|technology > separation techniques > centrifugation|
|Field of application|
|Description of techniques|
|Separation by gravity|
|Separation by centrifugal force|
|Tubular/disc bowl centrifuges|
|Solid bowl nozzle or valve type centrifuges|
|Conveyor Bowl/reciprocating conveyor/basket centrifuges|
dairy industry in the clarification of milk, the skimming of milk and whey, the concentration of cream, in the production and the recovery of casein, in the cheese industry, and in lactose and whey protein processing, etc. This processing technique is also used in beverage technology, vegetable and fruit juices, coffee, tea, beer, wine, soy milk, oil and fat processing/recovery, cocoa butter, and in sugar manufacturing etc.
sedimentation and skimming). In beer production, clarification of the hot wort is carried out in order to remove particles (hot trub) to get a clear wort. The commonly used equipment for wort clarification is the whirlpool, where wort trub particles are separated in tangential flow.
- Batch-wise: this occurs in a vessel containing a dispersion of solid particles with a higher density than the liquid. In time these heavier particles fall to the bottom of the vessel. If the height of the vessel is shortened and the surface increased, the sedimentation time can be reduced.
- Continuous: the liquid containing the slurried particles is introduced at one end of the process and flows towards an overflow. The sedimentation capacity of the vessel can be increased by adding baffle plates (horizontal or inclined).
- tubular/disc bowl centrifuges for separating of immiscible liquids
- solid bowl/nozzle valve discharge centrifuges, for clarifying liquids by the removal of small amounts of solids
- conveyor bowl/reciprocating conveyor centrifuges, for dewatering sludges (with a high solids content).
Typically, the disc bowl centrifuge is more widely used in the food industry as it can achieve a better separation due to the thinner layers of liquid formed. With the disc bowl centrifuge, the cylindrical bowl contains inverted cones or discs. The liquids only have to travel a short distance to achieve separation. These centrifuges operate at 2000 – 7000 rpm and have capacities of up to 150000 l/h. Disc bowl centrifuges are used to separate cream from milk, and to clarify oils, coffee extracts and juices or to separate starch from slurry.
wall whilst the liquid spills over the top of the bowl. Periodically the centrifuge has to be stopped to enable the cake to be removed. Liquors containing higher levels of solids, i.e. >3% w/w, can be separated using nozzle or valve discharge centrifuges. These centrifuges are a modified disc bowl centrifuge with a double conical bowl and enable the discharge of solids automatically. These types of centrifuges are used to treat oils, juices, beer and starches to recover yeast cells. They have capacities of up to 300000 l/h. A special type is the “bactofuge”, which is specially designed to separate micro-organisms from milk. Bacteria, and particularly spores, have a higher density than milk and the solids are called bactofugate.
tea slurries and to desludge oils. In the conveyor bowl centrifuge (decanter), the solid bowl rotates at 25 rpm faster than the screw conveyor. This causes the solids to be conveyed to one end of the centrifuge whereas the liquid fraction moves to the other larger-diameter end.
The reciprocating conveyor centrifuge is used to separate fragile solids (e.g. crystals from liquor). The feed enters a rotating basket through a funnel, which rotates at the same speed. This gradually accelerates the liquid to the bowl speed and thus minimises shear forces. Liquid passes through perforations in the bowl wall. When the layer of cake has built up it is pushed forward by a reciprocating arm.
The basket centrifuge has a perforated basket lined with a filtering medium, which rotates at 2000 rpm. Separation occurs in cycles, which last from 5 – 30 minutes. In the three stages of the cycle the feed liquor first enters the slowly rotating bowl, the speed is then increased and separation takes place, finally the speed of the bowl is reduced and the cake is discharged through the base. Capacities for this group of centrifuges are up to 90000 l/h.