Lesson Video: Fractional Distillation of Air Chemistry

In this video, we will learn how to describe the properties of gases in air and describe their separation by low-temperature fractional distillation.

17:50

Video Transcript

In this video, we will learn about the properties of the components of air, how to separate these components, and the reasons for doing this, in other words, the uses of different air fractions. Let’s start by briefly looking at the composition of air.

Our atmosphere is a mostly homogeneous mixture of gases, some particulate matter, and water vapor. Dry air, which does not contain water vapor or particulate matter, is composed of approximately 78 percent nitrogen gas by volume. In other words, about 78 percent of the volume of the atmosphere is made of nitrogen. 21 percent is made of oxygen, and the small fraction that remains is made of other gases. These other gases include argon, carbon dioxide, the noble gases neon, helium, krypton, and xenon, as well as gases like methane, nitrogen dioxide, which is a pollutant, and ozone, O3. So, this is the composition of dry air.

But remember though that air does in fact contain water vapor. There is more gaseous water or water vapor in the air at sea level and less further away from the surface of the earth. Water vapor is approximately a quarter of a percent by mass of the whole atmosphere. Now, let’s compare the main components of air according to the physical properties.

Let’s look at nitrogen, oxygen, carbon dioxide, and water vapor. Nitrogen gas with a triple bond is the least reactive of these four air fractions and has the highest average bond energy, 942 kilojoules per mole. This tells us that it requires a lot of energy to break this triple bond. The next highest average bond energy is in carbon dioxide. A value of 799 kilojoules per mole is for one of the carbon oxygen double bonds. This high value tells us that carbon dioxide is also fairly unreactive.

Oxygen gas, however, with a double bond is fairly reactive. It has a much lower average bond energy than one of the double bonds in carbon dioxide. And water is the most reactive of the four fractions. Its average bond energy is for one of the hydrogen oxygen single bonds. Only 459 kilojoules per mole are needed to break this bond. The density of these air fractions are unique to each fraction.

Density is usually measured in grams per centimeter cubed or kilograms per meters cubed. And the values for nitrogen, oxygen, and carbon dioxide are given at standard temperature and pressure, which are zero degrees Celsius and one bar. We can see that nitrogen has the lowest density then oxygen with a slightly higher density then carbon dioxide being the most dense of the three fractions.

Remember that water exists as a gas above its boiling point of 100 degrees Celsius. However, water can exist as a vapor below 100 degrees Celsius and even at zero degrees Celsius. But this depends on the conditions. Just above zero degrees Celsius, water is generally found as a liquid and its density is about one gram per centimeter cubed. There is a definite trend in the boiling points on this table for these fractions.

Liquid nitrogen boils into a gas at the lowest temperature of negative 196 degrees Celsius. Oxygen is converted into a gas from a liquid at negative 183 degrees Celsius, while carbon dioxide goes straight from a solid to a gas at minus or negative 79 degrees Celsius. In other words, carbon dioxide undergoes sublimation. So, technically, for carbon dioxide, we should not refer to it as boiling point but perhaps sublimation point.

These physical properties or physical characteristics are unique to each fraction, in other words, unique to each chemical substance. They help us identify what substance is present. For example, if you had a gas whose boiling point was negative 196 degrees Celsius, you could identify that gas as being nitrogen. The property of boiling point is very useful. It is what allows us to separate the components or fractions of air. Let’s investigate that a bit further.

To separate the fractions of air, the air is first filtered to remove solid particulates and dust, in other words, to purify the air. Air is then compressed under high pressure. A cooling process then begins. As the air gets cooler. It reaches a temperature of about six degrees Celsius. At this temperature, water vapor is mostly all condensed and filtered off. Further cooling to negative 79 degrees Celsius solidifies gaseous carbon dioxide to solid carbon dioxide. This is then removed.

The fractions that remain are nitrogen, oxygen, and a small amount of argon. When the cooling process reaches negative 183 degrees Celsius, oxygen gas liquefies. Then, at negative 186 degrees Celsius, argon liquefies. And at negative 196 degrees Celsius, nitrogen liquefies. The liquid oxygen, argon, and nitrogen is then further cooled to about negative 200 degrees Celsius. And this liquid mixture is fed into a special plant for fractional distillation.

Fractional distillation is a suitable method to separate different liquids from a mixture of liquids. It is a physical separation because no chemical bonds are broken. Substances are merely separated by virtue of the different boiling points.

The separation of nitrogen, argon, and oxygen happens in a fractionating column. The mixture enters the column at negative 200 degrees Celsius at the bottom. The column is cooler at the top and warmer at the bottom. As the liquid enters the bottom of the column, it warms up slowly. When it reaches a temperature of negative 196 degrees Celsius, which is nitrogen’s boiling point, the liquid nitrogen is converted to a gas, which then bubbles up the column and is collected at the top.

The mixture is further warmed until negative 186 degrees Celsius, which is argon’s boiling point. Argon is converted from a liquid to a gas and begins to bubble up the column. It is usually collected at its own special outlet point. However, many fractionating column diagrams don’t show a special outlet point for argon, so we’ll just simplify it and leave it off for now.

So, we know nitrogen gas boils off first and is collected at the top of the column. We know that argon gas boils off second and is collected at its own special outlet point. What remains in the column is liquid oxygen. It is warmed just a degree more to about negative 185 degrees Celsius to ensure that all the argon has left the mixture. And then, a valve at the bottom of the fractionating column is opened, and the liquid oxygen is allowed to run off and is collected.

