154 The hydrocyclone or liquid cyclone
In the hydrocyclone, or hydraulic cyclone, which is discussed extensively in the literature1-29-35-1, separation is effected in the centrifugal field generated as a result of introducing the feed at a high tangential velocity into the separator. The hydrocyclone may be used for:
(a) separating particles (suspended in a liquid of lower density) by size or density, or more generally, by terminal falling velocity;
- Figure 1.36. NEI Delta sizer ultrafine classifier
(b) the removal of suspended solids from a liquid;
(c) separating immiscible liquids of different densities;
(d) dewatering of suspensions to give a more concentrated product;
(e) breaking down liquid-liquid or gas-liquid dispersions; and
(f) the removal of dissolved gases from liquids.
In this section, the general design of the hydrocyclone and its application in the grading of solid particles, or their separation from a liquid, is considered and then the special features required in hydrocyclones required for the separation of immiscible liquids will be addressed. The use of cyclones for separating suspended particles from gases is discussed in Section 1.6.2.
General principles and applications in solids classification
A variety of geometrical designs exists, the most common being of the form shown in Figure 1.37, with an upper cylindrical portion of between 20 and 300 mm in diameter and a lower conical portion, although larger units are occasionally employed. The fluid is introduced near the top of the cylindrical section, and overflow is removed through a centrally located offtake pipe at the top, usually terminating approximately at the level corresponding to the junction of the cylindrical and tapered portions of the shell. Other configurations include an entirely cylindrical shell, a conical shell with no cylindrical
Overflow
Feed chamber
Feed
Overflow
Feed chamber
Feed
Vortex finder
Lining
Cone section
Apex valve
Tail piece
Vortex finder
Lining
Cone section
Apex valve
Tail piece
Underflow
Figure 1.37. Liquid cyclone or hydrocyclone portion and curved, as opposed to straight, sides to the tapered section. Generally, a long tapered section is preferred.
As flow patterns are influenced only slightly by gravitational forces, hydrocyclones may be operated with their axes inclined at any angle, including the horizontal, although the removal of the underflow is facilitated, with the axis vertical.
Much of the separating power of the hydrocyclone is associated with the interaction of the primary vortex which follows the walls, and the secondary vortex, revolving about a low pressure core, which moves concentrically in a countercurrent direction as shown in Figure 1.37. The separating force is greatest in this secondary vortex which causes medium sized particles to be rejected outwards to join the primary vortex flow in which they are then carried back towards the apex. It is this secondary vortex which exerts the predominant influence in determining the largest size or heaviest particle which will remain in the overflow stream. The tangential fluid velocity is a maximum at a radius roughly equal to that of the overflow discharge pipe or "vortex finder".
The flow patterns in the hydrocyclone are complex, and much development work has been necessary to determine the most effective geometry, as theoretical considerations alone will not allow the accurate prediction of the size cut which will be obtained. A mathematical model has been proposed by Rhodes et al.(36), and predictions of streamlines from their work are shown in Figure 1.38. Salcudean and Gartshore(37) have also carried out numerical simulations of the three-dimensional flow in a hydrocyclone and have used the results to predict cut sizes. Good agreement has been obtained with experimental measurements.
- Figure 1.38. Predicted streamlines in a hydrocyclone(36)
Near the top of the hydrocyclone there will be some short-circuiting of the flow between the inlet and the overflow, although the effects are reduced as a result of the formation of circulating eddies, often referred to as the mantle, which tend to act as a barrier. Within the secondary vortex the pressure is low and there is a depression in the liquid surface in the region of the axis. Frequently a gas core is formed, and any gas dispersed in the form of fine bubbles, or coming out of solution, tends to migrate to this core. In pressurised systems, the gas core may be very much reduced in size, and sometimes completely eliminated.
The effectiveness of a hydrocyclone as a separator depends primarily on the liquid flow pattern and, in particular, on the values of the three principal components of velocity (tangential, ut, axial ua, and radial ur) throughout the body of the separator. The efficiency with which fractionation takes place is highest at very low concentrations, and therefore there is frequently a compromise between selectivity and throughput in selecting the optimum concentration. Turbulent flow conditions should be avoided wherever possible as they give rise to undesirable mixing patterns which reduce the separating capacity.
Most studies of hydrocyclone performance for particle classification have been carried out at particle concentrations of about 1 per cent by volume. The simplest theory for the classification of particles is based on the concept that particles will tend to orbit at the radius at which the centrifugal force is exactly balanced by the fluid friction force on the particles. Thus, the orbits will be of increasing radius as the particle size increases. Unfortunately, there is scant information on how the radial velocity component varies with location. In general, a particle will be conveyed in the secondary vortex to the overflow, if its orbital radius is less than the radius of that vortex. Alternatively, if the orbital radius would have been greater than the diameter of the shell at a particular height, the particle will be deposited on the walls and will be drawn downwards to the bottom outlet.
The general characteristics of vortices have been considered in Volume 1, Chapter 2, where it has been shown that, in the absence of fluid friction, the relation between tangential fluid velocity ut and r is as follows:
(a) in a forced vortex (formed, for example, when a rotating member, such as an impeller in a pump or mixer, imparts a constant angular velocity m to the fluid):
(b) in a free (or natural) vortex, in which the energy per unit mass of fluid is constant throughout:
Laboratory measurements on hydrocylones have shown that, in the primary vortex, the relation between tangential velocity ut and radius r, is approximately of the form:
Thus, as might have been be expected, the primary vortex in the hydrocyclone is more akin to a free (n = 1) than to a forced (n = —1) vortex.
