Inner vortex separation

Air classification is perhaps the simplest fly ash processing option and is normally employed to improve the fineness of the ash (i.e., remove coarse particles). A typical cyclone classifier uses centrifugal force to separate fine particles from an air stream. The particles enter tangentially into a cylindrical chamber dispersed in an air stream and centrifugal force forces the coarser particles to the wall of the cylinder while the air stream and finer particles spiral to an inner vortex. The air exits from the inner core via an outlet port while the particles slide down the chamber walls and exit the bottom. 

When air classification is employed for ash beneficiation purposes, the mechanism is similar, but a high-performance cyclone is used. An example is shown in Fig. 8. Devices vary considerably in the feed arrangement used, but most employ some mechanism to effectively distribute the ash particles across the profile of the cyclone, such as a rotating cage or distribution plate. The velocity of the feed is controlled to produce the desired size separation whereby fine particles and most of the air migrate to the inner vortex while coarse ash exits the bottom. Air classifiers have been used for many years to control pozzolan fineness simply by removing coarse ash particles, particularly when LOI is not a problem. Air classification is usually not effective for reducing LOI, particularly if the carbon is fine. Some coarse carbon particles may be rejected with the coarse ash, but since the density of the carbon particles is lower than... 

This means that the separating ability of the inner vortex is 8 times that of the outer one. 

As illustrated in Figure 10.7, a cyclone consists of a vertical cylinder with a conical bottom, a tangential inlet near the top, and outlets at the top and the bottom, respectively. The top outlet pipe protrudes into the conical part of the cyclone in order to produce a vortex when a dust-laden gas (normally air) is pumped tangentially into the cyclone body. Such a vortex develops centrifugal force and, because the particles are much denser than the gas, they are projected outward to the wall flowing downward in a thin layer along this in a helical path. They are eventually collected at the bottom of the cyclone and separated. The inlet gas stream flows downward in an annular vortex, reverses itself as it finds a reduction in the rotation space due to the conical shape, creates an upward inner vortex in the center of the cyclone, and then exits through the top of the cyclone. In an ideal operation in the upward flow... 

Muschelknautz et al. (1996) also proposed a mechanistic model of cyclone operation. In this model, the gas can carry only a maximum amount of solids (called the critical loading). At any solids loading in excess of this critical loading, the solids are immediately separated from the gas at the inlet to the cyclone, as indicated in Fig. 5. The solids remaining in the gas are then separated in the cyclone barrel and in the inner vortex below the gas outlet tube as if the cyclone were operating at a lower solids loading. 

The cyclone is a mechanically simple, reliable device for the separation of PM from an air stream by the action of centrifugal forces. The centrifugal forces, resulting from the tangential velocity given to the dust-laden gas at the top of the cyclone, eject the particles in a circular, vortex motion toward the cyclone wall. These particles, because of their inertia forces, attempt to move toward the outside wall, from which they are led to a receiver or hopper. The clarified gas exits as an axial inner vortex through the top by way of the gas exit duct. 

A very fundamental characteristic of any lightly-loaded cyclone is its cut-point diameter or cut size, X50, produced by the spin of the inner vortex. This is the particle diameter that has a 50% probability of capture. As discussed elsewhere in this book, the cut size is analogous to the screen openings of an ordinary sieve or screen although, with a cyclone, the separation is not as sharp as that of a sieve. 

Here, we will determine the overall separation efficiency for saltation conditions, i.e. when Cg > Col- This efficiency includes the efficiency due to saltation in the inlet and the efficiency due to classification in the inner vortex. A portion of the incoming solids that is not collected by the former is collected by the latter, so that the total efficiency becomes (see also Chap. 9) ... 

If the above were not sufficient reason to do whatever is necessary to minimize excessive gas upfiow, it s worth pointing out that gas upflow, if sufficient, will affect the stability of the inner vortex and may shorten its effective length , thus reducing separation performance and possibly increasing erosion in the cone area as well as solids entrainment. 

If Co < CoL, then there is no mass loading effect and the comparatively simple method for computing the cyclone s separation performance, as described in Sects. 13.5.1 and 13.5.2, applies. Conditions that may lead to this scenario include a low liquid loading, Co, a very fine feed drop size distribution, and a large inner vortex cut-point diameter, X50 ... 

Irrespectively, the centrifugal force is proportional to the particle mass and, therefore, the cube of the particle diameter x. The drag force, which is due to the flow of gas from the outer to the inner part of the vortex, is proportional to x, at least when Stokes law applies which it often does in practice. The largest particles are therefore the easiest to separate. 

Mothes-Loffier All centrifugal separators. Combines migration to wall with interchange between inner and outer part of the vortex. Based on concept introduced by Dietz... 

We notice first that the end of the vortex is precessing around the inner wall of the separator at some precessional frequency / and precessional velocity vector Vp. For the case illustrated, this motion is counterclockwise (ccw) as viewed from above. However, superimposed on this motion is the vortex core spin or rotational velocity vector, vcs- This core spin velocity adds to the precessional velocity Vp at its top-most position (position A in illustration) producing the resultant velocity v. On the other hand, the core spin vector opposes the precessional velocity at the bottom-most position (position B). 

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