Scrubber adipic acid

Other Organic Acids. Experiments were also carried out using other organic acid buffers details can be found in (7). The three most carefully studied acids were succinic, adipic, and glycolic. Adipic acid is the most likely candidate for use as a buffer in commerical scrubbers. The order of inhibition power is... 

Laboratory Investigation of Adipic Acid Degradation in Flue Gas Desulfurization Scrubbers... 

The addition of adipic acid to limestone-based FGD wet scrubbers results in improved limestone utilization and enhanced S02 sorption kinetics. The use of adipic acid was first proposed by Rochelle (1) and has been tested by the EPA in pilot systems at the Industrial Environmental Research Laboratory, Research Triangle Park, North Carolina and at the TVA Shawnee Test Facility at Paducah, Kentucky. Adipic acid in the concentration range of 1,000-2,000 mg/1 has been found effective as a scrubber additive. During scrubber operation, however, adipic acid is lost from the system in the liquid and solid phase purge streams and by chemical degradation (2,3). 

Tests conducted at the Shawnee Test Facility indicated that adipic acid added to their limestone FGD scrubber did not degrade at pH s below 5. Since these unexpected but favorable results were important to the future application of adipic acid as an FGD additive, independent verification was desired. Radian was contracted by the EPA to carry out a systematic study of the effects of scrubber operating conditions on adipic acid degradation. 

The topics presented in this paper include a description of the bench-scale system, the experimental approach, and the results of degradation testing. Also included are the results of batch precipitation experiments designed to study coprecipitation of adipic acid in scrubber waste solids. 

At the start of each run, various materials were added to the hold tank. These materials included deionized water, adipic acid, calcium sulfite seed crystals, NaCl, MnS( H20, Fe2(S( )3, fly ash, etc. depending on the purpose of the test. The resulting thin slurry was circulated through the scrubber where it was contacted with flue gas. This procedure was continued for several hours, allowing time for the mass of solids in the hold tank to increase and the concentration of ionic species (particularly S03) to reach a constant level. At this point, a quantitative amount of slurry was withdrawn from the hold tank. 

The effect of liquid phase adipic acid concentration on adipic acid concentration in the scrubber solids, and... 

The extent of adipic acid coprecipitation is highly influenced by the oxidation fraction of the scrubber solids (sulfite versus sulfate),... 

The scrubber slurry pH does not significantly effect adipic acid coprecipitation (at least over the pH range tests). 

The batch precipitation tests show dramatic effects of adipic acid slurry concentration and solid phase oxidation fraction on coprecipitation of adipic acid in scrubber solids. Real world scrubbers would probably never operate at adipic acid concentrations as high as those tested and would also not likely ever produce pure phase calcium sulfite hemihydrate. Therefore, the magnitude of the results observed is somewhat a product of the laboratory test conditions. The results do, however, establish the potential importance of adipic acid coprecipitation and, hence, the need for analysis of scrubber solids for adipic acid when determining adipic acid chemical degradation rates by a mass balance calculation approach. 

Large amounts of adipic acid were coprecipitated or occluded in the scrubber solids. The concentration of adipic acid in the solids was a function of both the liquid phase adipic acid concentration and the oxidation fraction in the solids. 

Meserole, F.B. Lewis, D.L. and Kurzawa, F.T. "Further Study of Adipic Acid Degradation in FGD Scrubbers," Final Report, EPA-600/7-80-152, Environmental Protection Agency, Research Triangle Park, NC, August, 1980. 

Mass transfer enhancement depends on the buffering properties and diffusivity of the additive. Cavanaugh (14) measured effective pka values of buffer alternatives at scrubber conditions. Chang and Rochelle (16) developed a model of enhancement based on mass transfer with equilibrium reactions and experimentally demonstrated its effectiveness with acetic and adipic acid at 25°C. In this paper, we present experimental and model results at 55°C with several buffer alternatives (17). 

