Dipleg pipe

ABB Lummus s RTD consists of a two-stage reactor cyclone system (see Figure 9-2). The riser cyclones (the first stage) are hard-piped to the riser. Attached to the end of each riser cyclone dipleg is a conventional trickle valve as shown in Figure 9-3. Each trickle valve has a small opening to prevent catalyst defluidization, which can be a problem, especially during start-ups. 

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. 

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. 

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. 

Obviously, this situation could pose a problem as most of the solids attempting to exit the dust hopper would find it difficult to enter the dipleg. We could expect that many of the solids—especially the finer fractions—would become reentrained in the rising vortex core and exit out the overflow piping. This situation also could be expected to lead to gross solids recirculation with the hopper. 

Active aeration of diplegs is normally undesirable since the aeration piping will generally not hold up over an extended period in the rather severe operating environment in which they normally operate. 

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