How Packed Towers Work

The two features that should be maximized in packed beds are  

Structured packing has about 50 percent more open area than Raschig rings and two or three times their wetted surface area. Hence, structured packing has largely replaced packing in the form of rings, in many packed towers. 

In any type of packed tower, the liquid or internal reflux drips through the packing, and forms a thin film of liquid on the surface of the packing. Vapor percolates up through the packing, and exchanges heat and molecules with the thin film of liquid on the surface of the packing. 

In a trayed tower, vapor-liquid contact occurs, only on the 5 or 6 in above the tray deck, and the majority of the tower s volume is not used to exchange heat or mass between vapor and liquid. In a packed tower, the entire packed volume is used for this vapor-liquid contacting. 

In a trayed tower, the area used for the downcomer which feeds liquid to a tray, and the area used for draining liquid from a tray, are unavailable for vapor flow. In a packed tower, the entire cross-sectional area of the tower is available for vapor flow. 

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Traditionally, packings have not been used in distillation duties, except for small columns of less than 1 m diameter, and even less for high pressure service. The situation has changed over the last 10-15 years, partly through a better understanding of how packed towers work, particularly with respect to liquid distribution, but also through the development of new proprietary packings. 

In the 1904 edition there is, for example, a sample calculation of the heat balance on a Glover tower treated as an evaporator, which shows how inefficient it was then ( what a heat waster it is (23)), There is also a discussion on the efficiency of various packings, explaining in terms of surface areas why coke is 1.5 to 2 times more efiBcient than bricks (23, 26), But in general, Davis approach was still empirical the operations are described as procedures of practical utility, and are not based on fundamental physics. Neither the work of Osborne Reynolds nor dimensionless group theory had been assimilated yet into the profession. 

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