Influence of t-butanol and of pH on hydrodynamic and mass transfer parameters in an ozonation process

Ozone treatment is a complex process including mass transfer from gas to liquid before oxidation itself. In addition, oxidation proceeds via two synergistic pathways, i.e. molecular ozone oxidation and radical by-products oxidation. In order to study the relative importance of both pathways at the laboratory scale, the use of t-butanol (radical scavenger) is convenient, as it will inhibit the radical pathway. Meanwhile, this will modify the surface tension of the solution, then the bubbles characteristics and, finally, the mass transfer parameters. This study investigated the influence of t-butanol and of pH on both hydrodynamics and transfer parameters in a semi batch ozone bubble column. Image analysis was used to monitor the bubble shape and the slip velocity. The Sauter diameter and gas hold-up were determined in order to calculate the specific interfacial area (a). Besides, a procedure based on two mass balances for ozone allowed the simultaneous determination of the overall transfer coefficient (kLa), the self-decomposition kinetic rate constant and the liquid/gas solubility ratio. It was established that the addition of t-butanol at a concentration of 10−3 mol L−1 lead to smaller bubbles and increased the interfacial area by 30%. The overall transfer coefficient (kLa) was also increased by 30%, while the liquid film transfer coefficient (kL) remained almost constant (3.9 × 10−4 m s−1). This result was not consistent with the values obtained from Higbie's relation, which indicates that the liquid film coefficient (kL) should increase when t-butanol is added.