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.