High-temperature and isothermal distillation

In the distillation of substances of low volatility at atmospheric and reduced pressures a short column is all that is employed and its function is mainly to act as spray trap. Coarse packing, placed on a gauze support, is used plate columns are unsuited for such purposes. It is obvious that the column should be provided with a heating jacket and that no part of the apparatus, from the stiU pot to the condenser, should be unheated, so that there may be no partial condensation. The receivers should also be capable of being warmed. A useful measure is to heat the take-off valve by radiation from an infrared source or by a hot air blower (hair dryer), so as to prevent it from sticking. Better still is the use of magnetic valves. 

In laboratory high-temperature distillation the boiler is as a rule heated indirectly by an oil, Wood s metal or molten salt bath [111], controlled by a contact thermometer. (The maintenance of a constant high temperature in such a bath is, however, usually not an easy matter.) Table 46 shows the temperatures that are attainable with various baths (c/. section 7.7). 

A molten salt bath has the advantage that it can be used over a wide temperature range and that it does not give rise to objectionable vapours. However, the flask must be removed from the bath when heating is stopped and the adhering salt washed off with water. 

Apparatus for high-terape-rature distillation with boiling condenser (Stage) 

In isobaric distillation — the form normally practised — the pressure is kept constant and the distillate passes over with rising temperature. In isothermal distillation the still pot temperature is kept constant by a thermostat and the pressure is pro- 

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