The effect of superficial gas velocity on the bubble passing frequency in fully developed flow and the sparger regions has been illustrated in Figure 16(a-c) and Figure 17(a-c), respectively, for different solids loading (Cs = 0.0, 9.1 and 25 vol. %) and for the center and the wall region of the bubble column. As shown in the figures, in the absence of solids loading case (Cs = 0.0 vol. %), the center and wall regions exhibit a similar behavior where the bubble passing frequency increased with increasing the gas velocity and the difference between these two regions is a negligent. When adding the solids (Cs = 9.1 and 25 vol. %), the bubble passing frequency in the center region exhibits increase with the increase of the gas velocity, whereas, the wall region appears a slight increase.
Furthermore, the solids loading decreases the bubble passing frequency for all the velocities (Ug = 0.2, 0.3 and 0.45 m/s), and increases the gap in bubble passing frequency between the center and wall region progressively with increasing the gas velocity. To explain that, with the presence of the solids there are two mechanical parameters affect this variation in the bubble passing frequency. Second, increasing the obstruction by the internals structure against the large bubbles that leads to accumulate in the center region.
Therefore, the reduction in the bubble passing frequency in the wall is much more than the center region.
Figure 12. The effect of solids loading on the bubble rise velocity in fully developed flow region and Ug = 0.45 m/s; (a) H/D = 5; (b) H/D = 4; (c) H/D =
3
Figure 13. The effect of solids loading on the bubble rise velocity in sparger region and Ug = 0.45 m/s; (a) H/D = 5;
(b) H/D = 4; (c) H/D = 3
Figure 14. Gas velocity effect on the bubble rise velocity in fully developed
flow region and H/D = 5; (a) Cs = 0.0 vol. %; (b) Cs = 9.1 vol. %; (c) Cs = 25
vol. %
Figure 15. Gas velocity effect on the bubble rise velocity in sparger region and H/D = 5; (a) Cs = 0.0 vol. %; (b) Cs
= 9.1 vol. %; (c) Cs = 25 vol.
Meanwhile, the figures exhibit a slight impact of the aspect ratio on the bubble passing frequency where the increase in the aspect ratio decreases the bubble passing frequency. The results provide an evidence for the increase of bubble coalescence rate and the reason of the reduction in the local gas holdup in SBCR, which would be responsible for increased bubble velocity. The radial bubble passing frequency is controlled by the bubble slip velocity created by the turbulent dispersion and the net radial force, therefore like gas holdup, adding solids would affect the radial profile of bubble passing frequency as well [1]. According to Choi and Lee [62], the bubble passing frequency is a function for the bubble size, and bubble rise velocity as well as the intensity of the liquid turbulence.
Therefore, bubble passing frequency affects the transport phenomena (both mass and heat transfer), and hence, the conversion and selectivity will be affected. Worth to mention, the reduction in the numbers of bubbles in the wall region (i.e., the bubbles that rise upward), indicates to that the existence of solids enhances the liquid circulation.
5.7. BUBBLE SPECIFIC INTERFACIAL AREA
The bubble specific interfacial area is bubble surface area per unit volume of liquid/slurry-phase. It is a key parameter for the mass transfer phenomenon in the multiphase system, where the transfer of the species occur cross it from gas to liquid/slurry phase and vice versa [63]. Behkish [64] investigated the bubble properties and the volumetric liquid-side mass transfer coefficient (𝑘ℓ𝑎) in bubble and slurry bubble column operated under pressure (0.1-2.7 MPa) and temperature (323-453 K). He revealed that the liquid-side mass transfer coefficient was varied with the changing of the bubble interfacial area.
Figure 16. The effect of gas velocity and solids loading on the bubble passing frequency in the fully developed flow
region; (a) H/D = 5; (b) H/D = 4; (c) H/D = 3
Figure 17. The effect of gas velocity and solids loading on the bubble passing frequency in the sparger region; (a) H/D
= 5; (b) H/D = 4; (c) H/D = 3
Further, the bubble interfacial area is a characteristic to the degree of mixing in the multiphase. Kagumba and Al-Dahhan [1] reported that the profile of the bubble interfacial area was increased with gas velocity in a low range of Ug (0-0.1 m/s), while, at a high range of Ug (0.1-0.45 m/s) being less or slightly increased.. Their attribution was that the flow regime was transited from the transition to the churn turbulent regime. According to Kagumba and Al-Dahhan [1], the bubble interfacial area could be used as a parameter to demarcate the flow regime transition. Therefore, introducing the influence of the solids loading with the presence of internals and the gas velocity on the bubble interfacial area is essential to improve the SBCR performance, particularly, that the transfer coefficients are related significantly to the rate of reaction.
The effect of gas velocity and solids loading on the specific interfacial bubble area for the fully developed flow and the sparger regions has been exhibited in Figure 18 (a-c) and Figure 19 (a-c), respectively. The figures show a significant increase in the bubble interfacial area with the increase the superficial gas velocity in both regions the fully developed flow and the sparger. In the fact of matter, the spherical bubbles have a low surface area per unit volume. Meanwhile, as the superficial gas velocity increases, the shape of bubbles deforme and be more irregular. Thus, that could be the reason to attribute the increase in the bubble interfacial area with increase the superficial gas velocity.
According to Kagumba and Al-Dahhan [1], and Al-Naseri et al. [2], reported that the interfacial area mainly relates to the bubble passage frequency, and hence, the trends of bubble interfacial area in the center and wall regions of the bubble column, and for both regions the fully developed flow and the sparger are similar to that in the in Figure 16 and Figure 17. While, the solids loading appears a significant impact toward decrease the
interfacial bubble area, which could be explanted to increase the bubble size and reduce the bubble passing frequency. However, the wall region exhibits a low value for the bubble interfacial area by adding the solids, which is confirmation of the reducing in the numbers of the rising bubbles and low bubble size concertation.