In general the separation of mixtures into fractions or its constituents by suitable solvents is called extraction. Organic solvents are partly toxic, inflammable and/or explosive. These disadvantages lead to a lot of effort in solvent recovery and plant safety. Therefore, supercritical fluids are often used for extraction offering in parallel the possibility to design a highly selective process.

Supercritical fluid extraction is a very important field in high pressure technology. Uhde has been engaged in this arena since 1980 by developments, design and engineering. These activities gained a lot of experience and expertise and lead to numerous patent filings for equipment and processes.

The keyword SFE describes a variety of applications and processes treating very different products. The most prominent application is the recovery of extracts from natural raw materials. Besides the sole extraction other processes are used in industry like upgrading of raw materials, extractive purification, cleaning of raw materials, etc.

• Recovery of extracts from natural raw materials
• Upgrading of raw materials
• Extractive purification
• Cleaning

Traditionally Uhde is in close contact to universities and R&D institutes on national and international basis and collaborates in R&D programmes which are partly publicly funded. In parallel Uhde develops solutions on special customer demand. Most processes as well as numerous equipment features were developed in close co-operation with customers, in some cases Uhde is the sole process provider.

The different process applications are based on a common process principle i.e. variation of solubility between supercritical and gaseous state and will be explained for natural substances. Special process features are described thereafter.

Natural substances comprise of different constituents. Some ingredients (e.g. oils, aromas, active ingredients) are highly valuable and normally forming part of small amounts of the raw material only. By means of SFE these ingredients can be separated from the raw material. The feedstock can be of liquid or solid state and SFE allows for gentle treatment and recovery of pure extracts in a few process steps only.

SFE is commercially applied for highly valuable, pure and close to nature extracts. Industrial examples – among others - are found in the following fields:

• Food industry:
  SFE of hops, pepper, etc. from spices, herbs and other plant materials
• Pharmaceuticals industry:
  SFE of active ingredients (e.g. anti-oxidants) from natural raw materials
• Cosmetics industry:
  SFE of fats, essential oils and waxes from plant material

Due to the advantageous properties CO₂ is mostly applied under extraction conditions typically between 35 and 80 °C and pressures as high as 750 bar. The extractor volume varies from a few millilitres in lab scale to some cubic meters in industrial scale.

For solid feedstock SFE is performed batch-wise, whereas liquid feedstock is processed in counter-current columns.

The autoclave C is filled with solid raw material and closed. Liquid CO₂ flows from the collecting vessel D after sub-cooling in heat exchanger E4 to the pump P. CO₂ is compressed to the required extraction pressure and transferred to the heat exchanger E 1 where it is heated up to the extraction temperature and transferred to the extraction vessel. The substances to be extracted are dissolved in the CO₂ as they pass through the extraction vessel C. The CO₂, laden with the dissolved substances, is then fed to the separator S. By adjusting the pressure and/or temperature, the dissolving power of the CO₂ in the separator is reduced, thus causing the extracts to precipitate. This may take place in several stages, thereby allowing extract fractions of different qualities to be obtained. The gaseous CO₂ from the separator is liquefied in a refrigerated condenser and collected in the collecting vessel D. The use of several extraction vessels for the extraction of substances in the solid state permits virtually semi-continuous operation.

In SFE of liquid feedstock the extraction vessel is replaced by a counter-current column. The liquid raw material is pumped from a storage tank and fed into the column head. In the column the liquid flows to the bottom being in intense contact with the supercritical fluid that is being fed into the bottom part of the column. While the supercritical fluid and the feedstock are in counter-current in the column, extractable substances are transferred from the liquid phase into the supercritical phase and dissolved in the fluid. The further process is similar to SFE of solid matter.

Besides the extraction of natural substances numerous applications of SFE are known in industry similar to the process described.