Fractional Distillation, Azeotropes and Steam Distillation

1Oil from leaves without burning it

Horizon SOPYRWA in Musanze grows geranium, patchouli, lemongrass and eucalyptus as crops for essential oils. The oils are mixtures of volatile compounds that boil well above 100 °C; eucalyptol, the main compound in eucalyptus oil, boils at 176 °C. Heating leaves to such temperatures would scorch them and damage many of the compounds. Yet if steam is blown through the leaves, the oil comes over with the water below 100 °C and floats on top of it in the receiver. Raoult's law, and its failure for liquids that do not mix, explain both this trick and the more familiar fractional distillation.

2Boiling point–composition diagrams and fractional distillation

A mixture boils when its total vapour pressure equals atmospheric pressure. For an ideal mixture, Raoult's law gives both the boiling temperature of each liquid composition and the composition of the vapour that comes off. Plotting these at 101.3 kPa gives two curves (Figure 3.1): the liquid curve (the temperature at which a liquid of that composition boils) and, above it, the vapour curve (the composition of the vapour in equilibrium with it).

Follow the staircase. A liquid with $x(\text{benzene})=0.20x(\backslash text\{benzene\})\; =\; 0.20$ boils at 102 °C and gives vapour with $x=0.38x\; =\; 0.38$. Condense that vapour and boil it again: it boils at 96 °C and gives vapour with $x=0.60x\; =\; 0.60$. Two more steps give 0.79 and then 0.91. A fractionating column, packed with glass beads or rings, performs these steps automatically: rising vapour condenses on the cooler packing, the liquid runs down, meets hotter vapour and partly re-evaporates. The column is hottest at the bottom and coolest at the top, so the more volatile component (lower boiling point) reaches the top and distils first, and the less volatile one runs back into the flask. A longer column provides more theoretical plates and a better separation. The same principle separates crude oil into fractions and liquid air into nitrogen and oxygen.

Figure 3.1 Boiling point–composition diagram for benzene and methylbenzene at 101.3 kPa, calculated from Raoult's law. Starting from $x(\text{benzene})=0.20x(\backslash text\{benzene\})\; =\; 0.20$, four theoretical plates raise the benzene content of the distillate to 0.91.

3Azeotropes: where distillation stops

When a mixture deviates strongly from Raoult's law, its vapour-pressure curve has a maximum or a minimum (Lesson 2), and so its boiling-point curve has a minimum or a maximum. At that point the liquid and vapour curves touch: the vapour has the same composition as the liquid, and no further separation is possible.

• Minimum-boiling azeotrope (large positive deviation). Ethanol–water forms one at 95.6 % ethanol by mass, boiling at 78.2 °C, just below pure ethanol (78.4 °C). When a dilute ethanol solution, such as a fermented sugar solution, is fractionally distilled, the azeotrope collects at the top of the column, and water is left in the flask. Distillation alone can never give ethanol purer than 95.6 %. "Absolute" ethanol is made by removing the last water in other ways: drying with calcium oxide or molecular sieves, or adding a third liquid that changes the phase behaviour.
• Maximum-boiling azeotrope (large negative deviation). Nitric acid–water forms one at 68 % $\mathrm{H}\mathrm{N}{\mathrm{O}}_3\backslash mathrm\{HNO\_3\}$ by mass, boiling at 120.5 °C, higher than water (100 °C) or pure nitric acid (83 °C). Whatever the starting composition, the component in excess over the azeotrope distils off, and the residue in the flask approaches 68 % acid. This is why concentrated nitric acid is sold as about 68 %. Hydrochloric acid behaves in the same way: its azeotrope is 20.2 % HCl, boiling at 108.6 °C.

Figure 3.2 Boiling point–composition diagrams for two azeotropes (sketches, not to scale; the dip for ethanol–water is only 0.2 °C deep and is drawn larger). At the black dot the liquid (blue) and vapour (orange) curves touch.

4Steam distillation: immiscible liquids

When two liquids do not mix, each forms its own layer, and molecules of each escape from their own surface as if the other liquid were not there. Raoult's law does not apply: each liquid exerts its full vapour pressure, whatever the amounts, and the total vapour pressure is the sum:

$$p=p_A^{\circ }+p_{{\mathrm{H}}_2\mathrm{O}}^{\circ }p\; =\; p\_A^\{\backslash circ\}\; +\; p\_\{\backslash mathrm\{H\_2O\}\}^\{\backslash circ\}$$

The mixture boils when this sum reaches atmospheric pressure. Because both terms contribute, that happens below the boiling point of either liquid, and so below 100 °C (Figure 3.3). The temperature stays constant as long as both liquids are present, whatever their proportions.

In the vapour, the ratio of the amounts of the two substances equals the ratio of their vapour pressures, so the mass ratio in the distillate is

$$\frac{n_A}{n_{{\mathrm{H}}_2\mathrm{O}}}=\frac{p_A^{\circ }}{p_{{\mathrm{H}}_2\mathrm{O}}^{\circ }}\Rightarrow \frac{m_A}{m_{{\mathrm{H}}_2\mathrm{O}}}=\frac{p_A^{\circ }{M}_A}{p_{{\mathrm{H}}_2\mathrm{O}}^{\circ }{M}_{{\mathrm{H}}_2\mathrm{O}}}\backslash frac\{n\_A\}\{n\_\{\backslash mathrm\{H\_2O\}\}\}\; =\; \backslash frac\{p\_A^\{\backslash circ\}\}\{p\_\{\backslash mathrm\{H\_2O\}\}^\{\backslash circ\}\}\; \backslash qquad\backslash Rightarrow\backslash qquad\; \backslash frac\{m\_A\}\{m\_\{\backslash mathrm\{H\_2O\}\}\}\; =\; \backslash frac\{p\_A^\{\backslash circ\}\backslash ,M\_A\}\{p\_\{\backslash mathrm\{H\_2O\}\}^\{\backslash circ\}\backslash ,M\_\{\backslash mathrm\{H\_2O\}\}\}$$

Even though $p_A^{\circ }p\_A^\{\backslash circ\}$ is small, a large molar mass $M_{A}M\_A$ means that a useful mass of A comes over. Steam distillation is used for substances that are volatile, immiscible with water and would decompose at their normal boiling points: essential oils from leaves, flowers and peel, and in the laboratory to purify phenylamine or nitrobenzene. Measuring the mass ratio and the temperature also gives the molar mass of A.

Figure 3.3 Why bromobenzene (boiling point 156 °C) steam-distils at 95.3 °C. The total vapour pressure (orange) is the sum of the water (blue) and bromobenzene (green) curves, and it reaches 101.3 kPa about 5 °C before water alone would boil.