Ternary Diagram PDF

1

Chapter 9

Principles of Liquid-Liquid

Extraction (LLE)

Extraction is a process where one or more

liquid solute(s) are removed from the liquid-

phase feed (called “the first liquid phase”) by

another liquid phase (called “the second liquid

phase” or the “solvent”); since all materials [i.e.

the solute, the first liquid phase, and the second

liquid phase (or the solvent)] in this operation

are in liquid state, this operation is commonly

called the “liquid-liquid extraction (LLE)”

2

In addition to liquid-liquid extraction (LLE),

the liquid or solid solute(s) can be removed from

the solid-liquid mixture (i.e. the feed) by another

liquid (the solvent); such operation is called the

“solid-liquid extraction (SLE)”

It is believed that the extraction has a long

history, which can be dated back to the Roman

time; at the time, the Romans separated gold

and silver from molten copper using the molten

lead as a solvent

However, it was not until the early 1930’s

when the first large-scale LLE process was in

operation

3

Lazar Edeleanu (a Romanian chemist: 1861-

1941) removed aromatic and sulphur compounds

from liquid kerosene by the LLE process using

liquid sulphur dioxide at the temperature as low

as 10-20 oF as a solvent; by removing such com-

pounds from kerosene, it yields clean kerosene

suitable to be used as a fuel for residential lighting

The liquid-liquid extraction can be divided

into 2 main cases:

Case I: While the liquid solute(s) can be dis-

solved in both liquid phases (i.e. the first and the

second liquid phases), the first and the second

liquid phases are not dissolved in each other);

this kind of LLE is called the immiscible liquid-

liquid extraction

4

Case II: In this case, the liquid solute(s) is

(are) still dissolved in both liquid phases, and

some amounts of the first liquid phase are dis-

solved in the second liquid phase or vice versa;

this kind of LLE is called the miscible liquid-

liquid extraction

In reality, almost all liquid-liquid extraction

operations are the miscible LLE; however, for

simplification, especially in the introduction level,

it is usually assumed that the immiscible LLE is

valid

5

On the contrary to the distillation, the LLE

operation does not requires heat as a separating

agent; the separating agent for the LLE opera-

tion is the second liquid phase

Accordingly, the LLE operation can separate

two liquid phases from each other at low temper-

atures

This makes the LLE operation suitable for

separating materials that may decompose or

de-nature at elevated temperatures or for

separating materials whose boiling tempera-

tures (or volatilities) are close to each other

6

Examples of the uses of extraction are

The separation of penicillin from the broth

(the liquid phase obtained from biological

processes) using butyl acetate as a solvent

The separation of aromatic-ring hydrocar-

bons (e.g., benzene, toluene) from paraf-

fins using sulpholane as a solvent

The extraction of vanilla from the oxidised

liquors using toluene as a solvent

The separation of vitamin A from fish-liver

oil using propane as a solvent

The extraction of vitamin E from vegeta-

ble oil using propane as a solvent

(note that vanilla, vitamin A, and vitamin

E can be de-natured at moderate tempera-

tures)

7

It is important to note that, in many applica-

tions, the downstream process that separates sol-

vent from the solute(s) is more expensive than

the extraction operation itself

Hence, the selection of the solvent is very

critical for the extraction system

The ideal solvent (or the second liquid phase)

should

be capable of extracting the solute(s) from

the first liquid phase

not be dissolved in the first liquid phase

be easy to be separated from the solute(s)

by a simple and inexpensive separating

technique

8

Additionally, the solvent should also be

non-toxic

non-reactive or chemically stable

non-corrosive

non-flammable or non-explosive

readily available

inexpensive

environmentally friendly (i.e. “green”)

Note that, in the liquid-liquid extraction, the

ratio of the concentration (normally presented in

“mole fraction”) of the solute in the second liquid

phase ( )iy to that in the first liquid phase ( )ix is

called the “distribution ratio”:

id

i

yK

x=

9

The ideal solution should also yield a high

distribution ratio

A complete LLE process can be illustrated in

Figure 9.1

Figure 9.1: A complete LLE process

(from “Separation Process Engineering” by Wankat, 2007)

The feed containing the first solvent and the

solute is fed into the extraction unit (or the ex-

tractor)

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Another solvent (the second solvent) is also

fed into the extractor counter-currently to the

feed, to remove the solute from the first solvent

After the system reaches the steady state, the

mixture is separated into 2 distinguished phases:

The raffinate phase (or just the raffi-

nate), which comprises the first solvent

and the solute; note that the raffinate

phase is also called the carrier-rich product

(the carrier is, in fact, the first solvent)

