Falling films minimum wetting rate

The minimum wetting rate (MWR) is tbe lower stability limit of packings. It is the liquid load below which the falling liquid film breaks up, and the liquid shortage causes dewetting of the packing surface. The area available for mass transfer diminishes, and efficiency drops (Sec. 8.2.2 points on Fig. 8.16a). 

A significant problem in the use of falling film evaporation systems is that of maintaining the integrity of the liquid film. The minimum wetting rate is required to rewet any dry patches formed on the film and evaporators should be operated above this minimum condition. A review on this topic is given by Hewitt and Hall-Taylor [271]. Typical of the earlier work in this area is that of Hartley and Murgartroyd [367], who considered the situation illustrated in Fig. 15.145. 

FIGURE 15.146 Minimum wetting rate for a falling film in the presence of mass transfer systems water/propanol, forming an azeotrope (from Norman and Binus [368], with permission). 

If this minimum is not achieved, the perimeter of the tube will not be uniformly wetted. Once a falling film has been induced, however, a lower terminal flow rate can be realized without destroying the film. This minimum terminal rate can be predicted with the equation... 

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