ANS:B - biuret formation

Prilling of urea should be accomplished (in a sprayer) just above the melting point of urea with a minimum retention time; otherwise, it will result in substantially wet non-flowing and sticky product. Here's why:

1. Melting Point of Urea: The melting point of urea is relatively low, around 132.7°C (271°F). Prilling is a process where molten urea is sprayed into droplets, which solidify into spherical prills upon contact with cooler air. Operating just above the melting point ensures that the urea is in a molten state, facilitating the formation of droplets for prill formation.
2. Minimum Retention Time: Keeping the retention time (the duration the urea stays in the sprayer) to a minimum is important to prevent excessive exposure to heat. Prolonged exposure to high temperatures can cause urea to degrade and form undesired by-products, such as biuret.
3. Result of Prolonged Exposure: If the prilling process is not conducted properly, and urea is exposed to high temperatures for an extended period, it can lead to the formation of a substantially wet, non-flowing, and sticky product. This occurs due to the partial decomposition of urea and the formation of sticky compounds, which hinder the free flow of prills and affect the product quality.