Episode 35 : Design Approach to Dilute Phase Pneumatic Conveying For many years gases have been used successfully in industry to transport a wide range of particulate solids - from wheat flour to wheat grain and plastic chips to coal. The pneumatic transport of particulate solids is broadly classified into two flow regimes: dilute (or lean) phase and dense phase SAJJAD KHUDHUR ABBAS Ceo , Founder & Head of SHacademy Chemical Engineering , Al-Muthanna University, Iraq Oil & Gas Safety and Health Professional – OSHACADEMY Trainer of Trainers (TOT) - Canadian Center of Human Development

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SAJJAD KHUDHUR ABBAS
Ceo , Founder & Head of SHacademy
Chemical Engineering , Al-Muthanna University, Iraq
Oil & Gas Safety and Health Professional – OSHACADEMY
Trainer of Trainers (TOT) - Canadian Center of Human
Development
Episode 35 : Design Approach
to Dilute Phase Pneumatic
Conveying

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Design Approach to Dilute
Phase Pneumatic Conveying

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4. 1. Introduction
• For many years gases have been used successfully in
industry to transport a wide range of particulate solids -
from wheat flour to wheat grain and plastic chips to coal.
• The pneumatic transport of particulate solids is broadly
classified into two flow regimes: dilute (or lean) phase and
dense phase

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Dilute phase transport –
•high gas velocities (greater than 20 m/s),
•low solids concentrations (less than 1% by volume)
•low pressure drops per unit length of transport line (typically
less than 5 mbar/m).
•limited to short route, continuous transport of solids at rates of
less than 10 tonnes/hour
•the only system capable of operation under negative
pressure.
•solid particles behave as individuals, fully suspended in the
gas, and fluid-particle forces dominate.

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Fig. 1: Dilute phase transport

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dense phase transport
•low gas velocities (1-5 m/s),
•high solids concentrations (greater than 30% by volume)
•high pressure drops per unit length of pipe (typically greater than 20
mbar/m).
•particles are not fully suspended and there is much interaction
between the particles.

8. the boundary
• the boundary between dilute phase flow and
dense phase flow is not clear cut, and there are
as yet no universally accepted definitions of
dense phase and dilute phase transport. [see
defination from Konrad (1986)]
• In this chapter the "choking velocity” will be
used to mark the boundary in vertical pipelines
and the "saltation velocity” will be used to
mark the boundary in horizontal pipelines.

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-- 2: The choking velocity in vertical transport –
Fig. 3. Phase diagram for dilute phase vertical transport, showing the general
relationship between pressure gradient p / L and gas velocity for a vertical transport
line.
•In the region DE as
velocity↓ solids
concentration ↑ rapidly, and
a point is reached when the
gas can no longer entrain all
the solids.
•At this point a flowing, a
slugging fluidized bed is
formed in the transport line.
-known as "choking" and is
usually attended by large
pressure fluctuations
•UCH is defined as the
lowest velocity at which this
dilute phase transport line
can be operated at the
solids feed rate G1.

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Punwani (1976) :
(Note that the constant is dimensional and that S.I. units must be used).
Equation 1 represents the solids velocity at choking and includes the assumption
that the slip velocity USLIP
is equal to UT
(see section 4 below for definition of slip
velocity). Equations 1 and 2 must be solved simultaneously by trial and error to
give CH
and UCH
.
(Eq. 1 and Eq. 2)
where
ε CH is the voidage in the pipe at
the choking velocity UCH
ρp is the particle density
ρ f is the gas density
G is the mass flux of solids (= Mp /
A)
UT is the free fall or terminal
velocity, of a single particle in the
gas
(Note that the constant is
dimensional and that S.I. units
must be used).

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-- 3: The saltation velocity in horizontal transport --
Fig. 4: Phase diagram for horizontal pneumatic transport
At point D the gas
velocity is
insufficient to
maintain the solids
in suspension and
the solids begin to
settle out in the
bottom of the pipe.
The gas velocity at
which this occurs is
termed the
saltation velocity.

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The correlation of Rizk (1973),
Xs,g
is the solids loading = [mass flowrate of solids] / [mass flowrate of gas]
Mp
is the mass flowrate of solids
USALT
is the superficial gas velocity at saltation
where (The units are S.I.)
D is the pipe diameter
x is the particle size

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The total pressure drop across a length of transport line
has in general six components:
1. pressure drop due to gas acceleration
2. pressure drop due to particle acceleration
3. pressure drop due to gas-to-pipe friction
4. pressure drop related to solid-to-pipe friction
5. pressure drop due to the static head of the solids
6. pressure drop due to the static head of the gas
4. Pressure drop

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Steps in Dilute Phase Conveying Design
1. Material and Gas properties (density, size shape,
viscosity, etc.)
2. Specify desired conveying rate
3. Estimate pipe diameter (with a little practice you get a
feel for what works)
4. Calculate saltation velocity
5. Check loading (mass solids/mass gas)
• If > 10 to 15 then need larger diameter pipe
• If < 1 then need smaller diameter pipe

