Experimental and predictive research on solids holdup distribution in a CFB riser
Graphical abstract
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).
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