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

B O T T O M S

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If you dont know how to log on toAspen Plus, please see theGetting Started on Aspen Plusmanual.

Choose the Template Option

Click OK.

This window allows youto select a particularsimulation option. Forthis example, select theGeneral with EnglishUnits option. Also, makesure that the option in theRun Type box displaysFlowsheet

Click OK

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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 nodename, and your correct User name andPassword.

Click OK

Another window will appear indicatingthe connection was established.

Click OK.

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

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To start the simulation,click on the Columns tabfound in the selection ofpieces of equipment. Goto the down arrow to theright of the RadFracselection. Click on Fract1,the second picture fromthe left in the top row.Next, move the cursorinto the white area andclick on the desiredposition for the column.A prompt should appearasking to input the blockID, for this example Distwas chosen.

The Feed, Distill andBottoms streamsneed to be created.

To create thestreams, first click onthe MaterialStreams box at thebottom left corner ofthe window. Redand blue arrowsappear around thecolumn.A red arrow signifiesa stream that isrequired for a designspecification; bluearrows signify anoptional stream.

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Click on the redarrow on the left sideof the column to addyour feed stream. Forthis simulation thereis only one feedstream, if there weremore feed streamsuse the blue arrowon the left of thecolumn to add morestreams.

Enter Feed in theID box whenprompted for thissimulation.

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 streamDistill.

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

All the necessarystreams have beenadded and numericaldata can now beentered.

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Now we are readyto begin enteringthe necessarynumerical data.

Click on the Nextbutton to beginentering the data.This button is blueand is located inthe tool bar at thetop of the window.

Click OK.

If this box doesnt appear itis because your flowsheetisnt complete. A box willthen appear telling youwhat part of the flowsheetyou are missing.

This box should appearand a title for thissimulation can now beentered although is notmandatory for thissimulation. Click on theNext button to continueentering numerical data.

If this box doesnt appearyou can go to Setup onthe left-hand side of thebox and click on that.

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The Components box willbe the next to appear. Thisallows us to enter all of thecomponents that will bepresent within our system; inthis simulation they will beMethanol and Water.

Type Methanolunder the columnComponent ID andpress the Enter key.

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AspenPlus will nowsearch itsdatabase andattempt to match achemical namewith theComponent ID thatwas entered. Ifthis happens theother three boxes(Type, ComponentName, andFormula) will fillautomatically andother componentscan then beentered.

Enter Water inthe second rowunderComponent IDand hit Enter tofill the other threeboxes. We nowhave entered allthe componentsnecessary for thissimulation. Hitthe Next buttonto proceed.

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The next screento appear is thePropertySpecificationssheet. Thisallows us tochoose the basemethod that wewant to use forthe calculationsAspenPlus willperform. We willselect NRTL forthis simulation.

Click Next.

The next screen toappear will showthe interactionparameters for thecomponents in oursystem using thebase method weselected. If youare happy with thisnumbers (and wesure hope you are)click Next.

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A prompt screen will appearasking if we want to enter anymore data or change theproperty specifications.Since everything is good to go,click OK.

Now its time tobegin entering thephysical data for thesystem. The firstscreen that willappear will be for theFeed stream.Before we actuallystart entering datalets go over acouple of thedifferent availableoptions.

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There are 3 StateVariables that weare allowed tochoose fromTemperature,Pressure, orVapor Fraction. 2of these 3 must bespecified. Click onthe arrow on theright side ofTemperature orPressure Box toalternate betweenthe choices.

The units can bechanged for anyvariable by simplyclicking on thearrow on the rightside of the unitbox. This will bringup a list of differentunits to choosefrom. For thisexample we willuse degreesCelsius fortemperature andatmospheres forthe pressure.

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Change the units ofTemperature toCelsius and enter 25.Also set the pressurein the stream equal to1 atm.

As stated in theproblem, the feedflowrate to the columnneeds to be set at 100lbmol/hr. Enter thathere.

The next requiredinput involvessetting theComposition forthe feed stream.

Click on the arrowon the right handside of the box toview all of thedifferent options fordefining thecomposition.

Choose Mole-Frac.

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The problemstatement callsfor a 50/50 splitbetween the twocomponents inthe feed.

Enter 0.5 in bothboxes and thenhit Next.

The feed streamis now complete.

Now is the timewell all beenwaiting for,designing theactual column.The first step is toget at least arough estimatefor the requirednumber of stagesneeded.

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To do this,were going touse AspenPlusto generate a T-XY plot and useit to do a quickMcCabe-Thilediagram. At thetop of thescreen underTools, click onAnalysis,Property, andBinary.

Click on this arrowand select T-xy as thetype of analysis youwant performed. Thepressure must also beequal to the operatingpressure of yourcolumn so enter 1atm.