Organic acids are normally stable to oxidation, but laboratory and pilot plant results (18) have shown that adipic acid oxidizes in conjugation with sulfite oxidation in the scrubber. This paper reports oxidative degradation rate of adipic acid as a function of pH and Mn concentration (19). Results are also presented on sulfopropionic, sulfosuccinic, succinic, hydroxypropionic, and hydroxyacetic acids (20). 

Fumaric acid degraded 2 to 4 times faster than adipic acid at pH 5.5 with 1 mM Mn (FI). The carbon-carbon double bond is apparently more susceptible to oxidation. Therefore, unsaturated acids should be sulfonated to avoid degradation. Since fumaric acid does not sulfonate rapidly in the scrubber, it is not an attractive buffer alternative. 

This paper summarizes the results of tests conducted from July 1978 through March 1981 at the EPA, 10-MW equivalent, lime/limestone wet-scrubbing FGD test facility, during which adipic acid as an additive was tested and shown to be a powerful scrubber additive for improving SO2 removal. The optimum concentration of adipic acid is only 700 to 1500 ppm at a scrubber inlet pH of 5.2 or higher. SO2 removal efficiencies in excess of 90 percent and reliable operation were demonstrated in four long term, limestone/adipic acid runs. Factorial tests were also conducted to characterize SO2 removal as a function of gas and slurry flow rates, pH, and adipic acid concentration. Intermediate duration optimization runs and favorable economics are also reported. 

HOOC(CH2)4COOH, that buffers the pH in the scrubber slurry. In theory, any acid which is intermediate in strength between carbonic acid and sulfurous acid, and whose calcium salt is reasonably soluble, may be employed. However, adipic acid was selected because it is one of the most cost-effective organic acid buffers on a molar basis and is commercially abundant. Its main use is as a raw material in the manufacturing of nylon. 

A number of attractive features of adipic acid as a scrubber additive are presented below. 

Limestone Utilization. At a scrubber inlet pH of about 5.2, the corresponding limestone utilization is normally 80 percent or higher for an adipic acid-enhanced system, as compared to 65 to 70 percent in unenhanced limestone systems at an equivalent S0 removal. Thus the quantity of waste solids generated is reduced in an adipic acid-enhanced system. Higher limestone utilization also contributes to more reliable scrubber operation by reducing the fouling tendency. This increased reliability is a very attractive feature of adipic acid-enhanced systems, since reliability problems have historically plagued limestone FGD. 

Operating pH. With proper pH control and sufficiently high adipic acid concentration (sufficient buffer capacity), the scrubber performance is more stable, and steady outlet SO2 concentrations can be maintained, even with wide fluctuations of inlet SO2 concentrations. 

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. 

Total Dissolved Solids. Addition of adipic acid does not significantly increase the total dissolved solids in liquid as does magnesium. High total dissolved solids in liquid entrainment can increase particulate emissions and fouling tendencies of equipment downstream of the scrubber. 

The theoretical basis for the effect of adipic acid on the performance of lime and limestone scrubbers was first developed in detail by G. Rochelle in 1977 ( 3). In October 1977, EPA began an investigation of adipic acid with the 0.1 MW IERL-RTP pilot plant to determine its effectiveness as an additive to limestone scrubbers for improving S02 removal efficiency (4). Initial results demonstrated, as predicted by Rochelle, that adipic acid was indeed an attractive and powerful additive. 

The results of the tests showed adipic acid to be very effective in improving SO2 removal efficiency, even when operating at chloride levels as high as 17,000 ppm. A TCA scrubber, which removed 82 percent of the inlet SO2 without the additive, yielded 89 percent S02 removal with 700 ppm adipic acid, 91 percent removal with 1,000 ppm, and 93 percent removal with 2,000 ppm adipic acid. The limestone utilization was concurrently increased from 77 percent without the additive to 91 percent with 1,600 ppm adipic acid. The observed effects thus confirmed the theoretical expectations in all respects. In addition, the tests showed no serious inteference by adipic acid on the performance of the oxidizer, operating at pH 6.1. 

