PLATE TOWERS FOR GAS ABSORPTION

Bubble-cap columns or sieve trays, of similar construction to those described in Chapter 11 on distillation, are sometimes used for gas absorption, particularly when the load is more than can be handled in a packed tower of about 1 m diameter and when there is any 

L m is the molar rate of flow per unit area of solute free liquid, 

G m is the molar rate of flow per unit area of inert gas, it refers to the plate numbered from the bottom upwards (and suffix n refers to material leaving plate n), 

It may be assumed that dilute solutions are used so that mole fractions and mole ratios are approximately equal. Each plate is taken as an ideal unit, so that the gas leaving of composition yn is in equilibrium with the liquid of composition xn leaving the plate. 

A material balance for the absorbed component from the bottom to a plane above plate n gives  

---

The design of a plate tower for gas-absorption or gas-stripping operations involves many of the same principles employed in distillation calculations, such as the determination of the number of theoretical plates needed to achieve a specified composition change (see Sec. 13). Distillation differs from gas absorption in that it involves the separation of components based on the distribution of the various substances between a gas phase and a hquid phase when all the components are present in Doth phases. In distillation, the new phase is generated From the original feed mixture by vaporization or condensation of the volatile components, and the separation is achieved by introducing reflux to the top of the tower. 

---