Page 115 - Separation process engineering

Ambient pressure = 1 bar, Valid phases = Vapor – liquid, and Free water = No. When happy with this
page, click on the Next button. The next page lets you select the components. First, pick a binary mixture
of interest (use ethanol and water). Component ID is whatever you want to call it. Type should be
conventional. Then click on the component name box or hit ENTER. If you used ethanol and water as
the component IDs, Aspen will complete that row. If you used a different ID, such as E or W, give the
appropriate component name (ethanol or water). Then do the next component. Aspen Plus will
recognize these two components. If Aspen Plus does not fill in the formula, click on the Find button and
proceed. When done with components, click on Next. Note that Aspen Plus can be picky about the
names or the way you write formulas. Note: If you use W as component ID, Aspen will think this is
tungsten. Type water as component name instead.

Click Next. You will probably now see the Properties Specifications-Data Browser screen. You must
select the appropriate physical properties package to predict the equilibrium for your chemical system.
There is no choice that is always best. The choice is made through a menu item on the right side
labeled “Property Method.” You may need to click twice to get the complete menu. This choice is very
important (Carlson, 1996; O’Connell et al., 2009; Schad, 1998). If you pick the wrong model, your
results are garbage. A brief selection guide is given in Table 2-4.

We will try different models and compare them to data. [Note that data also needs to be checked for
consistency (Barnicki, 2002; O’Connel et al., 2009; Van Ness and Abbott, 1982).] First, try the IDEAL
model. Before allowing you to check the VLE data, Aspen (in flowsheet mode) requires you to
complete the input information. Thus, we need to continue. Left-click on the Next button after you have
selected a VLE model (IDEAL). If you get a data bank with binary parameters, left-click Next again.
When you get a box that says, “Required Properties Input Complete,” click either “go to next required
input step” or “modify required property specifications.” Assuming you are happy with what you have,
left-click on OK to go to next required input step.

You should now get a data browser for your input stream. Fill this out. Try a pressure of 1.0 bar, vapor
fraction of 0.4 (click on arrow next to Temperature and select vapor fraction), total flow of 100.0
kmol/h, ethanol mole fraction of 0.9, and water mole fraction of 0.1 (use menu under Composition to
select mole fraction). Then left-click on the Next button.

Note, always use mole fraction or mass fractions for the units for the composition of the stream (use
the menu). Other choices will often lead to inadvertent errors.
Fill out the conditions for the flash drum block (same pressure and vapor fraction as in the feed) and
click on the Next button. At this point you will probably get a screen that says input is complete and
asks if you want to run the simulation now. Don’t. Click Cancel.

4. Analysis of VLE Data

Aspen will now let us look at the VLE so that we can determine if it makes sense. To do this, go to the
menu bar to tools. Choose, in order, Analysis-Property-Binary and left-click. Click OK on box that says
you will disable the Interactive Load. On the menu under Analysis Type, pick Txy plot, make sure Valid
Phases menu is Vapor-Liquid, change pressure unit to bar, and then click on GO. Cancel or minimize
this plot to reveal the data table below it. Compare the vapor and liquid mole fractions to the ethanol
water data in Table 2-1. To generate a y-x plot, click on Plot Wizard and follow the instructions.
Compare the y-x plot to Figure 2-2. Note that ideal is very far off for systems such as ethanol and water
that have azeotropes. Since Aspen Plus is quite willing to let you be stupid in picking the wrong
properties package, it is your responsibility to check that the equilibrium data make sense. Later, we
will find that some VLE packages closely match the actual data.

5. Doing a Flash Run with Ideal Model