Editing Urea (section)

===Reactant recycling===
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.

====Conventional recycle processes====

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.<ref name="Meessen 2012" />

==== Stripping recycle process ====

The "conventional recycle process" for recovering and reusing the reactants has largely been supplanted by a [[Stripping (chemistry)|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.<ref name="Meessen 2012"/><ref name="Meessen 2014">{{cite journal |last=Meessen |first=Jozef |title=Urea synthesis |journal=Chemie Ingenieur Technik |publisher=Wiley |volume=86 | issue=12| date=2014 |issn=0009-286X |doi=10.1002/cite.201400064 |pages=2180–2189|doi-access=free }}</ref>

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.<ref name="Meessen 2012"/><ref name="Meessen 2014"/>