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Radial profiles of solids concentration and velocity in a very fine particle (36 μm) riser
POWDER TECHNOLOGY ELSEVIER
Powder
Technology96 i 1998 ~ 2 6 2 - 2 6 6
Radial profiles of solids concentration and velocity in a very fine particle (36 Ixm) riser Yao Wang, Fei Wei, Zhanwen Wang, Yong Jin *, Zhiqing Yu DGJa/tn;e;t: O/ f'lwmi~,d I:n~i;w~ rin~. "lsi;;.~,ktu; University, Beiji;~,, 1000,'~'4, ('hino Rcccixcd I ()ctobcr 1996: rexiscd 13 October 1997
Abstract Radial profiles of line particle ( 36 I~tn ) concentration and velocity in a riser of 140 mm inner diameler are investigated using a dual-beam optical density sensor and a laser Doppler velocimeter. According to the cross-sectional average solids lraction three different kinds of radial solids fraction profiles are present: unilk)rm, dense ring and aggregation. At high superficial gas velocity and low solids concentration, the radial proliles of line particle velocity obey a I/7 power law. The radial profiles of solids fraction and velocity become flatter as the particle size decreases from 54- to 36 I.tm. {~ 1998 Elsevier Science S.A. All rights reserved. Kern .rd.w Fines: Fluid,zeal beds: Circulatin~ beds: Solids ftactiun: Particle vclocil~
1. Introduction Circulating fluidized beds (CFBs) have been successfully' used as efficient gas-solid reactors in coal combustion and fluid catalytic cracl~ing ( F C C ) for many years. BEcausE the CFB has advantages of high throughput, good gas.-solids contact and large reduction of gas and solids backmixing, much attention has been paid to its hydrodynamics and mixing behavior. Information concerning radial profiles of particle concentralion and veh)city is essential for the understanding of the flow dynamics and design of CFBs. Studies on tile radial prolilcs of solids concentration and velocily suggest that strong solids aggre~alion occurs towards the wall region in the CFB. That is, tlqe local solids concentration is low in the central region and high in the wall region, and thc local particle velocity is high at the axis and drops sharply near the wall. The proliles of solids concentration are determined by the cross-sectional average solids concenlration, ha~ing no direct relation with Ihe operating conditions and particle sizes for Geldart groups A and B. A survey of lhe hydrodynamic studies on CFBs indicates that almost all the former investigations used Geldarl A or B particles [ 1-7 I. Few investigations have been found that USE particles smaller than 54 ~ m in the CFB. However. owing to the high surface area and large ratio of surface area to x olume. :i: (~orrcsponding aulhor. E mail: +l'.ce(Wnlail.tsinghua.cdu.cn 0032 5 9 1 0 / 9 8 / $ 1 9 . 0 0 ~ ) 1998 E l s c ' ~ i c r S c J c n c c S . A AllrJghl,,rc~,crvcd. PIIS()(}32 5 9 1 0 ( 9 7 1(]~,382 2
the fine particles have higher catalytic activity and better heat and mass transfer behavior. Although a great deal of ef|~rt has been put into fluidizing fine powders (Geldart C) for their powerful potential advantages, most of these studies were simply locused on the dense phase fluidization at very low gas velocities. In fact, many applications of fine powders are concerned with the dilute phase, such as chemical vapor deposition ( CVD ), ,jet cracking, jet drying, jet coating, and the chemical reaction synthesis of advanced materials. The large suH'ace area and high activity of line powders require that chemical reactions proceed under a low solid/gas ratio. To date, it appears that no study has been reported concerning the local hydrodynamics o1 line powder in a riser. In this paper, the radial solids fraction and velocity profiles in a fine particle riser- are studied using a dual-beam density sensor and a laser Doppler vclocimeter system. Special attenlion is given to the comparison of the hydrodynamic differences between conventional FCC catalyst and line FCC catalyst of 36 i~m.
2. Experimental apparatus and methods Fig. I shows a schematic flow diagram of the experinaental apparatus. The total height of the apparatus is I 1 m. The riser has an i.d. of 0.14 m and is 10.4 m in height. A specially designed solids recovery system is used to guarantee full recovery of the fine particles.
E Wang ct al./l'o,'der Tecfmofo~,,v 96 f 19Ua') 262 266 i
lion, i.e. the ratio o f local solids fraction to the cross-sectional average solids fraction, is plotted against the radial position. Three kinds of radial fl-action profiles are clearly shown in Fig. 2. These different shapes are largely decided by the difference in cross-sectional average solids fraction (different 4, regions). Within each region, the radial prolile is quite stable, varying neither with the superlicial gas velocity nor with the solids circuhition rate.
ii~.-
Downer
Riser
i
Air
263
Gas-solid -~. separator Measuring Storagetank Butterflyvalve
3.1. I. Un{/orm distribution re,,ion re, < 1.09~ )
Fig. 1. Experimenlalapparaln~,. The fluidized material is very fine FCC catalyst with a particle density of 1670 k g / m ~ and a mean size of 36 p,m. Its detailed properties are listed in Table 1. The superlicial gas velocity is measured by rotan]eters and the solids circulation rate is measured by a three-way wdvc and a measuring vessel. The gas velocity ranged fi-om 1 to 7 m / s and the solids circulation rate from 1 to 70 k g / m e s. The cross-sectional apparent average solids fraction is measured by the pressure gradient along the riser. A dual-beam optical density sensor is used to measure the radial solids fraction prolile. Because of the nonlinear rehltionship between the sensor signal and the local solids fraclion, the sensor must be calibrated by the pressure gradient. Details of the measuring and calibration method are reported elsewhere 18.9]. A TSI liber optic laser Doppler velocimeler ( LI)V ) is used to measure the local solids velocity of line particles. A frequency shifter is applied to detect both the upward and downward solids velocities. In order to obtain experimental data in the fully developed region, the measuring plane is 4 - 6 m above the riser bollom where the effects of the entrance and exit structures can he eliminated.
