Although urea has been commercially synthesized since 1920 major expansion in production did not occur until the 1950’s. The stimulus for this expansion, aside from the burgeoning market for nitrogenous fertilisers was the development of large production units for NH3 and, simultaneously, the availability of large quantities of almost costless CO2, the latter available from the production of H2 via the CO-shift reaction: CO + H2O ~ H2 + CO2
Of no less importance was the gradual improvement of process techniques to master the difficult technological problems associated with urea production (see Chapter I).
In the early processes the unreacted carbamate was not recycled (once-through process), and all of the NH3 from the carbamate decomposers was simply neutralized to produce ammon­ium sulfate, nitrate or phosphate. The chief disadvantage of such processes is that the production of urea is coupled to that of the by-products. To avoid this difficulty— to achieve and perfect the recirculation step— became the most characteristic line of develop­ment in urea process technology. In perfecting the recirculation step the aim was to maximize the NH3 recovery, the heat recovery (by obtaining the heat of carbamate association at as high a temperature as possible) and technical reliability and to minimize the amount of water returning to the reactor in the recycle stream (reduces conversion) and the utilities and investment costs.

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