Diffusion coefficients at infinite dilution in alcohol solvents at temperatures to 348 K and pressures to 17 MPa.
Creators: name: Cooper, Elizabeth.
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 1992
Subjects: subject: Engineering, Chemical.

description: A knowledge of diffusion coefficients in liquids is important from engineering as well as theoretical aspects. Molecular diffusivities can be used in predicting the mass transfer coefficients for the design of equipment for separation processes. The diffusion process may determine the rate at which these processes take place. The understanding of the diffusion process is important from a theoretical standpoint in helping us to understand the structure of liquids, the mechanism of diffusive transport and to test the validity of existing theories of diffusion. This work examined the effect of temperature and particularly of pressure on the diffusion coefficients at infinite dilution of the solute gases, ammonia, propane, propene and carbon dioxide in the alcohol solvents, methanol, ethanol, propanol and butanol, using the dynamic Taylor dispersion method at temperatures of 298.15, 323.15 and 348.15 K and pressures up to 17 MPa (2500 psig). Solvent densities and some solvent viscosities were also measured over a similar range. The use of a data acquisition system for the collection and subsequent analysis of data using an IBM computer ensured that the data for molecular diffusivity, density, and viscosity were accurate and reliable. Also, because Taylor's method is a dynamic one, it is a relatively rapid method of measurement. Although the measured diffusivities showed some differences from the diffusivities obtained from the literature, the diffusivities from this work are considered to be more reliable and accurate because of the experimental technique and type of analysis used. The measured densities and viscosities of the solvents showed excellent agreement with those reported in the literature. It was found that the molecular diffusivities as well as the densities tended to increase linearly with increasing pressure, and the preliminary results for the viscosities showed that the viscosities tended to increase linearly with increasing pressure. Although the temperature effect on diffusivity, density and viscosity was larger than the pressure effect, there was a statistically significant effect of pressure. It was found that the effects of hydrogen bonding, dipole moment, polarity, association, molecular mass, molecular size, shape and the resulting molecular interactions between the solute and the solvent molecules all appeared to influence the diffusion of the gases through the solvents. The diffusion coefficients of the solutes increased in the order ammonia, propane, propene, and carbon dioxide. The diffusion coefficients in the solvents increased in the order butanol, propanol, ethanol, and methanol. A generalized equation based on the RHS theory was developed for the systems studied in this work which requires only certain physical properties of the solute and the solvent and the temperature in order to predict the diffusion coefficient. This study provides the first systematic effort to generalize the RHS theory to predict infinite dilution coefficients of solutes and solvents of various polarities, dipole moments and degrees of association at high pressures.