A feed source is introduced to the spray drying system usually through a balance tank (figure 4). For some applications the ingredient is mixed in close proximity to the spray dryer while for large applications feed material is tinkered in and held in silos. The type of tank used in this application will depend upon the industry and product to be dried. Food and pharmaceuticals feed stock would normally be held in an enclosed tank to reduce contamination while some applications may need a jacketed tank to maintain heat to allowing a lowering of the product viscosity.
Feed materials fall into two categories, a ‘slurry’ where the solids are suspended in the feed liquid and a ‘solution’ where the solids dissolve in the liquid. From the feed tank, feed is pumped to the atomiser inside the spray dryer. The type of pump employed depends somewhat on the type of atomiser and the viscosity of the liquid feed.
Spray dryer feed pumps (figure 5) must be robust as they form a critical point in the process. Some of the points that may have an impact on your pump selection are the viscosity of the material, corrosive nature of the product, temperature, the pressure required and amounts to be pumped. Typical pumps for this duty include peristaltic pumps which are very often employed where small flow is required or low pressure applications and very good for pumping viscous materials. On larger low pressure systems progressive cavity pumps are often chosen for their robust nature. For high pressure nozzle applications piston pumps are often preferred as they give good performance at high pressure and are very reliable.
Feed solution is fed into a spray dryer through an atomising device (figure6) that breaks down the feed solution into individual spheres. Spheres or droplets give maximum exposure to heat accelerating the process of evaporation.
Atomisation of the feed into droplets can be induced by the spinning speed of a disc located at the end of a vertical shaft, by a pressurised nozzle or by a nozzle using compressed air. The spinning disc uses the shearing action of air at the disc’s edge to break down the liquid feed into a spray which leaves the disc in a horizontal pattern. The disc, running at very high speed is an expensive delicate mechanism mainly suited for drying slurries containing high percentages of solids.
The pressure nozzle uses pump pressure to force the liquid through special inserts in the nozzle body to produce the desired pattern and droplet size of spray. In comparison with the disc it is an inexpensive piece of equipment and gives much greater control over the powder properties.
The air (two fluid) nozzle relies on air pressure to atomise the liquid and is mainly used on small spray dryers and special co-counter current dryers designed for producing large diameter powders.