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The spray drying may be performed according to three different processes, depending on the requirements to the final powder:
SPRAY DRYING BY THE PNEUMATIC CONVEYING PROCESS
In this process the whey concentrate is fed to the atomization device, which may either be of the pressure nozzle type or a rotary atomizer. As earlier described the drying is effected by introduction of hot air through the air disperser into the drying chamber. The air takes up the evaporated water from the concentrate and then passes through the cyclones where the powder particles carried along are separated.
The main part of the powder particles is discharged from the conical bottom of the chamber. Here it is fed to the pneumatic conveying and cooling system, which also collects the above mentioned cyclone fraction.
The total amount of powder is separated in a cyclone, the powder is sifted and bagged off.
This type of plant is shown in Fig. 134 and comprises a falling-film evaporator and a spray drying plant with pneumatic conveying system. The powder from such a plant has the following characteristics:
Fig. 134 Evaporator, crystallization tanks, and spray dryer with pneumatic conveying system
This kind of powder is mainly used as animal fodder. The hygroscopic tendency is due to the lactose in the powder being in the so-called amorphous stage which means water absorbing and sticky. The outlet temperature and content of residual moisture must therefore be kept low. This means also that the solids content must be kept low. The conditions for this process are usually the following:
Spray drying of precrystallized whey concentrate can be performed under much more economical conditions, as the drying temperature and the solids content can be increased. The total drying costs can thus be reduced. As most of the lactose is crystallized, and therefore can absorb very little water, the powder will be of a much better quality, because the caking tendency has been reduced.
Typical processing conditions are as follows:´
SPRAY DRYING BY THE FLUID BED PROCESS
Installation of static integrated fluid beds and/or Vibro-Fluidizers for after-drying and cooling will improve the drying economy. The principle is that the spray dryer is operated in such a way that the powder is dried to a moisture content of 5-6% in the primary drying stage.
The excess moisture is removed in the Vibro-Fluidizer by a hot air stream drying the powder to the final moisture content. By introducing cold air to a later section of the Vibro-Fluidizer, the powder is cooled to the wanted temperature. The air used for the after-drying and cooling is passed through a cyclone where the powder carried along with the air is recovered and returned, together with the main cyclone fraction, to the atomizer for agglomeration. The agglomeration means that the powder will be free-flowing and the specific surface reduced, so that any moisture absorption is further reduced resulting in a powder with practically no caking tendency.
In Fig. 135 a flow sheet of the non-caking whey process is shown, including evaporator, crystallization tanks and spray dryer with static fluid bed and Vibro-Fluidizers. Typical drying conditions are:
Fig. 135 Evaporator, crystallization tanks, and spray dryer with Vibro-Fluidizer
SPRAY DRYING BY THE BELT PROCESS
Another process is the so-called belt process, or wet process, which is especially advantageous when powders with very good caking/hygroscopic properties are required (see also Drying of Permeate, page 271), or when drying lactic acid whey which is very thermoplastic, due to the high content of lactic acid being liquid in dry form, even at low temperature. Lactic acid whey powder is tremendously sticky during the spray drying process. A low drying/particle temperature is therefore necessary in order to avoid chamber deposits. Typical drying conditions are:
In order to avoid condensation in the cyclones, due to a low outlet temperature (ap-prox. 55ºC), hot air may be introduced into the cyclones thus reducing the relative humidity.
Bag filters - standard or CIP-able - are not recommended due to the high moisture/relative humidity in the exhaust air. Wet scrubbers are therefore recommended.
The high moisture content in the powder means that the crystallization will continue in the powder to an even greater extent than in the process featuring fluid beds only, as discussed on page 261. This crystallization is improved, if the wet powder is kept at the high moisture content for 5-10 min. This is done on a belt conveyor or a rotating disc between the chamber outlet and the Vibro-Fluidizer, where the final drying takes place, see Fig. 136. The powder will consist of big agglomerates which will have a low bulk density and extremely good instant and non-caking properties. The bulk density may be increased by a hammer mill treatment.
Fig. 136 Spray Dryer with belt for after-crystallization
SPRAY DRYING OF HIGH CONCENTRATED WHEY CONCENTRATE
In an attempt to improve drying economy, the solids content in the concentrate has been experimentally increased up to 70-75% TS. To avoid crystallization in the last effect, as the lactose becomes supersaturated, the evaporator is equipped with a specially designed high concentrator operating with recycling and at a higher boiling temperature. Further, the higher temperature is necessary to lower the viscosity of the concentrate to avoid blocked tubes.
When the concentrate has reached its final solids content, it is flash cooled, and the crystallization takes place instantaneously. Normal crystallization tanks are not necessary, only an intermediate tank with agitation. However, the high temperature during the long recycling time in the evaporator has its price. The whey proteins are denatured, and valuable nutritional values are lost. Further, the Maillard reaction will start resulting in a pronounced discolouring and bad flavour in the final powder.
Therefore, even though the economy is attractive the end result - the final powder - does not meet international standards.
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