What is Urea?
Urea or carbamide is an organic compound with the chemical formula (NH2)2CO. The molecule has two amine (- NH2) residues joined by a carbonyl (- CO -) functional group.
Urea serves an important role in the metabolism of nitrogen - containing compounds by animals and is the main nitrogen - containing substance in the urine of mammals. Being solid, colorless, odorless, neither acidic nor alkaline, highly soluble in water, and relatively non-toxic, Urea is widely used in fertilizers as a convenient source of nitrogen. Urea is also an important raw material for the chemical industry. The synthesis of this organic compound by Friedrich Wöhler in 1828 from an inorganic precursor was an important milestone in the development of chemistry.
The terms Urea and carbamide are also used for a class of chemical compounds sharing the same functional group RR'NCO-NRR', namely a carbonyl group attached to two organic amine residues. Example include carbamide peroxide, allantoin, and hydantoin. Ureas are closely related to biurets and related in structure to amides, carbamates, diimides, carbodiimides, and thiocarbamides.
Urea (NH2CONH2) is a leading nitrogen fertilizer worldwide. It is a stable, colorless, and odorless solid at room temperature that melts at 135°C. It is highly water soluble and will slowly hydrolyze in the presence of water to give ammonium carbamate, which slowly decomposes into ammonia and carbon dioxide.
In the pie chart in Figure 1, global ammonia consumption is depicted by commercial application. The chart suggests that urea fertilizer is, by far, the largest application-accounting for almost half of the total ammonia derivatives.
Industrial methods
For use in industry, Urea is produced from synthetic ammonia and carbon dioxide. Large quantities of carbon dioxide are produced during the manufacture of ammonia from coal or from hydrocarbons such as natural gas and petroleum-derived raw materials. Such point sources of CO2 facilitate direct synthesis of Urea.
The basic process, developed in 1922, is also called the Bosch-Meiser Urea process after its discoverers. The various Urea processes are characterized by the conditions under which Urea formation takes place and the way in which unconverted reactants are further processed. The process consists of two main equilibrium reactions, with incomplete conversion of the reactants. The first is an exothermic reaction of liquid ammonia with dry ice to form ammonium carbamate (H2N-COONH4):
2NH3 + CO2 ↔ H2N-COONH4
The second is an endothermic decomposition of ammonium carbamate into Urea and water:
H2N-COONH4 ↔ (NH2)2CO + H2O
Both reactions combined are exothermic. Unconverted reactants can be used for the manufacture of other products, for example ammonium nitrate or sulfate, or they can be recycled for complete conversion to Urea in a total-recycle process.
Urea can be produced as prills, granules, pellets, crystals, and solutions. Solid Urea is marketed as prills or granules. The advantage of prills is that, in general, they can be produced more cheaply than granules. Properties such as impact strength, crushing strength, and free-flowing behavior are, in particular, important in product handling, storage, and bulk transportation. Typical impurities in the production are biuret and isocyanic acid:
2NH2CONH2 → H2NCONHCONH2 + NH3
NH2CONH2 → HNCO + NH3
The biuret content is a serious concern because it is often toxic to the very plants that are to be fertilized. Urea is classified on the basis of its biuret content.
Laboratory preparation
Ureas in the more general sense can be accessed in the laboratory by reaction of phosgene with primary or secondary amines, proceeding through an isocyanate intermediate. Non-symmetric Ureas can be accessed by reaction of primary or secondary amines with an isocyanate.
Resource implies fossil fuel, minerals, metals, lime, and water
Waste implies air, water and solid emission coming out from the industry
Urea is synthesized from ammonia and CO2. This is the basis for all manufacturing technologies. In all CO2 stripping methods, ammonia and CO2 are fed into the synthesis reactor, and ammonium carbamate is produced. The second step consists of dehydration of carbamate to urea and H2O. This is a summary of the manufacturing process for urea.
Urea
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Urea Production
Urea is produced on a scale of some 100,000,000 tons per year worldwide.
Urea chemical structure
As the helices are interconnected, all helices in a crystal must have the same molecular handedness. This is determined when the crystal is nucleated and can thus be forced by seeding. The resulting crystals have been used to separate racemic mixtures.
Chemical Properties - Molecular and crystal structure
The Urea molecule is planar in the crystal structure, but the geometry around the nitrogens is pyramidal in the gas-phase minimum-energy structure. In solid Urea, the oxygen center is engaged in two N-H-O hydrogen bonds. The resulting dense and energetically favourable 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 basic compared to, say, formaldehyde. Urea's high aqueous solubility reflects its ability to engage in extensive hydrogen bonding with water.
By virtue of its tendency to form a 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 comprising of hydrogen-bonded Urea molecules. This behavior can be used to separate mixtures, e.g. in the production of aviation fuel and lubricating oils, and in the separation of paraffins.
