I n d . E n g . Chem. Res. 1988,27, 1277-1281 Gmehling, J.; Onken, U. Vapour-Liquid Equilibrium Data Collection; Dechema Chemistry Data Series; Dechema: Frankfurt, 1979. Gmehling, J.; Rasmussen, P.; Fredenslund, Aa. Znd. Eng. Chem. Process Des. Deu. 1982,21, 118. Kikic, I.; Alessi, P.; Rasmussen, P.; Fredenslund, Aa. Can. J. Chem. Eng. 1980, 58, 253. Kojima, K.; Tochigi, K. Prediction of Vapor-Liquid Equilibria by the ASOG Method; Kodansha-Elsevier: Tokyo, 1979. Macedo, E.; Weidlich, U.; Gmehling, J.; Rasmussen, P. Znd. Eng. Chem. Process Des. Dev. 1983, 22, 676. Magnussen, T.; Rasmussen, P.; Fredenslund, Aa. Znd. Eng. Chem. Process Des. Dev. 1981, 20, 331.
1277
Perry, R.; Green, D. Chemical Engineers Handbook, 6th ed.; McGraw-Hill: New York, 1984. Skjold-Jorrgensen, S.; Kolbe, B.; Gmehling, J.; Rasmussen, P. Znd. Eng. Chem. Process Des. Deu. 1979, 18, 714. Smensen, J.; Arlt, W. Liquid-Liquid Equilibrium Data Collection; Dechema Data Series; Dechema: Frankfurt, 1979; Vol V, Part 1. Zarkarian, J.; Anderson, F.; Boyd, J.; Pransnitz, J. Ind. Eng. Chem. Process Des. Deu. 1979, 18, 657.
Received f o r review June 9, 1986 Accepted December 15, 1987
Effects of a High-Velocity Jet on Fundamental Particle-Separation Characteristics Ji-Yu Zhang,+Bi-Jiang Zhang,?and Douglas C. Chitester*' I n s t i t u t e of Coal Chemistry, Academia Sinica, P.O. Box 165, T a i y u a n , S h a n x i , China, and Department of Energy, Pittsburgh E n e r g y Technology Center, P.O. Box 10940, Pittsburgh, Pennsylvania 15236
T h e high-velocity jet region of a jetted fluidized bed plays an important role in momentum, mass, and heat transfer in the bed. A two-dimensional, fluidized-bed cold model equipped with a highvelocity jet was constructed to study the role of this high-velocity region. Glass beads were used t o simulate ash agglomerates, and millet and catalyst powder were used t o simulate coal and char, respectively. The effect on particle separation of various operating parameters including jet geometry, jet velocity, and particle size and density was studied. From these experiments, some important information on particle separation was obtained. An optimal, jet-zone, geometric configuration was identified for efficient particle separation. A graphical representation of the particle-separation process for heterogeneous solids was developed. Also, empirical equations were developed to calculate initial-separation velocity and critical-separation velocity.
I. Introduction
Table I. Physical Properties of Experimental Materials mean particle bulk particle size particle density, density, material range, mm size, mm g/cm3 g/cm3 2.496 1.644 glass beads 1 1.60-2.50 2.050 2.930 2.481 1.604 glass beads 2 2.50-3.36 millet 0.90-1.60 1.250 1.352 0.824 0.497 1.978 1.168 catalyst 0.16-0.90
A jetted fluidized bed is a special type of fluidized bed that employs a vertical, high-velocityjet for spouting. This type of bed has been used successfully in some ash-agglomerating gasifiers, such as those developed by U-Gas (Sandstrom et al., 1977) and Westinghouse (Hartman et al., 1978). The characteristics of the high-velocity jet play an important role in momentum, mass, and heat transfer and in reaction rate (Zhang, 1982). To gain a clear understanding of the effects of the jet characteristics, particle-separation experiments were conducted in a cold model.
11. Experimental Section A. Equipment and Materials. Reference in this report to any specific commercial product, process, or service is to facilitate understanding and does not necessarily imply its endorsement or favoring by the United States Department of Energy. The experiments were conducted using a two-dimensionalfluidized bed that is 120 cm high, 40 cm wide, and 4 cm deep. Figure 1 shows the experimental apparatus. The diameter of the Venturi tube (Do) is 25.4 mm. The effective spread angle (a)of the tube can be varied from 12' to 45O, and the angle of the conical distributor (p) can be varied from 25" to 60'. The length between the Venturi and the conical distributor (Lo)can be varied from 0 to 160 mm. The conical distributor contains 0.8-mm-diameterholes, and the total hole fraction is approximately 0.005. There are pressure taps along the entire height of the bed at 80-mm intervals. These taps +Institute of Coal Chemistry. *Pittsburgh Energy Technology Center.
