## 3 Results and Discussion
Increasing the velocity further causes particles to move and rearrange, which is reflected in the depression seen in the pressure drop profile. Finally, the pressure drop attains a steady value at high flow rates as the bed of activated carbon becomes fully fluidized. The pressure drop in the initial portion of the pressure drop was correlated with the Ergun equation, which computes the effective hydrodynamic diameter of the resident solid particles. Its value was found to be 27.47 \(\upmu\)m, which is very close to 26.4 \(\pm\) 1.1 \(\upmu\)m obtained from the particle size analysis (Table 1). Although the early onset of fluidization was introduced in the bed at approximately 0.2 mm/s, the bed appeared to be fully fluidized only when the gas velocity is 1 mm/s. Figure 6:1 Effect of the addition of external particles on the fluidization hydrodynamics of the monocomponent bed and mixed bed of calcium hydroxide and sand. The effect of the mixing of external group B particles is shown in Figure 6:2 Here, we broadened the scope of our work by varying the fraction of external particles. Four different cases were considered by gradually increasing the amount of external particles by adding 25, 50, 75, and 150 g of sand in a bed containing 500 g of activated carbon. Evidently, the initial slope of the pressure drop profile remained largely unaffected by the particle mixing. However, a substantial change in the pressure drop profile for velocities higher than 0.2 mm/s was observed. The depression seen in the pressure drop profile was substantially attenuated even with the addition of 25 g of external particles. The trend is similar when a high proportion of external particles is added. This finding can be attributed to the elimination of non-homogeneities from the bed due to the addition of external particles. However, the pressure drop at low velocities was substantially higher when 150 g of external particles were added. The result is due to the bed contraction leading to a decrease in the bed void fraction, which is reflected in the high pressure drop owing to a strong dependence of the pressure drop on the bed void fraction [7]. However, as the velocity was increased further, the pressure drop decreased and became constant due to the complete fluidization. At high velocities, the low fraction mixed bed (25 g sand) did not show any improvement as the pressure drop fell as low as 600 Pa due to the gas bypassing caused by the formation of channels in the fluidized bed. Further addition of 25 g external particles nonetheless significantly improved the fluidization behavior at low and high velocities. The behavior at high velocities became comparable when the amount of external particles was increased from 75 g to 150 g. At low velocity, the pressure drop for 150 g of sand was high due to the bed contraction phenomenon [30; 31]. The high pressure drop at high velocity, when the bed was fully fluidized, was due to the addition of sand, leading to an increase in the effective weight of the bed. Figure 6:3 Fluidization behavior of the monocomponent bed of activated carbon at ambient conditions using nitrogen gas. Figure 12 shows the fluidization index for the bed of activated carbon. Evidently, the FI is below unity when the bed of activated carbon contains either no external particles or very small fraction. The addition of 50 g of eternal particles significantly enhanced the fluidization hydrodynamics as FI is very close to unity. Further addition of 25 g helps to improve the fluidization behavior of the mixed bed. However, not much improvement was observed when a total of 150 g of external particles were added to the bed. The \(\chi^{2}\) values were 0.1512, 0.6788, 0.0393, 0.0172, and 0.0572 for 0, 25, 50, 75, and 150 g of sand, respectively. In terms of the weight percentages, the corresponding values are 0, 5, 9, 13, and 23. Furthermore, the optimum fraction of external particles in our case was 13 wt%.