When the temperature of the vapor reaches 78-79°C (172-174°F), it is time to start collecting the main product. The temperature will stay stable at this level throughout most of the main run. The product can contain 90% ethanol if a very low heat setting is maintained and the column is well insulated.
Let's take a look at how the fractionating column affects the time taken to process the 20 liters of 40% ethanol. The pot still took just over 5 hours to produce 12 liters of 60% product (35ml/min at 750 watts. With the fractionating still, it takes around 2^ hours to collect just over 8 liters of 90% product. The same heat setting that produced 40 ml/min in the pot still produces 55ml per minute in the fractionating still. Half the time for a stronger and cleaner product! This is because the action in the column transfers heat from water to the more easily vaporized ethanol. Since the energy needed to vaporize water is 2.6 times greater than that for ethanol (540 cal/gm vs. 204), this heat transfer results in the production of more ethanol and less water. Given the two products (12 liters of 60% vs. 8 liters of 90%), the fractionating still is 2.3 times more energy efficient than the pot still! This theoretical calculation ignores the heat losses from the uninsulated pot still, which increases the relative efficiency of the fractionating still.
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