How to optimize powder production in the dairy sector?
Stefan Tötterman, Lead Process Control Specialist / 05 Nov 2018
Drying is the oldest method available for preservation of food products. The advantage by drying is not only that it inhibits the growth of micro-organisms, it also reduces the weight and makes the storage and transportation of the product cheaper. The most common powder products in the dairy sector are milk and whey powder. Drying of dairy powder products are mass transfer processes where water is removed by evaporation. Spray dryers are used for producing milk and whey powder from liquid or slurry by drying it fast with hot air. For successful drying and easy operation of the spray dryer, the drying air should be hot, dry and quickly moving through the dryer unit. This makes the drying process energy demanding and small changes in operating conditions usually have quite big impact on the production costs.
Most of the dryers are using heated filtered air, taken from the outside of the plant, as a drying air. If the drying air moisture is constant and if the dryer liquid feed quality is also stable, optimal drying parameters can be quite easily tuned. This is however seldom the situation. Since 1 kg of outside air might contain up to 25 g of water on a warm rainy day and almost nothing on a cold winter day, the amount of water fed to the dryer along with the air is varying significantly. Changes in the water content of the liquid feed can even be greater than this and together the variations can be very large. In order to operate the dryer economically in an optimal way and to get a stable product quality, the water balance of the dryer should be continuously monitored and the operational parameters of the dryer should be adjusted according to present conditions. Not too fast changes, but fast enough to react on changed outer conditions.
Different products behave in a different way in the dryer. One of the most important constraints to pay attention to is the product stickiness. The dryer should be continuously operated on the safe side of the sticky curves in order to maintain a long lasting production run without fouling the dryer. The stickiness of different products can easily be monitored in real-time when the water balance is calculated. The particle size sprayed into the dryer is another important factor. Small sized particles have bigger surface area in relation to the particle volume and therefore the moisture can easier be evaporated. On the other hand, too small particles in combination with too quick drying can cause case hardening as well as complicate the particle recovery from the dryer outlet air.
There is in other words a lot to take into account when optimizing a dairy powder production line. If the dryer feed is maximized subject to dynamically varying constraints and the dryer feed consists of an output of another unit (such as an evaporator), there is probably also a need to adjust the feed to the foregoing unit in order to maintain the mass balance of the whole line. This can also be challenging to accomplish using traditional methods due to slow dynamics and long time-delays.
This is a typical example of a process that can with advantage be both controlled as well as optimized using NAPCON Controller and Optimizer (based on Model Predictive Control). Moisture meters can be utilized for the calculation of the unit water balance. Online NIR analyzer results can be utilized as well for controlling the powder moisture but NAPCON Controller is also able to optimize and control a dryer without installed NIR analyzers. NAPCON solutions make use of historical data of normal and special production states in order to increase intelligence and improving performance.