Formation and decomposition of 1-nitrosopiperazine in the CO2 capture process
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Piperazine (PZ) is a cyclic diamine, which means it can absorb two moles of CO2 per mole of amine and potentially has a higher capacity for CO2 capture than monoethanolamine, the current solvent of choice for flue gas treatment. Nitrosamines are formed from the reaction between secondary or tertiary amines and nitrites or nitrogen oxides. Over 80% of nitrosamines are carcinogenic. The reaction of PZ and nitrite can form 1-nitrosopiperazine (also mononitrosopiperazine, MNPZ) and N-N,dinitrosopiperazine (DNPZ). Carcinogenicity of DNPZ is almost 20 times as that of MNPZ. There is also a possibility of nitrosamine formation of PZ in the CO2 capture process because of NOx in input flue gas, with the oxidative and thermal degradation products of PZ. Analytical methods were developed in order to perform kinetic studies of the reaction between a nitrite solution and PZ over a range of temperature from 20 to 150 °C at two different PZ concentrations, 8 and 2 mol/kg of solution, and three levels of CO2 loading, 0.3, 0.2, and 0.1 mole CO2/mole of alkalinity. At less than 75 °C, nitrite reacts with PZ and disappears during the reaction to an equilibrium concentration while at the higher temperature; the concentration of nitrite quickly decreases to a very low value. There is no evidence of DNPZ as a reaction product in all reaction conditions, but MNPZ is formed at the temperature greater than 75 °C. The MNPZ concentration approaches a maximum value consistent with the material balance and nitrite disappearance. By developing the time of reaction at the higher temperature a decomposition of MNPZ has been observed, by either the reverse of the formation reaction or decomposition to other compounds. By increasing the temperature, the maximum value of MNPZ concentration is achieved more quickly and the rate of MNPZ decomposition increases. Reactions follow the same trend at both PZ concentration and at the three different degrees of CO2 loading. A model has been established considering temperature, PZ concentration, and CO2 loading. The calculated activation energies of MNPZ production and decomposition were determined. MNPZ decomposition is more rapid than PZ degradation.