3.1.5 Air lock under different gas superficial velocities
The air lock happens as the total particles in the bed reduces and the
particles stops feeding into the bed. Some assumptions are made about
the appearance of the air lock in this paper.
(a) Air lock happens when the pressure drop equals to the particle
gravity in the feed tube
a. The cavity size is relatively smaller than the solid seal-height of
feed influence zone.
In the feed tube, the particles force balance is equation (7) in the
feed tube. The pressure p gas can be got by CPP
method in the bottom of the feed tube. In this paper, the height of the
feed tube is so high that the particle gravity always larger than the
pressure term. According to equation (22), whenu g=0.59 m/s, is about 5 kPa whilep gas 600 Pa . The air lock occurs not because
this reason in this paper.
(7)
b. The cavity size is relatively larger than the solid seal-height
In this condition, the gas short-circuit easily happens. It makes the
pressure in the low area of the feed tube increases greatly, which
promotes the appearance of the air lock. However, the pressure in the
bottom of the feed tube is generally not bigger than the pressure drop
of the bed. In this paper, the particle gravity will be larger than the
pressure term. For instance, according to equation (22), whenu g=0.59 m/s, is about 5 kPa whilep gas 2.5 kPa . The air lock occurs not because
this reason in this paper.
(b) Air lock happens when the solid flow rate has larger value in the
solid discharge tubes than that in the feed tube
The solid flow rate in the orifices is usually calculated by equation
(8) 33. Normally, the equivalent diameter of the solid
discharge tubes is smaller than the feed tube. In this sense, the bed is
filled with particles.
(8)
However, the pressure in the bed is greater than that in the up of the
solid feed tube and bottom of the solid discharge tubes. The particles
flow downward under positive pressure gradient in the solid discharge
tubes. The solid flow rate grows with the gas superficial velocity as
the pressure increases (Fig.9). However, the particles flow downward
under negative pressure gradient in the solid feed tube. The allowance
maximum solid flow rates reduces with the gas superficial velocity in
the solid feed tube for its negative pressure gradient.
In the feed tube, the gas phase exerts upward forces on the particles to
prevent it from moving; while it has downward forces in the feed
discharge tubes. When the gas superficial velocity grows, the particles
tends more easily to moving out and hardly flowing into the bed. At the
critical condition, the solid flow rates has larger value in the solid
discharge tubes than that in the feed tube. The total particles in the
bed decreases, the air lock appears. The air lock will be controlled by
using high feed tube (equation (7)) and large diameter of the feed tube
(equation (8)).
In the original bed (type RA), under high gas superficial velocity, the
pinning is so thick that the particles stops flowing into the bed
through the right feed tube. The air lock appears when the solid flow
rate in the left solid feed tube smaller than the summation in the left
and right discharge tubes. In the bed with baffles (type RB), the solid
flow rate in the left discharge tube has large value than the right
discharge tube. The air lock appears in the left area when the solid
flow rate in the left solid feed tube smaller than that in the left
discharge tube. The air lock is then alleviated in type RB. In
experiment, the gas critical superficial velocity is improved from 0.44
m/s to 0.59 m/s by the introduction of baffles.