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Characterising non-uniformities in gas-particle flow in the riser of a circulating fluidized bed
277
Powder Technology, 72 (1992) 277-284
Characterising nori-uniformities circulating fluidized bed M. J. Rhodes
in gas-particle
flow in the riser of a
and P. Laussmann
Department of Chemical Engineering, University of Bradford, Bradford 807
1DP (UK)
(Received February 3, 1992; in revised form April 14, 1992)
Abstract A sampling probe operating in a non-isokinetic manner was used to measure variations of local solids mass flux with radial, angular and axial position and time in the 152 mm i.d. riser of a circulating fluidixed bed cold model. Time-averaged solids flux profiles exhibit a typically flattened parabolic shape with a region of downward flow adjacent to the riser wall. These profiles do not generally exhibit axial symmetry and a minimum of six radial profiles measurements are found to be necessary to describe the flow pattern adequately. These angular non-uniformities are reproducible, difficult to eliminate and although their form is probably system dependent, it is thought that such non-uniformities are likely to exist in all such gas-particle flow systems to a greater or lesser extent. Detail of the nature of the flowing suspension near to the riser wall has been delineated by analysis of variations in local solids flux with time measured using an adaptation of the same sampling probe.
Introduction Parallel with the commercial development of circulating fluidized bed (CFB) technology details of its operating characteristics and fluid dynamics are emerging thanks to a persistent academic research effort throughout the world. In recent years the many studies have.improved our understanding of the flow structure of the gas-particle suspension in the CFB riser. Bier1 et al. [l], Rhodes and Geldart [2], Weinstein et al. [3] and others demonstrated the existence of strong radial non-uniformities in suspension concentration and solid flux and established that the fluid dynamic phenomena in the riser of the CFB were no different from those known to exist in petroleum industry transport reactors. It is now widely recognised that the CFB riser operates in a regime characterised by a rapidly rising dilute core surrounded by a slowly-falling suspension near to the walls. This was named refluxing dilute-phase transport by Rhodes [4], and in risers of circular cross-section has become known as core-annulus flow. Various techniques have been used to characterise the fluid dynamic conditions in the CFB riser. These have included optic fibre probes [S, 61 measuring particle velocity and suspension concentration profiles, X-ray photography measuring suspension concentration profiles [3], sampling probes measuring solids flux profiles [l, 7, 8, 9, lo], capacitance probes measuring suspension concentration profiles [ll] and the use of pressure fluctuations in characterising flow conditions [12]. In this study a
0032-5910/92/$5.00
recently developed non-isokinetic sampling probe was used to characterise the radial and angular non-uniformities in the riser of a cold model CFB. The authors believe that this same technique can be easily adapted to measuring in hot systems such as CFB combustors.
Experimental Circulating jluidized bed apparatus The circulating fluidized bed apparatus
used in this study is shown in Fig. 1. The riser has an internal diameter of 152 mm and consists of a 4 m section in aluminium resting on a PVC tee section connected to a 100 mm diameter L-valve which controls the flow of solids from the 305 mm id. reservoir into the riser. The exit of the riser tapers conically into a 100 mm i.d. bend connected to the inlet of the primary cyclone. The solids exit from this cyclone is connected to the top of the solids reservoir via a pneumatically operated diverter which is able to divert the entire flowrate of solids into a secondary fluidized column termed the measuring bed. Solids collected by the secondary cyclone (i.e. less than 1% of the total circulation) are returned to the solids reservoir unmeasured. Air leaving the gas exit of the secondary cyclone is cleaned by a bag filter. Solids collected in the measuring bed may be returned to the main system via a connection into the horizontal section of the L-valve whilst the apparatus is in operation.
0 1992 - Elsevier Sequoia. All rights reserved
278
Secondary
r
I Bag
The powder used in all experiments was FRFS, a non-porous alumina with a surface-volume mean size of 74.9 pm and a particle density of 2 456 kg mT3.
cyclone
Primary
cyclone
Measurement of solids fhx profiles
-
filter
/,I
Solids
inlet
Solids
reservoir’
1, x*Dl”erter
“a,“0
-
Measuring
Solids
bed
feeder
Riser Flexible
I Powder
outlet-
Butterfly
valve
Powder
outlet
pipe
I/lj
Fixed glass
-
bed of beads
2 Air from Rootes-type blower
Fig. 1. Schematic
--
of Cold Model Circulating
Fluidized
Bed.
The main air feed to the riser is delivered by a Rootes-type blower and passes through a distributor of glass beads at the bottom of the riser. Air velocity in the riser is measured by an orifice plate in the line from the blower and corrected for temperature and pressure in the riser. The solids circulation flux around the CFB loop is measured by diverting the entire solids flow from the base of the primary cyclone into the measuring bed. The solids collected in this way are fluidized and their mass determined by monitoring the pressure drop across the resulting fluidized bed.
