Gas-film coefficients for the volatilization of ketones from water

Abstract

Volatilization is a significant process in determining the fate of many organic compounds in streams and rivers. Quantifying this process requires knowledge of the mass-transfer coefficient from water, which is a function of the gas-film and liquid-film coefficients. The gas-film coefficient can be determined by measuring the flux for the volatilization of pure organic liquids. Volatilization fluxes for acetone, 2-butanone, 2-pentanone, 3-pentanone, 4-methyl-2-pentanone, 2-heptanone, and 2-octanone were measured in the laboratory over a range of temperatures. Gas-film coefficients were then calculated from these fluxes and from vapor pressure data from the literature. An equation was developed for predicting the volatilization flux of pure liquid ketones as a function of vapor pressure and molecular weight. Large deviations were found for acetone, and these were attributed to the possibility that acetone may be hydrogen bonded. A second equation for predicting the flux as a function of molecular weight and temperature resulted in large deviations for 4methyl-2-pentanone. These deviations were attributed to the branched structure of this ketone. Four factors based on the theory of volatilization and relating the volatilization flux or rate to the vapor pressure, molecular weight, temperature, and molecular diffusion coefficient were not constant as suggested by the literature. The factors generally increased with molecular weight and with temperature. Values for acetone corresponded to ketones with a larger molecular weight, and the acetone factors showed the greatest dependence on temperature. Both of these results are characteristic of compounds that are hydrogen bonded. Relations from the literature commonly used for describing the dependence of the gas-film coefficient on molecular weight and molecular diffusion coefficient were not applicable to the ketone gas-film coefficients. The dependence on molecular weight and molecular diffusion coefficient was in general U-shaped with the largest coefficients observed for acetone, the next largest for 2octanone, and the smallest for 2-pentanone and 3-pentanone. The gas-film coefficient for acetone was much more dependent on temperature than were the coefficients for the other ketones. Such behavior is characteristic of hydrogen-bonded substances. Temperature dependencies of the other ketones were about twice the theoretical value, but were comparable to a literature value for water. Ratios of the ketone gas-film coefficients to the gasfilm coefficients for the evaporation of water were approximately constant for all the ketones except for acetone, whose values were considerably larger. The ratios increased with temperature; however, the increases were small except for acetone. These ratios can be combined with an equation from the literaure for predicting the gasfilm coefficient for evaporation of water from a canal to predict the gas-film coefficients for the volatilization of ketones from streams and rivers.