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How to Process Vapor-Liquid Equilibria (VLE) Data

Here we will explain you how you make your comparisons with VLE data with activity coefficient models in ChemSep (LITE). We'll use binary VLE data on Acetone and Benzene measured 25 degrees Celcius as an example. It was published by A. Tasic, B. Djordjevic, D. Grozdanic, N. Afgan, D. Malic, in Chem. Eng. Sci. (1978), 33, pp. 189-197. Many binary VLE data sets published in literature are collected in the thermodynamic libraries. The most well known of these libraries is the DECHEMA data series of which there is also an online version, called Detherm. This library includes the data of the Dortmund Daten Bank (DDB).

We will start with a new file in ChemSep and enter the VLE Data in the comments section of the sep file. You can find the VLE data in our set of binary vle (bvle) files using the library indices of acetone, 1051, and that of benzene, 501. The data file is then called "bvle_1051_501_t25.txt". The order of the components is set to increasing normal boiling points. Since acetone boils at a lower temperature than benzene it comes first. The component indices are separated with an underscore since the uses of spaces in file names makes the use of commandline tools difficult. As a last part of the file we add an indicator for the conditions at which the measurements were taken, here at constant temperature of 25 C. We can use copy-paste for entering the data as text in the comments field:

# file=bvle_1051_501_t25.txt
# ref=ces33p189
# title=P-xy data for acetone (1051) / benzene (501) at 25 C in torr
# fullref=A.Tasic, B.Djordjevic, D.Grozdanic, N.Afgan, D.Malic, Chem.Eng.Sci. (1978), 33, pp. 189-197
p	x	y
95.05	0	0
116.65	0.082	0.248
137.2	0.185	0.416
156.3	0.307	0.55
169.3	0.401	0.632
182.1	0.505	0.703
191.2	0.59	0.755
203.2	0.704	0.824
213.4	0.8055	0.882
220.95	0.8955	0.936
222.35	0.909	0.944
225.05	0.932	0.958
230.4	1	1

Note that we separated the data here by tabs. ChemSep's import option for binary VLE data also allows spaces, comma's, or semicolons as separators. Important is that there is a line describing the type of data in each column. When data is only available in graphical form you can use the tool ScanIt to obtain the datapoints. ScanIt lets you pick the origin and two axis end points to determine the scale of the scan. After this you can select each point with the mouse and ScanIt will mark it and compute the x,y values of the original datapoint. With the "Copy data" button you can copy the data to the clipboard and from there to a sep file or a text file. Be sure to always include the complete reference to the original article besides a descriptive title. If possible, we refer to the publishers URL of the publication. We use specific set of journal abbreviations and refer to the volume and first page of the publication, in this case "ces33p189". The title and reference are put on lines that start with a "#" to indicate the line does not contain data. By including lines with the file name, short and full reference, and title, it is easier to automatically scan a lot of files for specific information. It takes a little effort to do this systematically for each file but the pay-back is huge when many of these small files are compiled into free online resources. If you send us your files without references they are of little use to us as we can not make them available to others.

Entering VLE data in a new file.

When we want to compare the VLE data to predicted values by thermodynamic models we need to define the components as well as make a model selection and provide the interaction parameters for the models. Select "Components" in the tree of the input navigator on the left hand side. This will open a pane where we can select the compounds. ChemSep shows the compounds in the default component library (a "PCD" file) and you can select any of these by double-clicking a compound. Typing a (part of a ) name in the "find" box will narrow down the list of components to only those with the particular string in the name (clicking the "advanced" checkbox allows for more sophisticated searched). We add the components in the order of their boiling point, first acetone and then benzene:

Selecting the components in the mxiture.

Next we click on the "properties" in the input navigation tree to select the thermodynamic models. To describe Vapor-Liquid equilibria ChemSep uses K-values, which define the ratio of the vapor to the liquid compositions (K=y/x). For ideal mixtures the K value equals the ratio of the component vapor pressure to that of the system. For nonideal systems such as the system at hand, this is insufficient, and an activity coefficient is needed to describe the non-ideality of the liquid mixture fugacity. The resulting model for the K-values is called the DECHEMA model, since it is also used in the DECHEMA data book series. As activity model we will first select a predictive model, UNIFAC, which will require no further input. As vapor pressure model we select the Antoine model.

Selecting the thermodynamic model(s).

To display our calculations in the same units of measure, select "units" in the navigator. Our experimental data was in degrees Celcius and torr (mm Hg). Since torr is not part of the standard list of pressure units you must type it in the edit box. ChemSep allows you to do so and thus compose all kinds of personal units of measure. ChemSep knows most chemical engineering units and all the prefices to scale them.

Specifying units of measure.

Now save the file under a descriptive name, for example "bvle_1051_501_t25.sep". Then go to Analysis - Phase diagrams - Binary menu option. This opens a window to generate our pxy diagram. Select the type of diagram (pxy) and phase equilibrium (VLE) as well the condition (temperature = 25C) and click on calculate to generate the phase diagram:

VLE model predictions.

