Sidorenko, I., & Rhodes, M. J. (2002) The influence of pressure on fluidized bed behaviour. Paper presented at the World Congress on Particle Technology 4, Sydney.

1. THE INFLUENCE OF PRESSURE ON FLUIDIZED
BED BEHAVIOUR
Igor Sidorenko and Martin J. Rhodes
CRC for Clean Power from Lignite
Department of Chemical Engineering, Monash University, VIC 3800 Australia
ABSTRACT
The influence of operating pressure, up to 2200 kPa, on the fluidization fundamentals of
Geldart Group A and B powders were studied using a vessel of diameter 146-mm and
fluidizing gases air and nitrogen. Electrical Capacitance Tomography (ECT) was used to
characterise the influence of operating pressure on the bubbling behaviour. The effect of
operating pressure on minimum fluidization and minimum bubbling velocities and bed
expansion was also investigated.
The minimum fluidization velocity decreased with pressure for both Group A and Group B
materials and the minimum bubbling velocity for the Group A material increased with
pressure. For the Group B material, increasing operating pressure caused an increase in the
bed voidage, but had no influence on the either the amplitude or the frequency of voidage
fluctuations. For the Group A material, increasing operating pressure caused a decrease in the
bed voidage, the amplitude and frequency of the voidage fluctuations.
INTRODUCTION
The behaviour of fluidized beds at high pressures is of much interest from both practical and
theoretical points of view. It is important for optimal design and operation of the high
pressure fluidized bed reactors such as those used in coal gasification and combustion
processes. The elevated pressure increases the power output per area unit, and hence reduces
the size and the capital cost of equipment. Until the middle of the 1970s, there was very little
information about the effects of temperature and pressure on the operation of fluidized beds.
One of the reasons for that was, and still is, the high cost of building and operating of the
high-pressure laboratory research rigs. Also, some of the measuring techniques, common at
ambient conditions, cannot be used under severe conditions of high pressure.
Capacitance imaging offers a tool to actually see inside of the fluidized bed and clarify many
of uncertain or controversial aspects of bubbling characteristics. The advantage of
tomographic imaging is that it is a non-invasive technique that can give measurement of
voidage fluctuations over the entire cross-section of a vessel. Although the tomographic
imaging is a measurement technique already used in the area of fluidization, it is still under
development and has not been used for measurements under pressure.

2. Many researchers (1-6) have studied how increased pressure affects the minimum fluidization
velocity and found that the minimum fluidization velocity decreases with pressure for coarse
powders and is independent of pressure for fine particles. The trends of varying pressure can
be predicted by the Ergun equation or numerous correlations, however, quite often the
absolute values of predictions based on the correlations are significantly in error.
The dependence of the minimum bubbling velocity on pressure and type of gas is still
relatively unknown and findings in the literature are somewhat controversial. The available
correlation for the minimum bubbling velocity by Abrahamsen and Geldart (7) predicts an
increase of the minimum bubbling velocity with pressure, however some experimental studies
reported in the literature provide contradictory results.
The pressure effect on the minimum bubbling velocity has been experimentally studied by a
number of researchers (8-10). Guedes de Carvalho et al. (8) found that the minimum bubbling
velocity was independent of pressure for fine particles and decreased a little with increase in
pressure for coarser ballotini. Those results are opposite to the predictions from the
Abrahamsen and Geldart correlation and the experimental results of other researchers.
With Group A materials, the bed expands uniformly without bubbling as the gas velocity is
increased up to the minimum bubbling velocity, when it reaches a maximum height, and
gradually collapses to a minimum height with further increase in gas velocity. Then bubbling
dominates and the bed expands again with increasing gas velocity. The results of experiments
presented in (10) indicate that the total bed expansion increases substantially with pressure
increase from 100 to 1500 kPa. Contrary to these results Subzwari et al. (11) found that
raising the gas pressure to 600 kPa has relatively little effect on bed expansion, but further
increase in pressure to 700 kPa causes significant expansion.
A number of researchers studied how bed expansion is affected by increasing pressure in
fluidized beds with coarse powders (12-14) and observed the increase of bed expansion with
pressure, however, Llop et al. (14) reported that bed expansion increased significantly with
pressure but this influence, very strong at low pressures, seemed to reach a maximum at
approximately 800 kPa and decreased thereafter up to 1200 kPa.
Although some data on the effect of pressure on the behaviour of fluidized beds have been
obtained during the past 25 years, areas remain where further experimental work and analysis
would be valuable.
EXPERIMENTAL EQUIPMENT AND PROCEDURE
In this study, experiments were conducted to investigate fluidization fundamentals of Geldart
Group A and B materials (15) at operating pressures up to 600 kPa using compressed air, and
up to 2000 kPa using industrial and ultra-pure nitrogen. The fluidized bed vessel was 146-
mm-dia. with walls in Perspex. This vessel was inserted into a pressure vessel, 2.38-m-high,
equipped with 5 glass observation ports and capable of operating at pressures up to 2500 kPa.
2