The extract phase (or just the extract),

which contains the second solvent and

the extracted solute; in some textbooks/

references, the extract phase is also called

the solvent-rich product

11

Note that the feeding point of the feed (the

first solvent and the solute) and solvent depends

on the density of each stream

The stream with higher density or specific

gravity (SG) is normally fed from the top of the

extractor, while the stream with lower density is

fed bottom-up; this flow scheme allows one stream

to be in good contact with another stream, thus

enhancing the separation efficiency

The resulting extract phase (or the loaded sol-

vent stream in Figure 9.1) is then passed into the

solvent recovery unit, in which the second solvent

is separated from the solute

12

An industrial example of the complete LLE

process can be illustrated in Figure 9.2

The feed containing acetic acid and water is to

be separated to yield glacial acetic acid (with the

concentration of acetic acid of, at least, 99.8 wt%)

by the LLE process

Note that the normal boiling points of acetic

acid and water are 118 and 100 oC, respectively

Although the difference in boiling tempera-

tures of these two species is relatively large, it is

not practical and economical to separate this

mixture by distillation

13

Figure 9.2: An illustration of a complete liquid-

liquid extraction process (from “Separation Process Principles” by Seader and Henley, 2006)

14

This is due mainly to the fact that the heat

capacity (or the specific heat: p

c ) of water is

relatively high; thus, it requires a considerable

amount of heat or energy to vaporise water

Additionally, since, by considering the normal

boiling temperatures of the two species in the

mixture, water is more volatile than acetic acid,

at steady state,

the vapour (or gas) phase contains a high

amount of water

the liquid phase contains a high amount of

acetic acid

To obtain the vapour phase, the liquid mixture

has to be re-boiled at the re-boiler

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Since the vapour phase contains a high amount

of water, it requires a high amount of heat load (at

the re-boiler) to vaporise the mixture containing

a high amount of water

Accordingly, the cost of providing heat/energy

for vaporising the mixture with a high amount of

water increases drastically, thus, rendering distil-

lation uneconomical

Hence, an alternative separation technique

must be considered, and it is found that, to sepa-

rate acetic acid from water, the LLE process

seems to be the promising choice

16

In Figure 9.2,

water in the feed is the first solvent (or

the carrier)

acetic acid in the feed is the solute

Note that the feed is fed from the top of the

L-L extraction unit

The recycle solvent, which contains mainly

ethyl acetate (~96%) is used as the second solvent

The second solvent is fed from the bottom of

the unit

Eventually, after the LLE process is at steady

state, two separate phases are settled and can be

divided into

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the extract phase, whose composition is as

follows:

o ethyl acetate 83.5%

o water 8.3%

o acetic acid 8.2%

(the solute)

(note that this phase is rich in ethyl ace-

tate—the second solvent or the solvent)

the raffinate phase, with the following com-

position:

o ethyl acetate 7.1%

o water 92.8%

o acetic acid 0.1%

(the solute)

(note that this phase is rich in water—the

first solvent or the carrier)

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Note that the concentration of acetic acid in

the feed is ~22%

The extract phase is then sent to the distilla-

tion column (the upper distillation column in

Figure 9.2) for further separation

In this distillation column (the upper one in

Figure 9.2), acetic acid is separated from ethyl

acetate and water

Since its boiling point is higher than those of

ethyl acetate (n.b.p. = 77 oC) and water, it

emerges from the distillation column as the bot-

tom product

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Ethyl acetate and water exit the column as the

top product; after being condensed at the conden-

ser ethyl acetate and water can be separated from

each other using a two-phase decanter

The recovered ethyl acetate can then be used

as the recycle solvent for the LLE process

It is noteworthy that, since the extract phase

contains a low amount of water (but a high

amount of ethyl acetate), the heating load, at the

re-boiler, reduces dramatically (note that the

heat capacity of ethyl acetate is much lower than

that of water)

20

The bottom product of the liquid-liquid ex-

tractor, or the raffinate phase, is sent to another

distillation column (the lower one in Figure 9.2)

