Sodium ammonia scrubbing

Without the commercial sale or internal consumption, it is necessary to find a non-polluting means of disposing of the reaction products. The reaction products from sodium and ammonia scrubbing are quite soluble... 

Tail gas emissions are controlled by improving the SO2 conversion efficiency and by scrubbing the tail gas. In a double-absorption process plant, a five-bed converter has 0.3 percent unconverted SO2, as compared to 0.5 percent for a four-bed converter. A Lurgi Peracidox scrubber may be used to remove up to 90 percent of the residual SO2 in the tail gas from a double-absorption plant. Hydrogen peroxide or electrolytically produced peroxymonosulfuric acid is used to convert the SO2 to H2SO4 in the Lurgi scrubber. Tail gas from single-absorption plants may be absorbed on activated carbon (Sulfacid) or scrubbed with ammonia or sodium sulfite (Wellman-Lord). 

Tail gas scmbbers are sometimes used on single absorption plants to meet S02 emission requirements, most frequently as an add-on to an existing plant, rather than on a new plant. Ammonia (qv) scrubbing is most popular, but to achieve good economics the ammonia value must be recovered as a usable product, typically ammonium sulfate for fertilizer use. A number of other tail gas scrubbing processes are available, including use of hydrogen peroxide, sodium hydroxide, lime and soda ash. Other tail gas processes include active carbon for wet oxidation of S02, molecular sieve adsorbents (see MoLECULARSIEVEs), and the absorption and subsequent release of S02 from a sodium bisulfite solution. 

Walker and Pennline [91] have described a process for the simultaneous removal of SO2 and NO f from flue gas by a combination of adsorption, electrodialysis, and additional regeneration of the scrubbing liquor. The process consisted of scrubbing the flue gas with either a solution of ammonium sulfate and iron(II) ethylenedi-aminetetraacetic acid (Fe-EDTA) or sodium sulfate and Fe-EDTA. The SO2 gas converts the ammonium sulfate to the bisulfite which is then passed through an electrodialysis unit to further dilute the bisulfite stream, which is eventually converted to the sulfate downstream by the addition of ammonia. The size of the Fe-EDTA-NO species formed from the adsorption of NO gas does not allow it to pass through either of the permselective membranes. 

In a 250-mL, two-necked flask flushed with nitrogen, 1.5 g of freshly prepared sodium amide (38 mmole) and 6.2 g of tetramethylphosphonium bromide (36 mmole) are suspended in 100 ml of tetrahydrofuran. The mixture is heated under reflux for at least 5 hours. The ammonia evolved is trapped in a scrubbing bottle containing 1 N H2SO4. Titration of the acid indicates that 36 mmole (100%) of NH3 are formed in the process. Filtration from the sodium bromide formed (3.2 g) and fractional distillation of the filtrate yields 2.9 g (90%), bp, 122°/760 torr. All isolation and purification procedures are also conducted under nitrogen, using glass apparatus with inert gas inlet cocks. 

After the scrubbing operation, the solution of salts is exposed to sodium hydroxide solution chemical reaction releases ammonia and steam for recycle. 

Scrubbing processes use various removal media including limestone, lime, dolomite, magnesium carbonate, sodium carbonate, ammonia, molten carbonate, or activated carbon. 

Final emission values of 100 ppm can be achieved by alkali scrubbing e.g. with sodium hydroxide, ammonia or urea solutions, but disposal of the nitrites and nitrates formed is problematic. 

With the double contact process it is unnecessary to purify the tail gases to reduce their sulfur dioxide content still further, whereas tail gases from single contact plants have to be purified. This can be realized either by scrubbing with ammonia or with an aqueous solution of sodium sulfite and sodium hydrogen sulfite (Wellman-Lord process), absorption on activated charcoal (sulfacid process from Lurgi) or by oxidative gas purification such as in the peracidox process (oxidation of sulfur dioxide with hydrogen peroxide or peroxomonosulfuric acid). 

Ammonia is then removed from the aqueous phase by distillation and either recovered by scrubbing with sulfuric acid to produce ammonium sulfate, which is sold as a fertilizer constituent, or it may be discharged to air. Economical reacidification of the aqueous ammonia recovery residue with flue gas and subsequent distillation allows recovery of both phenol and sodium carbonate (Eq. 14.27, 14.28). 

Recovery systems in which sulfur dioxide or elemental sulfur is removed from the spent sorbing material, which is recycled, are much more desirable from an environmental and sustainability viewpoint than are throwaway systems. Many kinds of recovery processes have been investigated, including those that involve scrubbing with magnesium oxide slurry, sodium sulfite solution, ammonia solution, or sodium citrate solution. One type of recovery system uses a solution of sodium sulfite to react with sulfur dioxide in the flue gas... 

Over two hundred dry or wet processes have been proposed for the removal of SO2 gases. Limestone and dolomite find extensive use in many of the desulfurization processes. Activated soda is considered as an attractive and more reactive alternate to limestone. In addition to calcium and sodium based processes, ammonia based, magnesia based, organic based scrubbing systems and catalytic processes have also been proposed for SQ2 removal. Calcium based dry processes have found wide application especially in fluidized bed combustors and in... 

Efforts have been made in some installations to recover the fluorine evolved in phosphate-rock processing as cryolite or other marketable products. Hie quantity of fluorine evolved in such operations is very large so that steps to recover it would appear warranted. A process developed by the Tennessee Valley Authority (TVA) is claimed to accomplish this (Anon., 1957). In the TVA process, the water used to scrub nodulizing-kiln exhaust gas is maintained at a pH of S to 6 by the continuous addition of ammonia, and the resultant rich liquor is treated to precipitate impurities and yield a valuable NH4F solution. The absorber solution is recirculated to bring its fluorine content up to about 35 g/liter. Hie rich solution is then treated with sufficient ammonia to raise the pH to 9, thus precipitating iron, silica, and part of the phosphorus. The precipitate is fiitered off, and the solution is thoi used to make cryolite (by adding sodium sulfiite and alum at a pH of 6) or aluminum fluoride. 

---