Ultrasonically Assisted Catalytic Extraction

Hielscher ultrasonic reactors are used in many industries to assist and improve the catalytic extraction processing (CEP) or so-called phase-transfer extraction (PTE). Catalytic extraction involves a heterogeneous immiscible phase system, such as liquid-liquid or liquid-solid. Ultrasonic high shear and cavitational forces improve dissolving rates of solutes significantly leading to a faster and more complete extraction. Additionally, this effect can be used to reduce the amount of solvent or acid used. As a proven technique, the ultrasonically assisted extraction is used increasingly due to an growing demand for environmental-friendly extraction techniques with shortened extraction time and reduced organic solvent consumption.

Catalytic Extraction/Phase Transfer Extraction – Fundamentals

The term “Catalytic Extraction Processing (CEP) or Phase Transfer Extraction (PTE) describes the liquid-liquid or solid-liquid distribution when the extraction and removal of analytes is focussed. Therefore, the liquid or solid diluent has to be dispersed/ emulsified into the solvent (liquid phase). By the term “extractant” only the active substance in the solvent is described (i.e. the homogeneous ‘organic phase’ which comprises the extractant, the diluent and/or the modifier) which is primarily responsible for the transfer of solute from the ‘aqueous’ to the ‘organic’ phase. [IUPAC]. The target matter, which is extracted is termed extract.
Traditional extraction methods such as soxhlet extraction, maceration, microwave, percolation, extraction under reflux and steam distillation, or turbo-extraction are often slow and inefficient and/or require a high amount of hazardous solvents resulting in a cost-intensive and time consuming process that is harmful to the environment.
Ultrasound is a proven alternative to conventional extraction methods providing a faster and more complete extraction with less or no hazardous solvents! Ultrasound is a powerful technique for green, evironmental-friendly processing.

Principle of Ultrasonically Assisted Catalytic Extraction

For the extraction of a substance, the immiscible phases must be mixed so that the substance to be extracted can be dissolved from the carrier phase into the solvent phase. Most commonly phase-transfer extractions are performed from a disperse phase into a continuous phase, which means droplets and particles need to be dispersed homogeneously into the solvent.
Power ultrasound is a well-known mixing and extraction technology that has several positive effects on the extraction process:

Benefits of Ultrasonic Extraction:

• break boundary layers
• overcome van-der-Waals forces
• move unsaturated liquid to contact surface
• reduce or eliminiate need for transfer agents
• reduce time, temperature and/or concentration
• less excess compared to volume required for full saturation
• less volume to be refined (e.g. by distillation, evaporation, drying)
• no continuously stirred reactors (CSR)
• save power
• no batching but inline processing
• use less acidic or cheaper solvent
• avoid solvents, use aqueous instead
• process high solids concentrations or high viscosity slurries
• green processing: environmental-friendly
• use organic acids, such as malic acid or citric acid
• avoid multistage extraction processes

Liquid-Liquid Extraction

Conventional Process: The liquid-liquid extraction is a partitioning method to extract substances from one liquid phase into another liquid phase based on the substances‘ relative solubilities in the two different immiscible liquid phases. The use of ultrasonics improves the rate at which the solute is transferred between the two phases by high performance mixing, emulsifying, and dissolving!
Liquid-liquid extraction is a separation technique to isolate and concentrate valuable components from an aqueous solution by using an organic solvent. The liquid-liquid extraction is often applied when a other separation techniques (e.g. distillation) are ineffective. Liquid-liquid extraction is used in the pharmaceutical & cosmetic (active compounds, APIs, fragrances), as well as food and agricultural industry, for organic and inorganic chemistry, petrochemical industry, and hydrometallurgy.

Problem: A common problem is the immiscibility of the liquid phases (solvent and diluent are immiscible), so that a proper mixing method is required. As an even mixing of both liquid phases promotes the phase transfer between diluent and solvent, a reliable dispersing or emulsification method is crucial. The finer the mixture and the higher the contact area between both phases, the better the soluent can travel from one liquid phase to another liquid phase. Conventional extraction processes mostly lack in the promotion of mass transfer so that the extraction process is slow and often incomplete. To improve the extraction, often excessive amounts of solvent are used, which makes the process expensive and environmentally polluting.
 

 
Solution: Ultrasonic Liquid-Liquid Extraction excels traditional liquid-liquid extraction techniques at various points:
Power ultrasound mixes two or more liquid phases reliable and easily together. By ultrasonication, droplets can be reduced to nano-size so that fine micro- and nano-emulsions are obtained. Thereby, the generated cavitational forces promote the mass transfer between the liquid phases. As sonication can be run in a continuous inline-system, large volumes and highly viscous liquids can be handled without problems.
But also micro extraction, e.g. for analytical purposes, can be improved by sonication, too (e.g. ionic liquid-based micro-extraction with ultrasonic emulsification).

