Buffer scrubbing

If the gas to be measured contains sulfur dioxide, it has to be scrubbed from the gas before oxidation of the reduced compounds can occur. The gas is scrubbed using an SO2 scrubber. This may contain citrate buffer solution (potassium citrate or sodium citrate). The collection efficiency of the sulfur diox ide may be as high as 99%. 

A typical colorimetric analyzer is illustrated schematically in Figure 6-8. Sample air is drawn at a metered rate into a contact column, where the air is scrubbed with a metered flow of potassium iodide buffered at a pH of 6.8. The reaction of oxidants with the potassium iodide solution produces the yellow triiodide ion (I, ). The colored solution flows to a colorimeter cell, where the absorbance of the triiodide ion is measured... 

Although promising, this system has not been further developed because of its sensitivity to pH variations and the need to buffer the feed (either via a front-end step where the feed acidity is reduced through denitration, or via the introduction of large amounts of carboxylic acid in the scrubbing solution). 

Experimental work with lime scrubbing has shown that sulfite scaling occurs in the scrubber bed when free hydroxide is introduced. By proper control of the pH of the spray slurry (less than 10) entering the scrubber, calcium sulfite scaling will be prevented within the scrubber. In the calcium carbonate system, the buffering action of the carbonate-bicarbonate couple (Reaction 5) maintains a system pH between 5 and 6 thus sulfite scaling is not encountered. 

Liquid scrubbing is also known as one of the most efficient methods for very fine-particle collection, but they are gathered in the form of a suspension that can be rather used for nasal, pulmonary, or parenteral delivery. The suspension stability is often a delicate issue because no chemical or biological degradation shall occur, nor particle decantation, which requires the use of complex mixtures, including a buffer (especially for biomolecules), preservatives (antimicrobial, antioxidant, etc.), and surfactants. Several systems have been described (37,86-88) ... 

When organic fuels are burned, carbon dioxide and water vapor are released along with various amounts of sulfur dioxide and nitrogen oxides. The sulfur and nitrogen oxides in the atmosphere are then further oxidized with the assistance of ultraviolet solar radiation when these gases are scrubbed from the air by precipitation, a dilute solution of sulfuric acid and nitric acid forms. Carbon dioxide itself hydrolyzes to carbonic acid and is important in the marine carbonate buffer system however, it is a weak organic acid and atmospheric concentrations typically lower the pH of distilled water only to about 5.7 (5-6). 

In the slurry scrubbing process, limestone dissolves at pH A to 6 and 55°C in both absorber and the hold tank/crystallizer. Because of HC1 accumulation from the flue gas, typical scrubbing solution contains 0.01 to 0.2 M CaCl2 C02 partial pressure can vary from near zero with forced oxidation to one atmosphere with CO2 evolution from the hold tank and is typically 0.1 atm in the absorber. Sulfite/bisulfite buffer can be present in concentrations up to 0.1 M. CaS03 and/or CaS04 crystallization must occur simultaneously with limestone dissolution. Buffer additives such as adipic acid should enhance both SO2 removal and CaC03 dissolution at concentrations of 3 to 10 mM (5). 

Limestone dissolution in throwaway scrubbing can be modeled by mass transfer. The mass transfer model accurately predicts effects of pH, Pcc>2> temperature, and buffers. For particles less than 10-20 pm, the mass transfer coefficient can be obtained by assuming a sphere in an infinite stagnant medium. This model underpredicts the absolute dissolution rate by a factor of 1.88, probably because it neglects agitation and actual particle shape. 

Buffer Additives for Lime/Limestone Slurry Scrubbing... 

Buffer additives are attractive for enhancing SO2 removal and/or CaC03 utilization in lime/limestone slurry scrubbing processes for flue gas desulfurization. This work was sponsored by EPA to provide experimental data on commercial synthesis, gas/liquid mass transfer enhancement, and oxidative degradation of useful buffer additives. 

Lime/limestone slurry scrubbing is the dominant commercial technology for flue gas desulfurization 0.). SO2 is absorbed at 50-55°C and pH 5.5-6.0 in an aqueous slurry of excess CaC03 and product solids. The CaS03/CaS04 product is disposed of as solid waste. With greater than 500-1000 ppm SO2 in the flue gas, SO2 absorption is controlled by liquid-film mass transfer resistance because of the limited solubility of SO2 gas and alkaline solids. Additives that buffer between pH 3 and pH 5.5 enhance S02 absorption by providing dissolved alkaline species for reaction with SO2 (8). 

Since forced oxidation converts sulfite to sulfate, it has an adverse effect on SO2 removal in an unenhanced lime system in which sulfite is the major SO2 scrubbing species. This is also true in MgO-enhanced lime and limestone systems in which the promotion of SO2 removal relies on an increased sulfite-bisulfite buffer. When adipic acid is used with lime, calcium adipate becomes a major buffer species therefore, both good SO2 removal and sulfite oxidation can be achieved using within-scrubber-loop forced oxidation. 

The reason for improved SO2 removal, particularly at low pH, is that forced oxidation eliminates bisulfite species, thereby reducing the SO2 vapor pressure at the gas-liquid interface and improving the SO2 mass transfer efficiency. This mechanism of improved SO2 removal holds true when sulfite is not a major scrubbing species and the SO2 removal does not depend on the sulfite-bisulfite buffer. In the case of Figure 8 and 9, calcium adipate is the major scrubbing reagent. 

Figure 128 shows the apparatus used by Braman and Tomkins which consists of a sample reaction chamber, U-trap, the flame emission type detector, and conventional type photometric readout and recording system. Inorganic and methyltin compounds in aqueous solution in the reaction chamber are reduced to stannane or the corresponding methylstannanes by treatment with sodium borohydride solution buffered at pH 6.5. Helium carrier gas scrubs the volatile stannanes out of solution and into the liquid nitrogen-cooled U-trap where they are frozen out. Upon removal of the liquid nitrogen and warming, the stannanes are separated and carried into the detector. 

Carbon dioxide has to be scrubbed from synthesis gas into carbonate-bicarbonate buffer solution using arsenite as a catalyst. Assume the subsequent data apply ... 

Organic acids increase SO2 removal efficimcy and produce other performance mhance-ments in a limestone FGD system by buffering the pH of the scrubbing slurry. Organic adds are used in relatively low concentrations, e.g., adipic acid is used in concentrations of 200-1,SOO ppm, DBA in concentrations of 200-2,000 ppm, and formate in concentrations of SOO to 5,000 ppm (Blythe et al., 1991 Moser and Owens, 1991). The concentrations of organic acids used in actual applications are often in the lower parts of the above ranges. Citric and acetic acids were used in the past however, their use is no longer common. 

To ensure that the process operates within the desired pH range and that neither the lower nor the upper pH limit specifications are exceeded, the scrubbing medium must be neutralized or buffered. Potassium or sodium hydroxide is commonly used. In most applications KOH is the preferred neutralizing agent because potassium salts purged from the system with the sulfur product are a more tolerable contaminant when the ultimate use of the sulfur product is in fertilizers. 

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