Search

The principles on which the performance of a full-scale fluidized-bed reactor may be inferred from that of a cold, scaled-down model are outlined and lead to a review of the scaling rules developed in the recent past. Dimensional analysis based on the Buckingham π-theorem is described as well as the alternative approach based on the governing equations of conservation of mass and momentum of fluidized particles. Examples are given of both rigorous and simplified sets of dimensionless groups appropriate to the scaling process and a description is given of the way they are applied to bubbling beds. This is followed by a consideration of the scaling relationships relevant to circulating fluidized-bed combustors where additional groups such as the Damkohler numbers can be applied. Work on the validation of the scaling rules is then described and leads to a section in which scaling is analysed in terms of the non-linear chaotic behaviour of fluidized beds. The chapter ends with a description of the application of the scaling rules to a scaled-down model of a thermal denitration reactor and its internal structure as revealed by X-ray analysis.

In the processes to be considered here the advantageous features of fluidized beds noted in the previous Chapter are again in evidence. The high degree of solids mixing in multi-component systems such as are used in the chloride process for titanium dioxide and the consequent isothermal nature of the reacting mixture are highly desirable features and are exploited to the full in reactor design. The ability to transfer fluidized solids between reactors is exploited in the treatment processes of uranium compounds leading to the production of uranium dioxide for use in nuclear reactors. These are described along with processes for the production of hydrogen chloride and ultra-pure silicon while fluid coking and sulphide ore roasting are touched on briefly. The fluidized-bed combustion of coal is treated in detail with sections on plant developments, combustion mechanisms, desulfurization and sulfation models. Coal gasification is also considered and the chapter ends with an extended section on the relatively new technique of chemical looping.

Thermal conversion processes, namely combustion, gasification and pyrolysis are presented in this chapter. The conversion mechanisms that take place in fluidized-bed reactors are explained with emphasis on materials in-feeding, devolatilization and volatile conversion, char conversion and particle attrition and elutriation. A comparison between conventional and unconventional fuels is made, with particular focus on gasification of waste fuels for energy generation. The operating parameters in gasification processes are discussed, with reference to feeding (feeding methods, number of feeding points and solid fuel feed size), bed depth, bed temperature, fluidization velocity, equivalent ratio, and inerts content. To conclude, examples of current technologies which employ fluid-bed combustors, pyrolysers and gasifiers in operation in the UK and around the world are discussed. Advanced thermal processes such as fluid-bed plasma for waste gasification are also touched upon.

This paper reports some experimental observations on the homogeneous expansion of three FCC catalysts with increasing temperature from ambient up to 650 °C. These catalysts were all characterized by a wide particle size distribution and by nonperfectly spherical particles. The Richardson–Zaki equation was used to describe the homogeneous expansion of these catalysts. Experimental expansion profiles were obtained experimentally with increasing temperature, and the parameters of the Richardson–Zaki equation, n and ut, were estimated. Experimental n and ut values were compared with predictions given, respectively, by the Richardson–Zaki n=n(Ret) correlations employed for liquids and by the drag equation for the settling of an individual particle, using the Stokes law. Experimental n and ut values were also compared with data reported in the literature. For all FCC catalysts, experimental n and ut values were found to be much higher than the predicted ones, with the discrepancy being greater at low temperatures. The homogeneous bed was assumed to be characterized by presence of clusters, whose diameter, dp*, was back-calculated from the ut* values. The values of dp*, which were obtained at temperatures above 100 °C, were found to be of the same order of magnitude as the surface average and volume average mean diameters, indicated as dS and dV, respectively. These findings underlined the importance of defining appropriate mean particle diameters to describe the particles' systems under study. Furthermore, parallel to fluidization tests, thermal analyses were carried out to investigate possible changes in the particles' physical properties as a function of temperature. Results from these tests were compared with fluidization findings in order to highlight the role of the interparticle forces on the stable behaviour of these materials.

