Influence of pressure on fluidization properties

Powder Technology 141 (2004) 137 – 154 www.elsevier.com/locate/powtec

Influence of pressure on fluidization properties Igor Sidorenko, Martin J. Rhodes * Cooperative Research Centre for Clean Power from Lignite, Department of Chemical Engineering, Monash University, P.O. Box 36, Clayton, Victoria 3800, Australia Received 27 August 2003; received in revised form 12 December 2003; accepted 25 February 2004 Available online 13 April 2004

Abstract An experimental study of the influence of pressure on fluidization phenomena was carried out in a bubbling fluidized bed over the range of operating absolute pressures 101 – 2100 kPa with Geldart A and B bed materials in an apparatus 15 cm in diameter. Bed voidage data was gathered using electrical capacitance tomography (ECT) and the resulting time series were analysed to give average bed voidage, average absolute deviation, average cycle frequency and Kolmogorov entropy. Bed collapse tests were also performed to establish the influence of operating pressure on dense phase voidage. For the Geldart B powder Umf was found to decrease slightly with pressure whilst voidage at Umf was unaffected and no change in behavior from Geldart B to A was observed. Analysis of the ECT results for the Geldart B powder showed no influence of pressure on cycle frequency and average absolute deviation, but an increase in mean bed voidage with pressure. For the Geldart A powder, no influence of pressure on Umf, Umb and emf was observed, though emb was found to increase slightly with pressure. Analysis of the ECT results for the Geldart A powder revealed a decrease in average bed voidage, cycle frequency and average absolute deviation with pressure. The trend in Kolmogorov entropy within increasing pressure was not entirely revealed over the range of velocities used, but results suggest a decrease. D 2004 Elsevier B.V. All rights reserved. Keywords: Fluidization; Pressure effect; Electrical capacitance tomography

1. Introduction Fluidized bed technology originated as the Winkler process for lignite gasification developed in the 1920s and the fluidization technique as it is known now was born from the successful development of the fluidized bed catalytic cracking process in the early 1940s. It has been known for some time that the use of higher operating pressures increases the rate of processes in fluidized beds and improves fluidization quality [1]. Although some data on the effect of pressure on the behavior of fluidized beds have been obtained during the past 50 years and several comprehensive review papers have been published [2 –5], areas remain where further experimental work and analysis would be warranted. In industrial practice fluidized bed reactors are mostly operated at superficial gas velocities well above the minimum fluidization velocities and, therefore, the minimum fluidiza-

* Corresponding author. Tel.: +61-3-9905-3445; fax: +61-3-99055686. E-mail address: [email protected] (M.J. Rhodes). 0032-5910/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.powtec.2004.02.019

tion velocity is not a quantity with a precise significance for industrial applications, however discussion on accurate prediction of the minimum fluidization velocity still seems to remain of much scientific interest. The effect of pressure on the minimum fluidization velocity has been studied by many researchers [6– 19]. The common finding is that the effect of pressure on the minimum fluidization velocity depends on particle size. The results of the experiments show a decrease in the minimum fluidization velocity with increasing pressure for particles larger than 100 Am (Geldart B and D materials); however, for fine Geldart A particles the minimum fluidization velocity is unaffected by pressure. Under ideal experimental conditions, Abrahamsen and Geldart [20] expected a very weak effect of pressure on the minimum bubbling velocity. However, this effect is still relatively unknown and findings in the literature are somewhat controversial. Sobreiro and Monteiro [18] found that the influence of pressure on the minimum bubbling velocity could be very different for different bed materials, but failed to explain their results. According to Jacob and Weimer [21], it has generally been found that with increasing pressure for systems of Geldart A powders the minimum

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bubbling velocity increases, however, the extent of the pressure effect and the reason for it is uncertain. There are also conflicting reports on the effect of increasing pressure on the bed voidage at minimum fluidization (emf) and minimum bubbling (emb) conditions. According to Bin [22], for industrial design purposes the assumption that emf does not change with pressure and that emf can be taken as that experimentally determined at atmospheric conditions is justified for the majority of solids. Several researchers reported that the voidage at minimum fluidization emf is essentially independent of pressure [11,18,19]. Yang et al. [23]suggested that the variation of the voidage at incipient fluidization caused by increase in pressure is negligible for Geldart B and D materials and expected an appreciable change with pressure for Geldart A. Weimer and Quarderer [24] observed only very small increase in bed voidage at minimum fluidization with increase in pressure for Geldart A materials and no change in voidage for Geldart B solids. Contrary to that, Olowson and Almstedt [16] observed a slight increase of the voidage at minimum fluidization with pressure for Geldart D particles and did not notice any dependence of the voidage emf on pressure for Geldart B particles. Rowe [25] presented some of his calculations which showed no measurable change in predicted minimum bubbling voidage, however, these predictions are not in agreement with experimental findings of other researchers [11,26,27] who generally observed an increase in minimum bubbling voidage with pressure. When fluidized beds operate in the bubbling regime, they consist of the dense (emulsion) phase and bubbles. The dense phase voidage is an important parameter in determining the fluidized bed performance because it is widely believed that it has a direct influence on chemical reactions in the fluidized bed. It affects the degree of gas – solid contact and heat and mass transfer. It is also believed that in Geldart A powders the equilibrium size of the bubbles may be controlled by the dense phase voidage [28]. The dense phase voidage is generally taken as being equal to its value at the minimum fluidization point for Geldart B and D materials, but can be higher for Geldart A powders [28,29]. A few methods have been used to measure the dense phase voidage in bubbling fluidized beds. For Geldart A bed materials the voidage can be measured by means of the bed collapse technique. This method has a few variations but generally it involves abruptly stopping the gas flow to a vigorously bubbling fluidized bed and measuring the rate of collapse of the bed surface (e.g. Refs. [30 –32]). Other techniques for measuring voidage in fluidized beds involve the use of capacitance probes, optical fibres, X-ray or g-ray attenuation and capacitance tomographic imaging [29,33]. A non-invasive technique for fast measurement of solids volume fraction in bubbling fluidized beds was pioneered at Morgantown Energy Technology Centre (METC) in the US and at the University of Manchester, Institute of Science and Technology (UMIST) in the UK. By the early 1990s both development groups had successfully

demonstrated the technique, which became known as the electrical capacitance tomography (ECT). In this work, the influence of pressure on fluidization phenomena, such as minimum fluidization velocity, minimum bubbling velocity, bed expansion and voidage was studied; and a novel diagnostic technique suitable for online monitoring of fluidized bed behavior was tested. Experiments were conducted in a bubbling fluidized bed at operating pressures up to 2100 kPa and ambient temperature with Geldart A and B materials [34]. Pressure probe, visual observation and the electrical capacitance tomography were used for characterizing the fluidized bed behavior at elevated pressure. Although the ECT technique has been used in fluidization research [35 –37], it has not been previously used to study the fluidized bed behavior in a pressurized environment.

2. Experimental 2.1. Equipment The process and instrumentation diagram of the pressurized fluidized bed used in this study of pressure effects on fluidization is given elsewhere [38]. The pressure vessel, capable of operating at pressures up to 2600 kPa, was 2.38 m high and was equipped with five glass observation ports. One of two 15-cm diameter plastic fluidized bed models was inserted into the pressure vessel and used to study physical behavior of gas-fluidized beds (Fig. 1). Both

Fig. 1. Pressurized fluidized bed apparatus used in this study.

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fluidized bed columns were identical and made of clear plastic; however the second column had a permanently mounted capacitance sensor described below. The bottom flange of the column was bolted to a carbon steel plenum chamber and distributor assembly, and the top flange was bolted to an expanded top freeboard for solids disengagement. The plenum chamber and distributor assembly were designed and manufactured according to recommendations given by Svarovsky [39]. Two types of distributor plates were specially made for the study. The first distributor was made from a sintered bronze plate with nominal pore size of 12 Am and the pressure drop through the plate of 4.9 kPa at a superficial gas velocity of 0.5 m/s. The second distributor was designed for pressure drop experiments for Geldart A materials so that the pressure drop was approximately 6.5 kPa at a superficial gas velocity of 0.01 m/s in accordance with recommendations by Svarovsky [39]. The assembled fluidized bed column was inserted into the pressure vessel and positioned vertically. Provisions were made in the design of the lid of the pressure vessel for charging/discharging of solids and inserting bed differential pressure probe. Fluidizing gas for experiments was supplied either from a building compressed air supply or from dedicated nitrogen and compressed air cylinders and metered by variable area flow meters. According to the German Standard VDI/VDE 3513 Part 2, the rotameters of the accuracy class 1.6 were properly compensated for difference in gas properties at different operating pressures and temperatures. The operating pressure in the fluidized bed was set with a backpressure control valve. The experimental facility was of an open circuit type so the same gas was used for pressurizing the system and for fluidizing the solids. The apparatus was pressurized to the desired operating pressure before the fluidizing gas flow rate was adjusted to a desired value. Bed pressure drop was measured using a simple differential pressure probe, having one leg at distributor level and the other in the freeboard, and connected to a pressure transducer and microprocessor-based data acquisition unit. The electrical capacitance tomography (ECT) system used in this work was a single plane system with driven guard drive circuitry of PTL300 type manufactured by Process Tomography. The required electrode pattern for this work was designed in accordance with PTL application note [40]. The maximum number of measurement electrodes (12) was used together with driven guard electrodes. The sensor was fabricated using standard printed circuit board design techniques from flexible copper-coated plastic laminate that was etched with the electrode pattern and wrapped tightly around the fluidized bed plastic vessel. The sensor consisted of 12 rectangular electrodes, 35 mm wide and 50 mm high. The centers of the electrodes were located at the height of 250 mm above the distributor plate, or approximately at a half of the static bed height used in the experiments.

