Information Technology Reference
In-Depth Information
CHAPTER SEVEN
FLASHES AND DECANTER
Aspen Plus's model library contains two different rigorous flash blocks that solve the
appropriate material, energy balance, and equilibrium equations. The Flash2 block is
designed to produce a single vapor phase and a single liquid phase that are in equi-
librium when the flash conditions are specified such that a multicomponent mixture's
state is in the two-phase region. Similarly, the Flash3 block is designed to produce one
vapor phase and two liquid phases in equilibrium for suitably specified process condi-
tions. The Flash3 block is also capable of solving a liquid - liquid equilibrium problem
under conditions where no vapor is produced. A similar block is Decanter, which is
designed to produce two liquid phases in equilibrium in the absence of a vapor phase.
The Flash2, Flash3, and Decanter blocks can be found in the model library under the
Separators tab.
7.1 Flash2 BLOCK
Figure 7.1 is a graphical depiction of a Flash2 block. The nomenclature used is as
follows:
F
represents the total feed flow, in moles/time;
f
i
the flow of component
i
in the feed, in moles/time;
V
the total vapor flow, in moles/time;
v
i
the flow of
component
i
in the vapor, in moles/time;
L
the total liquid flow, in moles/time;
l
i
the
flow of component
i
in the liquid, in moles/time; and
n
the number of components.
Including the flash temperature
T
f
and pressure
P
f
results in 2
n
+
2 independent
variables given the feed state. These are equilibrium temperature; flash pressure; two
total flows,
L
and
V
;and2
(n
−
1
)
component flows; or alternatively, 2
n
component
flows, excluding the total flows. Mole fractions are calculated from the independent
