Integrierte Vermeidung und Verminderung der Umweltverschmutzung

2.1.6 Concentration by heat (F)
2.1.6.1 Evaporation (liquid to liquid) (F.1)
2.1.6.1.1 Objective
Chapter 2
Evaporation is the partial removal of water from liquid food by boiling. For instance, liquid
products can be concentrated from 5 % dry solids to 72 %, or even higher, depending on the
viscosity of the concentrates. Evaporation is used to pre-concentrate food, to increase the solid
content of food, to change the colour of food and to reduce the water content of a liquid product
almost completely, e.g. as in edible oil drying.
2.1.6.1.2 Field of application
Evaporation is used in many FDM sectors. For example, it is used to process milk, starch
derivatives, coffee, fruit juices, vegetable pastes and concentrates, seasonings, sauces, sugar and
edible oil.
2.1.6.1.3 Description of techniques, methods and equipment
Steam, vapour, or exhaust gases from other drying operations, are usually used as the heating
medium. The latent heat of condensation is transferred to the liquid food to raise its temperature
to boiling point, to evaporate the water. The vapour is then removed from the surface of the
boiling liquid. Since food products are heat sensitive, it is often necessary to work at low
temperatures. This is achieved by boiling the liquid part under vacuum. Evaporation normally
occurs in the range of 50 to 100 °C, although it can be as high as 130 °C in the sugar industry.
In its simplest form, evaporation is carried out by boiling off water to the air, using immersed
electric heaters. However, in practice the most commonly used equipment is multistage shell
and tube evaporators, or plate evaporators. Shell and tube evaporators may be natural or forced
circulation, climbing or falling film types. Centritherm evaporators, wiped film evaporators
(WFE), thin film evaporators and vacuum pans are specially designed for the evaporation of
highly viscous products. Typical total solids concentrations for various types of evaporators are
shown in Table 2.3. The level of total solids in the outlet depends on the composition of the
product to be concentrated.
Evaporator type Total solids inlet (%) Total solids outlet (%)
Shell and tubes, multistage 5 – 25 40 – 75
Plates, multistage 5 – 25 40 – 75
Vacuum pans 60 – 70 80 – 85
Centritherm, single-stage 5 – 25 40 – 60
WFE, single-stage 40 – 50 70 – 90
Table 2.3: Typical total solids concentrations for various types of evaporators
[1, CIAA, 2002]
Multiple-effect evaporators are used when evaporation requires significant energy, e.g. in sugar
beet processing, starch production and the evaporation of milk and whey. These evaporators use
fresh steam or exhaust gases from other operations, and so recover or re-use energy, to boil off
water vapour from the liquid in the first effect. The evaporated water still has sufficient energy
to be the heat source for the next effect, and so on. A vacuum is applied in a multiple-effect
series to allow the water to boil off. The liquid being processed is passed from one evaporator
body through the others so it is subjected to multiple stages of evaporation. In this way, one unit
of steam injected in the first evaporator can remove three to six units of water from the liquid.
RHC/EIPPCB/FDM_BREF_FINAL January 2006 41