page_title: Azeotrope
section_title: Pressure swing distillation
hierarchical_section_title: Azeotrope / Separation of constituents / Pressure swing distillation
caption_attribution_description: Subject: Graph illustrating pressure-swing distillation Created with Gimp 2.2 on 14 April 2007 Status: Released to public domain
context_page_description: An azeotrope or a constant boiling point mixture is a mixture of two or more liquids whose proportions cannot be altered or changed by simple distillation. This happens because when an azeotrope is boiled, the vapour has the same proportions of constituents as the unboiled mixture. Because their composition is unchanged by distillation, azeotropes are also called constant boiling point mixtures.
  Some azeotropic mixtures of pairs of compounds are known, and many azeotropes of three or more compounds are also known. In such a case it is not possible to separate the components by fractional distillation. There are two types of azeotropes: minimum boiling azeotrope and maximum boiling azeotrope. A solution that shows greater positive deviation from Raoult's law forms a minimum boiling azeotrope at a specific composition. For example, an ethanol-water mixture on fractional distillation yields a solution containing at most 97.2% by volume of ethanol. Once this composition has been achieved, the liquid and vapour have the same composition, and no further separation occurs.
context_section_description: A hypothetical azeotrope of constituents X and Y is shown in the adjacent diagram. Two sets of curves on a phase diagram one at an arbitrarily chosen low pressure and another at an arbitrarily chosen, but higher, pressure. The composition of the azeotrope is substantially different between the high- and low-pressure plots – higher in X for the high-pressure system. The goal is to separate X in as high a concentration as possible starting from point A. At the low pressure, it is possible by progressive distillation to reach a distillate at the point, B, which is on the same side of the azeotrope as A. Note that successive distillation steps near the azeotropic composition exhibit very little difference in boiling temperature. If this distillate is now exposed to the high pressure, it boils at point C. From C, by progressive distillation it is possible to reach a distillate at the point D, which is on the same side of the high-pressure azeotrope as C. If that distillate is then exposed again to the low pressure, it boils at point E, which is on the opposite side of the low-pressure azeotrope to A. So, by means of the pressure swing, it is possible to cross over the low-pressure azeotrope.
  When the solution is boiled at point E, the distillate is poorer in X than the residue at point E. This means that the residue is richer in X than the distillate at point E. Indeed, progressive distillation can produce a residue as rich in X as is required.
  In summary:
  
  
   
  1. Low-pressure rectification (A to B)
  
  2. High-pressure rectification (C to D)
  
  3. Low-pressure stripping (E to target purity)
  
   
  Rectification: the distillate, or "tops", is retained and exhibits an increasingly lower boiling point.
  Stripping: the residue, or "bottoms", is retained and exhibits an increasingly higher boiling point.
   
  Note that both azeotropes above are of the positive, or minimum boiling type; care must be taken to ensure that the correct component of the separation step is retained, i.e. the binary phase-envelope diagram (boiling-point curve) must be correctly read.
  A mixture of 5% water with 95% tetrahydrofuran is an example of an azeotrope that can be economically separated using a pressure swing – a swing in this case between 1 atm and 8 atm. By contrast the composition of the water to ethanol azeotrope discussed earlier is not affected enough by pressure to be easily separated using pressure swings and instead, an entrainer may be added that either modifies the azeotropic composition and exhibits immiscibility with one of the components, or extractive distillation may be used.
caption: Azeotrope composition shift due to pressure swing.
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