Many chemical processes involve mass transfer between gas and liquid phases. A stirred tank is a common gas-liquid contactor. Gas is injected in to the tank via sparger or lance. Single or multiple impellers are used to increase the contact are between the gas phase and liquid phase. The shear introduced by the stirring action causes the bubbles to break-up increasing the contact surface or interfacial area. The interfacial are directly determines the mass transfer across the phases and thus the reaction rates. STAR-CCM+ can model the complex vessel geometry using polyhedral or trimmed mesh. An Eulerian multipase model is used to describe the phase behaviour which can incorporate mass transfer across the phases. The S-gamma model can also be used to describe abubble size distribution. The coalescene and break-up kernels in the S-gamma model can calcualte the change in mean bubble size due to coalescing and break-up processes.