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Design-Specification Manual:A How-to on Aspen Plus

MIXER

FEED

Pure Isobutane

100 Kmol/hr

PRODUCT95% (mol) isobutane

5% (mol) isobutene

RECYCLE

80% (mol) isobutane

20% (mol) isobutene

This is an example on how to use the Design-Spec mode on Aspen Plus to solve some

simple mass balances. This is especially useful when inlet stream conditions are partiallyunknown and some outlet conditions are known. A diagram is shown above to illustrate a

particular set up. There are two unknowns in this example problem²the flow rate of the

RECYCLE stream and the flow rate of the PRODUCT stream.



If you don't know how to login toAspen Plus, please see the "Getting

Started on Aspen Plus" manual.

Choose the Template option.

Click 'OK.'

This window allows you

select a particular simulaoption. For this example

select the "General with

Metric Units" option. Amake sure that the optionthe Run Type box displa

"Flowsheet."

Click 'OK.'



Each time a new file is created or an

existing file is opened this login windowappears.

Enter UNIX host for the server type,

sylvan.che for the node name, and your correct user name and password.

Click on 'OK.'

Another window will appear indicating the

connection was established. Click 'OK.'

Aspen Plus automatically assigns

labels to every stream and block.To turn this option off, click on the

Tools menu in the toolbar andselect Options.

In the Options window, click on the

folder tab labeled Flowsheet. Next,click on these boxes under the

"Stream and Block labels." Thecheck marks in the boxes should

disappear indicating that theseoptions are disabled.

Click 'OK' when through.



To start the flow simulation, click

on the down arrow to the right of the mixer selection at the bottomof the window. This will give you

options for the type of mixer youwill use. Click on the solid black

square mixer. Next, move thecursor into the white area and

click on the desired position for the mixer. A prompt should

appear asking you to input the block ID, for this example

³Mixer´ was chosen. You havenow created the Mixer for the

simulation.

The Feed, Recycle and Product

streams need to be created for thisexample.

To create the Feed and Recycle

streams, first click on the ³MaterialStreams´ box at the bottom left

corner of the window. Red and bluearrows appear around the mixer.

A red arrow signifies a stream that isrequired for a design specification;

blue arrows signify an optionalstream.



To change where the Feed streamenters the mixer, click and hold on

the red feed arrow; note how thecursor changes into a double

arrow. "Drag" the feed arrow to

its desired position (the top of themixer) and release the mouse button. To return the stream to it¶s

original position simply rightclick.

Once the Feed stream has been

moved, move the cursor awayfrom the mixer and click at the

desired location. The stream ID box will appear. Label the stream

"Feed." Click 'OK'. The Feedstream will be created.

The Recycle stream is created the

same way as the Feed. Click onthe "Material Streams" box at the

bottom left corner of the window.Red and blue arrows appear

around the mixer. The feed arrowon the mixer is now blue because

more than one inlet stream isn¶trequired. Click and hold on the

blue feed arrow and drag it to the bottom of the mixer before

releasing. Then move the cursor away from the mixer and click.

Label the new stream as

"Recycle". Your diagram shouldappear the same as the window tothe left.

Several more feed streams could be connected to the mixer using


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If this screen appears, enter in a

title for the simulation and click on the Next button.

If it doesn¶t you can find it byclicking on Setup, which is foundon the left hand side of the input

screen. However, completing theSetup form isn¶t manditory.

Once finished inputting a title,

Click Next.

In the Component Name boxenter the two components for this

problem: isobutane andisobutylene (isobutene). Note

that the data bank doesn¶t

recognize the name isobutene,hence isobutylene is entered because it is the same compound

and it is in the data bank. Enter these names in the component

name column. The names in thecomponent ID column are aliases.

Click Next.

Click Ok.

If this box doesn¶t

appear it is becauseyour flowsheet isn¶t

complete. A box wil

then appear telling yowhat part of the

flowsheet is missing.



At this screen, enter in NRTLfor the "base method´ as we

have done before in other simulations. This screen is

under the Specifications sub-directory of the Properties

section.

