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Gas discharge modes at a single horizontal nozzle in a two-dimensional fluidized bed
ELSEVIER
PowderTechnology90 (1997) 71-77
Gas discharge modes at a single horizontal nozzle in a two-dimensional fluidized bed C.S. Chyang, C.H. Chang, J.H. Chang Chemical Engineering Deparlmenl, Chung Fuan Christian University. Chung Li. Taiwan
Received24 October 1995;tgvised5 July 1996
Abstract The gas discharge modes, bubbling and jetting, formed at a single horizontal nozzle in a two-dimensional gas fluidized bed wereinvestigated by visual observation, The inside diameters of the nozzles used in this study were 3.0, 4.5, 6.0, 7.5 and 9.0 ram. The gas velocity through the nozzle was in the range from ! to 130 m/s. The ratio of the superficial velocity to the minimum fluidizing velocity was in the range from 0 to 4. Gas discharge modes were analyzed by high speed cine photography, and the effects of the nozzle diameter, gas velocity through the nozzle, particle diameter and the static bed height were studied. The results showed that the length of jet penetration increased with the gas velocity through the nozzle and the nozzle diameter. The phase diagram of bubbling, jetting and transiti~'~ zones were mapped by ufing a modified Froude number and the ratio of the nozzle diameter, Do, to the panicle size. Dp. Keywords: Two-dimensionalfluidizedbed; Horizontalnozzle; Gas discharge mode;Jet penetration length
1. Introduction
it is ~vcU known that the grid region in a gas fluidized bed reactor plays an important role in determining the chemical conversion, especially for a fast reaction, When the fluidizing gas flows through a multi-orifice distributor into a fluidized bed, it forms bubble plumes and permanent jets at the orifices. The gas discharge modes at the orifice significantly affect the hydrodynamics in the grid region and are thus a very important research aspect. For most industrial fluidized bed systems, tuyeres, bubble caps and nozzles .are usually mounted on the gas distributor plate instead of perforated plate. The gas flow is introduced into the bed via a horizontally directed jet. However, the research concerning file horizontal jet is limited. Based on the investigation of horizontal jets from a bubble cap distributor, Kozin and Baskakov [ 1] found that a circulation of particles was formed in the cap effect zone. For horizontal air jet penetration, the related correlations were presented by Zenz [2] and Shakhova [3] to predict the penetration depths. Merry [4] proposed a mathematical model to explain the hydrodynamic behavior of particles around a horizontal jet. The hydrodynamic behavior with the dragging zone around a horizontal jet was investigated by Xuereb et el. [ 5,6 ] in a two-dimensional bed. The shape and extent of voids
formed by a horizontal jet in a filree-dimensional fluidized bed were studied by Chen and Weinstein [ 71. In a previous study [ g]. the gas discharge mode ha.,~been studied by visual observation and spectral analysis ofpn:ssure fluctuations in the grid region. It is also found that most of the research concerning the gas discharge mode was concentrated on the vertical jets. In this study, the gas discharge mode, bubbling and jetting at a horizontal nozzle are investigated by visual observation. The effects of nozzle diameter, gas velocity through nozzle, superficial gas velocity, static bed height and particle diameter on the gas discharge modes are discussed.
2. Experimental A schematic diagram of file experimental apparatus is shown in Fig. !. All the experiments were conducted in a two-dimensional fluidized bed fabricated with acrylic plates. The cross-section of the bed was 380× 15 mm. Glass beads with mean diameters of 0.214, 0,545, 0.775 and 0.920 mm were used as the fluidizing material. A single horizontal nozzle mounted at the lateral wall was 100 mm above the distributor. The operating conditions and the characteristics of the nozzles, shape~: and dimensions, ate listed in Table 1. Gas velocity through the nozzle was in the range from 1 to 130 m / s . A packed bed of 0.214 mm glass beads was used
0032-5910/97/$17.00 Copyright© 1997ElsevierScienceS.A. All tights reserved PII S0032- 59 ! 0 ( 96 ) 03202-0
3-D 2.-D
3-D
Shak.hova [ 31 Zenz [ 2 ]
Merry [41
2-D
3-D 2-D
Xuereb et al. [ 5.6]
Chen and Weinstein [7] This work
2-D
2-D
3-D
Column type
0725 1.13 1.64
polystyrene
polystyrene polystyrene 0.059 0.214 0.545 0.775 0.920
0.605
polystyrene
FCC catalyst glass bead glass bead glass bead glass bead
0.4-0.63 0.85-1.5 4 0.05-2 0.17 0.18 0.1 t~ 0,18 0,33 0~33 0.33 0.18 0.33 0.78 2.0 0.41
refractory brick refractory brick copolymer sand null scale sand sand sand sand sand sand sand sand steel shot kale seeds polystyrene
6.4-12.7 3,0-9.0 3.0-9.0 3.0-9.0 3.0-9.0
8
4-..6 14.0 7.9 2.54 6.35 14.3 2.54 6.35 1&3 3.175 3.175 3.175 3.175
3.0-8.1
(ram)
(mm) 0.2-0.85 02-0.4 0.4-0.63
D,,
Dp
vanadium iron ore non ore
Bed material
Variables investigated
Kozin and Baskakov [ I ]
Reference
Table I Experiment condttions
1450 2600 2600 2600 2600
1020
1.2 1.177 i, 177 1.177 1.177
1.2
1.3 1.3 1.3 1,3
1.2
2640 2640 2640 7430 1000
!.3
1.2
1.2
( k g m -3 )
pg
2640
998 1013 t000 2600 2600
730 :.658 1680
(kg m ')
~
2.0 2.0 1.5 i.2 IA4.4 !.82.9 ! .22.7 1.44-2 1.71,9 4.44 i.0 1.0 1.0 1.0
2,0
2.0
I. 17 I 1.0 7.9
3,08.1
U/Umr
23--69 53-129 43-120 2 I-127 21-126
63-125 54-105
47-125
63-125
40-300 40-300 40--300 40-300 35-125
5-20
5-20
52-303 2.8-6.6 75-1 i 8
O- I I 0
Uo (ms-')
0.52 0.45 0.48 0.4". 0.49
0.4 200-300 60-t00 0.47 160-280 0.49 0.47 0.49 0.55
0.5
( I -- ~l
850 1130 100 i',M 100
I 10,0
200 200 200 200
30-120 790--4270 490-3660 70-120
65-800
H~ (mm)
20-190 80-170 80-140 20-t60 20--170
130-250
140-270
120-260
20-100 20-100 20-50 20-100 50-330
130-250
100-210
30-120 150-220 60- 100
L (ram)
~o
C,S. Chyang et aL /Powder Technology 90 f 1997) 71-77
l. 2, 3. 4. 5. I'. 8. 9. 10.
