Chemical Engineering August 2016 - 59

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30
25
20
15
10
5
Single-stage spray dryers
(retention time in seconds)
Fluidized bed dryers
(Retention time in minutes)
ture of a second-stage fluidized bed
results in a lower heat history of the
product. A second cooling section
can be added to the fluidized bed
stage to further cushion heat sensitive
products.
A spray dryer with an added circular
fluidized bed will gain the following
advantages:
* Lower heat history and higher
quality product
Constant
rate
Falling
rate
Time
FIGURE 2. This typical drying curve helps to illustrate how a second-stage dryer can optimize the drying
process
operating air temperatures, making
drying more energy efficient than
with spray drying alone, while reducing
overall heat history.
Operating advantages
Table 1 shows the operating advantages
of two-stage drying. The
inefficiency of a direct dryer (that
uses air to dry) is seen in the hot air
exhausted. By reducing the temperature
of the air exhausted or the
amount of air consumed, energy
consumption will be reduced. At
the same time, a lower spray-dryer
outlet temperature will increase the
temperature difference (∆T) and thus
increase the evaporation capacity.
For Example 1, as mentioned in
Table 1, the intermediate temperaInternal
(circular fluidized beds):
* Wet feed is distributed (seeded to start)
* Good localized mixing gives consistent
blended moisture
* Once-through design versus multiple passes
with external backmix
* Deeper bed in center
* Vibrator can be adjusted for plug flow or
backmix direction
Wet feed
Wet feed
Backmixer
Conveys in one direction
Dry powder
Semi-dried material
blends with wet feed
Dry powder
FIGURE 3. Some products may need re-drying and multiple back mixing passes, but internal back mixing,
inherent in circular fluidized beds, accomplishes drying with minimal heat history to the product
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ture of a two-stage dryer is 25 degrees
lower than that of the single
stage alone (170-145ºF) starting
with a spray-dryer inlet-air temperature
of 300ºF. The energy savings of
the two-stage drying is 24%, while
the productivity increase is 26%.
At higher temperatures, as represented
by Examples 2 and 3 in the
table, the results are less dramatic,
but still substantial.
Although the energy savings and
productivity gains justify the cost of
a second stage, so can the quality
improvements, depending on
the material being dried. Whereas
a spray-dried product is greatly affected
by the spray dryer's outlet
air temperature due to evaporative
cooling, the lower outlet air temperaExternal
(rectangular fluidized bed):
* Hot dry product is discharged into a conveying system
back into a paddle mixer that blends the sloppy feed
with the dry product
* Some product may be redried more than once
increasing the heat history
* Rectangular, vibrating fluidized bed requires external
backmixing for too-wet feeds
* Shallow bed in inlet section
* Conveys in one direction
Recycled dry
powder
* Increased capacity
* Higher energy efficiency
* Less buildup of material
* Handle stickier products
* Controlled particle size
* Larger window of operation
* Lower cost
* Can add an agglomeration step
* More uniform drying of larger,
difficult-to-dry, agglomerated
particles
Additional advantages of a twostage
spray and fluidized-bed dryer
combination include the following:
1. Direct top or bottom spray agglomeration
capability in the circular
fluidized-bed dryer
2. Dry larger particles via fines recirculation
over the nozzle atomizer
during spray agglomeration
3. Small footprint and plant space
when using circular, instead of
horizontal, fluidized bed as the
second stage
4. Fluidized bed exhaust is recirculated
into the spray dryer, gaining
energy efficiency
5. Wider window of operation with
cleaner spray-dryer
chamber
because of larger discharge port
to the fluidized bed
A caveat is that two-stage drying
may not be effective if the product
undergoes a sticky stage or encounters
flow issues at higher intermediate
moisture levels.
Internal vs. external backmixing
The circular fluidized-bed dryer's
internal backmixing, once-through
design, as described in Figure 3, allows
for a larger window of operation
for difficult-to-handle feed materials.
Other types of dryers use external
backmixing, where some material
may be re-dried more than once, increasing
the heat history.
The circular vibrating fluidizedbed
dryer, with internal backmixing,
thus can handle a wetter feed
than most other dryers. Internal
59
Moisture content, %
Warm up

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Chemical Engineering August 2016

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