3. The Geldart Group B material, silica sand, had a surface-volume mean of 195 mm, and
minimum fluidization velocity in air, experimentally determined at ambient conditions, of
3.15 cm/s. The Geldart Group A material, fluid cracking catalyst (FCC), had the surface-volume
mean of 93 mm and the minimum fluidization velocity in air, experimentally
determined at ambient conditions, of 0.28 cm/s.
The Electrical Capacitance Tomography (ECT) system used in this work is the Process
Tomography Ltd. PTL 300 unit based on design developed by the University of Manchester
Institute of Science and Technology (UMIST). The capacitance sensor was mounted to the
fluidized bed vessel in such a way that the sensor plane was situated 20 cm above the
distributor plate. Twelve guarded sensor electrodes mounted on the outside surface of the
fluidized bed vessel were connected via 24 sealed coaxial cables to a high-speed data
acquisition module DAM200 located outside the pressure vessel. A computer containing a
transputer motherboard was used to control the DAM200. The complete tomography system
was controlled by the PTL PCECT software.
The tomography system generated a cross-sectional image of the bed and showed the volume
fraction (the ratio of solids to gas) at sensor level. For each operating condition 16,000 frames
of data were logged and each frame was recorded by the ECT system at a frequency of
approximately 81 Hz. For each frame of measurements 66 capacitance values between
electrodes were recorded and the Linear Back-Projection algorithm was used for image
reconstruction. The output was in the form of 32x32 pixels matrix of normalized capacitance
values which was averaged to provide the average volume fraction. A single value between 0
for gas only and 1 for packed bed of solids was generated, representing the average solids
fraction for the entire cross-section of the bed. A true voidage value was then calculated and
presented in this study.
EXPERIMENTAL RESULTS AND DISCUSSION
The minimum fluidization velocity (Umf) was determined in a standard way from a plot of bed
pressure drop versus gas velocity. It is generally accepted that the minimum fluidization
velocity decreases with pressure for coarse powders and is independent of pressure for Group
A particles. Although the best method to determine Umf is by experiment, this is not always
easy, especially at different operating conditions.
The minimum fluidization velocity was found to decrease with pressure for both Group A and
Group B materials but the experimental values did not quite match with predictions of
empirical correlations available in the literature (16-18). In each case the experimental values
were lower than those predicted by empirical correlations. Some researchers (1,2 and 12)
found that values obtained from the Wen and Yu correlation were consistently smaller than
the experimental values.
3

4. It is accepted that in fluidised beds of coarse particles the minimum bubbling velocity (Umb)
is the same as the minimum fluidization velocity. Fine powders of Group A, however, have
the ability to expand smoothly and homogeneously without bubbling at much higher
velocities than the minimum fluidization velocity. Experimental determination of the
minimum bubbling velocity is subject to uncertainty since it is based usually on visual
observation of the first distinct bubbles. Another less subjective definition has been suggested
in N. Cheremisinoff and P. Cheremisinoff (19) as the velocity at which the maximum bed
height is observed.
Summary of our experimental results and their comparison with some correlations are
presented in Table 1 for Group A material and Table 2 for Group B material.
FCC Atmospheric pressure Elevated pressure (2000 kPa)
Umf, cm/s (Experiment) 0.28 0.20
Umf, cm/s (Wen-Yu’s correlation) 0.38 0.38
Umf, cm/s (Baeyens’ correlation) 0.46 0.38
Umb, cm/s (Visual observation) 0.39 0.47
Umb, cm/s (Maximum bed height) 0.54 0.55
Umb, cm/s (Abrahamsen-Geldart’s
correlation)
4
0.89 1.08
Table 1. Experimental and calculated minimum fluidization and minimum bubbling
velocities at atmospheric and elevated pressures for Group A material
Sand Atmospheric pressure Elevated pressure (1500 kPa)
Umf, cm/s (Experiment) 3.15 2.72
Umf, cm/s (Wen-Yu’s correlation) 3.29 3.02
Umf, cm/s (Thonglimp’s correlation) 3.65 3.30
Table 2. Experimental and calculated minimum fluidization velocities at atmospheric and
elevated pressures for Group B material
Time series of void fraction fluctuations in a fluidized bed measured by ECT were processed
by using the PTL PCECT software, Microsoft Excel and RRChaos software developed at
Delft University of Technology. The latter software has been designed to specifically analyse
experimental time series fluctuation in fluidized beds. This software calculates the relevant
time series statistics, such as average, average absolute deviation, number of cycles,
distribution of cycle frequencies, density distribution function and others.
Schouten and van den Bleek (20) proposed a test method for monitoring the quality of
fluidization using the short-term predictability of pressure fluctuations which combines
features of three types of time series analysis, statistical, spectral and chaos analysis. First, in
the test method the average absolute deviation is chosen as a statistical measure of the width
of the probability distribution function of the measured fluctuations. Second, the length of the
reconstructed points in state space is based on the average cycle time, which is related to the
average cycle frequency. And third, the comparison of the distributions of state space
distances through the degrees of predictability is a part of chaos analysis.