Now, we know about the fractions in air, their unique properties, and how to separate them. We need to now ask ourselves, why are the fractions of air separated? In other words, what use do we have for these different fractions? Nitrogen gas, we have seen, is inert and unreactive. For this reason, it is perfect to use as the atmosphere in food packaging and storage, for example, in ground coffee tins and in potato chip bags. It is the lack of oxygen gas and water vapor in these bags that prolongs the freshness of the food and helps prevent oxidation.

Nitrogen is one of the reactants in the Haber process, which is the industrial preparation of ammonia gas. Ammonia gas is a starting material for many other reactions. Liquid nitrogen, because it is so cold, is used to freeze dry foods and cryogenically freeze biological tissue samples. In the lab, nitrogen gas is used as an inert atmosphere over chemical reaction mixtures.

Oxygen gas is used in oxyacetylene torches in welding and metal cutting, as well as in medicine, for example, in breathing equipment. Carbon dioxide gas also has many uses. Some of them include use in fire extinguishers, to carbonate or make soft drinks fizzy, and in its solid form as dry ice, which is used to keep food cool. You may have seen this, for example, in mobile ice cream carts.

Argon is often used as the atmosphere in light bulbs as it is inert and unreactive, even when the filament gets white-hot. Argon lasers are used in eye surgery. And argon, like nitrogen, is used as an inert atmosphere in chemical reactions. It’s time to practice a problem.

The components of air can be separated in a fractionating column, as shown. (a) Fraction A is collected at the top of the column. Which range of temperatures must be maintained at the top of the column? (A) Negative 195 to negative 190 degrees Celsius, (B) negative 190 to negative 185 degrees Celsius, (C) negative 185 to negative 180 degrees Celsius, (D) negative 205 to negative 200 degrees Celsius, or (E) negative 200 to negative 195 degrees Celsius.

The question asks about separating the components of air. To separate the components of air, air must first be filtered to remove dust and particulates. It is then compressed under high pressure and cooled in stages. In the first stage, air is cooled to about six degrees Celsius, at which point almost all the water has condensed and this is removed by a filter. The air is now dry and water free. It is further cooled to negative 79 degrees Celsius, at which point carbon dioxide gas solidifies to solid carbon dioxide. The solid carbon dioxide is then easily removed.

The last stage in cooling is to cool the remaining components, which are nitrogen and oxygen, to negative 200 degrees Celsius, at which point they are both in the liquid form. In reality, there is a tiny fraction of argon also present in the mixture, but this fraction is less than one percent. And here, we will assume that the main fractions are nitrogen and oxygen.

The mixture of liquid nitrogen and liquid oxygen are then fed into a fractionating column, where they are separated by a process called fractional distillation. This process is similar to the process for separating the components of crude oil. The mixture is fed into the bottom of the column, where it is warmer. At the top of the column, it is cooler. The negative 200 degrees Celsius liquid mixture of nitrogen and oxygen is then warmed slowly.

When the temperature reaches negative 196 degrees Celsius, which is nitrogen’s boiling point, the nitrogen boils off and leaves the top of the column as fraction A. The temperature at the top of the column is maintained just above nitrogen’s boiling point, or just slightly warmer than nitrogen’s boiling point. It cannot be much warmer than this. Otherwise, oxygen might also boil off. Oxygen’s boiling point is negative 183 degrees Celsius. And so, the temperature range at the top of the column must be cooler than oxygen’s boiling point. So, the range of temperatures that must be maintained at the top of the column is negative 195 to negative 190 degrees Celsius.

Question (b) fraction B is collected at the bottom of the column. Which range of temperatures must be maintained at the bottom of the column? The answer options are the same as for part (a).

When all the nitrogen gas has left as fraction A at the top of the column, what is left is liquid oxygen. This is fraction B. We saw that oxygen’s boiling point is negative 183 degrees Celsius. And to prevent the oxygen from boiling off, the temperature at the bottom of the column is maintained between negative 190 and negative 185 degrees Celsius, which is cooler than oxygen’s boiling point.

Ensuring that the oxygen remains as a liquid, a valve or tap is then opened at the bottom of the fractionating column, and the oxygen in liquid form is easily collected. So, which range of temperatures must be maintained at the bottom of the column? The answer is negative 190 to negative 185 degrees Celsius, which is just cooler than oxygen’s boiling point.

Question (c) identify the major component of fraction A. (A) gaseous argon, (B) liquid nitrogen, (C) liquid oxygen, (D) gaseous nitrogen, or (E) gaseous oxygen.

We have seen that the major component of fraction A is gaseous nitrogen because nitrogen boiled off when the temperature in the column reached about negative 196 degrees Celsius. So, the major component of fraction A is gaseous nitrogen.

Question (d) identify the major component of fraction B. And the answer options are the same as for part (c).

We have seen that the major component of fraction B is liquid oxygen. Because often nitrogen comes off as a gas, the only component left is oxygen. It’s kept at a temperature cooler than its boiling point, in other words, cooler than negative 183 degrees Celsius, and easily drained out the bottom of the column by opening a tap or a valve. So, the major component of fraction B is liquid oxygen.

Let’s summarize what we’ve learned about the fractional distillation of air. We learned that the main components of air are nitrogen and oxygen gas. A very small percentage of air is composed of carbon dioxide, argon, and noble gases as well as water vapor. We saw that the fractions each have unique physical properties and that boiling point, in particular, is useful in order to separate these fractions. We learned about the process of fractional distillation used to separate nitrogen and oxygen. And lastly, we looked at the applications or uses of the different air fractions.

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