What is the Difference Between Molecular Distillation and Short Path Distillation

The key difference between molecular distillation and short path distillation is that molecular distillation uses extremely low vacuum pressure and it is a type of short path distillation technique, whereas short path distillation is an analytical technique and a type of distillation that allows the sample to travel a short distance at reduced pressure.

Molecular distillation is an analytical technique and a type of short path vacuum distillation that uses an extremely low vacuum pressure. This technique also uses a molecular still. Short path distillation is an analytical technique and a type of distillation that allows the sample to travel a short distance at reduced pressure.

CONTENTS

1. Overview and Key Difference
2. What is Molecular Distillation 
3. What is Short Path Distillation
4. Molecular Distillation vs Short Path Distillation in Tabular Form
6. Summary – Molecular Distillation vs Short Path Distillation 

What is Molecular Distillation?

Molecular distillation is an analytical technique and a type of short path vacuum distillation that uses an extremely low vacuum pressure. This technique uses a molecular still. The vacuum pressure that is used for this process is as low as 0.01 torr or below. This technique is important in separating components in a mixture, in purification processes, and in the concentration of natural products. Moreover, we can use molecular distillation for natural products, complex compounds, and thermally sensitive molecules such as vitamins and polyunsaturated fatty acids.

Molecular distillation involves short-term exposure of the distillate liquids to high temperatures under a high vacuum that can be observed in the distillation column. There is a small distance between the evaporator and the condenser, which is about 2 cm. In this process, fluids are in the free molecular flow regime. For example, the mean free path of the molecules is comparable to the size of the equipment.

Figure 01: Distillation at Laboratory

Usually, the gas phase does not exert significant pressure on the substance that is going to evaporate. As a result, the rate of evaporation does not depend on pressure. Further, the movement of the molecules through the column is in a straight line of sight. This is because these molecules do not create a continuous gas anymore. Therefore, it is necessary to have a short path between the hot surface and the cold surface. Typically, this is obtained by suspending a hot plate covered with a film of feed, which is placed next to a cold plate having a line of sight in between.

The major advantage of this process is the ability to avoid the problem of toxicity that occurs due to the solvent we are using in this technique as separating agents. Moreover, another important advantage is minimizing the losses due to thermal decomposition, which can be useful in a continuous feed process. This allows harvesting distillate without any break in a vacuum.

Industrially, molecular distillation is important in purifying oils, enriching borage oil in gamma linoleic acid, and recovering tocopherols from deodorizer distillate of soybean oil.

What is Short Path Distillation?

Short path distillation is an analytical technique and a type of distillation that allows the sample to travel a short distance at reduced pressure. This short distance is as short as 2 cm. Often, this type of short path distillation technique has a variety of names. The name depends on the manufacturer of the system and the compounds that are going to be distilled using this instrument. E.g. distillation involving the distillate travelling from one glass bulb to another (which does not need any condenser separating the two chambers).

Figure 02: Different parts of short path vacuum distillation apparatus; 1. Still pot, 2. Cold finger 3. Cooling water out 4. Cooling water in 5. Vacuum inlet 6. Distillate flask.

Often, short path distillation is useful for compounds that are not stable at high temperatures and also to separate and purify small amounts of the compound. The major advantage of using this analytical technique is that heating temperature can be considerably lower at reduced pressure compared to the boiling point of the liquids (at the given standard pressure value). Moreover, the distillate has to travel only a small distance before the condensation process. Using a short path can ensure the loss of only a small amount of compound on the sides of the apparatus. A good example is the Kugelrohr distillation process.

What is the Difference Between Molecular Distillation and Short Path Distillation?

The key difference between molecular distillation and short path distillation is that molecular distillation uses extremely low vacuum pressure and it is a type of short path distillation technique, whereas short path distillation is an analytical technique and a type of distillation that allows the sample to travel a short distance at reduced pressure.

The following table summarizes the difference between molecular distillation and short path distillation.

Summary – Molecular Distillation vs Short Path Distillation

Molecular distillation and short path distillation are important analytical techniques. The key difference between molecular distillation and short path distillation is that molecular distillation uses extremely low vacuum pressure and it is a type of short path distillation technique, whereas short path distillation is an analytical technique and a type of distillation that allows the sample to travel a short distance at reduced pressure.