Znd. Eng. Chem. Res. 1996,34, 4033-4039
4033
Study of Local Properties in Conical Spouted Beds Using an Optical Fiber Probe Martin Olazar,* Maria J. San Jose, Ricardo LLamosas, Sonia Alvarez, and Javier Bilbao Departamento de Ingenieria Quimica, Universidad del Pais Vasco, Apartado 644, 48080 Bilbao, Spain
The geometry of the spout, the local bed voidage, and the maps of velocities and trajectories of the particles have been studied in conical spouted beds by means of a probe composed of three optical fibers placed in parallel. The overall analysis of these properties allows for understanding solid circulation in the contactor and at the interface between the annular and spout interface. The influence on the local properties of the geometric factors of the contactor (angle and inlet diameter) and of the operating conditions (diameter of the glass spheres, stagnant bed height,and excess of air velocity above that corresponding to the minimum spouting) has been studied. The results are interesting for a rigorous modeling of gas and solid flow in the contactor. 1. Introduction
Conical spouted beds are a contact method of simple construction and easy design, which are especially suitable for the handling of solids with a wide particle size distribution or sticky solids that are difficult t o handle with other gas-solid contact techniques. They have a great capability for bed expansion, up t o the jet spouted bed regime, which increases their versatility for systems that require a high bed voidage together with a vigorous contact between the phases. In previous papers, the geometric factors and the design requirements for stability have been delimited and the operating conditions have been compared with those of other gas-solid contact techniques (Olazar et al., 1992). Original correlations have also been obtained for calculation of hydrodynamic properties (Olazar et al., 1993a; San Jose et al., 1992, 1993) and of correlations for the contactor design (Olazar et al., 1993b).Both the hydrodynamics of mixtures of different particle size (Olazar et al., 1993~)and their segregation have been studied. Segregation was determined by mixing indices obtained by following treatments similar to the conventional ones for fluidized beds (San Jose et al., 1994). The conical spouted beds have been used in the gasification of bituminous coals (Uemaki and Tsuji, 1986, 1991; Tsuji et al., 1989), and in the treatment of wood residues (Olazar et al., 1994). The progress in the use of conical spouted beds in applications such as catalytic reactions (interesting on the basis of the capability of these contactors in the handling of fine particles and of their versatility for operating with short gas contact times) requires the development of models for gas and solid flow that allow for a rigorous reactor design. With the aim of progressing in this subject, an original model for the gas flow has been proposed (Olazar et al., 1995), in which a uniform spout diameter with the longitudinal position has been considered. In the same way, for the sake of simplicity, in the application of the model the bed voidage is considered uniform in each one of the annular and spout zones. No models have yet been published in the literature t o describe the solid flow in conical spouted beds. In the case of cylindrical spouted beds, models that define the solid trajectories in the annular zone along stream tubes, which are experimentally determined using solid tracers and optical observation in half columns, have been proposed. In these experimental papers reported 0888-5885/95/2634-4033$09.0Ol0
by Epstein and Grace (1984) the incorporation of the solid t o the spout zone at different levels along the spout-annulus interface is quantified. Hook et al. (1992) have proposed a theoretical model to describe the gas and solid (Co304 catalyst supported on alumina) circulation patterns in a cylindrical spouted bed reactor used in an adiabatic regime for CO oxidation. The gas flow model in the annular zone is based on the vector form of the Ergun equation as proposed by Stanek and Szekely (1974), and the particle circulation patterns in the annular zone are determined by a minimum path-length analysis. Krzywanski et al. (1992) have proposed a multidimensional model for cylindrical spouted beds using the Ergun equation for the gas flow model in the annular zone and equations of soil mechanics for the solid flow model in the annular zone. In this paper the experimental study of local properties in conical spouted beds, which are interesting for the proposal of real models for gas and solid flow, has been approached. The properties studied are: (1)spout geometry, (2) local bed voidage in the annular and spout zones, and (3) the velocity and trajectory of particles. The technique used has been an optical fiber probe, and the experimental study, at pilot plant scale, has been carried out using glass spheres of different particle size in conical contactors of different geometry (angle, inlet diameter) and under different operating conditions (stagnant bed height and air velocity).
