Hydrocyclone
Hydrocyclones are centrifugal separators for liquid mixtures. Solid particles contained in suspensions are separated or classified using hydrocyclones. Likewise, emulsions are separated, such as. B. Oil-water mixtures.
Cyclones for gases are dealt with in the main article centrifugal separators .
construction
Apart from special forms, a hydrocyclone consists of the following parts:
- the upper, cylindrical segment with the
- tangential inlet,
- the lower, conical segment with the
- Underflow or apex nozzle and
- the vortex finder (or the overflow nozzle), in the form of a dip tube, which protrudes axially from above into the interior of the cyclone.
In this case, the terms "above" and "below" are based on the lower course (specifically heavier fraction) and the upper course (specifically lighter fraction). The actual positioning of a hydrocyclone is largely independent of this, so horizontally installed hydrocyclones are also used.
functionality
Due to the tangential entry into the cylindrical segment, the liquid is forced onto a circular path and flows downwards in a downward eddy. The tapering in the conical segment leads to a displacement of volume inwards and to a build-up in the lower area of the cone, which leads to the formation of an inner, upwardly directed vortex that escapes through the vortex finder or the upper flow opening. The aim is to separate the specifically heavier fraction (e.g. solids) on the wall of the cyclone and thus discharge it through the underflow, while the specifically lighter fraction escapes through the overflow.
The prevailing currents or flow velocities are thus:
- the vertical flow velocity, which is directed downwards in the outer area and upwards in the inner area. It thus feeds materials either to the upper or lower opening. The area or the area in which the vertical velocity component is equal to "zero" is referred to as the "locus of zero vertical velocity" (LZVV also mantle) or conical classification surface.
- the tangential flow velocity of the circular path, which, unlike in the case of a solid body rotation ( vortex flow ), increases with decreasing radius. The tangential speed is responsible for the acting centrifugal force .
- And the radial flow velocity, which results from the displacement of volume inwards. The flow force acting through them works against the centrifugal force.
In addition, there are turbulences and particle interactions, which also influence the separation.
The basic principle of the separation and classification effect is described by the interaction of centrifugal and flow forces. While the centrifugal force is stronger on large particles and these are thus separated outwards to the cyclone wall, the force of the flow on the particles (resistance force) is of great importance for small particles due to their higher specific surface area. Among other things, this led to the equilibrium orbit theory , in which each particle size is assigned a radius on which the forces acting are in equilibrium. With the help of this simple theory, an attempt was made to predict the separation effect ( separation grain size , degree of separation ). However, the actual conditions are far more complicated, so that it is not possible even with modern, numerical, software-based methods to predict the separation performance of a cyclone. So far, good results have only been achieved for suspensions with a low solids content (approx. 1% by volume). Further theories and approaches to describe the separation include the residence-time theory , the crowding theory and the turbulent two-phase flow theory .
See also the formation of gold soaps and meanders in river beds. Albert Einstein recognized and published the exact principle in 1926 based on the phenomenon that tea leaves collect in the middle when the tea is stirred against the centrifugal force , as a "friction-related secondary circulation" by slowing down the flow at the edge of the river bed (the problem is also known as " Schrödinger's tea leaves ” , the effect also influences the Ekman transport ).
Pressure difference
In the case of hydrocyclones, the pressure difference or the pressure loss means the pressure difference between the inlet and the overflow. The counter pressure in a hydrocyclone is created by the tangential speed and the resulting centrifugal force. The pressure difference to the inner region of the vortex thus also characterizes the velocity gradient from the inner to the outer regions of the vortex. A higher pressure difference thus also results in an increase in efficiency or a reduction in the size of the separating particles.
Separation anomalies
As already described, the separation effect is largely determined by the prevailing currents. This also leads to some anomalies that can occur in a classification.
This includes B. the distance of the degree of separation curve to the zero line, which is caused by a uniform dispersion of fine particles in the fluid. Normally the degree of separation curve for fine particles tends towards zero, since these are separated with the fine material. With hydrocyclones, however, the uniform dispersion of the fine grain ensures that these grain size classes are divided according to the division of the volume flow. In other words: fine grain is separated with the coarse grain in the same proportion as the volume flow between the upper and lower reaches (volume flow ratio).
Another anomaly that occurs is the so-called fish-hook effect, the origin of which has not yet been fully clarified. It relates to an increase in the degree of separation curve in the fine grain range, beyond the value of the volume flow ratio. Recent studies indicate that it is a matter of particle interactions in which fine particles are carried away in the “fairway” of coarse particles.
Short-circuit currents are currents that bypass the vortex and flow directly into the flow escaping through the vortex finder. This means that you can always carry part of the solid matter and coarse material with the fine material through the overflow. However, they can be influenced by structural changes to the vortex finder.
See also:
literature
- Ladislav Svarovsky: Hydrocyclones . Holt, Rinehart and Winston, London 1984, ISBN 0-03-910562-8 .
- Ladislav Svarovsky: Solid-Liquid Separation . 3rd edition Butterworth, London 1990, ISBN 0-408-03765-2 .
- Douglas Bradley: The Hydrocyclone (International Series of Monographs in chemical engineering; Vol. 4). Pergamon Press, London 1965.
- Christian H. Gerhart: Investigations into the separation behavior of hydrocyclones of low separation grain sizes, classification and sedimentation behavior as the cause of the fish-hook effect . Logos Verlag, Berlin 2001, ISBN 3-89722-662-6 (plus dissertation, University of Erlangen-Nuremberg 2001).
- Gabriele Mayer, Steffen Schütz, Manfred Piesche: Theoretical and experimental investigations on flow and separation behavior in hydrocyclones. Final report . Institute for Mechanical Process Engineering, University of Stuttgart 2005.
Individual evidence
- ↑ A. Einstein: The cause of the meander formation of the river formation and the so-called Baer's law , Naturwiss. 14 , 1926, 223-224, reviewed by Karl-Heinz Bernhardt : Tea cup cyclones and river meanders - Einstein classic. , Meeting reports of the Leibniz-Sozietät, 78/79 (2005), 81–95, (PDF file) .