Cyclone operating variables

Four experiments were carried out in the fluidised bed combustion pilot plant with a 16-18 hour test time and whose operation conditions were modified in order to obtain an efficient combustion process. These two conditions yield lower pollutant emissions. Two operation variables were changed mainly in this study feeding rate and air excess percentage. These two parameters, in turn, affect the others parameters. Table 2 shows the combustion operation conditions set in the experiments. In Table 3, the flue gas composition during the four tests is shown. The sampling flue gas temperature was 150 C, the probe, cyclone and filter was all held at the same temperature. The CO level emitted in Test 2 with a 40-60 air excess percentage is lower than in the other experiments. 

The product gas has a heating value of 4.4 to 6.5MJ/m depending upon the feedstock, moisture content and operational variables. The gas is most likely to be used in a close coupled boiler, kiln or dryer after particulate removal in a simple cyclone. It is desirable to keep the product gas hot, usually 200°F to 400°F, to minimize condensation of the tars and to maximize sensible heat transfer. 

Figure 6.13 Plot of operating variables for a particular cyclone operated to give constant overflow clarity. D - 88 mm, chalk in water, x = 10.6 pm, Rietema s design...

The approach used in this chapter is based on families of geometrically similar cyclones so that all design variables are omitted from the scale-up correlations (except the size of the underflow orifice which should be variable and is considered here to be an operating variable). 

It should be pointed out here that the scale-up procedure described here is specifically designed for separation or classification of fine particles. These are the particles predominantly used in the chemical industry and the approach, using dimensionless groups, is in common with the treatment of other unit operations in chemical engineering. The geometrical similarity concerns all internal dimensions of the individual cyclones except the size of the underflow orifice which is regarded here as an operational variable. 

Centrifugal demister or cyclone) devices rely on high velocities to remove liquid particles and substantial pressure drops are required in cyclone design to generate these velocities. Cyclones have a limited range over which they operate efficiently this is a disadvantage if the input stream flowrate is very variable. 

There is clearly a need to investigate the mechanism of attrition to relate it to the fracture properties of the solids, and to develop a realistic attrition index , similar to that used for abrasion in cyclones. Such an index would indicate the relative importance of operating conditions and design variables such as inlet velocity, feed solids concentration or cyclone diameter. This could then be used in scale-up to predict (or minimize) the effect of the shape, the particle size distribution or the hardness and strength of the feed solids, if known, may allow such predictions without any experimental tests. Generally, better understanding of attrition and its relation to abrasion may lead to better equipment design and operation. 

The key design variables are defined as those that are associated with the hydrocyclone dimensions, including cyclone diameter, D, inlet diameter, D, outlet (apex) diameter, D, and vortex finder diameter, D. In addition, the total length of the cyclone, L, the length of the cylindrical section, /, and the cone angle, 6, are also important. They all affect the operating performance of the hydrocyclone. 

The cyclone diameter or the diameter of the base of the cone is a primary design variable, and all other dimensions are usually related to it. For a given feed flow rate, the effects of cyclone diameter on the operating performance of the hydrocyclone can be described by proportionalities... 

Centrifugal sampler Cyclones (1) Battery operated (2) Small (hand size) (1) Allows for spatially variable samples (2) Elimination of sample transfer loss (1) Sampling accuracy affected by vortexing (higher counts, false higher efficiency rating) (2) Counts should be done manually (use of strip) (3) Flow rate only quantifiable theoretically (4) Questionable accuracy of the collected sample (5) Limitation on collection of small particles onto the collection surface... 

It should also be pointed out that the Eu number of the commercial cyclone is essentially constant and independent of variations in any of the variables comprising this dimensionless number, such as gas density or inlet velocity. Thus, if we were to know its pressure drop for any operating condition then, knowing also the gas density and its inlet velocity, we could quite easily... 

As this example tends to suggest, one does not normally have to account for the suppression of the X50 cut-point unless the cyclone is to be operated under high vacuum conditions (typically less than about 100 mm Hg) or with a high molecular weight gas. The influence of such variables is revealed by inspecting Eqs. (12.3.5), (12.3.4) and (12.3.6). 

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