Separation Process-II Awais Javaid

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Separation Process-II

Introduction In the field of Chemical Engineering – separation process is the transfer of

any mass that converts the substance mixture into distinctive product mixtures. “In some cases, a

separation may fully divide the mixture into its pure constituents. Separations are carried out based

on differences in chemical properties, or physical properties such as size, shape, mass, density, or

chemical affinity, between the constituents of a mixture, and are often classified according to the

particular differences they use to achieve separation.”

In most cases one single difference is used to attain a desired separation;

however multiple processes are implemented in combination to attain the desired solution. Apart

from a few cases, compounds and elements are found naturally in an impure state and these impure

materials needs to be separated for purification. For example; crude oil that occurs naturally is a

mixture of various hydrocarbons and impurities – purification process splits the mixture into

valuable natural gas, gasoline and chemical feedstock, these further need separation to render

further purer form.

Why we need to study separation process? Why does chemical engineering require the study of separation techniques?

Because separations are crucial in chemical engineering. A typical chemical plant is a chemical

reactor surrounded by separators, as diagramed in the schematic flow sheet of Figure 1.1. Raw

materials are prepurified in separation devices and fed to the chemical reactor; unreacted feed is

separated from the reaction products and recycled back to the reactor. Products must be further

separated and purified before they can be sold. This type of arrangement is very common. Since

separations are ubiquitous in chemical plants and petroleum refineries, chemical engineers must

be familiar with a variety of separation methods.

Figure. 1.1. Schematic flow sheet of a common industry

CONCEPT OF DRIVING FORCE AND EQUILIBRIUM: When two substances or phases are brought into contact with each other

(not in equilibrium), there is tendency for change to take place that will result in approaching to

equilibrium. The difference between the existing condition and the equilibrium condition is the

driving force causing this change e.g. concentration. The equilibrium stage concept is applicable

when the process can be constructed as a series of discrete stages in which the two phases are

Separation Process-II Awais Javaid

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contacted and then separated. The two separated phases are assumed to be in equilibrium with each

other.

Consider a vapor and a liquid that are in contact with each other as shown

in Figure 1.2. Liquid molecules are continually vaporizing, while vapor molecules are continually

condensing. If two chemical species are present, they will, in general, condense and vaporize at

different rates. When not at equilibrium, the liquid and the vapor can be at different pressures and

temperatures and be present in different mole fractions. At equilibrium the temperatures, pressures,

and fractions of the two phases cease to change. Although molecules continue to evaporate and

condense, the rate at which each species condenses is equal to the rate at which it evaporates.

Although on a molecular scale nothing has stopped, on the macroscopic scale, where we usually

observe processes, there are no further changes in temperature, pressure, or composition.

Figure. 1.2. Vapor-liquid contacting system

Mass transfer operations: When two phase of different compositions are brought into contact, a

transfer of components may occur from one phase to the other and vice versa as shown in figure

1.2. In most cases of mass transfer operations the two phases are only partially miscible. So that at

equilibrium there still exist two phases that can be separated from each (distillation, liquid-liquid

extraction, gas absorption, gas adsorption, etc.). Consequently the relative amount of component

transferred between phases are different, so that separation is achieved.

Repeating contacting and stage: Under approximate conditions, repeated contacting and separation of

phases can result to an almost complete separation of components.

Bases of separation process: Various chemical and physical properties of constituents of the mixture are

examined to determined greatest difference among component.

Factors keeping in view to select a separation process: 1. Chemical properties/physical properties

2. Energy requirement, cost and equipment

Separation Process-II Awais Javaid

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Liquid-Liquid Extraction

Definition: The separation of the component of a liquid mixture by treatment with a

solvent, in which one or more of the desired component of the liquid mixture is preferentially

soluble is known as liquid-liquid extraction or solvent extraction.

Why it is carried out? Solvent extraction is usually carried out in following cases;

1. When separation by distillation is in-effective or difficult the liquid-liquid extraction is

most suitable.

2. When the relative volatility is unity. (close boiling point mixture) A large amount of heat

is required in distillation.

3. Substances that cannot withstand the temperature of distillation even under vacuum. In

addition, for the cases where heating must be avoided.

Principle: Liquid-liquid extraction is based on the solubility difference rather than the

difference in the vapor pressure of the component to be separated out. It is based on the relative

solubility in the two phases.