2-D Bed Diatributor Windbox Nozzle Needle valve Pressure gauge Rotameter Ball valve Pressure regular.or Air tiller
12. 13 14 15.
Air Lank Air eompre'~sor Video L~ght s o u r c e
6.
73
as the fluidizing gas distributor. The superficial gas velocity within the bed is 0 to 1.4 times the minimum fluidizing velocity. The gas discharge mode was observed and recorded by using a video camera at various operating conditions.
3. Results and discussion
1~. Con~enaor
3.1, Gas discharge modes
.~4~___~: 1
I ]
l,
_~
~
L___I
io
Fig. 1. Schematic diagram of the experimental apparatus.
A permanent fame-like cavity stands in front of the orifice as shown in Fig. 2 is called a jet. 'Bubbles' are formed when a gas void is formed and then detached at the orifices, as shown in Fig. 3. When operating under low gas velocity, there is a clear and complete air pillar formed near the orifice which causes bubbles, it is found that bubble shape varies with the frequency of bubble formation. When particles are light and small or the nozzle diameter is large, the gas discharge mode at the orifice will be unstable. Such a case is called 'transition state'. From Fig. 4, it can be seen that there are two gas streams flowing upward and
•iii
Fig. 2+ Jet types at the horizontal nozzle for different experimental conditions. (a) Dr=0.920 ram, D,+=7.5 ram, H.~200 ram. Uo=67 m/s; (b) Dp ~ 0.545 ram. Do =7.5 ram, H,~-200 nun, Uo=27 m/s.
i
Fig. 3. Bubble types at the hotizon~ nozzle for different experimental conditions. (a) Dp~0.920 ram. D,,=7.5 r a m . / / , = 2 0 0 nun, U.,=8 m/s; ( b ) Dp = 0.545 ram. D,, =~7.5 ram. H, ~, 200 ram./3, ~=4 ml s.
C S. Chyang et al. / Powder Technology 90 (1997) 71-77
74
forward from the end of jet region. The extent of the jet penetration length can shrink sometimes after expansion. Generally speaking, if the gas plume is unstable, a bubble will emerge from the orifice but if it is ~t~ble, a jet will emerge
from the orifice. Usually, the gas discharge mode changes from bubble to jet when the gas velocity through nozzle is increased. The transition state between the bubbling region and jetting region can be easily seen. It can also be observed that some particles are entrained into the jet from the emulsion phase. Entrained particles can also be seen in the bubbles, jets, and jet ends but, due to the oscillation of jet boundary, particles cannot be observed precisely.
3.2. Horizontal jet penetration length 3.2.1. Effect of gas velocity through nozzle The effect of gas velocity through nozzle on the jet penetration length is shown in Fig. 5. The jet penetration length increases as gas velocity through nozzle increases. The effect of superficial velocity on the jet penetration length at various gas velocity through nozzle is shown in Fig. 6. Above a certain value of superficial velocity, the jet penetration curve peaks. The maximum points of those curves were found when the ratio of UIU,,,r was in the range 0.6-1. To ensure that the experiment was working under the maximum jet penetration length, the ratio of UIU~r equal to I was chosen. This ratio for UI Umf gave the maximum jet penetration length for all gas velocities used in this study.
3.2.2. Effect of nozzle diameter and shape
Fig. 4. Transitionslates at the horii~onlalnozzle for differentexperimental conditions. (a) Dr =0.214 ram, D,,= 7.5 mm, H, =200 ram, U, = 123 m/s; (h) Dp=0.214 ram. D,~=7.5 ram+H,= 200 mm, U,+=12 m/s.
For a given gas velocity through the nozzle, tile jet pene..2ration length increases with nozzle diameter, as shown in Fig. 7. The relationship between the different shapes of nozzles are listed ia Table 2. Fig. 8 shows that the effect of the shape of nozzle on jet penetration length is minimal. Although ntJw.erous investigators have considered the combinations of nozzle diameter and horizontal penetration length as a dimensioaless factor LID,,, we can state that the jet penetration length increases with the noTzle diameter and jet momentum, within a certain range of gas ~,elocity through the nozzle, in this study. 200
200
UI o o a
.D. ~,%. 8.0 * 7.6 O 9.0
r,
...-..
t
.~tuo,.3
o~
34
tSO
150
+
,~,ll-I 19 2?
g,.
