Falling-film column

A falling-film column (or wetted-wall column) is a particular chemical equipment used to achieve mass and heat transfer between two fluid phases (in general one gas phase and one liquid phase).

It is essentially formed by a vertical tube-shaped vessel: the liquid stream flows through the inner wall of the tube and the gas stream flows in correspondence of the centre of the tube.

Description
Schematics of a typical falling-film column.

In the most common case, the column contains one liquid stream and one gas stream. The liquid forms a thin film that covers the inner surface of the vessel,[1] instead the gas stream is normally injected from the bottom of the column, so the two fluids are subjected to a counter-current exchange of matter and heat, that happens in correspondence of the gas-liquid interface.

Sometimes, the same equipment is used to achieve the co-current mass and heat transfer between two immiscible liquids.[2]

Applications

Because of its easiness of modelling, falling-film column is generally used as laboratory equipment, for example to measure experimentally the values of transport coefficients.[1] A significant experiment was carried out in 1934 by Edwin R. Gilliland and Thomas Kilgore Sherwood that used a falling-film column to study the mass transfer phenomenon between a liquid phase and a gas phase, obtaining an experimental correlation between Sherwood number, Reynolds number and Schmidt number.[3]

It is not used at an industrial scale, because it has low surface area and liquid hold-up compared to other gas-liquid contactors (e.g. a packed column or a plate column).

References
  1. Binay K. Dutta, Principles of Mass Transfer and Separation Processes, Nuova Delhi, Prentice-Hall of India Private Limited, 2007, p. 91. ISBN 8120329902.
  2. Satoru Asai, Junichi Hatanaka , Toshiya Kimura , Hidekazu Yoshizawa, Mass transfer in wetted-wall column with cocurrent laminar liquid-liquid flow in Industrial & Engineering Chemistry Research, vol. 26, n. 3, ACS Publications, marzo 1987, pp. 483–488. DOI:10.1021/ie00063a014.
  3. Robert S. Brodkey, Harry C. Hershey, Transport Phenomena: A Unified Approach, Brodkey Publishing, 2003. ISBN 0972663584.
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