Spray drying is an advanced drying technology that is used in various industries. The development of the spray drying process is closely linked to the dairy industry and is require for longer shelf life of many food products. The origins of spray drying date back to the 1800s, but it was not until the 1850s that the process began to be used on an industrial scale. This method was further developed, and today a wide range of products are spraydried, with capacities ranging from a few kg/h to several dozen tons/h. Spray dryers are currently used for research and commercial purposes for drying agrochemical and biotechnological products, fine and heavy chemicals, dairy products, colorants, mineral concentrates and pharmaceuticals.
Spray-dried products can be divided into three groups, depending on the morphological structure of the particles (Walton & Mumford, 1999):
- skin-forming;
- porous;
- with crystalline structure.
As a rule of thumb, organic substances belong to the group of skin-forming materials, water-soluble inorganic substances to materials with crystalline structure, and water-soluble inorganic substances to porous materials. However, it is important to remember that this is only a general classification and that there are exceptions (Walton, 2000).
Skin-forming materials are spherical and have a relatively smooth surface, which is usually filled with gas. With this type of material, drying initially takes place on the surface of the droplet, resulting in a thin, hard outer layer known as the "skin" or "shell". The thickness of the skin is usually between 50 and 130 µm. During the process, the skin thickens and a solid or hollow particle is formed, depending on the material being dried. Hollow particles tend to collapse after drying, while solid particles retain their shape. At a higher drying temperature, around 200 °C, the skin is quickly formed, whereupon the gas trapped inside the particle ruptures and causes it to collapse. At this temperature, the thickness of the "crust" is between 30 and 50 µm. In some materials, secondary bubbles can form in the original particle. This is caused by a certain amount of residual moisture inside the particle.
Skin-forming materials include: Sodium silicate, sodium dodecyl sulfate (SDS), potassium nitrate, gelatin, skim milk, chicken eggs (Walton & Mumford, 1999) or maltodextrin (Zbiciński & Kwapińska, 2003).
Porous materials, also known as agglomerates, consist of individual particles bound by submicron dust or a binder. The particles generally have a regular, spherical shape. Drying of this type of material is achieved by gradual evaporation of moisture from the interior of the particles. The highly porous structure allows water vapor and gasses to flow freely from the inside of the particles to their surface. This explains the high degree of sphericity of the particles and the rare occurrence of irregularities on their surface.
If the initial particle size of the suspension is much larger than 1 µm, the particles tend to form a solid structure upon drying, while the resulting particles are hollow if the initial particle size is less than 1 µm.
In contrast to skin-forming materials, the morphology of porous materials practically does not depend on the drying temperature. Only the drying speed changes with the temperature. Porous materials include, among others: Silica, colloidal carbon, cocoa and some detergents (Zbiciński & Kwapińska, 2003).
Materials with crystalline structure are characterized by a highly ordered structure of their atoms or molecules. The solid phase is formed by the growth of crystals on nucleation centers on the droplet surface. In this type of material, the morphology of the particles depends largely on the type of substance. Sodium chloride, for example, forms large, cubic crystals, while sodium benzoate forms small, elongated crystals. Both solid and hollow particles can occur, and a relatively large shell thickness of the hollow particles is observed, namely 200-300 µm.
At a higher drying temperature, over 200 °C, a phenomenon analogous to that observed with skin-forming materials is observed, namely the disruption of the particle structure and the secondary formation of nucleation centers. In addition, materials dried at a higher temperature are characterized by much lower shell thickness of the empty particles, i.e. 50-100 µm
Materials with crystalline structure include, among others: Sodium chloride, sodium carbonate, zinc sulfate, sodium pyrophosphate, sodium benzoate, sodium formate (Walton & Mumford, 1999)
The research program presented for IFPRI assumes finding the relationship between the rheological properties of the solution as well as the drying speed on the morphology of the particles obtained by the spray drying method. For this purpose, in the second year of the project, the following tasks were assigned:
- Carrying out spray drying experiments on the semi-industrial scale.
- Analysis of the physicochemical properties of the obtained powder samples.
- Preparation of a monodisperse droplet generator to construct devices to measure drying kinetics.