Dispersion and Holdup in Multiphase Packed Bed Microreactors

Abstract

Liquid holdup and dispersion are reported for a column of 2 mm internal diameter, filled with 0.1 mm spherical particles, for multiphase flows with hydrocarbon liquid flow rates of 10-100 lL/min and nitrogen gas flow rates of 50-1000 lL/min using different tracers with varying diffusion coefficients and vapor pressures. It was found that the liquid holdup (liquid volume/external void volume) was between 0.65 and 0.85, with variations between different experiments and limited impact of flow rate on the holdup. The dispersion characteristics were very similar to single-phase dispersion. The particle Peclet number for dispersion was close to 0.2. This value was of the same order of magnitude -just a factor of two to three lower -as the value that was obtained without gas flow. Tracer volatility did cause the tracer to elude earlier, but did not cause significant additional dispersion. The results suggest that the fluid mechanical interaction between the gas and the liquid was very limited.

Volatile tracer dispersion in multi-phase packed beds

“…Bellos et al (2004) showed the first two cases: (1) the mean residence time and variance are the same for both tracers, (2) the mean residence time is the same but the variance is larger for the volatile tracer. Marquez et al (2008) reported the last case: (4) the mean residence time is shorter for the volatile tracer but the variance is the same as the one for a non-volatile tracer. We try to identify the relative rate of mass transfer versus flow for the four different cases performing RTD simulations over a wide range of Stanton values.…”

“…In previous work, we reported shorter residence times for the volatile compounds in a multi-phase micro-packed bed (Marquez et al, 2008). In this work, we present our modeling results for isothermal conditions and zero pressure drop, just to limit the parameter space.…”

“…Experimentally, we use accurate predictions for relevant parameters in a more narrow range limited by the equipment, based on our earlier work. The liquid hold-up f and Pé clet number based on the liquid interstitial velocity are calculated from the residence time distribution of a non-volatile compound in the liquid phase, as described in Marquez et al (2008). Note that the value of Pe l in this work is defined with a different reference velocity: we multiply the Pe l value obtained using Marquez et al (2008) by the liquid hold-up.…”

“…The liquid hold-up f and Pé clet number based on the liquid interstitial velocity are calculated from the residence time distribution of a non-volatile compound in the liquid phase, as described in Marquez et al (2008). Note that the value of Pe l in this work is defined with a different reference velocity: we multiply the Pe l value obtained using Marquez et al (2008) by the liquid hold-up. The distribution coefficient (H), defined as the ratio of the concentration in the liquid and that in the gas at vapor-liquid equilibrium, was calculated as follows:…”

“…The liquid flow that exits the first gas-liquid separator is analyzed with a refractive-index cell (R401, Waters Associates) in section E. The liquid flow that exits the second gas-liquid separator is collected for closing the tracer-mass balance. The gas flow that exits the second gas-liquid separator runs to a micro-volume thermal conductivity detector (Vici Valco Instruments Co. Inc.) for analysis in section F. Further details about the set-up design and tracer dispersion contribution in the liquid phase of the auxiliary equipment are described in Marquez et al (2008).…”