The solution having to be distilled, stored in a tank of plastic material, is sent to the distillation column by a metering pump after crossing a pre-heating exchanger. The distillation column is completely of glass and it consists of a reboiler, of a sieve-tray column and of a condenser with reflux head and valve. The bottom product and the distillate are collected into two tanks after being cooled by two heat exchangers.