Figure 9. Cohesive inter-particle forces induced by high temperatures: (a) cohesive inter-particle force \(F_{C}\) and (b) ratio\(\frac{F_{C}}{F_{N}}\).
The elevated temperature may influence the fluidization behavior in a fluidized bed via two aspects. On the one hand, it may change the gas properties, such as gas density and viscosity, which essentially affect the hydrodynamic interaction between particles and gas. On the other hand, it can alter the surface property of particles and thus cause varying cohesive inter-particle forces. Based on the above discussion, it can be concluded that for the silica particles used in this study, at relatively low temperature the effect of cohesive inter-particle forces plays a minor role, and so the change in gas properties due to elevated temperature is responsible for the fluidization behavior. At relatively high temperature, the change in particle surface properties which causes variation of cohesive inter-particle forces due to the variation in temperature becomes very important. In summary, both aspects make a coupled yet complicated contribution to the fluidization transition from Geldart B to A as observed in this work.

Conclusions

By the use of a recently developed high-temperature ECT, we first time observed that the onset of bubbles in a fluidized bed with silica particles, which typically manifest Geldart B fluidization at ambient temperature, but shift to a lower superficial gas velocity at a higher operation temperature. Further analysis of standard deviation of solids concentration derived from ECT images, as well as the pressure drop profile, suggests that \(U_{\text{mb}}/U_{\text{mf}}\) increases from 1.0 to around 1.2 as the increase in temperature from 20 to 600ยฐC. This indicates that there exists fluidization transition from Geldart B to A induced by high temperature for silica particles studied in this work and the visualization of homogeneous fluidization regime by ECT. Our results showed that the elevated temperature influences the fluidization behavior in a fluidized bed via two aspects: (1) changing gas properties, such as gas density and viscosity and (2) altering surface property of particles and thus cohesive inter-particle forces. At relatively low temperature the effect of cohesive inter-particle forces plays a minor role, and so the change in gas properties due to increase in temperature is responsible for the fluidization behavior. At relatively high temperature, the enhanced cohesive inter-particle forces because the change in particle surface properties becomes much more important. Both aspects make a coupled yet complicated contribution to the fluidization transition from Geldart B to A for silica particles. These results agree well with previous studies in the literature. The advantages of using high-temperature ECT to investigate the fluidization behavior include high temporal resolution, robust, reliable, and low cost. Therefore, high-temperature ECT is promising and is expected to open up a new approach to in-situ measurement of 3D fluidized beds under real industrial operation conditions.

Acknowledgments

We are grateful to the National Key Research and Development Program of China (Grant No. 2018YFB0604904), the National Natural Science Foundation of China (Grant No. 91834302) and the Newton Advanced Fellowship of the Royal Society, UK (Grant No. NA140308).

NOMENCLATURE:

\(P\)๏ผšpressure drop
S: normalized sensitivity matrix
G: normalized permittivity distribution
N: Number of pixels in measurement zone, 3228 in this study
e: Error vector
P: Function operator
p: Normalized permittivity
s: Pixel area
Q: Number of frames of each measurement, 20,000 in this study
STD: Standard deviation of average solids concentration
๐‘ˆ๐‘š๐‘ : Minimum bubbling velocity, cm/s
๐‘ˆ๐‘šf : Minimum fluidization velocity, cm/s
\(\text{Re}_{\text{mf}}\ :\) Reynolds number at incipient fluidization
\(\varnothing\ \): Sphericity
\(d_{p}\): Sauter diameter of silica
Ar: Archimedes number
\(P_{t}\): Pressure overshoot
\(P_{c}\): Fluidized bed pressure drop
\(H_{\text{mf}}\): Bed height at incipient fluidization
\(F_{c}\): Cohesive inter-particle forces
\(F_{N}\): Hydrodynamic force
Greek symbols
g: Normalized capacitance vector
๐›ผ: Optimal step length
\(\beta\): Solids concentration
\(\theta\): Solids concentration of a packed bed
\(\varepsilon_{\text{mf}}\ \): Void fraction at incipient fluidization
\(\rho_{g}\): Air density
\(\mu\) : Gas viscosity
\(\rho_{p}\): Silica density
\(\sigma_{t}\): Tensile yield strength