Department of Chemical Engineering
Unit Operations Laboratory
Batch Distillation Theory
This handout provides important information on filtration theory and related issues:
o
o
o
o
o
o
o
o
Batch Distillation Theory
Determination of Composition by Gas Chromatography
Computer Simulation
Degrees of Freedom/Sensitivity
Material and Energy Balances
Properties
Statistical Analysis
Important Remarks
Batch Distillation Theory
The theory is well developed for the prediction of distillation column operations. This section
provides an overview of some of the equations and theory associated with batch distillation. For
additional theory development, see Resources.
In batch mode, with no feed, vapor from the reboiler rises up the trays in the column and is
finally liquefied in the condenser. This condensed liquid is either drawn off or reintroduced into
the top of the column. Each tray of the column is assumed to be in vapor-liquid equilibrium.
Geankoplis (1993) (see Resources) provides excellent diagrams illustrating the flows within a
distillation column and the equilibrium on each tray.
Several approaches can be taken:
Graphical Methods (McCabe-Thiele method and Ponchon-Savart method)
Shortcut Methods (e.g. Fenske-Underwood-Gilliland method)
Rigorous plate-to-plate calculations (using computer software)
The graphical and shortcut methods are specifically for steady state operations. While they do
not apply directly to the unsteady operation of batch distillation (except for the special case of
total reflux), they can be useful in initially characterizing a column. For total reflux calculations,
the Fenske equation (part of the FUG Shortcut Method) can be used to determine the number of
plates required for a given separation. The rigorous plate-to-plate calculations can easily be
performed with available computer aided engineering flowsheet simulation programs or special
purpose distillation programs.
Modified: 4/19/2007
Kevin M. Sapp
Original Document:
Jullie Lawson
page 1/4
2007
Department of Chemical Engineering
Unit Operations Laboratory
Determination of Composition by Gas Chromatography
o You will be injecting 0.1 micro-liter samples
o At the beginning of each lab period, inject ethanol, isopropanol, and methanol standards.
Note the retention time for each of these peaks.
o Use the retention time for the standard peaks to identify peaks from your samples.
o Use the relative peak areas of the components in your sample to determine the mole
fractions of components in the sample.
Computer Simulation
Using your knowledge and resources on Batch Distillation Theory, develop a theoretical basis
for predicting dynamic behavior of the distillation column for your experimental conditions and
steady-state temperatures, pressures, flows, compositions, etc. for operation under total reflux
conditions. Use these results to plan your experimental runs then compare your actual results
with those predicted.
HYSYS is an excellent programming resource for computer simulation of distillation columns. A
tutorial is available online at http://hysys.che.ufl.edu/unit_op_lab_examples/index.html.
Dynamic Simulation
o Several computer programs are available to perform the dynamic calculations,
and return the tray-by-tray characteristics as a function of time.
o The HYSYS Plant flowsheet simulation package can be run in dynamic mode for
this purpose.
o Ideally, you would like to determine the following as a function of time:
distillate product composition
bottoms product composition
stage by stage composition profile
stage by stage flow profile
heat transfer rate to the condenser
heat transfer rate to the reboiler
o And determine the
effect of Murphree efficiency on the above
time to reach a specific distillate concentration
Modified: 4/19/2007
Kevin M. Sapp
Original Document:
Jullie Lawson
page 2/4
2007
Department of Chemical Engineering
Unit Operations Laboratory
Steady State Simulation (Total Reflux Only)
o Several computer programs are available to perform the dynamic calculations,
and return the tray-by-tray characteristics as a function of time.
o The HYSYS Plant flowsheet simulation package run in steady-state mode is
recommended for this.
o Ideally, you would like to determine steady-state values for:
distillate product composition
bottoms product composition
stage by stage composition profile
stage by stage flow profile
heat transfer rate to the condenser
heat transfer rate to the reboiler
o And determine the:
effect of Murphree efficiency on the above
Tray Efficiency Evaluation
Determining the efficiency of the trays in the column is an iterative trial and error
procedure. First the programs or simulation packages (e.g. in HYSYS Plant simulation)
must be setup to simulate the operation of the column. Next, the user guesses the tray
efficiencies and the computer performs a tray-by-tray calculation to determine the
concentration of the distillate and bottoms products. If the values of the calculated
distillate and bottoms concentrations equal to the values found experimentally, then the
guessed efficiencies are assumed to be correct. Otherwise, another estimate of the
efficiencies is made and the program should be executed again. This is repeated until the
predicted product compositions are the same as those determined experimentally.
Degrees of Freedom/Sensitivity
The main independent variables available for study in the experiments include:
Flow rates (feed, distillate, bottoms)
Reflux ratio
Steam flow rate
Condenser water flow rate
Consider effects of changes of these variables on key distillation performance indicators. Note
that there are limits to the control of these variables with the equipment available, and not all of
these variables will affect performance in a measurable way.
Modified: 4/19/2007
Kevin M. Sapp
Original Document:
Jullie Lawson
page 3/4
2007
Department of Chemical Engineering
Unit Operations Laboratory
Material and Energy Balances
Use the experimentally determined compositions and flow rates to determine material balances.
Also perform energy balances over the column, around the reboiler, and around the condenser,
determining duties and heat transfer coefficients as needed. Resolve any data discrepancies and
obtain closure on the material and energy balances.
Properties
Properties for ethanol and isopropanol are well known. Material and Safety Data Sheets for these
major components, as well as methanol, methyl isobutyl ketone, and ethyl acetate, which are
present in small amounts, are available for download on the Batch Distillation Safety webpage.
HYSYS DISTIL can be used to create vapor-liquid equilibrium curves for the binary system of
ethanol and isopropanol, as well as the ternary system of methanol, ethanol, and isopropanol.
Preparation of XY phase diagrams and HXY diagrams for the experimental conditions is
strongly recommended. Note the importance of selecting an appropriate property estimation
model.
Statistical Analysis
Apply appropriate statistical procedures to determine the apparent error in results.
Important Remarks
Whenever possible, measure variables several times and different ways to verify values.
Modified: 4/19/2007
Kevin M. Sapp
Original Document:
Jullie Lawson
page 4/4
2007
