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Distillation columns are one of the most commonly used thermal separation units and have been used for many centuries. Their operation is based on the difference in boiling temperatures of the liquid mixture components, and on recycling counter-current gas-liquid flow. The properly organized temperature distribution up the column results in different mixture compositions at different heights. While multi-component inter-phase mass transfer is a common phenomenon for all column types, the flow regimes are very different depending on the internal elements used. The two main types are a tray column and a packed column, the latter equipped with either random or structured packing. Different types of distillation columns are used for different processes, depending on the desired liquid holdup, capacity (flow rates), pressure drop etc, but each column is a complex unit, combining many structural elements.
A stage of the tray column typically combines the open channel flow, with weirs, downcomers and heat exchangers. Free surface flow over the tray is disturbed by gas bubbles coming through the perforated tray, and possible leakage of liquid dropping through the upper tray. Such a complex flow morphology is addressed in ANSYS CFX computational fluid dynamics software by either allocating more than two phases, or adjusting the two phase mixture model to the local flow conditions. A packed column is similar to a trickle-bed reactor, where liquid film flows down over the packing surface in contact with the upward gas flow. A small fragment of packing geometry can be accurately analysed assuming the periodic boundary conditions, which allows calibration of the porous media model for a big packing segment. Flow in other internal elements of the packed columns such as liquid collectors and distributors are modeled using either Eulerian approach (free surface flow) in collectors, particle model for droplets coming from distributors or Lagrangian particle transport. The latter approach is also used for modeling liquid droplets inside the absorption columns.