Conditions to be fulfilled for solvent extraction: a. Bring the feed mixture and the solvent into intimate contact by dispersing one phase into

other as droplets.

b. Separation of the resulting two phases that have different densities.

c. Removal and the recovery of the solute from the solvent phase in a relative pure form

(evaporation, crystallization or distillation).

d. Removal and recovery of the solvent from each phase usually by distillation.

Why distillation is preferred for a binary mixture? 1. Distillation is operationally simple then extraction because a binary mixture easily

separated out into two components in a column whereas for liquid-liquid extraction, in

addition to extraction, a distillation column is also required. Therefore the combined

operation is normally more complicated and expensive, in-spite of the fact that heating,

cooling required even when distillation is carried.

2. Very often, more than one distillation column required for the recovery of the solvent from

aqueous phase.

3. Phase separation of the two phases may not be easy if the difference in their density is

small and interfacial tension is low.

4. Loss of solvent may also occur in some cases.

Despite of all the limitation, solvent extraction is an attractive alternative in

some cases as discuss below;

i. Compounds to be separated have close boiling point.

ii. Separation of heat sensitive material e.g. antibiotics.

iii. Recovery of solute from very dilute solution

In addition extraction does offer great flexibility in choice of operating

conditions, since the type and amount of solution can be variant as well as the temperature.

Separation Process-II Awais Javaid

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Applications: Penicillin is recover from the fermentation broth by extraction with a solvent such as butyl

acetate, after lowering the pH to get a favorable partition co-efficient. Solvent is treated

with a buffered phosphate solution to extract the penicillin from the solvent and give a

aqueous solution, from which penicillin is eventually produced by drying.

To separate acetic acid from dilute aqueous solution.

To separate petroleum product that have different chemical structure but about same

boiling point. Lube oil (boiling point ˃300 oC) fractions are treated with solvent such as

phenol or furfural to extract the aromatic to get paraffinic and naphthenic.

Separation of benzene, toluene and xylene from petroleum fractions.

Extraction of phenol from aqueous waste (coal carbonization plant, phenolic resin plant).

Purification of the uranium fuels.

SINGLE STAGE BATCH PROCESS: In the single-stage batch process illustrated in Figure 1.3, the solvent and

solution are mixed together and then allowed to separate into the two phases — the extract E,

“containing the required solute in the added solvent” and the raffinate R, “the weaker solution with

some associated solvent”. With this simple arrangement mixing and separation occur in the same

vessel.

Figure 1.3. Single stage batch process

CONTINUES TWO STAGE OPERATION: A continuous two-stage operation is shown in Figure 1.4, where the mixers

and separators are shown as separate vessels. The feed is passed to a multistage extractor.

Figure 1.4. Continues two stage operation

Separation Process-II Awais Javaid

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Feed mixture is mixed in the solvent in the mixer 1 and then extract 1 and

raffinate 1 are separated in the separator 1. Raffinate 1 is again treated similarly in mixer 2 and

separator 2.

MULTIPLE CONTACT SYSTEM WITH FRESH SOLVENT: There are three main forms of equipment.

1. There is the mixer-settler as shown in Figure 1.4.

2. There is the column type of design with trays or packing as in distillation

3. There are a variety of units incorporating rotating devices such as the Scheibel and the

Podbielniak extractors.

In all cases, the extraction units are followed by distillation or a similar

operation in order to recover the solvent and the solute.

Dispersions of one phase into another as droplet: 1. Two phases must be brought into good contact to permit transfer of material and then

separation.

2. Extract is the layer of solvent plus extracted solute.

3. Raffinate is the layer from which solute has been removed.

4. The extract may be lighter or heavier than the raffinate and therefore it can be shown

coming from top of equipment or bottom of equipment in some cases.

EQUILIBRIUM DATA:

Distribution Law: The equilibrium condition for the distribution of one solute between two

liquid phases is conveniently considered in terms of the distribution law. Thus, at equilibrium, the

ratio of the concentrations of the solute in the two phases is given by;

R

E

C

Ck

Where, CE= concentration of solute in Extract and CR= concentration of

solute in the Raffinate. This concept is applicable in the following conditions;

1. This will apply accurately when both solvents are immiscible.

2. There is no association or disassociation of the solute.

3. If the solute form molecules of different molecular weight, then the distribution law holds

for each molecular specie.

The distribution law holds when there is no chemical reaction of the solute

with solvent.