Distilling water

The simplest distillation you can perform is to heat up water and then condense the vapor. This results in a distillate that contains only water, and nothing else. It will contain no salts or any other nonvolatile dissolved substances. Many people distil water at home, using a variety of small commercially produced water distillers. Industrially, enormous plants produce millions of gallons of distillate from seawater for irrigation and drinking water

Textbooks often present the classical survival still as a way to survive in the desert by distilling contaminated water. Let's discuss this simple but very inefficient design, and see how it can be improved.

Fig. 3-2 is a diagram of the simple survival still. It is merely two sheets of transparent plastic, one held down at its edges by stones and stretched over a pit lined with the other. Stones are put in the middle of the sheet to create a low point, and a pot or other collecting vessel is placed under it. Contaminated The simple Survival Still water or moist malcnaj in<c plants, is placed in the pit around the collecting vessel.

Sunshine passes through the transparent sheet and warms up the stones and liquid in the bottom of the pit, causing the contaminated water to evaporate. This vapor condenses on the sheet and drips from the low point into the collecting vessel. A good yield might be a couple of mouthfuls of distilled water each day, which is not enough to keep you alive in a hot desert.

This is a very small yield, especially with the considerable energy pouring down from the sun in desert areas. What's wrong with the design?

Actually, the flaw in the design is quite simple. The large quantities of water evaporate, but the condenser is a sheet of plastic sitting in the hot sun! The only cooling available comes from the wind, which may or may not blow. No wonder very little finds its way into the collection vessel, and most of the hot vapor escapes.

One simple improvement, shown in Fig. Fig.3-3, is to separate the heating action from the cooling. One way of doing this is to place the stones at one side, rather than in the middle. Then shade the side with the stones. The condenser will still be warm, but a lot cooler than if it were in the direct sunlight, which is now confined to the heater side. The larger or darker the shaded part, the cooler it will be. Fig. 3-3

If you have some contaminated water to spare (use your imagination on where that might come from!), you can make it even more efficient by moistening a thin layer of soil sprinkled on top of the "condenser" area of the sheet. Evaporation of this water will help cool the condenser sheet and produce even more drinking water inside.

That's just one way of improving the survival still. There are many other ingenious ones, and you should spend some time thinking about how you could make it more efficient (try a metal plate lying between the plastic and the weighting stones above the cup). This time will be well spent, because it will firmly fix the principles of distillation in your mind. Even very simple systems may be improved if you understand the process that's taking place, and you're not afraid to experiment.

Distillation Alcohol Experiment
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