Paul Richard Tortora

The Equivalent Electrical Permittivity of Gas-Solid Mixtures at Intermediate Solid Volume Fractions

Several mixture models are evaluated for their suitability in predicting the equivalent permittivity of dielectric particles in a dielectric medium for intermediate solid volume fractions (0.4 to 0.6). Predictions of the Maxwell, Rayleigh, Bottcher and Bruggeman models are compared to computational simulations of several arrangements of solid particles in a gas and to the experimentally determined permittivity of a static particle bed. The experiment uses spherical glass beads in air, so air and glass permittivity values (1 and 7, respectively) are used with all of the models and simulations. The experimental system used to measure the permittivity of the static particle bed and its calibration are described. The Rayleigh model is found to be suitable for predicting permittivity over the entire range of solid volume fractions (0-0.6).

Solids-Loading Measurements in a Gas-Solid Riser

Gas-solid multiphase flows are commonly used in chemical processing, petroleum fluid catalytic cracking, and other industrial applications. The distribution of the solid phase in gas-solid flows (generally in the form of small particles) is seldom uniform, but more commonly involves clusters, streamers, and core-annular distributions, depending on the flow orientation and the overall gas and solid flowrates and their ratio. For this reason, tomographic techniques are of great interest for measurement of cross-sectional solids distributions in such flows. The cross-sectional profiles of solids loading can be integrated to yield a cross-sectionally averaged solids loading. Determination of this averaged solids loading is needed to understand the axial variations of solids loading and its sensitivity to flow parameters and to optimize performance. A common technique for determining volume-averaged solids loading in vertical flows like the riser section of a circulating fluidized bed (CFB) is by measurement of the time-averaged axial pressure gradients along the riser axis (differential pressure or ΔP method). Neglecting acceleration and wall friction, the axial momentum balance simplifies to equate the multiphase hydrostatic pressure term with the pressure gradient along the axis. Many authors (e.g., Louge and Chang, 1990) have pointed out the neglected terms in this approach and generally show that ΔP is applicable in the special cases of no solids-loading gradient (fully developed flow) or small solids flux. A more generally applicable technique for measuring solids loading in gas-solid flows is gamma tomography. A gamma tomography system using a 100-mCi Cs-137 source collimated into a fan beam and an array of scintillation detectors, has been developed and implemented for application to a cold-flow (non-reacting) CFB. The CFB has a 14-cm-ID 6-m tall riser, and is currently operated with a multiphase mixture of air and fluid catalytic cracking (FCC) catalyst particles. Typical operating conditions include mean superficial gas velocities up to 7.4 m/s and solids fluxes up to approximately 100 kg/m2· s. Quantitative comparison of gamma- and ΔP-determined solids loadings was made over a range of operating conditions (combination of superficial gas velocity and solids flux). Results indicate that the differences between gamma and ΔP-determined cross-sectionally averaged solids loading are most pronounced near the base of the riser, where solids concentration is highest and the mixture is accelerating. Higher in the riser, the agreement is better. Additionally, the difference is larger in cases of higher superficial gas velocity. In addition, several studies were performed to design an electrical-impedance tomography (EIT) system for a gas-solid flow to collect data suitable for validating computational models. A two-electrode bulk impedance system was studied experimentally. The required accuracy, spatial resolution and temporal resolution of an EIT system are addressed, and modeling and reconstruction are discussed. Bulk solid volume fractions measured by the two-electrode system and by gamma-densitometry tomography are in general agreement. Experiments with the two-electrode system also show that the Maxwell-Hewitt relation, used to convert the mixture impedance to solid volume fraction, must be applied carefully, paying attention to the identity of the dispersed and continuous phases. The design of a 16-electrode system is also described.Copyright

Enhanced Productivity of Chemical Processes Using Dense Fluidized Beds

The work detailed in this report addresses Enabling Technologies within Computational Technology by integrating a “breakthrough” particle-fluid computational technology into traditional Process Science and Engineering Technology. The work completed under this DOE project addresses five major development areas 1) gas chemistry in dense fluidized beds 2) thermal cracking of liquid film on solids producing gas products 3) liquid injection in a fluidized bed with particle-to-particle liquid film transport 4) solid-gas chemistry and 5) first level validation of models. Because of the nature of the research using tightly coupled solids and fluid phases with a Lagrangian description of the solids and continuum description of fluid, the work provides ground-breaking advances in reactor prediction capability. This capability has been tested against experimental data where available. The commercial product arising out of this work is called Barracuda and is suitable for a wide (dense-to-dilute) range of industrial scale gas-solid flows with and without reactions. Commercial applications include dense gas-solid beds, gasifiers, riser reactors and cyclones.

Circulating fluidized bed hydrodynamics experiments for the multiphase fluid dynamics research consortium (MFDRC).

An experimental program was conducted to study the multiphase gas-solid flow in a pilot-scale circulating fluidized bed (CFB). This report describes the CFB experimental facility assembled for this program, the diagnostics developed and/or applied to make measurements in the riser section of the CFB, and the data acquired for several different flow conditions. Primary data acquired included pressures around the flow loop and solids loadings at selected locations in the riser. Tomographic techniques using gamma radiation and electrical capacitance were used to determine radial profiles of solids volume fraction in the riser, and axial profiles of the integrated solids volume fraction were produced. Computer Aided Radioactive Particle Tracking was used to measure solids velocities, fluxes, and residence time distributions. In addition, a series of computational fluid dynamics simulations was performed using the commercial code Arenaflow{trademark}.