Mean free path and molecular distillation

Thermally unstable compounds and gas molecules with high boiling points and low volatility are separated by molecular distillation. Molecular distillation separates molecules based on their mean free path. The vacuum is so adjusted that the mean free path between two molecules does not allow molecular collisions.

Simply it is a distillation method that does not allow the collisions between two molecules by increasing the mean free path of the molecules. The mean free path increases with increasing temperature. As the temperature increases, the average speed of each molecule will increase. It is the reason why the smell of hot sizzling food reaches several meters away from than smell of cold food. The mean free path increases with decreasing pressure of the gas and the diameter of the gas molecules.

Now imagine what happens?

When there are no / fewer molecular collisions the vapor pressure of the substance decreases. The substance distills at a lower temperature where it does not get decomposed, typical examples are oils and there are many heat-sensitive pharmaceutical products like vitamins

Molecular distillation and free path [ Background reading]

Molecular distillation ???????????????????????????????????????????????????????????????????????????????????????????????????????????????

This is a type of short-path vacuum distillation that uses a molecular still to achieve extremely low vacuum pressures of 0.01 torr or less. It's a method for separating, purifying, and concentrating natural substances, as well as complicated and thermally sensitive compounds like vitamins and polyunsaturated fatty acids. The distilled liquid is exposed to high temperatures for a brief time at a high vacuum (about 104 mmHg) in the distillation column, and the distance between the evaporator and the condenser is around 2 cm.

In molecular distillation, fluids are in the free molecular flow regime, i.e., the mean free path of molecules is comparable to the size of the equipment. The gaseous phase no longer exerts significant pressure on the substance to be evaporated, and consequently, rate of evaporation no longer depends on pressure.

Molecular distillation is used industrially for the purification of oils

Mean free path

At room temperature, gas molecules travel at a pace of several hundred metres per second. Even if we are closer to the room, the odour of an open perfume bottle takes a while to reach us. The reason for the delay is that the molecules' odour cannot get directly to us because it collides with surrounding air molecules and flows in a zigzag pattern. The mean free path (λ) is the average distance that a molecule travels between collisions. Using kinetic theory, we may determine the mean free path.

Expression of mean free path

The molecules of a gas are in random motion and collide with each other, according to kinetic theory postulates. A molecule moves along a straight path with a uniform velocity between two collisions. This approach is referred to as the mean free path.

Consider a collection of molecules, each of which has a diameter of d. The number of molecules per unit volume is denoted by n.

Assume that just one molecule is moving while the rest remain stationary, as shown in Figure.

If a molecule moves with an average speed v in a time t, the distance traveled is vt. In this time t, consider the molecule to move in an imaginary cylinder of volume πd2vt. It collides with any molecule whose center is within this cylinder. Therefore, the number of collisions is equal to the number of molecules in the volume of the imaginary cylinder. It is equal to πd2vtn. The total path length divided by the number of collisions in time t is the mean free path.

Mean free path = The distance [λ] travelled / No of collisions

1. Mean free path increases with increasing temperature. As the temperature increases, the average speed of each molecule will increase. It is the reason why the smell of hot sizzling food reaches several meters away from than smell of cold food.

2. Mean free path increases with decreasing pressure of the gas and diameter of the gas molecules.

Molecular distillation

The principle and the process

Molecular Distillation is a distillation process in which molecules travel a mean free path without coming into collision with one another by applying vacuum that increases the mean free path and is also condensed separately.

When liquids have low viscosity and density, as well as a long mean path, distillation is straightforward. The evaporating surface and the condensing surface should be separated by a minimum distance so that molecules travel through the condenser as soon as they leave the evaporating surface. Short path distillation is another name for molecular distillation. The intermolecular distance should be as large as possible, which can be accomplished by using a very high vacuum (0.1 to 1.0 pascal)

Principle

Falling film Molecular still

It consists of a vessel with a diameter of one meter and a jacket that wraps around it. The wiper is also linked to the vessel's wall. The rotor also connects the wipers to the revolving head. A wide diameter pipe connects the vacuum pumps. The condenser is mounted on the wall as well. Feeding (at the top) and collecting the distillate and undistilled liquid residue are also included (at the bottom)

The vessel walls are heated suitably by a heating jacket. The feed flows down the walls and is spread to a film by the polytetrafluoroethylene (PTFE) wipers which move about 3 m/s giving a film velocity of about 5 ms. Due to heat liquid film evaporates and vapors hit the condenser. There are no intermolecular collisions due to long mean free path. Collect the condensate as a product The residue is collected at the bottom of the vessel and it is re-circulated (through the feed line).

Credit: Google