In liquid-liquid extraction, as in gas absorption and distillation, two phases must be brought into contact to permit transfer of material and then be separated. Extraction equipment may be operated batchwise or continuous. The extract is the layer of solvent plus extracted solute and the raffinate is the layer from which solute has been removed. The extract may be lighter or heavier than the raffinate, and so the extract may be shown coming from top of the equipment in some cases and from the bottom in others. The operation may of course be repeated if more than one contact is required, but when the quantities involved are large and several contacts are needed, continuous flow becomes economical.
If the components of the original solution distribute differently between the two liquids, separation will result. The component balances will be essentially identical to those for , but there are two major differences which are the carrier phase is a liquid, not a solid, so the physical separation technique will change and two distinct phases develop, so the simplicity of uniform solution is lost.
Propionic acid is an important commercial product and extracting it out of aqueous solution is a growing requirement in fermentation based industries and recovery from waste streams.
OBJECTIVES
In experiment Part A, the objectives are to determine the distribution coefficient for the system organic solvent – propionic acid – water as well as to its dependence on concentration. Meanwhile, the objectives of experiment Part B are to demonstrate how a mass balance is performed on the extraction column, and to measure the mass transfer coefficient with the aqueous phase as the continuous medium.
THEORY
In dilute solution at equilibrium, the concentrations of the solute in the two phases are called the distribution coefficient or distribution constant K, as describe in the followings:
K = Y/X ..............(1)
where Y is the concentration of the solute in the extract phase whereas X is the concentration of the solute in raffinate phase. The distribution coefficient can also be expressed as the weight fraction of the solute in the two phases in equilibrium contact:
K = y’/x ................(2)
where y’ is the weight fraction of the solute in the extract and x is the weight fraction of the solute in the raffinate.
The rate at which a soluble component is transferred from one solvent to another will be dependent, among other things, on the area of the interface between the two immiscible liquids. Therefore it is very advantageous for this interface to be formed by droplets and films, the situation being analogous to that existing in packed distillation columns.
The theory for the system Trichloroethylene-Propionic acid-Water is as follows:
Let
Vw : Water flow rate, lt/s
Vo : Trichloroethylene flow rate, lt/s
X : Propionic acid concentration in the organic phase, kg/lt
Y : Propionic acid concentration in the aqueous phase, kg/lt
Subscripts : 1 : Top of column
: 2 : Bottom of column
Mass Balance :
Propionic acid extracted from the organic phase (raffinate).
=Vo(X1−X2) .................(3)
Propionic acid extracted by the aqueous phase (extract)
=Vw(Y1−0) ..................(4)
Therefore theoretically,
Vo(X1−X2) = Vw(Y1−0) ...................(5)
Mass transfer coefficient:
......................(6)
where Log mean driving force : (ΔX1-ΔX2) / ln (ΔX1/ΔX2)
ΔX1 : Driving force at the top of the column = (X2 - 0)
ΔX2 : Driving force at the bottom of the column = (X1-X1*)
where X1* and X2* are the concentrations in the organic phase which would be in equilibrium with concentrations Y1 and Y2 ( = 0.0) in the aqueous phase, respectively. The equilibrium values can be found using the distribution coefficient for the chemicals used (Assume that Y=KX relation holds at equilibrium for a constant K). Rate of acid transfer may be calculated using Equation (3) or (4) based on raffinate or extract phases, respectively.
APPRATUS
Experiment A
- 250 ml conical flask
- 250 ml measuring cylinder
- 250 ml separating funnel
- Pipette with rubber bulb
- Sodium Hydroxide solution ( 0.1M and 0.025M)
- Phenolphthalein
- Propionic acid
Experiment B
EXPERIMENTAL PROCEDURE
Experiment Part A
- 50ml of trichloroethylene is being mixed with 50ml water in conical flask. Then 2ml of propionic acid is added to the mixture.
- A stopper is placed and the mixture is shaken for 5 minutes.
- The mixture is then separated using the separation funnel.
Each of the bottom and upper samples is titrated against 0.1M NaOH
using phenolphthalein as the indicator.
Experiment Part B
- 100mL of propionic acid are added to 10 litres of trichloroethylene. The mixture is then filled into the organic phase tank (bottom tank).
- The level control is switched to the bottom of the column by keeping the bottom electrodes on (the S2 valve is switched on).
- The water feed tank is filled with 15 litres of clean demineralised water (the V13 valve was open). The water feed pump is started (valve S3) and the flow rate of water is regulated to the maximum by opening valve C1.
- The flow rate is reduced to 0.5 litre/min as soon as the water reaches the top packing.
- The metering pump (S4) is started.
- Steady conditions must be achieved by running the set up for 15-20 minutes. The flow rate is monitored during the period to ensure that they remain constant.
- Two or three batches of 30 ml sample are taken from the feed, raffinate and extract streams (valve V1).
- 10 ml of each sample is titrated against 0.025M NaOH using phenolphthalein as the indicator (to titrate the feed and raffinate continuous stirring using magnetic stirrer may be needed).
