Underflow Configurations

When faced with a cyclone performance problem it is almost always advisable to focus attention on the underflow configuration. The majority of performance problems are due, for one reason or the other, to the inability of particulates to properly discharge out the underflow openings. 

As with more conventional cyclones, a vortex stabilizer may also be installed in swirl tube type cyclones. Nevertheless, for reasons given above, one has to be most careful if the swirl tube is to be installed in a multicyclone configuration. One has to ensure that the underflow configuration can handle the flow maldistribution that always accompanies multicyclone configurations. Some examples of vortex stabilizers can be found in Chap. 13, which discusses demisting cyclones. 

Control philosophies applied to continuous countercurrent decantation (CCD) thick eners are similar to those used for thickeners in other applications, but have emphasis on maintaining the CCD circuit in balance. It is important to prevent any one of the thickeners from pumping out too fast, otherwise an upstream unit could be stai ved of wash liquor while at the same too much underflow could be placed in a downstream unit too quickly, disrupting the operation of both units as well as reducing the circuit washing efficiency. Several control configurations have Been attempted, and the more successful schemes... 

For those units utilizing a tertiary separator, regular measurement of the PSD and metal content of the underflow of this separator is also recommended. A shift in catalyst PSD to higher values is an indication of a potential loss problem. A decrease in fines content typically reflects a problem with collection equipment. Depending on the unit configuration, this can have a significant impact on catalyst circulation. 

Figure 1 presents the most widely used ASWWTP configurations for combined N and P removal. The A2/0 process presents a sequence of anaerobic reactors (to promote the growth of PAOs) followed by a sequence of anoxic to promote denitrification, and finally aerobic reactors. It has one internal and one external recycle stream. The internal recycle stream conducts a fraction of the nitrified liquor from the last aerobic to the P anoxic compartment, and the external recycle conducts a fraction of the sludge from the underflow of the sedimentation tank to the P compartment. In the UCT process, both recycle streams are feed to the anoxic zone and a second internal recycle stream is present from the anoxic to the anaerobic compartment. The modified UCT process has 2 internal recycles and 1 external one as in the original UCT process but the anoxic zone is divided into 2 zones. The external recycle is directed from the underflow of the decanter to the P anoxic zone. The P internal recycle stream conducts a fraction of the nitrified liquor from the aerobic to the 2 anoxic zone. Finally, the second internal recycle stream pumps a fraction of the mixed liquor from the P anoxic back to the anaerobic compartment. The Bardenpho process configuration has also an external... 

There are two basic types of standpipe configurations, the overflow standpipe (Fig. 6A) and the underflow standpipe (Fig. 6B). The overflow standpipe is so named because the solids overflow from the top of the fluidized bed into the standpipe, and there is no bed of solids above the standpipe. In the underflow standpipe, the solids are introduced into the standpipe from the underside, or bottom, of the bed or hopper, and a bed of solids is present above the standpipe. With this definition, a cyclone dipleg is classified as an overflow standpipe because there is no bed of solids above the entrance to the dipleg. 

Underflow fluidized standpipes in FCC units are operated in a vertical configuration, a completely angled configuration, or a hybrid configuration in... 

Figure 11.1.1 illustrates six common industrial underflow or seal configurations. Most industrial cyclone systems utilize one of these seal designs or some variant thereof. The primary purpose of these underflow sealing devices is to isolate, more or less, the cyclone from the conditions that exist downstream of the sealing device. Without such a seal, either gas would blow out the bottom of the cyclone or it would flow up the cyclone. Neither of these conditions is normally desirable although a controlled amount of gas blowdown may be specified in some instances for solids conveying purposes. 

Fig. 11.1.1. Conventional cyclone featming six common types of underflow seal configurations ...

---