The elu,triation of particles is an inevitable consequence of fluidized beds, and the equipment required to separate the gas and powder provides a significant contribution to the capital and operation costs of the process. In some applications the fine powder carried out is valuable and process economics cannot allow its loss; in others, emission of the particles constitutes a nuisance or a health hazard; in many applications all of these considerations apply.
Efficient gas cleaning and recovery of the solids form an essential part of the process requirements for the fluidized bed and are the subject of this chapter. Because cyclones are the most common device used in fluidized bed systems they receive the greatest attention, but some of the other techniques used as alternatives or in addition are also discussed briefly.
Solid-gas separation as a title may be interpreted to mean both'degassingof solids (as a direct analogy with solids dewatering in solid-liquid separation) andgas cleaning, i.e. dedusting of the gas. Only the latter is to be considered here and the terms solid-gas separation and gas cleaning will be used interchangeably because the termgas cleaning is quite commonly used, even in cases when the solids represent the product.
Three phases may be distinguished in any gas-cleaning operation: tranport of particles onto a surface (separation), collection of separated particles from the separation surface into discharge hoppers (or particle fixation), and the disposal of the collected material from the gas-cleaning equipment. The following account of equipment deals only with the first two phases.
197
8.2 PRINCIPLES OF PARTICLE SEPARATION AND CLASSIFICATION OF EQUIPMENT
Flowrate-pressure drop relationship; efficiency; economic criteria; suita-bility for different conditions (the nature of both the dust and the gas), solids concentration, method of disposal, reliability, etc.
8.3.1 Flowrate-pressure drop relationship
Most gas-cleaning devices have a fixed relationship between static pressure drop I1p and gas flowrate Q, depending on the configuration of the gas cleaner. Most frequently, the relationship is expressed in the same way as with other flow devices as:
In the first phase of gas cleaning, forces are applied to the particles in order to bring them to a collecting surface; the principles of particle separation are usually classified according to the nature of the forces involved. These may be:
(a) External forces due to fields of acceleration which are external to the gaseous suspension, such as gravity, electrostatic, or magnetic forces, or (b) Internal forces due to fields of effects which take place within the suspension itself, e.g. inertial or centrifugal forces, diffusion, coagu-lation, electrostatic effects of charged particles, thermophoresis, diffusio-phoresis, and piezophoresis.
The process of screening, in which particles are classified in relation to their ability to pass through an aperture in the screen, does not lend itself to the above classification, but its role in gas cleaning is relatively minor.
Gas-cleaning equipment often combines two or more of the above-mentioned principles in one unit; the classification of equipment therefore does not necessarily follow the same pattern. The most common classification is into four groups, as follows:
(a) Aeromechanical dry separators in which gravity and/or inertial effects prevail. This group includes (Svarovsky, 1981b) cyclones, settling chambers, intertial separators, dual vortex separators, and fan collectors (or 'mechanical cyclones'). Only the first will be considered in this chapter.
(b) Aeromechanical wet separators (scrubbers) which make use of diffusional and inertial effects.
(c) Electrostatic precipitators which depend on electrostatic and gravity forces.
(d) Filters which use inertial and diffusional effects.
Many gas-cleaning systems combine two or more of the above groups either by using different equipment in series or by combining these in a single unit.
The above classification of equipment will be followed in the sections on equipment and a separate section will be devoted to each of the four groups.
I1p
=
V2EuPg U2 (8.1)where Eu, the Euler number, is a resistance coefficient (analogous to Co in Eq. 6.2) which may be a function of the Reynolds number and other operational variables such as the feed concentration of solids, specific water consumption with scrubbers, etc.; Uis a characteristic velocity calculated from Q/A, whereA is a characteristic area in the separator (e.g. the cross-section of the cylindrical body in cyclones); andPg is gas density.
One exception in the application of Eq. (8.1) is in air filters, where the pressure drop is also a function of the amount of dust deposited on the filter.
8.3.2 Efficiency
Efficiency is an important criterion, since it governs the degree of cleaning. It is best expressed as gravimetric grade efficiencyG(d) (see Svarovsky, 1981a, for details of its evaluation). Comparison of typical grade efficiencies of aeromechanical dry (D) and wet (W) separators, electrostatic precipitators (E), and filters (F) is made in Fig. 8.1.
