Reversible chemical complexation

ATh = equilibrium constant for reaction fch = forward reaction rate constant for reaction (2.11). 

Reversible chemical complexation processes can be either equilibrium or mass transfer (rate) limited. For those in which equilibrium is the controlling, or design, mechanism, it is important that 

For those in which rate of formation of the complex is the limiting factor, 

The preferred usage is when the complexing agent interacts with the solute of interest and has little or no interaction with the other components of the feed stream. Separation processes based on reversible chemical complexation provide an enhancement in the solubility of the selected solute through the complexation reaction. This approach can provide high enhancement of capacities and selectivities for dilute solutes, especially when the solute feed concentration is below 10%. 

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In the case of non-dispersive extraction of cephalosporins by reversible chemical complexation taking place across hydrophobic microporous hollow fiber (MHF) modules and co-current flow of the two phases, the solute in the aqueous... 

Reactive absorption is probably the most widely applied type of a reactive separation process. It is used for production purposes in a number of classical bulk-chemical technologies, such as nitric or sulfuric acid. It is also often employed in gas purification processes, e.g., to remove carbon dioxide or hydrogen sulfide. Other interesting areas of application include olefin/paraffin separations, where reactive absorption with reversible chemical complexation appears to be a promising alternative to the cryogenic distillation (62). 

Koval CA, Drew SM, Spontarelli T, Noble RD. Concentration and removal of nitrogen and sulfur containing compounds from organic liquid phases using electrochemically reversed chemical complexation. Sep Sci Technol 1988 23 1389-1399. 

King, C. J., Separation processes based on reversible chemical complexation, in Handbook of Separation Process Technology (R. W. Rousseau, Ed.), Chap. 15. Wiley, New York, 1987. 

Discuss the applications of reversible chemical complexation to separations. 

Explain how an MSA used in reversible chemical complexation differs from that used in adsorption. What is the advantage of reversible chemical complexation over non-complexing separation processes ... 

Separation Processes Based on Reversible Chemical Complexation... 

Azhin, M., Kaghazchi, T. and Rahmani, M. (2008) A review on olefin/paraffin separation nsing reversible chemical complexation technology. Journal of Industrial and Engineering Chemistry, 14, 622-638. 

Reversible, chemical complexation can often be used to effect mass transfer (see Chapter IS). Munson and King provide a review of factors that influence the extraction of ethanol from aqueous solutions. Much of this information can be applied by analogy to other tystems. Synergism, which results from blending two or more solvents, can be exploited in some instances. For example, Wardell and King identified a synergism for the mixed solvem, trioctylamine and chloroform, when it is used to extract acetic acid from water. The rationale used to explain this synergism is that die trioctylamine-acetic acid adduct is more extractable into chloroform than acetic acid alone. 

The recovety of oiganic solutes (see Fig. 7.8-7) with higher molecular weights may be carried out by reversible chemical complexation. Distillation is sddom attractive in these cases due lo the low volatility of the solute and thermal decomposition. Typically, acid-base equilibria may be used in the desired separation. For example, the solute may extract under alkaline conditions in column 1, but strip under acidic conditions in column 2. Usually, the solute A, which is recovered in the extract pirase from column 2, will be more dilute than it was in the column 1 feed, but it will be separated from the B diluent as desired. Hence, the column 2 extract is often subjected to subsequent concentration (e.g., evaporation and/ or crystallization) to obtain the solute in the desired form. 

FIGURE 7.8-7 Conceptual flowsheet for solute recovery by reversible chemical complexation. 

The topic of this chapter is the application of reversible chemical complexation for removal and recovery of polar organic solutes from aqueous solution. There are some current examples of such separations, and many more are subjects of active research. One of the most important applications is the recovery of products of biochemical synthesis processes, such as fermentation and enzymatically catalyzed reactions. These separation problems range from recovery of commodity chemicals or fuel substances such as acetic acid and ethanol to isolation of much more complex pharmaceuticals and compounds produced by recombinant DNA and other recently developed biological techniques. Another important application is removal and recovery of polar organics from effluent or recycle water streams. 

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