,. uo
go
(m/s)
.0
,~,.~IOD
I
'+.+-
,+o
o.~,
o.',,
U/Umf
,.:'+
,.,,
Fig. 5, Effect o f gas velocity through the nozzle on the jet penetratson length
Fig. 6. Effect of superficial velocity on Ihc jet penetration length at various gas velocities through the nozzle. Dp=0.545 ram, H+~ 100 ram, D o =
with variousnozzle diameters.Dr =0.545 ram, H ~ 100 mm, U/U,,,= I.
7.5 mm.
75
C.S. Chyang et aL / Powder Technology 90 (1997) 71-77
L60
200 I
] Uo I
" 34 &
o
,,..-t 27
150
.._..100
~.'o
.__~
._
,.'o
1
~
o.o
o.o
lo.o
Do ( m m ) Fig~ 7. Effect of nozzle ~iameter on the jet penetration length. Dp = 0.545 nun, H,= 100 mm, UIUm~= I.
Dp ( m m ) Fig. 9. Effectof particlediameteron thejet penetrationlength.Do= 7.5 ram, H, = I00 nun. UI U,,r= i,
Table 2 Noa,zleshapes (ram) Circular (i.d.) Square Rectangular
120
1~ , i n o 40 o 70 ~' 100 150 * 200
o
3.0; 6.0; 7.5; 9.0 5.0 × 50 3.I × 9.0:9.0 × 3. !
0
o
8 o
.=.1 r~
200[ o°OaH°zZleo.0,3.15.3*5.36"O3.1,9.0dim~nai°nn'mm
~-~o~.i
Q
/~,t~,
"..--.
t3
O00
500 - - - - - -
i ~0
I .....
30
40
the
nozzle,detectedat differentstaticbed heights.Dp= 0345 mm./)0= 7.5 nun. UIO,,r= I.
J
~ 40
~
eo (m/s)
Fig, 10, Variation of jet penetration length with gas velocity through
//
/
I 10
1
..~
i 60
......
80
13o (m/s) Fig. 8, Effectof gas velocitythroughthe nozzleon thejet penetrationlength with variousnozzledimensions.Dp= 0.545 ram.H,= 100 ram. UIUmf-" 1.
3.2.3. Effect o/particle diameter Fig. 9 shows that jet penetration length decreases with particle diameter. However, there will be a significant difference in the jet penetration length when operating with particles with diameter larger than 0.775 ram. In Fig. 9, the significant difference in the jet penetration length also shows that different groups (Groups B and D) have a different effect on the jet momentum. 3.2.4. Effect of static bed height The jet penetration length was significantly affected by gas velocity through the nozzle when the bed height was lower than 100 mm. The experimental data, shown in Fig. 10, indi-
cates that the jet penetration length decreases with static bed height. However, ira very shallow bed is used, the gas plume pushes upwards to the surface of the very quickly. A suitable bed height needs to be chosen in order to get a fully-developed jet.
3.3. Comparison o f experimental data with correlations reported Experimental conditions for the investigation of horizontal jets are listed in Table 1. Numerous correlations have been proposed for predicting the jet penetration leng',k in ~he fiui-. dized bed. The typical correlations which can be used for predicting the penetration length of horizontal jets are listed in Table 3. There are various definitions of jet penetration length. The maximum jet penetration length was adopted by Zenz [ 2] and Merry [4]. Moreover~ Shakhova [ 3t calculated the jet length as the mean value of the maximum and minimum penetration length. In this study, the maximum jet penetration length was adopted as the jet length in all experiments.
76
CS. Chyang et aL /Powder Technology 90 (1997) 71-77
Table 3 Correlationsforjet penetrationlength
V,, ]
Shakhova [3]
L,~,.+__~..=781-:
Zenz 12]
0.044-'~o'*+ 1.57= 0.5 Iog(p~U~)
2do
shows the flow regimes of gas discharge modes at a single horizontal nozzle by using the data obtained with Group B particles. From the experimental data shown in Fig. 13, the boundary line between the bubbling regime and transitional state can be expressed by Eq. (2).
" "LPp "q g d o) ''2" J
2
04
Fr* =2.25 × 10s
-'+ <+r 1 d,_+'+5=+- L(t-e)Or,gd~_l ~ p /
Merry [4]
0.2
02
~d,,l
(2)
The boundary line between the transitional state and jetting regime can be expressed by Eq. (3). F r * = l . 7 0 × 10S(D ~-
0
150 o 0 o
°°
(3)
From the data shown in Fig. 13, one can find that the gas discharge mode changes from bubbling regime to jetting regime as the Froude number increases. In other words, the jetting types can be observed more often with a large Froude number and large nozzle diameter. Fig. 14 shows that the transitional state between the bubbling regime and jetting regime is not so obvious for the
0
0
,~100
2.94)
-2"2
200
1
z'o
+o
+'o
.'o
,oo
150
Uo ( m / s ) Fig. ! 1. Comparise.aof numerica/pedictionswith experimentaldata from jet penetrationlength; (!) Shakhova [3]; (2) Zenz [2]; (3) Merry [41. Experimental conditions: Dp=0.545 ram, t1,= 100 mm. Do=6.0 mm, Ut U,~f= I.
m
0 0
,-3 0
O
The compeaison of jet penetration lengths was calculated by the using various correlations listed in Table 3. From the experimental data obtained in this study, it can be shown that all the correlations underestimate the jet penetration length. Representative results are shown in Fig. 1 ! for small particles (Group B) and Fig. 12 for large particles (Group D). The differences can be explained by the various definitions of jet penetration length and the experimental conditions, it should be noted that the two correlations by Zenz and Merry could not be used in this study to predict the jet penetration length when gas velocities through the nozzle were less than 35 ms- t.
