Modelling of large-scale dense gas–solid bubbling fluidised beds using a novel discrete bubble model

Abstract

In order to model the complex hydrodynamic phenomena prevailing in industrial scale gas-solid bubbling fluidised bed reactors and especially the macro-scale emulsion phase circulation patterns induced by bubble-bubble interactions and bubble coalescence, a discrete bubble model (DBM) has been developed. In the DBM, the (larger) bubbles are modelled as discrete elements and are tracked individually during their rise through the emulsion phase, which is considered as a continuum. The DBM, originally developed for the description of gas-liquid flows, has been adapted to cope with bubbles with a diameter larger than the size of an Eulerian cell, which is required in view of the large bubble size distribution at higher gas flow rates. Moreover, a new drag model for a single bubble rising in a fluidised bed derived from empirical correlations has been implemented, as well as a simple model to account for bubble coalescence and break-up. The strong advantage of the DBM compared to other models previously reported in the literature for the description of large-scale fluidised beds is that it fully accounts for the two-way coupling between the bubbles and the emulsion phase, which enables direct computation of the emulsion phase velocity profiles. Comparison of the results of simulations ignoring bubble coalescence and simulations taking bubble coalescence properly into account demonstrated the significant effect of bubble coalescence on the large-scale circulation patterns prevailing in bubbling fluidised beds. The simulation results for the lateral profiles of the visible bubble flow rate have been compared qualitatively with experimental results reported by Werther [1974. Influence of the bed diameter on the hydrodynamics of gas fluidized beds. A.I.Ch.E. Symposium Series 70(141), [53][54][55][56][57][58][59][60][61][62]. The effect of the superficial gas velocity on the velocity and porosity profiles has been studied. In general, it can be concluded that the DBM is able to capture the salient features of the hydrodynamics of bubbling fluidised beds. However, further research is required to improve the closure equations for the bubble behaviour, bubble-bubble interactions and bubble coalescence and break-up to enable a complete quantitative description. ᭧

Comparison between two-fluid model simulations and particle image analysis & velocimetry (PIV) results for a two-dimensional gas–solid fluidized bed

“…A fourth approach can be followed using Bubble discrete models and phenomenological methods, in which only the most relevant macroscopic characteristics of the bed are reproduced, e.g. gross bubble behaviour and general motion of the emulsion phase (see, for example, Bokkers et al, 2006). As this approach is based on simplified models and phenomenological correlations, it is the less computationally expensive and, therefore, it is especially suitable for the simulation and optimisation of large scale fluidized beds supporting chemical reactions such as combustion or gasification.…”

Monitoring of particle motions in gas-solid fluidized beds by electrostatic sensors

“…The net downward transport of particles is only in the wall region in the fluidized bed, whereas the overall upward flow of particle motions occupies the remaining larger part of the cross section [24]. Furthermore, the magnitude of upward velocity is remarkably greater than that of the downward velocity [47,50]. Since the induced electrostatic voltage was simultaneously affected by spatial sensitivity of sensors and particle velocity, the measured correlation velocity was always positive in the dense-phase region in the fluidized bed, which means particles showed an overall upward movement in this region.…”

“…Since the induced electrostatic voltage was simultaneously affected by spatial sensitivity of sensors and particle velocity, the measured correlation velocity was always positive in the dense-phase region in the fluidized bed, which means particles showed an overall upward movement in this region. While for the dynamic bed level region, both the upward and downward particle motions were more vigorous [50]. The number of particles moving upward became fewer since more particles moved laterally due to the bubbles eruption at the bed surface [24].…”

A novel approach for modeling bubbling gas-solid fluidized beds

“…In contrast to those previous models where the bubbles are considered as spherical elements, in the present approach, the bubbles are considered as spherical caps which rise according to their size and local condition. Moreover, instead of computing the bubble trajectory by integrating the bubble velocity in time from empirical correlations, the bubbles are tracked individually according to Bokkers et al (2006), where the virtual mass force has been modified to account for the spherical cap (Kendoush, 2003).…”

“…However, the fact that both the bubble rise velocity and the trailing effect between rising bubbles are based on empirical correlations make the general use of DIBS difficult for modelling bubbling FB's. In contrast to the DIBS model, the DBM reported in Bokkers et al (2006), fully accounts for the two way coupling between the emulsion phase and the rising bubbles, which are considered as discrete spherical elements, that rise according to the second law of motion, moreover the model includes the bubblebubble and bubble-wall interaction to model the coalescence process. Nevertheless, the force resulting from bubble to bubble interaction due to wake effect is ignored, and the inclusion of the Eulerian solution for the dense phase increases the computational cost.…”