Basic Principle and Methodology of Distillation Under Reduced Pressure and Steam Distillation : Pharmaceutical Guidelines -->

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Basic Principle and Methodology of Distillation Under Reduced Pressure and Steam Distillation

Water's boiling point increases when the external pressure is increased, while its boiling point decreases when the external pressure is decreased.

Distillation Under Reduced Pressure


Water's boiling point increases when the external pressure is increased, while its boiling point decreases when the external pressure is decreased. The freeze-drying process makes use of this principle. Because their boiling points are so high, many compounds are not able to be distilled at atmospheric pressure. The compounds decompose at average boiling points. The boiling point of some of these materials can be reduced significantly by distilling them under reduced pressure, since the required temperature to boil the liquid can be lowered accordingly. If you reduce the external pressure by halving the boiling point by 10 °C. You can either raise the temperature of a liquid or decrease the pressure of a liquid to make it vaporize. As a result of the reduced pressure inside the distillation column, vacuum distillation is also known as vacuum distillation.

With vacuum distillation, the pressure inside the distillation column remains constant in order to lower the temperature necessary to vaporize the liquid. Heat-sensitive products, low-viscosity liquids, and fluids that foam or foul are the best candidates for this distillation method. Vacuum distillation involves the addition of vacuum pumps to the distillation system, allowing the column pressure to be reduced below atmospheric pressure. See the figure below for an example. Separation depends on the relative volatility of the materials at a given temperature and pressure, which requires careful pressure control. Separation can be negatively affected by changes in relative volatility.

As a result of this distillation technique, boiling is caused by the vapour pressure of a liquid exceeding the ambient pressure. Regardless of whether the mixture is heated or not during vacuum distillation, boiling occurs.


Distillation unit with vacuum consists of a distillation column, distillate condenser, and reboiler. To maintain a vacuum in distillation columns, vacuum pumps and regulators are used. By using these vacuum distillation columns, it is possible to distill many mixtures at temperatures that are much more economical.

Low-temperature distillation is also known as vacuum distillation. A distilled product, a purified product, or a remaining product can be the result of this distillation. Depending on what product is being distilled, the vacuum pressure involved in distillation may vary. Vacuum distillations of volatile substances such as those used in oil refineries, for instance, are likely to operate at higher pressures than 1 Torr, perhaps at 20-50 mmHg.

Steam Distillation


In steam distillation, miscible liquids are separated according to their volatilities. It is because of the low boiling point of these products that the constituents can easily vaporize. Different liquids exert different vapor pressures based on their specific temperatures. Natural products boil when they exceed atmospheric pressure, so the boiling point of the liquids occurs when that pressure exceeds that of the liquid. This causes the vapour pressure to increase overall.


Water is poured into the steam can. A small quantity of water is added to the non-aqueous liquid in a flask. The flask is then heated slightly. This causes steam to bubble up in the contents. Steam is mixed with vapors of the compound in the condenser, where they escape. It is an aqueous and non-aqueous liquid mixture that can be separated.

In this case, the bottom of the still is perforated or mesh. Water is boiled below the mesh to produce steam. Steam passes through the mesh, over which the materials are packed to be extracted. Once the vapour containing the oil has passed through the condenser, it is then condensed. A Florentine receiver collects the distillate. Oil and water are separated in Florence receivers based on their densities. Water can be recycled again without losing any volatile oil to the aqueous phase.

There are two types of Florida Receiver: Type-I is designed to separate heavy oils from lighter oils. Oil heavier than water is separated by Type-II. With Type-I receivers, oil is collected from the bottom of the vessel while water is collected from the top of the vessel. Type II receiver has a bottom siphon, which works when filling with water, whereas the top tap allows oil to flow out.
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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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