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Journal ArticleDOI

Residence time distribution of solids in multistage fluidisation

01 Jan 1982-Chemical Engineering Science (Pergamon)-Vol. 37, Iss: 9, pp 1371-1377
Abstract: The residence time distribution of the solids in multistage fluidisation, wherein downcomers for transferring the solids from stage to stage are provided to the horizontal perforated plates, is experimentally investigated covering a wide range in process variables using flat, baffle and spiral plates developed in the study. The RTD data is modelled using (i) the multiple parameter model, (ii) the diffusional mixing model and (iii) the fractional tank extension model when it is noted that over certain conditions of operation both spiral and baffle plates indicate near piston-flow for the solids without hampering the fluidisation characteristics.

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Topics: Residence time distribution (53%), Baffle (52%)
Citations
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Journal ArticleDOI
Qian Weizhong1, Liu Tang1, Wang Zhanwen1, Wei Fei1  +3 moreInstitutions (2)
Abstract: Methane decomposition over a Ni/Cu/Al 2 O 3 catalyst is studied in a two-stage fluidized bed reactor. Low temperature is adopted in the lower stage and high temperature in the upper stage. This allows the fluidized catalysts to decompose methane with high activity in the high temperature condition; then the carbon produced will diffuse effectively to form carbon nanotubes (CNTs) in both low and high temperature regions. Thus the catalytic cycle of carbon production and carbon diffusion in micro scale can be tailored by a macroscopic method, which permits the catalyst to have high activity and high thermal stability even at 1123 K for hydrogen production for long times. Such controlled temperature condition also provides an increased thermal driving force for the nucleation of CNTs and hence favors the graphitization of CNTs, characterized by high resolution transmission electron microscopy (HRTEM), Raman spectroscopy and XRD. Multistage operation with different temperatures in a fluidized bed reactor is an effective way to meet the both requirements of hydrogen production and preparation of CNTs with relatively perfect microstructures.

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101 citations


Journal ArticleDOI
TL;DR: It turns out that discretization of the reactor into a moderate number of segments already gives excellent numerical approximations to the continuous model, and in the diffusion limit a partial differential equation for the particle density p(t,x) is obtained.

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Abstract: In this paper we study stochastic models for the transport of particles in a fluidized bed reactor and compute the associated residence time distribution (RTD). Our main model is basically a diffusion process in [0,A] with reflecting/absorbing boundary conditions, modified by allowing jumps to the origin as a result of transport of particles in the wake of rising fluidization bubbles. We study discrete time birth-death Markov chains as approximations to our diffusion model. For these we can compute the particle distribution inside the reactor as well as the RTD by simple and fast matrix calculations. It turns out that discretization of the reactor into a moderate number of segments already gives excellent numerical approximations to the continuous model. From the forward equation for the particle distribution in the discrete model we obtain in the diffusion limit a partial differential equation for the particle density p(t,x) \[ \frac{\partial}{\partial t} p(t,x) =\frac{1}{2} \frac{\partial^2}{\partial x^...

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40 citations


Cites background from "Residence time distribution of soli..."

  • ...Krishnaiah, Pydisetty, and Varma [11] proposed a model with a combination of a mixed section with a stagnant one and short circuiting....

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Journal ArticleDOI
Zheng Zou1, Yunlong Zhao1, Hu Zhao1, Libo Zhang1  +3 moreInstitutions (1)
Abstract: The simulation of bubbling fluidized beds (BFB) residence time distribution (RTD) based on the structure-based drag model are conducted for the single and binary gas-solid phases systems, a comparison of computed results with experimental data proves that our model is applicable to both systems with better accuracy. The revised drag coefficient (Hd) increases with decreasing the gas velocity or increasing the particle diameter. The increase of the feed rate could improve the solids flow pattern to be close to the plug flow, while increasing gas velocity or bed height would lead to a wider RTD. The particles in the binary mixture are in more diffusion-oriented movement so as to have less MRT (mean residence time) than that of the single system. The coarse particles with longer MRT are simulated to accumulate into the bed bottom with a slower vertical velocity.

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18 citations


Journal ArticleDOI
M Prasad Babu1, Y. Pydi Setty1Institutions (1)
Abstract: Experimental investigation of RTD (residence time distribution) of solids is carried out in a single-stage fluidized bed provided with an internal, using uniformly sized particles and a binary solid mixture, varying gas flow rate, solids rate, bed height, dilution and the bed geometry. The effect of these variables on first and second moments as well as on F-curves has been determined. Using a binary solid mixture or an internal inside the bed is found to reduce backmixing of solids. The data is fitted to FTEM and the values of N obtained were compared for different variables.

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18 citations


Journal ArticleDOI
Zheng Zou1, Yunlong Zhao1, Hu Zhao1, Libo Zhang1  +3 moreInstitutions (1)
Abstract: The present work focuses on a numerical investigation of the solids residence time distribution (RTD) and the fluidized structure of a multi-compartment fluidized bed, in which the flow pattern is proved to be close to plug flow by using computational fluid dynamics (CFD) simulations. With the fluidizing gas velocity or the bed outlet height rising, the solids flow out of bed more quickly with a wider spread of residence time and a larger RTD variance ( σ 2 ). It is just the heterogeneous fluidized structure that being more prominent with the bed height increasing induces the widely non-uniform RTD. The division of the individual internal circulation into double ones improves the flow pattern to be close to plug flow.

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15 citations


References
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Journal ArticleDOI
J. Raghuraman1, Y.B.G. Varma1Institutions (1)
Abstract: A three-parameter mathematical model is suggested for expressing the residence time distribution in multistage systems. It is assumed that a fraction of the feed short-circuits each stage which comprises of an active backmix region and a dead region with cross-flow of the material between the two regions. The mass balance differential equations are solved using Laplace transformation. The computed results for different values of the parameters are presented and the model is compared with the available experimental data.

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25 citations


Journal ArticleDOI
Y. B. G. Varma1Institutions (1)
01 Sep 1975-Powder Technology
Abstract: Staging of a fluidisation column with horizontal screens reduces the axial mixing of the phases and limits the formation and growth of the bubbles. The pressure drop of the fluid and the flow patterns of the phases differ markedly with the mode of operation of the multistage fluidised bed. These aspects with respect to the theoretical models, the experimental data and the range of applicability are discussed along with the correlations to predict the characteristics of the bed.

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23 citations


Journal ArticleDOI
J. Raghuraman1, Y.B.G. Varma1Institutions (1)
Abstract: The cross-flow model ( Chem. Engng Sci. 1973 28 585), proposed by the authors for multistage systems, is matched with the experimental data taken subsequently on the residence time distribution of solids in a multistage fluidised bed. The model parameters are related to the plate geometry and the flow rates of the phases. The model is found to agree with the experimental data of the earlier investigators on single and multistage fluidised beds with a maximum root mean square deviation of 0·05. Based on the data, conditions for smooth fluidisation are suggested.

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15 citations


Journal ArticleDOI
J. Raghuraman1, Y.B.G. Varma1Institutions (1)

9 citations


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No. of citations received by the Paper in previous years
YearCitations
20211
20191
20173
20151
20091
20081