News Information Bulletin best@buchi Information Bulletin 15 2002 Evaporation & @Life Science

Rectification of a two component mixture of using a rotary evaporator A novel system is presented to fraction a mixture of two components based on a rotary evaporator. A packed column expands the application range to simple separation processes. www.buchi.com best@buchi EVAPORATION & LIFE SCIENCE 15 / 2002 Industrial

Rectification of a two component mixture of solvents using a rotary evaporator

Rotary evaporators are mostly used either to separate a 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 of two solvents with close is thus not possible. Hence, mixtures are disposed of, resulting in relatively high operating costs. A new glass assembly based on a packed column is presented to drastically increase the separation performance of a two fluid mixture. It was shown that a mixture of ethanol and water could be separated into more than 89% ethanol in the distillate and a remaining 6% in the water.

Author: Harry Brandenberger

Introduction Several processes work with mixtures of different solvents, such as extraction 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 pure components. x

x

x

29 71

Fig. 2: Boiling curve (black) and condensation curve (red) for a mixture of ethanol and water for different pressures. (X=Azeotropic point)

Fig 1.: x-y diagram of ethanol and water

Fig. 1 shows the x-y diagram of a mixture of ethanol and water. The x base is the composition of the liquid whereas the y base shows the composition of the vapour in thermodynamic equilibrium. So a mixture of 40% (w/w) ethanol is in equilibrium with the vapour of 74% (w/w) ethanol. It can be clearly seen that there is an at 96% (w/w) ethanol: the Distillation under vacuum has not only the advantage of a more composition of the liquid and the gaseous phase are equal. efficient energy balance due to lower boiling points, but can This means that even with an ideal thermal process, this mixture also change the position of the azeotropic point. Fig. 2 shows cannot be separated. its dependence on the boiling curve and the condensation curve best@buchi EVAPORATION & LIFE SCIENCE 15 / 2002

in the liquid vapour equilibrium diagram. The azeotrope is shifted and even disappears (Fig. 2). However, the boiling curve and condensation curve are so close that a purely thermal separation is extremely difficult.

So assuming that 50% (w/w) ethanol has to be separated into 4 two fractions of equal weight, the condensate has a concentration of 71% ethanol, whereas the residual liquid has a concentration of 29%.

A typical approach to increase the separation performance for these kinds of mixtures is to use a or rectification column. The principle consists in applying a counter- current stream of rising vapour and falling condensate from a head (Fig. 3). Due to the big surface in the column, 5 3 heat and mass transfer are very efficient. The rising vapour is accumulated with lower boiling solvent whereas the backflow of condensate is accumulated by the higher boiling solvent. The ratio of the head condenser is an important parameter to evaluate the performance of the column. 2

1

Fig. 4: Picture of the glass assembly with separation column

phase, the system was in a steady condition. The reflux ratio was in the range of 50%. Higher reflux rates lead to a better separation performance but longer separation times . Fig. 3: Schematic of packed column with mass transfer Results and Discussion Two different mixtures were investigated: ethanol / water as Experimental set-up a widely used mixture and methanol/ethanol as a ® The basic unit is a BÜCHI Rotavapor R-220 with a 20 litre thermodynamically ideal mixture. Tab. 1 shows the initial feed evaporating flask. The glass assembly consists of a distribution concentration and the distillation parameters: piece (1), a column -with a perforated tray (2), a packed bed consisting of glass Raschig rings (3), a head condenser (4), Initial mixture 47.1% (w/w) 50% (w/w) the main condenser (5) and the receiving assembly. The reflux ethanol methanol ratio is visually determined by counting the number of droplets in water in ethanol in the main condenser and the droplets of the head condenser. The ratio can be regulated by the cooling water throughput or Bath temperature 60°C60°C the temperature of the cooling medium, respectively, in the Pressure 163 mbar -> 280 mbar -> head condenser. 145 mbar *) 260 mbar *) Rotation speed 70 rpm 70 rpm The feed was filled into the evaporating flask and the distillation parameters, comprising the vacuum pressure, bath tem- *) Due to the batch process, optimal parameters change with the perature and rotation speed, were set. After a certain starting concentration in the flask best@buchi EVAPORATION & LIFE SCIENCE 15 / 2002

After distillation of approx. 50% of the weight (split into two Ethanol/Methanol fractions), the distillate and the residual liquid were weighed. The ethanol concentration was determined by an areometer Weight Concentration (density measurement method) and by the index of refraction Methanol (w/w) for the ethanol/methanol mixture. 1st Fraction 872 g 75.1 % Ethanol/Water 2nd Fraction 998 g 73.9 % Residue 2104 g 19.1 % Weight Concentration Ethanol (w/w) By transferring the data to the Txy-diagram of the ideal mixture, 1st Fraction 3308 g 90.2 % the number of theoretical fractionating trays can be graphically 2nd Fraction 325 g 89.6 % determined to be between 4.5 and 5.5. Residue 3993 g 6.5 % With the described separation system, it was possible to show The high concentration of ethanol in the distillate clearly show that with low investment costs, the rotary evaporator as a the increased separation performance due to the packed single-step distillation unit can be expanded to provide an column. Even the second fraction has a high concentration. efficient fractionating system corresponding to 4.5 - 5.5 This means that an initial mixture of 12.8% ethanol (determined theoretical trays. by the mass balance of ethanol) can be concentrated to 89.6%! By changing the reflux ratio of the head condenser, the ethanol concentration in the distillate could be increased up to 90.6%. www.buchi.com Quality in your hands

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