Measurements of Local Flow Characteristics in a Gas–Liquid–Solid Circulating Fluidized Bed with Immersed Telocentric Photography

Abstract

The gas bubble size, local phase holdup, and other local flow characteristic parameters of the gas–liquid–solid circulating fluidized bed play an important role in the design and operation of this type of reactor. However, due to the limitation of measurement techniques of three-phase fluidization, the local flow characteristics have not been well measured and studied. In recent years, telecentric imaging technology has been applied in other multiphase systems, and a new telecentric probe method has been used to measure the local flow characteristics of a gas–liquid–solid expanded bed. In this work, the measuring method was introduced to measure the local phase holdups and bubble size distributions of the three-phase circulating fluidized bed. Concurrently, the influence of the superficial liquid and gas velocities on the particle circulation rate in the three-phase circulating fluidized bed was studied. The method reliability of gas holdup and bubble size measurements was validated by a conductivity probe method. The results provided basic data of such a reactor for future flow numerical simulations and industrial applications.

Multibubble Behavior and Solid Circulation Rate in a Gas–Liquid–Solid Circulating Microfluidized Bed

“…The positive effect of superficial gas velocity on G S is significant. This is qualitatively consistent with the findings of Zhang et al 19 on macroscopic three-phase circulating fluidized beds. Increasing the superficial gas velocity generates more voids near the gas injection point, reducing the local pressure at the bottom of the riser and creating a higher pressure drop between the downer and the riser.…”

“…Regardless of the particle diameter, the bubble diameters expressed in the center of the interval follow a log-normal distribution, with a narrower size distribution for dispersed bubble flow regime and a broader distribution for coalesced bubble flow regime. Zhang et al 19 observed that the distribution of D b in a macroscopic circulating fluidized bed also follows a log-normal distribution. However, Li et al 5 found that the bubble sizes in a three-phase microexpanded bed with an inner diameter of 0.8 mm conform to normal and step distributions under dispersed bubble flow regime and coalesced bubble flow regime, respectively.…”

“…Once the circulating fluidization regime is reached, increasing superficial gas velocity leads to a greater frequency of bubbles, increasing the number of bubbles and therefore raising ε G . Conversely, increasing superficial liquid velocity reduces the residence time of bubbles in the riser, 19 resulting in a slight decrease in gas holdup. However, this effect is not as pronounced as increasing the gas velocity.…”

“…The experimental investigation on the hydrodynamics is a main method, and it depends on the advanced measuring techniques of multiphase flow. At present, many researchers based on invasive testing techniques, such as optical fiber probes, , conductivity probes, and telecentric camera probes, as well as noninvasive test techniques, such as ultrasound and high-speed camera, , determined the gas holdup, bubble size, and bubble velocity of macroscopic three-phase circulating fluidized beds. The pressure gradient method and the bed collapse method have also been used to determine the global gas holdup of macroscopic gas–liquid–solid circulating fluidized bed systems.…”

“…For the determination of the solid circulation rate, the focus is on accurate determination of the flow rate of particles. Traditional methods such as the accumulation methods have been widely applied in the study of macroscopic circulating fluidized beds. , Despite its simplicity in operation, this method has the potential to disrupt the steady-state fluidization in the riser. Other testing methods, such as magnetic tracing, radioactive particle tracing, impact solid flow meter, and electrical capacitance tomography, are also used to determine the solid circulation rate.…”