Study Of N-Nitrosopiperazine Formation Inpiperazine-based CO₂ Capture

In recent years,greenhouse gas caused climate change is the greatest environmental problems.CO2 is the main greenhouse gas resource due to burning of fossil-fuels.CO2 emissions should be mitigated to control the global warming.Amine-based chemical absorption is the most extensively studied technology for CO2 capture.Piperazine(PZ)has become a very promising amine for CO2 capture due to its excellent solvent properties and other characteristics,such as fast kinetic rate,high absorption capacity,and low resistance,etc.But unfortunately,PZ can be nitrosated by NOx in the flue gas and the nitrite in the solution to produce N-nitrosopiperazine(MNPZ),which is a carcinogenic compound.In order to reduce the accumulation of MNPZ in the PZ-based CO2 capture system and reduce the possibility of polluting the environment,it is necessary to study the pathways of forming MNPZ in the process of CO2 capture.In this paper,the effect of flue gas composition on the rate of MNPZ formation was investigated.The reaction kinetics of formation of MNPZ by nitrite in PZ solution was studied.The main findings are listed as follows:(1)The effects of each component on nitrite accumulation rate and MNPZ production rate were investigated by changing the concentration of NOx,O2 and CO2 in the simulated flue gas.The experimental results showed that the nitrite accumulation rate and the MNPZ production rate were linearly correlated with the NOx concentration.According to the experimental data,the nitrite accumulation rate was estimated at about 10?30 nmol·L·1·s-1 and the MNPZ production rate at about 5.0?15 nmol-L-1·s-1 when the concentration of PZ was 0.1 mol-dm-3,the CO2 concentration was 4%,the O2 concentration was 10%,and the NOx concentration was 100?300 ppmv,respectively.Even in the absence of CO2,the NOx in the flue gas can directly lead to the nitrosation of PZ,but the presence of CO2 will significantly increased the MNPZ formation rate.O2 was not directly involved in the reaction of PZ nitrosation,but can indirectly affect the rate of nitrite accumulation in PZ solvent and the nitrosation rate of PZ.(2)Under alkaline environmental conditions,formaldehyde can catalyze the reaction between PZ and nitrite to produce MNPZ.The formation rate of MNPZ decreased with the increasing of solution pH,but increased with the increasing of formaldehyde concentration.The kinetic model of MNPZ for the reaction of PZ with nitrite is (?).The activation energy is 78.30 k J/mol with a rate constant of 47.96 dm6·mol-2·s-1 at 50 o C.(3)In the MEA/PZ mixed amine system,the activation energy is 62.29 k J/mol with a rate constant of 74.27 dm6·mol-2·s-1 at 50 o C.The nitrosation of PZ and nitrite in the individual PZ solvent system is more susceptible to temperature than the MEA/PZ mixed amine system.