MDEA/Piperazine as a solvent for CO2 capture

The solvent blend methyldiethanolamine/piperazine (MDEA/PZ) has been investigated as an alternative for CO(2) capture from coal-fired power plants. MDEA/PZ offers advantages over monoethanolamine (MEA) and MDEA alone because of its resistance to thermal and oxidative degradation at typical absorption/stripping conditions. We measured thermal degradation rates of MDEA and PZ of -7 +/- 20 mmolal/day and -9 +/- 5 mmolal/day, respectively, in a loaded 7 m MDEA/2 m PZ solvent blend at 120 C. At 135 C, the PZ degradation rate in the loaded solvent blend is -44 +/- 2 mmolal/day, which follows the appearance of unidentified diamine compounds. When sparged with 98% O(2) at 55 degrees C, 7 m MDEA/2 m PZ with 1 mM Fe(2+) produced 0.011 +/- 0.001 mmoles formate/L-hr. At the same conditions, 7 m MDEA produced 0.024 +/- 0.007 mmoles formate/L-hr. We determined that the resistance to oxidative degradation follows the order: MDEA/PZ>MDEA>PZ. The formation of amides in oxidatively degraded samples can be as much as twice the amount of formate produced. In the absence of PZ, MDEA forms amides at an order of magnitude greater rate. The volatility of MDEA in 7 m MDEA/2 m PZ at 40 and 60 degrees C with low CO(2) loading is 6 to 11 ppm and 19 to 30 ppm, respectively. PZ activity decreases by nearly an order of magnitude in the solvent blend as loading of CO(2) is increased to a one-to-one ratio with PZ, giving a PZ volatility at 40 degrees C of 2 to 16 ppm. We calculated a CO(2) capacity of approximately 0.75 moles CO(2)/kg amine+water, as compared to a capacity of 0.5 moles CO(2)/kg amine+water for MEA under comparable conditions in an absorber/stripper configuration. (C) 2009 Elsevier Ltd. All rights reserved.