Determine the mass$-$transfer coefficient and the flux for mass transfer $($in kmol$/$m$2$

s$)$ from a

$30$ mm in diameter solid sphere of a volatile substance to a gas at $60\backslash$deg C and $2\mathrm{.}5$ atm flowing

at an average velocity of $0\mathrm{.}5$ m$/$s$.$ The diffusivity of the substance in the gas at $1$ atm and

$40\backslash$deg C is $210-6$ m$2$

$/$s and the vapour pressure of solid substance is $8$ mmH$2$O$.$ Density of the

gas at $60\backslash$deg C and $2\mathrm{.}5$ atm is $1\mathrm{.}2$ kg$/$m$3$

and viscosity $210-5$ Pa$$s$.$