2. Experimental Section The unit used has been described in detail in previous papers (Olazar et al., 1992, 1993a). In Figure 1a diagram of the equipment used and the arrangement of the optical fiber probe in the contactor are shown. Five conical contactors of poly(methy1 methacrylate) have been used, which have the following dimensions (geometry defined in Figure 1): column diameter D,, 0.36 m; base diameter Di,0.06 m; height of the conical section H,, 0.36, 0.40, 0.45, 0.50, and 0.60 m; angle of the contactor y , 45", 39", 36", 33", and 28"; gas inlet diameter Do, 0.03,0.04, and 0.05 m; stagnant bed height H,, between 0.10 and 0.30 m. The solids studied have been glass spheres (density = 2420 kg/m3)of the following particle diameters: 1,2, 3, 4, 6, and 8 mm. A vertical displacement device is provided for the probe, Figure 1. This device positions the probe in front
0 1995 American Chemical Society
4034 Ind. Eng. Chem. Res., Vol. 34, No. 11, 1995
Photodiodes
r
Light Source and Amplifiers
Contactor
\
/
\
h,-B A/D
Interface
Figure 1. Scheme of the equipment used and the arrangement of the optical fiber probe. To Receiver f -
From Light Source
To Receiver Front View
Side View
Figure 2. Arrangement of the three optical fibers in the probe.
of the contactor hole, at the level at which the measurement is to be carried out. The probe is manually placed in the radial position in the bed, through holes made in the contactor wall (every 20-mm height), as is shown in Figure 1. Graduation of the probe allows for setting the radial position in the bed. The probe, Figure 2, consists of a stainless steel encasing, graduated every millimeter, whose maximum and minimum external dimensions are 5 and 1.5 mm, respectively, which contains three optical fibers placed in parallel. The principle of the measurement is based on the emission of a light beam by the central fiber, which is reflected on the particles of the bed. Each time that a particle passes in front of the probe, it reflects light, which is successively collected by the two fibers placed above and below the emitting fiber. The shape of the probe avoids perturbations in the gas and solid flow. The maximum distance for light reception delimits the field range of the probe. By changing the light intensity, a field range of the same order as the particle size is attained, which permits a signal of the maximum clarity. The effective distance between the two fibers that receive the reflected light, which is an important parameter in calculating the particle velocity, has been experimentally determined on a rotating disc of known angular velocity (Benkrid and Caram, 19891, and a value of 4.3 mm was obtained. The delimitation of the interface between the spout zone and the annular zooe has been carried out as is shown in Figure 3. When the tip of the probe is in the annular zone, the corresponding signal is formed by wide peaks, due to the fact the particles are in contact and moving at a low velocity. When the tip of the probe reaches the spout zone, the signal registered, which is
ANNULAR ZONE SIGNAL
SPOUT ZONE SIGNAL
Figure 3. Signals of the optical fiber probe in the annular and spout zones of the bed.
formed by narrow and pronounced peaks, is the one corresponding to particles in movement at high velocity and in contact with each other. The precise point where the change from one signal to the other happens corresponds to the radial position of the annular zonespout zone interface. The measurements have been carried out every 20 mm of bed level and at radial positions of the probe tip every 2.5 mm. In this way, the interface between the spout zone and the annular zone is delimited with an experimental error of f1.25 mm. In previous papers where optical fibers have been used in fluidized beds (&in and Liu, 1982; Matsuna et al., 1983; Boiarski, 19851, a linear relationship has been found between bed voidage at the measuring point and the output signal of the fiber optic probe. This fact has been used in cylindrical spouted beds by He et al. (1994a). The intensity of the light reflected by the particles that pass in front of the fiber depends on the type or composition of the particle, on its size or size distribution, and on the bed voidage. For this reason a calibration has been carried out for this solid so that local bed voidage has been related to the probe signals, either in the spout zone or in the annular zone. The calibration procedure has been as follows: For the spout zone, and certain positions of the annular zone (fountain), in which the particles do not touch each other and the bed voidage is high, the solid is fed by means of a hopper to a 60-mm i.d column where the probe has been introduced at a given level. A linear relationship between the intensity of the reflected light (area under the curve corresponding t o the signal) and the volume fraction of the bed occupied by the particles, 1 - E , is obtained. By changing the solid flow rate, the bed voidage, E , is changed and the calibration curve is obtained. The bed voidage has been calculated from the solid flow rate in the feed, Q, with the following expression:
Q = pSv,(l - E )
(1)
In eq 1, the velocity of the particles along the longitudinal direction at the probe zone, vz,is calculated as is explained later.