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 fluid-bed stability of three fresh FCC catalysts has been investigated both experimentally and theoretically as a function of increasing temperature. Values of the voidage at minimum bubbling conditions were obtained from 20°C up to 650°C, and compared with predictions given by the Foscolo and Gibilaro particle bed model. The formulation of the drag force proposed by Foscolo and Gibilaro has been generalized in order to correctly describe the homogeneous expansion of the three FCC catalysts. Consequently, the procedure followed to obtain the Foscolo and Gibilaro stability criterion has been re-examined, and a generalized expression of their criterion is proposed. Predictions obtained from the generalized particle-bed model were found in agreement with experimental data. This paper discusses also the effect of the drag and buoyancy force in predicting correct trends for e mb . To this end, results obtained using the Jean and Fan model are reported. The physical origin of the stability of these catalysts is also discussed, and the role of the hydrodynamic forces and interparticle forces is examined.

This paper reports some experimental observations on the effect of temperature on the fluidization of three fresh FCC catalysts and an equilibrium (E-cat) FCC tested at ambient pressure and at temperatures up to 650°C. The bed collapse test was used as a quantitative test to characterise the fluidization behaviour. It provides a sensitive and discriminating means of assessing the changes in the materials' aeratability between ambient and high temperature fluidization. The dense phase collapse rates were obtained from the collapse profiles and the experimental values are reported in this paper. Fluidization of the three fresh FCC catalysts improved with increasing temperature, the deaeration rate of these catalysts decreased as temperature increased. On the other hand, the deaeration rate of the E-cat FCC increased with increasing temperature, as the catalyst became less aeratable due to the dominant role of the interparticle forces (IPF) over the hydrodynamic forces (HDFs). The characteristics of the surface of this catalyst changed as a function of temperature. Where changes in fluidization at high temperature were observed, the factors responsible were investigated. To this end thermo-mechanical analyses were carried out, and the results obtained are discussed. Comparisons of experimental dense phase collapse rates with theoretical predictions are also reported. Deaeration rates obtained for the fresh FCC catalysts are predicted fairly well from a modified form of the Richardson–Zaki equation and from Abrahamsen and Geldart's correlation. The experimental collapse rates obtained for the E-cat FCC could not be matched with theory. The physical properties of a highly porous Group A powder were changed deliberately in order to highlight under which conditions the fluidization behaviour is dominated by the IPFs. These results demonstrate how temperature can increase the effect of IPFs, causing a Group A material to behave in a similar manner to a Group C material. The experimental values obtained for the minimum fluidization velocity are reported and compared with prediction. Results obtained from the bed collapse test are also presented. The dense phase collapse rate is compared with prediction obtained using the modified form of the Richardson–Zaki equation. The results emphase that a purely hydrodynamic equation can predict the fluidization behaviour where the IPFs do not show a dominant role over the HDFs.

The influence of horizontally placed internals on the physical behaviour of a dense fluidized bed has been observed using an X-ray technique. The internals were sieve-like baffles with an aperture size two orders of magnitude larger than the diameter of the bed particles. Both a freely bubbling bed and single injected bubbles were observed. For the latter, the bubbles were made to emerge from a layer more opaque to X-rays into one more transparent. In this way, the flow pattern around the bubble and the wake could be observed. The results show that the baffles do not influence the mean bubble size or velocity significantly, confirming earlier indirect observations. It is also shown that the wake material is removed from the fluidization bubbles by the baffles. Results are given as statistics for bubble size and rise velocity as a function of the fluidization velocity with and without baffles present, and sequences of X-ray images showing the effect of the baffles.

Experiments have been performed in which a fluidized bed in a metal container was tilted and observed with X-rays. Previous experiments using such a system emphasized the importance of interparticle forces in determining bed behaviour. In the present work these forces were controlled through the use of a metal container and by varying the humidity of the fluidizing gas. It was found that, as previously reported, when the container was tilted the surface of the bed also tilted; however, unlike in the previous experiments, the angle of tilt of the bed surface was nearly always less than that of the container. The two angles were closer if there was moisture in the fluidizing air. The angle of tilt at which the bed failed decreased with increasing gas velocity. A theory to explain the behaviour of the tilting bed is proposed.

