Aspen Radfrac

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RadFrac for Dummies:

A How to Guide on Aspen Plus

This example will show how to use Radfrac on Aspen Plus to model distillationcolumns. The feed shown in the diagram above will consist of 50 lbmol/hr ofMethanol and 50 lbmol/hr of water. A purity of 99.5% is desired in both thebottoms and distillate product streams using a reflux ratio of 1.5.

DISTFEED

DISTILL

BOTTOMS



Radfrac © SDSM&T 2/35

If you don’t know how to log on toAspen Plus, please see the

“Getting Started on Aspen Plus”manual.

Choose the Template Option

Click “OK.”

This window allows you

to select a particularsimulation option. For

this example, select the“General with EnglishUnits” option. Also, make

sure that the option in theRun Type box displays“Flowsheet”

Click “OK”




Each time Aspen Plus is opened and anew file is created or an existing file isopened this login window appears.

Enter Unix host for the server type,sylvan.che.sdsmt.edu for the node

name, and your correct User name andPassword.

Click “OK”

Another window will appear indicating

the connection was established.

Click “OK.”

Aspen Plus automatically assigns labels toevery stream and block. To turn this option

off, click on the Tools menu in the toolbarand select Options.

In the Options window, click on the folder tablabeled “Flowsheet”. Next, click on theseboxes under the "Stream and Block labels."

The check marks in the boxes shoulddisappear indicating that these options aredisabled.

Click 'OK' when through.




To start the simulation,

click on the Columns tabfound in the selection ofpieces of equipment. Go

to the down arrow to the

right of the RadFracselection. Click on Fract1,

the second picture fromthe left in the top row.Next, move the cursor

into the white area andclick on the desiredposition for the column.

A prompt should appearasking to input the blockID, for this example “Dist”

was chosen.

The Feed, Distill andBottoms streams

need to be created.

To create the

streams, first click onthe “MaterialStreams” box at the

bottom left corner ofthe window. Redand blue arrows

appear around thecolumn.A red arrow signifies

a stream that isrequired for a designspecification; blue

arrows signify an

optional stream.




Click on the redarrow on the left sideof the column to addyour feed stream. Forthis simulation thereis only one feedstream, if there were

more feed streamsuse the blue arrowon the left of thecolumn to add morestreams.

Enter “Feed” in theID box whenprompted for this

simulation.

We next need to add theproduct streams. Onlyone of the red arrowsneeds to be chosen atthe top of the column.

Chose the bottom of thetwo red arrows at the topof the column. Thiscolumn will have a totalcondenser and thereforeliquid distillate. Labelthe top product stream“Distill”.

Next add the bottomproduct stream using thered arrow. Label theproduct “Bottoms”.

All the necessary

streams have beenadded and numericaldata can now beentered.




Now we are readyto begin entering

the necessarynumerical data.

Click on the “Next”

button to beginentering the data.This button is blue

and is located inthe tool bar at the

top of the window.

Click “OK”.

If this box doesn’t appear it

is because your flowsheetisn’t complete. A box willthen appear telling you

what part of the flowsheet

you are missing.

This box should appearand a title for thissimulation can now be

entered although is notmandatory for thissimulation. Click on the

Next button to continueentering numerical data.

If this box doesn’t appearyou can go to Setup onthe left-hand side of the

box and click on that.




The Components box willbe the next to appear. This

allows us to enter all of the

components that will bepresent within our system; in

this simulation they will be

Methanol and Water.

Type “Methanol”under the column“Component ID” and

press the Enter key.




AspenPlus will nowsearch its

database andattempt to match achemical name

with the

Component ID thatwas entered. If

this happens theother three boxes(Type, Component

Name, andFormula) will fillautomatically and

other componentscan then beentered.

Enter “Water” inthe second rowunder

“Component ID”

and hit Enter tofill the other three

boxes. We nowhave entered allthe components

necessary for thissimulation. Hitthe “Next” button

to proceed.




