Ammonia, $N{H}_3,$ is being selectively removed from an air $-N{H}_3$ mixture

by absorption into water. In this steady$-$state process, ammonia is transferred

by molecular diffusion through a stagnant gas layer $5mm$ thick and then

through a stagnant water layer $0\mathrm{.}1mm$ thick. The concentration of ammonia

at the outer boundary of the gas layer is $3\mathrm{.}42$mol$\%$ and the concentration at

the lower boundary of the water layer is essentially zero.

The temperature of the system is $288K$ and the total pressure is $1$atm.

The diffusivity of ammonia in air under these conditions is $0\mathrm{.}215c\frac{m^2}{s}$ and in

liquid water is $1\mathrm{.}77{10}^{-5}c\frac{m^2}{s}.$ Neglecting water evaporation, determine the

rate of diffusion of ammonia, in $k\frac{g}{m^2}-h.$ Assume that the gas and liquid are

in equilibrium at the interface.

The equilibrium data for ammonia over very dilute aqueous solution at

$288K$ and $1$atm can be represented by $($Welty et al$.,1984)$

$y_A=0\mathrm{.}121+0\mathrm{.}013ln{x}_A$

Hint: At steady$-$state, the

ammonia rate of diffusion

through the gas layer

must equal the rate of dif$-$

fusion through the liquid

layer.