Residence time distribution of the liquid in two‐phase cocurrent downflow in packed beds: Air/newtonian and non‐newtonian liquid systems
TL;DR: In this article, the liquid residence time distribution for two-phase downflow of air-water and non-Newtonian liquids through packed beds of porous and nonporous particles is presented.
Abstract: New data on the liquid residence time distribution for two-phase downflow of air-Newtonian and non-Newtonian liquids through packed beds of porous and non-porous particles are presented. The piston-dispersion-exchange model is used to describe the liquid flow. With porous particles the dynamic evolution of the tracer concentration in the particles is described in terms of diffusion phenomena. The axial dispersion is very important in the case of two-phase downflow of air-water (trickle flow regime) and air-CMC systems through fixed beds with porous particles, and is negligible in the case of non-porous particles. With the porous particles, a key value is the effective diffusion coefficient of the tracer in the pores of the particles.
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TL;DR: In this paper, state-of-the-art correlations for shear and velocity slip factors and Ergun single-phase flow bed constants (Blake−Kozeny−Carman and Burke−Plummer bed parameters) were developed.
Abstract: The original and extended Holub phenomenological models for pressure drop and liquid holdup in trickle flow regime systematically under-predicted frictional pressure drops at elevated pressure and at high gas throughputs. On the basis of an extensive historic trickle flow regime database and Ergun bed constants (over 4000 measurements from 34 references between 1959 and 1998), state-of-the-art correlations for shear and velocity slip factors and Ergun single-phase flow bed constants (Blake−Kozeny−Carman and Burke−Plummer bed parameters) were developed. The correlations involved combination of feed-forward neural networks and dimensional analysis. The shear and velocity slip factors were expressed as a function of the six most expressive dimensionless groups (ReL, ReG, FrL, WeL, XL, StL), whereas Blake−Kozeny−Carman and Burke−Plummer bed parameters were correlated to particle equivalent diameter, sphericity factor, bed porosity, and column diameter. These correlations fed into Holub's phenomenological mode...
79 citations
TL;DR: In this paper, the authors demonstrate experimentally the temperature shift of trickle-to-pulse flow regime transition, pulse velocity, two-phase pressure drop, liquid holdup and liquid axial dispersion coefficient.
49 citations
TL;DR: In this article, a comprehensive database containing 973 liquid axial dispersion coefficient measurements (DAX) for trickle-bed operation reported in 22 publications between 1958 and 2001 was used to assess the convenience of the few available correlations.
Abstract: Current correlations aimed at estimating the extent of liquid back-mixing, via an axial dispersion coefficient, in trickle-bed reactors continue to draw doubts on their ability to conveniently represent this important macroscopic parameter. A comprehensive database containing 973 liquid axial dispersion coefficient measurements (DAX) for trickle-bed operation reported in 22 publications between 1958 and 2001 was thus used to assess the convenience of the few available correlations. It was shown that none of the literature correlations was efficient at providing satisfactory predictions of the liquid axial dispersion coefficients. In response, artificial neural network modeling is proposed to improve the broadness and accuracy in predicting the DAX, whether the Piston–Dispersion (PD), Piston–Dispersion–Exchange (PDE) or PDE with intra-particle diffusion model is employed to extract the DAX. A combination of six dimensionless groups and a discrimination code input representing the residence-time distribution models are used to predict the Bodenstein number. The inputs are the liquid Reynolds, Galileo and Eotvos numbers, the gas Galileo number, a wall factor and a mixed Reynolds number involving the gas flow rate effect. The correlation yields an absolute average error (AARE) of 22% for the whole database with a standard deviation on the AARE of 24% and remains in accordance with parametric influences reported in the literature.
© 2002 Society of Chemical Industry
44 citations
TL;DR: In this article, a mechanistic film model was proposed for the description of trickle-bed reactor hydrodynamic parameters (two-phase pressure drop, total external liquid holdup) in the low interaction regime.
42 citations
TL;DR: In this paper, a comparative analysis is made for a trickle-bed reactor, a packed-bubble column, a three-phase fluidized bed and a slurry-bubble column with an active and moderately deactivating catalyst for the wet oxidation at high pressure and temperature of organic-containing aqueous wastes.
36 citations
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TL;DR: In this article, a 2.58-cm I.D. tube, packed with glass beads and granular CuO · ZnO catalyst or β-naphthol particles, was used as a trickle bed.
Abstract: Liquid holdup and mass transfer rates were measured in a 2.58-cm I.D. tube, packed with glass beads and granular CuO · ZnO catalyst or β-naphthol particles, and operated as a trickle bed. Gas-to-liquid (water) transport coefficients were determined from absorption and desorption experiments with oxygen at 25°C and 1 atm. Liquid-to-particle mass transfer was studied using β-naphthol particles.
Holdup and both mass transfer coefficients were unaffected by gas flow rate but increased with liquid rate. The data were correlated with equations that could be used for predicting mass transfer coefficients at high temperatures and pressures for use in the reaction studies reported in Part II.
222 citations
TL;DR: In this paper, residence time distributions in columns under trickle-flow conditions have been measured with the injection of a tracer, and the results indicated the occurrence of two different elementary mechanisms causing a spread in residence time, viz. an eddy diffusion process and mass exchange with stagnant areas.
145 citations
TL;DR: In this article, a review of the state of the art on backmixing in gas-liquid and gas-liquefied solid-reactor applications is presented, with a brief outline of various techniques for measuring residence time distribution (RTD) of various phases in a multiphase reactor is presented.
Abstract: This review evaluates the present state of the art on backmixing in gas-liquid and gas-liquid-solid reactors. A brief outline of numerous techniques for measuring residence time distribution (RTD) of various phases in a multiphase reactor is presented. This is followed by a brief description of differential and stagewise models for characterizing backmixing from RTD measurements. Both simple (that is, single-parameter axial dispersion model) and more complex (that is, two-, three-, or four-parameter models) models are evaluated. Backmixing characteristics of various gas-liquid columns such as trickle beds, spray columns, mechanically agitated columns, plate columns, fluidized bed columns, etc., are subsequently evaluated. The performance of a bubble column under various reaction conditions is analyzed. Criteria for the elimination of backmixing in packed-bed reactors are presented, and the effect of backmixing on the multiple steady states in a gas-liquid reactor is briefly reviewed. Finally, the scale-up problems associated with gas-liquid reactors with various degrees of backmixing and the recommendations for the future work in RTD and macromixing models are outlined.
131 citations
TL;DR: In this article, an expression analytique regoureuse de la distribution des temps de sejour dans un reacteur semi-infini a ecoulement piston avec dispersion axiale and echange de matiere avec des zones stagnantes est etablie.
128 citations
123 citations