Page 118 - Separation process engineering

butane, 44 mol% n-pentane, and 28 mol% n-hexane and is a liquid. The flash drum is at 1.0 atm, it is
adiabatic (heat duty = 0), and there is no heat exchanger. Choose an appropriate property method.
Initially, for the feed, set vapor fraction = 0 (a saturated liquid) and set the temperature. You want to
obtain V/F in the drum of 0.4000 (rounded off to fourth decimal). Find the feed temperature that gives
this value of V/F. (As you do runs, note that feed pressure for a saturated liquid increases as feed
temperature is increased. Why?) Once you find the correct feed temperature, remove the feed
specification that vapor fraction = 0 and specify a feed pressure that is 1.0 atm. above the pressure
reported by Aspen Plus when vapor fraction of feed = 0. Liquids that are saturated liquids (this is what
Aspen means by vapor fraction = 0) cannot be pumped easily. By raising the pressure, we make
pumping easy. Rerun simulation one last time to check that you have met all requirements. Expect to do

several Aspen Plus runs to solve this problem. Report the property method, feed temperature and
pressure, drum temperature, heat duty in drum, vapor and liquid flow rates, and x and y values.

Assignment to Hand In

Each group should write a one- to two-page memo addressed to the professor or teaching assistant from
the entire group of members. You may attach a few appropriate graphs and tables on a third page (do not
attach the entire Aspen results printout). Anything beyond three pages will not be looked at or graded
(this includes cover pages). The memo needs to have words in addition to numbers. Give a short

introduction. Present the numbers in a way that is clearly identified. Mention the graphs or figures you
have attached as backup information. (If a group member is absent for the lab and does not help in
preparation of the memo, leave his/her name off the memo. Attach a very short note explaining why this
member’s name is not included. For example, “Sue Smith did not attend lab and never responded to our
attempts to include her in writing the memo.”) Prepare this memo on a word processor. Graphs should be
done on the computer (e.g., Aspen or Excel). Proofread everything and do a spell check also!

Chapter 2 Appendix B. Spreadsheets for Flash Distillation

2.B.1 Regression of Binary VLE with Excel

McCabe-Thiele calculations are easiest to do on spreadsheets if the y versus x VLE data are expressed in
an equation. The form y = f(x) is most convenient for flash distillation and for distillation columns (see
Chapter 4) if stepping off stages from the bottom of the column up. The form x = g(y) is most convenient

for distillation columns if stepping off stages from the top down. Built-in functions in Excel will
determine polynomials that fit the data, although the fit will usually not be perfect. This will be illustrated
for fitting the ethanol-water equilibrium data in Table 2-1 in the form y = f (x ). (Note: An additional
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data point x = 0.5079, y = 0.6564 was added to the numbers in the table.) Enter the data in the
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spreadsheet with x values in one column and the corresponding y values in the adjacent column.
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Highlight all the x-y data. In Excel 2007 in the the menu bar, click on the Insert tab. Select a Scatter chart
and then select the icon showing “scatter with data points connected by smooth lines.” This creates a
figure of the data plotted as y versus x. If desired, you can add labels, grid lines, and other touches by
clicking on Layout in the menu and then following the instructions. If you are not familiar with these
techniques, try it step-by-step in Excel.

To find a polynomial that fits the data, first highlight the x-y data. Then go to the Excel tool bar and click
on Layout→Analysis→Trendline→More Trendline Options. Choose polynomial as type and in the menu
select the desired order of the polynomial. (You can try different orders to find which has the best fit.)
Make sure the boxes “Display Equation on Chart” and “Set intercept = 0” are checked. Then click Close.
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For the ethanol-water data at 1.0 atm, the best fit was a 6 -order polynomial. This result is: