Characteristics of Gas-Fluidized Beds in Different Flow Regimes

Abstract

Hydrodynamic characteristics among different flow regimes of gas fluidized beds are compared on the basis of experiments with fluidized catalytic cracking particles in a 76.2 mm diameter riser. Pressure and local voidage fluctuations were analyzed using both statistical and chaotic tools. The standard deviations of local voidage fluctuations are much lower in a high-density circulating fluidized-bed riser than in the bubbling and turbulent flow regimes, even for identical local time-mean voidages. A chaotic time series analysis can distinguish flow structures of the various flow regimes. Bifractal structures, characterized by two Hurst exponents, two correlation dimensions, and two Kolmogorov entropies, characterize the motions of the dilute and emulsion phases in the bubbling and turbulent flow regimes. The two-phase structure becomes less distinguishable with increasing gas velocity, eventually disappearing on reaching dilute phase transport. However, for high solids concentrations, the bifractal character persists, suggesting that particles may travel in two different forms. Radial profiles of chaotic parameters are relatively flat in the bubbling and turbulent flow regimes, but significantly nonuniform in dense suspension upflow. Flow behavior in the high-density riser and in the dense bottom region of a low-density riser operated in the fast fluidization regime differ from the bubbling and turbulent flow regimes, even when compared where there are equal local voidages.

Gas‐solids flow structures in a novel circulating‐turbulent fluidized bed

“…This broad distribution resulted in a better gas-solids contacting quality and further reduces gas by-passing, which occurred in the core region of conventional CFB risers as reflected by the narrow sharp peak distribution. 32 Third, moving outwards towards the wall, the low solids concentration peak disappeared and there was only a high-density peak with a long tail towards the left. For all three radial regions, there existed wide distributions of local solids concentration.…”

New investigation in regime transition from bubbling to turbulent fluidization

“…It is well accepted that with increasing gas velocity, a gas-solid fluidized bed goes through particulate fluidization, bubbling fluidization, turbulent fluidization, fast fluidization, and pneumatic transport (Lim et al, 1995). According to the experimental evidence available in the literature so far, a common way to classify the flow regimes is to determine the transition velocities by some measurement techniques and interpretation methods (Bai et al, 1996(Bai et al, , 1999Smolders and Baeyens, 2001). The transition velocities have been found to be influenced by many factors such as particle properties (Cai et al, 1990;Grace and Sun, 1991;Gauthier et al, 1999), solids inventory (Chen et al, 1993), column diameter (Kim et al, 2004), distributor design (Svensson et al, 1996;Ellis et al, 2004), operating temperature and pressure (Choi et al, 1998;Sathiyamoorthy and Horio, 2003), the measuring techniques and data analysis methods (Cui and Chaouki, 2004).…”

Characterization of Gas Spouted Beds Using the Rescaled Range Analysis

“…The applications of Hurst rescaled range analysis in multiphase flow is summarized in Table 1. Bai et al (1999b) Gas-solid fluidized bed 98.3 × 1.5 100 10 000 dP Bai et al (1999a) Riser 76.2 × 6.1 100 10 000 dP AP OP Kim and Han (1999) Circulating fluidized 50 × 2.5 120 3000 dP bed Kikuchi et al (2001) Circulating fluidized 10 × 10 1,000 32 768 HT bed Freitas et al (2002) Slot To summarize, the following findings are important: • The Hurst exponent appears to be a unique function of operating conditions or fluidization regimes. Thus it can be used to identify flow regime transitions; • There is a break or overlapping point of R/S curve in the Pox diagrams.…”