Browsing by Subject "Carbon dioxide capture"
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Item Amine oxidation in carbon dioxide capture by aqueous scrubbing(2013-05) Voice, Alexander Karl; Rochelle, Gary T.; Sexton, Andrew J; Reible, Danny D; Willson, Carlton G; Anslyn, Eric VAmine degradation in aqueous amine scrubbing systems for capturing CO₂ from coal fired power plants is a major problem. Oxygen in the flue gas is the major cause of solvent deterioration, which increases the cost of CO₂ capture due to reduced capacity, reduced rates, increased corrosion, solvent makeup, foaming, and reclaiming. Degradation also produces environmentally hazardous materials: ammonia, amides, aldehydes, nitramines, and nitrosamines. Thus it is important to understand and mitigate amine oxidation in industrial CO₂ capture systems. A series of lab-scale experiments was conducted to better understand the causes of and solutions to amine oxidation. This work included determination of rates, products, catalysts, and inhibitors for various amines at various conditions. Special attention was paid to understanding monoethanolamine (MEA) oxidation, whereas oxidation of piperazine (PZ) and other amines was less thorough. The most important scientific contribution of this work has been to show that amine oxidation in real CO₂ capture systems is much more complex than previously believed, and cannot be explained by mass transfer or reaction kinetics in the absorber by itself, or by dissolved oxygen kinetics in the cross exchanger. An accurate representation of MEA oxidation in real systems must take into account catalysts present (especially Mn and Fe), enhanced oxygen mass transfer in the absorber as a function of various process conditions, and possibly oxygen carriers other than dissolved oxygen in the cross exchanger and stripper. Strategies for mitigating oxidative degradation at low temperature, proposed in this and previous work are less effective or ineffective with high temperature cycling, which is more representative of real systems. In order of effectiveness, these strategies are: selecting an amine resistant to oxidation, reduction of dissolved metals in the system, reduction of the stripper temperature, reduction of the absorber temperature, and addition of a chemical inhibitor to the system. Intercooling in the absorber can reduce amine oxidation and improve energy efficiency, whereas amine oxidation should be considered in choosing the optimal stripper temperature. In real systems, 2-amino-2-methyl-1-propanol (AMP) is expected to be the most resistant to oxidation, followed by PZ and PZ derivatives, then methyldiethanolamine (MDEA), and then MEA. MEA oxidation with high temperature cycling is increased 70% by raising the cycling temperature from 100 to 120 °C, the proposed operational temperature range of the stripper. PZ oxidation is increased 100% by cycling to 150 °C as opposed to 120 °C. Metals are expected to increase oxidation in MEA and PZ with high temperature cycling by 40 - 80%. Inhibitor A is not expected to be effective in real systems with MEA or with PZ. MDEA is also not effective as an inhibitor in MEA, and chelating agents diethylenetriamine penta (acetic acid) (DTPA) and 2,5-dimercapto-1,3,4-thiadiazole (DMcT) are only mildly effective in MEA. Although MEA oxidation in real systems cannot be significantly reduced by any known additives, it can be accurately monitored on a continuous basis by measuring ammonia production from the absorber. Ammonia production was shown to account for two-thirds of nitrogen in degraded MEA at low temperature and with high temperature cycling, suggesting that it is a reliable indicator of MEA oxidation under a variety of process conditions. A proposed system, which minimizes amine oxidation while maintaining excellent rate and thermodynamic properties for CO₂ capture would involve use of 4 m AMP + 2 m PZ as a capture solvent with the stripper at 135 °C, intercooling in the absorber, and use of a corrosion inhibitor or continuous metals removal system. Reducing (anaerobic) conditions should be avoided to prevent excessive corrosion from occurring and minimize the amount of dissolved metals. This system is expected to reduce amine oxidation by 90-95% compared with the base case 7 m MEA with the stripper at 120 °C.Item Measuring and modeling aerosols in carbon dioxide capture by aqueous amines(2016-08) Fulk, Steven Michael; Rochelle, Gary T.; Bonnecaze, Roger T; Chen, Eric; Hildebrandt Ruiz, Lea; McDonald-Buller, ElenaPilot scale CO2 capture plants have shown that amine condensation onto seed nuclei results in very high amine emissions which are very difficult to control using traditional aerosol removal techniques. Aerosol emissions can be