Tests without forced oxidation also demonstrated the efficacy of adipic acid. Operating a TCA scrubber with 2,000 ppm adipic acid and 6 inches H2O pressure drop, 92 percent SO2 removal was obtained at a limestone utilization level of 88 percent. By comparison, only 75 percent SO2 removal would be expected in the pilot plant at these test conditions without the additive. At this adipic acid level, the unoxidized sludge filtered to 49 percent solids at lower adipic acid levels (1,500 ppm or less), the... 

The odor has been identified as that of valeric acid, CH3(CH2)3 COOH, an intermediate product formed by side reactions that degrade adipic acid at scrubber operating conditions. At Shawnee, this odor was rarely noticed and was not a problem. 

Limestone Long-Term Tests with Two Scrubber Loops and Forced Oxidation. The venturi/spray tower system was modified for two-scrubber-loop operation with forced oxidation as shown in Figure 2. Two tanks were used in the oxidation loop (venturi loop) air was injected to the first of these tanks through a simple 3-inch diameter pipe below the agitator. Adipic acid was dry-fed to the spray tower effluent hold tank. This was accomplished by manually adding one-pound increments hourly to maintain specified concentration, usually totaling only a few pounds per hour. A small screw feeder would serve the purpose in a full-scale plant. 

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

Limestone Long-Term Test with One Scrubber Loop and Without Forced Oxidation. Perhaps the most straightforward illustration of the effectiveness of adipic acid is demonstrated by a long-term limestone test conducted on the Shawnee TCA system, in which the additive was introduced without any system modifications. 

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. 

Average SO2 removal improved from 83 percent at 2,350 ppm average inlet SO2 concentration for the base case run, to 93 percent SO2 removal at the higher inlet SO2 concentration of 2,900 ppm with 615 ppm adipic acid, and to 97.5 percent removal at 2,750 ppm inlet SO2 with 1,305 ppm adipic acid. Thus, with 600 to 1,300 ppm adipic acid, SO2 removal improved by 10 to 15 percent over the base case removal of 83 percent at 50 gal/Mcf liquid-togas ratio and 7.0 scrubber inlet pH. 

Lime Test with One Scrubber Loop and Forced Oxidation. With-in-scrubber-loop forced oxidation in a single-loop scrubbing system would not be expected to give good SO2 removal for a lime scrubber because of the oxidation of the major scrubbing species, sulfite ion, into nonreactive sulfate ion. With adipic acid addition, however, satisfactory SO2 removal should be possible because calcium adipate becomes the major scrubbing species. In addition, the lower pH at which a lime/adipic acid system operates should facilitate sulfite oxidation. 

Table 5 lists the test results of such a lime run, Run 951-2E, using within-scrubber-loop forced oxidation with 1,330 ppm adipic acid. The system configuration used for this run was the same as that shown in Figure 3, except that the oxidizing air was injected into the effluent hold tank (oxidation tank). A single... 

Limestone Long-Term Test with One Scrubber Loop and Forced Oxidation. A one-scrubber-loop system has an inherent advantage over a two-scrubber-loop system in its simple design and lower capital and operating costs. If a simple one-loop limestone (or lime) system is operated with adipic acid, which offers the advantage of lower operating pH, then both good SO2 removal and sulfite oxidation can be achieved with minimum cost. 

This was illustrated in a long-term adipic acid-enhanced limestone run, Run 917-1A, conducted on the Shawnee spray tower system from December 26, 1980, to March 13, 1981. Figure 4 shows the flow diagram for this long-term run with forced oxidation using two series tanks in the slurry loop. Oxidation was forced in the first tank while fresh limestone was added to the second. Use of two tanks in series in a within-scrubber-loop forced oxidation system has several advantages over a single tank ... 

The run was controlled at a scrubber inlet pH of 5.0 to 5.1 and an adipic acid concentration of 1,300 to 1,700 ppm to obtain 90 percent or higher SO2 removal. The flue gas flow rate was varied between 20,000 and 35,000 acfm (5.4 to 9.4 ft/sec superficial gas velocity) according to a "typical daily boiler load cycle." The slurry flow rate was fixed at 2,400 gpm (L/G = 85 to 150 gal/Mcf). Slurry solids concentration was controlled at 15 percent. 

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