Rectification of two component solvents using rotary evaporator

1st April 2013

Rotary evaporators are mostly used to either separate a solvent from a non-evaporating product or to recycle the solvents for further use.

The process is basically a single step distillation where the generated vapour is condensed directly. The separation of mixtures with close to boiling points is thus not possible ­ hence mixtures are disposed of which results in relatively high operating costs.

Several processes work with mixtures of different solvents, such as extractions using ethanol/water or cleaning processes using isopropanol and water. The fluids of these mixtures cannot be separated with a single step distillation, as the boiling points are too close to get to a reasonable purity of the components for further use. So assuming that a 50 per cent (w/w) ethanol/water mixture has to be separated into two fractions of equal weight, the condensate has a concentration of 71 per cent ethanol, whereas the residual liquid has a concentration of 29 per cent.

A typical approach to separating these mixtures is to use a fractionating or rectification column. The principle consists in applying a counter-current stream of rising vapour and falling condensate from a head condenser. The vapour is accumulated with lower boiling solvent whereas the condensate is accumulated by the higher boiling solvent. The reflux ratio of the head condenser is an important parameter to evaluate the performance of the column.

The following set-up involved a Büchi Rotavapor R-220 with 20 litre evaporating flask. A new glass assembly based on a packed column was used to increase the separation performance of a two fluid mixture. The glass assembly consisted of a distribution piece, a column with perforated tray, a packed bed which consisted of glass Raschig rings, a head condenser and the receiving assembly.

A high ethanol concentration of up to 91 per cent in the distillate shows the increased separation performance due to the packed column. Even an initial mixture of 12.8 per cent ethanol can be concentrated to 89.6 per cent.

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Büchi Labortechnik AG is based in Flawil, Switzerland.




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