Lect Slide 7

7/29/2019 Lect Slide 7

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Chemical Engineering -

Separations 5

Lecture 7 – Separation of azeotropic

and close boiling mixtures

John Christy

Use of Mass Separating Agents

• Can be used for close boiling mixtures,homogeneous azeotropes and in order toreverse volatilities

• If a relatively volatile MSA is used, it will bechosen so that it forms an azeotrope (usuallymin boiling) with either or both of theoriginal components (Azeotropic Distillation)

• If a non-volatile solvent is used, it will bechosen to dissolve one componentpreferentially, allowing other component toleave in distillate (Extractive Distillation

Azeotropic distillation

• Add substance that forms minimum boiling

azeotrope with one or both components.

• Take azeotrope off top and separate by

decanting or by solvent extraction

• May get binary or ternary azeotrope, either

homogeneous or heterogeneous

Azeotropic distillation - examples• Azeotropes

– Benzene/cyclohexane (74°C) use acetone to formazeotrope with cyclohexane (53.1°C) and waterwash to recover acetone

– Ethanol/water – add benzene to form heterogeneousternary azeotrope (decant)

• Close Boilers

– Toluene/methylcyclohexane – add methanol to formazeotrope with mch. Gives methanol/toluene inbottoms (water wash)

– Butene/butadiene – add ammonia at 16bar – butenesform heterogeneous azeotropes with ammonia

Azeotropic distillation - examples

• Reverse volatilities

– Paraffins/aromatics

• add methanol to take paraffins overhead. Water wash

then distill

• Add nitromethane to take paraffins overhead. Cool

and decant


• Form heterogeneous ternary azeotrope withbenzene to separate ethanol/water



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• Form heterogeneous binary azeotrope of butene

with ammonia to separate butene/butadiene


• Form binary azeotrope of one component withacetone to separate benzene/cyclohexane, thenuse water to extract acetone from cyclohexane

Extractive distillation

• Add non-volatile substance/entrainer that

modifies relative volatility of the other

components.

• May reverse volatilities

• However, leads to higher T in reboiler

• Must separate entrainer by decanting,distillation or solvent extraction

Extractive distillation - examples

• Max boiling Azeotropes

– Nitric acid/water or hydrochloric acid/water –

extract water using sulphuric acid

• Close Boilers

– Toluene/methylcyclohexane – add phenol to

dissolve toluene. Gives mch overhead – distill

phenol/touene


• Reverse volatilities

– Paraffins/aromatics – add glycol to dissolve

aromatics – recover glycol with water wash

– Propene/propane in presence of nitrogen – use

acrylonitrile to reverse the volatilities of propane

and propene. Propene dissolves in acrylonitrile –

separate by flash followed by distillation


• Use glycol to dissolve aromatics, enabling paraffinsto leave overhead. Glycol recovered by liquidextraction with water



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Choice of MSA (Extractive)

• Substantially higher BPt from i and j.• Select entrainer from same homologous

series as one of components, but higher

molecular weight. Need to ensure it will not

form minimum boiling azeotrope with any

components.

• Miscible with both components

• MSA should form complex or H-bonds with

i and not j, therefore decrease αij.

Choice of MSA (Azeotropic)

• Boil within 30°C of other components• Large +ve deviation from Raoults Law.

• Soluble in at least one component

• Lower temperature in condenser should becompatible with use of cooling water

• Azeotropic composition and latent heatimportant as they govern the heat duty of the column

• MSA must break existing H-bonds betweeni and j or complexes of i and thereforeincrease αij.

Choice of MSA

• Entrainer should separate easily from

relevant component

• Preferably operate in a region where no

immiscible liquid phases form in the

column as this can lead to problems in

ensuring overflow of both phases• Entrainer must be thermally stable, non-

reactive, non-corrosive and cheap.

Heuristics for difficult separations

• Separations using MSA should be

considered difficult. i.e. do last.

• Consider MSA for systems not easily

separable by conventional distillation

• Azeotrope-forming

• Very close boiling or non-ideal with α<1.1or spread of components with different

structures, similar BPs

Heuristics for difficult separations• On adding MSA, recover in next separation

• Don’t use MSA to recover another MSA

• If multicomponent products specified,

favour sequences that produce these

products directly

• If components partially immiscible,

consider liq-liq extraction or decanting

Column Profiles

• Typical profile



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Column Profiles

• Zone 1: near top

• Much benzene, ethanol less volatile thanwater – effect ethanol/water separation

• Slight increase in water at feed plate due towater in feed

• Benzene has intermediate volatility

Column Profiles

• Zone 2: just below feed

• Ethanol and benzene almost constant, nofractionation (looks like misplaced feed)

• αWE =2.0 near pinch composition of benzene. αWE < 1.1 without benzene.

• Water stripping section

Column Profiles

• Zone 3: near bottom

• Ethanol and benzene binary separation

• Benzene more volatile than ethanol. StrictlyE/B min boiling azeotrope is stripped fromexcess ethanol

• E/B azeotrope is 45%E, 55%B (at 1atm)

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