Henry?s law constants and hydration equilibrium constants of n-hexanal and their temperature dependence as determined by the rectangular pulse method

The Henry's law constants of n-hexanal are needed to predict its rate of mass transfer between gas and aqueous phases. However, information on these constants is limited because the mass transfer of n-hexanal involves a reversible hydration reaction in water; therefore, Henry's law constants are difficult to obtain experimentally and cannot be deduced only from the gas-to-water equilibrium. Here, we conducted stimulus-response experiments using a rectangular input pulse and a double-mixing gas-liquid contactor to simultaneously determine Henry's law constants and hydration equilibrium constants for n-hexanal in deionized water, aqueous sodium chloride, or aqueous sodium sulfate in the temperature range of 5.3-22.5 degrees C. The change in the concentration of a mixture of n-hexanal in nitrogen gas after passing through the contactor was examined, and simulation-based optimization was conducted to optimize parameters that included Henry's law constants and hydration rate constants. In the simulation, convolution using the rectangular input pulse was used to determine the input signal, and perfect gas-phase mixing and two-film layer mass transfer were assumed. This approach, which we called the rectangular pulse method, was validated by performing similar investigations for toluene, ethyl acetate, and ethyl trifluoroacetate. As a result, Henry's law constants and rate constants for the reversible hydration reactions of n-hexanal and their temperature dependence were obtained, and the calculated effective Henry's law constant agreed with some, but not all, of the values reported in the literature. A potential reason for the disagreement was discussed from the viewpoint of long time-constants used for the hydration equilibrium. In addition, a salting effect by the aqueous salt solutions was found to affect the Henry's law constants, but not the hydration equilibrium constants. The overall liquid-side mass-transfer coefficient was found to be correlated with the diffusion coefficient, based on the Higbie penetration theory. (c) 2021 Elsevier Ltd. All rights reserved.