Best Fractional Distillation System by Goel Scientific Glass Inc. USA & CANADA: 1 Ultimate Guide
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Piyush Shah (Business Director)
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August 5, 2023
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What is Fractional Distillation?
Fractional distillation is the separation of various components from a liquid homogeneous mixture. Each component typically has a different boiling point and vapour pressure than the rest, so fractional distillation can be achieved by applying a temperature gradient.
The various components are condensed at different stages or levels of the setup, which can either be a small setup consisting of laboratory glassware and other equipment, or a large-scale industrial operation, such as in the oil and gas industry when refining petroleum into different types of fuel.
Fractional distillation is typically used when there are more than two components in a liquid mixture that need to be separated. The differences in the boiling points of the liquids make it possible to condense the liquids at various points of the fractionating column.
Both small-scale and large-scale laboratory fractionating columns have the same basic designs: The various types of vapours are condensed and collected at various levels corresponding to their volatilities.
What is the Purpose of Fractional Distillation?
Fractional distillation is very useful when separating more than two types of liquids from a homogeneous mixture. This method is particularly useful in separating liquids that have less than 25°C difference in boiling points. It can also be used to increase the purity of a particular distillate. During fractional distillation, fractions of the original mixture are evaporated and condensed at different levels as the temperature gradient changes.
Although an ordinary distillation method can also be used to separate mixtures consisting of more than two types of liquids, it would be tedious, time consuming, and impractical to redistill the same liquid mixture repeatedly at different temperatures just to separate the different components.
The elegant solution to this problem is using temperature gradients. This can be done either by using a fractionating column or separate distillation chambers in sequence. Here’s how the sequential distillations process works:
Sequential Simple Distillations
As you can see in the example, the molar purity of benzene distillate can be increased through a series of simple distillation steps. From 50% purity in the first flask, the distillate purity reached 97% in the last flask.
What is Fractional Distillation Used to Separate?
Fractional distillation is used to separate out several different liquid components from a mixture. It can be done either through a sequence of simple distillations or by using a fractionating column. Its most common application is the distillation process for crude oil, which refines it into different types of fuel and other petroleum products as the above image shows.
Fractional distillation can also be used for purifying water. Its other industrial uses include deriving highly concentrated and purified silicon from chlorosilanes. The electronics industry, which includes computer and mobile phone manufacturers, are dependent on this process. Without it, high quality semiconductor components would be difficult and expensive to manufacture.
The industrial process of extracting nitrogen, argon, and oxygen from the air also involves the large-scale fractional distillation of air. During this process, air is cooled down to −200°C in order for it to turn into liquid form. It’s then fractionally distilled to extract the various useful components. For example, oxygen can be separated from the mixture at the temperature of −183°C.
Stages of Fractional Distillation
As the name suggests, fractional distillation involves the separation of the “fractions” or components of a mixture. Each fractional component has its own boiling point, which is inversely proportional to its volatility.
The vapour pressure, on the other hand, is directly proportional to its volatility. This means that some vapours will condense at certain temperatures. The lightest and most volatile ones will be collected at the top while the heavier ones will settle at the bottom.
Fractional distillation has three basic stages, regardless of the type of liquid mixtures that will be distilled. These steps are evaporation, condensation, and finally, collection. These steps happen at various levels of temperature, meaning that the process produces multiple products or distillates at various levels.
Step 1: Heat and Evaporate
At this stage, the liquid mixture, such as crude oil, is heated in order to make it evaporate. The vapour is then directed to the fractionating column.
Step 2: Separate and Condense
As the vapour rises, the heaviest and least volatile components start to condense. Some return to the bottom while others are condensed at certain levels corresponding to lower temperatures compared to the temperature at the bottom. The more volatile components rise to the topmost portion of the column, where they condense.
Step 3: Collect and Cooldown
The distilled fractions are collected and cooled down into separate containers. These are the distinct products of the distillation process.
What is the Difference Between Distillation and Fractional Distillation?
In terms of the basic principles and basic steps involved, both simple distillation and fractional distillation are very similar. However, the main difference is in the details. The latter is more complex, consisting of sub-steps. The latter also produces multiple distillates as final products.
What is the Distillation Process?
Distillation is the process of purifying a substance, whereby pure substances are extracted from a mixture.
There are different types of distillation processes, including fractional distillation, simple, steam, and vacuum distillation.
Distillation has several commercial and industrial applications. For example, it can be used to distil wine. In fact, the well-documented use of distillation can be traced as far back as the 13th century, where it was used to distil alcohol from wine. It’s able to do this because the difference in the boiling point of water and ethanol makes it possible to isolate purer amounts of alcohol from wine.
