E3.7 Explosive Limits - Environmental Concerns

Example 3.7: An acetone-air vapor mixture is transported in a duct to a thermal type VOC incinerator. The flow from the dryer is 1000 acfm at 150 °F and 216 lb/h of acetone are evaporated. Determine the concentration of acetone in the duct. Is this a dangerous situation? Company policy requires that flammable vapor concentrations in duct be maintained at 25% of the LEL (lower explosive limit) or less. What can be done about this situation? The following data apply:

Molecular weight, M = 54, Boiling point, T_b = 56 °C,

Heating value, HV = 13120 Btu/lb, Lower explosive limit, LEL = 2.5% by volume,

Upper explosive limit, UEL = 12.8% by volume.

Solution

Acetone evaporation rate = 216 lb/h = 216/54 = 4 lbmol/h = 6.667´10^-2 lbmol/min

Specific volume of a gas at 150 °F,

V = 379(150 + 460)/(60 + 460) = 444.6 ft³/lbmol

Volumetric flow rate of acetone, υ = 6.667 ´ 10^-2(444.6) = 29.64 ft³/min

Volume fraction of acetone in air, C

C= (Volumetric flow rate of acetone)/(volumetric flow rate of air)

C = 29.64/1000 = 2.964% by volume

This concentration lies between LEL and UEL and any source of ignition could cause a fire or explosion.

Required concentration of acetone = 25% of LEL = 25%(0.025) = 0.625%

Ratio of actual concentration to required concentration = 2.964/0.625 = 4.724

Required flow rate = 4.724(current flow)

A dilution of 4.724 - 1 = 3.724 fold is required.

We need to mix 3724 ft³/min of air to the stream leaving the dryer.