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Solid mass fluxes in circulating fluidized beds
Powder Technology, 70 (1992) 197-205
197
Solid mass fluxes in circulating fluidized beds Blaine
Herb*,
Suisheng
Dou**,
Kemal
Tuzla
and John
C. Chen
Institute for Thermo-Fluid Engineering and Science, Lehigh University, Bethlehem, PA (USA)
Abstract
Experimental measurements of local solid mass flux were made in the riser component of two different circulating fluidized beds (CFBs). Tests were conducted in both a lab-scale riser measuring 5 cm i.d. and 2.7 m in height and a pilot-scale riser of 15 cm i.d. and 10.8 m height. FCC catalyst particles with a Sauter mean diameter of 68 pm and sand particles with mean diameters of 125 pm and 276 pm were used in the tests. The results showed the local time-average solid mass flux to vary with radial position, elevation, total solid mass flow, superficial gas velocity and mean particle diameter. Based on the solid mass flux and local solid concentration measurements, and on video studies, a physical description of the solid flow phenomena in CFBs is presented.
Introduction
Many industrial processes have evolved in which solid particles are contacted with a gas to achieve chemical or physical change within the gas or solid phase. Examples of these processes include solid particle drying, coal combustion, fluid catalytic reactions and calcining reactions. Depending on the relative velocity between the gas and the solid, gas and particle properties, and equipment size and geometry, several different vertical gas-solid flow regimes result. At superficial gas velocities above the particle terminal velocity and at relatively high solid mass fluxes, an entrained suspension of solids forms which is known as a circulating fluidized bed (CFB). Knowledge of the solids distribution and flow behavior in the CFB riser is the key to successful design and operation of CFB systems. The solids distribution governs the pressure drop occurring along the CFB riser and is directly related to the mean solids residence time within the riser. The solid concentration determines the gas-solid interfacial area per unit volume of the mixture, which is expected to directly affect gas-solid reaction and mass transfer rates. Furthermore, heat transfer coefficients measured at the riser wall and on submerged surfaces show strong dependence on the solid concentrations in the vicinity of the heat transfer surface [l]. Solid mass flux measurements are important to describing the solid flow behavior in CFBs. In addition, *Presently **Presently ceton, NJ.
at Air Products and Chemicals Inc., Allentown, PA. at Mobil Research and Development Corp., Prin-
erosion rates of exposed surfaces are expected to tie closely with the solid flow behaviour along these surfaces. Few measurements of solid mass fluxes in CFBs have been reported in the literature. Van Breugal et al. [2], using an isokinetic sampling probe in a 30 cm riser operating under dense-phase flow conditions, reported a parabolic solid flux profile with downward solid flow along the riser wall. Using a non-isokinetic sampling probe, Bier1 et al. [3] also measured strong radial variations in the solid mass flux. Combining these measurements with information on radial variations of time-average solid concentration obtained from X-ray measurements, they concluded that the CFB consisted of a dilute, upflowing solids core surrounded by a dense, downflowing solids annulus. More recently, Monceaux et al. [4] measured particle mass fluxes in the fully-developed region of the CFB in a 15 cm i.d. riser using fine catalyst particles. For area-average solid volume fractions below 0.05, normalized solid flux profiles at a given gas velocity were found to be independent of solids mass flux. At higher solid concentrations, the normalized solid flux profiles were no longer independent of total solid mass flow. The experimental findings’ supported their theoretical analysis which predicted the existence of similar solid flux profiles below some critical solid concentration. Rhodes et al. [5], using non-isokinetic sampling probes, measured solid mass flux at three elevations within the CFB to show the axial development of the solids flow profile. A core-annular flow model was proposed in which the solids flow and distribution was described by a net flow from the upflowing (uniform) core region to a (uniformly) downflowing annulus.
0 1992 - Elsevier
Sequoia. All rights reserved
198
The objective of this paper is to present a new collection of time-average solid mass flux measurements covering a range of operating conditions of importance to industrial CFB applications, and to provide insight into the solid flow behavior of CFB systems.
Experimental Experiments were carried out in two different CFB systems: a lab-scale CFB with a 5 cm i.d. riser, 2.7 m height, and a pilot-scale CFB system, shown in Fig. 1, having a cylindrical riser with an i.d. of 15 cm and a height of 10.8 m. As Fig. 1 shows, primary air and secondary air were introduced below and above a sintered metal distributor, respectively. Under normal test conditions, a dense bed was established between the distributor and the secondary air injection elevation. Above the secondary air injection point, the circulating fluidized bed was established and existed until the smooth right-angle exit to the cyclone. Axial positions within the riser were measured relative to the secondary air inlet location, above which the total air flow was established. Particles captured by the cyclone were returned through a downcomer leg which provided a particle storage hopper and standpipe combination for the measurement of solid recirculation rate. The return particles were horizontally transported to the CFB riser by means of secondary air flow. Thirty-three pressure taps were installed along the height of the riser. Each pressure tap line was purged at a constant rate to prevent particles from entering and blocking the pressure tap lines. The pressure tap line were manifolded to six differential pressure transducers to allow the pressure drop to be measured simultaneously across six segments of the riser. In a typical experiment, the desired superficial velocity was set by adjustment of both primary and secondary
CYCLONE .
aP TRANSDUCER ’
STANDPIPE
\ CFB RISER
(15
INSTRUhiENTATlON PARTICLE
STORAGE
CM ID)
Sampling probe and measurement method
To measure the axial component of the time-average solid mass flux locally within the CFB, a sampling apparatus and technique were developed. The sampling probe was designed to mount in the instrumentation ports and to traverse the riser i.d. Figure 2 depicts the design of the sampling probe tip. To ensure the accuracy of the upward solid flux measurements, the probe was designed for isokinetic sampling. With the probe tip facing upstream, isokinetic sampling requires that the gas velocity aspirated through the probe tip equals the local gas velocity that would exist at this point if the probe were not present. If the aspiration velocity is below the isokinetic velocity, the gas flow is deflected around the probe tip along with fine particles carried by the gas, which causes the solid sample collected to be too small. If the aspiration velocity exceeds the isokinetic velocity, the collected solid sample will be too large, since excess gas and solids are drawn into the probe tip. Under isokinetic sampling conditions, the static pressures measured inside and outside the probe tip are expected to be equal. Thus, inside and outside pressure taps made of 1.6 mm stainless tubing
PORTS VESSEL
T
PRESSURETAPS RETURN
air flows, and particles were charged to the riser at the desired rate by adjusting the solid feed valve. The solid recirculation rate (total solid mass flux in the riser) was measured by temporarily closing the butterfly valve above the particle storage vessel and measuring the time to accumulate a packed bed volume of 0.0032 m3 in the transparent standpipe. A separate measurement of the apparent packed bed density of the test particles was used to calculate the mass of solids collected. Due to the surge volume provided by the particle storage vessel, the normal feed rate of returned solids continued without interruption during this measurement procedure. The results presented in this paper were obtained for 68 pm FCC particles, and 125 pm and 276 pm sand particles with corresponding terminal velocities in ambient air of 0.27 m s-l, 0.74 m s-’ and 1.8 m s-l. Data were obtained for superficial gas velocities ranging from l-8 m s-l and solid recirculation rates in the range of 10-70 kg me2 s-’ in the CFB riser.
LEG (12.5
CM ID)
INSIDE
OUTSIDE
25
SOLID
STATIC
PRESSURE
ST&Tic
PRESSURE
FEED’VALVE SIGHT
GLASS
SECONDARY PRIMARY
1 -I
AIR AIR
Fig. 1. Schematic of CFB test loop.
