Accurate determination of binary gas diffusion coefficients by using laser-optical measurement methods and molecular dynamics simulations
Chemical and Thermal Process Engineering
Final Report Abstract
The main aim of the present project was the accurate determination of diffusion coefficients for selected binary gas mixtures consisting of methane (CH4), propane (C3H8) and carbon dioxide (CO2) by laser-optical experimental and theoretical methods. The experimental data served for the improvement and validation of theoretical approaches, but also improve the data situation for the design of processes and apparatus in chemical and energy technology. Holographic interferometry combined with a Loschmidt cell (HILC) provided experimental Fick diffusion coefficients D11 for the systems CH4-C3H8 and CO2-C3H8 at temperatures of 293 and 313 K and pressures between 0.05 and 0.5 MPa employing improved experimental boundary conditions and evaluation strategies. Adsorption-desorption processes superimposing the diffusion processes, however, were the reason that estimated expanded uncertainties of the obtained data are 5 and 7% for the two systems and, thus, concentration dependencies could not be resolved. As expected, D11 data for a given system increase significantly with increasing temperature and decreasing pressure and agree with the few available literature data published more than 40 years ago. Comparing the two systems investigated, D11 is larger for CH4-C3H8 at given temperature, pressure, and composition, which can be related to the smaller molecule size and polarity of CH4 compared to CO2. Dynamic light scattering (DLS) served for the measurement of diffusivities of all three binary systems in the denser hydrodynamic regime at temperatures between 283 and 363 K and at pressures between 0.5 and 12 MPa, where hardly any experimental data are available in the literature. In the present project, an evaluation strategy was developed allowing the consistent attribution of the signal contributions in the recorded temporal intensity correlation functions to either Fick diffusion coefficients D11, thermal diffusivities a or mixed diffusivities representing both D11 and a, yet with increased uncertainties that cannot be quantified reliably. With the help of this approach, D11 data could be obtained for supercritical and liquid states of the systems CH4-C3H8 and CO2-CH4, where most expanded uncertainties were about 1% and increased up to about 5% in some of the supercritical states studied. Critical slowing-down of the diffusion processes close to the plait critical point could be observed with the help of the measured D11 data. Self- and mutual diffusivities as well as viscosities especially in the gas and supercritical state were calculated by different approaches in molecular dynamics (MD) simulations. Maxwell-Stefan (MS) diffusivities and thermodynamic factors were computed separately as kinetic and thermodynamic contributions being combined to obtain Fick diffusion coefficients. In the MD simulation methodology developed to obtain self- and MS diffusivities, it could be shown that the Yeh-Hummer correction and the correction term suggested by Jamali et al. accounting for system-size effects in liquid systems can be transferred consistently to gaseous systems of varying density. Concentration-dependent MS diffusivities predicted with simulated self-diffusivities via the Darken-equation or with simulated MS diffusivities close to infinite dilution via the Vignesequation showed good agreement with directly simulated MS and Fick diffusivities up to pressures of 2.5 MPa. The comparable values found for MS and Fick diffusivities originate from the thermodynamic factor being close to unity in this region. For the calculation of the thermodynamic factor, the combination of the correction proposed by Ganguly and van der Vegt for the radial distribution functions and of the approach of Krüger et al. for the spatial integration of these functions turned out to provide an efficient and accurate route. In general, the use of simplified ab initio-derived force fields (FFs) developed in an associated parallel project has been proven to provide results that are in better agreement with experimental data compared to other effective FFs available in the literature for most of the states studied. Comparability of simulated Fick diffusion coefficients with experimental data from DLS was limited to few liquid states, where the deviation was close to combined experimental and statistical uncertainties. In the low-density gas phase, good agreement of Fick diffusivities obtained from MD simulations employing the ab initio-derived FFs, from HILC measurements and from ab initio calculations combined with the kinetic theory of gases was found.
Publications
-
Binary diffusion coefficients for gas mixtures of propane with methane and carbon dioxide measured in a Loschmidt cell combined with holographic interferometry, International Journal of Thermophysics 40, article 18 (2019)
P. Zangi, M. H. Rausch, A. P. Fröba
-
Enhancement of the predictive power of molecular dynamics simulations for the determination of self-diffusion coefficient and viscosity demonstrated for propane, Fluid Phase Equilibria 496, 69-79 (2019)
U. A. Higgoda, R. Hellmann, T. M. Koller, A. P. Fröba
-
Fick diffusion coefficients of binary fluid mixtures consisting of methane, carbon dioxide, and propane via molecular dynamics simulations based on simplified pair-specific ab initio-derived force fields, Fluid Phase Equilibria 502, article 112257 (2019)
U. A. Higgoda, C. J. Kankanamge, R. Hellmann, T. M. Koller, A. P. Fröba
-
Self-diffusion coefficient and viscosity of methane and carbon dioxide via molecular dynamics simulations based on new ab initio-derived force fields, Fluid Phase Equilibria 481, 15-27 (2019)
U. A. Higgoda, R. Hellmann, T. M. Koller, A. P. Fröba
-
Diffusivities of an equimolar methane-propane mixture across the two-phase region by dynamic light scattering, International Journal of Thermophysics 41, article 102 (2020)
M. Piszko, C. Giraudet, A. P. Fröba
-
Diffusivities of binary mixtures consisting of carbon dioxide, methane, and propane by dynamic light scattering, Journal of Chemical and Engineering Data 65, 1068-1082 (2020)
M. Piszko, K. Batz, M. H. Rausch, C. Giraudet, A. P. Fröba
