Verification of fluidized bed electrical capacitance tomography measurements with a fibre optic probe

Chemical Engineering Science 58 (2003) 3923 – 3934

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Veri cation of "uidized bed electrical capacitance tomography measurements with a bre optic probe T. Pugsleya;∗ , H. Tanfaraa , S. Malcusa , H. Cuib , J. Chaoukib , C. Wintersc a Department

of Chemical Engineering, The University of New Brunswick, Fredericton, New Brunswick, Canada E3B 5A3 of Chemical Engineering, Ecole Polytechnique de Montreal, C.P. 6079, succ. Centre-Ville, Montreal, Quebec, Canada H3C 3A7 c Centre for Therapeutic Research, Merck Frosst Canada and Co., 16711 Trans Canada Highway, Kirkland, Quebec, Canada H9H 3L1

b Department

Received 9 January 2003; received in revised form 20 May 2003; accepted 9 June 2003

Abstract Previous studies aimed at determining the spatial accuracy of electrical capacitance tomography (ECT) have employed phantoms placed within the ECT measurement space. No previous studies have compared ECT with a second independent measurement technique in an operating "uidized bed. In the present work, radial voidage pro les have been measured with ECT in the 0.14-m I.D. riser of a circulating "uidized bed (CFB) and in a bubbling "uidized bed with a 0.19-m I.D. The dynamic and time-averaged radial voidage pro les have been compared with measurements taken with a bre optic probe in the same riser and in a slightly narrower (0.15-m I.D.) bubbling "uidized bed. In spite of the intrusiveness of the latter technique, the time-averaged radial pro les in the CFB riser fall within 10% of each other when the CFB is operated at high-"ux conditions that lead to a very dense wall region. Iterative reconstruction of the ECT images is not needed in this case. Similar agreement is found between the two techniques in the bubbling "uidized bed, but o=-line iterative image reconstruction is clearly necessary in this "uidization regime. These results suggest that ECT, which is often described as a tomographic imaging technique with low spatial resolution, can in fact provide semi-quantitative time-averaged images of the cross-section of "uidized beds of diameter comparable to or less than that used here. ? 2003 Elsevier Ltd. All rights reserved. Keywords: Fluidization; Fluidized bed; Circulating "uidized bed; Electrical capacitance tomography; Fibre optic probe

1. Introduction Capacitance, bre optic and suction probes have found widespread use in "uidization research (Louge, 1997; Yates & Simons, 1994). Such probes must be inserted through an access port in the wall of the "uidized bed in order to collect data on the local "ow structure. Thus, they are intrusive and disturb the two-phase gas–solid "ow that they are intended to measure. Tomographic imaging is a non-intrusive alternative for hydrodynamic measurement in "uidized beds. Chaouki, Larachi, and Dudokovic (1997) provided an extensive review of the various tomography techniques available for measuring multiphase "ows. These systems may be classed as nuclear-based using ionizing radiation (e.g. X-ray, -ray, ∗ Corresponding author. New address: Department of Chemical Engineering, The University of Saskatchewan, 105 Maintenance Road, Saskatoon, SK, Canada S7N 5C5. Tel.: +1-306-966-4761; fax: +1-306-966-4777. E-mail address: todd [email protected] (T. Pugsley).

0009-2509/$ - see front matter ? 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0009-2509(03)00288-4

positron emission), nuclear-based but non-ionizing (e.g. NMR), and non-nuclear-based (e.g. electrical capacitance, microwave, and ultrasonic tomography). As summarized by Chaouki et al. (1997), nuclear-based tomographic techniques exhibit medium to high spatial resolution, but low temporal resolution. Electrical capacitance tomography (ECT) which is the most common non-nuclear technique applied to "uidized beds, has a high temporal resolution, but low spatial resolution. The low-resolution images produced by the ECT arise from the “soft- eld” nature of the electrical eld generated by the ECT electrodes. The non-uniformly distributed material inside a "uidized bed causes the eld lines to bend. This phenomenon tends to blur the interphase boundaries and leads to the lower spatial resolution. McKeen and Pugsley (2002) have recently demonstrated this blurring e=ect by placing various phantoms in the sensor eld of an ECT system mounted on a "uidized bed. Two earlier studies by Yang, Gamio, and Beck (1997) and Yang, Spink, York, and McCann (1999) with di=erent ECT con gurations and phantoms also demonstrated the blurring of the phantom boundaries.