UTOR
TAP
k5
ALL DlMENSlONS ARE IN MM
Fig. 2. Schematic of sampling probe tip.
TAP
199
are located 25 mm from the probe tip to detect the isokinetic sampling condition. Since gas must be drawn through the probe tip in order to collect a solid sample, isokinetic sampling is ‘only possible in the direction of the local gas flow. Isokinetic sampling of the downward solid flu component along the riser wall is not expected to be possible if the gas flow is upward. The tube through which the two-phase flow is drawn is made of 5 mm i.d. stainless tubing (1.6X lo-’ m2 area). The tip is machined to a 10” knife edge to introduce the least possible disturbance to the gas-solid flow. Figure 3 provides a schematic drawing of the sampling system used in conjunction with the sampling probe. The four-way valve may be switched in two different positions: one to sample the gas-solid mixture through the probe and the other to purge the probe tip with air. In the sampling position, gas-solid flow is drawn through the probe tip and led to the sample collection vessel, where the solids are disengaged and settle to the bottom of the collection vessel. The sampled gas flows through a porous filter, one of two rotameters where the gas flow is monitored, and continues to flow into an evacuated vessel. With the four-way valve in the purge position, a stream of purge air flows in the reverse direction through the sample jar and continues out through the probe tip to prevent particles from entering the probe. The horizontal run of tubing leading from the probe tip to the sample jar was minimized to prevent solid particles from settling out and blocking the probe. To detect the isokinetic condition, the pressure tap lines were connected to an inclined manometer capable of resolving pressure differences as small as 0.25 mm of water. When the pressure taps were not being used to measure the difference in static pressures, the manometer was bypassed to prevent pressure oscillations from blowing out the merian fluid and the pressure tap lines were purged to prevent solids from penetrating and blocking them. Figure 4 shows the results of a test of the probe’s ability to measure the local gas velocity in single-phase gas flow. With the probe oriented along the riser centerline and facing upstream, the local gas velocity
Fig. 3. Solids sampling apparatus.
10
6
b
.
PITOT
.
ISOKINETIC .
F E
+
6 .
am 4 : 2
0
0
2
10
4 “,[m/i,
Fig. 4. Single-phase
S
qualification
of isokmetic
sampling.
was measured isokinetically within +0.5 m s-l. blocking the sample line, the local gas velocity can calculated within + 1 m s-’ from a measurement the pressure rise at the probe tip due to stagnation the fluid using a standard pitot tube equation:
By be of of
VPitot
(1)
=
c(2uIPf)‘”
where VPitot= local gas velocity, AZ’= stagnation pressure rise, pr= fluid density, C = constant = 0.98-1.0. The excellent agreement between the local gasvelocity measurements obtained using the two measurement methods indicates that the probe was capable of detecting the isokinetic condition. With the probe facing upstream, the following procedure was used to obtain an isokinetic solid flux measurement. Initially, the probe and pressure taps were purged and the manometer was bypassed. Then the system was switched into the sampling mode. With the manometer bypass valve and the pressure tap purge valves temporally closed, the aspiration rate through the probe was adjusted to zero the pressure difference. Sampling was stopped by switching to the purge mode and the sampling vessel was removed, emptied and replaced. Then, the system was switched to the sampling mode while the collection of solids was timed. After a solid sample of N 100 g had accumulated in the collection vessel, the system was switched to the purge mode, the sampling time was recorded, and the collected solid sample was weighed. The solid mass flux through the probe tip was obtained from the following expression G s&c=MI(QJ
(2)
where Gs,,= = time-averagelocal solid mass flux, kg mm2 s-l, M,=mass of solids collected, kg, A,= collection area of probe tip, m2, t,= collection time, s. To qualify the isokinetic solid sampling technique for two-phase flow, a dilute gas-solid flow was generated in which the solids were observed to flow uniformly upwards through a transparent 5 cm i.d. acrylic riser with no downward solid flow along the wall. With the probe positioned to measure the upward solid mass flux along the centerline, solid mass fluxes of 285 pm
200 1.5 (a)
ACCEPTABLEAANGEOF SAMPLING VELOCITIES
B 7 $1 8
D Ug G, z d,,
v* = 3.9
I
= = = = =
0.05 m 3.9 m/s 13.1 kg/m2s 1.62 276 &m sand
0.5 3 v.,,“r-,s]
g
I2
.J
.
4-
00
32 F 1 5 %
-
_ ’ 0 _ -1 -2
- -.
-3
-
. ACCEPTABLE SAMPLING bP
.
1
. . .
Fig. 5. Qualification
of isokinetic
solid flux measurements.
0.6 6
P
0
-e
0.4
-
0.2
-
B 9 B
0 0
I
I
I
I
I
2
4
6
a
10
12
“aspWI
responding to probe pressure difference readings ranging from approximately - 2 to + 2mm H,O. A complete radial traverse indicated a relatively flat mass flux profile at this operating condition, as was expected. At higher solid flow rates (increased solid holdup), a downward flow of solids along the riser wall was observed. Therefore, to obtain an accurate measurement of the local time-averaged net solid mass flux, the solid fluxes in both the upstream and downstream directions were measured at a given position, and the difference between the two measurements (upward-downwards flux) yielded the net solid mass flux. A sensitivity study revealed that the downward component of the solid flux was sensitive to the aspiration velocity in the region near the wall where the downflow of solids was more significant. The increase in the measured downward solid flux with increasing aspiration velocity, shown in Fig. 6, was expected to result from drawing excess gas and low velocity solids through the probe tip. At the minimum aspiration velocity necessary to continually withdraw solids without blocking the sampling line, the pressure inside the probe tip was lower than the outside pressure. The downward flux measurements were taken at the minimum aspiration velocity to introduce the least disturbance to the gas flow while sampling solids. This sampling condition approaches isokinetic sampling and yields a downward flux measurement with best possible accuracy. The uncertainty in the measured local mass flux due to the uncertainties in the collection time, sample mass measurement and area of the probe tip was estimated to be less than 10% of the measurement. Added uncertainty in the net solid flux measured in the vicinity of the riser wall resulted from the uncertainty from measuring the downward component of the mass flux at this location.
Fig. 6. Sensitivity of net solid flux near the wall.
sand particles were measured at different aspiration velocities while maintaining the solid recirculation rate at approximately 14 kg mm2 s-l. Figure S(a) plots the mass flux normalized with respect to the solid recirculation rate (measured during the sampling interval) as a function of the aspiration velocity. For this dilutephase flow condition, a uniform solid mass flux profile was expected, having a normalized mass flux close to unity. As Fig. S(a) indicates, the upward flux was accurately measured over a range of aspiration velocities centered about the isokinetic point where the local flux at the centerline was found to approximately equal the solid recirculation rate measured in the return leg of the CFB. Figure 5(b) plots the pressure difference (I’,,,-Pin) measured as a function of aspiration velocity, and indicates that an accurate solid mass flux measurement can be attained with aspiration rates cor-
Results and discussion Net solid mass flux profile
The net solid mass flux profiles are constructed from separate time-average profiles obtained for both the upward and downward solid mass fluxes. Assuming that the upflowing and downflowing solid flux measurements included only solids that would have passed through the probes sampling area in the upward or downward direction, respectively, in the probes absence, the timeaverage net solid flux is given by the difference between the time-average up and downflowing solid flux measurements [2]. Two measurements of both the up- and downflowing solid fluxes were obtained at eight radial locations (thirty-two flux measurements in total) to determine the solid flux profile at a given operating condition and probe elevation.
