NON-CONVENTIONAL DISTILLATION PROCESS

RAJDEEP MALDEPARTMENT OF CHEMICAL TECHNOLOGY(Petrochemicals and Petroleum Refinery

Engineering)Post B.Sc B.Tech in Chemical Technology

University Of Calcutta

Distillation

Distillation is a process of separating components where more volatile substances in a mixture are separated from less volatile substances.

This separation process is carried out by vaporizing a liquid mixture or by condensing a vapour mixture.

Distillation is a Unit Operation or Physical separation method.

Principle :• Relative volatility • Boiling point of the components to be

separated.

Types of distillation:• Conventional distillation method• Non-Conventional distillationNon-Conventional distillation method can be classified as (a) Extractive Distillation (b) Azeotropic Distillation (c) Reactive Distillation etc.

To understand the Principle of Non-Conventional distillation process, we have to get a primary idea on some Parameters. i.e. as follows (a)Raoult’s law (Ideal Solution) (b)Deviation from Raoult’s law (Non-ideal ) (c)Azeotrope (d)Types of Azeotrope

Raoult’s LawThe partial pressure of any volatile component of a solution at any temperature is equal to the vapour pressure of pure component multiply by the mole fraction of that component in the solution.

In a solution of two miscible liquids (A & B) the partial pressure of component “A” (PA) in the solution equals the partial pressure of pure “A” (PA

o) times its mole fraction (xA)Partial Pressure of A in solution = PA = (PA

o) x (xA)Partial Pressure of B in solution = PB = (PB

o) x (xB) When the total pressure (sum of the partial pressures) is equal

to or greater than the applied pressure, normally Atmospheric Pressure (760 mm Hg), the solution boils

Ptotal = PA + PB = PAo xA + PB

o xB

If the sum of the two partial pressures of the two compounds in a mixture is less than the applied pressure, the mixture will not boil. The solution must be heated until the combined vapor pressure equals the applied pressure.

Raoult’s law is followed by only Ideal solution Vapour pressure of ideal binary solution of two

components A and B having different mole fractions are shown

in Fig-1

Fig-1 Vapour Press vs Composition

DEVIATIONS FROM RAOULT’S LAW (NON-IDEAL SOLUTIONS)

There are two types of deviations may possible from Raoult’s law

(i) Vap. Press curve of liq.pairs showing positive deviation (ii) Vap. Press curve of liq.pairs showing negative deviation

Deviations from Raoult’s law (cont.) Positive deviation :Cohesive (Attraction) forces between unlike components (say A-B) are weaker than those of the pure liq (say A-A or B-B)i.e. PA>PA

oxA ,PB>PBoxB and Ptotal >

PAo xA + PB

o xB

Negative deviation : Attraction forces between unlike molecules in solution (A-B) are stronger than those between like molecules (A-A and B-B)i.e. PA<PA

oxA ,PB<PBoxB and Ptotal < PA

o xA + PB

o xB

Azeotrope

An azeotropic mixture is a mixture of two liquids having same boiling point.

These mixture can’t be separated by simple fractional distillation because no change in composition of these mixture on boiling i.e. same boiling point.

These mixture is called Constant Boiling Point mixture.

These mixtures are formed by non-ideal solution

AZEOTROPE• Constant boiling mixture• Non-ideal solution• Cannot be seperated by simple distillation• Same composition in distillate and residue at minimum temperature.

TYPES OF AZEOTROPESBased on the boiling point on temperature-composition or press-composition diagram, Azeotrope can be classified as follows:

Positive azeotrope/ minimum boiling mixture• When boiling temperature of the azeotrope is less than that of the pure component.-minimum boiling mixture.• It is shown by those liq.pair which show positive deviation from ideal behavior.•Example – Ethanol (96%) and Water mixture (4%) Negative azeotrope/maximum boiling mixture.• Boiling temperature of the azeotrope is higher than

that of pure component• Liq pair which shows negative deviation• Example –Water (20.2%) and HCl(79.8%)

POSITIVE AZEOTROPE / MINIMUM BOILING MIXTURE

NEGATIVE AZEOTROPE /MAXIMUM BOILING MIXTURE

NON-CONVENTIONAL DISTILLATION PROCESS

PRINCIPLE OF NON-CONVENTIONAL DISTILLATION PROCESS

At low to moderate pressure, with the assumption of ideal-gas model for the vapor phase

Vapor-liquid phase equilibrium (VLE) of many mixture can be adequately describe by the

following Modified Raoult’s Law: i.e. yi P = xi γi Psat for i = 1, …, c [1] Where, yi = mole fraction of component i in vapor

phase , xi = mole fraction of component i inliquid phase, P = system pressure , P sat = vapor pressure of component i , γi = liquid-phase activity coefficient of component i

CONT.

