the adsorbent (Table A-1).
Table A-1 Examples of the types of absorption.
| Solute* | Absorbent | Absorption type |
|---|---|---|
| Ammonia | Water | Physical |
| Carbon dioxide | Ethanolamine | Chemical, reversible |
| Carbon dioxide | Sodium hydroxide (aq.) | Chemical, irreversible |
| Hydrochloric acid | Sodium hydroxide (aq.) | Chemical, irreversible |
| Hydrochloric acid | Water | Physical |
| Hydrogen sulfide | Sodium hydroxide (aq.) | Chemical, irreversible |
| Hydrogen sulfide | Olamine** | Chemical, reversible |
| Nitrogen oxides | Water | Chemical, reversible |
| Sulfur dioxide | Water | Physical and chemical*** |
| *Listed alphabetically by solute. **For example, monoethanolamine and diethanolamine. ***In addtion to dissolution (physical absorption), there is also the potential for chemical absorption by the occurrence of the reversible reaction: SO2 + H2O ↔ H2SO3 | ||
Absorption is the process by which a gas is distributed throughout an absorbent (liquid); depends only on physical solubility and may include chemical reactions in the liquid phase (chemisorption). The process is generally used to separate a higher-boiling constituent from other components of a system of vapors and gases. The absorption medium is usually an oil in the range of gas oil. Absorption is widely employed in the gas clearing (gads processing) industry for recovery or removal of constituents from natural gas stream and from cured oil refinery streams of natural gasoline from well gas and of vapors given off by storage tanks. The reverse of this, i.e., the transfer of the absorbed constituents (the stripping process) is accomplished by contacting the absorbent (liquid or solid) liquid with an inert gas such as nitrogen or steam. With aqueous waste streams the stripping gas is usually air.
Scrubbing is primarily concerned with the removal of particulate liquids or solids from gas streams by contacting the gas with a liquid scrubbing medium. That is, the particles are scrubbed out of the gas. However, if the gas also contains some soluble gaseous components, they can dissolve in the scrubber solution. Thus, the scrubber may also be an absorber, just as an absorber may also be a scrubber if the feed contains particulate matter as well as soluble components. Absorption and scrubbing are commonly used separation processes for preventing air pollution from industrial exhaust or stack gases. Stripping is used for both prevention of water pollution by treatment of wastewaters to remove soluble toxic gases and remediation of contaminated water at already existing hazardous waste sites.
Typically, absorption is achieved by dissolution (a physical phenomenon) or by reaction (a chemical phenomenon) and is different from adsorption (Table A-2).
Table A-2 Comparison of adsorption and absorption.
| Adsorption | Absorption | |
|---|---|---|
| Definition | Accumulation at the surface the solid or liquid | Accumulation in the bulk of solid or liquid |
| Characteristic | A surface phenomenon | A bulk phenomenon |
| Concentration | Different at surface to bulk | Same throughout |
| Reaction rate | Increases to equilibrium | Occurs at a uniform rate |
| Reaction type | Exothermic process | Endothermic process |
| Temperature | Unaffected by temperature | Not affected by temperature |
Chemical adsorption processes adsorb sulfur dioxide onto a carbon surface where it is oxidized (by oxygen in the flue gas) and absorbs moisture to give sulfuric acid impregnated into and on the adsorbent.
Liquid absorption processes (which usually employ temperatures below 50°C (120°F) are classified either as physical solvent processes or chemical solvent processes. The former processes employ an organic solvent, and low temperatures, or high pressure, or both enhance absorption; regeneration of the solvent is often accomplished readily. In chemical solvent processes, absorption of the acid gases is achieved mainly by use of alkaline solutions such as amines or carbonates. Regeneration (desorption) can be achieved by the use of reduced pressures and/or high temperatures, whereby the acid gases are stripped from the solvent.
If absorption is a physical process not accompanied by any other physical or chemical process, it usually follows the Nernst partition law in which the ratio of concentrations of solute species in two bulk phases in contact is constant for a given solute and bulk phases, i.e.:
The value of constant KN, the partition coefficient, is dependent upon temperature and the value is valid if concentrations are not too large and if the species x does not change its chemical or physical form in either phase-1 or phase-2.
In the case of gas absorption, the concentration a solute (c) in one of the phases can be calculated using the Ideal gas law (e.g., c = p/RT. Alternatively, partial pressure may be used instead of concentration.
The absorption oil has an affinity for the natural gasoline constituents. As the gas stream is passed through an absorption tower, it is brought into contact with the (lean) absorption oil which soaks up a high proportion of the liquid hydrocarbons. The rich absorption oil now containing the hydrocarbons exits the absorption tower through the base after which it is fed into lean oil stills,
