APV Dryer Handbook - Umbc

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Capacity can be adjusted to suit the output from the preceding process equipment
which may be hard to control and slow in its 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.
CLOSED CYCLE DRYING
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Figure 25. Spin flash dryer in closed cycle arrangement
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Once a decision has been made to increase feed solids prior to drying, the small
size and lower air flow 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 schematic shown in Figure 25 illustrates 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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