Distillation has been used for centuries to separate the components of liquid solutions through highly-selective heating and cooling. Multiple instruments are used to manipulate the differing thermodynamic properties of the fluidic compartments within a solution, encouraging mixtures to separate through boiling, evaporation, and condensation. This methodology has a broad range of applications but is conventionally associated with increasing the alcohol content of fermented beverages and purifying water sources for safe human consumption.
Short-path distillation is a distillation technique that involves the distillate travelling a short distance, often only a few centimeters, and is normally done at reduced pressure. A classic example would be a distillation involving the distillate travelling from one glass bulb to another, without the need for a condenser separating the two chambers. This technique is often used for compounds which are unstable at high temperatures or to purify small amounts of compound. With short path distillation, a decrease of boiling temperature is obtained by reducing the operating pressure. It is a continuous process with very short residence time. The advantage is that the heating temperature can be considerably lower (at reduced pressure) than the boiling point of the liquid at standard pressure, and the distillate only has to travel a short distance before condensing. A short path ensures that little compound is lost on the sides of the apparatus. The Kugelrohr is a kind of a short path distillation apparatus which often contain multiple chambers to collect distillate fractions.
What is short path distillation used for?
Short path and molecular distillation were developed in the 1930s and 1940s mainly in the UK and in the USA. In short path distillation, the produced vapours do not have contact with the liquid any more. The number of theoretical stages is 1 at a small distillate rate and may rise up to 2.2 at a higher evaporation rate. A first short path evaporator is used to distill off a fraction that meets the quality demands, but at a poor yield; too much of the light fraction remains in the residue. This residue is then distilled in a second still where the light fraction is removed completely unfortunately together with heavier components. This mixture is then recycled into the feed stream to the first short path evaporator. Short path distillation is used in the production of lactic acid, paraffin wax, monoglycerides, omega‐3 fatty acids, and monomers.
The Benefits of Short-Path Distillation
Short-path distillation is a compact purification method that is ideal for laboratory applications where minimal instrumentation footprint is essential. This low-pressure technique uses multiple flasks and comparatively short extraction feeds to separate condensate media across paths of just a few centimeters. Conventional distillation methods often require much larger apparatus to achieve the requisite purity levels for application specific requirements. Short-path distillation meanwhile can achieve distillate purity levels of up to 99% using multiple components in a smaller working space.