Whenever you areready, click on Go.

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AspenPlus shouldgenerate thisliquid/vaporequilibrium plot forMethanol and Waterat 1 atm. The X-axisis the molefrac ofMethanol, so theextreme left andextreme rightrepresent pure Waterand MeOHrespectively.

Notice that there areno azeotropesbetween the two soperfect separation istheoretically possible.

This diagram cangive us an estimateof how manyequilibrium stagesare necessary byusing the McCabe-Thiele method.

Unfortunately, wecannot show youthe actual drawingbut trust us; ittakes about 7equilibrium stages.

After the graph hasbeen made andlooked at you canclose all threewindows that arefor the graph.

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Enter 10 for theNumber of Stagesbecause ourcolumn will not beoperating at 100%efficiency.

We will optimizethis number a littlebit further down theroad.

Click on the arrowon the right handside of theCondenser box todisplay thecondensercandidates.

As mentioned earlierthis column will havea Total condenser.

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Click on the arrow onthe right hand side ofone of the operatingspecification boxes todisplay all of thealternatives. For thisexample, were goingto choose to set theReflux Ratio andDistillate rate sincemethanol is thecomponent we areworried aboutrecovering and it hasa lower boiling pointthan water.

Because there areno azeotropes, theDistillate Flowrateof methanol can beset to the samenumber of moles asthe entering flowrate,50 lbmol/hr.

The normal range ofReflux Ratios isfrom 1.25 - 1.8 formost applications.Enter 1.5 and pressNext to continue.

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The next screen asksto specify the FeedStage . If youre notcertain of the locationa safe bet is usuallysomewhere in themiddle.

This is another areathat will be optimizedlater.

For now, well enter 5and press Next.

We are now requiredto enter the Pressureat which the columnwill operate.

Change the units toatmospheres byclicking the arrow onthe right hand side ofthe units box, and typea 1 in the box on theleft.

If data is alreadyavailable for PressureDrop in the column, itcan be entered. Forthis simulation we willaddress the pressuredrop in the column alittle later on in ourprogram.

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Instead of clicking onNext, which wouldbring up a prompt torun the simulation,click on theEfficiencies folder.

Our column isntperfect but what reallyever is. Normalefficiencies range from50-80% and there areshortcut methodsfound in literature todetermine efficiency ofa column.

We wantMurphree-typeefficiency, so clickon the circle to theleft to highlight it.

Click on theVapor-Liquid tabto continue.

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Next were going toset the efficiencyfor one stage of thecolumn.

AspenPlus willthen be able tocalculate the otherstages efficienciesfrom our givenbenchmark.

Enter 5 for thestage and 0.7 forthe efficiency.

The next step isfor us to set thetype of trays thatour column willconsist of.

Click on the TraySizing folder onthe left to get tothis screen.

Click New tocontinue.

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A box will appear prompting youfor a section number.

Click OK to continue.

This screen allows us tochoose from a list ofpossible types of trayswithin our column.

Enter a 2 in the startingstage and a 9 in theending stage becausethere are 10 stages inour column as both thecondenser and reboilercount as stages.

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For this simulation,we are going tochoose Bubble-Captrays.

Click on the box tothe right of TrayType to see the listof available trays.Well leave thenumber of passesand tray spacing asthey are.

Click on the TrayRating folder on theleft. Click new tocontinue.

Another prompt willappear to choose asection number, clickOK.

Enter 2 and 9 againrespectively for thestarting and endingstages along withBubble cap for theTray Type.

An estimate for theDiameter of thetrays is nowrequired.Enter 1.5 ft.

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Click on theDesign/Pdrop tab tocontinue.

Click on the boxdirectly to the left ofUpdate sectionpressure profile tohave AspenPluscalculate thepressure drop in thecolumn.

Just one final detail andwell be ready to run oursimulation. Double-clickon the Setup folder,then click on ReportOptions and finally clickon the Stream tab.

Click on the box to theleft of Mole in theFraction BasisColumn. AspenPlus willnow report the Molefraction of each stream,99.5% is the goal.

Click Next.

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Heres the momentweve been waitingfor. Were ready torun our simulation.

Click OK.

The control panelwill appear displaythe iterations asAspenPlus findssolutions for oursimulation. Thecalculations shouldcomplete normally.

Click on the bluefolder with the checkmark to view theresults.

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Click on Streams toview the results foreach of the threestreams.

Use the arrows on theleft and bottom of thestream report to lookthrough all of theresults. We achieved94.2% purity in bothproduct streams. Notquite up to the spec of99.5%, lets find outwhat we can optimizeto meet our goal.

Double-click on theblue Blocks folderand then also onDist.

Next, go to the topmenu bar and clickon Plot and thenPlot Wizard.

Plot Wizard takesthe profilesAspenPlusgenerated for thecolumn and turnsthem into pretty littlecharts.

