Phase equilibria for ternary liquid systems of (water + levulinic acid + cyclic solvent) at T = 298.2 K: Thermodynamic modeling

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Abstract

(Liquid + liquid) equilibrium (LLE) data of the solubility curves and tie-line end compositions are presented for mixtures of {water (1) + levulinic acid (2) + methylcyclohexanol, or cyclohexanone, or cyclohexyl acetate (3)} at T = 298.2 K and P = (101.3 ± 0.7) kPa. Among the studied C6 ring-containing cyclic solvents, methylcyclohexanol and cyclohexyl acetate give the largest distribution ratio and separation factors for extraction of levulinic acid. A solvation energy relation (SERLAS) has been used to estimate the (liquid + liquid) equilibria of associated systems containing components capable of a physical interaction through hydrogen bonding. The tie-lines were also predicted using the UNIFAC-original model. The reliability of both models has been analyzed against the LLE data with respect to the distribution ratio and separation factor. SERLAS matches LLE data quite accurately, yielding a mean error of 3.6% for all the systems considered.

Introduction

In the chemical industry, various selective solvent systems have been tested to improve the efficient separation of carboxylic acids from aqueous fermentation solutions [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. Especially, extractive recovery of acetic acid from fermentation broth and wastewater including lower than 10% (w/w) acid concentrations has received increasing interest [7], [8]. However, it is desirable to use a low boiling solvent that have to be distilled so long as no azeotropes appear. The physical extraction of hydrophilic acids through hydrogen bonding is still challenging because such systems show extremely nonideal behaviour.

Regarding the technical and economic merits of low boiling solvents during the regeneration by distillation, the selection of C6 ring-included extracting agents from various classes of polar, protic or nonprotic type was made. They all have higher boiling temperatures than water and lower in comparison with levulinic (4-oxopentanoic) acid. As continuation of the previous study [9], [10], the present work aims to produce new (liquid + liquid) equilibrium (LLE) data for the extraction of levulinic acid from water using methylcyclohexanol, cyclohexanone, and cyclohexyl acetate (cyclohexyl ethanoate) as proton-donating and -accepting cyclic solvents of lower vapor pressure than water and higher than levulinic acid. LLE data for extraction of a carboxylic acid from water through a cyclic solvent are scarce in the literature [3], [9], [10], [11]. It has not been found dependable LLE results for the present ternaries in the literature.

In this study, the properties and (liquid + liquid) equilibria of associated ternary mixtures have been estimated through a solvation energy relation (SERLAS) [9]. The model incorporates the solvatochromic parameters [12], [13] with the thermodynamic factors derived from the UNIFAC-Dortmund model [14] in a relation including expansion terms and two correction factors. The LLE data have been determined for each of the systems (water + levulinic acid + methylcyclohexanol, or cyclohexanone, or cyclohexyl acetate) at T = 298.2 K. The distribution data were also correlated using the UNIFAC-original model [15], [16], and compared with the predictions through SERLAS.

Section snippets

Experimental

Methylcyclohexanol, cyclohexanone, and cyclohexyl acetate (99.5%, g.c.), as well as levulinic acid of analytical grade were supplied by Fluka. All the chemicals were used as received without further purification. Mass-fractions of impurities detectable by g.c. were found to be <0.002.

The binodal (solubility) curves and the mutual solubility of the (water + cyclic solvent) binaries were determined by the cloud point method in an equilibrium glass cell with a water jacket to maintain isothermal

Distribution behaviour of levulinic acid

The compositions of mixtures on the binodal curve and the mutual binary solubilities of water and cyclic solvent at T = 298.2 K are given in FIGURE 1, FIGURE 2, FIGURE 3, in which wi denotes the mass-fraction of the ith component. Table 1 tabulates the experimental tie-line compositions of the equilibrium phases, for which wiandwi refer to the mass fractions of the ith component in the aqueous and solvent phases, respectively. The experimental and calculated tie-lines through UNIFAC-original

Correlation of LLE data by SERLAS

In the previous study [9], the properties (Pr) of ternary liquid systems, such as the separation factor (S=(x2/x2)/(x1/x1)) and the modified distribution ratio (DM={(x2+x3)/(1-x3)}/{(x2+x3)/(1-x3)}), were estimated using a SERLAS. x″ and x′ designate solvent-rich and water-rich compositions of water (1), acid (2) and solvent (3), respectively. The model includes a part accounting for the properties at the composition limit of acid x2 = 0 (Pr0), and an expansion term with respect to the

Conclusions

LLE data for the three ternary mixtures {water (1) + levulinic acid (2) + cyclic solvent (3)} were determined at T = 298.2 K. It is apparent from the distribution data that the separation of levulinic acid from water by extraction with a C6 ring-containing cyclic solvent is feasible (table 2 and FIGURE 1, FIGURE 2, FIGURE 3, FIGURE 4, FIGURE 5). The distribution of levulinic acid in the (water + cyclic solvent) two-phase system is better for methylcyclohexanol and cyclohexyl acetate than cyclohexanone.

Acknowledgments

This work was supported by the Research Fund of Istanbul University. The author is also grateful to Meric Senol.

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