Characterization of Dynamics of Binary Gas-Solid Flow of Particles with Different Density and Shape Using Digital Image Analysis

Owing to their excellent mixing characteristics, gas-solid fluidized beds are used in several industrial processes like coal and, biomass gasification and combustion. These industrial processes involve mixtures of particles with different size, density and, shape (eg. coal and inerts, coal and ash, biomass and inert used in gasification process). The difference in the physical properties (size, density, and shape) of particles leads to a non-uniform mixing and segregation phenomena at a particular range of superficial gas velocity and bed composition. As the local fluidization behaviour governs the mixing and segregation phenomena, mass and heat transfer mechanisms, and eventually the performance of the fluidized bed reactor, it is important to characterize the dynamics of binary gas-solid flow. Therefore, time-and space-resolved phase volume fraction measurements are important to comprehend the dynamic characteristics of flow, which can elucidate the effects arising due to the mixing of particles that differ in size, density, and shape.

The mixing and segregation phenomena of binary gas-solid flow was investigated by several researchers and the main focus of their work was to predict the bed dispersed height, minimum fluidization velocity (U_mf), and flow regimes. Goldschmidt et al. (2003) and Olaofe et al. (2013) quantified the dynamics of mixing and segregation phenomena of binary system differing in the size through percentage of segregation and average bed height of solid phases with respect to time. However, the dynamics of mixing and segregation of binary system differing in density and shape is not reported in the open literature. In addition to the measurements of time-evolution of percentage of segregation and average bed height of solid phases, it is also important to measure the bubbling characteristics (bubble size distribution and bubble rise velocity) because of its significance in influencing and understanding the flow behaviour. These detailed measurements are required in understanding and characterizing the dynamics of fluidization behaviour of binary system differing in density and shape and to develop and to rigorously validate the Eulerian CFD models to simulate the binary gas-solid flow.

The present work is carried in a pseudo 3-D (rectangular) fluidized bed because of its advantage of visualizing the flow behaviour. The objectives are (i) to measure the local individual solid volume fraction (α_s) of two binary systems, one differing only in density (of same size) and another one only in shape (of same density) from the grey scale images obtained by Digital Image Analysis technique, (ii) to characterize the dynamics of mixing and segregation behaviour using the local α_s measurements and (iii) to develop a methodology to measure the bubble size distribution and bubble rise velocities for a wide range of bed compositions and superficial gas velocities.

Experimental Set-up and Methodology

The experiments were performed in a rectangular column with height (H) of 1 m, width of 0.2 (m) and depth of 0.02 (m) with an initial static bed height of 25 cm. Air was used as the fluidizing medium and glass beads (GB) (d_p = 2.26 mm; ρ_p = 2500 kg/m³), mustard seeds (MS) (d_p = 2.18 mm; ρ_p = 720 kg/m³), cylindrical glass particles (CG) (d_p = 2 mm; L = 10 mm; ρ_p = 2500 kg/m³) were used as the solid phases. The experimental images were recorded with a high–speed monochromatic camera at 640 × 800 pixel² resolution and at a rate of 400 fps. The background was illuminated with a LED light and two solid phases were colored black and white to create a difference in the color intensities of the three phases. As a part of post-processing, the grey-scale images obtained are converted into RGB images and the local area fraction of three phases (two solids and one gas) was acquired using the image processing toolbox in MATLAB. The dynamics of fluidization of binary system-1 (GB: MS) and binary system-2 (CG: GB) in the weight ratio of 70%:30%, 50%:50%, and 30%:70% solids was investigated for superficial gas velocities (U_G) in the range 1.1-2 U_mf of dense phase where segregation and mixing of the particles was visually observed. Further details of the experimental set-up, methodology and the calibration procedure to identify all three phases (two solids and gas) from grey scale image will be presented in the full manuscript.

Results and Discussion

The time-evolution of percentage of segregation and average bed heights of individual solid phases are calculated from the RGB image and calculation criteria based on Goldschmidt et al. (2003) was used. For binary system-1 (GB: MS = 70:30 wt. %) at a superficial gas velocity of 1.73 m/s (1.1 U_mf of GB), it was observed that the time-evolution of average bed height of GB reached 15 cm from an initial static bed height of 25 cm after 25s, which elucidates that the lower portion of the bed is segregated and dominated by GB particles. This is due to the operating U_g is just above the U_mf of dense phase i.e., GB. The time-evolution of percentage segregation calculated was found to reach 60 % after 25 s and thereafter further change was not observed in the segregation pattern of the solid phases. The calculated quantities showing the final time required to attain the segregation and average dispersed bed heights reached by individual solid phases are in good agreement with the visual observations.

For binary system-2 with particles differing in shape, for a bed composition of CG: GB = 70:30 wt. %, segregation was not observed at lower U_G due to the channelling as the bed is dominated by CG particles. Therefore, experiments were performed at a higher U_G of 2.78 m/s where mixing was observed. In addition, from the information of the bubble area fraction and displacement of bubble with respect to time and axial location, bubble size distribution and bubble rise velocities are calculated and compared with binary system-1 at U_G = 2.78 m/s. The detailed measurements and analysis of the effect of U_G, and bed composition on dynamics of mixing and segregation, bubble size distribution and bubble rise velocity of the two binary systems will be presented in the full manuscript.

Conclusions

Local solid phase area fraction measured using high-speed imaging was used to characterize the segregation of binary gas-solid flow at lower gas velocities and fluidization behaviour at higher gas velocities. The effects due to the difference in solid phase physical properties (density and shape) and mixture compositions on dynamics of mixing and segregation of binary gas-solid flow are investigated for a wide range of superficial gas velocities. The dynamics of mixing and segregation were quantified in terms of time-evolution of percentage of segregation and average bed heights of individual phases. In addition to this, to compare the fluidization behaviour of binary gas-solid flow differing in density and shape, bubble size distribution and bubble rise velocities are measured at higher gas velocities. Detailed analysis of the aforementioned measurements will be presented in the full manuscript. The present work will be useful to understand the differences in dynamic behaviour of binary gas-solid fluidization caused due to difference in density and shape of particles. Further, the comprehensive set of experimental data presented will be used to validate the Eulerian CFD models.

Acknowledgments

One of the authors (Sirisha Parvathaneni) gratefully acknowledges the NETRA of NTPC Limited for providing the research fellowship.

References

Goldschmidt, M.J.V., Link, J.M., Mellema, S. and Kuipers, J.A.M. Power Technology, Vol. 138, 135 – 159 (2003).

Olaofe, O.O., Buist, K.A., Deen, N.G., van der Hoef, M.A. and Kuipers, J.A.M. Power Technology, Vol. 246, 695 – 706 (2013).