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Spatial resolution of phantom images generated by ECT may be improved by implementing an iterative oNine image reconstruction algorithm. Yang et al. (1997, 1999) proved this qualitatively while McKeen and Pugsley (2002) reported a quantitative comparison. The latter study found relative errors between 4% and 10% when the known phantom diameter and the diameter of the phantom in the reconstructed image were compared. The relative error depended on the number of iterations used for reconstruction and the position of the phantom in the measurement space. The quantitative comparison of McKeen and Pugsley (2002) suggests that ECT has the potential to become a valuable non-intrusive measurement tool for "uidization research that can provide insight into not only the dynamic behaviour of the "uidized bed, but also images of the cross-sectional distribution of solids and gas that are semi-quantitative. However, no study has ever attempted to independently con rm the spatial resolution of either the on- or o=-line reconstructed images generated by an ECT system applied to an actual operating "uidized bed. We address this lack of knowledge in the present work by comparing the images obtained from an ECT system applied to a conical "uidized bed and a circulating "uidized bed (CFB) riser, with measurements made with a bre optic probe in the same beds. Our aim is to con rm the semi-quantitative nature of ECT images and the need for o=-line reconstruction, depending on the "uidization regime. 2. Experimental 2.1. Fluidized bed units ECT and bre optic measurements were made on two "uidized beds. The rst is a bench scale "uidized bed of

conical cross-section. It is a self-contained unit that takes air in from its surroundings and supplies it to the bed via a built-in 0.75-kW fan. The actual "uidized bed vessel is fabricated from Plexiglas and supported within a cast aluminium support that locks the bed into place on the distributor. Fig. 1(a) is a schematic diagram showing the dimensions of the conical Plexiglas vessel. With the given dimensions of the bed, the calculated angle of the conical section with the vertical has been determined to be 21◦ . The Plexiglas cone is mounted on a Dutch-weave screen mesh distributor. A "at-plate ori ce meter, designed according to ISO standards with an ori ce diameter of 0:014 m, was installed inside the pipe connecting the fan to the "uidized bed windbox. The ori ce pressure drop was used to calculate the bed supercial gas velocity based on the cross-sectional area of the base of the cone. The conical "uidized bed shown in Fig. 1(a) was tted with the ECT electrodes. When these electrodes were in place, they covered the entire outer surface of the vessel. This made it impossible to use any other measurement technique in conjunction with the ECT on the same vessel. Therefore, a second Plexiglas conical "uidized bed of similar dimensions was used for the bre optic probe measurements (Fig. 1(b)). This "uidized bed was slightly taller and narrower than the rst, resulting in an angle of 13◦ with the vertical. This cone was instrumented with two measurement ports, corresponding (approximately) to the centreline locations of the sensors of the ECT unit. These ports were located 7 and 12 cm vertically above the distributor and allowed the insertion of the bre optic probe into the bed. Probe measurements were taken at 1-cm intervals extending outward from the bed centreline in the positive and negative radial directions, as illustrated in Fig. 1(b). 0.250

0.265

0.105 0.160 Radial measurement positions at 1-cm intervals 0.290 0.215 -r

+r

0.120 0.070

0.115

(a)

0.115

(b)

Axial positions of measurement ports

Fig. 1. Schematic diagrams of conical "uidized beds, all dimensions in m: (a) ECT-equipped conical "uidized bed; (b) taller, narrower cone used for bre optic probe measurements.

T. Pugsley et al. / Chemical Engineering Science 58 (2003) 3923 – 3934

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Table 1 Sieve analysis of placebo granulate

Fig. 2. Flow diagram of the CFB system.

In addition to the bench scale "uidized bed, ECT and bre optic probe measurements were taken in the riser of a CFB. Fig. 2 is a simpli ed "owsheet of the CFB apparatus. The CFB riser is 7-m tall with an inside diameter of 0:14 m and a smooth exit at the top. A primary and a secondary cyclone arranged in series separate the gas from the solids. After a sloped transition region downstream of the primary cyclone outlet, the solids enter a standpipe of 0:19 m inside diameter. The solids feeding system that returns the solids from the standpipe to the riser, consists of an aerated annular bed with an internal diameter of 0:76 m. This solids feeder, which is a unique feature of this CFB system in comparison to others, has been described in detail by Malcus, Chaplin, and Pugsley (2000). The aerated solids in the annular bed "ow through eight equally spaced 1.5-cm-diameter openings at the bottom of the riser, where they contact the air fed from the blower. Fluidizing air from the blower is humidi ed by countercurrent contact with water in a packed bed to reduce static electricity build-up. The riser super cial gas velocity is measured with an ori ce plate located upstream of the riser inlet. The riser solids mass "ux was initially measured with a permeable butter"y valve located in the standpipe. Through the course of experimentation, it was found that the radial pressure drop in the annular bed as measured by two pressure taps inserted into the bottom of

Sieve opening dpi (m)

xi

0 –75 75 –105 105 –149 149 –210 210 –297 297– 420 420 –590 590 –850 850 –1190 1190 –1680 1680 –2380 2380 –3360

