(354e) Triethyl Citrate Production from a Citrate Salt

Triethyl citrate is a colorless, odorless, citric acid derivative used as a stabilizing food additive and pharmaceutical and polymer plasticizer. Citric acid esters are commonly obtained by direct esterification of citric acid and alcohols (e. g. ethanol, butanol), using acid catalysts. The process occurs in a series-parallel reaction scheme, where the monoalkyl citrate is initially obtained, followed by the formation of the diester, and finally Trialkyl citrate. In the case of reacting with ethanol; methyl, diethyl and triethyl citrate (TEC) are the reaction products.

The Citric acid that is the raw material for these esters is produced by fermentation. The downstream processing for its purification is complex and depending on the separation technology, it may require precipitation with calcium, extraction with amines or solvents, chromatographic separation, evaporation and several crystallization steps. All these processes are energy intensive and make citric acid an expensive raw material for the production of commodities such as the plasticizers.

Alternatively, this work evaluated the production of TEC from a calcium salt usually obtained during the precipitation process. Calcium citrate is a fine-white solid, insoluble in water and in ethanol.

TEC production was carried out by an acidification process of the calcium salt in an alcoholic medium using sulfuric acid. During the process, a first reaction step of ion exchange mechanism occurred (reaction between sulfuric acid and calcium citrate to produce calcium sulfate and citric acid in solution), and then the simultaneous esterification of the dissolved citric acid occurred. The latter reaction is catalyzed by the sulfuric acid remaining in the reaction mixture.

The experimental study was carried out using the Box-Benhken method where three variables were considered:  Temperature, ethanol to calcium salt molar ratio, and sulfuric acid loading. Experiments were carried out in agitated test tubes under isothermal conditions. To reproduce a batch experiment, for each experimental condition, several tubes were loaded with identical initial compositions, but removed from the isothermal system at different times. This allowed to obtain a kinetic profile to follow reaction performance. After calcium sulfate removal, the reacting media was sampled and analyzed by using liquid chromatography.

Results indicated that TEC can be obtained directly from the calcium salt, avoiding the purification of citric acid  for the traditional esterification route. A kinetic model of the process was established and it can be used for process up-scaling.