Solid-liquid equilibrium in the system 2-keto-L-gulonic acid + sodium-2-keto-L-gulonate + hydrochloric acid + sodium chloride + water
Graphical abstract
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 and , 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: , , 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 and that are calculated from the experimental liquid phase composition in SLE as follows:
from the mass fractions and the molar masses 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.
References (23)
Microbial processes for ascorbic acid biosynthesis: a review
Enzym. Microb. Technol.
(1990)- et al.
Biotechnological approaches for L-ascorbic acid production
Trends Biotechnol.
(2002) - et al.
Recovery of carboxylic acids produced by fermentation
Biotechnol. Adv.
(2014) - et al.
Application of electrodialysis to the production of vitamin C
Chem. Eng. J.
(2000) - et al.
Solid-liquid equilibrium in the system 2-keto-L-gulonic acid + sodium-2-keto-L-gulonate + water
Fluid Phase Equilib.
(2018) - et al.
Structural investigation on sodium-2-keto-L-gulonate-monohydrate
J. Mol. Struct.
(2012) - et al.
The prediction of mineral solubilities in natural waters: the Na-K-Mg-Ca-H-Cl-SO4-OH-HCO3-CO3-CO2-H2O system to high ionic strengths at 25∘C
Geochem. Cosmochim. Acta
(1984) - R. Dümpelmann, T. Keglevic, Process for the Conversion of the Sodium Salt of 2-Keto-L-Gulonic Acid to the Free Acid, US...
- J. De Troostembergh, I. Debonne, W. Obyn, C. Peuzet, Process for the Manufacture of 2-Keto-L-Gulonic Acid, EP 1417324...
- W. Ning, Z. Tao, C. Wang, S. Wang, Z. Yan, G. Yin, Fermentation Process for Producing 2-Keto-L-Gulonic Acid, EP 0278447...