Effect of Fine Bubbles on Flow Properties in Bubble Column With Suspended Solid Particles

Abstract

Flowproperties in a dissolver for direct coal liquefaction were simulated by using a cold bubble column of 12 cm i.d. To generate small gas bubbles in the column, a surfactant was added to tap water and a gas distributor with fine holes was employed. Whenglass spheres of 44 and 113 /nn dia. were suspended in the presence of the surfactant, the gas holdup was 1.5-4 times that for the tap water system, and the axial dispersion coefficient of liquid showed a minimum at superficial gas velocities of 3-5 cmà"s~*. Themeansettling velocity of solid particles, vp, wasaffected by the quality of flow in addition to gas velocity and terminal velocity of solid particles. Most values of vp obtained in this experiment were larger than those for the tap water system where no fine gas bubbles were generated. Experimental equations for the correlation of vp are presented. An introduction of secondary gas in the homogeneous bubble flow regime effectively increased the axial liquid mixing. This suggests that in the design of a dissolver quenching gas injection is important to attain spontaneous discharge of ash particles from the top of the vessel.

Heat Transfer and Hydrodynamics in a Three-Phase Slurry Bubble Column

“…The difference disappeared with increasing operating pressure. Koide et al ( 1979) and Ueyama et al (1980) Yu and Kim (1988) observed that bubble chord length in the column center was higher than that near the walL…”

“…(Hikita et al, 1980;Miller, 1980;Fan et al, 1985;Reilly et al, 1986;Wachi et al, 1987;Prakash, 1991;Del Pozo, 1992;Gandhi, 1997 (Akita and 'fahida, 1973;Kato et al, 1973;Grover et al. 1986;Hatate et al, 1986;Morooka et al, 1986;Ozturk et al, 1987 2)The design of gas and liquid sparger plays a vital role in limiting the magnitude of the erron. Some gas is capable of flowing into the column after the gas has been hirned off due to higher pressure f o n d inside the distributor before the gas is tumed off.…”

“…nie gas holdup was detennined from the summation of ail gas bubbles in contact with the probe. Yu and Kim (1988) used a U-shaped optical fiber probe to masure the gas holdup, bubble velocity, and bubble chord length.…”

“…The presence of electrolyte or impurities also leads to a higher gas holdup (Hi-kita et al, Kelkar et al, 1984;Morooka et al, 1986;Sada et al, 1986a). Sada et al (1986a) also noted that the presence of suspended solids particles has a much smaller influence on gas holdup in electrolyte solutions than in non-electrolyte solutions.…”

“…A number of researchers have investigated the effects of particle size and concentration on gas holdup (Kat0 et al, 1973;Kara et al, 1982;Sada et al, 1986a;Morooka et al, 1986). The influence of particle size has b e~ found to depend on a nurnber of factors including flow regime, gas velocity, liquid properties and siuny concentration.…”