Chapter 6, Problem 6.106P

Air at 25°C and 1 atm with a relative humidity of 25% is to be dehumidi?ed in an adsorption column packed with silica gel. The equilibrium absorptivity of water on silica gel is given by the expression¹⁹

   $X*\left(\text{kg}\text{water/100}\text{kg}\text{silica}\text{gel}\right)=12.5\frac{p_{{\text{H}}_{\text{2}}\text{O}}}{p_{{\text{H}}_{\text{2}}\text{O}}^{*}}$

where $p_{{\text{H}}_{\text{2}}\text{O}}$is the partial pressure of water in the gas contacting the silica gel and $p_{{\text{H}}_{\text{2}}\text{O}}^{*}$is the vapor pressure of water at the system temperature. Air is fed to the column at a rate of 1.50 L/min until the silica gel is saturated (i.e., until it reaches equilibrium with the feed air), at which point the ?ow is stopped and the silica gel regenerated.

(a) Calculate the minimum amount of silica gel needed in the column if regeneration is to take place no more frequently than every two hours. State any assumptions you make.

(b) Brie?y describe this process in terms that a high school student would have no trouble under- Standing. (What is the process designed to do, what happens within the column, and why is regeneration of the column packing necessary?)