Solvent extraction extractor design

Initial Extraction Technique Continuous extraction apparatus was employed, including an extractor designed to contain the starting plant materials, a distillation flask to hold the solvent mixture, the flask being equipped with a reflux condenser, a drip device to facilitate the removal of the volatilized mixture from the condenser and to percolate it through the continuous extractor, and a Soxhiet type return. Means for heating the continuous extraction system were provided. 

During recent years pilot scale equipment, smaller than the prototype pilot plants described and capable of operating with exceedingly high efficiency, has been designed. Such equipment as the York-Scheibel solvent extraction tower and the Podbielniak countercurrent centrifugal mixer and extractor are typical. Data from this equipment may be correlated with commercial performance. 

A small-scale chain conveyor type of extractor was developed in the 1940s at Iowa State University with the intent of using trichloroethylene solvent to extract soybeans. Crown Iron Works of Minneapolis, Minnesota, licensed the technology and extractor design, and supplied several continuous solvent extraction plants in 1951 using the new extractor and trichloroethylene solvent (6). The meal from these plants proved detrimental to animals, so the plants were either closed or converted to petroleum-based solvents (7). The chain-type extractor apparatus continued on and is the basis of the modern Crown Iron Works Model III extractor used today. 

An extraction system set up to take advantage of solvent reuse is designated as countercurrent extraction. A battery of batch extractors is provided and the solvent is used to treat the contents of each extractor in succession. Each time a batch of miscella is drained from an extractor, it is sent to another extractor containing material previously extracted with a richer miscella. 

Accelerated solvent extraction as implemented in commercial equipment is basically discrete in nature, so it is rarely coupled to other operations of the analytical process. In fact, only in two reported applications was the static mode coupled on-line to other operations such as chromatographic separation, preconcentration and detection. Both used custom extractors as the compact design of the commercial models precluded their adaptation. 

For oleaginous materials having a low oil content (18-20%), such as soybean and rice bran, solvent extraction is often applied for oil recovery. Hexane is widely accepted as the most effective solvent used today. Most of the extractors currently used are designed as countercurrent flow devices. The solid material flows in an opposite direction of solvent-oil miscella with an increasing oil concentration. The miscella containing around 25-30% oil after extraction is subjected to solvent distillation to recover the oil. The extracted solid material, commonly known as white flakes, is also conveyed to the desolventizing process. 

Some solvent-extraction techniques are relatively difficult to effect using conventional laboratory apparatus. For example, the classical penicillin G extraction in which acidified broth is contacted with a water-immiscible solvent can only be operated effectively using continuous-flow methods because of the poor stability of the product at low pH values. This extraction can be reproduced on the bench scale using the AKUFVE apparatus, which was designed for extraction studies in the nuclear industry (12,13). Selective extraction may involve the use of a solvent in which the product has a poor partition coefficient. Countercurrent extractors are mostly process scale devices but the smallest four-stage extractor produced by Robatel could be considered a bench scale. It has a throughput of 50-100 mL/min. 

---