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In accordance with PTL recommendations, the measurement and guard electrodes were connected to the data acquisition module by 1.5-m-long screened coaxial cables, terminated in miniature coaxial connectors. The capacitance sensor had to be positioned inside the pressure vessel and, since the data acquisition module was not designed for operation at elevated pressure, it was located outside of the pressure vessel. Therefore, it became necessary to direct 24 coaxial cables and an earth wire out of the pressure vessel without gas leaks and pressure loss. The detail of how this was achieved is given in Sidorenko [41]. Compressed air from a centralized building supply was usually used for experiments when operating pressure did not exceed 600 kPa. For other experiments that were usually run at higher operating pressure, industrial grade nitrogen supplied from a bank of 12 gas cylinders was used. Geldart A FCC catalyst and Geldart B silica sand were used as bed solids. Geldart A FCC catalyst with the Sauter mean diameter of 77 Am contained 10.1% of fines less than 45 Am and had the particle density of 1330 kg/m3. Particle size distributions were measured using a Malvern Instruments Mastersizer and particle density was determined by using a mercury intrusion analysis. Seven kilograms of the powder were used as the bed material for experiments to observe the minimum fluidization and minimum bubbling conditions, which gave a static bed height of 53.5 cm. For experiments using the ECT system, 5 kg were used, and the bed height was 44 cm. For bed collapse experiments, 6.56 kg of the catalyst were used, which gave the bed height of 49.7 cm. Geldart B silica sand of the narrow size distribution had the Sauter mean diameter of 203 Am and the particle density of 2650 kg/m3. Nine kilograms of the powder were used as the bed material for experiments to observe the minimum fluidization and minimum bubbling conditions, which gave a static bed height of 40 cm. For experiments using the ECT system, 10 kg were used, and the bed height was 42 cm. The instruments downstream of the fluidized bed were protected by a line filter capable of removing particles down to 0.3 Am. Simple filter checks after each experiment indicated that the fines carryover was negligible. 2.2. Minimum fluidization velocity A single set of experiments was carried out in order to determine the minimum fluidization velocity, minimum bubbling velocity, bed expansion and voidage at minimum fluidization and minimum bubbling conditions; and the details of the experimental procedure are given here. Bed pressure drop measurements were obtained for both increasing and decreasing gas velocities. The minimum fluidization velocity is taken as the velocity at the intersection point of the line corresponding to the constant bed pressure drop in the fluidized state and the extrapolated straight line of the packed bed region. There is no agreed procedure for determining the precise point of the minimum fluidization

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velocity and, usually, the two straight lines are obtained as the gas flow rate is gradually reduced in increments from a vigorously bubbling state. In this case, a slightly larger value of the minimum fluidization velocity is obtained. That makes sense for industrial applications as it provides a maximum experimental value for the minimum fluidization velocity and, therefore, the lowest limit for potential defluidization of a process. According to Svarovsky [39], better reproduction of results can be obtained by allowing the bed to mix first by bubbling freely before turning the gas flow rate down to zero and then taking pressure drop measurements, while increasing gradually the gas flow rate. However, various researchers have described three different points representing the minimum fluidization velocity on the increasing gas velocity curve for Geldart A materials, as reviewed by Fletcher et al. [42]. A certain procedure was followed for preparing the bed for the experiments. Prior to each experiment at ambient conditions, the pressure vessel was open and the air supply pressure was set to the gauge pressure of 100 kPa. The bed was fluidized at superficial gas velocities well above the onset of fluidization for at least 15 min, allowing bed solids to fully mix. To achieve an initial packed bed of similar structure in each experiment, the air supply was then slowly turned off by closing the flow control valve. In high-pressure experiments the same gas was used for pressurizing the system and then for fluidizing the bed material. Therefore, prior to each experiment, the backpressure controller was set to a pre-determined operating pressure and the system was pressurized to that level first. At the same time, the gas passed through the fluidized bed, which was properly fluidized for much longer than at the ambient conditions. Then the gas supply was slowly turned off, allowing the bed to settle in the pressurized environment. In actual experiments, measurements of the pressure drop were taken for both increasing and decreasing gas velocities. After each gas velocity change, the pressure in the system was allowed to stabilize for at least 10 min and then pressure drop measurements were recorded at a frequency of 1 Hz for at least 3 min. For each experiment a straight line was drawn from the origin through the series of bed pressure drop points until it crossed the horizontal line corresponding to bed weight per unit area (there was generally good agreement between measured data and this horizontal line). The point of intersection was taken as the value of minimum fluidization velocity at a given experimental condition. Where measurements showed a hysteresis effect between increasing and decreasing gas velocities, both intersection points were established as lower and upper values of the minimum fluidization velocity at a given experimental condition. The difference between the lower and upper values was generally small, and for comparison between experiments at different operating pressures the average of these two points was taken as the minimum fluidization velocity. These experimental curves are presented in Sidorenko [41].

2.3. Minimum bubbling velocity The minimum bubbling velocity was measured by visual observation of the point at which the first bubbles appeared and by examination of the bed height variation with gas velocity. Visual observation of the fluidized bed behavior provides a common and simple but subjective way of determining the minimum bubbling velocity. When the gas flow is gradually increased, the gas velocity at which the first distinct bubbles appear on the bed surface is recorded. Alternatively, the gas velocity is noted at which bubbling stops when the gas flow is decreased. According to Geldart [28] and Svarovsky [39], the average of the two values provides the minimum bubbling velocity, or more appropriate values can be taken during decreasing gas flow. In this work, measurements of the minimum bubbling velocity were taken for both increasing and decreasing gas flow. At each gas velocity the bed height was recorded and, based on visual observation, general behavior of the bed was noted. The first bubbles were not confused with the small channels, which sometimes appeared and resembled small volcanoes. Genuine bubbles usually appeared in several places on the bed surface and were about 10 mm in diameter; while the channels were smaller in diameter and usually stayed in one place. The minimum bubbling velocity was also determined less subjectively as the velocity at which the maximum bed height was observed [43]. 2.4. Bubbling bed voidage Experiments using the ECT equipment were carried out at operating pressures ranging from 300 to 1900 kPa, using Geldart A FCC catalyst as bed material, and ultra-pure nitrogen and compressed air as fluidizing gases. Another series of experiments was conducted at operating pressures ranging from ambient to 1700 kPa, using Geldart B silica sand as bed material, and industrial nitrogen and compressed air as fluidizing gases. Before commencing experiments, the ECT system was calibrated by filling the sensor with the two reference materials in turn and by measuring the inter-electrode capacitances at the two extreme values of relative permittivity (empty vessel and packed bed). The packed bed was formed each time under relevant pressure and following the same procedure (vigorous fluidization followed by gradual defluidization). Since some drift in the very sensitive electronic capacitance measuring circuitry was observed after a few hours of fluidization, it became necessary to carry out such a calibration twice a day and check its accuracy before and after each experiment. In actual experiments at pre-determined operating pressure, the gas velocity was an experimental variable. After each gas velocity change, the pressure in the system was allowed to stabilize for at least 10 min. When both pressure

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and gas flow became stable, the tomographic data were logged on an ECT dedicated computer. The ECT system generated a cross-sectional image of the bed and showed the solids volume fraction (the ratio of solids to gas) at sensor level. Under each set of operating conditions, the behavior of the bed was also characterized by visual inspection and recording of bed height versus gas velocity. 2.5. Bed collapse method Bed collapse experiments were carried out at room temperature and the operating pressures ranging from atmospheric to 1600 kPa, using 6.56 kg of Geldart A FCC catalyst as bed material, and industrial nitrogen as fluidizing gas. The experimental program was based on a ‘‘singlevented’’ technique given by Geldart and Wong [44]. The bed collapse experiment was carried out as follows: the gas flow rate was set at a predetermined level so that the bed was bubbling, and the average bed height was recorded. The gas supply was quickly isolated and the bed collapse was recorded on a video camera at a rate of 25 frames per second. The plenum chamber was not vented simultaneously. The combined internal volume of the chamber and the pipeline downstream of the isolation valve was estimated to be equal to 0.00115 m3. Since the bed height fluctuated due to bubbling, five repeat tests were made and an average bed height was taken. Similar tests were also carried out at different predetermined gas flow rates. Because of restrictions to our gas supply, we used initial velocities in the range 0.054 m/s at atmospheric pressure to 0.008 m/s at 1600 kPa—somewhat below the 0.04 m/s recommended by Geldart [45] and the higher values used by Geldart (0.08 m/s in Geldart and Wong [44] and 0.1 m/s in Geldart and Xie [30]).

