내부순환유동층에서의 기체 bypassing 및 고체순환속도

남궁 원, Guy Christophe, Legros Robert

Ecole Polytechnique

Gas Bypassing and Solids Circulation Rate in an Internally Circulating Fluidized Bed

Namkung Won, Guy Christophe, Legros Robert

Dept. of Chemical Engineering, Ecole Polytechnique, P.O. Box 6079 St. "Centre-Ville", Montreal, Quebec, H3C 3A7, CANADA

Introduction

The internally circulating fluidized bed (ICFB) has been widely used for various gas-solid reactions such as gasification of coal[1] and biomass, thermal treatment of industrial solid wastes[2], and catalytic oxidative coupling [3]. One of its major advantages is the possibility to control gas and solids residence time independently due to the use of a draft tube.

The solids circulation rate (Gs) which influences the residence time of particles and the solid holdup in the riser is an important design parameter and the most critical variable in predicting the performance. Therefore, information of Gs is needed to predict the performance of reactor. In spite of its simplicity, pressure drop measurement provides very useful hydrodynamics information on the riser. Changing the gas velocity (Ug) or Gs affects directly the hydrodynamic characteristics in the riser, which also affects the pressure drop. This means that if the pressure drop is known in the riser, one may predict Gs at a given experimental conditions. Until now, theoretical studies on prediction of Gs based on the pressure drop are rare. Gas bypassing which represents the fraction of annulus gas flowing to the riser, is the main controlling parameter of the Gs. Actual gas velocities in the riser and annulus sections may be different from the injected gas velocities due to gas bypassing between the riser and annulus sections. Gas bypassing enhances the circulation rate of solids from annulus to riser and can be controlled by the inlet geometry of the draft tube. Therefore the hydrodynamic properties, such as solids circulation rate and gas bypassing must be determined to provide the necessary information for the design of an ICFB [4].

In this study, the effect of operation variable and bed temperature on Gs and gas bypassing has been determined and a model was proposed to predict Gs using values of pressure drop in the riser.

Experimental

Experiments were carried out in an ICFB pilot scale reactor as shown in Fig. 1. The reactor consists of two concentric cylinders and a natural gas burner vertically located at the base of the inner tube called the riser. The inner diameter and height of riser located in the center of reactor are 0.078 and 1.5 m, respectively. A 20 kW burner is installed at the bottom of the riser to provide heat to the reactor by natural gas combustion. An impact-type gas-solid separator is placed at the top of the riser. The inner diameter and height of external cylinder are 0.355 and 2.5 m, respectively. Annulus air provided at the base of the outlet tube is used to improve solid motion in the annulus region and to facilitate solid circulation to the riser. Solids from the annular region re-enter the riser through 6 perforated holes in a steel ring at the riser bottom. The reactor is insulated with layers of refractory and ceramic wool. The average particle diameter and density of sand particle used as a bed material are 185 m and 2600 kg/m3, respectively. Pressure taps (1.32 m apart) are mounted at the riser wall to measure pressure drop in the riser

Fig.  The internally circulating

fluidized bed pilot scale unit

Results and discussion

Pressure drops measured in the riser of recirculating fluidized bed are the sum of static head, particle acceleration and friction loss terms. It has been found that the contribution of gas solid friction to the total pressure can be neglected when compared to the acceleration and solids static head components [5,6]. Neglecting the friction loss term, one writes:

[eqn: -{dp} over dz =rho_s (1- epsilon)g +rho_s (1- epsilon ) V_p {dV_p over dz}]                            (1)

The momentum balance on particle is written as:

[eqn: V_p dV_p over dz = 3 over 4 C_D rho_g over {d_p rho_s } ( U_g over epsilon - V_p )^2 +g {rho_g - rho_s } over rho_s]                     (2)

Vp is the cross sectional average upward particle velocity. The drag coefficient CD is written in general form as [6] :

[eqn: C_D = K over Re_p^0.6]                                                      (3)

In the acceleration regime, Rep remains in the Stokes regime (Rep < 100), and K can be obtained by applying the following limiting condition:

[eqn: z = L_acc , V_p = V_{p{inf}} , ~dV_p over dz = 0]                                    (4)

