An Overview Of The Fluidized Bed Dryer

The fluidized bed dryer (FBD) is widely accepted in the pharmaceutical industry and is used during the granulation process to dry fine powder granules before these go forward to the rotary tabletting machine. Unlike other types of drying oven, the FBD is fast acting and rapidly removes moisture from powder particles to a level that is acceptable for tablet formulation.

How it works

The principle behind the FDB is that if a gas is forced upwards through a bed of powder, the particles will lift and become suspended in the gas, providing the gas flow rate is greater than the settling rate of the particles. Under these conditions the solid powder takes on physical characteristics more like that of a boiling liquid than a solid, and is said to be fluidized. In a fluidized bed dryer, hot air is used as the fluidizing gas and it is this that dries the powder granules.

There are several designs of fluidized bed dryer that are available to pharmaceutical companies. The simplest form consists of a stainless steel chamber with a removable perforated bottom or bowl. The granules to be dried are placed in the bowl and hot air is forced upwards through the perforated bowl. Air from the surrounding atmosphere is drawn into the equipment and filtered before being heated and forced through the granules with one or more fans.

It is common to load the granules into the drying chamber in bags that are permeable to air. These help to contain the powder, which can otherwise easily escape if an operator forgets to close the wrong vale during the process. Once the hot air flow is established, the bags expand and the granules within them become fluidized.

Air that leaves the top of the particles may pass through another filter, before being recirculated through the bottom of the dryer. Or for some products, the air may simply pass through once. The flow rate and circulation are important parameters that need to be controlled to ensure uniform and rapid drying of the granules. Air that leaves the dryer passes through another filter, thereby ensuring that particles of product do not escape into the workplace.

Although the simple fluidized bed dryer is widely used in the pharmaceutical industry for batch processing, more sophisticated fluidized bed dryers are also available for continuous processing of powder. In a continuous dryer, the flow of air is controlled along the length of the dryer to maximise fluidisation, enabling very wet and sticky materials to be handled. As the material passes along the dryer it gradually loses moisture until the target dryness is achieved, at which point it passes into a cooling zone. Here the hot air is replaced by cool ambient air, which reduces the product temperature to the desired level.

Challenges with the Fluidized Bed Dryer

There are some issues that can arise with fluidized bed dryers despite their widespread use in the pharmaceutical industry. For example, particles may become electrostatically charged and be difficult to discharge from the dryer; sometimes rapidly dried material can start to burn or decompose if accurate control of temperature is not maintained. Filters may also become blocked, especially the outlet filter. It is necessary therefore to maintain and check the equipment regularly to ensure that it performs reliably for all production runs.

At LFA we do not provide Fluidized Bed Dryers, however in developing our articles section we have tried to cover a broad range of topics that may be of interest to our customers and readers in general. If you have an idea for a subject that you would like us to cover then please get in contact.

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