This article n o t subject t o U.S. Copyright.
are valved so that any one tap can be selected for measurement of differential pressure between it and either the primary or secondary collector. These collectors are beneath the Venturi tube. In the experiments, glass beads were used to simulate ash agglomerates. Millet particles and catalyst powder were used to simulate coal particles and fine coal ash, respectively. Some physical properties of these experimental materials are shown in Table I. B. Procedure. At the beginning of an experiment, a quantity of air equivalent to one-fourth to one-half of the total fluidizing air was injected through the Venturi tube. The balance of the air was injected through the conical distributor. The solid materials were then fed individually into the bed. The superficial gas velocity (VfJ at this point was sufficient to prevent separation of the solids. After allowing the bed to come to a steady-state condition, the amount of gas injected through the Venturi tube was gradually decreased. At lower jet velocities ( UJ), particle separation was observed. The separation process was studied at various jet velocities to determine the discharge rate, the size distribution of the separated particles, and the separation efficiency. Also, pressure drops were measured across different sections of the bed to compare bed densities. Experiments were conducted in beds with several different static heights (Ho)using nozzles with
Published 1988 by t h e American Chemical Society
1278 Ind. Eng. Chem. Res., Vol. 27, No. 7, 1988 B Well fluidized bed
A Poorly fluidized bed .=IS', 8 = 2 5 O , I = S2
I l/i
TiME
-
Figure 2. Typical pressure-fluctuation curves.
Air
Figure 1. Experimental apparatus. Table 11. Values of Exwrimental Variables variable values um, m / s 1.04-1.12 12.0, 15.0, 18.0, 23.5, 30.0, 45.0 a,deg 25.0, 30.0, 38.0, 45.0, 53.0, 60.0 8, deg 330, 400, 450, 520, 600 Ho,mm 0, 39, 69, 130, 160 Lo, mm UJ, m/s 10.0-20.0 cb, %
c,,
%
10.0-30.0 0.0-100.00
several different geometries. The weight percent of all glass beads in the bed [i.e., particles to be separated (C,)] and the weight percent of the coarser glass beads in the particles to be separated [Le., effective particles (C,)] were varied in the experiments. Table I1 shows the values used for all of the variables in this experiment. 111. Observations a n d Analyses
A. Flow Characteristics. When gas was injected through the Venturi a t a high velocity into the fluidized bed, a torch-shaped jet and a jet zone were formed. The jet quickly collapsed and formed a large bubble, which became larger as it rose swiftly up the bed and burst through the surface. The frequent formation of large bubbles from the collapse of the jet and a strong effect on the quality of fluidization and, thus, on the gas-solid contacting in the bed. The quality of fluidization diminished with increased initial bed height. Also, the movement of the bed surface was well represented by fluctuations in total bed pressure drop. Two typical pressure drop fluctuation curves are shown in Figure 2. Certain characteristics of these curves, such as average pressure drop fluctuation fluctuation frequency (f), and average pressure drop (AP), are related to the quality of fluidization. At higher values of AP,, fluidization is less uniform, and at higher values of A.P and f , fluidization is more uniform. If I represents the nonuniformity index, with larger values of I indicating less uniformity or poorer fluidization quality, then the following equation can be formulated (Morse, 1951): 100APf I=(1)
(e,),
2APf
Generally, the value of I in these experiments was relatively high. This is reasonable because the high-velocity jet causes spouting in the bed and thus less uniformity. Also, in these experiments, the density of the bottom portion of the bed was consistently higher than the density
4
4
4
"IJ
"SJ
"CJ
Initial
Stable
Critical
Figure 3. Graphical representation of the particle-separation process.
