dr saad al-shahraniche 334: separation processes ternary system most practical situations...

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


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.


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.



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.


Triangular Diagrams

Ternary systems are represented on two types of triangular diagrams:

1. Equilateral triangles



2. Right Triangles




(Solute)

Original solvent

Extractive solvent

Feed .

Mixture [50% Acetic + 20 H2O + 30%vinyl acetate


b) Liquid-liquid Equilibrium tie lines (LLE Tie lines)

Different chemical systems give different types of triangular diagrams.



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.



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.


M


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.



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.



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.




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).


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)


The difference is due to the accuracy of reading composition from the diagram


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.



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).



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.



Extractor/stripper process.



1. Mixer/ Settler

Horizontal gravity-settling vessel.Mixing vessel with variable-speed turbine agitator



2. Spray column



Extract

3. Packed column

Single-section cascade

Two-section cascade

Dual solvent with two-section cascade



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



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.



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)



Rearranging these two equations, we obtain:

Solving for the ratio FAIFB, we have:

or



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.

dr saad al-shahraniche 334: separation processes ternary system most practical situations...

Documents

solvent phase

liquid phases

raffinate component

solvent rich phase

original solvent component

raffinate phase

extractive solvent component

raffinaterich phase