Electrical Capacitance Volume Tomography for Characterization of Gas-Solid Slugging Fluidization with Geldart Group D Particles under High Temperatures

Industrial & Engineering Chemistry Research Accepted Manuscript (2018)

Abstract

A 3-dimensional ECVT sensing technique is applied to imaging complex slugging phenomena of a gas-solid 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 inter-particle forces, internal friction of particles and gas velocity in addition to the wall stress.

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Investigation of core-annular flow in an industrial scale circulating fluidized bed riser with electrical capacitance volume tomography (ECVT)

Powder Technology, March 2018, Pages 524-535

Abstract

There is a paucity of riser data for industrial scale units, particularly with regard to the solids fraction. This is especially true for detailed spatially distributed values. To alleviate this problem, NETL installed a 0.445 m tall electrical capacitance volume tomography (ECVT) sensor 9.66 m from the gas distributor of its industrial size (15.45 m high and 0.3 m diameter) circulating fluidized bed (CFB) cold model. A series of tests were conducted to investigate the gas-solid flow behavior using high density polyethylene (PPE) solids. Static electricity was successfully minimized using Larostat and humidification. Time averaged radial solid fractions profiles are presented and discussed. The time and spatially averaged solid fractions measured by the ECVT agree well with estimates from the pressure drop. The annular thickness was measured and found to increase with increases in the solids flow rate and decrease with increases in the gas velocity. Comparisons of the annular thickness and solids fraction as determined from the ECVT unit were compared to existing correlations. The average error ranged from 13% to 275% which is not surprising since the literature correlations were developed from data on much smaller units and for significantly different particles.

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ECVT imaging and model analysis of the liquid distribution inside a horizontally installed passive cyclonic gas–liquid separator

Chemical Engineering Science 141 (2016) 231–239

Abstract

Passive cyclonic gas–liquid separators (PCGLSs) have unique advantages compared with other types of gas–liquid separators. In this study, liquid distribution inside a horizontally installed PCGLS is investi-gated using electrical capacitance volume tomography (ECVT). The liquid distribution is measured under various gas and liquid flow rates. It is found that the inlet liquid flow rate is the dominating factor influencing the liquid distribution. The angular velocity of the liquid layer is found to be proportional to the liquid flow rate when no gas is injected. The center axis of the gas core is observed to be lower than the center axis of the separator cylinder, and it will drift towards the separator’s center axis with increasing liquid flow rate. A mechanistic model is derived, and flow field parameters are solved ana-lytically to qualitatively explain this phenomenon. The calculated gas core positions match the ECVT images well.

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ECVT Imaging of 3D Spiral Bubble Plume Structures in Gas‐Liquid Bubble Columns

The Canadian Journal of Chemical Engineering Volume 92, Issue 12, pages 2078–2087, December 2014

Abstract

Electrical Capacitance Volume Tomography (ECVT) is a 3D, real‐time imaging technique that is recently developed to image the multiphase flow behaviour in columns of regular or irregular geometries. In this study, the ECVT measurements are conducted to obtain phase holdups and flow structures in bubble columns of a straight‐cylinder shape and a tapered‐cylinder shape with porous and orifice gas distributors. The phase holdups obtained by the ECVT in a straight‐cylinder bubble column with porous gas distributor are verified by other measurement techniques and correlation equations reported in the literature. The flow regimes of the bubble columns are also characterized by the ECVT. Particularly, a dynamic 3D flow structure, which is uniquely represented by the spiral motion of the bubble plumes occurring within the heterogeneous regime of the gas‐liquid flow in a straight‐cylinder shape bubble column with perforated distributor, is captured for the first time instantaneously over the entire flow field with the ECVT. Other flow behaviour such as the converging flow pattern of the gas bubbles in a tapered‐cylinder shape bubble column is also revealed by the ECVT.

