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 a stripper, and many total recycle urea plants have converted to a stripping process. 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. 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 desigened to be short. biuret is formed when two molecules of urea combine with the loss of a molecule of ammonia. 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. biuret is undesirable in urea fertilizer because it is toxic to crop plants to varying degrees, but it is sometimes desirable as a nitrogen source when used as a in animal feed. isocyanic acid hnco and ammonia nh3 results from the thermal decomposition of ammonium cyanate +−, which is in chemical equilibrium with urea: co(nh2)2 → +− → 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. 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 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 1990's, and zirconium or zirconium-clad titanium tubing in the 2000's. finishing urea can be produced in solid forms (prills, 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 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 supress caking. other shaping techniques such as pastillization (depositing uniform-sized liquid droplets onto a cooling conveyor belt) are also used. liquid forms 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. in uan accounts for 80% of the liquid fertilizers in the us.