Liquid multi-stage extraction

Multi-stage extraction is used to achieve a higher efficiency of separation, in which the product is almost completely removed from the raffinate. The solvent is split up into several portions and fed to a series of mixers and settlers. The disadvantage of this method is the need to use large volumes of solvent. A more complicated system, called countercurrent, multi-stage extraction, uses a series of mixers and settlers arranged as before, but the feed liquid and pure solvent are passed through the system in opposite directions, that is counter-currently. Continuous countercurrent operation may be carried out by means of spray columns, packed columns (similar to those used in distillation), plate columns, or, sometimes, bubble cap columns. 

Figure 2.5 Possible technological solutions to bioprocess problems a) Fed-batch culture b) Continuous product removal (eg dialysis, vacuum fermentation, solvent extraction, ion exchange etc) c) Two-phase system combined with extractive fermentation (liquid-impelled loop reactor) d) Continuous culture, internal multi-stage reactor e) Continuous culture, dual-stream multi-stage reactor f) Continuous culture with biomass feedback (cell recycling). (See text for further details).

Rectiflow A multi-stage, liquid-liquid extraction process for removing non-paraffinic components from lubricating oils. Furfural has been used as the solvent. Developed and used by the Shell Petroleum Company in the 1940s, subsequently abandoned. 

Cui, M., Song, F., Zhou, Y., Liu, Z., and Liu, S. (2000). Rapid identification of saponins in plant extracts by electrospray ionization multi-stage tandem mass spectrometry and liquid chromatography/tandem mass spectrometry. Rapid Commun. Mass Spectrom. 14, 1280-1286. 

The determination of Type-A and Type-B trichothecenes in wheat and stmcture elucidation by means of multi-stage positive-ion LC-APCl-MS" on an ion-trap instrument was reported [100]. The analytes were liquid extracted from wheat. After SPE clean-up, the extract was separated on a 125x2-mm-ID C,8 column with a hnear gradient of 25-98% methanol in water at a flow-rate of 250 pl/min. Confirmation of identity was done by retention time and fragmentation pattern in MS", while quantitation was based on peak areas in the mass chromatograms of [M+H]" or of abundant fragments. Typical LOQ range from 10 to 100 pg/kg in wheat. 

Fig, 2,8 Depiction of a single-stage extraction in a triangle diagram 2.1.1.3.2 Multi-Stage Liquid-Liquid Extraction... 

In the laboratory, a multi-stage liquid-liquid extraction can be performed by a simultaneous distillation-extraction process according to Likens-Nickerson [29] (Fig. 2.10). Here, the liquid matrix with the solute in one flask is evaporated together with an immiscible solvent in a second flask. Extraction takes place in the vapour phase where an intensive distribution of both phases is ensured. The condensed vapours from the two phases are separated via a siphon using their different densities and their reintroduction into the original flasks. As the distillation process is continued, extraction is repeated until the solute is exhausted in the original matrix. This method is very useful when traces of non-volatile solutes are present, which are only partly miscible... 

It is also worth considering multi-stage counter-current extraction rather than merely a single stage of mixing and separation, in which case it is important to ascertain whether or not a system contains a solutrope. This corresponds to an azeotrope in distillation and presents a barrier to advance in a liquid/liquid extraction. It can be spotted from a change of slope from positive to negative on the tie lines. 

If our main interest is in the total volumetric mass transfer between the liquids, the role of shear rate and blend time is relatively minor. However, if we are interested in the bubble-size distribution, and we often are because that affects the settling time of an emulsion in a multi-stage co-current or counter-current extraction process, then the change in macro- and micro-rates on scale up is a major factor. Blend time and circulation time are usually not a major factor on scale up. 

Sun F, He Q, Shi P, Xiao P, Cheng Y (2007) Characterization and identification of triterptaioid saponins in crude extracts from Clematis spp. by high-performance liquid chromatography/ electrospray ionization with multi-stage tandem mass spectrometry. Rapid Commun Mass Spectrom 21 3743-3750... 

The advantage of such a method of vitamin Bn production over the traditional methods is that the cells are used repeatedly the vitamin is isolated from the culture liquid without separating the biomass, so that extraction and multi-stage purification of vitamin Bn is unnecessary. 

---