[5] liquid liquid extraction (1)

7/31/2019 [5] Liquid Liquid Extraction (1)

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Ch. E. 4232 Fall 2011 Dr. Hedden

Pre-Lab Notes: Liquid-Liquid Extraction

LLE is a separation technique based upon transfer of a solute between two immiscible liquids.

The key principle underlying LLE is often that the solubility of the solute differs significantly between the liquids,providing a thermodynamic driving force for transfer from one phase to the other.

Any two immiscible liquids may be used, but it is common to use water and an organic solvent.

In a small-scale chemistry laboratory, batch LLE is often performed using a separatory funnel:

The funnel is shaken vigorously, while taking care tovent any gases or vapors formed.

The shaking enhances the surface area of contact betweenthe phases, while the agitation enhances mass transfer fromone phase to the other by providing convection.

The shaken mixture is allowed to stand until the two phases

separate into layers (hopefully without forming an emulsion!)

Care is taken to recover the extract phase, which is thought to contain a high concentration of the solute. The extractphase may be either the top or the bottom phase!

The extraction may be repeated by shaking the remaining raffinate phase with more solvent two or three additional timesuntil most of the solute is extracted. The raffinate phase is usually discarded at this point.

After possible additional washing or drying steps, the solute is generally recovered by distillation of the solvent, or bycrystallization from solution followed by filtration, to yield a concentrated solute.


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An example of a lab-scale LLE procedure

Suppose we wish to extract caffeine (a solid) from a cup of tea (mostly water) to determine the caffeine content of thebeverage. We might follow a procedure like this:

1) The (cooled) cup of tea is shaken with methylene chloride (CH2Cl2), an organic solvent, in a separatory funnel.

2) Much of the caffeine goes into the CH2Cl2 phase, but other organic compounds (e.g. proteins) tend to remain in thewater phase.

3) The CH2Cl2 phase (extract) is recovered and saved.

4) The extraction of the raffinate (water) phase is done two more times, and the CH2Cl2 phase is recovered and savedeach time.

5) The CH2Cl2 phase is dried over anhydrous sodium sulfate three times to remove residual H2O.

6) The solvent is removed by rotary evaporation, and the caffeine crystals (and possibly other compounds) are recoveredand dried in an oven.

Needle-like caffeine crystals Caffeine at 400X in apolarized light microscope


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Typical Co-current Flow LLE Process Flowchart

http://www.enterprise-europe-network.ec.europa.eu/src/request/pictures/Photos%20Syrris%20TO.JPG

Counter-current flow is also commonly used in LLE systems (like in our lab!)


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2) Next, consider another scenario. Suppose the solution had instead been a thermally sensitive organic compoundin water, rather than caffeine. We can't distill off the water off to recover the solid because we can't heat it withoutdecomposition. Other options would be evaporation of the water without heating (takes too long) or freeze drying (alsonot very fast). However, CH2Cl2 is much easier to remove by distillation at a significantly lower temperature. So theLLE provides a way to get rid of residual water, while speeding up recovery of the thermally sensitive compound.

3) Suppose we are planning to discharge a large amount of wastewater from a chemical plant into a river. The problemis that there is a small amount of a dissolved solid or liquid, which is environmentally unfriendly. We could use LLE tostrip out the contaminant before discharging the wastewater.

4) Finally, consider a case where we are manufacturing a new pharmaceutical, which is synthesized in a veryexpensive organic solvent. We wish to recover the pharmaceutical and recycle the solvent. Fortunately, thepharmaceutical is water-soluble, so we use LLE to recover it and the expensive organic solvent (raffinate) is recycledback into the process. The raffinate is not always garbage, especially on an industrial scale!

5) Note that when there is a choice between LLE and distillation, choosing LLE often saves energy.

Why did we need to do LLE in the first place?

There are many reasons to use LLE, but here are a few of the common ones.

