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In the very first processes the feed of ammonia and carbon dioxide passed once through the synthesis section
and the nonconverted NH3 was neutralized
with acids (e.g. nitric acid) to produce ammonium salts as
coproducts of urea production.
The main disadvantages of the once-through processes are the
large quantity of ammonium salt formed as coproduct and the
limited amount of overall carbon dioxide conversion that could
be achieved.
Once-through processes have almost all been replaced by total-recycle
processes, where essentially all of the nonconverted ammonia
and carbon dioxide are recycled to the urea reactor.
The Urea Ammonium Nitrate (UAN) plants do however, partly
use the once-through concept.
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UREA 
