Evaporation of accumulated multicomponent liquids from fibrous filters

Fibrous filters are used for environmental and occupational mist sampling and industrial mist collection. If compounds in the filtered droplets are volatile or semi-volatile, they may evaporate into the gas passing through the filter. In sampling applications, failure to properly account for evaporation of collected mist will lead to mist concentration estimates that are low. In control applications, volatilization of filtered droplets may release vapors that are harmful to workers or the public. Also, vapor emitted from mist filters may recondense as a hazardous aerosol or on surfaces to pose a safety or housekeeping problem. Droplets collected by fibrous filters coalesce into larger drops that reside on the fibers. Results from a numerical model developed to predict evaporation from these drops agree favorably with experimental data. Measurements and numerical predictions show that a gas stream leaving a wetted fibrous filter can be saturated with the vapor of semi-volatile compounds retained on the filter. In some situations, the model indicates that the gas stream will be saturated before it passes 0.1 mm into a wetted filter. If the liquid retained on a filter is a pure compound, the vapor concentration leaving the filter is constant when initially clean air passes through. If the liquid is multicomponent, the downstream vapor concentration in previously uncontaminated air will decrease with time as the more volatile components evaporate preferentially. Fluctuations in incoming mist and vapor concentrations can enhance evaporation because more retained liquid will volatilize when incoming vapor concentrations are low.