Click Next.

Aspen will now ask you for the

FEED stream information. (Notehow the title bar indicates which

input form you are in.) Enter a

temperature and a pressure of your choice. Next, enter theknown flow rate of isobutane in

the Total Flow box (make surethat you are using the right units).

Also, Aspen Plus requires somecomposition info for the feed

stream. For this example, in theComposition box, select the

Mole-Frac and input 1 in the

This screen is asking you if you areready to go to the next required step.

Click Ok.



Value box for isobutane.Click on the Next button.

The next screen will be the input

screen for the Recycle stream.(Look at the title bar if you are

unsure of which form you are in.)Once again enter in a temperature

and pressure of your choice aswell as the known mole fractions.

Since we don't know the flow rateof this stream, just enter in a

guess as was done here. Theguess does not matter because we

will design-spec will iterate tofind this value later.

Enter in a guess.

After all the information isentered, click the Next button

Continue entering information and clicking the Next button until you arrive at this screen.Here is where Aspen will ask you if you want to run a simulation. At this point, we don't

want to run the simulation so click the Cancel button.

This screen is asking for the flash

options for the mixer, however, if you look in the lower left hand of

the window it says that the input iscomplete. Therefore, it isn¶t

necessary to input anything on thisscreen. Simply click next.



The next step is to click on the Data button from the toolbar. Go to Flowsheeting options

and then Design-Spec (shown below). This will take you to the Design-Spec mode inAspen Plus.

This is the first window of the

Design-Spec mode in Aspen. Wewant to create a new design-spec

so click on the New button.



Aspen will now ask you to name avariable. You can give a name of

your choice or choose the defaultname that Aspen has provided.

Click OK after choosing a name.

When you enter a variable, the

screen at the left will show theinput page for the variable. In the

Design-Spec mode, you mustdefine the ³specified variable´ as

well as the ³manipulatedvariable´. In this example, the

unknown flow rate of the recyclestream will be the manipulated

variable. The know composition,mole fraction, of the product

stream will be the variable

specified.

It is necessary to createtwo new variables. Thevariables created here are

the flowrate of theRecycle stream and the

flowrate of the Productstream because those are

both unknown.

To create a new variableclick on the New button.

Here you can enter a namefor the variable. In this

example Recycl was

chosen.



The next step is to enter in thetype of variable. If you click on

the arrow you will see a list of choices. The manipulated

variable is given the arbitraryname, ³RECYCL´ It represents

the unknown flow rate of therecycle stream. The type of

variable is inputted as³Stream-Var´ which stands for

stream variable. Use this choicewhen your variable is a property

of the stream, i.e. temperature, pressure, flow rate, etc.

After entering the type of variable,

Aspen will then ask for the locationof the variable. In this case, we needto define the name of the stream

associated with this variable. Enter ³Recycle´ for the stream location.

The Substream cell will then appear.The default is ³mixed´ and should be

left unchanged. Finally, if we definethe type of variable as a Stream-Var,

we also need to specify the propertyvariable. In this case the variable is

a molar flow rate, so we enter MOLE-FLOW for the last cell. At

this point, the screen should becomplete and we can move on to the

next screen.

Before moving on, we need to

define the specified variable that isthe mole-fraction of isobutane in

the product stream. Do this bycreating a new variable the same

way as the RECYCL variable wascreated.

At left is the input screen for the

mole fraction variable.

After both variables are entered, goto the Spec folder.



Here are the results. The Recycle stream flow rate came out to be 33.78 Kmol/hr. Onecan also notice that Aspen calculated the product stream flow rate as well. This was

originally an unknown. Even though we didn¶t define this variable, Aspen still calculatesthe mass balance simultaneously along side the Design-spec. Click on the ³>>´ button at

the top.

If you create a Design-Spec andrun a simulation, you should

always check this screen. Thiswill tell you if Aspen converged

on a value or not.If convergence was not achieved,

it usually means you need tochange the limits of your

manipulated variable.

design spec10

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