0 o
O0
O~
Fr* = 2p+02 U,.p 31~D~g
2
~"
i
ZO
,_
•
40
Uo (m/s)
0 ~I
i
l
, 80
Eq{3)
/
0 0 0 0~0~"~
0 0 0 090~J
1 o o Qj~rma8
P..
,°.
°
°°07 aO
I0 '
lO *
....
I
0 O0
•, i O
This represents the balance between the inertial force of gas at the nozzle and the gravity force of particles. Fig. 13
i
U / U m r = I..
~q(21
(l)
,1 k 6u
Fig. 12. Comparisonof numericalpedietionswith experimentaldata from jet penetrationlength; (1) Shakhova[31; 12) Zenz [2]; (3) Merry 14]+ Experimental conditions: Dp=0.920 ram, H,: 100 nun, D,=7.5 ram,
3.4. Flow regimes o f the gas discharge mode
In order to integrate the effects on the gas discharge modes of particle size, nozzle diameter and gas velocity through nozzle, two pmameters were used to map the flow regimes. They are the ratio of nozzle diameter to particle size and a modified Froude number.The modified Froude number was defined as
0
0
,~,~lO0
~D
IO •
!
l0 '
FF° Fig. 13. Flow regimesof the gas discharge mode for horizontal nozzles, Group B particles:D~,= 0.214 and 0.545 mm.
c.s. Chyang et al. / Powder Technology 90 (1997) 71-77
to group the bubbling, jetting and transition zone types. The phase diagram of bubbling, jetting, and transition zone can be distinguished easily. In this phase diagram, the bubbling type is observed at low Froude numbers and the jetting type at high Froude numbers. From the experimental data obtained in this study, it can be stated that the dominate factor for gas discharge mode from a horizontal nozzle is the inertial force of the gas flow through the nozzle. The gas discharge mode will be changed from bubble to jet while the gas velocity through nozzle (inertia force/moment) is increased. Between these two regions, the transition state is observed.
q(41
,~.
'~
A
~
~
&
o/
/L~
o
o
ci
o
cI ~ O r ' l n O
[] o o o o o c o
cl LI
onr~Q
,1..... 2 _ _ / ~ ? .... o o o . . . . . . . . . . i0 j
10 '
A
20 "
Fr" Fig. 14. Flow regimes of the gas discharge mode for horizontal nozzles. Group D particles:Dp-- 0.775 and 0.920 mm. Group D particles. The boundary line between these two regimes can be represented by Eq. (4). F r * = l . 4 8 X 10~-~p-1.79)
77
(4)
It is found that the jetting phenomenon become unstable while DolDp increases. For a certain value of Do, a decrease in Dp will lead to bubble formation. However, when particle size and Froude number increase, jet formation can be observed more readily.
5. List ofsymbols Do
Fr*
H~ L U
nozzle diameter (ram) ~u'ticle di~,¢-:-:: :: ~ r ~ ) modified l:ro~.,~c ,.~-~-.~.:, static bed height (ram) jet penetration length (ram) primary fluidizating velocity ( m s - ~) minimum fluidizating velocity ( m s - ~) gas velocity through the nozzle ( m s - x)
Greek letters
pg pp p~ •
gas density (kg m - j ) solid particle density (kg m -3) gas density at nozzle (kg m -3) bed voidage
4. Conclusions
In this study, the gas discharge modes at a single horizontal nozzle in a two-dimensional fluidized bed were investigated by visual observation. It is found that a maximum value of the jet penetration length occurs at the superficial velocity around 0.6--1.0 times minimum fluidization velocity. The length of jet penetration increases with gas velocity through the nozzle and nozzle diameter, and decreases with particle diameter and static bed height. I~y using a modified Froude number as the horizontal axis and DolDp as the vertical axis, a phase diagram can be drawn
References
[ I | V.E. Kozinand A.P. Ba~kp_knv.lnL Chem. Eng.. 8 (1968) 257. [2] F.A. Zcnz, ln~t~ Chem. Eng. Syrup. Set., 30 (1968) 136. [3] N.A. Shakhova,lnzh. Fiz. ZIt. 14 (1968) 61. 14] J.M.D. MeJry,Trans. Inst. Chert I~ng., 49(1971) 189. [5] C. Xuereb, C.C. Laguerie and T. Baron, Powder Technol..64. ( 1991 ) 271[6l C. Xuereb, C.C. Laguerie and T. Baron, Powder Technol..67 ( 1991 ) 43. [7] L. Chen -andH. Weinst¢in,A/C/E,/., 12 (1993) 1901. [8} C.C. Huang and C.S. Ch~.mg, Jpn. J. Chem. Eng., 5 ( 1991 ) 633.
