Fractional Distillation and Azeotropes

Introduction

Simple distillation takes advantage of unequal liquid and vapour compositions. Fractional distillation repeats that vapour–liquid equilibration many times to improve separation. Its limit is especially clear at an azeotrope, where the equilibrium vapour and liquid have the same composition. Understanding the phase diagram explains both why a column works and why a tall column cannot always deliver a pure component.

Core explanation

For a nonazeotropic binary liquid mixture, the vapour above a liquid is usually enriched in the more volatile component. Condense that vapour, vaporise it again, and the next vapour may be richer still. A fractionating column organises many such contacts between rising vapour and descending liquid. An ideal equilibrium contact is called a theoretical plate or stage; a real tray or section of packing may not achieve ideal equilibrium. The number of effective stages, heat input and reflux ratio together control separation performance.

Reflux is condensed overhead liquid returned down the column. It contacts rising vapour, allowing mass and heat transfer. More reflux often improves separation for a given number of stages, at an energy and operating cost. The reboiler supplies vapour at the bottom, while a condenser removes heat at the top. The column is not a series of isolated equilibrium flasks; material and energy balances connect its stages, so realistic design requires more than simply reading one tie line.

On a constant-pressure T–x–y diagram, each equilibrium stage links a liquid composition x and vapour composition y at the same temperature. Repeated contacts step across the two-phase relationships toward the volatile end. If the bubble and dew curves meet at an interior composition, x=y there. This is an azeotrope. At fixed pressure an ordinary distillation step at that composition produces vapour of the same composition as the liquid, so there is no incremental enrichment through equilibrium vaporisation. A finite or even idealised infinite sequence of ordinary stages cannot cross that composition by the same fixed-pressure VLE route.

A minimum-boiling azeotrope has a boiling-temperature minimum at its special composition; its vapour pressure has a corresponding maximum at fixed temperature under the usual interpretation. A maximum-boiling azeotrope has a temperature maximum and corresponding lower pressure tendency. Strong positive departures from Raoult-law behaviour often lead to minimum-boiling azeotropes, while sufficiently strong negative departures can yield maximum-boiling ones. These are tendencies involving activities and intermolecular interactions, not a universal rule from a single hydrogen-bond label.

Ethanol and water provide a familiar minimum-boiling azeotrope at ordinary pressure. Its exact composition changes with pressure, so a quoted value belongs to specified conditions. Ordinary fractional distillation at fixed atmospheric pressure cannot make arbitrarily pure ethanol starting from an ethanol–water feed. Alternative processes may change pressure, add an entrainer, use adsorption or membranes, or apply another separation principle. Such methods do not violate equilibrium; they change the equilibrium constraints or introduce additional components.

“Boils at a constant temperature” alone is not a sufficient definition of azeotropy. A pure liquid also has a fixed boiling point at fixed pressure. The distinguishing binary-mixture condition is equality of coexisting liquid and vapour compositions, x i=y i. Nor does an azeotrope imply that all compositions are inseparable; distillation can enrich feed mixtures on either side toward the azeotrope or another product endpoint.

Step-by-step reasoning

Mark liquid x and vapour y on a fixed-pressure VLE diagram. For a feed away from an azeotrope, compare the two compositions to see which component rises in vapour. Represent repeated equilibration as sequential stages. Locate any interior point where x=y; recognise that enrichment stalls there under the same pressure and ordinary distillation conditions.

Visual explanation

On a T–composition chart, sketch bubble and dew curves touching at an interior minimum or maximum. Horizontal equilibrium ties become shorter as they approach that meeting point. At the azeotrope the tie-line length is zero because both phases have the same composition, even though distinct liquid and vapour can still coexist.

Real-world analogy

Climbing a staircase can move one floor at each step, but if the staircase ends at a locked landing, adding more identical steps cannot take you through the wall. A distillation column adds equilibrium stages; an azeotrope removes the composition difference that drives those stages. Other separation methods are like taking a different route, not extending the same staircase.

Real-world example

Industrial solvent recovery may use a fractionating column to enrich a volatile solvent, then a drying unit to remove the residual component near an azeotropic limit. The column's contribution follows VLE enrichment, while the drying unit uses a different selectivity. Process engineers specify pressure because the azeotrope and relative volatility can shift with it.

Why?

Why does x=y block ordinary distillation? Condensing vapour does not change its composition when it is already identical to its parent liquid. Reboiling that condensate at the same pressure reproduces the same composition. Repetition cannot create a composition difference absent from each equilibrium stage.

Common misconception

An azeotrope is not a chemical compound with fixed molecular formula; it is a particular equilibrium mixture composition at specified pressure. “More plates always reach a pure component” is also false when an azeotropic barrier lies along the proposed ordinary distillation path.

Worked example

At one pressure, a binary mixture at x A=0.40 produces equilibrium vapour y A=0.65. One ideal evaporation–condensation step enriches A from 0.40 to 0.65. Suppose at x A=0.80 the diagram instead shows y A=0.80. At that composition condensation and revaporisation do not enrich A further. The latter state is an azeotropic composition at the specified pressure, not proof that A and B have become a new pure substance.

Quick check

1. What equality identifies an azeotrope in a binary vapour–liquid equilibrium diagram? Answer: The coexisting vapour and liquid have the same composition, y A=x A (and consequently y B=x B), at the stated pressure or temperature.

Exam focus

Define theoretical plate as an equilibrium stage, not a literal metal plate. Show whether the azeotrope is a temperature minimum or maximum on a fixed-pressure diagram. State the pressure when discussing azeotropic composition and explain the separation limit through x=y, not through an unsupported claim about column height.

Advanced insight

Relative volatility approaches unity at a binary azeotrope. The Gibbs–Duhem relation constrains activity-coefficient behaviour, and thermodynamic consistency links extremal pressure or boiling temperature with x=y under ordinary binary VLE. Pressure-swing distillation may work when the azeotropic composition moves sufficiently with pressure, but it requires separate equilibrium and process analysis.

Summary

Fractional distillation uses repeated equilibrium vapour–liquid contacts, often aided by reflux, to enrich components. An azeotrope is a mixture composition where x=y at specified conditions; ordinary fixed-pressure distillation cannot enrich through it. Minimum- and maximum-boiling azeotropes appear as corresponding extrema on T–composition diagrams.

Practice questions

1. A liquid has x A=0.30 and its vapour y A=0.55. Which phase is richer in A? Answer: The vapour, so condensing it can provide an A-enriched liquid in an ideal equilibrium step. 2. At x A=0.72, a binary VLE diagram gives y A=0.72. What is special about this composition? Answer: It is an azeotropic composition at the specified conditions, so an ordinary equilibrium evaporation step gives no enrichment. 3. Does increasing reflux eliminate an azeotrope at fixed pressure? Answer: No. Reflux changes contact and energy usage but cannot change the fundamental equality of liquid and vapour compositions at the azeotrope. 4. How does a minimum-boiling azeotrope appear on a fixed-pressure temperature diagram? Answer: The bubble and dew curves meet at an interior temperature minimum, with equal liquid and vapour compositions there.