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Checklist

Distillation at Atmospheric Pressure, Under Vacuum, or Under High Vacuum? A Decision-Making Checklist for Choosing the Right Process

Substances containing components with different boiling points can be separated, concentrated, or purified using thermal separation/distillation. A variety of criteria determine whether distillation can be performed at normal pressure (= atmospheric distillation), or whether vacuum distillation (e.g., thin-film distillation/thin-film evaporation) or perhaps even high-vacuum distillation (= molecular distillation/short-path distillation) is required. In addition to pressure, there are other decision criteria (scraper/possibility of horizontal distillation/possibility of combining with a column). The following checklist helps in determining the correct process:
 

In these cases, high-vacuum distillation (short-path distillation = molecular distillation) is absolutely necessary:

Pressures up to: 10⁻³ mbar
Objective: thermal protection + removal of heavy ends, colorants, odor and flavor compounds, oxidation products, impurities, and contaminants

  • The product is highly sensitive to heat (decomposition begins even at relatively low temperatures, well below the boiling point)
  • The residence time during distillation/evaporation must be minimal (in the range of seconds)
  • High-boiling impurities must be removed (distillation/evaporation of substances with very high boiling points, e.g., 500–600 °C, is possible under atmospheric conditions)
  • Strict legal or regulatory limits require extremely low residual levels of the impurities to be removed—such as permissible concentrations of volatile organic compounds (VOCs)
  • Odor and taste neutralization of natural pigments
    (chlorophyll, beta-carotene, anthocyanins → removal of off-flavors)
  • MOSH/MOAH separation – short-path distillation enables the lowest achievable residual levels of MOSH/MOAH through extremely low pressures – a level of purity that cannot be achieved with any other thermal separation process
  • Refining of high-quality edible oils (e.g., separation of free fatty acids)
  • Concentration of emulsifiers, mono- and diglycerides
  • Purification of lactic acid to separate salts (ions), ash, and catalysts. The highest degrees of purity can be achieved, up to ultra-pure lactic acid that meets the highest standards, including a quality test in the form of a heat stability test. This lactic acid can be used for the production of PLA (polylactic acid).
  • VOC removal (volatile organic compounds)
  • Color optimization of natural substances, resins, and esters
  • Removal of high-molecular-weight byproducts in specialty chemicals
  • Extraction of cannabinoid oils/hemp oil
  • Recovery of valuable substances from by-products (e.g., extraction of natural tocopherol/vitamin E from deodorization distillates from edible oil production)

Thin-film distillation is recommended in the following cases

(thin-film distillation/wiped film distillation offers, in addition to pressure reduction, the best options for processing a highly viscous feed. This is primarily due to the special properties of the wipes and the turbulent product film they generate. Furthermore, a wide variety of configurations are possible: with or without a column, horizontal or vertical, reactive distillation, etc.)
Pressure range: Anything between atmospheric pressure (1 bar) and as low as 1 mbar is possible

  • The product has a boiling point too high for atmospheric distillation/evaporation
  • This product is temperature-sensitive
  • The feed is viscous, yet a thin film is essential for optimal heat transfer. For non-homogeneous products, horizontal thin-film distillation with longer residence times is particularly suitable
  • Low-boiling-point substances should be removed (moisture, solvents, monomers, etc.)
  • Switching from batch processes to a continuous process to improve efficiency. This eliminates the need for regular restarts (including refilling, cleaning, and inspection)
  • Deposits must be prevented using wiper technology optimized for the application
  • Evaporation with a high turndown ratio without product recirculation
  • Recovery of valuable materials from by-products (e.g., lecithin extraction during sunflower oil and soybean oil refining)
  • Viscous products (oils, waxes, polymers, APG/soap, formaldehydes)
  • Inhomogeneous substances (alcohol and water extracts from herbs)
  • Separation of omega-3 fatty acids EPA and DHA (fish oil refining)
  • Evaporation of the following substances as a preliminary step for degassing and preconcentration in a multi-stage distillation process that subsequently also utilizes flash evaporation:
    • Water
    • Solvents
    • Other low-boiling substances
    • Distillation of corrosive substances (lactic acid)
    • Monomers from prepolymers
  • VOC-removal
  • Lecithin dehydration
  • Glycerin concentration
  • Use of the thin-film evaporator as a reboiler for rectification or fractionation columns
     

In these cases, atmospheric distillation/evaporation is sufficient

(conventional column, batch, or continuous)

  • The product is temperature-stable
  • No quality issues (color/odor)
  • No risk of thermal decomposition
  • Standard chemical products without sensitivity
  • Separation of conventional solvents
    • Ethanol/water (up to azeotropic limits)
    • n-hexane / cyclohexane (with restrictions)
  • Dehydration/degassing of non-critical feed streams
  • Distillation of robust intermediates
  • Concentration of stable chemicals