Re-Interpretation of fluidization

Fluidization has been widely used in a number of industrial processes as an effective mean for providing good mixing and contact of the gas and solid phases, as well as good heat transfer. Applications include coal combustion, chemical, petrochemical and metallurgical processes, CO₂ capture. This attractive feature is achieved by solids circulation within the bed, in which particles are driven by gas, or bubbles, and transported around the bed. A different circulation pattern will give different heat/mass transfer rate, different mixing efficiency, different solid/gas contact and different residence time of solids and gas. Despite the extensive work carried out since 1960s, many uncertainties still remain, and the interpretations reported in literature are frequently conflict. For example, Grace and Harrison (1969) were among the first to systematically and quantitatively investigate the spatial distribution of bubbles in a two-dimensional bed by means of photography. They suggested that the bubbles with small sizes were uniformly distributed in a layer close to the distributor and then shafted inwards to the central region of the bed with the increase in the bed height, resulting in the reduction in concentration of bubbles in the region near the walls. This was used as a basis by Darton et al. for the popular model of bubble coalescence. However, different observations were reported by Werther and Molerus (1973) when they investigated the bubble spatial distribution in three-dimensional beds containing quartz sand, glass spheres and spherical copper powder under various operating conditions by using capacitance probes. In this chapter, we will report our recent funding in both bubbling fluidization and circulation fluidization by using non-invasive positron emission particle tracking (PEPT) technique. We will report the three flow structures that we funded in three dimensional fluidized beds, and discuss the impact of bed materials, operational parameters on the flow structure, solid gas pattern. We will discuss the operational conditions and flow structure on solid mixing, solid gas contact, solid and bubble residence time in both bubbling fluidized bed and circulation fluidized beds, and will discuss the distinction between the different regimes in circulating fluidized beds. The results will benefit academic and commercial/industrial researchers in combustion and fluidization engineering worldwide.