Effects of particle shape, density, and size on a distribution of phase holdups in a gas-liquid-solid circulating fluidized bed riser

Electrical resistance tomography and optical fiber probe were employed to study the effects of particle shape, size, and density on the distribution of phase holdups in the riser section (5.97 m tall) of a gas-liquid-solid circulating fluidized bed (GLSCFB) at four axial locations. Saline water was used as conductive liquid phase while air was the gas phase. Two different sizes of spherical shape glass beads and two different sizes of irregular shape lava rock particles were used as the solid nonconductive phase. Because of the tendency of gas bubbles to move in the central region of the riser, gas holdup was higher in this region and decreased gradually toward the wall region for all four types of particles. Consequently, liquid and solid holdups were lower in the central region compared to the wall region. Under the same auxiliary fluidization rate, larger particles had low solids circulation rate (low particle velocity) compared to smaller size particles due to less fluidization in the distribution region. Gas and solids holdups for the smaller particles were found higher compared to larger particles. Shape (sphericity) appeared to have minimum influence on the phase holdups. The decreasing hydrostatic pressure on rising gas bubbles resulted in gas bubble expansion leading to higher concentration of solid particles in wall regions at higher axial locations. The particle size and shape appeared to have a direct impact on gas bubble breakage and coalescence, and in turn also on phase holdup distributions at different heights along the riser. Solids holdup decreased with increasing liquid velocity at all axial locations for all types of particles. The nonuniformity of solids holdup in the central region decreased as the flow structure was developing along the riser.