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Fluidized Bed Dryers And FBD Finger Bags – An Overview

Drying in the pharmaceutical industry involves the extraction of liquids such as water and/or organic solvents from materials or granules. Drying can be carried out in a conventional heated oven through which air is passed. A more efficient means of drying is to employ a fludized bed dryer (FBD) in which the powdered material or granules are fluidized in an upward flowing stream of hot dry air. The FBD offers a rapid and convenient way of drying powders and is widely used in the pharmaceutical industry for drying granular formulations in which samples are held in porous cloth finger bags.

Operation of the fluidized bed dryer

Several types of fluidized bed dryer are available. The simplest form consists of a stainless steel chamber with a removable perforated bottom or bowl. The granules to be dried are placed inside the bowl, suspended over the bowl on a wire mesh or inside small finger bags that are suspended above the bowl. Air from outside the chamber is filtered and heated before a fan forces the air upwards through the perforated bowl.

The air velocity is increased with time to a point where the frictional drag on the particles is equal to the gravitational force. The granules rise in the container due to the high air velocity and then fall back in random motion, like a boiling liquid. In this fluidized state, the granules are partially suspended in the air stream.

The hot air that swarms over the granules extracts any absorbed liquid from the particles, and since in the fluidized state all the particles are in the same gaseous environment, the process allows the granules to dry equally.

When particles are entrained in bags, the granules tend to stick to the inner surface of the bag which has to be shaken when it comes to removing all of the dried granules.

FBD Finger Bag Filter

The FBD finger bag filter has a finger-like shape which enables a large number of bags to be loaded into the drying chamber, thereby making best use of the chamber footprint and enabling a high drying efficiency. The effectiveness of the drying process also depends on the materials used to construct the finger bag and a variety of fabrics can be employed.

Finger bags can be used for drying material in agriculture, chemical industry, and industrial processing as well as pharmaceuticals. Note that the bags are often referred to as finger bag filters as the fabric used to make the bag effectively filters the air and thereby prevents large contaminant particles from entering the fludizied powder. The choice of fabric to be used is governed by the type of granular material that is to be dried. Example fabrics include cotton, anti-static synthetic polymers (e.g., nylon and polypropylene), and satin. Other factors to be considered when selecting finger bags, include air permeability, micron size of the material and size of the granules, and the ability to dissipate static change. As different companies have different requirements for FDB finger bags, there is no one-size-fits all, and, since the fabric is intrinsically weak, bags should be tested regularly to ensure that they meet the quality standards necessary for pharmaceutical product drying, as well as being cost effective. Good finger bags have the following attributes:

  • Material is abrasion and wear resistant
  • Durable construction
  • High surface area
  • Has excellent particle retention and high flow rate combination

At LFA we do not sell drying ovens or Fluidized Bed Dryers (FBD). However we do know a little about them and would be more than happy to answer any questions you have. The reason why we do not supply this is simple and twofold. A) We have not found a good and reliable source. B) We only sell products that we are so familiar with we could take them apart with our eyes closed. That is why we limit ourselves to what we are best at, Desktop Pill Presses, Rotary Pill Presses and Mixers.
If you have any questions please do get in contact.

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