What is Urea 46?

In common with most commercial nitrogen fertilizers, urea is manufactured from anhydrous ammonia (NH3).

The high analysis of urea—46% N—is the main reason for the low cost of this form of nitrogen fertilizer. Freight costs and storage and handling are all lower than with lower analysis fertilizers such as ammonium nitrate (34-0-0) or ammonium sulfate (21-0-0).
Urea 46% Nitrogen a white crystalline solid containing 46% nitrogen, is widely used in the agricultural industry as an animal feed additive and fertilizer. 
Fertilizer urea can be purchased as prills or as a granulated material.

In the past, it was usually produced by dropping liquid urea from a “prilling tower” while drying the product. The prilled formed a smaller and softer substance than other materials commonly used in fertilizer blends. Today, though, considerable urea is manufactured as granules. Granules are larger, harder, and more resistant to moisture. As a result, granulated urea has become a more suitable material for fertilizer blends.

Urea, also known as carbamide, is an organic compound with chemical formula CO(NH2)2. This amide has two –NH2 groups joined by a carbonyl (C=O) 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.

It is a colorless, odorless solid, highly soluble in water, and practically non-toxic (LD50 is 15 g/kg for rats). 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.

Friedrich Wöhler’s discovery, in 1828, that Urea can be produced from inorganic starting materials, was an important conceptual milestone in chemistry. It 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 vitalism, which stated that only living things could produce the chemicals of life.

Urea is an inexpensive form of nitrogen fertilizer with an NPK (nitrogen-phosphorus-potassium) ratio of 46-0-0. Although urea is naturally produced in humans and animals, synthetic urea is manufactured with anhydrous ammonia.

Although urea often offers gardeners the most nitrogen for the lowest price on the market, special steps must be taken when applying urea to the soil to prevent the loss of nitrogen through a chemical reaction.

Urea was first produced industrially by the hydration of calcium cyanamide but the easy availability of ammonia led to the development of ammonia/carbon dioxide technology. This is a two-step process where the ammonia and carbon dioxide react to form ammonium carbamate which is then dehydrated to urea.

In the process, ammonia and carbon dioxide are fed to the synthesis reactor which operates around 180-210oC and 150 bar pressure. The reaction mixture containing ammonia, ammonium carbamate, and urea is first stripped of the ammonia and the resultant solution passes through a number of decomposers operating at progressively reduced pressures. Here the unconverted carbamate is decomposed back to ammonia and carbon dioxide and recycled to the reactor.

The urea solution is concentrated by evaporation or crystallization, and the crystals can be melted to yield pure urea in the form of prills or granules. Prills are made by spraying molten urea from the top of a high tower through a counter-current air stream. Granular urea is formed by spraying molten urea into a mixture of dried urea particles and fines in a rotating drum.

Urea processes fall into two categories: external solution recycle systems, and internal solution stripping systems. In the former, energy is saved by high carbon dioxide conversion rates while the latter reduces net energy requirements by optimizing heat recovery.