Search

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 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.

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 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.

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 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 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

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.

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.

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 ...

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.