Distillation is a physical method of assorting mixtures depending upon the difference in the boiling point of the component substances. In simple words, the working principle of distillation is to heat a mixture at a specific temperature, collect the hot vapors, and condense to separate the component substance. In short, a highly volatile compound is separated from a less-volatile or non-volatile compound by using distillation.
Today, it is one of the most popular technique implemented for purification and separation of a mixture. There are several methods of distillation depending on the procedure and the instrument setup. Following are the common types.
Simple distillation is a procedure by which two liquids with different boiling points can be separated. Simple distillation (the procedure outlined below) can be used effectively to separate liquids that have at least fifty degrees difference in their boiling points. As the liquid being distilled is heated, the vapors that form will be richest in the component of the mixture that boils at the lowest temperature. Purified compounds will boil, and thus turn into vapors, over a relatively small temperature range (2 or 3°C); by carefully watching the temperature in the distillation flask, it is possible to affect a reasonably good separation.
As distillation progresses, the concentration of the lowest boiling component will steadily decrease. Eventually the temperature within the apparatus will begin to change; a pure compound is no longer being distilled. The temperature will continue to increase until the boiling point of the next-lowest-boiling compound is approached. When the temperature again stabilizes, another pure fraction of the distillate can be collected. This fraction of distillate will be primarily the compound that boils at the second lowest temperature. This process can be repeated until all the fractions of the original mixture have been separated.
- Simple distillation involves heating the liquid mixture to the boiling point and immediately condensing the resulting vapors.
- This method is only effective for mixtures wherein the boiling points of the liquids are considerably different (a minimum difference of 25oC).
- The purity of the distillate (the purified liquid) is governed by Raoult’s law.
Fractional distillation is the process of taking a chemical mixture and using heat to separate out the various components in that mixture. When you think of this process, the first word that should come to mind is separation. In other words, as a chemist in the laboratory, you would use this process when you are interested in isolating one or more compounds present in a mixed sample containing as few as two and up to an endless amount of compounds.
Because heat is used in this separation technique, boiling points play a very important role in fractional distillation. Essentially, you are able to determine what given component is separated out from the mixture by its boiling point. Remember that boiling point is a physical property. Specifically, it is the temperature where a phase change occurs from liquid to vapor. Before we go over the process of fractional distillation, let’s look at some examples where this process is commonly used.
Fractional distillation is often used to separate mixtures of liquids that have similar boiling points. It involves several vaporization-condensation steps (which takes place in a fractioning column). This process is also known as rectification. The apparatus required to perform a fractional distillation on a mixture is listed below.
- Round-bottom flask or distilling flask
- A source of heat, which can be a fire or a hot bath.
- Receiving flask to collect the condensed vapors
- Fractioning column
- Thermometer to measure the temperature in the distilling flask
- Standard Glassware.
Steam distillation is carried out by passing dry steam through the plant material whereby the steam volatile compounds are volatilized, condensed and collected in receivers. Steam distillation has been in use for essential oil extraction for many years. Hydrosteam distillation is carried out when the perfumery plant material is susceptible to direct steam. In this technique the plant material is supported on a screen or a perforated grid placed at some distance above the bottom of the still. Distillation is carried out with low pressure steam which replaces the volatile compounds from the intact plant material.
- Steam distillation is often used to separate heat-sensitive components in a mixture.
- This is done by passing steam through the mixture (which is slightly heated) to vaporize some of it. The process establishes a high heat-transfer rate without the need for high temperatures.
- The resulting vapor is condensed to afford the required distillate.
- The process of steam distillation is used to obtain essential oils and herbal distillates from several aromatic flowers/herbs.
Vacuum distillation is the technique used to separate higher boiling fractions of crude oil. The underlying theory and the process are analogous to those used to separate the lighter fractions in the atmospheric distillation process. The difference between the two physical separation methods is that atmospheric distillation occurs under atmospheric pressure, whereas vacuum distillation occurs at a significantly reduced pressure, thus reducing the boiling point of a substance. Hence, high boiling components can be boiled at lower temperatures, without the risk of cracking. Vacuum towers are much shorter than atmospheric towers in order to minimize the pressure differential from top to bottom.
Generally, the pressure used in vacuum distillation is in the range of 50 to 100 mmHg, although some lubricating oil stocks may require even lower pressure operating conditions . Fractions recovered from the vacuum distillation process usually are divided into gas oil, lubricating oil, and asphalt. The combination of the atmospheric and vacuum distillation processes is an important first step in converting crude oil into useful and economically valuable products.
- Vacuum distillation is ideal for separating mixtures of liquids with very high boiling points.
- In order to boil these compounds, heating to high temperatures is an inefficient method. Therefore, the pressure of the surroundings is lowered instead.
- The lowering of the pressure enables the component to boil at lower temperatures. Once the vapor pressure of the component is equal to the surrounding pressure, it is converted into a vapor.
- These vapors are then condensed and collected as the distillate. The vacuum distillation method is also used to obtain high-purity samples of compounds that decompose at high temperatures.