Solid/Liquid Extraction

GENERAL INFORMATION

Solid/Liquid extraction or leaching processes have a very long tradition and are widely used in the food, pharmaceutical and chemical industries. The basic idea is to dissolve components out of a solid material with a liquid solvent. Examples in every day’s life are the preparation of coffee or tea dissolving flavors with hot water out of grinded coffee beans or tea leaves. For pharmaceutical applications also other solvents as e.g. alcohols and paraffines or liquified gases as carbon dioxide and propane are commonly used to extract flavors and pharmaceutical ingredients from natural, mainly plant based, material. In the chemical industries almost any solvent incl. the a.m. liquified gases can be found as extracting solvent. In metallurgical applications also acids are commonly used for the leaching process. A suitable solvent does not only provide a high solubility for the component of interest but should also be easy to separate from the extracted component.

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THE BASIC PROCESS STEPS

Solid/liquid extraction systems from De Dietrich Process Systems not only enable the extraction process itself but also permit the preparation of the solvent and the preconcentration of the extract. If temperatures sensitive substances are extracted special care has to be taken to avoid elevated temperatures throughout the process. This is very often the case when extracting flavors and ingredients from natural raw material. The complete extraction process can be divided in 3 procedures:

• Preparation of the solvent
• Extraction process itself
• Preconcentration of the extract

The final purification of the extract is commonly realized downstream from the solid/liquid extraction unit.

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PREPARATION OF THE SOLVENT

The consistence and temperature of the solvent has an important impact on the yield and product quality. One possibility to purify a solvent is its evaporation. This can be realized with electrical heating jackets for spherical vessels or with steam or heat transfer fluids for immersion heater or jacketed vessels. The subsequent condensation of the vapors provides a purified solvent. Its temperature of the condensate depends on the orientation of the condenser. An uprising condensation delivers the solvent at boiling temperature whereas a down coming version at lower temperatures. An additional heat exchanger permits further temperature adjustments of the solvent.

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EXTRACTION

Prior to the batch extraction the solids have to be charged into the extraction device. This can be a stirred vessel or a receiver with an internal retainer, support or filter for the solids.

The flow of the solvents through the solids is mainly done in 4 different semi-batch modes:

• Trickle bed – trickling of the solvent from the top of the solids towards the bottom of the receiver.
• Underflow extraction – the solids are flooded with solvent passing from the top to the bottom of the receiver.
• Overflow extraction – the solids are flooded with solvent passing from the bottom to the top of the receiver.
• Soxhlet – repeated cycles of filling from the top and draining through the bottom of the receiver.

After the extraction process the solids have to be taken out, the receiver been cleaned and reloaded with fresh solids for the next extraction process. The product of the extraction process is solvent containing the extract and guided into a receiver.

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