Chemical Engineering October 2014 - 62

Solids Processing
bar
2.5
flyash, alumina and terephthalic
acid. All of these bulk solids can now
be conveyed at rates in excess of 100
ton/h and conveying distances longer
than 1,000 m.
The relationship between flow
pattern (mode of conveying) and
key operating variables (such as
pressure drop, gas velocity and solids
flowrate) for a given conveying
layout is best represented by the
system state diagram (Figure 2;
often referred to as the Zenz plot).
An alternate approach for the state
diagram - one that plots the gas
flux versus solids flux with lines of
constant conveying pressure - has
also been used by some researchers.
In the original Zenz plot, the pressure
drop across a section (horizontal
or vertical), was plotted against
superficial gas velocity on a log-log
scale. Extending this approach to
represent the entire conveying system,
today the overall pressure drop
is typically plotted against terminal
gas velocity on a linear scale.
For blower selection, where air
flowrate must be specified, the use of
gas flowrate on the abscissa (x-axis)
comes in handy. In any pneumatic
conveying system, each combination
of bulk material characteristics
and specific pipe-routing schematic
will have its own unique conveying
characteristics, hence a system-specific
state diagram. Such a diagram
for any given conveying system will
also depend on conveying length
and routing configuration, since the
overall pressure drop is used for the
ordinate (y-axis). This diagram is,
however, independent of the feeding
technology (for instance, whether a
blow tank or rotary valve is used).
In the system state diagram (Figure
2), the lowest curve represents
the pressure drop characteristics
of single-phase (gas) flow in a conveying
line. Here, the pressure
drop is proportional to the square
of gas velocity. When solids are introduced
into the conveying line,
additional energy is required to
overcome losses due to friction, wall
impacts, and to initially accelerate
the particles and then reaccelerate
them after bends or vertical lifts.
These losses manifest themselves
Figure 2. The
state diagram of
pneumatic conveying
shows the relationship
between key
operating variables
(such as conveying
rate, gas velocity and
pressure drop), and
provides demarcation
of corresponding
pneumatic conveying
modes
Dense-phase conveying
Dunes Uncontrolled
plug
formation
2.0
1.5
m
*
1.0
0.5
∆P
10
20
30
40
Terminal
gas velocity (v), m/s
as additional pressure drop, which
increases with increasing solids
flowrate. If the conveying gas velocity
is sufficiently high, then stable
dilute-phase conveying conditions
will prevail, where all particles are
fully suspended in the conveying
gas (Figure 1, top).
As the gas velocity (or gas flowrate)
is reduced, the pressure drop
continues to decrease, even though
solids flowrate remains constant.
Correspondingly, the flow pattern
in the conveying line changes from
fully suspended flow to stratified
flow with a higher concentration of
particles in the lower section of the
pipe. Eventually, the particles begin
to fall out of suspension and begin
to roll, slide and move along the
bottom of the pipe. The gas velocity
corresponding to this state of flow is
called the saltation velocity, and the
corresponding pressure drop shows
a minimum in the characteristic
curve. Operating conditions to the
left of the pressure drop minimum
will result in additional settling of
particles (saltation), which leads to
sluggish conveying behavior and
may cause temporary plugging of
the conveying line along with intense
line vibrations.
This unstable zone (shown in Figure
2) separates the dilute-phase
from the dense-phase areas in the
state diagram. If the conveying
pressure required for stable conditions
exceeds the available pressure
from the blower, compressor or compressed
plant air supply, then the
conveying system will stall or plug.
60 ChemiCal engineering www.Chemengonline.Com oCtober 2014
With further decreasing gas velocity,
the zone of stable dense-phase
conveying is reached - to the left of
the unstable zone (that is, at lower
gas velocities but significantly
higher pressures). The conditions
that produce stable, dense-phase
conveying are much more limited
compared to those that produce dilute-phase
conveying.
Finally, the line shown furthest to
the left in Figure 2 represents the
termination of dense-phase conveying
in the form of a stationary plug.
The zone of stable, dense-phase
conveying is wedged between the
unstable region and the conveying
limit. The pressure required
to move a slug of solids is significantly
higher than that required for
dilute-phase conveying at the same
conveying rate - thus, most practical
applications require the use
of compressors.
Dense-phase conveying is often
referred to as " slow-motion conveying. "
Not all high-pressure conveying
systems will actually operate in
dense-phase mode or in slow-motion
conveying. One must pay particular
attention to the system design, line
stepping (that is, increasing the
line diameter along the conveying
length to reduce local gas velocity)
and velocity profile, and manage
the conveying gas, to achieve densephase
conveying.
Conveying characteristics may
be approximated for preliminary
evaluations by comparison with
materials that have " similar " conveying
behavior. For example, the
D
configuration
v
50 m/s
For a given fixed
system
s = 20 t/h
10 t/h
5 t/h
1 t/h
m
*
s = 0
Dilute-phase conveying
Strands + Particle
cloud
Conveying pressure drop (∆P), bar
Unsteady zone
Plugging limit for
dilute-phase conveying
i
l
Conveying
mit
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Chemical Engineering October 2014

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