Sponsored by Charles Ross & Son Company
Agitated vacuum drying is an efficient method for removing volatile components from plastics, ceramics, metals, specialty chemicals and other products. Due to the reduced pressure within the dryer, the liquid’s boiling point is lower than in atmospheric conditions and drying can proceed at lower temperatures. Vacuum drying is therefore ideal for processing heat-sensitive materials without risk of thermal degradation. It also allows manufacturers to recover costly solvents or safely dispose of any harmful volatiles removed from the closed system.
The combination of deep vacuum, gentle heat and agitation achieves significantly shorter drying times compared to high heat alone. It is not uncommon for atmospheric oven and tray drying operations to run for many hours, sometimes overnight, in order to reach very low moisture levels. The impact on energy costs and production rates is tremendous.
Vacuum ovens and tray dryers offer better drying rates than their atmospheric counterparts but agitated vacuum dryers are even more effective. In these systems, materials in direct contact with the dryer’s heated surfaces are continuously renewed, ensuring that drying is not only fast but also uniform.
Another advantage of agitated vacuum dryers is the wide range of feed forms that they can accommodate – from slurries and pastes, to granules, pellets and powders. A liquid feed material, for example, may undergo a series of relatively swift changes in physical state – from slurry to a viscous paste, and from a paste to a dry powder.
The characteristics of the feed, intermediate and final dried product are all important considerations when selecting the right equipment for a particular application. Process engineers are faced with many options in today’s market but the three most common and superior technologies are the Vertical Blender/Dryer, Cylindrical Blender/Dryer and Double Planetary Mixer.
Vertical Blender/Dryers feature a slow-turning auger screw which orbits around a conical vessel. The screw gently lifts material upward as it advances along the vessel walls. At the same time, materials at the upper most level of the batch cascade slowly back down into regions opposite the moving auger screw. The constant exchange of materials between the heated sidewalls and center of the blender accelerates the drying process.
A main issue with agitated dryers is the risk of product attrition or generation of fines. This concern is well minimized on a Vertical Blender/Dryer. The blending action it imparts is very thorough but gentle enough even for the most delicate of applications.
The low impact agitation of this dryer is also ideal for highly abrasive materials which would quickly abrade horizontal systems like ribbon or paddle-type blender/dryers. On the other hand, shear input can be increased in a Vertical Blender/Dryer when necessary. For instance, products that contain undesired lumps benefit from the addition of a high-speed chopper assembly.
During the drying cycle, hot water, oil or steam is recirculated through the vessel jacket. Due to the lower processing temperatures made possible by the vacuum environment, there is a reduced risk of creating hot spots on the vessel walls where product could bake.
Advantages of Vacuum Drying
in a Ross Vertical Blender
- Highly effective moisture removal. Ross Vertical Blender/Dryers are commonly used for drying materials to less than 0.10 percent moisture.
- Fully top-supported screw agitator. Without a bottom support bearing to get in the way, dried materials are discharged quickly and completely. This design also allows the machine to be operated with very little maintenance.
- Low energy consumption. The Vertical Blender/Dryer consumes up to 50 percent less power than a comparable horizontal blender. Low horsepower to volume ratio, low processing temperatures and short cycle times all contribute to the economical design of this blender/dryer.
- Customization and flexibility. Ross can tailor almost any feature of the Vertical Blender/Dryer to suit a specific application and ensure long service life. Standard models range from 1 to 500-cu.ft., each capable of handling volumes as low as 10 percent of the rated capacity without destroying friable or delicate products.
- Fast drying rates on many proven applications including hygroscopic plastic resins, polymer fillers, fibers, pigment powders and more.
Sample Application: Polymer Powders
A manufacturer of binder resins uses a Ross Vertical Blender for vacuum drying polymer powders from 35 percent to <1 percent moisture. The thermoplastic resin was previously being dried in a V-Cone Tumble Blender and transferred to trays for curing in an oven. The powders were then milled to disperse agglomerates that have re-formed during curing. This three-step process took 24 hours to complete.
