Self-diffusion in compressed supercritical water

Abstract

The self-diffusion coefficients D of compressed supercritical water have been measured as a function of pressure in the temperature range 400 to 700 °C using the NMR spin–echo technique. The experimental diffusion data were compared to theoretical predictions based on a dilute polar gas model using a Stockmayer potential for the evaluation of collision integrals and a temperature dependent hard sphere diameter. The empirical expression ρDαTn, where n = 0.76, fits the experimental data to within±10% over the entire density and temperature region studied. The value of the exponent n = 0.76 agrees favorably with the n values found for diffusion of other gases. The product ρD is density independent under isothermal conditions which indicates that two-body collisions dominate the diffusion behavior. This finding is in agreement with our earlier results for proton relaxation in compressed supercritical water which were analyzed using a binary collision approximation and a collision modulated spin–rotation interaction described by a single correlation function which is an exponential function of time. The hydrodynanic Stokes–Einstein equation appears to hold at densities above ρc. As expected, the diffusion data cannot be described in terms of a hard sphere model but an empirical fit to lnρD = (A/T)+B, where A and B are constants, represents the experimental data well over the range of temperatures and densities studied.

Density dependence of rotational and translational molecular dynamics in liquids studied by high pressure NMR

“…The low temperature limit of these data was set by the short deuteron spin-spin relaxation times, which combined with the low self-diffusion coefficient do not permit a reliable determination of D with the experimental setup used. The self diffusion results published in the more recent papers (261)(262)(263)(264)(265)(266)(267) agree in the region of overlap within the precision claimed by the different groups. Weing/irtner t35) has recently critically reassessed all older data published.…”

Methane and methanol oxidation in supercritical water: Chemical kinetics and hydrothermal flame studies

“…In applying these mechanisms to supercritical water environments, a hypothesis is made that reactions are dominated by gas-phase, free radical chemistry rather than the ionic chemistry of liquids (Holgate and Tester, 1993). This hypothesis is supported by examining several properties of supercritical water at typical process conditions: the ionic dissociation constant of water is about 10-20, or six orders of magnitude lower than liquid water (Marshall and Franck, 1981); the dielectric constant, a measure of molecular association, is an order of magnitude lower than liquid water (Uematsu and Franck, 1980); and SCWO transport coefficients resemble gas-phase more than liquid-phase properties (Lamb, et al, 1981). Further support of the hypothesis comes from experimental evidence of free-radical reactions in SCWO reactions (Antal, et al, 1987), as well as from past successes in applying models based on the hypothesis (Holgate, 1993).…”

“…As an example, diffusion coefficients at our experimental conditions are two orders of magnitude smaller than those for atmospheric flames (Reid, et al, 1987). Additionally, suggested correlations (Lamb, et al, 1981) for diffusion coefficients at supercritical densities show a different (i.e., Arrhenius) dependence on temperature than is used to derive Equation 3.1 (D -7'1.67). Models that employ Arrhenius-type diffusion coefficients predict significant changes in flame structure (Bechtold and Margolis, 1992).…”

“…For the reactants diffusing toward each other, D is estimated as twice the experimentally measured self-diffusion coefficient of water (2~1.5~10-3 cm2 SI) (Lamb, et al, 1981). The Leonard-Jones diameters of hydrogen and oxygen are approximated as 2.92 and 3.54 A.…”

Hawaii Integrated Biofuels Research Program: Final Subcontract Report, Phase III

“…These properties include the solvent's dielectric constant (Franck, 1970), electrolyte conductance (Quist, 1970;Marshall, 1968), ion product (Marshall, 1981;Quist et al, 1965), transport properties (ie. viscosity (Bruges, 1969), diffusivities (Lamb et al, 1981), ion mobil i ties (Franck et ale , 1968) ) , hydrogen bonding characteristics (Franck, 1970) and solute/solvent enhancement factor (Franck, 1983). with these unusual properties, the reaction rates may be enhanced while maintaining the selectivity and it may be possible to dissolve reactants and catalyst in a single, homogeneous fluid phase so that interphase mass transfer is limited (Subramanlam and McHugh, 1986).…”