Efficiencies, extraction

In a simple liquid-liquid extraction the solute is partitioned between two immiscible phases. In most cases one of the phases is aqueous, and the other phase is an organic solvent such as diethyl ether or chloroform. Because the phases are immiscible, they form two layers, with the denser phase on the bottom. The solute is initially present in one phase, but after extraction it is present in both phases. The efficiency of a liquid-liquid extraction is determined by the equilibrium constant for the solute s partitioning between the two phases. Extraction efficiency is also influenced by any secondary reactions involving the solute. Examples of secondary reactions include acid-base and complexation equilibria. 

In the simplest form of liquid-liquid extraction, the only reaction affecting extraction efficiency, is the partitioning of the solute between the two phases (Figure 7.20). In this case the distribution ratio and the partition coefficient are equal. 

A solute, S, has a Kd between water and chloroform of 5.00. A 50.00-mL sample of a 0.050 M aqueous solution of the solute is extracted with 15.00 mL of chloroform, (a) What is the extraction efficiency for this separation (b) What is the solute s final concentration in each phase (c) What volume of chloroform is needed to extract 99.9% of the solute ... 

The fraction of solute present in the organic phase is, therefore, 0.600. Extraction efficiency is the percentage of solute successfully transferred from its initial phase to the extracting phase. The extraction efficiency is, therefore, 60.0%. 

In Example 7.14 a single extraction results in an extraction efficiency of only 60%. If a second extraction is carried out, the fraction of solute remaining in the aqueous phase, ( is given by... 

For the extraction described in Example 7.14, determine (a) the extraction efficiency for two extractions and for three extractions and (b) the number of extractions required to ensure that 99.9% of the solute is extracted. 

Thus, the extraction efficiencies are 84.0% with two extractions and 93.6% with three extractions. 

Plot of extraction efficiency versus number of extractions for the liquid-liquid extractio scheme in Figure 7.20. 

The value for D given by equation 7.29 can be used in equation 7.25 to determine extraction efficiency. 

The extraction efficiency, therefore, is almost 75%. When the same calculation is carried out at a pH of 5.00, the extraction efficiency is 60%, but the extraction efficiency is only 3% at a pH of 7.00. As expected, extraction efficiency is better at more acidic pHs when HA is the predominate species in the aqueous phase. A graph of extraction efficiency versus pH for this system is shown in Figure 7.23. Note that the extraction efficiency is greatest for pHs more acidic than the weak acid s piQ and decreases substantially at pHs more basic than the pi A- A ladder diagram for HA is superimposed on the graph to help illustrate this effect. 

Plot of extraction efficiency versus pH of the aqueous phase for the liquid-liquid extractor of the molecular weak acid in Example 7.16. 

Typical plot of extraction efficiency versus pH for the liquid-liquid extraction of a metal ion by metal chelator. 

A sample contains a weak acid analyte, HA, and a weak acid interferent, HB. The acid dissociation constants and partition coefficients for the weak acids are as follows Ra.HA = 1.0 X 10 Ra HB = 1.0 X f0 , RpjHA D,HB 500. (a) Calculate the extraction efficiency for HA and HB when 50.0 mF of sampk buffered to a pH of 7.0, is extracted with 50.0 mF of the organic solvent, (b) Which phase is enriched in the analyte (c) What are the recoveries for the analyte and interferent in this phase (d) What is the separation factor (e) A quantitative analysis is conducted on the contents of the phase enriched in analyte. What is the expected relative erroi if the selectivity coefficient, Rha.hb> is 0.500 and the initial ratio ofHB/HA was lO.O ... 

Efficiency. Since only 35 to 50% of fired duty is absorbed in the radiant section, the flue gas leaving the radiant chamber contains considerable energy that can be extracted efficiently in the convection section of the furnace. In the convection section, the feed is preheated along with dilution steam to the desired crossover temperature. Residual heat is recovered by generating steam. The overall thermal efficiency of modem furnaces exceeds 93%, and a value of 95% is not uncommon. 

The effect of different pai ameters such as temperature, pressure, modifier volume, dynamic and static extraction time on the SFE of the plant were investigated. The orthogonal array experimental design method was chosen to determine experimental plan, (5 ). In this design the effect of five parameters and each at five levels were investigated on the extraction efficiency and selectivity [4]. 

Extraction (discussed in Chapter 5) uses the selective adsorption of a component in a liquid to separate specific molecules from a stream. In application extraction may be coupled with its cousins, extractive distillation and azeotropic distillation, to improve extraction efficiency. Typical refinery extraction applications involve aromatics recovery (UDEX) and lubricants processing (furfural, NMP). Extractive distillation and azeotropic distillation are rarely employed in a refinery. The only... 

E. M. Ean as and S. R. Rissato, Influence of temperature, pressure, modifier and collection mode on superaitical CO2 extraction efficiencies of diuron from sugar cane and orange samples , J. Microcolumn Sep. 10 473-478 (1998). 

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