Thermal Diffusivity and Fick Diffusion Coefficient in Mixtures of Hydrogen and Methane by Dynamic Light Scattering

Mixtures of hydrogen (H-2) and methane (CH4) are given in many technical applications where accurate thermophysical property data are required for the design and optimization of corresponding processes. This work evaluates the accessibility of the thermal diffusivity a and the Fick diffusion coefficient D-11 in gaseous binary mixtures of H-2 and CH4 by dynamic light scattering (DLS). The investigations are performed at temperatures T and pressures p of (293, 333, 363, and 393) K and (5, 10, and 15) MPa with varying CH4 mole fractions x(CH4) of (0.05, 0.3, 0.6, and 0.8). For all thermodynamic states investigated, only one hydrodynamic mode was observable by DLS. The assignment of the single related diffusivity to either a, D-11, or a mixed diffusivity D-mix representing both a and D-11 is performed by considering D-11 calculated by the Chapman-Enskog kinetic theory, experimental D-11 literature data, a predicted by using two different approaches, and calculations of the so-called Rayleigh ratio. The findings indicate that DLS gives access to a at high x(CH4), D-11 at low x(CH4), and D-mix at x(CH4) approximate to 0.3. All data are summarized in the form of correlations providing a and D-11 as a function of T, p, and x(CH4).