Countercurrent extractions

In Section 2.1.3 we discussed extractions from the viewpoint of one substance being transferred from one phase to another or the separation of two solutes by selective extraction. When we have a system in which the distribution constants (K) or distribution coefficients (ftc) differ by 103, we can only recover the extracted solute in about 97% purity. Continued extractions will increase yield but not purity. Good separations, with high purity, of two or more solutes can be achieved when there is a difference in the thermodynamic behavior of the various solutes, that is, a difference in the distribution constants (K) or coefficients (Kc). A measure of this degree of separation is the separation factor, a, for pairs of solutes which is defined as 

If there are other equilibria (intraphase) involved then the K values are replaced by the distribution ratio, D. For extractions, it makes little difference which solute appears in the numerator because the a term is usually defined in such a manner that it will have a numerical value 1. Separation factor applied to chromatographic systems is expressed as a ratio of retention data (see Section 1.4)  

V = adjusted retention volume. The component which is more re-tamed (greater value of Vi) is placed m the numerator. 

Before proceeding, the author wishes to point out that the separation factor, a, as defined above is the term used by most people working in chromatography and the term recommended by the IUPAC. Readers will encounter statements to the contrary in some references (5,7). 

When explaining chromatography theory on the basis of a discontinuous model we make three assumptions  

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The theory behind countercurrent extractions is outlined in Appendix 6. 

Let s assume that the solute to be separated is present in an aqueous phase of 1 M HCl and that the organic phase is benzene. Because benzene has the smaller density, it is the upper phase, and 1 M HCl is the lower phase. To begin the countercurrent extraction the aqueous sample containing the solute is placed in tube 0 along with a portion of benzene. As shown in figure A6.1a, initially all the solute is present in phase Lq. After extracting (figure A6.1b), a fraction p of the solute is present in phase Uq, and a fraction q is in phase Lq. This completes step 0 of the countercurrent extraction. Thus far there is no difference between a simple liquid-liquid extraction and a countercurrent extraction. 

Figure A6.1 and Table A6.1 show how a solute s distribution changes during the first four steps of a countercurrent extraction. Now we consider how these results can be generalized to give the distribution of a solute in any tube, at any step during the extraction. You may recognize the pattern of entries in Table A6.1 as following the binomial distribution...

Fraction of Solute Remaining in Tube r After Extraction Step n for a Countercurrent Extraction... 

Progress of a countercurrent extraction for Example A6.1 after 30 steps. 

Two solutes, A and B, with distribution ratios of 9 and 4, respectively, are to be separated by a countercurrent extraction in which the volumes of the upper and lower phases are equal. After 100 steps, determine the 99% confidence interval for the location of each solute. 

Since the two confidence intervals overlap, a complete separation of the two solutes cannot be achieved in a 100-step countercurrent extraction. The complete distribution of the solutes is shown in Figure A6.4. 

For the countercurrent extraction in Example A6.2, calculate the recovery and separation factor for solute A if the contents of tubes 85-99 are pooled together. 

From Example A6.2 we know that after 100 steps of the countercurrent extraction, solute A is normally distributed about tube 90 with a standard deviation of 3. To determine the fraction of solute in tubes 85-99, we use the single-sided normal distribution in Appendix lA to determine the fraction of solute in tubes 0-84 and in tube 100. The fraction of solute A in tube 100 is determined by calculating the deviation z (see Chapter 4)... 

Fig. 6. Countercurrent extraction showing the equiUbtium stages (horizontal dashed lines) where A and B are immiscible.

Coalescence and Phase Separation. Coalescence between adjacent drops and between drops and contactor internals is important for two reasons. It usually plays a part, in combination with breakup, in determining the equiHbrium drop si2e in a dispersion, and it can therefore affect holdup and flooding in a countercurrent extraction column. Secondly, it is an essential step in the disengagement of the phases and the control of entrainment after extraction has been completed. 

The De Danske Sukkerfabriker (DDS) diffuser extractor (Fig. 6) is a relatively simple version of this family of machines, employing a double screw rotating in a vessel mounted at about 10° to the horizontal. The double screw is used to transport the soHds up the gradient of the sheU, while solvent flows down the gradient. Equipment using a single screw in a horizontal sheU for countercurrent extraction of soHds under pressure has been described (19). 

Commercial soy protein concentrates typically contain 70 to 72% cmde protein, ie, nitrogen x 6.25, dry wt basis. Soy protein isolates are prepared from desolventhed, defatted flakes. A three-stage aqueous countercurrent extraction at pH 8.5 is used to disperse proteins and dissolve water-soluble constituents. Centrifugation then removes the extracted flakes, and the protein is precipitated from the aqueous phase by acidifying with HCl at pH 4.5. 

Fig. I. Schematic flow diagram of a countercurrent extraction unit for the continuous separation of two REE or two groups of REE.

Two ions a and b can be separated by countercurrent extraction as long as the ratio of the distribution coefficients, that is, the separation factor Q, is not unity ... 

The purified acid is recovered from the loaded organic stream by contacting with water in another countercurrent extraction step. In place of water, an aqueous alkafl can be used to recover a purified phosphate salt solution. A small portion of the purified acid is typically used in a backwashing operation to contact the loaded organic phase and to improve the purity of the extract phase prior to recovery of the purified acid. Depending on the miscibility of the solvent with the acid, the purified acid and the raffinate may be stripped of residual solvent which is recycled to the extraction loop. The purified acid can be treated for removal of residual organic impurities, stripped of fluoride to low (10 ppm) levels, and concentrated to the desired P2 s Many variations of this basic scheme have been developed to improve the extraction of phosphate and rejection of impurities to the raffinate stream, and numerous patents have been granted on solvent extraction processes. 

The yield of furfural from xylose is improved by countercurrent extraction with tetraJin (Schoenemann, Proc. 2d Europ. Symp. Chem. React. Eng., Pergamon, 1961, p. 30). 

Extraction from Aqueous Solutions Critical Fluid Technologies, Inc. has developed a continuous countercurrent extraction process based on a 0.5-oy 10-m column to extract residual organic solvents such as trichloroethylene, methylene chloride, benzene, and chloroform from industrial wastewater streams. Typical solvents include supercritical CO9 and near-critical propane. The economics of these processes are largely driven by the hydrophihcity of the product, which has a large influence on the distribution coefficient. For example, at 16°C, the partition coefficient between liquid CO9 and water is 0.4 for methanol, 1.8 for /i-butanol, and 31 for /i-heptanol. 

Figure 46. Simplified schematic of countercurrent extraction process C-contactor, S- separator.

Proteins (BSA or ovomucoid, OVM) have also been successful in the preparative resolution of enantiomers by liquid-liquid extraction, either between aqueous and lipophilic phases [181] or in aqueous two-phase systems (ATPS) [123, 180]. The resolution of d,l-kynurenine [180] and ofloxacin and carvediol [123] were performed using a countercurrent extraction process with eight separatory funnels. The significant number of stages needed for these complete resolutions in the mentioned references and others [123, 180, 189], can be overcome with more efficient techniques. Thus, the resolution of d,l-kynurenine performed by Sellergren et al. in 1988 by extraction experiments was improved with CCC technologies 10 years later [128]. 

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