Ind. Eng. Chem. Res., Vol. 34, No. 11, 1995 4035 The bed voidage has been calculated by means of an image treatment system (Peiias, 1993),which quantifies the number of particles a t the column zone where the probe is located. The results are identical t o those obtained using eq 1. The intensity of the light reflected has been calculated from the area under the voltage vs time curve, by means of an integration program incorporated into the general program for data treatment, MATLAB 4.2. The calibration of bed voidage in the annular zone has consisted of the measurement in moving beds in a column of 60-mm i.d., using different particle sizes (of known bed voidage). With each bed, measurements have been carried out under different situations: packed beds, beds loosened to the maximum, and partially loosened beds. In this way, bed voidage values in the 0.38-0.53 range have been obtained. The average relative error of the measurement, which has been repeated three times a t each point, is 4%. On the other hand, it has been proven that the position of the probe in the bed does not affect the resulting calibration curve. The light reflected by the particles and received by the optical fibers of the probe is sent t o two analogical channels, Figure 1. From a statistical analysis, by means of the cross-correlation function (incorporated into MATLAB 4.2 program), the signals with a correlation coefficient higher than 90% are accepted (which indicates that the same particles pass in front of both fibers). From the effective distance between the two receiving fibers (de = 4.3 mm) and the delay time between the two signals, z (time corresponding to the maximum value of the cross-correlationfunction), it can be ascertained whether the displacement is upward or downward (positive or negative time delay), and the velocity of the particle along the longitudinal direction can be calculated:
vz = d$r
(2)
3. Results 3.1. Geometry of the Spout. The general shape of the spout is shown in Figure 3, which corresponds to a system adopted as an example ( y = 45"; Do = 0.03 m; d, = 4 mm; Ho= 0.18 m; u = 1.02ums). Qualitatively, the shape of the spout zone is similar in all the systems studied and has a pronounced expansion near the inlet of the contactor, which is followed by a neck and then expands toward the fountain. It is noteworthy that the average diameter of the spout is much higher than the inlet diameter. The spout shape in Figure 3 has certain similarities with those of the cylindrical spouted beds of conical base observed by Mathur and Epstein (1974) but with the peculiarity that the expansion near the contactor inlet and towards the fountain is more pronounced in the conical spouted beds. There are differences between the results of this paper and the previous observations in the relevant literature for conical spouted beds. Mukhlenov and Gorshtein (1965) using a piezoelectric probe determined that the spout continually expands from the contactor base. Goltsiker (19671, from experiments carried out in two dimensional contactors, observed a pronounced neck between two expansion zones. This spout shape is the one proposed as typical in conical spouted beds (Romankov and Rashkovskaya, 1968) and is similar to the one observed in the present paper, in which the follow-
De= 0.03m Ho=0.18m
0 ' 0
1
I
0.10
0.20
44 &=O.l8m dp=4mm De=0.09 m 0.04 0.05
1.0
0
0.10
0.20
44 Figure 4. Effect of the contactor geometric factors on the spout diameter. a, spout diameter vs bed level, for different values of the contactor angle; b, spout diameterhnlet diameter ratio vs bed level, for different values of the inlet diameter.
