J.A. Hogendoorn

New absorption liquids for the removal of CO2 from dilute gas streams using membrane contactors

A new absorption liquid based on amino acid salts has been studied for CO2 removal in membrane gas–liquid contactors. Unlike conventional gas treating solvents like aqueous alkanolamines solutions, the new absorption liquid does not wet polyolefin microporous membranes. The wetting characteristics of aqueous alkanolamines and amino acid salt solutions for a hydrophobic membrane was studied by measuring the surface tension of the liquid and the breakthrough pressure of the liquid into the pores of the membrane. The dependence of the breakthrough pressure on surface tension follows the Laplace–Young equation. The performance of the new absorption liquid in the removal of CO2 was studied in a single fiber membrane contactor over a wide range of partial pressures of CO2 in the gas phase and amino acid salt concentrations in the liquid. A numerical model to describe the mass transfer accompanied by multiple chemical reactions occurring during the absorption of CO2 in the liquid flowing through the hollow fiber was developed. The numerical model gives a good prediction of the CO2 absorption flux across the membrane for the absorption of CO2 in the aqueous amino acid salt solutions flowing through the hollow fiber.

Approximate solution to predict the enhancement factor for the reactive absorption of a gas in a liquid flowing through a microporous membrane hollow fiber

Approximate solutions for the enhancement factor (based on the traditional mass transfer theories) for gas–liquid systems with a liquid bulk have been adapted to situations where a liquid bulk may be absent and a velocity gradient is present in the mass transfer zone. Such a situation is encountered during the absorption a gas in a liquid flowing through a hollow fiber. The explicit solution of DeCoursey [Chem. Eng. Sci. 29 (1974) 1867] for a second-order irreversible reaction has been used as a representative sample of the approximate solutions available in literature. It was chosen because of the accuracy of its predictions and the simplicity in use. The solution of DeCoursey was adapted, but still has limitations at long gas–liquid contact times. Under these conditions, the actual driving force for mass transfer of the gas phase species may not be identical for physical and reactive absorption. Also for these situations, there may be a significant depletion of the liquid phase reactant at the axis of the fiber (which is considered to be analogous to the liquid bulk in traditional mass transfer models). A criterion has been proposed for the applicability of the adapted DeCoursey’s approximate solution for a second-order irreversible reaction. Within the range of applicability, the approximate solution has been found to be accurate with respect to the exact numerical solution of the mass transfer model as well as the experimentally determined values of enhancement factor in a single hollow fiber membrane gas–liquid contactor. The single hollow fiber membrane contactor that has been used in this study has potential for use as a model gas–liquid contactor. This contactor can thus be used, along with the present approximate solution of the enhancement factor, to obtain the physicochemical properties of a reactive gas–liquid system from the experimental absorption flux measurements or vice versa, as described in the present work.

Approximation for the enhancement factor applicable to reversible reactions of finite rate in chemically loaded solutions

A new explicit relation is proposed for the prediction of the enhancement factor for reversible reactions of finite rate in chemically loaded solutions which also allows for unequal diffusivities. The relation for the enhancement factor is not based on an approximation of the absorption process, but is derived from a similarity which can be observed between the results of the approximation for an irreversible (1,1) order reaction given by, for example, DeCoursey (surface renewal model), and the exact numerical results. The present relation combines the solution of DeCoursey (1974 Chem. Engng Sci. 29, 1867?1872) for irreversible finite rate reactions, and the solution of Secor and Beutler (film model, 1967 A.I.Ch.E. J. 13, 365?373) for instantaneous reversible reactions. The diffusivity ratios in the solution of Secor and Beutler (1967) were replaced by the roots of these ratios in order to adapt the enhancement factors to the penetration theory. In general, this adaptation of the solution of Secor and Beutler gave reasonably good results, however, for some situations with unequal diffusivities deviations up to 20% were found. The results of the present approximation were for various reactions compared to the numerical enhancement factors obtained for the model based on the Higbie penetration theory. Generally, the agreement was reasonably good. Only 26 of 2187 preselected simulations (1.18%) had a deviation which was larger than 20%, while the average deviation of all simulations was 3.3%. The deviations increased for solutions with a substantial chemical loading in combination with unequal diffusivities of the components. For reactions with a kinetic order unequal to unity, the Ha number had to be multiplied by a factor, ??, so that Ea = ??H aA in the regime 2 < HaA Ea,?. This factor agreed well with the factor given by Hikita and Asai (1964, Int. Chem. Engng 4, 332?340) in their dimensionless number