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.

A physical model is suggested to explain the occurrence of a shell of varying porosity around a gaseous bubble rising in a fluidized bed. According to the model, this phenomenon is a direct consequence of the necessity to maintain the balance of forces acting on solid particles in the vicinity of the bubble. Both volume forces due to gravity, drag and inertia and surface forces caused by the particulate pressure are shown to be essential. The agreement between model predictions and experimental data is good apart from some difference in the particle concentration at the edge of the bubble void. Requirements of the mass, momentum and energy conservation at this edge regarded as a surface of discontinuity result in conclusions concerning the particle flux through the bubble surface and the solid concentration inside the bubble.

Measurements of bubble rise velocities in two gas fluidised beds have been compared with numerical simulations of spherical bubble-wake regions rising through Newtonian liquids. The comparisons have been used to elucidate the theological nature of the dense phase of gas fluidised beds. The investigation sheds new light upon results obtained in the past by other workers, who did not have the benefit of the powerful numerical simulation techniques of today.It was found that the assumption of a Newtonian viscosity coefficient of 0.8 Pa s (values around this have often been suggested in the literature) yields bubble rise velocities of the right order of magnitude. Nevertheless, the experimentally determined variation in bubble rise velocity with size cannot be explained fully on basis of a Newtonian rheological model, but indicates shear-thickening behaviour of the dense phase. A discussion, also of the effect of bubble Reynolds number and wall effects upon the dense phase flowpattern around rising bubbles, and comparison with other work is provided.

A comprehensive, up-to-date review is presented of predictive relationships for the terminal, free-fall velocity of solid particles falling in an infinite Newtonian fluid. The study explores accuracy of the implicit and explicit equations in terms of the drag coefficient and the terminal velocity. Problems of predicting the terminal velocity of non-spherical, isometric as well as non-isometric, particles is discussed.

The flow dynamics of a gas—solid mixture were investigated for downward flow of polyvinyl chloride powder in a vertical tube fitted with a fluidized bed on the top. Powder was charged from the top bed to the vertical lean flow part through perforated plates. Radially non-uniform distributions of powder flux, solid holdup and particle velocity were determined by powder sampling and photographic observation of the tube. A prediction method for the flow dynamics, including the choking phenomenon and the effect of scale-up, is presented in terms of a development of two-phase turbulent circulation theory incorporated with the turbulent kinematic viscosity.

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.

A freely-bubbling fluidized bed containing horizontal banks of tubes was observed by means of X-rays. The number of rows of tubes, the distances between them, the distance of the first row above the distributor and the fluidizing gas velocity were all varied, the size distribution of bubbles rising through the tube banks being recorded on cine film for subsequent frame-by-frame analysis. In many cases interaction between bubbles and tubes caused the bubbles to break up into two or more “daughter” bubbles, the sizes of which were determined by the ratio of the diameter of the tube to that of the parent bubble as well as by the horizontal separation between the tubes. Following break-up at a row of tubes bubbles grew by coalescence before reaching the next row above, whereupon further break-up occurred. From measurements with a single row of tubes a correlation was developed for the breakage rate and this was combined with an expression for bubble growth to give predictions of bubble size at any point in a bed fitted with tubes. The predictions were found to be in good agreement with the measurements made in the multitubular bed.

[1] Experiments were carried out to study the fluidization behaviour of ignimbrite at high temperature and while being mechanically agitated. Geldart group C behaviour occurs up to 565°C when the material is subjected to increasing gas flow (without agitation) from the loosely packed state. In contrast, even gentle mechanical agitation inhibits channelling and results in group-A type behaviour with homogeneous (non-bubbling) expansions of up to 30–40%. Bed collapse tests exhibit group-C behaviour at room temperature, group-A behaviour at 200–565°C, and transitional behaviour at 55°C. Both elevated temperature and mechanical agitation greatly increase the fluidizability of ignimbrite. It is inferred that a combination of high temperature and shear during transport will promote Geldart group A behaviour in pyroclastic flows.

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.

