Experimental Investigation of Liquid-Liquid Extraction: Distribution and Mass Transfer Coefficients in Trichloroethylene, Propionic Acid, and Water System

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Liquid-liquid extraction, also recognized as solvent extraction or partitioning, is a technique employed to segregate compounds based on their relative solubility in two immiscible liquids—typically water and an organic solvent like propionic acid. This method involves the transfer of a substance from one liquid phase to another, commonly conducted in chemical laboratories using a separator funnel. Often performed post-chemical reactions as part of the work-up process, liquid-liquid extraction enables the separation of a substance from a mixture by selectively dissolving it in a suitable solvent.

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Essentially, it is a means of isolating a soluble compound from an insoluble one.

The fundamental principle underlying extraction is the introduction of a solution to another solvent that is immiscible with the original. This added solvent is also soluble with a specific solute in the solution, resulting in the formation of two phases due to density differences. The choice of solvent is crucial, as it should attract the solute more than the original solution.

Consequently, mass transfer of the solute occurs from the solution to the solvent. Although additional steps are required to separate the extracted solute and the solvent, these separation costs may be preferable compared to other methods like distillation, particularly when extraction is applicable.

A typical extraction column features two input streams and two output streams. The inputs consist of a solution feed at the top containing the solute for extraction and a solvent feed at the bottom that extracts the solute from the solution. The solvent, now containing the extracted solute, exits from the top as the extract stream.

Meanwhile, the solution, with minimal solute remaining, exits from the bottom as the raffinate. Further separation of these output streams may be necessary through additional processes.

In this laboratory experiment, the focus was on liquid-liquid extraction, a fundamental technique for separating compounds based on their solubility in immiscible liquids. The apparatus used included separator funnels and liquid-liquid extraction columns, and the objective was to determine distribution coefficients and mass transfer coefficients for a system involving Trichloroethylene (Vo), Propionic acid (X), and Water (Vw).

Experimental Setup and Procedure

The first experiment involved the use of separator funnels to separate solutions of different solubility and densities. NaOH titrations were performed with concentrations of 0.1M and 0.025M. Distribution coefficients (K and K') were calculated using the concentrations of solute in the extract (Y) and raffinate (X). The values obtained were then utilized in the second experiment.

In the second experiment, a liquid-liquid extraction column was employed to obtain feed, raffinate, and extract samples. Again, NaOH titrations were performed with different concentrations. The mass transfer coefficients (MTC) were determined based on the rates of acid transfer and the log mean driving force, considering equilibrium concentrations obtained from the first experiment.

Theory and Equations

The distribution coefficient, K, represents the concentration ratio of solute in the extract to that in the raffinate (K = Y/X). Additionally, the weight fraction form (K' = y*/x) was considered. The mass balance equations for the organic and aqueous phases were expressed as:

Vo(X1−X2)=Vw(Y1−0)

The mass transfer coefficient (MTC) was defined as the rate of acid transfer divided by the volume of packing and the mean driving force. The log mean driving force (ΔX1-ΔX2) / ln (ΔX1/ΔX2) was calculated using the driving forces at the top and bottom of the column.

Experimental Results and Calculations

The values for distribution coefficients from the first experiment were utilized to find equilibrium concentrations (X1* and Y1) in the second experiment. These concentrations, along with the initial concentrations, were applied in the mass balance equation. The log mean driving force was then computed for determining the mass transfer coefficient.

Experiment A:

Ensure that valves V6 and V11 are in the closed position.
Activate valves S1, C3, and S3.
Adjust the feed flow rate (C1) to its maximum.
Turn on valve S4.
When water reaches the top of the column, set C1 to 300 cc/min.
Allow 20 minutes for the process to proceed.
Extract 100 mL samples from the raffinate, feed, and extract.
Extract 10 mL from each sample in Experiment A and add 3 drops of phenolphthalein.
Perform titration on each sample using 0.1 M NaOH.
Repeat steps 1 and 2 using 0.025 M NaOH.
Perform two titrations for each mole of NaOH.

Experiment B:

Combine 50 mL of water, 50 mL of organic solvent, and 1 mL of propanoic acid in a conical flask; shake thoroughly.
Repeat step 1 using 1.5 and 2 mL of propanoic acid.
Observe the formation of two layers and extract 10 mL from each upper and lower layer.
Add 3 drops of phenolphthalein to each sample.
Conduct titrations on each sample using 0.1 M and 0.025 M NaOH.

EXPERIMENT A

Concentration of NaOH (M)	Raffinate	Feed	Extract
0.1 M NaOH	1.3 mL	0.1 mL	0.1 mL
0.025 M NaOH	1.0 mL	0.6 mL	0.4 mL

EXPERIMENT B

Volume of Propionic acid (mL)	Volume of NaOH (mL)		Concentration of NaOH (M)
Volume of Propionic acid (mL)	Upper	Bottom	Concentration of NaOH (M)
1.0	12.8	1.7	0.1 M
1.5	27.0	6.3
2.0	27.7	7.6
1.0	37.9	8.6	0.025 M

Distribution Coefficient (K) Formula: $K = X Y$ Where $Y$ is the concentration of solute in the extract phase, and $X$ is the concentration of solute in the raffinate phase.

Finding Mass Transfer Coefficient:

Mass Balance Equation: $M_{1} V_{1} = M_{2} V_{2}$ Where $M_{1}$ is the concentration of NaOH, $M_{2}$ is the concentration of propionic acid, $V_{1}$ is the volume of NaOH, and $V_{2}$ is the volume of propionic acid.
Rate of Acid Transfer: $Rate of acid transfer = V_{w} (Y_{1} - 0)$
Equation for Mass Transfer: $V o (X_{1} - X_{2}) = V_{w} (Y_{1} - 0)$
Distribution Coefficient Equation: $K = X ^{*} Y ^{1}$
Log Mean Driving Force: $Log mean driving force = l n ( Δ X ^{2} Δ X ^{1} ) Δ X ^{1} - Δ X ^{2}$ Where $Δ X_{1} = X_{2} - 0$ and $Δ X_{2} = X_{1} - X_{1 *}$
Packing Dimension:
- Length = 1.2 m
- Diameter = 50 mm
- Packing Volume ( $V$ ) = $π r^{2} L$ = $π \times (0.025 m)^{2} \times (1.2 m)$ = $2.36 \times 1 0^{-3} m^{3}$ = 2.36 L
Mass Transfer Coefficient: $Mass transfer coefficient = Volumes of packing \times mean driving force Rate of acid transfer$