(534d) Design of a Reactive Distillation Column: Comparison Between Trays and Packings

Paola Bastidas, Julio Vargas, Álvaro Orjuela, Iván Gil, Gerardo Rodríguez

Grupo de Procesos Químicos y Bioquímicos. Departamento de Ingeniería Química y Ambiental, Universidad Nacional de Colombia-Sede Bogotá, Cr. 30 45 – 03, Bogotá, Colombia

During the last decade, there has been an increasing application of reactive distillation technology in industry, due to the economic benefits that it offers. For example, the production rate and the conversion are improved by the constant removal of the generated products, and the capital costs are reduced since less units operations are required. This growing interest demands deeper study, in order to have more efficient processes.  

The key to have efficient separation-reaction process is the proper design and selection of the hardware to be implemented inside the column. The current process simulators are powerful tools that allow comparing different types of internals, that is, to study the effect of the equipment geometry to achieve given separation and reaction grades. That is the case of the Aspen Plus software, where rigorous design of reactive distillation columns is possible through the non-equilibrium (NEQ) model, which has in to account the effect of heat and mass transfer velocities in the global and phase balances, and assumes that the equilibrium is reached only at the interphase.

In this work, the design of a reactive distillation column to produce isoamyl acetate is done with a NEQ model approach. Comparative studies between using trays and packings are developed through process simulations using Aspen Plus^®software. In both cases, NRTL-HOC thermodynamic model is used to describe the phase equilibrium, and an equilibrium kinetic model using Amberlyst 35 as catalyst is implemented to describe the reaction behavior. A stream containing acetic acid, amyl alcohol, amyl acetate and water is feed in the lower zone of the reactive section.  At the same time an excess of amyl alcohol is fed at the top of that section.

For the same column configuration, better results are obtained with packings than with trays. When using packings as internals, the isoamyl acetate purity is higher than with trays, as well as the global conversion. According to the global heat transfer rate, the feed streams vaporize when enter to the column, with a stronger tendency when using packings than when using trays. This change of energy in the reactive zone promotes the reaction in the desired direction, due to its endothermic nature. Because of the vaporization is weaker in the case of the tray column, the temperature profile goes down in this zone, making that one of the reactants be in the phase vapor rather than in the liquid vapor, which leads to a lower conversion than in the case of the packed column.

It was encountered also, that in the reactive zone were the higher tray efficiency and the lower HETP value, for the tray and packed column, respectively. That is, the reactive zone is the most efficient zone, mainly because that is the zone with bigger differences between the vapor and liquid temperatures.