The rate of oxidation of hydrogen sulphide by oxygen to elemental sulphur over NaX and NaY zeolites and the adsorption of sulphur

The rate of oxidation of H2S by O2 over synthetic sodium faujasite zeolites to produce elemental sulphur has been studied at partial sulphur l

Mass transfer in a gas-solid packed column at trickle flow

The height of an overall transfer unit has been evaluated in a gas—solid packed column at trickle flow by measuring column performance during steady state adsorption experiments. Results have been interpreted with an extraction model: mass transfer and axial dispersion in both phases. Using Bodenstein numbers for the gas and solid phases from a previous investigation the height of a true transfer unit has been calculated.\ud \ud The column was filled with dumped Pall rings, the solid phase was a freely flowing catalyst carrier, and the gas phase was air at ambient conditions containing freon-12 as adsorbing component.\ud \ud At low gas velocities column performance is entirely determined by axial dispersion but at higher gas velocities mass transfer limitations become important. For conditions of practical importance the height of a true transfer unit corresponds to 4 – 9 Pall ring layers

Axial dispersion of gas and solid phases in a gas—solid packed column at trickle flow

Axial dispersion of gas and solid phases in a gas—solid packed column at trickle flow, a promising new countercurrent operation, was evaluated using residence time distribution (RTD) experiments. The column was packed with dumped Pall rings, the gas phase was air at ambient conditions and the solid was a porous catalyst carrier.\ud \ud The RTD experiments for the solid phase were carried out using the “perfect pulse method”, while for the gas phase the “imperfect pulse method” was used. The model parameters were calculated by the methods of moments and various parameter optimization methods.\ud \ud At a given solid flow rate axial dispersion of the gas phase decreases with increasing gas velocity and is strongly dependent upon solid mass flux. Axial dispersion of the solid phase is approximately independent of the gas velocity and it is reduced if the solid mass flux is increased. For conditions of practical importance, 2 – 5 and 5 – 15 Pall ring layers correspond to the height of a mixing unit in the gas and solid phase, respectively

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

Hydrodynamic behaviour of a gas—solid counter-current packed column at trickle flow

Trickle flow of a more or less fluidized catalyst through a packed column is a promising new gas—solid counter-current operation. The hydrodynamic, behaviour of such a column, filled with dumped PALL rings, has been investigated, while some results have been obtained with RASCHIG rings and cylindrical screens as packing. The solid used was a microspherical catalyst carrier. Pressure drop, hold-up, loading and flooding were evaluated and compared with literature data for gas—liquid systems. The behaviour is analogous although the absolute magnitude is different.\ud \ud Pressure drop is low, up to 50% of the solid being carried by the packing. A correlation for the pressure drop, which is mainly caused by suspended particles, has been derived. At low gas velocities particle velocity is constant, whilst near flooding the slip velocity between gas and solid reaches a constant value. Using empirical values for particle velocity and slip velocity, hold-up, loading and flooding can be predicted. Scaling-up problems still need to be investigated. Results on mass transfer, axial dispersion of both phases and solid spread factors will be published later.\u

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

The influence of transport phenomena on the fluidized bed combustion of a single carbon particle

The burning rate and temperature of the carbon particles are known to affect the efficiency of a fluidized bed combustor, and also the emission levels of undesired noxious components. The main results of an extensive study on the fluidized bed combustion behaviour of a single carbon particle [1] are summarized. Calculations have been carried out with a newly developed transient model, the ASPC model, and also with the much simpler progressive conversion model. Besides, many experiments have been performed in a lab-scale fluid bed construction to measure the burning rate and temperature of individual carbon particles for various conditions. From the comparison between experimental results and model predictions it has been overall concluded that the ASPC model is especially useful in i) describing the complex behaviour of progressive carbon conversion for the regime of combustion controlled by carbon reactivity plus intraparticle oxygen diffusion, and ii) estimating the conditions for which transition to the regime of external mass and heat transfer control occurs. Accurate prediction of the carbon particle burning rate and temperature is only possible for the latter combustion regime

The gas-solid trickle-flow reactor for the catalytic oxidation of hydrogen sulphide: a trickle-phase model

The oxidation of H2S by O2 producing elemental sulphur has been studied at temperatures of 100–300°C and at atmospheric pressure in a laboratory-scale gas-solid trickle-flow reactor. In this reactor one of the reaction products, i.e. sulphur, is removed continuously by flowing solids. A porous, free-flowing catalyst carrier has been used which contains a NaX zeolite acting as a catalyst as well as a sulphur adsorbent. In order to describe mass transfer in the trickle-flow reactor, a reactor model has been developed in which a particle-free, upflowing gas phase and a dense, downflowing gas-solids suspension, the so-called trickle phase, are distinguished. Within the trickle phase, diffusion of the reactants parallel to reaction in the catalyst particles takes place. The mass transfer rate from the gas phase to the trickle phase has been evaluated by the reaction of H2S with SO2, which is a much faster reaction than the reaction with O2. From the experiments and from the reactor model calculations it appears that for the H2S-O2 reaction no mass transfer limitations occur at temperatures up to about 200°C, whereas at 300°C gas-phase mass transfer and diffusion within the dense solids suspension offer resistance to reaction

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. \ud \ud ¿ enhanced mass transfer by physical adsorption on small particles.\ud \ud ¿ enhanced mass transfer by fast homogeneous reactions in the slurry, due to inert particles,\ud \ud ¿ enhanced mass transfer by homogenous reaction in the liquid with dissolving particles,\ud \ud ¿ enhanced mass transfer due to reactive particles and\ud \ud ¿ enhanced mass transfer due to catalytic particles in heterogeneous reactive systems.\ud \ud Prospective areas for additional research are identified