Electrical Capacitance Volume Tomography for Characterization of

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Electrical Capacitance Volume Tomography for Characterization of Gas-Solid Slugging Fluidization with Geldart Group D Particles under High Temperatures Dawei Wang, Mingyuan Xu, Qussai Marashdeh, Benjamin Straiton, Andrew Tong, and Liang-Shih Fan Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04733 • Publication Date (Web): 19 Jan 2018 Downloaded from http://pubs.acs.org on January 19, 2018

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Industrial & Engineering Chemistry Research

Electrical Capacitance Volume Tomography for Characterization of Gas-Solid Slugging Fluidization with Geldart Group D Particles under High Temperatures Dawei Wang1, Mingyuan Xu1, Qussai Marashdeh2, Benjamin Straiton2, Andrew Tong1* and Liang-Shih Fan1* (1)William G. Lowrie Department of Chemical and Biomolecular Engineering, The Ohio State University, Columbus, OH, 43210 (2)Tech4Imaging LLC, Columbus, OH, 43235 * Correspondence can be addressed to either one of us Andrew Tong: [email protected] Liang-Shih Fan: [email protected]

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Abstract A 3-dimensional ECVT sensing technique is applied to imaging complex slugging phenomena of a gassolid fluidized bed under ambient and elevated temperature conditions. The study indicates that the time interval between rising slugs decreases with an increase in the gas velocity, reaching a nearly steady time interval value of about 1 second between two slugs when the gas velocity is ~1.7 m/s above the minimum fluidization velocity. The fluidized bed behaves as a bubbling fluidized bed at low gas velocities. In slugging regime, the slug rise velocity increases with the gas velocity. A mechanistic analysis of forces around the dense phase solid particles suggests that the relationship between the slug rise velocity and the gas velocity for the square-nosed slugging bed is not strictly linear and is highly related to the interparticle forces, internal friction of particles and gas velocity in addition to the wall stress.


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1. Introduction Slugging fluidization represents one of the major gas-solid fluidization regimes. In the slugging fluidized bed, the gas bubbles grow to a size comparable to the diameter of the fluidized bed. The growth of the bubbles is due to the rapid coalescence of the bubbles as they rise through the bed from the gas distributor. The slugging fluidization regime occurs when the length to diameter aspect ratio of the fluidized bed is large. The minimum aspect ratio for slugging to occur substantially decreases when coarse or large particles are used. Stewart and Davidson described two axisymmetric forms of slugging fluidized bed based on the specific particle size employed, i.e. round-nosed slugs and square-nosed slugs1. Roundnosed slugs occur when fine particles are used as the fluidized particles. In the round-nosed slug, the solids trickle downward through the annular regions of the gas slug. Square-nosed slugs occur when coarse particles are used in a small diameter bed, resulting in significant particle bridging and “locking” effects between particles and the bed wall. These gas slugs fill the entire cross-section of the bed where the solids can be observed raining down through the slugs. In addition to the above two axisymmetric forms of slugs, asymmetric slugs, or wall slugs/half slugs can occur in a fluidized bed with rough walls, large particle diameter to bed diameter ratios, and/or high superficial gas velocities. Stewart and Davidson proposed that the rise velocity for axisymmetric slug can be given as

U sl = k1 (U g − U mf ) + k2 gD

(1)

where Usl is the slug velocity; Ug is the gas superficial velocity; Umf is the minimum fluidization velocity of bed material; g is the gravitational acceleration; and D is the bed diameter 1. For round-nosed slugs, the constant, k2, was experimentally obtained as 0.35 for the gas-solid slugging system which is identical to that obtained for gas-liquid slugging systems. Ormiston et al. suggested the constant, k1, is to be equal to 1, which was experimentally verified by Stewart2, 3. The maximum height of solids in a slugging fluidized bed relative to the height under the minimum fluidization condition can then be derived as4



