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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
Azeotropic distillation - examples
• Form heterogeneous ternary azeotrope withbenzene to separate ethanol/water
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Azeotropic distillation - examples
• Form heterogeneous binary azeotrope of butene
with 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 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
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 propane
and 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
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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)