nitrogen-release fertilizer. [17] Urea has the highest nitrogen content of all solid nitrogenous fertilizers in common use. Therefore, it has a low transportation cost per unit of nitrogen nutrient. The most common impurity of synthetic urea is biuret, which impairs plant growth. Urea breaks down in the soil to give ammonium ions (NH+4). The ammonium is taken up by the plant through its roots. In some soils, the ammonium is oxidized by bacteria to give nitrate (NO−3), which is also a nitrogen-rich plant nutrient. The loss of nitrogenous compounds to the atmosphere and runoff is wasteful and environmentally damaging so urea is sometimes modified to enhance the efficiency of its agricultural use. Techniques to make controlled-release fertilizers that slow the release of nitrogen include the encapsulation of urea in an inert sealant, and conversion of urea into derivatives such as urea-formaldehyde compounds, which degrade into ammonia at a pace matching plants' nutritional requirements.
Urea is a raw material for the manufacture of formaldehyde based resins, such as UF, MUF, and MUPF, used mainly in wood-based panels, for instance, particleboard, fiberboard, OSB, and plywood.
Urea can be used in a reaction with nitric acid to make urea nitrate, a high explosive that is used industrially and as part of some improvised explosive devices.
Urea is used in Selective Non-Catalytic Reduction (SNCR) and Selective Catalytic Reduction (SCR) reactions to reduce the NOx pollutants in exhaust gases from combustion from diesel, dual fuel, and lean-burn natural gas engines. The BlueTec system, for example, injects a water-based urea solution into the exhaust system. Ammonia (NH3) produced by the hydrolysis of urea reacts with nitrogen oxides (NOx) and is converted into nitrogen gas (N2) and water within the catalytic converter. The conversion of noxious NOx to innocuous N2 is described by the following simplified global equation: [25]
When urea is used, a pre-reaction (hydrolysis) occurs to first convert it to ammonia:
Being a solid highly soluble in water (545 g/L at 25 °C), [2] urea is much easier and safer to handle and store than the more irritant, caustic and hazardous ammonia (NH3), so it is the reactant of choice. Trucks and cars using these catalytic converters need to carry a supply of diesel exhaust fluid, also sold as AdBlue, a solution of urea in water.
Urea in concentrations up to 10 M is a powerful protein denaturant as it disrupts the noncovalent bonds in the proteins. This property can be exploited to increase the solubility of some proteins. A mixture of urea and choline chloride is used as a deep eutectic solvent (DES), a substance similar to ionic liquid. When used in a deep eutectic solvent, urea gradually denatures the proteins that are solubilized. [26]
Urea in concentrations up to 8 M can be used to make fixed brain tissue transparent to visible light while still preserving fluorescent signals from labeled cells. This allows for much deeper imaging of neuronal processes than previously obtainable using conventional one photon or two photon confocal microscopes. [27]
Urea-containing creams are used as topical dermatological products to promote rehydration of the skin. Urea 40% is indicated for psoriasis, xerosis, onychomycosis, ichthyosis, eczema, keratosis, keratoderma, corns, and calluses. If covered by an occlusive dressing, 40% urea preparations may also be used for nonsurgical debridement of nails. Urea 40% "dissolves the intercellular matrix" [28] [29] of the nail plate. Only diseased or dystrophic nails are removed, as there is no effect on healthy portions of the nail. [30] This drug (as carbamide peroxide) is also used as an earwax removal aid. [31]
Urea has also been studied as a diuretic. It was first used by Dr. W. Friedrich in 1892. [32] In a 2010 study of ICU patients, urea was used to treat euvolemic hyponatremia and was found safe, inexpensive, and simple. [33]
Like saline, urea has been injected into the uterus to induce abortion, although this method is no longer in widespread use. [34]
The blood urea nitrogen (BUN) test is a measure of the amount of nitrogen in the blood that comes from urea. It is used as a marker of renal function, though it is inferior to other markers such as creatinine because blood urea levels are influenced by other factors such as diet, dehydration, [35] and liver function.