3. Experimental results and discussion 3.1. Minimum fluidization velocity Many researchers have previously studied the pressure effect on the minimum fluidization velocity and found that the minimum fluidization velocity is not affected by pressure for fine Geldart A powders and decreases with pressure increase for coarse materials. Numerous correlations for predicting the minimum fluidization velocity have been suggested with the most widely used correlation proposed by Wen and Yu [46]. However, this popular correlation was based only on data obtained at atmospheric pressure. Four similar correlations based on experiments carried out at elevated pressures have also been proposed in the literature [6,9,15,17]. All the correlations are based on the Ergun [47] equation, and are simplified by assuming certain fixed values for the bed voidage at the minimum fluidization and constant particle shape factor. Previously it was quite often found that the absolute values of predictions based on the corre-

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lations, available in the literature, were significantly in error [12,14,16]. One of the better methods for prediction of the minimum fluidization velocity, based on using the Ergun [47] equation, was suggested by Werther [48] and has been used in the present work. Values of the minimum fluidization velocity experimentally determined at ambient conditions and the voidage at minimum fluidization, described in the next section, were used to calculate a characteristic particle diameter udp from the Ergun equation. Following this method, which is described in Sidorenko and Rhodes [4], the minimum fluidization velocity values at various operating pressures were calculated. A comparison between the experimentally measured values of the minimum fluidization velocity and the calculated values from the Ergun [47] equation and the available correlations [6,9,15,17,46] is shown in Figs. 2 and 3. For a Geldart A material, Fig. 2 shows that within the studied pressure range the minimum fluidization velocity was independent of the operating pressure, which is in line with observations of other workers (e.g. Refs. [9,11,18,26,49,50]). Many researchers (e.g. Refs. [6 – 19]) have observed the decrease in the minimum fluidization velocity with increasing pressure for Geldart B materials. Our results also show a slight decrease in minimum fluidization velocity with increasing pressure in the range up to 1600 kPa. Depending on the physical properties of particles, the correlations agree with the experimental results with variable success. For the FCC catalyst, Fig. 2 shows that the Ergun [47] equation and the correlations of Nakamura et al. [15], and to some extent of Wen and Yu [46] predict the minimum fluidization velocity values very well. However, the correlations of Chitester et al. [9], and to larger extent of Saxena and Vogel [17] and Borodulya et al. [6] overestimate the minimum fluidization velocity values. Similar results can be observed for the silica sand Geldart B material in Fig. 3, where only the Ergun [47] equation agrees well with the experimental results and all the correlations more or less overestimate the minimum fluidization velocity values. 3.2. Voidage at minimum fluidization conditions Knowing the mass of the bed solids m and the bed crosssectional area A, the experimental values of the voidage at minimum fluidization emf were determined, using measured bed height Hmf values corresponding to the minimum fluidization velocity at both increasing and decreasing gas velocity. The average of the two values was used as a parameter for comparison at various operating pressures. It was experimentally found that the voidage at minimum fluidization was essentially independent of pressure for:


Geldart A FCC catalyst (emf = 0.42) in a pressure range 101 –1100 kPa.
Geldart B silica sand (emf = 0.50) in a pressure range 101 –1600 kPa.

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Fig. 2. Variation of the minimum fluidization velocity with pressure for Geldart A FCC catalyst. Comparison of experimental values (clear squares) with predictions from the Ergun equation [47,48] (line 4) and correlations of Borodulya et al. [6] (line 1); Chitester et al. [9] (line 3); Nakamura et al. [15] (line 5); Saxena and Vogel [17] (line 2); and Wen and Yu [46] (line 6).

Other workers [11,18,19] also observed the independence of the voidage at minimum fluidization on pressure. Yang et al. [23] suggested, however, that the variation of the voidage at minimum fluidization caused by a pressure increase would be very small, if at all, for coarse materials and substantial for Geldart A powders. That was not observed in the present study. Weimer and Quarderer [24] carried out their experiments at much higher pressures and did not observe any voidage change for a Geldart B material and noticed only very small increase in the voidage at minimum fluidization for Geldart A materials.

3.3. Minimum bubbling velocity The silica sand was found to exhibit Geldart B behavior, with coincidence of minimum bubbling and minimum fluidization velocities, for all pressures up to 1600 kPa. This result is in agreement with the findings of King and Harrison [11] at pressures up to 2500 kPa, but at odds with the findings of Sobreiro and Monteiro [18] and Sciazko and Bandrowski [51,52], who were able to establish separate values for the minimum bubbling and minimum fluidization velocities within the similar pressure range, as well as at

Fig. 3. Variation of the minimum fluidization velocity with pressure for Geldart B silica sand. Comparison of experimental values (clear squares) with predictions from the Ergun equation [47,48] (line 6) and correlations of Borodulya et al. [6] (line 2); Chitester et al. [9] (line 3); Nakamura et al. [15] (line 4); Saxena and Vogel [17] (line 1); and Wen and Yu [46] (line 5).

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higher pressures up to 3500 kPa. These results are also contrary to those of Varadi and Grace [53], obtained within the pressure range from atmospheric to 2170 kPa. However, it should be noted that the often-cited work of Varadi and Grace [53] is only a preliminary study carried out in a twodimensional bed. FCC catalyst showed a typical Geldart A behavior, expanding without bubbling within a certain gas velocity range at ambient conditions. At all the experimental conditions the height of the homogeneously expanded bed of the FCC catalyst was at least 0.6 m. In all cases the minimum bubbling velocity values, determined visually, coincided for both increasing and decreasing gas flow and were equal to values, obtained from the maximum bed height. Only one correlation for determining the minimum bubbling velocity has been proposed [20]. This equation is not commonly used on its own as Abrahamsen and Geldart [20] combined it with another correlation for the minimum fluidization velocity [54] and proposed a more popular relation: l0:523 expð0:716FÞ Umb 2300q0:126 g ¼ Umf dp0:8 g0:934 ðqp  qg Þ0:934

ð1Þ

Thus, Geldart A behavior corresponds to the velocity ratio being more than unity. The minimum bubbling to minimum fluidization velocity ratios calculated from the experimental values of the minimum bubbling and minimum fluidization velocities and compared to the predictions of Eq. (1) are presented in Fig. 4. As can be seen in Fig. 4, Eq. (1) predicts an increase in the velocity ratio with increasing pressure but the accuracy of prediction is quite low. It overestimates the

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experimental values for the FCC catalyst which show no pressure effect on the velocity ratio over the range considered. Within the higher pressure range (2140 to 6900 kPa) and in a smaller column (2.7 cm in diameter), Crowther and Whitehead [55] fluidized 50 g of fine synclyst particles with a high density gas (CF4) and found that the minimum bubbling velocity was equal to 0.008 m/s at 2140 and 2820 kPa, and increased to 0.009 m/s at 4180 kPa and to 0.011 at 5540 kPa. In another study within the pressure range similar to present work, Richardson [56] fluidized Perspex particles at pressures between atmospheric and 1400 kPa and reported a ‘‘slight increase’’ in the velocity ratio from 2.2 to 2.8. However, when Richardson [56] fluidized similarly sized phenolic resin particles, the velocity ratio trend was not obvious. It should be noted that, since both the minimum fluidization and minimum bubbling velocities are very low, a small variation of just 0.5 mm/s in either of them could position the velocity ratio anywhere in the range between two and three, as can be illustrated by substantial error bars in Fig. 4. 3.4. Voidage at minimum bubbling conditions Abrahamsen and Geldart [20] correlated the maximum non-bubbling bed expansion ratio in the following way: l0:115 expð0:158FÞ Hmb 5:50q0:028 g ¼ Hmf dp0:176 g0:205 ðqp  qg Þ0:205

ð2Þ

According to this correlation, the maximum bed expansion ratio should slightly increase with pressure. A compar-

Fig. 4. Variation of the minimum bubbling to minimum fluidization velocity ratio with pressure for Geldart A FCC catalyst. Comparison of experimental values (clear squares) with predictions from the Abrahamsen and Geldart [20] correlation.

