Distillation and gas absorption are the prime and most common gasliquid mass-transfer operations. Other operations that are often performed in similar equipment include stripping (often considered part of distillation), direct-contact heat transfer, flashing, washing, humidification, and dehumidification.
The most common types of contactors by far used for these are tray and packed towers. These are the focus of this subsection. Other contactors used from time to time and their applications are listed in Table 14-4.
In this subsection, the terms gas and vapor are used interchangeably. Vapor is more precise for distillation, where the gas phase is at equilibrium. Also, the terms tower and column are used interchangeably.
A crossflow tray (Fig. 14-17) consists of the bubbling area and the downcomer. Liquid descending the downcomer from the tray above enters the bubbling area. Here, the liquid contacts gas ascending through the tray perforations, forming froth or spray. An outlet weir on the downstream side of the bubbling area helps maintain liquid level on the tray. Froth overflowing the weir enters the outlet down-comer. Here, gas disengages from the liquid, and the liquid descends to the tray below. The bubbling area can be fitted with numerous types of tray hardware. The most common types by far are:
Sieve trays (Fig. 14-18a) are perforated plates. The velocity of upflowing gas keeps the liquid from descending through the perforations (weeping). At low gas velocities, liquid weeps through the perforations, bypassing part of the tray and reducing tray efficiency. Because of this, sieve trays have relatively poor turndown.
Fixed valve trays (Fig. 14-18b) have the perforations covered by a fixed cover, often a section of the tray floor pushed up. Their performance is similar to that of sieve trays.
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