201
Figure 7 shows sample upward and downward solid fluxes plotted against the normalized radial position in the riser. The profiles were measured at an elevation of 1.5 m in the 15 cm i.d. riser. FCC particles having a mean diameter of 87 pm were flowing at 20 kg me2 s-’ with superficial air velocity of 2.4 m s-l. The normalized radius is + 1 at the inner tube wall next to the test port, 0 at the riser centerline. The normalized upward solid flux profile is parabolic and the maximum solid flux at the riser centerline attains a value close to three times larger than the cross-sectional average solid flux. The downward solid flux profile is also parabolic in shape. Within the central core of the riser the downward solid flux is relatively small in comparison to the upward flux; however, it is important to note that a downward flow of solids persists into the central core of the riser. As the wall is approached, the magnitude of the downward solid flux increases sharply and exceeds the magnitude of the upward solids flux near the wall. This suggests the predominance of downward solids flow in an annulus in the vicinity of the riser wall. The net upward solid flux profile is obtained by subtracting the upward and downward components of the solid mass flux. Variation with radial position
Figure 8 shows sample measurements of both local, time-average solid mass flux (A) and local, time-average solid concentration (0) plotted against normalized radial position. Solid concentrations were obtained by time-averaging dynamic solid volume fraction measurements obtained using a capacitance probe technique [6]. The data were obtained at an elevation 1.5 m above the secondary air inlet using 87 pm FCC particles flowing at 20 kg mm2 s-l with a superficial gas velocity of 2.4 m s-l. The net local solid flux measurements are represented by (A). The cross-sectional average net solid mass flux obtained by integrating the profile over the riser cross-section (16 kg m-’ s-l) agreed reasonably with the independent total solid flux mea-
Ug = 2.4 m/S Gs = 20.5 kglm*s dp = 87 pm FCC Z= 1.5m
-25
r/R
Fig. 8. Radial variation
1
r/R
Fig. 7. Upward
and downward
solid mass fluxes.
and mass flux.
surement (20 kg mm2 s-l). This helped to support our method for measuring the downward flux component. As in Fig. 7, the net solid flux profile is parabolic with net upflow in the riser core and net solids downflow along the riser wall. The mass flux profile decreases gradually in the core region and then drops sharply upon entering the downflowing annulus. It is important to note that the magnitude of the local solid flux deviates significantly from the cross-sectional average value. In the center of the riser, the local flux is more than twice the magnitude of the average net solid upflow. Near the riser wall the downward flux is comparable to the average net solid upflow (20 kg me2 s-l), but oppositely directed. At this operating condition, an annulus, defined by the presence of net solids downflow, that is approximately 1 cm thick and occupies 27% of the riser crosssectional area was detected. Based on an integration of the net solid flux profile across the core, the upflow through the core at this operating condition exceeds the measured net solid throughput by 12%. The mass of solids flowing up through the core per unit mass of solids flowing down through the annulus is given by: M,IM,=
0.5
of solid concentration
(MJMJI(1
-MC/M,)
and attains a value of 9 for this case. Since negligible solid downflow is expected at the exit to the riser, the mass flow of solids through the core is expected to equal the solid throughput at the riser exit. From the measurement location (z = 1.5 m) to the riser exit, the average rate of change of the net solid upflow through the core with elevation is 0.005 kg m-l s-‘. This quantity represents the net rate of solid transfer from the upflowing core to the downflowing annulus. Since mass is conserved, this is also the rate of increase in solid downflow in the annulus with decreasing elevation in this section of the riser.
202
As Fig. 8 also illustrates, the solid concentration (0) clearly increases from the riser centerline toward the wall. The radial concentration profile is also flatter in the riser core (r/R < 0.7) and undergoes a sharp increase near the riser wall. The local solid fraction also shows significant radial variations about the area-average value of 0.037. Using the equation of continuity applied to the solid phase, the local time-average solid particle velocity can be obtained from the local solid mass flux and concentration measurements. The solid particle velocity is given by: I&c = Gs.~ocl(~scs)
(4)
where K,lOc= local time-average particle velocity, m s-l, G s,loc=local net time-average solid flux, kg m-’ s-l, ps = solid particle density, kg rnm3, es = local time-average solid volume fraction. Figure 9(a) plots the local time-average solid particle velocity calculated using the data in Fig. 8. The shape of the solid velocity profile is similar to the solid mass flux profile. Solid particles in the riser centerline experience a net upward flow, with calculated average particle velocities exceeding 1 m s-l. As the riser wall is approached, the time-average particle velocity decreases, first gradually and then more rapidly. Close to the riser wall, the FCC particles flow downward with an estimated speed of 0.4 m s-l. Hartge et al. [7] have 2 (a) 1.5 1 F E 0.5
&_=3.4m_s
3 0 U =2.4m/s Gg = 20.5 kglm2s 1=1.5m d,,= 87pm FCC -1 EI 0.5 0
-0.5
1
directly measured solid particle velocity profiles in a 40 cm i.d. riser using fiber optic probes and obtained similar profiles. The dashed line drawn at 0.4 m s-l represents the value of the mean solid particle velocity defined by:
(5) In Fig. 9(b), an estimate of the local particle slip velocity is plotted. The local gas velocity is assumed to be fairly uniform and is approximated by V,l( 1 - E,). This plot indicates that the local slip velocity increases from the centerline toward the riser wall. For this operating condition, the estimated slip velocity varies from four to eight times the particle terminal velocity (U,=O.3 m s-l). The relative velocity between the gas and the solid is a strong function of radial position. The above information on the radial variations of the solid flow and concentration behavior provides supporting evidence for a core-annular flow structure within the CFB riser. The core region is dominated by a relatively dilute suspension of solids that flows upward at relatively high speeds. The detection of a downward solid flux component in the core does suggest the slight presence of downflowing solids in this region. Thus, the core may be comprised of disperse upflowing solids with intermittent downflowing solid particle clusters. The annular region in the vicinity of the wall is not a thin layer but is hundreds of particle diameters in thickness. This annulus is characterized by a denser gas-solid mixture with downward flow on a time-average basis. The detection of an upward solids flux component in this region suggests the occasional presence of upflowing solids. High speed video photographs taken through a transparent section of the riser revealed a persistent downflow of strands of particles along the riser wall which was occasionally interrupted and temporarily replaced by a disperse upflowing mixture. In Fig. 10, the FCC particle mass flux data in Fig. 8 are replotted to illustrate how the flow of solids through a given increment in radius of the riser varies with radial position. Due to the cylindrical geometry
r/R 10
(W
Ug = 2.4 m/s G, = 20.5 kgimzs z = 1.5 m d,,=87pmFCC
/
U
0.5
1
r/R
Fig. 9. Radial variation of solid particle and slip velocities.
-3+ 0 r/R
Fig. 10. Solid mass flow per unit thickness vs. radial position.
