Zymurgy Bob's Distillation Facts

Understanding and Predicting the Potstill Run

Although it's perfectly possible to do a still run with no idea of what will happen, when it will boil at what temperature, what the percentage of alcohol by volume (%ABV) will come out of the still, and how long distillation will take, it's good insight to use your knowledge of distillation to predict some of these things. With that in mind, let's look at what we know, and how that will allow us to predict the still run.

The first two inputs are the amount of wash, and whether it will be a stripping run or a spirit run. Very generally, a stripping run will take far less time than a spirit run, and the more wash, the more time it takes to distill it.

Knowing the %ABV of the wash is a big predictor of the numbers a still run will produce. Let's say, for example, that we know the wash %ABV to be 10%. Using the graph in Figure 1, locate 10% on the bottom line of the graph.

Follow the vertical line up from that 10% label all the way up to the blue curve, the boiling point
temperature curve, and note the point where they cross. You'll see that the 10% line, the blue curve
** and the horizontal line from the 93C label** all pass through that point. That tells us that
your 10% ABV wash will boil at 93 C.

Now follow that same horizontal line that represents 93C to the right, until it meets the red curve, the vapor temperature/%ABV curve. From this new point on the red curve, follow the vertical line down to the %ABV scale across the bottom of the graph, and note that your vertical line represents a vapor (and distillate) ABV of 53%.

So, if you put a 10%ABV wash in your boiler, it will start boiling at 93C and the first distillate will be 53%ABV, a significant increase in ethanol concentration. You can see from this that concentrating ethanol in the distillate (one of the most important parts of distillation) is built into the physics of the process, at least in the early parts of the still run.

While we don't always have the luxury of knowing the wash %ABV before we start distilling, specific gravity data acquired during the fermenting of the beer, wine, or sugar wash can be used to make a very good guess about that wash %ABV. Specific gravity, another name for density, can (and should) be measured by the brewer using a common winemaker's hydrometer, available at any homebrew supply store.

The hydrometer is a simple glass bulb and stem that floats in your fermenting wash, and measures specific gravity by how low or high it floats in that wash. A numbered scale inside the glass can be read at the surface of the liquid to get the numeric value of the specific gravity (SG). During a fermentation, the SG will usually be between the numbers .990 and 1.200. The SG of the wash at the time fermentation is started is the Original Gravity, or OG.. When the fermentation is finished, the wash SG is now the FG, the Final Gravity. The approximate %ABV of the final wash can be calculated by subtracting the FG number from the OG number, and multiplying that number by 131. If the OG is 1.084 and the FG is 1.008, then OG-FG=.076. .076 times 131 = 9.96, so the wash %ABV is about 10%ABV.