suppressed by adjusting operating conditions such that drops evaporate, or, alternatively, grow to a size that can be efficiently captured by low cost methods. The effects of operating conditions on aerosol growth were investigated by experimental measurement and numerical modeling with sensitivity analyses. Total particle densities and particle size distributions (PSDs) were measured using a custom-built phase Doppler interferometer (PDI) on bench and pilot scale CO2 absorbers. Seed nuclei were generated using vaporized H2SO4, gaseous SO2, and flue gas from a coal-fired power plant. PSDs were used to calculate the aerosol amine concentration when compared to total phase (gas and aerosol) measurements collected by FTIR. The effects of operating conditions on aerosol growth were simulated in a combined heat and mass transfer model coded in MATLAB®. Aerosol transport equations included corrections for surface curvature and transport length scale regimes. Absorber and water wash models were simulated using Aspen Plus®. Inlet CO2 is crucial in creating supersaturation in the absorber; the loading difference between the aerosol and bulk solvent creates an amine driving force for condensation. Aerosols grow faster in non-intercooled columns due to differences in solvent composition (CO2 loading) and temperature. H2O condensation is the primary growth mechanism in the water wash. Reducing the water wash amine concentration and providing additional residence time leads to more aerosol growth. Doubling the water wash height results in a 13.7 % increase in the final aerosol diameter for a generic 8 m PZ absorber. Similar to some other volatile amines, PZ forms 1–5 μm aerosols because its amine volatility is a strong function of CO2 loading. The amine concentration in measured aerosol distributions, calculated by PDI/FTIR comparison, was one-to-two orders of magnitude lower than the bulk solvent. SO2 forms aerosol with PZ. 65 % of injected SO2 leaves in the aerosol phase. Therefore, SO2 polishing scrubbers are essential and systems should not be designed for simultaneous absorption of CO2 and SO2.Item Thermal degradation and oxidation of aqueous piperazine for carbon dioxide capture(2011-05) Freeman, Stephanie Anne; Rochelle, Gary T.; Maynard, Jennifer A.; Reible, Danny D.; Katz, Lynn E.; Critchfield, JamesAbsorption-stripping with aqueous, concentrated piperazine (PZ) is a viable retrofit technology for post-combustion CO2 capture from coal-fired power plants. The rate of thermal degradation and oxidation of PZ was investigated over a range of temperature, CO2 loading, and PZ concentration. At 135 to 175 °C, degradation is first order in PZ with an activation energy of 183.5 kJ/mole. At 150 °C, the first order rate constant, k1, for thermal degradation of 8 m PZ with 0.3 mol CO2/mol alkalinity is 6.12 × 10-9 s-1. After 20 weeks of degradation at 165 °C, 74% and 63%, respectively, of the nitrogen and carbon lost in the form of PZ and CO2 was recovered in quantifiable degradation products. N-formylpiperazine, ammonium, and N-(2-aminoethyl) piperazine account for 57% and 45% of nitrogen and carbon lost, respectively. Thermal degradation of PZ likely proceeds through SN2 substitution reactions. In the suspected first step of the mechanism, 1-[2-[(2-aminoethyl) amino]ethyl] PZ is formed from a ring opening SN2 reaction of PZ with H+PZ. Formate was found to be generated during thermal degradation from CO2 or CO2-containing molecules. An analysis of k1 values was applied to a variety of amines screened for thermal stability in order to predict a maximum recommended stripper temperature. Morpholine, piperidine, PZ, and PZ derivatives were found to be the most stable with an allowable stripper temperature above 160 °C. Long-chain alkyl amines or alkanolamines such as N-(2-hydroxyethyl)ethylenediamine and diethanolamine were found to be the most unstable with an allowable stripper temperature below 120 °C. Iron (Fe2+) and stainless steel metals (Fe2+, Ni2+, and Cr3+) were found to be only weak catalysts for oxidation of PZ, while oxidation was rapidly catalyzed by copper (Cu2+). In a system with Fe2+ or SSM, 5 kPa O2 in the inlet flue gas, a 55 °C absorber, and one-third residence time with O2, the maximum loss rate of PZ is expected to 0.23 mol PZ/kg solvent in one year of operation. Under the same conditions but with Cu2+ present, the loss rate of PZ is predicted to be 1.23 mole PZ/kg solvent in one year of operation. Inhibitor A was found to be effective at decreasing PZ loss catalyzed by Cu2+. Ethylenediamine, carboxylate ions, and amides were the only identified oxidation products. Total organic carbon analysis and overall mass balances indicate a large concentration of unidentified oxidation products.