Urea has also been studied as an excipient in Drug-coated Balloon (DCB) coating formulation to enhance local drug delivery to stenotic blood vessels. [36] [37] Urea, when used as an excipient in small doses (~3 μg/mm2) to coat DCB surface was found to form crystals that increase drug transfer without adverse toxic effects on vascular endothelial cells. [38]
Urea labeled with carbon-14 or carbon-13 is used in the urea breath test, which is used to detect the presence of the bacterium Helicobacter pylori (H. pylori) in the stomach and duodenum of humans, associated with peptic ulcers. The test detects the characteristic enzyme urease, produced by H. pylori, by a reaction that produces ammonia from urea. This increases the pH (reduces the acidity) of the stomach environment around the bacteria. Similar bacteria species to H. pylori can be identified by the same test in animals such as apes, dogs, and cats (including big cats).
Amino acids from ingested food (or produced from catabolism of muscle protein) that are used for the synthesis of proteins and other biological substances can be oxidized by the body as an alternative source of energy, yielding urea and carbon dioxide. [46] The oxidation pathway starts with the removal of the amino group by a transaminase; the amino group is then fed into the urea cycle. The first step in the conversion of amino acids into metabolic waste in the liver is removal of the alpha-amino nitrogen, which produces ammonia. Because ammonia is toxic, it is excreted immediately by fish, converted into uric acid by birds, and converted into urea by mammals. [47]
Ammonia (NH3) is a common byproduct of the metabolism of nitrogenous compounds. Ammonia is smaller, more volatile, and more mobile than urea. If allowed to accumulate, ammonia would raise the pH in cells to toxic levels. Therefore, many organisms convert ammonia to urea, even though this synthesis has a net energy cost. Being practically neutral and highly soluble in water, urea is a safe vehicle for the body to transport and excrete excess nitrogen.
Urea is synthesized in the body of many organisms as part of the urea cycle, either from the oxidation of amino acids or from ammonia. In this cycle, amino groups donated by ammonia and L-aspartate are converted to urea, while L-ornithine, citrulline, L-argininosuccinate, and L-arginine act as intermediates. Urea production occurs in the liver and is regulated by N-acetylglutamate. Urea is then dissolved into the blood (in the reference range of 2.5 to 6.7 mmol/L) and further transported and excreted by the kidney as a component of urine. In addition, a small amount of urea is excreted (along with sodium chloride and water) in sweat.
In water, the amine groups undergo slow displacement by water molecules, producing ammonia, ammonium ions, and bicarbonate ions. For this reason, old, stale urine has a stronger odor than fresh urine.
The cycling of and excretion of urea by the kidneys is a vital part of mammalian metabolism. Besides its role as carrier of waste nitrogen, urea also plays a role in the countercurrent exchange system of the nephrons, that allows for reabsorption of water and critical ions from the excreted urine. Urea is reabsorbed in the inner medullary collecting ducts of the nephrons, [48] thus raising the osmolarity in the medullary interstitium surrounding the thin descending limb of the loop of Henle, which makes the water reabsorb.
By action of the urea transporter 2, some of this reabsorbed urea eventually flows back into the thin descending limb of the tubule, [49] through the collecting ducts, and into the excreted urine. The body uses this mechanism, which is controlled by the antidiuretic hormone, to create hyperosmotic urine — i.e., urine with a higher concentration of dissolved substances than the blood plasma. This mechanism is important to prevent the loss of water, maintain blood pressure, and maintain a suitable concentration of sodium ions in the blood plasma.
The equivalent nitrogen content (in grams) of urea (in mmol) can be estimated by the conversion factor 0.028 g/mmol. [50] Furthermore, 1 gram of nitrogen is roughly equivalent to 6.25 grams of protein, and 1 gram of protein is roughly equivalent to 5 grams of muscle tissue. In situations such as muscle wasting, 1 mmol of excessive urea in the urine (as measured by urine volume in litres multiplied by urea concentration in mmol/L) roughly corresponds to a muscle loss of 0.67 gram.
In aquatic organisms the most common form of nitrogen waste is ammonia, whereas land-dwelling organisms convert the toxic ammonia to either urea or uric acid. Urea is found in the urine of mammals and amphibians, as well as some fish. Birds and saurian reptiles have a different form of nitrogen metabolism that requires less water, and leads to nitrogen excretion in the form of uric acid. Tadpoles excrete ammonia, but shift to urea production during metamorphosis. Despite the generalization above, the urea pathway has been documented not only in mammals and amphibians, but in many other organisms as well, including birds, invertebrates, insects, plants, yeast, fungi, and even microorganisms. [51]
Urea can be irritating to skin, eyes, and the respiratory tract. Repeated or prolonged contact with urea in fertilizer form on the skin may cause dermatitis. [52]
High concentrations in the blood can be damaging. Ingestion of low concentrations of urea, such as are found in typical human urine, are not dangerous with additional water ingestion within a reasonable time-frame. Many animals (e.g. camels, rodents or dogs) have a much more concentrated urine which may contain a higher urea amount than normal human urine.
