Characterization of Particle Flow In Inclined Standpipes

Characterization of particle flow in inclined standpipes

Lilian de Martín and J. Ruud van Ommen

Delft University of Technology, Department of Chemical Engineering, Product & Process Engineering, Julianalaan 136, 2628 BL Delft, The Netherlands

In several chemical processes, such as fluid catalytic cracking (FCC), the presence of several coupled elements (risers, standpipes, cyclones…) complicates to have a steady operation. Two main limitations are encountered in these systems that are associated with the standpipes.

The first one is commonly found operating at high catalyst recirculation rates. In this condition, an excess of gas can be transferred down the standpipe with the solids. When this occurs, the regime in the standpipe changes and there is no longer a well-defined dense phase [1]. At this point, the apparent density and consequently the pressure build-up in the standpipe decreases, leading in turn to the air go up the standpipe and a decrease of the collector efficiency [2]. This undesired situation can be avoided increasing the pressure drop across the standpipe by increasing the apparent density of the solid.

The second one comes from the bubbles formed in the standpipe due to the aeration required to maintain the solid circulating. The maximum catalyst flow rate that the standpipe can handle is limited by the size of these bubbles [3]. This limitation is especially a problem when operating with inclined standpipes because the rising bubbles become large at low solids flow rate and low aeration

In this work we have characterized the catalyst circulation features in a circulating fluidized bed (CFB) equipped with an inclined standpipe by means of visual observation, pressure drop and pressure fluctuations. We have specially focused on inclined standpipes because they are quite often used in industry, but seldom studied in academia. Moreover, the hydrodynamics behavior in inclined standpipes is quite different from that in vertical standpipes or horizontal transport lines.

The tests were carried out in a CFB consisting in a riser 0.265 m diameter and 5 m tall. The standpipe has an inclination of 45^o, 0.09 m diameter and it is made of Perspex. The solid fluidized is FCC catalyst with an average particle size of 60 μm.

[1] J. R. Grace, A. A. Avidan and T. M. Knowlton. Circulating fluidized beds. Springer 1996.

[2] D. Geldart, N. Broodryk and A. Kerdoncuff. Studies on the flow of solids down cyclone diplegs. Powder Technology, 76 (1993) 175-183.

[3] W.-C Yang. Handbook of fluidization and fluid-particle system. CRC Press 2003.