PowderTechnology90 (1997) 71-77
Gas discharge modes at a single horizontal nozzle in a two-dimensional fluidized bed C.S. Chyang, C.H. Chang, J.H. Chang Chemical Engineering Deparlmenl, Chung Fuan Christian University. Chung Li. Taiwan
Received24 October 1995;tgvised5 July 1996
Abstract The gas discharge modes, bubbling and jetting, formed at a single horizontal nozzle in a two-dimensional gas fluidized bed wereinvestigated by visual observation, The inside diameters of the nozzles used in this study were 3.0, 4.5, 6.0, 7.5 and 9.0 ram. The gas velocity through the nozzle was in the range from ! to 130 m/s. The ratio of the superficial velocity to the minimum fluidizing velocity was in the range from 0 to 4. Gas discharge modes were analyzed by high speed cine photography, and the effects of the nozzle diameter, gas velocity through the nozzle, particle diameter and the static bed height were studied. The results showed that the length of jet penetration increased with the gas velocity through the nozzle and the nozzle diameter. The phase diagram of bubbling, jetting and transiti~'~ zones were mapped by ufing a modified Froude number and the ratio of the nozzle diameter, Do, to the panicle size. Dp. Keywords: Two-dimensionalfluidizedbed; Horizontalnozzle; Gas discharge mode;Jet penetration length
1. Introduction
it is ~vcU known that the grid region in a gas fluidized bed reactor plays an important role in determining the chemical conversion, especially for a fast reaction, When the fluidizing gas flows through a multi-orifice distributor into a fluidized bed, it forms bubble plumes and permanent jets at the orifices. The gas discharge modes at the orifice significantly affect the hydrodynamics in the grid region and are thus a very important research aspect. For most industrial fluidized bed systems, tuyeres, bubble caps and nozzles .are usually mounted on the gas distributor plate instead of perforated plate. The gas flow is introduced into the bed via a horizontally directed jet. However, the research concerning file horizontal jet is limited. Based on the investigation of horizontal jets from a bubble cap distributor, Kozin and Baskakov [ 1] found that a circulation of particles was formed in the cap effect zone. For horizontal air jet penetration, the related correlations were presented by Zenz [2] and Shakhova [3] to predict the penetration depths. Merry [4] proposed a mathematical model to explain the hydrodynamic behavior of particles around a horizontal jet. The hydrodynamic behavior with the dragging zone around a horizontal jet was investigated by Xuereb et el. [ 5,6 ] in a two-dimensional bed. The shape and extent of voids
formed by a horizontal jet in a filree-dimensional fluidized bed were studied by Chen and Weinstein [ 71. In a previous study [ g]. the gas discharge mode ha.,~been studied by visual observation and spectral analysis ofpn:ssure fluctuations in the grid region. It is also found that most of the research concerning the gas discharge mode was concentrated on the vertical jets. In this study, the gas discharge mode, bubbling and jetting at a horizontal nozzle are investigated by visual observation. The effects of nozzle diameter, gas velocity through nozzle, superficial gas velocity, static bed height and particle diameter on the gas discharge modes are discussed.
2. Experimental A schematic diagram of file experimental apparatus is shown in Fig. !. All the experiments were conducted in a two-dimensional fluidized bed fabricated with acrylic plates. The cross-section of the bed was 380× 15 mm. Glass beads with mean diameters of 0.214, 0,545, 0.775 and 0.920 mm were used as the fluidizing material. A single horizontal nozzle mounted at the lateral wall was 100 mm above the distributor. The operating conditions and the characteristics of the nozzles, shape~: and dimensions, ate listed in Table 1. Gas velocity through the nozzle was in the range from 1 to 130 m / s . A packed bed of 0.214 mm glass beads was used
0032-5910/97/$17.00 Copyright© 1997ElsevierScienceS.A. All tights reserved PII S0032- 59 ! 0 ( 96 ) 03202-0
3-D 2.-D
3-D
Shak.hova [ 31 Zenz [ 2 ]
Merry [41
2-D
3-D 2-D
Xuereb et al. [ 5.6]
Chen and Weinstein [7] This work
2-D
2-D
3-D
Column type
0725 1.13 1.64
polystyrene
polystyrene polystyrene 0.059 0.214 0.545 0.775 0.920
0.605
polystyrene
FCC catalyst glass bead glass bead glass bead glass bead
0.4-0.63 0.85-1.5 4 0.05-2 0.17 0.18 0.1 t~ 0,18 0,33 0~33 0.33 0.18 0.33 0.78 2.0 0.41
refractory brick refractory brick copolymer sand null scale sand sand sand sand sand sand sand sand steel shot kale seeds polystyrene
6.4-12.7 3,0-9.0 3.0-9.0 3.0-9.0 3.0-9.0
8
4-..6 14.0 7.9 2.54 6.35 14.3 2.54 6.35 1&3 3.175 3.175 3.175 3.175
3.0-8.1
(ram)
(mm) 0.2-0.85 02-0.4 0.4-0.63
D,,
Dp
vanadium iron ore non ore
Bed material
Variables investigated
Kozin and Baskakov [ I ]
Reference
Table I Experiment condttions
1450 2600 2600 2600 2600
1020
1.2 1.177 i, 177 1.177 1.177
1.2
1.3 1.3 1.3 1,3
1.2
2640 2640 2640 7430 1000
!.3
1.2
1.2
( k g m -3 )
pg
2640
998 1013 t000 2600 2600
730 :.658 1680
(kg m ')
~
2.0 2.0 1.5 i.2 IA4.4 !.82.9 ! .22.7 1.44-2 1.71,9 4.44 i.0 1.0 1.0 1.0
2,0
2.0
I. 17 I 1.0 7.9
3,08.1
U/Umr
23--69 53-129 43-120 2 I-127 21-126
63-125 54-105
47-125
63-125
40-300 40-300 40--300 40-300 35-125
5-20
5-20
52-303 2.8-6.6 75-1 i 8
O- I I 0
Uo (ms-')
0.52 0.45 0.48 0.4". 0.49
0.4 200-300 60-t00 0.47 160-280 0.49 0.47 0.49 0.55
0.5
( I -- ~l
850 1130 100 i',M 100
I 10,0
200 200 200 200
30-120 790--4270 490-3660 70-120
65-800
H~ (mm)
20-190 80-170 80-140 20-t60 20--170
130-250
140-270
120-260
20-100 20-100 20-50 20-100 50-330
130-250
100-210
30-120 150-220 60- 100
L (ram)
~o
C,S. Chyang et aL /Powder Technology 90 f 1997) 71-77
l. 2, 3. 4. 5. I'. 8. 9. 10.
