Absorption (chemistry)

Absorption in chemistry is the uptake of a substance into the bulk of another material. The substance taken up is called the absorbate, and the receiving material the absorbent. In chemical engineering, the term commonly refers to transferring a component of a gas mixture into a liquid solvent. Absorption may involve physical dissolution, a chemical reaction, or both. Its defining distinction from adsorption is bulk uptake rather than accumulation at an interface. (goldbook.iupac.org)

Terminology and scope

Absorption and adsorption describe where an absorbed or adsorbed substance resides, not simply whether chemical bonding occurs. An adsorbed substance is concentrated at a surface; an absorbed substance enters the receiving phase. Both processes can occur in the same system. Sorption is the broader term used when the mechanism is unspecified, difficult to distinguish, or includes both processes. A polymer sorbent, for example, may operate through surface adsorption or bulk absorption. (media.iupac.org)

The word absorption also denotes the transfer of energy from electromagnetic radiation to matter. That usage, central to spectroscopy, is distinct from the material-uptake process discussed here. Radiation absorption concerns energy transfer; chemical absorption concerns the uptake of matter. (goldbook.iupac.org)

Physical and chemical absorption

Physical absorption

In physical absorption, a gas component dissolves in a liquid without relying on a chemical reaction to retain it. Its uptake depends on the gas–solvent equilibrium and on transport between the phases. Important properties include equilibrium solubility, diffusivity, density, and viscosity. Lower temperatures generally favor the physical absorption of gases, although the behavior must be established for the particular gas–solvent system. (nepis.epa.gov)

Physical solvents are used in high-pressure gas processing, including removal of carbon dioxide and other acid gases. The solvent’s affinity for the target component and the operating conditions determine its usefulness; a liquid that absorbs one gas effectively need not be suitable for another. (netl.doe.gov)

Chemical absorption

In chemical absorption, dissolution is coupled to a reaction in the receiving liquid. Reaction consumes dissolved material and can sustain further uptake from the gas. Alkaline scrubbing solutions, for example, absorb acid gases and convert them into salts. Water containing sodium hydroxide, sodium carbonate, or calcium hydroxide is used for this purpose. (nepis.epa.gov)

Reaction does not eliminate transport limitations. A fast reaction may leave gas-to-liquid transfer as the controlling step, whereas a slower reaction requires explicit treatment of chemical kinetics. Absorption accompanied by reaction is therefore a coupled transport-and-reaction process, rather than an alternative to physical transfer across the interface. (epa.gov)

Equilibrium and Henry’s law

For a dilute dissolved gas, the relationship between equilibrium liquid concentration and gas-phase partial pressure is often represented by Henry’s law. One convention is

where $c_A^{*}$ is the equilibrium molar concentration of dissolved species $A$, $p_A$ is its partial pressure, and $H_{\mathrm{s}}^{cp}$ is the concentration–pressure form of the Henry’s law solubility constant. At a fixed temperature, this expression predicts greater equilibrium uptake as partial pressure increases. Henry’s law is a limiting relationship for dilute solutions, not a universal description at arbitrary concentration or pressure. (old.goldbook.iupac.org)

Henry’s constants are reported using several conventions. Solubility constants and volatility constants express reciprocal relationships, and their numerical values depend on the concentration and pressure units used. Temperature dependence must also be included when applying data measured under different conditions. (iupac.org)

If a dissolved gas dissociates or reacts, its total concentration in the liquid is not necessarily the concentration of the molecular species appearing in Henry’s law. Calculations must then account for chemical equilibria as well as physical dissolution. Ignoring this distinction can produce incorrect estimates of absorption capacity. (nist.gov)

Mass transfer and absorber operation

Equilibrium determines the relationship between gas and liquid compositions, but it does not by itself determine how quickly absorption proceeds. Mass transfer depends on contact between the phases, transport properties, and the departure of operating compositions from equilibrium. Absorber calculations therefore combine equilibrium data with flow rates and transport behavior. A high equilibrium capacity alone does not establish that a particular apparatus will achieve the required removal. (epa.gov)

Industrial absorbers create extensive gas–liquid contact using wetted packing, trays, or droplets:

• Packed columns distribute liquid over packing while gas passes through the remaining spaces.
• Tray columns bring the phases into contact on successive trays.
• Spray towers disperse liquid as droplets into a gas stream.

Countercurrent packed columns introduce liquid near the top and gas near the bottom. The liquid flows downward while the gas flows upward. Crossflow configurations are also used. (epa.gov)

Liquid distributors help wet packing uniformly, while mist eliminators reduce liquid-droplet carryover in the outlet gas. Spray towers are mechanically simple, but their comparatively limited mass-transfer capability generally favors applications involving readily absorbed gases. (epa.gov)

Operating flow rates have practical limits. Insufficient liquid flow can leave packing inadequately wetted; excessive gas flow can obstruct downward liquid movement and cause flooding. Pressure drop raises fan-energy requirements. Heat released by dissolution or reaction can change the temperature profile and absorption performance. (epa.gov)

Applications, regeneration, and limitations

Absorption is used to purify gas streams, recover valuable components, and control gaseous emissions. Wet scrubbers commonly remove inorganic gases, including ammonia, sulfur dioxide, and hydrogen chloride. Absorption also recovers soluble organic vapors, although emission-control applications depend strongly on finding an appropriate solvent and managing the resulting liquid. (nepis.epa.gov)

Solvent absorption is an important approach to carbon capture. Carbon dioxide is transferred from a gas into a liquid carrier by physical or chemical absorption. Relevant performance characteristics include absorption capacity, tolerance to impurities, and the energy required to regenerate the solvent. (netl.doe.gov)

A loaded absorbent may be regenerated and recycled, treated to remove reaction products, or discarded and replaced. Regeneration is a separate process requirement: capturing a component from a gas does not, by itself, provide a usable recovered product or resolve waste handling. Scrubber monitoring consequently includes liquid flow, outlet gas composition, pressure differential, and, where relevant, pH or absorbent concentration. (epa.gov)

Absorption systems can suffer from foaming, fouling, corrosion, solvent contamination, and solvent loss. These effects can reduce separation performance and equipment reliability. Selecting an absorbent therefore involves more than maximizing uptake: compatibility with the feed, selectivity, regeneration, and contamination control also matter. (netl.doe.gov)

References

1. Air Pollution Control Technology Fact Sheet: Packed Bed Packed Tower Wet Scrubbernepis.epa.gov
2. Patented Solvent Technology Offers Efficient Process for CO2 Capturenetl.doe.gov
3. Henry’s Law Constantsiupac.org
4. The Solubility of Gases in Liquidssrdata.nist.gov
5. Avoid Common Pitfalls when using Henry’s Lawnist.gov
6. Chapter 1 Section 5.2 — Wet Scrubbers for Acid Gasepa.gov
7. Monitoring by Control Technique — Wet Scrubber For Gaseous Controlepa.gov