Heidolph LABOROTA 20 automatic - Manual (Page 72)

72
E
REMARKS ABOUT DISTILLATION
1. General
Using rotational evaporators for thermal breakdown of substances needs some basic
considerations about optimizing the distillation process. Parameters stipulated below are to be
considered for making optimal settings.

Rotational speed, evaporator flask
Increasing rotational speed will increase rate of distillation. Shorter distillation time preserves
thermal stability of your substances to be distilled.

Difference between heating bath and vapor temperatures:
This temperature difference should not be less than 20° C to maintain proper rate of distillation.
As long as your substances in distillation allow, you may even chose a higher temperature
difference.
Rule of thumb: doubling actual temperature difference doubles rate of distillation.

Temperature difference between vapor temperature and coolant:
To maintain adequate condensation rates, we recommend a 20° C temperature difference
between the solvent vapor temperature and coolant. A low temperature difference will affect
solvent recovery. For substances with high evaporation heat, you should increase temperature
difference.
2. Select vacuum control mode
2.1. p
const
vacuum control mode
The p
const
mode includes the following features:

One of the most important modes of control

Selective distillation of different substances within the scope of their physical properties

Distillation of solvent mixtures need manual pressure settings

High recovery rates

Requires knowledge of thermodynamic substance data
first choice for all standard processes

For operation & parameter settings refer to page 90

Pressure (set vacuum) and hysteresis

p (set

p) need to be set

Select pressure p as to locate boiling point of the substance to be distilled between bath
temperature and coolant temperature, make reference to General Notes. For more information
refer to table and chart on page 75.
 
p (hysteresis) value defines max. pressure rise in mbar to open the vacuum valve.
Extended open valve times results in high rates of solvent loss due to extensive gas
transfer.
Low-boiling substances require a higher

p-value (5 – 10 mbar), high-boiling substances a lower
value (1
– 5 mbar)
2.2. T
auto
vacuum control mode