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ison between the maximum bed expansion ratio determined experimentally and that predicted from Eq. (2) is shown in Fig. 5. As can be seen in Fig. 5, the effect of pressure on the maximum bed expansion within the pressure range 101 – 1100 kPa was found to be negligible for the Geldart A material used in this work. Eq. (1) overestimates the experimental values for the FCC catalyst. The experimental results and predictions from Eq. (2), however, contradict the experimental results presented by Piepers et al. [26] who reported a substantial increase in the total bed expansion with pressure increase up to 1500 kPa. Values of the voidage at minimum bubbling emb were determined from bed height measurements. In this study, the bed voidage at minimum bubbling for FCC catalyst was found to increase from 0.47 at ambient conditions to 0.49 at 1000 kPa. Similar small increases caused by pressure was noticed for some Geldart A materials by other workers [11,18,49,57 – 59]. Piepers et al. [26] observed slightly greater increase in the minimum bubbling voidage for 59Am sized catalyst within the similar pressure range (from 0.58 at atmospheric pressure to 0.63 at 1500 kPa). 3.5. Bubbling bed voidage at elevated pressure For FCC catalyst and silica sand the ECT system generated two types of data:


time series image frames of the local solids volume fraction distribution at a given horizontal level in the form of a 32  32 pixels matrix;
time series data points of the average solids volume fraction representing the whole bed at a given horizontal level.

It is accepted that the tomographic images obtained from the capacitance measurements are usually of relatively low resolutzion (e.g. Refs. [60 – 62]). A linear back-projection algorithm, supplied as standard with the PTL300 system, is mathematically simple and fast because the image reconstruction process is reduced to matrix – vector multiplication. However, the images are only approximate and suffer from blurring. Although, image resolution and accuracy can be improved by employing an iterative linear back-projection algorithm, this and other reconstruction algorithms are still under development [61,63 –65]. According to the Process Tomography Application Note [66], the linear back-projection algorithm will always underestimate areas of high permittivity and overestimate areas of low permittivity. The images produced by this method will always be approximate, since the method spreads the true image over the whole sensor area. Because the image has been spread out over the whole area, the magnitude of the image pixels will be less than the true values. However, the sum of all of the pixels will approximate to the true value, and the linear back-projection algorithm is therefore useful for calculating the average voidages. At each operating pressure, measurements of relative solids volume fraction were taken at a number of superficial gas velocities above the minimum fluidization velocity. At each given gas velocity, 16,000 frames of data were logged over more than 3 min of dynamic operation and recorded by the ECT system at the rate of approximately 81 frames per second. The output, which was in the form of 32  32 pixels matrix was averaged and provided for each frame the average volume fraction as a single value between 0 (gas) and 100 (packed bed). From 16000 values of the relative voidage and the sample frequency the relevant time series statistics were calculated.

Fig. 5. Variation of the maximum bed expansion ratio with pressure for Geldart A FCC catalyst. Comparison of experimental values (clear squares) with predictions from the Abrahamsen and Geldart [20] correlation.

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Three parameters were selected for further analysis— average, average absolute deviation and average cycle frequency. Although some analysis techniques were originally proposed for pressure fluctuations time series, in this work they have been applied to the analysis of the voidage fluctuations resulting from the ECT measurements. According to Schouten et al. [67], the average absolute deviation of the data points from the solids volume fraction average value is a robust estimator of the width of time series around the mean. According to Zijerveld et al. [68], the average absolute deviation is comparable with the standard deviation and shows similar dependence on operating conditions. The average cycle frequency is the reciprocal of the average cycle time, which is defined as the average time to complete a full cycle after the first passage through the time series average. These three parameters make it possible to compare results between measurements at various gas velocities and different operating pressures, since their values are unambiguous and can be readily calculated. The time series data were also used in the reconstruction of an attractor from which the Kolmogorov entropy was estimated for use in chaos analysis. 3.5.1. Average total bed voidage The experimental values of the average relative solids volume fraction were expressed in percentage points, and under the existing operating conditions fluctuated somewhere between 100 in a non-fluidized state and 55 in a bubbling regime. The value of 100 was established during the calibration procedure and corresponded to a packed bed. Based on a known packed bed height, the packed bed voidage was calculated to be equal to 0.476 for the FCC catalyst and 0.463 for the silica sand. For both materials, calibration procedures were completed at various operating

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pressures and the packed bed height, and therefore voidage, did not vary with pressure. Fig. 6 shows the measured influence of superficial gas velocity and operating pressure on the average bed voidage for FCC catalyst. Average bed voidage reduces with increasing gas velocity above the minimum bubbling velocity of 0.006 m/s and up to about 0.03 m/s, and generally increases thereafter. This result is consistent with the findings of other researchers at ambient pressure [44,45,69,70] who found that at superficial gas velocities up to 0.03 m/s the bed voidage decreases with increase in gas velocity. It might be expected that the total bed expansion would increase if more gas is put into the fluidized bed as bubbles. However, according to Geldart et al. [69], the reduction in the bed expansion just above the minimum bubbling velocity in Geldart A powders occurs because the dense phase volume in the bubbling bed is reduced more rapidly than the bubble hold-up increases. According to Geldart and Radtke [70], this reduction in dense phase voidage is due to small interparticle forces which make the powder slightly cohesive and are continually disrupted by the bubbles passage and increase in powder circulation. The effect of operating pressure on the cross-section averaged total bed voidage for the Geldart A FCC catalyst can be clearly seen in Fig. 6. This diagram shows that any pressure increase is accompanied by the reduction of the average bed voidage. The ECT system used in this work can only provide information about the total bed voidage, consisting of the dense phase voidage and bubbles volume held up within the bed and averaged across one cross-sectional plane. Therefore, based on these results alone, we cannot determine whether the decrease in the bed voidage with the increase in operating pressure, seen in Fig. 6, is caused by a reduction

Fig. 6. Variation in average bed voidage with gas velocity for FCC catalyst over a range of operating pressures from 300 to 1900 kPa (lines are used to guide the eye).

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in dense phase voidage or volume of bubbles in the bed, or both. However, the bed collapse experiments (see below) demonstrate that for FCC catalyst the dense phase voidage increased with the increase in pressure. The decrease in the bubbling bed voidage or expansion with increase in pressure, found in this work from the ECT dynamic measurements, can therefore be explained by the reduction in volume of bubbles held up within the bed at higher operating pressures. This might be in line with the observations of other researchers [24,59,71 –75], who reported that fluidization became smoother at higher pressures and attributed that to smaller bubbles in Geldart A powders at increased pressure. However, in the present work, because of the ECT limitations in image generating, no conclusion on the volume or size of individual bubbles could be reached. The results of the ECT experiments with the Geldart B silica sand are different from those for the FCC catalyst. As can be seen in Fig. 7, a considerable increase in the bed voidage with increasing gas velocity takes place at each operating pressure. This is expected as more gas is put into the fluidized bed as bubbles. In comparison to Geldart A powders, the interparticle forces in Geldart B materials are negligible compared with the hydrodynamic forces acting in the fluidized bed. Fig. 7 also shows that increase in operating pressure produces an increase in average bed voidage, consistent with the trends reported for Geldart B powders by other workers [8,9]. However, some unexplained changes in bed expansion at higher pressures were observed by Olowson and Almstedt [76] and Llop et al. [13,77], who fluidized silica sand and found that the bed expansion strongly increased with pressure up to a maximum at approximately 800 kPa and then stayed constant or even slightly decreased

thereafter at pressures up to 1600 kPa. Although in the present work, no obvious decrease in total bed voidage was observed at higher operating pressures, it was found that the pressure influence was stronger at lower pressures (see Fig. 7). Olowson and Almstedt [78] found in their fluidization work with Geldart B sand that the bubble size is determined by a complex balance between splitting and coalescence, and an increase in pressure may cause either increase or decrease in bubble size, depending on the location in the bed, gas velocity and the pressure level. In a comprehensive summary and analysis of previous research on bubble size under pressurized conditions, applicable for the pressurized fluidized bed combustion, Cai et al. [79] reached the conclusion that in fluidized beds of coarse materials the bubble size decreases with increasing pressure except when gas velocity is low. At low gas velocities, there is a dual effect of pressure on bubble size, i.e. there is an initial increase in bubble size in the pressure range less than 1000 kPa and then a size decrease with further increase in pressure. The bed voidage results, obtained from the dynamic ECT data in this study, are therefore in accordance with the views of Olowson and Almstedt [78] and Cai et al. [79]. 3.5.2. Average absolute deviation In bubbling fluidized beds the main source of signal (pressure or voidage) fluctuations originates from the formation of bubbles. The intensity of the signal can be measured and compared in a statistical manner by using the average absolute deviation. Therefore it was expected that the average absolute deviation in the bed voidage would steadily increase with increasing gas velocity when more and more gas is put into the bed as bubbles. This was indeed the case for both Geldart A and Geldart B materials, as can be seen in Figs. 8 and 9.