U g − U mf H max = 1+ H mf 0.35 gD

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(2)

where H is the height of the bed. The equation was verified with over 60 sets of experimental data. For square-nosed slugs, however, the slug rise velocity is approximately half of that in the round-nosed slugs5. This is mainly due to the fact that the upward movement of the interface between the dense phase particle flow and the dilute phase slug is largely caused by the particles raining down uniformly through the disperse regions. A large percentage of gas percolates through the slow moving particulate region. The upward drag force generated by the relative velocity between the gas and the dense phase particles is balanced by the gravity, high interparticle forces and shear stresses at the wall1. Thus, the slug rise velocity of the square-nosed slugs is much smaller than that of the round-nosed slugs. Due to the limited availability of the experimental measurement methods, characterization of the slugging phenomena was mainly conducted through visualization by measuring the slug rise velocity, slug length, inter-slug spacing and slug frequency5-7. Noordergraaf et al. experimentally studied the square-nosed slugging fluidization by measuring pressure fluctuations and found a narrowed frequency spectrum in the pressure fluctuation during the transition from the bubbling regime to the slugging regime8. The frequency for the square-nosed slug increases approximately in proportion to the square root of the bed diameter and is inversely proportional to the initial bed height, but independent of the particle type. Cho et al. analyzed the slugging fluidization of polyethylene particles using pressure drop signals based on the classical statistical methods, and concluded that the slugging frequency decreases with the gas velocity9. To date, the slugging fluidized bed characterization has been conducted mainly for ambient temperature conditions largely due to the difficulty in the flow measurement under high temperature conditions. High temperature fluidization, however, is common in industrial operation. With recent advances in the sensor development and image construction techniques for the electrical capacitance volume tomography (ECVT) for 3-D multiphase flow imaging, it is now possible to use the ECVT to obtain in a real time the multiphase flow phenomena at high temperatures10-13. ECVT is one of the instrumentations that have been developed with the capability for advanced process applications for multiphase flow imaging in real time 4 ACS Paragon Plus Environment

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or near real time speeds. The ECVT sensors of various sizes, with up to 60 in internal diameter (ID), have been used in process imaging14. Different applications of ECVT to image flows in complex geometries have also been available. As examples, ECVT was used to image gas-solid flows in a 90° exit pipe15 and gas-liquid flows in a tapered column16. The ability of ECVT to image with rates up to hundreds of frames per second also enabled real-time visualization of fast speed flows such as in a packed bed flow17. More recently, ECVT sensors were used to image 3-dimensional velocity vector fields18. ECVT sensors have been successfully applied to visualize cold flow process at different settings and conditions. They have also been used to develop new process models, verify existing models, and optimize process variables19, 20. Although ECVT was developed initially for imaging cold flow processes, recent tests have indicated feasibility of using capacitance sensors at high temperatures. Advances in material development have made it possible for ECVT sensor to utilize non-conductive materials that can withstand the harsh high temperature and high pressure conditions to encase and protect the capacitance sensors. The sensors used in this study are composed of a large set of capacitive plates (i.e. 24 plates) placed around the sensing domain. Because the sensor plates are distributed non-invasively around the processes columns, they allow the measurement of independent changes in electrical capacitance between all possible plate combinations. These capacitance changes stem from variations in electric (dielectric and conductive) properties of different species within the imaging domain. Such variations can be caused by a change in the phase concentration in a multiphase flow, a temperature variation in a reacting flow, or a change in the chemical composition such as in the ionization in flames. In ECVT, an image reconstruction algorithm maps back changes in electrical capacitance into a 3-dimensional visualization of the underlying process flow causative of such capacitance variations. The ECVT sensing technology, along with the developed reconstruction algorithms, has been successfully applied to systems for imaging cold flow processes, the result of these mappings being, for example, the volumetric and dynamic images of multiphase flow concentrations. ECVT is a natural enhancement of Electrical Capacitance Tomography (ECT), which has been researched and advanced for many years. It is a common notion that capacitancebased tomography is limited to one or two phase applications, as capacitance is a single metric. The 5 ACS Paragon Plus Environment


