Information Technology Reference
In-Depth Information
CHAPTER FIVE
FLOWSHEETING AND MODEL
ANALYSIS TOOLS
Two of the most useful tools in Aspen Plus are the Sensitivity study and the Design
specification, both located on the Data Browser. These tools work hand-in-hand and
are used when a design specification is used to specify a block output variable. For
example, the file Chapter Five Examples/mixer5.bkp presents a mixer block with three
feeds. When all input streams are specified properly, the block will calculate the com-
position and state of the output stream. Figures 5.1 and 5.2 show the graphic setup
and the solution. Note that the product, stream 4, is a straightforward summation on a
componential basis, of all the feeds, excluding an energy balance. Suppose, however,
that in the example one requires that the composition of a specific component, say
ethanol, is to be 0.5 mole fraction. It will be necessary to increase the flow of stream
2. This is easy to do with a hand calculation, but if this block were in the middle of
a process flow diagram, it might not be a trivial task. The Sensitivity study permits
the specification of a trial range of flow rates for stream 2, which may be plotted
against the resulting ethanol composition. Once an approximate solution is obtained,
by plotting the results and checking the location of the target composition, the Design
specification can be employed to provide a means for Aspen Plus to solve for the
exact value of the flow rate of stream 2, which satisfies the specification desired. In
setting up both the Sensitivity study and the Design specification, it may be necessary
to employ the Fortran language capability embedded in several blocks and available
on a stand-alone basis in the Calculator block.
5.1 INTRODUCTION TO FORTRAN IN ASPEN PLUS
Fortran (Formula Translation) is a very old but rich language which has traditionally
been used for scientific applications. Details of the language are very complex and can
