Page 269 - Separation process engineering

(Note: Do minimum number of simulations to answer these questions. Do not optimize.)
3. Which tray gives the largest column diameter with sieve trays when one uses the originally
specified feed stage? Aspen Tray #__________ Column diameter = ____________ meters

[Use the default values for number of passes (1), tray spacing (0.6096 m), minimum downcomer
area (0.10), foaming factor (1), and over design factor (1). Set the fractional approach to
flooding at 0.7. Use the Fair design method for flooding.]

4. Which components in the original problem are the key components (label light and heavy keys)?
___________________________________________________________
5. Change one specification in the operating conditions (keep N, feed location, feed flow, feed
composition, feed pressure, feed temperature or fraction vaporized constant) to make methanol
the light key and ethanol the heavy key.

What operating parameter did you change, and what is its new value? _________

Temperature of condenser = ________ K, temperature of reboiler = ________ K
Distillate product mole fractions: ___________________________________________
Bottoms product mole fractions: ___________________________________________

Chapter 6 Appendix. Computer Simulations for Multicomponent Column
Distillation

Lab 4. The methods to start the process simulator are discussed in Labs 1 and 2 (appendix to Chapter 2)

and specific directions for distillation are discussed in Lab 3 (appendix to Chapter 4).
I. Multicomponent distillation. Aspen Plus uses the same calculation methods for binary and
multicomponent distillation, but there are some differences. Draw a distillation column with a partial
condenser using RADFRAC. To do this, take a vapor distillate product instead of a liquid distillate
product. Then in the Block for the column, choose the following from the menu: Calculation Type:
Equilibrium, Number of Stages: 40 (this is initial setting only), Condenser: Partial-Vapor, Reboiler:
Kettle, Valid Phases: Vapor-Liquid, Convergence: Standard. If you get an error message in part 1a,
chances are you have a liquid distillate in the drawing and are trying to take a vapor distillate product
(that is what Condenser: Partial-Vapor requires). Redraw your column with a vapor distillate stream.
The following system converges and allows you to explore the effect of many variables. The feed

consists of: n-butane, 20.0 kmol/h, n-pentane, 30.0 kmol/h, and n-hexane, 50.0 kmol/h. Pick an
appropriate VLE package to use and check the equilibrium predictions (e.g., K values should be
reasonably close to predictions made with the DePriester chart—but note that the correlation should
be more accurate than the DePriester charts). In part 1a use a saturated vapor feed at 1 bar. Initially try
a column pressure of 1 bar. A column with 40 stages with the feed on number 20 and L/D = 3 are more
than sufficient for this separation.

a. Adjust the column settings to make the butane and pentane exit from the top of the column and the
hexane from the bottom. This can be done by setting D (or equivalently B) so that an external mass
balance will be satisfied if a perfect split is achieved (e.g., butane recovery in distillate of 100%,
pentane recovery in distillate of 100% and hexane recovery in bottoms of 100%). Note the
compositions and temperatures of the distillate and bottoms. Check the heat loads in the condenser
and reboiler.
b. Change the settings so that the butane exits the top and the pentane and hexane the bottoms. (Change D
or B to do this.) Under these conditions you should get an error message that the column dried up.
Change the feed to a saturated liquid feed (V/F = 0 in feed), reinitialize (in tool bar click on Run and