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23 November 2001

Secrets of Simple Distillation

by Shawn Carlson

Odds are you won’t be a citizen scientist long before you’ll need to purify a liquid that’s tainted with a contaminant. You might need to separate volatile organics from some a tincture of molecular soup, purify water, or concentrate a dilution of alcohol. If so, get thee to a distillery.

The basic process behind distillation is simple. At a given temperature and pressure different substances generally have different vapor pressures. Distillation uses this fact to separate the materials in a liquid. Just heat the soup. The molecules with the larger vapor pressures will evaporate from the surface in greater numbers than those of lower vapor pressures. When condensed, the resulting liquor will have more of the high vapor pressure molecules than did the original broth. Simple enough, right?

Unfortunately, in the lab, as with the rest of life, things are rarely as simple as they ought to be. Since all substances have at least some vapor pressure within the still, distillation does not separate the molecules as some techniques in chemistry do, it merely alters their relative concentrations. This means that two substances with similar vapor pressures can be quite difficult to separate and this has lead to the development of advanced methods called "fractional distillation" which carry out many distillation steps simultaneously within a single still. I’ll describe such systems in detail in coming weeks. Worse, even a simple distillation system can be a pain to assemble and use. So to get you started, I’ve developed a system that is both inexpensive and easy to build and operate.

First, you’ll need a source of heat. If I’m not working with flammable materials I prefer an open flame, like a camp stove or an alcohol lamp as I described last week. However, whenever I need to make anhydrous alcohol, purify a solvent or work with some other ignitable vapor I always use an electric heater and so should you. You can buy portable hot plates at any department store. However, its often just as easy to wrap the boiler with heating cord that you plug into the wall and control with a dimmer switch. You’ll find these at Omega Engineering at www.omega.com.

Simple stills can be cobbled together from humble materials. I fashioned my first still from an empty soda can, some flexible copper tubing, and an old peanut butter jar. The can serves as the boiler, the jar as the receiver and the copper tubing is the core of the condenser; the part of the still that condenses the vapor back into a liquid. Copper promotes condensation by rapidly conducting the heat from the vapor. One then needs to pull the heat away from the coil. Unfortunately, fans don’t have enough cooling power to do the job by themselves so most homespun stills rely on evaporative cooling by covering the copper with wet rags. To make sure they stay wet, most folks find a way to drip water onto the cloth continuously. (My grandpa Don taught me to distill solvents in this way when I was ten years old.)

These wet condensers work just fine, but they can be quite messy. I usually get water all over my lab before I’m done. To avoid the inevitable headaches, I have devised another method–a makeshift evaporative-regenerative cooler that requires only a small fan to cool. It’s pretty simple and I’ll explain just how it works in a moment. But first, since I don’t have any illustrations for this column, let me describe how to build the condenser to give you a good mental picture.

First, carefully uncoil about twenty feet of 3/8 inch OD flexible copper tube. (Make sure you ask for "refrigerator tubing" at the hardware store, because most people buy this stuff to repair refrigerator coolant systems. When I asked for "flexible copper tube" at my local Home Depot the sales assistant looked at me as though I was speaking Dutch.)

Now, wrap the entire length with a thick layer of paper towels. For reasons that will become clear in a moment, the goal here is to convert your thin tube into a much thicker one-- at least two inches in diameter. I cover the towels as I go with an outer-wrap of several rolled gauze bandage to keep the paper in good contact with the copper tube.

Next, unroll twenty feet of aluminum foil beneath the assembly and then thoroughly soak the paper with isopropyl (rubbing) alcohol. To keep the fluid from evaporating, fold the foil up over the whole assembly, and roll the foil down from the edge to create a rolled seam. Then smear a liberal coating of Vaseline on the tube where it emerges from the foil. Pinch down the aluminum-foil at these points and secure the ends with rubber bands. (Unlike most other substances, rubber actually pulls tighter as it warms and this keeps the aluminum foil snug against the copper as the pipe is warmed by the vapor.)

