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 counter-current flow to the gas. The heat-exchanger tubes are have an outside diameter of 2.5 cm and a length of 0.8 m. Determine: (a) The time required to reach the maximum thickness of BA of 1.25 cm. (b) The number of tubes needed The properties of solid BA are given below: Thermal conductivity = 0.163 W mK; density = 1316 kg m; Mw = 122.12 g mol; AH=85 kJ mol; Heat capacity of gas = 1.09 J g K1; Vapor pressure of BA can be correlated with temperature by following equation: log[Pv (torr)] 11.956-4409/T, T in Kelvin. =