Ammonia Absorption into the Water in a Packed Tower: VOF

Description: Absorption is a method of separating the components of a gas mixture by bringing it into contact with a liquid solvent, relying on differences in solubility to draw one or more gas components into the liquid phase — a process refineries commonly use to strip ammonia out of a gas stream. This project simulates ammonia absorption from air inside an absorption tower using ANSYS Fluent, tracking how the gas and liquid streams interact as they pass through the column.

Methodology: The 3D vertical tower geometry, built in Design Modeler, has an ammonia-laden airflow entering at 0.43 m/s through a bottom nozzle and exiting from the upper section, while liquid water enters at 0.0332 kg/s through a top nozzle and exits from the lower section — the two streams meeting and interacting as they move in opposite directions through the chamber. The domain is meshed in ANSYS Meshing with an unstructured grid of 478,882 elements, and the VOF multiphase model is used to resolve the interaction between the one-percent-ammonia air stream and the water solvent.

Analysis: Post-processing includes 2D and 3D contours of pressure, velocity, turbulent viscosity, density, and the volume fractions of water, air, and ammonia, giving a full picture of how each phase behaves throughout the tower. The volume fraction contours show the ammonia concentration steadily declining in the gas phase as it rises through the chamber, while the corresponding water-phase contours show a matching increase in absorbed ammonia content as the liquid descends and contacts the rising gas — direct confirmation that mass transfer is occurring at the gas-liquid interface rather than the two streams simply passing by one another. The velocity and pressure fields further show how the counter-current arrangement of the two nozzles sets up the internal flow pattern that sustains this prolonged contact, which is central to the tower's separation performance. Taken together, the results confirm that the water phase effectively absorbs the ammonia and separates it from the gas stream, illustrating the core mechanism that makes packed absorption towers effective for gas purification in industrial process design.