Molecular Distillation And Short Path Distillation

Molecular distillation equipment is a highly efficient distillation device that separates mixtures by exploiting differences in the mean free path of molecular motion. Its principles, functions, and effects are as follows:

I. Principle

Molecular distillation is based on differences in the mean free path of molecular motion. In a high vacuum environment (typically <1 Pa), when a liquid mixture is heated, the molecules escape the liquid surface (evaporate). Different types of molecules have different mean free paths (the average distance a molecule travels between two collisions).

Light molecules (low boiling point, small molecular weight) have long mean free paths and can escape collisions with surrounding molecules, reaching the condensation surface and being captured.

Heavy molecules (high boiling point, large molecular weight) have short mean free paths and collide with other molecules before reaching the condensation surface, returning to the liquid surface.

By setting an extremely short distance (typically 0.5-5 cm) between the condensation surface and the evaporation surface, only the light molecules reach the condensation surface, thereby achieving separation of the mixture.

II. Main Functions

1. Low-Temperature Separation: Under high vacuum, the boiling point of liquids is significantly lowered (far below the boiling point at atmospheric pressure), enabling the separation of heat-sensitive substances (such as vitamins, essential oils, and enzymes) at low temperatures, preventing their decomposition or deterioration due to heat.

2. High-Efficiency Purification: It can separate near-boiling mixtures, high-viscosity substances (such as oils and resins), or mixtures of volatile and non-volatile components that are difficult to process using conventional distillation, achieving high purification precision (capable of obtaining high-purity monomers).

3. Continuous Operation: The equipment can be designed for continuous feeding, separation, and discharge, making it suitable for large-scale industrial production and improving separation efficiency.

III. Main Applications

1. Pharmaceutical and Chemical Industry: Purification of heat-sensitive drugs such as antibiotics, vitamins, and active ingredients in traditional Chinese medicines, removing impurities and solvents.

2. Food Industry: Separation of omega-3 fatty acids from fish oil, purification of plant essential oils (such as rose oil and lemon oil), and removal of harmful substances (such as plasticizers and heavy metals) from oils and fats. 

3. Fine Chemicals: Preparation of high-purity monomers (such as polymer monomers), separation of fragrances, and purification of cosmetic raw materials.

4. Environmental Protection: Treatment of waste oils and solvents for recovery and purification, reducing pollutant emissions.

In short, molecular distillation equipment, with its low temperature, high efficiency, and high vacuum, plays an irreplaceable role in the separation and purification of heat-sensitive, high-purity substances, and is widely used in the pharmaceutical, food, and chemical industries.

Molecular distillation and short-path distillation are two closely related but distinct concepts. The core differences between the two are reflected in their definition scope, equipment structure, and application scenarios, as follows:

1. Essential Definitions

Molecular distillation: A separation principle that uses the mean free path differences of different molecules to achieve separation under a high vacuum environment (usually <1 Pa). Its core principle is to achieve purification by exploiting the characteristics of molecular motion (light molecules have a long free path and can reach the condensation surface, while heavy molecules have a short free path and cannot reach it), emphasizing the "separation mechanism based on molecular motion." Short-path distillation is a type of equipment structure that refers to a distillation apparatus with an extremely short distance between the condensing and evaporating surfaces (typically less than 5 cm, close to the molecular mean free path). Its core design is the "short-path condensation" structure, which aims to reduce collisions between molecules on the evaporating and condensing surfaces, thereby improving separation efficiency.

2. Different inclusion relationships

Short-path distillation is a typical implementation of molecular distillation technology, namely:

All short-path distillation equipment operates based on the principle of molecular distillation (because only at a short distance can molecules minimize collisions, meeting the vacuum and free path requirements of molecular distillation);

However, molecular distillation technology is not exclusively achieved using short-path distillation equipment (theoretically, other structures can also achieve molecular distillation as long as they meet the conditions of high vacuum and molecular free path difference). However, in practice, short-path structures are almost always used, so the two are often used interchangeably).

3. Differences in Equipment Structural Details

The "short-path" in short-path distillation specifically refers to the extremely close distance between the condensing surface and the evaporating surface (for example, in a wiped-film short-path distillation, the condensing column is inserted directly into the evaporating chamber, a distance of only 1-3 cm). This design minimizes collisions between molecules during their movement, ensuring that light molecules efficiently reach the condensing surface and represents a "best practice" for molecular distillation principles.

Generally speaking, molecular distillation equipment that does not strictly limit the distance between the condensing surface and the evaporating surface (such as some early falling-film devices) may result in some molecular collision losses due to the slightly greater distance, resulting in slightly lower separation efficiency, but still falls within the scope of molecular distillation.

4. Subtle Differences in Application Scenarios

Molecular distillation, as a principle, is applicable to all applications requiring low-temperature separation of heat-sensitive, high-boiling-point substances (such as pharmaceuticals and essential oil purification).

Due to its structural advantage of "short-path distillation," short-path distillation offers higher separation efficiency and lower energy consumption, making it more suitable for industrial production with high-purity requirements (such as purification of high-end chemical raw materials and vaccine ingredients). It is currently the mainstream equipment form for molecular distillation technology.

Summary

Core Difference: Molecular distillation is a separation principle based on the free path of molecules, while short-path distillation is an equipment structure based on short-path condensation.

Relationship: Short-path distillation is a typical application of the molecular distillation principle. Almost all industrial-grade molecular distillation methods use a short-path structure. Therefore, in practical applications, the two are often regarded as "different expressions of the same technology." However, strictly speaking, short-path distillation is a highly efficient implementation of molecular distillation.