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multiphase problem addressed here presents an additional challenge in that it is subjected to high and varying temperatures (up to 650°C) that require independent metrics to fully analyze the flow. Capacitance, by itself, is a single variable and so can only determine one unknown (permittivity distribution). To extend applications of ECVT to high temperature flows, temperature sensors are used as an additional metric where the system can adjust its reconstruction algorithms based on sensed temperatures. A calibration procedure records the sensor response to a packed bed of the particles under examination, and for various temperatures. Those calibrations are then used to adjust the image reconstruction algorithm accordingly. In this study, the ECVT is used to measure the slugging phenomena, specifically, the shape, size, and rise velocity of gas slugs at high temperatures. Geldart Group D particles are used as the fluidized particles. The temperature effects on the slugging fluidization regime are also discussed. 2. Experimental Setup The present section provides the design of the high temperature fluidized bed apparatus and the operating conditions used for the parametric studies of the slugging fluidized bed. 2.1 Fluidized Bed Configuration The experimental setup is shown in Figure 1. The slugging fluidized bed consists of a stainless steel column with an I.D. of 76.2 mm and a height, excluding the solids disengaging section, of 0.76 m. A relatively short column is used in the experiment due to the fact that maintaining a uniform temperature over a long column in the experiments is difficult. With the column height used in this study, it is adequate to examine the slugging phenomena while maintaining the bed temperature as uniform as possible. To ensure a short column height does not induce the end effect, another column made of the acrylic materials with a height of 2 m is used to test this effect under the ambient temperature condition. Fluidization in this column is conducted based on flow visualization. Experiments conducted under the ambient temperature conditions indicate that the results of the slug rise velocity is identical between the acrylic column from flow visualization and the stainless steel column from ECVT. These experiments thus ascertain that the height of 0.76 m designed for the stainless steel column is feasible. 6 ACS Paragon Plus Environment

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Dense alumina spherical particles of 6.35 mm in diameter with a density of 3,500 kg/m3 are placed at the bottom of the fluidized bed, serving as gas distributor. The fluidizing particles are of spherical iron oxide particles with an average diameter of 1.5 mm and a density of 2,500 kg/m3. The dielectric constant of the materials is the key multiphase field parameter in the ECVT measurement technique. This technique measures the dielectric property distribution in the flow field, which is a direct reflection of the flow field particle distribution. For iron oxide particles used in this study, the dielectric constant increases slowly with temperature up to 540°C, then experiences a sharp change above this temperature. If the temperature of the fluidized bed is not uniform, the dielectric constants of the particles thus vary, leading to an incorrect reconstruction of the flow field particle distribution. The minimum fluidization velocity of the bed is obtained by directly measuring the pressure drop with pressure transducers under various temperature conditions. As examples, the minimum fluidization velocity is obtained as 0.89 m/s at the ambient temperature, as 0.95 m/s at 300 oC, as 0.94 m/s at 400 oC, and as 0.88m/s at 650oC. In the stainless steel column experiments, the initial bed height under the fixed bed condition is 0.40 m. Air is used as the fluidizing gas supplied from an air compressor with an air supply pressure of 791 kPa (abs). An enlarged section with a diameter of 154 mm and a height of 0.3 m is installed on top of the 76.2 mm section to reduce the gas velocity and disengage the particles from the gas flow, so that there is no particle loss during the experiment. The fluidized bed reactor is operated at the ambient pressure. An ECVT sensor is installed in the middle section of the fluidized bed with the top of the ECVT sensor located at the height of the initial bed height of the fluidized bed. Ceramic semi-cylindrical heaters surround the 76.2mm ID column sections except the ECVT sensor region to provide radiant heating for the fluidized bed and maintain the bed temperature constant during the measurements. Direct electric gas preheaters raise the fluidizing gas temperature up to the operating temperature of the bed prior to entering the reactor. Thermocouples are placed above and below the sensor region to verify the uniformity of the temperature throughout the fluidized bed prior to capacitance measurement. Two mass flow controllers are used to control the air flow rate to the fluidized bed each with a range of 0-1,000 slpm. A differential pressure transmitter (Omega, PX419-005DWU5V) ranging 7 ACS Paragon Plus Environment


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from 0-25 kPa is used to measure the pressure change across the ECVT sensor region to compare the results against the reconstructed image information.

2.2 ECVT Test Apparatus

Figure 1. (Left) Photo of assembled ECVT test apparatus. (Right) Diagram of process flow vessel and accessories of the ECVT system

The sensor, as shown in Figure 1, consists of 24 (6 plates/layer x 4 layers) circular-arc-shaped capacitance plates of 47.6 mm tall and 47.2 mm wide. The adjacent capacitance plates are spaced 5.7 mm from each other for electrical isolation and a center-to-center distance of 102.3 mm is maintained for capacitance plates opposite of each other. The ECVT sensor section is lined with an alumina and silica based castable ceramic material, UTRAFLO 70 M, provided by Allied Mineral Products, Inc. The ceramic lining provides electrical and thermal insulation for the capacitance plates from the solid fluidization media and made the internal diameter of the sensor section consistent with the upper and lower heating zones, i.e.


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76.2 mm. Conductive wire leads of ~1.6 mm in diameter are connected to the capacitance plates and drawn out of the metal outer lining. Ceramic insulation bushing are used to provide electrical insulation between the capacitance wire leads and the outer metal lining and compression fittings are used to prevent gas from leaking through wire leads and bushings.

2.3 ECVT Data acquisition and Image Reconstruction The ECVT data acquisition starts with a calibration stage which takes into account the range of dielectric constant and conductivity of the gas and solid phases in the system. Calibration is completed in two stages. In the first stage, the driver amplitude of the excitation signal from sensor plates is set by taking measurements of the empty and full sensor regions where empty is air and full is the packed bed of the solid fluidization media. This ensures a maximum digital quantization resolution while also ensuring no information is lost due to clipping. In the second stage of the calibration, the signal balance of the sensor plates is set by offsetting the signal phase until the received signal matches the transmitted signal during data collection from the empty sensor region. This ensures that the ceramic lining of the sensor which physically isolate the sensor plates from the flow material no longer contributes to the change in signal from excite to receive plates. After calibration, the normalization stage occurs. For the purposes of imaging and other data processing which require the comparison of the relative capacitance values between plate pairs, all data must be normalized between 0 and 1 based on the maximum and minimum signal amplitudes from each plate pair respectively. Similar to calibration, data is collected for every plate pair for an empty and full bed. All data collected after this step is then normalized according to Equation (3) as given below for each unique plate pair.

cij =

measured valueij - empty valueij full valueij - empty value ij

(3)

In this equation, c is the capacitance value; i is the frame number; j is the voxel number with empty indicating the air phase and full indicating the packed bed of iron oxide particles. 9 ACS Paragon Plus Environment


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The calibration and normalization values stored in the data acquisition boards are valid for collecting data within a set of the temperature range based on the dielectric properties of the materials being monitored in the flow. Because calibration typically deals more with changes in the structure of the sensor rather than the contents of the flow system, calibration does not need to be redone as the temperature of the system changes due to the robust mechanical design of the sensor. Normalization, however, is entirely based on the contents of the flow. Because the dielectric changes drastically over the temperature range, a new normalization is performed while the system is close to the temperature that the materials will be at during the capacitance measurement acquisition. This correction allows the system to still provide accurate imaging and solids hold up information at high temperatures. The 24 capacitance plates equates to 276 unique capacitance measurements acquired during each frame. The capacitance measurements combined with the ECVT algorithm were then used to construct a 3dimensional distribution profile of the solids volume fraction for each recorded frame, which were then used to render a dynamic image of the slugging fluidized bed under different operational conditions of gas velocities and temperatures. 2.4 Test Procedure There are 72 parametric test conditions conducted to analyze the characteristics of slugging fluidized bed performance with four operating temperatures, i.e. 25°C, 300°C, 400°C and 650°C, and 18 different superficial gas velocities under each operational temperature. The superficial gas velocity for different temperatures is defined as the volumetric gas flow rate under the given fluidization temperature divided by the cross-sectional area of the bed. The range of the superficial gas velocity was from 0.25 m/s to 2.1 m/s above the minimum fluidization velocity of the particles under the operational temperature. The experiments under different operational conditions were repeated for several times to verify its correctness. A total of over 300 tests were conducted. The fluidized bed was first heated to a desired temperature with a designated temperature increment using the external ceramic heater lining. The mass flow controllers were used to maintain the superficial gas 10 ACS Paragon Plus Environment