Typical profiles of the components ut, ur and ua of the liquid velocity in a hydrocyclone, as given by Kelsall(38) and Svarovsky(34) , are shown in Figure 1.39. These profiles are
- (a) (b)
Figure 1.39. Typical velocity distributions in a hydrocyclone1-38-1. (a) axial (b) radial (c) tangential (broken line LZVV is the locus of zero axial velocity)
Figure 1.39. Typical velocity distributions in a hydrocyclone1-38-1. (a) axial (b) radial (c) tangential (broken line LZVV is the locus of zero axial velocity)
quite critically dependent on the geometry of the separator. Entry velocities, corresponding to ut near the walls at that level, will be up to about 5 m/s.
Qualitatively similar velocity profiles are set up within gas cyclones, discussed in Section 1.6.2, which are extensively used for the removal of suspended solids from gases. In this case, the velocities are generally considerably higher.
For suspended particles, the axial and tangential velocities will differ little from those of the liquid. As already suggested, however, in the radial direction, the particles will tend to rotate at the radius at which the centrifugal force is balanced by the drag force of the fluid on the particle. This means that the radius of the particle orbit in a liquid will increase with both particle size and density, or more particularly with terminal falling velocity. As a result, the larger and denser particles will move selectively towards the walls of the hydrocyclone and will be discharged at the apex of the cone. For a sharp fractionation to be achieved, the residence times of the particles must be sufficiently long for them to attain their equilibrium orbits within the hydrocyclone(39), Velocity profiles do in practice tend to be flatter than those predicted by equation 1.44, especially in large units where turbulence may develop.
Design Considerations
Hydrocyclones are 20-500 mm in diameter, with the smaller units giving a much better separation. Typical values of length to diameter ratios range from about 5 to 20. Because of the very high shearing stresses which are set up, flocs will be broken down and the suspension in the secondary vortex will be completely deflocculated, irrespective of its condition on entry. Generally, hydrocylones are not effective in removing particles smaller than about 2-3 ^m.
Because separating power is greatest in hydrocyclones of small diameter, the cut size being approximately proportional to the diameter of the cylindrical shell raised to the power of 1.5, it is common practice to operate banks of small hydrocyclones in parallel inside a large containing vessel. Furthermore, this procedure also makes scale-up easier to carry out. With units connected only in parallel (or as single units), however, it is not possible to control the compositions of the overflow and underflow independently, and therefore some form of series-parallel operation is often employed. This is an important consideration when the hydrocyclone is used for thickening a suspension. In this case, there is the requirement to produce an underflow of the appropriate solids concentration and for the overflow to be particle-free, and these two conditions cannot usually be satisfied in a single unit. In thickening, the particle concentrations are high and little classification by size occurs.
In general, the performance of the hydrocyclone is improved by increasing the operating pressure, and the principal control variable is the size of the orifice on the underflow discharge. Several theoretical and practical studies have been made in an attempt to present a sound basis for design, and these have been described Bradley(29) and Savrovsky(30 35) among others, although they are generally not entirely satisfactory, and some design charts and formulae have been given by Zanker(40) . In practice, tests with the actual materials to be used are desirable for the evaluation for the various parameters. Since systems are usually scaled-up by increasing the number of units in parallel, it is seldom necessary to carry out tests on large units.
The optimum design of a hydrocyclone for a given function depends upon reconciling a number of conflicting factors and reference should be made to specialist publications. Because it is simple in construction and has no moving parts, maintenance costs are low. The chief problem arises from the abrasive effect of the solids; materials of construction, such as polyurethane, show less wear than metals and ceramics.
Effect of non-Newtonian fluid properties
There have been very few studies of the effects of non-Newtonian properties on flow patterns in hydrocyclones, although Dyakowski et al.(41) have carried out numerical simulations for power-law fluids, and these have been validated by experimental measurements in which velocity profiles were obtained by laser-doppler anemometry.
Liquid-liquid separations
For the separation of two liquids, the hydrocyclone is normally operated at a pressure high enough to suppress the formation of the gas core by appropriate throttling of the outlets on the underflow and overflow streams. Hydrocyclones are now used extensively for separating mixtures of oil (as the less dense component) from water. A wide variety of designs has been developed to cope with different proportions of the light and heavy components. Colman(42) has described a plant for cleaning up large quantities of sea or river water following a oil spill, the proportion of the light component may be very low (2-3 per cent). A separator with an enlarged cylindrical top section may be fitted with two tangential inlets, and the conical section may be very long with an included angle as small as 1-2°. The gradual taper leads to the formation of a secondary vortex of very small diameter to accommodate the relatively small proportion of the lighter (oil) phase. By using this configuration, it has been possible to recover a high proportion (up to 97 per cent) of the oil in the feed. In this case, the correct setting of the orifice on the underflow discharge is of critical importance.
Liquid-gas separations
Hydrocyclones are used for removing entrained gas bubbles from liquids, and the extracted gas collects in the gas-core of the secondary vortex before leaving through the vortex finder. Nebrensky(43) points out that because of the low pressure in the region close to the axis, they will also remove dissolved gases(43).
Applications
Hydrocyclones are now being used for a very wide range of applications and are displacing other types of separation equipment in many areas. They are compact and have low maintenance costs, having no moving parts, and have substantially lower liquid holdups than gravity-driven separators. Since any aggregrates tend to be broken down in the high shear fields, they usually give cleaner separations. However, they are relatively inflexible in that a given unit will operate satisfactorily over only a narrow range of flowrates and particle concentrations. This is not usually a serious drawback as hydrocyclones are usually operated with banks of small units in parallel, and their number can be adjusted to suit the current flowrate.
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