A pair of evaporators will be arranged in series to remove water from a salt solution $($i$.$e$.,$ the salt solution that exits the first evaporator will be the feed to the second evaporator$).$ The first evaporator will be operated at conditions sufficient to evaporate water at a rate of $9$ kmol$/$h$.$ The second evaporator will be operated at conditions sufficient to evaporate water at a rate of $4\mathrm{.}5$ kmol$/$h$.$ You may assume that no salt exits either unit via evaporation. The salt solution that exits the first evaporator $($i$.$e$.,$ the feed to the second evaporator$)$ flows at a rate of $635$ kg$/$h and contains a $50/50$ mixture $($by mass$)$ of water and salt.

a$)$ Before doing any calculations, draw and fully label a flow chart for this process $($including all knowns$/$unknowns and relevant system boundaries$).$

b$)$ A correctly drawn flow chart will include $3$ system boundaries$-$ the overall, evaporator $1,$ and evaporator $2.$ Perform a degree of freedom analysis for each system boundary and specify an order in which the problem could be solved to determine all unknowns.

c$)$ Calculate the composition $($mass fraction$)$ of the overall feed stream $($i$.$e$.,$ into the first evaporator$).$

d$)$ Calculate the composition $($mass fraction$)$ of the exiting salt solution.