3.2.3 Cavity under different gas superficial velocities
In the Appendix , the cavity is also derived in the radial bed. The final conclusions (equations or data) are listed here. Its formation can also be analyzed qualitatively by the particle normal forces inx and z directions.
Particle normal force in x direction
According to equation (11-13), when the cavity appears before pinning, the critical pressure drop of radial bed is computed by equation (11); otherwise, it can be calculated by equation (12). The pressure drop of inner/outer areas of the bed is calculated, the cavity occurs when it is bigger than above critical pressure drop (equation (13)). In both the centrifugal and centripetal beds, with the increasing of the position of baffle, the pinning is firstly controlled and then promoted in the inner area; while it is eliminated in the outer area.
As shown in Fig.11, with an increasing of the radial position of baffles, the required and real critical pressure drop of cavity increases in the inner area while decreases in the outer area. However, the changeable amplitudes of these two pressure drops are different in inner and outer areas. The cavity is hardly observed under high . In the centrifugal bed, with the increasing of the position of baffle, the cavity is firstly controlled and then promoted in the inner area; while it is contrary in the outer area. In the centripetal bed, with an increasing of the position of baffle, the cavity is firstly prompted and then eliminated in the inner area; while it is contrary in the outer area. Fig.11 indicates that the cavity is well controlled when the baffles put in the (r -r 1)/(r 2-r 1)≈0.4.
(11)
(12)
(13)
Particle normal force in z direction
The particle normal force in z direction is mainly influenced by the gas axial velocity. The cavity is eliminated under large particle normal force in z direction or small gas axial velocity. According to Fig.12, compared to type CA, the maximum gas axial velocity reduces when setting the baffles in type CB. When the gas-solid baffles increases, the gas axial velocity increases in the inner area; while it decreases in the outer area. In other words, higher position of baffles, easier cavity appears in the inner area and harder occurs in the outer area. In particular, the maximum gas axial velocity is smallest when (r -r 1)/(r 2-r 1)=0.3 in the bed with baffles, which decreased by 40 % from 0.3 m/s to 0.18 m/s. The maximum gas axial velocity has almost the same values between 183 m3/h without baffles (type CA) and 549 m3/h with baffles (type CB). The critical gas flow rate of cavity will be improved in type CB.
3.2.4 Air lock under different gas superficial velocities
The air lock may occur for the same reasons given in section 4.1.4. In this paper, consider the structural difference of the radial and rectangular bed, the maximum pressure drop of the semi-centrifugal radial bed when Q =564 m3/h (0.7 kPa) is relatively smaller than the rectangular bed whenu g=0.53 m/s (2.2 kPa).
(a) In the feed tube, the pressure drop equals to the particle gravity.
When air lock occurs, whether the cavity size bigger than the solid seal-height or not, equation (7) is satisfied under this assumption. Compared to the rectangular bed, the pressure term becomes smaller, the equation is hardly satisfied.
(b) In the feed tube, the solid flow rate has a larger value in the solid discharge tubes than that in the feed tube.
In the radial bed of type CA and CB, the solid flow rate also increases with gas flow rate in the solid discharge tubes for the positive pressure gradient. The allowance maximum solid flow rate decreases in the solid feed tubes for the negative pressure gradient. However, this assumption is also hardly met by some easier methods, e.g., using high height and large equivalent diameter of the solid feed tube.