3. Results and discussions 3. I. Radial solids./)action profile
In order to compare radial solids fraction proliles under different operating conditJtons, the reduced radial solids frac-
When the solids fraction is very low. the radial solids fraction prolile is rather flat, as shown in Fig. 3. This form of prolile is termed the uniform distribution region. A sharp rise of solids fi'aclion exists at the radial position very close to the wall which indicates that only a small amount of solids may accumulate in the wall region. As oas and solids flow in the dilute phase, the distance between particles is largely increased and the interparticle forces greatly decrease so that adhesive behavior tends to reduce and even vanish. Fig. 3 plots the profile and compares it with that o f 54 bun FCC catalyst, and the difference between them is apparent. As the nlean size of particle reduces flOlll 54 to 36 i~in, particle accumulation towards the wall region also decreases, resulting in a nluch Ilattcr solids fraction prolile in the riser. This difference indicates that the solids moving mechanisms for 54 and 36 I,tm solids are quite different. Changing the operating conditions does not appear to affect the shape of this profile when the cross-sectional a\ crage solids fraction is less thai] 1~/~.The fl)llowing correlation is proposed by regressing the experilnental data: (::, -- = 0.905
q- (
I / R ) It*
1)
The curves m Figs. 2 and 3 are the results predicted by the correlation, which are in good agreement with the experimental data. This uniform solids fraction profile will greatly benelii the line particle riser reactor. It indicates less solids aggregation and good gas-solids contact, which implies good heal and mass transfer in the fine particle r i s e r - - very important l])r rapid reactions. hi addition, we investigated the equation given by, Biet al. [ 1 I I. By, this equation,when the cross-sectional average solids fraction is very low less than 1~{ ), the radial solids
Table 1 Properties of the line panicles d r,
d,,+/dvl
dr,,'/dp/~
dr,+ +/drl
i ~m)
Particle density kg!i]/" I
Ratio e l surface to
30 54
1.5
2.3
3.5
670 398
U, (m/s)
voJunle ( I]/ili
Fine FCC C o m . FCC
G,,,, (m/s)
16(~ 111
i )
7.43 × 10 ~ 1.40xlO ~
0.215 0.286
Partich: size distribution d( ~,. O. I )
~'/( v, 0.5 )
d( v, 0.9!
( b `-II] )
( b till }
( b ull )
12.0(i 31.91
3(~.18 64.98
fJ4,43 tJ4.97
264
F. Wanq, et of. / Pon'der Teclmologv 96 (1998j 262-266 3.0
2.5 ~s Ug (m/s) Gs(kglm2.S)A 2.0 I~" 1.5
4,
0.64%
2.51
2.75
~
1.87%
5.16
85.0 / O /
"
6'10°/°
2'37
//~';
66"2
~
I
• 0.0
0.0
~ ~
i
0.4
0.6
0.0
0.8
Ug(m/s) Gs(kgtm2s) ~s dp(~m)
g
t~
o.8
2.51
2.75 0.64% 36
3 82
3.21
'=1'-
6.47
0.6
0.0
66.69
4.542
37.64
•4"
1.87%
5.162 85.0 ?./ar.~
/~
/
@ & ~ O •
•
i
0.2
'
I
I
I
0.4
0.6
0.8
1.0
r/R Fig. 4. Dense ring di~tribulion of solids t'raction.
I
0.22% 36
1.01 0.20% 36 Tung's equation -- r'/R(calculated by Bi et al)
,' i' ',""
0.2
4.198
1.44%
vary significantly, with the operating conditions. A correlation of the profile is proposed by, regressing the experimental data:
1.2
~,
1.53%
'
0.0
1.0
r/R Fig. 2. The thlcc types of I-adial solids fraction prolilcs.
1.0
• •
0.5
dp=36 P.m
I
Ugl (m/s) Gs(kg/m2.s)
dp=36 vm
1.0
"/'/
0.2
2.0
----" 1.5
/cir.
o.s
~'.s
2.5
0.4
0.6 0.8 1.0 r/R Fig. 3. Unifornl radial solids fraction prolilc.
fraction profile has a very narrow wall region, which is consistent with our experimental results.
