Automatic Control of Flotation Columns - Advanced Control and Supervision of Mineral Processing Plants - page 266

standard addition method might imply an important propagation error. However,
this method might be useful for estimating the gas hold-up in the froth zone where
the siphon method cannot be used.
The third method available for measuring the gas hold-up uses an array of passive
sensors and sonar processing algorithms (sonar array) originally developed for fluid
flow rate measurements and later adapted for measuring the amount of air entrained
in a slurry pipe [43]. This sensor was developed by CIDRA Corporation (USA) and
is commercialized under the name of SONARtrac ®1 . This technology can simulta-
neously measure the velocity of fluid flow and naturally generated acoustic waves.
The gas hold-up is determined from the propagation velocity of the acoustic waves
(speed of sound) via a nonlinear relationship specific to each case.
The principle of the sonar array instrument is based on the detection, tracking
and measurement of the mean velocities of coherent disturbances traveling in the
axial direction of a pipe. These disturbances are grouped into three major categories:
disturbances conveyed by the flow, acoustic waves in the fluid, and vibrations trans-
mitted via the pipe walls. Each disturbance class travels at a given velocity. For
example, the flow will convey turbulent eddies, density variations, or other fluid
characteristics at the rate of the fluid flow. Liquid-based flows rarely exceed 9 m/s.
Acoustic waves in the fluid will typically have a minimum velocity of 80 m/s and a
maximum velocity of 1500 m/s. The third group, pipe vibrations, travels at veloc-
ities that are several times greater than the acoustic waves. Thus each disturbance
class may be clearly identified and accurately measured. The sequence of events
leading to the actual gas hold-up measurement are as follows:

(a) Acoustic waves are generated by spargers, rotors, or other sources. (b) These
acoustic waves enter the gas hold-up meter along with the air bubbles. (c) The

Figure 6.7 McGill two-cell sensor

1

SONARtrac is a registered trademark of CiDRA Corporation.