ARR - Annual Report

This is the third phase of the research performed on agglomeration of particles in fluidized bed systems including both theoretical and experimental work on aggregating fluidized beds. The report contains an updated review on high temperature agglomerating fluidized beds with emphasis on fluidized bed conbustors. The report also includes a detailed comparison between predicted and measured minimum gas velocities necessary to keep a bed of sticky (wet) or sintered granules in the fluidized state.

The experimental procedure using a dilatometer to measure the apparent surface viscosities of sintering particles is described together with measurements to determine agglomerate strength. These quantities must be known before any theoretical model which predicts defluidization in fluidized beds can be applied. Furthermore, a theoretical model based on the growth of vertical channels in a defluidized bed to predict refluidization of a defluidized (packed bed) is described. Future theoretical and experimental work on the project is also outlined.

Friction losses were determined for titanium dioxide suspensions at concentrations of 17.5, 22.3, 25.3 and 33.9 percent by volume, flowing through 1 inch and 2 inch standard 45 deg. and 180 deg. bends, and 90 deg. bends of various radii of curvature, and also through gate and globe valves and Venturi meters. Pertinent suspension properties, which were measured, included particle size distribution, suspension pH, zeta potential, sedimentation behavior and sediment volume and rheological behavior. The rheological behavior was shear-thinning at low shear rates, and shear- thickening at high shear rates. Preliminary analysis and correlation of the rheological data included determination of the power-law parameters over appropriate ranges of shear rate, which were used to define a generalized Reynolds number for correlation of the friction loss data.

Introduction

The knowledge of strain distributions of granular materials is required in various kinds of particle handling processes in chemical, ceramic, food processing and other related industries. However, it is very difficult to obtain such a knowledge as compared with the case of fluids. If we want to obtain a velocity distribution of a gas or liquid flow, we can utilize the Navier-Stokes equation. We have had, however, no basic equations for the corresponding calculation of granular materials. The aim of this paper, is therefore, to derive a new equation for granular materials like the Navier-Stokes equation for fluids.

I will derive a new simple equation for obtaining the strain-distribution based on the classical hydrodynamics. Then, the equation will be applied to simple one-dimensional problem, and compared with the experimental results.

SUMMARY The objectives of our project are to establish an industrial estimation method for any powder bed yield loci and to develop a design procedure for an industrial powder process by use of the yield loci. In this report, research works on the phenomena of powder, in which fluidization, packing structure, adhesion and agglomeration, mechanical yield etc. are contained, are systematically reviewed. Examples of the application of the method for estimating powder bed yield loci to actual particulate processes are discussed. Finally, the transmission of particle bulk density in powder is described qualitatively and experimentally.

Preamble

The work reported here falls into three distinct areas. Part A provides a review of attrition and an appraisal of existing test methods. Part B presents the results of work on the behaviour of alumina particles in the annular attrition cell and in single particle crushing experiments and associated tests. Part C is concerned with developing insight into attrition kinetics by initial tests in the annular shear cell. The immediate goal was a clarification of the effect of molecular weight on the attrition of high density poly- ethylene.

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.

SYNOPSIS

During this first year a preparation procedure for stable model suspensions was established. Preliminary rheological data suggest the presence of a deformable stabiliser layer. There seems to exist an analogy between oscillatory and steady state data.

The yielding of weakly flocculated suspensions starts at low strains (10 -2 - 1O'3) both in steady state and oscillatory flow. In the former it occurs before the stress overshoot is reached. Time effects suggest a kinetic contribution in yielding. The structural breakdown proceeds according to different patterns in different materials thus complicating the modelling.

Data reduction schemes for dielectric probing of structure have been suggested on the basis of data for carbon black suspensions. The application of transient dielectric data in studying thixotropic time scaler is demonstrated.