Flow conditions in the CFB riser were characterised by the measurement of solids mass flux profiles, and mean suspension concentration. Solids flux profiles were measured using a non-isokinetic sampling probe developed by the authors. This probe is described in detail elsewhere [lo, 131 and so only a brief description of its construction, operation and capabilities is given here. The sampling probe itself and its associated sampling system are shown in Figs. 2 and 3. Previous work by the authors determined that the form of the probe shown in these figures, and in particular the shape of the probe tip, was optimum for accuracy and reproducibility. The sampling system has three modes; purge, standby and sampling. In the purge mode all sampling lines are purged into the riser to prevent ingress of powder. The system is switched to standby just before the sample is to be taken. In the standby mode solids are drawn from the riser into the standby collector, whilst the suction rate is adjusted to that required using needle valve V5 and the rotameter. Once the suction rate is set, the system is switched into sampling mode. After the timed sample period the system is briefly switched back into standby mode before being returned to purge status. This sequence of operations was found to reduce errors at the start and finish of the sampling period. The probe works on the following principle. It has been demonstrated [lo] that, under the conditions typical of a CFB riser, isokinetic sampling is strictly speaking not possible and not necessary. These authors demonstrated that an accurate measure of local solids flux in such conditions could be obtained by taking samples with the probe pointing upstream and then downstream at the same
I I
-Q_--
81eel
tubing 15.0 mm O.D.. 4.0
I/ --p_I
E
:: ?
1
De)tall Of
the
probe
Up
Fig. 2. Details of the Sampling Probe.
mm I.D.1 PoinM Indlcstlng the orlentetlon of the probe Up.
279
Sampling
Sampling
probe
1 he
Sample
collector
/
Needle
valve
purge air
Electronic pressuremeter
Stand
To
vacuum
-
by collector
Filter
Fig. 3. The Sampling System.
In this study each radial profile of solids flux was built up from a series of at least 11 individual net flux measurements. At each of the two axial positions, studied H, and H4, six radial profiles at angular positions Al-A6 were measured. These positions are shown in Fig. 4. Individual measurements of net solids flux were verified by comparing the value of cross-sectional mean solids flux obtained by integration of the solids flux profiles with the value obtained by external measurement (by diversion to the measuring bed). Because of the lack of axial symmetry in the flow in the riser, radial profile measurements at six angular positions had to be incorporated in the integration in order to achieve good agreement with the external measurement of crosssectional mean solids flux. This is demonstrated by the data given in Table 1. Measurement of pressure fluctuations
Angular
Positions
Fig. 4. Locations for Insertion of the Sampling Probe in the Riser.
suction velocity. These authors showed that the difference between these two sampling rates was independent of suction velocity at the probe tip provided that the suction velocity was sufficient to maintain the contents of the sampling lines in dilute flow. From the difference between the upward and downward sampling rates at a point, one can calculate the net upward solids mass flux at that point.
It has been demonstrated by the authors [13] that the pressure drop measured along a horizontal section of the sampling line from the non-isokinetic sampling probe is directly proportional to the probe solids sampling rate provided steady dilute conditions are maintained in the sampling lines. Since the local flux at a point in the riser fluctuates with time, so the instantaneous sampling rate and, hence, the pressure drop P, across the sampling line fluctuates as well. It is possible therefore to obtain qualitative information about changes in local solids flow with time from an analysis of these pressure drop fluctuations. This information, together with measurements of local suspension concentration are used here to build up a picture of the changes in local flow structure with radial position in the riser.
280 TABLE external
1. Comparison measurements
Probe used
P2 P2 P2 P2 P2 P2 Pl Pl Pl
-6
of values of cross-section
Superficial gas velocity U (m s-‘)
(1-c) 10-z
4.0 4.0 4.0 2.8 2.8 2.8 2.8 2.8 2.8
0.08
5.0 10.0
0.50
30.2 30.3 30.2 30.6 31.3 30.2 30.2
from
the
riser
G (measured
externally)
(kg m -* s-l)
60
40
20 Distance
ap
1.54 1.29 1.04 1.92 1.54 1.29
1
0
at mean solids flux derived from probe measurements
wall
X
in 10e3m
Fig. 5. Typical Radial Profile of Solids Mass Flux.
