Measurement of the coefficient of transverse dispersion in flow through packed beds for a wide range of values of the Schmidt number

Experimental values of the coefficient of transverse dispersion (DT) were measured with the system 2-naphthol/water, over a range of temperatures between 293K and 373K, which corresponds to a range of values of viscosity (μ) between 2.83×10−4 Ns/m2 and 1.01×10−3 Ns/m2 and of molecular diffusion coefficient (Dm) between 1.03×10−9 m2/s and 5.49×10−9 m2/s. Since the density (ρ) of water is close to 103 kg/m3, the corresponding variation of the Schmidt number (Sc=μ/ρDm) was in the range 1000 – 50. More than 200 experimental values of the transverse dispersion coefficient were obtained using beds of silica sand with average particle sizes (d) of 0.297 and 0.496mm, operated over a range of interstitial liquid velocities (u) between 0.1mm/s and 14mm/s and this gave a variation of the Reynolds number (Re=ρduε/μ) between 0.01 and 3.5.Plots of the dimensionless coefficient of transverse dispersion (DT/Dm) vs. the Peclet number (Pem=ud/Dm) based on molecular diffusion bring into evidence the influence of Sc on transverse dispersion. As the temperature is increased, the value of Sc decreases and the values of DT/Dm gradually approach the line corresponding to ‘gas behaviour’ (i.e. Sc ≈ 1), which is known to be well approximated by the equation DT/Dm=1/τ+ud/12Dm, where τ is the tortuosity with regard to diffusion.