Two-stage drying involves spray drying to a moisture content which is, for normal milk powders, about 2 - 5% higher than the required final moisture content. Subsequent fluid bed drying then removes the excess of moisture. The outlet temperature from the spray dryer is about 15 - 25°C lower than with a single-stage process. Consequently the surrounding air temperature at the critical drying stage and particle temperature are correspondingly lower as well. Therefore two-stage drying allows an increase of the inlet temperature and/or feed concentration above such values, which would simply be impossible in the single-stage process. This contributes further to economy improvement.
The completion of moisture removal is carried out by additional fluid bed drying. In this method, warm drying air is supplied gradually to meet the needs of the rate of diffusion to secure the completion of drying. The temperature of the powder, which in this case is anyhow relatively low, remains low and continues to decrease. It only begins to rise again when moisture content approaches its final value. However, no heat damage takes place under these conditions as the inlet air temperature to the fluid bed is too low to cause this.
The second drying stage conducted in the fluid bed requires of course energy input, but in spite of the specific heat consumption being relatively high, after-drying of powder by fluid bed requires only 30 - 50% of that energy, which would have been required if the same drying had been conducted in the first or spray drying stage. Thus, in comparison with single-stage drying, if all other parameters remain the same, the two-stage drying method requires at least 10% less heat. Under certain circumstances considerably more savings are possible by increasing the air inlet temperature and feed concentration. Apart from improved heat economy the plant capacity is also increased.
Two-stage drying has its limitations, but it can be applied to such products as skim milk, whole milk, pre-crystallized whey concentrates, caseinates, whey protein concentrates and similar powders. The level of moisture of the powder leaving the first drying stage is limited by the thermoplasticity of the wet powder, i.e. by its stickiness. With increasing moisture content the temperature at which the powder starts to be sticky (so-called sticking temperature) decreases. The sticking temperature is defined as the temperature at which the powder starts to stick to a warm metal surface and forms deposits and lumps. It depends on the powder composition. The components contributing to the stickiness and thereby to lowering of the sticking temperature are amorphous lactose, lactic acid, sucrose and other carbohydrates.
For skim milk and whole milk powder the moisture content of the powder leaving the spray dryer should be no higher than 7 - 8%. This is to ensure that the powder is continuously discharged under gravity into the fluid bed without lumps and that the chamber remains reasonably free of wall deposits. Any mechanical treatment of wet powder is undesirable as it will create hard lumps. Therefore the only type of drying chamber which is suitable for application of twostage drying techniques has a reasonably steep cone with a separate outlet for the drying air.
The two-stage drying techniques can be applied both for the production of non-agglomerated and agglomerated powders. Agglomeration requires special features which will be discussed later. However, even two-stage dried powder produced without these special means for agglomeration, is always slightly agglomerated and consequently has a lower bulk density. Nevertheless, agglomerates formed due to the high powder moisture content at the chamber outlet are very fragile and are broken down by pneumatic transport or by blow line transport to storage silos. After such treatment the bulk density is usually higher than that obtainable by single-stage drying.
Two-stage drying is very suitable for production of agglomerated powders by separating the non-agglomerated particles from the agglomerates (i.e. collecting the cyclone fractions and reintroducing these fine fractions, so-called fines, into the wet zone around the atomization device). The agglomeration is in this case much stronger since it takes place when the primary particles have much higher moisture content than they would under the same conditions in single-stage drying. For processing of whey the two-stage drying method is possible only with pre-crystallized whey concentrate. Pre-crystallisation transforms a great part of amorphous lactose (which is a component contributing to stickiness) into a-lactose mono-hydrate. Generally, products containing a high amount of amorphous lactose or other carbohydrates are difficult to treat by two-stage processing. It has to be decided on a case to case basis bytesting whether two- stage drying is feasible or not.