Results and discussion Solids jlux profiles
Individual solids flux profiles were approximately parabolic in form with a negative flux in the annular region near to the wall. A typical radial profile is shown in Fig. 5. Downward fluxes near to the wall reached a maximum of 10.5 times mean flux (i.e. a reduced flux of -10.5) and the maximum measured reduced flux at the axis of the riser was +5.0. However, the flow in the riser was generally non-axi-symmetric and radial profiles at six angular positions were required in order to describe the flow pattern adequately. Examples of the measured non-uniformities are presented here in
with values obtained
Gs (averaged over N positions) (kg m -2 s-l)
by
N
5.7 9.4 26.4 30.0 27.2 27.4 22.1 23.2 26.7
two forms; as plots of local flux against angular position with radial position as a parameter (Figs. 6(a) and (b)) and as contours of equal solids flux at a given axial position (Figs. 7 (a) and (b)). Considering first the variations in local solids flux, GSL, with radial and angular position shown in Fig. 6, it is clear that the solids flux in the riser does not exhibit axial symmetry. The angular non-uniformity is greatest at r/R values between 0.9 and 1.0, that is, in the downflow regions near to the riser wall. Although some similarities in angular variation in flux under different conditions are apparent, there seems to be no obvious pattern. One measure of the degree of angular non-uniformity is found by calculating the cross-sectional and time averaged solids flux GSL, ANAv from the radial profiles at each angular position, assuming axial symmetry. In the case of true axial symmetry the ratio GSL, ANAv/ G, will equal unity for each angular position. The greater the degree of angular non-uniformity, the greater the deviation of GsL, ANAv/GS from unity. Also, values of G sL, ANAv/GS< 1.0 indicate angular positions with high downward flux and values of G,,, ANAv/GS> 1.0 indicate positions where downward flux is low. Figure 8 shows how the value G,,, ANAv/GS varies with flow conditions and angular position. This figure indicates that the angular non-uniformities follow a certain pattern and, hence, are not purely random. Studying the contour of equal solids flux plotted in Figs. 7 (a) and (b) certain conclusions can be drawn. The profiles tend to flatten with increasing gas velocity and increasing height in the riser. In both cases this flattening is associated with a decrease in mean solids concentration. The form of the angular non-uniformity (i.e. displacement of the peak of the solids flux profiles towards angular position, A2) is common for all the conditions shown. In general, the lack of axial symmetry is thought to be associated with the way in which the solids and gas were fed into the riser. Other factors which are believed
281 10 -
Flow
6
conditions:
u
q
2.8
m/s;
Gs
E 30
kg/m2s;
A3
Ad
Height
H2
6
2
0 ii 0
-2
u 0” 2
-4
P
a
-6
-6
-10 ’ Al
Al’
A2
(4
A2’
Angular
Al
Al’
Position
10
Flow
6
6
-
conditions:
u = 4.0
m/s;
Gs - 30
kg/m*s;
Height
H4
I
I”
Al @)
Fig. 6. Variation
Al’
A2
A2’
Angular
A3
A3’
Al
Al’
Position
in the Local Solids Flux with Angular
and Radial Position
to influence symmetry are imperfections in the inside surface of the riser and the type of exit to the riser. Based on experience of operating a CFB with a larger diameter riser, the authors believe that the magnitude of angular non-uniformities may increase with scale. This is supported by the observations of Azzi et al. [14], on industrial scale risers.
in the Riser.
Pressure jhtuations
The fluctuations in sampling line pressure drop PI measured with the probe pointing downstream together with the associated solids flux profile at U=2.8 m s-‘, G, = 30 kg me2 s- ’ and angular position A2 are shown in Fig. 9. The corresponding suspension concentration profile measured using an optic fibre probe is shown
282
(a)
u :
2.8
GS:
30
Al
Al
A2’
A2’
(b)
m/s
Fig. 7. Contours
m/s
G.: 30 kg/m% Height H4
kg/m%
Height H2 II- &lap = 1.54
u = 4.0
et al. [15] in their work using high speed video. The optic fibre probe measurements of Fig. 10 indicate that in this region of smoother downflow the suspension concentration is considerably lower than that at the wall. Moving further away from the wall (plots (iv) and (v)), amplitude of pressure fluctuations is seen to increase once more around the region where downward flux gives way to upward flux before diminishing once more toward the riser axial centre (plot (vi)). Thus a local maximum in the amplitude of fluctuations of the suspension flow occurs at the point where GS,=O. and this maximum is associated with a local maximum in suspension concentration measured by the optic fibre probe. These observations are summarised schematically in Fig. 11.
x
(l-
1o-2
El.,:
0.5
x 1o.2
of Equal Solids Flux.
in Fig. 10. Generally speaking, the plots of Fig. 9 indicate a decrease in the amplitude of pressure fluctuations and, hence, solids flux variations in moving from the wall towards the riser centre. However, closer inspection reveals other useful information. In plots (i) and (ii), the strong low frequency component is thought to be associated with the unsteady downward motion of packets of solids observed through the transparent wall of a CFB riser under these conditions [15]. Moving away from the wall this low frequency disappears (plot (iii)), indicating a steadier, smoother downward flows of solids. This phenomenon was observed by Rhodes
Conclusions A non-isokinetic sampling probe has been used to characterise radial and angular non-uniformities in the riser of a cold model circulating fluidized bed operating in the refluxing transport or core-annulus flow regime. Radial variations in solids flux are thought to be characteristic of the fluid dynamics of such a gas-particle system. Deviations from axial symmetry which are thought to be apparatus dependent are difficult to eliminate and are expected to increase with scale. In a 152 mm diameter riser these deviations have been
l :
In
”
x v
1.0
\
. v
x x .
z
P .
D
;
x . 0
. : .
.
9 $
: x
x
0.5
t 7
i
. -1.0
’ Al
Al’
A2
A2’
Angular
Fig. 8. Variation
of GsL mav/G,
A3
Position
with Angular Position.