1) Let's use the caffeine example to illustrate the first fundamental reason for needing LLE: separating mixtures of solids.

The caffeine is mixed together with a whole bunch of proteins, tannins, and other organic molecules in the original solution(cup of tea). We are only interested in recovering and weighing the caffeine, so some kind of separation process isneeded to remove the other solids from the product.

Batch LLE vs. Continuous

Most industrial LLE processes are actually done on a continuous basis, and on a much larger scale than theseparatory funnel technique described earlier. As chemical engineers, we are most concerned with the operation ofcontinuous-flow LLE processes where the feed is added continuously, and the raffinate is discarded continuously.

Let's take a look at some examples of commercial-scale LLE processes and the equipment used therein.


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Source: KPMS Extraction Group website, http://www.liquid-extraction.com/industrial-applications.htm


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Unit Ops Lab LLE Equipment & Experiment Goals

Hypothetical Situation: Your consulting company (a.k.a. Unit Ops. lab group) has been assigned to

clean up an organic waste stream that contains a substantial volume fraction of propionic acid.While the acid is not especially toxic as organic compounds go, it has an especially strong andnoxious odor, which you will learn to love while completing this lab experiment (not). The reason forthe LLE unit is to recover the expensive organic solvent ($75 per liter!), which will be recycled backinto your process after further purification by distillation.

Experiment goals: We will be using water to extract the propionic acid from an organic solvent, n-

propyl bromide (nPrBr) using a continuous, counter-current flow extraction column that is packedwith Raschig rings. We wish to determine the final concentration of propionic acid in both theaqueous (extract) and organic (raffinate) phases, and to determine a mass transfer coefficient forthe extraction system, which will be useful in process modeling for design of a large-scale LLEcolumn in a new plant we're designing.

Additional Exercises: We will finish the experiment by distilling the nPrBr to remove any residual

propionic acid and/or water for future use due to its high cost (though we would likely not do anadditional distillation step in an industrial scale operation- why?).

During the course of this experiment, we will also learn how to calibrate a gear pump to determineflow rate vs. stroke length.


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LLE Apparatus Manufactured by Armfield Ltd.


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Counter-Current Extraction Column: Which Phase is Continuous?

* Our setup

Which phase has higher specificgravity?

In which solvent should the solubilityof propionic acid be higher?


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Selected Theoretical Principles of LLE

Distribution coefficients.

Suppose we have a solute (S) that is dissolved in a mixture of two immiscible liquids. Although thesolute might be preferentially soluble in one phase or the other, it's unusual to find that 100% of it isdissolved in either phase. Rather, the solute will be distributed between the phases. Thedistribution coefficient (K) is defined as:

ext

s

raff

s

C

CK

where Csraff is the concentration of S in the raffinate and

Csext is the concentration of S in the extract

The important observation regarding K in a continuous LLE operation is that it isn't necessarily aconstant. K depends on concentration, especially if the solute concentration is high. For dilutesolutions of the solute, and assuming steady state conditions, however, we can generally

assume that K will reach a constant value.

Mass Balance. As with many continuous-flow chemical engineering operations, it is essential toperform a mass balance on the system to fully understand its operation. You will be performing amass balance on the LLE experiment as a part of your final report. Here, we will apply massbalances to analyze both batch and continuous versions of a LLE operation.

(note: K is dimensionless)


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A. Batch LLE Mass Balance.

We start from V0 liters of solvent #1 containing ms grams of solute. The solution is shaken in aseparatory funnel or processed in a batch extraction unit using Ve liters of an extraction solvent

(#2). The two solvents are assumed to be completely immiscible, or close enough so that wecan neglect mixing of the two liquids. Assume the distribution coefficient K is known, and thatequilibrium is reached after shaking. What is the concentration of S in each phase?

