(143d) Tributyl Citrate Production Via Reactive Distillation

Over the last decades, a market shift towards the consumption of biobased plasticizers has been observed. Such trend obeys a generalized concern about toxicity, biodegradability and sustainability of traditional petroleum derived plasticizers (mainly phthalates). Tributyl citrate (TBC) and Acetyl Tributyl citrate are two well-known bioderived plasticizer which has been used as substitutes for various phthalic esters. TBC production is generally accomplished by direct esterification of citric acid and butanol in a semi-batch process. This reaction is limited by chemical equilibrium, so excess of butanol and water removal are common strategies to improve reaction conversion at the industrial scale. Such operating conditions makes the process energy and mass intensive, with low efficiency and costly.  

Taking into account the above, this work focuses on the evaluation of a reactive distillation system for the production of TBC. A pilot scale reactive distillation system was used to evaluate the process performance under different conditions. A 7m tall 3” diameter stainless steel reactive distillation unit with structured packing similar to KATAPAK SP-11 with Amberlyst 70 as catalyst in the reactive sections was used. Pall rings were used as internals in the rectifying and striping sections. The column was coupled with a top decanter to allow removing produced water when need it. The column performance was studied at local atmospheric pressure (74 kPa), using unreacted and pre-reacted mixtures of different molar ratios of butanol and citric acid, under different feed loadings and at different reflux ratios.

Under different operating condition citric acid conversion was found to be close to 100% and a selectivity towards TBC above 90% was reached. A mixture of TBC and Butanol was withdrawn as the bottoms product to avoid thermal degradation of the citric species. With this butanol content, the temperature at the reboiler was kept below 426 K. Experimental results were compared to the simulated operation using a Radfrac module included in Aspen Plus. The necessary reaction kinetics and thermodynamics for the reactive system were obtained experimentally and correlated with suitable models (Power law kinetics and UNIQUAC activity based model). An equilibrium stage model was used during simulations, and it was enhanced by including Murphree efficiency, catalysts effectiveness factor and heat losses. A good agreement between experimental and simulation data were observed, indicating that the model can be used for up-scaling the operation.