Benefits of Ultrasonic Extraction:
Powerful ultrasonic forces – generated by low frequency/ high power ultrasound – helps to

• reshape droplets
• avoid emulsion transfer agents or amphiphillic catalysts
• avoid use of detergents or surfactants
• avoid amphiphillic catalsts, detergents or surfactants
• generate turbulent unstable emulsions without surfactant layers

Solid-Liquid Extraction Improved by Ultrasonics

Goal of the solid-liquid extraction or solid-phase extraction (SPE) is it to separate analytes, which are dissolved or suspended in a liquid mixture, and to isolate them from a matrix according to their physical and chemical properties. Therefore, the isolate is eluted from the sorbens with aid of an appropriate solvent. The extracted substance is called elute.
Conventional SPE techniques are the maceration, soxhlet extraction, percolation, combination of reflux and steam distillation, or high speed mixing/ turbo-extraction. The solid-liquid extraction is a common procedure to separate compounds in biology, chemistry as well as in the food, pharmaceutical and cosmetic industry. The extraction of metals is also known as leaching.
Problem: Conventional SPE techniques are known as time-consuming and require relatively large quantities of solvents that are mostly environmentally hazardous and polluting. High process temperatures can even lead to the destruction of thermal sensitive extracts.
Solution: With an ultrasonically assisted solid-liquid extraction, the common problems of traditional SPE can be normally overcome. As sonication provides a fine distribution of the solids in the solvent phase, a larger interfacial boundary is available so that the mass transfer of the target substance into the solvent is improved. This results in a faster and more complete extraction whilst the solvent use is reduced or completely avoided (use water as liquid phase instead). By the application of power ultrasound, the solid-phase extraction can be carried out more efficient, economic, and environmetal-friendly. Due to the reduction or avoidance of polluting or hazardous solvents, ultrasonic extraction can be considered as environmental-friendly green process. Economically, the process costs are reduced due to the savings of energy, solvent, and time.

Ultrasonic Solvent Extraction

In case of a solvent extraction, a solvent (e.g. an organic solvent) is used to dissolve and separate a compound from a another liquid (e.g. an aqueous phase). Generally, the more polar solutes dissolve in the more polar solvent, and the less polar solutes in the less polar solvent. Using solvent extraction, it is possible to separate oxidized thiophenes (sulfoxides, sulfones) from an oil phase using acetonitrile or other polar solvents. Solvent extraction is also used to to extract materials, such as uranium, plutonium, or thorium from acid solutions into organophosphate tri-n-butyl phosphate (PUREX process).
Reduce your solvent use: The use of ultrasonics minimizes the use of solvents in the process and optimizes the product load in the solvent. It also leads to a faster and more complete extraction.
Click here to read more about the Ultrasonically-Assisted Oxidative Desulphurization!

Ultrasonically-Assisted Soxhlet Extraction

Soxhlet extraction is a solid-liquid extraction technique frequently used synthetic and analytical laboratories. The Soxhlet extraction is mainly applied when a substance has only a limited solubility in a solvent, and the impurity is insoluble in that solvent.
Ultrasound can be very successfully combined with the Soxhlet extraction resulting in increased yields and shorter extraction time.
Please click here to learn more about the ultrasonically assisted Soxhlet extraction!

Extraction in Melts using Sonication

Liquid-Liquid extractions can be performed in mixtures where either one or both liquid phases are melts, such as molten salts or molten metals, such as mercury. Powerful inline sonication in ultrasonic flow cell reactors allow for processing even liquids with high viscosities such as melts.

Ultrasonically-Assisted Leaching

Leaching describes the use of acids, solvents or hot water to selectively dissolve a solute from an inert insoluble solid carrier. Leaching is often used in mining to extract metals from ores.
Benefits of Ultrasonic Leaching:

• wash small orifices of porous materials
• overcome selectivities of membranes
• destroy solids, delaminate and deagglomerate solids
• remove of passive layers
• removal of oxide layers
• wet all material surface in particular for high surface tension liquids
• shear thinning

Click here to learn more about ultrasonic leaching!

Hielscher Sonicators for Any Production Volume

Sonication at lab, bench-Top and production scale: All Hielscher ultrasonic devices are built to run 24h/7d, even the ultrasonic lab homogenizers can process considerable volumes either in batch or flow-through mode. The bench-top and industrial ultrasonicators are designed and built at industrial grade so that high volumes and high viscosities can be processed without problems – even under demanding condition such as high pressures and high temperatures (e.g. in combination with supercritical CO2, for extrusion processes etc.). Hielscher’s robust ultrasonicators are capable to handle solvents, abrasive liquids, and corrosives. Suitable accessories make it possible to adapt the ultrasonic system optimally to the extraction process requirements. For installation in hazardous environments, ATEX or FM rated explosion-proof ultrasonic systems are available.
Thereby, Hielscher robust and powerful sonicators and the broad range of accessories enables to sonicate materials such as hot water/ liquids, acids, metal melts, salt melts, solvents (e.g. methanol, hexane; organic, polar solvents e.g. acetonitrile).

Flowchart: Stages of ultrasonic phase transfer extraction