3.1.2. Dense rin f distribution region ( l . 5 q < E, < 5 q J As the cross-sectional average solids fraction increases ow,'r 1.5%, a great change is noticed in the reduced radial solids fraction profiles. Solids accumuhite near the wall region and a dense ring forms around the radial position r / R = 0.94. as shown in Figs. 2 and 4. For this reason it is named the dense ring distribution region which is similar to the radial solids fraction profile in a downer [ 10). The radial positions of the dense ring in both the riser and downer are the same, which indicates that a similar llow mechanism may exist. This form of profile in a riser does not yet appear lo have been reported in the literature where larger particles were used. This shows the complexity of the flow mechanism in the gas solids flow of the riser. As the solids conceniration increases, particle interactions with each other and the wall increase, and so does the solids aggregation in the riser, which results in the change of the solids fraction prol~le. Although the radial solids Iraction profile becomes steeper than in the uniform distribution region, it is still flatter than for larger particles, as reported by Tung et al. 19]. When the cross-sectional average solids flaction is between 1.5~?~ and 5~)b, the dense ring profile also does not
E,
-0.82+48(1
r/R)exp[--22(I-r/R)l
(2)
The comparison of the correlation prediction with experimental data is shown in Figs. 2 and 4. A fairly good agreement is achieved. 3.1.3. Aggregation distribulion region (e, > 6~)~:) With the average solids fraction further increased, the dense ring disappears and more solids accumulate at the wall region, as shown in Fig. 5. The solids fraction is rather low in the central area, increases gradually with the radial position moving towards the wall, and then reaches its maximum at the wall. This profile is the same as for the 54 p~m particles, which indicates that the strongest solids aggregation in the riser occurs at high solids fractions. The following correlation can describe the radial solids fiaction profile in this region very well: e , = 0 . 2 5 + 1 . 6 5 s i n s ( 7~r r )
(3)
~:,
As shown in Figs. 3-5, in any region, when the superficial gas velocity and solids circulation rate vary within a certain range, the radial solids fraction profiles remain unchanged at a constant cross-sectkmal average solids fraction. This result 3.0 dp{um)
2.5
I~ 2.0
+
I
~.
1.5
54 36 54 36
Ug(mts)~ s Gs(kg/m2.s) 3.6 3,5 44 2.4
8.8% 6 0% 63% 61%
102.0 78.0 1320 66.95
(
--
1.0
0.5 0.0
r
0.0
0.2
0.4
0.6 0.8 1.0 r/R Fig. 5. Aggregation distribulhm of solids fraction.
E B,'(m:,,el al. / Po 'd~'r7k'c/ 7o/o:W96 (1998) 262 266 shows that the average solids fraction is a key parameter in determining the radial solids fraction profile, and the gas velocity and solids circulation rate have no direct relation with the profile. The same results for larger particles were reported by Tung et al. [9]. However, as the solids size is reduced to 36 ~m, the radial solids fraction profile becomes more complex and can not be described by a unified correlation. The solids aggregation mechanism may' undergo a change for these 36 Ixm fine particles in a relatively dilute phase, being more uniform along the radial direction. This change in solids aggregation for fine particles reveals that the particle size has a large effect on the hydrodynamics e l the riser. The unifl)rmity of line particles in the dilute phase will cause good gas-solid contact efficiency and high mass and heat transfer rates in riser reactors. Extensive studies on line particle behavior are required to reveal the complicated Ilow mechanism and to make full use of fine particles. At high solids fractions, the solids aggregation approaches that of larger particles.
3.2. Radial profile qf tlu" /?article l'elocilv In order to analyze the differences of the radial particle velocity profiles under different operating conditions, the reduced panicle velocity, i.e. the ratio of the local particle velocity to the superficial gas velocity, is plotted againsl the radial position, as shown in Figs. 6 and 7. Apparently, the particle velocity is unevenly distributed along the radial direction with a maximum value at the axis, and gradually decreasing toward the wall. The particle velocity will become flatter when the superficial gas velocity is raised a n d / o r the average solids fraction is decreased. While the average solids fraction is below 0.3(/c and the superficial gas velocity is above 4.8 m / s , the radial particle velocity profile obeys the 1/7 power law of turbulent flow. described by 1,'7
Integration of the particle velocity along the radial direction shows that the average particle velocity is equal to the superficial gas velocity measu:ed by the rotameter. This observa-
2.0 -
265 ....
1/3.5
power
1/7
power
13
" -~------=,,~
m 1.2
Ug(m/sl Es Gs(kg;rn2a~;~~;.O~--
0.8
. •
oo,.,o 4.7, 02~°~o = ~ 4.7s 0.70°/. 47 7
•~,
4.71 1.30%
[]
0.4
,=, '=',U !
]
|
78.3
.L
dp =36 ~ m
0.0
i 0.0
r-
0.2
0.4
i
I
0.6
0.8
-r
1.0
fiR
Fig. 7. Influence of average solids fraction on the radial panicle ~elocity profiles lion is evidence that line particles can completely follow the gas movement under this condition. It seems that this minimal velocity will rise with an increase of the particle size a n d / o r the solids concentration. The smalter a particle is, the stronger its ability to f o l l o w gas movement. When the particle size increases by a certain amount, it is difficult to observe this ability under practical operating conditions. For 54 I~m particles, in the experimental range up to U,== 9 m/s, the minimum gas velocity following the 1/7 power law has not yet been reported. Because the LDV system can not work at high solids concentrations, only particle velocities under a solids fraction of 1.3q~ can be obtained precisely. As shown in Fig. 7. when the gas velocity is held constant, lhe particle velocity profile becomes steeper than in the dilute region. This may be caused by' the solids aggregation at higher solids concentrations. However, compared with the result for larger particles under the same conditions, it is flatter, especially in the wall region, as shown in Fig. 8. As we know, solids backmixing, significant in a conventional riser, has bad effects on conversion and production in a chelnical reactor. Although the degree of solids backmixing in a circulating fluidized bed has been largely reduced compared with that in a turbulent bed, experiments have proven that for 54 g m particles the axial Peclet number of particles is about 4 in CFBs, far from plug flow, and it does not vary 2.0
1.6
.
