Elsevier

Powder Technology

Volume 344, 15 February 2019, Pages 830-841
Powder Technology

Experimental and predictive research on solids holdup distribution in a CFB riser

https://doi.org/10.1016/j.powtec.2018.12.082Get rights and content

Highlights

  • Measure solids holdup of flow development region in a CFB riser

  • Provide some new experiment data on solids holdup distribution

  • Present new correlations for estimation of the solis holdup distribution

Abstract

The present work focuses on the axial and radial distribution of the solids holdup in the flow development region of a CFB riser. The effects of operating conditions (superficial gas velocity and solids circulation rate) and particle properties (particle density and size) on solids holdup are investigated by an optical fiber probe (PC6M). The solids holdup is higher in the lower section than that in the upper section of the riser, and is lower in the center than in the wall region of the riser. Increasing the solids circulation rate or decreasing the gas velocity results in the increment of the solids concentration and radial non-uniformity. With the increase of the particle density and size, the solids concentration increases, which is more evident at the lower part of the riser. Empirical correlations for predicting the cross-sectional averaged solids holdup and local solids holdup are developed respectively based on lots of experimental data. A good agreement is obtained between the predicted results and experimental data of the present work and many other data reported in the literature.

Introduction

Because of the advantage of highly efficient gas-solid contact, excellent mass/heat transfer, and the compact reactor structure, circulating fluidized beds (CFB) covered lots of the key industrial processes [1], especially where catalysts are rapidly deactivated and require regeneration, such as fluidized catalytic cracking (FCC), Fischer-Tropsch (F-T) synthesis, and methanol-to-olefins (MTO) process. Meanwhile, the design and scale-up of industrial reactors, expansion of industrial application scope, and process intensification have put forward higher requirements for the knowledge about gas and particles hydrodynamics.

Solids holdup is one of the most important parameters in gas-solid systems, which governs the gas-solid contact efficiency, heat and mass transfer, and the performance of chemical reaction [2,3]. According to existing research results, the axial profile of the solids holdup may be a linear distribution [4,5], an exponential-shaped [4,6], an S-shaped [2,6,7], or a C-shaped [[8], [9], [10], [11]], which depends on many factors, such as the operating conditions, the solid properties, the solid inventory, and the reactor geometry. According to Bai and Kato [5], in the riser which has a smooth exit and a weak restriction entrance, increase in the solids circulation rate tends to make a continuously increase in the solids holdup at each axial height under a constant superficial gas velocity. Once the solids circulation is increased to saturation carrying capacity (Gs*), a typical S-shaped solids holdup distribution starts to form, and the solids holdup distribution at the dense and dilute regions seems to change little with increasing solids circulating rate, although the dense region height continues to grow. In addition, the non-uniformity of the radial distribution of the solids concentration is an inherent property of the gas-solid flow in the riser. The particle content in the central region of the riser remains low and relatively constant, and increases significantly toward the wall [12,13]. The core-annulus flow pattern [[14], [15], [16], [17], [18], [19]] with a rapid up flowing dilute core region surrounded by a relatively dense annulus has been observed by many researchers. With increasing gas velocity and decreasing solid circulation rate, the solids concentration decreased [20] and the non-uniformity of radial solids holdup profiles tend to reduce [21].

In addition to the operating conditions, particle property is another crucial factor for the gas-solid hydrodynamics in the riser reactors. Both the mean particle size and the size distribution have a considerable effect on particle distribution and riser performance [7,[22], [23], [24], [25]]. Decreasing particle size causes an increase in the height of the acceleration region and solids circulation rate [26] and a reduction of the non-uniformity [21]. More spherical particles lead to higher solids concentration, no matter in acceleration or fully developed region [27]. However, there is no consensus about the effect of particle density on solids holdup. The results of Xu and Zhu [27] showed that with the increase of particle density, the solids concentration is higher nearly along the entire riser, which is different with the finding of Bai et al. [7], Mastellone and Arena [25], and Qi et al. [3]. In their opinion, increasing particle density leads to lower solids concentration in the fully developed region of the riser.

During the past few decades, a number of correlations for the prediction of solids distribution in the CFB riser have been put forward based on a large number of experimental data. Xu et al. [28], Qi et al. [3], and Bai and Kato [5] have listed the major correlations available in literatures. Table 1, Table 2 make a summary and supplement on it. According to the function of these formulas, they can be divided into two categories. One of them expounds the relationship between local solids concentration and cross-sectional averaged solids concentration. Based on the cross-sectional average solids holdup, the local solids holdup is regarded as a unique function of radial position and independent of bed diameter, particle density, measured plane, and particle size distribution [28,29]. The other kind of correlations are used to calculate the averaged solids holdup in the dense region, the fully developed region (the dilute region), or exit region of the riser. Most correlations that predict the averaged solids holdup are based on the operating conditions [5,9,30], gas and particle properties [3,31], and reactor configuration [3,32], but the related parameters and the scope of application are quite different from each other in previous literatures. Also, it's worth noting that the solids holdup in the dense region or dilute region is regarded as a constant value, so the effect of riser height on the solids concentration has not taken into consideration. Compared with the research on the stable dense region and the dilute region (fully developed flow region), there are few empirical formulas for the axial development stage in both cases of Gs < Gs* and Gs ≥ Gs*. Therefore, new correlations still need to be proposed to obtain the local solids holdup and the cross-sectional averaged solids holdup conveniently and accurately.

In this study, an optical fiber [6,12,27] named PC6M was used in a gas-solid CFB riser to gain new knowledge about the influence of operating conditions and particle properties on the solids holdup in the entire riser, especially in the axial development region. The axial and radial solids distributions were systematic investigated and new empirical correlations were provided to better predict the cross-sectional averaged and local solids holdup in the CFB riser.

