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An equation was developed to estimate the evaporation rate of a volatile liquid in a flowing airstream and tested against experimental data.The equation requires only the molecular weight, vapor pressure, air velocity, pool size, ambient pressure, and liquid temperature, or estimates of these quantities, to approximate the overall evaporation rate in mass/time/unit area.The equation was developed by solving the partial differential equations describing the mass balance of a differential element above the liquid and includes fits of the diffusion coefficient based on classical kinetic theory of gases. The experiments tested the evaporation rate of 13 different compounds at different temperatures and airflow rates, and regression fits were made of the data. Both the regression fit and the theoretical equation fit the data well within an order of magnitude. The theoretical equation thus provides an approximation of the evaporation rate for low vapor pressure (i.e., 35 Torr or 5% of the ambient pressure) liquids with a minimum of information about the liquid.
Keywords: airstream, evaporation rate, volatile liquid
Evaporation can be defined as the change of state of a liquid into a gas at the cost of a specific amount of energy. The rate at which the process occurs depends not only on the properties of the liquid, but also on the physical conditions of the surrounding environment such as (air) temperature, air velocity, direction and turbulence, etc. Thus a precise calculation of the evaporation rate is impossible, but a reasonable estimate may be made given some general assumptions. Equations that provide this estimate were also developed by Mackay,(ll) Gray,(3) Bishop,(4) and Liss,") among others. In the present work an equation is derived using the solution of a partial differential equation describing a simple model for the evaporation process. The model treats a differential element of space above a pure liquid pool with a flat surface, and arrives at an equation that uses only the molecular weight, vapor pressure, liquid temperature, pressure, liquid pool size, and air velocity as terms in a general equation, thus providing a reasonable estimate given a minimum of information. Such an equation is of great use to the Environmental Protection Agency (EPA) in making exposure estimates and also to the engineer for making an order of magnitude estimate of...