The other product from the ultrafiltration is the permeate, usually
containing about 4.5% lactose. The permeate can be used for fermenting,
production of glucose-galactose or lactylurea, or concentrated, crystallized
and then dried.
The concentration can be done either in a hyperfiltration plant and an
evaporator, or in an evaporator alone to 60% solids. Special attention should
however be paid to the Ca3(PO4)2 complex, as
it is a well-known fact that it will precipitate at high temperatures,
especially if the concentration is high and the pH is right.
If the Ca++ is not inactivated, the precipitation will occur as
deposits on the heat surface in the evaporator, especially in the first effect.
It is therefore necessary to make an intermediate cleaning of the evaporator
after 6-8 hours with acid.
In the industry the Ca-phosphate is precipitated in the whey/permeate by
adjusting the pH to 7.2 by addition of caustic (Mg(OH)2 or NaOH, see
also page 267), after which the product is heated to ≈80ºC. The precipitated
Ca-phosphate can then be removed by centrifugation or membrane filtration. The
discharged product can be dried and sold as "milk calcium".
After the decalcium process the product can be evaporated and dried
directly, but it is recommended to "neutralize" it by adding citric or acidic
acid, as it will otherwise give problems with deposits in the drying
chamber.
It is not possible to dry this concentrate direct due to the hygroscopic
tendency of the resulting powder, and crystallization is necessary. This is
carried out exactly as described under the crystallization of whey concentrate,
and also the drying is equal to the drying of whey. However, three different
spray drying processes should be mentioned:
1. The Dry Process
2. The Wet Process
3. The TIXOTHERM™
Process
DRY PROCESS
In the so-called "dry process" the
precrystallized permeate concentrate is dried in a Compact dryer (see Fig.
140a). Rotary atomization is preferred, as the atomizer wheel offers the best
alternative, as it can handle products containing crystals without wear.
Further rotary atomization is ideal for concentrates with high
viscosity.
The moisture content in the integrated fluid bed, however, is too
low (≈5% free moisture) to allow for further crystallization of the lactose.
The mobility of water needed to form lactose crystals is simply not present.
Higher moisture content in the powder in the fluid bed, needed for the
crystallization to proceed (made possible because of the supersaturation of the
lactose during the drying process) will make the fluidization of the powder in
the fluid bed impossible, and any attempt to make it happen will result in
drying out of the water, so that a post-crystallization will not take place.
Fig. 140a Compact
dryer for permeate - Dry process
However, the resulting powder will exhibit normal good hygroscopic and
caking tendencies, and the drying economy will be acceptable. As always, focus
has to be on powder emission. Cyclones can be used followed by wet scrubbers
using water as scrubbing medium, or normal bag filters. Due to the relative low
moisture content in the exhaust air, needed for the process to work at all, it
can also be passed through a CIP-able bag filter (see page 111). All powder
will thereby be collected as firstgrade powder, and powder emission is
avoided.
WET PROCESS
As permeate powder is used throughout the
world, also in countries where the climate is humid, the powder will absorb
moisture from the ambient, if the lactose is not totally crystallized. As
explained under the "dry process" the lactose cannot reach a 100%
crystallization due to lack of "mobile water" during the process. The remaining
amorphous lactose will therefore absorb moisture from the air, when the bag is
opened, if the ambient air has a high enough moisture content. The powder will
cake, and lumping will be the result leaving an unattractive product to the
end-user.
The belt or wet process for whey powder, as described on page 261, is
therefore often applied to permeate as well. The permeate is evaporated in a
multi-stage evaporator, preferably including a calcium phosphate
precipitation/separation process to allow for a 20 h run without intermediate
cleaning (see page 247). Typically the concentrate reaches a 60-65% TS before
flash cooling to 30ºC followed by further cooling and crystallization in
crystallization tanks during 16-24 hours before spray drying.
The actual drying is typically done in a Tall-Form spray drying plant using
inlet temperatures of 150-160ºC, depending on ambient conditions and an outlet
temperature of ≈55ºC, close to the dew point (read high moisture content) of
the outlet air. The low outlet air temperature results in a moisture content in
the powder leaving the drying chamber as high as 8-10%. The moist and
relatively cold powder is discharged onto a slowly moving timing belt or disc
with a residence time of 10-20 min.
