$6\mathrm{.}26.$ Recovering and reusing organic solvents $($rather than discharging the solvents in waste streams$)$ is an important part of the operation of most chemical plants. The magnitude of these recovery efforts can be staggering: in recent years the Eastman Chemical Company used $3\mathrm{.}6$ billion pounds of solvents and recovered $3\mathrm{.}5$ billion pounds $(97\%).$ Eastman's installation of a $$26$ million acetone$-$recovery system reduced acetone emissions by $50\%$ in the division that had been responsible for most of these emissions. ${?}^8$

In an acetone$-$recovery process, a gas stream containing $20\mathrm{.}0$mole$\%$ acetone and the remainder nitrogen leaves a chemical plant at $90C$ and $1$atm. The stream is cooled at constant pressure in a condenser, enabling some of the acetone vapor to be recovered as a liquid. The nitrogen and uncondensed acetone are discharged to the atmosphere.

$($a$)$ Give two major benefits of recovering the acetone.

$($b$)$ Two cooling fluids are available$-$cooling$-$tower water at $20C$ and a refrigerant at $-35C.($At the latter temperature, the vapor pressure of acetone is essentially zero.$)$ For each fluid, calculate the percentage acetone recovery acetone condensed$/$mol acetone fed to condenser