Urea can cause algal blooms to produce toxins, and its presence in the runoff from fertilized land may play a role in the increase of toxic blooms. [53]
The substance decomposes on heating above melting point, producing toxic gases, and reacts violently with strong oxidants, nitrites, inorganic chlorides, chlorites and perchlorates, causing fire and explosion. [54]
Urea was first discovered in urine in 1727 by the Dutch scientist Herman Boerhaave, [55] although this discovery is often attributed to the French chemist Hilaire Rouelle as well as William Cruickshank. [56]
Boerhaave used the following steps to isolate urea: [57] [58]
In 1828, the German chemist Friedrich Wöhler obtained urea artificially by treating silver cyanate with ammonium chloride. [59] [60] [61]
This was the first time an organic compound was artificially synthesized from inorganic starting materials, without the involvement of living organisms. The results of this experiment implicitly discredited vitalism, the theory that the chemicals of living organisms are fundamentally different from those of inanimate matter. This insight was important for the development of organic chemistry. His discovery prompted Wöhler to write triumphantly to Jöns Jakob Berzelius:
In fact, his second sentence was incorrect. Ammonium cyanate [NH4]+[OCN]− and urea CO(NH2)2 are two different chemicals with the same empirical formula CON2H4, which are in chemical equilibrium heavily favoring urea under standard conditions. [62] Regardless, with his discovery, Wöhler secured a place among the pioneers of organic chemistry.
Uremic frost was first described in 1865 by Harald Hirschsprung, the first Danish pediatrician in 1870 who also described the disease that carries his name in 1886. Uremic frost has become rare since the advent of dialysis. It is the classical pre-dialysis era description of crystallized urea deposits over the skin of patients with prolonged kidney failure and severe uremia. [63]
Urea was first noticed by Herman Boerhaave in the early 18th century from evaporates of urine. In 1773, Hilaire Rouelle obtained crystals containing urea from human urine by evaporating it and treating it with alcohol in successive filtrations. [64] This method was aided by Carl Wilhelm Scheele's discovery that urine treated by concentrated nitric acid precipitated crystals. Antoine François, comte de Fourcroy and Louis Nicolas Vauquelin discovered in 1799 that the nitrated crystals were identical to Rouelle's substance and invented the term "urea." [65] [66] Berzelius made further improvements to its purification [67] and finally William Prout, in 1817, succeeded in obtaining and determining the chemical composition of the pure substance. [68] In the evolved procedure, urea was precipitated as urea nitrate by adding strong nitric acid to urine. To purify the resulting crystals, they were dissolved in boiling water with charcoal and filtered. After cooling, pure crystals of urea nitrate form. To reconstitute the urea from the nitrate, the crystals are dissolved in warm water, and barium carbonate added. The water is then evaporated and anhydrous alcohol added to extract the urea. This solution is drained off and evaporated, leaving pure urea.
Ureas in the more general sense can be accessed in the laboratory by reaction of phosgene with primary or secondary amines:
These reactions proceed through an isocyanate intermediate. Non-symmetric ureas can be accessed by the reaction of primary or secondary amines with an isocyanate.
Urea can also be produced by heating ammonium cyanate to 60 °C.
In 2020, worldwide production capacity was approximately 180 million tonnes. [69]
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.
The basic process, patented in 1922, is called the Bosch–Meiser urea process after its discoverers Carl Bosch and Wilhelm Meiser. [70] The process consists of two main equilibrium reactions, with incomplete conversion of the reactants. The first is carbamate formation: the fast exothermic reaction of liquid ammonia with gaseous carbon dioxide (CO2) at high temperature and pressure to form ammonium carbamate ([NH4]+[NH2COO]−): [17]
The second is urea conversion: the slower endothermic decomposition of ammonium carbamate into urea and water:
The overall conversion of NH3 and 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 °C) needed for the second is compensated for by conducting the process under high pressure (140–175 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.
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.