ing main differences have been found: the neck near the inlet is less pronounced and the expansion toward the fountain is more pronounced than that pointed out by Goltsiker (1967). In Figure 4 the effect of the geometric factors of the contactor on the spout geometry is shown. The results correspond t o u = 1.02ums. In Figure 4a, in which the spout diameter has been plotted against the bed level, for three values of the contactor angle, it is observed that the longitudinal position of the spout neck is nearer the contactor inlet as the contactor angle increases. In Figure 4b, in which the evolution of the ratio between the spout diameter and the contactor inlet diameter, D$ Do, is analyzed along the longitudinal position in the contactor, for three values of inlet diameter, it is observed that the position of the neck is further from the inlet as the inlet diameter increases. The great influence of the inlet diameter on the general expansion of the spout is noteworthy. The spout diameter in the upper part of the bed is three times the contactor inlet diameter, for an inlet diameter of Do = 0.03 m (DdDi =
lh). The influence of the operating conditions (particle diameter, air velocity, and stagnant bed height) on the magnitude of the spout neck, Figure 5, is less pronounced than the influence of the geometric factors, as previously commented. The position of the neck is nearer the contactor inlet as the particle diameter decreases, Figure 5a, as the stagnant bed height decreases, Figure 5b, and as the air velocity decreases (always above the minimum spouting velocity, urns), Figure 5c. The results of Figure 5a,b correspond to u = 1.O2um,. 3.2. Bed Voidage. As an example of the results, in Figure 6 the longitudinal and radial profiles of local bed voidage in the bed have been plotted, for one of the
4036 Ind. Eng. Chem. Res., Vol. 34, No. 11, 1995 0.10
I
I
Ds(m)
1
F
a
L -
O
Y =33' DO=0.03m &,=0.20m
0
7 = 36' D.= 0.03m dp= 4 mm Ha=0.18m 0.10
0
0.20
0.20
0.10
(m)
44 Y =33' D. = 0 . 0 3 ~ 1 dp=4mm
DS (4
Y =36" D. = 0.03 m d , = 4 mm H a =0.18 m 1
O L
0
Ds(m) -
-
0.10
1
Y =33. D0=0.04m dp=4mm HO= 0.18 m
c
0
I
0.20 z(m)
0.04
0
0.08 r(m)
Figure 6. Local bed voidages for one of the systems studied: a, longitudinal profiles; b, radial profiles. -
1.0 1
i 0.5
I\
Y =36'
D. = 0.03 m 0 ' 0
1
0.10
d , = 4 mm H. = 0.18 m
1
0.20 z(m)
Figure 5. Effect of the operating conditions on the spout diameter. a, spout diameter vs bed level, for different values of the particle diameter; b, spout diameter vs bed level, for different values of the stagnant bed height; c, spout diameter vs bed level, for iliflerent values of the air velocity.
systems studied ( y = 36"; Do= 0.03 m; d, = 4 mm; Ho = 0.18 m; u = 1 . 0 2 ~ ~ ~Each ) . curve in Figure Sa corresponds to a radial position in the contactor and each curve in Figure 6b to a longitudinal position. In Figure 6a it is observed that the bed voidage at the contactor axis (for r* = r/R = 0) decreases almost linearly from values near unity at. the contactor base up t o a value of 0.83 a t the surface of the bed. In other radial positions, the bed voidage passes through a minimum value, and then, for a higher level in the bed, the bed voidage passes through a maximum value. As the radial coordinate increases, the minimum and the maximum of the bed voidage correspond to higher levels in the bed. It is noteworthy that the positions for r* = 0.5 and 0.95 both correspond to the annular zone and that, in addition to the previously discussed effect on the longitudinal position, the difference between the value of bed voidage in these two positions of the annular zone is very important. The influence of the radial position on the bed voidage, Figure 6b, is even more important than that
0 '
0
I
I
0.04 0.08
I
I
0.12
0.16
0.20
z(m) Figure 7. Average bed voidage a t each longitudinal position in the annular and spout zones.
on the longitudinal position. At longitudinal positions near the contactor base the bed voidage decreases with the radius in a very pronounced way. The decrease is attenuated in the upper half of the bed (for z = 0.12 and z = 0.18 m), and except a t the base of the contactor, the bed voidage is uniform in positions near the contactor wall. Although the information in Figure 6 can be useful for a rigorous design of the operation in conical spouted beds when the local bed voidage is used, the knowledge of the average bed voidage at each longitudinal position in the annular and spout zones, Figure 7, is also interesting for a more simplified design. These results have been obtained by averaging those of Figure 6a. It is observed in Figure 7 that the decrease in the average bed voidage in the spout zone with bed longitudinal position attenuates down to a minimum value of 0.7 a t the upper surface of the bed. The decrease in the average bed voidage in the annular zone with the longitudinal position is nearly linear from 0.67 at the 0.02-m bed level down to a minimum value of 0.47 at the upper surface of the bed.
