Vacuum Drying Benefits and Limitations

Vacuum drying equipment is applicable for many applications across most industrial sectors such as chemical, pharmaceutical, food, plastics and metal powders.

There are many advantages with vacuum drying, but like most things in life, understanding the limitations and how to apply each design is equally important for successful implementation.

For most applications there are two main generic in-direct heat dryer types to consider;

  • Tumble Designs such as Rotary Cone (RCVD) and
  • Agitated Designs such as Rotary Paddle (RVPD)

S2 Engineering manufacture, supply and install both generic types of vacuum dryers and this article explains the differences.

Vacuum drying equipment typically used for batch operation as it removes water or removes and recovers solvents from a moist material. The equipment is also sometimes used to change a material's molecular and physical chemistry (called a phase change) in specialised operations such as chemical reactions and polymer solid stating.

A vacuum dryer is typically used for separating a volatile liquid by vaporisation from a powder, cake, slurry, or other moist material. This process is fundamentally thermal and doesn't involve mechanically separating the liquid from the material, such as in filtration or centrifugation.

Unlike a direct-heat dryer, in which the material is immersed directly into the heating media (usually a hot gas stream) and is dried by convection, a vacuum dryer is an indirect-heat dryer. That is, the heat is transferred to the material as it contacts the dryer's heated surface, drying the material by conduction.

Understanding this distinction is essential for grasping the advantages and limitations of vacuum drying, as well as for selecting a vacuum dryer that efficiently and economically achieves your process goals. To understand how vacuum operation aids drying we need to start with a simplified drying equation:

Q = U A ΔT, ..... where

  • Q is the total heat (in British thermal units [BTU])
  • U is the overall heat transfer coefficient (in BTU/[ft2/0F]),
  • A is the effective heat transfer surface area (in square feet), and
  • ΔT is the temperature difference between the liquid's boiling point (that is, vaporisation temperature) and the heating media's temperature (in degrees Fahrenheit).

The process objective of the drying equipment is to achieve an effective heat transfer (Q) to the material so that its liquid content is vaporised. Most often, the material's properties and the dryer type effectively establish the U and A values for the process. 2 So when using a dryer, your focus turns to maximising the ΔT value to increase the Q value.

Here vacuum drying provides a unique advantage. By controlling atmospheric pressure, the vacuum dryer increases the effective ΔT for a given process. That is, vacuum drying simple reduces the boiling point - or vaporisation temperature - required for removing the liquid.

By controlling pressure and the heat introduced to the dryer, you can significantly increase the effective ΔT and thus dry the material faster than at normal atmosphere. For this reason, a vacuum dryer is especially suited to drying a heat-sensitive material that degrades above a given temperature and would otherwise require a lengthy drying cycle. Examples of such materials are vitamins, antibiotics, and many fine chemicals.

The closed-system design required for achieving and maintaining the low-pressure atmosphere inside the dryer also provides advantages for processing a hazardous material. Examples include toxic chemicals or solvents and explosive materials. The vacuum dryer safely contains and condenses the hazardous vapours from such substances without any threat to your workplace environment or outside atmosphere. With some hazardous materials, you can provide further protection by using inert gas to limit the oxygen level in the vacuum dryer.

When comparing a vacuum dryer with a direct-heat dryer, such as a direct-heat rotary dryer or fluid bed dryer, keep some limitations in mind. The vacuum dryer almost inherently operates in batch mode because of the dryer's sealing requirements. But depending on your industry's practices, this may not be a problem. For example, if you need to identify and trace individual lots of your products, batch operation is probably preferable. Batch drying also permits greater process versatility and can be more easily adapted to changing manufacturing practices. But if your vacuum dryer is part of a continuous process, you'll need to install surge hoppers and other material handling equipment before the dryer to create a hybrid batch-continuous operation.

Another vacuum dryer limitation is related to the equipment's heat transfer mode. A vacuum dryer's upper temperature limit, typically about 315 deg C (600° F) is lower than that of a direct-heat dryer. The rate at which material temperature can be raised in a vacuum dryer is also limited. This is because the in direct heat vacuum dryer is limited by the surface area available for heat transfer, unlike a direct-heat dryer, which is limited only by the hot gas volume in the drying chamber.

S2 Engineering will help in the selection of the dryer. For further information please contact the S2 Engineering Technical Team.

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