Section snippets

CFB system

A schematic diagram of the CFB system used in this study is shown in Fig. 1. The system mainly consists of a riser (0.15 m-ID and 4.8 m-high), a solids storage tank (1.5 m-ID), and a solids circulation tube (0.8 m-ID). The CFB system is constructed by Plexiglas for visual observation with only small portions made of stainless steel. The riser is 4.8 m-high (the expanding part is 0.7 m) and 0.15 m-ID (the expanding part is 0.2 m). A branched pipe distributor (see Fig. 1 (a)) made of stainless

Axial profiles of solids holdup

For a given gas-solid flow system, the hydrodynamics is only influenced by the superficial gas velocity and solids circulation rate [50]. Fig. 3 shows the axial distribution of the averaged solids holdup in the CFB riser under different operating conditions. The cross-sectional averaged solids holdups are obtained by integral method of the local solids holdup at every radial position based on the cross-sectional area. The axial profiles in Fig.3 is mainly exponential, and the typical S-shaped

Conclusion

Numerous measurements are carried out in a 0.15 m-ID and 4.8 m high CFB riser to investigate the axial and radial solids holdup profiles by using particle fluid voidmeter PC6M. The objective of this study is to understand the effect of local riser position (axial and radial), operating conditions, and particle properties on solids holdup in the flow development region.

The results show that solids holdup is non-uniform in both axial and radial directions under all operating conditions. The

Nomenclature

    Ar

    Archimedes number (=dp3ρgg(ρp − ρg)/μg2), (−)

    D

    Riser diameter (m)

    Dc

    Crossover diameter (cm)

    dp

    Particle diameter (m)

    FrD

    Froude number based on riser diameter (=Ug/(gD)0.5) (−)

    Frt

    Particle Froude number (=Ut/(gD)0.5) (−)

    Gs

    Actual solids circulation rate (kg/m2s)

    Gs

    Saturation carrying capacity (kg/m2s)

    Gideal

    Ideal solid circulation rate if the entire solid at the upper section of the riser circulated out of the riser (kg/m2s)

    g

    Gravitational acceleration (m2/s)

    H

    Riser height (m)

    h

    Projected roof height (cm)

    R

Acknowledgements

The authors gratefully acknowledge the financial support of the National High-Tech R&D Program of China (2011AA05A204).

References (67)

  • S.W. Kim et al.

    Flow behavior and regime transition in a high-density circulating fluidized bed riser

    Chem. Eng. Sci.

    (2004)
  • A.T. Harris et al.

    Characterisation of the annular film thickness in circulating fluidised-bed risers

    Chem. Eng. Sci.

    (2002)
  • H. Zhang et al.

    The voidage in a CFB riser as function of solids flux and gas velocity

    Procedia Eng.

    (2015)
  • D. Geldart

    The effect of particle size and size distribution on the behaviour of gas-fluidised beds

    Powder Technol.

    (1972)
  • G.K. Khoe et al.

    Rheological and fluidization behaviour of powders of different particle size distribution

    Powder Technol.

    (1991)
  • P. Wang et al.

    Impact of particle properties on gas solid flow in the whole circulating fluidized bed system

    Powder Technol.

    (2014)
  • J. Xu et al.

    Effects of particle properties on flow structure in a 2-D circulating fluidized bed: Solids concentration distribution and flow development

    Chem. Eng. Sci.

    (2011)
  • G. Xu et al.

    Two distinctive variational regions of radial particle concentration profiles in circulating fluidized bed risers

    Powder Technol.

    (1999)
  • W. Zhang et al.

    Radial voidage profiles in fast fluidized beds of different diameters

    Chem. Eng. Sci.

    (1991)
  • G.S. Patience et al.

    Scaling considerations for circulating fluidized bed risers

    Powder Technol.

    (1992)
  • M.J. Rhodes et al.

    Similar profiles of solid flux in circulating fluidized-bed risers

    Chem. Eng. Sci.

    (1992)
  • F. Wei et al.

    Profiles of particle velocity and solids fraction in a high-density riser

    Powder Technol.

    (1998)
  • Y. Wang et al.

    Radial profiles of solids concentration and velocity in a very fine particle (36 μm) riser

    Powder Technol.

    (1998)
  • R. Wong et al.

    Modelling the axial voidage profile and flow structure in risers of circulating fluidized beds

    Chem. Eng. Sci.

    (1992)
  • T.S. Pugsley et al.

    A predictive hydrodynamic model for circulating fluidized bed risers

    Powder Technol.

    (1996)
  • Y. Alghamdi et al.

    A correlation for predicting solids holdup in the dilute pneumatic conveying flow regime of circulating and interconnected fluidised beds

    Powder Technol.

    (2016)
  • X. Qi et al.

    Hydrodynamic similarity in circulating fluidized bed risers

    Chem. Eng. Sci.

    (2008)
  • Z. Wang et al.

    Hydrodynamic characteristics of multi-stage conversion fluidized bed (MFB)

    Fuel Process. Technol.

    (2013)
  • M. Das et al.

    Axial voidage profiles and identification of flow regimes in the riser of a circulating fluidized bed

    Chem. Eng. J.

    (2008)
  • H. Zhu et al.

    Characterization of fluidization behavior in the bottom region of CFB risers

    Chem. Eng. J.

    (2008)
  • H. Zhu et al.

    Comparative study of flow structures in a circulating-turbulent fluidized bed

    Chem. Eng. Sci.

    (2008)
  • A.S. Issangya et al.

    Further measurements of flow dynamics in a high-density circulating fluidized bed riser

    Powder Technol.

    (2000)
  • C.M.H. Brereton et al.

    Microstructural aspects of the behaviour of circulating fluidized beds

    Chem. Eng. Sci.

    (1993)
  • Cited by (0)

    View full text