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 130°C 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 heatexchanger tubes are of the type in Example 17.15. Some properties of benzoic acid are given in Exercise 17.37. In addition,

kc of solid benzoic acid = 1.4 cal/h-cm-^oC

ρc of solid benzoic acid = 1.316 g/m³

Determine the number of tubes needed and the time required to reach the maximum thickness of benzoic acid of 1.25 cm.