Spray dryers are industrial devices used to rapidly dehydrate liquids and produce reliable powders. The starting liquid can be either a reaction (a strong, dissolved substance) or an insoluble, reliable substance suspended in a liquid – often referred to as a suspension. Typical examples of spray drying include instant coffee, powdered milk, paints, egg powder, resins, and detergents. It is used in both the food and pharmaceutical industries.
In its basic form, the spray dryer handles the entire process of receiving liquid raw materials at one end, filling bags or other containers at the other end, and transporting them. In larger plants, various upstream and downstream processes are usually added to reduce potential processing costs. For commodities such as milk, it usually makes economic sense to incorporate evaporation systems into the feed technology to increase the dry matter content of the feed to an acceptable level. Spray drying programs come in many shapes and sizes and look very different, but they all work the same way.
Spray drying is an evaporation process, which is also the most common strategy for producing powders specifically designed for heat-sensitive materials. A spray dryer essentially consists of a large chamber through which very hot air (or other gases) flows, a feed pump, and an atomizer. The powder produced is usually collected in a cyclone drum.
The feed is fed directly into the spray dryer through an atomizer that separates the feed liquid into individual spheres. Nozzles are typically used to produce the smallest droplets possible to maximize heat transfer and evaporation rate of the potable water. Droplet size can range from 20 to 180 microns, depending on the nozzle. Drying chambers are the equivalent of storage silos, which are large open containers whose volume is determined by the time the food ingredients to be dried are exposed to heat.
The raw material (usually air) is typically heated by a burner as it is fed into the main chamber. The reaction liquid supplied is atomized into droplets in the chamber and, when combined with hot air, evaporates to form a powder. When the visible powder particles are combined with the very hot fuel, they solidify into one scenario: a hard outer surface and a molten inner layer.
As the combination of chamber and hot air reduces the size of the particles, the liquid from the relaxation is attracted to the particles. It is usually advantageous if a small amount of moisture remains in the powder particles at the end of the process, known commercially as residual moisture. The time required for the particles to settle in the chamber is referred to as the residence time. Compared to other drying methods, spray dryers can dry solutions very quickly. They also help maximize profits and simplify the process by processing solutions and suspensions into dry powder in a single step.
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