Article pubs.acs.org/IECR
Simultaneous Absorption and Oxidation of Nitric Oxide and Sulfur Dioxide by Aqueous Solutions of Sodium Persulfate Activated by Temperature Yusuf G. Adewuyi* and Nana Y. Sakyi Chemical, Biological and Bioengineering Department, North Carolina Agricultural and Technical State University, Greensboro, North Carolina 27411, United States ABSTRACT: The absorption−oxidation of nitrogen oxide (NO) induced by aqueous solutions of sodium persulfate (Na2S2O8) in the presence of SO2 has been studied in a bubble column reactor operated in semibatch mode. The effects of Na2S2O8 concentration (0.01−0.20 M), temperature (23−70 °C), 1550 ppm gas-phase SO2, and solution pH on NO removal (1000 ppm gas-phase concentration) were investigated. The presence of SO2 dramatically improved NO gas absorption and oxidation while it was itself completely removed. The NO fractional conversions in the presence of SO2 ranged from 77% to 83%, with the greatest effect occurring at lower temperatures (23 and 30 °C). While persulfate concentration of 0.1 M appeared optimal for aqueous NO removal, both in the absence and presence of SO2, significant improvements in NO removal were observed for persulfate concentrations of >0.05 M but antagonistic effects were observed with concentrations of 98% purity) was obtained from Acros Organics, Morris Plains, NJ; 5.0 N sulfuric acid was obtained from LabChem, Inc., Pittsburgh, PA; phosphate buffer (a mixture of NaOH and KHPO4, pH 7.0 ± 0.02 at 24 °C) was obtained from Fisher Scientific, Pittsburgh, PA; sodium hydroxide (pellets, >97%), ACS reagent, was obtained from 11703
dx.doi.org/10.1021/ie401649s | Ind. Eng. Chem. Res. 2013, 52, 11702−11711
Industrial & Engineering Chemistry Research
Article
Figure 1. Dependence of 1000 ppm NO conversion on initial Na2S2O8 aqueous concentration at 50 °C in the presence and absence of ∼1550 ppm SO2 at times ≥1000 s.
Figure 2. NO concentration profiles for 0.1 M Na2S2O8 in the presence and absence of ∼1550 ppm SO2 at different temperatures: (a) 23, (b) 30, (c) 40, (d) 50, (e) 60, and (f) 70 °C.
3. RESULTS AND DISCUSSION
scrubber is determined from the percent fractional removal of feed gas, NO, or SO2; i.e., the fractional conversion defined as
Several sets of experiments at various temperatures (23, 30, 40, 50, 60, and 70 °C (±1 °C)) and persulfate (0.02, 0.05, 0.10, and 0.20 M (±0.5%)) concentrations were conducted to investigate the aqueous removal of NO (1000 ppm initial gasphase) in the absence and presence of 1550 ppm SO2 and the effect of solution pH. The efficiency (Ef) of the aqueous
⎛ [gas]out ⎞ Ef = 1 − ⎜ ⎟ ⎝ [gas]in ⎠
(4)
where [gas]in and [gas]out ([gas]out = (gas)in × (1 − conversion)) are steady-state calibrated gas concentration values as recorded from the FTIR analyzer in parts per million 11704
dx.doi.org/10.1021/ie401649s | Ind. Eng. Chem. Res. 2013, 52, 11702−11711
Industrial & Engineering Chemistry Research
Article
(ppm) of NO and SO2 at the inlet and outlet of the bubble column reactor, respectively. 3.1. Effect of SO2 on NO Absorption and Oxidation. SO2 is always present with NO with concentrations of ∼2000 ppm and ∼800 ppm for SO2 and NOx, respectively, in typical flue gas systems.36 In previous separate and combinative studies using peroxomonosulfate solutions and ultrasound to simultaneously absorb and oxidize NO (50−1040 ppm) and SO2 (52− 4930 ppm), it was determined that low to moderate concentrations of SO2 (990−2520 ppm) increased the overall fractional conversion of NO while complete removal of the SO2 itself was observed for all concentration range studied.4,12 To test the effectiveness of the aqueous persulfate for NO scrubbing in the presence of SO2 and understand its effects, several experiments were conducted with a NO−SO2−N2 gas blend containing initial SO2 and NO gas-phase concentrations of ∼1550 ppm and ∼1000 ppm, respectively. Several sets of experiments were also conducted at different temperatures (23−70 °C (±1 °C)) and persulfate concentrations (0.02, 0.05, 0.1, and 0.2 M (±0.5%)) to investigate the effect of NO removal in the presence and absence of SO2. Figure 1 illustrates the steady-state (at times ≥1000 s) fractional conversion of NO, as a function of persulfate concentration at 50 °C. As shown in this figure, the NO conversion in or without the presence of SO2 increased sharply with persulfate up to ∼0.10 M, beyond which changes are insignificant. It can be observed that solutions of lower persulfate concentrations absorbed significantly less NO gas than higher concentrations when SO2 was present, and a substantial increase in conversion occurred when the persulfate concentration was increased from 0.05 M to 0.10 M, compared to the case in the absence of SO2. The results of this study indicate that, while the absorption capacity of the 0.10 M Na2S2O8 scrubbing solution with or without SO2 is sufficient to maintain a constant absorption rate throughout the experiment without being significantly depleted, SO2 further improved NO conversion at higher persulfate concentration (≥0.10 M). At 0.10 and 0.20 M Na2S2O8 and in the presence of SO2, conversions of up to 80% and 74%, respectively, were observed, at 50 °C, compared to 52% and 61%, respectively, in the absence of SO2. The slight decrease in NO conversion with increase in persulfate concentration from 0.1 M to 0.20 M is possibly due to self-recombination and intercombination of oxidative sulfate and hydroxyl (SO4•− and •OH) free radicals resulting from higher production of sulfate radicals, bceause of the combined effects of higher temperatures and higher persulfate concentrations, in addition to the competition between NOx and S(IV) species for oxidative species. While persulfate concentration of 0.10 M appeared optimal for aqueous NO removal both in the absence and presence of SO2, significant improvements in NO removal were observed for persulfate concentrations exceeding 0.05 M but antagonistic effects were observed with concentrations of