Fig. 7. Variation in average bed voidage with gas velocity for silica sand over a range of operating pressures from atmospheric to 1700 kPa.

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Fig. 8. Variation in average absolute deviation in bed voidage with gas velocity for FCC catalyst over a range of operating pressures from 300 to 1900 kPa (lines are used to guide the eye).

For the Geldart A FCC catalyst (Fig. 8) average absolute deviations of bed voidage was found to increase steadily with gas velocity, as expected, and decrease with increasing operating pressure—this effect being stronger at lower pressure, as was found for average bed voidage. The decrease in average absolute deviations of bed voidage with increasing operating pressure is consistent with smaller bubble size at higher pressures. For the Geldart B silica sand, as can be seen in Fig. 9, increasing operating pressure from ambient to 2100 kPa has practically no effect on the average absolute deviation in bed voidage and bubbling behavior of the bed. Once again, the fluidized bed remains in the bubbling state within the range of gas velocities studied, as can be seen

from the increase in the average absolute deviation with increasing gas velocity. 3.5.3. Average cycle frequency The dependence of the measured average cycle frequency on gas velocity and operating pressure for the Geldart A FCC catalyst and the Geldart B silica sand is shown in Figs. 10 and 11, respectively. For the Geldart A FCC catalyst the average cycle frequency is found to be variable at low gas velocities, but tend towards a steady value once the bed was freely bubbling (Umf is approximately 0.003 m/s and Umb is approximately 0.006 m/s). Similar large differences between different pressure fluctuations measurements at velocities

Fig. 9. Variation in average absolute deviation in bed voidage with gas velocity for silica sand over a range of operating pressures from atmospheric to 2100 kPa.

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Fig. 10. Variation in average cycle frequency with gas velocity for FCC catalyst over a range of operating pressures from 300 to 1900k Pa (lines are used to guide the eye).

near to Umf and Umb were observed at ambient conditions by van der Stappen et al. [80] and Daw and Halow [81]. It can be seen from Fig. 10 that average cycle frequency decreases from 4.5 Hz at 300 kPa to 2.3 Hz at 1900 kPa. We note that the influence of pressure on the average cycle frequency is stronger at low pressures, as was the case for average bed voidage and average absolute deviation in voidage. According to Bai et al. [82], experimental results presented by van der Stappen et al. [83] suggest that higher cycle frequencies correspond to more complex dynamic systems. Therefore, the ECT experimental results, presented in Fig. 10, show that increasing the operating pressure leads

to a less dynamic fluidized bed system in line with other researchers’ observations of smoother fluidization for Geldart A materials at high pressure, which is generally attributed to smaller bubbles. For the Geldart B silica sand, in the intermediate regime near the minimum fluidization and at gas velocities below approximately 0.04 m/s, the average cycle frequency is very sensitive and the differences between different measurements are quite large, as shown in Fig. 11. When the bubbling bed is developed, the average cycle frequency of voidage fluctuations is about constant at a value of about 3.2 Hz at all the operating pressures tested in the range

Fig. 11. Variation in average cycle frequency with gas velocity for silica sand over a range of operating pressures from atmospheric to 2100 kPa (lines are used to guide the eye).

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from atmospheric to 2100 kPa. Similarly, the constant value of the average cycle frequency of pressure fluctuations of about 3.3 Hz in a bubbling bed was reported by van der Stappen et al. [80] who fluidized Geldart B polystyrene particles at ambient conditions. From Fig. 11, it can be observed that in the fully developed bubbling regime the average cycle frequency is practically independent of pressure. 3.5.4. Deterministic chaos analysis Recently it has been suggested that the irregular behavior of the fluidized bed dynamics is due to the fact that the fluidized bed is a chaotic non-linear system. According to some researchers in this area [68,84], chaotic systems are governed by non-linear interactions between the system variables and all methods of non-linear chaos analysis are based on the construction of an attractor of the system in state-space. The attractor forms a fingerprint of the system and reflects its hydrodynamic state. The most common method to characterize the attractor is the evaluation of the correlation dimension and the Kolmogorov entropy, where the correlation dimension expresses the number of degrees of freedom of the system and the Kolmogorov entropy is a measure of the predictability of the system and the sensitivity to the initial state. The Kolmogorov entropy, K, is a number expressed in bits/s and can be calculated from a time series of only one characteristic variable of the system, solids volume fraction in this case. Generally, the Kolmogorov entropy is small in cases of more or less regular behavior, and large for very irregular dynamic behavior such as fluctuations in turbulent flow. The average absolute deviation is a measure of the characteristic length and the average cycle frequency is a

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measure of the time scale in the time series, and both invariants were used in the reconstruction of the attractor, from which the Kolmogorov entropy was calculated. Since for the Geldart B material studied, both invariants were found to be practically independent of pressure, it is expected that the Kolmogorov entropy for this material would be also more or less unaffected by pressure. In order to observe the dynamic processes in fluidized beds and be able to apply fully a chaotic time series analysis, Daw and Halow [81] recommend acquiring time series fluidized bed pressure drop or voidage measurements at an optimum sampling rate of 200 Hz. Schouten and van den Bleek [85] have found that in order to sufficiently accommodate the chaos analysis attractor in its state space, the required sampling frequency should be at approximately 100 times the average cycle frequency. Based on the average cycle frequency of 3.2 Hz for the bubbling bed of the Geldart B sand, the required sampling frequency should be more than 300 Hz. However, the ECT system used in this work was not capable of sampling frequency above 90 Hz. Taking into consideration this limitation of the equipment, it was accepted that the chaos analysis of the experimental data might not be complete. According to Kuehn et al. [86], a number of points per cycle in the voidage time series from the ECT measurements is rather low which may lead to an over-estimation of the Kolmogorov entropy. The Kolmogorov entropy calculated from the results of the ECT experiments with the Geldart A material is presented in Fig. 12. As can be seen in Fig. 12, at minimum bubbling (Umbg0.006 m/s), the Kolmogorov entropy seems to have a minimum, generally less than 5 bits per second, which is lower than in the freely bubbling state. This is in good

Fig. 12. Kolmogorov entropy as a function of gas velocity for FCC catalyst over a range of operating pressures from 300 to 1900 kPa (lines are used to guide the eye).

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agreement with results of van der Stappen et al. [80], who qualitatively explained this in the following way—at gas velocities where the first bubbles appear, the bubbles are independent of each other and rise through the bed with no or minor interaction. This behavior is relatively simpler and more periodic compared to the developed bubbling state, where bubbles are influencing each other in a more complex way. In the freely bubbling state, the Kolmogorov entropy is expected to settle at a certain value, which could be as high as 32 at the operating pressure of 300 kPa or as low as 12 at 1900 kPa. However, because of the equipment limitations previously explained, Fig. 12 shows possibly not the absolute values but merely trends. Examination of Fig. 12 alone does not yield any trends. However, we could reasonably make the comparison with the results for average cycle frequency (Fig. 10), where the trend established itself at gas velocities beyond 0.03 m/s. The variation in Kolmogorov entropy with operating pressure may therefore be revealed by the final data point in each series—i.e. Kolmogorov entropy decreases with increasing pressure. When the Kolmogorov entropy decreases, the fluidized bed hydrodynamics become more predictable. Zijerveld et al. [68] explain this in terms of an increase in bubble size at the bed surface. This is in agreement with Schouten et al. [87], who related the Kolmogorov entropy K to the bubble size db as follows: K¼