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. [17]
The "conventional recycle process" for recovering and reusing the reactants has largely been supplanted by a 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. [17] [72]
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. [17] [72]
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. [17]
Biuret is formed when two molecules of urea combine with the loss of a molecule of ammonia.
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. [17] Biuret is undesirable in urea fertilizer because it is toxic to crop plants to varying degrees, [73] but it is sometimes desirable as a nitrogen source when used in animal feed. [74]
Isocyanic acid HNCO and ammonia NH3 results from the thermal decomposition of ammonium cyanate [NH4]+[OCN]−, which is in chemical equilibrium with urea:
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. [17]
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 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 steels in the 1990s, and zirconium or zirconium-clad titanium tubing in the 2000s. [17]
Urea can be produced in solid forms (prills, granules, pellets or crystals) or as solutions.
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 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. [17]
Solutions of 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. [17] UAN accounts for 80% of the liquid fertilizers in the US. [75]
Ammonia is an inorganic chemical compound of nitrogen and hydrogen with the formula NH3. A stable binary hydride and the simplest pnictogen hydride, ammonia is a colourless gas with a distinctive pungent smell. Biologically, it is a common nitrogenous waste, and it contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to fertilisers. Around 70% of ammonia produced industrially is used to make fertilisers in various forms and composition, such as urea and diammonium phosphate. Ammonia in pure form is also applied directly into the soil.
Nitrogen is a chemical element; it has symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at seventh in total abundance in the Milky Way and the Solar System. At standard temperature and pressure, two atoms of the element bond to form N2, a colourless and odourless diatomic gas. N2 forms about 78% of Earth's atmosphere, making it the most abundant chemical species in air. Because of the volatility of nitrogen compounds, nitrogen is relatively rare in the solid parts of the Earth.
A fertilizer or fertiliser is any material of natural or synthetic origin that is applied to soil or to plant tissues to supply plant nutrients. Fertilizers may be distinct from liming materials or other non-nutrient soil amendments. Many sources of fertilizer exist, both natural and industrially produced. For most modern agricultural practices, fertilization focuses on three main macro nutrients: nitrogen (N), phosphorus (P), and potassium (K) with occasional addition of supplements like rock flour for micronutrients. Farmers apply these fertilizers in a variety of ways: through dry or pelletized or liquid application processes, using large agricultural equipment, or hand-tool methods.
Ammonium is a modified form of ammonia that has an extra hydrogen atom. It is a positively charged (cationic) molecular ion with the chemical formula NH+4 or [NH4]+. It is formed by the addition of a proton to ammonia. Ammonium is also a general name for positively charged (protonated) substituted amines and quaternary ammonium cations, where one or more hydrogen atoms are replaced by organic or other groups. Not only is ammonium a source of nitrogen and a key metabolite for many living organisms, but it is an integral part of the global nitrogen cycle. As such, human impact in recent years could have an effect on the biological communities that depend on it.
Ammonium nitrate is a chemical compound with the formula NH4NO3. It is a white crystalline salt consisting of ions of ammonium and nitrate. It is highly soluble in water and hygroscopic as a solid, although it does not form hydrates. It is predominantly used in agriculture as a high-nitrogen fertilizer.
Carbonation is the chemical reaction of carbon dioxide to give carbonates, bicarbonates, and carbonic acid. In chemistry, the term is sometimes used in place of carboxylation, which refers to the formation of carboxylic acids.
Ammonium bicarbonate is an inorganic compound with formula (NH4)HCO3. The compound has many names, reflecting its long history. Chemically speaking, it is the bicarbonate salt of the ammonium ion. It is a colourless solid that degrades readily to carbon dioxide, water and ammonia.
In organic chemistry, a carbamate is a category of organic compounds with the general formula R2NC(O)OR and structure >N−C(=O)−O−, which are formally derived from carbamic acid. The term includes organic compounds, formally obtained by replacing one or more of the hydrogen atoms by other organic functional groups; as well as salts with the carbamate anion H2NCOO−.
Ammonium sulfate (American English and international scientific usage; ammonium sulphate in British English); (NH4)2SO4, is an inorganic salt with a number of commercial uses. The most common use is as a soil fertilizer. It contains 21% nitrogen and 24% sulfur.
Sulfamic acid, also known as amidosulfonic acid, amidosulfuric acid, aminosulfonic acid, sulphamic acid and sulfamidic acid, is a molecular compound with the formula H3NSO3. This colourless, water-soluble compound finds many applications. Sulfamic acid melts at 205 °C before decomposing at higher temperatures to water, sulfur trioxide, sulfur dioxide and nitrogen.
