Thermal effects in reactive liquid chromatography

doi:10.1016/j.ces.2007.02.033

Chemical Engineering Science

Volume 62, Issues 18–20, September–October 2007, Pages 5674-5681

https://doi.org/10.1016/j.ces.2007.02.033 Get rights and content

Abstract

Thermal effects in unsteady-state liquid phase chromatographic reactors were investigated experimentally in a thermally insulated column. In the case of an exothermic esterification reaction catalyzed by an acidic ion-exchange resin, a self-amplifying positive thermal wave was found to develop in the reactor. This improved the reactor performance considerably when compared to isothermal conditions: both conversion of reactants and the mole ratio of the desired and undesired products in a product fraction increased. The heat of reaction, enthalpy of adsorption, and heat of mixing all contribute to the thermal behavior of the reactor. The coupling between the concentration fronts and thermal waves was elucidated by means of numerical simulations. The solid phase to fluid phase heat capacity ratio was found to be an important parameter because it affects the magnitude and propagation velocities of the thermal waves.

Introduction

In reactive chromatography, chemical reactions and chromatographic separation of the products are combined into a single unit operation. The aim of the method is to increase conversion of the reactants and product purity. Characteristic to reactive chromatographic processes is their transient behavior. When carried out batchwise in a single column unit, pulses of reactants are eluted through the reactor. In a continuous simulated moving bed reactor (Mazzotti et al., 1996), periodic switching of the inlet and outlet ports in the direction of fluid flow generates a periodic, quasi-steady state with moving concentration fronts in each column.

Owing to the dynamic nature of the process, there is continuous generation or consumption of heat due to enthalpies of adsorption, chemical reaction, and mixing. In addition, thermal effects can also originate from viscous heat dissipation, especially in HPLC columns packed with fine particles (Brandt et al., 1997). Such phenomena should be taken into account in scale-up of the reactor concept because increasing the column diameter eventually renders the system nearly adiabatic. Until recently (Sainio, 2005), thermal effects in liquid phase chromatographic reactors have been overlooked and the literature is limited to isothermal conditions. Meurer (1999) briefly discussed the factors involved in non-isothermal operation, but thermal effects were considered to be small and no experimental or numerical investigation was carried out. This is in contrast with reactive gas–solid separation processes, where thermal effects are commonly included in mathematical modeling (Yongsunthon and Alpay, 1999, Xiu et al., 2002).

The aim of this work is to demonstrate how thermal effects can significantly affect conversion and separation in non-isothermal liquid phase chromatographic reactors. Experimental results for esterification of acetic acid with ethanol, catalyzed by an acidic ion-exchange resin, are shown. Influences of adsorption and mixing enthalpies on the thermal behavior of the reactor are illustrated with data from non-reactive experiments. Numerical simulations are presented to further elucidate the coupling between concentration and temperature waves in the reactor. The results demonstrate that—analogous to gas–solid reactors (Glöckler et al., 2003)—the solid phase to fluid phase heat capacity ratio is a major factor in non-isothermal unsteady-state liquid phase chromatographic reactors.

Section snippets

Materials and methods

A sulfonated poly(styrene-co-divinylbenzene) cation-exchange resin in $H^{+}$ form was used as the stationary phase in the chromatographic reactor experiments. KEF76 resin (Finex Oy, Finland) has a mesoporous structure and a moderate level of cross-linking (see Table 1). Esterification of acetic acid with ethanol served as a model reaction. Reagent grade chemicals were used as received in all experiments. The properties of the chemicals, shown in Table 2, yield a reaction heat $Δ_{rxn} H = - 4.6 kJ / mol$ and a

Experimental results and discussion

An important design objective for the esterification of acetic acid with ethanol in a chromatographic reactor is to minimize the water content in the product fraction containing ethyl acetate. Typical experimental results, as are displayed in Fig. 1, show that the application of a chromatographic reactor facilitates this objective. Analysis of the concentration profiles in similar experiments under isothermal conditions has been given by Mazzotti et al. (1997) and is not repeated here.

As seen

Theory

The dynamic behavior of a non-isothermal chromatographic reactor is best analyzed by mathematical modeling. However, the phase equilibrium behavior of the esterification reaction mixture in presence of an elastic ion-exchange resin is complex, and several temperature and concentration-dependent parameters would be required to describe it accurately. In addition, the heat of mixing in such a non-ideal multicomponent system is difficult to estimate. The aim here is to demonstrate the coupling

Numerical results and discussion

The aim here is to demonstrate the coupling between the concentration fronts and thermal waves as well as to identify parameters that influence the reactor performance. For this purpose, a reversible reaction $A ⇌ B + C$ and linear isotherms are considered. The eluent is regarded inert and non-adsorbing. It is further assumed that the reactant A is eluted between the products B and C, and that separation of the products is possible. Although complete conversion and separation is possible also with

Conclusion

It was shown experimentally that a moving, self-amplifying thermal wave can develop in a liquid phase chromatographic reactor under adiabatic conditions. For an exothermic reaction the thermal wave is positive (i.e., higher temperature result compared to the initial state) and can significantly enhance the reactor performance when it moves together with the reactive front. An increase of 90% in the ethyl acetate to water mole ratio was observed when the chromatographic reactor was operated

Notation

$c$	liquid phase concentration, $mol m^{- 3}$
$C_{p}$	heat capacity, $J {kg}^{- 1} K^{- 1}$
$C_{pm}$	molar heat capacity, $J {mol}^{- 1} K^{- 1}$
$E_{act}$	activation energy, $kJ {mol}^{- 1}$
$F$	phase ratio, dimensionless
$Δ_{rxn} G$	Gibbs energy of reaction, $kJ {mol}^{- 1}$
$Δ_{ads} H$	enthalpy of adsorption, $kJ {mol}^{- 1}$
$Δ_{rxn} H$	enthalpy of reaction, $kJ {mol}^{- 1}$
$K$	slope of linear adsorption isotherm, dimensionless
$K_{a}$	reaction equilibrium constant, $mol m^{- 3}$
$k_{+}$	forward reaction rate constant, $s^{- 1}$
$L$	column length, m
$M$	molar mass, $kg {mol}^{- 1}$
$n_{B}$	molar amount of product B in a pure product fraction, mol
$q$	solid phase

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Thermal effects in reactive liquid chromatography

Abstract

Introduction

Section snippets

Materials and methods

Experimental results and discussion

Theory

Numerical results and discussion

Conclusion

Notation

Journal of Chromatograpy A

Chemical Engineering Science

Chemical Engineering Science

Chemical Engineering Science

Chemical Engineering Science

Chemical Engineering Sciecne