Encyclopedia of Renewable Energy. James G. Speight

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      Amine Washing

Amine washing (more correctly olamine washing) of a gas stream involves the chemical reaction of the amine with any acid gases with the liberation of an appreciable amount of heat, and it is necessary to compensate for the absorption of heat. Amine derivatives such as ethanolamine (monoethanolamine, MEA), diethanolamine (DEA), triethanolamine (TEA), methyldiethanolamine (MDEA), di-isopropanolamine (DIPA), and diglycolamine (DGA) have been used in commercial applications (Table A-19). Amine washing is the primary process for sweetening sour natural gas and is quite similar to the processes of glycol dehydration and removal of natural gas liquids by absorption.

      The primary process for sweetening sour natural gas is quite similar to the processes of glycol dehydration and removal of natural gas liquids by absorption. In this case, however, amine (olamine) solutions are used to remove the hydrogen sulfide (the amine process).

Table A-19 Amines (olamines) used for gas processing.

      In the process, the sour gas is run through a tower, which contains the olamine solution. There are two principal amine solutions used, monoethanolamine (MEA) and diethanolamine (DEA). Either of these compounds, in liquid form, will absorb sulfur compounds from natural gas as it passes through. The effluent gas is virtually free of sulfur compounds, and thus loses its sour gas status. Like the process for the extraction of natural gas liquids and glycol dehydration, the amine solution used can be regenerated for reuse.

      As currently practiced, acid gas removal processes involve the selective absorption of the contaminants into a liquid, such as an olamine (Table A-19), which is passed countercurrent to the gas. Then, the absorbent is stripped of the gas components (regeneration) and recycled to the absorber. The process design will vary and, in practice, may employ multiple absorption columns and multiple regeneration columns.

      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 absorption is enhanced by low temperatures, or high pressure, or both. 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 pressure and/or high temperature, whereby the acid gases are stripped from the solvent.

      Regeneration of the solution leads to near complete desorption of carbon dioxide and hydrogen sulfide. A comparison between monoethanolamine, diethanolamine, and diisopropanolamine shows that monoethanolamine is the cheapest of the three but shows the highest heat of reaction and corrosion; the reverse is true for diisopropanolamine.

      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 chemistry can be represented by simple equations for low partial pressures of the acid gases:

At high acid gas partial pressure, the reactions will lead to the formation of other products:

      The reaction is extremely fast, the absorption of hydrogen sulfide being limited only by mass transfer; this is not so for carbon dioxide.

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

      Ammonia

      Ammonia is a valuable chemical that is often produced from renewable sources (biomass) that contain nitrogen. It is recoverable from both the liquid and gas streams and can be readily separated from the liquid, by an ammonia still. The fixed salts must be treated with lime or caustic in a lime leg. The gaseous ammonia may be absorbed with sulfuric acid to produce ammonium sulfate, processed to anhydrous ammonia, or destroyed by combustion.

      The standard procedure for the manufacture of ammonium sulfate from a gas stream involves several steps. The gas is first cooled to approximately 32°C (90°F) in an appropriate condensation system. Any tar, which is a very troublesome contaminant of ammonium sulfate (and vice versa), condenses and, in addition, much of the water containing approximately 25% of the ammonia (NH₃), primarily as ammonium (NH₄+) salts, also condenses. This water is rendered basic (lime treatment), thereby converting the ammonium ion to ammonia, which is recovered by being stripped off in a lime still and placed back in the coal gas stream. The coal gas stream is heated to above its dew point (approximately 65°C; 150°F), and the ammonia is adsorbed in 5 to 10% sulfuric acid solution contained in a lead-lined saturator at a temperature of 50 to 60°C (120 to 140°F); ammonium sulfate crystals precipitate from the sulfuric acid solution.

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