Mass transfer accompanied with complete reversible chemical reactions in gas-liquid systems: an overview

In many processes in the chemical industry mass transfer accompanied with reversible, complex chemical reactions in gas-liquid systems are frequently encountered. In point of view of design purposes it is very important that the absorption rates of the transferred reactants can estimated sufficiently accurate. Moreover, mass transfer phenomena can also affect substantially important process variables like selectivity and yield. Therefore large amounts of research effort has been invested in describing these processes and in the development of models that can be used for the calculation of the mass transfer rates and other parameters.\ud \ud The description of the absorption of a gas followed by a single first order irreversible reaction is simple and straightforward. For all mass transfer models, e.g. film, penetration and surface renewal respectively, this process can be analytically solved. For other processes however, only for a limited number of special cases analytical solutions are possible and therefore numerical techniques must be used for the description of these phenomena. Besides numerically solved absorption models the mass transfer rates often can be calculated satisfactory accurate by simplifying the actual process by means of approximations and/or linearizations. In this paper an overview will be given of the absorption models that are available for the calculation of the mass transfer rates in gas-liquid systems with (complex) reversible reactions. Both numerically solved and approximated models respectively will be treated and conclusions on the applicability and restrictions will be presented. Also perspectives and white spots will be indicated

Kinetics of CO2 with primary and secondary amines in aqueous solutions - II. Influence of temperature on zwitterion formation and deprotonation rates

The kinetics of the reaction of CO2 with various alkanolamines (MEA, DGA, DIPA, DEA, MMEA) in aqueous solutions has been studied as a function of temperature. Also kinetic data at 303 K were obtained for the reaction between CO2 and the cyclic amine morpholine in aqueous solutions. All observed phenomena can be explained very satisfactorily with the zwitterion mechanism proposed by Caplow. With respect to the temperature influence on the overall reaction rate for primary and secondary amines, two classes can be distinguished: when the zwitterion formation is rate determining a significant temperature influence is observed whereas only a slight temperature dependence is observed when the zwitterion deprotonation is rate determining. All kinetic experiments were interpreted with the aid of a numerically solved absorption model which describes gas absorption accompanied by reversible chemical reactions. For last reversible reactions like those in the present study, only in this way reliable reaction-rate data can be deduced from absorption experiments. The Brønsted relationship between the zwitterion-formation rate constant and the acid dissociation constant of the alkanolamine, as proposed by Versteeg and van Swaaij (1988a), seems to be valid over a wide range of temperatures and for a great variety of alkanolamines. This relationship is not valid for cyclic amines like MOR

On the kinetics between CO2 and alkanolamines both in aqueous and non-aqueous solutions—I. Primary and secondary amines

The reaction between CO2 and primary and secondary alkanolamines (DEA and DIPA) has been studied both in aqueous and non-aqueous solutions (ethanol and n-butanol) at various temperatures. Reaction kinetics have been established by chemically enhanced mass transfer of CO2 into the various solutions. The experiments were performed in a stirred vessel operated with a horizontal interface which appeared to the eye to be completely smooth. The reaction can be described with the zwitterion-mechanism originally proposed by Caplow (1968) and reintroduced by Danckwerts (1979). Literature data on the reaction rates can be correlated fairly well with this mechanism. As all amines react with CO2 in a reversible way, and the mass transfer models used for the interpretation of the experimental data neglect this reversibility and take only the forward reaction rate into account, the influence of the reversibility is studied. With the aid of numerical mass transfer models (Versteeg et al., 1987b,c) the experimental method with its underlying assumptions have been verified and the applicability and the limits of this method were determined. Special attention has been paid to the influence of small amounts of impurities (amines) on the measured mass transfer rates. A Brønsted relationship exists between the second-order rate constant, k2, for the formation of the zwitterion and the acid dissociation constant of the alkanolamine

Mass transfer characteristics in structured packing for CO2 emission reduction processes

Acid gas treating and CO2 capture from flue gas by absorption have gained wide importance over the past few decades. With the implementation of more stringent environmental regulations and the awareness of the greenhouse effect, the need for efficient removal of acid gases such as CO2 (carbon dioxide) has increased significantly. Therefore, additional effort for research in this field is inevitable. For flue gas processes the ratio of absorption solvent to gas throughput is very different compared to acid gas treating processes owing to the atmospheric pressures and the dilution effect of combustion air. Moreover, in flue gas applications pressure drop is a very important process parameter. Packing types are required that allow for low pressure drop in combination with high interfacial areas at low liquid loading per square meter. The determination of interfacial areas in gas-liquid contactors by means of the chemical method (Danckwerts, P. V. Gas-liquid reactions; McGraw-Hill: London, 1970) has been very frequently applied. Unfortunately, many of the model systems proposed in the literature are reversible and therefore this condition possibly is not met. Versteeg et al. (Versteeg, G. F.; Kuipers, J. A. M.; Beckum, F. P. H.; van Swaaij, W. P. M. Chem. Eng. Sci. 1989, 44, 2292) have demonstrated that for reversible reactions the conditions for the determination of the interfacial area by means of the chemical method are much more severe. In a study by Raynal et al. (Raynal, L.; Ballaguet, J. P.; Berrere-Tricca, C. Chem. Eng. Sci. 2004, 59, 5395), it has been shown that there is a dependency of the interfacial area on the packing height. Unfortunately, most model systems used, e.g., CO2-caustic soda (as used by Raynal et al.), are much more complex and consist of (a set of) reversible reaction(s). The natures of these systems make the conditions at which the interfacial area can be determined much more severe and put more limitations on the process conditions and experimental equipment than a priori can be expected. Therefore, an extended absorption model is required to determine the conditions at which the interfacial area can be measured without detailed knowledge of the values of the liquid-side mass transfer coefficient, k1, beforehand.

