ARCHIVED REPORTS XR0012553 - Laserfiche WebLink

Home Browse Search

5 t _r . uuqq � rrY. n •N�.,r..r a '. ' a. b fi- •Y' Attachment 1 . CALCULATIONS OF ATMOSPHERIC/WATER CONCENTRATIONS Calculations that were used to determine if an explosive atmosphere can be created in the sewer system utilize the Henry's Law relationship. The Henry's Law constant can be :�. expressed by the following relationship: � . H = X N` where: H = the Henry's Law constant P = the partial pressure of the compound of interest in the gas phase x = the concentration of the compound of interest in the water Atmospheric partial pressures can be expressed in atm, mm Hg, Ya, or psia. Concentrations in water can be expressed as m/thole, gm/m, or mole fraction. As a result of the different units in which each concentration can be expressed, the value of the Henry's constant can be presented in several unit systems. The most common (andthe b9e used by the EPA) see+ns to be the units of atmospheres per m /mole. r� Henry's constant values are determined experimentally when a given amount of the compound of interest is dissolved in water in an enclosed vessel. The system is allowed to some to equilibrium and the concentration of the compound in the gas phase and in the water is determined. Since the Henry's Law const-ant changes with temperature, all deter inations are correct only for a given temperature (usually 25 C). A rule ' of thumb is that the Henry's Law constant will double with a M 250 C rise in temperature. For the Henry's Law cal:."ulation to be applicable to the sewer system, several assumptions must be made: 1. Solution concentrations must be very dilute (less than 1% mole fraction). 2. The sewer temperature must be equivalent to the temperature at which the experimental data used to calculate the constant was collected. 3. The solubility of the compound is low enough so that there is no significant change in the Henry's Law constant with a change in compound concentration. Prediction of atmospheric concentrations of organics in ,.the sewer would require calculations involving extremely complicated mass transfer equations and the assumption of -� estimated kinetic parameters. Since the atmospheric concentratipn over a water solution of these compounds cannot become any higher than the equilibrium concentration, calculations based on Henry's Law will . be worst case predictions of the atmospheric concentration. It is expected that in fact the system will not be at equilibrium and that actual atmospheric concentrations will be much lower than +^� estimated here.