203
of the riser tube, this quantity varies differently with radial position than the local solid mass flux. The area of ring-shaped element having radius r and differential thickness & is given by: A,=2m(&)
(6)
The mass flow of solids per unit thickness is given by: G,,~J(~)
= 277G,,,
(7)
Figure 10 shows that the mass flow per unit thickness (kg m-l s-l) does not vary monotonically. It first increases with increasing radius in the core region and then decreases as the wall is approached. The upward flow of solids per unit thickness of the riser reaches a maximum value within the core region, and at this operating condition the maximum occurs at rlR=0.6. Furthermore, a large percentage of the upflowing solids pass through the outer part of the core region bounded by 0.2
r/R
Fig. 11. Variation of solid mass flux with radial position.
at the lower elevation. To compensate for the increased amount of solid downflow in the annulus near the wall, a higher net upward solid flux through the central core region is required. The solid flux profile at the lower elevation indicates a slightly higher positive net solid flux in the core region for 0.4 < rlR < 0.8. The change in the shape of the solid flux and concentration profiles with changing elevation provides insight into how the two-phase flow develops with increased riser elevation. By integrating the mass flux profile across the area of the annulus, the average mass flow in this region can be determined. As Fig. 11 shows, the downward flow of solids in the annulus has decreased with increasing elevation. This experimental information is consistent with the concept proposed by Bolton and Davidson [8] and Rhodes et al. [5] of net particle transport occuring within the CFB from the core region to the wall region. Bolton and Davidson’s analysis suggests that the downward flow of solids in the annulus of the CFB decays exponentially with increasing height in the riser. Figure 12 plots the axialvariation of the cross-sectional average solid particle concentration inferred from pressure gradient measurements obtained for operating conditions similar to those for the data in Fig. 11. Close to the base of the riser, the solids concentration decays with increasing elevation. The gas-solid flow continues to develop throughout this height as the particles introduced at the base of the riser experience a net upward acceleration. Above this region, fully-developed flow conditions are attained and the average solid particle concentration becomes independent of elevation. From this plot it can be seen that the 1.5 m elevation lies in the developing flow region and at 5.5 m fully-developed conditions are approached. The solid concentration and flow behavior in the fullydeveloped region of the riser are not expected to depend on height or solid feed configuration, but are expected to depend on particle properties, gas velocity, solid loading and tube diameter. Within the developing re-
0
2
4
2tml
6
a
10
Fig. 12. Axial development of solid concentration profile.
doubles the mass flowing up through the core. The net downward solid flux in the annulus does not appear to have changed significantly with this two-fold increase in the total solids mass flow; however, it is important to note that small changes in the net flux measurements in the wall region can significantly affect the integrated flux measurements. Effect of gas velocity
Fig. 13. Effect of solid flow rate on solid
Fig. 14. Effect of gas velocity on solid mass
Figure 14 presents two solid mass flux profiles obtained at an elevation of 5.5 m using the 125 pm sand test particles. The total solid mass flux was held at approximately 40 kg me2 s- ’ for two different superficial gas velocities, 4.2 and 6.0 m s-l. As the gas velocity is lowered, the upward flow of solids through the core region increases and the downflow of solids in the annulus also increases to maintain a constant total mass throughput. The downward flow of solids along the riser wall acts as a recycle stream. Decreasing the gas velocity causes an increase in the particle recycle rate through reverse flow within the riser. Measurements of bed pressure drop over the same section of the riser reveal that this increased recirculation results in increased suspension density. Since the mass flow is constant and the holdup increases, the mean residence time for particles within the riser also increases. These measurements show that U, can be used to influence the extent of recirculation, downflow along the wall, and particle residence times in the riser. In addition, the distribution of particle residence times is expected to change with gas velocity. Particles in the core region should pass quickly up through the riser, while particles near the wall region may tend to descend along the riser wall and eventually be re-entrained. Variation with riser diameter
1 r/R Fig. 15. Effect of riser diameter 0
on solid mass flux.
gion, the solid mass flux and concentration profiles are expected to depend on the solid feeding arrangement. In particular, the momentum (upward velocity component) of the incoming solids is expected to influence the solids distribution and flow behavior in the developing region. Effect of solid mass flow
Figure 13 plots time-average solid mass flux profiles obtained at an elevation of 5.5 m for a gas velocity of 4.2 m s-l and for two different 125 pm sand mass flows (20.9 and 43.6 kg me2 s-l). A significant increase in the upward solid flux in the riser core resulted from the total solid mass flow increase, which more than
Figure 15 compares mass flux profiles obtained in two different CFB risers having inside diameters of 5 and 15 cm, respectively. Reynold’s numbers calculated for single-phase air flow were 11300 (5 cm) and 42 000 (15 cm), indicating turbulent gas flow. The solid flux profile for the larger diameter riser was obtained in the fully-developed flow region at an elevation of 5.5 m (z/D=37). The solid flux profile for the smaller riser was taken at an elevation of 2.1 m (z/D =42). The normalized solid mass fluxes are plotted against the normalized radial positions. A similar radial dependence within the central core region is shown for both CFB systems for the selected operating conditions.
Conclusions
Measurements of the time-average local solid mass fluxes in two CFB systems having diameters of 5 and
205
15 cm provide clear evidence of the core-annular flow structure in CFB risers. The central core region is dominated by a dilute upflowing suspension of solids flowing at high velocities. The presence of a downward solid flux component in the core region suggests that particle clusters flow intermittently downward within this region. Most of the upflowing solids were found to pass through the outer core region having less than half of the riser cross-sectional area. The annulus, having a thickness on the order of centimeters, is occupied by a dense, downflowing solid suspension having a relatively low velocity. The detection of an upward solid flux in the annulus and video photographs indicate that the downflowing solid strands may be intermittently interrupted by a disperse upflowing mixture. The ratio of solid upflow through the core to downflow through the annulus was found to decrease with increasing elevation and increasing gas velocity, indicating reduced solids transport from the upflowing core to the downflowing annulus. A fundamental understanding of the nonuniform solid flow behavior in CFB systems is critical to successful modeling of industrial CFB processes.
Acknowledgement This work was supported by Air Products and Chemicals, Inc. and Lehigh University.
List of symbols
4 4 G, G s,loc
probe sampling area, Sauter mean particle total solid circulation net local solids mass
m* diameter, pm rate, kg m-’ s-l flux, kg mm2 s-l
solids mass flux down through annulus, kg S-l
solid flow up through core, kg s-l total solid flow rate, kg s-l sample collection time, s superficial gas velocity, m s-’ Pitot tube velocity, m s-l mean solids velocity, m s- ’ local solids velocity, m s- ’ axial distance above the secondary air inlet, m Greek letters
solid volume fraction, cross-sectional average solid volume fraction,
?J ,hP
difference between inside and outside pres. sures, Pa
solid particle density, kg mm3
Ps References
S. Dou, B. Herb, K. Tuzla and J. C. Chen, presented at AIChE Annual Meeting, San Francisco, 1989. J. W. Van Breugal, J. M. Stein and R. J. de Vries, Proc. Inst. Mech. Eng., 184 (1970) 18. T. W. Bier], L. J. Gajdos, A. E. McIver and J. J. McGovern, U.S. Dept. of Energy Report No. EE2449-11, 1980. L. Montceauz, M. Azzi, Y. Molodtsof and J. F. Large, in P. Basu (ed.), Circulating Fluidized Bed Technology, Pergamon, New York, 1986, pp. 185-210. M. J. Rhodes, P. Laussman, F. Villian and D. Geldart, in P. Basu and J. P. Large (eds.), Circulating Fluidtied Bed Technology ZJ Pergamon, New York, 1988, pp. 155-164. B. Herb, K. Tuzla and J. C. Chen, Fluidization Vr, Engineering Foundation, New York, 1989, pp. 65-72. E. U. Hartge, Y. Li and J. Werther, in P. Basu and J. P. Large (eds.), Circulating Fluidized Bed Technology II, 1988, Pergamon, New York, pp. 153-160. L. W. Bolton and J. F. Davidson, in P. Basu and J. P. Large (eds.), Circulating Fluidized Bed Technology ZI, Pergamon, New York, 1988, pp. 139-146.
