What we know about Distillation
Distillation seems like a wonderful purifier to most people.
But if you consider the reality described herein, it will become
obvious to you that practical distillation falls far short of the ideal.
What's wrong with distillation?
This discussion is intended to bring to you an intuitive feel for the practical
shortcomings of distillation. The main shortcoming is that there can be
significant carryover of stuff from the starting mixture. This carryover is
the reason that makers of distilled beverage alcohol describe on the labels
of their bottles “Triple Distilled, Fifteen Times Filtered” or some other
combination of numbers. Start by asking yourself the questions “If distillation
makes things pure, why would they need to do it three times, and to filter it
so many times? What are the filters removing?”
The Ideal Simplified Concept
If you want to purify water, you turn it into a gas, leaving behind all the
dissolved solids and higher boiling temperature liquids, and then condense the
pure water vapor gas back into a liquid by bringing it into contact with a
chilled surface. Droplets form, run down the surface, and into your
collection container. You can even remove lower boiling temperature liquids
from the water by pre-heating it and exposing it to a stream of warm air.
The volatile liquids evaporate and are not carried into your boiling chamber,
where they could end up in your purified water.
Other Factors Complicate Things
While distillation is a purifying process, it is not really that simple. It
does not inevitably produce a pure result, but it does tend to reduce the
concentration of many impurities from various substances being distilled.
And how effectively it does this depends on many different factors. It might
be more meaningful to consider an inverse perspective, and say that
distillation can increase the concentration of a desired substance.
Distillation works with substances that evaporate. It is effective at
reducing the presence of impurities that are dissolved in the starting
material. (It could be entirely useless if the impurities have the same vapor
pressures and boiling points as the desired end product.) Reducing the presence
of dissolved solids is often a goal when using distillation. Separating a
mixture of liquids with different boiling temperatures is the most widely
employed use of distillation – as in “refining” crude oil, making gasoline and
other important products. With careful distillation process design, the
condensed liquid, called the distillate, will be much more pure than the
Often, the goal of achieving a desired outcome must be tempered with a
dose of “reality physics.” Sometimes the physics of mixtures prevents
complete separation of two substances, such as ethyl alcohol and water,
where the most concentrated distillate can only be about ninety five percent
alcohol, and achieving that can require multiple distillation cycles.
When a substance doesn’t evaporate fast enough for practical distillation
at room temperature, heat must applied, and the substance can be brought
to its boiling point. (The boiling point is that temperature and pressure
where adding more heat does not raise the temperature of the substance any
more, but results, instead, only in making some of the material change from
liquid to vapor.) Boiling can produce copious amounts of vapor, and can
sometimes even be a rather violent phenomenon when heat is added rapidly.
To condense a pure gas into a liquid one merely has to bring it into contact
with a surface that is colder than the boiling temperature of the material.
The actual temperature doesn’t much matter, although if it is lower than the
material’s melting temperature (also the freezing point) it would become a
To separate a mixture into individual components, the vaporized mixture can
be brought into contact with a series of surfaces that are cooled below the
boiling temperatures of the various materials. The first condensing surface
temperature must be held lower than the boiling temperature of the material
with the highest boiling temperature, but also must be above the boiling
temperature of the material with the next to highest boiling temperature.
This way, only one substance will condense out of the mixture of gasses
onto the first condensing surface. The temperatures of each successive
condensing surface would then be maintained to condense another material
while allowing the rest of the mixture to pass by as a gas. This is how
oil refineries work.
Heat causes the molecules of a liquid to move more rapidly, which increases
their ability to overcome the forces that make them stick together in liquid
form. When they break free of these forces they become a gas, a vapor.
The colder a substance is, the more slowly the molecules are moving on average.
They are not, however, all moving with the same speed, however. How much
tendency there is for something to turn into a gas is called the material’s
vapor pressure. For any particular material, he vapor pressure increases as
the temperature of the material increases. Even as a solid, some molecules
are breaking free and becoming a gas, just by chance.
The production of vapor is accelerated by adding heat to a liquid. The molecules
speed up, and the liquid will even boil if enough heat is added. Boiling is
characterized by the sudden formation of bubbles of vapor inside the volume of
The bubbles in a boiling liquid form and rise to the top surface of the liquid,
where they emerge, carrying along a film, of the liquid that is boiling, held
together by surface tension. As the top of the bubble of gas rises above the
surface, the liquid in the film above the bubble drains back down, making the
film thinner, until at last the film is too thin to hold back the pressure of
the rising gas bubble. At this time the bursting of the film releases the gas
vapor and sets into motion several actions.
