Chemical Engineering October 2014 - 66

Solids Processing
the saltation velocity. For stepped
systems, one must also maintain
the gas velocity above saltation conditions
at each step location. Consider
these factors:
1. Saltation velocity is a function
of material properties and solids
flowrate
2. The gas-delivery volume of the
air mover (except for the plant
network supply) depends on the
system pressure drop or the conveying
pressure
3. Absolute pressure at the pickup
location affects the superficial
gas velocity as per the Ideal Gas
Law
4. Air leakage at the feeder (such as
through rotary airlocks) depends
on conveying pressure and must
be compensated for
By taking all these factors into account,
one may calculate the gasflowrate
setpoint that is required
to maintain optimal gas velocity at
the pickup location. The function of
an automatic air-control system is
to determine this setpoint and send
the control signal to the hardware
in the field. The next challenge is
to identify the necessary hardware
to achieve the desired flowrate at
pickup (Figure 6). For blower or
compressor air movers, three main
types of control configurations can
be found in practice, namely:
1. Bleeding off conveying gas downstream
of the air mover (blower
or compressor) to adjust the gas
flowrate to the desired value
2. Using flow-control valves or
(variable) Laval nozzles (sonic
nozzles) to set the flowrate by
partial circulation of compressed
air back to the suction side of the
air mover (blower or compressor)
3. Adjusting the speed of the air
mover (blower or compressor)
and changing the actual flowrate.
The
gas-bleeding approach may the
simplest option; however, it may
compromise energy efficiency by
venting previously compressed air.
And, this approach is not suitable
for conveying gases other than air,
and for systems using fans as air
movers. While the same energy considerations
apply for gas recirculation,
the control of the pickup velocity
using a control valve or Laval
nozzle tends to be a more reliable
approach and is commonly implemented
in large-scale systems. Airmover
speed control is the most energy-efficient
approach of the three
options outlined above, but this approach
may be limited by motor size
and permissible air-mover speed
(see Table 1 for guidance).
Calculation of pickup velocity.
Velocity at the pickup can be estimated
by performing a mass balance
of conveying gas and correcting
for pressure and temperature
conditions at the pickup location.
[]
m˙ f pickup
[]
Vpickup
= m˙ f blower inlet
[]
m˙ f feeder leakage
[]
f
A
[]= m˙ f /()
=× ××3πη
p
(2)
pickup
Where:
˙mf = The mass flowrate of the gas,
kg/s
Fw dg Vrad
pp
Fc dP Rn π 5 400/,
=× ×× ×32 3
f = The density of the gas, kg/m3
A = The flow cross-section of the
pipe, m2
[]
m˙ f pickup
= m˙ f blowerinlet
[]
Dense-phase conveying
The key objective of an air-control
(or air-management) system in
dense-phase systems is to maintain
the operating point within the
stable zone (Figure 7). The stable
region is bounded by the conveying
limit on the low end, and the unstable
region on the high end of gas
flowrate. These limits are derived
from the state diagram (Figure 2),
and the maximum available pressure
comes from air mover characteristics.
Even though the specifics
of air-control systems remain the
know-how of conveying-system suppliers,
a general outline on how such
a system works is provided below.
The basis for determining the
permissible operating window and
the respective boundary lines come
from the pneumatic system calculation
and the system state diagram
(shown in Figure 2). These calculations
are performed for the respective
line routing, and minimum as
well as maximum conveying rates,
64 ChemiCal engineering www.Chemengonline.Com oCtober 2014
Air
mover
FT= flow transmitter
PT= pressure transmitter
PC= pressure-control valve
TT = temperature transmitter
VFD= variable-frequency drive
Figure 6. Some general concepts for
controlling pickup velocity in pneumatic
conveying systems are presented here
[]
m˙ f bleed
[]
m˙ f feederleakage
for a given material. The acceptable
range of operating parameters
within the gas-management system
is taken from these limits by applying
some safety margins resulting
from practical experience. Thus, the
acceptable operating window or the
stable zone is demarcated by the
green lines shown in Figure 7.
When starting the operation of
the conveying system for a solids
flowrate equal to ˙mS2 initially a
conveying gas flowrate according
to the operating Point A (as shown
in Figure 7) is set by the programmable
logic controller (PLC). This
corresponds to maximum acceptable
terminal conveying velocity at
minimum permissible capacity for
the shortest conveying distance.
The flowrate, temperature and
conveying pressure are measured
to calculate the actual pickup velocity.
This initial pickup velocity is
at the upper limit of the operating
window. Then the air flowrate will
be reduced in increments, and the
[]
m˙ f bleed
(1)
a. Gas bleed
FT
Air
mover
b. Control valve or
Laval (sonic) nozzle
PC
PT
Air
mover
c. Air-mover speed
control
VFD
PT
TT
FT
Control valve/
variable laval
nozzle
TT
FT
PT
TT
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Chemical Engineering October 2014

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