P.N. Rowe

A model for a segregating gas fluidised bed

A differential equation model is derived to describe particle segregation in a binary mixture of solids fluidised by gas. The model, which is formulated to conform with qualitative descriptions of the mechanism of segregation, yields solutions displaying the unusual features of the steady—state solids—concentration profiles found experimentally and enables appraisals to be made of the relative importance of the contributary mechanisms.

Prediction of bubble size in a gas-fluidised bed

Abstract It is shown that existing data on bubble size (when this is not restricted by the vessel dimensions) can be described by where (U — Umf) is the excess gas flow, h is the height above the distributor and h0, a measure of the initial bubble size, characterises the distributor. h0 is effectively zero for a porous plate but may be more than a meter for large tuyeres.

The division of gas between bubble and interstitial phases in fluidised beds of fine powders

Abstract The division of gas flow between the two phases of a fluidised bed of commercial catalyst powder has been determined from measurements of interstitial phase voidage and bed height over a wide range of gas velocities. Voidage measurements were made by comparing the X-ray absorption of the interstitial phase of the freely bubbling catalyst with that of a calibration wedge containing the same material. X-Ray photography was also used in the measurement of bed height. Three batches of catalyst powder containing different amounts of ‘fines’ were examined. The results clearly demonstrate that increasing the fines content of a fluidised powder leads to an increase in the relative proportion of gas flowing interstitially at all fluid velocities. Furthermore the interstitial gas flow is shown to be up to 25 times greater than the minimum fluidisation value for the powder containing the highest proportion of fines. The implications of this for the two-phase theory of fluidisation and for the design and operation of fluidised bed reactors is discussed.

A model for desulphurisation with limestone in a fluidised coal combustor

Abstract Kinetic expressions used to describe the reactions of deactivating catalyst particles are applied here to the sulphation of calcined limestone in a fluidised bed coal combustor where reactivity diminishes as pores fill with sulphate. Following closely the recent work of Fieldes and Davidson it is shown how experiments with batchwise addition of limestone to the combustor may be used to derive the two reaction rate constants of the system, ks, and kd, and how these may be incorporated into a two-phase model of a fluidised bed reactor to obtain predictions of desulphurisation efficiency under conditions of continuous operation. The resulting equation for SO2 retention, η, may be simplified to: η = 1 − [ 1 1 + K β ] where K is a function of limestone type and operating conditions and β is the calcium-to-sulphur mole ratio. The predictions of this equation are shown to be in agreement with experimental results.

Single bubbles injected into a gas fluidised bed and observed by X-rays

Bubbles were provoked in a gas fluidised bed by injecting a pulse of air and were then observed by X-rays. For a given quiescent bed, the volume of bubble produced, VB by injecting a volume, Vin was found to be described by, where A is a constant of proportionality. The initiating volume, Vin 0 decreased with increasing bed flow rate whilst the constant A generally increased. With one of the two materials used (Ballotini), both constants, the bubble shape and, to a slight degree, velocity depended on the previous history of the bed. In spite of these differences it is concluded that, although it may be difficult to predict the size of bubble produced by injection, once generated it behaves in a very similar manner to a spontaneously formed one.

The effect of pressure on the flow of gas in fluidized beds of fine particles

Abstract The division of gas flow between the bubble and dense phases of fluidized beds of six different types of Group A powders has been studied at pressures of up to 20 bar using surface collapse and X-ray absorption measurements. It was found that with these fine powders as pressure increases at constant volumetric gas flowrate so the size and hold-up of bubbles decrease while their frequency increases. Contrary to previous measurements the average bubb velocity appears to decrease with increasing pressure. The dominant mode of bubble break-up in all the powders was found to be division from the rear, contrast to that observed with Group B powders at atmospheric pressure. Interstitial phase voidage was found to increase with increasing pressure. The results are interpreted in terms of a model which assumes a difference between the voidages, and hence the gas flow, of powder in the wakes behind

The measurement of emulsion phase voidage in gas fluidized beds of fine powders

Abstract The results of two different experimental methods, X-ray absorption and bed collapse, for the determination of the emulsion-phase voidage of gas fluidized beds are compared. It is shown that good agreement exists between them over the range of gas velocities studied, but that possible differences in voidage in an axial direction are only revealed by the X-ray technique. The bed collapse method applied to a Geldart Group A powder confirms that there is little difference between the surface settling rates of a bubbling bed and of a uniformly expanded bed of the same voidage.

Particle-to-liquid mass transfer in fluidised beds

Abstract Nelson and Galloways theory of particle to fluid mass transfer in dense systems of fine particles is re-examined and modified slightly to make it applicable to liquid fluidised beds. The resulting expression agrees with published data although these are inadequate to test it critically.

An analysis of the distribution of flow between phases in a gas fluidised bed

Abstract A new and comprehensive theory is developed to describe the division of gas between the bubble and interstitial phases of a fluidised bed, something not satisfactorily predicted by existing theories. It is based on the hydrodynamic models of Davidson, Harrison and Murray and distinguishes between bubbles with and without clouds. It makes no assumptions about the value to be attributed to the average dense phase porosity but requires an expression relating it to permeability. An example of application is given using data obtained from a bed of silicon carbide particles of mean diameter 262 μm fluidised by air. In this case, the dense phase porosity and the interstitial gas velocity decrease with height. Near the distributor a large proportion of the bubbles are small, slow moving and therefore without clouds but this proportion decreases sharply with increase in height.

A model for chemical reaction in the entry region of a gas fluidised-bed reactor

Abstract Contrary to what was often imagined, gas enters a bed of fluidised particles in the form of bubbles as if entering a true liquid. This fact has been unambiguously shown by X-ray cine photograhy over a wide range of conditions which additionally reveals a substantially constant bubbling frequency of 8 per second at each point of entry. This fact leads to a simple hydrodynamic model that describes the history of contact between gas and particles during the brief formation process which, combined with chemical kinetics, explains the high rate of reaction commonly observed in a shallow entry region. The model predicts that conversion is unlikely to be changed by alterations to the distributor design.