Bubbles rising through fluidized beds at velocities several times superficial velocities contribute to solids backmixing. In micro-fluidized beds, the walls constrain bubble sizes and velocities. To evaluate gas-phase hydrodynamics and identify diffusional contributions to longitudinal dispersion, we injected a mixture of H2, CH4, CO, and CO2 (syngas) as a bolus into a fluidized bed of porous fluid catalytic cracking catalyst while a mass-spectrometer monitored the effluent gas concentrations at 2 Hz. The CH4, CO, and CO2 trailing RTD traces were elongated versus H2 demonstrating a chromatographic effect. An axial dispersion model accounted for 92% of the variance in the data but including diffusional resistance between the bulk gas and catalyst pores and adsorption explained 98.6% of the variability. At 300 °C, the CO2 tailing disappeared consistent with expectations in chromatography (no adsorption). H2 and He are poor gas-phase tracers at ambient temperature. We recommend measuring the RTD at operating conditions.
The hydrodynamics of gas-phase fluidized beds is non-ideal due to high velocity. Micro-fluidized beds have distinct flow patterns because of the wall and the diameter constrains bubble velocity. We measured the gas phase RTD in a 8 mm ID quartz tube loaded with fluid catalytic cracking catalyst (FCC). We devised a feed manifold to introduce a 4-component tracer gas as a bolus pulse. The FCC separated the gases based on diffusivity like chromatography. At ambient temperature, the trailing edge of CO, CH4, and CO2 have extended tails and an axial dispersion model accounts only for 92 % of the variance. We developed a model to characterize the tailing that includes diffusion from the bulk gas to the FCC pores and adsorption-desorption of the gas on the catalyst. This model accounted for 98.6 % of the variance in the RTD. At 300°C the tailing disappeared consistent with expectations in chromatography.