ANS:B - With increase in NH₃/CO₂ ratio, urea yield decreases for a given temperature, pressure and total feed rate.

In urea production via the urea synthesis reaction (NH3 + CO2 → NH2CONH2), the NH3/CO2 ratio plays a crucial role in determining the yield of urea. Here's an explanation of why increasing the NH3/CO2 ratio can lead to a decrease in urea yield, assuming constant temperature, pressure, and total feed rate:

1. Stoichiometry of the Reaction: The urea synthesis reaction between ammonia (NH3) and carbon dioxide (CO2) has a stoichiometric ratio of 2 moles of NH3 per mole of CO2 to produce one mole of urea (NH2CONH2). The balanced chemical equation is: 2𝑁𝐻3+𝐶𝑂2→𝑁𝐻2𝐶𝑂𝑁𝐻2+𝐻2𝑂2NH3​+CO2​→NH2​CONH2​+H2​O Therefore, an NH3/CO2 ratio higher than 2:1 implies an excess of NH3 relative to CO2, which may lead to unreacted NH3 remaining in the system after the reaction.
2. Equilibrium Considerations: The urea synthesis reaction is reversible and governed by chemical equilibrium. According to Le Chatelier's principle, increasing the concentration of one reactant (NH3) relative to another (CO2) will shift the equilibrium position of the reaction to the left, favoring the formation of reactants over products. As a result, a higher NH3/CO2 ratio tends to decrease the extent of the urea synthesis reaction and reduce the yield of urea.
3. Incomplete Reaction: Excess NH3 in the reaction mixture can lead to incomplete conversion of CO2 to urea. When the NH3/CO2 ratio is increased beyond the stoichiometric ratio, there may not be enough CO2 available to react with all the NH3 present, resulting in unutilized NH3 and reduced urea formation.
4. Economic Considerations: Maintaining the NH3/CO2 ratio within the optimal range close to the stoichiometric ratio is essential for maximizing urea yield and minimizing production costs. Excessive NH3 usage can lead to increased operating costs associated with NH3 production and handling, without a corresponding increase in urea yield.

Overall, while NH3 is a critical reactant in urea synthesis, maintaining the NH3/CO2 ratio within the appropriate range is crucial for achieving high urea yields and efficient production processes. Deviating from the optimal ratio can result in incomplete reactions, reduced urea yields, and increased production costs.