PERFORMANCE EVALUATION OF DRYING CHARACTERISTICS IN BFB DRYERS
6.2 HYDRODYNAMIC BEHAVIOUR
In order to investigate the bed hydrodynamics, preliminary experiments were carried out to calculate the pressure drop along the height of the three dryers. Experiments were carried out with three inlet air velocities (1.1, 1.6 and 2.1 m/s) and four-bed inventories (1, 2, 2.5 and 3 kg) and the results were compared. Numerical simulations were also carried out to compare the pressure drop between sand and paddy particles with the same operating parameters. The pressure drop of the two conical dryers was compared with the results of Thant et al. (2018) [123]. Furthermore, the effect of a spiral on pressure drop was investigated in the two conical dryers. Finally, the effect of cone angle with a spiral on pressure drop was investigated, and results were compared. In order to study the effect of the spiral on pressure drop, only one set of parameters was considered, i.e. mp = 2.5 kg and U0 = 1.6 m/s. For all investigations, the
difference in pressure drop was measured using water-filled differential U-tube manometers.
Twelve pressure taps separated by 10 cm were inserted above the distributor plate along the height of the three dryers, and the differential height was measured between two successive pressure taps. From the differential height, the pressure drops were calculated.
Figure 6.1 shows the variation of experimental and numerical pressure drops between two consecutive pressure taps along the height of the conical dryer (α = 10°) at different superficial air velocities (1.1, 1.6 and 2.1 m/s) and a constant bed inventory (mp = 2 kg).
Figure 6.1: Pressure drop along the height of a conical dryer with 10° cone angle at different superficial air velocities
It was observed from the figure that there was a decrease in pressure drop with an increase in the height of the dryer for all velocities. At a particular height of the dryer, the pressure drop was lower for higher superficial velocity. In Fig. 6.1, it was also observed that at dryer heights of 30 cm to 45 cm, the pressure drop was steeper for lower superficial velocity because the increasing air velocity continues to raise the gas volume fraction, which reduces the concentration of the bed material. Figure 6.1 also concludes that the effective bed height for increased superficial velocity is higher. Furthermore, the pressure drop between paddy and sand particles was compared numerically along the height of the conical dryer at a 10° cone angle with the operating parameter of air velocity. In this investigation, two air velocities, such as 1.1 m/s and 1.6 m/s, were considered. The bed height of both materials was taken as 10 cm.
The size of paddy and sand particles were taken as 2.5 mm and 0.435 mm, respectively.
Nonetheless, the influence of humidity and paddy moisture content was not taken into account.
Similarly, the experimental pressure drop of paddy was also compared with the numerical simulation. As a result, the graph of the pressure drop as a function of dryer height is shown in Figs. 6.2 (a) and (b).
(a) (b)
Figure 6.2: Comparison of pressure drop (a) between sand and paddy particles and (b) between experimental and 3-D simulation
It was observed from figure 6.2 (a) that the pressure drop for the sand particles was higher, as its density was higher than that of the paddy particles. Despite the fact that sand particles have a smaller particle size than paddy particles, the pressure drop for sand particles was higher due to their higher density. But the trend of pressure drops along the height of the dryer for both the particles remains the same. In Fig. 6.2 (b), the pressure drop along the height of the dryer for the three air velocities between experimental and simulation conditions were found to be in good agreement.
Pressure drops along the dryer's height were also investigated for varying bed inventories, as shown in Fig. 6.3.
Figure 6.3: Pressure drop along the height of a conical dryer with 10° cone angle at different bed inventories
It was observed that pressure drop between two successive distances decreases with dryer height for all inventories. This is because the concentration of bed particles is higher at the
bottom of the conical fluidized bed dryer, and it decreases with the height of a dryer. The figure also shows that the pressure drop increases as bed inventory increases for a constant superficial air velocity. Static pressure is defined as the weight per unit of cross-sectional area, and an increase in bed inventory increases the weight of bed inventory per unit of cross-sectional area.
Furthermore, the effect of mixing sand particles with paddy on pressure drop along the height of the conical dryer having a cone angle of 10° is investigated. Figure 6.4 shows the effect of mixing sand and paddy on pressure drop along the height of the conical dryer at a 10° cone angle.
Figure 6.4: Effect of mixing of sand with paddy on pressure drop in a conical dryer with 10° cone angle
The prime intention of mixing was to shorten the drying time. The hydrodynamic behaviour of the fluidized bed dryer must be understood in order to investigate the drying characteristics. To achieve a constant bed inventory of 2 kg, 0.4 kg of sand was mixed with 1.6 kg of paddy. The figure reveals that the pressure drop increased when sand was mixed with paddy for the same amount of bed inventory. The explanation for this is that the density of the sand particles is higher than that of the paddy, so the interphase momentum influence is less. Similar to the previous studies, the effect of a spiral on pressure drop in the two conical dryers was investigated, and the results are presented in Figs. 6.5 and 6.6.
Figure 6.5: Effect of a spiral on pressure drop along the height of a conical dryer with 5 cone
angle
Figure 6.6: Effect of a spiral on pressure drop along the height of a conical dryer with 10 cone
angle
The pressure drops increase with the use of a spiral for both dryers, as seen from the figures.
The reason for this behaviour is the vigorous agitation of particles along the height of dryers.
The reason may also be due to the increased expansion and contraction of particles to the wall of the dryers. The effect of cone angle on pressure drop without and with a spiral is shown in Figs. 6.7 (a) and (b).
(a) (b)
Figure 6.7: Effect of cone angle on pressure drop, (a) without a spiral and (b) with a spiral In Fig. 6.7 (a), the results of pressure drop for two conical dryers were compared with the result of Thant et al. (2018) [123]. The work of Thant et al. (2018) [123] shows vigorous fluctuation in pressure drops, whereas the current experiments show a gradual pressure drop with the height of the dryer. The reason for this characteristic can be attributed to good particle mixing and less fluctuation in particle compression and expansion to the wall. Pressure drop in conical
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.