0.016 0.054 0.179 0.202 0.157 0.063 0.042 0.021 0.026 0.036 0.054 0.068

dp; avg: =

220 m

the annular bed was directly proportional to the solids mass "ux. Thus, a correlation between the solids mass "ux and the radial pressure drop was established that allowed us to determine the solids mass "ux on-line without disturbing the system. The CFB riser consists of several interchangeable "anged Plexiglas sections, one of which is equipped with the ECT sensor. Measurements were taken with the ECT sensor at a single axial location, 1:6 m above the riser inlet. The system was then shut down and the ECT measurement section interchanged with a section containing a measurement port through which bre optic measurements could be made at the same axial location. The accurate on-line metering of the air and solids mass "ux ensured that the operating conditions could be re-established for the purpose of comparing the two measurement techniques. Radial measurements with the bre optic probe in the CFB riser were taken at 1-cm intervals in a manner similar to that used in the conical "uidized bed. 2.2. Solid materials A placebo pharmaceutical granulate mixture was used in the conical "uidized bed. The size distribution of the placebo granulate mixture (Table 1) was obtained from sieve analysis. The granulate mixture exhibited a wide particle size distribution, with particles ranging from 40 m to greater than 3 mm. This is typical of granulate mixtures that are fed batch-wise to "uidized bed dryers in the pharmaceutical industry. The granulate mixture had a particle density of 1100 kg=m3 . This was calculated based on helium pycnometry experiments at the Merck Frosst laboratories and an estimate of the particle void volume supplied by Merck Frosst. The minimum "uidization velocity was determined by "uidizing the granulate mixture in another laboratory-scale "uidized bed of cylindrical cross-section and found to be 0:03 m=s.

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Table 2 FCC catalyst particle size analysis Diameter range dpi (m)

xi

¡ 25:4 25.4 –29.5 29.5 –34.1 34.1–39.5 39.5 – 45.8 45.8–53.0 53.0 – 61.5 61.5 –71.5 71.5 –82.5 82.5 –96.0 96.0 –111 111–129 129 –150 150 –173 173–201 201–233 233–270 270 –313 313–362 362– 420 420 – 487 487–564

0 0.1 0.4 1.1 2.6 5.0 9.3 15.5 17.6 16.0 11.2 7.5 4.4 2.8 2.1 1.5 1.0 0.7 0.5 0.3 0.2 0.1

dp; avg: =

89 m

Fig. 3. Schematic diagram of the ECT sensor installed on a cylindrical section of the CFB riser, all dimensions in m.

2.4. ECT system

FCC catalyst was used as the test material in the CFB riser. The size distribution of the FCC catalyst particles (Table 2) was measured with a Malvern particle size analyser and gave a Sauter mean diameter of 89 m. The FCC catalyst particle density, based on mercury porosimetry measurements carried out at Ecole Polytechnique, was 1550 kg=m3 . The minimum "uidization velocity of the FCC catalyst was determined theoretically by applying the equation developed by Baeyens as cited in Geldart (1986). The theoretical value was 0:0057 m=s. 2.3. Experimental operating conditions In the bench scale conical "uidized bed, the super cial gas velocity was varied between 0.25 and 0:75 m=s, which was well in excess of the minimum "uidization velocity of 0:03 m=s. The static bed height ranged from 0.12 to 0:17 m above the gas distributor. Fibre optic probe measurements could be taken for all static bed heights when the probe was inserted through the instrumentation port located 7 cm above the distributor. When the probe was inserted through the port located 12 cm above the distributor, measurements could only be taken at the highest static bed height. Only at this bed height was the probe entirely immersed in the solids. In the CFB riser, the super cial gas velocity was xed at 4:7 m=s, while the solids mass "ux was varied between 148 and 264 kg=(m2 s).

An ECT system has three main components: the sensor, the sensing electronics and the computer. The sensor consists of a set of electrodes that have been etched into a copper-coated plastic laminate and mounted symmetrically on the outside of an insulating vessel. The ECT systems utilized in the present work were custom designed and fabricated for both the conical bed and the CFB by Process Tomography Ltd., UK. Both are twin-plane systems, which means that two electrode levels exist within the ECT sensor. Each plane consists of eight electrodes, all having a length of 4 cm. The electrodes of the two planes are immediately adjacent to each other resulting in an average centre-to-centre distance of 4 cm between the two sensor planes. Fig. 3 is a schematic of the ECT electrodes installed on the cylindrical CFB riser. The electrodes on the conical "uidized bed are identical in terms of length and number. The ECT systems are equipped with radial and axial guards. The presence of the radial guards mitigates the dominance of the adjacent interelectrode capacitances (Jaworski & Bolton, 2000), while the axial guards maintain a parallel measurement plane across the vessel cross-section. Huang, Xie, Thorn, Snowden, and Beck (1992) described the measurement principle and the basic circuitry of an ECT system. Capacitance measurements are initiated by applying an electric potential to one of the electrodes while maintaining the others at ground. The excited electrode discharges to the seven grounded electrodes. A sequence of rapid, computer-controlled switching follows in which the charged electrode is varied until all unique electrode