Lattice Kinetics of Diffusion-Limited Coalescence and Annihilation with Sources

We study the 1D kinetics of diffusion-limited coalescence and annihilation with back reactions and different kinds of particle input. By considering the changes in occupation and parity of a given interval, we derive sets of hierarchical equations from which exact expressions for the lattice coverage and the particle concentration can be obtained. We compare the mean-field approximation and the continuum approximation to the exact solutions and we discuss their regime of validity.Comment: 24 pages and 3 eps figures, Revtex, accepted for publication in J. Phys.

On the kinetics between CO2 and alkanolamines both in aqueous and non-aqueous solutions—II. Tertiary amines

The reaction between CO2 and tertiary alkanolamines (MDEA, DMMEA, TREA) has been studied in aqueous solutions at various temperatures. Also the absorption of CO2 in a solution of MDEA in ethanol has been studied. Reaction kinetics have been established by chemically enhanced mass transfer of CO2 into the various solutions. The experiments were performed in a stirred vessel with a horizontal interface which appeared to the eye to be completely smooth. The reaction of CO2 with tertiary amines can be described satisfactorily with the base-catalysis mechanism proposed by Donaldson and Nguyen (1980). Also attention has been paid to the influence of reversibility and small amounts of impurities (primary and secondary amines) on the measured mass transfer rate. For the reaction rate constant, k2, of the reaction between carbon dioxide and tertiary amines exists a Brønsted relation. There is a linear relation between the logarithm of k2 and pKa at 293 K

Modelling the kinetics of transesterification reaction of sunflower oil with ethanol in microreactors

Transesterification reaction of vegetable oil with ethanol leads to ethyl esters, used to date for applications principally in food and cosmetic industry. To open the application field to biofuels (to substitute current fuels resulting from fossil resources), the process efficiency has to be developed to be economically profitable. In this work, the sunflower oil ethanolysis was performed in a micro-scaled continuous device, inducing better control for heat and mass transfer in comparison with batch processes. Moreover, this device ensures kinetic data acquisition at the first seconds of the reaction, which was not feasible in a conventional batch process. These data were used to model occurring phenomena and to determine kinetic constants and mass transfer coefficients. A single set of these parameters is able to represent the evolution of the reaction media composition function of time for five ethanol to oil molar ratios (6.0, 9.0, 16.2, 22.7 and 45.4). The model was validated in reaction and diffusion mode. Finally, it was subsequently used to simulate reactions with other operational conditions and to propose other process implementation

Kinetics of the thermal degradation of Erica arborea by DSC: Hybrid kinetic method

The scope of this work was the determination of kinetic parameters of the thermal oxidative degradation of a Mediterranean scrub using a hybrid method developed at the laboratory. DSC and TGA were used in this study under air sweeping to record oxidative reactions. Two dominating and overlapped exothermic peaks were recorded in DSC and individualized using an experimental and numerical separation. This first stage allowed obtaining the enthalpy variation of each exothermic phenomenon. In a second time, a model free method was applied on each isolated curve to determine the apparent activation energies. A reactional kinetic scheme was proposed for the global exotherm composed of two independent and consecutive reactions. In fine mean values of enthalpy variation and apparent activation energy previously determined were injected in a model fitting method to obtain the reaction order and the preexponential factor of each oxidative reaction. We plan to use these data in a sub-model to be integrated in a wildland fire spread model

The determination of drug stability by HPLC assay of degradation products.

This work evaluates the advantages in drug stability testing of following the decomposition by analysis of decomposition products. Conventional methods of drug stability testing are criticised and the limitations are shown to be largely a result of analysing for un-decomposed drug. Using simulated analytical results incorporating various levels of precision, the use of product concentration is shown to be capable of producing conventional rate constants in shorter times, by utilising smaller extents of reaction than use of intact drug analysis. The simulation is applied to single, parallel, consecutive and reversible reactions. The findings of the simulated decompositions are supported by practical decomposition studies on several drugs. HPLC with ultraviolet detection is used as a single analytical method for both reactant and product. Aspirin and diiodoaspirin represent single-decomposition product systems; tetracycline is examined as a drug decomposition involving parallel, consecutive and reversible reactions. Nafimidone is studied to establish the advantages and limitations of product and reactant measurements, where all decomposition products have not been identified. The oxidation of 5-hydroxymethylfurfural is also examined to determine the usefulness of product analysis in limit testing and in establishing reaction pathways. In all cases where product identity is known, it is shown that the initial rate method employing product concentrations provides more rapid determination of rate constants. It is suggested that reaction order is overemphasized in shelf-life determination of drugs and that the initial rate method with analysis of product can minimise - and in certain cases, eliminate - the need for temperature stressing to determine shelf-life. HPLC is shown to be very generally applicable in product measurement. New criteria for stability-indicating assays that allow use of the initial rate method are demonstrated for the above drugs, and for succinylsulphathiazole, diphenhydramine and chloramphenicol. The separations obtained are described in terms of current ion pairing ideas