Initial concentration of solute in solvent #1 =ms/V0

Initial concentration of solute in solvent #2 = 0

Total mass of solute V0Csraff + Ve Cs

ext = V0Csraff + VeCs

raff/ K = ms

Final concentration of solute in solvent #1 = Csraff

Final concentration of solute in solvent #2 = Csext = Cs

raff/ K

Mass balance: total mass of solute is conserved.

Csraff = ms/ (V0 +Ve/ K) Cs

ext = Csraff / K = ms/ (KV0 +Ve)

Here, we have assumed that the volume of each phase remains constant during extraction. Thatassumption is only valid if the solute was dilute in the first place. What if it had been concentrated?


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Multiple stage batch extraction

Suppose we extract the raffinate n times using Ve liters of fresh solvent. The residual concentration ofsolute found in the raffinate after nextractions with clean solvent is given by:

n

e

snnraff

sKVV

mVc

/0

1

0

,

This expression assumes:

- the volume of the raffinate phase does not change appreciably during extraction- the same volume of fresh solvent (Ve) is used each time- K is constant (concentration-independent)- the solvents are completely immiscible

where ms was the mass of the solute before the 1stextraction was done

You will derive this equation as a HW exercise.


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B. Now consider the operation of a continuous-flow, counter-current LLE column like the one wehave in the lab.

Csext

ext

V(liters/min.)

Csraff

raffV (liters/min.)

feed

csraff,o aqueous

pure water

Csext, o =

0 Q: At steady state, can we safely use the distribution coefficient K

to calculate csext

from csraff,o

? Why or why not?

wV

fV

In our lab experiment, only is difficult to measure directly. Theflow of the (organic) raffinate phase turns on and off periodically due tothe level sensor/solenoid valve mechanism. However, we cancalculate a value for this flow rate using the mass balance equation.

raffV

A mass balance on the column yields the concentrations in theoutlet streams in terms of the inlet concentrations and volumetricflowrates. You will derive an expression for from the mass

balance as a homework exercise and include it in your lab report.

raffV

We will want to determine the rate of mass transfer of solute from theorganic phase to the water phase in units of (g solute)/L/min. Thisrate is simply given by the rate of acid removal in the water stream:

ext

ext

s

VC overall rate of mass transfer into water phase =


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Mass Transfer Coefficient in a Counter-current LLE Column

A continuous-flow extraction differs from a single-stage batch extraction in that we cannotgenerally assume that the two phases are in equilibrium with each other at an arbitrary point

in the column. Thus, we need some average measure of the driving force for mass transfer,in the absence of a simple concentration differential.

The log mean driving force for mass transfer is given by

21

21

/ln CC

CCLMDF

where C1 = Csfeed-Cs* driving force at top of columnC2 = Cs

raff-0 driving force at bottom of column

Using the LMDF, we can calculate a mass transfer coefficient (ka) based on the raffinate phase:

Cs* is the hypothetical concentration of solute (in the raffinate) that would be inequilibrium with the extract phase, which is given by:

KCCext

ss *

LMDFV

CVka

col

ext

sext

(note: "a" is the interfacial area between thephases, which we cannot easily measure, so wereport the product ka instead). We have used therate of mass transfer calculated on the previouspage to get the numerator of this equation.Vcol is the volume of the column.


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LLE Operating Line and Equilibrium Line

The operating line for the LLE column relates the concentration of acid in the extract (water)phase to the concentration in the organic (raffinate) phase at all points along the column.However, we can only measure concentrations at the top of the column and the bottom of thecolumn. Thus, we have only two data points available for construction of the operating line, and wehave to assume it's linear.

Cs

ext(g/L)

Csraff (g/L)

bottom of column: Csext =0 and Cs

raff = determined by titration

top of column:Cs

ext = determined by titration,

and Csraff

= feed concentration

The equilibrium line goes through the origin,and if we assume Cs

ext/Csraff = constant, it will

be a straight line. What is its slope based upon

K, the distribution coefficient?

[5] liquid liquid extraction (1)

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