1:5ðU  Umf ÞD db2

ð3Þ

According to this equation, the Kolmogorov entropy decreases with an increase in bubble size. Based on this and the trends presented for the Geldart A material in Fig. 12, it appears that the bubble size increases with increasing pressure. However, this is contradictory to the previous research and the results of this study presented in Fig. 6. As previously mentioned, generally it has been reported that fluidization becomes smoother with high pressure, which has been attributed to smaller bubbles at increased pressure. Many workers found that increasing the operating pressure causes bubble size to decrease in Geldart A materials [24,58,71 – 75]. A possible explanation might be that Eq. (3) was developed based on the pressure fluctuations obtained during the experiments with coarse Geldart B polyethylene particles (0.56 mm) and silica sand (0.4 mm) carried out at ambient conditions. As previously mentioned, it was found that bubbling behavior in fluidized beds of coarse Geldart B materials differs noticeably from that in fluidized beds of Geldart A powders even at ambient conditions. Even with the Geldart B materials, there are some apparently conflicting reports coming from the same school. For example, Zijerveld et al. [68] found that in the bubbling bed the Kolmogorov entropy decreases with superficial gas velocity and decreases with an increase in bubble size. Earlier van der Stappen et al. [80] found that in a freely

bubbling bed the Kolmogorov entropy settles at a value of about 17 bits per second. In another related study [86], higher gas velocity and more turbulent hydrodynamics was found to cause an increase of the Kolmogorov entropy, though bubble size in Geldart B powders is known to increase with gas velocity. Obviously, deterministic chaos theory can provide numerical and graphical tools that can quantify and visualize the fluidized bed hydrodynamics, however, it is still in its early days and much more work has to be done. It is necessary to verify further the correlations and incorporate other particle systems. So far, it seems that the chaos analysis method has been mainly applied to scale-up of the dynamic behavior of the fluidized bubbling reactors and fluidization regime transitions in the circulating fluidized beds [68,87]. 3.6. Bed collapse tests Bed collapse experimental results had been obtained in a form of video recording, from which the plots of bed height decrease with time were constructed. The results of the second intermediate stage when the bed surface falls at a constant rate of collapse velocity Uc, until the bed height approaches a certain critical height Hc, are plotted in Fig. 13. It should be noted that the error bars for the curve at atmospheric pressure are based on the total of 20 experiments, five for each of four initial fluidization gas velocities. Similar collapse curves for a FCC catalyst at various operating pressures were observed by Foscolo et al. [49]. The straight line portions of the bed collapse curves were extrapolated back to time zero and it was assumed that the intercept with the bed height axis gives the height which the dense phase would occupy in the bubbling bed. Abrahamsen and Geldart [45] correlated the dense phase expansion (Eq. (4)) which includes the gas density effect. 2:54q0:016 l0:066 expð0:09ÞF Hd g ¼ 0:043 Hmf dp0:1 g0:118 ðqp  qg Þ0:118 Hmf

ð4Þ

According to this correlation, the dense phase expansion should slightly increase with increasing the operating pressure. A comparison between the dense phase expansion determined experimentally from the bed collapse tests at various operating pressures and that predicted from the correlation by Abrahamsen and Geldart [45] is presented in Fig. 14. For comparison with the maximum non-bubbling bed expansion for this material shown in Fig. 5, the scale of both graphs is the same. As can be seen in Fig. 14, the dense phase expansion linearly increases with increasing pressure within the studied range 101 – 1600 kPa. Compared to the predictions of the correlation by Abrahamsen and Geldart [45], the experimental results show a slightly higher rate of increase, and therefore, a stronger influence of operating pressure than the factor qg0.016 included in the correlation.

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Fig. 13. Bed surface collapse curves for FCC catalyst at various operating absolute pressures.

Similar results were observed by other workers [24,26]. As noted by Barreto et al. [88], the extensive experiments of Abrahamsen and Geldart were carried out with air at atmospheric pressure and the effect of gas density arose as a consequence of the correlation method. According to the correlation, it was expected that a pressure increase from atmospheric to 1500 kPa should cause only 4.5% increase in the dense phase height for a FCC catalyst fluidized with nitrogen [26]. However, their experimental results demonstrated a much higher 21% increase. Weimer and Quarderer [24] also found that the correlation proposed by Abrahamsen and Geldart [45] largely underestimated the effect of pressure on the dense phase voidage for a Geldart A powder.

Bed collapse experiments are primarily intended to obtain a direct value of the dense phase voidage. Once the initial heights of the dense phase of the bed had been determined from the collapse curves obtained at various operating pressures (Fig. 13), the experimental values of the dense phase voidage ed were calculated. The experimental values of all the voidages—dense phase voidage, settled bed voidage, minimum bubbling voidage and minimum fluidization voidage—can now be compared (Fig. 15). As can be seen in Fig. 15, within the studied pressure range, the dense phase voidage was found to be higher than the voidage at minimum fluidization and to increase steadily with increasing pressure for a Geldart A powder.

Fig. 14. Variation of the dense phase expansion with pressure for FCC catalyst. Comparison of experimental values (clear squares) with predictions from the Abrahamsen and Geldart [45] correlation.

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Fig. 15. Variation with pressure of dense phase voidage (ed), settled bed voidage (es), minimum fluidization voidage (emf) and minimum bubbling voidage (emb) for FCC catalyst.

This is in line with the observations of other researchers [24,26,49,58,71].

4. Conclusions A comprehensive experimental study of the influence of pressure on fluidization phenomena, such as minimum fluidization and minimum bubbling velocities, bed expansion and voidage was carried out in a bubbling fluidized bed over the range of operating absolute pressures 101 – 2100 kPa with Geldart A and B bed materials. As expected, it was found that the minimum fluidization velocity decreased slightly with increasing pressure for a Geldart B material and was independent of pressure for a Geldart A material. In the pressure range studied in this work, bed voidage at minimum fluidization was found to be practically independent of pressure for both Geldart A and Geldart B materials. Although it has been implied that with pressure increase Geldart B solids could demonstrate Geldart A type behavior, in this work, we observed no shift in behavior over the pressure range studied. For the Geldart A powder, the minimum bubbling velocity and the maximum bed expansion ratio were found to be practically unaffected by increasing pressure. Predictions using existing correlations for the minimum bubbling velocity and the maximum bed expansion ratio were not in agreement with the experimental values. In line with observations of other researchers, bed voidage at minimum bubbling was found to increase slightly within the studied pressure range. Electrical capacitance tomography was used to provide a time series analysis of fluctuations of the average solids

volume fraction, from which a quantitative description of the bubbling bed dynamics as a function of operating pressure was obtained. For Geldart A catalyst, it was found that average bed voidage decreased with increasing operating pressure. For Geldart B sand, however, the opposite trend was observed. The intensity of the voidage fluctuations caused by the formation of bubbles was measured and compared by using the average absolute deviation. For Geldart A material, increase in operating pressure gave rise to a decrease in the average absolute deviation in bed voidage, suggesting more uniform bubbling at elevated pressure. For Geldart B material, increasing pressure up to 2100 kPa had no effect on the average absolute deviation in voidage fluctuations. For Geldart A material, the average cycle frequency decreased with increasing operating pressure. This suggested that increasing the operating pressure lead to a less dynamic fluidized bed system in line with other researchers’ observations of smoother fluidization for Geldart A materials at high pressure. For Geldart B solids, the average cycle frequency was found to be independent of pressure. Bed collapse tests were carried out at various pressures up to 1600 kPa, using Geldart A solids. The dense phase voidage values derived from these experiments were found to be higher than the values of voidage at minimum fluidization and were found to increase steadily with increasing pressure. References [1] M.I. Fridland, The effect of pressure on productivity and on entrainment of particles in fluidized bed equipment, Int. Chem. Eng. 3 (4) (1963) 519 – 522. [2] J.S.M. Botterill, Fluidized bed behaviour at high temperatures and

I. Sidorenko, M.J. Rhodes / Powder Technology 141 (2004) 137–154

[3]

[4] [5]

[6]

[7]

[8]

[9] [10]

[11] [12]

[13]

[14] [15]

[16] [17]

[18] [19]

[20]

[21]

[22] [23]

[24]

