PERFORMANCE EVALUATION OF DRYING CHARACTERISTICS IN BFB DRYERS
6.3 DRYING CHARACTERISTICS
bed was found to be lower at different subsequent taps than the pressure drop of Thant et al.
(2018) [123]. The pressure drop in both cases (with and without a spiral) decreases with the increase in cone angle [Figs. 6.7 (a) and (b)]. For the same mass of particles, as the cone angle increases, the area of the cross-section along with the height of the dryer increases, causing the concentration of paddy per unit area of the cross-section to decrease. Because of the different cross-sectional areas, the static bed height of particles differs for all dryers when the same amount of bed inventory is used. As a result, the pressure drop was found to be lower in a dryer with a large cone angle. From the figures, it can also be revealed that the effective height of pressure drop was lower for the larger degree of cone angle. This was because the velocity gradient along the height of a conical dryer increases as the cone angle increases due to the increase in cross-sectional area along the height. As a result, the paddy particles were unable to expand to a higher height. Hence, the effective height of the pressure drop was lower for the larger degree of cone angle conical dryer.
reduction in drying time prevailed when the inlet air temperature increased from 55 to 65°C.
The effect of superficial air velocity on drying characteristics is also shown in Fig. 6.9.
Figure 6.9: Effect of superficial air velocity on drying characteristics
For the comparison, three different superficial velocities of air were considered: 1.1, 1.6 and 2.1 m/s. However, the paddy inventory and temperature were kept at 2.5 kg and 55°C, respectively. It was revealed from the figure that as the superficial air velocity increases, the drying time decreases. The drying time of paddy grains was approximately 22.5 mins for the highest value of air velocity. However, for a superficial air velocity of 1.1m/s, it takes nearly 37 mins to dry the paddy grains. This is due to the fact that at high values of superficial air velocity, more heat is carried by the flowing air and vigorous mixing of air and paddy particles prevails, resulting in high heat transfer between air and paddy grains. As a result, drying time was reduced at high-velocity values. Apart from this, the effect of bed inventory on drying characteristics is also presented in Fig. 6.10.
Figure 6.10: Effect of bed inventory on drying characteristics
For that, the bed temperature and superficial velocity of air were kept constant at 55°C and 1.6 m/s, respectively. It was observed from the figure that as the bed inventory was increased with constant velocity and inlet air temperature, the drying time increased. This is because when the bed inventory increases at a constant velocity, the turbulence of particles and air decreases which is accompanied by the decreasing fluctuation ratio, resulting in less vigorous mixing of particles and the air. The distribution of paddy per unit cross-sectional area was more for an increased amount of bed inventory, and thus, heat transfer between air and paddy was lower at a constant velocity. The effect of mixing sand with paddy on drying characteristics is shown in Fig. 6.11.
Figure 6.11: Effect of mixing of sand with paddy on drying characteristics
It is seen from the experimental results that mixing sand with paddy takes less time to reduce the moisture content. The sand particles have a higher heat capacity than paddy particles, allowing them to retain more heat from hot inlet air, increasing interphase heat transfer.
Moreover, conduction heat transfer also increases between particles, as the thermal conductivity of sand is higher than that of paddy particles. Furthermore, due to the smaller size of sand particles than paddy particles, the sand particles are easily exhaled by the hot air, and as they are in contact with the paddy particles, the sand particles transfer heat to the paddy particles. As a result, the drying time was reduced when sand particles were mixed with paddy particles. When the sand was combined with paddy particles, the drying time was reduced by 13.33%. The drying characteristics plots in terms of moisture content vs drying time graph with spiral and without a spiral are presented in Figs. 6.12 and 6.13.
Figure 6.12: Effect of a spiral on drying characteristics in a conical dryer with 5 cone
angle
Figure 6.13: Effect of a spiral on drying characteristics in a conical dryer with 10 cone
angle
The results showed that the incorporation of a spiral improves the drying characteristics of both dryers. The particles were in direct contact with the spiral inside the dryer, resulting in unsteady-state heat conduction. However, for a dryer without a spiral, heat transfer takes place between particles and heated air only. Furthermore, the spiral increases the turbulence in mixtures and makes the heat transfer rate faster. Hence, drying time was reduced in the case of the dryer with a spiral. When a spiral was used, there was a nearly 16.67% reduction in drying time. The effect of cone angle on drying characteristics without and with a spiral is shown in Figs. 6.14 (a) and (b).
(a) (b)
Figure 6.14: Effect of cone angle on drying characteristics, (a) without a spiral, and (b) with a spiral In Fig. 6.14 (a), the result of two conical dryers was compared with Thant et al. (2018) [123].
It is seen from Figs. 6.14 (a) and (b) that a higher cone angle reduces the drying time for the same amount of bed inventory. When the same amount of inventory was used at a higher value
of cone angle, the static bed height was less. Due to the lower static bed height, the turbulence of particles is more for a conical dryer with a higher cone angle which enhances the heat transfer between the fluid phase and particulate phase. Hence, faster is the drying rate. It was observed that a higher cone angle dryer with a spiral had a drying time of 25 mins. However, a conical dryer with a smaller cone angle takes almost 30 mins to dry, i.e. 20% more drying time than that of the larger cone angle conical dryer. On the other hand, the cylindrical dryer takes nearly 34 mins to get the desired drying.
6.4 ENERGY CONSUMPTION