197
Solid mass fluxes in circulating fluidized beds Blaine
Herb*,
Suisheng
Dou**,
Kemal
Tuzla
and John
C. Chen
Institute for Thermo-Fluid Engineering and Science, Lehigh University, Bethlehem, PA (USA)
Abstract
Experimental measurements of local solid mass flux were made in the riser component of two different circulating fluidized beds (CFBs). Tests were conducted in both a lab-scale riser measuring 5 cm i.d. and 2.7 m in height and a pilot-scale riser of 15 cm i.d. and 10.8 m height. FCC catalyst particles with a Sauter mean diameter of 68 pm and sand particles with mean diameters of 125 pm and 276 pm were used in the tests. The results showed the local time-average solid mass flux to vary with radial position, elevation, total solid mass flow, superficial gas velocity and mean particle diameter. Based on the solid mass flux and local solid concentration measurements, and on video studies, a physical description of the solid flow phenomena in CFBs is presented.
Introduction
Many industrial processes have evolved in which solid particles are contacted with a gas to achieve chemical or physical change within the gas or solid phase. Examples of these processes include solid particle drying, coal combustion, fluid catalytic reactions and calcining reactions. Depending on the relative velocity between the gas and the solid, gas and particle properties, and equipment size and geometry, several different vertical gas-solid flow regimes result. At superficial gas velocities above the particle terminal velocity and at relatively high solid mass fluxes, an entrained suspension of solids forms which is known as a circulating fluidized bed (CFB). Knowledge of the solids distribution and flow behavior in the CFB riser is the key to successful design and operation of CFB systems. The solids distribution governs the pressure drop occurring along the CFB riser and is directly related to the mean solids residence time within the riser. The solid concentration determines the gas-solid interfacial area per unit volume of the mixture, which is expected to directly affect gas-solid reaction and mass transfer rates. Furthermore, heat transfer coefficients measured at the riser wall and on submerged surfaces show strong dependence on the solid concentrations in the vicinity of the heat transfer surface [l]. Solid mass flux measurements are important to describing the solid flow behavior in CFBs. In addition, *Presently **Presently ceton, NJ.
at Air Products and Chemicals Inc., Allentown, PA. at Mobil Research and Development Corp., Prin-
erosion rates of exposed surfaces are expected to tie closely with the solid flow behaviour along these surfaces. Few measurements of solid mass fluxes in CFBs have been reported in the literature. Van Breugal et al. [2], using an isokinetic sampling probe in a 30 cm riser operating under dense-phase flow conditions, reported a parabolic solid flux profile with downward solid flow along the riser wall. Using a non-isokinetic sampling probe, Bier1 et al. [3] also measured strong radial variations in the solid mass flux. Combining these measurements with information on radial variations of time-average solid concentration obtained from X-ray measurements, they concluded that the CFB consisted of a dilute, upflowing solids core surrounded by a dense, downflowing solids annulus. More recently, Monceaux et al. [4] measured particle mass fluxes in the fully-developed region of the CFB in a 15 cm i.d. riser using fine catalyst particles. For area-average solid volume fractions below 0.05, normalized solid flux profiles at a given gas velocity were found to be independent of solids mass flux. At higher solid concentrations, the normalized solid flux profiles were no longer independent of total solid mass flow. The experimental findings’ supported their theoretical analysis which predicted the existence of similar solid flux profiles below some critical solid concentration. Rhodes et al. [5], using non-isokinetic sampling probes, measured solid mass flux at three elevations within the CFB to show the axial development of the solids flow profile. A core-annular flow model was proposed in which the solids flow and distribution was described by a net flow from the upflowing (uniform) core region to a (uniformly) downflowing annulus.
0 1992 - Elsevier
Sequoia. All rights reserved
198
The objective of this paper is to present a new collection of time-average solid mass flux measurements covering a range of operating conditions of importance to industrial CFB applications, and to provide insight into the solid flow behavior of CFB systems.
Experimental Experiments were carried out in two different CFB systems: a lab-scale CFB with a 5 cm i.d. riser, 2.7 m height, and a pilot-scale CFB system, shown in Fig. 1, having a cylindrical riser with an i.d. of 15 cm and a height of 10.8 m. As Fig. 1 shows, primary air and secondary air were introduced below and above a sintered metal distributor, respectively. Under normal test conditions, a dense bed was established between the distributor and the secondary air injection elevation. Above the secondary air injection point, the circulating fluidized bed was established and existed until the smooth right-angle exit to the cyclone. Axial positions within the riser were measured relative to the secondary air inlet location, above which the total air flow was established. Particles captured by the cyclone were returned through a downcomer leg which provided a particle storage hopper and standpipe combination for the measurement of solid recirculation rate. The return particles were horizontally transported to the CFB riser by means of secondary air flow. Thirty-three pressure taps were installed along the height of the riser. Each pressure tap line was purged at a constant rate to prevent particles from entering and blocking the pressure tap lines. The pressure tap line were manifolded to six differential pressure transducers to allow the pressure drop to be measured simultaneously across six segments of the riser. In a typical experiment, the desired superficial velocity was set by adjustment of both primary and secondary
CYCLONE .
aP TRANSDUCER ’
STANDPIPE
\ CFB RISER
(15
INSTRUhiENTATlON PARTICLE
STORAGE
CM ID)
Sampling probe and measurement method
To measure the axial component of the time-average solid mass flux locally within the CFB, a sampling apparatus and technique were developed. The sampling probe was designed to mount in the instrumentation ports and to traverse the riser i.d. Figure 2 depicts the design of the sampling probe tip. To ensure the accuracy of the upward solid flux measurements, the probe was designed for isokinetic sampling. With the probe tip facing upstream, isokinetic sampling requires that the gas velocity aspirated through the probe tip equals the local gas velocity that would exist at this point if the probe were not present. If the aspiration velocity is below the isokinetic velocity, the gas flow is deflected around the probe tip along with fine particles carried by the gas, which causes the solid sample collected to be too small. If the aspiration velocity exceeds the isokinetic velocity, the collected solid sample will be too large, since excess gas and solids are drawn into the probe tip. Under isokinetic sampling conditions, the static pressures measured inside and outside the probe tip are expected to be equal. Thus, inside and outside pressure taps made of 1.6 mm stainless tubing
PORTS VESSEL
T
PRESSURETAPS RETURN
air flows, and particles were charged to the riser at the desired rate by adjusting the solid feed valve. The solid recirculation rate (total solid mass flux in the riser) was measured by temporarily closing the butterfly valve above the particle storage vessel and measuring the time to accumulate a packed bed volume of 0.0032 m3 in the transparent standpipe. A separate measurement of the apparent packed bed density of the test particles was used to calculate the mass of solids collected. Due to the surge volume provided by the particle storage vessel, the normal feed rate of returned solids continued without interruption during this measurement procedure. The results presented in this paper were obtained for 68 pm FCC particles, and 125 pm and 276 pm sand particles with corresponding terminal velocities in ambient air of 0.27 m s-l, 0.74 m s-’ and 1.8 m s-l. Data were obtained for superficial gas velocities ranging from l-8 m s-l and solid recirculation rates in the range of 10-70 kg me2 s-’ in the CFB riser.
LEG (12.5
CM ID)
INSIDE
OUTSIDE
25
SOLID
STATIC
PRESSURE
ST&Tic
PRESSURE
FEED’VALVE SIGHT
GLASS
SECONDARY PRIMARY
1 -I
AIR AIR
Fig. 1. Schematic of CFB test loop.
