Information Technology Reference
In-Depth Information
where
F
is the degrees of freedom,
C
the number of components, and
P
the num-
ber of phases. For a single-phase system,
F
2; therefore, two specifications are
required. For an
n
-component system,
F
=
n
+
1. Since
n
−
1 mole fractions need
to be specified and the last calculated by the sum of the mole fractions equal to 1,
only two additional specifications are required. In both cases these are usually, but not
necessarily, temperature and pressure.
When two phases in equilibrium are involved, only one degree of freedom is avail-
able. For example, for a one-component stream such as a saturated liquid, specification
of the temperature fixes the (vapor) pressure. But in such circumstances it is neces-
sary to state the fraction of the mixed stream that is vapor (or liquid). For a saturated
liquid the
V/F
specification would be a very small number, such as 0.00001. For a
multicomponent stream the situation is identical and it is necessary to specify either
temperature or pressure and the vapor or liquid fraction.
If the process contains tear (recycle) streams, they will not be treated as required
input, and pushing the Next button may not suffice. Typically, Aspen Plus will assign
zeros as starting values to the variables that are to be converged, but if a user wishes
to provide starting values, the stream name under the streams list in the Data Browser
can be clicked and a display analogous to Figure 1.15 will appear.
=
1.10 BLOCKS
When all stream input has been completed, pushing the Next button will result in
the appearance of the first input form for a block that requires data. Details of block
input are addressed in other chapters. After the data input forms for the first block are
completed, pushing the Next button will produce the forms for the next block in the
process until all the block data have been entered.
The browser list in Figure 1.15 shows some additional topics that may be appropriate
for a user's simulation. These will not appear when the Next button is pushed; however,
they may be selected by clicking on the subject. For example, clicking the Convergence
entry permits the selection of the convergence method and parametric default values.
These subjects are addressed in other chapters. When data entry for all blocks and
supplementary data entry is completed, selecting the Next button produces a dialogue
to enable execution. All input can be reviewed, prior or after execution, by selecting
the
or
keys above the Flash Options tab near the top of Figure 1.15 and selecting
input from the drop-down list between the double arrows.
1.11 VIEWING RESULTS
In preparation for viewing the results, prior to execution of a simulation, a user may
customize reports by selecting Report Options, as shown in Figure 1.16. Customization
is available under each of the tabs. The stream tab illustrates the variety of options
available for selecting the stream information to be displayed.
At the conclusion of a simulation, the run control panel is usually displayed. If
not, it can be viewed from the drop-down list under the main menu item View. This
contains a brief summary of the execution of each block and a list of error messages. It
is important that users check and correct any errors that might have occurred. When a
process contains tear streams, errors that occur during the iterations will be presented.
