Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 13

When gaseous products are produced from a source, the gas stream will invariably contain unwanted constituents that must be removed. This not only applies to natural gas streams and gas streams produced during the refining of the crude oil (as well as to gas streams produced from the other fossil fuels) but also to the alternative fuel industry where gas streams are produced from feedstocks such as biomass and waste. One such process for removing the unwanted constituents is the absorption process.

Absorption is a physical or chemical process by which molecules enter a liquid or solid bulk phase and is different to adsorption in which a chemical is subject to the surface forces of 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 K_N, 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, where the mixture is heated to a temperature above the boiling point of the absorbed hydrocarbons but below that of the oil. This process allows for the recovery of approximately 75% v/v of butanes, and 85 to 90% v/v percent of pentanes and higher boiling hydrocarbons from the stream.

The process above can be modified to improve its effectiveness, or to target the extraction of specific hydrocarbons. In the refrigerated oil absorption method, where the lean oil is cooled through refrigeration, propane recovery can be upwards of 90% v/v and around 40% v/v of any of ethane present in the gas stream. Extraction of higher molecular weight hydrocarbons approaches 100% v/v using this process.

See also: Adsorption, Gas Cleaning, Gas Processing, Gas Treating.