The next screen

to appear is thePropertySpecifications

sheet. This

allows us tochoose the base

method that wewant to use forthe calculations

AspenPlus willperform. We willselect NRTL for

this simulation.

Click “Next.”

The next screen toappear will showthe interaction

parameters for thecomponents in oursystem using the

base method weselected. If you

are happy with thisnumbers (and wesure hope you are)

click “Next.”




A prompt screen will appearasking if we want to enter any

more data or change theproperty specifications.

Since everything is good to go,click “OK.”

Now it’s time tobegin entering the

physical data for thesystem. The firstscreen that will

appear will be for the

Feed stream.Before we actually

start entering datalet’s go over acouple of the

different available

options.




There are 3 “StateVariables” that we

are allowed tochoose fromTemperature,

Pressure, or

Vapor Fraction. 2of these 3 must be

specified. Click onthe arrow on theright side of

Temperature orPressure Box toalternate between

the choices.

The units can be

changed for anyvariable by simply

clicking on thearrow on the rightside of the unitbox. This will bring

up a list of differentunits to choosefrom. For this

example we willuse degreesCelsius for

temperature andatmospheres for

the pressure.




Change the units ofTemperature to

Celsius and enter 25.Also set the pressurein the stream equal to

1 atm.

As stated in the

problem, the feedflowrate to the columnneeds to be set at 100

lbmol/hr. Enter that

here.

The next requiredinput involves

setting theComposition forthe feed stream.

Click on the arrowon the right hand

side of the box toview all of thedifferent options for

defining thecomposition.

Choose “Mole-

Frac.”




The problemstatement calls

for a 50/50 splitbetween the twocomponents in

the feed.

Enter 0.5 in both

boxes and thenhit “Next.”

The feed stream

is now complete.

Now is the timewe’ll all beenwaiting for,

designing theactual column.The first step is to

get at least arough estimate

for the requirednumber of stages

needed.




To do this,we’re going to

use AspenPlusto generate a T-XY plot and use

it to do a quick

McCabe-Thilediagram. At the

top of thescreen underTools, click on

Analysis,Property, and

Binary.

Click on this arrow

and select T-xy as thetype of analysis youwant performed. The

pressure must also beequal to the operatingpressure of your

column so enter 1atm.

Whenever you are

ready, click on “Go.”




AspenPlus shouldgenerate this

liquid/vaporequilibrium plot forMethanol and Water

at 1 atm. The X-axis

is the molefrac ofMethanol, so the

extreme left andextreme rightrepresent pure Water

and MeOHrespectively.

Notice that there areno azeotropesbetween the two so

perfect separation is

theoretically possible.

This diagram can

give us an estimateof how manyequilibrium stages

are necessary byusing the McCabe-Thiele method.

Unfortunately, wecannot show you

the actual drawingbut trust us; ittakes about 7

equilibrium stages.

After the graph has

been made andlooked at you canclose all three

windows that are

for the graph.




Enter 10 for theNumber of Stages

because ourcolumn will not beoperating at 100%

efficiency.

We will optimize

this number a littlebit further down the

road.

Click on the arrowon the right handside of the

Condenser box todisplay thecondenser

candidates.

As mentioned earlier

this column will have

a Total condenser.




Click on the arrow onthe right hand side of

one of the operatingspecification boxes todisplay all of the

alternatives. For this

example, we’re goingto choose to set the

Reflux Ratio andDistillate rate sincemethanol is the

component we areworried aboutrecovering and it has

a lower boiling point

than water.

Because there areno azeotropes, the

Distillate Flowrateof methanol can beset to the same

number of moles asthe entering flowrate,50 lbmol/hr.

The normal range ofReflux Ratios is

from 1.25 - 1.8 formost applications.Enter 1.5 and press

“Next” to continue.




The next screen asksto specify the Feed

Stage. If you’re notcertain of the locationa safe bet is usually

somewhere in the

middle.

This is another areathat will be optimizedlater.

For now, we’ll enter 5

and press “Next.”

We are now required

to enter the Pressureat which the columnwill operate.

