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__Distillation, stripping, absorption and extraction columns__

**This color indicates a
link available via Clarkson University.
If you are off campus, login using your Clarkson ID and PW.**

**History and types of
distillation**

__Design methods and information__

**Errors in modeling distillation****Residue curves for ternaries (see p 13-56 in Perry's Chemical Engineers' Handbook (7th Edition)).****Vapor pressure calculator****Phase diagrams for some non-ideal mixtures****Ternary azeotropes ; Location using Aspen SPLIT****Overcoming azeotropes ; Selecting Entrainers for Azeotropic Distillation****Non-equilibrium Modeling of Distillation (by Clarkson’s own Professor Taylor et al.)****Modeling Reactive Absorption****Component trapping****Design of Distillation Column Control Systems****Column pressure control****Dividing-wall distillation columns**

**Capacity and pressure drop for packed columns**__Packing__**, tower internals and demisting software.****Download free from ACS Industries.**__Vacuum distillation of lube oils__**: request paper 022 (in subject line) from techrequest@distillationgroup.com****Condensers for vacuum distillation**__HAZOP analysis of overpressure in a distillation column__**Pumps for reflux and reboilers Because the condenser for large columns is normally at ground level, the pressure drop across the reflux pump should be approximately the height of the column times the liquid density and gravity. Don’t forget to include an accumulator.**__Accumulator sizing.__

__Simulation using Aspen Plus__

**Many tutorials are available on-line.**

·
**Tutorial
on flash separation using Aspen Plus**

·
**Tutorial
on distillation using Aspen Plus**

·
**Use
of Aspen Plus to create binary & ternary phase diagrams, find azeotropes, make residue curves**

__Simulation using HYSYS and UniSim__

·
__Defaults__**:
There are several defaults that are not true for real columns:**

·
**Linear
pressure change through column based on specified top and bottom pressures.**

·
**10
trays**

·
**For
distillation, feed on tray 5**

·
**1.5
m diameter**

·
**100%
efficiency**

__Procedure for distillation calculations:__

**1.
****For the desired separation, select the
heavy and light key components, i.e. the split where you want lower boiling
components above the light key to go almost entirely overhead and the higher
boiling components below the heavy key to go almost entirely out in the
bottoms. You may want to use a “Short
Cut Distillation” unit first to do approximate calculations that will give you
an idea of what the number of trays, reflux ratio, and feed tray should be. Alternately, you can use Heuristics to estimate the required number of trays and
reflux ratio. **

**2.
****If
you have non-condensables such as nitrogen and oxygen in a distillation
column, it may be necessary to use a partial condenser with the vent rate
specified in order to avoid an extremely low temperature in the condenser. Sometimes it is helpful to use a simple flash
vessel before the column in order to eliminate most of the non-condensables.**

**3. **__Pressures:__**Unless the feed is 100% vapor,
set the bottom pressure to the nominal feed pressure. (This is because the feed pipe normally
starts near the reboiler height, and then goes up to the
feed tray causing a static head loss, so the pressure of the feed is actually
smaller.) Use heuristics
to estimate the pressure drop from tray to tray. To get the overhead pressure, multiply the
number of trays by the pressure drop / tray and then add this to the bottom
pressure.**

**4.
****Start
with a reflux ratio and a number of trays greater than those from short cut,
heuristics, or McCabe Thiele diagram.
Remember that the separation increases with increasing reflux ratio and
number of trays. The optimal feed tray
can be estimated by looking at how the temperature and composition vary from
tray to tray. For example, if the feed
is saturated liquid, the best feed tray is probably that with nearly the same
liquid composition.**

**5.
****Track the calculations using the Design/Monitor page. Add additional specifications to monitor the
results, but you must have only enough of these “active” that the degrees of
freedom is 0. Convergence may be easier
to obtain by specifying product flows rather than compositions. However, if your design requires a recycle
stream flow specifications will cause problems.
To avoid these problems, change the column flow rate specifications to
something else, such as compositions.**

**6.
****Once you have a converged column that gives your desired results,
use the tray-sizing
utility to get the column diameter and pressure drop. Since this pressure drop will likely differ from
that estimated in step 3 above, you will have to change the pressure
specifications for the column.**

**7.
****Size
the reboilers and condensers ****For
heat exchangers that condense steam or boil water fed at saturation, the
temperature of the water-steam is fixed by the pressure and is a constant. The flow rate of the water-steam is
calculated by dividing the Q by the latent heat of evaporation at that pressure. If the boiler feed water (bfw)
is below saturation, calculation of the heat exchanger should be broken into
two parts -- one to heat the bfw up to the saturation
temperature and a second to evaporate all of it. Similarly, if superheated steam is fed, for
calculation purposes break the heat exchanger into one that cools the steam to
saturation and a second that condenses it to saturated water. **

**8.
****Size
the pumps for reflux and reboilers**

·
__Procedure for
liquid-liquid extractors, strippers and absorbers__**: If you encounter a
zero-diagonal matrix error message, cut the number of trays to 2. Then, without resetting the calculations,
increase the number of trays until you reach the number desired. This should avoid the matrix inversion
problem. To efficiently obtain the
desired separation, you will have to experiment with changing the column
pressure and the flow rate of the extraction, stripping or absorbing agent. Do not use the tray-sizing
utility to find the diameter or pressure drop of a LLE column. Instead use Perry’s Handbook or search sites
given at ****General. For supercritical fluid extraction, see Supercritical Fluid Extraction.**

*Last
revised January 13, 2012. Please email
questions, comments and suggestions to W.R. Wilcox*

*Disclaimer: The material on these
pages is intended for instructional purposes by Clarkson University students
only. Neither Clarkson University nor
Professor Wilcox is responsible for problems caused by using this information.*

**Wilcox
home ****×**** ChE
design home ****×**** Profession ****×**** General ****×**** Properties ****×**** Equipment ****×**** Separation ****×**** HYSYS &
UniSim ****×**** Costs ****×**** Safety ****×**** Case studies ****×**** Excel
****×**** MATLAB ****×**** Data Analysis**