Mixing and Elutriation of Fines in a Fluidised Bed

Publication Reference: 
02-08
Author Last Name: 
Davidson
Authors: 
L Hailu J F Davidson
Report Type: 
ARR
Research Area: 
Powder Flow
Publication Year: 
1991
Publication Month: 
09
Country: 
United Kingdom

EXECUTIVE SUMMARY

The main objective of the project is to study the mixing and elutriation of fines in a bed containing mostly coarse particles, eg. 40 /,um or 70 um fines in a bed of 480 um coarse particles.

In this year’s work programme, experiments were carried out to investigate the following:

(i) Qualitative study of the mechanisms of fines transport in the particulate phase of a two - dimensional bed.

(ii) Measurement of gas flow through a two - dimensional bubble.

(iii) Measurement of the axial and radial distributions of fines in a bubbling cylindrical bed.

Photographic evidence showed that there are two mechanisms mainly responsible for the upward transport of fines (i) fines carried up within the bubble void and (ii) fines moving with the wake of the bubble. Both mechanisms contribute to the ejection of fines from the bed surface; however, depending on the ratio of the fines terminal velocity to the minimum fluidisation velocity of the coarse particles, one of the mechanisms is more dominant than the other. If the ratio is much less than unity then mechanism (i) is dominant but if the ratio is greater than unity then mechanism (ii) is dominant. The presence of fines within the bubble void can make a significant contribution to the overall gas conversion in a fluidised bed reactor. Thus the presence of fines needs to be considered when designing fluidised bed reactors.

The concentration of fines within the bubble void is a function of the velocity of flow through the bubble which is proportional to the incipient fluidising velocity of the coarse material. The gas flow through a two - dimensional bubble was measured using a Laser Doppler Velocimeter, and the throughflow velocity was found to be about the same as the minimum fluidisation velocity.

The axial and radial distributions of 70 um fines in a bed of 480 um particles were measured in a 0.28 m diameter bed in batch and continuous modes of operation. In both cases it was found that the concentration of fines is higher at the top and lower at the bottom of the bed. The existence of such a profile indicates that the upward fines transport rate is primarily induced by bubbles; the bubbles are effective in transporting fines upwards as compared with the slow downwards movement of coarse particles over the whole cross section of the bed. As would be expected, the radial fines concentration gradient is much smaller than the axial concentration gradient.

It is postulated that there are three mechanisms by which fines mix in the bed (i) upwards mainly in the wake and void of the bubble (ii) downwards in the bulk circulation (iii) downwards by eddy diffusion. These mechanisms give rise to the observed concentration profile with high fines concentration at the top of the bed which promotes elutriation of fines from the bed surface.

For the future, attention will be focused on developing theoretical work based on the above mixing mechanisms in order to predict the fines concentration profile and the fines elutriation rate.

Theory arising from the above should be helpful in interpreting the effect of temperature on elutriation, to be measured at Bradford.