Fluidized bed hydrodynamic modeling of CO₂ in syngas: Distorted RTD curves due to adsorption on FCC

Abstract

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 H 2 , CH 4 , CO, and CO 2 (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 CH 4 , CO, and CO 2 trailing RTD traces were elongated versus H 2 demonstrating a chromatographic effect. An axial dispersion model accounted for 92% of the variance but including diffusional resistance between the bulk gas and catalyst pores and adsorption explained 98.6% of the variability. At 300 C, the CO 2 tailing disappeared consistent with expectations in chromatography (no adsorption). H 2 and He are poor gas-phase tracers at ambient temperature. We recommend measuring RTD at operating conditions.