2-D Bed Diatributor Windbox Nozzle Needle valve Pressure gauge Rotameter Ball valve Pressure regular.or Air tiller
12. 13 14 15.
Air Lank Air eompre'~sor Video L~ght s o u r c e
6.
73
as the fluidizing gas distributor. The superficial gas velocity within the bed is 0 to 1.4 times the minimum fluidizing velocity. The gas discharge mode was observed and recorded by using a video camera at various operating conditions.
3. Results and discussion
1~. Con~enaor
3.1, Gas discharge modes
.~4~___~: 1
I ]
l,
_~
~
L___I
io
Fig. 1. Schematic diagram of the experimental apparatus.
A permanent fame-like cavity stands in front of the orifice as shown in Fig. 2 is called a jet. 'Bubbles' are formed when a gas void is formed and then detached at the orifices, as shown in Fig. 3. When operating under low gas velocity, there is a clear and complete air pillar formed near the orifice which causes bubbles, it is found that bubble shape varies with the frequency of bubble formation. When particles are light and small or the nozzle diameter is large, the gas discharge mode at the orifice will be unstable. Such a case is called 'transition state'. From Fig. 4, it can be seen that there are two gas streams flowing upward and
•iii
Fig. 2+ Jet types at the horizontal nozzle for different experimental conditions. (a) Dr=0.920 ram, D,+=7.5 ram, H.~200 ram. Uo=67 m/s; (b) Dp ~ 0.545 ram. Do =7.5 ram, H,~-200 nun, Uo=27 m/s.
i
Fig. 3. Bubble types at the hotizon~ nozzle for different experimental conditions. (a) Dp~0.920 ram. D,,=7.5 r a m . / / , = 2 0 0 nun, U.,=8 m/s; ( b ) Dp = 0.545 ram. D,, =~7.5 ram. H, ~, 200 ram./3, ~=4 ml s.
C S. Chyang et al. / Powder Technology 90 (1997) 71-77
74
forward from the end of jet region. The extent of the jet penetration length can shrink sometimes after expansion. Generally speaking, if the gas plume is unstable, a bubble will emerge from the orifice but if it is ~t~ble, a jet will emerge
from the orifice. Usually, the gas discharge mode changes from bubble to jet when the gas velocity through nozzle is increased. The transition state between the bubbling region and jetting region can be easily seen. It can also be observed that some particles are entrained into the jet from the emulsion phase. Entrained particles can also be seen in the bubbles, jets, and jet ends but, due to the oscillation of jet boundary, particles cannot be observed precisely.
3.2. Horizontal jet penetration length 3.2.1. Effect of gas velocity through nozzle The effect of gas velocity through nozzle on the jet penetration length is shown in Fig. 5. The jet penetration length increases as gas velocity through nozzle increases. The effect of superficial velocity on the jet penetration length at various gas velocity through nozzle is shown in Fig. 6. Above a certain value of superficial velocity, the jet penetration curve peaks. The maximum points of those curves were found when the ratio of UIU,,,r was in the range 0.6-1. To ensure that the experiment was working under the maximum jet penetration length, the ratio of UIU~r equal to I was chosen. This ratio for UI Umf gave the maximum jet penetration length for all gas velocities used in this study.
3.2.2. Effect of nozzle diameter and shape
Fig. 4. Transitionslates at the horii~onlalnozzle for differentexperimental conditions. (a) Dr =0.214 ram, D,,= 7.5 mm, H, =200 ram, U, = 123 m/s; (h) Dp=0.214 ram. D,~=7.5 ram+H,= 200 mm, U,+=12 m/s.
For a given gas velocity through the nozzle, tile jet pene..2ration length increases with nozzle diameter, as shown in Fig. 7. The relationship between the different shapes of nozzles are listed ia Table 2. Fig. 8 shows that the effect of the shape of nozzle on jet penetration length is minimal. Although ntJw.erous investigators have considered the combinations of nozzle diameter and horizontal penetration length as a dimensioaless factor LID,,, we can state that the jet penetration length increases with the noTzle diameter and jet momentum, within a certain range of gas ~,elocity through the nozzle, in this study. 200
200
UI o o a
.D. ~,%. 8.0 * 7.6 O 9.0
r,
...-..
t
.~tuo,.3
o~
34
tSO
150
+
,~,ll-I 19 2?
g,.