A3’
Al
Al’
283
PLIPd
PLiPd
WI
(ii)
(iI
3600
3600
2400
36
set
u : 2.6
m i’
G i 30 Height
kg
Angular
-11
40
20 Dlstanos
from
the
rlrer
60 wall
X In mm
porltlon
A2
I , ’ 80
I
0
mea 6-l
Ii4
-
Fig. 9. Fluctuations in the Sampling Line Pressure Drop: Variation of Amplitude and Frequency with Radial Position in the Riser.
surements of local suspension density by optic fibre probe have enabled a detailed picture of the radial changes in suspension flow structure to be assembled. g =
0.06
l
s
Acknowledgements
s 2 z
l 0.04
-
This research was carried out with the support of the Science and Engineering Research Council.
l
-
l
List of symbols
01
0
20
Distance
40
from
the
60
riser
wall .X
Fig. 10. Radial Variation in Time-Averaged centration (Measured by Optic Probe).
60
in mm
Suspension Con-
adequately characterised by measurement of radial flux profiles at six different angular positions. Measurement of the fluctuations in the pressure drop along a section of sampling line together with mea-
cross-sectional and time-averaged solids flux calculated from external measurements local time-averaged solids flux G SL G SL.ANAV cross-sectional and time-averaged flux calculated from a single radial solids flux profile assuming axial symmetry, r-R --- 1
GS
G
sL ANAV- R2,rr s
r-0
G,,2rrr
dr
284 Appearance
Distancefrom the riserwallX in mm
Fig. Il. Schematic
G SL,
RAV
Description
of Variations
-
in Flow Structure
mean solids flux at a given radial position calculated as the arithmetic mean of GSL values measured at each of six angular positions. G SL.RAV=
+L,AI+GsL.AZ+...+
G SL. A6 1
GSI
cross-sectional and time averaged solids flux calculated from six solids flux profiles r-R 1 GsI= G u_.,RAV 2rrdr R27T s
PI
sampling line pressure drop radius of riser radial distance from riser axis superficial gas velocity
r-0
R r
u
References
T. W. Bierl, L. J. Gajdos, A. E. McIver and J. J. McGovern, Studies in Support of Recirculating Bed Reactors for the Processing of Coal, Carnegie-Mellon University, Pittsburgh, PA, USA, prepared for the US Department of Energy Contract No. EX-C-76-01-2449, 1980. M. J. Rhodes and D. Geldart, Powder Technol., 53 (1987) 155.
I
Local MainFlow Dlrectlor
Fluctuations
I
with Radial Position
in the Riser.
3 H. Weinstein, M. Shao and M. G. Schnitzlein, in P. Basu (ed.), Circulating Fluidtied Bed Technology, Pergamon Press, Oxford, 1986, pp. 201-206. 4 M. J. Rhodes, Chem. Eng. Res. Des., 67 (1989) 30. 5 E. U. Hartge, Y. Li and J. Werther, in P. Basu (ed.), Circulating Fluidized Bed Technology, Pergamon Press, Oxford, 1986, p. 153. 6 M. Horio, K. Morishita, 0. Tachibana and N. Murata, in P. Basu and J. F. Large (eds.), Circulating Fluidired Bed Technolog~ II, Pergamon Press, Oxford 1988, p. 147. 7 L. Monceaux, M. Azzi, Y. Molodtsof and J. F. Large, in P. Basu (ed.), Circulating Fluidized Bed Technology, Pergamon Press, Oxford, 1986, p. 185. 8 L. W. Bolton and J. F. Davidson, in P. Basu and J. F. Large (eds.), Circulating Fluidized Bed Technology II, Pergamon Press, Oxford, 1988, p. 139. 9 R. Bader, J. Findlay and T. M. Knowlton, in P. Basu and J. F. Large (eds.), Circulating Fluidized Bed Technology II, Pergamon Press, Oxford, 1988, p. 123. 10 M. J. Rhodes, P. Laussmann, F. Villain and D. Geldart, in P. Basu and J. F. Large (eds.), Circulating Fluidized Bed Technology II, Pergamon Press, Oxford, 1988, pp. 155-164. 11 C. Brereton and L. Stromberg, in P. Basu (ed.), Circulating Fluidized Bed Technology, Pergamon Press, Oxford, 1986, pp. 133-144. 12 M. G. Schnitzlein and H. Weinstein, Chem. Eng. Sci., 43 (1988) 2605. 13 M. J. Rhodes and P. Laussmann, Powder Technol., 70 (1992) 141. 14 M. Azzi, P. Turlier, J. F. Large and J. R. Bernard, in P. Basu, M. Horio and M. Hasatani (eds.), Circulating Fluidized Bed Technology III, Pergamon Press, Oxford, 1991, pp. 189194. 15 M. J. Rhodes, H. Mineo and T. Hirama, in P. Basu, M. Horio and M. Hasatani (eds.), Circulating Fluidized Bed Technology III, Pergamon Press, Oxford, 1991, pp. 171-176.