A process is under development for the steam gasification of biomass to produce a hydrogen-rich gas for use with a fuel cell to generate electricity on a local scale. A pilot plant is currently under construction in southern Italy operated with a circulating fluidized bed, and to predict the fluid dynamic conditions within the plant, a cold laboratory rig was built according to existing scaling laws, and experimental studies were carried out. In this paper, we present the experimental results concerning the solids residence time of particles introduced into the system and the particle mixing in the “gasifier” section of the model. Both parameters are of fundamental importance for the operation of the pilot plant as they determine the performance of the gasification process. It is shown that the biomass particles spend sufficient time in the gasifier to be fully gasified, and an equation is derived to predict the mean residence time of the biomass particles as a function of the dimensionless mass turnover ...

In order to investigate the potential of a four-compartment interconnected fluidized bed for the combustion of biomass, the behaviour of cylindrical pellets in a bed material of glass ballotini was characterized as a function of the operational parameters. This involved (a) studying the distribution of pellets over the compartments after a given fluidization time, (b) measuring the pellet circulation time and (c) following the dynamics of a radio-active pellet using Positron Emission Particle Tracking (PEPT). The circulation rate of the bed material itself was also measured to relate it to that of the pellets. The results show that control of the material circulation rate can be achieved either by varying the excess gas velocity in the two vigorously fluidized ('fast') compartments or the aeration rate in the two slow ones, the latter option being the better. The fast compartments were found to act as ideal mixers, while little mixing took place in the slow ones at the aeration rates used. Segregation of pellets relative to the bed material was observed, mainly in the slow compartments, in some cases leading to pellets getting stuck low in the slow beds. It was found that the best option for avoiding problems with segregation and controlling the circulation rate of both pellets and bed material was to aerate the slow beds at superficial velocities under the minimum fluidization velocity. The draw-back in doing so is that dead-spaces will develop in these beds. (C) 1999 Elsevier Science S.A. All rights reserved.

The object of the work described here was to elucidate the effects of operation under pressure on the physical behaviour of gas fluidized beds. Extensive measurements of various bubble properties such as size, shape and rise velocity in beds of coarse powders (mean particle diameters of 184 μm and 450μm) operated at pressures of up to 81 bar were made by photographing the images created by irradiation of the bed with X-rays, and analysing the bubble silhouettes thereby obtained. Most of the results presented here are averages of some 200 individual measurements. Experimental evidence to support the following picture of the effect of pressurization on the behaviour of freely bubbling gas fluidized beds is presented. Both bubble interaction (tendency to coalesce) and the incidence of bubble splitting increase with increasing pressure; the two are intimately connected. The nett results are a decrease in bubble size with increasing pressure over most of the pressure range and an increase in the tendency for b...

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.

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.

Experimental results from an X-ray study of gas bubbles entering a fluidized bed of two differently sized powders are presented and their significance for chemical reactions taking place in the distributor region of a fluidized bed reactor are examined using a simple two-phase model.

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

An experimental study conducted by University College London in which low concentrations of sulfur dioxide were removed from an air stream at 750/sup 0/F by reaction with cupric oxide supported on alumina in a 3.9-in. fluidized bed demonstrated that the most favorable conditions for fluidized-bed SO/sub 2/ removal from a flue-gas stream require the bed to be as deep and hot as possible and operated at the lowest gas flow rates compatible with the convenient removal of reaction heat. As the temperature increases, the reactor efficiency increases at a decelerating rate; above 730/sup 0/F, the experimental results agree well with theoretical values. Breakthrough times are insensitive to temperature.

Bed collapse experiments are interpreted by a three-phase model of a gas-fluidized bed. Application of the model leads to a relationship between bed voidage in the sedimentation zone of the collapse and the properties of the emulsion and bubble phases of the bed prior to the collapse. Experimental results show that for beds of small particles fluidized under pressure the flow of gas through the emulsion phase is higher than that expected on the basis of existing theories, but it is shown that average rise velocities of bubbles may be obtained from results of bed collapse measurements.