Now we want to make a comparison of the model predictions with the experimental data. For this we must load the data either from a file or from the comments section. For both you must click the "file open" icon next to data. In our case we must select the "read from comments field" file type and select to open any file. This loads the following information onto the data line (all in one line):

We see the data is processed: the pxy identifier is repeated and then used for identifying each data item, separated by comma's. The data points themselves are separated by semicolons. Note that the original data did not have units, so no conversions are done. The diagram now has the points shown as entered:

Comparing experimental data with UNIFAC/antoine model predictions.

To quantify the deviations between the model and the experimental points we can compute the standard deviations by clicking the "Data Dev." button. This computes for each point the pressure q and vapor fraction z that is in equilibrium with the measured liquid composition x and the specified temperature 25 C. ChemSep adds these to the the data line:

It also computes the relative standard deviations (only for the binary points) and adds them to the title of the diagram. Here we see that the UNIFAC+Antoine models predict vapor compositions (sy) with 0.72% and the pressure (sp) with 1.01%. This is actually very good. Actually most of the deviation in the pxy diagram is caused by the Antoine model not capable of predicting the exact pure component bubble pressures. You can download the sep file by clicking the following link: bvle_1051_501_t25.sep.

Calculating standard deviations in y and p.

Once you are happy with the way your VLE diagrams looks like use the Plot button to generate a GNUplot diagram. From GNUplot you can bring the plot into any other windows program supporting MS meta-graphics, such as Word and powerpoint, by pressing Alt-Space in GNUplot and selecting Options - Copy to Clipboard.

VLE diagram in GNUplot

Though the fit of the predictive UNIFAC model looks very good, we see that close to pure acetone the dew and bubble point lines start to curve at a decreasing slope, indicating that we are close to an azeotrope. Actually, the data does not seem to have this tendency. When we go back to the thermodynamic model selections and select the UNIQUAC model and load the parameters from the library,

Selecting the UNIQUAC activity coefficient model and interaction parameters

we will obtain a fit that is less good - deviations in pressure increase by more than a factor two - but the curvature of the dew and bubble point lines is correct:

UNIQUAC fit of the VLE data

Thus, lower relative deviations of pressure and mole fractions aren't necessarily indicative for a qualitative better description of the binary VLE. This is due to the errors in the pure component vapour pressures. Errors over half of the concentration range are introduced when a pure component bubble point temperature is off by just one or two degrees! We leave it to the reader as an exercise to select different models for the vapor pressure to see how the predictions can vary.

Finally some useful tips:
  1. write up a title with the binary in question, e.g. "Methanol (1) / DiMethyl Carbonate (2) 1 atm"
  2. include a complete reference with URL, e.g.
    Hu-Ping Luo, Wen-De Xiao, Kai-Hong Zhu, FPE 175 (2000) pp. 91-105
    Isobaric vapor–liquid equilibria of alkyl carbonates with alcohols
  3. make sure the same units are set as the tables in the publications, e.g. "K" and "kPa"
  4. include your name & date: "entered by H.Kooijman 21apr2014"
  5. include the type of data followed by data lines, e.g.
    356.34 0.0447 0.2258
    352.98 0.0815 0.3297
    349.13 0.1319 0.4282
    344.33 0.2254 0.5493
    343.17 0.2610 0.5810
    340.76 0.3654 0.6472
    339.49 0.4439 0.6844
    338.88 0.4987 0.7071
    338.45 0.5610 0.7264
    337.87 0.6214 0.7374
    337.49 0.6756 0.7669
    336.99 0.8283 0.8402
    337.00 0.8522 0.8534
    337.03 0.8761 0.8679
    337.15 0.9356 0.9187
    337.52 0.9794 0.9686
  6. separate multiple data sets with an empty line, ~, and #. This way ChemSep can assign them different colors:
             5.033  .1579  .9732
             8.412  .2652  .9924
             1.862  .0350  .8782
             3.930  .0728  .9437
             7.584  .1058  .9758
             0.310  .0000  .0000
             1.793  .0150  .4570
             3.378  .0230  .7880
             1.228  .0000  .0000
             3.654  .0330  .6213
             5.102  .0317  .7053
  7. pick the components in the same order of light to heavy, based on their normal boiling points. Name the file "bvle_CID1_CID2_DataSet.sep" where CID1 and CID2 are the ChemSep component library indices. The DataSet can be the range of the temperatures (with units) covered e.g. "T178-411K" or pressure (with units) e.g. "p1atm".
  8. pick appropriate thermodyamic models, i.e. activity coefficients for Tr less than 0.8 and else EOS
  9. when making a binary VLE diagram, compute it at all the temperatures or pressures in the data set by listing them all separated by spaces , e.g. "177.59 210.93 244.26 277.59"
  10. try to adapt the axis settings such that they match the figures from the publication
  11. try fitting the BIPs of the thermodynamic model selected unless this is a GC method (e.g. PSRK or UNIFAC)
  12. if you make a screenshot of the BVLE, do not change the size of the binary phase diagram panel! Keep it as is and export in the PNG format with the same name as the sep file.

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