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).