Won Namkung

SOLIDS FLOW CHARACTERISTICS IN LOOP-SEAL OF A CIRCULATING FLUIDIZED BED

The hydrodynamics of solids (FCC) recycle in a loop-seal (0.08 m) at the bottom of the downcomer (0.08 m-I.D.x4.0 m-high) in a circulating fluidized bed (0.1 m-I.D.x 5.3 m-high) have been determined. Solid flow rate through the loop-seal increases linearly with increasing aeration rate. At the same aeration rate, the maximum solid flow rate can be obtained at a loop-seal height-to-diameter ratio of 2.5. The effects of solid inventory, solid circulation rate and gas velocity on pressure balance around the CFB have been determined. At a given gas velocity and solid circulation rate, pressure drops across the downcomer and loop-seal increase linearly with increasing solids inventory in the bed. At a constant solid inventory, pressure drops across the riser and the downcomer increase with increasing solid circulation rate but decrease with increasing gas velocity in the riser. The obtained solid flow rate has been correlated with pressure drop across the loop-seal.

Gas Backmixing in a Circulating Fluidized Bed

Abstract The gas backmixing characteristics in a circulating fludized bed (0.1 m i.d., 5.3 m high) of fluid catalytic cracking (FCC) particles have been determined. The gas backmixing coefficient has been determined based on the axial dispersion model by using two tracer gases (He and CO2) in a circulating fluidized bed (CFB). The obtained gas backmixing coefficients by using the absorbed CO2 gas overestimates the gas backmixing coefficient 60–62% in the dense region, 49–62% in the transition region and 2.4–19% in the dilute regulates compared to the values obtained by the non-adsorbed He tracer gas. The gas backmixing coefficient increases with increasing the ratio of particle to gas velocities (Up/Ug). In the upper dilute region, the core annulus model is proposed to determine the gas backmixing coefficients and the mass transfer coefficients between the core and the annulus regions. The gas backmixing coefficient and mass transfer coefficient increase with increasing Up/Ug.

Solid Recycle Characteristics of Loop-seals in a Circulating Fluidized Bed

Solid recycle characteristics through a conventional and a newly developed loop-seal (0.08 m i.d.) system are determined in a circulating fluidized bed of FCC or silica sand particles. In the loop-seal developed in this paper, gas was injected downward tangentially to the wall of the loop-seal to increase solids mass flux with stable flow. For conventional loop-seal, solids mass fluxes increase linearly with increasing aeration rate but it reaches a maximum value. At the same aeration rate with different aeration locations (0.1-0.6 m) in a conventional loop-seal, a maximum solids mass flux is seen at a height to diameter ratio of 2.5. For the newly developed loop-seal, mass fluxes of FCC and sand particles are higher and more stable than in conventional loop-seal at the same aeration rate. The solid mass fluxes obtained have been correlated with the aeration rate and Archimedes number.

Radial gas mixing in a circulating fluidized bed

Abstract The effects of gas velocity, solid circulation rate, the secondary air injection ratio, and secondary injection type on the radial gas-mixing coefficient (Dr) in a circulating fluidized bed (CFB) (0.1 m i.d., 5.3 m high) have been determined. It increases with increasing solid circulating rate and the secondary air ratio (SAR), but decreases with increasing gas velocity. Based on the present and previous studies, Dr increases with increasing column diameter. The effect of secondary air injection type on the radial gas-mixing coefficient is more pronounced with the tangential than with the radial air injection. The radial gas-mixing coefficient in terms of Peclet number has been correlated based on the isotropic turbulence theory.

Gas backmixing in the dense region of a circulating fluidized bed

The gas backmixing characteristics in a circulating fluidized bed (0.1 m-IDx5.3-m high) have been determined. The gas backmixing coefficient (Dba) from the axial dispersion model in a low velocity fluidization region increases with increasing gas velocity. The effect of gas velocity onDba in the bubbling bed is more pronounced compared to that in the Circulating Fluidized Bed (CFB). In the dense region of a CFB, the two-phase model is proposed to calculate Dbc from the two-phase model and mass transfer coefficient (k) between the crowd phase and dispersed phase. The gas backmixing coefficient and the mass transfer coefficient between the two phases increase with increasing the ratio of average particle to gas velocities (Up/Ug).

Radial gas mixing characteristics in a downer reactor

In a downer reactor (0.1 m-I.D.x3.5 m-high), the effects of gas velocity (1.6-4.5 m/s), solids circulation rate (0–40kg/m2s) and particle size (84, 164 Μm) on the gas mixing coefficient have been determined. The radial dispersion coefficient(Dr) decreases and the radial Peclet number (Per) increases as gas velocity increases. At lower gas velocities, Dr in the bed of particles is lower than that of gas flow only, but the reverse trend is observed at higher gas velocities. Gas mixing in the reactor of smaller particle size varies significantly with gas velocity, whereas gas mixing varies smoothly in the reactor of larger particle size. At lower gas velocities, Dr increases with increasing solids circulation rate (Gs), however, Dr decreases with increasing Gs at higher gas velocities. Based on the obtained Dr values, the downer reactor is found to be a good gas-solids contacting reactor having good radial gas mixing.

Hydrodynamic characteristics of a FCC regenerator

The hydrodynamic and gas mixing characteristics have been determined in a FCC regenerator (0.48 m I.D.x3.4 m high) with FCC particles. Solids holdup in the dense bed decreases with increasing gas velocity, but it increases in the freeboard region. The bubble/void fraction increases with an increase along the bed height at a given gas velocity and increases with increasing gas velocity at a constant bed height. Backmixed tracer gas at the wall region is higher than that at the center region of the bed. The gas backmixing coefficient decreases with increasing gas velocity.