Before the whey is concentrated and dried, it must undergo pre-treatment, including pasteurisation and cooling. This must be carried out just as carefully as for milk. If this is not done immediately and properly, fermentation continues, generating lactic acid, which will make the whey extremely difficult to dry and produce a very sticky and hygroscopic product. Since 70-74% of the whey solids consist of lactose, it is important that the lactose is crystallised prior to the drying process in order to obtain a powder with the best possible properties.
For dairy applications, including evaporation of whey, the preferred type of evaporator is the tubular, falling-film evaporator.
These are based on vertical calandrias with tubes, normally with a diameter of 50 mm and up to 18 m in length. Liquid is pumped through the pre-heater to the pasteuriser and enters the top of the calandria in which it is evenly distributed to the falling-film tubes. A liquid film is formed along the internal tube walls and as the liquid falls, the evaporation begins. The heating medium is steam/vapour. At the bottom of the calandria, the concentrate is extractedusing a concentrate pump.
The product is separated from the vapour in the dynamic separator connected to the static separator. By using multi-stage operation with thermal vapour recompression (TVR) where vapour from one effect (stage) is used as a heating medium for the following effect, the energy consumption is substantially reduced. The primary energy source of TVR is live steam. When electricity is available at low cost, the principle of mechanical vapour recompression (MVR) may be used. In this system, most of the energy for the evaporation process comes from the compressor and there is almost no need for steam. To bring the TS content from 6% to 55-62%, the whey is pre-concentrated in an MVR pre-evaporator followed by a TVR finisher or, alternatively, in a multi-stage TVR evaporator. The choice of evaporator is mainly determined by the price of steam versus the price of electricity.
For spray drying of whey a large number of tiny crystals are required. This crystallisation is obtained by cooling the concentrate, immediately after evaporation in a flash cooler to a temperature of 30-45°C depending on concentration. Then, the product is slowly cooled in a crystallisation tank provided with a slow-moving reversing agitator.
Drying of the concentrated whey takes place in a two- or three-stage dryer with internal and external fluid beds. In some cases, the external fluid bed is replaced by a cooling and conveying system.
In the spray drying process, the product is fed to the atomiser system and sprayed downwards in the drying chamber co-currently with the hot drying air. The required degree of moisture in powder is reached in the fluid beds. From the integrated fluid bed, the powder continues to the external fluid bed for cooling and final crystallisation of lactose. The typical inlet drying air temperature is 170-190°C.
Reverse osmosis (RO) is another important method for the concentration of whey. The difference between evaporation and reverse osmosis is that reverse osmosis takes place at low temperatures (typically below 35°C) and consumes less energy. The process takes place at high pressure (over 30 bar).
The maximum obtainable concentration factor depends on the processing temperature, the pre-treatment of the feed product, fouling, etc. When whey is concentrated by RO, it will at some point become uneconomical to continue the concentration because of a low flux and investments increased membrane area. Typical viable TS levels are:
- Sweet whey, 21%
- Acid whey, 17%
Evaporation and reverse osmosis compared
The main difference between evaporation and reverse osmosis is that evaporation removes water by the application of heat which turns the water into steam, followed by condensation of the vapour, whereas reverse osmosis is a simple filtration process requiring mechanical energy. Using reverse osmosis means that it is unnecessary to turn the water into steam.
With regard to the solids levels that can be obtained, RO is limited by the osmotic pressure of the concentrated solution and for whey, the maximum level is approx. 25-28% TS. Evaporation, on the other hand, is only limited by the solubility of the solutes at the evaporation temperature and thus a TS level of 60-65% is obtainable.
The key parameter in a comparison between the two processes is the energy consumption, but other operating costs must, of course, be considered, e.g., the consumption of cleaning chemicals, the cost of membrane exchange and membrane rejection, as well as the purity of the condensate and permeate. Investment costs in the actual plant and possible building costs in the case of evaporators are also important parameters. In each specific case, SPX FLOW is able to calculate and design the optimum solution: evaporation only, reverse osmosis only, or a combination of the two systems.