5. Although this analysis technique was originally proposed for pressure fluctuations time series,
we have applied it to the analysis of voidage fluctuations resulting from the ECT
measurements. The average absolute deviation Dx of the data points from the void fraction
average value is a robust estimator of the width of time series around the mean. The average
cycle frequency is the reciprocal of the average cycle time, which is defined as the average
time to complete a full cycle after the first passage through the time series average. Those
parameters should make it possible to compare results between different measurements since
their values are unambiguous and can be readily calculated.
The experimental results presenting average bed voidage, average absolute deviation of bed
voidage and average cycle frequency for Group A catalyst are shown in Figures 1-3, and those
for Group B silica sand are presented in Figures 4-6. The bed expansion results for the Group
A material in the bubbling regime in Figure 1 indicate that bed voidage decreases with
increasing pressure. As can be seen in Figure 1, for each pressure, bed voidage decreases first,
then increases with increasing gas velocity. The average absolute deviation of bed voidage is
less ambiguous; it increases as expected with increasing gas velocity and decreases with
increasing pressure for fine particles, as can be seen in Figure 2. Figure 3 shows that, once the
bed is steadily bubbling, the average cycle frequency of bed solids volume fraction
fluctuations do not change with increasing gas velocity. Increasing pressure causes a decrease
in the frequency of bed fluctuations.
The results of the experiments with the Group B material are different from those for the fine
powder. Figure 4 indicates that bed voidage for larger particles increases with increasing
operating pressure, and the average absolute deviation in bed voidage is independent of
pressure (Fig.5) as is the average cycle frequency (Fig.6).
CONCLUSIONS
The minimum fluidization velocity was found to decrease with pressure for both Group A and
Group B materials and the minimum bubbling velocity for the Group A material increased
with pressure. In each case the experimental values were lower than those predicted by
empirical correlations available in the literature.
This study also found that, for the Group B material, increasing operating pressure caused an
increase in the voidage of the fluidized bed, but had no influence on the either the amplitude
or the frequency of fluctuations in bed solids volume fraction. For the Group A material,
increasing operating pressure caused in general a decrease in voidage of the fluidized bed as
well as a decrease in both the amplitude and frequency of the voidage fluctuations.
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 Co-operative
Research Centres Scheme. Special thanks are also given to Akzo Nobel Chemicals
5
Pty Ltd for providing a Group A material.