When γi = 1, the mixture is said to be ideal and equation 1 simplifies to Raoult’s Law.

Non-ideal mixtures (γi ≠ 1) can exhibit either positive (γi > 1) or negative deviations (γi < 1) from Raoult’s law.

In many highly non-ideal mixtures these deviations become so large that the pressure- composition (P-x, y) and temperature-composition (T-x, y) phase diagrams exhibit a minimum or maximum azeotrope point.

In the content of T - x, y phase diagram, these points are so called minimum boiling azeotrope (where the boiling temperature of the azeotrope is less than that of the pure component) or maximum boiling azeotrope (the boiling temperature of the azeotrope is higher than that of the pure components).

About 90% of the known azeotropes are of the minimum variety. At these minimum and maximum boiling azeotrope, the liquid phase and its equilibrium vapor phase have the same composition, i.e.,

xi = yi for i = 1, …, c [2]

EXTRACTIVE DISTILLATION Extractive distillation is a vapor-liquid

process operation that uses a third component or solvent or Entrainer.

The extractive agent creates or enhances the volatility difference between the components to be separated.

The extractive agent and the less volatile component flow to the bottom of the distillation column, where the extracted component is recovered by a subsequent distillation.

The non-extracted species are distilled to the top of the extractive distillation tower.

A mixture of components A and B which are close boiling. In order to separate the components by extractive distillation, a suitable non-volatile solvent, E, is introduced near the top of the column. The solvents preferentially associates one of the component, say B, and there by increases the relative volatility of the other component, A. A stream of relatively pure A is obtained as the top product of the column. The mixture of B and E leaves at the bottom and is further processed to recover the solvent for recycle and to separate the product B. The solvent E is called an entrainer since it entrains or extracts one of the component.

Process flow diagram showing an extractive distillation apparatus

CHOICE OF SOLVENT / ENTRAINEREntrainer / Solvent - is an agent, which affects the volatility of one of the azeotrope constituents more than another. The most important factors in the selection of

solvent are Solvent Selectivity and Solvency. Selectivity refers to the solvents ability to

enhance the separation of the key component(s) in the mixture. It is expressed as the ratio of the distribution coefficients of the two components in the solvent.

Solvency is the concentration ratio of the entrained component B in the solvent phase and raffinate phase.

CONTD..

The solvent should be low-cost, non-corrosive, non-toxic, non-reactive and should have low latent heat.

The solvent should not form an azeotrope with any of the component(s) but it should definitely have more affinity to for the less volatile component.

APPLICATION OF EXTRACTIVE DISTILLATION

Extractive distillation is widely used in Petrochemicals industry. It is used for Separation of butadiene from a mixture of

butane, butene and small quantities of other unsaturated hydrocarbon can be separated by using extractive distillation. Low polarity solvents like furfural, N-methyl pyrrolidone (NMP) etc are good entrainers for butadiene.

Separation of Benzene from the mixture of benzene-cyclohexane during the production of cyclohexane from benzene via hydrogenation. Here propylene glycol is used as a Solvent/Entrainer.

The basic configuration for LLE requires four units, in contrast to the two required for ED

ED systems now use better heat integration.

Smaller and fewer pieces of equipment

Enhanced mass transfer devices

Liquid-liquid Extraction vs Extractive distillation

AZEOTROPIC DISTILLATION Azeotropic distillation is any of a range of techniques used

to break an azeotrope in distillation. Azeotropic distillation usually refers to the specific

technique of adding another component, called solvent or entrainer.

Entrainer is added to the mixture for generating new or lower boiling azeotrope (minimum boiling azeotrope) or new upper boiling azeotrope (maximum boiling azeotrope).

Example- (i) minimum boiling azeotrope or positive azetrope is 95.6% ethanol and 4.4% water(by mass), ethanol boils at 78.4ºC and water boils at 100ºC but the azeotrope boils at 78.2ºC.

(ii) maximum boiling azeotrope or negetive azeotrope is hydrochloric acid at a concentration of 20.2% and 79.8% water( by mass).HCl boils at -84ºC and water at 100ºC but the azeotrope boils at 110ºC.