Powder Technology, 72 (1992) 277-284
Characterising nori-uniformities circulating fluidized bed M. J. Rhodes
in gas-particle
flow in the riser of a
and P. Laussmann
Department of Chemical Engineering, University of Bradford, Bradford 807
1DP (UK)
(Received February 3, 1992; in revised form April 14, 1992)
Abstract A sampling probe operating in a non-isokinetic manner was used to measure variations of local solids mass flux with radial, angular and axial position and time in the 152 mm i.d. riser of a circulating fluidixed bed cold model. Time-averaged solids flux profiles exhibit a typically flattened parabolic shape with a region of downward flow adjacent to the riser wall. These profiles do not generally exhibit axial symmetry and a minimum of six radial profiles measurements are found to be necessary to describe the flow pattern adequately. These angular non-uniformities are reproducible, difficult to eliminate and although their form is probably system dependent, it is thought that such non-uniformities are likely to exist in all such gas-particle flow systems to a greater or lesser extent. Detail of the nature of the flowing suspension near to the riser wall has been delineated by analysis of variations in local solids flux with time measured using an adaptation of the same sampling probe.
Introduction Parallel with the commercial development of circulating fluidized bed (CFB) technology details of its operating characteristics and fluid dynamics are emerging thanks to a persistent academic research effort throughout the world. In recent years the many studies have.improved our understanding of the flow structure of the gas-particle suspension in the CFB riser. Bier1 et al. [l], Rhodes and Geldart [2], Weinstein et al. [3] and others demonstrated the existence of strong radial non-uniformities in suspension concentration and solid flux and established that the fluid dynamic phenomena in the riser of the CFB were no different from those known to exist in petroleum industry transport reactors. It is now widely recognised that the CFB riser operates in a regime characterised by a rapidly rising dilute core surrounded by a slowly-falling suspension near to the walls. This was named refluxing dilute-phase transport by Rhodes [4], and in risers of circular cross-section has become known as core-annulus flow. Various techniques have been used to characterise the fluid dynamic conditions in the CFB riser. These have included optic fibre probes [S, 61 measuring particle velocity and suspension concentration profiles, X-ray photography measuring suspension concentration profiles [3], sampling probes measuring solids flux profiles [l, 7, 8, 9, lo], capacitance probes measuring suspension concentration profiles [ll] and the use of pressure fluctuations in characterising flow conditions [12]. In this study a
0032-5910/92/$5.00
recently developed non-isokinetic sampling probe was used to characterise the radial and angular non-uniformities in the riser of a cold model CFB. The authors believe that this same technique can be easily adapted to measuring in hot systems such as CFB combustors.
Experimental Circulating jluidized bed apparatus The circulating fluidized bed apparatus
used in this study is shown in Fig. 1. The riser has an internal diameter of 152 mm and consists of a 4 m section in aluminium resting on a PVC tee section connected to a 100 mm diameter L-valve which controls the flow of solids from the 305 mm id. reservoir into the riser. The exit of the riser tapers conically into a 100 mm i.d. bend connected to the inlet of the primary cyclone. The solids exit from this cyclone is connected to the top of the solids reservoir via a pneumatically operated diverter which is able to divert the entire flowrate of solids into a secondary fluidized column termed the measuring bed. Solids collected by the secondary cyclone (i.e. less than 1% of the total circulation) are returned to the solids reservoir unmeasured. Air leaving the gas exit of the secondary cyclone is cleaned by a bag filter. Solids collected in the measuring bed may be returned to the main system via a connection into the horizontal section of the L-valve whilst the apparatus is in operation.
0 1992 - Elsevier Sequoia. All rights reserved
278
Secondary
r
I Bag
The powder used in all experiments was FRFS, a non-porous alumina with a surface-volume mean size of 74.9 pm and a particle density of 2 456 kg mT3.
cyclone
Primary
cyclone
Measurement of solids fhx profiles
-
filter
/,I
Solids
inlet
Solids
reservoir’
1, x*Dl”erter
“a,“0
-
Measuring
Solids
bed
feeder
Riser Flexible
I Powder
outlet-
Butterfly
valve
Powder
outlet
pipe
I/lj
Fixed glass
-
bed of beads
2 Air from Rootes-type blower
Fig. 1. Schematic
--
of Cold Model Circulating
Fluidized
Bed.
The main air feed to the riser is delivered by a Rootes-type blower and passes through a distributor of glass beads at the bottom of the riser. Air velocity in the riser is measured by an orifice plate in the line from the blower and corrected for temperature and pressure in the riser. The solids circulation flux around the CFB loop is measured by diverting the entire solids flow from the base of the primary cyclone into the measuring bed. The solids collected in this way are fluidized and their mass determined by monitoring the pressure drop across the resulting fluidized bed.
