Integrierte Vermeidung und Verminderung der Umweltverschmutzung

Chapter 2
2.1.9.3.5 Boiler feed-water
In the FDM sector, steam is generated using boilers with working pressures of up to
about 30 bar. For power generation with steam turbines, higher steam pressures are needed.
Steam is used for the sterilisation of tanks and pipelines. Another application is UHT treatment
with direct steam injection. Sometimes steam injection is used for heating the product or for
adjusting the water content of the raw material. In all these cases, more or less direct contact
between steam and the food product is possible so drinking water quality is required.
Requirements generally depend on the working pressure and temperature of the boiler and the
conductivity [68, European confederation of organisations for testing, 1984]. The higher the
pressure and temperature, the higher the quality requirements are. This makes extensive water
treatment necessary, e.g. removal of iron, softening and chemical conditioning. The quality of
the boiler water is controlled by the quality of the boiler feed-water. The frequency of boiler
water blowdown also controls the quality.
It is important that boiler feed-water does not cause scaling in the boiler or corrosion of the
steam system. This means that boiler feed-water has to have a very low hardness and be deaerated.
Returned condensate can also be used as boiler feed-water supplemented by suitably
treated make-up water.
2.1.9.4 Vacuum generation (U.4)
2.1.9.4.1 Objective
Vacuums are used primarily to reduce the temperature at which operations take place, thereby
reducing potential deterioration in the quality of the material being processed or, in the case of
edible oil processing, to avoid unwanted oxidation of the product during processing at higher
temperatures.
2.1.9.4.2 Field of application
A vacuum is applied to many unit operations in the FDM sector, e.g. drying, evaporation,
neutralisation and filtration.
2.1.9.4.3 Description of techniques, methods and equipment
There are three basic systems for producing a vacuum, i.e. steam jet ejectors, reciprocating
pumps, and rotary vacuum pumps.
A steam jet ejector, which can produce absolute pressures down to 1 or 2 mm Hg
(133 or 267 Pa), consists of a steam nozzle that discharges a high velocity jet across a suction
chamber connected to the equipment. The gas is entrained in the steam and carried into a
venturi-shaped diffuser that converts the velocity energy of the steam into pressure energy. The
steam and vaporised material from the ejector are condensed either directly in a spray of water,
e.g. barometric condensers, or indirectly with surface type condensers or especially condensed
under low temperature conditions, e.g. ice condensation below -20 ºC.
With barometric condensers, the cooling water can be used on a once-through basis or
recirculated, e.g. in a closed-circuit. In edible oil processing, for saponification of ffa, this is
done over cooling towers, e.g. under high pH conditions. With indirect condensers, the
condensate can be recovered. The size of the condenser depends on the cooling temperature
used, and this also controls the amount of steam required. Chilling (see Section 2.1.7.1) or
freezing (see Section 2.1.7.2) systems can be used to enable operation at low temperature,
thereby reducing steam usage.
56 January 2006 RHC/EIPPCB/FDM_BREF_FINAL