Distillation separates liquid mixture components based on differing boiling points. The liquid is heated until the most volatile component vaporizes, which is then cooled and condensed back into a purified liquid, known as the distillate. Vacuum distillation is a specialized form of this technique that achieves separation by operating the entire system under a pressure significantly lower than standard atmospheric pressure. This modification allows for the effective purification of substances that would otherwise be difficult or impossible to separate using traditional methods. The technique is widely employed when the compounds in a mixture are either extremely sensitive to heat or possess very high boiling points.
How Pressure Controls Boiling Point
A liquid begins to boil when its vapor pressure is equal to the external pressure. Under normal conditions at sea level, this external pressure is the atmospheric pressure, which is approximately 760 Torr or one atmosphere. Heating a liquid increases the kinetic energy of its molecules, raising the internal vapor pressure until it matches the external atmospheric pressure, at which point boiling occurs.
By creating a vacuum within the distillation apparatus, the external pressure is drastically reduced to a fraction of atmospheric pressure. If the external pressure is lowered, the liquid’s vapor pressure does not need to reach such a high level to match it, meaning a lower temperature is required for the liquid to boil. For instance, water normally boils at 100°C at sea level, but under a deep vacuum, its boiling point can drop below 40°C. This principle is applied to substances with high boiling points, allowing them to vaporize at temperatures far below normal atmospheric conditions.
Required Equipment for Vacuum Distillation
The apparatus for vacuum distillation is an adaptation of a standard distillation setup. The most distinctive component is the vacuum pump, which is plumbed into the system to continuously evacuate air and non-condensable gases, thereby maintaining the low-pressure condition. A distilling flask holds the initial mixture and is heated to vaporize the desired component.
The vapor then travels through a specialized column, which can be packed or simple, before reaching the condenser. This condenser is cooled to convert the purified vapor back into a liquid state. Finally, the liquid distillate is collected in a receiving flask, which must be sealed to the rest of the system to maintain the integrity of the vacuum. All glassware joints must be tightly sealed, often with specialized grease, to prevent air from leaking into the system and collapsing the vacuum.
Following the Distillation Procedure
The operational process begins by assembling the completely sealed distillation system. Before any heat is applied, the vacuum pump is activated to slowly reduce the pressure inside the apparatus to the desired operating level. This evacuation step is performed first to prevent the rapid, uncontrolled boiling or “bumping” of the liquid.
Once the pressure is stabilized and monitored by an attached gauge, controlled heat is gently applied to the mixture in the distilling flask. The application of heat must be gradual to achieve a smooth rate of vaporization. Unlike distillation at atmospheric pressure, a magnetic stir bar or capillary tube is often used for agitation instead of traditional boiling stones, as the latter do not function reliably under a vacuum.
As the liquid begins to boil, the vapor rises, and the thermometer measures the temperature of the vapor, which corresponds to the boiling point of the component being purified. The vapor then passes into the condenser, where it cools and turns into pure liquid droplets that flow down and are collected in the receiving flask.
Practical Applications and Benefits
Vacuum distillation is preferred over simple distillation because of its ability to separate compounds without exposing them to damaging high temperatures. Purification of heat-sensitive compounds is a major advantage, as they would otherwise decompose or degrade before reaching their atmospheric boiling point. This is important in the pharmaceutical industry for purifying active ingredients and in the food industry for extracting delicate essential oils and flavors from plant material.
The technique is also highly effective for separating compounds that possess extremely high boiling points, often exceeding 200°C, which would require excessive energy and specialized heating equipment under normal pressure. A prominent industrial example is the refining of crude oil, where a vacuum tower is used to separate heavy lubricating oils and residues from the crude mixture after the lighter components have been removed. By significantly lowering the necessary operating temperature, vacuum distillation improves product purity and integrity while also reducing energy consumption.