Chemical Engineering June 2018 - 39

Facts At Your Fingertips
Pneumatic Conveying Flow Patterns
Department Editor: Scott Jenkins
Figure from Ref. [1]
neumatic conveying is a ubiquitous
mode of conveying
bulk solids in a wide range
of industries, including chemicals,
plastics, grain, food, agriculture,
mining, power generation and cement,
among others. The flow patterns
within conveying lines depend
on the properties of the materials
being moved, the pressure drop
across the conveying route and the
velocity of the conveying gas. This
one-page reference provides information
about plotting relationships
between these variables.
P
Zenz plot
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 1; often called a Zenz plot).
Today's versions of the Zenz diagram
typically plot the overall pressure drop
against terminal gas velocity on a linear
scale. In any pneumatic conveying
system, each combination of bulk
material characteristics and specific
pipe-routing schematics will have its
own unique characteristics, hence a
system-specific state diagram. Overall
pressure drop is used for the ordinate
(y-axis). This diagram is independent
of the feeding technology.
In the system state diagram, the
lowest curve represents the pressure
drop characteristics of singlephase
(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 as additional pressure
drop, which increases with rising
solids flowrate.
Saltation velocity
If the conveying gas velocity is sufficiently
high, then stable dilute-phase
38
conveying conditions
will prevail,
where all particles
are fully suspended
in the conveying
gas. As the gas velocity
(or gas flowrate)
is reduced, the
pressure drop continues
to decrease,
even though the
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.
0.5
∆P
010
20
30
40
Terminal gas velocity (v), m/s
FIGURE 1. The state diagram of pneumatic conveying shows the relationships
between key operating variables
much more limited compared to
those that produce dilute-phase
conveying. Finally, the line shown
furthest to the left in Figure 1 represents
the termination of densephase
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
Dense phase conveying
This unstable zone (shown in Figure
1) 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. With further decreases
in 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
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.
n
Editor's note: This " Facts at your Fingertips " column
was adapted from the following article (Reference 1):
Wilms, H. and Dhodapkar, S., Pneumatic conveying: optimal
system design, operation and control, Chem. Eng.,
Oct. 2014, pp. 59-67.
CHEMICAL ENGINEERING WWW.CHEMENGONLINE.COM
JUNE 2018
D
configuration
v
50 m/s
For a given fixed
system
bar
2.5
Dense-phase conveying
Dunes Uncontrolled
plug
formation
2.0
1.5
m
*
1.0
s
= 20 ton/h
10 ton/h
5 ton/h
1 ton/h
m
*
s = 0
Dilute-phase conveying
Strands + Particle
cloud
Conveying pressure drop (∆P), bar
Unsteady zone
Plugging limit for
dilute-phase conveying
l
Conveying
i
mit
http://WWW.CHEMENGONLINE.COM
Chemical Engineering June 2018

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