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Electrical capacitance volume tomography for imaging of pulsating flows in a trickle bed

Chemical Engineering Science 119 (2014) 77–87

Abstract

Experimental results of the air–water pulsating flows in a trickle bed column were obtained using the electrical capacitance volume tomography (ECVT) system. Detailed 3-D pulse structures in both the fully developed and the transient conditions were illustrated. Pulse frequency, pulse traveling velocity, average liquid holdup and liquid holdup inside the gas-rich and liquid-rich regions, respectively, were measured. Based on a simplified model, the linear liquid velocities inside the gas-rich and liquid-rich regions were estimated. The results revealed that the gas flow rate was the most important parameter in controlling the pulsating flow properties. Discussion on the physical nature of the pulsating flow was also given.

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Fluid bed characterization using Electrical Capacitance Volume Tomography (ECVT), compared to CPFD Software’s Barracuda

Powder Technology 250 (2013) 138–146

Abstract

Being able to accurately predict the performance and operation of multiphase flow systems continues to be a significant challenge. In order to continue the advancement of knowledge and to develop better models, a 10 cm diameter fluidized bed of 185 μm glass beads was used along with an Electrical Capacitance Volume Tomography (ECVT) sensor and high speed pressure measurements. Three dimensional images of the gas–solid flow structure were obtained and analyzed as well as frequency information from the high speed pressure transducers. The experimental data was then compared to four computational models performed with CPFD Software’s Barracuda code exploring different techniques to handle the perforated distributor plate.

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Bubbling fluidized bed characterization using Electrical Capacitance Volume Tomography (ECVT)

Powder Technology Volume 242, (2013), Pages 40-50

Abstract

Understanding the fundamentals of gas–solid fluidized beds and, in general, multiphase flows has been a significant task since the conception of gas–solid fluidization and fluid particle systems. Various measurement techniques have been applied in an attempt to better understand the fundamentals of the complex gas–solid flow structures that form in fluidized beds. This information may potentially provide a better design, scale-up, and operation of these systems as well as lead to accurate performance predictions of multiphase flow systems. Electrical Capacitance Volume Tomography (ECVT) has now reached a point of development where these multiphase flow structures can be imaged accurately and reliably in three dimensions at good resolutions and sampling rates to provide significant insight into the internal gas–solid flow structures. A 10 cm ECVT sensor was used in order to investigate the bubble behavior of a 10 cm diameter bubbling fluidized bed (BFB) of 185 micron glass beads at various fluidization velocities. Three dimensional images of gas–solid flow structures as well as time average vertical and radial solid fraction profiles are presented in this paper, and average bubble diameter and bubble frequency are discussed and compared to various correlations available in the published literature.

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A comparison of magnetic resonance imaging and electrical capacitance tomography: An air jet through a bed of particles

Powder Technology 227 (2012) 86–95

Abstract

Magnetic resonance imaging (MRI) and electrical capacitance volume tomography (ECVT) have been com-pared for the visualisation of a jet of air issuing from a distributor provided with a single orifice and support-ing a bed of poppy seeds contained in a vertical tube of 50 mm diameter. The mean diameter of the seeds was 1.2 mm: the orifice was 4 mm in diameter. MRI and ECVT images were acquired in three dimensions for flow rates of air such that, when divided by the cross-sectional area of the tube, they gave superficial velocities below that required for minimum fluidisation. The ECVT images were found to be of lower contrast (as well as resolution) than the MRI, owing to smoothing inherent to the ECVT reconstruction process. An adap-tive threshold method is developed to recover the 3D structure of the jet from the ECVT results. This method describes the smoothing in the ECVT images by a point-spread function that can be objectively defined by comparison with the MRI results. Using the new adaptive threshold method, the maximum discrepancy in the measured jet length by ECVT was an overestimation by 7 mm in comparison with the MRI results. The discrepancies in the measured maximum jet widths appeared to be ca. 2 mm.