8.3 GENERAL CHARACTERISTICS OF EQUIPMENT
There are several factors affecting the choice of gas-cleaning equipment for any particular application. These are:
Particle size d(}km) )0
Figure 8.1 Typical grade efficiency curves of dry separators (D), wet separators (W), electrostatic precipitators (E), and filters (F).
The most important disadvantage is their relatively low efficiency for very fine particles which leads to their frequent role as a pre-cleaner.
Cyclones are now used in many different fields of technology, but their most extensive application still remains in gas cleaning, where they are employed for the separation of relatively coarse dusts.
There are basically two main designs of cyclones available: the reverse-flow cyclones and the 'uniflow' cyclone., the former being more frequently used.
In both types, tne inlet gas is brought tangentially into a cylindrical section and a strong vortex is thus created inside the cyclone body. Particles in the flow are subjected to centrifugal forces which move them radially outwards, towards the inside cyclone surface on which the solids separate. The two types of cyclone differ in the direction in which the clean gas leaves the main body, as is shown in the following.
A typical reverse-flow cyclone (Fig. 8.2) consists of a cylindrical section joined to a conical section and the clean gas outlet is through a pipe which extends some distance axially into the cyclone body through the top. The gas inlet may be tangential, spiral, helical, or axial. Different inlet types lead to some variations in performance, but the effect is not strong and, in most cases, the tangential inlet is preferred for its simple construction.
8.3.3 Economic criteria
Economic criteria consist of the capital and running costs of the dust-arresting plant. Capital cost is normally expresse~ per thousand cubic metres. of cleaned gas per hour; it may be further spht mto the cost of the ~on~tr~ctlO.n material cost of labour erection, design, etc. There are other cntena 10this
" 3 3) 'fi fl
category, such as specific volume of the plant (m /1,000 ~ , s~ecl.c oor area taken (m2/1,OOO m3), etc., the importance of which vanes with different
applications. .
Running costs include cost of power, mamtenance, water, etc;. ~ower needed for running the plant consists of the power for pumps, electnclty for cleaning (electrostatic precipitators), and also the power for blowing the gas through the plant.
Whenever the total power requirements consist solely of the po~er needed for passing the given flowrate Qthrough the separator, the theoretical power can be calculated from the product of the required pressure dropI:1pand gas flowrate: Q x I:1p (W). To allow relative comparisons .b.etween differe~t separators, the theoretical power may be expressed as speclfi~energy per UOit flowrate, and this is usually in watt-hours per thousand cubiC metres. Thus;
each newton per square metre of pressure drop represents 0.28 Wh/l,OOOm or in practical engineering units, 1 mm w.g. (column of water) represents 2.73 Wh/1,000 m3• The actual power can only be derived from th~ theoretical power requirements if the efficiency of the fan and the electnc motor are known or assumed.
8.3.4 Suitability for different conditions
There are a number of other factors, such as gas temperature and humidity, the cohesiveness and abrasiveness of the dust, reliability, limits in dust concentration, etc., which may exert an overiding influence on the final choice.
8.4 CYCLONES
The advantage of cyclones, and indeed all aeromechanical dry separators, include:
simple design, low capital cost,
suitability for higher temperatures, low energy consumption,
product is dry, reliability,
Helical Axial Spiral Three other types of gas inlet available
1 Gas inlet 2 Cylindrical port 3 Conical port 4 Gas outlet 5 Top cover
The outer vortex created by the tangential entry is helical, moving downwards - particles in the flow settle into a dust layer on the wall and this is pushed down into the apex. Hence, the removal of the dust from the collection surface is due to the gas flow and not to gravity; gravity has been found to have little effect on the separation efficiency of cyclones.
The outer vortex reverses its axial direction in the apex and creates the inner vortex going upward, which carries the gas into the outlet pipe.
As the vortex reaches very far into the apex, it is advisable not to put a rotary valve or a sliding valve there; it is better to leave the dust outlet clear and use a discharge hopper underneath the cyclone with the necessary valve on the hopper outlet. There is evidence of the vortex reaching even into the hopper itself, but the layer of dust reentrainment is much reduced by using a discharge hopper.