Flow conditions in the CFB riser were characterised by the measurement of solids mass flux profiles, and mean suspension concentration. Solids flux profiles were measured using a non-isokinetic sampling probe developed by the authors. This probe is described in detail elsewhere [lo, 131 and so only a brief description of its construction, operation and capabilities is given here. The sampling probe itself and its associated sampling system are shown in Figs. 2 and 3. Previous work by the authors determined that the form of the probe shown in these figures, and in particular the shape of the probe tip, was optimum for accuracy and reproducibility. The sampling system has three modes; purge, standby and sampling. In the purge mode all sampling lines are purged into the riser to prevent ingress of powder. The system is switched to standby just before the sample is to be taken. In the standby mode solids are drawn from the riser into the standby collector, whilst the suction rate is adjusted to that required using needle valve V5 and the rotameter. Once the suction rate is set, the system is switched into sampling mode. After the timed sample period the system is briefly switched back into standby mode before being returned to purge status. This sequence of operations was found to reduce errors at the start and finish of the sampling period. The probe works on the following principle. It has been demonstrated [lo] that, under the conditions typical of a CFB riser, isokinetic sampling is strictly speaking not possible and not necessary. These authors demonstrated that an accurate measure of local solids flux in such conditions could be obtained by taking samples with the probe pointing upstream and then downstream at the same
I I
-Q_--
81eel
tubing 15.0 mm O.D.. 4.0
I/ --p_I
E
:: ?
1
De)tall Of
the
probe
Up
Fig. 2. Details of the Sampling Probe.
mm I.D.1 PoinM Indlcstlng the orlentetlon of the probe Up.
279
Sampling
Sampling
probe
1 he
Sample
collector
/
Needle
valve
purge air
Electronic pressuremeter
Stand
To
vacuum
-
by collector
Filter
Fig. 3. The Sampling System.
In this study each radial profile of solids flux was built up from a series of at least 11 individual net flux measurements. At each of the two axial positions, studied H, and H4, six radial profiles at angular positions Al-A6 were measured. These positions are shown in Fig. 4. Individual measurements of net solids flux were verified by comparing the value of cross-sectional mean solids flux obtained by integration of the solids flux profiles with the value obtained by external measurement (by diversion to the measuring bed). Because of the lack of axial symmetry in the flow in the riser, radial profile measurements at six angular positions had to be incorporated in the integration in order to achieve good agreement with the external measurement of crosssectional mean solids flux. This is demonstrated by the data given in Table 1. Measurement of pressure fluctuations
Angular
Positions
Fig. 4. Locations for Insertion of the Sampling Probe in the Riser.
suction velocity. These authors showed that the difference between these two sampling rates was independent of suction velocity at the probe tip provided that the suction velocity was sufficient to maintain the contents of the sampling lines in dilute flow. From the difference between the upward and downward sampling rates at a point, one can calculate the net upward solids mass flux at that point.
It has been demonstrated by the authors [13] that the pressure drop measured along a horizontal section of the sampling line from the non-isokinetic sampling probe is directly proportional to the probe solids sampling rate provided steady dilute conditions are maintained in the sampling lines. Since the local flux at a point in the riser fluctuates with time, so the instantaneous sampling rate and, hence, the pressure drop P, across the sampling line fluctuates as well. It is possible therefore to obtain qualitative information about changes in local solids flow with time from an analysis of these pressure drop fluctuations. This information, together with measurements of local suspension concentration are used here to build up a picture of the changes in local flow structure with radial position in the riser.
280 TABLE external
1. Comparison measurements
Probe used
P2 P2 P2 P2 P2 P2 Pl Pl Pl
-6
of values of cross-section
Superficial gas velocity U (m s-‘)
(1-c) 10-z
4.0 4.0 4.0 2.8 2.8 2.8 2.8 2.8 2.8
0.08
5.0 10.0
0.50
30.2 30.3 30.2 30.6 31.3 30.2 30.2
from
the
riser
G (measured
externally)
(kg m -* s-l)
60
40
20 Distance
ap
1.54 1.29 1.04 1.92 1.54 1.29
1
0
at mean solids flux derived from probe measurements
wall
X
in 10e3m
Fig. 5. Typical Radial Profile of Solids Mass Flux.
