## 4 Experimental Results and Discussion
### Effect of Bed Particle Size  
In this section effect of particle size on the hydrodynamic characteristics of a fluidized bed was investigated. Pressure drop variation with superficial air velocity was investigated with a varying particle size of bed material. Figure 8 describes the pressure drop variation across the bed with superficial air velocity for different distributor plates at fixed particle size bed material. Three different types of bed material were used with different particle sizes, such as 250, 590, and 850. Bed pressure drop across the bed increased for coarse bed particle size and decreased by employing smaller particles. This effect is due to the formation of relatively larger bubbles in coarse particle bed. When ascending along the fluidization column, these larger bubbles create a larger and longer wake behind them. So due to the formation of this larger suction area formation behind bubbles, many large and small eddies are generated, resulting in form drag due to eddy dissipation and, consequently, loss of total energy of the fluidizing medium. Many researchers also found a similar trend both computationally and experimentally that by increasing particle size, bed pressure drop tend to increase [33].  
Conversely, the most probable description of less pressure drop across the bed for smaller-sized bed particles is that the small particles need less pressure to rise. This explanation is also illustrated by [PERSON] [34] in his work on hydrodynamics in a fluidized bed dryer. This effect can also be enlightened by inter-particle forces effects on air bubbles entrainment. Such as, when air passes through smaller particles, particle packing density may decrease due to continuous loading of fluidizing medium and consequently lead to  
Figure 7: Experimental observation of initial bed height for perforated plate distributor.  
particle-particle loose interaction. Hence smaller size air bubbles entrainment possibly will occur [35]. Therefore, early fluidization results in lower minimum fluidization and lower pressure drop for smaller particles. For example, [PERSON] [36] revealed similar information computationally about the pressure drop rise with the particle size increase and vice versa. On the other hand, additional bed expansion was also observed in smaller particles due to the higher uplift of smaller particles for a similar air flow rate.  
Figure 8: _Cont._  
Moreover, minimum fluidization velocity also alters with the change in particle size. The figure above shows that minimum fluidization velocity decreases for smaller particles, as smaller particles are susceptible to fluidization at lower volumetric flow rates [37]. This effect mainly happened due to premature vibrations of smaller particles due to smaller bubbles' entrainment and approaching the bed surface, leading to early initial minimum fluidization velocity. Furthermore, by observing the above graphs of bed particle size with different distributor plates, it can be revealed that the highest-pressure drop is observed with a 45\({}^{\circ}\) slotted plate with all three-bed particle sizes. However, minimum fluidization velocity is found to be highest for 45\({}^{\circ}\) slotted plates and lowest for perforated plates, and intermediate for novel hybrid plate distributors, as shown in Figure 8a-c. Therefore, using smaller particles with a 45\({}^{\circ}\) slotted plate is advantageous in reducing the fluidized bed's overall pressure drop and minimum fluidization velocity. Also, a novel hybrid plate would be recommended for a broader range of particle size particles, such as a mixture of smaller and larger particles, as intermediate pressure drop and minimum fluidization velocity has found for novel hybrid plate.