Dr Saad Al-ShahraniChE 334: Separation Processes
Ternary System
Most practical situations involving liquid-liquid equilibrium involve
three or more components.
Our attention is with three component systems. In this process, a
solute is removed from a feed stream by contacting it with a solvent.
The solute is quite soluble in the solvent, while the other component
in the feed is less soluble.
Liquid-Liquid Equilibrium
Dr Saad Al-ShahraniChE 334: Separation Processes
Terminology
Solute ≡ Component (1)
Original solvent ≡ Component (2)
Extractive solvent ≡ Component (3)
x1S, x2
S and x3S are the composition of the three components in (solvent
rich phase) 1,2,3 respectively.
x1R, x2
R and x3R are the composition of the Three components in the
(raffinate phase) 1,2,3 respectively.
Liquid-Liquid Equilibrium
Feed
(component +original solvent)
Extractive solvent
solvent-rich phase
(x1S1, x2
S1 , x3S1)
Raffinate-rich phase
(x1R1, x2
R1 , x3R1)
The solvent phase is rich in solvent and preferentially soaks up component 1 (the solute), which we are trying to separate from the other component in the feed (component 2, raffinate).
The raffinate phase is the liquid phase which is rich in the component 2 (raffinate) and from which the solute (component 1) is being removed.
Dr Saad Al-ShahraniChE 334: Separation Processes
Liquid-Liquid Equilibrium
The original feed is usually a mixture of solute (component 1) and raffinate (component 2).
The solvent-rich phase contains mostly solvent (component 3) and solute (component 1) and only a small amount of raffinate (component 2)
The raffinate-rich phase contains mostly solute (component 1) and raffinate (component 2), but also possibly some small amount of solvent.
Dr Saad Al-ShahraniChE 334: Separation Processes
Liquid-Liquid Equilibrium
Dr Saad Al-ShahraniChE 334: Separation Processes
Triangular Diagrams
Ternary systems are represented on two types of triangular diagrams:
1. Equilateral triangles
Liquid-Liquid Equilibrium
Dr Saad Al-ShahraniChE 334: Separation Processes
2. Right Triangles
Liquid-Liquid Equilibrium
Dr Saad Al-ShahraniChE 334: Separation Processes
Liquid-Liquid Equilibrium
(Solute)
Original solvent
Extractive solvent
Feed .
Mixture [50% Acetic + 20 H2O + 30%vinyl acetate
Dr Saad Al-ShahraniChE 334: Separation Processes
b) Liquid-liquid Equilibrium tie lines (LLE Tie lines)
Different chemical systems give different types of triangular diagrams.
Liquid-Liquid Equilibrium
Dr Saad Al-ShahraniChE 334: Separation Processes
The phase boundary, called the solubility line, is the solid line. Within the two-phase region,
liquid-liquid equilibrium lines (the dashed lines) connect compositions of the two phases that are in equilibrium with each other
The left side of the phase boundary gives the compositions of the raffinate-rich liquid phase (xj
R).
The right side of the phase boundary gives the compositions of the solvent-rich liquid phase (xj
S).
The LLE tie-lines and the equilibrium phase boundary are normally found by laboratory experimentation.
A mixture that has an overall composition inside the two-phase region will split into two liquid phases with compositions given at the two ends of the LLE tie-line.
Liquid-Liquid Equilibrium
Dr Saad Al-ShahraniChE 334: Separation Processes
A conjugate line can be used to locate the tie-lines.
From point A on-the left phase boundary, the other end of the tie-line is found by drawing a horizontal line to the conjugate line.
A vertical line is then drawn from the point M intersection to the right phase boundary. The point of intersection of this line and the right phase boundary (point B in the figure) is the other end of the tie-line.
Liquid-Liquid Equilibrium
M
Dr Saad Al-ShahraniChE 334: Separation Processes
As the system becomes richer in solute, the tie-lines get shorter and ultimately become just a point at the plait point P. Outside the two-phase region, a single, homogeneous liquid phase exists.
Effect of Temperature on solubility
Usually, the solubility increases as the temperature increases, for this reason, most liquid-liquid extraction systems operate at low temperatures and some times even require refrigeration.
Pressure, on the other hand, has little effect on solubility.
Liquid-Liquid Equilibrium
Dr Saad Al-ShahraniChE 334: Separation Processes
if we specify only one concentration of one liquid phase, all the other concentrations can be immediately determined from the phase diagram For example, if we fix the concentration of component 1 in the
raffinate-rich phase (x1R), we can read from the diagram:
1. The concentration of component 3 in the raffinate-rich phase (x3R),
by using the left side of the solubility curve.
2. The concentrations of components 1 and 3 in the solvent-rich
phase that is in equilibrium with the raffinate-rich phase, by going to
the other end of the LLE tie-line. The concentrations x1S and x3
S are
read from the right side of the solubility curve.
Liquid-Liquid Equilibrium
Dr Saad Al-ShahraniChE 334: Separation Processes
Example: Thirty thousand kg/hr of a ternary mixture of 19 weight percent isopropyl alcohol (IPA), 41 weight percent toluene, and 40 weight percent water are fed into a, decanter operating at 25°C. the figure gives the LLE data for the system. Determine the compositions and flow rates of the two liquid streams leaving the decanter.
Liquid-Liquid Equilibrium
Dr Saad Al-ShahraniChE 334: Separation Processes
Liquid-Liquid Equilibrium
The solvent-rich phase is 23 percent
IPA and 74 percent water.
The overall compositions of the feed
(z1 = 19 percent and z2 = 40
percent) are located on the diagram.
