A vertical thermosiphon reboiler is required for a column. The liquid at the base of the column is essentially pure n-butane. A vapor rate of 5 kg/s is required. The pressure at the base of the column is 20.9 bar. Saturated steam at 5 bar will be used for heating.

Estimate the number of 25 mm outside diameter, 22 mm inside diameter, 4 m long, tubes needed. At 20.9 bar, the saturation temperature of n-butane is 117°C and the heat of vaporization is 828 kJ/kg.