(265f) Measurement of Gas Velocities in Gas/Solids Flows Using Time-of-Flight Ultrasound

Gas phase velocity profiles determine the residence time distribution in gas/solid reactors. Coupling between the gas and solids phases can produce large departures from ideal flow models, resulting in effects such as down-flow near the walls of the vessel. An ability to measure gas velocities non-invasively would allow improved models of these reactors to be designed and would aid attempts to optimize the performance of existing units. We present measurements of gas phase velocities in a moderately loaded gas/solid flow using time-of-flight (TOF) ultrasound. A method was developed using the transmission of high energy, phase shift encoded signals and the application of a pulse compression algorithm after subsequent reception to resolve difficulties presented by the attenuation of the ultrasound, the interference from acoustic emissions in the system, and the inaccurate determination of the TOF. With this technique we were able to measure TOF to within 2.5 microseconds, in systems consisting of up to 9 volume percent FCC catalyst particles in upward flowing air, using commercial narrowband transducers. Measurements will be presented of axial gas velocities in a vertical riser, the solid phase being either FCC catalyst or glass beads having a mean diameter of 150 microns. The use of encoded pulses allows many transmitters to operate simultaneously. This feature makes possible the construction of fast arrays for ultrasonic tomography that can estimate complete velocity profiles at high frame rates. We describe our front-end system for high-speed tomography applications and present data taken with the system from a small 4-transducer array.