RESULTS
Experiment A
Experiment B
SAMPLE CALCULATION
Experiment Part A
Calculation of concentration (Organic)
1) Concentration in Organic Layer, Y (5ml Propionic Acid)
Formula: M1V1 = M2V2
M1 = Concentration of NaOH (moles)
V1 = Volume of NaOH (ml)
M2 = Concentration of Propionic acid (moles)
V2 = Volume of Propionic acid (ml)
(0.1)(0.3ml) = M2(5ml)
M1 = 0.006M
2) Concentration in Organic Layer, Y (3ml Propionic Acid)
(0.1)(2.5ml) = M2(3ml)
M2 = 0.083M
3) Concentration in Organic Layer, Y (1ml Propionic Acid)
(0.1)(22.6ml) = M2(1ml)
M2 = 2.26M
Calculation of concentration (Aqueous)
1) Concentration in Aqueous Layer, X (5ml Propionic Acid)
(0.1)(1.7ml) = M2(5ml)
M1 = 0.034M
2) Concentration in Aqueous Layer, X (3ml Propionic Acid)
(0.1)(14ml) = M2(3ml)
M2 = 0.467M
3) Concentration in Aqueous Layer, X (1ml Propionic Acid)
(0.1)(7.9ml) = M2(1ml)
M1 = 0.79M
Calculation of Distribution Coefficient, K :
1) Distribution coefficient (5ml Propionic Acid)
Formula: K = Concentration of the solute in the extract phase, Y
Concentration of the solute in the raffinate phase, X
K = 0.034M
0.006M
K = 5.67
2) Distribution coefficient (3ml Propionic Acid)
Formula: K = Concentration of the solute in the extract phase, Y
Concentration of the solute in the raffinate phase, X
K = 0.467M
0.083M
K = 5.63
3) Distribution coefficient (1ml Propionic Acid)
Formula: K = Concentration of the solute in the extract phase, Y
Concentration of the solute in the raffinate phase, X
K = 0.79M
2.26M
K = 0.35
Experiment Part B
Concentration of Propionic Acid at feed:
Formula: M1V1 = M2V2
M1 = Concentration of NaOH (moles)
V1 = Volume of NaOH (mL)
M2 = Concentration of propionic acid (moles)
V2 = Volume of propionic acid (ml)
(0.1)(7.2ml) = M2(10ml)
M2 = 0.072M
Concentration of Propionic Acid at Raffinate
Formula: M1V1 = M2V2
(0.1)(3.1ml) = M2(10ml)
M2 = 0.031M
Concentration of Propionic Acid at Extract
(0.1)(17ml) = M2(10ml)
M2 = 0.17M
The flow rate of aqueous and organic phase = 0.0033 L/s
Mass Balance:
Propionic Acid extracted from the organic phase (raffinate)
= Vo (X1 - X2)
= 0.0033 (0.072 – 0.0031)
= 0.000135 kg/s
Propionic Acid extracted by the aqueous phase (extract)
= Vw (Y1 - 0)
= 0.0033 (0.17 - 0)
= 0.000561 kg/s
To calculate the X1*, calculate the average distribution coefficient from experiment A
K = 5.67 + 5.63 + 0.35
3
K = 3.883
K = Y1
X1*
3.883 = 0.17
X1*
X1* = 0.0438
Log mean driving force =
= (0.0031)-(0.072 – 0.0438)
ln
= 0.0114
Mass transfer coefficient (based on the raffinate phase)
=
= 3.883
x 0.0114
= 144561
DISCUSSION
This experiment is conducted basically to achieve the main objectives which are to determine the distribution coefficient and therefore the mass transfer coefficient, which is based on the concentration of the solvent.
As what has been practised in the experiment, the mixture of trichloroethylene-propanoic acid-water is separated by using separator funnel. When a compound is shaken in a separator funnel with two immiscible solvents, the compound will distribute itself between the two solvents. The bottom layer contains more water and the upper layer contains more propanoic acid as water is denser than the solvent.
Regarding the result, we can see the decrease in distribution coefficient when the amount of solvent added is decreasing. This shows that the greater the amount of solvent added, the higher the increase in distribution coefficient.
Nevertheless, there might be some errors that happened during the experiment. First, errors might be done while taking the reading of the burette. Supposedly, the eye of the observer should be parallel to the meniscus level. Likewise, it is preferable to put a white paper just behind the level in order to aid in reading the meniscus. Therefore, a different reading will lead to different values of calculation from the correct one.
Second, all of the instruments used during this experiment might not be handled properly. Since the quantity of the measuring instruments is limited, the members of the group needed to wash all the measuring cylinders required to perform the titration. Therefore, the added solutions are inevitably mixed with the water left after being washed. This may also lead to some errors in the calculations.
CONCLUSION
The main objectives of this experiment are achieved successfully, where the distribution coefficient for solution with 5mL, 3mL and 1mL of propanoic acid added are 5.67, 5.63 and 0.35 respectively. Likewise, the mass transfer coefficient is 144561. From experiment B, the concentration of propanoic acid appears to be the highest in extract phase. From experiment A, the distribution coefficient value increases as the volume of propanoic acid added as an extracting solvent is increased.
RECOMMENDATIONS
There are some recommendations to make sure this experiment would attain more accurate and precise results in the future:
- Make sure the apparatus are in good condition before conducting the experiment.
- All the color change after the titration should approximately about the same color on each other.
- Be concerned with the eyes should always be at the same level when taking the reading of pipette and burette.
- For Experiment A and B can be repeated on titration with 0.025M NaOH.
References
- http://en.wikipedia.org/wiki/Liquid-liquid_extraction
- http://www.cheresources.com/extraction.shtml
-
Geankoplis, C.J. (2003). Transport Processes and Separation Process Principles. (4th edition). Pearson Prentice Hall.
-
McCabe, W. L., and J. C. Smith, (1956). Unit Operations of Chemical Engineering. New York: McGraw-Hill Companies.
APPENDICES
