52,385 research outputs found
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
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¿ enhanced mass transfer by physical adsorption on small particles.\ud
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¿ enhanced mass transfer by fast homogeneous reactions in the slurry, due to inert particles,\ud
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¿ enhanced mass transfer by homogenous reaction in the liquid with dissolving particles,\ud
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¿ enhanced mass transfer due to reactive particles and\ud
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¿ enhanced mass transfer due to catalytic particles in heterogeneous reactive systems.\ud
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Prospective areas for additional research are identified
An experimental study of diffusion and convection of multicomponent gases through catalytic and non-catalytic membranes
Diffusion of binary and ternary gases through catalytic and non-catalytic membranes has been studied experimentally at atmospheric pressure. These experiments were conducted in a modified Wicke-Kallenbach diffusion cell consisting of two continuously stirred gas volumes separated by a membrane. The equipment was suitable to measure fluxes of components through the membrane in the absence of gas-to-membrane mass transfer limitations.\ud
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Transport through a porous membrane has been measured and compared with the results of the dusty-gas model, which has been used to predict transport through a membrane. With independently determined input parameters this model turned out to be able to predict the transport of a multicomponent gas mixture through a membrane within a few percent (< 5%). The Fick model extended with a convective trnasport contribution was not able to produce similar results as obtained from the dusty-gas model, especially when an overall pressure gradient was present over the membrane.\ud
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In order to demonstrate the occurrence of surface effects, dynamic transport of a binary gas was studied in a similar experimental setup as described by Novák et al. In this setup the transport of gas mixtures containing helium, argon and nitrogen was in good agreement with the model simulations. For transport of carbon dioxide and propane through a γ-Al2O3 coated membrane, adsorption phenomena were observed, but no substantial surface mobility was detected at temperatures ranging from 293 to 433 K
Dynamic modeling of three-phase upflow fixed-bed reactor including pore diffusion
The dynamics of a three-phase upflow fixed-bed reactor are investigated using a non-isothermal heterogeneous model including gas–liquid and liquid–solid mass transfer and diffusion/reaction phenomena inside the catalyst. The partial differential and algebraic equations involving three integration variables (time and two space coordinates) are solved via discretization of the spatial coordinates coupled with the Gear method. For a multistep hydrogenation on a shell catalyst, the model exhibits significant effects of the external and above all internal resistance to hydrogen transfer but also non-trivial internal hydrocarbons concentration profiles. A simplified model is compared with the extended one and with experimental data in transient regime. In the investigated conditions—hydrocarbons in large excess—the diffusion of hydrocarbons appears to be actually not limiting, so that the simplest model predicts accurately the transient reactor behavior
Catalytic Wet Air Oxidation of Aqueous Organic Mixtures
Catalytic Wet Air Oxidation (CWAO) has been investigated for the treatment of water contaminated by 4-hydroxybenzoic acid (4HBA) and equimolar mixture of phenol-4HBA. Both batch measurements for kinetics determination and continuous
fixed bed operation have been performed on the same Activated Carbon (AC). After a fast initial deactivation AC was proved stable and efficient at moderate
temperature and oxygen pressure, like for phenol degradation.
The kinetic study in the case of highly adsorbing material as AC may require complex approach to account for the variation of adsorbed reactants during batch
oxidation. Adsorption isotherms at reaction temperature and with aged AC have been obtained according to Langmuir equation and used in 4HBA mass balance to
derive more significant kinetic parameters. At high catalyst loading and relatively low pollutant concentration, the variation of 4HBA during the batch may be even higher on the solid than in the aqueous phase
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
Sorption kinetics for the removal of aldehydes from aqueous streams with extractant impregnated resins
The sorption kinetics for the removal aldehydes from aqueous solutions with Amberlite XAD-16 and MPP particles impregnated with Primene JM-T was investigated. A model, accounting for the simultaneous mass transfer and chemical reaction, is developed to describe the process. It is based on the analogy to the diffusion and reaction in a stagnant liquid sphere, but corrected for the porosity and particle properties influencing the diffusion. The developed model describes the kinetic behavior of the process in the low concentration region rather well. However, in the high concentration region, larger discrepancies are observed. Initially, the influence of the flow rate was investigated to eliminate the effect of the external mass transfer. The influence of the particle morphology was investigated for both physical and reactive sorption. Physical sorption experiments were used to determine the factor τ that takes the particle properties influencing the diffusion into account. It was shown that the diffusion is faster in XAD-16 than in MPP impregnated systems. Reaction rate constant kx was determined by fitting the model to the experimental data. Sorption of benzaldehyde appears to be significantly slower (kx ~ 10−4 l/mol s) than the sorption of pentanal (kx ~ 10−3 l/mol s) due to the slower chemical reaction. The influence of the particle size was investigated for the sorption of pentanal with XAD-16. It was observed that the particle size does influence the diffusion term, but does not have an effect on the reaction rate. On the other hand, the extractant loading influences the reaction rate slightly in the low concentration region, whereas the initial concentration of the solute has more pronounced effect
Model system studies with a phase separated membrane bioreactor
The operation and evaluation of a bioreactor designed for high intensity oxygen transfer in a microgravity environment is described. The reactor itself consists of a zero headspace liquid phase separated from the air supply by a long length of silicone rubber tubing through which the oxygen diffuses in and the carbon dioxide diffuses out. Mass transfer studies show that the oxygen is film diffusion controlled both externally and internally to the tubing and not by diffusion across the tube walls. Methods of upgrading the design to eliminate these resistances are proposed. Cell growth was obtained in the fermenter using Saccharomyces cerevisiae showing that this concept is capable of sustaining cell growth in the terrestial simulation
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