.
.
.
.
.
.
.
.
.
.
.
1.5-
12 o~
::~ 0.8
1.0
O.
D.
>
>
Ug 0.4
0.0 0.0
•
3.49
26
•
4.27
&0
7= 0.999
•
4.78
5.3
d p = 3 6 b~ m
~ 0.2
r 0.4
i 0.6
7--0.8
0.5=
0.0
dpO.rn) Ug(m/s
r/R
Fig. (~. Influence of L/,, on the radial parlicle velocity prolilc~;.
.
36
4.7
99.3%
47.7
[
•
54
4.3
99.5%
35.0
II
0.0
i 0.2
I 0.4
i 0.6
f 0.8
-0.5
.0
.
•
1.0
r/R
Fig. ~. Comparison of solids velocity profilesof dffrerem parti~:k.'s.
266
). W~m,, ~,~~d. /I'¢*~rder Tedmotocq % t/9%'j 262 266
with operating conditions [8]. The non-uniformily of the particle velocity and solids fraction profiles is the mosl important reason lk~r solids backmixing. By using fine powder, the uniformity can be improved, thus axial solids backmixing may largely be decreased. The conditions will approach plug llow. which will benefit fine particle riser reactors by improving their efficienc). The change in lhe radial profiles of solids conccntration and velocity for the fine particle riser raises a question: is the conventional 54 i,tm FCC catalyst widely used in commercial units, ideal f o r a riser? At least from the hydrodynamic p o i n t of view, it is far from optimal. Our resulls show IMI detailed studies of the efl'ecl of particle size on the fluidization behavior are urgently needed to provide more information on this ve U inlportant issiie.
R d( \ . O I )
Uf gnl[
U. Vp
radius of core region of riser (m) radius of riser (m) particle size under 10% of volume fraction (gm) superlicial gas velocity ( m / s ) minimal fluidizing velocity ( m/s ) terminal velocily ( m / s ) local particle velocity ( m/s )
(;reelc lelter,s {-
local voidage cross-sectional average solids fraction local solids fraction
6~
References 4. Conclusions
Three kinds of r,:tdial solids fraction proliles are loLind in a riser with 36 >m line particles: uniform, dense ring and aggregation. Compared with the results for 54 >m particlcs, the radial concentration distribution has been improved with the decrease m the particle size. At high gas velocities and low solids fractions, the radial proliles of the line particle velocity obey the 1/7 power law of turbulent flow, which is much flatler than for 54 bun parlicles. The impix)vement of flow uniformily of line particles may have many good effects on the gas and solids backmixing and mass and heat lransfer in a riser. The FCC catalyst should be optimized with regard to particle size, even though the normal 54 bun FCC catalyst is widely used in comn3ercial planls.
5. List of s y m b o l s
dp F
particle size ( I~m ) solids circulating rate ( k g / m : s ) radial coordinate (rn)
H. Weinstein, M.-J. Sllao. M. Schnitziein, in: K. Oster,2aard, M . A SOfellNCn ( Eds. ), Fluidizalilm V. Engineering l:oundation, New Y o r k I CJ~fl, pp, 329 336, M. Horio, K. Morishila. (). T~lchibana. N. Murala. in: P. Basu, J.F I,argo iEds.). CircuMing Fluidized Bed "TecMology 11, Pergamon Toronto. IggN, pp. 147 154, RJ. l)ry, Powder Technol, 49 f 1987) 37 44. R. Bader. J. Findlay. T.M. Knob, lion. in: P. Basu, J.F. Large ( E d s . ) ('irculating Fluidizcd Bed 'lechnology II. Pergamon. Toronlu, I t)NN pp. 127-137. li.U. Harlage, 1). Rensner. ,I. ~erthef, in: P. Basu. ,I.F. I,arge ( Eds. ) (TirculatJng FMdizcd Bed "fecMolo.gy 11, Pergalnon. Toronto, lC)F.S. pp, 165-180.
W. Nowak. H. Mine< Y. Malsunlura, R. Yamazaki. K. Yoshida, in: K. Yoshida. S. Morouka I [kts. ), Prec. Asian Conf. FMdized Bed and Three-Phase Rcaclors, Tok\ o. ,l@an. I ~J88, pp. 16(I-17(). 71 Y.I_. Yang, Y..lin, Z.(,,). "l'u. Z.W. Wmlg, D.R. Bai. hi: P. Basu. M Horio. M. Ha:,alani ( l:ds. i. ('irculating Fhlidizcd Bed Technology Ill. Pcrgainon, Toronllt, I 0 ~) I. pp. 201 206. Y. ('heng, I Indergradualc Thcsi,. Tsinghua Univer:,i b . Bcijing, China. I O94. Y..I. Tung. J.lt. IJ. M. Kwauk. in: M. Kwauk, I). Kunii (F, ds.L Fluidizalion "g8 Science :uld Tcchnohlg,~, ( Conf. Papers 3fd (Thina ,lapan Syrup. [:luidiz~l/km, Bci.iing, ('hinla. 19gN). Science Press. klcijing. 1!,1814.pp. 13c) 145. l l01 Y.I,. Yang, l)ocloral Thesi<,. 'fsinghua I,Jnivcrsitv, Beijing, China, 1991.
l i i l H.T. B i, J Zhou, S.-Z. Qi n. ,1.R. Grace. Can. J. Chem. ling., 74 ( Ic)06 i 811 814.
