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(2001j 5ú 18¢ 7>, 2001j 7ú 24¢ j)

Mass Transfer Characteristics of Three-Phase Circulating Fluidized Beds

Sa-Jung Kim, Yong-Jun Cho, Yong Kang^†, Sang-Done Kim* and Sung-Hee Park**

Department of Chemical Engineering, Chungnam National University, Daejeon 305-764, Korea

*Department of Chemical Engineering, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Korea

**Department of Chemical Engineering, Woosuk University, Cheonju 565-701, Korea (Received 18 May 2001; accepted 24 July 2001)

º £

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Abstract−Characteristics of gas-liquid volumetric mass transfer has been investigated in a riser of three-phase circulating fluidized bed(0.102 m ID and 3.5 m in height). Effects of gas(0.01-0.09 m/s) and liquid(0.12-0.43 m/s) velocities, solid circu- lation rate(2-8 kg/m²s) and fluidized particle size(1.0-3.0 mm) on the volumetric gas-liquid mass transfer coefficient have been examined. The mass transfer coefficient has been recovered from the concentration profile of dissolved oxygen in the axial direction of the riser by means of axial dispersion model. Tap water, filtered and compressed air and glass bead whose density is 2,500 kg/m³ were employed as a liquid, gas and solid phase, respectively. It has been found that the gas velocity is a important factor to determine the value of mass transfer coefficient; the mass transfer coefficient increases with increasing gas velocity but it dose not change considerably with the variation of liquid velocity in the beds of relatively large particle(d_P: 1.7-3.0 mm), although the k_La value increases slightly with increasing U_L in the beds of 1 mm glass bead. The value of mass transfer coef- ficient increases with increasing solid circulation rate as well as particle size. The mass transfer coefficient has been correlated as a function of operation variables as well as dimensionless groups.

Key words : Three-Phase Circulating Fluidized Bed, Gas-Liquid Mass Transfer, Mass Transfer Coefficient, Axial Dispersion- Model, Correlation

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†To whom correspondence should be addressed.

E-mail: kangyong@hanbat.cnu.ac.kr

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(1)

at

(2)

at

(3)

VB, Pe^*º Peclet >‚ ULL/D_Zε_Lf ?š ;~>– St^*º Stanton >

‚ (kLa)L/U_Lf ?š ;~B. ‚Þ xº ZNö ^š¢, Ò C^* º Ϛֲ~ ï;³ê¢ ¾æÞ. V¢B, "Úê Ú*–šöB ç ß&~ »OË æ*~ æzö Vž Ϛֲ~ ·j G;~  (1)‚

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1 Pe^* ---d²C

dx² --- dC

---dx

– +St^*(C^*–C)=0

x 0 C C₀ 1 Pe^* ---dC

---dx

x=0

+ , =

=

x 1 dC ---dx

x=0

0 , =

=

Fig. 1. Schematic diagram of a three-phase circulating fluidized bed.

1. Riser 19. Flowmeter

2. Down comer 10. Control valve

3. Hopper 11. Sampling vessel

4. L/S separator 12. DO meter 5. Pressure tap 13. N₂ tank 6. Butterfly valve 14. G/L distributor 7. Compressor 15. Liquid reservoir

8. Control valve 16. Pump

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 ê>ö ~º 'Ëj Fig. 5ö ¾æÚî. š âöB " > ®

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Fig. 2. Typical example of dissolved oxygen concentration profile in the axial direction of a column(G_S=2 kg/m²s, d_P=1 mm).

Fig. 3. Effects of U_G on k_La in the riser of three-phase circulating fluid- ized beds.

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Fÿ[~ çß&öB VÚ 5 ‡Ú~ F³, ÚFÿ«¶~ B~³ê Fig. 4. Effects of U_L on k_La in the riser of three-phase circulating fluid-

ized beds.

Fig. 5. Effects of G_S on k_La in the riser of three-phase circulating fluid- ized beds.

