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You are reading the e-book Milk Powder Technology by Vagn Vestergaard.
The wet scrubber has been known for many years in the chemical industry. It is based on the venturi scrubber principle. When introduced in the dairy industry, it proved especially efficient, i.e. an efficiency very close to 100 %.
The droplet separator is designed according to the well-known cyclone principles, however with a modified outlet, resulting in a minimum liquid level, thereby minimizing bacteria growth, and a design reducing foam building. However, use of antifoam agents is required, depending on the product being dried.
The principle in the venturi wet scrubber is the following: - See Fig. 58
The outlet air from the spray dryer containing powder particles is accelerated to a high velocity in the venturi inlet, where also the liquid is injected through full-cone nozzles. Due to different velocities between the air/particles and the liquid droplets, they will collide, and the powder will dissolve in the liquid droplets. Passing through the subsequent diffuser the dissolving process will continue simultaneously with a certain pressure recovery of the air/droplet mix.
Passing through the separator, air and liquid are separated again. The air leaves through the centre duct and the liquid - having reached the wet bulb temperature ≈45 ºC - through the bottom outlet for further processing or recycling depending on what system is selected.
Two different flow systems can be used:
Recurcultion with water
According to above description of the principle, water is recycled by means of a centrifugal pump. The flow is controlled by a valve, and the actual flow can be seen on a flow meter. The level is kept constant in the separator by a tank with level control simultaneously ensuring addition of water to add up for the evaporation taking place in the scrubber. The evaporation takes place due to the air temperature from the dryer, which is typically 90-95 ºC (for one-stage dryers), being cooled to the wet bulb temperature (≈45 ºC ) at the same time evaporating the water. See Fig. 59.
As the temperature of the water continues to be around 45 ºC , bacteria growth must be expected after some time. Even though the scrubbing liquid is added up due to the evaporation, the liquid will after 4-6 hours contain bacteria and should be used only as animal feed.
However, a pasteurization and cooling is recommended before sending the liquid to the farmers. Evaporation and drying are also carried out in some cases, but the resulting powder is still used for animal fodder.
Single pass with milk or whey
The scrubber can also operate in a single-pass system with milk or whey as the scrubbing liquid, see Fig. 59a. This is especially advantageous as the evaporation, which will always take place, now takes place from the product which is then preconcentrated a little. The solids increase will naturally also comprise the powder particles picked up from the air. Cold milk or whey is pumped into the preheating system of the evaporator as discussed earlier. When the milk or whey has reached 45 ºC (wet bulb temperature) in the preheater, it is passed to the wet scrubber, where as mentioned it is slightly preconcentrated simultaneously with the cleaning of the exhaust air from the dryer.
The product is returned to the rest of the preheaters of the evaporator and is finally pasteurized prior to the evaporation and drying.
As temperatures in the scrubber offer favourable conditions for bacteria, a bacteriological growth must be expected. Single pass with the milk and whey is therefore not used, but only mentioned as an example of how the overall dryer efficiency can be improved and approached (see Formula (17) page 129), if the inlet air at the same time is preheated by condensate from the last effects of the TVR evaporator (see page 169).
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