Short path distillation is very high vacuum distillation. Operating pressure at distillation surface in short path distillation unit (SPDU) could be as low as 10–6 bar (1 mbar). Short path distillation provides the distillation at the minimum possible temperature which is desirable for heat sensitive products. In distillation, maximum boiling point is of the bottom product. If the normal boiling point of any product of distillation is higher than the decomposition temperature or cracking temperature, distillation cannot be carried out at atmospheric pressure. Reduction in absolute pressure results in the reduction in distillation temperature or boiling temperature. In lower pressure range, more reduction in boiling temperature of pure component is observed for a small reduction in operating pressure. For example, boiling point of stearic acid is reduced by 0.2°C when operating pressure is reduced from 251 mbar to 250 mbar. But, its boiling point is reduced by 3.8°C when operating pressure is reduced from 5 mbar to 4 mbar. In both cases, reduction in operating pressure is 1 mbar but in lower pressure range reduction in boiling point is quite high. Hence, reduction of pressure in lower pressure range is important as it creates considerable reduction in boiling point. For a food grade product (for example, orange juice) and for a highly heat sensitive product (for example, vitamin E, i.e., a–tocopherol) reduction in boiling point means better recovery. Maximum recovery is obtained at the minimum boiling temperature. Decreases in pressure results in the increase of vapour volume per unit mass and in turn it results in increase in vapour velocity. In the lower pressure range, even low vapour fl ow rate or non-condensable fl ow rate results in relatively higher value of pressure drop.
In a used oil recycling plant, the short path distillation unit is used to separate and purify base oils from contaminants like aromatic hydrocarbons and moisture. The feed (used oil) is preheated and introduced into the short path evaporator. Under reduced pressure, the lighter and more volatile components vaporize and condense on the nearby condenser. The purified base oil is collected as the distillate, while contaminants remain as residue. This process allows for the recovery of high-quality base oils suitable for reuse.
This method is particularly advantageous in the recycling industry due to its ability to handle feedstocks with complex compositions, efficiently separating valuable components while minimizing degradation.