A mixture of methane and air is capable of being ignited only if the mole percent of

methane is between $5\%($lower$)$ and $15\%($upper flammability limits$).$ A mixture containing

$9\mathrm{.}0$ mole $\%$ methane in air flowing at a rate of $700$ kg$/$h is to be diluted with pure air to

reduce the methane concentration to the lower limit$.$

a$.$ Calculate the required flow rate of air in kmol$/$h$.$

b$.$ Calculate the percent, by mass, of oxygen in the product gas.

c$.$ Calculate the flow rate of an $47\%$ methane stream that would have to be added to

the original stream to prevent the mixture from igniting.

d$.$ Qualitatively explain why there are upper and lower flammability limits$.$ It can be

helpful to think about this from a safety and$/$or hazards perspective.