Can explain why CO2 conversion is decreased if we operate Urea Reactor with reduced load

Can any one explain why CO2 conversion is decreased if we operate Urea Reactor with reduced load (say 50 % load). What would be effect on residence time in urea reactor if reduced plant load to (say 50%).

(Note that on reduction of Urea plant top/bottom delta is decreased)
t is proven that increase in residence time will lead to better CO2 conversion in Urea reacter. At low load usually we are not able to control other parameters which are needed to improve conversion of CO2 to Urea.

As you know that Urea production takes place in two steps:

Step:1) CO2 + 2NH3 < = > NH4CO2NH2 (AKA Carbamate) + Heat (100% complete) (Exothermic Reaction)
Step:2) NH4CO2NH2 <=> NH2CONH2 (Urea) + H2O - Heat (50% to 66% depending on various factors) (Endothermic Reaction)
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Overall) CO2 + 2NH3 < = > Urea + H2O + Heat (Exothermic Reaction)

the things that can affect the conversion of Limiting reactant (CO2 in our case) are
1) NH3 / CO2 ratio: (Higher this N/C ratio, higher would be the conversion)
2)H2O /CO2 ratio: (Higher this H/C ratio would would encourage higher Conversion)
3) Operating pressure: (Increased pressure will encourage higher conversion rate)
4) Operating Temperature: (Though overall reaction is exothermic, however as step:2 in conversion is the rate defining step, therefore high temperature favors conversion)

At low load plant operation, usually, if any of the above mentioned factors is not right, it will directly affect the conversion of CO2 to Urea.
Normally at low load load operation, we are unable to control H/C in Urea reactor (owing to dilution of recycle Carbamate solution) which causes reduction in reactor pressure, hence causing equilibrium temperature drop, ultimately causing drop in conversion.