Powder
Technology96 i 1998 ~ 2 6 2 - 2 6 6
Radial profiles of solids concentration and velocity in a very fine particle (36 Ixm) riser Yao Wang, Fei Wei, Zhanwen Wang, Yong Jin *, Zhiqing Yu DGJa/tn;e;t: O/ f'lwmi~,d I:n~i;w~ rin~. "lsi;;.~,ktu; University, Beiji;~,, 1000,'~'4, ('hino Rcccixcd I ()ctobcr 1996: rexiscd 13 October 1997
Abstract Radial profiles of line particle ( 36 I~tn ) concentration and velocity in a riser of 140 mm inner diameler are investigated using a dual-beam optical density sensor and a laser Doppler velocimeter. According to the cross-sectional average solids lraction three different kinds of radial solids fraction profiles are present: unilk)rm, dense ring and aggregation. At high superficial gas velocity and low solids concentration, the radial proliles of line particle velocity obey a I/7 power law. The radial profiles of solids fraction and velocity become flatter as the particle size decreases from 54- to 36 I.tm. {~ 1998 Elsevier Science S.A. All rights reserved. Kern .rd.w Fines: Fluid,zeal beds: Circulatin~ beds: Solids ftactiun: Particle vclocil~
1. Introduction Circulating fluidized beds (CFBs) have been successfully' used as efficient gas-solid reactors in coal combustion and fluid catalytic cracl~ing ( F C C ) for many years. BEcausE the CFB has advantages of high throughput, good gas.-solids contact and large reduction of gas and solids backmixing, much attention has been paid to its hydrodynamics and mixing behavior. Information concerning radial profiles of particle concentralion and veh)city is essential for the understanding of the flow dynamics and design of CFBs. Studies on tile radial prolilcs of solids concentration and velocily suggest that strong solids aggre~alion occurs towards the wall region in the CFB. That is, tlqe local solids concentration is low in the central region and high in the wall region, and thc local particle velocity is high at the axis and drops sharply near the wall. The proliles of solids concentration are determined by the cross-sectional average solids concenlration, ha~ing no direct relation with Ihe operating conditions and particle sizes for Geldart groups A and B. A survey of lhe hydrodynamic studies on CFBs indicates that almost all the former investigations used Geldarl A or B particles [ 1-7 I. Few investigations have been found that USE particles smaller than 54 ~ m in the CFB. However. owing to the high surface area and large ratio of surface area to x olume. :i: (~orrcsponding aulhor. E mail: +l'.ce(Wnlail.tsinghua.cdu.cn 0032 5 9 1 0 / 9 8 / $ 1 9 . 0 0 ~ ) 1998 E l s c ' ~ i c r S c J c n c c S . A AllrJghl,,rc~,crvcd. PIIS()(}32 5 9 1 0 ( 9 7 1(]~,382 2
the fine particles have higher catalytic activity and better heat and mass transfer behavior. Although a great deal of ef|~rt has been put into fluidizing fine powders (Geldart C) for their powerful potential advantages, most of these studies were simply locused on the dense phase fluidization at very low gas velocities. In fact, many applications of fine powders are concerned with the dilute phase, such as chemical vapor deposition ( CVD ), ,jet cracking, jet drying, jet coating, and the chemical reaction synthesis of advanced materials. The large suH'ace area and high activity of line powders require that chemical reactions proceed under a low solid/gas ratio. To date, it appears that no study has been reported concerning the local hydrodynamics o1 line powder in a riser. In this paper, the radial solids fraction and velocity profiles in a fine particle riser- are studied using a dual-beam density sensor and a laser Doppler vclocimeter system. Special attenlion is given to the comparison of the hydrodynamic differences between conventional FCC catalyst and line FCC catalyst of 36 i~m.
2. Experimental apparatus and methods Fig. I shows a schematic flow diagram of the experinaental apparatus. The total height of the apparatus is I 1 m. The riser has an i.d. of 0.14 m and is 10.4 m in height. A specially designed solids recovery system is used to guarantee full recovery of the fine particles.
E Wang ct al./l'o,'der Tecfmofo~,,v 96 f 19Ua') 262 266 i
lion, i.e. the ratio o f local solids fraction to the cross-sectional average solids fraction, is plotted against the radial position. Three kinds of radial fl-action profiles are clearly shown in Fig. 2. These different shapes are largely decided by the difference in cross-sectional average solids fraction (different 4, regions). Within each region, the radial prolile is quite stable, varying neither with the superlicial gas velocity nor with the solids circuhition rate.
ii~.-
Downer
Riser
i
Air
263
Gas-solid -~. separator Measuring Storagetank Butterflyvalve
3.1. I. Un{/orm distribution re,,ion re, < 1.09~ )
Fig. 1. Experimenlalapparaln~,. The fluidized material is very fine FCC catalyst with a particle density of 1670 k g / m ~ and a mean size of 36 p,m. Its detailed properties are listed in Table 1. The superlicial gas velocity is measured by rotan]eters and the solids circulation rate is measured by a three-way wdvc and a measuring vessel. The gas velocity ranged fi-om 1 to 7 m / s and the solids circulation rate from 1 to 70 k g / m e s. The cross-sectional apparent average solids fraction is measured by the pressure gradient along the riser. A dual-beam optical density sensor is used to measure the radial solids fraction prolile. Because of the nonlinear rehltionship between the sensor signal and the local solids fraclion, the sensor must be calibrated by the pressure gradient. Details of the measuring and calibration method are reported elsewhere 18.9]. A TSI liber optic laser Doppler velocimeler ( LI)V ) is used to measure the local solids velocity of line particles. A frequency shifter is applied to detect both the upward and downward solids velocities. In order to obtain experimental data in the fully developed region, the measuring plane is 4 - 6 m above the riser bollom where the effects of the entrance and exit structures can he eliminated.
