Piperazine Degradation in Pilot Plants

Abstract

Concentrated, aqueous piperazine (PZ) has shown promise as a solvent for use in amine scrubbing of post - combustion flue gas to reduce CO2 emissions. Compared with 1-ethanolamine (MEA), PZ has a greater CO2 absorption rate, capacity, and thermal stability. These properties should reduce the overall energy requirements of a CO2 capture facility operating with PZ solvent compared to one operating with MEA.Between 2009 and 2011, 4 campaigns were run at the pilot plant at the Separations Research Program (SRP) of the University of Texas using an inventory of 8 m PZ. Synthetic flue gas consisting of CO2 mixed with air was used at a flow rate equivalent to a 0.1 MW coal-fired power plant. Due to the high oxygen content of the synthetic flue gas, substantial oxidative degradation was anticipated in this campaign. Another campaign was run at “Pilot Plant 2” (PP2) using PZ. This pilot plant used a slipstream of real flue gas drawn from a coal-fired power plant. Solvent samples were collected during campaigns at both SRP and PP2.The operating CO2 capacity of the SRP PZ solvent decreased approximately 10% compared to fresh, undegraded 8 m PZ. The heat of CO2 absorption and the CO2 absorption rate at 40°C (absorber operating conditions) did not change significantly, but greater temperature dependence was observed at higher temperature. These property changes indicate that changes in the solvent have occurred, but that the resulting solvent is not significantly worse than clean piperazine.It is known from bench-scale studies that PZ can degrade into a variety of products, including ethylenediamine (EDA), formate, N-formylpiperazine (FPZ), acetate, and 1-(2-aminoethyl)-piperazine (AEP). These products were observed in the degraded solvents of both pilot plants, in small concentrations compared to bench-scale results.As a secondary amine, PZ could potentially react with nitrite to form the known carcinogens N-nitrosopiperazine (MNPZ). Nitrite could accumulate in the solvent either through absorption of NO2 from the flue gas or degradation of the amine solvent. The synthetic SRP flue gas contained only ambient atmospheric levels of NOx. MNPZ was observed in the SRP solvent at a low concentration of 0.09 mmol/kg. The flue gas used in PP2 was drawn from a coal boiler and was treated by selective catalytic reduction to reduce NOx concentration. Samples collected at regular intervals from PP2 show that after an initial spike up to 2.87 mmol/kg early in the campaign, MNPZ settled at a steady state concentration of around 0.9 1.2 mmol/kg. It is hypothesized that thermal degradation of the MNPZ in the stripper prevented further accumulation beyond these levels.