Application of the Maxwell–Stefan theory to the transport in ion-selective membranes used in the chloralkali electrolysis process

The results of a fundamental mass transport model based on the Maxwell–Stefan approach are compared to experimental data obtained by Akzo-Nobel for a Dupont Nafion ion-selective membrane as used in chloralkali electrolysis processes. The main problem in the application of the Maxwell Stefan based mass transfer model to the chloralkali electrolysis process is a lack of available diffusivities for the membrane. Estimation of these diffusivities in the membrane based on a method presented by Wesselingh et al. (1995. Chem. Engng J., 57, 75–89) gave unrealistic high membrane potential drops. Therefore, another method was followed. First, a sensitivity analysis was carried out which resulted in a reduced set consisting of the dominating Maxwell–Stefan diffusivities. First estimates of these remaining diffusivities were determined for single layer sulfonic and a carboxylic membranes. With a slight adjustment of the values of the diffusivities obtained for the separate sulfonic and carboxylic layers, the performance parameters of the DuPont Nafion membrane could be predicted well for a reference experiment. These diffusivities also proved to be suitable for other anolyte strengths. However, for other catholyte strengths and current densities these diffusivities (even after a correction for the water uptake according to the method of Wesselingh et al. (1995. Chem. Engng. 5., 57, 75–89)) did not result in a good agreement between the simulated and experimentally observed performance parameters. Only after a correction of the diffusivities the simulations yielded approximately the same performance parameters as experimentally observed. From this it can be concluded that although a fundamental model is used in order to describe the mass transfer in a membrane, a single set of diffusivities is not sufficient in order to obtain the experimentally observed performance parameters at different process conditions. At this moment there is not enough knowledge on the exact phenomena taking place in the membrane in order to predict the necessary corrections of the diffusivities a priori. As long as there are no theoretically founded and reliable relations available to predict the Maxwell–Stefan diffusivities in a membrane (or accurate experimental data for these diffusivities) only a semi-empirical method as used in this study can serve as a basis for a further progress in the development of an existing (in this case DuPont Nafion) membrane.

The absorption of carbon monoxide in COSORB solutions : absorption rate and capacity

Absorption rate experiments and equilibrium experiments were carried out for the COSORB reaction at 300 K. The equilibrium data at 300 K could reasonably well be described with the following relation: [...] Determination of the kinetics and mechanism of a chemical reaction by means of absorption experiments is possible in the fast reaction regime, where the absorption rate is independent of the mass transfer. In the present study it turned out to be impossible to eliminate the influence of mass transfer, although the experiments were carried out at very low CO partial pressures. This made the determination of the kinetics and mechanism of the forward reaction of the COSORB reaction not possible. The conditions of the present experiments were such that it can be concluded that for absorption equipment design the COSORB reaction can be regarded as instantaneous with respect to mass transfer

Experimental study of the absorption of acid gases in porous particles impregnated with aqueous alkanolamide solutions

In the present study the absorption behaviour and the equilibria of CO2 and H2S in porous α- and γ-alumina particles impregnated with inert solution and alkanolamine solutions were investigated. From the equilibrium data it could be concluded that for CO2 and H2S also adsorption on the internal surface of liquid filled γ-alumina particles occurs. The experimental absorption rate data were compared with theoretical data obtained with an absorption model, which describes the mass transport accompanied by chemical reaction in porous particles impregnated with reactive solutions. The CO2 and H2S alkanolamine data necessary for this model were taken from literature, while the tortuosity and porosity were experimentally determined. The model could describe the experimentally observed absorption rates satisfactorily. Discrepancies mainly occurred for the process conditions where equilibrium was approached. These deviations probably must be attributed to the calculation of the equilibrium compositions (no correction for non-idealities) and the accuracy of the available thermodynamic data.

Mass transfer accompanied by reversible chemical reactions in an inert porous sphere impregnated with a stagnant liquid

The absorption of gaseous components in porous particles impregnated with a reactive liquid phase has been studied theoretically. A model that describes this absorption process has been developed in which it is assumed that the porous particles act as a liquid support and are chemically inert. The model is based on the description of diffusion and reaction in a stagnant liquid sphere, and validated with several asymptotic analytical solutions. Both reactions instantaneous with respect to the mass transfer rate and reactions with finite reaction rates can be applied. For the instantaneous reactions an approximation for the dimensionless total accumulation is presented if the diffusivities of the absorbed gas and the other components are unequal. For reactions with finite rates different absorption regimes can be recognized in the time?flux diagrams. The simultaneous absorption of two gases which both react in the liquid-impregnated particles, e.g. H2S and CO2 in aqueous amine solutions, has also been studied. The results of the simulations of the simultaneous absorption of two gases indicate that selective removal of one of the gas components with porous particles impregnated with a reactive liquid seems feasible.