3. Results and discussions 3. I. Radial solids./)action profile
In order to compare radial solids fraction proliles under different operating conditJtons, the reduced radial solids frac-
When the solids fraction is very low. the radial solids fraction prolile is rather flat, as shown in Fig. 3. This form of prolile is termed the uniform distribution region. A sharp rise of solids fi'aclion exists at the radial position very close to the wall which indicates that only a small amount of solids may accumulate in the wall region. As oas and solids flow in the dilute phase, the distance between particles is largely increased and the interparticle forces greatly decrease so that adhesive behavior tends to reduce and even vanish. Fig. 3 plots the profile and compares it with that o f 54 bun FCC catalyst, and the difference between them is apparent. As the nlean size of particle reduces flOlll 54 to 36 i~in, particle accumulation towards the wall region also decreases, resulting in a nluch Ilattcr solids fraction prolile in the riser. This difference indicates that the solids moving mechanisms for 54 and 36 I,tm solids are quite different. Changing the operating conditions does not appear to affect the shape of this profile when the cross-sectional a\ crage solids fraction is less thai] 1~/~.The fl)llowing correlation is proposed by regressing the experilnental data: (::, -- = 0.905
q- (
I / R ) It*
1)
The curves m Figs. 2 and 3 are the results predicted by the correlation, which are in good agreement with the experimental data. This uniform solids fraction profile will greatly benelii the line particle riser reactor. It indicates less solids aggregation and good gas-solids contact, which implies good heal and mass transfer in the fine particle r i s e r - - very important l])r rapid reactions. hi addition, we investigated the equation given by, Biet al. [ 1 I I. By, this equation,when the cross-sectional average solids fraction is very low less than 1~{ ), the radial solids
Table 1 Properties of the line panicles d r,
d,,+/dvl
dr,,'/dp/~
dr,+ +/drl
i ~m)
Particle density kg!i]/" I
Ratio e l surface to
30 54
1.5
2.3
3.5
670 398
U, (m/s)
voJunle ( I]/ili
Fine FCC C o m . FCC
G,,,, (m/s)
16(~ 111
i )
7.43 × 10 ~ 1.40xlO ~
0.215 0.286
Partich: size distribution d( ~,. O. I )
~'/( v, 0.5 )
d( v, 0.9!
( b `-II] )
( b till }
( b ull )
12.0(i 31.91
3(~.18 64.98
fJ4,43 tJ4.97
264
F. Wanq, et of. / Pon'der Teclmologv 96 (1998j 262-266 3.0
2.5 ~s Ug (m/s) Gs(kglm2.S)A 2.0 I~" 1.5
4,
0.64%
2.51
2.75
~
1.87%
5.16
85.0 / O /
"
6'10°/°
2'37
//~';
66"2
~
I
• 0.0
0.0
~ ~
i
0.4
0.6
0.0
0.8
Ug(m/s) Gs(kgtm2s) ~s dp(~m)
g
t~
o.8
2.51
2.75 0.64% 36
3 82
3.21
'=1'-
6.47
0.6
0.0
66.69
4.542
37.64
•4"
1.87%
5.162 85.0 ?./ar.~
/~
/
@ & ~ O •
•
i
0.2
'
I
I
I
0.4
0.6
0.8
1.0
r/R Fig. 4. Dense ring di~tribulion of solids t'raction.
I
0.22% 36
1.01 0.20% 36 Tung's equation -- r'/R(calculated by Bi et al)
,' i' ',""
0.2
4.198
1.44%
vary significantly, with the operating conditions. A correlation of the profile is proposed by, regressing the experimental data:
1.2
~,
1.53%
'
0.0
1.0
r/R Fig. 2. The thlcc types of I-adial solids fraction prolilcs.
1.0
• •
0.5
dp=36 P.m
I
Ugl (m/s) Gs(kg/m2.s)
dp=36 vm
1.0
"/'/
0.2
2.0
----" 1.5
/cir.
o.s
~'.s
2.5
0.4
0.6 0.8 1.0 r/R Fig. 3. Unifornl radial solids fraction prolilc.
fraction profile has a very narrow wall region, which is consistent with our experimental results.
