Q. We are using a vacuum oven to dry our wet-cake intermediate. At 120 °C, it takes 26 hours on average to dry the material from 80% to 10% moisture. Can we shorten our drying time, possibly by agitating the product to improve heat transfer?
A. Your instinct is right—agitating the material while heating it under vacuum accelerates the drying process. Mixing provides a mechanism for constantly exposing fresh material to areas where the rate of heat transfer is highest at the heated surfaces of the vessel.
A double planetary mixer has been proven to be successful in this application. The mixer is equipped with two identical blades that rotate on their own axes while also orbiting on a common axis. After only 36 revolutions, in about one minute, the blades contact virtually every point in the vessel. This mixing action is very efficient, thorough, and predictable.
The double planetary mixer is a relatively low-speed device, which is an advantage in vacuum drying applications where generation of fines should be avoided. Its gentle blending action can adequately turn over the batch contents, whether they are in the form of a thin slurry, a sticky paste, a wet granulation, or dry, fine powders. The mix vessel can be removed from under the mixer for full access to the agitators and the product. Clean design features such as highly polished wetted parts, a sealed and purged gearbox assembly, flush discharge valve and sanitary charge ports all contribute to minimizing the risk of contamination. If liquids need to be added and mixed into the batch prior to the drying stage, atomizing spray nozzles may be installed on dedicated ports for controlled and spill-free addition.
Vacuum-rated double planetary mixers are supplied in capacities as small as ½ pint and as large as 750 gal. The straightforward scalability of these systems allow R&D scientists to conserve expensive ingredients while following a repeatable, consistent mixing and drying process that can be easily replicated in pilot and production scale.
Drying times vary depending on the raw materials, product quantity, temperature, vacuum level, final moisture content, and other variables. However, cycle times in the range of 1–2 h are fairly common, especially for laboratory batches. Testing is recommended to confirm drying time for a specific application.
—Christine Banaszek, application engineer, Charles Ross & Son Company.
If you have a problem with your equipment or process, an industry expert may have the solution. Please send your question to Jennifer Markarian, editor of Equipment and Processing Report, and we may be able to provide an answer in a future issue. All questions will remain anonymous.