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 ≈45șC - through the bottom
outlet for further processing or recycling depending on what system is
selected.
Two different flow systems can be used:
- Recirculation with water
- Single-pass with milk or whey
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 90-95șC (for
one-stage dryers), being cooled to the wet bulb temperature (≈45șC ) at the
same time evaporating the water. See Fig. 59.
As the
temperature of the water continues to be around 45șC , 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 45șC (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.
The evaporation process in the scrubber will be discussed on page 173.
As temperatures in the scrubber offer favourable conditions for
bacteria, a bacterio-logical 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).