Hydrodynamics and Mass Transfer in Gas‐Liquid‐Solid Circulating Fluidized Beds

Abstract

Although extensive work has been performed on the hydrodynamics and gas-liquid mass transfer in conventional three-phase fluidized beds, relevant documented reports on gas-liquid-solid circulating fluidized beds (GLSCFBs) are scarce. In this work, the radial distribution of gas and solid holdups were investigated at two axial positions in a GLSCFB. The results show that gas bubbles and solid particles distribute uniformly in the axial direction but non-uniformly in the radial direction. The radial nonuniformity demonstrates a strong factor on the gas-liquid mass transfer coefficients. A local mass transfer model is proposed to describe the gas-liquid mass transfer at various radial positions. The local mass transfer coefficients appear to be symmetric about the central line of the riser with a lower value in the wall region. The effects of gas flow rates, particle circulating rates and liquid velocities on gas-liquid mass transfer have also been investigated.

Multi-Scale Mass Transfer Model for Gas-Solid Two-Phase Flow

“…The demonstration agrees with the experimental observations of Li (1998) and Wang et al (2003) and can be interpreted as follows. First, the gas-solid contact can be improved by the dynamic movement of clusters, which decreases the cluster shield effect and provides more effective gas-solid contacting area for the mass transfer.…”

“…In order to prevent the fluidizing-gas air from being saturated by the stimulated naphthalene, a kind of inert resin-styrene particles (density: 1298 kg=m 3 ) was mixed with the naphthalene spheres (density: 1162 kg=m 3 ). This also made the particle mixture easily fluidizable (Li, 1998). The tested naphthalene particles had two different sauter mean diameters, 0.50 and 0.30 mm, and their corresponding size distributions are shown Table I.…”

“…A few works had discussed the reason (Kunii and Levenspiel, 1991;Kato et al, 1970), and one recognized cause for the scattering was thought to be the influence of flow structure's heterogeneity, which induces, for example, gas bypass and back-mixing to consequently cause the mass transfer to differ from case to case (Li, 1998). Thus, Halder and Basu (1988) had treated the slip velocity between fluid and particles as an important parameter in analyzing the gas-solid mass transfer rate occurring in the fast fluidized beds.…”

Characteristics of Three-Phase (Gas–Liquid–Solid) Circulating Fluidized Beds

“…Considering the effects of operating variables on ε S , it was found that the value ε S increased with increasing U G , d P or G S but decreased with increasing U L or μ L in TPCFBs as shown in Figures 3 and 4 (Cho et al, 2001a;Liu et al, 2003;Vatanakull et al, 2003;Razzak et al, 2007Razzak et al, , 2009). Regarding the G S in TPCFBs, Kim et al (2015) suggested Eq.…”

“…It has been noted that the axial and radial distributions of ε S are relatively uniform in TPCFBs as well as LSCFBs at fully developed flow conditions, as shown in Figure 2, comparing with those of conventional fluidized beds. This could be due to the rapid liquid velocity which is higher than the terminal velocity of fluidized solid particles (Cho et al, 2001a;Zheng et al, 2002;Liu et al, 2003;Vatanakull et al, 2003;Cao et al, 2009). Effects of operating variables on the values of ε S and ε G in TPCFBs and LSCFBs are summarized in Figures 3-5.…”