(88b) Heat Integrated Reactive Distillation Using External Side Reactors for Synthesis of Tri-Ethyl Citrate

Organic acids, typically produced by fermentation of carbohydrate feedstocks, constitute an important class of biorenewable platform chemicals that can be further converted to useful products. Tri-ethyl citrate (TEC), produced from citric acid and ethanol, is currently gaining of attention as a non-toxic, biocompatible plasticizer that can be used in place of petroleum-based phthalate compounds. Phthalates have potential carcinogeneity and overall health concerns to the extent that they have been banned in Europe from use in children’s toys and human contact applications. At present, the potential of TEC is limited by the lack of large-scale, energy efficient and economic production.

Esters are advantageously produced using reactive distillation, where simultaneous reaction and separation take place in the same process unit. However, the hardware design of standard reactive distillation columns exhibits challenges for slow chemical reactions such as tri-ethyl citrate formation, because high liquid holdup and large quantities of catalyst are required inside the reactive distillation column to obtain complete conversion. To overcome this problem and  make the synthesis of citrate esters economically competitive with their petroleum-based counterparts,  we have developed and demonstrated advanced reactive distillation concepts involving heat integration and external  side reactors.   AspenPlus process analysis has been performed using available kinetic data for esterification of citric acid with ethanol to integrate a pre-reactor as well as side reactors, which can have larger catalyst volumes and operate at any desired temperature, with a fractionation column.  Heat integration is examined for energy efficiency and maintaining temperatures in side reactors to optimize catalyst utilization. The concept of integrating side reactors with distillation has been validated experimentally using the reactive distillation facility at Michigan State University with a pre-reactor and one side reactor coupled to a distillation column. An 85% conversion of acid groups was obtained in this configuration when the pre-reactor and side reactor were operated at 110^oC using Amberlyst-15 as a catalyst. Using AspenPlus simulation for the case of multiple side reactors, greater than 98% conversion of citric acid groups can be obtained.