T. Pugsley et al. / Chemical Engineering Science 58 (2003) 3923 – 3934

combinations complete the charging/sensing sequence. For the eight-electrode systems used in this investigation, 28 unique capacitance measurements are obtained for each image per measurement plane. The measurement frequency is 100 Hz (i.e. one image of the vessel cross-section at each plane is captured every 10 ms). The measured capacitance is related to the electrical permittivity of the bed material. A calibration speci c to the solid material under investigation is, therefore, required prior to use of the ECT system. For the purpose of image reconstruction (as described in the section on ECT image reconstruction in this paper) the software supplied by Process Tomography Ltd. divides the image viewing area into a square matrix of 32 × 32 pixels. The actual image as viewed by the ECT software consists of only 812 of the 1024 available pixels that are contained within the circular sensing area. The resolution of the imaged cross-section is therefore 812 pixels, which corresponds to a pixel area of 0:19 cm2 for the 0:14 m I.D. riser. Including the 4-cm height of the electrodes, the spatial resolution of the ECT system applied to the CFB riser is 0:77 cm3 =pixel. In the case of the conical "uidized bed, the bed diameter varies with height in the cone, so the spatial resolution varies as well. The centre of the lower measurement plane is vertically located 0:11 m above the distributor, where the bed diameter is 0:19 m. The centre of the upper measurement plane is located 0:15 cm above the distributor, where the bed diameter is 0:22 m. The spatial resolution is therefore 1:4 cm3 =pixel at the lower plane and 1:9 cm3 =pixel at the upper plane. 2.5. Fibre optic probe Fibre optic probes have been commonly regarded as e=ective tools to measure local voidage in "uidization. The measurement principle of bre optic probes is based on either forward or backward scattering of light. The latter approach is more commonly used in "uidization experiments because it is simpler and less intrusive (Louge, 1997). In order to attain high accuracy of measurement, one should rst design a reasonable bre optic probe, and then adopt an e=ective probe calibration procedure. However, common bre optic probes (parallel) have two problems: the presence of a blind region and an unde ned measurement volume. Proper calibration procedure is also often not available due to diRculty in reaching various standard two/three phase "ow systems. These problems limit the accuracy of the bre optic probe. Cui, Moustou , and Chaouki (2001) have recently addressed these issues by comparing four probe designs and developing a new calibration method. For all three parallel- bre optic probes tested, instantaneous values of the normalized voltage signal were shown to exceed one with a high frequency, which introduces wrong information. This phenomenon was mainly due to the presence of blind regions. The fourth generation probe design, known as the cross- bre optic probe and depicted in Fig. 4 (Reh &

3927

emitting optical fibre window

defined measurement volume

receiving optical fibre Fig. 4. Schematic diagram of the cross-optical 1991).

bre probe (Reh & Li,

Li, 1991), does not have a blind region and the measurement volume is well de ned (less than 2 mm3 ). This probe exhibited a superior response to instantaneous voidage "uctuations in comparison to earlier generation probes. This was true in both dense and dilute "uidized beds. The bre optic probe was calibrated through two consecutive steps. First, the bre optic probe was inserted into "uidized beds to test both the empty and the minimum "uidization states successively; the aim was to achieve the two extreme voltage responses to the two extreme voidages. Second, the bre optic probe’s response curve to various voidages between these two extremes was calibrated by using the probe to measure a series of mixtures of particles and transparent polymer beads with di=erent particle concentrations (Cui et al., 2001). The bre optic probe was also validated by comparing pressure drop measurements with an integrated solid holdup from a radial distribution pro le obtained with the bre optic probe. A di=erence of less than 5% was observed at the same operating conditions. Considering the superior response of the probe to voidage "uctuations in a "uidized bed and the advances made in probe calibration, the cross- bre optic probe appears to be the most suitable device for use in the independent veri cation of ECT measurements and so has been used for that purpose in this study. A PV-4A particle velocity analyzer (Institute of Process Engineering, Chinese Academy of Sciences) was used as a light source and to record time series of instantaneous local voidage at various operating conditions. To ensure the validity and repeatability of sampled signals, at a given radial position in the CFB riser or conical "uidized bed, 32,000 data points were collected at sampling frequency of 488 Hz. All data were analyzed to determine the time-averaged voidage and the standard deviation at each radial position.

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Fig. 5. Illustration of the 32 × 32 pixel array representing the ECT image cross-section, the perimeter of the CFB riser within that array, and the pixels used to generate radial voidage pro les from the ECT images.