6. REFERENCES
1. Knowlton T M (1977) High-pressure fluidization characteristics of several particulate solids,
primarily coal and coal-derived materials. AIChE Symposium Series 73(161), pp.22-28.
2. Saxena S C and Vogel G J (1977) The measurement of incipient fluidisation velocities in a bed of
coarse dolomite at temperature and pressure. Trans IChemE 55, pp 184-189.
3. King D F and Harrison D (1982) The dense phase of a fluidised bed at elevated pressures. Trans
6
IChemE 60, pp 26-30.
4. Rowe P N, Foscolo P U et al. (1982) Fine powders fluidised at low velocity at pressures up to 20
bar with gases of different viscosity. Chemical Engineering Science 37(7), pp 1115-17.
5. Nakamura M, Hamada Y et al. (1985) An experimental investigation of minimum fluidization
velocity at elevated temperatures and pressures. The Canadian Journal of Chemical Engineering
63, pp 8-13.
6. Olowson P A and Almstedt A E (1991) Influence of pressure on the minimum fluidization
velocity. Chemical Engineering Science 46(2), pp 637-40.
7. Abrahamsen A R and Geldart D (1980) Behaviour of gas-fluidized beds of fine powders, Part I.
Homogeneous expansion. Powder Technology 26, pp 35-46.
8. Guedes de Carvalho J R F, King D F et al. (1978) Fluidisation of fine particles under pressure.
Fluidization, (Cambridge University Press)
9. Piepers H W, Cottaar E J E et al. (1984) Effects of pressure and type of gas on particle-particle
nteraction and the consequences for gas-solid fluidization behaviour. Powder Technology 37, pp
55-70.
10. Jacob K V and Weimer A W (1987) High-pressure particulate expansion and minimum bubbling
of fine carbon powders. AIChE Journal 33(10), pp 1698-12.
11. Subzwari M P, Clift R et al. (1978) Bubbling behaviour of fluidised beds at elevated pressures.
Fluidization, (Cambridge University Press)
12. Chitester D C, Kornosky R M et al. (1984) Characteristics of fluidization at high pressure.
Chemical Engineering Science 39(2), pp 253-61.
13. Kawabata J, Yumiyama M et al. (1981) Characteristics of gas-fluidised beds under pressure.
Journal of Chemical Engineering of Japan 14(2), pp 85-89.
14. Llop M F, Casal J et al. (1995) Incipient fluidization and expansion in fluidised beds operated at
pressure and temperature. Proc Fluidization VIII, Tours (Engineering Foundation, New York)
15. Geldart D (1973) Types of gas fluidization. Powder Technology 7, pp 285-292.
16. Wen C Y and Yu Y H (1966) A generalized method for predicting the minimum fluidization
velocity. AIChE Journal 12(3), pp 610-12.
17. Baeyens J and Geldart D (1974) Predictive calculations of flow parameters in gas fluidised beds
and fluidization behavior of various powders. Proc Int Symp Fluidization and its Applications,
Toulouse.
18. Thonglimp V, Hiquily N et al. (1984) Vitesse minimale de fluidisation et expansion des couches
fluidisees par un gaz. Powder Technology 38(3), pp 233-53.
19. Cheremisinoff N P and Cheremisinoff P N (1984) Hydrodynamics of gas-solids fluidization. pp
137-206. (Gulf Publishing, Houston)
20. Schouten J C and van den Bleek C M (1998) Monitoring the quality of fluidization using the
short-term predictability of pressure fluctuations. AIChE Journal 44(1), pp 48-60.

7. 7
0.66
0.64
0.62
0.60
0.58
1000 kPa
0 0.02 0.04 0.06 0.08
gas velocity, m/s
bed voidage
1800 kPa
11400 kPa
600 kPa
200 kPa
Figure 1. Variation in bed voidage with gas velocity for Group A material (FCC) over a range
of operating pressures from 200 to 1800 kPa
0.016
0.012
0.008
0.004
0
1000 kPa
200 kPa
0 0.02 0.04 0.06 0.08
gas velocity, m/s
average absolute deviation
in bed voidage
1800 kPa
1400 kPa
600 kPa
Figure 2. Variation in average absolute deviation in bed voidage with gas velocity for Group
A material (FCC) over a range of operating pressures from 200 to 1800 kPa
5
4
3
2
1
0 0.02 0.04 0.06 0.08
gas velocity, m/s
average cycle frequency, Hz
200 kPa
600 kPa
1000 kPa
1400 kPa
1800 kPa
Figure 3. Variation in average cycle frequency with gas velocity for Group A material (FCC)
over a range of operating pressures from 200 to 1800 kPa

8. 8
0.68
0.64
0.60
0.56
0.52
0.48
0.44
0.00 0.05 0.10 0.15
gas velocity, m/s
bed voidage
ambient
100 kPa
200 kPa
400 kPa
500 kPa
1000 kPa
1600 kPa
Figure 4. Variation in bed voidage with gas velocity for Group B material (sand) over a range
of operating pressures from ambient to 1600 kPa
0.10
0.08
0.06
0.04
0.02
0.00
0.00 0.05 0.10 0.15
gas velocity, m/s
average absolute deviation
in bed voidage
ambient
100 kPa
200 kPa
400 kPa
500 kPa
1000 kPa
1600 kPa
Figure 5. Variation in average absolute deviation in bed voidage with gas velocity for Group
B material (sand) over a range of operating pressures from ambient to 1600 kPa
5
4
3
2
1
0.05 0.10 0.15
gas velocity, m/s
average cycle frequency, Hz
ambient
100 kPa
200 kPa
400 kPa
500 kPa
1000 kPa
1600 kPa
Figure 6. Variation in average cycle frequency with gas velocity for Group B material (sand)
over a range of operating pressures from ambient to 1600 kPa