It is classified as positive azeotrope or minimum boiling azeotrope.Here as an entrainer cyclohexane or benzene is used. A minimum boiling azeotrope of water and cyclohexane leaves as the overhead vapour.The condensate is phase separated as usual and cyclohexane phase is sent back as reflux. Dehydrated ethanol leaves the bottom of the column. The water rich phase flows to the cyclohexane stripper. The recovered hydrocarbon is recycled.

AZEOTROPIC DISTILLATION FOR ETHANOL WATER SYSTEM

Uses of pure Ethanol (anhydrous)

Why produce 100% ethanol using azeotropic distillation when standard distillation gives 96% anyway?

• Pure ethanol is used as a solvent for laboratory and industrial application- paint and varnishes.

• Pure ethanol is used as a motor fuel alcohol ( gasohol)

Water could react with other chemicals that the solvent is mixed with and engines are only suitable for anhydrous ethanol.

USES OF AZEOTROPIC DISTILLATION

Seperation of isobutanol and water

Seperation of benzene and cyclohexane

However, the main application of azeotropic distillation is to separate ethanol and water mixture, also called dehydration of ethanol.

EXTRACTIVE DISTILLATION VS AZEOTROPIC DISTILLATION

ED method is used when mixture boiling pt. is very close to each other.

Entrainer, which is used for ED process, is used to create a difference of relative volatility b/w the component(s) of mixture.

For ED, additional component (Entrainer) appears mostly at the bottom of the column.

Example-Separation of Butadiene from the mixture of butane, butene or other unsaturated hydrocarbon.

AD is used for the mixture of close boiling component.

Entrainer, which is used for AD process, forms a new lower boiling azeotrope with one or more components of the feed.

For AD, additional component (Entrainer) appears in appreciable amount at the top of the column.

Example- Dehydration of Ehanol by using cyclohexane or benzene as entrainer.

Extractive Distillation Azeotropic Distillation

REACTIVE DISTILLATION

Reactive distillation involves a chemical reaction and product separation in the same column or vessel.

Add reactant &/or catalyst to cause a reversible/selective reaction with one of the feed components.

Example- Production of methyl tertiary butyl ether (MTBE), a synthetic octane-booster produced by the reaction between iso-butene and methanol.

COMPARISON BETWEEN EXTRACTIVE, AZEOTROPIC AND REACTIVE DISTILLATION

Extractive Distillation 

Azeotropic Distillation 

Reactive Distillation

Higher boiling solventeg- NMP, Propylene glycol

 

Add entrainer that forms lower boiling point (min boiling AZ) or upper boiling point (max boiling AZ) eg.-Benzene, cyclohexane etc

Add reactant &/or catalyst to cause a reversible/selective reaction with one of the feed components

Enters near top of column 

Added near top or bottom depending upon the AZ is min/max BP

Reaction and distillation occur in same vessel 

Interacts with other components to affect volatility or activity coefficients

Entrainer forms a new and lower boiling AZ with one or more of the components in the feed.

 

Example – Butadiene extraction from butane, butene and other unsaturated hydrocarbon.

Example- Dehydration of ethanol

Example- Production of methyl tertiary butyl ether (MTBE) by the reaction between iso-butane and methanol.

ADVANTAGES AND DISADVANTAGES OF NON-CONVENTIONAL DISTILLATION PROCESS

Advantages : Allowing the separation of chemicals that cannot feasibly

be separated by conventional distillation, such as systems containing azeotropes or pinch points.

Improving the economics of the separation by saving energy and increasing recovery.

Disadvantages : Larger diameter column required to allow for increased

vapour volume due to the azeotropic agent. An increase in control complications compared with

simple distillation.

REFERENCES : 1. Principle of Mass Transfer and Separation

Process by B.K.Dutta 5th edition 2. Unit operations in Chemicals engineering by

Mc-Cabe-Smith 7th edition 3.http://

www.cheresources.com/content/articles/separation-technology/extractive-5.distillation-an-in-depth-look

4.http://www.rccostello.com/distil/distileqp.htm 5.http

://www.differencebetween.net/science/chemistry-science/differences-between-azeotropic-and-extractive-distillation/

6.https://en.wikipedia.org/wiki/Azeotropic_distillation

7.https://en.wikipedia.org/wiki/Extractive_distillation

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