Vaporization

The alcohols in the charge begin to vaporize around specific temperatures. If by themselves they would be:

• Acetone 56.5°°C (134°F)
• Methanol (wood alcohol) 64°C (147°F)
• Ethyl acetate 77.1°C (171°F)
• Ethanol 78°C (172°F)
• 2-Propanol (rubbing alcohol) 82°C (180°F)
• 1-Propanol 97°C (207°F)
• Water 100°C (212°F)
• Butanol 116°C (241°F)
• Amyl alcohol 137.8°C (280°F)
• Furfural 161°C (322°F)

Once together, a mixture of several of them will be slightly different however. You no longer get them coming off separately, but always as a mixture. Fortunately for us though, each will tend to dominate around its boiling point temperature, thus we know whats "mostly" coming off at that point. By tracking the temperature of the vapor, you have a fairly good idea when you're collecting the ethanol you're after (78-82 °C), vs when it is starting to get lean and you're into the higher alcohols.