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1. A large flowsheet should be divided into a subset of smaller flowsheets.
2. Each subset should have its own appropriate properties.
3. Initially, in solving a subset, basic blocks without energy balances should be
employed and converged.
4. All initial attempts to converge should use Aspen Plus defaults (i.e., Wegstein's
method). The Wegstein approach can solve a wide variety of problems.
5. Initial convergence should be with loose specifications.
6. As the development proceeds, the basic blocks should be replaced, one or two
at a time, with rigorous blocks, and energy balances turned on.
7. The rigorous blocks should be tested on a stand-alone basis with feeds from the
streams calculated using the basic blocks prior to integration.
8. When a subset is executed using rigorous blocks, the independent variables from
all tear streams that are feeds to the blocks should be transferred from the con-
verged initial flowsheet where the basic blocks were used.
9. If the Aspen Plus-selected tear streams within the subsets are not suitable, new
tear streams should be identified. Convergence blocks should be defined for the
new tear streams. New calculation sequences may also be required.
10. When the subsets are completed, assembly of a fully rigorous flowsheet should
proceed one subset at a time with intermediate testing. Tear streams may need to
be changed. A gradual tightening of specifications is required. This process con-
tinues until the combined subsets have been integrated and the entire flowsheet
has converged.
12.2 EXAMPLE: THE PRODUCTION OF STYRENE
A flowsheet of a simplified version of the commercial process for the production of
styrene by catalytic dehydrogenation of ethylbenzene is shown in Figure 12.1. The
process description is as follows.
9
K
2
G
7
F
15
8
H
10
J
6
11
C
12
B
D
A
3
4
5
13
E
1
14
Figure 12.1
Modified styrene production flowsheet.