The Wöhler synthesis is the conversion of ammonium cyanate into urea. This chemical reaction was described in 1828 by Friedrich Wöhler. It is often cited as the starting point of modern organic chemistry. Although the Wöhler reaction concerns the conversion of ammonium cyanate, this salt appears only as an (unstable) intermediate. Wöhler demonstrated the reaction in his original publication with different sets of reactants: a combination of cyanic acid and ammonia, a combination of silver cyanate and ammonium chloride, a combination of lead cyanate and ammonia and finally from a combination of mercury cyanate and cyanatic ammonia.
Biuret is a chemical compound with the chemical formula HN(CONH2)2. It is a white solid that is soluble in hot water. A variety of organic derivatives are known. The term "biuret" also describes a family of organic compounds with the chemical formula R1R2N−C(=O)−N(R3)−C(=O)−NR4R5, where R1, R2, R3, R4 and R5 are hydrogen, organyl or other groups. Also known as carbamylurea, it results from the condensation of two equivalents of urea. It is a common undesirable impurity in urea-based fertilizers, as biuret is toxic to plants.
Isocyanic acid is a chemical compound with the structural formula HNCO, which is often written as H−N=C=O. It is a colourless, volatile and poisonous substance, with a boiling point of 23.5 °C. It is the predominant tautomer and an isomer of cyanic acid (aka. cyanol).
Metabolic wastes or excrements are substances left over from metabolic processes (such as cellular respiration) which cannot be used by the organism (they are surplus or toxic), and must therefore be excreted. This includes nitrogen compounds, water, CO2, phosphates, sulphates, etc. Animals treat these compounds as excretes. Plants have metabolic pathways which transforms some of them (primarily the oxygen compounds) into useful substances.
The Kjeldahl method or Kjeldahl digestion (Danish pronunciation:[ˈkʰelˌtɛˀl]) in analytical chemistry is a method for the quantitative determination of a sample's organic nitrogen plus ammonia/ammonium. (NH3/NH4+). Without modification, other forms of inorganic nitrogen, for instance nitrate, are not included in this measurement. Using an empirical relation between Kjeldahl nitrogen and protein, it is an important method for indirectly quantifying protein content of a sample. This method was developed by Johan Kjeldahl in 1883.
The chemical element nitrogen is one of the most abundant elements in the universe and can form many compounds. It can take several oxidation states; but the most common oxidation states are -3 and +3. Nitrogen can form nitride and nitrate ions. It also forms a part of nitric acid and nitrate salts. Nitrogen compounds also have an important role in organic chemistry, as nitrogen is part of proteins, amino acids and adenosine triphosphate.
Carbamic acid, which might also be called aminoformic acid or aminocarboxylic acid, is the chemical compound with the formula H2NCOOH. It can be obtained by the reaction of ammonia NH3 and carbon dioxide CO2 at very low temperatures, which also yields ammonium carbamate [NH4]+[NH2CO2]−. The compound is stable only up to about 250 K (−23 °C); at higher temperatures it decomposes into those two gases. The solid apparently consists of dimers, with the two molecules connected by hydrogen bonds between the two carboxyl groups –COOH.
Sable Chemical Industries Limited is the sole manufacturer of ammonium nitrate (NH4NO3) in Zimbabwe.
Ammonium carbamate is a chemical compound with the formula [NH4][H2NCO2] consisting of ammonium cation NH+4 and carbamate anion NH2COO−. It is a white solid that is extremely soluble in water, less so in alcohol. Ammonium carbamate can be formed by the reaction of ammonia NH3 with carbon dioxide CO2, and will slowly decompose to those gases at ordinary temperatures and pressures. It is an intermediate in the industrial synthesis of urea (NH2)2CO, an important fertilizer.
The Bosch–Meiser process is an industrial process, which was patented in 1922 and named after its discoverers, the German chemists Carl Bosch and Wilhelm Meiser for the large-scale manufacturing of urea, a valuable nitrogenous chemical.
The compound H2N-CO-NH2 has the retained name 'urea', which is the preferred IUPAC name, with locants N and N′, as shown above the structure below. The systematic name is 'carbonic diamide', (…).
{{cite journal}}
: Missing or empty |title=
(help)