3.1.2. Dense rin f distribution region ( l . 5 q < E, < 5 q J As the cross-sectional average solids fraction increases ow,'r 1.5%, a great change is noticed in the reduced radial solids fraction profiles. Solids accumuhite near the wall region and a dense ring forms around the radial position r / R = 0.94. as shown in Figs. 2 and 4. For this reason it is named the dense ring distribution region which is similar to the radial solids fraction profile in a downer [ 10). The radial positions of the dense ring in both the riser and downer are the same, which indicates that a similar llow mechanism may exist. This form of profile in a riser does not yet appear lo have been reported in the literature where larger particles were used. This shows the complexity of the flow mechanism in the gas solids flow of the riser. As the solids conceniration increases, particle interactions with each other and the wall increase, and so does the solids aggregation in the riser, which results in the change of the solids fraction prol~le. Although the radial solids Iraction profile becomes steeper than in the uniform distribution region, it is still flatter than for larger particles, as reported by Tung et al. 19]. When the cross-sectional average solids flaction is between 1.5~?~ and 5~)b, the dense ring profile also does not
E,
-0.82+48(1
r/R)exp[--22(I-r/R)l
(2)
The comparison of the correlation prediction with experimental data is shown in Figs. 2 and 4. A fairly good agreement is achieved. 3.1.3. Aggregation distribulion region (e, > 6~)~:) With the average solids fraction further increased, the dense ring disappears and more solids accumulate at the wall region, as shown in Fig. 5. The solids fraction is rather low in the central area, increases gradually with the radial position moving towards the wall, and then reaches its maximum at the wall. This profile is the same as for the 54 p~m particles, which indicates that the strongest solids aggregation in the riser occurs at high solids fractions. The following correlation can describe the radial solids fiaction profile in this region very well: e , = 0 . 2 5 + 1 . 6 5 s i n s ( 7~r r )
(3)
~:,
As shown in Figs. 3-5, in any region, when the superficial gas velocity and solids circulation rate vary within a certain range, the radial solids fraction profiles remain unchanged at a constant cross-sectkmal average solids fraction. This result 3.0 dp{um)
2.5
I~ 2.0
+
I
~.
1.5
54 36 54 36
Ug(mts)~ s Gs(kg/m2.s) 3.6 3,5 44 2.4
8.8% 6 0% 63% 61%
102.0 78.0 1320 66.95
(
--
1.0
0.5 0.0
r
0.0
0.2
0.4
0.6 0.8 1.0 r/R Fig. 5. Aggregation distribulhm of solids fraction.
E B,'(m:,,el al. / Po 'd~'r7k'c/ 7o/o:W96 (1998) 262 266 shows that the average solids fraction is a key parameter in determining the radial solids fraction profile, and the gas velocity and solids circulation rate have no direct relation with the profile. The same results for larger particles were reported by Tung et al. [9]. However, as the solids size is reduced to 36 ~m, the radial solids fraction profile becomes more complex and can not be described by a unified correlation. The solids aggregation mechanism may' undergo a change for these 36 Ixm fine particles in a relatively dilute phase, being more uniform along the radial direction. This change in solids aggregation for fine particles reveals that the particle size has a large effect on the hydrodynamics e l the riser. The unifl)rmity of line particles in the dilute phase will cause good gas-solid contact efficiency and high mass and heat transfer rates in riser reactors. Extensive studies on line particle behavior are required to reveal the complicated Ilow mechanism and to make full use of fine particles. At high solids fractions, the solids aggregation approaches that of larger particles.
3.2. Radial profile qf tlu" /?article l'elocilv In order to analyze the differences of the radial particle velocity profiles under different operating conditions, the reduced panicle velocity, i.e. the ratio of the local particle velocity to the superficial gas velocity, is plotted againsl the radial position, as shown in Figs. 6 and 7. Apparently, the particle velocity is unevenly distributed along the radial direction with a maximum value at the axis, and gradually decreasing toward the wall. The particle velocity will become flatter when the superficial gas velocity is raised a n d / o r the average solids fraction is decreased. While the average solids fraction is below 0.3(/c and the superficial gas velocity is above 4.8 m / s , the radial particle velocity profile obeys the 1/7 power law of turbulent flow. described by 1,'7
Integration of the particle velocity along the radial direction shows that the average particle velocity is equal to the superficial gas velocity measu:ed by the rotameter. This observa-
2.0 -
265 ....
1/3.5
power
1/7
power
13
" -~------=,,~
m 1.2
Ug(m/sl Es Gs(kg;rn2a~;~~;.O~--
0.8
. •
oo,.,o 4.7, 02~°~o = ~ 4.7s 0.70°/. 47 7
•~,
4.71 1.30%
[]
0.4
,=, '=',U !
]
|
78.3
.L
dp =36 ~ m
0.0
i 0.0
r-
0.2
0.4
i
I
0.6
0.8
-r
1.0
fiR
Fig. 7. Influence of average solids fraction on the radial panicle ~elocity profiles lion is evidence that line particles can completely follow the gas movement under this condition. It seems that this minimal velocity will rise with an increase of the particle size a n d / o r the solids concentration. The smalter a particle is, the stronger its ability to f o l l o w gas movement. When the particle size increases by a certain amount, it is difficult to observe this ability under practical operating conditions. For 54 I~m particles, in the experimental range up to U,== 9 m/s, the minimum gas velocity following the 1/7 power law has not yet been reported. Because the LDV system can not work at high solids concentrations, only particle velocities under a solids fraction of 1.3q~ can be obtained precisely. As shown in Fig. 7. when the gas velocity is held constant, lhe particle velocity profile becomes steeper than in the dilute region. This may be caused by' the solids aggregation at higher solids concentrations. However, compared with the result for larger particles under the same conditions, it is flatter, especially in the wall region, as shown in Fig. 8. As we know, solids backmixing, significant in a conventional riser, has bad effects on conversion and production in a chelnical reactor. Although the degree of solids backmixing in a circulating fluidized bed has been largely reduced compared with that in a turbulent bed, experiments have proven that for 54 g m particles the axial Peclet number of particles is about 4 in CFBs, far from plug flow, and it does not vary 2.0
1.6
.