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Electrical Capacitance Volume Tomography Imaging of Three-Dimensional Flow Structures and Solids Concentration Distributions in a Riser and a Bend of a Gas−Solid Circulating Fluidized Bed

Ind. Eng. Chem. Res. 201251 (33), pp 10968–10976

Abstract

Electrical capacitance volume tomography (ECVT) is a newly developed imaging technique that can quantify three-dimensional (3D) multiphase flows in a complex, geometric flow field. In this study, the 3D phase distribution images inside a gas−solid circulating fluidized bed (CFB) are obtained using ECVT. Specifically, measurements are made at a riser section and a 90° bend-shape riser exit section of the CFB. Inside the vertical riser, a symmetric core−annulus structure with a low solids holdup in the riser center along with a high solids holdup near the riser wall is observed. The average volume solids holdup and the thickness of the annulus decrease with the superficial gas velocity. A core−annulus flow structure is formed both in the vertical and horizontal parts of the bend. The annulus structure is noncentro-symmetric in the horizontal part of the bend. The solids holdup in the annulus near the top wall area in the bend is higher than that in other locations of the annulus. At a higher superficial gas velocity in the riser, the centrifugal acceleration increases due to high solids velocity in the bend, and more solids are separated to the outside of the bend from the main stream. A “reversed-S” shape solids holdup distribution along the diagonal line is also observed. The solids holdup increases and then decreases from the outer corner to the center of the bend, which indicates that a relatively dilute region is formed near the outer corner of the bend.

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Horizontal gas and gas/solid jet penetration in a gas–solid fluidized bed

Chemical Engineering Science 65 (2010) 3394–3408

Abstract

In this paper, the real time, dynamic phenomena of the three-dimensional horizontal gas and gas/solid mixture jetting in a 0.3 m (12 in) bubbling gas–solid fluidized bed are reported. The instantaneous properties of the shape of the jets and volumetric solids holdup are qualified and quantified using the three-dimensional electrical capacitance volume tomography (ECVT) recently developed in the authors’ group. It is found that the horizontal gas jet is almost symmetric along the horizontal axis during its penetration. As the jet width expands, the total volume of the gas jet increases. A mechanistic model is also developed to account for the experimental results obtained in this study. Comparison of jet penetration length and width between the model prediction and ECVT experiment shows that both the maximum penetration length and the maximum width of the horizontal gas jet increase with the superficial gas velocity. When the horizontal gas jet coalesces with a bubble rising from the bottom distributor, it loses its symmetric shape and can easily penetrate into the bed. For the horizontal gas/solid mixture jet penetration in the bed, the tail of the jet at the nozzle shrinks and the jet loses its jet shape immediately when the jet reaches its maximum penetration length, which are different from the characteristics exhibited by the gas jet. The solids holdup in the core region of the gas/solid mixture jet is higher than that in the gas jet. The penetration length of the horizontal gas/solid mixture jet is also larger than that of the gas jet.

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Comparison of ECVT and MR Measurements of Voidage in a Gas-Fluidized Bed

Ind. Eng. Chem. Res. 2009, 48, 172–181

Abstract

This paper reports the first quantitative comparison of magnetic resonance (MR) and electrical capacitance volume tomography (ECVT) on a 50 mm diameter gas-fluidized bed of silica-alumina catalyst support particles (dp ) 58 µm). ECVT data were acquired at a temporal resolution of 12.5 ms and a nominal spatial resolution of 2.5 mm × 2.5 mm × 4.5 mm. Snapshot MR data were acquired at a temporal resolution of 26 ms and a spatial resolution of 1.9 mm × 1.9 mm in the transverse plane and 1.9 mm × 3.8 mm in the axial plane. The particles were doped with water to produce a detectable signal with MR. The two techniques are demonstrated to produce quantitatively comparable time-averaged measurements of the voidage. The bubble frequencies measured from the snapshot images using both techniques were found to be in good agreement. However, the signal intensity inside the gas bubbles was more accurate when measured with MR, and the wake structure could be more clearly resolved using MR. This was attributed to the effect of the smoothing, or point spread function, of the ECVT measurements. An initial estimate of the smoothing in the ECVT has been performed by assuming a Gaussian point spread function.

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