(736b) Kinetics of the Solid-Liquid Transesterification to Produce Sucrose Esters Using Sodium Stearate As Contacting Agent

Sucrose esters (emulsifier food ingredient E473) are value-added and entirely biobased surfactants used in niche applications in alimentary products, cosmetics and pharmaceuticals. Being green chemicals they have interesting properties such as rapid biodegradability, biocompatibility, and biocide potential for certain microorganisms [1]. Sucrose esters are produced by alkaline transesterification of sucrose and fatty acid methyl esters (FAME) at temperatures between 90 and 180°C, and under vacuum conditions to remove the methanol produced. The main challenge of this reaction is incompatibility of reactants, namely sucrose is a highly polar solid, while FAME are highly non-polar liquids.

One of the processing alternatives proposed to overcome the incompatibility of the reactants is the use of emulsifiers to provide an intensive contact and enhance the solubility of sucrose in FAME [2–5]. The main emulsifiers used are fatty acid soaps (mono- and divalent). However, in this heterogeneous reaction, solid-solid-liquid interfaces are present between the catalyst (usually K₂CO₃), sucrose and FAME, respectively. Due to complexity of the system, there are few available models capable of describing the reaction kinetics of the process. Two different reaction mechanisms have been proposed to explain interactions between the solid sucrose and the solid catalyst that lead to formation of the active species during the solvent-free transesterification. It was assumed that such interactions lead to formation of solid sucrate, which further reacts with FAME forming a sucrose monoester [6]. An alternative more recent explanation suggests that the solid sucrose dissolved to a certain extent in FAME interacts with the latter on the catalyst surface giving sucrose monoester [7]. According to that study solubility of sucrose in FAME is enhanced by temperature and presence of the contacting agents.

In this work we have revisited sucrose transesterification process, focusing on fundamental understanding of the reaction mechanism and development of a suitable kinetic model. The advanced model considers that sucrose is solubilized in FAME and that solubility is enhanced by the presence of sucrose monoester and temperature elevation. Based upon these assumptions, a kinetic model of the reaction was derived and validated in the laboratory scale.

The experiments conducted in this work revealed that sucrose dissolution up to the saturation limit was quickly reached, therefore mass transfer limitations were neglected. On the other hand, the interactions between the free sucrose and the solid catalyst were analyzed at different catalyst concentrations. The observed non-linear behavior of the reaction rate with the catalyst concentration was attributed to solid-liquid interactions. The experimental evidences strongly support that the reaction order towards the catalyst concentration is consistent with the theoretical one for solid spherical particles [8]. A complete set of experiments to validate the proposed kinetics was carried out in a batch reactor with 0.1 L volume using sodium stearate (NaS) as an emulsifier, and K₂CO₃ as a catalyst. The consumed FAME and produced sucrose esters of different esterification degree were quantified using high performance liquid chromatography (HPLC). The kinetic parameters were adjusted using the experimental data obtained under different operating conditions: temperature 120-160°C, 7 wt.% NaS, sucrose/FAME molar ratio 0.5-1.5, and the catalyst concentrations ranging between 1 to 5 wt.%.

The obtained data validated the proposed reaction mechanism, displaying a good agreement with the kinetic model. Thus, the developed model can be used in conceptual process design and in preliminary economic evaluations of sucrose esters production in a solvent-free reaction system.

References

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[7] R. Zhao, Z. Chang, Q. Jin, W. Li, B. Dong, X. Miao, Heterogeneous base catalytic transesterification synthesis of sucrose ester and parallel reaction control, Int. J. Food Sci. Technol. 49 (2014) 854–860. doi:10.1111/ijfs.12376.

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