Thermal Degradation of Aqueous Piperazine for CO2 Capture. 1. Effect of Process Conditions and Comparison of Thermal Stability of CO2 Capture Amines

Abstract

The effect of process conditions on the rate of thermal degradation of concentrated, aqueous piperazine (PZ) was investigated. At 150 °C, 8 m (m) PZ degrades with a first order rate constant, k1, of 6.1 × 10–9 s–1. Thermal degradation of 8 m PZ with 0.3 mol CO2/mol alkalinity demonstrated an Arrhenius dependence on temperature with an activation energy of 184 kJ/mol. Degradation at 175 °C was negligible with no dissolved CO2, while the k1 increased from 65 to 71 × 10–9 s–1 at 0.1 to 0.4 mol CO2/mol alkalinity and decreased to 24 × 10–9 s–1 at 0.47 mol CO2/mol alkalinity. In an industrial system with a simple stripper, losses due to thermal degradation are expected to be 0.043 mmol PZ/mol CO2 captured. In the case of a 2-stage flash, losses are expected to be only 0.0086 mmol PZ/mol CO2 captured. A Maximum Estimated Stripper Temperature (MEST) was calculated for a variety of amines to provide the same thermal degradation rate of MEA at 120 °C based on first order rate constants for amine loss during thermal degradation and the expected Arrhenius dependence on temperature for all amines. Substituted and unsubstituted 6-member amine rings were found to be the most thermally stable.