drives off the absorbed acid gases and water.
Other processes include the Alkazid process for removal of hydrogen sulfide and carbon dioxide using concentrated aqueous solutions of amino acids. The hot potassium carbonate process decreases the acid content of natural and refinery gas from as much as 50% to as low as 0.5% and operates in a unit similar to that used for amine treating. The Giammarco-Vetrocoke process is used for hydrogen sulfide and/or carbon dioxide removal. In the hydrogen sulfide removal section, the reagent consists of sodium or potassium carbonates containing a mixture of arsenite derivatives (such as sodium arsenite, NaAsO2) and arsenate derivatives (such as sodium arsenate, Na3AsO4); the carbon dioxide removal section utilizes hot aqueous alkali carbonate solution activated by arsenic trioxide or selenous acid or tellurous acid.
Molecular sieves are highly selective for the removal of hydrogen sulfide (as well as other sulfur compounds) from gas streams and over continuously high absorption efficiency. They are also an effective means of water removal and thus offer a process for the simultaneous dehydration and desulfurization of gas. Gas that has excessively high water content may require upstream dehydration, however.
The molecular sieve process is similar to the iron oxide process. Regeneration of the bed is achieved by passing heated clean gas over the bed. As the temperature of the bed increases, it releases the adsorbed hydrogen sulfide into the regeneration gas stream. The sour effluent regeneration gas is sent to a flare stack, and up to 2% of the gas seated can be lost in the regeneration process. A portion of the gas stream may also be lost by the adsorption of hydrocarbon components by the sieve.
See also: Alkazid Process, Gas Cleaning, Gas Processing, Gas Treating.
Acid Gas Scrubbing – Basic Solid or Solution
Sulfur dioxide is an acid gas and thus the typical sorbent slurries or other materials used to remove the sulfur dioxide from the flue gases are alkaline. The reaction taking place in wet scrubbing using a limestone (CaCO3) slurry produces calcium sulfite (CaSO3):
When wet scrubbing with a lime [Ca(OH)2] slurry, the reaction also produces calcium sulfite:
When wet scrubbing with a magnesium hydroxide [Mg(OH)2] slurry, the reaction produces magnesium sulfite (MgSO3):
In some designs, the calcium sulfite is oxidized to produce calcium sulfate (gypsum, CaSO4.2H2O):
Seawater is also used to absorb sulfur dioxide; the SO2 is absorbed in the water and when oxygen is added reacts to form sulfate ions (SO4-) and free protons (H+) which result in the release of carbon dioxide from the carbonates in the seawater:
See also: Gas Cleaning, Gas Processing, Gas Treating.
Acid Gas Scrubbing – Wet Scrubbers
To promote maximum gas-liquid surface area and residence time, a number of wet scrubber designs have been used in wet flue gas desulfurization systems, including spray towers, venturi scrubbers, plate towers, and mobile packed beds.
Scale buildup, plugging, or erosion affect the dependability and absorber efficiency of flue gas desulfurization systems and the trend has been to use simple scrubbers such as spray towers instead of more complicated ones. The configuration of the tower may be vertical or horizontal, and flue gas can flow co-currently, counter-currently, or cross-currently with respect to the liquid. The chief drawback of spray towers is that they require a higher liquid-togas ratio requirement for equivalent sulfur dioxide removal than other absorber designs.
A wet scrubber is a form of pollution control technology and is a device that removes pollutants from gas streams. In a wet scrubber, the polluted gas stream is brought into contact with the scrubbing liquid, by spraying it with the liquid, by forcing it through a pool of liquid, or by some other contact method, so as to remove the pollutants.
Scrubbers can be designed to collect particulate matter and/or gaseous pollutants. Wet scrubbers remove dust particles by capturing them in liquid droplets. Wet scrubbers remove pollutant gases by dissolving or absorbing them into the liquid. Any droplets that are in the scrubber inlet gas must be separated from the outlet gas stream by means of another device referred to as a mist eliminator. Also, the resultant scrubbing liquid must be treated prior to any ultimate discharge or being reused in the plant.
There are numerous configurations of scrubbers and scrubbing systems, all designed to provide good contact between the liquid and polluted gas stream. Examples include a venturi scrubber and the mist eliminator for a venturi scrubber is often a cyclone separator whereas the packed tower design has the mist eliminator built into the top of the tower.
The ability of a wet scrubber to collect small particles is often directly proportional to the power input into the scrubber. Low-energy devices such as spray towers are used to collect particles larger than 5 micrometers. To obtain high efficiency removal of 1 micrometer (or less) particles generally requires high-energy devices such as venturi scrubbers or augmented devices such as condensation scrubbers. Additionally, a meticulously designed and operated entrainment separator or mist eliminator is important to achieve high removal efficiencies. The greater the number of liquid droplets that are not captured by the mist eliminator the higher the potential emission levels.
Wet scrubbers that remove gaseous pollutants are referred to as absorbers. Good gas-to-liquid contact is essential to obtain high removal efficiencies in absorbers. A number of wet scrubber designs are used to remove gaseous pollutants, with the packed tower and the plate tower being the most common.
If the gas stream contains both particle matter and gases, wet scrubbers are generally the only single air pollution control device that can remove both pollutants. Wet scrubbers can achieve high removal efficiencies for either particles or gases and, in some instances, can achieve a high removal efficiency for both pollutants in the same system. However, in many cases, the best operating conditions for particles collection are the poorest for gas removal.
In general, obtaining high simultaneous gas and particulate removal efficiencies requires that one of them be easily collected (i.e., that the gases are very soluble in the liquid or that the particles are large and readily captured) or by the use of a scrubbing reagent such as lime (CaO) or sodium hydroxide (NaOH).
For particulate control, wet scrubbers (also referred to as wet collectors) are evaluated against fabric filters and electrostatic precipitators (ESPs).
Wet scrubbers have the ability to handle high temperatures and moisture and in wet scrubbers, the inlet gases are cooled, resulting in smaller overall size of equipment. Also, wet scrubbers can remove both gases and particulate matter and can neutralize corrosive gases. On the other hand, wet scrubbers are subject to corrosion and there is the need for entrainment separation or mist removal to obtain high efficiencies and the need for treatment or reuse of spent liquid.
See also: Gas Cleaning, Gas Processing, Gas Treating.