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velocity above the minimum fluidization velocity throughout the heating process. The gas preheaters were also engaged to assist in maintaining a uniform temperature profile through the fluidized bed operation. When the temperature of the bed reached the desired temperature, the fluidizing gas was turned off, the ECVT sensor was calibrated for packed bed condition. Once the sensor calibration was completed, the superficial gas velocity was adjusted to a desired value. Measurements from the ECVT sensor were initiated after the bed temperature reaches a constant value and the pressure fluctuation reaches the same variation pattern. The frame acquisition frequency of the ECVT sensor was set at 80 Hz. Approximately 3,000 frames, which corresponds to about 40 seconds of measurement, were collected during each measurement. After each measurement, the gas velocity was increased for the next measurement. When all the measurements under the given temperature were conducted, the gas was shut down, and the bed were emptied by opening the valve at the bottom. The temperature of the bed was maintained at the same as the test condition for the calibration of the ECVT sensor under the empty condition.

3. Results and Discussion The ECVT sensor measurements and reconstruction algorithm results were first compared against the differential pressure measurements to verify the dynamic behavior observed from the capacitance sensors consistent with the pressure data. The reconstructed 3-dimensional images and solids volume fraction profiles were then used to analyze the slugging properties of the fluidized bed under various superficial gas velocities and operating temperatures.

3.1. Comparison Between ECVT and Pressure Transducer Measurements The pressure drop across the fluidized bed is a function of the solids holdup and bed height. The pressure drop and its fluctuation are a direct reflection of the solids holdup in a fluidized bed. The solids holdup measured from the ECVT sensor was compared against the pressure drop through the length of the ECVT sensor section, as shown in Figure 2. Figure 2(a) is the comparison under ambient temperature with a gas flow rate of 450 slpm, corresponding to a superficial gas velocity of 1.56 m/s. Figure 2(b) is under the 11 ACS Paragon Plus Environment


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condition of 400°C with a gas flow rate of 360 slpm, corresponding to a superficial gas velocity of 2.63 m/s. The figures show the variation of the solids holdup obtained from the ECVT that match well with the pressure drop fluctuation signals. When the pressure drop was low, the ECVT measured a low solids holdup. When the pressure drop was high, the ECVT measured a high solids holdup. The recurring evolution of the solids holdup from the ECVT images indicated the recurring slugs flowing through the bed, which matched very well with the pressure drop profile in terms of duration and frequency of each slug. With the increase of the gas velocity due to the rise in bed temperature in Figure 2(b), both the pressure drop and the ECVT measurements captured the increase in slug frequency, further confirming the solids holdup profile measured by the ECVT is in agreement with the pressure drop profile. It should be noted that in the slugging fluidized bed, the friction between the solid particles and the wall contributes also significantly to the bed pressure drop. When the measuring section is in a dense phase mode, the measured pressure drop will be significantly higher than the total weight of the particles. Thus, the range of the normalized pressure drop shall be higher than the range of the normalized solids holdup, which can be observed from Figure 2.


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(a) Ambient temperature with the gas flow rate of 450 slpm Average solids holdup from ECVT

Pressure drop

1 0.9 0.8

normalized value

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0

2

4

6

8

10

12

14

16

18

20

Time (s) (b) 400 C temperature with the gas flow rate of 360 slpm o

Figure 2. Comparison of ECVT measurement and pressure drop profile obtained from pressure transducers 13 ACS Paragon Plus Environment


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3.2. Reconstruct of 3-Dimensional Image of the Slugging Fluidized Bed Figure 3 (left) illustrates the reconstructed image of the slugging fluidized bed at 25°C and superficial gas velocity of 2.07 m/s, which corresponds to 1.18 m/s above minimum fluidization velocity of the particles. The solids volume fraction measurements given in the right graphs of Figure 3 were recorded from four layers of ECVT sensor assembly as shown in Figure 1. It can be seen that the recurring changes of the solids volume fraction at different heights of the bed are slightly staggered, which indicates the rise of the gas slug through the fluidized bed column.

Figure 3. Reconstructed slug image (left) and solids volume fraction measurements along the height of the sensor region with frames (right). Solids holdup acquisition frame rate is 80 Hz

Figure 4 is the rendered images of the 3-dimensional solids volume fraction distribution at a certain moment for different operating temperatures of the fluidized bed. The 3-dimensional image includes the solids volume fraction distributions of three horizontal planes (bottom, middle and top of the ECVT sensor zone) and two vertical planes which are perpendicular to each other. The superficial gas velocities of all the 4 cases were at ~1.2 m/s above minimum fluidization velocity of the bed under the given operational conditions.


