Staged Extraction

In extraction (qv), the distribution coefficient value is the slope of the equiUbrium line. In practice, the slope of the operating line is set at a value somewhat less than the distribution coefficient to provide driving force and fix the required theoretical extraction stages at some reasonable number. 

A theoretical or equihbrium stage is a device or combination of devices that accomplishes the effect of intimately mixing two immiscible liquids until equilibrium concentrations are reached, then physically separating the two phases into clear layers. Crosscurrent extraction (Fig. 15-4) is a cascade, or series of stages, in which the raffinate R from one extraction stage is contacted with additional fresh solvent S in a subsequent stage. 

The above covers most conventional mixers there is another class of mixers, called pump-mix impellers, where the impeller serves not only to mix the fluids, but also to move the fluids through the extraction stages. These are speciahzed designs, often used in the metals extraction industries. For these types of impellers, a knowledge of the power characteristics for pumping is required in addition to that for mixing. For a more detailed treatment of these special cases, the reader is referred to Lo et al. 

Multistage counter-current liquid-liquid extraction is a typical continuous process. Fig. 121 shows a schematic diagram of the process. The slope of the line depends on the volumetric ratio of the aqueous and organic phases. The aqueous and organic phases move through the system as two counter currents. The raffinate from a specific extraction stage moves on to the next stage, to be mixed with a relatively fresh portion of the extract, i.e. extractant that was previously mixed with a low-concentration aqueous solution. 

Another undesirable modification is removal of some amino acids residues from the C-terminus and/or the N-terminus by microbial exoproteases. Care needs to be taken during the fermentation and extraction stages to minimize proteolytic damage, and any nibbled molecules should be removed during purification. 

Here the extraction is carried out continuously in a single, perfectly mixed, extraction stage as shown in Fig. 3.34. It is assumed that the outlet flow concentrations, Xi and Yi, achieve equilibrium and that density variations are negligible. 

Figure 3.35. Information flow diagram for the continuous equilibrium extraction stage.

Quantitative analysis of multicomponent additive packages in polymers is difficult subject matter, as evidenced by results of round-robins [110,118,119]. Sample inhomogeneity is often greater than the error in analysis. In procedures entailing extraction/chromatography, the main uncertainty lies in the extraction stage. Chromatographic methods have become a ubiquitous part of quantitative chemical analysis. Dissolution procedures (without precipitation) lead to the most reliable quantitative results, provided that total dissolution can be achieved follow-up SEC-GC is molecular mass-limited by the requirements of GC. Of the various solid-state procedures (Table 10.27), only TG, SHS, and eventually Py, lead to easily obtainable accurate quantitation. 

Opportunities for reducing effluent flows and water use are also present in the bleaching plant. Acid filtrates from hypochlorite or chlorine dioxide stages can be used as dilution and wash water for the first bleaching stage. Similarly, second extraction stage filtrates can be used as dilution and... 

Study the effect of the extraction stage on reactor performance by varying the magnitudes of the the mass transfer coefficient Ka, the equilibrium distribution ratio m, the recycle ratio R, the relative reactor and extraction volumes and solvent flowrate. 

This is represented by a single well mixed, constant volume, continuous flow extraction stage. 

---