Counter-current liquid-liquid extraction
Theoretical background
The organic extraction procedure is the most commonly used in laboratory work, and used in
the following cases :
- to obtain an organic compound from an aqueous solution with an organic solvent immiscible
with water,
- to extract acidic or basic additives, inorganic salt beside an organic compound dissolved in
an organicsolvent with water
- to separate organic acids or bases from other organic compounds, such that the mixture
dissolved in an organic solvent isextracted with a dilute aqueous solution of inorganic bases
or acids.
Extraction is based on the phenomenon, that material to be recovered is partitioned between
two immiscible solvents. That partitioned material at a temperature can be described by its
partition coefficient which is the fraction of the concentrations in the two phases at
equilibrium, and independent of the actual concentration of the compound. From the partition
laws it can be deduced that using a given amount of extracting solvent in several smaller
portions, the extraction will be done at higher efficiency than if the total amount of the
extracting solvent would be used in a single step.
The quality of the extraction process is characterized by the extraction efficiency, which is the
fraction of the material separated by extraction:
mseparated
R
mall
where R is the efficiency of extraction, mseparated is the amount of material separated and mall is
the original amount of material, which is actually the sum of the amount of residual material
in the solvent and in the sample solution as well.
Mass balances for a single-stage, single-contact liquid-liquid extraction:
FS  M  ER
F  x F  S  yS  M  x M  E  y  R  x
E RM

R ME
F MS

S FM
Picture of the equipment::
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switch for
stirrer
motor
relief
tube
extractor
water inlet
emulsion
organic
solvent
pump
water pump
organic
solvent
inlet
magnetic stirrer
Sample collector
Task: extraction of oil from oil-water emulsion with an organic solvent (dichloromethane)
using different operational parameters (flow rates)
1. Calibration of the feed pumps following the instructions of the supervisor. Detemining
the flow rate for both pumps using different combinations of settings for stroke
(amplitude) and frequency (4-4 combinations for both pumps, eg. 40-40, 40-80, 80-40,
100-100). Weigh 2 dry Erlenmeyer-flasks (one for water and one for dichloromethane)
on the tara scale. Insert the outlets of the pumps into the flasks, set the amplitude first
(upper knob) then start pump by setting the frequency (lower knob), and at the same
time start the stopper. Sample collecting time can be about 2-3 minutes for lower
speeds, 1 minute for highest speed. Weigh the mass of the filled flasks and calculate
the flow rate for both pumps in g/sec and cm3/sec units (the density of
dichloromethane is 1.32 g/cm3).
2. Fill the extractor first with dichloromethane up to the top of the water inlet, then fill up
with water fully (when starts to overflow at the relief tube). Start the strirrer motor.
3. Prepare 1 liter aqueous emulsion from olive oil with a concentration of 1 %(m/m)
using tap water. Use the magnetic stirrer to maintain the emulsion by vigorous
strirring.
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4. Calculate how much time is needed to feed 100 cm3 dichloromethane into the
extractor for each 4 amplitude-frequency combinations. Weigh the 4 round bottom
flasks (record the mass without the cork flask support ring).
5. Place the suction tube of the water pump into the strirred emulsion carefully. Start the
water pump with a higher speed and wait until the oil appears in the extractor. Then
set the same amplitude-frequency combination for both pumps, start the stopper and
collecting the sample by opening the valve at the bottom of the equipment. Maintain
the emulsion+ dichloromethane input and sample collection for that time you have
calculated before for that pump speed. Try to set the flow rate of the sample same as
provided by the dichloromethane pump (remember, the target volume is 100 cm3). As
time expires stop both pumps and the sampling valve, weigh the mass of the collected
sample in the flask. If the collected volume is much less than 100 cm3 (132 g net), get
some supplementary sample without starting any pumps. Repeat the pumpingsampling process for the remaining 3 speed combinations.
6. Stop the stirrer motor, empty the extractor via the sampling tube: organic phase into
the bottle labelled „waste dichloromethane”, water phase into the drain.
7. Evaporate the dichloromethane from each 4 sample using the rotary evaporator.
Weigh the mass of the oil remained in the flasks and calculate the oil recovery
efficiency for each parameter. Construct the material balance for the whole process.
Do not forget to calculate with the amount of the emulsion input (using the calibration
data and time periods calculated before).
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