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.