The percentage of fuel in a fuel$-$air mixture below which the mixture cannot be ignited is called the lower flammability limit $($LFL$),$ sometimes referred to as lower explosion limit $($LFL$).$ There is an upper flammability limit $($UFL$)$ also known as the upper explosion limit $($UEL$).$ For example, the LFL of propane in air is $2\mathrm{.}3$ mole $\%C_3{H}_8$ and the UFL is $9\mathrm{.}5$mole$\%.$ Thus, if the percentage of propane in a propane$-$air mixture is greater than $2\mathrm{.}3$mole$\%$ and less than $9\mathrm{.}5$ mole $\%,$ the gas mixture can ignite if exposed to a flame or spark.

A mixture of propane in air containing $4\mathrm{.}03$ mole $\%C_3{H}_8($fuel gas$)$ is fed to a combustion furnace. If there is a problem in the furnace, a stream of pure air $($dilution air$)$ is added to the flue mixture prior to the furnace inlet to make sure that ignition is not possible.

$($i$)$ Draw and label a flowsheet $($block flow diagram$)$ of the fuel gasdilution air mixing unit, assuming that the gas entering the furnace contains propane at the LFL$,$ and do the degree$-$of$-$freedom analysis

$($ii$)$ If propane flows at a rate of $150mol{C}_3\frac{H_8}{s}$ in the original fuel$-$air mixture, what is the minimum molar flow rate of the dilution air?

$($iii$)$ How would the actual dilution air feed rate probably compare with the value calculated in $($ii$)?$