Immiscible extraction

In discussing the first two problems listed above, what we are really interested in, of course, is determining how the variables involved influence the rate of extraction we can obtain in an agitated vessel. The rate of mass transfer of solute from the solution to be extracted into an immiscible extraction solvent is usually described by some such relation as... 

The codeine that occurs naturally in small amounts in opium is isolated from the aqueous morphine alkaloid mother liquors by immiscible extraction with a nonaqueous solvent. Dilute sulfuric acid is employed to extract the codeine sulfate from the nonaqueous solvent. This solution is evaporated, crystallized, and recrystallized. The alkaloid is precipitated from a sulfate solution by alkali and purified, if necessary, by alcoholic crystallization. It is converted into the phosphate by solution in phosphoric acid, evaporation, crystallization, centrifugation, and drying. 

The polarity of some l-alkyl-3-methylimidazolium ionic liquids has been probed using the solvatochomic dye Nile Red (Figure 1(d)), and found to be comparable to that of lower alcohols.31 As such, polar organic solvents like dichloromethane and diethylether are miscible with ionic liquids, solvents of low polarity show partial miscibility, and nonpolar solvents are essentially immiscible. Extractants such as crown ethers can be used to extract cations such as Na+, Cs+, and Sr2+ from ionic liquids.32... 

Some extraction systems are such that the solvent and diluent phases are almost completely immiscible in each other. Hence, separation yields an extract phase essentially free of diluent and a raffinate phase that is almost pure diluent. This greatly simplifies the characterization of the system. When partial miscibility for an extraction process is very low, the system may be considered immiscible and application of McCabe-Thiele analysis is appropriate. It is important to note that McCabe-Thiele analysis for immiscible extraction applies to a countercurrent cascade. The McCabe-Thiele analysis for immiscible extraction is analogous to the analysis for absorption and stripping processes. Consider the flow scheme shown in Figure 5.23,... 

In liquid-liquid systems, a chemical reaction is encountered for three distinct purposes. Firstly, the reaction may be a part of the process, such as nitration and sulphonation of aromatic substances, alkylation, hydrolysis of esters, oximation of cyclohexanone, extraction of metals and pyrometallurgical operations involving melts and molten slag. Secondly, a chemical reaction is deliberately introduced for separation purposes (e.g. removal of dissolved acidic solutes from a variety of hydrocarbons). Finally, the yield and the rate of formation of many single phase reactions are affected and often can be favourably increased by the deliberately controlled addition to the reaction system of an immiscible extractive phase, whose major purpose is to extract the product from the reactive phase. Such operations are sometimes referred to as "extractive reactions" and have been discussed previously in some detail (14-17). 

Extraction is a process where one or more solutes are removed from a liquid by transferring the solute(s) into a second liquid phase. The second liquid phase, the solvent, is a mass separating agent that must be recovered later. The two liquid phases must be immiscible (that is, insoluble in each other) or partially immiscible. In this chapter we discuss extraction equipment, immiscible extraction, partially miscible extraction, and equipment design. The separation is based on different solubilities of the solute in the two phases. Since vaporization is not required, extraction can be done at low tenperature and is a gentle process suitable for unstable molecules such as proteins or DNA. 

The McCabe-Thiele analysis for dilute immiscible extraction is very similar to the analysis for dilute absorption and stripping discussed in Chapter 12. It was first developed by Evans (1934) and is reviewed by Robbins (1997). In order to use a McCabe-Thiele type of analysis we must be able to plot a single equilibrium curve, have the energy balances automatically satisfied, and have one operating line for each section. 

Most solvent-diluent pairs that are essentially completely immiscible become partially miscible as more solute is added (see Section 13.71. This occurs because appreciable amounts of solute make the two phases chemically more similar. Since Eq. (13r2) is usually valid only for very dilute systems, we can usually analyze immiscible extraction systems using constant total flow rates. Thus, the fifth assunption we usually make is... 

This process is analogous to constant level batch distillation fsection 9.3.1 and the analysis for immiscible extraction is very similar to the analysis in that section. If we assume that the level in the mixed tank is constant, then the overall mass balance becomes in = out, and for dS kg of entering solvent,... 

In this chapter, we considered what extraction is used for, developed McCabe-Thiele and Kremser methods for immiscible extraction, and explored methods for ternary partially miscible extraction systems. At the end of this chapter, you should be able to satisfy the following objectives ... 

Apply the McCabe-Thiele and Kremser methods to immiscible extraction... 

A5. If the extract and raffinate phases are totally immiscible, extraction problems can still be solved using triangular diagrams. Explain how and describe what the equilibrium diagram will look like. 

Various commercial extractors are available. The steam codistillation extractor allows simultaneous condensation of a steam distillate and an immiscible extraction solvent, e.g., dichloromethane. The steam distillator extractor passes a sample steam distillate through an immiscible lighter-than-water solvent such as hexane. 

Dispersive liquid—liquid microextraction (DLLME) enhances the effective surface area of the LLE, reaching the equilibrium in a shorter time [15,16]. DLLME is based on the use of an immiscible extractant solvent and a disperser solvent, which is miscible in both phases, aqueous and organic solvent. Extractant and disperser are mixed and injected rapidly into the sample, producing a turbulent mixture due to the formation of small droplets of the extractant throughout the aqueous sample. 

In many cases the main substance will separate out in crystalline form as the extract cools. Separation of carboxylic acids, other rather strong acids (e.g., halogenated phenols), weak acids, and neutral compounds can be achieved by shaking the water-immiscible extract consecutively with aqueous solutions of sodium hydrogen carbonate, sodium carbonate, and sodium hydroxide. In some cases buffer solutions are best used. The aqueous solutions are then acidified and extracted with ether. The operations should be carried out rapidly using ice-cooled solutions. Acetone extracts may be fractionated in the same way if first diluted with 4-5 times their volume of ether. 

Another reactive separation processes studied for ethyl lactate production is the catalytic extractive reaction (Figure 20.4.7). In this case, the esterification is performed in a biphasic liquid solvent system composed by a reactive polar liquid phase which contains the esterification constituents lactic acid, eflianol and catalyst, and an extractive organic solvent selective of the ester. Therefore, ethyl lactate should preferably be dissolved in the extractive organic phase shifting, in this way, the reaction equilibrium to ester formation. The immiscible extractive solvent is an aromatic or other solvent like toluene, benzene or diethyl ether, among others. Nevertheless, it has also been used an immiscible solvent based on fatty acid methyl ester, but in this case, the procedure represents a method to produce an organic biosolvent and not just ethyl lactate as solvent. 

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