(560jd) Tributyl Citrate Production from Calcium Citrate: Solid-Liquid Reaction Kinetics

Plasticizers are chemical compounds used to enhance the flexibility, softness and durability of plastic materials. Phthalates are the most widely used plasticizer with large consumption in the manufacturing of PVC products. Recently, increasing concerns have appeared regarding the use of phthalates as additives in plastic for bottles, food packaging, toys and medical devices. As the plasticizers are not chemically bounded to the polymeric matrix, they can migrate from the polymeric materials. Thus, several studies have demonstrated that phthalates represent a risk for human health and the environment as a result of their toxicity and low-biodegradability. As a result of these problems, phthalates have been banned in many countries for sensitive applications, boosting the demand for non-toxic, biodegradable, and biobased plasticizers.

Among the variety of biobased plasticizers, citric acid esters have been considered promising phthalates substitutes because they offer a similar plasticizing performance. Among citrate esters, tributyl citrate (TBC) and acetyl tributil citrate (ATBC) have received a special attention, since they have been successfully used in many industrial applications. Nevertheless, currently production costs make TBC limits its potential to become a major player in the global plasticizers market. Regularly, TBC is synthetized through semi-batch esterification of citric acid with a large excess of butanol to overcome equilibrium limitations, using homogeneous acid catalyst and long residence times. As TBC production costs largely depend on raw materials costs, the use of lower costs feedstock would highly improve process economics.

During citric acid purification, there is a neutralization step where an intermediate calcium salt is precipitated from the culture broth. This stream is subjected to filtration, drying, water suspension, and acidification to recover the citric acid in aqueous solution. Later the acid is purified by ion exchange, adsorption, crystallization steps, filtration, and drying. Thus, it is expected that TBC production costs could be reduced by using the intermediate calcium solution as feedstock.

Then, the aim of this work is to explore the use of calcium citrate as raw material in the production of TBC. In the proposed process, dried calcium salt is dispersed in butanol and the insoluble mixture is acidified with sulfuric acid, allowing the simultaneous extraction and esterification of citric acid. This involves a solid-solid-liquid reaction where solid calcium citrate reacts with sulfuric acid to produce solid calcium sulfate, and citric acid in alcoholic solution. Then, dissolved citric acid reacts with butanol to produce butyl citrates owing to the presence of sulfuric acid (i.e. esterification catalyst). The water produced from esterification can be up taken by the solid calcium sulfate, thus shifting equilibrium to the esters side.

In order to evaluate the feasibility of the proposed process, a solid-solid-liquid reaction model was developed to describe the simultaneous acidification-esterification stage. Initially, the esterification kinetics of pure citric acid with butanol using sulfuric acid as catalyst was experimentally measured. A Box-Behnken experimental design was carried out, exploring the kinetic effects of reaction temperature, acid\alcohol feed ratio, and catalyst loading. In parallel, experiments were carried out to determinate the chemical equilibrium constant under the operating conditions.

Subsequently, the solid-liquid reaction experiments were performed in batch reactors using calcium citrate as reactant. The salt was dispersed in a solution of butanol and sulfuric acid. An experimental design was accomplished evaluating the kinetic effects of temperature and feed reactant ratio. Then, based upon the experimental observation of both reaction steps, a solid-solid-liquid reaction model was constructed to describe reaction mechanisms. Microscopy images help elucidating and developing the corresponding solid reaction model. Finally, the experimental data were used to regress a kinetic model and a good agreement with observations was obtained. Then, the regressed model can be further used for process design and up-scaling.