Integrierte Vermeidung und Verminderung der Umweltverschmutzung

Chapter 2
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. Cakes can also be removed without stopping the
centrifuge. 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 which enables the automatic discharge of solids. These types of
centrifuges are used to treat oils, juices, beers and starches to recover solid parts, e.g. yeast cells.
They have capacities of up to 300000 l/h. A special type is the bactofuge, which is a high speed
centrifugal process which separates bacterial cells and spores from milk.
The conveyor bowl or decanter, reciprocating conveyor and basket centrifuges are used when
the feed contains high levels of solids or sludges. They are used to recover animal protein, e.g.
precipitated casein from skimmed milk, vegetable proteins, to separate coffee, cocoa and tea
slurries and to remove sludge from oils. In the conveyor bowl centrifuge, 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 solids 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 to 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 are up to 90000 l/h.
A typical hydrocyclone comprises a conical section terminating in a cylindrical section. The
hydrocyclone is fitted with a tangential inlet and closed with an end plate, with an axially
mounted overflow pipe or vortex finder. The end of the conical portion terminates in a circular
apex opening. During operation, the slurry is forced, under pump pressure, through the
tangential inlet and this produces a strong swirling motion. The fraction of the slurry contains
the lighter particles discharged through the overflow, and the remaining slurry and solids are
discharged through the underflow opening.
2.1.3.5 Filtration (C.5)
2.1.3.5.1 Objective
Filtration is the separation of solids from a suspension in a liquid by means of a porous medium,
screen or filter cloth, which retains the solids and allows the liquid to pass through. Air filtration
is discussed in Sections 4.4.3.7.1, 4.4.3.7.2 and 4.4.3.7.3.
2.1.3.5.2 Field of application
Filtration is used in the FDM sector to clarify liquid products by the removal of small amounts
of solid particles with the subsequent recovery of the filtrate, e.g. for wine, beer, oils and syrups,
and to separate a liquid from a significant quantity of solid material to obtain the filtrate or cake,
or both, e.g. for fruit juices or beer.
2.1.3.5.3 Description of techniques, methods and equipment
The technique operates either by pressure filtration, i.e. applying pressure to the feed side, or
by vacuum filtration, i.e. applying a vacuum to the filtrate side.
The two main types of pressure filtration are the plate and frame filter press and the leaf filter.
20 January 2006 RHC/EIPPCB/FDM_BREF_FINAL