Elsevier

Chemical Engineering Journal

Volumes 198–199, 1 August 2012, Pages 561-570
Chemical Engineering Journal

Absorption and desorption mass transfer rates in chemically enhanced reactive systems. Part II: Reverse kinetic rate parameters

https://doi.org/10.1016/j.cej.2012.03.051Get rights and content

Abstract

The forward and reverse kinetic rate parameters have been determined for CO2 absorption and desorption mass transfer processes in aqueous 2.0 M MDEA solutions at temperatures of 298.15, 313.15, and 333.15 K and the loading of CO2 ranging from 0 to 0.8. The derived kinetic rate parameters have been based on the results of experimental work in a controlled environment in a batch operated stirred tank reactor.

In a continuous effort to describe the fundamentals of gas–liquid desorption processes [1], [2], it has within applied experimental conditions been shown that; (1) the forward and reverse kinetic rate parameters derived by an analytical relation based on the Higbie penetration theory are within 25% of those numerically derived by a system of partial differential equations based on the Higbie penetration theory. The analytical relations were based on reversible reactions of finite rate in solutions of different CO2 loadings and diffusivities, (2) the reaction order of the forward reaction in solutions of different CO2 loadings is close to unity, and in agreement with the proposed reaction mechanism, (3) Arrhenius type of equations already developed for correlation of forward kinetic rate parameters were further modified in order to sufficiently correlate reverse kinetic rate parameters. These types of equations thus form a tool for the correlation and prediction of reverse kinetic rate parameters for engineering purposes and (4) the experimentally determined forward and reverse kinetic rate parameters were accordingly found to be related by an overall temperature dependent chemical equilibrium constant.

Highlights

► Forward and reverse kinetic rate parameters have been experimentally determined. ► Kinetic rate parameters were analytically and numerically derived. ► The reaction order of the forward reaction was close to unity. ► Arrhenius equations were modified to correlate reverse kinetic rate parameters. ► Kinetic rate parameters were related by a chemical equilibrium constant.

Introduction

In previous work [1], [2], it has been established that the liquid phase mass transfer coefficients for non-reactive systems and the chemical enhancement factors for reactive systems are identical for absorption and desorption mass transfer processes given identical operating conditions for the two processes. It is usually assumed and accepted that the forward and reverse kinetic rate parameters are related to each other by an overall chemical equilibrium constant at specific operating conditions; however, measurements of reverse kinetic rate parameters are very limited in the open literature, despite the industrial importance of desorption as a unit operation. Absorption and desorption mass transfer rates of CO2 in aqueous methyldiethanolamine (MDEA) solutions have previously been reported by Cadours et al. [3], Glasscock et al. [4], Jamal et al. [5], [6], Mshewa and Rochelle [7], Pacheco [8], Ramachandran et al. [9], Shi and Zhong [10], and Xu et al. [11]. In these publications, the absorption and desorption mass transfer rates are given at different experimental conditions, e.g. temperatures, liquid concentrations, CO2 partial pressures, etc., but not necessarily the reverse kinetic rate parameters. Jamal et al. [5], [6] experimentally measured the absorption and desorption mass transfer rates and numerically solved a comprehensive system of differential equations describing each individual species throughout the film in order to calculate the absorption and desorption kinetic rate parameters. The development of numerical techniques usually requires efforts, and it would be desirable to experimentally determine reverse kinetic rate parameters via simple analytical relations, as is usually carried out for determination of forward kinetic rate parameters. The purpose of the current work is to:

  • 1.

    determine forward and reverse kinetic rate parameters from experimental results obtained in previous work [1], [2]. The kinetic rate parameters are to be derived from simplified analytical relations based on the Higbie penetration theory and a numerically solved system of differential equations also based on the Higbie penetration theory describing each species exactly throughout the film. The results are to be compared to one another in order to investigate the possibility of using a simple analytical relation for the determination of reverse kinetic rate parameters from experimental work at different conditions;

  • 2.

    determine the order of the reaction of CO2 with aqueous MDEA at different temperatures and CO2 loadings in order to verify the proposed reaction mechanism;

  • 3.

    determine whether the experimentally determined reverse kinetic rate parameters can be correlated by Arrhenius type of equations, similar to the Arrhenius type of equations used to correlate forward kinetic rate parameters;

  • 4.

    determine whether the experimentally determined forward and reverse kinetic rate parameters can be related by a temperature dependent overall chemical equilibrium constant.

Section snippets

Experimental procedure

The reversible reaction of finite rate of aqueous MDEA with CO2 was applied for the determination of the forward and reverse kinetic rate parameters:MDEA+CO2+H2OMDEAH++HCO3-with the following reaction rate equation:RCO2=k2cCO2cMDEA-k-2cMDEAH+cHCO3-where c is the concentration, k is the kinetic rate parameter, the subscripts 2 and −2 refer to the respective forward and reverse reactions, and the concentration of water was set to unity. The current work treats an aqueous 2.0 M MDEA solution, and

Diffusivities of CO2 in CO2 loaded aqueous 2.0 M MDEA solutions

The experimentally determined mass transfer coefficients of N2O, the dimensionless Re, Sc, and Sh numbers, and the diffusivities of N2O and CO2 in CO2 loaded aqueous 2.0 M MDEA solutions are given in Table 2 at different temperatures and CO2 loadings. The densities and viscosities were taken from Weiland et al. [13]. The determined diffusivities are seen to decrease with increased CO2 loadings and increase with increased temperatures, the similar pattern seen for the mass transfer coefficients.

Analytical and numerical solutions of mass transfer theories

Conclusion

The forward and reverse kinetic rate parameters have been determined for CO2 absorption and desorption mass transfer processes in aqueous 2.0 M MDEA solutions at temperatures of 298.15, 313.15, and 333.15 K and the loading of CO2 ranging from 0 to 0.8. The derived kinetic rate parameters were based on the results of experimental work carried out in a controlled environment in a batch operated stirred tank reactor.

Forward and reverse kinetic rate parameters have been derived from simple analytical

Acknowledgments

This research has been carried out in the context of the CATO-2 program. CATO-2 is the Dutch national research program on CO2 Capture and Storage technology (CCS). The program is financially supported by the Dutch government (Ministry of Economic Affairs) and its consortium partners.

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    Present address: Statoil Research Centre, NO-3908 Porsgrunn, Norway.

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