a+ primer design spec

8/3/2019 A+ Primer Design Spec

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Primer II for Using Aspen PlusProcess Simulation Software DesignCalculations

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 firstprimer.

Aspen can vary column parameters to optimize reflux ratios, mole recoveries, etc. It can also automaticallyoptimize 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 SpecificationsWithin Aspen, design specifications that can be automatically targeted appear in Table 1

Table 1 Summary of Aspen Design specifications

Design Specification Specifies

Stream purity Purity of product, internal, or decanter stream(s)

Component recovery Recovery of component (s) in product stream(s) based on feed stream(s)Flow rate Flow rate of product, internal, or decanter stream(s)

Component ratio Ratio of flow of component(s) of an internal stream to flow of

component (s) of internal, feed, or product stream(s)

Stage temperature Temperature on a specific stageStream property value Property value of a product or internal stream

Stream property difference Difference of two property values based on product or internal stream(s)

Stream property ratio Ratio of two property values based on product or internal stream(s)Distillate flow rate Distillate flow rate

Bottoms flow rate Bottoms flow rate

Reflux flow rate Reflux flow rate from condenserBoilup flow rate Boilup flow rate

Reflux ratio Ratio of reflux flow rate to distillate flow rate

Boilup ratio Ratio of boilup flow rate to bottoms flow rate

Condenser duty Condenser dutyReboiler 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, severaldesign specifications provide a complete design. The specific example here assumes a 50:50 mixture of n-butanol 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 equilibriumstages 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 1 Data entry dialog box for design specifications.

1. Select New in the Design Specifications dialog box under the Block node of the Data Browser treesection (see Figure 1).

2. Accept the default name of 1 for this specification3. 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 theappropriate 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 n-

butanol in the bottoms.


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Figure 2 Dialog 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 Varynode 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 DataBrowser.


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2. Select the design parameter to be varied from the Adjusted Variable drop-down box, in this case RefluxRatio.

3. Enter the range over which the design parameter (Reflux Ratio) may be varied to meet the DesignSpecifications. In this example, the reflux ratio may vary from 0.5 to 25.

Figure 3 Dialog 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 designspecification after the computations complete (see next section).

Design CalculationThe 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% purealcohol 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 thedesign 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 orthe reflux ratios, Aspen provides an automatic method to analyze this tradeoff. The NQ Curves node of theBlocks 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 4 NQ 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 setthe 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 boxThis 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 numberof 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 19.5with 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 givennumber 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 vsoperating costs that is taller towers vs. higher heat/cooling demands in these towers.


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Figure 5 Example 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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