How to Make Short-Path Distillation

Short-path distillation is very suitable for the extraction and concentration of heat-sensitive and high-boiling substances. The equipment consists of a cylinder with a heating jacket, a rotor and a built-in condenser. The rotor mount is equipped with a scraping device and a splash-proof device.

The built-in short-path distillation condenser is located in the center of the evaporator and the rotor rotates between the cylinder and the condenser. The short path distillation consists of an externally heated vertical cylinder, a central central condenser, and a wiper that rotates between the distiller and the condenser.

At the beginning of the distillation, the material is fed from the top of the evaporator and evenly distributed over the heating surface through the material liquid distributor on the rotor. The scraper scrapes the material into a thin, turbulent liquid film that is propelled downwards. During this process, the light molecules that escape from the heated surface hardly collide through a short path, condense in the built-in condenser to form a liquid. Then light molecules flow down the condensing duct and pass through a discharge pipe located at the bottom of the evaporator. Heavy molecules are collected in a circular channel under the heating channel, then pass through the side of the tube and flow out.

Components of Short-Path Distillation

Chiller: The term “chiller” can be miss-leading when talking about short path. A vapor can be recondensed at a temperature that is at least 20 degrees colder than what is was boiled off at. For example, a compound boiled off at 400F could be recondensed at 380F. The 380F temp is “colder” than the 400F that the compound boiled off at but, this is by no means actually cold. When doing short path distillation, the temp range will be about 100C to 250C (212F to 482F) on the heating mantle. This means the short path condenser will be set between 40C and 60C. We prefer 50C (122F) for a starting point (temp differentials regarding specific fractions will be discussed in an advanced post). This means we do not neccessarily need a “chiller”. We just need a device that will keep our desired 50C temperature during the process. This 50C temp will keep our short path from clogging and will be “cold” enough to easily recondense any vapors that travel into the path of the condenser.

Vacuum Pump: When doing a distillation process, it is imperative that you have a chemically resistant vacuum pump that can acheive extreme depth. The pump should be able to achieve at least 50 micron but the deeper the better. The preferred type of pump for distillation is a rotary vane (oil) pump. Rotary vane pumps offer the best vacuum depth compared to other types of pumps (specialty pumps such as diffusion pumps will be discussed in a later post). Short path distillation can be very chemically abrasive to your vacuum pump. For this reason it is recommended to change your oil before every distillation. This will allow you the best depth and keep your vacuum pump running at peak performance.

Cold Trap: The cold trap plays the important role of catching any vapors that pass through the condenser. This protects and prolongs the lifespan of your vacuum system.

Short Path: The Short path itself is made up of 4 major glass components and the heating mantle. These components are: the boiling flask, the short path head, the receiving cow, and the receiving flasks. The heating mantle is similar to a heat bath on the rotovap. This is how you will heat the mixture inside the boiling flask. The head is the key to distillation. Inside the head there are small indentations called “vigreaux”. These create contact points for the vapors to bump as they travel up into the head. They create “theoretical plates” needed for distillation where vapors will become purified before entering the condenser path. The head also encompasses the condenser where the vapors will be pulled to re-condense into that beautiful golden color we all want to see. The three receiving flasks correspond to the three main fractions (single cows, swapable adapters, and head packing will be discussed in an advanced short path post).

short path distillation
Short-Path Distillation

Why use Short-Path Distillation

For meeting the need of concentrating and extracting substances from mixtures in many fields, people invented several distillation devices for achieving their destinations. Among the distillation devices invented, short path distillation is one of the most popular in the marketing. Short-path distillation is a high-vacuum distillation process in which the feedstock is heated in a short-path evaporator. The material was scraped into a film and the light components were evaporated under high vacuum and condensed on a built-in condenser. The separated light and heavy components are continuously discharged under high vacuum. Molecular distillation or vacuum short-path distillation is an more effective method when conventional distillation does not achieve the desired effect. In the distillation operation, short path distillation can achieve higher yields and more stable production control.

Complete Short-Path Distillation Kit

01. Evaporation equipment

Evaporation equipment is the key part of the whole equipment. It is in charge of evaporating solvents, which is to extract solvents from liquid sample. In this part, there are two parts, which are heating mantle and evaporation flask. Heating mantle is to improve the evaporation flask’s temperature till solvents’ boiling point in order to extract it. The temperature of heating mantle is adjustable and users can adjust temperature through control panel.

02. Receiving equipment

There are two different receiving flasks here in the whole equipment. Because short path distillation is to separate two solvents, then it is necessary to set two different containers.

03. Condensing equipment

Liquid solvents are easier than gas solvents to collect. So there is condensing equipment working here to cool the gas solvents down. In this part, low temperature coolant circulation pump and condenser work together to cool gas solvents down.

04. Vacuum pump

In the whole process, the equipment works under vacuum at the whole time. Vacuum pump provides vacuum environment for the reacting equipment.

Short path distillation process
Short path distillation process

Short-Path Distillation Process

1. Molecules diffuse from the liquid body to the evaporation surface.

Generally, the diffusion rate in the liquid phase is the main factor controlling the rate of molecular distillation, so the thickness of the liquid layer should be reduced as much as possible and the flow of the liquid layer should be strengthened.

2. Free evaporation of molecules on the surface of the liquid layer.

The evaporation rate increases with the increase of temperature, but the separation factor sometimes decreases with the increase of temperature. Therefore, the economical and reasonable distillation temperature should be selected on the premise of the thermal stability of the material to be processed.

3. Molecules fly from the evaporation surface to the condensation surface.During the evaporation of vapor molecules from the evaporation surface toward the condensation surface, they may collide with each other or with air molecules remaining between the two sides.

4. Molecule condensation on the condensation surface. As long as there is enough temperature difference between the hot and cold sides (generally 70~100 °C), the form of the condensing surface is reasonable and smooth, it is considered that the condensation step can be completed in an instant, so it is very important to choose a reasonable condenser form.

You can also refer to:

Short path distillation procedure

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