From this point on, an almost pure azeotropic mix of ethanol and water will be coming through and may be collected as the main product. The temperature should remain absolutely stable throughout the collection of the main product, and you can increase the rate of collection to around 4 drops per second.
We give guidelines in terms of drops per second, but the size of a drop is affected by many things - its composition, its temperature, and the size of the tube it is dripping from. The real measure is the production rate, remembering that we want to collect the main body of ethanol at 90% reflux. 750 Watts will produce about 50 ml of 95% ethanol per minute, so you want to be collecting between 4 and 5 ml per minute to establish a reflux ratio of 90%.
The best way to measure this is to establish a slow flow, begin collecting it in a small graduated cylinder, and measure the time it takes to collect 10 ml. Keep adjusting the valve until it takes two to two and a half minutes. When you are satisfied with the collection rate, count the number of drops you get in 10 seconds. Do this three or four times, and take the average.
In the future, you can reset the still to the same 90% reflux ratio by adjusting the valve until you have the correct number of drops falling in 10 seconds. This adjustment may be made quite rapidly.
In our experience, the correct rate of collection is found at about 35 drops in 10 seconds - not far off the 4 drops we recommended above! For the best accuracy, though, you should make this measurement yourself and calibrate your system.
Calculating on the basis that the 20 liters put in the boiler contains 8 liters of ethanol, it will take around 14 hours to collect all of it. You could get slightly better purity if you reduced the draw-off rate to 2 drops a second, but the run would take twice as long.
To achieve the highest purity, you will take the slow road, and you probably won't want to watch the still for 16-18 hours, not to mention the extra attention required near the end, checking for the onset of tails. Fortunately, all the toxic compounds and heads have been removed, so it's perfectly safe to stop the run at any time and resume it later. You will have to equilibrate the column again, but since the most volatile component is now ethanol, it will only take an hour at 100% reflux. Once equilibrium is achieved, you can start collecting product immediately.
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