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Distillation

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 original liquid.

Reality Physics

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.

Condensing

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 solid.

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.

Boiling

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 liquid.

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.

Droplets

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.

Mist

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.

So What?

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 additive.

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 genera Acetobacter and Gluconobacter.

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.

Distilled Vinegar

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”.

Conclusion

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
http://lorien.ncl.ac.uk/ming/distil/distil0.htm