ARR - Annual Report
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Summary
This is the fourth phase of a research program to study the agglomeration of particles in fluidized bed systems. The report contains an updated review of the literature on high temperature agglomerating fluidized beds as well as equilibrium shapes of liquid bridges between particles. It also includes a comparison between predicted and measured minimum gas velocities necessary to keep a bed of sintered granules in the fluidized state.
The use of a dilatometer to measure the minimum sintering temperature of potentially agglomerating particles is described together with measurements to determine agglomerate strength. Different materials such as glass, polymers, coal and inorganic salts were used during the experiments.
The correlation between the sintering temperature and other thermodynamic properties of these materials, as observed from a differential scanning calorimeter test, was determined experimentally. A new original theory concerning the strengthening of a liquid bridge between two particles due to relative motion (viscous dissipation) is proposed. Future theoretical and experimental work on the project including a means to verify the above theory are also outlined.
Abstract
This 1984 IFPRI Annual Report, covering the period from September 1, 1983 to April 15, 1985, presents the results of studies involving the characterization, the rheology and the flow in straight pipe as well as pipeline transitions of concentrated aqueous laterite suspensions. The report consists of two parts. The first part presents the results of comprehensive bench-scale tests whose aim was to probe the limits of the properties of laterite suspensions, and to explore some of the means to control them. Experimental data on sedimentation rates, zeta potential, 'in-situ' particle aggregate size, and yield stress for concentrated laterite suspensions were taken and have been correlated with suspension pH. The second part of the report presents the results of large scale pipeline flow experiments involving concentrated laterite suspensions. The experimental work in this part consisted of property characterization and rheology measurements for various concentrations of laterite suspensions, and also tests on laminar and turbulent flow of these suspensions through straight pipe and through pipeline bends, fittings and flow control and flow metering devices. The results of the experiments have been analyzed, and are presented as correlations of zeta potential, yield stress and stress-rate of strain dependence as functions of suspended solids concentration, as friction factor - generalized Reynolds number correlations for straight pipe flow, and as correlations of friction loss with generalized Reynolds number for flow through bends, fittings and valves.
Introduction
Sorting is synonymous to the separation of two- or multicomponent mixtures into its individual components. The newly produced materials being the better and the more valuable, the less they are contaminated with components i. e. materials of the other kind. The report mainly covers the dry sorting of two component mixtures by means Of a combination of two classification methods i. e. sieving and air classification. A method which has been developed at Clausthal.
The method can be used for coarse materials only, that is for particulate mixtures coarser than approximately one millimeter. Sorting in this size range becomes more and more important however, since it may be used for the recycling of waste or secondary products. The process described may, on the other hand, also be used for the dry softing of minerals such as coal or ore where water is scarce or not available in sufficient quantities, and also in countries with average temperatures below freezing point, and it may replace wet methods as used up until now in mineral Processing. The combined sorting process sorts mixtures into individual components with high throughputs, reasonable costs of investment and low specific costs.
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:
- The construction and commissioning of an aerated flow rig.
- The performance of some preliminary experiments in the rig, whose results are evaluated in this report.
- The completion of the theoretical work undertaken previously.
Also included is a proposal for the continuation of the work.
During the second year systematic measurements were performed on sterically stabilised suspensions of extreme particle sizes and covering a wide concentration range. A scaling law was found to hold up to moderate concentrations. The dependence of one of the scaling parameters on particle size was established. The scaling law was tested for 3 temperatures and two suspending fluids. Deviations from hard sphere behaviour were found, which are This effect was characterized by means of oscillatory measurements. attributed to stabiliser layer deformability. Under some conditions anomalous behaviour was encountered. For weakly flocculated systems time-dependent behaviour (i.e. thixotropy) was investigated. From experiments involving stepwise changes in shear rate some conclusions about the kinetics were obtained. The kinetics of recovery small amplitude oscillatory measurements. in effect of the after cessation of flow were investigated using preceding shear rate was found and information about the nature of the developing structure could be derived.
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.