Results and discussion Solids jlux profiles
Individual solids flux profiles were approximately parabolic in form with a negative flux in the annular region near to the wall. A typical radial profile is shown in Fig. 5. Downward fluxes near to the wall reached a maximum of 10.5 times mean flux (i.e. a reduced flux of -10.5) and the maximum measured reduced flux at the axis of the riser was +5.0. However, the flow in the riser was generally non-axi-symmetric and radial profiles at six angular positions were required in order to describe the flow pattern adequately. Examples of the measured non-uniformities are presented here in
with values obtained
Gs (averaged over N positions) (kg m -2 s-l)
by
N
5.7 9.4 26.4 30.0 27.2 27.4 22.1 23.2 26.7
two forms; as plots of local flux against angular position with radial position as a parameter (Figs. 6(a) and (b)) and as contours of equal solids flux at a given axial position (Figs. 7 (a) and (b)). Considering first the variations in local solids flux, GSL, with radial and angular position shown in Fig. 6, it is clear that the solids flux in the riser does not exhibit axial symmetry. The angular non-uniformity is greatest at r/R values between 0.9 and 1.0, that is, in the downflow regions near to the riser wall. Although some similarities in angular variation in flux under different conditions are apparent, there seems to be no obvious pattern. One measure of the degree of angular non-uniformity is found by calculating the cross-sectional and time averaged solids flux GSL, ANAv from the radial profiles at each angular position, assuming axial symmetry. In the case of true axial symmetry the ratio GSL, ANAv/ G, will equal unity for each angular position. The greater the degree of angular non-uniformity, the greater the deviation of GsL, ANAv/GS from unity. Also, values of G sL, ANAv/GS< 1.0 indicate angular positions with high downward flux and values of G,,, ANAv/GS> 1.0 indicate positions where downward flux is low. Figure 8 shows how the value G,,, ANAv/GS varies with flow conditions and angular position. This figure indicates that the angular non-uniformities follow a certain pattern and, hence, are not purely random. Studying the contour of equal solids flux plotted in Figs. 7 (a) and (b) certain conclusions can be drawn. The profiles tend to flatten with increasing gas velocity and increasing height in the riser. In both cases this flattening is associated with a decrease in mean solids concentration. The form of the angular non-uniformity (i.e. displacement of the peak of the solids flux profiles towards angular position, A2) is common for all the conditions shown. In general, the lack of axial symmetry is thought to be associated with the way in which the solids and gas were fed into the riser. Other factors which are believed
281 10 -
Flow
6
conditions:
u
q
2.8
m/s;
Gs
E 30
kg/m2s;
A3
Ad
Height
H2
6
2
0 ii 0
-2
u 0” 2
-4
P
a
-6
-6
-10 ’ Al
Al’
A2
(4
A2’
Angular
Al
Al’
Position
10
Flow
6
6
-
conditions:
u = 4.0
m/s;
Gs - 30
kg/m*s;
Height
H4
I
I”
Al @)
Fig. 6. Variation
Al’
A2
A2’
Angular
A3
A3’
Al
Al’
Position
in the Local Solids Flux with Angular
and Radial Position
to influence symmetry are imperfections in the inside surface of the riser and the type of exit to the riser. Based on experience of operating a CFB with a larger diameter riser, the authors believe that the magnitude of angular non-uniformities may increase with scale. This is supported by the observations of Azzi et al. [14], on industrial scale risers.
in the Riser.
Pressure jhtuations
The fluctuations in sampling line pressure drop PI measured with the probe pointing downstream together with the associated solids flux profile at U=2.8 m s-‘, G, = 30 kg me2 s- ’ and angular position A2 are shown in Fig. 9. The corresponding suspension concentration profile measured using an optic fibre probe is shown
282
(a)
u :
2.8
GS:
30
Al
Al
A2’
A2’
(b)
m/s
Fig. 7. Contours
m/s
G.: 30 kg/m% Height H4
kg/m%
Height H2 II- &lap = 1.54
u = 4.0
et al. [15] in their work using high speed video. The optic fibre probe measurements of Fig. 10 indicate that in this region of smoother downflow the suspension concentration is considerably lower than that at the wall. Moving further away from the wall (plots (iv) and (v)), amplitude of pressure fluctuations is seen to increase once more around the region where downward flux gives way to upward flux before diminishing once more toward the riser axial centre (plot (vi)). Thus a local maximum in the amplitude of fluctuations of the suspension flow occurs at the point where GS,=O. and this maximum is associated with a local maximum in suspension concentration measured by the optic fibre probe. These observations are summarised schematically in Fig. 11.
x
(l-
1o-2
El.,:
0.5
x 1o.2
of Equal Solids Flux.
in Fig. 10. Generally speaking, the plots of Fig. 9 indicate a decrease in the amplitude of pressure fluctuations and, hence, solids flux variations in moving from the wall towards the riser centre. However, closer inspection reveals other useful information. In plots (i) and (ii), the strong low frequency component is thought to be associated with the unsteady downward motion of packets of solids observed through the transparent wall of a CFB riser under these conditions [15]. Moving away from the wall this low frequency disappears (plot (iii)), indicating a steadier, smoother downward flows of solids. This phenomenon was observed by Rhodes
Conclusions A non-isokinetic sampling probe has been used to characterise radial and angular non-uniformities in the riser of a cold model circulating fluidized bed operating in the refluxing transport or core-annulus flow regime. Radial variations in solids flux are thought to be characteristic of the fluid dynamics of such a gas-particle system. Deviations from axial symmetry which are thought to be apparatus dependent are difficult to eliminate and are expected to increase with scale. In a 152 mm diameter riser these deviations have been
l :
In
”
x v
1.0
\
. v
x x .
z
P .
D
;
x . 0
. : .
.
9 $
: x
x
0.5
t 7
i
. -1.0
’ Al
Al’
A2
A2’
Angular
Fig. 8. Variation
of GsL mav/G,
A3
Position
with Angular Position.
