J.G. Yates

Effects of temperature and pressure on gas-solid fluidization

Abstract The objective of this paper is to review experimental and theoretical studies of gas-solid fluidization at elevated temperatures and pressures. The survey begins with the low velocity end of operations in the region between minimum fluidization velocity and minimum bubbling velocity and shows how correlations established at ambient temperature and pressure for these two quantities may be used to calculate their values at super-ambient conditions. The application of purely hydrodynamic fluid-bed stability criteria to account for the transition from the non-bubbling to the bubbling state is described and compared with the expected effect of interparticle forces on this transition. The effects of temperature and pressure on the dynamics of gas bubbles in powders of Groups A, B and D are considered next and areas of uncertainty in current theories of bubble motion are highlighted. Correlations for jet penetration are then discussed and recommendations made as to the most reliable of these. Circulating fluidized beds (CFBs) operated at high velocity are then considered and it is shown that many of the observed effects in these systems at superambient conditions can be accounted for in terms of changes in the value of the terminal fall velocity, u t , of the bed particles. The effects of changes in u t on entrainment, elutriation and choking are also considered. The effect of increased pressure in enhancing bed-to-surface heat transfer coefficients in beds of Group A powders is shown to be due to the suppression of bubbling while in beds of Group B materials the enhancement is through an increase in the gas convective component of the transfer coefficient. The small amount of work carried out on heat transfer in CFB combustors is reviewed. Pressure effects on the combustion of char in bubbling beds are considered in terms of an established two-phase theory model and it is concluded that the increased rate of solids burn-out at high presures is due to an increase in the value of the local Sherwood number thereby increasing the rate of mass transfer of oxygen to the surface of the burning particle. The important question of sintering leading to defluidization at elevated temperatures is then examined and attention drawn to the current lack of broadly based mechanistic models to account for and predict the phenomenon. The state of the art in the area of scaling relationships is reviewed and it is shown that while the scaling laws for bubbling beds are by now reasonably well established the same is not so for CFBs, indicating a major area for further work.

Self-segregation of high-volatile fuel particles during devolatilization in a fluidized bed reactor

Abstract The interaction between fuel particles and incipiently bubbling gas fluidized beds during devolatilization has been investigated by X-ray imaging. The fuel consisted of a ligneous biomass ( Robinia pseudoacacia ) reduced into millimeter-sized particles and doped with lead nitrate in order to make particles visible upon X-ray irradiation. A purposely designed single-particle-injector was used to impulsively introduce fuel particles one at a time at a given depth into the fluidized bed. Experiments highlighted three main features of the phenomenology, namely: (a) the formation of ( endogenous ) volatile matter bubbles around devolatilizing fuel particles; (b) the uprise of endogenous bubbles; and (c) the uprise of fuel particles closely associated to endogenous bubble motion. Bubble and particle trajectories and bubble cross sections as functions of time were worked out in order to assess fuel particle segregation times and endogenou s bubble growth rate. The choice of operating under incipient bubbling conditions enabled thorough assessment of interactive processes establishing between gas-emitting particles and the fluidized suspension. The formation, growth and motion of endogenous volatile bubbles and the associated motion of the fuel particle could be characterized without the perturbation caused by exogenous gas bubbles (i.e. bubbles formed under freely bubbling conditions). This represents a first step towards the characterization of the interaction between gas-emitting particles and freely bubbling beds.

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.

Hydrodynamic influences on particle breakage in fluidized beds

Abstract Experimental data are provided for the attrition of particles in a gas-fluidized bed. Results are interpreted using mechanistic models which relate the rate of attrition of the bed material to the hydrodynamic forces in the bed and the mechanical strength of the material. Two models are developed based on low-energy impacts in the bulk of the bed and high-energy collisions near the distributor. The reasonable agreement between the theory and the measurements indicates the attrition of bed material is caused by particle—particle and/or particle—wall impacts in the core of the bed.

Fine particle effects in a fluidized-bed reactor

Abstract Recent experimental work has shown that when the concentration of fine particles (45 μm) in a fluidized bed is increased the gas flow pattern within the bed is altered, more gas being caused to flow through the emulsion phase and less through the bubble phase. In the case of a catalytic reaction system this effect should result in an increase in conversion for a given throughput of reactant and the work described here was carried out to test this hypothesis. Experimental results are presented for the catalytic oxidative dehydrogenation of butene-1 in fluidized beds of catalyst containing 0, 16 and 27% fines and it is shown that conversion does in fact increase with increasing fines content. The system can be modelled on the basis of emulsion-phase gas flows that are in excess of the minimum fluidization flow, agreement between model predictions and experimental results being quite satisfactory.

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.

Multiplicity of the steady state in fluidised bed reactors — I.: Steady state considerations

Abstract Multiple steady states are known to be possible in many types of chemical reactor due to the non-linear dependence of reaction rate on either temperature or reactant concentration. It is reasoned in this study that multiple steady states can also exist in a gas-solid fluidised bed reactor. The calculations are based on the two-phase theory of fluidisation and on the important assumption that the interstitial phase gas in completely mixed. Both thermal and concentration multiplicites are shown to be possible.

High temperature effects on the dense phase properties of gas fluidized beds

Abstract This paper reports some of the results obtained from an extensive experimental campaign aimed to study the influence of temperature on the fluidization behaviour of solid materials. The fluidization behaviour of a wide range of materials was investigated from ambient conditions up to 650°C. The aim of this work was to highlight the conditions under which the role of the hydrodynamic forces (HDFs) or interparticle forces (IPFs) were dominant, in order to make predictable the fluidization behaviour at elevated temperatures. To this end, the fluidization behaviour of three fresh FCC catalysts was studied. An E-cat FCC catalyst, which contained process residuals, was examined without performing pre-treatments prior to fluidization tests. Furthermore, a highly porous silica catalyst was doped with increasing amount of potassium acetate (KOAc), 1.7, 7 and 10 wt.%, and a sample of glass ballotini was doped with 0.1 wt.% of KOAc. This was done in the attempt of modifying their surface characteristics, thus triggering changes in their fluidization behaviour with increasing temperature. The measured pressure drop across the bed and deaeration tests was used to highlight changes in the fluidization behaviour as a function of temperature. The standardized collapse time (SCT) was obtained from the collapse profiles and was used to distinguish between systems of powders dominated by HDFs and IPFs. Results obtained from analytical techniques such as thermomechanical analysis (TMA), Gas Chromatography Mass Spectrometry analysis (GCMS) and scanning electron microscope (SEM) are also discussed, these techniques were used to investigate physical changes in the particles with increasing temperature.

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.