Five factors that influence solvent extraction rates

One of the most common separation methods used in food, chemical, environmental and nutraceutical industries is solid-liquid extraction. The technique is used as a basic step in the processing and analysis of many foods or natural products, for the recovery of active constituents or for the removal of undesirable compounds from raw materials. In environmental labs, solid-liquid extraction is used as a sample preparation step for analysis of environmental contaminants, such as dioxins, PAH and PCBs.

In solid-liquid extraction, certain compounds are dissolved out from a solid matrix by a liquid solvent forming a solution. The solvent used should have suitable properties to dissolve the current substances from the insoluble permeable solid. This is important for quantification purposes and to increase process yield.

The removing soluble fraction is called the solute and the resulting solution of the solute is called the extract. Normally, the solvent is recovered and reused in the extraction process.

Factors that influence the rate of extraction

During solid-liquid extraction, it is important to maintain constant fluid flows, pressures and temperatures and to provide sufficient contact time between the solvent and the solids. Factors, such as particle size of the solid, the solvent, the nature of the compound to be extracted, as well as the raw material to be processed should be considered when devising the most appropriate extraction strategy.

1.??????Solvent - A solvent with a similar polarity to that of the analyte should be chosen. Lower viscosity is beneficial for faster diffusion rates in the solvent phase. Ideally, the solvent should be nontoxic, nonreactive, nonflammable, environmentally friendly and affordable.

?2.??????Particle size – Grinding before solvent extraction promotes an increase of the contact area between the solvent and the solid matrix. Extraction efficiency increases with decreasing particle size.?However, smaller particle sizes also result in lower drainage rates and may create problems if liquid circulation is hindered.

?3.??????Sample Humidity – the water in the solid material can compete with the extraction solvent for the solute’s dissolution, so in most cases the material is dried under conditions that do not cause degradation of the compounds.

?4.??????Temperature – the solubility and diffusivity of the material being extracted increases with increasing temperature, hence improving the rate of extraction. An upper temperature limit should be set to avoid undesirable degradation reactions and to ensure the solvent remains in a liquid state.

5.??????Solvent agitation - Agitation of the solvent increases turbulent diffusion and material transfer rates.