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Steps Continued
6. Calculate pressure drop
• May require iteration
• If too large, may need to gradually increase pipe sizes
(telescoping)
• Do not let velocity drop below saltation velocity
6. Size blower

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Components sum to total pressure drop
Filter
Blower Hopper
Other
Components
Cyclone
Dust collector
Filter
Valves
1. Blower, silencer, inlet filter
2. Acceleration
3. Horizontal pipe section
4. Pipe bends
5. Vertical pipe section
1.
2. 3.
4.
5.
( )
blower
filtervertbendhorizacc
P
PPPPPP
∆=
∆+∆+∆+∆+∆−=∆

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Acceleration
• Gas (outside of blower) and
particles must accelerate up to
operating velocity
– Conveying gas velocity
– Gas density
– Particle velocity
– Pipe cross sectional area
– Conveying rate (mass/time) =
– Solids loading =
















+=
+=
∆+∆=∆ −−
g
p
c
gg
c
ps
c
gg
solidsagasaaccel
V
V
g
V
Ag
Vw
g
V
PPP
µ
ρ
ρ
21
2
2
2
2
gV
gρ
pV
A
sw
AVppρ
µ
g
s
w
w

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Must relate Vp and Vg
• Vg – Vp = slip velocity
• Empirical relation by
Hinkle (1954)
– dp = particle diameter
(meters)
– Particle density in
kg/m3
• In English units
– dp in feet
– density in lbm/ft3
5030
04401 ..
. pp
g
p
d
V
V
ρ−=
5030
12301 ..
. pp
g
p
d
V
V
ρ−=

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Bends
• Major source of wear/attrition
• Pressure loss associated with re-acceleration of gas and
solids
• Bends usually specified by a ratio of the bend radius to
pipe diameter R/D
• Typical in conveying R/D = 6 to 12
R
D

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Bends
• Chambers and Marcus (1986)
• R/D >= 6, B = 0.5
• R/D = 4, B = 0.75
• R/D = 2, B = 1.5 ( )
c
gg
bend
g
V
BP
2
1
2
ρ
µ+=∆
=µ
g
s
w
w

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Horizontal Pipe
( )
c
gg
zhoriz
g
V
D
L
fP
2
4
2
ρ
µλ+=∆• Pressure loss
associated with frictional
losses by gas and solids
• L = length of horizontal
section
• = solids friction
factor
•Use experimental data
if available
zλ
=µ
g
s
w
w

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Solids Friction Factor
• For dp > 500 microns
• For dp < 500 microns
• where
10
25086030
0820
.
...
.








= −−
p
sz
d
D
FrFrµλ
10
250130
12
.
..
.








= −−
p
sz
d
D
FrFrµλ
gD
V
Fr
g
2
=
gD
U
Fr t
s
2
= tU = Terminal
Velocity
=µ
g
s
w
w

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Vertical
• Pressure loss accounts for frictional losses
plus static head (gravity)
( )
cc
gg
zvert
g
g
z
g
V
D
L
fP ∆++=∆ 0
2
2
4 ρ
ρ
µλ
( )
pp
s
pg
VA
w
ρ
ε
ρεερρ
−=
−+=
1
10 Bulk density
Void fraction of gas phase
(recall for fluidized beds
voidage is typically 0.4 to 0.7)

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Top Ten Tips for Reliable Design & Operation of
Pneumatic Conveying Systems
Lyn Bates, Shrikant Dhodapkar, George Klinzing
1. Allow a ‘reasonable’ horizontal conveying length, (15 to
20 pipe diameters), before the first bend to allow the
bulk material to accelerate. Use of flexible hose at
the pickup should be kept to minimum. Excessive
length of flexible hose, often in the form of a coil, is
the worst pickup pipe configuration. Inability to
provide proper configuration at the pickup will result
in plugging condition at gas velocities higher than
saltation velocity.
2. Avoid conveying line layout with bends or elbows placed
back to back. This will inevitably causes excessive
pressure drop and premature line plugging.

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3. Consider stepping conveying lines (increasing the
pipe diameter) to prevent excessive velocity at the
end of the line. Be sure to maintain the minimum
required Froude number at the step location, otherwise
the material will salt out of the suspension. Properly
stepped systems
result in more efficient systems with lower
degradation and wear. Using ISO pipes or
tubes allows for more choices in diameters.
4. Conveying lines should not be routed like utility
(compressed gas / steam) lines which follow
the contours of a building. Minimizing the number
of bends or directional change results in
higher capacity, lower degradation, less erosive
wear and more reliable flow.

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5. Electrostatics effects in conveying systems can be
minimized by increasing the relative
humidity of the conveying air to more than 70%.
6. ‘More air’ can be ‘Less transfer capacity’ in dilute
phase systems. Larger solids and gas
frictional losses caused by higher gas velocities
can absorb more energy than the extra input
of energy. There is an optimum gas flow rate for
a given lean phase flow system.

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7. Install sufficient ports or couplings in the systems for
pressure measurement during troubleshooting.
Pressure measurement is a convenient method to
measure the pulse of a conveying system. Take
some time to generate the base line data for an
existing conveying system. It comes in handy for
future troubleshooting.
8. Proper venting of rotary airlock/feeder in a positive
pressure system is critical for reliable
operation. Air leakage due to clearances and
returning pockets can result in reduced flow or
unsteady feed rate. The leakage can be vented in
a number of ways. The vent system should
be designed much akin to a conveying system with
sufficient gas flow and minimal bends.

