Industrial Extractor Technologies: Methods & Applications

Fundamentals of Solid-Liquid Extraction

Key Applications in Various Industries

• Extraction of sugar from sugar beets
• Coffee decaffeination
• Extraction of essential oils from herbs and botanicals
• Recovery of oils from seeds and other materials
• Obtaining pectin (for jelly) from fruit fractions

Commonly Used Solvents

• Water
• Hexane
• Heptane
• Ether
• Chloroform
• Benzene
• Ethanol

Industrial Extractor Equipment Designs

Static Bed Extractors

These extractors operate with a stationary solid bed through which the solvent passes.

Simple Contact Extractors

These operate discontinuously and are simple in design. They consist of a double-bottom tank where the solid material is placed. A liquid solvent is sprayed over the solid and percolates through it by gravity. The solvent, now containing the extracted solute, is drained and can be recirculated. This type is typically used for small-scale operations.

Multiple Contact Extractors

These systems involve several simple contact units connected in series. Heaters are often placed between units to enhance extraction efficiency. They are commonly used for extracting coffee, tea, oils, and sugar from sugar beets.

Moving Bed Extractors

In these systems, the solid material moves continuously or semi-continuously relative to the solvent flow.

Bollmann Extractor

The Bollmann extractor resembles a large wheel equipped with multiple baskets, all housed within an airtight container subjected to steam. The solid material (solute) is placed in these baskets. Solvent is introduced via showers, percolating through the solid to extract the solute. The solvent-solute mixture is collected from the bottom.

Rotocel Extractor

The Rotocel extractor is a cylindrical tank internally divided into multiple cells by a slowly rotating assembly. The outer tank is stationary. Solid material is fed into the cells, and solvent is sprayed onto it. After extraction, the depleted solid is discharged. This type is frequently used for oilseed extraction (e.g., soybeans) and sugar beet processing.

Hildebrandt Extractor

This extractor consists of two vertical cylindrical towers of different heights, connected at their base by a short, horizontal cylinder. Inside the towers, a screw conveyor (worm screw) with perforated fins transports the solid material. The solid is charged into the shorter tower and conveyed upwards. Solvent is introduced into the taller tower and flows counter-currently to the solid. It is used for applications like extracting oil from soy flakes and sugar from sugar beets.

Supercritical Fluid Extractors (SFE)

Supercritical Fluid Extraction (SFE) is a modern separation technique applicable to both solid and liquid materials. It employs solvents that are typically gases at ambient conditions but behave as supercritical fluids at specific temperatures and pressures above their critical point.

Principle of Supercritical Fluid Extraction

Under supercritical conditions, a substance exists in a state where distinct liquid and gas phases do not exist. These fluids possess unique properties beneficial for extraction: high density (like liquids, enabling good solvent power) and low viscosity (like gases, allowing better penetration into the solid matrix and improved mass transfer). The absence of surface tension also contributes to their efficiency.

Carbon Dioxide (CO2) as a Preferred Solvent: CO2 is widely used in SFE due to its advantages: it is non-toxic, non-flammable, inert, readily available, inexpensive, and its critical point (31.1 °C and 73.8 bar) is easily achievable. It can be safely vented after use and leaves no solvent residue in the extracted product.

SFE Equipment and Process

SFE equipment typically consists of:

• An extraction tank where supercritical conditions are maintained. The material to be extracted is placed in this tank.
• An expansion tank (or separator) which is kept at a lower pressure.

During the process, the supercritical fluid dissolves the target compounds from the material in the extraction tank. The resulting solution then flows to the expansion tank. Here, the pressure is reduced, causing the supercritical fluid to revert to its gaseous state and release the extracted compounds (the extract). The gas can then be re-compressed, its temperature adjusted, and recycled back to the extractor for further use.

Common Applications of SFE

• Removal of cholesterol and triglycerides from food products like eggs and milk fat.
• Decaffeination of coffee and tea.
• Production of concentrated hop extracts for the brewing industry, often eliminating bitter tastes.
• Extraction of essential oils, flavors, and fragrances from natural products.

Limitations of SFE Technology

• The primary limitation is the high capital cost for equipment due to the high-pressure systems required.