3. ECT image reconstruction and analysis The simplest and fastest approach for ECT image reconstruction is the linear back projection (LBP) algorithm (e.g. Xie et al., 1992; Yang et al., 1997). As the name implies, LBP assumes that the permittivity distribution of the gas– solid suspension over the "uidized bed cross-section (which may be used to calculate the voidage distribution) is linearly related to the set of measured capacitance values. The LBP algorithm was initially developed for the hard eld lines of X-ray tomography systems. However, due to the soft- eld nature of the electrical eld lines in an ECT system, the linear relationship does not hold and the LBP algorithm leads to errors in the calculated permittivity distribution. It follows that, if the erroneous permittivity distribution calculated by the LBP algorithm is used to re-calculate the capacitance data, then the re-calculated capacitance data will not correspond exactly to the original measured capacitance values. This discrepancy has been exploited by Yang et al. (1997) to develop what they call the iterative LBP algorithm. They take the di=erence between the calculated capacitance values and the original data and create an error image. This error image is added to the present calculated image to produce a new calculated image. This process is repeated until a satisfactory image is obtained that can be much sharper than the original LBP image, depending on the number of iterations. Process Tomography Ltd., UK, included, along with the ECT hardware, a MATLAB program containing the LBP and iterative LBP reconstruction algorithms that was used in this study. For data collected in the CFB riser, the LBP algorithm provided suRcient accuracy, as the results of the next section will con rm. However, the iterative LBP algorithm was required to produce semi-quantitative images in the conical "uidized bed. To extract instantaneous and time-averaged voidage data from the ECT images for comparison with voidage data obtained from the bre optic probe, the ECT images were analyzed in the following manner. As discussed previously, the ECT image reconstruction software divides the image into a 32 × 32 pixel array. The gray-shaded pixels shown

in Fig. 5 represent those pixels that were used to obtain radial voidage pro les in the CFB riser. For a given radial location, the arithmetic average of the voidage values corresponding to the eight shaded pixels at four di=erent angular positions (every 90◦ ) was used as the voidage at that radial location. As indicated in Fig. 5, the pixels were chosen at the wall such that they were not located directly at the edge of the ECT electrodes, since at the electrodes, permittivities were displayed that corresponded to the packed bed voidage, which was not realistic. The identical approach was used for obtaining radial voidage pro les from ECT images of the conical "uidized bed, with the exception of the manipulation of pixels near the electrode edges. Since the conical bed was very dense at the wall, the high permittivity readings near the electrode edges did not in"uence the voidage pro les. 4. Results and discussion 4.1. Comparison between ECT and >bre optic probe measurements in the CFB riser 4.1.1. Time-averaged radial voidage pro>les Fig. 6a compares the time-averaged radial voidage pro le measured with the bre optic probe to the radial voidage pro le from ECT measurements at a solids mass "ux of 148 kg=(m2 s) and a riser super cial gas velocity of 4:7 m=s. There is some discrepancy between the two curves at all radial locations, but this di=erence does not exceed 10%. At the wall, the voidage obtained from the ECT system is about 4% greater than that based on the bre optic probe. This di=erence decreases to about 1% near the riser centreline. When the solids mass "ux is increased to 264 kg=(m2 s) with the super cial gas velocity remaining constant, Fig. 6b illustrates that the discrepancy between the two measurement techniques remains at about 1% near the riser centreline, and is even less near the wall. Malcus et al. (2000) have shown that, as the solids mass "ux is increased from 148 to 264 kg=(m2 s), a dense bed with a cross-sectionally averaged voidage of about 0.8 is formed

T. Pugsley et al. / Chemical Engineering Science 58 (2003) 3923 – 3934 1.00 0.95 0.90 0.85

Voidage (-)

0.80 0.75 0.70 0.65

ECT fibre optic probe

0.60 0.55 0.50 0.0

0.2

0.4

(a)

0.6

0.8

1.0

0.6

0.8

1.0

r/R (-) 1.00 0.95 0.90 0.85

Voidage (-)

0.80 0.75 0.70 0.65 0.60

ECT fibre optic probe

0.55 0.50 0.0

(b)

0.2

0.4

r/R (-)

Fig. 6. Comparison of radial voidage pro les measured with ECT and bre optic probe in the CFB riser containing FCC catalyst: (a) riser solids mass "ux = 148 kg=(m2 s), super cial gas velocity = 4:7 m=s; (b) riser solids mass "ux = 264 kg=(m2 s), super cial gas velocity = 4:7 m=s.

at the base of the riser. The radial voidage pro les of Figs. 6a and b show that as this dense bed is formed, the greatest decrease in local voidage occurs between a dimensionless radial position of 0.5 and the riser wall. We conclude that the improved accuracy of the ECT system over that radial range is due to the decrease in local voidage. This nding is consistent with the statement of Dyakowski, Edwards, and Williams (1997) who noted that the ECT system works better in dense beds due to the increase of the signal-to-noise ratio associated with an increase of the volume fraction of the high-permittivity material. Dyakowski et al. (1997) further reported that the accuracy of ECT is inversely related to the distance from the electrodes, with the system able to resolve a minimum size of about 0.2% of the pipe cross-section near the pipe wall

