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

An extensive 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 particles. 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. Stable operation was demonstrated over a wide range of catalyst fluxes up to 60 kg/s-m² and an imposed superficial gas velocity of 0.04 to 0.15 m/s in the standpipe. However, increasing flow instabilities and catalyst flow problems were observed as the fines content less than 44 microns decreased below 7 to 8% by weight in the circulating catalyst.

Helium tracer gas testing indicated that some degree of gas bypassing is occurring in the 6 m tall standpipe, even at the 8% fines content. Research at PSRI[1] has shown that the pressure ratio across a fluidized bed greater than 1.05 was expected to promote gas bypassing in deep beds. The pressure ratio across the standpipe operating in this test unit was approximately 1.38 and gas bypassing would occur, which was accounted for the root cause of instabilities in standpipe operation. The study also showed that the higher fines contents in the catalyst helps reduce the tendency for gas bypassing[2] to occur for a given bed height.

[1] Karri, S. B. R., A. Issangya and T. M. Knowlton, Gas bypassing in deep fluidized beds, in Fluidization XI, U. Arena, R. Chirone, M. Miccio and P. Salatino, eds., 515 – 521, 2004.

[2] Issangya A., Karri, S. B. R. and T. M. Knowlton, Why Gas bypassing occurs in deep fluidized beds of Geldart Group A particles and how to prevent it, 10th Int. Conf. on Multiphase Flow in Industrial Plant, Tropea (VV), Italy, , XI-XXXI, 2006.