Laboratory preparation
Ureas in the more general sense can be accessed in the laboratory by reaction of phosgene with primary or secondary amines, proceeding through an isocyanate intermediate. Non-symmetric Ureas can be accessed by reaction of primary or secondary amines with an isocyanate.
Resource implies fossil fuel, minerals, metals, lime, and water
Waste implies air, water and solid emission coming out from the industry
Urea is synthesized from ammonia and CO2. This is the basis for all manufacturing technologies. In all CO2 stripping methods, ammonia and CO2 are fed into the synthesis reactor, and ammonium carbamate is produced. The second step consists of dehydration of carbamate to urea and H2O. This is a summary of the manufacturing process for urea.
Urea Usages
Agriculture
More than 90% of world production of Urea is destined for use as a nitrogen-release fertilizer. Urea has the highest nitrogen content of all solid nitrogenous fertilizers in common use. Therefore, it has the lowest transportation costs per unit of nitrogen nutrient.
Many soil bacteria possess the enzyme, Urease, which catalyzes the conversion of the Urea molecule to two ammonia molecules and one carbon dioxide molecule, thus Urea fertilizers are very rapidly transformed to the ammonium form in soils.
Among soil bacteria known to carry Urease, some ammonia-oxidizing bacteria (AOB), such as species of Nitrosomonas are also able to assimilate the carbon dioxide released by the reaction to make biomass via the Calvin Cycle, and harvest energy by oxidizing ammonia (the other product of Urease) to nitrite, a process termed nitrification. Nitrite-oxidizing bacteria, especially, Nitrobacter, oxidize nitrite to nitrate, which is extremely mobile in soils and is a major cause of water pollution from agriculture. Ammonia and nitrate are readily absorbed by plants, and are the dominant sources of nitrogen for plant growth. Urea is also used in many multi-component solid fertilizer formulations. Urea is highly soluble in water and is, therefore, also very suitable for use in fertilizer solutions (in combination with ammonium nitrate: UAN), e.g., in 'foliar feed' fertilizers. For fertilizer use, granules are preferred over prills because of their narrower particle size distribution which is an advantage for mechanical application.
Because of the high nitrogen concentration in Urea, it is very important to achieve an even spread. The application equipment must be correctly calibrated and properly used. Drilling must not occur on contact with or close to seed, due to the risk of germination damage. Urea dissolves in water for application as a spray or through irrigation systems.
In grain and cotton crops, Urea is often applied at the time of the last cultivation before planting. In high rainfall areas and on sandy soils (where nitrogen can be lost through leaching) and where good in-season rainfall is expected, Urea can be side- or top-dressed during the growing season. Top-dressing is also popular on pasture and forage crops. In cultivating sugarcane, Urea is side-dressed after planting, and applied to each ratoon crop. In irrigated crops, Urea can be applied dry to the soil, or dissolved and applied through the irrigation water. Urea will dissolve in its own weight in water, but it becomes increasingly difficult to dissolve as the concentration increases.
Dissolving Urea in water is endothermic, causing the temperature of the solution to fall when Urea dissolves.
As a practical guide, when preparing Urea solutions for fertigation (injection into irrigation lines), dissolve no more than 30 kg Urea per 100 L water. In foliar sprays, Urea concentrations of 0.5% – 2.0% are often used in horticultural crops. Low-biuret grades of Urea are often indicated. Urea absorbs moisture from the atmosphere and therefore is typically stored either in closed/sealed bags on pallets, or, if stored in bulk, under cover with a tarpaulin.
As with most solid fertilizers, storage in a cool, dry, well-ventilated area is recommended.
Chemical industry
Urea is a raw material for the manufacture of many important chemical compounds, such as
Various plastics, especially the Urea-formaldehyde resins.
Various adhesives, such as Urea-formaldehyde or the Urea-melamine formaldehyde used in marine plywood.
Potassium cyanate, another industrial feedstock.
Explosive
Urea can be used to make Urea nitrate, a high explosive which is used industrially and as part of some improvised explosive devices.
Automobile systems
Urea is used in SNCR and SCR reactions to reduce the NOx pollutants in exhaust gases from combustion from Gas Oil, dual fuel, and lean-burn natural gas engines.
The BlueTec system, for example, injects water-based Urea solution into the exhaust system. The ammonia produced by the hydrolysis of the Urea reacts with the nitrogen oxide emissions and is converted into nitrogen and water within the catalytic converter.
Laboratory uses
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, a type of ionic liquid.
Urea can in principle serve as a hydrogen source for subsequent power generation in fuel cells. Urea present in urine/wastewater can be used directly (though bacteria normally quickly degrade Urea.)