ABSTRACT
Chapter 1 Introduction
II. Solid-catalyzed gas-phase reactions Fluid catalytic cracking
1.3 Hydrodynamics of circulating fluidized bed
1.3.1 Macro-scale hydrodynamics
The event/structures which occurs at reactor scale is classified as macroscale phenomenon.
Macro scale hydrodynamics can be analyzed as axial and radial flow structures which largely depend on operating conditions.
Axial flow structure
At the outset of CFB research, axial and radial distribution of solids was the peculiarity of the CFB. The cross sectional averaged axial solid volume fraction follows ‘S’-shaped profile which consists of dense bottom region, dilute top region and in between transition region. Later other kind of profiles viz, exponential, linear, ‘C’-shaped curves are also reported in CFB. The axial solid hold up profile depends on solid circulation rate, total solids inventory, particle size and density, solids inlet configuration, secondary air injection and the level of solids reintroduction into the riser (Lim et al., 1995). Riser entry and exit configurations also affect the axial flow structure (Fan and Zhu, 1998).
Ratio of the solid circulation rate to the saturation carrying capacity of gas also influences the shape of axial solid holdup profile (Li et al., 1988). If the solid circulation rate is more than the saturation carrying capacity, S-shape or linear solid holdup profiles are observed.
However, if solid circulation rate is lower than the saturation carrying capacity of the gas (entered solids are immediately entrained from the system), exponential profile is observed.
In case of restricted/tapered exits, the solid volume fraction increases significantly at the top section of the riser which leads to C-shaped solid holdup profile in CFB (Martin et al.,
1992). Yang et al. (1988) have classified regimes in CFB based on axial solid holdup profiles. These are dilute bed, fast fluidized bed and dense transport bed where axial solid holdup profiles are exponential, S-shaped and linear respectively.
Axial solid holdup profile also depends on geometrical and operational parameters.
Increasing the solid circulation rate or decreasing the superficial gas velocity increases the cross-sectional averaged axial solid fraction in riser, primarily in the bottom dense region (Bai et al., 1992; Issangya et al., 1999; Wang et al., 2014). Similarly, increase in particle size and/or solid density, increases the solid fraction in the bottom region however at the same time slight decrease in solid holdup at the top region is also observed (Bai et al., 1992). It has also been noted that with increase in riser diameter cross-sectional averaged axial solid holdup decreases, particularly in the bottom region.
Acceleration zone is defined as zone extending from the bottom of the riser to the point where the solids attain constant velocity (Weinstein and Li, 1989) or where pressure gradient is constant (Arena et al., 1986; Bai et al., 1990). Acceleration length may occupy 1/3 to 2/3 length of the riser. In other words, solids attain approximately constant average velocity after 1/3 to 2/3 length of the riser. However, most of the acceleration occurs in transition zone (90 to 95%), suggesting as the boundary of the accelerating zone for all the practical purposes (Arena et al., 1986; Bai et al., 1990; Parssinen and Zhu, 2001; Wang et al., 2014). Acceleration length or axial particle velocity development varies with distributor, solid circulation rate and riser diameter (Yan et al., 2008; Bai et al., 1990; Yan et al., 2008; Wang et al., 2014). It increases with the solid circulation rate and riser diameter and decreases with superficial gas velocity.
Radial flow structure
The radial distribution of solids is one of the most critical aspects of the CFB. Most of the researches have reported, ‘core - annular’ structure where dilute core (low solid fraction) exist at the center and dense annulus region is found near the wall (Bader et al., 1988;
Rhodes et al., 1988; Bolton and Davidson, 1988; Brereton and Grace, 1993). The thickness of annular region increases with an increase in diameter (Werther, 1994; Zhang et al., 1995) and height of the riser (Kim et al., 2004). It is also a function of cross sectional averaged solid volume fraction (Bi et al., 1996). Further, the radial diffusion of solids from low concentration (at center) to high concentration (at wall) is also observed which has been explained by various theories given by several researchers (Berruti and kalogerakis, 1989, Bolton and Davidson, 1988, Li et al., 1991 etc.). However, none of these theories are well accepted due to their limited applicability.
In literature, several authors have also reported uniform solid holdup at the centre region of the riser which increases dramatically near the wall. The center dilute region can be extended up to 70 – 85% of the column radius (Hartge et al., 1988; Parssinen and Zhu, 2001). Parabolic and M shaped radial voidage profile are also widely reported (Weinstein et al., 1986; Hartge et al., 1988; Bai et al., 1991; Kato et al., 1991; Herb et al., 1992; Zhou et al., 1994). The shape of radial profile changes significantly with the operating conditions (Zhang et al., 1991; Brereton and Grace, 1993; Lim et al., 1995; Issangya et al., 2000; Wang et al., 2014). It has also been reported that superficial air velocity and solid circulation rate greatly influences the solid holdup near the wall (Issangya et al., 2000; Wang et al., 2014).
Relative increase in solid hold up near the wall and simultaneous decrease in dilute central core region is observed for an increase in the solid circulation rate or decrease in the superficial gas velocity. In the high solid flux condition (Gs>700 Kg/m2s), dilute core region shrinks up to 20% of the radius (Wang et al., 2014).
Mean solid velocity along the radius is positive in the dilute core and negative near the wall. Negative velocity near the wall greatly depends on the operating conditions. However, in case of the dense suspension upflow or dilute flow, mean axial velocities of solids are reported positive all along the column. Several profiles for mean axial solids velocity, parabolic, horizontal S, power law, boltzman function, linear etc., are reported in literature.
The shape of the solid axial velocity strongly depends on the operating condition i.e.
superficial gas velocity and solid circulation rate.
Lastly, flow development in the riser depends on the operating conditions, diameter of riser, distributor, inlet and exit configuration. The length required for flow development decreases with the increase in the superficial gas velocity and increases with the increase in solid circulation rate. Flow develops faster in the core region compared to the annulus.
In high density riser, three or four distinct radial profiles of velocity are observed in the riser up to flow development. Mostly parabolic or horizontal S shaped profile is observed in the bottom or distributor controlled region. In the middle section steep or linear profile is observed and at the top parabolic profile is observed.