W.P.M. van Swaaij

On the Kinetics between CO2 and Alkanolamines both in Aqueous and Non-Aqueous Solutions. An Overview

Alkanolamine solutions are frequently used as solvent for the removal of acid compounds from industrial gases (Kohl and Riesenfeld, 1979). Depending on the process requirements, e.g., selective removal of H2S, CO2-bulk removal, several options for alkanolamine based treating solvents with varying compositions of the solution have been proposed. In this paper an overview is presented of the mechanisms that have been proposed in literature and the kinetic data for the various reactions are critically evaluated. Conclusions on the applicability and restrictions are discussed along with perspectives. In addition white spots in the present knowledge are indicated. The reaction between CO2 and primary/secondary amines both in aqueous and non-aqueous solutions can be described over a wide range of conditions and amine concentrations with the zwitterion-mechanism as originally proposed by Caplow (1968) and reintroduced by Danckwerts (1979). All published results, both non-aqueous and aqueous solutions, amine-prom...

A study on the reaction between CO2 and alkanolamines in aqueous solutions

Literature data on the rates of reaction between CO2 and alkanolamines (MEA, DEA, DIPA, TEA and MDEA) in aqueous solution are discussed. These data induced us to carry out absorption experiments of CO2 into aqueous DEA, DIPA, TEA and MDEA solutions from which the respective rate constants The results for DEA and DIPA were analysed by means of a zwitterion-mechanism which was derived from the mechanism originally proposed by Danckwerts [1 The reaction rate of CO2 with aqueous TEA and MDEA solutions shows a significant base catalysis effect which is also reported by Donaldson and Nguy

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 Bronsted relationship exists between the second-order rate constant, k2, for the formation of the zwitterion and the acid dissociation constant of the alkanolamine.

High-temperature membrane reactors : potential and problems

The most recent literature in the field of membrane reactors is reviewed, four years after an analogous effort of ours (Saracco et al., 1994), describing shortly the potentials of these reactors, which now seem to be well established, and focusing mostly on problems towards practical exploitation. Since 1994, progress has been achieved in several areas (sol–gel deposition of defect free sol–gel derived membranes, reduction in thickness of Pd membranes, synthesis of zeolite membranes) whereas stagnation was noticed in some others (high-temperature sealing of membranes into modules, scaling-up of membrane reactor, etc.). As a result, despite the still increasing research efforts, industrial application does not seem to be round the corner, yet. However, several non-permselective membrane reactor opportunities with currently available membranes might pave the way for more sophisticated applications.

Mass transfer in gas-liquid slurry reactors

A critical review is presented on the mass transfer characteristics of gas?liquid slurry reactors. The recent findings on the influence of the presence of solid particles on the following mass transfer parameters in slurry reactors are discussed: volumetric gas?liquid mass transfer coefficients (kLa, kGa), liquid-side mass transfer coefficients (kL and kS) and specific gas?slurry contact area (a). The second part of this paper reviews the recent progress in our knowledge and understanding of the enhancement of gas?slurry mass transfer due to the presence of solids. Five different cases are distinguished, i.e. ? enhanced mass transfer by physical adsorption on small particles. ? enhanced mass transfer by fast homogeneous reactions in the slurry, due to inert particles, ? enhanced mass transfer by homogenous reaction in the liquid with dissolving particles, ? enhanced mass transfer due to reactive particles and ? enhanced mass transfer due to catalytic particles in heterogeneous reactive systems. Prospective areas for additional research are identified.

Microporous hollow fibre membrane modules as gas-liquid contactors. Part 1: Physical mass transfer processes. A specific application: mass transfer in highly viscous liquids

Gas-liquid mass transfer has been studied in a membrane module with non-wetted microporous fibres in the laminar flow regime. This new type of gas/liquid contactor can be operated stabily over a large range of gas and liquid flows because gas and liquid phase do not influence each other directly. Therefore foam is not formed in the module, gas bubbles are not entrained in the liquid flowing out of the reactor and the separation of both phases can be achieved very easily. These phenomena often limit the applicability of conventional contactors, e.g. a bubble column which was also studied in the present work. The large mass transfer area of a bundle of small fibres offers the possibility of creating a compact gas/liquid mass exchanger. However, owing to the small channels in and around the fibres the flow of either gas or liquid becomes laminar which reduces the mass transfer capacity of the module. Therefore the mass transfer coefficients in the laminar flow regime were determined experimentally. For mass transfer determined by the transport in the liquid phase it was found that the active mass transfer area is equal to the total membrane area, regardless the porosity of the fibre. For processes with liquid flowing through the fibres, the influence of fibre diameter, diffusivity in the liquid, liquid viscosity and liquid velocity on mass transfer can be correlated extremely well with the Graetz-Leveque solution derived for the analogous case of heat transfer. For liquid flowing around regularly packed fibres mass transfer was described satisfactory with a correlation derived from a numerical solution for the similar heat transfer problem [Miyatake and Iwashita, Int. J. Heat Mass Transf., 33 (1990) 416]. Correlating mass transfer in liquid flowing around irregularly packed fibres was not possible because of the undefined dimensions of the different channels between the fibres.

Microporous hollow fibre membrane modules as gas-liquid contactors. Part 2. Mass transfer with chemical reaction

Absorption determined by mass transfer in the liquid is described well with the Graetz-Leveque equation adapted from heat transfer. The influence of a chemical reaction on the mass transfer was simulated with a numerical model and tested on the absorption of CO2 in a hydroxide solution. Absorption determined by mass transfer in the gas phase and the pores of the membrane was also analysed experimentally and numerically. It was found that the gas phase concentration profile is established at a very short distance from the entrance of the fibre. This results in a constant Sherwood number along the fibre. A module coated with a very thin silicone rubber layer showed absorption rates comparable to the uncoated membranes. If absorption liquids are used which wet the membranes, resulting in leaky membranes, such a coating can enlarge the application of microporous hollow fibre membrane module.

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 Bronsted relation. There is a linear relation between the logarithm of k2 and pKa at 293 K.

Determination of mass transfer rates in wetted and non-wetted microporous membranes

The mass transfer resistance of microporous membranes placed between a gas and a liquid phase was studied for both wetted and non-wetted membranes. It was found that the mass transfer coefficient can be described according to the film model in which the porosity of the membrane and the tortuosity of the pores is incorporated. For the non-wetted membranes (mean pore diameter of 0.1 um), the Knudsen and continuum diffusion must be taken into account. No difference was observed in the values of the liquid-phase mass transfer coefficients between systems with and without a membrance at the gas?liquid interface, despite the different hydrodynamic situation at the interface. The influence of a chemical equilibrium reaction on the mass transfer through a wetted membrane was analysed mathematically (two-film concept). With this model the tortuosity factor of the membrane was calculated from experimentally determined fluxes.

Current hurdles to the success of high-temperature membrane reactors

High-temperature catalytic processs performed using inorganic membranes have been in recent years a fast growing area of research, which seems to have not yet reached its peak. Chemical engineers, catalysts and materials scientists have addressed this topic from different viewpoint in a common effort. Despite the amount of work already carried out, the direct application of inorganic membrane reactors in the process industry is still limited because of a large number of technical and economic drawbacks. The opportunities of this novel type of reactor compared with conventional reaction and separation systems have been emphasised in several former reviews. In the present contribution attention is focused on the major hurdles along the way of exploitation of these possibilities: high costs. low permeability, synthesis of defect-free permselective years, instability of membranes and catalysts, sealing of membranes into modules, etc. An assessment of what has been already accomplished in the attempt to overcome these limitations is given, stressing the gaps still to be filled and pointing the way towads future efforts to rach this goal.