UTOR
TAP
k5
ALL DlMENSlONS ARE IN MM
Fig. 2. Schematic of sampling probe tip.
TAP
199
are located 25 mm from the probe tip to detect the isokinetic sampling condition. Since gas must be drawn through the probe tip in order to collect a solid sample, isokinetic sampling is ‘only possible in the direction of the local gas flow. Isokinetic sampling of the downward solid flu component along the riser wall is not expected to be possible if the gas flow is upward. The tube through which the two-phase flow is drawn is made of 5 mm i.d. stainless tubing (1.6X lo-’ m2 area). The tip is machined to a 10” knife edge to introduce the least possible disturbance to the gas-solid flow. Figure 3 provides a schematic drawing of the sampling system used in conjunction with the sampling probe. The four-way valve may be switched in two different positions: one to sample the gas-solid mixture through the probe and the other to purge the probe tip with air. In the sampling position, gas-solid flow is drawn through the probe tip and led to the sample collection vessel, where the solids are disengaged and settle to the bottom of the collection vessel. The sampled gas flows through a porous filter, one of two rotameters where the gas flow is monitored, and continues to flow into an evacuated vessel. With the four-way valve in the purge position, a stream of purge air flows in the reverse direction through the sample jar and continues out through the probe tip to prevent particles from entering the probe. The horizontal run of tubing leading from the probe tip to the sample jar was minimized to prevent solid particles from settling out and blocking the probe. To detect the isokinetic condition, the pressure tap lines were connected to an inclined manometer capable of resolving pressure differences as small as 0.25 mm of water. When the pressure taps were not being used to measure the difference in static pressures, the manometer was bypassed to prevent pressure oscillations from blowing out the merian fluid and the pressure tap lines were purged to prevent solids from penetrating and blocking them. Figure 4 shows the results of a test of the probe’s ability to measure the local gas velocity in single-phase gas flow. With the probe oriented along the riser centerline and facing upstream, the local gas velocity
Fig. 3. Solids sampling apparatus.
10
6
b
.
PITOT
.
ISOKINETIC .
F E
+
6 .
am 4 : 2
0
0
2
10
4 “,[m/i,
Fig. 4. Single-phase
S
qualification
of isokmetic
sampling.
was measured isokinetically within +0.5 m s-l. blocking the sample line, the local gas velocity can calculated within + 1 m s-’ from a measurement the pressure rise at the probe tip due to stagnation the fluid using a standard pitot tube equation:
By be of of
VPitot
(1)
=
c(2uIPf)‘”
where VPitot= local gas velocity, AZ’= stagnation pressure rise, pr= fluid density, C = constant = 0.98-1.0. The excellent agreement between the local gasvelocity measurements obtained using the two measurement methods indicates that the probe was capable of detecting the isokinetic condition. With the probe facing upstream, the following procedure was used to obtain an isokinetic solid flux measurement. Initially, the probe and pressure taps were purged and the manometer was bypassed. Then the system was switched into the sampling mode. With the manometer bypass valve and the pressure tap purge valves temporally closed, the aspiration rate through the probe was adjusted to zero the pressure difference. Sampling was stopped by switching to the purge mode and the sampling vessel was removed, emptied and replaced. Then, the system was switched to the sampling mode while the collection of solids was timed. After a solid sample of N 100 g had accumulated in the collection vessel, the system was switched to the purge mode, the sampling time was recorded, and the collected solid sample was weighed. The solid mass flux through the probe tip was obtained from the following expression G s&c=MI(QJ
(2)
where Gs,,= = time-averagelocal solid mass flux, kg mm2 s-l, M,=mass of solids collected, kg, A,= collection area of probe tip, m2, t,= collection time, s. To qualify the isokinetic solid sampling technique for two-phase flow, a dilute gas-solid flow was generated in which the solids were observed to flow uniformly upwards through a transparent 5 cm i.d. acrylic riser with no downward solid flow along the wall. With the probe positioned to measure the upward solid mass flux along the centerline, solid mass fluxes of 285 pm
200 1.5 (a)
ACCEPTABLEAANGEOF SAMPLING VELOCITIES
B 7 $1 8
D Ug G, z d,,
v* = 3.9
I
= = = = =
0.05 m 3.9 m/s 13.1 kg/m2s 1.62 276 &m sand
0.5 3 v.,,“r-,s]
g
I2
.J
.
4-
00
32 F 1 5 %
-
_ ’ 0 _ -1 -2
- -.
-3
-
. ACCEPTABLE SAMPLING bP
.
1
. . .
Fig. 5. Qualification
of isokinetic
solid flux measurements.
0.6 6
P
0
-e
0.4
-
0.2
-
B 9 B
0 0
I
I
I
I
I
2
4
6
a
10
12
“aspWI
responding to probe pressure difference readings ranging from approximately - 2 to + 2mm H,O. A complete radial traverse indicated a relatively flat mass flux profile at this operating condition, as was expected. At higher solid flow rates (increased solid holdup), a downward flow of solids along the riser wall was observed. Therefore, to obtain an accurate measurement of the local time-averaged net solid mass flux, the solid fluxes in both the upstream and downstream directions were measured at a given position, and the difference between the two measurements (upward-downwards flux) yielded the net solid mass flux. A sensitivity study revealed that the downward component of the solid flux was sensitive to the aspiration velocity in the region near the wall where the downflow of solids was more significant. The increase in the measured downward solid flux with increasing aspiration velocity, shown in Fig. 6, was expected to result from drawing excess gas and low velocity solids through the probe tip. At the minimum aspiration velocity necessary to continually withdraw solids without blocking the sampling line, the pressure inside the probe tip was lower than the outside pressure. The downward flux measurements were taken at the minimum aspiration velocity to introduce the least disturbance to the gas flow while sampling solids. This sampling condition approaches isokinetic sampling and yields a downward flux measurement with best possible accuracy. The uncertainty in the measured local mass flux due to the uncertainties in the collection time, sample mass measurement and area of the probe tip was estimated to be less than 10% of the measurement. Added uncertainty in the net solid flux measured in the vicinity of the riser wall resulted from the uncertainty from measuring the downward component of the mass flux at this location.
Fig. 6. Sensitivity of net solid flux near the wall.
sand particles were measured at different aspiration velocities while maintaining the solid recirculation rate at approximately 14 kg mm2 s-l. Figure S(a) plots the mass flux normalized with respect to the solid recirculation rate (measured during the sampling interval) as a function of the aspiration velocity. For this dilutephase flow condition, a uniform solid mass flux profile was expected, having a normalized mass flux close to unity. As Fig. S(a) indicates, the upward flux was accurately measured over a range of aspiration velocities centered about the isokinetic point where the local flux at the centerline was found to approximately equal the solid recirculation rate measured in the return leg of the CFB. Figure 5(b) plots the pressure difference (I’,,,-Pin) measured as a function of aspiration velocity, and indicates that an accurate solid mass flux measurement can be attained with aspiration rates cor-
Results and discussion Net solid mass flux profile
The net solid mass flux profiles are constructed from separate time-average profiles obtained for both the upward and downward solid mass fluxes. Assuming that the upflowing and downflowing solid flux measurements included only solids that would have passed through the probes sampling area in the upward or downward direction, respectively, in the probes absence, the timeaverage net solid flux is given by the difference between the time-average up and downflowing solid flux measurements [2]. Two measurements of both the up- and downflowing solid fluxes were obtained at eight radial locations (thirty-two flux measurements in total) to determine the solid flux profile at a given operating condition and probe elevation.
