What is Distillation?

Distillation is a process for separating mixtures. It relies on the fact that different substances have different boiling points. When a mixture is heated, the substance with the lowest boiling point will vaporize. This vapor is then condensed and collected.

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It’s possible to keep repeating this process to collect increasing concentrations of ethanol. This is called fractional distillation.

Composition of the Distillate

As a liquid mixture heats up, molecules of the more volatile component begin to break away from the solution. They move upward into the vapor phase and then condense back to the liquid state in the condenser. This is the distillate. The composition of the distillate can be controlled by altering the reflux ratio.

In the example given in Figure 5.13, a distillation curve is plotted with molecular composition on the x-axis and boiling point on the y-axis. The left side of the curve represents a sample that is pure compound A, and the right side of the curve represents a sample that contains both compound A and compound B. The boiling points of A and B are marked on the curve (bp A, bp B). The curve illustrates that, as the system cools, the concentration of the liq phase begins to rise, while that of the vapor remains constant. This is because, as the vapor evaporates, it becomes richer in component B.

As distillation continues, the composition of the vapor and liq phases continue to shift until they reach the two phase region shown in Figure 5.16. At this point, the liq phase is 0.62 and the vapor is 0.94 with a ratio of 0.82:1. Once again, as the vapor boils over, it becomes richer in B, and the remaining liquid becomes richer in A.

Purification

The process of distillation separates two or more substances in the liquid state, exploiting differences in their physical properties. It can also be used to purify liquids. The liquids must be miscible (they form a solution when mixed together) for this to work. If the mixture is comprised of two compounds with very different vapor pressures, the one with the lower vapor pressure will evaporate more readily. The resulting vapor is then concentrated in the condenser, and the concentrated liquid is known as the distillate.

The composition of the vapors and liquid changes during the distillation process, because the more volatile component boils off earlier in the process. The resulting liquid phase is richer in the less volatile compound. This is clearly illustrated in a vapor-liquid equilibrium diagram.

Laboratory scale distillation is almost always run as a batch distillation. The device used consists at least of a reboiler or pot in which the source material is heated, a condenser in which the heated vapor cools back to the liquid state, and a receiver to collect the concentrated distillate.

A variation on this is continuous distillation, in which the sources, vapors, and distillate are continuously replenished. This allows for more detailed control of the separation process. A column is often added to a batch distillation system, to improve the separation by increasing the surface area on which the vapors and condensate come into contact with each other.

Refining

When a miscible mixture is heated, the components with lower boiling points vaporize first and separate from the remainder of the liquid. The vapors condense and collect in a separate container, known as a distillation apparatus. The composition of the liquid and vapor at any given temperature is known from a published vapor-liquid equilibrium diagram, which shows both the bottom curve (the liquid in the flask) and the top one (the distillate).

A high degree of purity can only be achieved by using a column-type distillation apparatus, where a series of heated plates separated by partitions provide different boiling ranges for each component. This allows the vapors of each of these compounds to be collected separately, with each fraction containing a particular chemical substance.

As the hot vapors rise up the distillation apparatus they cool and, by Raoult’s law, some of their constituents will be recondensed as a fresh new layer of vapor. This is then heated again, and each vaporization-condensation cycle yields a purer and pure solution of the more volatile compound.

As a result, distillation is used to produce a variety of fuel oils and other commercial products that can be used for transportation, electrical power generation, heating and in thousands of industrial applications. Refineries further process the distillate into a variety of more useful and saleable products through processes such as cracking, reforming, alkylation, isomerization and hydrotreating.

Applications

Distillation is used extensively in industry, from the production of alcoholic beverages to petroleum refining. The process is based on the principle that different substances have different boiling points, so that by heating a mixture of liquids, those with lower boiling points will evaporate first and be condensed into a purified liquid.

Some compounds have extremely high boiling points, so to avoid decomposition they are boiled under vacuum instead of atmospheric pressure, a technique known as vacuum distillation. This can be done using a simple apparatus such as a vacuum flask, but is more commonly employed in industrial facilities using large-scale distillation towers (also known as fractionation columns).

The ability to separate the components of a liquid mixture based on their boiling point is important because it allows a wide range of materials to be separated and purified. Among these are organic substances, such as a variety of plants and their extracts, which can be separated by steam distillation; synthetic chemicals, for example, formaldehyde and phenol, which are frequently separated by distillation; and many types of petroleum products, from fuel to lubricating oil.

A variant of distillation, called codistillation, is used to isolate immiscible mixtures in which the components have different partial pressures. This is achieved using a Dean-Stark apparatus, for example, to remove water from synthesis products; or the Perkin triangle apparatus, which uses means via a series of taps to allow fractions to be isolated without being removed from either the heat source or the vacuum.