LITMIR.BIZ

      At increased temperatures (205 to 370°C, 400 to 700°F), zinc oxide has a rapid reaction rate, therefore providing a short mass transfer zone, resulting in a short length of unused bed and improved efficiency.

      Removal of larger amounts of hydrogen sulfide from gas streams requires a continuous process, such as the Ferrox process or the Stretford process.

      The Ferrox process is based on the same chemistry as the iron oxide process except that it is fluid and continuous. The Stretford process employs a solution containing vanadium salts and anthraquinone disulfonic acid. Most hydrogen sulfide removal processes return the hydrogen sulfide unchanged, but if the quantity involved does not justify installation of a sulfur recovery plant (usually a Claus plant), it is necessary to select a process that directly produces elemental sulfur.

      The processes using ethanolamine and potassium phosphate are now widely used. The ethanolamine process, known as the Girbotol process, removes acid gases (hydrogen sulfide and carbon dioxide) from liquid hydrocarbons as well as from natural and from refinery gases. The Girbotol process uses an aqueous solution of ethanolamine (H₂NCH₂CH₂OH) that reacts with hydrogen sulfide at low temperatures and releases hydrogen sulfide at high temperatures. The ethanolamine solution fills a tower called an absorber through which the sour gas is bubbled. Purified gas leaves the top of the tower, and the ethanolamine solution leaves the bottom of the tower with the absorbed acid gases. The ethanolamine solution enters a reactivator tower where heat drives the acid gases from the solution. Ethanolamine solution, restored to its original condition, leaves the bottom of the reactivator tower to go to the top of the absorber tower, and acid gases are released from the top of the reactivator.

      The process using potassium phosphate is known as phosphate desulfurization, and it is used in the same way as the Girbotol process to remove acid gases from liquid hydrocarbons as well as from gas streams. The treatment solution is a water solution of potassium phosphate (K₃PO₄), which is circulated through an absorber tower and a reactivator tower in much the same way as the ethanolamine is circulated in the Girbotol process; the solution is regenerated thermally.

      Moisture may be removed from hydrocarbon gases at the same time as hydrogen sulfide is removed. Moisture removal is necessary to prevent harm to anhydrous catalysts and to prevent the formation of hydrocarbon hydrates (such as C₃H₈.18H₂O) at low temperatures. A widely used dehydration and desulfurization process is the glycolamine process, in which the treatment solution is a mixture of ethanolamine and a large amount of glycol. The mixture is circulated through an absorber and a reactivator in the same way as ethanolamine is circulated in the Girbotol process. The glycol absorbs moisture from the hydrocarbon gas passing up the absorber; the ethanolamine absorbs hydrogen sulfide and carbon dioxide. The treated gas leaves the top of the absorber; the spent ethanolamine-glycol mixture enters the reactivator tower, where heat 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

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