Gamma-densitometry Tomography Research Articles

Radial profiles of solids loading and flux in a gas–solid circulating fluidized bed

An electrical-impedance tomography (EIT) system has been developed to non-invasively measure radial voidage profiles in the riser of a pilot-scale circulating fluidized bed (CFB), yielding quantitative information that is validated by comparison to a gamma-densitometry tomography (GDT) system. EIT and GDT were applied to the CFB riser (14-cm inner diameter, 5.77-m height) containing fluid catalytic cracking particles in air. For all cases, the average and near-wall voidages from EIT and GDT agreed to within 0.03 and 0.07, respectively. This good agreement suggests that, where feasible, EIT can be used in place of GDT, which is advantageous since EIT systems are often safer, less expensive, and faster than GDT systems. The results also compared well to two correlations for radial voidage profile from the literature. Finally, a procedure for determining radial solids flux profiles from radial voidage profiles using an additional correlation [M.J. Rhodes, X.S. Wang, H. Cheng, T. Hirama, B.M. Gibbs, Similar profiles of solids flux in circulating fluidized-bed risers, Chemical Engineering Science 47 (1992) 1635–1643] was investigated. It was found that the accuracy of this correlation strongly depends on the voidage and/or solids flux measurement at the riser center.

Quantitative measurement of solids distribution in gas–solid riser flows using electrical impedance tomography and gamma densitometry tomography

An electrical impedance tomography (EIT) system has been developed to non-invasively measure particle distributions in the riser of a pilot-scale circulating fluidized bed (CFB). Although EIT systems have often been applied to yield qualitative information about gas–solid flows, the present EIT system yields quantitative information that is validated by comparison to a gamma densitometry tomography (GDT) system. EIT and GDT were applied to the CFB riser (14-cm inner diameter, 5.77-m height) containing fluid catalytic cracking particles in air. The flows examined were annular with a dilute core and had average and near-wall solids volume fractions up to 0.25 and 0.66, respectively. For all cases, the average and near-wall solids volume fractions from EIT and GDT agreed to within 0.03 and 0.07, respectively. This good agreement suggests that, where feasible, EIT can be used in place of GDT, which is advantageous since EIT systems are often safer, less expensive, and faster than GDT systems.

Capacitance measurements of solid concentration in gas–solid flows

Several methods of solid concentration measurement were implemented in a gas–solid multiphase flow to validate an electrically based solid concentration measurement device called a bulk impedance ring. Spatially averaged gamma-densitometry tomography and differential pressure measurements are compared and contrasted with the electrically based method. Experiments were performed on the riser of a pilot-scale circulating fluidized bed at Sandia National Laboratories, which circulates equilibrium fluid catalytic cracking particles with air. It was found that the mixture model used to convert the electrical impedance obtained by the bulk impedance ring to solid concentration must be applied carefully. Use of the Maxwell–Hewitt relation requires that the continuous and dispersed phases in the multiphase flow be identified correctly for the mixture model to work. Temporally averaged solid concentrations obtained from gamma-densitometry tomography and differential pressure measurements agree with solid concentrations obtained from the bulk impedance ring. Temporally resolved data from the bulk impedance ring and differential pressure measurements show good correlation. These results have validated the bulk impedance ring and show the feasibility of building a multiple-electrode electrical-impedance tomography system.

Three-phase material distribution measurements in a vertical flow using gamma-densitometry tomography and electrical-impedance tomography

Experiments are presented in which electrical-impedance tomography (EIT) and gamma-densitometry tomography (GDT) measurements are combined to simultaneously measure the solid, liquid, and gas radial distributions in a vertical three-phase flow. The experimental testbed is a 19.05-cm diameter bubble column in which gas is injected at the bottom and exits out the top while the liquid and solid phases recirculate. The gas phase is air and the liquid phase is deionized water with added electrolytes. For the solid phase four different particle classes are investigated: 40–100 μm and 120–200 μm glass particles ( 2.41 g/cm 3 ), and 170–260 μm and 200–700 μm polystyrene particles ( 1.04 g/cm 3 ). Superficial gas velocities of 3–30 cm/s and solid volume fractions up to 0.30 are examined. For all experimental conditions investigated, the gas distribution shows only a weak dependence on both particle size and density. The average gas volume fraction as a function of superficial gas velocity can be described to within ±0.04 by a single curve. For most cases, the solid particles appear to be uniformly dispersed in the liquid.

Validation of electrical-impedance tomography for measurements of material distribution in two-phase flows

A series of studies is presented in which an electrical-impedance tomography (EIT) system is validated for two-phase flow measurements. The EIT system, developed at Sandia National Laboratories, is described along with the computer algorithm used for reconstructing phase volume fraction profiles. The algorithm is first tested using numerical data and experimental phantom measurements, with good results. The EIT system is then applied to solid–liquid and gas–liquid flows, and radial phase volume fraction profiles within a circular cross-section are determined. These profiles are compared to results from an established gamma-densitometry tomography (GDT) system. In the solid–liquid flows, the average solid volume fractions measured by EIT are in good agreement with nominal values. In the gas–liquid flows, average gas volume fractions and radial gas volume fraction profiles from GDT and EIT are also in good agreement.

Gamma-densitometry tomography of gas holdup spatial distribution in industrial-scale bubble columns

Gamma-densitometry tomography (GDT) experiments have been performed to measure gas holdup spatial variations in two bubble columns: a 0.19 m inside diameter Lucite column and a 0.48 m inside diameter stainless steel vessel. Two-phase air/water flows were examined. Horizontal scans at one vertical position in each column were made for several air flow rates. An axisymmetric tomographic reconstruction algorithm based on the Abel transform has been used to calculate the time-averaged gas holdup radial variation. Integration of these profiles over the column cross section has yielded area-averaged holdup results, which have been compared with volume-averaged gas holdups determined from differential pressure measurements and from the rise in the air/water interface during gas flow. The results agree reasonably well.