,. uo
go
(m/s)
.0
,~,.~IOD
I
'+.+-
,+o
o.~,
o.',,
U/Umf
,.:'+
,.,,
Fig. 5, Effect o f gas velocity through the nozzle on the jet penetratson length
Fig. 6. Effect of superficial velocity on Ihc jet penetration length at various gas velocities through the nozzle. Dp=0.545 ram, H+~ 100 ram, D o =
with variousnozzle diameters.Dr =0.545 ram, H ~ 100 mm, U/U,,,= I.
7.5 mm.
75
C.S. Chyang et aL / Powder Technology 90 (1997) 71-77
L60
200 I
] Uo I
" 34 &
o
,,..-t 27
150
.._..100
~.'o
.__~
._
,.'o
1
~
o.o
o.o
lo.o
Do ( m m ) Fig~ 7. Effect of nozzle ~iameter on the jet penetration length. Dp = 0.545 nun, H,= 100 mm, UIUm~= I.
Dp ( m m ) Fig. 9. Effectof particlediameteron thejet penetrationlength.Do= 7.5 ram, H, = I00 nun. UI U,,r= i,
Table 2 Noa,zleshapes (ram) Circular (i.d.) Square Rectangular
120
1~ , i n o 40 o 70 ~' 100 150 * 200
o
3.0; 6.0; 7.5; 9.0 5.0 × 50 3.I × 9.0:9.0 × 3. !
0
o
8 o
.=.1 r~
200[ o°OaH°zZleo.0,3.15.3*5.36"O3.1,9.0dim~nai°nn'mm
~-~o~.i
Q
/~,t~,
"..--.
t3
O00
500 - - - - - -
i ~0
I .....
30
40
the
nozzle,detectedat differentstaticbed heights.Dp= 0345 mm./)0= 7.5 nun. UIO,,r= I.
J
~ 40
~
eo (m/s)
Fig, 10, Variation of jet penetration length with gas velocity through
//
/
I 10
1
..~
i 60
......
80
13o (m/s) Fig. 8, Effectof gas velocitythroughthe nozzleon thejet penetrationlength with variousnozzledimensions.Dp= 0.545 ram.H,= 100 ram. UIUmf-" 1.
3.2.3. Effect o/particle diameter Fig. 9 shows that jet penetration length decreases with particle diameter. However, there will be a significant difference in the jet penetration length when operating with particles with diameter larger than 0.775 ram. In Fig. 9, the significant difference in the jet penetration length also shows that different groups (Groups B and D) have a different effect on the jet momentum. 3.2.4. Effect of static bed height The jet penetration length was significantly affected by gas velocity through the nozzle when the bed height was lower than 100 mm. The experimental data, shown in Fig. 10, indi-
cates that the jet penetration length decreases with static bed height. However, ira very shallow bed is used, the gas plume pushes upwards to the surface of the very quickly. A suitable bed height needs to be chosen in order to get a fully-developed jet.
3.3. Comparison o f experimental data with correlations reported Experimental conditions for the investigation of horizontal jets are listed in Table 1. Numerous correlations have been proposed for predicting the jet penetration leng',k in ~he fiui-. dized bed. The typical correlations which can be used for predicting the penetration length of horizontal jets are listed in Table 3. There are various definitions of jet penetration length. The maximum jet penetration length was adopted by Zenz [ 2] and Merry [4]. Moreover~ Shakhova [ 3t calculated the jet length as the mean value of the maximum and minimum penetration length. In this study, the maximum jet penetration length was adopted as the jet length in all experiments.
76
CS. Chyang et aL /Powder Technology 90 (1997) 71-77
Table 3 Correlationsforjet penetrationlength
V,, ]
Shakhova [3]
L,~,.+__~..=781-:
Zenz 12]
0.044-'~o'*+ 1.57= 0.5 Iog(p~U~)
2do
shows the flow regimes of gas discharge modes at a single horizontal nozzle by using the data obtained with Group B particles. From the experimental data shown in Fig. 13, the boundary line between the bubbling regime and transitional state can be expressed by Eq. (2).
" "LPp "q g d o) ''2" J
2
04
Fr* =2.25 × 10s
-'+ <+r 1 d,_+'+5=+- L(t-e)Or,gd~_l ~ p /
Merry [4]
0.2
02
~d,,l
(2)
The boundary line between the transitional state and jetting regime can be expressed by Eq. (3). F r * = l . 7 0 × 10S(D ~-
0
150 o 0 o
°°
(3)
From the data shown in Fig. 13, one can find that the gas discharge mode changes from bubbling regime to jetting regime as the Froude number increases. In other words, the jetting types can be observed more often with a large Froude number and large nozzle diameter. Fig. 14 shows that the transitional state between the bubbling regime and jetting regime is not so obvious for the
0
0
,~100
2.94)
-2"2
200
1
z'o
+o
+'o
.'o
,oo
150
Uo ( m / s ) Fig. ! 1. Comparise.aof numerica/pedictionswith experimentaldata from jet penetrationlength; (!) Shakhova [3]; (2) Zenz [2]; (3) Merry [41. Experimental conditions: Dp=0.545 ram, t1,= 100 mm. Do=6.0 mm, Ut U,~f= I.
m
0 0
,-3 0
O
The compeaison of jet penetration lengths was calculated by the using various correlations listed in Table 3. From the experimental data obtained in this study, it can be shown that all the correlations underestimate the jet penetration length. Representative results are shown in Fig. 1 ! for small particles (Group B) and Fig. 12 for large particles (Group D). The differences can be explained by the various definitions of jet penetration length and the experimental conditions, it should be noted that the two correlations by Zenz and Merry could not be used in this study to predict the jet penetration length when gas velocities through the nozzle were less than 35 ms- t.