A3’
Al
Al’
283
PLIPd
PLiPd
WI
(ii)
(iI
3600
3600
2400
36
set
u : 2.6
m i’
G i 30 Height
kg
Angular
-11
40
20 Dlstanos
from
the
rlrer
60 wall
X In mm
porltlon
A2
I , ’ 80
I
0
mea 6-l
Ii4
-
Fig. 9. Fluctuations in the Sampling Line Pressure Drop: Variation of Amplitude and Frequency with Radial Position in the Riser.
surements of local suspension density by optic fibre probe have enabled a detailed picture of the radial changes in suspension flow structure to be assembled. g =
0.06
l
s
Acknowledgements
s 2 z
l 0.04
-
This research was carried out with the support of the Science and Engineering Research Council.
l
-
l
List of symbols
01
0
20
Distance
40
from
the
60
riser
wall .X
Fig. 10. Radial Variation in Time-Averaged centration (Measured by Optic Probe).
60
in mm
Suspension Con-
adequately characterised by measurement of radial flux profiles at six different angular positions. Measurement of the fluctuations in the pressure drop along a section of sampling line together with mea-
cross-sectional and time-averaged solids flux calculated from external measurements local time-averaged solids flux G SL G SL.ANAV cross-sectional and time-averaged flux calculated from a single radial solids flux profile assuming axial symmetry, r-R --- 1
GS
G
sL ANAV- R2,rr s
r-0
G,,2rrr
dr
284 Appearance
Distancefrom the riserwallX in mm
Fig. Il. Schematic
G SL,
RAV
Description
of Variations
-
in Flow Structure
mean solids flux at a given radial position calculated as the arithmetic mean of GSL values measured at each of six angular positions. G SL.RAV=
+L,AI+GsL.AZ+...+
G SL. A6 1
GSI
cross-sectional and time averaged solids flux calculated from six solids flux profiles r-R 1 GsI= G u_.,RAV 2rrdr R27T s
PI
sampling line pressure drop radius of riser radial distance from riser axis superficial gas velocity
r-0
R r
u
References
T. W. Bierl, L. J. Gajdos, A. E. McIver and J. J. McGovern, Studies in Support of Recirculating Bed Reactors for the Processing of Coal, Carnegie-Mellon University, Pittsburgh, PA, USA, prepared for the US Department of Energy Contract No. EX-C-76-01-2449, 1980. M. J. Rhodes and D. Geldart, Powder Technol., 53 (1987) 155.
I
Local MainFlow Dlrectlor
Fluctuations
I
with Radial Position
in the Riser.
3 H. Weinstein, M. Shao and M. G. Schnitzlein, in P. Basu (ed.), Circulating Fluidtied Bed Technology, Pergamon Press, Oxford, 1986, pp. 201-206. 4 M. J. Rhodes, Chem. Eng. Res. Des., 67 (1989) 30. 5 E. U. Hartge, Y. Li and J. Werther, in P. Basu (ed.), Circulating Fluidized Bed Technology, Pergamon Press, Oxford, 1986, p. 153. 6 M. Horio, K. Morishita, 0. Tachibana and N. Murata, in P. Basu and J. F. Large (eds.), Circulating Fluidired Bed Technolog~ II, Pergamon Press, Oxford 1988, p. 147. 7 L. Monceaux, M. Azzi, Y. Molodtsof and J. F. Large, in P. Basu (ed.), Circulating Fluidized Bed Technology, Pergamon Press, Oxford, 1986, p. 185. 8 L. W. Bolton and J. F. Davidson, in P. Basu and J. F. Large (eds.), Circulating Fluidized Bed Technology II, Pergamon Press, Oxford, 1988, p. 139. 9 R. Bader, J. Findlay and T. M. Knowlton, in P. Basu and J. F. Large (eds.), Circulating Fluidized Bed Technology II, Pergamon Press, Oxford, 1988, p. 123. 10 M. J. Rhodes, P. Laussmann, F. Villain and D. Geldart, in P. Basu and J. F. Large (eds.), Circulating Fluidized Bed Technology II, Pergamon Press, Oxford, 1988, pp. 155-164. 11 C. Brereton and L. Stromberg, in P. Basu (ed.), Circulating Fluidized Bed Technology, Pergamon Press, Oxford, 1986, pp. 133-144. 12 M. G. Schnitzlein and H. Weinstein, Chem. Eng. Sci., 43 (1988) 2605. 13 M. J. Rhodes and P. Laussmann, Powder Technol., 70 (1992) 141. 14 M. Azzi, P. Turlier, J. F. Large and J. R. Bernard, in P. Basu, M. Horio and M. Hasatani (eds.), Circulating Fluidized Bed Technology III, Pergamon Press, Oxford, 1991, pp. 189194. 15 M. J. Rhodes, H. Mineo and T. Hirama, in P. Basu, M. Horio and M. Hasatani (eds.), Circulating Fluidized Bed Technology III, Pergamon Press, Oxford, 1991, pp. 171-176.