of the top portion. This was because of the quick rising and coalescence of bubbles, which became very large in the top portion of the bed. This observation agrees with that of Behie et al. (1970). B. Particle Separation. The characteristics of the high-velocity jet have a major effect on the fluidized-bed particle-separation process. When the jet velocity ( U J )is less than the terminal velocity (Ut) for a particle of a particular size, the particles that are larger than this size will be discharged through the nozzle. If the jet velocity is gradually decreased from a velocity at which the entire bed is suspended, particle discharge from the nozzle occurs in three distinct stages. These are illustrated in Figure 3. A t the initial-separation velocity (UIJ),particle discharge ,begins. At the stable-separation velocity (UsJ), particles are discharged in a stable, pulsating manner. At the critical-separation velocity ( UcJ), particles are discharged in an unstable, irregular manner. When UJ is between U, and UCJ, the separation efficiency (q) is high, and the average discharge rate is essentially constant. For this reason, the jet velocity range between VI, and UCJ is defined as the stable-separation-velocityrange (AU,). If the jet velocity is lower than UcJ, slugging will occur and diminish separation efficiency. Also, ash sintering will occur a t this condition. For efficient, trouble-free operation of an ash-agglomerating gasifier, V, must be lower than VI, and higher than UcJ. C. Optimum Nozzle Geometry. The optimum nozzle geometry is one that allows highly efficient, stable particle separation to occur while maintaining the bed in a wellfluidized condition. The geometric configuration parameter, C,, which is defined by
c, = CMdCMv I ~
can be used to determine the optimum configuration. The maximum discharge coefficient, C M d , is defined as 100 times the discharge rate at UJ = UCj(Gd), divided by the discharge rate due to gravity (Gdo) (Zhang et al., 1980), as shown in the following equation:
(3) The stable-separation-velocity coefficient, CMv,is defined as the difference between the initial-separation velocity
Ind. Eng. Chem. Res., Vol. 27, No. 7, 1988 1279
E
16
-
15
-
L 0 Ho 4 0 0 m m A Ho=450mm 0 Ho=520mm V Ho=600mm
xH 14 13
Figure 4. Graphical illustration of the effects of a and p on C,. 15
I
I
I
I
I
I
I
1.4
P E
14
-
13
-
i,
3) 12
0 Ho=330mm
-
A H, = 4 5 0 mm
I. ,I
L,
VO
1
I
/
14
c,
Figure 6. Graphical illustration of the effects of Hoand C b on UIJ,
V H, = 6 0 0 m m
mm
Figure 5. Graphical illustration of the effect of Loon I , W , and C,.
and the critical-separation velocity, divided by the critical-separation velocity, as shown in the following equation:
(4) Therefore, CMd represents the separability with a particular nozzle diameter, and CMvrepresents the range of jet velocities that assure stable separation. High values of Cs indicate a well-fluidized bed, a high separation efficiency, a wide stable-separation range, and a high discharge rate. Figures 4 and 5 show the effect of the important geometric parameters on Cs as determined in this study. These figures indicate that the optimum values for Lo,CY, and P are 0, 15-18', and 35-45O, respectively. D. Other Important Separation Parameters. In addition to the geometric parameters, several other parameters play important roles in the separation process. These include the physical properties of the particles, the static bed height (Elo),the superficial gas velocity (Uf),and the concentration of effective particles in the bed (Cbe). The concentration of effective particles in the bed is equal to the concentration of particles in the bed to be separated (cb) multiplied by the fraction of these particles that are easily and effectively separated (Ce). Particles approximately 3 mm in diameter and larger are easily separated (Sandstrom et al., 1977; Hartman et al., 1978; Zhang, 1976, 1982; Jequier et al., 1960; Grace and Matsen, 1980; Zhang et al., l983,1984,1985a,b). Equation 5 illustrates the above relationship: Cbe = c b c e (5) Figures 6 and 7 show the effects of Hoand Cbe on UIJ and UCJ,respectively. Note that with a constant Cbe, and UcJ increase with an increase in Ho.Also, with a constant Ho, UIJ and UCJincrease with an increase in Cbe. Based on the data obtained in this experiment, the following empirical equations were derived:
UIJ= 0.9158Ut exp(0.3010Ho+ 8.0924Cbe)10-3 (6) UcJ = 0.7828Ut exp(0.2274Ho+ 10.2122Cb,)10-3 (7)
12
0
C, ;30% !
I
0
5
C,
;65 %
I
I
I
I
10
15
20
25
30
Cber %
I
0
I
25
I
50
c,,
I
75
I
100
%
Figure 8. Separation diagram.