In this distillation, water and acetic acid is

separated from ethyl acetate

Since the boiling temperature of ethyl acetate

is much lower than those of water and acetic acid,

it is rather easy to separate it from the other two

species by distillation; additionally, as a high puri-

ty of the bottom product, which contains mainly

water with only a very small amount of acetic

acid, is not required, a high heating load at the

re-boiler is not necessary

21

From the example as illustrated in Figure 9.2,

it is evident that the LLE process alone does not

yield an ultimate separation; to obtain a highly

purified product, further separation process is

required

However, the LLE process makes further sepa-

ration much easier, cheaper, less energy-consum-

ing, and more efficient

Several types of liquid-liquid extraction equip-

ment are employed in various industries, which

can be divided into

Mixer-Settler

For this type of L-L extractor, two liquid

phases are mixed in the first unit, called a mixer

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The resulting mixture is then sent to the

subsequent unit, where the extract and the raffi-

nate phases are allowed to be settled and sepa-

rated into two distinguished phases

Figures 9.3 illustrates the mixer-settler

unit

Figure 9.3: A mixer-settler


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Spray Column

A spray column, as shown in Figure 9.4, is

the simplest and oldest type of L-L extractor

Figure 9.4: A spray column (from “Separation Process Principles” by Seader and Henley, 2006)

24

In this type of L-L extractor, either light

or heavy phase/stream or both can be sprayed or

dispersed into small droplets, using nozzles

Normally, there are no plates (stages) or

packings in the column; thus, the throughputs are

large, meaning that the contact time between

two phases is relatively short

Accordingly, the extraction efficiency for

this type of extractor is rather low

Hence, despite its low cost, the spray co-

lumn is rarely used for industrial applications

25

Packed Column

The configuration of the packed column is

similar to that of the spray column, except that,

for the packed column, packings are packed

within the extractor column, as shown in Figure

9.5

Note that the spaces at the top and the

bottom of the column are the settling zones for

the light and the heavy phases, respectively

Plate Column

In the plate-column type L-L extractor, as

depicted in Figure 9.6, sieve trays (or plates) are

added into the column

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Figure 9.5: A packed-column type L-L extractor

(http://lewis.armscontrolwonk.com)

The plates enhances the contact between

the light and the heavy phases, thus improving

the extraction efficiency

27

Figure 9.6: A plate-column type L-L extractor

(http://www.cheresources.com)

The principal purpose of the addition of

the trays or plates is the same as per the distil-

lation column, in which the plates allows the

contact between the down-coming liquid phase

and the rising vapour phase

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Columns with Mechanical Agitation

To increase the contact and mass transfer

between the light and the heavy phases, a device

that enables mechanical agitation within the co-

lumn of the L-L extractor is added to the system,

as illustrated in Figure 9.7

Figure 9.7: A Rotating Disc Contactor (RDC)

(http://www.liquid-extraction.com)

29

The L-L extracting unit as depicted in

Figure 9.7 is commonly called the “rotating disc

contactor”

The discs or the rotors are generally ro-

tated radially, thus enhancing the mixing within

the stage (or between the two stators, as shown

in Figure 9.7)

Normally, the discs and the stators have

holes, through which the light and the heavy

phases flows

For some instances, the discs may be agi-

tated in the vertical direction

30

Note, once again, that the spaces at the

top and the bottom of the column are the settling

zones for the light and the heavy phases, respec-

tively

The advantages and disadvantages of various

types of L-L extractors are summarised in Table

9.1

Table 9.1: Advantages and disadvantages of

different liquid-liquid extraction equipment

Type Advantages Disadvantages

Mixer-Settler Good contacting

Can handle wide

flow ratio between

the feed and the

solvent

High efficiency

Can be scaled up

easily

Large hold-up time

High power cost, as

inter-stage pumping

is required

High investment

cost

Large space require-

ment

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Table 9.1: Advantages and disadvantages of

different liquid-liquid extraction equipment

(cont.)

Type Advantages Disadvantages

Continuous,

counter-current

flow contactors

(with no mecha-

nical agitation);

e.g., spray,

packed, and

plate columns

Low investment cost

Low operating cost

Simple construction

Large throughput

Cannot handle high

flow ratio

Scale-up is rather

difficult

Continuous,

counter-current

flow contactors

(with no mecha-

nical agitation);

e.g., RDC

Good dispersion and

mixing

High efficiency

Reasonable cost

Scale-up is relatively

easy

Moderate to large

throughput

Cannot handle high

flow ratio

Cannot handle

emulsifying systems

32

In addition to counter-current cascades or

counter-current extraction, which is the most

common extraction scheme, other configuration

of the extraction cascades can also be employed

One type of such other cascades is a cross-

flow cascade, as illustrated in Figure 9.8

Figure 9.8: A cross-flow L-L extraction unit


Note that, in this type of extraction cascade,

the fresh solvent is fed to each stage

33

In addition to liquid-liquid extraction (LLE)

