Introduction
Gas-solids fluidized bed reactors operated at high temperature are
widely used in industrial processes, including methanol-to-olefins
(MTO), fluid catalytic cracking (FCC), coal combustion, metallurgy, and
polymerization, because of their good performance in heat and mass
transfer.1-7 However, it is discovered that elevated
temperature could change fluidization behavior and, in severe situation,
cause defluidization in fluidized beds.1-6 For
example, particle agglomeration in polymerization fluidized bed reactors
induced by high temperature would lead to the reduction of fluidization
quality and sometimes undesired reactor shutdown.1-2In a metallurgy process, cohesion between particles would be enhanced
due to high temperature, which makes the particles be stuck together and
difficult to be fluidized.6 Therefore, it is very
important to understand the change in fluidization behavior, especially
fluidization transition in fluidized beds, induced by elevated
temperature.
Geldart classified gas-solids fluidization into four different groups
according to the density and size of particles.8 In
the past decades, researchers have investigated the influence of
temperature on fluidization transition between different Geldart groups.
Because of the lack of effective measurement techniques at high
temperature, most of the previous work was carried out based on indirect
measurement, e.g. pressure or pressure drop across a fluidized bed. For
instance, Botterill et al.9 studied fluidization of
sand between 380 and 2320 μm up to 960oC by measuring
pressure drop across a fluidized bed, and found that for small solids
between 380 and 530 μm the void fraction at incipient fluidization
increases with temperature. Lucas et al.10 confirmed
the increase in the minimum fluidization void fraction with elevated
temperature for particles with a narrower size distribution based on
pressure drop across a fluidized bed. Lettieri et
al.11 measured pressure drop across a fluidized bed
and standard collapse time and compared the fluidization behavior of FCC
catalysts from ambient conditions to 650°C. They found that the increase
in temperature affects both hydrodynamic and inter-particle forces and
thus causes the fluidization transition from Geldart A to C in the
fluidized bed. Shabanian and Chaouki12 investigated
fluidization of coarse particles at high temperature from 700 to
1000oC by pressure measurement, and revealed that the
bubbling fluidization of coarse particles at high temperature is
principally impacted by the varying gas density if the cohesive
inter-particle forces are negligible. They also pointed out that the
change in physical and/or physico-chemical properties of fluidized
particles and gas due to an increase in temperature should be
considered. In addition to pressure measurement, Cui et
al.13 used an optical fiber probe to measure the local
void fraction and average void fraction of dense phase to investigate
fluidization transition from Geldart A to B for FCC particles in a range
of 25-420oC. They showed that the increase in
temperature can cause fluidization transition from Geldart A to B, which
they attributed to the enhanced inter-particle attractive force and
reduced inter-particle repulsive force. However, both pressure
transducers and optical fiber probe provide indirect measurement and
cannot visualize flow regime transition.
Visualization of fluidized beds at high temperature is critical for
studying fluidization transition, especially for the onset of bubbles in
a fluidized bed, as the minimum bubbling fluidization is widely accepted
as the point, at which the first visual bubble appears. Raso et al.
designed a two-dimensional (2D) fluidization facility in an electrically
heated refractory furnace,14 and applied a video
camera to record the fluidization processes, up to
900oC. They confirmed the looser stable structure in
the fluidized bed even at zero gas velocity induced by enhanced
inter-particle forces at high temperature.14 Later,
Formisani et al. further explored the origin of the increase in the void
fraction of a packed bed with temperature, and argued that it is clearly
related to a variation in inter-particle forces with temperature and
classical correlation can be directly used if the dependence of void
fraction on temperature is correctly accounted for.15However, these results were obtained by visualization of 2D fluidized
beds, and may differ from the fluidization occurred in three-dimensional
(3D) fluidized beds, where it is difficult to obtain optical images.
Recently, Chirone et al. used an X-ray imaging system together with
pressure measurement to study the effect of temperature up to
500oC on the minimum fluidization velocity of
different Geldart powders (B, A and C) in a 3D fluidized bed of 140 mm
in diameter.16 The X-ray images can capture flow
structure, such as gas channels.16 However, the low
temporal resolution and high cost of X-ray imaging system hinder its
application in industry.
Electrical capacitance tomography (ECT) is based on capacitance
measurement, and is a visualization technique widely used to measure the
hydrodynamics of 3D fluidized beds, in particular, solids concentration
and distribution, bubble size and bubble rise
velocity.17-20 However, ECT has been mainly used at
ambient temperature because of the challenges in making high-temperature
ECT sensors and in dealing with the effect of temperature on capacitance
measurements and hence image reconstruction. In 2015, we successfully
developed high-temperature ECT sensors, which can withstand up to
1000°C,21-22 and showed that the high-temperature ECT
can work well with fluidized beds up to 800°C.23Recently, Wang et al. extended the application of high-temperature ECT
to measure a slugging fluidized bed of Geldart D powder up to
650oC.24
This paper describes first time high-temperature ECT applied to study
fluidization of silica particles with mean diameter of 222 μm and
density of 2650 kg/m3, which is typically Geldart B
powder under ambient condition. It is shown that high-temperature ECT
can visualize the onset of bubbles and fluidization transition of silica
particles from Geldart B to A at elevated temperature. Analysis shows
that the results agree well with previous studies in literature and the
transition is due to enhanced cohesive inter-particle forces induced by
high temperature.
Experimental set-up
Figure 1 shows the experimental set-up, i.e. a fluidized bed equipped
with a high-temperature ECT sensor, which can work up to
800oC.23 The fluidized bed is made
by a quartz tube of 48 mm inner diameter and 2 mm thick wall. Dried and
filtered air is controlled by a mass flowmeter with the range of
0~30 L/min, and supplied to the fluidized bed through a
porous plate gas a distributor of 1 mm thick. Silica particles were used
in experiments, which were first calcined at 600°C for four hours to
stabilize the physical and chemical properties. The Sauter mean diameter
of silica particles is 222 μm, which was measured using a particle size
analyzer (Mastersizer 3000, Malvern Instruments Ltd., UK). Figure 2
shows the size distribution and typical scanning electron microscope
image (SEM) of silica particles. The circularity of silica particles is
0.6144 which is measured by analyzing the SEM images. The sphericity of
silica particles is replaced by circularity following
Kanada.25 The physical properties and elementary
compositions were measured by a Philips Magix-601 X-ray fluorescence
spectroscopy (XRF) (see Table 1).