Distillation. Absorption

Tray columns are used to create contact between a gas phase and a liquid phase, bringing them as close as possible to equilibrium. The greater the contact surface and the greater the turbulence between the phases, the greater the transfer of matter and heat between them.

Tray columns were developed for distillation, an operation in which resistance to material transfer is greater in the vapor phase than in the liquid phase. Trays are therefore designed to ensure dispersion of the vapor phase within the liquid. Tray columns are used with equal success in distillation and absorption.

In the vast majority of cases, steam pressure energy is used directly to disperse the phases one into the other. In all cases, gravity is used for the movement of the liquid phase, which always takes place from top to bottom.

Apart from a few special devices, trays can be divided into two main categories (figure 1 ):

• cross-flow trays with overflow ;

• countercurrent trays without overflow.

In crossflow trays, the rising gas phase passes through the liquid, which moves horizontally across the tray. An outlet dam maintains a layer of liquid on the tray. The liquid flows by gravity from one tray to the next via a pipe (overflow). Countercurrent trays have no outlet weir or overflow; liquid and steam flow through the same orifices.

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