[25]

pressures, in: L.K. Doraiswamy, A.S. Mujumdar (Eds.), Transport in Fluidized Particle Systems, Elsevier, Amsterdam, 1989, pp. 33 – 70. T.M. Knowlton, Pressure and temperature effects in fluid – particle systems, in: W.C. Yang (Ed.), Fluidization Solids Handling and Processing: Industrial Applications, Noyes Publications, Westwood, 1999, pp. 111 – 152. I. Sidorenko, M.J. Rhodes, Pressure effects on gas – solid fluidized bed behavior, Int. J. Chem. React. Eng. 1 (2003) R5. J.G. Yates, Review article number 49. Effects of temperature and pressure on gas – solid fluidization, Chem. Eng. Sci. 51 (2) (1996) 167 – 205. V.A. Borodulya, V.L. Ganzha, V.I. Kovensky, Gidrodynamika i teploobmen v psevdoozhizhenom sloye pod davleniem (in Russian), Nauka i Technika, Minsk, 1982. R. Bouratoua, Y. Molodtsof, A. Koniuta, ‘‘Hydrodynamic characteristics of a pressurized fluidized bed’’. Paper presented at 12th International Conference on Fluidized Bed Combustion, La Jolla, 1993. S. Chiba, J. Kawabata, T. Chiba, Characteristics of pressurized gasfluidized beds, in: N.P. Cheremisinoff (Ed.), Encyclopedia of Fluid Mechanics, Gulf Publishing, Houston, 1986, pp. 929 – 945. D.C. Chitester, R.M. Kornosky, L.S. Fan, J.P. Danko, Characteristics of fluidization at high pressure, Chem. Eng. Sci. 39 (2) (1984) 253 – 261. M.A. Gilbertson, D.J. Cheesman, J.G. Yates, Observations and measurements of isolated bubbles in a pressurized gas-fluidized bed, in: L.S. Fan, T.M. Knowlton (Eds.), Fluidization IX, Engineering Foundation, New York, 1998, pp. 61 – 68. D.F. King, D. Harrison, The dense phase of a fluidized bed at elevated pressures, Trans. Inst. Chem. Eng., (1) (1982) 26 – 30. T.M. Knowlton, High-pressure fluidization characteristics of several particulate solids, primarily coal and coal-derived materials, AIChE Symp. Ser. 73 (161) (1977) 22 – 28. M.F. Llop, J. Casal, J. Arnaldos, Incipient fluidization and expansion in fluidized beds operated at high pressure and temperature, in: J.-F. Large, C. Laguerie (Eds.), Fluidization VIII, Engineering Foundation, New York, 1995, pp. 131 – 138. A. Marzocchella, P. Salatino, Fluidization of solids with CO2 at pressures from ambient to supercritical, AIChE J. 46 (5) (2000) 901 – 910. M. Nakamura, Y. Hamada, S. Toyama, A.E. Fouda, C.E. Capes, An experimental investigation of minimum fluidization velocity at elevated temperatures and pressures, Can. J. Chem. Eng. 63 (1) (1985) 8 – 13. P.A. Olowson, A.E. Almstedt, Influence of pressure on the minimum fluidization velocity, Chem. Eng. Sci. 46 (2) (1991) 637 – 640. S.C. Saxena, G.J. Vogel, The measurement of incipient fluidization velocities in a bed of coarse dolomite at temperature and pressure, Trans. Inst. Chem. Eng. 55 (3) (1977) 184 – 189. L.E.L. Sobreiro, J.L.F. Monteiro, The effect of pressure on fluidized bed behavior, Powder Technol. 33 (1982) 95 – 100. C. Vogt, R. Schreiber, J. Werther, G. Brunner, Fluidization at supercritical fluid conditions, in: M. Kwauk, J. Li, W.C. Yang (Eds.), Fluidization X, United Engineering Foundation, New York, 2001, pp. 117 – 124. A.R. Abrahamsen, D. Geldart, Behavior of gas-fluidized beds of fine powders: part I. Homogeneous expansion, Powder Technol. 26 (1980) 35 – 46. K.V. Jacob, A.W. Weimer, High-pressure particulate expansion and minimum bubbling of fine carbon powders, AIChE J. 33 (10) (1987) 1698 – 1706. A.K. Bin, Minimum fluidization velocity at elevated temperatures and pressures, Can. J. Chem. Eng. 64 (5) (1986) 854 – 857. W.C. Yang, D.C. Chitester, R.M. Kornosky, D.L. Keairns, A generalized methodology for estimating minimum fluidization velocity at elevated pressure and temperature, AIChE J. 31 (7) (1985) 1086 – 1092. A.W. Weimer, G.J. Quarderer, On dense phase voidage and bubble size in high pressure fluidized beds of fine powders, AIChE J. 31 (6) (1985) 1019 – 1028. P.N. Rowe, The dense phase voidage of fine powders fluidised by gas

[26]

[27]

[28]

[29]

[30]

[31] [32] [33]

[34] [35] [36]

[37]

[38]

[39] [40] [41] [42]

[43]

[44]

[45]

[46] [47] [48] [49]

153

and its variation with temperature, pressure and particle size, in: J.R. Grace, L.W. Shemilt, M.A. Bergougnou (Eds.), Fluidization VI, Engineering Foundation, New York, 1989, pp. 195 – 202. H.W. Piepers, E.J.E. Cottaar, A.H.M. Verkooijen, K. Rietema, Effects of pressure and type of gas on particle – particle interaction and the consequences for gas – solid fluidization behavior, Powder Technol. 37 (1984) 55 – 70. C. Vogt, R. Schreiber, J. Werther, G. Brunner, ‘‘The expansion behavior of fluidized beds at supercritical fluid conditions’’. Paper presented at World Congress on Particle Technology 4, Sydney, 2002. D. Geldart, Single particles, fixed and quiescent beds, in: D. Geldart (Ed.), Gas Fluidization Technology, Wiley, Chichester, 1986, pp. 11 – 32. J.G. Yates, Experimental observations of voidage in gas fluidized beds, in: J. Chaouki, F. Larachi, M.P. Duducovic (Eds.), Non-Invasive Monitoring of Multiphase Flows, Elsevier, Amsterdam, 1997, pp. 141 – 160. D. Geldart, H.Y. Xie, The collapse test as a means for characterizing Group A powders (preprints), in: J.-F. Large, C. Laguerie (Eds.), Fluidization VIII, Engineering Foundation, Tours, 1995, pp. 263 – 270. J.R. Grace, Agricola aground: characterization and interpretation of fluidization phenomena, AIChE Symp. Ser. 88 (289) (1992) 1 – 16. K. Rietema, The Dynamics of Fine Powders, Elsevier, London, 1991. M. Louge, Experimental techniques, in: J.R. Grace, A.A. Avidan, T.M. Knowlton (Eds.), Circulating Fluidized Beds, Blackie, London, 1997, pp. 312 – 368. D. Geldart, Types of gas fluidization, Powder Technol. 7 (1973) 285 – 292. J.S. Halow, P. Nicoletti, Observations of fluidized bed coalescence using capacitance imaging, Powder Technol. 69 (1992) 255 – 277. Y.T. Makkawi, P.C. Wright, ‘‘Application of process tomography as a tool for better understanding of fluidization quality in a conventional fluidized bed’’. Paper presented at 2nd World Congress on Industrial Process Tomography, Hannover, 2001. S.J. Wang, T. Dyakowski, C.G. Xie, R.A. Williams, M.S. Beck, Real time capacitance imaging of bubble formation at the distributor of a fluidized bed, Chem. Eng. J. 56 (3) (1995) 95 – 100. I. Sidorenko, M.J. Rhodes, ‘‘The use of Electrical Capacitance Tomography to study the influence of pressure on fluidized bed behaviour’’. Paper presented at 6th World Congress of Chemical Engineering, Melbourne, 2001. L. Svarovsky, Powder Testing Guide: Methods of Measuring the Physical Properties of Bulk Powders, Elsevier, London, 1987. Engineering design rules for ECT sensors (Wilmslow, Process Tomography), 2001a. I. Sidorenko, Pressure Effects on Fluidized Bed Behaviour, PhD thesis, Monash University, Clayton, 2003. J.V. Fletcher, M.D. Deo, F.V. Hanson, Fluidization of a multi-sized Group B sand at reduced pressure, Powder Technol. 76 (1993) 141 – 147. P. Harriott, S. Simone, Fluidizing fine powders, in: N.P. Cheremisinoff, R. Gupta (Eds.), Handbook of Fluids in Motion, Science, Ann Arbor, 1983, pp. 653 – 663. D. Geldart, A.C.-Y. Wong, Fluidization of powders showing degrees of cohesiveness: II. Experiments on rates of de-aeration, Chem. Eng. Sci. 40 (4) (1985) 653 – 661. A.R. Abrahamsen, D. Geldart, Behavior of gas-fluidized beds of fine powders: part II. Voidage of the dense phase in bubbling beds, Powder Technol. 26 (1980) 47 – 55. C.Y. Wen, Y.H. Yu, A generalized method for predicting the minimum fluidization velocity, AIChE J. 12 (3) (1966) 610 – 612. S. Ergun, Fluid flow through packed columns, Chem. Eng. Prog. 48 (1952) 89 – 94. J. Werther, Stromungsmechanische grundlagen der wirbelschichttechnik (in German), Chem. Ing. Tech. 49 (3) (1977) 193 – 202. P.U. Foscolo, A. Germana, R. Di Felice, L. De Luca, L.G. Gibilaro,

154

[50]

[51]

[52]

[53]

[54]

[55]

[56]

[57] [58]

[59]

[60]

[61]

[62]