At this point, you’ve got a sealed container that will retain the alcohol vapors fairly well. When hot vapor from the boiling flasks enter copper tube, the narrowness of the channel ensures good thermal contact with the gas–it draws a lot of heat from the gas over a relatively short distance. This heat is conducted away by the alcohol which begins evaporating more rapidly from the surface of the ace bandage. These evaporated molecules carry with them a lot of energy. These then run into the aluminum foil and warm it. That’s the evaporative part of the system. To engage the regenerative part, use a fan to blow air over the foil. This conducts the heat away and causes the alcohol vapors to re-condense and fall as a liquid back onto the paper towels. Distributing the heat over a larger surface area allows the fan to cool the tube more efficiently. Basically, you’ve built a pipe that is narrow at the center to rapidly pull heat from the vapor and that rapidly transports that heat to a larger surface can efficiently transfer that heat to the air flowing over. You’ll find the system extremely simple and effective way to cool your condenser.

Next, wrap the construction into a coil around a convenient mandrel. I’ve used the bottom of an office trash can, but my preferred form is a cardboard "Quick Tube" used by builders to pour concrete support pillars. You’ll find them in a variety useful diameters at any well-stocked hardware store. The coils allow you to position the entire length of the tube into the air stream of a single fan.

The rest of the still can be put together in just a few minutes. If you don’t have an Erlenmeyer flask, go to the SAS on-line store and buy one. While you’re waiting on delivery, you can make do with a soda can. First, after thoroughly washing the soda can, add the liquid you wish to distill and rotate the pull-tab so that it’s centered over the opening in the can’s lid. Then cut off a piece of a plastic playing card so that it fits snugly under the pull-tab and completely blocks the can’s opening. Next, mark the card at the center of the hole in the pull-tab. Now remove the card, and punch a hole just large enough to accept the end of the flexible copper tubing. Finally, reposition the card piece centered on the hole and affix the card in place using a silicon sealant or a dollop of melted candle wax.

Place a gentle 90-degree bend in one end of the copper tubing and insert it through the hole in the card. The pull-tab will help hold the tube steady. Add a dollop of silicon sealant around where the card and copper tubing meet to insure a good seal and wait for it to set. (Obviously, this will require you to replace the card after each distillation.) Now, place the can on the burner and secure the condenser so that neither can move. Make sure the far end is over a sink and position the peanut butter jar to collect the distillate. Turn on your fan, turn up the heat and distill away.

Using this set up I’ve distilled a liter of water and managed to collect 98 percent of the fluid. That’s about as good as you could expect to do with a professional-grade water-cooled unit.

Of course, part of the cooling also comes from the fact that the alcohol isn’t perfectly sealed in and so some vapor does escape can carries it’s excess thermal energy with it. After a few months my condenser needs to be recharged with alcohol. That’s easily done by soaking the coil overnight in alcohol.

Now, just in case you’ve got something foolish and illegal in mind, let me warn you… DO NOT USE THIS SIMPLE STILL TO MAKE DRINKING SPIRITS. You’d quite likely poison yourself. That’s because ethyl alcohol isn’t the only byproduct that yeasts make when they metabolize sugar. They also manufacture small quantities of other alcohols including various amyl alcohols and lipids (amyl alcohols together with the fatty acids make "fusel oils," a toxic industrial solvent used to dissolve wax), and carbohydrates. These are only minor contaminates in wine and beer and have no effect on your health. But simple distillation can concentrate some of these poisons over a thousand or even a million-fold. Ever heard of someone going blind because they imbibed "poisoned" Moonshine? Now you know why. Making brandy or other liquors requires fractional distillation and a much more complex system than the one described here. So please, if you choose to consume distilled spirits, get them from your local liquor store, not homemade in your basement.

One more thing. Copper tube has its limitations. You can’t expose it to acids, for example. I’ll take up other options, together with fractional distillation, in future installments.