Organic esters are commercially important bulk chemicals used in a gamut of industrial applications. Traditional routes for the synthesis of esters are energy intensive, involving repeated steps of reaction typically followed by distillation, signifying the need for process intensification (PI). This study focuses on the evaluation of PI concepts such as reactive distillation (RD) and distillation with external side reactors in the production of organic acid ester via esterification or transesterification reactions catalyzed by solid acid catalysts. Integration of reaction and separation in one column using RD is a classic example of PI in chemical process development. Indirect hydration of cyclohexene to produce cyclohexanol via esterification with acetic acid was chosen to demonstrate the benefits of applying PI principles in RD. In this work, chemical equilibrium and reaction kinetics were measured using batch reactors for Amberlyst 70 catalyzed esterification of acetic acid with cyclohexene to give cyclohexyl acetate. A kinetic model that can be used in modeling reactive distillation processes was developed. The kinetic equations are written in terms of activities, with activity coefficients calculated using the NRTL model. Heat of reaction obtained from experiments is compared to predicted heat which is calculated using standard thermodynamic data. The effect of cyclohexene dimerization and initial water concentration on the activity of heterogeneous catalyst is also discussed. Continuous pilot scale reactive distillation runs were conducted to demonstrate the technical feasibility of cyclohexyl acetate formation and to exemplify the opportunities for heat integration in RD operation. Based on these preliminary runs, process configurations and conditions suitable for high conversions of cyclohexene are suggested. The experimental data obtained at steady state are compared with results obtained from simulations performed using the RADFRAC column module in Aspen Plus. The concept of distillation with an external side reactor was evaluated in a process involving the transesterification of methyl stearate and 1-butanol, yielding butyl stearate. An activity-based kinetic model for this reaction using AmberlystTM 15 as catalyst was developed. The kinetic model includes etherification reactions occurring at reaction temperatures greater than 90°C, producing butyl methyl ether and dibutyl ether. Kinetic parameters from this database were used in modeling the distillation column with external side reactors. Process simulation using Aspen Plus describes column performance as a function of operating conditions, number of side reactors utilized, requirement for a pre-reactor, location of side draws and re-entry points in the column. The column configuration that maximizes conversion of the methyl ester to its butyl counterpart is presented.In addition to evaluation of the PI concepts, using butyric acid as a model compound, the effect of factors such as alcohol carbon chain length and type of solid acid catalyst on esterification reaction rate was investigated. In summary, advanced process concepts for continuous production of organic esters have been examined. Chemical kinetic data from laboratory scale experiments has been used in developing computational models of these concepts using Aspen Plus. The combination of process simulations and pilot-plant experiments have identified process configurations and conditions that lead to more efficient organic esters production with potential commercial interest.
Read