3929

and about 2% of the cross-section at the pipe centre. This characteristic of ECT also contributes to the better agreement with the optical bre probe measurement near the riser wall. It is also possible that the inverse relationship between ECT resolution and distance from the electrodes is responsible for the insensitivity of the ECT local voidage measurement near the riser centreline to the change in solids mass "ux, as shown in Fig. 6b. However, the same insensitivity is observed with the bre optic probe. Hence, it appears that there is little variation in the voidage at the riser centreline, even when the solids mass "ux is nearly doubled. The improved resolution of the densely "owing catalyst suspension near the riser wall is consistent with the phantom experiments of McKeen and Pugsley (2002) in a vessel of identical cross-sectional area. They placed a high-permittivity phantom (a packed bed of FCC catalyst contained in a 4.2-cm I.D. paper tube) inside the otherwise empty vessel. The quality of the ECT-generated image of the phantom was poor when the phantom was located at the centre of the vessel, but was vastly improved when the phantom was positioned at the vessel wall. It is also noteworthy that McKeen and Pugsley (2002) achieved an accurate image of the high-permittivity phantom at the vessel wall with 10 – 20 iterations of the reconstruction algorithm. The images of the CFB riser presented in Figs. 6a and b correspond to no iterations, i.e. simple LBP. It is possible that the agreement between the ECT measurement and the bre optic probe measurement could be slightly improved by performing a small number of iterations on the LBP-generated images. However, the work of McKeen and Pugsley (2002) found that when a high permittivity phantom was introduced near the wall of an otherwise empty vessel, iterative reconstruction introduced a ring-like structure of high permittivity into the image that did not exist in reality. They recommended that care must be taken to not introduce this artefact into images of dilute gas–solids "ow through image reconstruction. Considering this nding and the small gains to be realized given the reasonable accuracy of the LBP images of Figs. 6a and b, iterative image reconstruction was not attempted for the CFB riser. 4.1.2. Analysis of instantaneous radial voidage data The instantaneous voidage signals from the ECT and bre optic probe have been analysed by plotting the standard error as a function of radial position. The standard error accounts for the di=erent sample sizes collected using the two measurement techniques by dividing the standard deviation of the time-averaged voidage at a given radial position by the square root of the sample size. The radial pro les of standard error are presented in Figs. 7a and b for riser solids mass "uxes of 148 and 264 kg=(m2 s), respectively. The trend of increasing standard error with radial position is evident in Fig. 7a using both measurement techniques. This re"ects increasing variability in the "ow as the riser wall is approached. The same trend is apparent from the

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T. Pugsley et al. / Chemical Engineering Science 58 (2003) 3923 – 3934 -3

1.0x10

-4

9.0x10

-4

8.0x10

-4

ECT fibre optic probe

Standard Error (-)

7.0x10

-4

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-4

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1.8x10

32,000 data points have been collected with the bre optic probe to generate the time-averaged radial voidage proles reported in this study, it seems unlikely that additional sampling is necessary. The larger standard error of the ECT measurement in Fig. 7b casts further doubt on this explanation, unless there are unknown interactions between the riser operating conditions and the required sample size. A second and more plausible explanation for the observations of Fig. 7 is that the di=erences in standard error are due to a combination of the disturbance of the gas–solid "ow by the bre optic probe and the relatively large measurement volume of the ECT (0:77 cm3 ) compared with the bre optic probe (¡ 2 mm3 ). Con rmation of these e=ects would require systematic experimentation with di=erent-sized bre optic probes in risers of di=ering diameter and/or in a riser equipped with an ECT sensor consisting of a di=erent number of electrodes of di=ering dimensions. Such an experimental program is beyond the scope of the present study.

-5

1.6x10

4.2. Comparison between ECT and >bre optic probe measurements in the conical bed

-5

1.4x10

-5

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1.2x10

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-5

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Fig. 7. Comparison of radial standard error pro les measured with ECT and bre optic probe in the CFB riser containing FCC catalyst: (a) riser solids mass "ux = 148 kg=(m2 s), super cial gas velocity = 4:7 m=s; (b) riser solids mass "ux = 264 kg=(m2 s), super cial gas velocity = 4:7 m=s.