Ind. Eng. Chem. Res., Vol. 34, No. 11, 1995 4037
''
14
Y = 33" D.= 0.05m
a
(m/s) 12
10 6 6
4 2
n
0.05
0
0.15
0.10
0.20 z (m)
S-1
b
1
Y = 33'
I
Figure 8. Local particle velocities at different longitudinal positions of the bed for one of the systems studied ( y = 33";Do= 0.03 m; d, = 2 mm; H,= 0.23 m; u = 1 . 0 2 ~ ~ ~ ) .
3.3. Velocity and Trajectory of the Particles. In Figure 8 the local velocities a t 10 longitudinal positions of the bed are shown, as an example, for one of the systems studied ( y = 33"; D o= 0.03 m; d, = 2 mm; Ho = 0.23 m; u = 1.O2um,). In the spout zone the upward longitudinal velocity is maximum at the contactor axis, decreases as the radial position in the contactor increases, and is practically zero a t a position that corresponds to the spout-annular interface. In the annular zone the downward velocity has a maximum at an intermediate radial position. Previously, Rovero et al. (1985) have also determined, in cylindrical spouted beds, that due to the wall friction the velocity near the wall is lower than the velocity in the central region of the annular zone. The particles that circulate toward the base of the contactor at positions near the contactor wall accelerate as they approach the base of the contactor, so that the maximum downward velocity is attained at the contactor base and close to the wall. This result has also been observed in conical-cylindrical spouted beds (He et al., 1994b) but is more pronounced in the totally conical contactors studied in this paper. In all the systems studied, qualitatively similar velocity profiles to those of Figure 8 have been observed. It is noteworthy that the solid velocity profile in both the annular and spout zones is very pronounced near the inlet (at a level one-tenth of the total bed height). As the longitudinal position in the bed increases, the velocity profiles in the two zones become flatter. These results are qualitatively similar to those obtained by Kmiec (1980) in conical-cylindrical spouted beds and by Uemaki and Tsuji (1992) in jet-spouted beds. It must be pointed out that the study on the solid velocity profiles in the fountain is beyond the scope of this paper, as the results obtained have a difficult interpretation and require a special analysis by combining the results obtained using the optical fiber probe described in this paper with the results of an image treatment technique (Pefias, 1993). The solid velocity distribution in the spout zone can be appreciated in more detail in Figure 9, where the velocity profiles, longitudinal (Figure Sa) and radial (Figure 9b), have been plotted for one of the systems studied ( y = 33"; Do= 0.05 m; d, = 4 mm; Ho= 0.18 m; u = 1 . 0 2 ~ ~In ~ Figure ). 9a it is observed that when the radial position is higher, the longitudinal position corresponding to the maximum solid velocity, vz,is also
0.01
0
0.03
0.02
r(m) Figure 9. Local particle velocities for one of the systems studied: a, longitudinal profiles; b, radial profiles.
-
vz
0
(m/s) -0.5
3.0
-1.0
c
-1.5
2.0 Y
1.0
= 33'
-2.0
= 4 mm H.= 0.18 m
d,
0 '
0
I
0.04
I
I
-2.5 1
'
-3.0
0.08 0.12 0.16 0.20
z (m) Figure 10. Average solid velocities in the annular and spout zones vs the longitudinal position along the bed.
higher. The solid velocity decreases with the radial position for all the longitudinal positions in the bed, as is shown in Figure 9b. In Figure 10 the average solid velocities in the annular and spout zones have been plotted vs the longitudinal position along the bed. The upward particle velocity along the spout decreases linearly with the longitudinal position in the bed. The descending particles in the annular zone have a slight acceleration at first, and then they reach a velocity which linearly increases towards the bottom of the bed. With a practical view t o design, the particle trajectories in the spout zone are totally vertical. Nevertheless, in the annular zone the solid velocity has an important radial component, which has its maximum value a t the spout zone-annular zone interface and its minimum value a t the contactor wall, where the direction of the velocity vector is imposed by the angle of the wall. From the values of the vertical component of the solid velocity, vz,the values of the radial component, Y,.,can
4038 Ind. Eng. Chem. Res., Vol. 34, No. 11, 1995
Figure 12. Map of the velocity vector for one of the systems studied ( y = 33”; Do = 0.03 m; d, = 2 mm; Ho = 0.23 m; u = 1.02u,,).
