Sedimentation and centrifugation are used to separate immiscible liquids and solids from liquids. The separation is carried out by the application of either natural gravity or centrifugal forces.
Field of application
Centrifugation is typically used in the 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.
Description of techniques
Centrifugation is used to separate mixtures of two or more phases, one of which is a continuous phase. The driving force behind the separation is the difference in density between the phases. By using centrifugal forces the separation process is accelerated. The necessary centrifugal forces are generated by rotating the materials. The force generated depends on the speed and radius of rotation. In raw milk for example, the skimmed milk is the continuous phase, the fat phase is a discontinuous phase formed of fat globules with diameters of some microns, and a third phase consists of solid particles, hairs, udder cells, straw etc. When the differences in density are large and time is not a limiting factor separation can take place by gravity (known as 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.
a) Separation by gravity
• 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).
b) Separation by centrifugal force
Centrifuges are classified into three groups:
• 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).
c) Tubular/disc bowl centrifuges
A tubular bowl centrifuge consists of a vertical cylinder, which rotates between 15000 – 50000 rpm, inside a stationary casing. It is used to separate immiscible liquids, e.g. vegetable oil and water or solids from liquid. The two components are separated into annular layers, with the denser liquid solid setting nearer to the bowl wall. The two layers which are then discharged separately. 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.
d) Solid bowl nozzle or valve type centrifuges
A solid bowl centrifuge is the simplest solids/liquid centrifuge and is useful when small amounts of solids must be removed from large volumes of liquid. It consists of a rotating cylindrical bowl. Liquor is fed into the bowl; the solids settle out against the bowl 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.
e) Conveyor Bowl/reciprocating conveyor/basket centrifuges
These centrifuges are used when the feed contains high levels of solids (sludges). They are used, for example, to recover animal and vegetable proteins (i.e. precipitated casein from skimmed milk), to separate coffee, cocoa and 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.