Sidorenko, I., & Rhodes, M. J. (2003) Observation of particle movement near the fluidised bed wall at different pressures. Paper presented at CHEMECA conference, Adelaide.

1. Observation of Particle Movement Near the Fluidised Bed Wall at Different Pressures*
I Sidorenko and M J Rhodes
CRC for Clean Power from Lignite
Department of Chemical Engineering, Monash University, VIC 3800
ABSTRACT
The influence of operating pressure on the motion of particles near the fluidised bed wall surface was studied using luminescent pigment as bed solids in a vessel of diameter 146mm. A pulse of bright light transmitted from outside of pressure vessel via fibre optics was used to illuminate a 7mm diameter region of the bed particles adjacent to a transparent vessel wall. After illumination these particles showed an afterglow for several seconds which was recorded on digital videotape. Typically the illuminated particles remained visible as a bright spot decreasing in intensity with time and in bubbling fluidised bed the spot shifted along the wall surface. Digital image analysis of the movement of the spot gave its statistically determined velocity along the surface and the decay in luminosity defined the particle exchange frequency in the direction perpendicular to the wall surface. Understanding of the effect of pressure on the motion of particles at the wall permits better understanding of the effect of pressure on bed-to-surface heat transfer.
INTRODUCTION
One of the attractive features of bubbling fluidised beds from a design viewpoint is their excellent heat transfer property. Gas-to-particle heat transfer is normally very efficient due to the high surface area of the particulate phase. More interesting is heat transfer between the bed material and solid surfaces such as walls or immersed surfaces.
In bubbling fluidised beds the total bed-to-surface heat transfer coefficient is presented as a sum of three independent components – particle convective, gas convective and radiative [1]. The main factors in the heat transfer between a surface and a fluidised bed are movement of particles close to the surface and their residence time at the surface. The good heat transfer properties of fluidised beds are the result of the high heat capacity of bed particles and their mobility. At temperatures below 873K, when the radiative heat transfer component is negligible, the particles in the bulk of the fluidised bed exchange heat by the gas phase conduction and stay in the bulk of the bed long enough to reach the same temperature as their neighbouring particles.
When some of the particles are swept into close proximity with the heat transfer surface, there is a high local temperature gradient between the surface and the particles. The longer the particles stay at the surface, the more their temperature approaches the surface temperature which leads to reduction in the local temperature gradient and the effective rate of heat transfer. Highest rates of heat transfer between a surface and a fluidised bed are obtained therefore, when there is rapid exchange of material between the adjacent to the heat transfer region and the bulk of the bed, i.e. when particle residence times at the surface are very short. However, it is common for vertical surfaces to become covered by the downward return flow of solids. In 1953 Toomey and Johnstone [in 1] described the now well-known appearance of particle motion close to the wall, in which there is upward flow of particles through the centre

2. of the column induced by bubbles, and comparatively slow downward flow at the wall. In general, material adjacent to the wall is only occasionally disturbed by rising bubbles and slugs which penetrate to the wall.
For particles less than 500μm, pressure effect on the bed-to-surface heat transfer is considered to be negligible [1, 2]. According to Borodulya et al. [3], the weak dependence of the conductive – convective heat transfer coefficient on pressure in fluidised beds of small (less than 1mm) particles is a well-known experimental fact. However, according to [4], the heat transfer clearly depends on pressure within the range of particle sizes from 50μm to 1mm and the influence of particle motion on the heat transfer should manifest itself in a similar pressure dependence.
EXPERIMENTAL EQUIPMENT AND PROCEDURE
Experiments were conducted to investigate particle motion near to the fluidised bed wall at operating absolute pressures up to 2100kPa in a high-pressure fluidisation cold model facility described elsewhere [5]. The fluidised bed vessel was 146mm in diameter with transparent walls in Perspex. This vessel was inserted into a 2.38m high pressure vessel, equipped with five 100mm in diameter glass observation ports.
A pulse of bright light from a conventional photoflash with a simple adaptor was transmitted from outside of the pressure vessel via fibre optics and illuminated a 7mm diameter region of the bed material adjacent to a transparent fluidised bed vessel wall. A special 3mm diameter fibre optic light guide was designed to be able to withstand pressure inside of the pressure vessel. Using plumbing compression couplings, the light guide assembly was sealed through a pressure vessel flange. One end of the fibre optic was supported with a simple arrangement inside the pressure vessel and positioned next to the transparent bed wall in such way, that it was at approximately mid-height of the bed level and could be clearly seen through one of the observation windows. Direct observation of particle motion along the bed wall surface was possible through one of the observation ports. Experimental observation of the motion of particles near the wall surface was based on the method described in [4].
The luminescent pigment selected for filming particle motion near to the bed wall was inorganic luminescent pigment for visual effects Lumilux® with chemical composition of ZnS:Cu. It was in the form of relatively spherical particles with a surface-volume mean diameter of 62μm, and six kilograms of the pigment were used for charging the fluidised bed.
After illumination, the 7mm diameter spot, consisting of a cluster of particles, showed an afterglow for several seconds. A conventional digital video camera was mounted in front of the observation window and covered with some lightproof fabric. All the remaining observation ports were blocked so that it was completely dark inside and only the illuminated spot on the black background was visible through the camera viewfinder (Figure 1).