[63] [64] [65] [66] [67] [68]

[69] [70]

I. Sidorenko, M.J. Rhodes / Powder Technology 141 (2004) 137–154 An experimental study of the expansion characteristics of fluidized beds of fine catalysts under pressure, in: J.R. Grace, L.W. Shemilt, M.A. Bergougnou (Eds.), Fluidization VI, Engineering Foundation, New York, 1989, pp. 187 – 194. P.N. Rowe, P.U. Foscolo, A.C. Hoffmann, J.G. Yates, Fine powders fluidized at low velocity at pressures up to 20 bar with gases of different viscosity, Chem. Eng. Sci. 37 (7) (1982) 1115 – 1117. M. Sciazko, J. Bandrowski, Effect of pressure on the minimum bubbling velocity of polydisperse materials, Chem. Eng. Sci. 40 (10) (1985) 1861 – 1869. M. Sciazko, J. Bandrowski, Effect of pressure on the minimum bubbling velocity of polydisperse materials. Reply to comments, Chem. Eng. Sci. 42 (2) (1987) 388. T. Varadi, J.R. Grace, High pressure fluidization in a two-dimensional bed, in: J.F. Davidson, D.L. Keairns (Eds.), Fluidization, Cambridge Univ. Press, Cambridge, 1978, pp. 55 – 58. J. Baeyens, D. Geldart, Predictive calculations of flow parameters in gas fluidized beds and fluidization behavior of various powders, in: H. Angelino, J.P. Couderc, H. Gibert, C. Laguerie (Eds.), Fluidization and its Application, Societe de Chimie Industrielle, Toulouse, 1974, pp. 263 – 273. M.E. Crowther, J.C. Whitehead, Fluidization of fine particles at elevated pressure, in: J.F. Davidson, D.L. Keairns (Eds.), Fluidization, Cambridge Univ. Press, Cambridge, 1978, pp. 65 – 70. J.F. Richardson, Incipient fluidization and particulate systems, in: J.F. Davidson, D. Harrison (Eds.), Fluidisation, Academic Press, London, 1971, pp. 25 – 64. K. Godard, J.F. Richardson, The behaviour of bubble-free fluidised beds, IChemE Symp. Ser. 30 (1968) 126 – 135. J.R.F. Guedes de Carvalho, Dense phase expansion in fluidized beds of fine particles. The effect of pressure on bubble stability, Chem. Eng. Sci. 36 (2) (1981) 413 – 416. J.R.F. Guedes de Carvalho, D.F. King, D. Harrison, Fluidization of fine particles under pressure, in: J.F. Davidson, D.L. Keairns (Eds.), Fluidization, Cambridge Univ. Press, Cambridge, 1978, pp. 59 – 64. M. Byars, ‘‘Developments in Electrical Capacitance Tomography’’. Paper presented at 2nd World Congress on Industrial Process Tomography, Hannover, 2001. T. Dyakowski, L.F.C. Jeanmeure, A.J. Jaworski, Applications of electrical tomography for gas – solids and liquid – solids flows—a review, Powder Technol. 112 (2000) 174 – 192. Y.T. Makkawi, P.C. Wright, Optimization of experiment span and data acquisition rate for reliable electrical capacitance tomography measurement in fluidization studies—a case study, Meas. Sci. Technol. 13 (2002) 1831 – 1841. T. Dyakowski, personal communication, Clayton, 2002. O. Isaksen, A review of reconstruction techniques for capacitance tomography, Meas. Sci. Technol. 7 (3) (1996) 325 – 337. O. Isaksen, J.E. Nordtvedt, A new reconstruction algorithm for process tomography, Meas. Sci. Technol. 4 (3) (1993) 1464 – 1475. Generation of ECT images from capacitance measurements (Wilmslow, Process Tomography) 2001b. J.C. Schouten, F. Takens, C.M. van den Bleek, Estimation of the dimension of a noisy attractor, Phys. Rev., E 50 (3) (1994) 1851 – 1861. R.C. Zijerveld, F. Johnsson, A. Marzocchella, J.C. Schouten, C.M. van den Bleek, Fluidization regimes and transitions from fixed bed to dilute transport flow, Powder Technol. 95 (1998) 185 – 204. D. Geldart, N. Harnby, A.C.-Y. Wong, Fluidization of cohesive powders, Powder Technol. 37 (1984) 25 – 37. D. Geldart, A.L. Radtke, The effect of particle properties on the

[71]

[72]

[73]

[74]

[75]

[76]

[77]

[78]

[79]

[80]

[81]

[82]

[83]

[84]

[85]

[86]

[87]

[88]

behavior of equilibrium cracking catalysts in standpipe flow, Powder Technol. 47 (1986) 157 – 165. G.F. Barreto, J.G. Yates, P.N. Rowe, The effect of pressure on the flow of gas in fluidized beds of fine particles, Chem. Eng. Sci. 38 (12) (1983) 1935 – 1945. I.H. Chan, C. Sishtla, T.M. Knowlton, The effect of pressure on bubble parameters in gas-fluidized beds, Powder Technol. 53 (1987) 217 – 235. D.F. King, D. Harrison, The bubble phase in high-pressure fluidized beds, in: J.R. Grace, J.M. Matsen (Eds.), Fluidization, Plenum, New York, 1980, pp. 101 – 108. P.N. Rowe, H.J. MacGillivray, A preliminary X-ray study of the effect of pressure on a bubbling gas fluidised bed, in: Fluidised Combustion: Systems and Applications, vol. IV-1, Institute of Energy, London, 1980, pp. 1 – 9. M.P. Subzwari, R. Clift, D.L. Pyle, Bubbling behavior of fluidized beds at elevated pressures, in: J.F. Davidson, D.L. Keairns (Eds.), Fluidization, Cambridge Univ. Press, Cambridge, 1978, pp. 50 – 54. P.A. Olowson, A.E. Almstedt, Influence of pressure and fluidization velocity on the bubble behavior and gas flow distribution in a fluidized bed, Chem. Eng. Sci. 45 (7) (1990) 1733 – 1741. M.F. Llop, J. Casal, J. Arnaldos, Expansion of gas – solid fluidized beds at pressure and high temperature, Powder Technol. 107 (2000) 212 – 225. P.A. Olowson, A.E. Almstedt, Hydrodynamics of a bubbling fluidized bed: influence of pressure and fluidization velocity in terms of drag force, Chem. Eng. Sci. 47 (2) (1992) 357 – 366. P. Cai, M. Schiavetti, G. De Michele, G.C. Grazzini, M. Miccio, Quantitative estimation of bubble size in PFBC, Powder Technol. 80 (1994) 99 – 109. M.L.M. van der Stappen, J.C. Schouten, C.M. van den Bleek, Application of deterministic chaos theory in understanding the fluid dynamic behavior of gas – solids fluidization, AIChE Symp. Ser. 89 (296) (1993) 91 – 102. C.S. Daw, J.S. Halow, Evaluation and control of fluidization quality through chaotic time series analysis of pressure-drop measurements, AIChE Symp. Ser. 89 (296) (1993) 103 – 122. D. Bai, A.S. Issangya, J.R. Grace, Characteristics of gas-fluidized beds in different flow regimes, Ind. Eng. Chem. Res. 38 (3) (1999) 803 – 811. M.L.M. van der Stappen, J.C. Schouten, C.M. van den Bleek, ‘‘Deterministic chaos analysis of the dynamical behaviour of slugging and bubbling fluidized beds’’. Paper presented at 12th International Conference on Fluidized Bed Combustion, La Jolla, 1993. F. Johnsson, R.C. Zijerveld, J.C. Schouten, C.M. van den Bleek, B. Leckner, Characterization of fluidization regimes by time-series analysis of pressure fluctuations, Int. J. Multiph. Flow 26 (4) (2000) 663 – 715. J.C. Schouten, C.M. van den Bleek, Monitoring the quality of fluidization using the short-term predictability of pressure fluctuations, AIChE J. 44 (1) (1998) 48 – 60. F.T. Kuehn, J.C. Schouten, R.F. Mudde, C.M. van den Bleek, B. Scarlett, Analysis of chaos in fluidization using electrical capacitance tomography, Meas. Sci. Technol. 7 (3) (1996) 361 – 368. J.C. Schouten, M.L.M. van der Stappen, C.M. van den Bleek, Scaleup of chaotic fluidized bed hydrodynamics, Chem. Eng. Sci. 51 (10) (1996) 1991 – 2000. G.F. Barreto, G.D. Mazza, J.G. Yates, The significance of bed collapse experiments in the characterization of fluidized beds of fine powders, Chem. Eng. Sci. 43 (11) (1988) 3037 – 3047.