The compositions of the two liquid
phases are read off the diagram at
the two ends of the LLE tie-line.
The raffinate-rich phase is 14
percent IPA and 2 percent water
(the rest being toluene).
Dr Saad Al-ShahraniChE 334: Separation Processes
Solving the last two equations simultaneously gives
S = 15833 kg/h
R = 14176 kg/h
IPA in = (30000)(0.19) = 5700 kg/h
IPA out = S(0.23) + R(0.14)
= (15833)(0.23) + (14176)(0.14) = 5625 kg/h
Total mass: 30000 = S + R
Water = (30000)(0.4) = S(0.74) + R(0.02)
Liquid-Liquid Equilibrium
The difference is due to the accuracy of reading composition from the diagram
Dr Saad Al-ShahraniChE 334: Separation Processes
Liquid-Liquid Extraction
In liquid-liquid extraction, a liquid of two or more components to be
separated is contacted with a second liquid phase, called the solvent,
which is immiscible or partially miscible with one or more components
of the liquid feed.
The simplest liquid-liquid extraction involves only a ternary system.
The feed consists of two miscible components, the carrier (C) and the
solute (A). Solvent (S) is a pure component. Components (C,S) are at
most only partially soluble in each other. Solute (A) is soluble in (C)
and completely or partially soluble in S.
During the extraction process, mass transfer of (A) from the feed to the solvent occurs, with less transfer of (C) to the solvent, or (S) to the feed.
Dr Saad Al-ShahraniChE 334: Separation Processes
Liquid-Liquid Extraction
Liquid-liquid extraction is used to separate components in situations where:
1. Relative volatilities are quite close to unity ( < 1.1), making distillation very costly. (Distillation requires tall towers due to the existence of many trays, and high energy consumption because of high reflux ratios.)
e.g. A mixture of benzene and cyclohexane. The normal boiling points of these organics are 80.1°C and 80.7°C, respectively, making their separation by distillation impractical
2. Thermally sensitive components will not permit high enough temperatures to produce a vapor-liquid system at reasonable pressures (pressures greater than 10-50 mm Hg).
Dr Saad Al-ShahraniChE 334: Separation Processes
Liquid-Liquid Extraction
EQUIPMENT
Different mechanical devices are used in liquid-liquid extraction such as:
1. The simplest is a mixer/settler, or decanter, in which the two liquid phases are separated.
2. Plate towers, packed towers, and mechanically agitated mixers (rotating disk contactors)
the number of stages tends to be much smaller than in distillation columns. This is due to the larger settling times required for liquid-liquid separation because of the small density differences between the liquid phases.
Liquid-liquid extraction columns are sometimes operated in a pulsed mode.
Dr Saad Al-ShahraniChE 334: Separation Processes
Liquid-Liquid Extraction
Extractor/stripper process.
Dr Saad Al-ShahraniChE 334: Separation Processes
Liquid-Liquid Extraction
1. Mixer/ Settler
Horizontal gravity-settling vessel.Mixing vessel with variable-speed turbine agitator
Dr Saad Al-ShahraniChE 334: Separation Processes
Liquid-Liquid Extraction
2. Spray column
Dr Saad Al-ShahraniChE 334: Separation Processes
Liquid-Liquid Extraction
Extract
3. Packed column
Single-section cascade
Two-section cascade
Dual solvent with two-section cascade
Dr Saad Al-ShahraniChE 334: Separation Processes
Liquid-Liquid Extraction
Dr Saad Al-ShahraniChE 334: Separation Processes
Liquid-Liquid Extraction
Dr Saad Al-ShahraniChE 334: Separation Processes
Liquid-Liquid Extraction
GRAPHICAL MIXING RULES
If we have two streams that contain three components and mix them together.
Let one of these streams be stream A with flow rate FA (kg/h) and composition
x1A, x2
A and x3A (weight fractions of components 1,2, and 3), and let the other be
stream FB with corresponding composition x1B, x2
B and x3B . The mixed stream
leaving the mixer will have a flow rate FM and composition x1M, x2
M and x3M . A
flow diagram is as follows:
FA
x1A, x2
A , x3A
FB
x1B, x2
B , x3B
FM
x1M, x2
M , x3M
Dr Saad Al-ShahraniChE 334: Separation Processes
Liquid-Liquid Extraction
To determine the location of the mixture composition on a graph, since there are three components, only two coordinates are needed to completely specify the composition of any stream. We can use either right or equilateral triangular plots.
If we use right-triangular plot. locate point A with coordinates (x1A, x2
A ) and point B with coordinates (x1
B, x2B). The point M with coordinates (x1
M, x2M ) representing
the mixture will lie some place on the graph.
Dr Saad Al-ShahraniChE 334: Separation Processes
Liquid-Liquid Extraction
After mixing point M is supposed to lie on a straight line joining the A and B points. If we can show that the angles and in the figure are equal, then M must lie on a straight line between A and B.
The total mass balance for the system is
Component balances for components 1 and 2 are
and
(1)
(2)
(3)
Dr Saad Al-ShahraniChE 334: Separation Processes
Liquid-Liquid Extraction
Rearranging these two equations, we obtain:
Solving for the ratio FAIFB, we have:
or
Dr Saad Al-ShahraniChE 334: Separation Processes
Liquid-Liquid Extraction
These two ratios are the tangents of the angles and , hence, tan = tan . Therefore, = , and we have proven that the line AMB is a straight line.
The coordinates of the point M can be solved for analytically by using equations (1), (2), and (3). Alternatively, M can be located graphically where the distance from the point A to the point M divided by the distance from the point M to the point B is equal to the ratio FB/FA.