Plant Safety, Explosive Limits:

Waste gases usually consist of a hydrocarbon or a mixture of hydrocarbons in air. They must be in such high concentration that ignition is not possible until more air is added. Such mixtures are said to be above the upper explosive limit (UEL). Mixtures that have such low hydrocarbon concentrations that they cannot be ignited, are said to be below the lower explosive limit (LEL). Most hydrocarbon air mixtures are of this type.

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

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

C Specific volume of a gas at 150 °F,

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

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

C Volume fraction of acetone in air, C

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.

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

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

C 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.