Columnar seal for particle processing system
A backflow blocker prevents backflow of gas to a lower pressure first stage of a system for processing particles from a downstream higher pressure second stage of the system. The blocker comprises an upright duct connecting the first stage to the second stage. The duct is at least partly filled with a column of particles passing from the first stage to the second stage. By properly selecting the duct dimensions and by controlling the flow rate of particles into and from the duct, the velocity of downwardly moving particles in the duct can be made to exceed the velocity at which the gasses flow through the particles in the duct. This prevents gasses from flowing from the lower pressure downstream stage to the higher pressure upstream stage.
1. A gas backflow blocker for use in a process treating a particle mass passing in a substantially continuous flow from a first stage of the process to a downstream second stage, said first stage to be held at a lower pressure than that of the second stage, wherein the particle mass has a density D, a first stage pressure of P h , and a second stage pressure of P l , and comprising:
a) an upright duct having an upper end and a lower end, for holding a column of the particle mass flowing therethrough;
b) first and second flow restrictors mounted at the upper and lower duct ends respectively, said first flow restrictor for receiving particles flowing from the first stage and limiting particle flow into the duct from the first stage and said second flow restrictor receiving flow from the duct and limiting particle flow from the duct into the second stage, at least one of said flow restrictors having a particle mass flow rate set by a flow rate signal;
c) a particle level sensor mounted on the duct adjacent the upper end and providing a level signal indicating the level of particles within the duct; and
d) a controller receiving the level signal and responsive thereto, providing a flow rate signal to the one of said flow restrictors to regulate particle flow through said flow restrictor to approximately maintain a predetermined particle mass level H within the duct at the level sensor, said controller regulating H to approximately satisfy the equation H≧(P h −P l )/D.
2. The blocker of claim 1 , wherein the particle level sensor is mounted at an elevation of approximately H=1.3×(P h −P l )/D above the lower end of the duct.
3. The blocker of claim 1 , wherein the first flow restrictor is of the type whose flow rate is set by the flow rate signal.
4. The blocker of claim 1 wherein the first flow restrictor comprises a rotary gate whose speed of rotation is controlled by the flow rate signal.
5. The blocker of claim 1 , wherein the second flow restrictor flow rate is set by the flow rate signal.
6. The blocker of claim 5 wherein the first flow restrictor comprises a rotary gate whose speed of rotation is controlled by the flow rate signal.
7. The blocker of claim 1 , wherein the interior of the duct has an anti-stick surface.
8. A gas backflow blocker for use in a process treating a particle mass passing in a substantially continuous flow from a first stage of the process to a downstream second stage, said first stage to be held at a lower pressure than that of the second stage, wherein the pressure difference between the first and second stages creates a predetermined gas velocity V g within and relative to the particle mass in the duct, comprising:
a) an upright duct having an upper end and a lower end, for holding a column of the particle mass flowing therethrough;
b) first and second flow restrictors mounted at the upper and lower duct ends respectively, said first flow restrictor for receiving particles flowing from the first stage and limiting particle flow into the duct from the first stage and said second flow restrictor receiving flow from the duct and limiting particle flow from the duct into the second stage, at least one of said flow restrictors having a particle mass flow rate set by a flow rate signal, and wherein at least one of the first and second flow restrictors provides a volume flow rate providing a downward velocity V p of the particle mass in the duct, with V p >V g ;
c) a particle level sensor mounted on the duct adjacent the upper end and providing a level signal indicating the level of particles within the duct; and
d) a controller receiving the level signal and responsive thereto, providing a flow rate signal to and to regulate particle flow through said flow restrictor to approximately maintain a predetermined particle mass level H within the duct at the level sensor.
9. The blocker of claim 8 , wherein at least one of the first and second flow restrictors is set to provide a volume flow rate providing a downward velocity V p of the particle mass in the duct, with V p >0.8 V gf where V gf is a fluidization velocity causing the particle column to lose compactness.
10. The blocker of claim 8 , wherein the controller provides a flow rate signal to one of said flow restrictors creating the volume flow rate providing the downward velocity V p of the particle mass in the duct.
11. A gas backflow blocker for use in a process treating a particle mass passing in a substantially continuous flow from a first stage of the process to a downstream second stage, said first stage to be held at a lower pressure than that of the second stage, comprising:
a) an upright duct having an upper end and a lower end, for holding a column of the particle mass flowing therethrough;
b) first and second flow restrictors mounted at the upper and lower duct ends respectively, said first flow restrictor for receiving particles flowing from the first stage and limiting particle flow into the duct from the first stage and said second flow restrictor receiving flow from the duct and limiting particle flow from the duct into the second stage, at least one of said flow restrictors having a particle mass flow rate set by a flow rate signal;
c) a particle level sensor mounted on the duct adjacent the upper end and providing a level signal indicating the level of particles within the duct, wherein the distance between the lower end of the duct and the location of the particle sensor is greater than (P h −P l )/D, where P h and P l are the gas pressures at the first and second stages of the process respectively, and D is the effective mass density of the particles in the duct; and
d) a controller receiving the level signal and responsive thereto, providing a flow rate signal to the one of said flow restrictors to regulate particle flow through said flow restrictor to approximately maintain a predetermined particle mass level H within the duct at the level sensor.