Another major application of distillation is in the petroleum industry, where fractional distillation is used to purify crude oil into different types of commercial fuels, like gasoline and diesel.
What is Distillation?
The distillation process generally involves three main steps:
- The conversion of the desired liquid from a mixture into vapour
- The condensation of the purified liquid
- The collection of the condensed liquid
Specific types of distillation processes may have several more stages, such as the fractional distillation of crude oil (more on that below).
In general, heat, corresponding to the boiling point of the desired liquid, is applied to the mixture that will be distilled. In other instances, pressure is lowered to optimise the distillation process.
Below is an illustration of a basic laboratory distillation setup:
In this example, freshwater is the liquid being extracted. As heat from the Bunsen burner flame is applied to the flask, which contains seawater, the water boils and evaporates. The water vapour is cooled and redirected by the condenser to the Erlenmeyer flask, drip by drip.
The flowing water in the outer cooling tube surrounding the inner condensing tube facilitates the condensation of the water vapour. Since salt is solid and has a very high boiling point, it doesn’t evaporate with the water. As a result, the distillate is freshwater.
This process is known as the desalination of water, and it’s used on a large-scale in some countries that have freshwater scarcity, such as those in the Middle East.
How Does the Process of Distillation Work?
The distillation process works by ‘exciting’ the molecules of the liquid to be distilled by heating it, thus encouraging evaporation. It’s crucial that the precise boiling point of the intended distillate is known.
Furthermore, the temperature of the liquid mixture must be meticulously monitored, and the temperature must be maintained at or very close to the precise boiling point of the intended distillate. Otherwise, other impurities may evaporate along with the desired distillate.
In mixtures where there are dissolved solids, like salt solutions, it’s relatively easy to isolate the liquid solvent from the mixture. This is because liquid solvents like water generally have lower boiling points than their solutes. In simple cases, such as in the distillation of alcohol from wine, the temperature must simply be maintained at the boiling temperature of the intended distillate.
For instance, ethanol (the alcohol in wines) has a boiling point of 78.37°C. This means that the wine mixture itself, which is mostly water, should not be allowed to boil at 100°C (the boiling point of water). Otherwise, the water will evaporate along with the alcohol. As much as possible, the temperature of the mixture must be maintained at the precise boiling point of the intended distillate.
Once the vapour has been collected, it must be allowed to cool down at room temperature for it to condense into liquid. It’s then collected in a container either for storage or further processing.
Often, distillates are not 100% concentrated, and may still contain some amounts of impurities, particularly the original solvent. Therefore, if you want to have a distillate with higher purity, further distillation is necessary.
Is Distillation a Chemical or Physical Process?
Distillation is a physical process because it involves a phase change from liquid to gas (vapour), and then back again to liquid. Generally, no chemical change is intended to occur during the process of distillation. That said, some incidental or accidental chemical reactions may occur during the process of distillation. The risk of this happening increases as the scale becomes larger.
For example, the distillation of flammable liquids may result in combustion or even explosion. In some cases, liquids may also react with the components of the distillation equipment. Some liquids may even react with the oxygen or impurities in the air once vaporised. Watch this case study presentation about an accidental explosion at a vinyl chloride monomer (VCM) distillation plant.
What Are the Different Types of Distillation?
There are several different types of distillation, owing to its range of industrial applications. The exact step-by-step process and equipment details of each type are beyond the scope of this article, but we’ll outline the generalised process and concepts of each one.
1. Fractional Distillation
As we mentioned earlier on, fractional distillation is commonly used in the petroleum industry to separate the many components of crude oil. This is necessary because the boiling points of these components are too close to each other that simple distillation would not work.
Fractional distillation is a multistage process that separates various types of fossil fuels, like kerosene and heavy oil. The temperatures at various stages are precisely controlled. The illustration below depicting fractional distillation of fossil fuels:
2. Simple Distillation
Simple distillation is used when the boiling points of the liquids in a mixture are very different. Typically, it’s used to distil liquids that have at least 100°C difference in boiling points. As the name suggests, it’s relatively easy to separate these liquids because there is sufficient room for temperature variations that will not affect one of the liquids in terms of evaporation.
3. Steam Distillation
As you can tell by the name, steam distillation uses steam to distil the desired liquids. The steam itself isn’t the distillate, but just a means to extract the distillates. It’s commonly used for extracting the essential oils of flowers. See the illustration below for the basic setup:
4. Vacuum Distillation
Vacuum distillation is the distillation process used for liquids with very high boiling points under normal atmospheric pressure. It’s also used for liquids that decompose under normal atmospheric pressure. As pressure is reduced, the boiling point is also reduced, thereby making it easier to evaporate the liquid. It’s commonly used in refining petroleum products.
