Information Technology Reference
In-Depth Information
D1
FD1
DIST1
6
3
B1
SP1
5
4
D
HT3
FD
HT1
DIST
1
DUPL
Q1
DUP
7
9
B
2
8
D2
SP2
HT2
DIST2
FD2
PUMP
B2
Figure 14.2
Shared reboiler - condenser flowsheet with reference column.
When implementing this approach it would be prudent first to have developed a
model of each column without energy integration to facilitate setting up specifications
for the degrees of freedom. An example of such a reference column, Dist, is illustrated
in Figure 14.2. This example employs a benzene -toluene distillation as its basis, and
the demonstration of the energy savings uses two columns each, virtually identical to
the reference column. Although the original column, Dist, is not part of the scheme, it is
convenient to have it available for debugging and as an aid to setting the specifications
for the degrees of freedom. In this example the reboiled absorber operates at a higher
pressure than the refluxed stripper, to facilitate heat transfer between the bottoms of
the refluxed stripper and the reboiled absorber's distillate. Unifac is used merely as a
convenience to provide estimated vapor - liquid equilibrium and is not recommended
for a real application. When the model is executed using Aspen Plus's defaults, it
does not converge. Initial estimates were provided for both secondary column feeds to
facilitate convergence. Details are given at Chapter Fourteen Examples/ExampleOne.
The difference in performance in columns Dist and Dist2 is due to the higher pressure
in Dist2. The higher pressure reduces the relative volatility of the benzene - toluene
system.
The energies used in the reference column (Dist) are as follows:
Condenser duty:
2
,
005
,
480 Btu/hr
Reboiler duty: 2,019,100 Btu/hr
Heat transferred through stream Q1: 1,831,371 Btu/hr
Reboiler energy use saved (Dist1): 91.3%
Condenser energy use saved (Dist2): 100%
−
The reboiler duty will still need to be provided for the source column, Dist2, and
the sink column, Dist1, will still require that its condenser duty be provided. This type
of configuration for several columns is analogous to a multieffect evaporator.
