Static hydrocyclones

FIGURE 3.47. Liquid-liquid static hydrocyclone separation liner. 

Static hydrocyclones require a minimum pressure of 100 psi to produce the required velocities. Manufacturers make designs that operate at lower pressures, but these models have not always been as efficient as those that operate at higher inlet pressures. If a minimum separator pressure of 100 psi is not available, a low-shear pump should be used (e.g., a progressive cavity pump) or sufficient pipe should be used between the pump and the hydrocyclone to allow pipe coalescence of the oil droplets. As is the case with flotation units, hydrocyclones do not appear to work well with oil droplets less than 10-20 pm in diameter. 

Advantages of static hydrocyclones include that (1) they have no moving parts (thus, minimum maintenance and operator attention are required), (2) their compact design reduces weight and space... 

The major difference between static and dynamic hydrocyclones is that in the dynamic hydrocyclone an external motor is used to rotate the outer shell of the hydrocyclone, whereas in a static hydrocyclone the outer shell is stationary and feed pressure supplies the energy to accomplish separation of oil from water (no external motor is required). 

Time is a critical variable because the magnetite settles if the mixture is held too long in a static condition. Use of hydrocyclones makes possible the separation in a fraction of a minute (21). These devices impart centrifugal force to the system, thereby permitting a separation to be made at a specific gravity less than that required in static, heavy-Hquid separations. 

Clarification may be partially achieved using batteries of small-diameter hydrocyclones (see above) to remove particles down to about 50 pm (Couch, 1991). Complete water clarification requires settling in cones and static thickeners, in conjunction with chemical dosing to promote flocculation and rapid settling. 

B.5.2.3 Hydrocyclone This device has been widely used in various industries such as pulp and paper and mineral processing. The separation/classification of particles in a hydrocyclone is also based on centrifugal force differences between particles of varying size/density. Unlike the centrifuge, a hydrocyclone is a static device. The principle of cell separation by hydrocyclone is shown in Figure 7B.9. 

Hydrocyclones In the range of 80-90% External Shear damage due to vortices interaction Good Possible Robust static device. No special maintenance required Well established designs from reputed manufacturers Multiple cyclones in parallel are a feasible altemalive. High harvest rates/ smaQ cyclones result in shear damage... 

This section describes a new and simple experimental method for obtaining the reduced cut size and the rest of the reduced grade efficiency curve of an operating separator. The method relies on feeding a known and fiiUy characterized slurry to the separator under test, and on measuring only two solids concentrations (in the feed and in the overflow), one static pressure differential (or some other flow rate-dependent variable) and the slurry temperature. These measurements are best done and logged by a personal computer, and, for hydrocyclones, have to be repeated at two different pressure settings. 

Due to the vortex flow in the hydrocyclone, the static pressure in the flow increases radially outward. This centrifugal static head is primarily determined by the distribution of both, the tangential fluid velocities and the suspension densities, within the flow and it constitutes the major contribution to the total pressure loss across an operating hydrocyclone. 

This relationship was then used to calculate the tangential velocities from static pressure measurements in different places within hydrocyclones run with clean liquids. Driessen and many others following him thus deduced the general expression for tangential velocity profiles in the outer vortex given previously in equation 6.1, where n is an empirical exponent, usually from 0.6 to 0.9. Note that for a free vortex in inviscid flow n =, while in a forced vortex (solid body rotation) = 1. 

Figure 6.5 The anaiogy between the static head in a gravity tank (a) and the centrifugal head in a hydrocyclone (b) (note that the hydrocyclone is shown with its axis horizontal)...

Each theory in this category offers a relatively simple correlation for the static pressure drop and the cut size of a hydrocyclone described by a few (but often not all) dimensions. The theories fall into two main groups the equilibrium orbit theory and the residence time theory. 

Since the early 1980s, hydrocyclones have been used in produced water treatment to de-oil the water prior to discharge. Hydrocyclones used to de-oil the water are referred to as "hquid-liquid de-oiling" hydrocyclones. Liquid-liquid hydrocyclones are further classified as static or dynamic hydrocyclones. 

As shown in Figure 3.51, a dynamic hydrocyclone consists of a rotating cylinder, axial inlet and outlet, reject nozzle, and external motor. The rotation of the cylinder creates a "free vortex." The tangential speed is inversely proportional to the distance to the centerhne of the cyclone. Since there is no complex geometry that requires a high pressure drop, dynamic imits can operate at lower inlet pressures (approximately 50 psig) than static imits. In addition, the effect of the reject ratio is not as important in dynamic imits as it is in static units. 

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