HYDRODYNAMIC CORRELATIONS FOR THE DESIGN OF CONICAL SPOUTED BEDS WITH OPEN-SIDED DRAFT-TUBE
Advancing the chemical engineering fundamentals
Multifase Flows (T2-5P)
Keywords: Conical spouted bed, draft-tube, hydrodynamics, stability
Spouted beds with fully conical geometry combine the features of the cylindrical spouted beds (such as the capacity for handling coarse particles, small pressure drop, cyclic movement of the particles and so on) with those inherent to their geometry, such as stable operation in a wide range of gas flow-rates. This versatility in the gas flow-rate allows for handling particles of irregular texture, fine particles and those with a wide size distribution and sticky solids, whose treatment is difficult using other gas-solid contact regimes. Moreover, operation can be carried out with short gas residence times (as low as milliseconds) in the dilute spouted bed (1).
In the past, such spouted beds have been widely used for drying, granulating, and mixing, but attempts have recently been made to make better use of their flexibility by using them for gasification, pyrolysis, combustion and three phase reactions (2).
A crucial parameter that limits the scaling up of spouted beds is the ratio between the inlet diameter and particle diameter. In fact, the inlet diameter should be smaller than 20-30 times the average particle diameter in order to achieve spouting status. The usual solution in conventional spouted beds is the use of a draft-tube. Nevertheless, the hydrodynamics with this tube is different to that of plain beds
A hydrodynamic study of conical spouted beds with draft-tube has been carried in a previous paper (3). In this one, conical spouted beds with open-sided draft-tubes have been studied in order to check its potential application in the handling of fine particles. Different types of tubes have been used. Thus, draft-tubes of different diameter (lower or higher than the inlet diameter), different opening wide, length above the bed, ring(s) at different levels and closed and open upper outlet have been used. Based on this study, stable operation regimes have been delimited.
Beds provided with open-sided draft-tube require a higher velocity to open the spout than beds with a solid draft-tube. Likewise, operating pressure drop is also higher. The values of these paramters are intermediate between those for plein beds and solid draft-tube spouted beds. Nevertheless, the solid circulation rate (turbulence) is much higher with open-sided draft-tube, which is due to the solid cross-flow from the annulus into the spout. Aireation of the annulus is also improved with this type of device.
Based on dimensional and statistical analysis, correlations have been determined for the minimum spouting velocity, operation pressure drop and peak pressure drop.
Literature
1. Olazar, M.; San José, M. J.;. Zabala, G.; Bilbao, J. Chem. Eng. Sci., 49, 4579(1994)
2. Lu, W.J., Chang, C.M., Chem. Eng. Sci., 49, 1465(1994).
3. H. Altzibar, S. Alvarez, M.J. San José, R. Aguado, J. Bilbao, M. Olazar, in Fluidization XII, (in press), (2007)
Presented Tuesday 18, 13:30 to 15:00, in session Multifase Flows (T2-5P).