Radial distribution of ions in pores with a surface charge

A sorption model applicable to calculate the radial equilibrium concentrations of ions in the pores of ion-selective membranes with a pore structure is developed. The model is called the radial uptake model. Because the model is applied to a Nafion sulfonic layer with very small pores and the radial uptake model is based on the assumption that continuum equations are applicable, the model is used near its limits of fundamental validity. However, the results indicate that the calculated profiles with the radial uptake model are realistic and similar to literature results (e.g. [J.R. Bontha, P.N. Pintauro, J. Phys. Chem. 96 (1992) 7778; J.R. Bontha, P.N. Pintauro, Chem. Eng. Sci. 49 (1994) 3835]). The membrane microstructure parameters (surface charge density and pore diameter) have been determined by fitting the sorption of sodium as predicted by the radial uptake model to the sorption of sodium as predicted by the so-called modified Pitzer model [J.H.G. Van der Stegen, A.J. van der Veen, H. Weerdenburg, J.A. Hogendoorn, G.F. Versteeg, Fluid Phase Equilibria 157 (1999a) 181]. This modified Pitzer model has proven to be able to predict volume averaged sorption of ions in a sulfonic membrane layer. Via the introduction of a component dependent correction factor in the radial uptake model, the sorption of ions other than sodium could also be fitted to the volume averaged sorption data as predicted by the modified Pitzer model. The correction factors were in the order of magnitude of 0.05–10, and dependent on the concentration of sodium. The necessity of the application of correction factors for the ions other than sodium may have been induced by the assumption that: • the applicability of continuum equations in the model is justified and/or; • the activity coefficients in the radial uptake model are equal to unity. It was observed that due to the preferential sorption of iron near the pore wall, the pore surface charge could be shielded, resulting in a decrease of the preferential selectivity of the membrane for sodium. However, such a phenomenon does not occur in the operating range of the chloralkali process, where the sorption of iron inside the membrane is proportional to its external concentration.

New Absorption Liquids for the Removal of CO2 from Dilute Gas Streams using Membrane Contactors

A new absorption liquid based on amino acid salts has been studied for CO2 removal in membrane gas–liquid contactors. Unlike conventional gas treating solvents like aqueous alkanolamines solutions, the new absorption liquid does not wet polyole5n microporous membranes. The wetting characteristics of aqueous alkanolamines and amino acid salt solutions for a hydrophobic membrane was studied by measuring the surface tension of the liquid and the breakthrough pressure of the liquid into the pores of the membrane. The dependence of the breakthrough pressure on surface tension follows the Laplace–Young equation. The performance of the new absorption liquid in the removal of CO2 was studied in a single 5ber membrane contactor over a wide range of partial pressures of CO2 in the gas phase and amino acid salt concentrations in the liquid. A numerical model to describe the mass transfer accompanied by multiple chemical reactions occurring during the absorption of CO2 in the liquid 9owing through the hollow 5ber was developed. The numerical model gives a good prediction of the CO2 absorption 9ux across the membrane for the absorption of CO2 in the aqueous amino acid salt solutions 9owing through the hollow 5ber. ? 2002 Elsevier Science Ltd. All rights reserved.

Continuous Gas Separation with Liquid Impregnated Particles in Gas-Solid Reactors

The absorption of a gas in liquid filled porous particles in gas-solid reactors was studied both theoretically and experimentally. In the theoretical study a micro mode!, describing mass transport accompanied with reaction inside the particles, was implemented in the macro balance for several asymptotic operation modes. The theoretical study showed that the gas separation can be carried out very efficiently with the liquid filled porous particles, especially for the countercurrent mode of operation. The results of the simulations for the removal of H2S from a gas stream also containing CO2 showed that a very selective absorption process can be obtained. For these kind of selective absorption processes the simulations showed that the residence time of the particles is a crucial parameter In the experimental part of this contribution the absorption of CO2 in porous y-alumina particles filled with water or 2M aqueous solutions of tertiary - or primary alkanolamines was investigated. Experiments were carried ...