In the 'uniflow' cyclone, often also called the 'straight-through' cyclone (Fig. 8.3), the axial direction of the vortex is not reversed but the flow continues in the same direction and the gas leaves through an outlet pipe which is at the end opposite to the inlet. While in the reverse-flow cyclone the separated solids are in powder form, the uniflow cyclone only functions as a concentrator, i.e. the separated solids leave the unit still suspended in gas which is usually 5 to 10 per cent of the main flow. A second-stage separator has to be provided to treat the 'underflow' if the solids are required in powder form.
Figure 8.4 Schematic diagram of a multi-cyclone arrangement.
-~--f~-*-.•..•••.Particles with small gas flow (5 to 10 per cent)
ments, the axial, vane inlet being most commonly employed. Figure 8.4 gives a schematic diagram of a typical multi-cyclone unit.
Multi-cyclones are generally more elaborate in construction and therefore more expensive than single cyclones; they are also more liable to abrasion and blocking of the dust discharge orifice due to the smaller diameter of the individual units. The latter disadvantage makes multicyclones unsuitable for cohesive dusts and leads to a lower limit in feed concentration of dust.
8.4.1 Flow characteristics
The static pressure drop measured between the inlet and the gas outlet of a cyclone is usually proportional to the square of gas flowrate Q: this means that the resistance coefficient defined as the Euler number, Eu, in Eq. (8.1) is practically constant for a given cyclone geometry or 'design', independent of the cyclone body diameter. The characteristic velocity Ucan be defined for gas cyclones in various ways but the simplest and most appropriate definition is based on the cross-section of the cylindrical body of the cyclone, so that:
U- 4Q
- 1T'D~
It can be shown theoretically (see Eq. 8.6 below) that for a given pressure drop the cut size of a cyclone is proportional to the square root of the cyclone diameter; i.e. the smaller the cyclone the higher will be its efficiency. It is therefore theoretically sound to build multi-cyclone arrangements which use several smaller units in parallel, with common inlet and exit manifolds. Great care must be taken in designing the systems to ensure even distribution of both the gas and the solids between the individual cyclones, because, if this is not achieved, the resulting blockages (and even backflow in some units) reduce the overall efficiency and the advantage of using multi-cyclones may thus be completely lost. Generally, the overall grade efficiency of any multi-cyclone is never as good as that of the individual multi-cyclone units.
Both reverse-flow and uniflow cyclones are used in multi-cyclone
arrange-where Qis the gas flowrate and Dc is the cyclone inside diameter.
The two other alternatives to the definition of characteristic velocity, the average inlet or outlet velocities, are not recommended because neither of them would lead to a sensible comparison of different designs; it can be argued that the cyclone body diameter is the most important dimension,
determining the manufacturing costs, the space occupied, headroom, etc. As an example to demonstrate the superiority of the definition of body characteristic velocity in Eq. (8.2) consider two cyclones, identical in diameter and in al1 other dimensions except their gas inlet and outlet diameters. One has a large inlet and smal1outlet while the other has a sman inlet and large gas outlet: the relative size of the two openings may be such as to result in an identical pressure drop - flowrate relationship for both cyclones. Using the body velocity defined in Eq. (8.2), the resistance coefficient Eu in Eq. (8.1) would be the same for both cyclones; this is to be expected as the cyclones are of the same size and give identical flowrates for the same pressure drops. If, however, either the inlet or the gas outlet velocities are used (and some authors still insist on using those), the resistance coefficients thus obtained would be very much different for the two cyclones, thus apparently favouring strongly (and wrongly) one of the two designs depending on which of the two alternative definitions ofUis used.
The resistance coefficient Eu increases a little at high pressure drops and reduces at high concentrations, but this can usually be compensated for by plant adjustment, and clean air data are normally taken in design calcu-lations.
As can be seen from Eq. (8.3), the separation efficiency is described there only by the cut sizedso and no regard is given to the steepness of the grade efficiency curve. If the whole grade efficiency curve is required in design or performance calculations, it may be generated around the given cut size using plots or analytical functions of a generalized grade efficiency function available from the literature or from previously measured data. The knowledge of the exact form of the grade efficiency is usually not critical in solid-gas separation applications because only total mass recovery is of interest, and this is not much affected by the shape of the curve.
Consequently, very little is known about how the shape of the grade efficiency curve is affected by operating pressure drop, cyclone size or design, and feed solids concentration.
In powder classification applications, however, including the case of de-gritting, the shape of the curve determines the amount of the 'misplaced' material, such as the amount of grit reporting to the gas outlet.