Fig. 6. Effects of d_P on k_La in the riser of three-phase circulating fluid- ized beds(U_G=0.03 m/s, G_S=2 kg/m²s).

Ò Fÿ«¶ ’V~ æzö Vž VÚ-‡Ú êšöB~ ¦b bî

ç&j  (4)f ?š ’† > ®î.

k_La=0.263U_G^0.275U_L^0.017G_S^0.155d_P^0.097 (4)

š ~ ç&ê>º 0.944‚ Fig. 7ö º :f ?š þÖ"f ¾

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š¾ ®¢ vªš –~ ìbæ‚ OÂ~š†(isotropic turbulence theory)j 'Ï~[14] bî* ê>f –·æ>f~ &ê¢ ZNö

b‚  (5)f ?š ¾æâ > ®î.

St*=k_La(L/U_L)=3.990(d_P/D)^0.130[U_L/(U_G+U_L)]^−1.088 (5)

 (5)º Fig. 8öB º :f ?š ç&ê> 0.913b‚ þÖ"f

¾ ¢~~&.

4. Ö †

âç B~Fÿ[~ çß&öB VÚ-‡Ú ¦bbî*ê>ö &‚

þ' ’Ö" VÚ-‡Ú ¦b bî* ê>¢ áV *š »OË ªÖ Κ ¾ 'Ï>î. çß&öB VÚ-‡Ú ¦b bî* ê>(kLa) º VÚF³~ Ã&ö V¢ Ã&~&, ‡ÚF³~ æzö V¢Bº

– æz¢ šæ p~.

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k_La=0.263U_G^0.275U_L^0.017G_S^0.155d_P^0.097

St*=k_La(L/U_L)=3.990(d_P/D)^0.130[U_L/(U_G+U_L)]^−1.088

6 Ò

 ’º ‚“"Ò Ï'V.’(1999-1-307-006-3)ö ~š >

¯>îb– æ Òö 6Òãî.

ÒÏV^

C : concentration of oxygen [mol/m³] C^* : saturated concentration of oxygen [mol/m³] C₀ : initial oxygen concentration [mol/m³] D_Z : axial dispersion coefficient [m²/s]

d_P : particle diameter [mm]

G_S : solid circulation rate [kg/m²s]

k_La : liquid phase volumetric mass transfer coefficient [1/s]

L : axial distance from the distributor [m]

Pe^* : Peclet number(=U_LL/D_Zε_L) [-]

St^* : Stanton number[=(k_La)L/U_L] [-]

U_G : superficial gas velocity [m/s]

U_L : superficial liquid velocity [m/s]

U_t : particle terminal velocity in a liquid medium [m/s]

ÒšÊ ^¶

ε_L : liquid phase holdup

^^ò

1. Kim, S. D. and Kang, Y.: Chem. Eng. Sci., 52, 3639(1997).

2. Liang, W., Wu, Q., Yu, Z., Jin, Y. and Wang, Z.: Can. J. Chem. Eng., 73, 656(1995).

3. Shin, K. S., Cho, Y. J., Kang, Y. and Kim, S. D.: HWAHAK KONG- HAK, 39, 91(2001).

4. Han, S., Zhou, J., Loh, K. C. and Wang, Z.: Chem. Eng. Sci., 70, 9 (1998).

5. Liang, W. G., Wu, Q. W., Yu, Z. Q., Jin, Y. and Bi, H. T.: AIChE J., 41, 267(1995).

6. Yang, W. G., Wang, J. F., Chen, W. and Jin, Y.: Chem. Eng. Sci., 54, 5293(1999).

7. Zhu, J. X., Zheng, Y., Karamanev, D. G. and Bassel, A. S.: Can. J.

Chem. Eng., 78, 82(2000).

8. Kim, S. H., Cho, Y. J., Song, P. S., Kang, Y. and Kim, S. D.: HWA- Fig. 7. Comparison between the experimental and calculated values of

k_La.

Fig. 8. Comparison between the experimental and calculated values of the Stanton number.

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