Thermodynamics is important for the design of amine scrubbing CO₂ capture processes. CO₂ solubility and amine volatility in aqueous amines were measured at high temperature and pressure. A rigorous thermodynamic model was developed for MEA-CO₂-H₂O in Aspen Plus®. CO₂ solubility at 80-190°C was obtained from total pressure measurements. Empirical models as a function of temperature and loading were developed for CO₂ solubility from 40 to 160°C in aqueous monoethanolamine (MEA), piperazine (PZ), 1-methylpiperazine (1MPZ), 2-methylpiperazine (2MPZ), PZ/2MPZ, diglycolamine® (DGA®), PZ/1MPZ/1,4-dimethylpiperazine (1,4-DMPZ), and PZ/methyldiethanolamine (MDEA). The high temperature CO₂ solubility data for MEA is comparable to literature and compatible with previous low temperature data. For MEA and PZ, amine concentration does not have obvious effects on the CO₂ solubility. The heat of CO₂ absorption derived from these models varies from 66 kJ/mol for 4 m (molal) PZ/4 m 2MPZ and to 72, 72, and 73 kJ/mol for MEA, 7 m MDEA/2 m PZ, and DGA. The heat of absorption estimated from the total pressure data does not vary significantly with temperature. At 0-0.5 loading ([alpha]), 313-413 K, 3.5-11 m MEA (mol fraction x is 0.059-0.165), the empirical model of MEA volatility is ln(PMEA/xMEA) = 30.0-8153/T-2594[alpha]²/T. In 7 m MEA with 0.2 and 0.5 loading, PMEA is 920 and 230 Pa at 120°C. At 0.3-0.5 loading, the enthalpy of MEA vaporization, -[Delta]Hvap,MEA, is about 70-73 kJ/mol MEA. At 0.25-0.4 loading, 313-423 K, 4.7-11.3 m PZ (x is 0.078-0.169), the empirical model of PZ volatility is ln(PPZ/xPZ) = -123+21.6lnT+20.2[alpha]-18174[alpha]²/T. In 8 m PZ with 0.3 and 0.4 loading, PPZ is 400 and 120 Pa at 120°C, and 2620 and 980 Pa at 150°C. At 0.25-0.4 loading, -[Delta]Hvap,PZ is about 85-100 kJ/mol PZ at 150°C and 66-80 kJ/mol PZ at 40°C. [Delta]Hvap,PZ has a larger dependence on CO₂ loading than [Delta]Hvap,MEA in rich solution because of the more complex speciation/reactions in PZ at rich loading. Specific heat capacity of 8 m PZ is 3.43-3.81 J/(g•K) at 70-150°C. Two new thermodynamic models of MEA-CO₂-H₂O were developed in Aspen Plus® starting with the Hilliard (2008) MEA model. One (Model B) includes a new species MEACOOH and it gets a better prediction than the other (Model A) for CO₂ solubility, MEA volatility, heat of absorption, and other thermodynamic results. The Model B prediction matches the experimental pKa of MEACOOH, and the measured concentration of MEACOO-/MEACOOH by NMR. In the prediction the concentration of MEACOOH is 0.1-3% in 7 m MEA at high temperature or high loading, where the heat of formation of MEACOOH has effects on PCO₂ and CO₂ heat of absorption. Model B solved the problems of Model A by adding MEACOOH and matched the experimental data of pKa and speciation, therefore MEACOOH may be considered an important species at high temperature or high loading. Although mostly developed from 7 m MEA data, Model B also gives a good profile for 11 m (40 wt%) MEA. / text

Amine 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. / text

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