.
.
.
.
.
.
.
.
.
.
.
1.5-
12 o~
::~ 0.8
1.0
O.
D.
>
>
Ug 0.4
0.0 0.0
•
3.49
26
•
4.27
&0
7= 0.999
•
4.78
5.3
d p = 3 6 b~ m
~ 0.2
r 0.4
i 0.6
7--0.8
0.5=
0.0
dpO.rn) Ug(m/s
r/R
Fig. (~. Influence of L/,, on the radial parlicle velocity prolilc~;.
.
36
4.7
99.3%
47.7
[
•
54
4.3
99.5%
35.0
II
0.0
i 0.2
I 0.4
i 0.6
f 0.8
-0.5
.0
.
•
1.0
r/R
Fig. ~. Comparison of solids velocity profilesof dffrerem parti~:k.'s.
266
). W~m,, ~,~~d. /I'¢*~rder Tedmotocq % t/9%'j 262 266
with operating conditions [8]. The non-uniformily of the particle velocity and solids fraction profiles is the mosl important reason lk~r solids backmixing. By using fine powder, the uniformity can be improved, thus axial solids backmixing may largely be decreased. The conditions will approach plug llow. which will benefit fine particle riser reactors by improving their efficienc). The change in lhe radial profiles of solids conccntration and velocity for the fine particle riser raises a question: is the conventional 54 i,tm FCC catalyst widely used in commercial units, ideal f o r a riser? At least from the hydrodynamic p o i n t of view, it is far from optimal. Our resulls show IMI detailed studies of the efl'ecl of particle size on the fluidization behavior are urgently needed to provide more information on this ve U inlportant issiie.
R d( \ . O I )
Uf gnl[
U. Vp
radius of core region of riser (m) radius of riser (m) particle size under 10% of volume fraction (gm) superlicial gas velocity ( m / s ) minimal fluidizing velocity ( m/s ) terminal velocily ( m / s ) local particle velocity ( m/s )
(;reelc lelter,s {-
local voidage cross-sectional average solids fraction local solids fraction
6~
References 4. Conclusions
Three kinds of r,:tdial solids fraction proliles are loLind in a riser with 36 >m line particles: uniform, dense ring and aggregation. Compared with the results for 54 >m particlcs, the radial concentration distribution has been improved with the decrease m the particle size. At high gas velocities and low solids fractions, the radial proliles of the line particle velocity obey the 1/7 power law of turbulent flow, which is much flatler than for 54 bun parlicles. The impix)vement of flow uniformily of line particles may have many good effects on the gas and solids backmixing and mass and heat lransfer in a riser. The FCC catalyst should be optimized with regard to particle size, even though the normal 54 bun FCC catalyst is widely used in comn3ercial planls.
5. List of s y m b o l s
dp F
particle size ( I~m ) solids circulating rate ( k g / m : s ) radial coordinate (rn)
H. Weinstein, M.-J. Sllao. M. Schnitziein, in: K. Oster,2aard, M . A SOfellNCn ( Eds. ), Fluidizalilm V. Engineering l:oundation, New Y o r k I CJ~fl, pp, 329 336, M. Horio, K. Morishila. (). T~lchibana. N. Murala. in: P. Basu, J.F I,argo iEds.). CircuMing Fluidized Bed "TecMology 11, Pergamon Toronto. IggN, pp. 147 154, RJ. l)ry, Powder Technol, 49 f 1987) 37 44. R. Bader. J. Findlay. T.M. Knob, lion. in: P. Basu, J.F. Large ( E d s . ) ('irculating Fluidizcd Bed 'lechnology II. Pergamon. Toronlu, I t)NN pp. 127-137. li.U. Harlage, 1). Rensner. ,I. ~erthef, in: P. Basu. ,I.F. I,arge ( Eds. ) (TirculatJng FMdizcd Bed "fecMolo.gy 11, Pergalnon. Toronto, lC)F.S. pp, 165-180.
W. Nowak. H. Mine< Y. Malsunlura, R. Yamazaki. K. Yoshida, in: K. Yoshida. S. Morouka I [kts. ), Prec. Asian Conf. FMdized Bed and Three-Phase Rcaclors, Tok\ o. ,l@an. I ~J88, pp. 16(I-17(). 71 Y.I_. Yang, Y..lin, Z.(,,). "l'u. Z.W. Wmlg, D.R. Bai. hi: P. Basu. M Horio. M. Ha:,alani ( l:ds. i. ('irculating Fhlidizcd Bed Technology Ill. Pcrgainon, Toronllt, I 0 ~) I. pp. 201 206. Y. ('heng, I Indergradualc Thcsi,. Tsinghua Univer:,i b . Bcijing, China. I O94. Y..I. Tung. J.lt. IJ. M. Kwauk. in: M. Kwauk, I). Kunii (F, ds.L Fluidizalion "g8 Science :uld Tcchnohlg,~, ( Conf. Papers 3fd (Thina ,lapan Syrup. [:luidiz~l/km, Bci.iing, ('hinla. 19gN). Science Press. klcijing. 1!,1814.pp. 13c) 145. l l01 Y.I,. Yang, l)ocloral Thesi<,. 'fsinghua I,Jnivcrsitv, Beijing, China, 1991.
l i i l H.T. B i, J Zhou, S.-Z. Qi n. ,1.R. Grace. Can. J. Chem. ling., 74 ( Ic)06 i 811 814.