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Figure 4. Reconstructed images of the solids holdup for different temperatures Figure 5 shows the series of reconstructed 2-dimensional images of solid volume fraction profiles through the center-line of the fluidized bed under different operational temperatures and gas velocities. In each set, 6 frames in a constant time interval were illustrated to capture the evolution of a slug moving through the ECVT sensor. It can be seen that the ECVT worked very well under different temperatures and different gas velocities. In Figure 5(a), where the temperature of the fluidized bed is 300 oC and Ug-Umf is 0.44 m/s, a relatively small slug is detected. The 6 frames shown in Figure 5(a) are with a time interval of 0.19 s. At the beginning, the slug bed starts to rise into the ECVT sensor region, showing the dense phase regime in the upper section and the dilute phase regime at the bottom. Gradually, the slug moves upwards with more region changing into dilute phase. Then, the entire slug enters into the ECVT sensor section, with both the top and bottom sections of the region are in dense phase regime while the middle section is very dilute. The slug then gradually moves out of the ECVT sensor region, and finally disappears from the observation of ECVT sensor. The ECVT sensor region are all in the dense phase regime. By contrast, Figure 5(b) shows a very big slug under a high gas velocity condition with Ug-Umf at 1.81 m/s. The operational temperature is 650oC. The 6 frames shown in Figure 5(b) are with a time interval of 0.38 s. It can be seen from the frames 2-5 of Figure 5(b) that, after the slug head moving out of the ECVT sensor region, there is a long time period that the entire ECVT sensor region is in the dilute phase regime. The



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tail of the slug appears in the ECVT sensor region at about 1.9 s after the slug head. Under both conditions, the ECVT sensor can reconstruct the detailed structure of the slug very well. It can also be seen from Figure 5 that, the developed slugs under these conditions are in a square-nosed shape that spans the horizontal cross section of the column evenly with no significant head and tail sections. The square-nosed shape of the slug is mainly due to the significant bridging effect from using Geldart group D particles and to the high height-to-diameter aspect ratio used in the fluidized bed column.

(a) Reconstructed slug images (300oC, Ug-Umf = 0.44 m/s, time interval 0.19 s)


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(b) Reconstructed slug images (650oC, Ug-Umf = 1.81 m/s, time interval: 0.38 s) Figure 5. Reconstructed slug images in series showing the evolution of slug passing through the ECVT sensor. 3.3. Time Intervals between two slugs under ambient temperature By counting the numbers of the recurring changes in the average volume fraction of the solids through a certain height of the bed over the period time of ECVT measurement, the average time intervals between two successive slugs can be obtained. Figure 6 shows the relationship of the time intervals between slugs versus the superficial gas velocity used under ambient temperature, i.e. 25°C. The measurements for each gas velocity were repeated eight times, each on separate days. The error bar shown in the figure represents the range of the experimental data obtained from the different measurements. From Figure 6, when the superficial gas velocity was less than 0.5 m/s above the minimum fluidization velocity, the average time between slugs increased with increasing gas velocity. However, when the superficial gas velocity was at 0.5 m/s above the minimum fluidization velocity, the time interval between two slugs 17 ACS Paragon Plus Environment


reached to a maximum value of about 2.2s and thereafter decreased with increasing superficial gas velocity until it reached a steady time interval of about 1 second at a superficial gas velocity of approximately 1.7 m/s above the minimum fluidization velocity. The 1 second time interval between two slugs obtained in this study is in agreement with the conclusions given from the previous studies for a slugging fluidized bed with 0.05-0.20 m ID2. The results indicate that under the slugging conditions, increasing the gas velocity increases the slug rise velocity and under a large gas velocity, the time interval between the two slugs are constant.

2.8 2.6 2.4 Time Interval, S

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2.2 2 1.8 1.6 1.4 1.2

Bubbling

1 0

0.5

1 Ug-Umf, m/s

1.5

2

Figure 6. Time intervals between slugs for different Ug-Umf at the ambient temperature (25 oC)

Further investigation into the abnormally high slugging frequency at low superficial gas velocities (i.e. Ug-Umf

Electrical Capacitance Volume Tomography for Characterization of

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