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 TIXOTHERMTM 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.