The residence time distribution of the gas phase in circulating fluidized beds (CFB)

CFB reactors are increasingly used for both gas catalytic and gas/solid reactions. Gas flow patterns in the riser are important in their design and subject of the present research. A literature review on the gas mixing in the riser highlights the contradictory results of previous investigations for CFB risers: some investigators neglected back-mixing of gas, whereas others report a considerable amount of back-mixing. The present paper reports experimental findings obtained in a 0.1 m I.D. riser, for a wide range of combined superficial gas velocities and solid circulation fluxes. The gas flow mode (plug vs. mixed) is strongly affected by the operating conditions, however with a dominant mode within a specific (U,G)-range. Sand was used as bed material. The operating conditions varied from 5.5 to 8.3 m/s as superficial gas velocity, and 40 to 170 kg/m2 s as solid circulation flux. In order to determine the residence time in the riser, a pulse of propane was injected at the bottom of the riser and detected at its exit. The cumulative response curves [F(t)] define: an average residence time (t50) obtained for F(t) = 0.5 the different slope of the curves, with a steeper one corresponding with a more pronounced plug flow, and expressed in terms of a deviation, σ. These parameters (t50 and σ) are used to define the gas flow mode. A quantitative comparison of experimental results with known RTD-models is inconclusive although the occurrence of both mixed flow and plug flow is evident, and (U,G)-dependent. The experimental results can be expressed in design equations: M. Van de Velden et al. t50 = 0.57 (U-Utr) G with t50 in s/m riser height σ = 0.87 (U-Utr) G The comparison of predicted and experimental results is fair. The results give clear indications concerning the gas mixing mode: significant deviations of tpf,gas, the residence time of the gas in the empty riser, and t50 already stress deviation from plug flow; high values of σ (e.g. ≥ 0.1) indicate a significant gas back mixing. Small values of σ are obtained at moderate values of U-Utr and high values of G. Operations at higher excess gas velocities correspond with higher σ-values (σ ≥ 0.25), thus proving the increasing deviation of gas plug flow and growing importance of gas back mixing. This strong (U,G)-dependency explains the contradictory literature findings, since experiments with similar systems might favor plug flow or mixing as function of the (U,G)-values. The approach is finally used to tentatively assess the conversion of the gas in the riser, as illustrated for a simple gas/catalytic first order reaction. The calculations show that the reaction yields, for the specific case, are favored by high velocities, whereas higher G-values at a given U reduce the conversion.