Elsevier

Fluid Phase Equilibria

Volume 495, 1 September 2019, Pages 21-27
Fluid Phase Equilibria

Solid-liquid equilibrium in the system 2-keto-L-gulonic acid + sodium-2-keto-L-gulonate + hydrochloric acid + sodium chloride + water

https://doi.org/10.1016/j.fluid.2019.05.005Get rights and content

Abstract

The reactive solid-liquid equilibrium (SLE) in the quinary system 2-keto-l-gulonic acid (HKGA) + sodium-2-keto-l-gulonate (NaKGA) + hydrochloric acid (HCl) + sodium chloride (NaCl) + water was studied experimentally at 298 K and ambient pressure. The precipitation of three solid species was observed: HKGA monohydrate (HKGAH2O), NaKGA monohydrate (NaKGAH2O), and NaCl. A thermodynamic model based on the Pitzer model to calculate the activity coefficients in the liquid phase for the SLE was developed and unreported Pitzer parameters were fitted to the experimental data of this work. The equilibrium constant for the dissociation of HKGA and the solubility products of HKGAH2O and NaKGAH2O at 298 K were calculated from experimental data, whereas the equilibrium constant for the autoprotolysis of water and the solubility product of NaCl were adopted from literature data. The agreement between the experimental data and the results from the model is excellent, both regarding the liquid phase composition and the solid species in solid-liquid equilibrium.

Introduction

2-Keto-l-gulonic acid (HKGA) is a key intermediate in the production process of l-ascorbic acid (vitamin C). Since vitamin C is an important product for many industries, such as food, animal nutrition, and health industry, HKGA is produced in large scale. HKGA is mainly produced by fermentation [[1], [2], [3]]. A crucial step thereby is the separation and purification of the products during downstream processing. Crystallization is an interesting option for the downstream processing of HKGA [4,5], but requires information on the solid-liquid equilibrium (SLE) in the considered systems. The solids that are crystallizing from the fermentation broth in the HKGA process depend to a large extent on pH value and salt load, both parameters are strongly adjusted during the process. For instance, during the fermentation, the decrease of the pH value due to the formation of the acid HKGA has to be compensated by addition of basic components that often contain sodium, such as sodium carbonate (Na2CO3) [6,7] or sodium hydroxide (NaOH) [5,8,9]. At the resulting neutral pH values, not the acid HKGA, but its sodium salt sodium-2-keto-l-gulonate (NaKGA) is obtained as solid during crystallization [4,5,10]. If one wants to crystallize the acid HKGA directly, the pH value has to be lowered prior to crystallization, e.g. by the addition of hydrochloric acid (HCl). A model system that can be used for describing the process steps mentioned above is the system 2-keto-l-gulonic acid (HKGA) + sodium-2-keto-l-gulonate (NaKGA) + hydrochloric acid (HCl) + sodium chloride (NaCl) + water. It is the goal of the present work to provide experimental data and a thermodynamic model of the SLE in this system.

Several subsystems of this system have been studied in the literature. The SLE in the system HCl + NaCl + water has been extensively studied in experiments [[11], [12], [13], [14], [15], [16]]. Furthermore, in a previous work of our group, the solid-liquid phase diagrams in the system HKGA + NaKGA + water were studied at temperatures in the range 275–313 K and ambient pressure [10]. It was found that both HKGA and NaKGA crystallize exclusively as monohydrates HKGAH2O and NaKGAH2O, respectively, at the studied conditions, which is in line with other reports in the literature [4,5,17]. In that previous work, a thermodynamic model for describing the SLE in the system HKGA + NaKGA + water was developed, which is based on an extended version of the Debye-Hückel theory. This simple model was shown to give an excellent description of the experimental data and was also successfully applied to describe the SLE in the related system HKGA + vitamin C + water [18]. However, this model was found to be not satisfactory for mixtures with high concentrations of HCl. To elucidate the SLE in the quinary system HKGA + NaKGA + HCl + NaCl + water, the ternary subsystems HKGA + HCl + water, NaKGA + NaCl + water, HKGA + NaCl + water, and NaKGA + HCl + water were studied experimentally in the present work at 298 K and ambient pressure. Three different solid species were observed: HKGAH2O, NaKGAH2O, and NaCl. A thermodynamic model for describing the SLE in the quinary system was developed. The Pitzer model was used to calculate the activity coefficients in the liquid phase.

Section snippets

Chemicals

Anhydrous 2-keto-l-gulonic acid (HKGA) was obtained from Cargill. Sodium hydroxide (NaOH) pellets were purchased from Sigma-Aldrich. Hydrochloric acid (HCl, 0.32 g/g) and sodium chloride (NaCl) were obtained from Carl Roth. As solvent, ultra pure water was used, which was produced with a Milli-Q purifier from Millipore (Elix Essential 5). Table 1 gives an overview of all chemicals used in this work, including the chemicals for the preparation of the eluents for the analysis by ion

Model

A scheme of the model of the reactive solid-liquid equilibrium is shown in Fig. 1. The model considers the dissociation of HKGA, NaKGA, HCl, and NaCl as well as the autoprotolysis of water in the liquid phase. NaKGA, HCl, and NaCl are strong electrolytes and are therefore considered to be fully dissociated in the liquid phase. By contrast, HKGA is a weak electrolyte and, hence, present in the dissociated (H+ and KGA) and undissociated (HKGA) form in the liquid phase. The equilibrium constant

Results and discussion

In Table 2, the experimental results of this work for the composition of the liquid phase in mass fractions w and the coexisting solid phases in SLE are summarized. Table 2 also contains the corresponding Jänecke indices J(Na+) and J(Cl) that are calculated from the experimental liquid phase composition in SLE as follows:J(Na+)=100wNa+/MNa+wNa+/MNa++wH+/MH+J(Cl)=100wCl/MClwCl/MCl+wKGA/MKGA

from the mass fractions wi and the molar masses Mi of the species i. Table 2 also contains

Conclusion

2-Keto-l-gulonic acid (HKGA) is the key intermediate in the l-ascorbic acid process and is mainly produced by fermentation. Crystallization is a promising method to separate and purify HKGA from fermentation broths, which are diluted aqueous solutions containing electrolytes. The presence of electrolytes and the pH value of the solution have a significant influence on the solubility of HKGA. Furthermore, different crystal species can precipitate from the solution depending on the composition of

Funding sources

This work was supported by the European Community’s Framework Program for Research and Innovation Horizon 2020 (2014-2020) [grant agreement number 637077].

Declarations of interest

None.

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