Residence time distribution in fluidized beds: diffusion,dispersion, and adsorption

The residence time distribution (RTD) characterizes the flow patterns of tracer particles in continuous systems to identify phenomena like channelling, dispersion, dead volume, and back-mixing. Understanding the hydrodynamics in experimental reactors is imperative to derive reaction kinetics. Here, we applied the RTD methodology to examine the relationship between the physico-chemical properties of powders and gas flow patterns in an 8 mm diameter quartz micro-fluidized bed. H₂ and He tracers egressed the reactor sooner than Kr, O₂, CO, CH₄, CO₂ demonstrating that gas diffusivity is a factor to consider when choosing a tracer gas. Intraparticle porosity is a second factor that delays the RTD curve: the residence time was 7% greater for virgin vanadium pyrophosphate catalyst (VPP) with a lower particle density than an equilibrated VPP. These results suggest that RTD experiments are sensitive enough to detect phenomena like catalyst sintering and pore blocking due to coke in situ. The RTD curve of CO and CH₄ tracers with a bed of fluid catalytic cracking catalyst (FCC) had a noticeable trailing edge (tail). For the CO₂ tracer, the leading edge of the RTD curve was delayed, the peak height was 50% shorter, and the tail was extended demonstrating adsorption phenomena like a chromatographic effect. The residence time of Kr, with a similar gas diffusivity as CO₂ was also longer compared to CO or CH₄, but the tail was shorter than for CO₂, which confirms the adsorptive nature of the FCC surface with CO₂. A bolus-pulse input function detected these hydrodynamic anomalies with a greater certainty than a Heaviside-step input.