Dipleg sealing

The coal is crushed in a hammer mill, dried, and then screened to —16 + 80 mesh. About 500 lb. of coal are charged to a hopper, which is connected at the bottom to the pretreater by a screw feeder. The feed enters the pretreater about 6 inches above the distributor plate. Feed rates of up to 100 lb./hr. can be attained. A 3-in. diameter overflow pipe controls the bed height. The overflow collects in a receiver and is periodically dumped into drums. Fines from the bed were originally returned to the bed by an internal cyclone with a dipleg sealed in the bed, but tar tended to build up in the cyclone and caused the reactor pressure to increase. At present, a heated external cyclone with a collector pot is installed and operates much more smoothly. 

For external cyclones, people often measure the pressure drop from a point immediately before the cyclone inlet to a point immediately above the cyclone in the gas outlet tube. This measurement does not include the contraction pressure drop as gas flows from the freeboard of the bed into the exit gas line from the fluidized bed. For dipleg seal height calculations, this pressure drop should be taken into account. It is better to measure the total cyclone pressure drop as the difference between the pressure in the freeboard and the pressure in the gas outlet tube. 

In the KBR system, as with the ABB Lummus design, the riser cyclones are hard-piped to the riser. The diplegs of both the riser cyclone and the upper reactor cyclone are often sealed with catalyst. This minimizes the carry-under of reactor vapors into the reactor housing and maximizes the collection efficiency of the riser cyclones. 

No trickle or flapper valves are used on the first stage. The riser cyclone diplegs terminate with a splash plate (Figure 9-4A). The upper reactor cyclone diplegs use conventional trickle valves. Sealing the upper reactor cyclone diplegs with about two feet of catalyst provides... 

The catalyst must be fluidized to provide an effective seal for the diplegs. Fluidization is critical without it, the diplegs cannot discharge the catalyst and will plug, with possible massive carry-over to the main fractionator. To ensure this uniform fluidization, the system uses an additional steam distributor. 

Dipleg is the part of a cyclone separator that provides a barometric seal between the cyclone inlet and the cyclone solid outlet. 

A seal pot is essentially an external fluidized bed into which the cyclone fines discharge via a straight dipleg (Fig. 23A). The solids and gas from the cyclone and the fluidizing gas for the seal pot are diseharged via a... 

As with the loop seal, the V-valve will not operate if the solids in the upflow (diverging) section are not fluidized. When the solids in the diverging section are fluidized, solids flow from the standpipe, through the aperture and diverging section, into the fluidized bed. As with the loop seal, the upflow section of the V-valve prevents pressure surges from causing the dipleg to blow. 

The top of the fluidized bed has a more or less clearcut surface, but some solids are entrained and a certain freeboard is necessary to minimize this. To avoid catalyst loss and elutriation, the exit stream flows through a two-stage cyclone. The catalyst is fed back to the bed through pipes called diplegs , which have a seal at their bottom for preventing leakage of bed fluid. 

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