Powder Flow

This report describes work done at the Cambridge University Department of Chemical Engineering during the first year of an IFPRI contract. Most of the year’s work was spent on two aspects of the project, a literature survey and the development of a computer program to predict the rate of deaeration of a powder in a baker. The report on the former has been completed and is attached with this report. The results of the computer analysis have been written up in the form of a “final report” which it is hoped to distribute to IFPRI members by the end of next month.

In the present report we confine ourselves to details of the other aspects of the work during the year. This report therefore consists of the following sections:

1. Introduction
2. Experimental measurement of stress-strain curves.
3. A report of some small flowrate measurements to investigate the effects of interstitial pressure gradient on flowrate.
4. Future work.
5. The current technical and fiscal status of the project.

ABSTRACT

When a fine material is loaded into a bunker, air can become trapped between the particles and is gradually expelled as the material consolidates. Two effects need to be considered when analysing the deaeration process; the changing density of the gas as the pressure falls and the changing voidage of the material as it consolidates.

If only the first factor is important and the voidage of the material in the bunker uniform, the theoretical analysis of the deaeration process is straightforward. For modest initial pressures, the pressure at the base of the hopper decays with a half-life of about 0.3H2E/k pa, where k is the Darcy's Law constant, H the height of the fill, E the void fraction and pa is atmospheric pressure.

When consolidation is important, the calculations are very much more difficult due to the changing height of the top surface. A detailed numerical analysis has been performed and the predicted values of the half-life of the deaeration process are presented graphically for a number of cases.

A detailed description of the theoretical analysis and of the computer program is included for the benefit of those wishing to extend the work.

Abstract

Many problems concerning the behaviour of particulate solids in a vessel remain unsolved. One of them is the phenomenon of wall pressure increase arising from vibrations or impacts on the wall of the vessel. An example is the insulation of low temperature tanks whore perlite powder is packed in the central annular space of double walled tanks holding liquefied gas. Tank wall expansion or contraction, caused by temperature changes whenever liquefied gas is added or removed, packs the perlite powder more densely and to quantify the wall pressure increase and to elucidate the mechanism of the phenomenon, so, that this can be taken into account in the design of storage tanks and vessels.

In the present study the wall pressure increases due to slight impacts on the front wall of a rectangular vessel containing particulate solids have been calculated by using a Janssen type equation which has been modified by introducing an angle between the direction of the major principal stress and the horizontal axis. This was then checked against experimentally obtained data.

The results indicate that the increase in wall pressure is probably due to the change in the angle of the principal stress in the powder bed and that this, in turn, comes from the change of the arrangement of the particles.

SUMMARY

This report consists of an account of work done under IFPRI contract in Cambridge University during the year to November 1984. The principal achievements during that time are:

1. The construction and commissioning of an aerated flow rig.
2. The performance of some preliminary experiments in the rig, whose results are evaluated in this report.
3. The completion of the theoretical work undertaken previously.

Also included is a proposal for the continuation of the work.

Summary

An experimental rig to characterise the flow of a powder when aerated has been designed, built and commissioned.

Preliminary results on two powders supplied by IFPRI members have shown interesting results which could be very significant in improving the understanding of the flow of aerated powders. Powders behave as powders at low strain rates, even at high aeration, but become liquid like at aeration rates less than that required for minimum fluidisation when the strain rate is higher. Even a little aeration has a large effect on the magnitude of the transmitted stress.

The objectives of our project are to establish an industrial estimation method for any powder yield locus and to develop a design procedure for industrial powder processes by use of the yield locus. The complete powder properties at any possible porosity under given conditions may be necessary to design particulate processes.

However, it is very difficult or almost impossible in time, technically or economically. We can point out the importance of a new problem "how to estimate powder property from the design standpoint of particulate process". Then, we will discuss a historical review on the birth of this concept of powder property estimation.

Next, the problem of "how the estimation method will be developed to be effective and inevitable in the design procedure, will be explained mainly by use of our method. And we will outline the design method of particulate process on the basis of the estimation method.

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

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.

Chapters 4 and 5

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.

Complementary to the already existing surveys offered by myself to IFPRI on "Air Classification" and the "Sorting by Sieving and Air Classification" a third survey report has been prepared on "Sorting in Gas Fluidised Beds". Sorting in a fluidised bed may be achieved either in a counterflow or a crossflow sorting machine. The present report describes in detail the relevant theoretical background necessary for the precalculation of a fluidised bed sorting process. Most of it belonging to the standard theories used in fluidised bed reactor design. It was clearly found, that the existing theories describing the behaviour of a fluidised bed with respect to changes in the size distribution of the particles forming the bed, their shape or the influence of the void ratio is inadequate and needs further theoretical and experimental consideration. The precalculation of a fluidised bed used for sorting furthermore suffers from the fact, that the settling behaviour of the heavy that is the sink product cannot be described accurately enough. This is mainly due to the fact, that the influence of the above mentioned variables on the apparent density of the fluidised bed is not very well known. Therefore in actual practice bench scale experiments must yield the relevant data. Bench scale experiments have been performed with four different bed materials and several different materials to be sorted. The last part of the report summarises equipment as invented and suggested in the literature for the sorting in gas fluidised beds. This report finalises my work in classification and sorting.

EXECUTIVE SUMMARY

An experimental and theoretical study of the agglomeration phenomenon which causes destabilization of certain low and high temperature fluidized beds was performed. A theoretical model was proposed to determine the conditions under which defluidization occurs in fluidized beds in which cohesion forces between granules arise due to the presence of sticky fluids and/or high temperatures. Bonding mechanisms between particles such as solid-liquid bridges, viscoelastic flattening and high temperature sintering were all considered. The model, which predicts breakup of aggregates by bubble motion, was compared to limiting fluidization-defluidization (quenching) experiments performed by the authors and others. An experimental method to measure surface softening of small particles heated to high temperatures was developed by using a dilatometer to measure the surface viscosity of the particles from rate of deformation data. Experimental methods to determine the minimum sintering temperatures of a variety of granules were also presented. Lastly, experiments were performed to study the dynamic strength of a liquid bridge between two spheres coated with a liquid and moving away from one another. It was shown that the strength of the dynamic bridge was at least one order of magnitude larger than the corresponding strength of the static bridge between the two spheres. This result accounts for the relatively high gas velocities necessary to keep a bed of sticky particles in continuous fluidization.