Factors Favoring Trays

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. 

Trays vs. Random Packings The following factors generally favor trays compared to random packings, but not compared to structured packings. 

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. 

The main distillation devices are the tray and packing. Both trays and packing come in different types. Device selection requires balancing many competing factors. Table 1 lists the factors that favor each type of equipment. Final equipment selection is usually service specific. 

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. 

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