As the concentrate with the saturated lactose solution is dried, it becomes
supersaturated again; however, the lactose will not crystallize during the
spray drying process. The time is simply too short, but as the powder is
discharged onto the timing belt at a high moisture content, the relative cold
powder with supersaturated lactose will start to crystallize, as there is
enough "mobile water" and time for the crystallization process to proceed.
When the powder has been finally crystallized on the belt or the disc, it is
discharged into a Vibro-Fluidizer for final drying and cooling.
The exhaust
air at ≈55ºC from the drying chamber contains a high moisture content in g
H2O/kg dry air. Special attention should therefore be paid to the
exhaust system.
Air exhaust through cyclones and wet scrubbers is an
alternative often used, depending on type of dryer, especially the air
extraction design. However, special attention should still be paid to the
powder extraction from the cyclones, as the moist fines particles are sticky,
and normal rotary valves will not work. Venturis with a positive air/powder
extraction at the base of the cyclone should therefore be established. The
exhaust air can also be passed through a CIP-able bag filter, provided the
dryer design is so that the fines amount can be controlled to avoid clogging of
the filter bags.
Even though it has been argued earlier that products with crystals are best
atomized by a rotary atomizer, a dryer with nozzles is chosen for the "wet
process", see Fig. 140b. This is because the wet permeate powder is very
sticky, and any obstructions to the air/powder movement in a traditional drying
chamber with the air exhaust duct protruding into the chamber will result in
powder deposits. The laminar downward air flow in the dryer combined with
nozzle atomization is therefore chosen at the cost of nozzle wear.
Fig. 140b
Tall-form dryer for permeate - Wet process
TIXOTHERM™ PROCESS
Realizing that the price for permeate
powder is low many attempts have been made to develop a process involving low
investment, low energy consumption and reduced space requirements.
The TIXOTHERM™ process, developed by GEA Niro (patent pending) offers these
advan-tages, however, it has only been tested on a pilot plant, but industrial
size plants are already being designed. See Fig. 140c.
Fig. 140c The
TIXOTHERM™ Process
The process is simple and requires the following equipment:
Evaporator: (increasing the solids content from 6%
to 60% - no flash cooling and pre-crystallization)
TVR Multi-effect or MVR solutions can be chosen. To ensure 20 h production a
decalcification process should be included.
Paddle Dryer/Evaporator: (increasing the solids
content to 85%).
Here a ROSINAIRE™ paddle dryer is selected. It consists of a double jacketed
horizontal tube heated by means of steam. A shaft in the centre with paddles is
moving the concentrate slowly forward, while the evaporation takes place. To
remove the vapour a counter-current air stream is established, which is passed
through a wet scrubber for cleaning, and together with the evaporation itself
this keeps the temperature of the product low, and discolouring is avoided. As
the lactose will be super-saturated, a spontaneous crystallization will
occur.
A viscosity increase will take place partly due to the increase in the
solids content and crystallization, but due to the tixotropic nature of the
product the vigorous mechanical treatment in the ROSINAIRE™ will keep the
product fluent.
When the product is discharged at ≈85%TS it is like a paste.
Screw Conveyor/Cooler:
The next processing step is a holding/curing step taking place in a screw
conveyor with two augers. The screw conveyor is double jacketed and chilled
water is circulated to cool the product. This is done for the following
reasons:
- The lactose becomes super-saturated again and further crystallization is
obtained as the holding time is long enough to obtain almost 100%
crystallization
- Browning of the product is avoided
- The viscosity is increasing, facilitating the following disintegration in
the fluid bed.
Fluid Bed Drying:
At the discharge, the product exhibits a texture suitable for fluid bed
drying. The product is therefore passed on to a combined back-mix/plug-flow
fluid bed similar to the lactose fluid bed - see below.
Milling of the final product is recommended.
The result of the above process is a saving of 30% energy and 50% building
requirements.
No pre-crystallization tanks are needed, the high-concentration step takes
place at atmospheric pressure, and spray drying is not necessary.