There is a percentage of fuel in a fuel—air mixture called the lower flammable limit (LFL), below which the mixture cannot be ignited. There is also an upper flammable limit (UFL), a percentage of fuel above which ignition will not take place. The LFL and UFL are collectively known as the flammability limits of the fuel. The LFL and UFL of propane in air at 1 atm are, respectively, 2.05 mole % C₃H₈ and 11.4 mole% C₃H₈. If the mole percent of propane in a propane—air mixture is between 2.05% and 11.4%, the gas mixture will burn explosively if exposed to a flame or spark; if the percentage is outside these limits, the mixture is safe—a match may burn in it but the flame will not spread. If the percentage of propane is below the LFL, the mixture is said to be too lean to ignite; if it is above the 1.JFL the mixture is too rich to ignite.

(a) Which would be safer to release into the atmosphere—a fuel—air mixture that is too lean or too rich to ignite? Explain.

(b) A mixture of propane in air containing 4.03 mole% C₃H₈ is the feed to a combustion furnace. If there is a problem in the furnace, the mixture is diluted with a stream of pure air to make sure that it cannot accidentally ignite. If propane enters the furnace at a rate of 150mol C₃H₈/s in the original fuel—air mixture, what is the minimum molar flow rate of the diluting air?

(c) The actual diluting air molar flow rate is specified to be 130% of the minimum value. Assuming the fuel mixture (4.03 mole% C₃H₈) enters the furnace at the same rate as in part (b) at 125°C and 131 kPa and the diluting air enters at 25°C and 110 kPa, calculate the ratio m3 diluting air/m³ fuel gas and the mole percent of propane in the diluted mixture.

(d) Give several possible reasons for feeding air at a value greater than the calculated minimum