Change the units toatmospheres by

clicking the arrow onthe right hand side ofthe units box, and type

a 1 in the box on theleft.

If data is alreadyavailable for PressureDrop in the column, it

can be entered. Forthis simulation we willaddress the pressure

drop in the column alittle later on in our

program.




Instead of clicking on“Next”, which wouldbring up a prompt to

run the simulation,click on theEfficiencies folder.

Our column isn’tperfect but what really

ever is. Normalefficiencies range from50-80% and there are

shortcut methodsfound in literature todetermine efficiency of

a column.

We wantMurphree-type

efficiency, so clickon the circle to theleft to highlight it.

Click on theVapor-Liquid tab

to continue.




Next we’re going toset the efficiencyfor one stage of the

column.

AspenPlus will

then be able tocalculate the otherstage’s efficiencies

from our givenbenchmark.

Enter 5 for thestage and 0.7 for

the efficiency.

The next step isfor us to set the

type of trays thatour column willconsist of.

Click on the TraySizing folder on

the left to get tothis screen.

Click “New” to

continue.




A box will appear prompting youfor a section number.

Click “OK” to continue.

This screen allows us tochoose from a list of

possible types of trayswithin our column.

Enter a 2 in the starting

stage and a 9 in theending stage because

there are 10 stages inour column as both thecondenser and reboiler

count as stages.




For this simulation,we are going to

choose Bubble-Captrays.

Click on the box to

the right of TrayType to see the list

of available trays.We’ll leave thenumber of passes

and tray spacing asthey are.

Click on the TrayRating folder on theleft. Click “new” to

continue.

Another prompt willappear to choose a

section number, click“OK”.

Enter 2 and 9 againrespectively for thestarting and ending

stages along with

Bubble cap for theTray Type.

An estimate for theDiameter of the

trays is nowrequired.

Enter 1.5 ft.




Click on theDesign/Pdrop tab to

continue.

Click on the box

directly to the left of

Update sectionpressure profile to

have AspenPluscalculate thepressure drop in the

column.

Just one final detail andwe’ll be ready to run our

simulation. Double-clickon the Setup folder,then click on Report

Options and finally clickon the Stream tab.

Click on the box to the

left of Mole in theFraction Basis

Column. AspenPlus willnow report the Molefraction of each stream,

99.5% is the goal.

Click “Next.”




Here’s the momentwe’ve been waitingfor. We’re ready to

run our simulation.

Click “OK.”

The control panelwill appear display

the iterations asAspenPlus findssolutions for our

simulation. Thecalculations shouldcomplete normally.

Click on the bluefolder with the check

mark to view the

results.




Click on Streams toview the results for

each of the threestreams.

Use the arrows on the

left and bottom of thestream report to look

through all of theresults. We achieved94.2% purity in both

product streams. Notquite up to the spec of99.5%, let’s find out

what we can optimize

to meet our goal.

Double-click on the

blue Blocks folderand then also onDist.

Next, go to the topmenu bar and click

on Plot and thenPlot Wizard.

Plot Wizard takesthe profilesAspenPlus

generated for thecolumn and turnsthem into pretty little

charts.




A welcoming screen willappear, click “next” to

move onto this screen.The first plot we want tolook at is the liquid

composition inside the

column.

Click on Composition

and hit “Next”.

Click on the doublearrows pointing to the

right to select bothcomponents to be inthe plot.

Next click the circlenext to Liquid under

“Select Phase.”

Clicking on “Next”

again brings updifferent plot options.Nothing on the next

set of options needs tobe changed.

Click “Finish” to view

the plot.




This plot can tell us acouple of different

things.

First off, there are no

level spots in the

diagram so we don’thave any extra stages.

Also, notice the littledip in the middle on

stage 5, our feedstage. This meansthat we could possibly

increase ourseparation bychanging the location

of the feed tray.

These are two areas

where we could

optimize our column.

Let’s start to optimize

by changing thenumber of stages inour column.

Close all the resultsfolders and click on

the Blocks and Distfolders respectively toreturn to the column

Setup screen.