3. Figure 1. An example of the illuminated cluster of particles as seen on a video camera
Under fixed bed conditions the illuminated spot was still visible after three minutes. When the bed was fluidised, the illuminated spot shifted along the wall surface while its shape deformed and its luminosity decreased. However, the illuminated particles stayed in close proximity as a cluster, and the spot remained a single identifiable object until it disappeared. When an image disappeared from view, it could be assumed that, depending on gas velocity, the image either moved along the surface out of the camera view or its brightness diminished.
Experiments were carried out at different gas velocities up to fifteen times the minimum fluidisation velocity Umf under various pressure conditions. At predetermined operating pressure and gas velocity, an experiment proper consisted of illuminating a small cluster of particles with the photoflash and filming the spot until it disappeared. At each gas velocity, at least ten separate flashes were recorded; and the fate of each independent light pulse was analysed separately using image analysis techniques.
DATA ANALYSIS
The images of the illuminated spot were captured in a black-and-white mode by a digital video camera at a rate 25 frames per second. Digital videos were downloaded on a dedicated computer and edited. Image analysis was then carried out in order to quantify the statistics of particle movement near to the wall surface. Videos were first processed in a way that all the frames were extracted and separate frames were organised in stacks for each light pulse. For each experimental condition, stacks of frames were analysed, and each pixel of the image was characterised by its X and Y coordinates and luminosity, expressed in dimensionless form on a greyscale, where 0 is black and 255 is white. From filming a reference ruler in daylight it was determined that on linear scale each pixel was equal to 0.25mm.
At the packed bed conditions, it was observed that the cluster of illuminated particles remained visible as a still spot decreasing in intensity with time. The initial step in digital data analysis involved discrimination of illuminated spots from the rest of the bed material. In general, the procedure for this image identification involves examination of the greyscale values histogram for a normal image consisting of both illuminated spot and dark background. Since the lighting conditions were uniform and image contrast was quite high, the accurate detection of the image boundary was possible using the global thresholding method [6].
On each frame, the following data for the illuminated spot was obtained – image area in mm2; mean, maximum and minimum luminosity within the image threshold on a greyscale; and X– Y coordinates of the centre of gravity of the image. More than 160 thousand files were processed and the results were organised for separate light pulses at each gas velocity. Further statistical analysis was performed, where for each gas velocity data from ten flashes were averaged, plotted and nonlinear regression was applied. Since the detection of clusters depended on their visibility, it could be possible that the disappearance of images resulted from loss of material luminosity with time or possibly from clusters mixing. A simple

