Cyclone separators solids loading

As cyclones are less effec tive as the particle size decreases, secondary collection units are frequently required, i.e., filters, elec trostatic precipitators, and scrubbers. When dry collec tiou is not required, ehmiuatlou of cyclones is possible if allowance is made for heavy solids loads in the scrubber (see Gas-Sohds Separations see also Sec. 14). 

The load characteristic of circulating fluidized bed boilers is closely related to the gas-solid flux through the combustor. Thus, choice of coal size and the proper gas-solid separator deserves attention. Generally, the fraction of particles below 0.1 mm is smaller than 10%, which could be removed only by a cyclone separator, while the major fraction of coarser particles could be easily separated by inertia or gravity (Li and Kwauk, 1981). The diagram of ash balance for a circulating fluidized bed boiler, shown in Fig. 22, can... 

Cristea, E. D., Malfa, E., and Coghe, A. "3-D Numerical Computation and Validation of High Solid Loading Flow Inside a Gas Cyclone Separator." ASME Fluids Engineering Division Summer Meeting, June 21-25,... 

The liquid-solid hydrocyclone, shown schematically in Fig. 3.4-3, functions like a gas-solid cyclone. The hydrocyclone is also known as a hydroclone. The primary independent parameters that influence the ability of a hydrocyclone to make a separation are size and geometry of the hydrocyclone, particle size and geometry, solids loading, inlet velocity, split between overflow and underflow, density differential, and liquid viscosity. A reasonable estimate of Ae particle cut diameter (50% in underflow and overflow) d o) is given by the following dimensionless relationship, developed initially by Bradley ... 

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. 

Hoffman et al. (1995) studied the cyclone vortex length and found that separation efficiency was less for cyclones with their natural vortex ending in the cone instead of the barrel. They also reported that the vortex length (swirl intensity) deereased with increasing solids loading and was a strong function of the eyelone length. Akiyama and Marui (1989), in... 

Unlike gross or overall efficiency, a cyclone s x-50 cut-point and grade-efficiency curve are true measures of its intrinsic separation potential. These properties are independent of the size distribution of the feed, at least under low solids loading conditions. We shall return to this issue later. 

In general, wall friction has a profound effect on the flow in cyclones and on cyclone performance. We shall discuss this in Chap. 5, when investigating the effect of cyclone body length on cyclone pressure drop and separation efficiency. We shall also discuss the effect of friction in Chap. 9 when looking at the effect of solids loading, which also turns out to be a consequence of the effect of the solids on the wall friction. 

Table 5.4.1. Models for cyclone separation efficiency at low solids loadings...

In addition to the normal feed particle size distribution that enters the cyclone, Muschelknautz and co-workers (Muschelknautz and Trefz, 1990,1991 Trefz, 1992) have modified their earlier cyclone model (or models) to include an inner feed or inner feed particle size distribution . The concept here is that, if the inlet solids loading exceeds the limit loading , some portion of the incoming feed solids will quickly separate out but those that don t will have a somewhat finer particle size than that feeding the cyclone. It is this... 

Cyclone designers have long known that the separation efficiency of tangential inlet cyclones improves with increasing solids loading. Even so, the exact mechanism for this improvement is still not established beyond doubt, in spite of many investigations of the topic. 

We will be returning briefly to the topic of solids loading in swirl tube separators later. Here, we wish to note that a difference exists between the behavior of cyclones with tangential inlets and swirl tubes equipped with inlet vane assemblies, so that the results shown in these figures cannot be applied to swirl tubes. 

Both the models of Smolik and Zenz predict cyclone separation efficiency as a function of loading purely from knowledge of the efficiency at low loading and the loading itself. Physical and operational factors, such as cyclone geometry and size, solids size distribution and density, inlet velocity and other operating conditions, are not included in these models, and the effect of these parameters is thus not thought to be of primary importance. In the Muschelknautz model, on the other hand, the inlet velocity, the cyclone dimensions, and the mean size and density of the inlet solids all feature. 

Fig. 9.1.3. The graphical model of Zenz for estimating the effect of solids loading on cyclone separation efficiency. 1 grain/ft = 2.29 g/m ...

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