A desublimation unit of the heat-exchanger type is to be sized for the recovery of 200 kg/h of benzoic acid (BA) from a gas stream containing 0.8 mol% BA and 99.2 mol% N₂. The gas enters the unit at 780 torr at 130C and leaves without pressure drop at 80°C. The coolant is pressurized cooling water that enters at 40°C and leaves at 90°C, in countercurrent flow to the gas. The heat-exchanger tubes are of the type in Example 17.15. In addition to benzoic-acid properties in Exercise 17.37: k_c of solid benzoic acid = 1.4 cal/h-cm-C and ρc of solid benzoic acid = 1.316 g/cm³. Determine the number of tubes needed and the time required to reach the maximum thickness of benzoic acid of 1.25 cm.

Example 17.15

A desublimation heat exchanger is to be sized for recovery of 100 kg/h of naphthalene (N) from a gas stream, where the other components are noncondensable. The heat-exchanger tubes are 1 m long with an outside diameter of 2.5 cm. Tube spacing is such that N can build up to a maximum thickness of 1.25 cm. The gas enters the unit at 800 torr and 80°C with a mole fraction for N of 0.0095. The water coolant flows through the inside of the tubes countercurrently to the gas. Cooling water enters at 25°C and exits at 45°C. Determine the number of tubes needed and the time required to reach the maximum thickness if the gas leaves at 60°C.