Fractional Distillation: the Secret to Making Moonshine

Fractional distillation is the chemical process of separating a mixture into its component parts.  We all know that moonshine, a.k.a rotgut, is distilled liquor produced against the law.  Perfume is also distilled from plants and dead animals by extracting the essential oils using fractional distillation.  Crude oil is also made usable by purifying it with fractional distillation.  Even water can be purified and desalinated by fractional distillation.

Distillation relies on the fact that different components boil at different temperatures. In essence, fractional distillation is the technique of taking advantage of these differences in boiling temperatures so that only one compound is boiling off of the mixture at any given time.

The mash

It all starts with the mash.  To make rotgut, or even commercial liquor, you need to have some fermenting, rotting mash in a bucket.  Take a bucket, fill it with water, berries, hops, corn, barley, rice, sugar, wine, or anything else that has sugar a healthy yeast can eat.  Cover the bucket with a cloth and let sit a week or more.  The yeast will eat the sugar and excrete alcohol until the mash is nice and musty and fermenting.  After several days, you’ll have something alcoholic that probably tastes like rot.

Batch distillation

The simplest form of distillation is batch distillation.  In this process, the mash is heated up until it begins boiling.  The boiling point of water is 100 degrees Celsius.  The boiling point of alcohol is around 78 degrees Celsius.  The mash will most likely start to boil at some temperature between that of alcohol and that of water.

The gas released by the boiling liquid is a mixture of all the compounds in the mash.  However, the compounds in the gas are not present in the same proportions as they were in the liquid.  The compounds with the lowest boiling points (and highest vapor pressures) will be present in higher concentration in the gas than they were in the liquid.  In laymen’s terms, substances with lower boiling points tend to boil off first.

In our example, let’s say the mash is originally 10% alcohol and 90% water.  The gas that evaporates when this liquid boils will be somewhere near 55% alcohol, since alcohol is more volatile than water and will vaporize more readily than water.

If you can capture this gas and condense it into a liquid again, you would have a stronger drink than the original mash.

And that’s exactly what the condenser in a distillation apparatus does.  It is usually a tube that passes through a bath of cold water.  The condenser tube cools the gas that passes through it so that it changes back into its liquid phase.  This more alcoholic liquid, the distillate, is then siphoned off into another container.

The result of this process, the distillate, can either be drunk straightaway, aged in a barrel, or put through the entire batch distillation process again a second or third time to produce even stronger drinks.

Since the gas that evaporates when you boil any mixture always has a higher concentration of the most volatile substances than the liquid did, you can be sure that each time you distill an alcoholic liquid, the gas produced has a higher concentration of alcohol than did the starting liquid.  So each time you distill the liquid, you produce a more alcoholic liquid as the distillate.

Looking at a chart of the relationship between alcohol concentration and boiling point, you can see that a liquid of 10% alcohol will boil at about 93 degrees Celsius.  If you follow horizontally from the liquid curve at 93 degrees to the vapor curve of the graph, you will see that the vapor released from the boiling of this liquid will be 55% alcohol.

The less pure the distillate (i.e. the fewer times it has been distilled), the more it has a characteristic flavor that results from all the other substances that are present in the mash.  These impurities are important for the flavor of many liquors, such as whiskey and brandy.  Some impurities, types of alcohol molecules historically found in low-quality moonshine, can make you go blind.

Each compound in the liquid mixture has a particular temperature range at which it evaporates out of the boiling liquid.  If you know the temperature range at which predominately ethanol, drinking alcohol, boils out of the mash, then you can make sure that you only collect distillate that is produced in that temperature range.  This is a good way to avoid going blind.

Fractional distillation

The problem with batch distillation, the simplest type of distillaton, is that it is time consuming and you get less and less liquid each time you distill.  If you wanted to produce a high alcohol liquor, you would need to distill the liquid multiple times.  At the end, you would have spent a lot of time producing a very small amount of booze.

A reflux distillation apparatus is the creative solution to this problem.  Reflux distilleries effectively distill the liquid multiple times all in one go.  The secret is the fractionating column.

Just as in a batch distillation, a fractional distillation has some boiling liquid that evaporates as gas, gets condensed, and siphoned off.  All the parts are the same, except that with fractional distillation, the fractionating column allows the liquid to evaporate and condense multiple times all within one tube, effectively purifying the liquid much more than the simple batch distillery could.

Think of it this way.  The air immediately above the boiling liquid is very hot.  Remember, the boiling point of a 10% alcohol liquid is about 93 degrees Celsius.  The air at the top of the fractionating column, further away from the hot liquid, is a bit cooler.  As the gas evaporates off of the liquid it travels up the column, cooling down along the way the further it gets from the heat source.  At some point, the gas cools down enough that it condenses on the wall of the column, changing back into liquid.  The liquid starts to drip down the wall of the column.

When the gas condenses on the side of the column and starts to drip back down, it heats up as it gets closer and closer to the boiling liquid.  Now remember that the gas that formed this condensed liquid had a higher concentration of alcohol than the original liquid, because alcohol is more volatile, and evaporates more readily. To be exact, the gas is 55% alcohol.  So this condensed liquid is also more alcoholic than the original.

Since it has a higher percentage of alcohol and since alcohol has a lower boiling temperature than water, the boiling point of this condensed liquid is lower than that of the the original liquid.  Using our alcohol/boiling point chart, you can see that the boiling point of a 55% alcohol liquid is only about 82 degrees Celsius.  So as the condensed liquid drips down the column, heating up along the way, it will reach its new boiling point, 82 degrees, before it reaches the bottom of the column, which as you know is a whopping 93 degrees.

So the condensed liquid will re-evaporate off of the side of the column before it reaches the bottom.  And when it re-boils, you know that the percent alcohol in the evaporated gas is higher than that of the liquid, because alcohol evaporates more readily than water.  To be exact, the gas that evaporates from a 55% alcohol solution will be around 83% alcohol.

So the newly evaporated gas has an even higher concentration of alcohol than did the condensed liquid that spawned it.  This new evaporated gas will again travel up the column, cool down along the way, and eventually condense and start dripping down the side again.  However, the boiling point of this new liquid is even lower than the last time since it has a higher concentration of alcohol.  It is 79 degrees Celsius. That means that it will re-boil off of the side of the column even more easily than last time.  It does not have to drip as far down the column before it reaches its new boiling point.  And the gas from that re-boiling will be even more alcoholic than last time.

This cycle repeats itself an infinite number of times, until the gas in the column is effectively divided into sections.  The gas lingering near the top of the column is very highly concentrated alcohol, around 96%, with possibly a few impurities of molecules that have a very similar boiling point to alcohol.  The gas at the bottom of the column has less alcohol and more water and other lower boiling point impurities.

If you have your fractional distillation apparatus set up such that the condenser tube is connected to the fractionating column just at the point along the column where the temperature of the gas in the column is that of the exact boiling point of alcohol, you will be siphoning off the most purely alcoholic part of the vapor.  The condenser will cool down the gas so it becomes liquid, and this liquid you can then enjoy as a very nice and strong, pure alcoholic spirit.

Served with pickles, honey, and bread, it just may be delicious.