Integrierte Vermeidung und Verminderung der Umweltverschmutzung

Chapter 2
Additional energy can be saved by recompressing the vapour using a thermal vapour
recompressor (TVR) or a mechanical vapour recompressor (MVR). However, this can lead to
steam contamination, making it unsuitable for return to the boilers and, therefore, increasing the
waste water load. In the final stage, the vapour may be condensed by cooling with cooling
water. Some of the vapours can be drawn off the evaporators to be used as heat sources for other
process requirements. The condensate may be of such a quality that it may even be fed to other
processes as process water.
During processing, product compounds gradually deposit on the heat-exchange surfaces. This
can affect the efficiency of the heat-exchange and lead to heat losses in the system. These
deposits may be inorganic or organic, depending on the product. The installation must,
therefore, be cleaned at regular intervals to prevent too high a heat loss occurring with a
subsequent loss of product quality.
2.1.6.2 Drying (liquid to solid) (F.2)
2.1.6.2.1 Objective
Drying is defined as the application of heat under controlled conditions to remove the water
present in liquid foods by evaporation to yield solid products. It differs from evaporation, which
yields concentrated liquid products. The main purpose of drying is to extend the shelf-life of
foods by reducing their aw.
2.1.6.2.2 Field of application
Typical applications for drying techniques include dairy products, e.g. milk, whey and creamers,
coffee, coffee surrogates, tea, flavours, powdered drinks, and processed cereal-based foods.
2.1.6.2.3 Description of techniques, methods and equipment
Two different principles can be applied for drying; hot air drying and surface drying by heat
conduction through a heat transfer system.
In hot air drying, hot air is used as the heating medium and is in direct or indirect contact with
the liquid product. The heat transferred from the hot air to the product causes evaporation of the
water content.
In surface drying by heat conduction through a heat transfer system, the heating medium is not
in contact with wet food but separated from it by a heat transfer surface. The heat is transferred
by conduction through the surface, and by convection from the hot surface to the food product
for evaporating and removing water from the food. This has two main advantages compared to
hot air driers; less air volume is required and, therefore, thermal efficiency is higher, and the
process may be carried out in the absence of oxygen.
The driers used are spray, roller, and vacuum band and vacuum shelf driers.
In spray drying, the material to be dried is suspended in air, i.e. the liquid is converted into a
fog-like mist or atomised liquid, providing a large surface area. The atomised liquid is exposed
to a flow of hot air in a drying chamber. The moisture evaporates quickly and the solids are
recovered as a powder consisting of fine, hollow, spherical particles. Air inlet temperatures of
up to about 250 °C or even higher, depending on the type of product, are used, but due to
evaporation, the air temperature drops very rapidly to a temperature of about 95 °C, which is the
outlet temperature of the air. The product temperature is 20to 30 °C below the air outlet
temperature. Heating the drying air can be accomplished by steam or by direct gas-fired air
heaters or by indirect heaters fired by gas, liquid or solid fuels. Spray drying is applied on a
large scale in the dairy industry and for drying coffee.
42 January 2006 RHC/EIPPCB/FDM_BREF_FINAL