and solid-liquid extraction (SLE), when there are

two solid species mixed together, and we want to

separate one solid species from another, a tech-

nique called a “washing” may be used

In the washing process, the soluble solid is

extracted from the insoluble one using a solvent

For example, we can separate salt from a mix-

ture of sand and salt, using pure water as a sol-

vent

In the washing process, water and the mixture

of sand and salt are mixed together in a mixer

34

During the mixing, salt is extracted from the

sand + salt mixture by the fact that only salt can

be dissolved in water

The resulting mixture is then allowed to be

settled in a settler (a settling tank) or a thickener,

in which the mixture is divided into 2 phases:

the overflow phase, which is the solution

of solvent and soluble solid (i.e. the salt

(in water) solution for this Example)

the underflow phase, which comprises

insoluble solid (i.e. sand) and the solution

of solvent and soluble solid (i.e. the salt

solution)

35

The main purpose of this operation (i.e. the

washing) is to have as pure insoluble solid as

possible

The washing process can be illustrated in

Figure 9.9

Figure 9.9: The washing process


36

From Figure 9.9,

O and U represents the flow rates of the

overflow and underflow phases, respectively

j

x and j

y are the concentrations of soluble

solute (solid) in the underflow and over-

flow phases, respectively

Note that the washing can be categorised as

the extraction, as the solute is extracted from the

carrier in the similar manner to that of the LLE

process, except that the solute and the carrier

are solid

A similar process to the washing, which still

employs the principle of the LLE process, is the

“leaching”

37

On the contrary to the washing, in which the

main goal is to obtain the insoluble solid with a

high purity, the main purpose of the leaching is

to obtain as many soluble solid (solute) in the

solvent as possible

Examples of the leaching include:

the making of liquid coffee from coffee

beans (เมล็ดกาแฟ) using hot water as a

solvent—in an espresso machine

the leaching of liquid tea from tea leaves

using hot water as a solvent

38

the recovery of soybean oil (a source of

bio-diesel) from soybean seeds using an

organic compound (normally hexane) as a

solvent

The general configuration for the leaching is

as depicted in Figure 9.10

Figure 9.10: A schematic diagram of washing


39

In Figure 9.10,

solid

F = the flow rate of insoluble solid

solv

F = the flow rate of solvent

j

Y = the concentration of the soluble

solid in the solvent

j

X = the concentration of the soluble

solid in the insoluble solid

Recently, a number of research works reveal

that, by replacing the liquid solvent with the

super-critical fluid, the efficiencies of the ex-

traction (either liquid or solid), the washing, and

the leaching are enhanced substantially

40

The super-critical fluid is the fluid whose tem-

perature and pressure are higher than the critical

point (c

T & c

P ) as shown in Figure 9.11

Figure 9.11: A phase (P-T) diagram that

indicates the super-critical region


41

The critical temperatures ( )cT , pressures ( )c

P

and densities ( )cr of selected pure substances are

illustrated in Table 9.2:

Table 9.2: The critical temperatures, pressures,

and densities of selected substances

Substance ( )o Cc

T ( ) atmc

P ( ) g / mLcr

CO2

Propane (C3H8)

Water

31

97

374

72.8

41.9

217.7

0.47

0.22

0.32

An interest of using super-critical fluid (SCF)

as a solvent for extracting compounds from

either solid (in the SLE process) or liquid (in the

LLE process) is increasing the past two decades

42

This may be because of the following reasons:

The densities and the viscosities of SCFs

are lower than those of liquids, thus result-

ing in low pressure drops during the

extraction process

The diffusivities (especially in solids) of

SCFs are much higher than those of liquids

(~10 folds)

There is no phase boundary, which results

in high mass transfer rates

Note that, of many SCFs available, super-

critical CO2 is found to be the most popular SCF;

this may be because super-critical CO2

43

is readily available

is inexpensive

is non-toxic

can be used at low temperatures

To separate the extracted solute from the

mixture of SCF + solute, it can be done easily

by lowering the system’s pressure, as shown in

Figure 9.12

For example, for napthalene dissolved in

super-critical CO2 (at a high pressure, e.g., 150

atm), when the system’s pressure is dropped to

~50 atm, the solubility of napthalene in super-

critical CO2 is reduced from ~10-1–10-2 to ~10-4–

10-5 (or by ~1,000 folds)

44

Figure 9.12: A regeneration of super-critical fluid

(SCF) by a pressure-swing system


Note also that super-critical CO2 is commonly

used in food and pharmaceutical industries, as it

can easily be separated completely from the de-

sired products (by lowering the system’s pressure

as described recently)

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