14
BÜCHI
Rotavapor
R-3000
5.3 Selecting the conditions of distillation
Rule of thumb
For optimum conditions of distillation, the energy transferred
to the distillation from the bath must be removed again by the
condenser. To guarantee this relationship it is advisable to
apply the following rule of thumb:
Cooling water
Vapor
Bath
Difference
D
T1 Difference
D
T2
20 °C
40 °C
60 °C
20 °C 20 °C
How do you obtain these conditions?
•
Have the cooling water (no warmer than 20 °C) flow through
the condenser at a rate of around 40 – 50 l / h
•
Select the working vacuum so the boiling point of the sol-
vent is 40 °C. Consult the following solvent table for the
corresponding pressure.
•
Set the bath temperature to 60 °C.
Advantages of a bath temperature of 60 °C
•
You can handle the evaporation flask without risk of scalding
•
Water does hot evaporate from the heating bath at such
a high rate
•
The heating bath energy is utilized at a good level of
efficiency
Optimization of the distillation
Do not make any adjustments until the heating bath reaches
a temperature of 60 °C.
Depending on the solvent to be distilled, it may be necessary
to optimize the distillation by making certain adjustments. The
goal must be: Condensation of solvent between 2/3 to 3/4 of
the available condensing coils.
Basically there are two ways to optimize the distillation:
1. Slowly lower the pressure further (increase the vacuum)
This lowers the boiling point (and hence the vapor tempe-
rature) and raises the
D
T2 value. The result is a higher
efficiency of distillation.
or
2. Raise the bath temperature
This also raises the
D
T2 value (resulting in a higher efficiency
of distillation). At the same time, however, raising the bath
temperature increases the amount of heat loss from the
bath due to the bigger differential between the bath tem-
perature and the ambient temperature.
5 Operating the Rotavapor
Figure 12: Conditions of distillation
D
T (min. 20 °C)
D
T (min. 20 °C)
1
2