Figure 11. Outline of the differential volume element in the annular zone. be calculated and, consequently, the velocity vector at any position in the contactor. The calculation of the radial components is carried out by means of a mass balance in a differential volume element of the annular zone, such as the one defined in Figure 11.
The boundary conditions are a t the interface (for r = rs):
at the wall (for r = rw):
vz = 0; v, = u,
v,./vz = tan(y/2)
(4)
(5)
a t the wall in the upper surface of the bed (for z = H and r = rw): E , = E , (6) In Figure 12 the map of the velocity vector for the system of Figure 8 is shown. For a better appreciation, the moduli of the velocity vector in the annular zone have been multiplied by 8. From the solid trajectories it is observed that the incorporation of the solids to the spout is not uniform along the bed level, as has been considered by some authors for cylindrical spouted beds (Thorley et al., 1959; Lefroy and Davidson, 1969; Epstein and Grace, 1984). Two zones of higher solid contribution to the spout are observed, which approximately correspond t o the positions of higher spout expansion. 4. Conclusions
The optical probe used and the programs for signal treatment are suitable for determining the local properties of conical spouted beds in a wide range of geometric factors and operating conditions. The study carried out as a whole on spout geometry, local properties, and particle trajectory map allows for understanding the solid circulation in the contactor. The information obtained is interesting for consideration in the gas flow
model (as the simplification of uniform properties, assumed in previous papers, is avoided) and for establishing the bases of a solid flow model that takes into account the solid transport from the annular zone t o the spout. The spout has two zones of pronounced expansion and a geometry that is sensitive t o the geometric factors (contactor angle and inlet diameter) and that is also sensitive, although to a lesser degree, to the operating conditions (particle diameter, stagnant bed height, and air velocity). The expansion of the spout is more pronounced than that observed for cylindrical beds and must be taken into account in the gas flow model where the assumption of considering the spout diameter similar to the inlet diameter can give way t o important errors. There is also low uniformity in the local bed voidage, the radial bed voidage non-uniformitynear the contactor base being especially pronounced in both the spout zone and the annular zone. The upward particle velocity in the spout has a very pronounced maximum at the contactor axis, which attenuates with the longitudinal position in the bed. In the annular zone the maximum value of the solid downward velocity corresponds to an intermediate position in the annular zone, in the upper half of the bed. The position of the maximum is displaced towards the wall as the solid descends towards the contactor base. This solid acceleration at the wall in the contactor base is more pronounced in conical spouted beds than in conical-cylindrical spouted beds. The solid trajectories show a nonuniform incorporation to the spout of the particles descending in the annular zone. There is a preferential transport at the interface zones where the spout is more expanded.
Acknowledgment
This work was carried out with financial support from the University of the Basque Country/Euskal Herriko Unibertsitatea (Project No. 069.310-3B14U92). Nomenclature
D,,Di,Do= diameter of the column (or the cylindrical part
of the contactor), of the contactor base, and of the gas inlet, respectively, m
Ind. Eng. Chem. Res., Vol. 34,No. 11, 1995 4030 de = effective distance between the two receiving fibers,
mm d, = particle diameter, mm H, H,,H, = heights of the developed bed, of the conical section, and of the stagnant bed, respectively, m Q = solid flow rate, kg s-l R = contactor radius at z height, m r, z = cylindrical coordinates r* = dimensionless radial position, rlR r8 = spout radius, m rw = radial position of the contactor wall at level z, m S = cross-sectional area, m2 u = air velocity, m s-l urns= minimum spouted bed velocity, m s-l ur = radial component of the solid velocity at the interface, m s-l Greek Letters e, ea, eo = bed voidage, bed voidage in the annular zone, and loose bed voidage, respectively .Z = average bed voidage y = contactor angle, degrees p = solid density, kg m-3 t = delay time between two signals, s v,., v, = components of particle velocity in r and z directions, m s-1 vz = radially-averaged particle velocity in z direction, m S-1
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Received for review December 29, 1994 Revised manuscript received J u n e 6, 1995 Accepted June 15, 1995@ IE940773U
Abstract published in Advance A C S Abstracts, September 15, 1995. @