Under Configuration,

change the number ofstages from 10 to 13while leaving the feed

stream on stage 5 for

the time being.




We also need tochange the number

of stages withrespect to our TraySizing and Tray

Rating.

Go into the 1 below

each folderrespectively andchange the ending

stage to 12.

We’re all set up, so

click “Next” to runthe simulation again.

Click “OK” to

continue.

Upon completion clickon the blue folder

again and go underResults Summaryand then Streams.

The results show thatwe now have 94.9%

purity in each of theproduct streams.Moving in the right

direction but still notup to the desiredspecs.

Let’s take a look at theLiquid Composition

diagram…




Double-click on theblue Blocks folder

and then on the blueDist folder to look atthe results for the

column.

Go to Plot at the top

of the screen and clickon Plot Wizard.

Hit “Next” through thewelcoming screen andclick on the

Composition plot.

Choose to plot Both

Components in theLiquid phase and hit

“Finish.”

From this plot it isobvious that our feed

stage is a little bit toolow.

We want thecomposition to besmooth lines without

any humps or bumpsin the middle.

Close the Resultsfolder and let’s go

adjust our feed stage.




Back under theBlocks/Dist folders,

click on Setup and thenthe Streams tab at thetop of the screen.

Change your feed platefrom 5 to 6 and click

next.

Once again check your

Liquid Compositiondiagram using PlotWizard as done in

previous steps.

Is the hump gone from

the middle? If notcontinue this procedureuntil the curves are

smooth. (Hint: 9 might

be the answer)

After you’vedetermined the

optimum feed platelocation (HINT: 9) yourplot should look like

this.

Next check your

stream results to see ifwe have achieved thedesired purity in our

Methanol Stream.




Unfortunately we stillhaven’t reached the

desired purity in ourproduct streams. Butwe did increase the

purity by 3% to 97.5%

by optimizing thelocation of the feed

plate.

Since the reflux ratio

is given at 1.5 in theproblem statement,the only other option is

to continue to increasethe number of stages

in our column.

Return to the Setupscreen under the

Blocks/Dist folder.

Click on the

Configuration tab atthe top and increase thenumber of stages in the

column.

Also, go under the Tray

Sizing and Tray Ratingfolders and change theending stage on each to

one stage less than theactual number of stages

that you entered here.




Next click on theStreams tab and

change the locationof the feed plate.

Click on “Next” to

run the simulation,then check the liquid

composition diagramto be sure that it is inthe optimum

position. If it is,check the results.

Continue thisprocedure until youmeet the given purity

spec.

There’s the magicnumber, 99.5%!

The final configurationof the column has 21

bubble-cap trays withthe feed plate onstage 16.

Now that we have ourcolumn set up, let’s

look at a couple of theother options that are

available.




AspenPlus has alsocalculated the

Diameter of thecolumn.

On the Results

page, click onBlocks, Dist,

Tray Sizing, and 1.

Next, click on the

Results tab at thetop of the screen.This screen shows a

calculated diameterof around 2 feet for

our column.

There are a couple of

different ways that wecan look at the pressure

drop within the column.The first is by clickingon the Tray Ratingfolder and opening 1.

Click on the Results tabat the top and use the

arrows on the right tomove down until theSection Pressure Drop

is shown.

Since this is the only

section, the pressuredrop for our column is

about 4.5 psi. Byclicking on the “psi” boxyou can also change

the pressure units.




The other way we canobserve the pressure

drop is by using PlotWizard to create agraph.

Go to Plot Wizard andclick on the Pressure

graph.

Choose the desired

units and click “Finish.”

A graph similar to this

one should be createdshowing the pressureprofile in the column.

The temperature profilecan also be created in a

similar manner.

Congratulations! You have

finished the race and you’reprobably not even half as tired

as this guy, he looks beat.

Now you can enter

components, feed streams,

column data and generate

results for all of your own

simulations. Feel free to brag

to all of your friends now

about your superior knowledge

of Radfrac, you’ve earned it.

You are truly a champion.

Peace and Love,

Bj




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