Fluid Bed Coatings

An alternative approach to pan coatings that provides the mixing and drying characteristics required by the film coating process is to use fluid bed technology. Fluid bed technology helps in achieving fast and uniform coating using air to mix, coat and dry the substrate at the same time. This technology originated from fluid bed dryers used for wet powder material drying operation. Fluid bed equipment was modified to coat tablets as an alternative innovation. However, this modification did not become popular for film coating tablets. The major concern during tablet coating using fluid bed equipment was high friability and compromise in core tablet appearance due to physical stress imparted during the fluidization process, compared to the much gentler pan coating process.

Fluid bed coating equipment later became popular for coating multiparticulate systems such as beads and nonpareil seeds.  The figure below shows diagrammatic representation of the fluid bed coating process by a bottom spray method which is widely used to film coat multiparticulate systems.

                                                        FBP Wurster Page1

Bottom Spray Fluid Bed Coating (Würster Coating)

  • The drying inlet air is passed upwards through the bottom perforated plate into the fluid bed chamber.
  • This air passes inside and outside of the central cylinder (Würster column), which houses a spray gun perpendicular to bottom plate, and parallel to the Würster column.
  • The air is taken out of the equipment from the exhaust filters mounted on the top of the equipment.
  • The material to be coated is loaded in the fluid bed chamber and fluidized.
  • The inlet air is maintained at a certain velocity and temperature to help in both fluidization of the material as well as its drying during the coating operation.
  • The height of the Würster column is adjusted such that pellets above the bottom plate (referred to as the Down-bed) are pulled in the Würster column due to the Venturi effect and pass through the liquid spray of coating solution from the spray gun positioned parallel to the column within.
  • When the material being coated reaches the expansion chamber (referred to as the Up-bed), its velocity drops and it falls by gravity around the Würster column and is then recycled back to the coating zone by Venturi effect.

Examples of Fluid Bed Technology Processes Include:

Würster Process (Bottom spray)

  • Most commonly used type of fluid bed process for multiparticulate coating.
  • In this process, there is a concurrent (same direction) movement of powder particles or pellets and liquid spray.
  • The coating process occurs within the Würster column.

Granulator Process (Top spray)

  • Most commonly used type of fluid bed process for granulation.
  • In this process, there is a counter current (opposite direction) movement of powder particles or pellets and liquid spray.

Rotor Process (Tangential spray)

  • Originally designed for the preparation of granulates (pellets), this process has also become adapted for the film coating of multiparticulates.
  • In this process, the powder particles or pellets move in a helical fashion due to spinning rotor disk on the bottom of the equipment. The liquid is sprayed within the moving powder or pellets.

The existence of these three processing concepts has resulted in the major suppliers of fluid bed equipment offering all three as standard inserts for a basic fluid bed processing unit.

Wurster spray patterns Page2

The schematic diagram above illustrates the salient features of the three basic approaches. Of the three designs, the Würster (Bottom spray) has been most commonly used fluid equipment for multiparticulate coating applications such as modified release coatings.

A summary of advantages, limitations and applications for each process is given in the table below:

FBP Table page

Product Literature