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Abstract:
Scale-up of chromatographic columns and reactors processing liquid phases is typically based on increasing the column diameter while maintaining the linear flow velocity constant. Chemical engineers’ long experience with conventional steady state fixed-bed reactor operation tells that such a scale-up strategy can, in case of exothermic reactions, lead to formation of hot spots. In addition, periodic operation of non-isothermal gas phase adsorptive reactors has been investigated in detail. It is therefore rather surprising that the thermal behaviour of periodically operated liquid phase chromatographic reactors has received only little attention in the literature [1–3].
The purpose of this study was to investigate under which conditions can nonisothermal operation of single column (batch) and simulated moving-bed (continuous) chromatographic reactors be advantageous or disadvantageous. A theoretical analysis was carried out by using the equilibrium theory and numerical simulations, and an experimental study by using laboratory scale reactors.
Thermal effects in a liquid phase chromatographic reactor originate from reaction enthalpy, adsorption enthalpy, and enthalpy of mixing the liquids. Under nonisothermal conditions, the propagation velocities of concentration fronts can differ significantly of those under isothermal conditions because concentration and thermal waves (and shocks) are coupled through the enthalpy of adsorption. Therefore, the liquid phase to solid phase heat capacity ratio becomes an important factor.
Esterifications of acetic acid with methanol and ethanol to produce methyl acetate and ethyl acetate were used as the model reactions. Amberlyst 15 (20 wt-% crosslink density) and Finex CS16G (8 wt-% cross-link density) ion-exchange resins in H+ form were chosen as the stationary phases. Reactor experiments were carried out in thermally insulated (adiabatic) columns of 1.6 and 2.6 cm in diameter and 20 to 50 cm in height, fitted with 5 to 8 thermocouples along the main axis of the column. Experiments under temperature controlled (isothermal) conditions were carried out as a reference.
As an example of the results, the development of a positive thermal wave in the column (higher temperature than in the feed) was recorded in the front of a wide pulse injection. The thermal wave had a self-amplifying nature because the moving reaction front travels at approximately the same velocity. Thermal effects in chromatographic reactors can therefore be much more pronounced than in conventional fixed-bed reactors. Using the Finex resin, for example, the observed maximum temperature increase in the thermal wave was approximately 25 K under conditions where the temperature rise in a conventional fixed-bed reactor operated at steady state was only 6 K.
References
1. Migliorini, C., Wendlinger, M., Mazzotti, M. Morbidelli, M., Temperature gradient operation of a simulated moving bed unit, Industrial & Engineering Chemistry Research 40 (2001) 2606-2617
2. Sainio, T., Ion-exchange resins as stationary phase in reactive chromatography, Doctoral dissertation, Acta Universitatis Lappeenrantaensis 218, Lappeenranta, 2005
3. Meurer, M., Dynamische Simulation chromatographischer Simulated-Moving-Bed Flüssigphasen-Reaktoren, VDI Verlag GmbH, Düsseldorf, 1999
