Electrical-impedance tomography for the quantitative measurement of solids distributions in gas -solid riser flows.

Abstract

An electrical-impedance tomography (EIT) system was constructed to non-invasively measure distributions of solids particles in the riser of a gas-solid circulating fluidized bed (CFB), intended for the development and validation of computational models of multiphase flows. EIT systems have often been applied to gas-solid flows, mainly yielding qualitative data, but here an EIT system is validated against a reliable gamma-densitometry tomography (GDT) system, yielding quantitative data. The systems were applied to a pilot scale CFB with a 14 cm inner-diameter, 5.77 m high riser circulating fluid catalytic cracking catalyst with air. Overall solids volume fractions were generally less than 25 percent, and the flows examined were annular with a dilute core. Radially symmetric solids volume fraction profiles in the form of fourth-order polynomials were produced by EIT and GDT and mostly agree within uncertainty. The EIT system is considered validated against GDT and can therefore be used in place of the slower, more expensive system. The EIT system can measure a solids distribution in approximately one second with low error, a significant improvement over the GDT system which requires several minutes. EIT overpredicts the solids concentration near the riser walls in comparison to GDT because of a bias error but this is within uncertainty. The bias error is +6% at the riser walls and -4% at the riser center. Both EIT and GDT underpredict the solid loading at the center of the riser, in many cases predicting zero loading where the literature indicates solids volume fractions of a few percent which is within the systems' uncertainty. An invasive method may be needed to measure the solids loading in this region. Mixture models used to relate solids volume fraction to electrical permittivity were examined during the design of the EIT system and assessed for their suitability. The Rayleigh mixture model was found to be suitable for use with the EIT system. It was also found that the permittivity of relatively dense distributions of particles (solids volume fraction greater than 40 percent) is sensitive to the particle arrangement, which has implications for the application of EIT to dense gas-solid flows.