Primer II for Using Aspen Plus® Process Simulation Software – Design
Calculations
This is the second primer for using Aspen Plus® software. It focuses on using design features of the simulation
software and builds on the first Primer. It does not review startup or most other issues discussed in the first
primer.
Aspen can vary column parameters to optimize reflux ratios, mole recoveries, etc. It can also automatically
optimize the number of stages required to minimize, for example, the reboiler duty or molar reflux ratio. This
primer briefly reviews how the problem can be set up for these types of calculations.
Design Specifications
Within Aspen, design specifications that can be automatically targeted appear in Table 1
Table 1 Summary of Aspen Design specifications
Design Specification
Stream purity
Component recovery
Flow rate
Component ratio
Stage temperature
Stream property value
Stream property difference
Stream property ratio
Distillate flow rate
Bottoms flow rate
Reflux flow rate
Boilup flow rate
Reflux ratio
Boilup ratio
Condenser duty
Reboiler duty
Specifies
Purity of product, internal, or decanter stream(s)
Recovery of component (s) in product stream(s) based on feed stream(s)
Flow rate of product, internal, or decanter stream(s)
Ratio of flow of component(s) of an internal stream to flow of
component (s) of internal, feed, or product stream(s)
Temperature on a specific stage
Property value of a product or internal stream
Difference of two property values based on product or internal stream(s)
Ratio of two property values based on product or internal stream(s)
Distillate flow rate
Bottoms flow rate
Reflux flow rate from condenser
Boilup flow rate
Ratio of reflux flow rate to distillate flow rate
Ratio of boilup flow rate to bottoms flow rate
Condenser duty
Reboiler duty
Notably absent from this list is the number of stages, which is discussed later. The design specification is
entered in the Design Specification node within the block that defines a distillation column. Commonly, several
design specifications provide a complete design. The specific example here assumes a 50:50 mixture of nbutanol and isobutanol and uses UNIQUAC thermodynamics models. An atmospheric-pressure column with a
total condenser and with a distillate to feed ratio of 0.5 and a nominal reflux ratio of 15 should be specified if
you want to make quantitative comparisons with this example. The column in this example has 25 equilibrium
stages with the feed on stage 15.
In this example, mole recovery specifies the amount of a feed component recovered in the distillate and bottoms
feeds, respectively. To make such specification:
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Figure 1Data entry dialog box for design specifications.
1. Select New in the Design Specifications dialog box under the Block node of the Data Browser tree
section (see Figure 1).
2. Accept the default name of 1 for this specification
3. Select an appropriate design specification in the next dialog box (Figure 2) from Table 1, in this case the
second choice in molar units, or Mole Recovery.
4. Enter the target value for this recovery (0.975) as the target specification.
5. Select the component on which to base this recovery (isobutanol) and the stream to which it corresponds
(2 – or distillate) by clicking on the Components and Streams tabs respectively and selecting the
appropriate information (or click on N-> to be guided to these sheets automatically).
A second design specification (Design Specification 2) should be similarly constructed for 97.5% recovery of nbutanol in the bottoms.
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Figure 2Dialog box for specifying the specifications for Design Specs 1.
Varying Parameters
The parameters to vary to meet the design specifications previously entered in the program appear in the Vary
node of the Block tree. At least one and no more than two varying parameters are specified in this node. These
must be selected from the two operating specifications established in the column Setup dialog box. In this case,
reflux ratio and distillate to feed ratio where chosen as the operating specifications in the Setup dialog box.
Reflux ratio will vary as the program attempts to meet the design specifications of 97.5% molar purity in each
of the distillate and bottoms feeds. The following procedure outlines how to make such specifications:
1. Create a new Vary object by selecting New in the Vary dialog box under the Block node of the Data
Browser.
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2. Select the design parameter to be varied from the Adjusted Variable drop-down box, in this case Reflux
Ratio.
3. Enter the range over which the design parameter (Reflux Ratio) may be varied to meet the Design
Specifications. In this example, the reflux ratio may vary from 0.5 to 25.
Figure 3Dialog box for specifying the parameter and its range of variation that should be varied to meet the design
specifications.
The Results tab on this dialog box will display the actual reflux ratio used in the program to meet the design
specification after the computations complete (see next section).
Design Calculation
The program is now prepared to vary the reflux ratio to meet the design specifications. To initiate this
calculation, press the next button (N->). The program will either meet the design specification or come as close
as it can to meeting it within the constraints of the varying parameter. That is, it will either produce 97.5% pure
alcohol streams or it will come as close as it can by varying the reflux ratio from 0.5 to 25. In this example, it
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meets the specification with a reflux ratio of 19.5. The resulting recoveries appear under the Results tabs of the
design specifications (97.5 in both cases) and the resulting reflux ratio appear under the Results tab of the Vary
specification (19.5 in this case).
Optimizing Stages vs. Heat Demand
Since column design often involves varying the number of stages vs. the reboiler or condenser heat demand or
the reflux ratios, Aspen provides an automatic method to analyze this tradeoff. The NQ Curves node of the
Blocks section of the Data Browser tree for a given column enables these calculations. Begin with the column
specifications in the same stage as it was left after varying Reflux Ratio to meet design specifications. Figure 4
illustrates the dialog box used to specify parameters for NQ Curves calculations.
Figure 4NQ Curve object specification in Aspen.
The primary steps in entering these data are as follows:
1. Create a new NQ Curve object by selecting New in the initial dialog box.
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2. Accept the default name of 1.
3. In the dialog box (Figure 4), enter the lower and upper limits of the number of stages you wish to
consider in this optimization. The upper limit may not exceed the number entered in the column Set Up
dialog box when the column was first established. For this example, increase this number to 30. Then set
the lower and upper limits to 25 and 30. In this case, the calculations consider columns ranging from 25
to 30 stages.
4. Enter a step size (number of stages by which the column optimization will change) in the indicated box.
This is normally either 1 or 2 – 1 in this case.
5. Specify the stream number that represents the feed stream.
6. Finally, select a design objective, called the objective function (see below).
In the classical design approach, the objective function is the reboiler duty (or occasionally the condenser duty)
as a function of the number of stages. However, conceptually similar optimizations can be performed by
combinations of heat duties or by varying reflux or other parameters. The NQ Curves object systematically
varies the number of stages and the feed stage location, finding an optimum feed stage location for each number
of stages modeled. It selects the optimum design based on the design objective (objective function) specified in
this box. That is, it will find the least number of stages and the optimum feed location based on minimizing the
reboiler duty, the reflux ratio, etc.
The program is now prepared to make these calculations. Do so by pressing the next button (N->). The results
of the calculations appear under the results node of the NQ Curves object, Figure 5 illustrating typical results.
The summary data included here (accessed from the Basic Results tab in the Results node of the NQ Curves
object) indicate that a column with 25 stages has an optimal feed location at Stage 13 and a reflux ratio of 19.5
with the indicated reboiler and condenser duties. Alternatively, a similar separation occurs with 26 stages and
the feed (optimized) at Stage 15 and a reflux ratio of 15.3. The sequence continue through 30 stages, with the
feed at Stage 17 and a reflux ratio of 9.1. Each of these calculations includes an optimal feed stage for the given
number of total stages. With such data, designers can select either taller columns or higher reflux ratios to
accomplish a given separation. Generally, the tradeoff between these two depends primarily on capital vs.
operating costs – that is taller towers vs. higher heat/cooling demands in these towers.
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Figure 5Example of optimized number of stages, feed location, and resulting condenser and reboiler duties and reflux ratios
during an NQ Curves calculation.
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