Vacuum distillation

vacuum distillation ,its purpose and its uses and advantages and disadvantages of this process....

1. Outline: 1- Definition 2- Principle 3- Purpose 4- Construction And Diagram 5- Working 6- Advantages And Disadvantages 7- Uses 8- Conclusion VACUUM DISTILLATION

2. Definition: Vacuum distillation is a method of distillation whereby the pressure above the liquid mixture to be distilled is reduced to less than its vapor pressure(usually less than its atmospheric pressure) causing evaporation of the most volatile liquid(s) ( those with the lowest boiling points). Principle: * Liquid boils when vapor pressure equal to the atmospheric pressure, i.e., pressure on its surface. If the external pressure is reduced by applying vacuum , the boiling point of liquid is lowered. * Therefore, the liquid boils at a lower temperature. This principle is illustrated using an example of water. Water boils at an 100 degree Celsius at an atmospheric pressure is 101.31 kPa ( 760mmHg). At 40 degree Celsius , the vapor pressure of water is approximately 9.33kPa (70 mmHg). Hence , the external pressure is reduced to 9.33kPa (70mmHg) where water boils at 40 degree Celsius. The net result is the increase in rate of mass transfer into vapor.

3. Purpose: The purpose of vacuum distillation is that it allows the purification of compounds not readily distilled at ambient pressures or simply to save time or energy. This technique separates compounds based on difference in boiling points. This technique is used when the boiling point of the desired compound is difficult to achieve or will cause the compound to decompose. A reduced pressure decreases the boiling point of compounds. Construction & Diagram: 1- It consists of a double- neck Distillation flask knows as “Clasien flask”. 2- Thick walled glass apparatus with interchangeable standard glass joints are used for vacuum distillation.

4. 3- In one of the necks of the Claisen flask, a thermometer is fitted. The second neck prevents splashing of the violently agitated liquid. 4- Bumping occurs readily during vacuum distillation. Placing a fine capillary tube in the second neck of the flask can prevent bumping. 5- The capillary tube is dipped in the boiling liquid, so that a stream of air bubbles is drawn out. 6- Water bath or oil bath is used for heating. 7- The Claisen flask is connected to a receiver through a condenser. 8- Vacuum pump is attached through an adapter to the receiver. A small pressure gauge (manometer) should be inserted between the pump and the receiver.

5. Working: Liquid to be distilled is filled ½ to 2/3 volume of flask. Small piece of porcelain are added to avoid the process of bumping. Then capillary tube and thermometer are placed in the flask. Required vacuum is applied through vacuum pump. The contents are heated and temperature of liquid raises and liquid get vaporized rapidly due to vacuum. Now these vapors pass through condenser and condensate is collected in receiver.

6. Advantages Disadvantages Using vacuum is often preferable for following reasons: Faster processing time. Effective processing of higher boiling point solvents without igniting them or causing thermal breakdown. Vacuum distillation can improve a separation by increasing capacity, yield and purity. Disadvantages of vacuum distillation are as follows: In vacuum distillation, persistent foaming occurs. This may be overcome by adding capryl alcohol to the liquid or by inserting a fine air capillary tube in the second neck of the Claisen flask. The steam of air is drawn in and breaks the rising foam. The above method is not suitable for the preparation of semisolid or solid extracts by distillation under vacuum.

7. Applications: The applications of vacuum distillation are as follows: Preventing degradation of Active constituents like Enzymes, Vitamins, Glycosides, Alkaloids. 2- Changing physical forms. Example: Cascara Sagrada Tablets at atmospheric pressure form dense compact residue , but at reduced pressure form light porous mass. 3- This type of distillation is used in oil industry. 4- Separation of impurities. 5- Separation of thermolabile (heat sensitive) substances. 6- Extracts or crude drugs are prepared by this process. Conclusion: Some materials breakdown at high temperature. With the use of vacuum distillation column, the compounds can be separated at lower temperature without effecting the components. Purification of compounds through vacuum distillation is more efficient.