Hydronamic study of gas-liquid co-current bubble column reactors at low superficial gas velocities.
Sasol's Research and Development Division has identified several proprietary gas-liquid reactions where very low superficial gas velocities « 0.8 cmls) are required to obtain desired conversions in a bubble column reactor. A review of existing literature has shown that research in bubble column reactors is typically conducted in the superficial gas velocity range of 1 - 40 cmls. Traditionally bubble column reactors are designed via the application of empirical correlations which are only valid under specific conditions. There is a danger of under or over design if incorrect nonadjustable parameters such as liquid dispersion coefficients, mass transfer coefficients and gas hold-up values are used. To this extent, a hydrodynamic study was undertaken at superficial gas velocities lower than 0.8 cmls, to determine whether existing correlations are valid in this little investigated superficial gas velocity regime. Three bubble column reactors were designed and set up to perform hydrodynamic studies: • 22 cm inner diameter QVF glass column, 190 cm tall • 30 cm inner diameter 304 stainless-steel column, 200 cm tall • 30 cm inner diameter QVF glass column, 80 cm tall All measurements were undertaken in an air/water system. Gas hold-up measurements revealed that at the investigated gas flow rates, the gas hold-up was less than 1 % and as such was not investigated extensively. Partition plates were installed into the bubble columns and residence time distribution measurements were undertaken. The bubble columns were found to behave identically to the well known tanks in series model (Levenspiel, 1962). Liquid dispersion coefficients were measured via two methods. Batch liquid measurements were undertaken via the method of Ohki and Inoue (1970) and continuous liquid residence time distribution measurements were also undertaken. Data reduction was performed for both methods using the axial dispersion model to regress the liquid dispersion coefficient EL_ Both methods yielded equivalent results. The effect of distributor plate geometry on EL was also investigated and proved not to affect EL. It was found that existing literature correlations developed at higher superficial gas velocities failed to accurately predict the measured dispersion coefficients obtained in this study_ Correlation of the EL values with column diameter and superficial gas velocity showed EL to be a weak function of diameter as compared to existing correlations. This will have a significant effect on scale-up to larger column diameters.