Factors Favoring Packed Columns

The main factors favoring packed columns are (1) very corrosive applications, where plastic or ceramic packings are favored over trays, which are almost always constructed of metal (2) low pressure drop requirement, which is easier to achieve with packings than with trays (3) small-diameter columns, because trays require access for inspection and maintenance and (4) foaming systems, which are easier to handle in packed towers. 

Most separations can be performed either with trays or with packings. The factors below represent economic pros and cons that favor each and may be overridden. For instance, column complexity is a factor favoring trays, but gas plant demethanizers that often use one or more interreboilers are traditionally packed. 

Absorption and stripping are usually conducted in packed columns or in trayed towers. Packed columns are preferred when (1) the required column diameter is less than 60 cm (2) the pressure drop must be low, as for a vacuum service (3) corrosion considerations favor the use of ceramic or polymeric materials and/or (4) low liquid holdup is desirable. Trayed towers are preferred when (1) the liquid/gas ratio is very low, and (2) frequent cleaning is required. If there is no overriding consideration, cost is the major factor to be taken into account when choosing between packed columns and trayed towers for absorption or stripping. 

PCBs do not volatize easily. For example, at atmospheric pressure, the distillation range of PCBs chlorinated at 42% (Aroclor 1242) is from 325° to 336°C. For Aroclor 1254, it is from 365 to 390°C and for Aroclor 1260, from 385 to 420°C. This is another factor favoring the use of capillary columns which are better suited than packed columns for eluting non volatile compounds. 

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