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[virat_chem05]

hi, i want to know that why the prills of urea is spherical after falling down in prilling tower.......?

[asad daud]

The answer is very simple.
Any liquid drop will acquire the shape of a sphere when allowed to fall freely.
This is due to the surface tension of a liquid, an inward force which is pulling the molecules at the surface towards the bulk of the liquid.
Before the prilling, urea is in the molten state (Its temp is above its melting point).
This molten Urea is converted into tiny droplets by passing it through a seive. Now days spining buckets are used. These drops, due to their surface tension, acquire the shape of spheres as they fall from the top of a prill tower, against the current of rising air in the shaft of the prill tower.
Molten urea loses heat to this air and solidify, keeping the same spherical shape.