Gamma Tomography System for Determination of Solids Loading in Gas-Solid Flows: Comparison with Axial Pressure Gradient Method

T J O’Hern1, S M Trujillo1, and P R Tortora2

1Engineering Sciences Center, Sandia National Laboratories, Albuquerque, NM, 87185, tjohern@sandia.gov

2University of Michigan, Ann Arbor, MI, 48109

ABSTRACT

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 solids cross-sectional 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 ∆P 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, 1990) have pointed out the neglected terms in this approach, and generally show that ∆P 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 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 circulating fluidized bed. The CFB has a 14-cm-ID riser, approximately 6 m tall, 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 92 kg/m2·s.

Quantitative comparison of gamma- and ∆P-determined solids loadings is made over a range of operating conditions (combination of superficial gas velocity and solids flux). Results indicate that the difference between gamma and ∆P-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.

Keywords Gamma Tomography, Circulating Fluidized Bed, Gas-Solid, Pressure