The Stokes number Stksodefined in Eq. (8.3) is usually constant for a given cyclone design (i.e. a set of geometric proportions relative to the cyclone diameter Dc), when the cyclone is used to separate granular material at feed concentrations of less than about 5 g/m3.
Particle size d is best measured as the equivalent Stokes diameter by sedimentation or elutriation methods. This equivalence is based on the assumption that, if a spherical particle and an irregularly shaped particle settle at the same velocity (in gravity or centrifugal fields), they will separate at the same efficiency. This assumption does not hold for flat or needle-shaped particles which assume different orientation in a cyclone than under gravity or centrifugal settling. Problems are also encountered when the particles undergo the separation process in an agglomerated state and the agglomerates are subsequently redispersed into single particles before particle size analysis.
8.4.2 Efficiency of separation
The second dimensionless group which characterizes the separation perfor-mance of a family of geometrically similar cyclones is the Stokes number Stkso defined as:
where dso is the cut size (equiprobable size). The cut size and its relationship to separation efficiency are defined below.
In general, the separation efficiency of gas cyclones depends on particle size (it is then called the 'grade' efficiency) and increases from zero for ultra-fine particles to 100per cent for very coarse particles (see curve D in Fig. 8.1).
The particle size recovered at 50 per cent. efficiencyis referred to 'as the 'cut' sizedso and can be understood as equivalent to the aperture size of an ideal screen that would give the same separation performance as the cyclone. The total solids recovery in a particular case then depends on the grade efficiency (or cut size) which characterizes the cyclone operated under given conditions and on the size, density, shape, and dispersion of the particles (i.e. the characte ristics of the feed material).
As the grade efficiency does not rise very steeply with increasing particle size, some particles in the feed coarser than the cut size will pass through the cyclone while some particles finer than the cut size will be separated.
8.4.3 Reverse-flow cyclone designs
Equations (8.1), (8.2), and (8.3) form the basis of gas cyclone design and scale-up. There is a whole host of different cyclone designs available today, which are usually divided into two main groups according to their geometrical proportions relative to the body diameter: the 'high efficiency' designs and the 'high rate' designs. Table 8.1 gives the geometry of the two best-known cyclone designs (refer to Fig. 8.2 for dimensions). Note that only the reverse-flow cyclone is considered here because that is the type used most widely in industrial practice.
The so-called 'high efficiency' cyclones are characterized by relatively small inlet and gas outlet orifices, and a long body, and give high recoveries. The 'high rate' designs give medium recoveries but offer low resistance to flow so
Table8.1 Cyclone proportions for two different designs based on cyclone diameter Dc = 1 (see Fig. 8.2)
Proportion relative
to diameter A B C E F L K M
Cyclone Dc type
Stairmand, H.E." 4.0 2.5 1.5 0.375 0.5 0.2 0.5 0.5
Stairmand, H. R.b 4.0 2.5 1.5 0.575 0.875 0.375 0.75 0.75
• High efficiency cyclone
bHigh flowrate cyclone.
ponding values of Eu and Stksoare plotted for several commercial and other well-known designs. The points are well scattered but a line can be drawn through them to show a general trend. The line drawn in Fig. 8.5 can be described by the following approximate equation:
R[
2Eu =
--Stkso
This equation may be used for estimates of cut size of unknown cyclone designs (of 'reasonable' proportions) from the cyclone flow characteristics and is intended for guidance only .
Note that the scale-up of cyclones based on Eu and Stkso works well for near ambient conditions but also predicts the performance reasonably well at high absolute pressures and high temperatures; this means that there is no effect of high pressures and temperatures other than that accounted for in the definitions of Eu and Stksoon gas viscosity and density.
that a unit of given size will give much higher air capacity than a high efficiency design of the same body diameter. The high rate cyclones have large inlets and gas outlets, and are usually shorter. In order to prevent the incoming jet of air impinging on the gas outlet pipe, the inlet is spiral (wrap-round type) while the high efficiency units can (and often do) have to a simple tangential entry.
It is interesting to find that, for well-designed cyclones, there is a direct correlation between Eu and Stkso: high values of the resistance coefficient
It is interesting to find that, for well-designed cyclones, there is a direct correlation between Eu and Stkso: high values of the resistance coefficient