Ind. Eng. Chem. Res. 1990,29, 1327-1333
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Extraction of Carboxylic Acids with Amine Extractants. 2. Chemical Interactions and Interpretation of Data Janet A. Tamadat and C. Judson King* Department of Chemical Engineering and Lawrence Berkeley Laboratory, University of California, Berkeley, California 94720
The results of the mass action law analysis of part 1are combined with the results from spectroscopic studies to analyze chemical interactions involved in the complexation of carboxylic acids with amine extractants in various diluents. Infrared spectroscopic studies were performed on anhydrous solutions of succinic, maleic, and fumaric acids to compare the effect of rigidity and configuration on complex stoichiometry. Dicarboxylic acids that form intramolecular hydrogen bonds are inhibited from forming (1,2) and (2,2) complexes. (The notation (p,q) denotes p , the number of acid molecules, and q, the number of amine molecules, in the complex.) The results from the mass action law analysis and spectroscopic studies of previous investigators indicate that formation of the (1,l) complex involves ion-pair or hydrogen-bond formation between the acid and the amine, while (2,l) complex formation involves hydrogen bonding between the carboxyl of the second acid and the carboxylate of the first. The effects of acid and diluent on the magnitude of the (1,l)equilibrium constant are discussed. In part 1 of this work, a mass action law analysis was used to study the effects of acid and diluent on the extraction of carboxylic acids with amine extractants. The stoichiometries of the complexes and equilibrium constants for complex formation were inferred from fitting batch extraction data. The mass action law analysis reveals patterns of extraction behavior, but it is limited to examining the net results of a complicated mixture of interactions. In part 2, the results of spectroscopic experiments of previous investigations and from the current work are analyzed to elucidate the structures of the complexes on a molecular level. These findings are combined with the results of the mass action law analysis to develop an understanding of the nature of the chemical interactions involved in acid extraction by amine extractants.
Experimental Methods Anhydrous solutions of 0.05 mol/L maleic and fumaric acids (Aldrich Chemical Co.) with 0.05, 0.10, and 0.20 mol/L Alamine 336 (Henkel Corp.) in chloroform (Mallinkrodt Corp., spectral grade) and solutions of 0.05 mol/L Alamine 336 in chloroform with 0.05, 0.025, and 0.113 mol/L succinic acid were prepared by adding measured quantities of acid crystals directly to the dry solvent phase. A small fraction, estimated visually as malonic > maleic = succinic > lactic > acetic. The effects of temperature on extraction of succinic and lactic acids by Alamine 336 with chloroform and methyl isobutyl ketone (MIBK) diluents have been measured. Enthalpies and entropies of complex formation have been derived from the results and are interpreted in terms of the differences in interactions among the species involved. Two approaches for regeneration through back-extraction into an aqueous phase are considered. These involve changes in the equilibrium relationship through a swing of temperature and a swing of diluent composition, respectively. The factors underlying the utility of each are explored and contrasted. The two approaches may be used in combination. In the production of carboxylic acids, processes such as fermentations produce multicomponent, aqueous solutions with product acid concentrations typically 10% w/w at most, and usually substantially less. Subsequent separation, purification, and concentration of these acids is difficult because of the high affmities of the acids for water. Distillation of dilute, nonvolatile acids involves large energy consumption for the heat of vaporization of water, which must be taken overhead. Furthermore, distillation cannot fractionate among nonvolatile acids. The low aqueous activity of carboxylic acids results in low distribution coefficients of acids into conventional solvents. Thus, solvent extraction with conventional solvents would require very high solvent flow rates and result in substantial dilution of the acid. Long-chain tertiary amines are effective extractants for carboxylic acids (Kertes and King, 1986). The strong interaction between the acid and amine allows for formation of acid-amine complexes and thus provides for high equilibrium distribution ratios. Additionally, the high affinity of the organic base for the acid gives selectivity for the acid over other nonacidic components in the mixture. This reaction is reversible, enabling recovery of the acid and recycle of the solvent. There are two steps to a practical extractive separation and recovery process. The first is extraction of the acid to produce an acid-loaded extract and a relatively acid-free aqueous raffnate. The second step transfers the acid from the solvent into a product phase and regenerates the extractant mixture which is recycled back to the extractor. Regeneration is a crucial step for development of a practical operation.
* T o whom correspondence should be addressed. Department of Chemical Engineering, E25-342, Messachusetts Institute of Technology, Cambridge, MA 02139. t Current address:
0888-5885/90/2629-1333$02.50/0
Water coextraction, Le., water that enters the organic phase with the solute, may also affect process economics. For example, it may be necessary to recover pure acid from an aqueous solution produced from the extract during regeneration. This work examines coextraction of water with carboxylic acid-amine complexes and two means of regeneration using back-extraction-temperature swing and diluent swing. Some of the chemistry involved in the effect of temperature on extraction is also discussed. McCabe-Thiele Diagram. A McCabe-Thiele operating diagram for dilute solutions (Figure 1)outlines the general form of an extractionlback-extraction acid recovery process (King, 1980). During the extraction step, the equilibrium curve must lie above the operating line. A low solvent flow rate corresponds to a relatively large slope of the operating line. Thus, high equilibrium distribution of acid into the solvent phase is necessary in order to achieve good product recovery at reasonable solvent flow rates. During regeneration, the situation is reversed; the equilibrium curve must lie below the operating line. The slope of the regenerator operating line should be lower than that for extraction, corresponding to low water flow rates, if the product is to be concentrated overall with substantial solute recovery. To achieve this, the equilibrium distribution of the acid into the organic phase must be low. Therefore, it is important to effect a downward shift in the equilibrium line between the extraction and regeneration stages. Such processes a r e known as "swing" processes.
Experimental Section Batch experiments were performed for the extraction of succinic and lactic acids by Alamine 336 (Henkel Corp.) in chloroform and methyl isobutyl ketone (MIBK) as diluents at various temperatures as described in part 1 (see also Tamada and King (1989)). Extraction experiments without amine were also performed at various tempera0 1990 American Chemical Society
