Chemical Engineering -Separations 5

Lecture 7 – Separation of azeotropicand 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 boilingazeotrope with one or both components.

• Take azeotrope off top and separate bydecanting or by solvent extraction

• May get binary or ternary azeotrope, eitherhomogeneous 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 form

azeotrope 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 washthen distill

• Add nitromethane to take paraffins overhead. Cooland decant

Azeotropic distillation - examples

• Form heterogeneous ternary azeotrope withbenzene to separate ethanol/water

Azeotropic distillation - examples

• Form heterogeneous binary azeotrope of butenewith ammonia to separate butene/butadiene

Azeotropic distillation - examples

• 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 thatmodifies relative volatility of the othercomponents.

• 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 – distillphenol/touene

Extractive distillation - examples

• 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 propaneand propene. Propene dissolves in acrylonitrile –separate by flash followed by distillation

Extractive distillation - examples

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

Choice of MSA (Extractive)• Substantially higher BPt from i and j.• Select entrainer from same homologous

series as one of components, but highermolecular weight. Need to ensure it will notform minimum boiling azeotrope with anycomponents.

• 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 be

compatible with use of cooling water• Azeotropic composition and latent heat

important as they govern the heat duty ofthe 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 thecolumn as this can lead to problems inensuring 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<1.1

or spread of components with differentstructures, 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 theseproducts directly

• If components partially immiscible,consider liq-liq extraction or decanting

Column Profiles

• Typical profile

Column Profiles

• Zone 1: near top• Much benzene, ethanol less volatile than

water – effect ethanol/water separation• Slight increase in water at feed plate due to

water in feed• Benzene has intermediate volatility

Column Profiles

• Zone 2: just below feed• Ethanol and benzene almost constant, no

fractionation (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. Strictly

E/B min boiling azeotrope is stripped fromexcess ethanol

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