Azeotropic distillation In chemistry , azeotropic distillation is any of a range of techniques used to break an azeotrope in distillation . In chemical engineering , azeotropic distillation usually refers to the specific technique of adding another component to generate a new lower-boiling azeotrope that is heterogeneous , such as the example below with the addition of benzene to water and ethanol . Example - distillation of ethanol/water A common distillation with an azeotrope is the distillation of ethanol and water . Using normal distillation techniques , ethanol can only be purified to approximately 96% ( hence the 96% ( 192 proof ) strength of some commercially available grain alcohols ) . Once at a 96/4% ethanol/water concentration the vapor from the boiling mixture is also 96/4% . Further distillation is therefore ineffective . Some uses require a higher percentage of alcohol , for example when used as a gasoline additive . The 96/4% azeotrope needs to be `` broken '' in order to refine further . Material separation agent One method is the addition of an `` MSA '' , a material separation agent . The addition of benzene to the mixture changes the molecular interactions and eliminates ( `` breaks '' ) the azeotrope . The drawback is that another separation is needed to remove the benzene . Pressure-swing distillation Another method , pressure-swing distillation , relies on the fact that an azeotrope is pressure dependent . It also depends on the knowledge that an azeotrope is not a range of concentrations that can not be distilled , but the point at which activity coefficients are crossing one another . If the azeotrope can be `` jumped over '' , distillation can continue , although because the activity coefficients have crossed , the water will boil out of the ethanol . To `` jump '' the azeotrope , the azeotrope can be moved by altering the pressure . Typically , pressure will be set such that the azeotrope will be closer to 100% concentration . For ethanol , that may be 97% . Ethanol can now be distilled up to 97% . It will actually be distilled to something slightly less , like 96.5% The 96.5% alcohol is then sent to a distillation column that is under a different pressure , one that pulls the azeotrope down , maybe to 96% . Since the mixture is already above the 96% azeotrope , the distillation will not get `` stuck '' at that point and the ethanol can be distilled to whatever concentration is needed . Molecular sieves Main article : molecular sieve For the distillation of ethanol for gasoline addition , the most common means of breaking the azeotrope is the use of molecular sieves . Ethanol is distilled to 96% , then run over a molecular sieve which absorbs water from the mixture . The concentration is now above 96% and can be further distilled . The sieve is heated to remove the water and reused . See also Extractive distillation Batch distillation Distillation Extractive distillation Fractional distillation References ^ Kister , Henry Z. ( 1992 ) . Distillation Design , 1st Edition , McGraw-Hill . ISBN 0-07-034909-6 . Categories : Distillation | Industrial processes | Chemical engineering | Unit operations In other languages : Español | Français Kister , Henry Z. Distillation Design 1st Edition McGraw-Hill 1992 ISBN 0-07-034909-6 molecular sieve 