## 2 Experimental
#### Characterization of Solid Particles  
Two different particle samples belonging to the Geldart group C classification were used in the present work. One was a high-surface-area porous activated carbon (Avonchem, Cheshire, UK), which is commonly used as an adsorbent for a wide variety of environmental applications. As shown in Figure 2, its particle size analysis reveals a wide size distribution, varying in the range of 0.2 to 200 \(\upmu\)m. The volume-mean diameters for three differently-sized analysis runs were 26.14, 25.41, and 27.55 \(\upmu\)m. The SEM image of the activated carbon is shown in Figure 3.  
Another group C particle sample used was much smaller than calcium hydroxide with a mean particle size of 1.08 \(\pm\) 0.23 \(\upmu\)m. Its SEM image, shown in Figure 4, reveals a wide size distribution owing to agglomeration.  
Figure 3: SEM image of activated carbon at a magnification of 20,000\(\times\).  
Figure 2: Particle size analysis of the powdered activated carbon sample.  
The sample of external inert particles, i.e., sand, was obtained by carefully sieving field sand in the lab and collecting the sample retained between 300 \(\upmu\)m and 212 \(\upmu\)m sieves, thus obtaining sample with a mean size of 256 \(\upmu\)m. Its SEM image is shown in Figure 5.  
The physical properties and the theoretical minimum fluidization velocities of all the particle samples are presented in Table 1. To compute the theoretical values of the minimum fluidization velocities, the pressure drop evaluated using the Ergun equation was equated with the fluidized bed pressure drop as follows:  
\[\frac{150\mu U_{mf}}{d_{p}^{2}}\frac{\left(1-\epsilon_{mf}\right)}{\epsilon_ {mf}^{3}}+\frac{1.75\rho_{f}U_{mf}^{2}}{d_{p}\epsilon_{mf}^{3}}=g\Big{(}\rho _{p}-\rho_{f}\Big{)} \tag{1}\]  
Figure 4: SEM images of calcium hydroxide at a magnification of 1000\(\times\).  
Figure 5: SEM images of inert sand samples used at magnification of 100\(\times\).  
where subscript '_mf_' signifies the incipient fluidization conditions, \(d_{p}\) is the particle diameter, \(\mu\) is the gas viscosity, \(\varepsilon\) the bed void fraction, \(U_{m\!f}\) is the minimum fluidization velocity, \(g\) is gravitational acceleration, and \(\rho_{p},\rho_{f}\) are particle and fluid densities, respectively. The Geldart classification of different particle samples used is presented in Table 1 and Figure 6. The resident solid particles of the fluidized bed belong to group C, whereas the external inert sand particles belong to group B. As mentioned earlier, the selection of group B particles for mixing with group C particles will force the mixture fluidization behavior like group A, which shows a particulate fluidization behavior. The selection of group C particle samples greatly differed from one another as the larger activated carbon lies at the A-C boundary, whereas the smaller calcium hydroxide was as much as 25 times smaller as compared with its larger counterpart.  
The fluidized bed was first loaded with 500 g group C particles. The initial static bed height was 300 \(\pm\) 5 mm for the case of calcium hydroxide. On the other hand, it was 225 \(\pm\) 3 mm for the bed of activated carbon.