ECT measurements at the higher "ux condition in Fig. 7b, although the overall variability of the "ow has decreased from Fig. 7a, as indicated by the lower values of the standard error at all radial positions. On the other hand, the standard error pro le based on the bre optic probe measurements plotted in Fig. 7b shows a di=erent trend, whereby standard error increases to a maximum at a dimensionless radial location of approximately 0.85 and then decreases to the riser wall. Brereton and Grace (1993) have previously documented this behaviour in a CFB riser using sand as the test material and operating at lower "uxes. There are two possibilities to explain the results presented in Fig. 7. In the case of Fig. 7a, it is possible that the sample size needs to be larger for the bre optic probe. This would reduce the standard error, bringing it more in line with the standard error from the ECT system. However, since

4.2.1. Time-averaged radial voidage pro>les Figs. 8a–c illustrate the basic trends observed with increasing gas velocity and static bed height in the conical "uidized bed. The gures also show the importance of iterative image reconstruction and the in"uence of the number of iterations on the ECT measurement in this bed. At a static bed height of 13 cm and a super cial gas velocity of 0:25 m=s, the bre optic probe measures a wave-like radial voidage pro le. This complex pro le could not be captured by the ECT, regardless of the number of iterations attempted in the oNine image reconstruction algorithm. Visual observations at these conditions suggest that small bubbles were moving slowly upward through the bed. It would appear that these conditions present a lower limit to the accuracy of the ECT. Fig. 8a does indicate that in order to get at least a qualitative picture of the "uidized state of the bed under such conditions, only LBP should be utilized. Even a small number of iterations introduce a centralized region of relatively high voidage that does not appear to be realistic when compared with the bre optic measurement. The situation changes when the gas velocity is increased to 0:6 m=s at a xed static bed height of 13 cm (Fig. 8b). Visual observations under such conditions found the bed to be vigorously "uidized and to exhibit a spout-like eruption from the centre of its upper surface. Consistent with these observations, the bre optic probe measured a central region of high voidage inside the bed. With 100 –200 iterations, the ECT image showed a similar feature. It is clear from Fig. 8b that image reconstruction using the LBP algorithm is inaccurate for these conditions. A similar conclusion is drawn for a static bed height of 15 cm and a super cial gas velocity of 0:6 m=s (Fig. 8c). In both Figs. 8b and c, the pro le measured by the bre optic probe tends to show

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a broader distribution about the centreline of the bed than that measured by the ECT system. This is likely due to the slightly di=erent geometries of the two conical beds used in the study. Fig. 8 demonstrates that for "uidization at high velocities in the conical bed containing pharmaceutical granulate iterative image reconstruction is needed. This requirement is consistent with the ndings of McKeen and Pugsley (2002) in a cylindrical bed of FCC catalyst. When they placed a hollow cylindrical phantom in the centre of a packed bed of catalyst, an accurate reconstructed image of the “void” could be obtained with 500 iterations. In their conclusions, McKeen and Pugsley (2002) suggested that, due to the boundary of a real bubble in an operating "uidized bed being more di=use than the sharp boundary of the phantom void immersed in

the packed bed, less iterations would be suRcient to generate accurate images of the bubbling bed with ECT. This statement is supported by the present work. It is interesting that, even though the time-averaged voidage pro les in the CFB riser and the conical "uidized bed are qualitatively similar, i.e. a dilute central core surrounded by a dense annular region, iterative reconstruction is needed for the conical bed. The key di=erence between the two cases is the steeper voidage gradient near the wall of the CFB riser. Figs. 8b and c indicate an annular region of constant voidage that is nearly 4 cm thick. The voidage in this region is about 0.5. The wall voidage in the CFB riser is 0.55 – 0.75, depending on the solids mass "ux, and rapidly decreases with decreasing radial position. The presence of the thicker, denser annular region in the conical bed

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appears to introduce signi cant distortion of the electrical eld lines emanating from the ECT electrodes. Iterative image reconstruction is needed to correct the resultant blurring of the interphase boundaries. If the gas velocity to the conical bed was increased and provision made to capture and recirculate entrained solids, then the bed would operate in a manner similar to the CFB riser (apart from the obvious geometrical di=erences). The above discussion on the need for iterative reconstruction suggests that less iteration would be needed at the higher velocities. This is an important conclusion since the conical "uidized beds used in the pharmaceutical industry for drying wet granulate operate at super cial gas velocities two to three times greater than the highest velocity studied in the conical bed in the present work. Hence, at the velocities where the pharmaceutical industry operates their beds on a regular basis, fewer iterations would be needed to generate an accurate image of the bed cross-section. This is a valu-

able result in the event that ECT is applied as a monitoring and control tool for "uidized bed dryers. It must also be pointed out that the in"uence of bed diameter is important; it is unknown if the conclusion connecting higher velocity "uidization regimes with the need for fewer iterations would hold in a larger vessels. The practical limitations of ECT in this regard still need to be established. 4.2.2. Analysis of instantaneous radial voidage data Figs. 9a–c present the radial pro les of standard error for the same operating conditions as Figs. 8a–c and for 200 iterations in the iterative reconstruction algorithm. As was the case for the radial standard error pro les in the CFB riser, discrepancies are apparent between the standard errors determined from the two measurement techniques. This is particularly noticeable near the wall of the vessel for all conditions as well as in the central portion of the bed for the high-velocity, high-static bed height conditions of Fig. 9c.

