Chemical Engineering October 2014 - 68

diagnoSTic and conTrol STraTegy for denSe phaSe SySTem
baSed on preSSure flucTuaTionS meaSuremenT
Solids Processing
acteristics of individual bulk solids.
Since large-capacity, dense-phase
conveying systems are exposed to
considerable pipe forces, another
appropriate criterion to use during
system operation is to take steps to
minimize shock loads in the piping
system. Typically, large pipe forces
coincide with strong pressure fluctuations.
Thus, the variability in
conveying pressure can be used as a
stability criterion and the gas flowrate
can be adjusted further within
the operating window by minimizing
pressure fluctuations.
With a control system that uses
pressure fluctuations to reduce pipe
forces, it is important to allow the
system to run smoothly and not to
overreact too quickly to respective
pressure fluctuations. The control
system should not react to individual
pressure spikes, but it should
analyze the overall pressure trends
and nature of fluctuations before
prescribing a control action (Figure
9). The pressure signals can be
analyzed using time series analysis
methods (for example, Fourier
transform and auto-correlation).
Due to inconsistency in conveying
behavior and temporary pressure
peaks reducing the actual gas velocity
and thus influencing the stability
of the conveying system, some
additional control mechanisms are
required to respond to these inconsistencies.
An increase in conveying
pressure can, for example, be
caused by a higher humidity resulting
from a dryer malfunction. An increased
conveying pressure over an
extended period of time must be answered
by an increase in conveying
gas flowrate to increase the velocity
and to reduce the pressure to a
lower value. Such control strategies
(see Figure 9) may be implemented
into an air-control system.
All the controls described so far
References
1. Wilms, H., Air Management Control Systems
for Dense-Phase Conveying Systems,
Proceedings of the AIChE Annual Meeting,
Dallas, Tex., November 1999.
2. Agarwal, A., and Dhodapkar, S., Debottleneck
Pneumatic Conveying Systems, Chem.
Eng., April 2004, pp. 38-44.
3. Klinzing, G.E., Dhodapkar, S.V., Apparatus
for facilitating solids transport in a pneupressure
measurement
∆P
t
∆P
∆Pmax
t
∆P
∆Pmax
t
∆P
∆Pmax
t
∆P
∆Pmax
t
Pressure fluctuations increase and exceed
acceptable limits with trend towards lower
total pressure (reduced flowrate, low solidsto-air
ratio):
* approaching instable zone
* reduce conveying gas flowrate to return
to stable operating window
Note: Dashed lines represent acceptable operating limits
Figure 9. This figure provides a comparison of various diagnostic and control
strategies for dense-phase systems, based on pressure-fluctuation measurements
and analysis. Smart air-control systems should incorporate these strategies
(see Figure 6) are meant to adjust
the conveying gas flow by setting respective
flow valves. Gas flow from
a compressor, however, can also be
adjusted by changing the speed
of the compressor via a variablefrequency
drive (VFD). While this
technology has traditionally been
relatively expensive, the use of VFD
control has become more economical
in recent years. Additionally, no
extra pressure drop from the gas
management system has to be overcome,
making this concept also a viable
option for system upgrades.
Line purge
When making a change in grade
or destination of a pneumatic conmatic
conveying line and associated method,
U.S. Patent US 5252007 A.
4. FEM 2581 Standard: General Characteristics
of Bulk Products with Regard to their
Classification and their Symbolization, Federation
Europeenne de la Manutention, November
1991.
5. FEM 2481 Standard: Principal Special Characteristics
of Bulk Products Transported in
Pneumatic Conveyors, Federation Europeenne
de la Manutention, July 1997.
66 ChemiCal engineering www.Chemengonline.Com oCtober 2014
veying system it is important to
clean the line of residual solids.
This is rather simple for most dilute-phase
systems since the individual
particles are carried by the
gas stream. Thus, it only takes a
certain period of operation until
the line is " blown " clean. However,
problems do occur when using special
bends, since these can create a
pocket of solids or exhibit a wider
cross-section, thus requiring additional
gas flow for line cleaning.
However, in dense-phase conveying
systems the conveying gas flow is
lower than the settling velocity and
thus is not sufficient to clean the
conveying line without a considerably
larger gas flowrate. This ad6.
Zenz, F.A., and Othmer, D.F., " Fluidization
and Fluid-Particle Systems, " Reinhold Publishing
Corp., New York, 1960.
7. Krambrock, W., Dichtstromforderung (in
German) . Chem.-1ng.-Tech., 54 (1982) No. 9,
pp. 793-803.
8. Klinzing, G.E., Marcus, R.D., Rizk, F., Leung,
L.S., " Pneumatic Conveying of Solids, " 2nd
Ed., Chapman & Hall, 1997.
Pressure fluctuations decrease to acceptable
limits, stable operating conditions:
* no more adjustments needed
* maintain current conveying gas flowrate
Pressure fluctuations within acceptable
limits, but trend towards higher total pressure
(increased flowrate, humidity):
* approaching plugging limit
* increase conveying gas flowrate
Pressure fluctuations exceed permissible
range or total pressure exceeds limits,
strong fluctuations, no trend:
* close to instable operating conditions
* reduce conveying gas flowrate to leave
instable zone
control strategy
Pressure fluctuations within acceptable
limits, no trend, no big spikes
* stable operating conditions
* no adjustment of conveying gas flow
needed
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