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A welcoming screen willappear, click next tomove onto this screen.The first plot we want tolook at is the liquidcomposition inside thecolumn.

Click on Compositionand hit Next.

Click on the doublearrows pointing to theright to select bothcomponents to be inthe plot.

Next click the circlenext to Liquid underSelect Phase.

Clicking on Nextagain brings updifferent plot options.Nothing on the nextset of options needs tobe changed.

Click Finish to viewthe plot.

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This plot can tell us acouple of differentthings.

First off, there are nolevel spots in thediagram so we donthave any extra stages.

Also, notice the littledip in the middle onstage 5, our feedstage. This meansthat we could possiblyincrease ourseparation bychanging the locationof the feed tray.

These are two areaswhere we couldoptimize our column.

Lets start to optimizeby changing thenumber of stages inour column.

Close all the resultsfolders and click onthe Blocks and Distfolders respectively toreturn to the columnSetup screen.

Under Configuration,change the number ofstages from 10 to 13while leaving the feedstream on stage 5 forthe time being.

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We also need tochange the numberof stages withrespect to our TraySizing and TrayRating.

Go into the 1 beloweach folderrespectively andchange the endingstage to 12.

Were all set up, soclick Next to runthe simulation again.

Click OK tocontinue.

Upon completion clickon the blue folderagain and go underResults Summaryand then Streams.

The results show thatwe now have 94.9%purity in each of theproduct streams.Moving in the rightdirection but still notup to the desiredspecs.

Lets take a look at theLiquid Compositiondiagram

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Double-click on theblue Blocks folderand then on the blueDist folder to look atthe results for thecolumn.

Go to Plot at the topof the screen and clickon Plot Wizard.

Hit Next through thewelcoming screen andclick on theComposition plot.

Choose to plot BothComponents in theLiquid phase and hitFinish.

From this plot it isobvious that our feedstage is a little bit toolow.

We want thecomposition to besmooth lines withoutany humps or bumpsin the middle.

Close the Resultsfolder and lets goadjust our feed stage.

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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 clicknext.

Once again check yourLiquid Compositiondiagram using PlotWizard as done inprevious steps.

Is the hump gone fromthe middle? If notcontinue this procedureuntil the curves aresmooth. (Hint: 9 mightbe the answer)

After youvedetermined theoptimum feed platelocation (HINT: 9) yourplot should look likethis.

Next check yourstream results to see ifwe have achieved thedesired purity in ourMethanol Stream.

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Unfortunately we stillhavent reached thedesired purity in ourproduct streams. Butwe did increase thepurity by 3% to 97.5%by optimizing thelocation of the feedplate.

Since the reflux ratiois given at 1.5 in theproblem statement,the only other option isto continue to increasethe number of stagesin our column.

Return to the Setupscreen under theBlocks/Dist folder.

Click on theConfiguration tab atthe top and increase thenumber of stages in thecolumn.

Also, go under the TraySizing and Tray Ratingfolders and change theending stage on each toone stage less than theactual number of stagesthat you entered here.

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Next click on theStreams tab andchange the locationof the feed plate.

Click on Next torun the simulation,then check the liquidcomposition diagramto be sure that it is inthe optimumposition. If it is,check the results.

Continue thisprocedure until youmeet the given purityspec.

Theres the magicnumber, 99.5%!

The final configurationof the column has 21bubble-cap trays withthe feed plate onstage 16.

Now that we have ourcolumn set up, letslook at a couple of theother options that areavailable.

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AspenPlus has alsocalculated theDiameter of thecolumn.

On the Resultspage, click onBlocks, Dist,Tray Sizing, and 1.

Next, click on theResults tab at thetop of the screen.This screen shows acalculated diameterof around 2 feet forour column.

There are a couple ofdifferent ways that wecan look at the pressuredrop within the column.The first is by clickingon the Tray Ratingfolder and opening 1.

Click on the Results tabat the top and use thearrows on the right tomove down until theSection Pressure Dropis shown.

Since this is the onlysection, the pressuredrop for our column isabout 4.5 psi. Byclicking on the psi boxyou can also changethe pressure units.

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The other way we canobserve the pressuredrop is by using PlotWizard to create agraph.

Go to Plot Wizard andclick on the Pressuregraph.

Choose the desiredunits and click Finish.

A graph similar to thisone should be createdshowing the pressureprofile in the column.

The temperature profilecan also be created in asimilar manner.

Congratulations! You havefinished the race and youreprobably not even half as tiredas this guy, he looks beat.Now you can entercomponents, feed streams,column data and generateresults for all of your ownsimulations. Feel free to bragto all of your friends nowabout your superior knowledgeof Radfrac, youve earned it.You are truly a champion.

Peace and Love,

Bj

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