Figure 40. Spin flash dryer in closed-cycle arrangement.
the same powder moisture. This provides an increase in thermal efficiency.
Air velocity through the cross section of the drying chamber is an important design factor and is determined in part by the final particle size that is required. A lower velocity will tend to reduce the final dried particle size carried out of the chamber. The major factor, however, is the stability of the very complex bed that must neither settle back into the air distributor nor blow out of the top of the chamber. Once the maximum velocity has been determined by test work for a given product, the diameter of a drying chamber can be selected to provide the desired water evaporation rate.
Capacity can be adjusted to suit the output from the preceding process equipment, which may be difficult to control and slow in response time. This is achieved by a cascade control from a feed vat level sensor to the inlet temperature controller set-point. The feed vat sometimes can be oversized to accept the batch discharge from a preceding filter press while allowing the dryer to operate continuously.
Once a decision has been made to increase feed solids prior to drying, the small size and lower airflow requirements of the spin flash dryer make it practical to design the system as a closed-cycle dryer with nitrogen as the drying medium. This type of system can be used to dry a solvent-based powder, allowing complete recovery of the solvent.
The simplified Figure 40 schematic shows a possible configuration of a closed-cycle spin flash dryer. The operating process as described earlier, is extended, with exhaust gas from the baghouse being scrubbed and cooled in a condenser (13) using cooled solvent from an external plate heat exchanger (15) as the scrubbing medium. Recovered solvent is bled off at (16), downstream from the scrubber recirculation pump (14) at a controlled rate based on scrubber liquid level.
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