The first action is the fragmentation of the film, and the pieces forming tiny
droplets of liquid, pulled into spherical shape by surface tension. The droplets
have a range of sizes, some small enough to remain suspended as a mist, while
others are large enough to fall back to the surface of the liquid.
Meanwhile, with the pressure of the vapor that was forming the bubble released while
the bubble was only about halfway out of the liquid, there is a hemispheric void in
the surface of the liquid. Of course gravity wants to make the surface of the fluid
level, so the sides begin to move inward toward the center, pushed by all the liquid
beside the bubble. This happens quite rapidly, and when the inrushing liquid meets
in the center, the collision forces a jet of the liquid to rise above the surface.
Once again surface tension enters the picture and begins to constrict the long thin
jet, trying to pull it into spherical droplets. Typically there is at least one main
droplet large enough to fall back to the surface of the liquid, sometimes several,
along with numerous smaller droplets, some of which are small enough to remain suspended
as a mist above the surface of the liquid.
Now, we have left a number of droplets falling back to the surface of the liquid. These
droplets fall with enough force to splash at least on a small scale when the strike the
liquid surface. Splashing forces material to the side as well as downward, resulting in
a cuplike depression with a fringe of tiny droplets around the edge of the cup like a
fringe. Many of these tiny droplets are small enough to remain suspended as a mist,
joining the mist droplets produced from the bubble film and the initial jet.
Finally the cup shaped depressions caused by the falling droplets collapse, forming more
jets much like the bubble’s original jet, but smaller. All this from one bursting vapor bubble.
The mist above the liquid rises along with the volumes of vapor coming from the bursting
bubbles. The vapor travels to the condenser, pushed along by the pressure of the gas created
by boiling the liquid. The mist is some of the original liquid, containing all the stuff
that was present when the liquid was being heated to a boil.
Picture the droplets, small enough to float in the moving vapor, but huge by comparison to
the scale of molecules. So whatever is in the original liquid will also be in the mist.
The mist droplets contact the condenser surface and join with the forming condensate liquid
and together they flow down into the collection container.
Industrial distillation equipment can be made with mist reduction stages between the
vaporizer and the condenser, but they cost money, require maintenance, and reduce the
efficiency of the of the apparatus, so are economically a challenge.
How much we care about this depends on what we are trying to accomplish with distillation
of the liquid. One of the important uses of distillation is to increase the concentration
of alcohol to be used for drinking by humans, and increasingly for use as a motor fuel
One other important purpose of alcohol distillation is to produce the intermediate material
for making acetic acid, more familiarly known as vinegar. Vinegar is produced from alcohol
by one additional stage of oxidation, performed by acetic acid producing bacteria of the
As mentioned earlier, the carryover of mist in the distillation processes results in a
liquid that is not really pure, but is rather more concentrated than at the start. For
beverage producers, this material includes leftover stuff from the starting material.
Even if you were to start with purified sugar and grow yeast in it, you must add other
nutrients for the yeast to live and work. The yeast thus effectively pollute the mixture
with their remains as well as the alcohol they make.
Starting with Wheat – Grain Neutral Spirits
If you start with wheat, it is crushed, cooked and enzymatically treated to convert the
starch into sugars that the yeast can process. Since wheat is about fifteen per cent
protein, there are quite a lot of protein fragments present in the liquid after cooking
and fermentation. To keep from clogging the boiling vessel, the liquid can be filtered
before distillation. Filtration, however, will not remove the small fragments of protein,
just the biggest pieces of it. So this means that along with the alcohol, there is plenty
of wheat protein in the liquid sent to the still, and also that there is some of this
protein left in the resultant product, called Grain Neutral spirits.
Grain Neutral Spirits made from wheat is frequently the feedstock for a vinegar making
process, and since filtration cannot remove molecules the size of a few connected amino
acids, there is plenty of wheat protein in the end product “Distilled Vinegar”.
Now, wouldn’t it be nice if distillation was really what we have all been taught that
it was in junior high science class? Something almost magical!
More details about distillation