Chemical Engineering December 2014 - 53

Solids Processing
143
2.36
Pres-out
Press
80
8.03
56
143
1.12
77
1.70
225
1.99
67
0.80
Recycle
Screen2
77
2.98
Product
FIGURE 3. With a new process configuration and optimized parameters, the process
now produces 57% more potassium chloride while meeting equipment constraints
and product quality requirements
pacities are not closely approached
and there is an opportunity for the
product flowrate to be increased.
The feed solution flowrate can be
increased 144% (from 160 to 230
m.t./h). The impact of this change
can be seen in Figure 2, where only
the compacting and sizing section
is shown.
With this change, the press is now
over its capacity of 144 m.t./h at 163
m.t./h due to a high recycle flowrate
of fines from the second screen
which is the result of over grinding
by the two mills. Here, there is
an opportunity to adjust the impeller
speeds of the mills to minimize
the recycle stream so that capacity
constraints of the press are met.
Instead of using trial-and-error, a
flowsheet simulator and its optimization
tool can be used to find the
optimal impeller speeds that would
be needed to minimize the recycle
flowrate while meeting the capacity
constraints on the three pieces
of equipment.
The optimization study shows
that the optimal impeller speeds are
14 and 8 m/s for the two mills, while
the initial speeds were both 15 m/s.
With these changes applied, the
recycle rate is minimized, but it is
still very large for the process. The
change reduces the recycle flowrate
from 100 m.t./h to 79 m.t./h and the
flowrate to the press is now below
the constraint. A preliminary cost
estimation of the capital and utility
costs of the entire process amount
to $15.5 million and $0.39 million,
respectively.
While the recycle flowrate was
already minimized under the
given conditions, it can be further
reduced by addressing the second
mill, which is at capacity. Additionally,
a portion of the particles
exiting the press might already
be at the correct particle size and
thus, a screen with three decks
could separate the particles that
are not too small or too large and
direct them to the product stream.
These changes to the structure of
the process were applied and the
new configuration can be seen in
Figure 3.
By adding these changes, the recycle
stream is reduced to 66 m.t./h,
the press now has spare capacity,
and the feed flowrate can be increased.
By further minimizing the
recycle flowrate by adjusting the
impeller speeds of the three mills
and increasing the potassium chloride
solution flowrate, the product
flowrate is maximized. With the
optimized impact velocity and the
alternative design it is possible to
produce 57% more potassium chloride.
The final process modifications
can be seen in Figure 3. Using preliminary
cost estimation, the different
process configurations can be
compared. With the adjustments
made in this example the process
now produces 46% more revenue
while the capital costs are only 15%
higher (which includes the new
mill) and the utility costs are only
4% higher.
52 ChemiCal engineering wwww.Chemengonline.Com DeCemBer 2014
Screen
33
0.81
7.72
56
7.72
113
7.72
Summary
Name
Feed Solution Flow
Solid Product Flow
Press Inlet Flow
Recycle Flow
Crusher 1 Inlet Flow
Crusher 2 Inlet Flow
Impeller speed 1st mill
OPTIM.VAR_VAL
Impeller speed 2nd mill
OPTIM.VAR_VAL
Value
290
76.5125
143.143
79.9534
56.284
9
8.88269
5.75
5.75
5.73698
Units
m.t./h
m.t./h
m.t./h
m.t./h
m.t./h
m/s
m/s
m/s
Further modification and optimization
may be necessary for
the rest of the process. For instance,
an analysis could be done
around the dewatering and drying
section of the flowsheet. The
energy use associated with the
hydrocyclone and the decanter
centrifuge, as well as the dryer,
could be minimized.
Understanding how the PSD
changes throughout a process -
especially one involving recycle
streams, such as those in the compacting
and sizing section of the
potassium chloride example given
- can help users capitalize on opportunities
to achieve increased
throughput, and reduced energy
and capital costs. Without flowsheet
simulation and the rigorous peervalidated
models available today,
many potential improvements to
the process may go unnoticed and
the benefits of reduced costs, increased
throughput and improved
quality may go unseen.
n
Edited by Suzanne Shelley
Reference
1. Hartge, E.U., Pogodda, M., Reimers, C., Schwier,
D., and others: Flowsheet Simulation of Solids
Processes, KONA Powder and Particle
Journal, Vol. 24 (2006) 146-158; www.jstage.
jst.go.jp/article/kona/24/0/24_2006017/_article
Author
Jennifer
Dyment is the
product marketing manager
for Aspen Plus and
its associated products
and features at Aspen
Technology (20 Crosby
Dr., Bedford, MA 01730;
Phone: 781-221-4286;
Email: Jennifer.Dyment@
aspentech.com). She
holds a B.S.Ch.e. from
Dartmouth College.
Claus Reimers is the
product management director
for solids modeling
and heat exchanger design
and rating at Aspen
Technology (20 Crosby
Drive, Bedford, MA
01730; Phone: 781-2216437;
Email: claus.reimers@aspentech.com).
Prior
to joining Aspen
Technology, Reimers was
a founder and co-CEO at
SolidSim Engineering
GmbH for four years. He holds a Ph.D. in chemical
engineering from Hamburg-University of
Technology and has published several articles
on simulation and optimization of complex solids
processes by use of flowsheet simulation
techniques.
http://www.jstage http://jst.go.jp/article/kona/24/0/24_2006017/_ar http://wwww.Chemengonline.Com
Chemical Engineering December 2014

Table of Contents for the Digital Edition of Chemical Engineering December 2014