In the hydrodynamic model, the radial non-uniformity or the core-annulus flow structure are lumped into the parameter K, the fully developed velocity and the voidage [2]. By replacing K, the momentum Equation (2) can be written as follows

[eqn: V_p dV_p over dz = {g(rho_s - rho_g )} over rho_s LEFT [ LEFT ( { U_g over {(1 - G_s over {rho_s V_p })} -V_p} over {U_g over epsilon_inf - V_{p inf}} RIGHT )^1.4 -1 RIGHT ]]                  (5)

with the initial condition

[eqn: z = 0, ~ V_p -> 0]                                                      (6)

The values of Vp and Gs at a given Ug are needed to solve eq. (5). Gs at first is obtained by using pressure drop measured in the riser. Eq. (5) is then solved numerically using a Runge-Kutta method to obtain Vp(z). Solid holdups along the axial position can be calculated using Vp(z) as follows:                  [eqn: epsilon (z) ~=~ 1- G_s over {rho_s V_p (z) }]                                              (7)

Fig. 2 shows the comparison of the Gs in the riser predicted from the model with experimental data at higher temperature. The values of Gs calculated (dash line) by neglecting acceleration term overestimate the experimental ones because the riser is operated in the accelerating zone at higher velocity. Therefore, the accelerating term on pressure drop must be considered at high velocities. As can be seen, when the acceleration component is considered, the model can predict well Gs at high temperature.

Fig.  Comparison of Gs predicted from

the model with experimental data.

Fig.  Effect of riser gas velocity on Gs.

The effect of temperature on Gs at different gas velocities is shown in Fig. 3. Gs increases with increasing temperature at identical experimental conditions. The increase of Gs with temperature may arise from two causes. One is due to the increase of voidage with increasing temperature. As the temperature increases, packing condition of the bed becomes looser and looser. Therefore, as temperature increases, the solid movement in the bed is enhanced so that Gs increases with increasing temperature. The other cause is the increase of viscosity with temperature.

The effect of annulus velocity (Ua) and superficial riser gas velocity (Ug) on gas bypassing is shown in Fig. 4. The bypassing fraction of the annular gas is defined as the vol. % of the annular gas bypassing to the riser. As

Fig.  Fraction of gas bypassing as a function of : (a) annulus velocity         (b) riser gas velocity.

can be seen, Ua and Ug do not affect the gas bypassing fraction. Instead, the value of gas bypassing at high temperature (400 - 800 oC) varies around an average value of 87 %. This means that most of the gas fed in the annulus region flows to the riser through the orifices. Although, most annular gas bypasses to the riser, Ug is hardly affected by gas bypassing because the amount of annular gas injected is much smaller than that of riser gas injected. In the study, the variation of Ug due to the gas bypassing is below 5 %. The gas residence time is then controlled in the annulus and riser regions, independently.

Conclusions

In the ICFB, gas bypassing and solids circulation rate have been determined under a range of temperature (25 - 800 oC). Using the data of pressure drop in the riser of an ICFB, solids circulation rates (Gs) are predicted. The values of Gs predicted from the model which account for acceleration term agree well with the experimental ones. Gs increases with increasing annulus gas velocity, but is independent of riser velocity. The Gs increases with increasing temperature. Bypassing of annulus gas from the annulus to the riser is not largely affected by variation of annulus and riser gas velocities. As temperature increased from 25 to 400 oC, gas bypassing increased from 59.4 % to 87 %, while further increase of temperature did not affect gas bypassing.

References

[1] Kim, Y. J., Lee, J.M. and Kim, S. D., Fuel 76, 1067 (1997).

[2] Mukadi, L., Guy, C. and Legros, R., Can. J. Chem. Eng., 77, 420 (1999).

[3] Mleczko, L. and Marschall, K. L., Can. J. Chem. Eng. 75, 610 (1997).

[4] Song, B.H., Kim, Y.T. and Kim, S.D., Chem. Eng. J., 68, 115 (1997).

[5] Louge, M. and Chang, H., Powder Technol. 60, 197 (1990).

[6] Pugsley, T. S. and Berruti, F., Powder Technol. 89, 57 (1996).