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9. Product damage and wear at bends is very material
dependent. Blind Tee's usually have much
merit, but cause a greater pressure drop than long
radius bends. Mitered elbows can be
a good compromise in some cases.
10. Minimum conveying velocity is a function of
conveying rate. Make sure that the gas velocity
at the pickup is greater than the saltation
velocity at the highest solids flow rate. Safety
margin must be allowed for non-optimal line
configuration at the pickup (short acceleration
length, bends etc.)

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Top Ten Tips for Dense Phase Pneumatic Conveying
1. The term “dense phase” is often misused in practice. For example,
many so-called “dense-phase” systems are found to be operating in dilute-
phase (or suspension flow).Also, many researchers and designers define
“dense-phase” as a flow mode with solidsloading greater than 10 or 15.
Many different types of dense-phase have been developed over the past
few decades to take advantage of certain product properties (e.g. air
retention, deaeration, permeability, cohesion, particle size distribution). In
most cases, “densephase”can simply be considered as some form of non-
suspension flow that occurs at some time at any section along the pipeline.
Using solids loading as an indicator of flow mode can be misleading (e.g.
solids loading is a mass concentration parameter that depends on the
mass or density of the particles; some dilute phase systems are operating
at a solid loading greater than 40 and some dense phase systems less
than 10). Ensure that the choice of a flow mode (and system) is based
on the product properties (rather than an imprecise definition or
misleading solids loading), and that the selected or supplied flow
mode is confirmed during commissioning.

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2. Coarse/granular materials that can be conveyed in dense phase (viz. low-
velocity slugflow) exhibit an unstable operating zone in between (high-velocity)
dilute phase and (low velocity) dense phase conveying. The dense phase
regime is bound by a high-velocity (unstable zone) boundary and a low-velocity
(blockage) boundary. Make sure the operating point (gas flow rate, solids
flow rate) falls well within these bounds at all locations in the system and
for all pipeline configurations (if applicable).
3. The minimum and maximum conveying rates in a process must be
defined upfront. Compared with dilute phase, the dense phase regime can be
more limiting and sensitive to variations in air flow and/or conveying rate. For
some materials, a reduction in solids flow rate can shift the operating point into
the unstable zone, thereby causing severe instability (line vibrations and
pressure spikes).
4. Bench top characterization tests (wall friction, permeability, deaeration, etc.)
are helpful for preliminary screening of a material’s suitability for dense phase
conveying. Dense phase conveying performance can be quite sensitive to
variations in material property (particle size, size distribution, shape, density,
moisture, cohesion, etc.). It is highly recommended that pilot scale or full
scale testing be conducted on representative material, especially for new
or different products where no prior experience is available.

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5. Using a conventional or “off-the-shelf” pipeline, not all materials
can be conveyed reliably in dense phase. Some materials can be
conveyed in single or multi slug/plug mode, some in fluidized moving
bed type flow, while others can only be conveyed in dilute phase.
Not selecting the right flow mode for a particular material or the right
operating condition for a given flow mode can result in excessive
pressure spikes, system shutdown, unstable vibrations and/or
pipeline blockages. For materials that do not have a natural
tendency for conventional dense phase conveying, consider
specialized systems with controlled and regulated gas injection
or bypass pipeline technology. Also, ensure proper dense
phase flow actually is achieved during commissioning.

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6. Feeder gas leakage can be a significant fraction (up to 50%) of total
gas consumption. The gas leakage at the feeder (esp. rotary valves)
must be considered in design calculations and compensated
appropriately. Ensure proper venting at the feeder to avoid
feeding problems that may result from the gas blowback.
7. Use a gas flow control system for multi-product and multi-
destination systems to ensure that the operating point is
maintained within the stable operating zone. Also, ensure
that the gas flow control system provides a constant gas mass flow rate
for the full range of operating pressures and pressure fluctuations.
Numerous control logic schemes are available from various vendors or
can be designed by reputable consultants.
8. In dense phase systems, gas expansion can be significant between
feed point and destination. This will result in a corresponding increase in
gas velocity, and a possible transition from dense to dilute phase flow
along the conveying pipeline. For high pressure drop systems (7 psi
or 50 kPa and higher), consider stepping the line diameter to
reduce the velocity and maintain dense phase conditions.

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9. The motion of slugs and stresses generated within the
conveying line during directional changes (bends or diverter
valves) results in significantly higher stresses on pipe
supports as compared to dilute phase systems. It is
essential to work closely with an experienced vendor to
design and install proper pipeline supports to prevent
excessive deflection and line movement, and reduce the
prospect of fatigue failure.
10. To purge a dense phase line clean, a controlled increase
in gas velocity may be required. A proper purge control
sequence may need to be designed and tested to avoid
unnecessary product degradation and/or pipeline
blockage. The dust collector must be designed to
handle the peak gas flow rates.

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