Instabilities in an Underflow Standpipe Operating in a Bubble-Upflow Regime

Testing was conducted to understand the operation of an underflow, bottom-constrained vertical 25-cm-diameter standpipe operating in a bubble upflow regime with Geldart Group A FCC equilibrium catalyst. Standpipe operation was studied over a range of solids mass fluxes, amount of external aerations at the bottom of the standpipe with slide valve controlling the catalyst circulation rates. The effects of both aeration amount and catalyst flux on standpipe operation in a bubble upflow regime were investigated. Test results yielded the following findings:

Demonstrated stable operation of the standpipe over a range of catalyst fluxes up to 60 kg/s-m² and an superficial gas velocity of 0.04 to 0.15 m/s in the standpipe. Under these conditions, the bubbles tend to flow upward against downflowing catalyst,

Stable operation was only achieved when the catalyst with fines content less than 44 microns, was increased to at least 7 to 8% by weight in the circulating catalyst. Increasing flow instabilities and catalyst flow problems were observed as the fines content of the catalyst decreased below these levels,

Helium tracer gas testing indicated that gas by-passing is occurring in the 6 m tall standpipe, even at the 8% fines content. Research at PSRI have shown that the pressure ratio across a fluidized bed has a direct impact on the probability that gas by-passing will occur in deep beds. Gas bypassing in the standpipe was the root cause of instabilities in standpipe operation,

For low fines catalyst, catalyst flow became unstable at very low solids flux in the standpipe, when the bottom aeration gas flow rate was increased to provide superficial gas velocity exceeding 0.08 m/s in the standpipe. The differential pressures (∆Ps) above and below the air sparger increased sharply and remained high for 10 to 20 seconds before returning to near normal for a short period.