Bubble properties and mixing characteristics were investigated in an Airlift Bubble column with an internal circulation of liquid. In order to reduce the high circulation velocity and obtain a redispersion of coalesced gas bubbles, two types of redistributor ( sieve and disk ) were used in this work. They were installed in a draft tube, 50cm above the bottom and their free surface areas were varied 6.5, 13, 19.5, 26% respectively.
The effects of gas velocity ( 2-20cm/s ), liquid velocity ( 0-2cm/s ), and presence of redistributor on mixing time and circulation time have been determined by the salt( KCl )-base-impulse method, and local holdup in each section of the airlift column was measured by manometric technique.
The effects of operating variables on bubble properties at different bed height ( 30 - 105 cm ) have been determined by using an electro-resistivity probe method.
The experimental results in the present investigations can be summarized as follows;
(1) The presence of redistributor not only increased the gas holdup in the draft tube, mixing time and circulation time but also decreased bubble sizes. The experimental correlation can be described as follows;
ε_d = $0.020 (exp (-0.077 U_L)) U_G^{0.731} (A_f/A_d)^{-0.182}$
t_c = $6.514 (exp (0.037 U_L)) U_G^{-0.275} (A_f/A_d)^{-0.895}$
t_m = $38.245 (exp (0.001 U_L)) U_G^{-0.168} (A_f/A_d)^{-0.532}$
(2) Without redistributor the vertical bubble length and its rising velocity increased with gas velocity and height, but decreased slightly with liquid velocity.
L = $0.610 U_L^{-0.033}; U_G^{0.438} H^{0.101}$
U_b = $84.521 U_L^{-0.006} U_G^{0.219} H^{0.056}$
(3) The pressure drop for co-current upward bubbly air-water flow through various sieve plates has been measured. The discharge coefficient was estimated to be from 0.61 to 0.75 over a range of gas void fraction, 0. - 0.3.
(4) The Disk type of redistributor is effective to redisperse coalesced bubbles than the sieve type, especially, the disk type with 19.5% free surface area has given the best result.
