Issue link: https://www.e-digitaleditions.com/i/1355344
April 2021 / 21 process efficiency, which conserves resources while ensuring a consistent material quality level. A contact dryer can be used in drying and evapora- tion processes with various bulk materials. Whether a vertical or horizontal dryer design is more suitable for the respective application depends on the material and the material's process parameter requirements. Drying material horizontally A horizontal contact dryer constantly mixes the material being dried so that many different material particles touch the dryer's heating elements. The dryer has a horizontally oriented cylindrical mixing vessel. Within the vessel are mixing blades attached to arms on a centered shaft, as shown in Figure 2. The dryer unit and blades are heated by internal steam or ther- mal oil. The material enters the dryer's top inlet and is dried and moved simultaneously throughout the ves- sel body by the mixing blades. The moisture extracted from the dried material is removed through a vapor filter. When the material is done drying, it exits out the dryer's bottom outlet and is conveyed to the next step in the process. The dryer blades' movement along with their arrangement allow for short material residence time within the unit as new material is constantly touching the heated mixing blades throughout the drying pro- cess. This leads to even material heating and drying and efficient energy use. While the horizontal contact dryer, as shown in Figure 3, can be used for batch and continuous pro- cesses, it's best suited for continuous processes, such as egg yolk extraction. For example, if an upstream rotary pressure filter or indexing belt filter constantly constantly contacting the heating elements, the heat- ing surfaces are used to their maximum efficiency. As a result, the temperature of all the material contained in the dryer is gradually increased to the desired level without any sharp rises or drops in temperature, which can ultimately damage the material. By mixing the material during the drying process so that the same material doesn't only touch one part of the heating elements, the material's tempera- ture doesn't get too high or too low. This is a major advantage over stationary drying methods where the material's temperature has the tendency to rapidly increase or decrease due to limited contact with the heating element. Contact drying is especially useful when your application involves sensitive materials, such as spices, or materials that must reach a target temperature, such as with calcination processes. Addi- tionally, since the materials are dynamically circulated, meaning the material doesn't stay stagnant or flow in a rigid pattern in the dryer, dead zones or areas of low material movement are avoided. This eliminates indi- vidual particles being heated to excess temperatures, which can lead to damaged material. This same steady temperature change effect can also be applied to the dryer air's humidity level. Continuous material movement means that the material's tempera- ture remains steady throughout the drying process as does the material's humidity level. This also means that the residual moisture content is consistent throughout the material, resulting in a reliable and high-quality end product. With consistent material temperature and moisture content levels, the drying process isn't only shorter than with stationary drying but also requires less energy. Contact drying leads to an increase in FIGURE 1 A horizontal contact dryer (left) is shown in comparison to a vertical contact dryer (right). FIGURE 2 The horizontal dryer's operating principle makes it well- suited for continuous processes.