ChemSepTutorial_FlashCalculations02

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ChemSep Tutorial: Phase Equilibrium Calculations

Ross Taylor and Harry Kooijman

In this document we illustrate the use of ChemSepby considering its use to solve some exampleproblems posed in thermodynamics textbooks. The examples considered below are

1. !stimating K"values from the #eng"Robinson !$uation of %tate &this is !xample '(.) in

Introductory Chemical Engineering Thermodynamics* by +.R. !lliott and ,.T. -ira rentice"Hall* '/.

). ,reating a phase diagram for methanol"ben0ene &this is !xample '(.1 in the same book/.

Tutorial 1: K-values from the Peng-Robinson Equation of State

The bubble point pressure of an e$uimolar mixture of nitrogen&'/ and methane&)/ at a temperatureof '(( K is to be calculated using the #eng"Robinson e$uation

2e begin by starting ChemSepand selecting nitrogen and methane from the databank.

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3ext select the Operationtab and choose lash

The next step is to select the appropriate thermodynamic models. It is beyond the scope of this

tutorial to explain how to choose appropriate models &it must suffice to note that the properselection of thermodynamic models is of central importance in phase e$uilibrium calculations/.

The screen shot below shows the models selected for this exercise.

3o enthalpy model is needed for this exercise &we are not interested in that calculation this time/*but we selected the #eng"Robinson model there as well.

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The next step is to specify the composition of the mixture. This is done in the feed panel

The feed panel asks for component flows* but what matters to us is the composition and not theflows. 4ccordingly* we simply set the component flows e$ual to the corresponding mole fractions.

3ote that the feed pressure and temperature are not that important in this calculation. 2e will setthe pressure at which the calculation is to be done in the 5lash %pecifications panel.

The first task is to select the type of flash calculation that we wish to perform. There are severaldifferent flash types available in ,hem%ep. To determine the bubble point pressure we need tospecify the temperature and vapor flow as indicated by the image below.

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2e now set the temperature &to the desired value of '(( K/ and the vapor flow to 0ero&corresponding to a bubble point calculation/.

The problem specifications now are complete and we can proceed to solve the problem. 5irst wesave the file &using the ilemenu 7 note that the file name appears on the blue bar at the top ofthe ,hem%ep window/ and then click on the green arrow head icon to start the calculations.

This will bring up the solve window 7 which very soon &this is an easy problem/ looks like this

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,lick on !oneto close the window and bring up the results panel.

2e from this screen shot that the bubble point pressure has been estimated to be (.8)9 :#a&!lliott and -ira report that the pressure is close to (.8)1 :pa but their calculation is not convergedas tightly as is the one here/.

The K"values at the specified temperature &and calculated pressure/ can be found in another table

The K"values given by !lliott and -ira are 1 1.890K = and 2 0.1073K = &remember that their

calculation is not fully converged/.

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Tutorial ": Phase !iagram for #ethanol $ %en&ene from a Cubic Equation of State


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%pecify the pressure at which the phase diagram is to be determined &on the lash Specificationspanel/ and the vapor flow &here set to 0ero to indicate that we are doing a bubble point calculation/.

%ave the file &under the ile/ menu and %olve it &click on the green arrow head icon and then clickon !onein the calculation window/ to bring up the results page

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This page tells us that &according to the #R !


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This brings up the following screen

,lick on )''and select ee' component 1 flo*as shown below.

Repeat this step and select ee' component 1 flo*a second time &there will be warning that youshould not select the same variable twice/. ,lear this warning because this is exactly what youmust do at this stage. The top portion of the screen now looks like this


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It does not* of course* make sense to vary the same variable more than once at the same timeD weare now going to change the second one. ,lick on the box above where you can read 11&whichmeans 5low '* component '/. Retype the string so that it reads 1"&for 5low '* component )/.

In addition* change the start and end values of both variables as shown in the image below

3ow* when the parametric study is run it will vary the amount of methanol in the feed &from one to0ero/ and the amount of ben0ene in the feed &from ( to '/ in such a way that the total amountremains the same &'/.

The next step is to select the results variables. 5or a Txy diagram we need the temperature andthe mole fraction of methanol in the vapor phase. >se the drop down list in the center part of the#arametric study window to select these variables and so that this part of the window appears asshown below

The E sign must be replaced by the number for methanol &' in this case/. ,lick on that box andreplace the E sign by '

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2e are almost ready to run the parametric study. 5irst* select the number of steps we want tocover the range of mole fractions. !nter this number in the space provided near the top of thewindow.

3ow click Run&near the foot of the window/. ChemSepwill now run through a series of closelyrelated problems as it varies the specified variables &feed mole fractions in this case/ over thespecified range &("' and '"( in this case/. 2hen completed you will see the window looksomething like this

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2e can plot the results in several different ways using either the plotting capabilities built intoChemSepor by using an alternative software package.

5irst* we show how to display the Txy diagram using the plotting capabilities built into ChemSep. Ifwe click on the the Plot button ChemSepwill plot the parametric study results using the firstvariable that was varied on the F axis &that would be the mole fraction of methanol here/ and all

other variables on the y"axis &that includes both the mole fraction of methanol in the vapor and thetemperature/. ,onse$uently the diagram will not be exactly what we want to see &which is why wedonGt show it here/. Instead of clicking on Plotwe suggest that you click on the E'it Plotbutton Dthis will bring up the panel shown below.

3ote the lowest part of this panel headed !ata sets. ChemSepuses the labels #%x &where x is anumber/ to refer to the columns of results obtained in the #arametric %tudy. 4s already notedabove ChemSepis programmed to display #%) &the mole fraction of methanol in the vapor/ as afunction of #%' 43 #%6 &temperature/ as a function of #%'. 2e need to change these settings.,lick in the #lot F and #lot cells and retype the entries so that this section appears as shownbelow.

3ext* set the axis limits &the $uick way is to click on )uto )+isbut we would recommend adjustingat least the x"axis limits to vary from ("'/. The plot and axis titles can also be changed &click in theappropriate cells and type the names that you prefer/. In the screen shot below we show that theplot title was removed* The axis labels were changed to better represent the variable names* the

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data set labels were deleted* the default plot colors were changed and the plotting of points wassuppressed &leaving only the line joining the points/.

5inally* we click on the !ispla(button near the top of this panel to bring up the Txy diagram in aseparate window.

It is also possible to add actual experimental data to this plot* detailed instructions on how to dothis can be found on the ChemSepweb site

www.chemsep.comJdownloadsJsourceJ-!LataJhowtoLvleJhowtoLvle.html

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http://www.chemsep.com/downloads/source/VLE_Data/howto_vle/howto_vle.htmlhttp://www.chemsep.com/downloads/source/VLE_Data/howto_vle/howto_vle.html


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4nother option is to use some other package that can create plots such as a spreadsheet program.,lick on the Cop( !atabutton* open a spreadsheet program and paste the data into an openworksheet. Here we use OpenOfficeCalc&free from www.openoffice.org/ to illustrate.

,olumn , contains the mole fraction of methanol in the li$uid* column has the vapor molefraction and the temperature is in column !.

3ow we can use the plotting capabilities of OpenOfficeCalc to create the Txy diagram shownbelow.

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( (.) (.8 (.B (.1 ' '.)

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T+( !iagram for #ethanol - %en&ene

#ole fraction methanol

Temperature,K-
http://www.openoffice.org/http://www.openoffice.org/

ChemSepTutorial_FlashCalculations02

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