Spray drying, the choice dehydration method for the food and pharmaceutical industries, employs an atomiser and a stream of hot gas to produce dry powders, granules or agglomerates from a liquid or slurry. The atomiser disperses liquid feed into fine droplets from which the liquid evaporates, leaving behind solids. Products dried using this process include powdered milk, cake mix, instant coffee, detergent, dyestuff, ceramic powder and yeast extract.
Advantage: Speed and Versatility
Spray drying's one-step ability to complete the drying process within seconds gives it an edge over other industrial drying techniques. In the food industry, fast drying plays a vital role in ensuring minimum overall flavour loss. The process operates on basic principles and lends itself to automation. The process is versatile and adaptable to a wide range of industries and their feedstock and product specifications. Virtually any feedstock that can be pumped -- solutions, suspensions, slurries, melts, pastes, gels -- can likewise be spray dried.
Advantage: Quality Control
Spray drying produces powders of controllable particle size and overall quality. Other characteristics manipulated during spray drying include bulk density, degree of crystallinity and residual solvent levels. Being able to control these parameters makes the process desirable for industries like pharmaceuticals, where the optimum absorption of a drug depends greatly on particle size. In dyestuff production, powders of uniform consistency ensure their convenient dispersion into paint suspensions. The food industry puts a premium on moisture content, which determines a product's shelf life.
Disadvantage: High Capital and Overhead Costs
Spray drying racks up huge costs from the equipment required and from its continuous operation. Main and auxiliary equipment are equally expensive, regardless of atomiser type and dryer capacity. Spray dryers generally have low thermal efficiencies, due to the large volume of hot air that circulates in the chamber without contacting the particles. Additionally, spray dryers that utilise two-fluid nozzles require compressed gas for atomising. Without factoring in labour and maintenance costs, the high energy and pressure requirements already add greatly to the overhead costs.
Disadvantage: Maintenance Issues
Spray dryer maintenance mostly involves issues with the nozzle used. One-fluid and two-fluid nozzles are particularly prone to clogging and abrasion at the nozzle mouth. Rotary disc atomisers, due to the number of moving parts in direct contact with the powders, suffer from internal corrosion. Finally, issues with powders sticking to the internal chamber walls further contribute to cleaning costs and profit losses.