4. analysis showed that the light source had enough power to sufficiently illuminate a spot and the material had enough luminosity such that the illuminated object could still be observed by the end of each test.
The effect of particle motion in a bed, fluidised at gas velocity U equal to four times Umf, on cluster maximum luminosity compared to that of packed bed is illustrated in Figure 2. In both cases the luminosity decay was found to be exponential in time, however, the rate of decay was much higher in the fluidised bed. Based on such large difference between the natural material luminosity decay and the luminosity loss during fluidisation, it was assumed that under the experimental conditions the luminosity decay in a fluidised bed is only caused by the fact that the illuminated cluster particles move away from the wall and are replaced by fresh particles. 020406080100050100150200250packed bedfluidized bedtime (s) image brightness on greyscale
Figure 2. Decay of maximum image luminosity in a packed bed compared to a fluidised bed (U = 4Umf) at ambient conditions
In all cases the luminosity decay was found to be exponential in time. A number of linear and nonlinear regression models was tested and it was found that a one phase exponential decay model [7, 8] predicted the experimental data very well. The following equation was used for the decay model:
(1) 0()exp()ththLLLLkt−=−−
Where the function of luminosity on a greyscale (L) starts at an initial level of span (L0–Lth) above constant plateau (Lth) and decays with time (t) to the plateau (Lth) at a rate constant (k). The value of the plateau (Lth) was determined by the threshold applied to the images during the processing.
As illustrated in Figure 2, the increased rate of decay in luminosity in a fluidised bed, compared to that in a fixed bed, is attributed to the fact that the originally illuminated particles move away from the wall during the fluidisation process and are replaced by particles coming

5. from the rest of the bed. Based on the rules for conditional probability, Molerus and Wirth [4] deduced an equation for the exchange frequency of particles f perpendicular to a solid surface. In this work, the rate constant k corresponds to the particle exchange frequency f defined by Molerus and Wirth [4]. The mean residence time of illuminated particles near to the wall was deduced by Molerus and Wirth [4] as a reciprocal of the particle exchange frequency f. The mean residence time τ, was established in this work as a reciprocal of rate constant k.
PARTICLE MOTION AT VARIOUS PRESSURES
As can be seen in Figure 3, mean residence times measured close to the wall decrease with increasing gas velocity in the range three to ten times Umf, and become practically independent of gas velocity in vigorously bubbling fluidised bed. The values of the mean residence times settle at below 0.1s, which means that during the experiments the illuminated images disappeared within only three frames on video and could not be measured with higher accuracy. Considering the experimental error, it is not possible to establish any significant pressure effect on particle residence times at the wall in a vigorously bubbling bed. 0.000.010.020.030.040.050.00.10.20.30.40.5101kPa500kPa700kPa900kPa1700kPa1900kPa2100kPagas velocity (m/s) mean residence time at wall (s)
Figure 3. Mean time between particle illumination and departure from the wall versus gas velocity at various absolute operating pressures
As previously mentioned, the highest rates of heat transfer between a surface and a fluidised bed are obtained when particle residence times at the surface are very short. Therefore, from Figure 3 it can be estimated that the typical mean residence times measured close to the wall at maximum heat transfer are practically independent of pressure and approximately equal to 0.1s. This, however, does not agree well with the experimental data presented by Molerus and Wirth [4], who observed the following mean residence times of illuminated particles with the size of 50μm at vertical solid surfaces in a fluidised bed at maximum heat transfer – 1.28s at 100kPa, 0.35s at 500kPa, 0.52s at 1000kPa and 0.45s at 2000kPa. Contrary to their claim that the heat transfer clearly depends on pressure and expectations of a similar pressure dependence on particle motion, there is no particular trend in their data, but the magnitude is considerably higher than the values measured in the present work.
Figure 3 shows that at superficial gas velocities below 0.015m/s, the mean residence times slightly decrease with pressure, and therefore, the particle convective heat transfer component

6. is expected to increase slightly with elevated pressure. However, with further increase in gas velocity the mean residence times reach a minimum value and are not influenced by pressure. Thus, the particle convective heat transfer coefficient does reach a maximum in a bubbling bed, which must be dependent only on particle physical properties and not on operating pressure or superficial gas velocity despite the fact that this maximum occurs at a certain gas velocity.
CONCLUSIONS
The highest rates of surface-to-bed heat transfer are obtained when there is rapid exchange of material between the adjacent to the heat transfer region and the bulk of the bed, i.e. when particle residence times at the surface are very short. For Geldart A solids mean residence times measured close to the wall decreased with increasing gas velocity and became practically independent of gas velocity in vigorously bubbling fluidised bed.
This observation of the pressure effect is in line with findings of other workers [1, 9-11] who did not observe much variation in the particle convective heat transfer for small particles at pressures above atmospheric. This, however, does not agree well with Molerus and Wirth [4].
ACKNOWLEDGEMENTS
We gratefully acknowledge the financial support received for this research from the CRC for Clean Power from Lignite, which is established under the Australian Government’s Cooperative Research Centres Scheme.
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