determined largely by the slip velocity between the gas and liquid phases. The value of kLaGL is also affected by the physical properties of the gas–liquid system, the velocities of the gas and the liquid, the lyophobicity of the solid and the solids holdup. The axial dispersion coefficient in either the gas or liquid phase characterizes the amount of mixing with a single parameter, Dax,G or Dax,L. The axial dispersion within a column may be quantified by performing tracer experiments. A pulse of non-transferring tracer is introduced at the phase inlet and the residence time distribution of this pulse is obtained at the reactor outlet. The outlet concentrations are modeled with an axial dispersion model as given in Eqs. (1) and (2) with the value of kLaGL set to zero; and for co-current upflow of gas and liquid: