Wurster coater
| The basic concept embodied in a
Wurster coater [also known as air suspension coater] is to separate the
particles from one another in a heated air (gas) stream and spray a
coating formulation onto the particles while they are suspended. |
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| The process takes a specially
modified fluid bed that is divided into two zones by a partition. The
inner area (green zone) is a high
velocity zone that separates the particles and transports them past the
spray nozzle. After passing the nozzle, the particles enter the expanded
area (red zone), slow down and fall back into
the outer section of the fluid bed product bowl. The operating
parameters are adjusted so that the coating dries while the particles are
suspended in order to prevent agglomeration from occurring when they enter
the fluidized bed (blue zone).
The dry, coated particles remain fluidized just enough to allow them to continue moving towards the bottom of the bowl. When the particles reach the bottom they are drawn back into the high velocity air stream and the cycle is repeated. This process is continued until the desired coating thickness has been achieved.
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| The conical shaped expansion
chamber (red zone) is typically sized to reduce the air
velocity by 50% below that of the lower portion of the chamber. This helps
to keep the finer particles out of the upper filter region.
Wurster coaters are particularly suited to scale-up from lab-scale to production systems. Critical design parameters include chamber geometry, air velocity and mass flow ratio of air to product. Typically, the depth of the fluidized bed is kept constant in order to maintain similar coating depth and particle density over the scale-up range. |
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