201
Figure 7 shows sample upward and downward solid fluxes plotted against the normalized radial position in the riser. The profiles were measured at an elevation of 1.5 m in the 15 cm i.d. riser. FCC particles having a mean diameter of 87 pm were flowing at 20 kg me2 s-’ with superficial air velocity of 2.4 m s-l. The normalized radius is + 1 at the inner tube wall next to the test port, 0 at the riser centerline. The normalized upward solid flux profile is parabolic and the maximum solid flux at the riser centerline attains a value close to three times larger than the cross-sectional average solid flux. The downward solid flux profile is also parabolic in shape. Within the central core of the riser the downward solid flux is relatively small in comparison to the upward flux; however, it is important to note that a downward flow of solids persists into the central core of the riser. As the wall is approached, the magnitude of the downward solid flux increases sharply and exceeds the magnitude of the upward solids flux near the wall. This suggests the predominance of downward solids flow in an annulus in the vicinity of the riser wall. The net upward solid flux profile is obtained by subtracting the upward and downward components of the solid mass flux. Variation with radial position
Figure 8 shows sample measurements of both local, time-average solid mass flux (A) and local, time-average solid concentration (0) plotted against normalized radial position. Solid concentrations were obtained by time-averaging dynamic solid volume fraction measurements obtained using a capacitance probe technique [6]. The data were obtained at an elevation 1.5 m above the secondary air inlet using 87 pm FCC particles flowing at 20 kg mm2 s-l with a superficial gas velocity of 2.4 m s-l. The net local solid flux measurements are represented by (A). The cross-sectional average net solid mass flux obtained by integrating the profile over the riser cross-section (16 kg m-’ s-l) agreed reasonably with the independent total solid flux mea-
Ug = 2.4 m/S Gs = 20.5 kglm*s dp = 87 pm FCC Z= 1.5m
-25
r/R
Fig. 8. Radial variation
1
r/R
Fig. 7. Upward
and downward
solid mass fluxes.
and mass flux.
surement (20 kg mm2 s-l). This helped to support our method for measuring the downward flux component. As in Fig. 7, the net solid flux profile is parabolic with net upflow in the riser core and net solids downflow along the riser wall. The mass flux profile decreases gradually in the core region and then drops sharply upon entering the downflowing annulus. It is important to note that the magnitude of the local solid flux deviates significantly from the cross-sectional average value. In the center of the riser, the local flux is more than twice the magnitude of the average net solid upflow. Near the riser wall the downward flux is comparable to the average net solid upflow (20 kg me2 s-l), but oppositely directed. At this operating condition, an annulus, defined by the presence of net solids downflow, that is approximately 1 cm thick and occupies 27% of the riser crosssectional area was detected. Based on an integration of the net solid flux profile across the core, the upflow through the core at this operating condition exceeds the measured net solid throughput by 12%. The mass of solids flowing up through the core per unit mass of solids flowing down through the annulus is given by: M,IM,=
0.5
of solid concentration
(MJMJI(1
-MC/M,)
and attains a value of 9 for this case. Since negligible solid downflow is expected at the exit to the riser, the mass flow of solids through the core is expected to equal the solid throughput at the riser exit. From the measurement location (z = 1.5 m) to the riser exit, the average rate of change of the net solid upflow through the core with elevation is 0.005 kg m-l s-‘. This quantity represents the net rate of solid transfer from the upflowing core to the downflowing annulus. Since mass is conserved, this is also the rate of increase in solid downflow in the annulus with decreasing elevation in this section of the riser.
202
As Fig. 8 also illustrates, the solid concentration (0) clearly increases from the riser centerline toward the wall. The radial concentration profile is also flatter in the riser core (r/R < 0.7) and undergoes a sharp increase near the riser wall. The local solid fraction also shows significant radial variations about the area-average value of 0.037. Using the equation of continuity applied to the solid phase, the local time-average solid particle velocity can be obtained from the local solid mass flux and concentration measurements. The solid particle velocity is given by: I&c = Gs.~ocl(~scs)
(4)
where K,lOc= local time-average particle velocity, m s-l, G s,loc=local net time-average solid flux, kg m-’ s-l, ps = solid particle density, kg rnm3, es = local time-average solid volume fraction. Figure 9(a) plots the local time-average solid particle velocity calculated using the data in Fig. 8. The shape of the solid velocity profile is similar to the solid mass flux profile. Solid particles in the riser centerline experience a net upward flow, with calculated average particle velocities exceeding 1 m s-l. As the riser wall is approached, the time-average particle velocity decreases, first gradually and then more rapidly. Close to the riser wall, the FCC particles flow downward with an estimated speed of 0.4 m s-l. Hartge et al. [7] have 2 (a) 1.5 1 F E 0.5
&_=3.4m_s
3 0 U =2.4m/s Gg = 20.5 kglm2s 1=1.5m d,,= 87pm FCC -1 EI 0.5 0
-0.5
1
directly measured solid particle velocity profiles in a 40 cm i.d. riser using fiber optic probes and obtained similar profiles. The dashed line drawn at 0.4 m s-l represents the value of the mean solid particle velocity defined by:
(5) In Fig. 9(b), an estimate of the local particle slip velocity is plotted. The local gas velocity is assumed to be fairly uniform and is approximated by V,l( 1 - E,). This plot indicates that the local slip velocity increases from the centerline toward the riser wall. For this operating condition, the estimated slip velocity varies from four to eight times the particle terminal velocity (U,=O.3 m s-l). The relative velocity between the gas and the solid is a strong function of radial position. The above information on the radial variations of the solid flow and concentration behavior provides supporting evidence for a core-annular flow structure within the CFB riser. The core region is dominated by a relatively dilute suspension of solids that flows upward at relatively high speeds. The detection of a downward solid flux component in the core does suggest the slight presence of downflowing solids in this region. Thus, the core may be comprised of disperse upflowing solids with intermittent downflowing solid particle clusters. The annular region in the vicinity of the wall is not a thin layer but is hundreds of particle diameters in thickness. This annulus is characterized by a denser gas-solid mixture with downward flow on a time-average basis. The detection of an upward solids flux component in this region suggests the occasional presence of upflowing solids. High speed video photographs taken through a transparent section of the riser revealed a persistent downflow of strands of particles along the riser wall which was occasionally interrupted and temporarily replaced by a disperse upflowing mixture. In Fig. 10, the FCC particle mass flux data in Fig. 8 are replotted to illustrate how the flow of solids through a given increment in radius of the riser varies with radial position. Due to the cylindrical geometry
r/R 10
(W
Ug = 2.4 m/s G, = 20.5 kgimzs z = 1.5 m d,,=87pmFCC
/
U
0.5
1
r/R
Fig. 9. Radial variation of solid particle and slip velocities.
-3+ 0 r/R
Fig. 10. Solid mass flow per unit thickness vs. radial position.
