This report is concerned with one aspect of the problem of flooding, namely the deaeration of powders in bins. In chapter 2 we review the literature on flooding and conclude that the consensus of opinion links flooding with the aeration properties of the material. Most authors maintain that materials become aerated as a result of rapid filling or the collapse of an arch or rat-hole. It is important to determine for how long the material will remain in the aerated state.
Chapter 3 contains a theoretical analysis of deaeration and a computer program has been written which enables detailed prediction of the half-life of the deaeration process. Also a simple formula is presented from which a rough estimate of the half-life can be found directly. The theory predicts that fine materials in industrial sized bins can remain aerated for periods of the order of days and thus present a flooding hazard.
The details of the experiments performed to test the predictions and the nature of the materials used are given in chapters 4 and 5 and the experimental results are presented and analysed in chapter 6. It was found that of the eight materials investigated, the five coarsest obeyed the model whereas the finer materials deaerated more rapidly than predicted. The materials failing to obey the predictions were all Geldart type-C materials which are known to channel badly on fluidisation. It is therefore not surprising that the model, which assumed homogeneous behaviour, fails in these cases.
We conclude therefore that we are able to predict the aeration properties of a material/bin combination and thus assess its flooding potential for all but the finest (type-C) materials. This work, unlike most previous work on flooding, suggest that flooding does not depend only on the nature of the material, but that bin dimensions are also important.