Situation: Water at 25 [C] flows down a wetted-wall tower, forming a 4-[mm]-thick film. The inside diameter of dry tower is 12 [cm] and its height is 84 (cm). Air with low humidity (25 % relative humidity) enters the bottom of the tower and absorbs humidity (water) as it goes up. The air flow rate (on a dry basis) is 0.04 [kg BDA/s]. Assume that the temperature is 25 [C] throughout the tower (in both phases). Relevant properties at 25 [C]: The vapor pressure of water is 23.8 (mm Hg) and its molar mass is 0.01802 [kg/mol). The diffusion coefficient of water in air is 0.253 [cm2/s). The dynamic and kinematic viscosity of air are 18.6 [uPa's) and 15.7 [cSt]. respectively. Requirements: Please do the following: (a) Use a differential mass balance to show that the expression for the concentration of water in air, in [kg/m"), as a function of axial coordinate z, where z is zero at the bottom, is: 4k z C.(z)=CW. 1 -expl dv (b) Estimate the average mass-transfer coefficient using the following correlation: Sh, 0.023 ReSCM () Estimate the outlet concentration of water, in [kg/m?) and its relative humidity. c