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The reasons for the observed discrepancies are not clear, but as discussed above with respect to the instantaneous voidage measurements in the CFB riser, it seems likely that they are due to a combination of the disturbance of the "ow by the insertion of the bre optic probe and the larger measurement volume of ECT. The latter is the most plausible explanation for the low standard error in the centre of the bed obtained from ECT in Fig. 9c. As the "ow at the centre of the bed becomes very dilute at the higher gas velocity (see Fig. 8c), small particle clusters will be present. Given the larger measurement volume of the ECT system, it will not be sensitive to the passage of these clusters. The bre optic probe, on the other hand, can measure the presence of the particle clusters, which shows up as increased variability in the centre of the bed in Fig. 9c. The discrepancy between the dynamic signals of the ECT and the bre optic probe closer to the wall of the vessel is not as easily explained. The increase in the standard error of the ECT signal seen at r=R ≈ ±0:4 can be rationalized by considering that this radial location corresponds approximately to the interface between the central dilute region and the denser wall region seen in the time-average voidage pro les of Figs. 8a–c. Hence, the sudden increase in the standard error at this radial position re"ects the di=use, "uctuating nature of this interface. This increase is not seen in the bre optic probe signal which may be due to the disturbance of the "ow by the probe or still unresolved issues surrounding the blind region in the dense portion of the bed. As was the case for the dynamic "uctuations in the CFB riser, more research is needed to completely understand the reasons for the observed discrepancies. 5. Conclusions Independent measurements of the voidage pro les inside a CFB riser and a conical bubbling "uidized bed have been successfully carried out with a bre optic probe and an ECT system. The time-averaged pro les obtained from these two measurement techniques are in agreement, except at the lowest velocity in the bubbling bed. Fibre optic probes have been used in "uidized bed systems for several years and are a well-established measurement technique. Even though they are intrusive and can only make point measurements in one direction, bre optic probes, particularly the fourth-generation design used here, are generally seen as accurate devices for quantifying the two-phase "ow. Hence, the good agreement of ECT with the bre optic measurements leads us to conclude that ECT can be used to obtain semi-quantitative information on the time-averaged voidage distribution inside a "uidized bed. The need for iterative ECT image reconstruction depends on the regime of "uidization. For the relatively dilute gas– solid "ow with a dense wall layer in the CFB riser, iterative reconstruction is not necessary; simple LBP reconstruction is suRcient. However, for the dense "uidization in the conical bed, iterative image reconstruction was clearly needed

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to obtain a radial voidage pro le that was similar to that obtained by the bre optic probe. We found approximately 100 iterations to be suRcient, but it must be pointed out that the number of iterations required is likely highly dependent on the operating conditions and the bed geometry. The in"uence of bed diameter is particularly important. The present work cannot comment directly on the maximum practical vessel diameter to which ECT can be applied and still obtain semi-quantitative results. To answer this question, experiments similar to those presented here would need to be repeated in several "uidized beds of larger diameter. However, our measurements in the CFB riser show that at high-density conditions, most of the solids accumulate next to the riser wall. The high signal-to-noise ratio associated with the lower voidage of the suspension near the wall, as well as the close proximity to the ECT sensors combine to maintain ECT accuracy. Thus, it seems reasonable to project that ECT could be used to obtain accurate voidage measurements in the wall region of CFB risers of much larger diameter than that used in the present study, perhaps greater than 0:30 m. The same is likely also true of the conical "uidized bed. All measurements presented here were taken at the lower plane of the ECT where the bed diameter is 0:19 m. With iterative reconstruction, semi-quantitative images in a bed of twice that diameter seem feasible. This would allow ECT to be used on conical "uidized beds of the size used for clinical scale production of drug products in the pharmaceutical industry. A comparison of the instantaneous voidage data obtained from the ECT and bre optic probe measurements was made by plotting the radial pro les of the standard error. Unlike the time-averaged plots, agreement between the standard error pro les from the two devices was poor. It seems likely that the disagreement is due to a combination of disturbance of the "ow by the probe and the relatively larger measurement volume in the case of the ECT. Systematic research is needed with probes of varying sizes and ECT sensors of varying diameters and/or number and size of electrodes to better understand the di=erences in the dynamic behaviour arising from the analysis of the signals from the two devices. Acknowledgements The nancial support of the Natural Sciences and Engineering Research Council of Canada (NSERC) through their Research Grant, CRD Grant, and Equipment Grant programs made this work possible. The support of Merck Frosst Canada & Co. is also gratefully acknowledged.

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