Gas Hold-up and Average Velocity of Gas Bubbles in Gas-bubbles Columns

Abstract

These experimental studies were carried out to make clear the effects of variable factors, such as gas velocity, liquid velocity, column diameter, column length, hole diameter, number of holes and the properties of the liquid, on the gas holdup and the average velocity of gas bubbles in gasbubble columns.The apparatus used in the experiments is shown in Fig. 1 and 2. The gas holdup was calculated from experimental values of hydrostatic pressure and from Eq.(6).The results obtained are summerized as follows:(1) On increasing the gas velocity, flow regions became divided roughly into three regions:(a) a region of separated gas bubble and foam flow, where (1-ψ) increased linearly, (b) a transition region, where small bubbles coalesced to produce large bubbles and the rate of increase of (1-ψ) decreased gradually and (c) a turbulent region, where (1-ψ) approached a constant value.(2) From the values of holdup at each height of the column, the column was roughly devided into two sections a section of the foam layer and dead space, l, and another section in which gas and liquid were uniformly distributed, l0. By equating this relationship, Eq.(10) for the average gas holdup was derived.For air-water system (ψ0-ψ′) l′ was correlated as shown in Fig. 7, from which experimental equations, Eqs.(11) and (12) were obtained.As shown in Fig. 8-12, in the separated bubble flow region the gas holdup, (1-ψ0), was a function of gas velocity, liquid velocity, and hold diameter and independent of the column diameter, number of holes, and the arrangement of the holes. In the turbulent region, however, (1-ψ0) was chiefly dependent on the gas velocity and liquid velocity, and independent of others.(3) For the average velocity of bubbles Eqs.(15) and (18) were derived, respectively.The average velocity of bubbles rising through a cross-section in a column l0 was correlated as shown in Figs. 13-15, from which it was ascertained that Eq.(17) held good for the experimental results ranging from parallel flow to counter flow of gas-liquid mixtures. Furthermore, Eq.(19) was obtained from Eqs.(18), (10), (15) and (17).