Process Technology: An Introduction - Haan A.B. 2015

8 Absorption and stripping
8.1 Introduction

In absorption (also called gas absorption, gas scrubbing, and gas washing) a gas mixture is contacted with a liquid (the absorbent or solvent) to selectively dissolve one or more components by transfer from the gas to the liquid. The components transferred to the liquid are referred to as solutes or absorbate. The operation of absorption can be categorized on the basis of the nature of the interaction between absorbent and absorbate into the following three general types:

· (1) Physical Solution — In this case, the component being absorbed is more soluble in the liquid absorbent than are the other gases with which it is mixed but does not react chemically with the absorbent. As a result, the equilibrium concentration in the liquid phase is primarily a function of partial pressure in the gas phase and temperature.

· (2) Reversible Reaction — This type of absorption is characterized by the occurrence of a chemical reaction between the gaseous component being absorbed and a component in the liquid phase to form a compound that exerts a significant vapor pressure of the absorbed component.

· (3) Irreversible Reaction — In this case, a reaction occurs between the component being absorbed and a component in the liquid phase which is essentially irreversible.

The use of physical absorption processes is usually preferred whenever feed gases are present in large amounts at high pressure and the amount of the component to be absorbed is relatively large. On the other hand, absorption processes with simultaneous chemical reaction are always preferred when the components to be separated from feed gases are present in small concentrations and at low partial pressures.

Gas absorption is usually carried out in vertical countercurrent columns as shown in Fig. 8.1. The solvent is fed at the top, whereas the gas mixture enters from the bottom. The absorbed substance is washed out by the lean solvent and leaves the absorber at the bottom as a liquid solution. Usually the absorption column operates at a pressure higher than atmospheric, taking advantage of the fact that gas solubility increases with pressure. The loaded solvent is often recovered in a subsequent stripping or desorption operation. After preheating the rich solvent is transported to the top of a desorption column that usually operates under lower pressure than in the absorption column. This second step is essentially the reverse of absorption in which the absorbate is removed from the solvent. Desorption can be achieved through a combination of methods:

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Fig. 8.1: Basic scheme of an absorption installation with stripping for regeneration.

· (1) flashing the solvent to lower the partial pressure of the dissolved components;

· (2) reboiling the solvent, to generate stripping vapor by evaporation of part of the solvent;

· (3) stripping with an inert gas or steam.

Wide use is made of desorption by pressure reduction because the energy requirements are low. After depressurization, stripping with an inert gas is often more economic than thermal regeneration. However, because stripping is usually not perfect, the absorbent recycled to the absorber contains species present in the vapor entering the absorber. The desired purity of the gas determines the final costs of desorption. The necessary difference between the partial pressure of the absorbed key component over the regenerated solution and the purified gas serves as a criterion for determining the dimensions of the absorption and desorption equipment. The lean solvent, devoid of gas, flows through a heat exchanger, where part of the heat needed for heating the rich solvent is recovered, and then through a second heat exchanger, where it is cooled down to the desired temperature before returned to the absorption column. Usually a small amount of fresh solvent should be added to the column to replenish the solvent which was partly evaporated in the desorption column or underwent irreversible chemical reactions which take place in the whole system.