Urea, also known as carbamide, is an organic compound with the chemical formula CO(NH₂)₂. It is a colorless, odorless solid highly soluble in water. Living organisms, particularly mammals, produce urea as a byproduct of protein metabolism, while industrially it is widely manufactured due to its versatile properties.
Industrial Production of Urea
The industrial synthesis of urea primarily relies on the reaction of ammonia and carbon dioxide, a process known as the Bosch-Meiser urea process. This method, patented in 1922, remains the standard for large-scale production due to its reliance on readily available and inexpensive reagents. The multi-stage process begins with raw material production.
Ammonia, a key precursor, is typically synthesized through the Haber-Bosch process, which combines nitrogen and hydrogen gases under high temperature and pressure using a catalyst. Carbon dioxide, the other essential raw material, is often obtained as a byproduct from this ammonia synthesis or other industrial operations. This co-location of urea and ammonia plants is common, optimizing resource use.
The core of urea synthesis involves two main equilibrium reactions that occur under specific conditions of high temperature and pressure within a reactor. First, liquid ammonia and gaseous carbon dioxide react exothermically to form ammonium carbamate. This initial step typically takes place at pressures around 140-175 bar and temperatures of approximately 190°C.
Following the formation of ammonium carbamate, a slower, endothermic decomposition reaction converts it into urea and water. The conditions are a compromise, as high temperature for urea formation can negatively affect initial carbamate formation, but high pressure helps counteract this. Unreacted ammonia and carbon dioxide are recovered and recycled, a crucial aspect of modern urea plants that maximizes efficiency by ensuring nearly all reactants are converted.
After synthesis, the urea solution undergoes purification to remove impurities and unreacted materials. The solution is then concentrated, often through vacuum evaporation, to remove excess water and increase urea content, typically resulting in a 70% by weight solution.
The final stage involves forming solid urea, usually as prills or granules. Prills are solidified droplets, while granules are larger, harder particles produced by accreting liquid urea onto seed particles. Granulation offers improved handling characteristics and crushing strength compared to prills, making granular urea more suitable for bulk storage and blending.
Biological Synthesis in Living Organisms
Urea is naturally produced in the bodies of many living organisms, particularly mammals and some fish, as a means of detoxifying ammonia. This biochemical pathway, known as the urea cycle or ornithine cycle, converts highly toxic ammonia, a byproduct of protein and amino metabolism, into less harmful urea for excretion.
Protein metabolism generates ammonia, which can be detrimental if it accumulates in high concentrations. The liver is the main organ responsible for carrying out the urea cycle, converting this toxic ammonia into urea. Approximately 80% of the nitrogen excreted by humans and mammals is in the form of urea, demonstrating its role in nitrogenous waste removal.
The urea cycle is a series of five enzymatic reactions that span two cellular compartments within liver cells: the mitochondrial matrix and the cytosol. The initial steps occur in the mitochondria, where ammonia and carbon dioxide are combined to form carbamoyl phosphate. This reaction consumes two molecules of ATP and is catalyzed by carbamoyl phosphate synthetase I.
Carbamoyl phosphate then reacts with ornithine to form citrulline, a reaction catalyzed by ornithine transcarbamylase. Citrulline is subsequently transported out of the mitochondria into the cytosol for the remaining steps. In the cytosol, citrulline combines with aspartate to form argininosuccinate, which then cleaves to produce arginine and fumarate.
Finally, arginine is hydrolyzed by the enzyme arginase to yield urea and regenerate ornithine. The ornithine is then transported back into the mitochondria, continuing the cycle. The carbon atom in urea originates from carbon dioxide, while the two nitrogen atoms come from ammonia and aspartate. The urea produced then enters the bloodstream, travels to the kidneys, and is ultimately excreted in the urine.
Important Uses of Urea
Urea’s diverse properties make it valuable across numerous industries. Its most significant application globally is its use as a nitrogen fertilizer in agriculture. Urea boasts the highest nitrogen content of all solid nitrogenous fertilizers, typically around 46%, making it a cost-effective choice for supplying essential nutrients to crops. When applied to soil, urea breaks down to release ammonium ions, which plants can absorb for growth.
Beyond agriculture, urea serves as a raw material in the production of various plastics and resins. Urea-formaldehyde resins are a prominent example, widely used as adhesives in the wood and furniture industries for products like particleboard and plywood. These resins are known for their durability and moisture resistance.
Urea is also utilized as a supplement in animal feed, particularly for ruminant animals like cattle and sheep. Its nitrogen content can help meet a significant portion of these animals’ protein requirements. This leverages urea’s ability to provide a nitrogen source convertible into protein.
Urea also finds use as a de-icing agent, offering an alternative to traditional road salts. It is considered less corrosive to surfaces like concrete and metal compared to chloride-based de-icers. While effective at temperatures down to approximately -3°C, its performance diminishes at lower temperatures.
An increasingly important application for urea is in diesel exhaust fluid (DEF). DEF, typically a 32.5% solution of urea in deionized water, is injected into the exhaust stream of diesel vehicles. This helps to convert harmful nitrogen oxide (NOx) emissions into harmless nitrogen gas and water vapor through a catalytic reaction.