what is the criteria for fractional distillation?

Fractional distillation is done to seprate 2 or miscible liquids for which difference in boiling point is less than  25 K

The mixture of two or more liquids with different boiling points is heated to a high temperature..

The mixture boils, forming vapor

The vapor enters the bottom of a long column that is filled with trays or plates.

• The trays have many bubble caps in them to allow the vapor to pass through.
• There is a temperature difference across the various trays , they are hot at the bottom and cool at the top
• The vapor rises in the column.
• As the vapor rises through the trays in the column, it cools.
• When a substance in the vapor reaches a height where the temperature of the column is equal to that substance's boiling point, it will condense to form a liquid.
• The substance with the lowest boiling point will condense at the highest point in the column; substances with higher boiling points will condense lower in the column.
• The trays collect the various liquid fractions.