203
of the riser tube, this quantity varies differently with radial position than the local solid mass flux. The area of ring-shaped element having radius r and differential thickness & is given by: A,=2m(&)
(6)
The mass flow of solids per unit thickness is given by: G,,~J(~)
= 277G,,,
(7)
Figure 10 shows that the mass flow per unit thickness (kg m-l s-l) does not vary monotonically. It first increases with increasing radius in the core region and then decreases as the wall is approached. The upward flow of solids per unit thickness of the riser reaches a maximum value within the core region, and at this operating condition the maximum occurs at rlR=0.6. Furthermore, a large percentage of the upflowing solids pass through the outer part of the core region bounded by 0.2
r/R
Fig. 11. Variation of solid mass flux with radial position.
at the lower elevation. To compensate for the increased amount of solid downflow in the annulus near the wall, a higher net upward solid flux through the central core region is required. The solid flux profile at the lower elevation indicates a slightly higher positive net solid flux in the core region for 0.4 < rlR < 0.8. The change in the shape of the solid flux and concentration profiles with changing elevation provides insight into how the two-phase flow develops with increased riser elevation. By integrating the mass flux profile across the area of the annulus, the average mass flow in this region can be determined. As Fig. 11 shows, the downward flow of solids in the annulus has decreased with increasing elevation. This experimental information is consistent with the concept proposed by Bolton and Davidson [8] and Rhodes et al. [5] of net particle transport occuring within the CFB from the core region to the wall region. Bolton and Davidson’s analysis suggests that the downward flow of solids in the annulus of the CFB decays exponentially with increasing height in the riser. Figure 12 plots the axialvariation of the cross-sectional average solid particle concentration inferred from pressure gradient measurements obtained for operating conditions similar to those for the data in Fig. 11. Close to the base of the riser, the solids concentration decays with increasing elevation. The gas-solid flow continues to develop throughout this height as the particles introduced at the base of the riser experience a net upward acceleration. Above this region, fully-developed flow conditions are attained and the average solid particle concentration becomes independent of elevation. From this plot it can be seen that the 1.5 m elevation lies in the developing flow region and at 5.5 m fully-developed conditions are approached. The solid concentration and flow behavior in the fullydeveloped region of the riser are not expected to depend on height or solid feed configuration, but are expected to depend on particle properties, gas velocity, solid loading and tube diameter. Within the developing re-
0
2
4
2tml
6
a
10
Fig. 12. Axial development of solid concentration profile.
doubles the mass flowing up through the core. The net downward solid flux in the annulus does not appear to have changed significantly with this two-fold increase in the total solids mass flow; however, it is important to note that small changes in the net flux measurements in the wall region can significantly affect the integrated flux measurements. Effect of gas velocity
Fig. 13. Effect of solid flow rate on solid
Fig. 14. Effect of gas velocity on solid mass
Figure 14 presents two solid mass flux profiles obtained at an elevation of 5.5 m using the 125 pm sand test particles. The total solid mass flux was held at approximately 40 kg me2 s- ’ for two different superficial gas velocities, 4.2 and 6.0 m s-l. As the gas velocity is lowered, the upward flow of solids through the core region increases and the downflow of solids in the annulus also increases to maintain a constant total mass throughput. The downward flow of solids along the riser wall acts as a recycle stream. Decreasing the gas velocity causes an increase in the particle recycle rate through reverse flow within the riser. Measurements of bed pressure drop over the same section of the riser reveal that this increased recirculation results in increased suspension density. Since the mass flow is constant and the holdup increases, the mean residence time for particles within the riser also increases. These measurements show that U, can be used to influence the extent of recirculation, downflow along the wall, and particle residence times in the riser. In addition, the distribution of particle residence times is expected to change with gas velocity. Particles in the core region should pass quickly up through the riser, while particles near the wall region may tend to descend along the riser wall and eventually be re-entrained. Variation with riser diameter
1 r/R Fig. 15. Effect of riser diameter 0
on solid mass flux.
gion, the solid mass flux and concentration profiles are expected to depend on the solid feeding arrangement. In particular, the momentum (upward velocity component) of the incoming solids is expected to influence the solids distribution and flow behavior in the developing region. Effect of solid mass flow
Figure 13 plots time-average solid mass flux profiles obtained at an elevation of 5.5 m for a gas velocity of 4.2 m s-l and for two different 125 pm sand mass flows (20.9 and 43.6 kg me2 s-l). A significant increase in the upward solid flux in the riser core resulted from the total solid mass flow increase, which more than
Figure 15 compares mass flux profiles obtained in two different CFB risers having inside diameters of 5 and 15 cm, respectively. Reynold’s numbers calculated for single-phase air flow were 11300 (5 cm) and 42 000 (15 cm), indicating turbulent gas flow. The solid flux profile for the larger diameter riser was obtained in the fully-developed flow region at an elevation of 5.5 m (z/D=37). The solid flux profile for the smaller riser was taken at an elevation of 2.1 m (z/D =42). The normalized solid mass fluxes are plotted against the normalized radial positions. A similar radial dependence within the central core region is shown for both CFB systems for the selected operating conditions.
Conclusions
Measurements of the time-average local solid mass fluxes in two CFB systems having diameters of 5 and
205
15 cm provide clear evidence of the core-annular flow structure in CFB risers. The central core region is dominated by a dilute upflowing suspension of solids flowing at high velocities. The presence of a downward solid flux component in the core region suggests that particle clusters flow intermittently downward within this region. Most of the upflowing solids were found to pass through the outer core region having less than half of the riser cross-sectional area. The annulus, having a thickness on the order of centimeters, is occupied by a dense, downflowing solid suspension having a relatively low velocity. The detection of an upward solid flux in the annulus and video photographs indicate that the downflowing solid strands may be intermittently interrupted by a disperse upflowing mixture. The ratio of solid upflow through the core to downflow through the annulus was found to decrease with increasing elevation and increasing gas velocity, indicating reduced solids transport from the upflowing core to the downflowing annulus. A fundamental understanding of the nonuniform solid flow behavior in CFB systems is critical to successful modeling of industrial CFB processes.
Acknowledgement This work was supported by Air Products and Chemicals, Inc. and Lehigh University.
List of symbols
4 4 G, G s,loc
probe sampling area, Sauter mean particle total solid circulation net local solids mass
m* diameter, pm rate, kg m-’ s-l flux, kg mm2 s-l
solids mass flux down through annulus, kg S-l
solid flow up through core, kg s-l total solid flow rate, kg s-l sample collection time, s superficial gas velocity, m s-’ Pitot tube velocity, m s-l mean solids velocity, m s- ’ local solids velocity, m s- ’ axial distance above the secondary air inlet, m Greek letters
solid volume fraction, cross-sectional average solid volume fraction,
?J ,hP
difference between inside and outside pres. sures, Pa
solid particle density, kg mm3
Ps References
S. Dou, B. Herb, K. Tuzla and J. C. Chen, presented at AIChE Annual Meeting, San Francisco, 1989. J. W. Van Breugal, J. M. Stein and R. J. de Vries, Proc. Inst. Mech. Eng., 184 (1970) 18. T. W. Bier], L. J. Gajdos, A. E. McIver and J. J. McGovern, U.S. Dept. of Energy Report No. EE2449-11, 1980. L. Montceauz, M. Azzi, Y. Molodtsof and J. F. Large, in P. Basu (ed.), Circulating Fluidized Bed Technology, Pergamon, New York, 1986, pp. 185-210. M. J. Rhodes, P. Laussman, F. Villian and D. Geldart, in P. Basu and J. P. Large (eds.), Circulating Fluidtied Bed Technology ZJ Pergamon, New York, 1988, pp. 155-164. B. Herb, K. Tuzla and J. C. Chen, Fluidization Vr, Engineering Foundation, New York, 1989, pp. 65-72. E. U. Hartge, Y. Li and J. Werther, in P. Basu and J. P. Large (eds.), Circulating Fluidized Bed Technology II, 1988, Pergamon, New York, pp. 153-160. L. W. Bolton and J. F. Davidson, in P. Basu and J. P. Large (eds.), Circulating Fluidized Bed Technology ZI, Pergamon, New York, 1988, pp. 139-146.