Simulation trials confirmed that the drying and curing steps can be done in a Ross Vertical Blender/Dryer by following the specified temperature profiles for each stage. The constant agitation prevents agglomerates from forming, therefore eliminating the downstream milling step.
Another popular vacuum dryer configuration is the Cylindrical Blender/Dryer equipped with a ribbon or paddle agitator. Heat transfer is extremely efficient since the jacket surrounds the entire cylinder and end plates. Cylindrical Blender/Dryers are ideal for products that would persistently hang up on the trough of an ordinary U-shaped horizontal blender.
Compared to the rotating screw auger of the Vertical Blender/Dryer, the agitator in a Cylindrical Blender/Dryer is better suited for applications that benefit from more vigorous mixing and a faster exchange of product in contact with the heated surfaces.
One major advantage to the Cylindrical Blender/Dryer is that it can accommodate high temperatures. The gearbox and all bearings are located outside of the drying chamber and are not subjected to the same temperature as the materials being processed. This feature allows for easy lubrication, simple maintenance and long service life.
Sample Application: Plastic Additives
A 150-cu.ft. Ross Cylindrical Blender/Dryer is being used for removing solvent from a powdered chemical that is added to plastics to enhance their performance. Temperature in the dryer is tightly controlled – too low a temperature will not effectively remove most of the solvent but too high a temperature can cause undesired changes in color and other properties. The Cylindrical Blender/Dryer holds a deep vacuum (>50 torr) during processing, also a critical factor in ensuring predictable drying times and consistent product quality.
Double Planetary Mixers
Double Planetary Mixers are commonly used for batching viscous formulations such as rubber adhesives, silicone sealants, and highly-filled composites. These devices move material by rotating two identical blades on their own axes as they orbit around the batch on a common axis. The mixing action is very thorough regardless of product rheology. Aside from their primary use in compounding high viscosity formulations, Double Planetary Mixers are also frequently utilized as vacuum drying equipment.
Heat transfer in a Double Planetary Mixer is very effective because the blades continuously advance along the periphery of the jacketed mix vessel, removing material from the walls and transporting it to the interior. This type of agitated vacuum dryer is well suited for applications that first need to be mixed intimately prior to solvent removal.
Such a process may occur in several stages: mixing and coating of solids with a liquid binder, application of heat and vacuum while continuing to agitate the mixture, followed by a physical transition from low-viscosity slurry to viscous paste to dry powder.
Sample Application: Filter Cake Material
A manufacturer of custom formulated plastics used to dry a filter cake material in a Ribbon Blender. The feed, which contained 28 percent moisture and had a clay-like consistency, came from a precipitation process in the production of PVC. After drying for several hours in the Ribbon Blender, it transforms into free-flowing particulates. In the initial stages, the product tends to stick to the periphery of the ribbons and relies on hot air blown into the head space to continue drying until it falls off as nodules or granules.
Testing showed that a vacuum-rated Double Planetary Mixer is able to dry the material faster and produce finer, more flowable powders. The mixing action of the planetary blades breaks up large globs and promotes even heating as the batch transitions from wet to dry.
Some useful techniques
- Make sure you have the right type of vacuum pump to handle the level of vacuum and the operating conditions that your process requires. For example, a rotary vane pump may allow you to draw a deeper vacuum (29.5-29.8” Hg) than a liquid ring pump. However, a liquid ring pump may better accommodate condensate from the batch. Use the appropriate filter and condenser before the pump to protect it from contaminants.
- When removing large quantities of vapor from the batch, purging the vessel headspace with a stream of dry air or nitrogen may help accelerate drying time and prevent premature condensation within the blender or mixer.
- Run the agitator at different speeds to confirm the ideal settings that would maximize heat transfer without damaging the product. If complete discharge is an absolute requirement, the Vertical Blender/Dryer is recommended over horizontally agitated dryers.
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