Liquid-Liquid Extraction Theory - Chemical Engineering

Department of Chemical Engineering
Unit Operations Laboratory
Liquid-Liquid Extraction Theory
This web-page provides important information on liquid-liquid extraction theory and related
issues:
o Liquid-Liquid Extraction Theory
o Determination of Composition by Titration Method
o Computer Simulation
o Degrees of Freedom/Sensitivity
o Material and Energy Balances
o Properties
o Statistical Analysis
o Important Remarks
Liquid-Liquid Extraction Theory
The theory is well developed for the prediction of liquid-liquid extraction column operations.
This section provides an overview of some of the equations and theory associated with liquidliquid
extraction. For additional theory development, see Resources.
When LLE is performed, different phases form—the continuous phase and the dispersed phase.
When an experiment is performed, the column will be first filled with water. This is the
continuous phase. Then the butyl acetate/acetone feed stream will be introduced as the dispersed
phase.
Several approaches can be taken in theoretical analysis:
• Graphical (triangular phase diagram)
• Shortcut Methods (Absorption factor method)
• Rigorous plate-to-plate calculations
The graphical and shortcut methods are specifically for steady state operations. The rigorous
plate-to-plate calculations can easily be performed with available computer aided engineering
flowsheet simulation programs or special purpose distillation programs.
There are several measures that can be used to determine the performance of a column. The most
traditional are the "number of transfer units" (NTU) and the "number of equivalent theoretical
plates" (NETP). The NTU method is theoretically more satisfying in that the theory behind it is
based on continuous contact mass transfer models. The NETP has no theoretical basis and is
evaluated empirically.
NTU/HTU Approach
Modified: 4/19/2007 Kevin M. Sapp page 1/4
Original Document: Jullie Lawson © 2007

Department of Chemical Engineering
Unit Operations Laboratory
One approach to evaluating the NTU is based on the absorption factor. The absorption factor
method assumes that Henry's Law is valid, which is usually only true for (very) dilute solutions.
If the NTU is evaluated using the absorption factor method, is essential that the Henry's Law is
validated, as it is a fundamental assumption (and more importantly, it isn't really true for this
experiment).
NTP/HETP Approach
The procedure for determining the HETP ("height equivalent to a theoretical plate") can be more
involved. One approach is to use the phase diagrams and step off the number of theoretical
plates (NTP) to achieve the LLE separation observed and dividing this number into the height of
the packing.
An alternate method of determining the equivalent NTPs is to use a computer model. Go to the
Computer Simulation section of this document for an outline of this approach.
Determination of Sample Concentration by Titration
Titration of samples will occur in a solution of hydroxylamine hydrochloride (HCl) with a 0.1 M
sodium hydroxide (NaOH) titrant. Use caution when handling the hydroxylamine HCl solution,
as it is highly corrosive. The chemical mechanism of the titration is available in the Titration
Handout written by Young Seok Kim. A step-by-step titration procedure is available on the
Liquid-Liquid Extraction Operation web page.
From the following data, the volume percent of acetone in samples can be calculated:
1. initial pH of hydroxylamine HCl solution (~3.5)
2. initial level of 0.1 N NaOH in the burette
3. pH of solution with sample
4. final level of 0.1 N NaOH in the burette
5. final pH of the titrated solution (as a check that you stopped the titration at the end point)
Next, calculate the volume of 0.1 M sodium hydroxide solution required to bring the pH of the
solution back to the initial pH. The volume percent of acetone in the sample is equal to the
NaOH volume multiplied by 0.78. The chemistry behind this titration is explained in the
Titration Mechanism handout.
Computer Simulation
Modified: 4/19/2007 Kevin M. Sapp page 2/4
Original Document: Jullie Lawson © 2007

Department of Chemical Engineering
Unit Operations Laboratory
Using your knowledge and resources on Liquid-Liquid Extraction Theory, develop a theoretical
basis for predicting steady-state temperatures, pressures, flows, compositions, etc. for your
experimental conditions. Use these results to plan your experimental runs then compare your
actual results with those predicted.
Following the NTP/HETP method, several simulations (using different numbers of theoretical
plates) are made to predict the performance of the column. The simulation exit concentration
values are used to calculate the separation factors which are plotted as a function of the number
of theoretical plates. The resulting plot is used to determine the equivalent NTP (this may include
a fractional plate). The "height equivalent to a theoretical plate" (HETP) is then just the height
of the column packing divided by the NTP.
HYSYS is an excellent programming resource for computer simulation of liquid-liquid
extraction columns. Use of a Three-Phase Separator is recommended. Aspen Plus is useful in
creating phase diagrams for liquid-liquid extraction column design and analysis. Other computer
programs are available as well.
Degrees of Freedom/Sensitivity
The main independent variables available for study in the experiments include:
• Flow rates (water feed, butyl acetate/acetone feed)
• Column size and packing
Consider effects of changes of these variables on key liquid-liquid extraction 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.
Material and Energy Balances
Use the experimentally determined compositions and flow rates to determine overall and
component material balances. It is particularly important to perform an acetone mass balance
during operation in order to check for the steady-state condition. Resolve any data discrepancies
and obtain closure on the material balances.
Properties
Properties for acetone, butyl acetate, and water are well known. Material and Safety Data Sheets
for these major components are available for download on the web page.
Modified: 4/19/2007 Kevin M. Sapp page 3/4
Original Document: Jullie Lawson © 2007

Department of Chemical Engineering
Unit Operations Laboratory
HYSYS DISTIL can be used to create a ternary phase diagram for the system of water, acetone,
and butyl acetate. Preparation of XY phase diagrams for the experimental conditions is strongly
recommended. Note the importance of selecting an appropriate property estimation model.
Use available correlations to estimate the mass diffusivities.
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.
Check and double-check valves and settings. If the column malfunctions and there is an overflow
or spill, there can be considerable effort required to clean up the laboratory.
Modified: 4/19/2007 Kevin M. Sapp page 4/4
Original Document: Jullie Lawson © 2007