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 new method by which to determine the particle density of porous aeratable powders has been developed. It depends on measuring the expansion of a gas-fluidized bed of the powder in the region of gas velocity between the points of minimum fluidization and minimum bubbling and applying the Richardson-Zaki equation to the measurements. Excellent agreement is found between the results of the new method and those obtained by other, generally less convenient, techniques.

This contribution deals with the size of bubbles forming in a fluidized bed above an isolated jet streaming from a nozzle into the bed and expressed in terms of the bubble diameter dB . It is shown that the relation proposed by Rowe (1976) for a freely bubbling fluidized bed can be modified to enable a prediction to be made of bubble sizes above an isolated submerged jet. Bed superficial velocity Ub calculated from the constant area Sb of the fluidized bed along its height is substituted by the superficial velocity Uc calculated using the effective cross-sectional area of the heterogeneous jet Sc' taken as the horizontal cross-section of the cloud. The modified equation: has been verified experimentally. The variable z in the equation denotes the vertical height above a point where a hypothetical bubble would have zero diameter.

A slugging-bed reactor has been used to study the catalytic oxidation of o-xylene. Predictions of conversion based on published theoretical reactor models have been compared with the experimental results and it has been found that the model of Raghuraman and Potter suitably modified to account for reaction in the pre-slugging region of the bed gives the closest agreement. A further slight modification of the model to account for, possibly, reaction on particles inside slugs would however seem to be necessary.

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.

The purpose of the work described here was to study the interaction between horizontal tubes and gas bubbles injected singly into an incipiently fluidized bed of alumina powder. The bed was observed directly with X-rays and the bubble-tube interactions recorded on cine film for subsequent analysis. Various geometrical arrangements of tubes were investigated and both the horizontal and vertical distances between tubes were varied. In this way tube arrangements that led to bubble break-up and the resulting effect on the volumetric flow of bubble gas were established. The results indicate that under some circumstances horizontal tubes cause split bubbles to recombine thus nullifying the beneficial effects of splitting. In general when a bubble is split by a tube the sum of the volumes of the daughter bubbles is less than the volume of the parent indicating the some of the original bubble gas leaks into the emulsion phase of the bed during the splitting process. Furthermore the amount of leakage increases as ...

The two phase model of a freely bubbling gas-fluidised bed in which chemical reaction is occuring has been described in detail by Partridge and Rowe. It is based on knowledge of the physical behaviour of the system and of the chemical kinetics. The isomerisation of butene-1 to a mixture of cis- and trans-butene-2 is an ideal reaction with which to test the model for it is comparatively simple kinetically and reasonably well understood. The paper describes the kinetics and how the data was obtained. Experiments are described in which varying mixtures of butene-1 and inert nitrogen were passed through a bed of silica-alumina catalyst which it fluidised and in which the composition of the exit gas was determined. The results are described very well by the proposed model and confirm several of its details.

Previous studies have demonstrated the influence of the delayed addition of reactants on the selectivity of competing reactions. A gas fluidised bed may be considered to be a system in which delayed addition to the dense phase occurs through the agency of the rising gas bubbles and the effect of this on the selectivity of competing reactions of the first and second order is investigated in this paper: The results show clearly that in most cases selectivity to the first order product, P 1 , is enhanced by low interphase gas exchange rates. Furthermore there is an optimum value of this exchange rate for every set of kinetic parameters. Two phase model calculations based on realistic bubble size distributions show that the selectivity to P 1 is increased by the presence immediately above the distributor of relatively large bubbles.

This paper describes stimulus-response experiments with a fluidised bed of porous cracking catalyst in which two tracer gases, one adsorbable and one non-adsorbable, were used. It is shown that the moments of the response curves are strongly dependent on the degree of adsorption of the tracer gas, and that with an adsorbable tracer the dense phase gas is almost completely mixed at gas velocities in excess of 3.3 μ mf .

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.

Etablissement d'une relation elementaire entre la taille d'une bulle, l'energie cinetique des particules solides et l'erosion d'un tube, a l'interieur d'un lit fluidise