Chemical Engineering October 2014 - 64

Solids Processing
increases along the conveying line.
As shown in Figure 3, the conveying
mode changes along the conveying
line due to differences in local
gas velocity while the solids-loading
ratio remains constant. Solids-loading
ratio, therefore, is not a reliable
indicator of conveying mode.
The use of secondary air injection
is one way to maintain a stable
mode of dense-phase conveying for
Class II bulk solids. It should be
noted that addition of secondary
air injection results in an increase
in gas and material velocity along
with a decrease in solids loading
along the conveying line. Depending
on the conveying characteristics,
the secondary air-injection points
can be of discrete (single point) or
continuous (longitudinal air injection)
design. Preferably, the air injection
should only be activated " on
demand " - that is, when required
due to build-up of a plug that is too
long or too dense.
Constant or uncontrolled secondary
air injection leads to a continuous
increase in gas flow and thus
gas velocity, with the potential of
changing conveying mode from
dense-phase into dilute-phase conveying.
The maximum length of the
slugs is determined by the distance
between the injection points. These
secondary air-injection points also
serve to disintegrate long slugs into
smaller ones, and this is required
for stable operation of dense-phase
systems with Class II materials.
This mode of air injection allows
the dense-phase conveying systems
to operate reliably even in the unsteady
zone between dense- and
dilute-phase conveying.
In order to keep the velocity of
the conveying gas in a desirable
range (for each mode of conveying),
the conveying line may need to be
stepped to increasing line diameters
along the conveying length
(Figure 4). Upon stepping the conveying
line, the gas velocity will
drop back into the desirable limits
at each point where the conveying
pipe is transitioned to a larger
diameter. The proper location for
these transitions must be carefully
determined to maintain the desired
m
*
m
*
m
*
s
F
P
D
v
Plugs, dunes
Stands
Particle cloud
Figure 3. Shown here is the typical change of conveying mode along a conveying
line with a constant diameter. This is a result of progressive expansion of the conveying
gas
a.
b.
c.
d.
Plug
Dune
Strand
Secondary air injection
Line step
Air bleed off
a. Constant diameter system
25
20
15
10
5
b. Constant diameter with secondary air injection
c. Stepped line
d. Constant diameter with air bleed off
b
a
Line step
Cloud/entrained
F = constant
s = constant
D = constant
m
*
d
c
100
200
Conveying distance, m
Figure 4. This figure provides a comparison of velocity profiles between various
designs of dense-phase systems
velocity profile.
Another option for optimizing the
velocity profile without stepping the
line is to use controlled leakage of
conveying gas at single or multiple
locations [3]. The air-leakage approach
is equivalent to stepping the
line as far as the velocity distribution
is concerned, since the expansion
of the air (and thus the volumetric
flowrate) is compensated by
the bleed-off. However, air-leakage
increases the solids-loading ratio.
It becomes evident that the stabila.
Constant diameter system
ity of a pneumatic conveying system
must be evaluated mainly by two
criteria: First, the availability of a
62 ChemiCal engineering www.Chemengonline.Com oCtober 2014
b. Constant diameter with secondary air injection
c. Stepped line
sufficiently high conveying pressure
of the conveying gas to keep
the material moving, and second a
stable velocity profile along the conveying
line in accordance with the
expected conveying mode and wear
characteristics. For high-capacity,
dense-phase systems, the minimization
of shock loads from slugs
impacting bends or other slugs may
become a third criterion.
Figure 5 defines the acceptable
velocity profile ranges for dilutephase
and dense-phase conveying.
As shown for the dilute-phase case
(red line), the extrapolation for the
250-mm-dia. conveying line beyond
300
400
Conveying gas velocity (v), m/s
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Chemical Engineering October 2014

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