GAS/SPRAY MIXING
The
technique used in spray dryers to mix the hot gas with the
atomized spray is critical
to the success of the spray drying process, not only for
evaporation, but for particle size control, product density,
and heat degradation. The variables that affect how the
spray is mixed with the hot gas depends upon the spray dryer
configuration and the orientation and velocity of the inlet
entry point.
SPRAY DRYER CONFIGURATION
Co-current Flow
- Both
the spray and gas flow downward through the dryer
- The
feed is sprayed into the hottest gas, increasing the
instantaneous rate of drying.
- Can
be used with all atomization techniques.
- Can
be configured for the most turbulent gas/spray mixing,
increasing the instantaneous rate of drying.
- Can
be configured for the slowest mixing, which can provide
the narrowest of particle size distributions.
- Performance
generally not affected by production rate or product
changes, as airflows can be changed with little effect on
particle trajectory time.
- Best
choice for heat sensitive products because the driest
particles are exposed to the lowest temperatures.
Counter-current Flow
- The
feed sprays down and the gas flows up through the spray
dryer.
- Sometimes
used for the production of large particle sizes because
the up-flow of air slows the particle “fall time”,
allowing for extra drying time.
- The
feed is sprayed into the coolest gas, decreasing the
instantaneous rate of drying and directionally producing a
higher density product.
- Can
be used only with pressure nozzle or two-fluid nozzle
atomization techniques.
- Performance
affected by production rate or product changes if
temperature profile is important to product quality.
- Product
degradation or burning can occur because the driest
particles are exposed to the highest gas temperatures.
- Inlet
or outlet gas bustle increases fabrication costs.
Mixed Flow (Fountain Flow)
- The
gas flows down and the feed sprays up, then comes down
with the gas.
- Sometimes
used for the production of large particle sizes because
the particle trajectory is increased, allowing for extra
drying time, and decreasing the overall spray dryer height
required.
- The
feed is sprayed into the coolest gas, decreasing the
instantaneous rate of drying and directionally producing a
higher density product.
- Can
be used only with pressure nozzle or two-fluid nozzle
atomization techniques.
- Performance
generally not affected by production rate or product
changes, as airflows can be changed with little effect on
particle trajectory time.
- Product
degradation or burning can occur because the driest
particles are exposed to the highest gas temperatures.
MIXING TECHNIQUE
With
each spray dryer configuration, the mixing technique can vary
from slow, parallel flow to fast, turbulent flow. Each
technique has its merits; the choice is a function of the
application objectives. If evaporation (drying) is the
only real objective, a fast mixing technique is used. If
other objectives such as particle size are important, a slow
mixing technique should be used.
Slow Mixing
- Used
when minimization of fines are important (narrow particle
size distribution) because the velocity of gas at the
point of atomization is lowest.
- Uses
full diameter perforated plates for gas distribution.
- Higher
product density is possible.
- Generally
requires a taller spray dryer
- Can
be used only with pressure or two-fluid nozzle atomization
Fast Mixing
- Used
when instantaneous drying rate is more important than a
narrow particle size distribution.
- Typically
uses a vane ring at the gas inlet to induce a cyclonic
rotation of the gas.
- Produces
a broader particle size distribution, because the high
turbulence further atomizes the droplets.
- Can
be used with all atomization techniques (required for
centrifugal atomization).
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