Encyclopedia of Renewable Energy. James G. Speight
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contacted with sub-cooled liquid or where steam is injected into saturated gas. In the latter scrubber, the movement of water vapor toward the cold water surface carries the particles with it (diffusiophoresis), while the condensation of water vapor on the particles causes the particle size to increase, thus facilitating collection of fine particles. The foam scrubber is a modification of a wet scrubber in which the particle-laden gas is passed through a foam generator where the gas and particles are enclosed by small bubbles of foam.
Electrostatic precipitators operate on the principle of imparting an electric charge to particles in the incoming airstream, which are then collected on an oppositely charged plate across a high voltage field. The dust cake is then collected from the plate by striking it with rappers. The dust collection efficiency is a strong function of dust resistivity. In general, particles in a gas stream are charged by a high-voltage discharge electrode and collected at collection plates of opposite polarity. Particles of high resistivity create the most difficulty in collection. Because resistivity varies with temperature, this can be an important parameter. Conditioning agents such as sulfur trioxide have been used to lower resistivity. Other important parameters include design of electrodes, spacing of collection plates, minimization of air channeling, and collection-electrode rapping techniques (used to dislodge particles). Techniques under study include high-voltage pulse energization to enhance particle charging, electron-beam ionization, and wide plate spacing. Electrical precipitators are capable of high efficiencies >99% under optimum conditions, but performance is still difficult to predict in new situations.
See also: Gas Cleaning, Gas Processing, Gas Treating, Pollution Control.
Adip Process
The Adip process is regenerative amine process for the removal of hydrogen sulfide and carbon dioxide from natural gas, refinery gas, and synthesis gas. The process uses aqueous solutions of the secondary amine, diisopropanolamine, or the tertiary amine, methyl di-ethanolamine. Amine concentrations up to 50% w/w can be applied. Hydrogen sulfide can be reduced to low sulfur levels. The process can also be applied to remove hydrogen sulfide, carbon dioxide, and carbonyl sulfide (COS) from liquefied petroleum gas or natural gas liquid to low levels.
The Adip-X process is a regenerative amine process that is highly suitable for bulk and deep removal of carbon dioxide from gas streams. The process uses aqueous solutions of the tertiary amine, methyl diethanolamine, and an additive.
See also: Gas Cleaning, Gas Processing, Gas Treating.
Adsorption
Adsorption (sometimes referred to as physisorption) is a process that occurs when a gas or liquid solute accumulates on the surface of a solid or a liquid (adsorbent), forming a film of molecules or atoms (adsorbate). It is different from absorption, in which a substance diffuses into a liquid or solid to form a solution. Adsorption is the process by which the gas is concentrated on the surface of a solid or liquid to remove impurities; carbon is a common adsorbing medium which can be regenerated upon desorption. The term sorption encompasses both processes, while desorption is the reverse process.
Adsorption differs from absorption in that the adsorption process is not a physical-chemical phenomenon in which the gas is concentrated on the surface of a solid or liquid to remove impurities. Usually, carbon is the adsorbing medium, which can be regenerated upon desorption. The quantity of material adsorbed is proportional to the surface area of the solid, and, consequently, adsorbents are usually granular solids with a large surface area per unit mass. Subsequently, the captured gas can be desorbed with hot air or steam, either for recovery or for thermal destruction.
The number of steps and the type of process used to produce specification-quality gas and liquids most often depend upon the source and makeup of the wellhead production stream. In some cases, several of the steps may be integrated into one unit or operation, performed in a different order or at alternative locations, or not required at all.
If the attractive forces are chemical in nature and result in the formation of a chemical bond between the solid adsorbent and the adsorbed molecules, the adsorption is called chemical adsorption or chemisorption. In the latter case, the removal of the chemisorbed material to regenerate the adsorbent is more difficult and may require chemical techniques to reactivate the spent adsorbent. For this reason, most adsorbents are operated as physical adsorbents with regeneration by reducing pressure, raising temperature, or flushing with solvents. However, even physical adsorbents are often slowly deactivated over many adsorption regeneration cycles by chemisorption due to unknown trace contaminants in either the adsorbent or the feed stream. At some specific deactivation level, the adsorbent must be either reactivated or discarded.
Two general types of adsorbents are commonly used. The first, of which activated carbon is an example, has a low-energy surface and is selective for non-polar molecules. These can be hydrocarbons and other organic compounds of low polarity such as ether derivatives, ketone derivatives, ester derivatives, and halogenated hydrocarbon derivatives. This type of adsorbent is useful for recovery of organic materials from wastewaters and exhaust gases. The other type of adsorbent, which consists of activated clay minerals, alumina minerals, and silica gel, has a higher energy surface that is more selective for polar molecules.
These materials are usually not effective for water solutions because the water selectively covers all the adsorption sites to the exclusion of everything else. However, they do find use for removing polar impurities such as water, oxidation products, or naturally occurring impurities like sulfur and nitrogen compounds from organic process streams. They are also useful for cleaning slightly contaminated waste streams to permit recycling of the major components. Zeolites and molecular sieves are special cases which depend on pore size and structure, i.e., access to the internal surface, for their selectivity rather than on adsorbent surface energy. They can be made from specially treated carbons as well as from natural or synthetic crystalline materials.
See also: Absorption, Adsorption, Adsorption Process, Chemisorption, Gas Cleaning, Gas Processing, Gas Treating, Physisorption.
Adsorption Isotherm
The adsorption process is studied through the development of an adsorption isotherm which relates the amount of adsorbate (x) adsorbed on the surface of adsorbent (m) and pressure at constant temperature. Different adsorption isotherms have been developed by Freundlich, Langmuir, and by means of the Brunauer, Emmett, and Teller (BET) theory. Simply, the adsorption process can be represented as:
Freundlich Adsorption Isotherm
Freundlich gave an empirical expression representing the isothermal variation of adsorption of a quantity of gas adsorbed by unit mass of solid adsorbent with pressure:
In this equation, x is the mass of the gas adsorbed on mass, m, of the adsorbent at pressure p; k and n are constants whose values depend upon adsorbent and gas at particular temperature. This isotherm establishes the relationship of adsorption with lower pressures but is not always suitable for high-pressure situations.
Langmuir Adsorption Isotherm
The Langmuir adsorption isotherm is based on several assumptions, one of which is that dynamic equilibrium exists between adsorbed gaseous molecules and the free gaseous molecules:
In this equation, A(g) is the unabsorbed gaseous molecule, B(s) is unoccupied metal surface, and AB is adsorbed gaseous molecule from which a relationship between the number of active sites of the surface undergoing adsorption and pressure can be derived: