Maximum yield of urea production

[Apoorva Sampat_11CHE1073]

In literature, it has been stated that the yield of urea production is maximum at a ratio, somewhat higher than that of the stoichiometric ratio (NH3/CO2) of reaction i.e. 2. Whereas a large excess of ammonia results in a reduced yield. Thus there exists an optimum for achieving this maximum yield.

 Is there any physio-chemical reason for existence of such an optima?

[Vishwanath Dalvi]

Please could you cite the source of your statements.  Then we can explore it more efficiently. 

[Apoorva Sampat_11CHE1073]

Sir,

     The following statement has been made in Chapter 6 of 'Chemical Process Technology by Moulijn' : 

'Thermodynamically, both the carbon dioxide conversion and the urea yield as a function of temperature go through a maximum, which usually lies between 450 and 480 K at practical conditions. Increasing the NH3/CO2 ratio leads to a higher carbon dioxide conversion. The yield on urea has a maximum at an NH3/CO2 ratio of somewhat above the stoichiometric ratio. A large excess of ammonia results in a reduced yield. In current practice, an NH3/CO2 ratio of 3–5 mol/mol is used.'