Yang Fluidization, Solids Handling, and Processing

780 Fluidization, Solids Handling, and Processing

6.0CONES, DUST HOPPERS AND EROSION

Cones are attached to cyclone barrels simply as a transition to the diameter of the so-called dipleg pipe necessary to allow exit of the stream of collected solids (Zenz, 1975). The cone is irrelevant to the cyclone’s performance; many installations in specific applications, as well as smaller units operating in parallel in multi-clones, are seldom provided with cones. Ideally, a conventional cone should be tapered to the angle of repose of the bulk collected solids under the centrifugal acceleration field within the cyclone. Such sophistication is generally disregarded and the cones sloped at 60 to 80 degrees from horizontal. The steeper this slope, the less the possibility of erosion within the upper end of the dipleg at the point where the stream of collected solids cascades from the edge of the cone’s apex and impacts the dipleg wall.

Three patents (MacLean, et al., 1982) assigned to Texaco filed in the latter half of 1980 resulted from an in-house review of all observed instances of erosion of cyclones in Texaco’s FCC units. The patents specify that to minimize erosion and maximize collection efficiency, the length of the cyclone relative to its barrel diameter is to conform to the relationship:

H/D = 4.49 - 1.09 (Ao/Ai)

However, the eroded regions which led to this relationship were all located in the so-called dust hoppers and not in the cyclone cones. Texaco’s personnel failed to realize that the erosion in the dust hoppers is due to impingement by the stream leaving the edge of the cone’s apex. The simple solution to such erosion is to eliminate the dust hoppers. There are numerous cyclones without dust hoppers operating in the process industries, including FCC installations, which are free of erosion in their cones and dipleg pipes provided that they “contain” the natural vortex length free of touching the cone wall. It is, however, imperative that in fabrication a cyclone cone always be rolled as opposed to being formed on a brake. As illustrated in plan and isometric views in Fig. 6, the condensed stream of collected solids spiralling down on the surface of the cone impinges the metal past any bend (resulting from fabrication on a brake) at an angle of attack steeper than 0 degrees. This has been evidenced in numerous instances as a spiral pattern of eroded holes and parallels the

784 Fluidization, Solids Handling, and Processing

contained fluidized bed or where the cyclone is external but close-coupled to the wall of a large vessel as in the case of fluidized bed combustors. If the entering stream is not uniformly dispersed, as induced in Fig. 10 by a preceding elbow, collection efficiency will be poorest in arrangement Fig. 10(a) and exhibit a decrease in efficiency with increase in either loading or inlet velocity. This is attributable to the more densely concentrated stream of solids entering adjacent to the gas discharge tube. The effect is more pronounced with a preceding downflow elbow, as in Fig. 10(a), than with an upflow elbow as in Fig. 10(b), presumably because in Fig. 10(b) there is an outlet tube length over which some particles may have a greater chance to reach the barrel wall rather than all hug the gas discharge tube and escape under its bottom edge drawn in with the exiting gas spiral.

Figure 10. Effect of preceding elbows.

Three generic forms of cyclone can be characterized by their inlet configurations as illustrated in Fig. 11. At equal gas inlet or gas outlet velocities, the tangential generally yields the highest collection efficiency. At very high inlet particle loadings, differences between the three forms