0 o
O0
O~
Fr* = 2p+02 U,.p 31~D~g
2
~"
i
ZO
,_
•
40
Uo (m/s)
0 ~I
i
l
, 80
Eq{3)
/
0 0 0 0~0~"~
0 0 0 090~J
1 o o Qj~rma8
P..
,°.
°
°°07 aO
I0 '
lO *
....
I
0 O0
•, i O
This represents the balance between the inertial force of gas at the nozzle and the gravity force of particles. Fig. 13
i
U / U m r = I..
~q(21
(l)
,1 k 6u
Fig. 12. Comparisonof numericalpedietionswith experimentaldata from jet penetrationlength; (1) Shakhova[31; 12) Zenz [2]; (3) Merry 14]+ Experimental conditions: Dp=0.920 ram, H,: 100 nun, D,=7.5 ram,
3.4. Flow regimes o f the gas discharge mode
In order to integrate the effects on the gas discharge modes of particle size, nozzle diameter and gas velocity through nozzle, two pmameters were used to map the flow regimes. They are the ratio of nozzle diameter to particle size and a modified Froude number.The modified Froude number was defined as
0
0
,~,~lO0
~D
IO •
!
l0 '
FF° Fig. 13. Flow regimesof the gas discharge mode for horizontal nozzles, Group B particles:D~,= 0.214 and 0.545 mm.
c.s. Chyang et al. / Powder Technology 90 (1997) 71-77
to group the bubbling, jetting and transition zone types. The phase diagram of bubbling, jetting, and transition zone can be distinguished easily. In this phase diagram, the bubbling type is observed at low Froude numbers and the jetting type at high Froude numbers. From the experimental data obtained in this study, it can be stated that the dominate factor for gas discharge mode from a horizontal nozzle is the inertial force of the gas flow through the nozzle. The gas discharge mode will be changed from bubble to jet while the gas velocity through nozzle (inertia force/moment) is increased. Between these two regions, the transition state is observed.
q(41
,~.
'~
A
~
~
&
o/
/L~
o
o
ci
o
cI ~ O r ' l n O
[] o o o o o c o
cl LI
onr~Q
,1..... 2 _ _ / ~ ? .... o o o . . . . . . . . . . i0 j
10 '
A
20 "
Fr" Fig. 14. Flow regimes of the gas discharge mode for horizontal nozzles. Group D particles:Dp-- 0.775 and 0.920 mm. Group D particles. The boundary line between these two regimes can be represented by Eq. (4). F r * = l . 4 8 X 10~-~p-1.79)
77
(4)
It is found that the jetting phenomenon become unstable while DolDp increases. For a certain value of Do, a decrease in Dp will lead to bubble formation. However, when particle size and Froude number increase, jet formation can be observed more readily.
5. List ofsymbols Do
Fr*
H~ L U
nozzle diameter (ram) ~u'ticle di~,¢-:-:: :: ~ r ~ ) modified l:ro~.,~c ,.~-~-.~.:, static bed height (ram) jet penetration length (ram) primary fluidizating velocity ( m s - ~) minimum fluidizating velocity ( m s - ~) gas velocity through the nozzle ( m s - x)
Greek letters
pg pp p~ •
gas density (kg m - j ) solid particle density (kg m -3) gas density at nozzle (kg m -3) bed voidage
4. Conclusions
In this study, the gas discharge modes at a single horizontal nozzle in a two-dimensional fluidized bed were investigated by visual observation. It is found that a maximum value of the jet penetration length occurs at the superficial velocity around 0.6--1.0 times minimum fluidization velocity. The length of jet penetration increases with gas velocity through the nozzle and nozzle diameter, and decreases with particle diameter and static bed height. I~y using a modified Froude number as the horizontal axis and DolDp as the vertical axis, a phase diagram can be drawn
References
[ I | V.E. Kozinand A.P. Ba~kp_knv.lnL Chem. Eng.. 8 (1968) 257. [2] F.A. Zcnz, ln~t~ Chem. Eng. Syrup. Set., 30 (1968) 136. [3] N.A. Shakhova,lnzh. Fiz. ZIt. 14 (1968) 61. 14] J.M.D. MeJry,Trans. Inst. Chert I~ng., 49(1971) 189. [5] C. Xuereb, C.C. Laguerie and T. Baron, Powder Technol..64. ( 1991 ) 271[6l C. Xuereb, C.C. Laguerie and T. Baron, Powder Technol..67 ( 1991 ) 43. [7] L. Chen -andH. Weinst¢in,A/C/E,/., 12 (1993) 1901. [8} C.C. Huang and C.S. Ch~.mg, Jpn. J. Chem. Eng., 5 ( 1991 ) 633.