Scrubbing adipic acid-enhanced

Burbank, D.A., and Wang, S.C., "Test Results on Adipic Acid-Enhanced Lime/Limestone Scrubbing at the EPA Shawnee Test Facility", presented at the Industry Briefing on EPA Lime/ Limestone Wet Scrubbing Test Program, Raleigh, N.C., Dec. 5, 1979. 

Adipic Acid-Enhanced Lime/Limestone Test Results at the EPA Alkali Scrubbing Test Facility... 

Adipic acid-enhanced limestone scrubbing has lower projected capital and operating costs than unenhanced limestone or MgP-enhanced limestone scrubbing. This is due primarily to the reduced limestone consumption at the lower operating pH, the reduced grinding cost, and the reduced quantity of waste sludge generated. 

Figure 2. Flow diagram for adipic acid-enhanced scrubbing in the venturi/spray tower system with two scrubber loops and forced oxidation.

Lime tests with One Scrubber Loop and Without Forced Oxidation. Tests with adipic acid in lime scrubbing also were impressive in enhancing S02 removal, both on the venturi/spray tower and TCA systems. Table 4 shows some typical results of adipic acid-enhanced lime tests from the Shawnee TCA without forced oxidation. The flow diagram for these tests is shown in Figure 3. 

Bleed stream oxidation of unenhanced lime or limestone slurry is usually not feasible because the pH rise caused by the residual alkali in the oxidation tank makes it difficult to redissolve the solid calcium sulfite. With adipic acid-enhanced limestone scrubbing, however, this constraint is removed because of the low operating pH and low residual alkali in the bleed slurry. Thus, the oxidation tank can be maintained at a low pH for good sulfite oxidation, while achieving high SO2 removal efficiency with a sufficiently high concentration of adipic acid in the scrubber liquor. 

As shown In Table 10, both the total capital investment and the first-year revenue requirement are the lowest for adipic acid-enhanced limestone scrubbing at low pH (Case 4). The total capital investment is reduced by 4.8 percent, and the first year revenue requirement reduced by 5.8 percent for the limestone/ adipic acid/low pH case (Case 4), compared with the conventional... 

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). 

A primary objective of the EPA alkali wet scrubbing test program during the last several years has been to enhance SO2 removal and improve the reliability and economics of lime and limestone wet scrubbing systems by use of adipic acid as a chemical additive. 

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. 

Chloride Effect. The effectiveness of adipic acid is not adversely affected by chlorides, as is the effectiveness of MgO in an MgO-enhanced process. Tests at the IERL-RTP pilot plant showed that SO2 removal efficiency obtained with 17,000 ppm chloride in the scrubbing liquor was not significantly different from that obtained without chloride under similar levels of adipic acid concentration. Thus, use of adipic acid is especially attractive for systems with a very tightly closed liquor loop. 

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