In eq 6 and 7, Ut is the terminal velocity of the largest particle to be separated (Zhang, 1976). The maximum deviation between experimental values and those calculated by using eq 6 and 7 is less than 5%. The equations are accurate over a range of Cb from 10% to 30% and at any velocity that maintains a completely fluidized bed. When Cb is lower than lo%, separation is not accomplished, and when Cb is higher than 30%, slugging in the jet zone occurs and subsequently retards the separation process. When Cb is higher than 40%, slugging becomes dominant. This causes sintering in the gasifier (Sandstrom et al., 1977; Hartman et al., 1978; Zhang, 1976, 1982; Jequier et al., 1960; Grace and Matsen, 1980; Zhang et al., 1983, 1984, 1985a,b). E. Separation Diagram. Figure 8 presents a diagram of the particle separation process for heterogeneous solid mixtures in a fluidized bed containing a high-velocity jet (Zhang et al., 1985a). There are four zones in the diagram.
1280 Ind. Eng. Chem. Res., Vol. 27, No. 7, 1988 20r.
'
I
I
I
'
I
I
I
I
I
1
.-E E
\
m
$
00.3
0.5
0.7
0.9
1.1
1.3
1.5
1.7
AUIs,m/s Figure 9. Relationship between Wand AU1, when A& is less than 0
A%.
20
40
60
00
100
usc % Figure 11. Graphical illustration of the effect of USc on 'le. I
this occurs is called the discharge-rate change point of the initial stable velocity difference (AU,,). The value of AUIs at which this point occurs is related to H, as shown in the following equation: AUs, = 2.2092 - 0.001422Ho (9)
.-c
E
\
cs,
i
At values of AUIs equal to or below AUs,, the discharge rate can be calculated by using W = 9.333AUIs - 2.799 (10) Wz9.33 AUIs
'1.3
I5
1.7
1.9
2.1
2.3
- 2.799 2.5
2.7
AuIs,m/s Figure 10. Relationship between Wand AU,, when A& is greater than AUsr.
Zone I is the normal-fluidization zone, where particle separation does not occur. Zone I1 is the stable-separation zone. I t is subdivided into two area, zone I1 and zone II*. Zone I1 is the perfect-operation area, where separation efficiency is high and separation is stable over a wide range of jet velocities. Zone 11* is the nonperfect-operation area, where high-separation efficiency can be maintained over a much narrower range of jet velocities than in the perfect-separation zone and the separation process may not be as stable. Zone I11 is the substable-separation zone, where pulsating discharge of solids occurs, and the separation efficiency is decreased. Zone IV is the irregularseparation zone, where the particle separation process becomes very unstable and separation efficiency is significantly reduced. The separation diagram is a very useful tool in the identification of desirable operating conditions in a given gasifier. F. Stable-DischargeRate. The stable-discharge rate ( W )is defined as the weight of particles separated per unit time when operating in the stable-separation zone. Since agglomerates are constantly forming and increasing in size in a gasifier, they exist in a distribution of sizes. This distribution has an effect on the separation process. In this experiment, at a constant UJ, W increased with an increase in C b and also with an increase in The effect of C b on was greater than the effect of Ho. The initial stable velocity difference (AUls) is defined by
w
In eq 8, Uj must be in the stable-separation range. Figures 9 and 10 show the relationship between W and AUls as plots of the data obtained in this work. Note that, a t a certain value of AUls, W suddenly increases a t a much greater rate with an increase in AUls. The point at which
At values of AUls above AUS,, the discharge rate can be calculated by using W = 122AUls + 0.16Ho - 251.7 (11)
G . Effective Separation Efficiency. Because agglomerates that are approximately 3 mm or larger are mostly ash and have a small carbon content (5%), it is most desirable to remove these particles from the gasifier. The weight percent of these particles in the total discharge from the gasifier represents the effective separation efficiency (7,). Figure 11 shows the effect of the ratio of velocity differences (Us,) on qe a t different values of C,. The following equation defines Us,:
When C, is less than 50%, qe decreases significantly with a decrease in Us,. When C, is more than 50%, qe changes little with changes in Usc, and the value of q, is greater than 85% in the range of values of US, from 25% to 85% regardless of the values of Ho and C,. The following equation for calculating qe when C, is 50% or higher and Cb is 15% or higher was derived from the data obtained in this work and is applicable a t all values of Ho: 8, =
79.48
+ 0.571Usc - 4.805 X 10-3Usc2
(13)
The average deviation between experimental values of qe and values calculated by using eq 13 is less than 1.7%. Figure 12 shows the q, for all of the data obtained in this work. The only values of qe that are significantly low are for conditions with C, lower than 40% and Us, lower than 20%. This is in the substable-separation zone. In the perfect-operation zone, qe is usually higher than 80%. In actual gasifiers, more complete gasification occurs at higher values of qe, Thus, this is an important design parameter. IV. Conclusions A high-velocity jet greatly affects the hydrodynamics of a fluidized bed. It results in large bubbles and accentuated fluctuation of the bed pressure drop. It also creates two
Ind. Eng. Chem. Res., Vol. 27, No. 7 , 1988 1281
ae
70 -
o
0
0
60-
50 40-
0 Ce = 10% Ce = 20% 0 C e = 30% A C e = 40% ‘ I C e : 50% Ce = 60% Ce = 70% V Ce 85% A Ce = 100% 0
-
-
C, = concentration of effective particles (>3 mm) in the bed to be separated, wt % CMd = maximum discharge coefficient at the critical separation velocity CMv = stable-separation velocity coefficient Cs = configuration flow parameter, 1 / s clp = particle diameter, mm d, = average particle diameter, mm f = frequency of fluctuation in bed pressure drop, l / s Ho = static bed height, mm I = nonuniformity index, s Lo = length between Venturi and conical distributor, mm APf = average bed pressure drop fluctuation, mmHzO = average bed pressure drop, mmHzO U,, = critical separation velocity, m/s U, = supercritical gas velocity, m/s UIJ = initial separation velocity, m/s UJ = j e t velocity, m/s Us, = ratio of velocity differences Us, = stable-separation velocity, m/s Ut = terminal velocity of largest particles to be separated, m/s AUIS = initial-stable-velocity difference, m/s AUsj = stable-separation-velocity range, m/s AUs, = initial-stable-velocity difference at the point of increased stable discharge rate, m/s W = stable discharge rate, g/min Greek Symbols a = effective spread angle of Venturi tube, deg /3 = cone angle of distributor, deg p b = bulk density, g/cm3 pp = particle density, g/cm3 7 = separation efficiency, w t % 7, = effective separation efficiency, wt %
Literature Cited Behie, L. A.; Bergougnou, M. A,; Baker, C. G. Can. J. Chem. Eng. 1970, 48(2), 158. Grace, J. R.; Matsen, J. M. Fluidization; Plenum: New York and London, 1980; p 429. Hartman, H. F.; Belk, J. P.; Reagan, D. E. “Low BTU Coal Gasification Processes”. In Selected Process Descriptions; Oak Ridge Nationril Laboratory: Oak Ridge, TN, 1978; Vol. 2, A-139-151. Jequier, L.; Longchambon, L.; van de Putte, G. J . Znst. Fuel 1960, 33, 584. Morse, R. D. Chem. Eng. Prog. 1951, 47, 199. Sandstrom, W. A,; Rehmat, A. G.; Bair, W. G. “Coal Processing Technology”. Chem. Eng. Prog. 1977,3, 180. Zhang, J.-Y. Chem. Eng. (China) 1976, 1 , 18. Zhang, J.-Y. Huagong Jixie 1982, 6, 39. Zhang, J.-Y.; Mao, X.-B.; Chen, R.-Y.; Wang, S.-X.; Zhang, B.-J. Ranliao Huazue Xuebao 1983,4, 44. Zhang, J.-Y.; Mao, X.-B.; Zhang, B.-J. Huagong Xuebao 1984,3, 1. Zhang, J.-Y.; Yan, Y.-M.; Mao, X.-B.; Zhang, B.-J. Huagong Jixie 1985a, 12, 4, 26. Zhang, J.-Y.; Yan, Y.-M.; Mao, X.-B.; Zhang, B.-J. Huagong Jixie 1985, 12, 5 , 30. Zhang, J.-Y.; Yang, G.-L.; Yang, S.-P.;Shu, S.-G.; Li, H.-Z. Huagong Xuebao 1980, 3, 229. Receiued for review September 8, 1986 Accepted January 12, 1988
