|
Abstract:
|
The solvent 7 molal (m ) methyldiethanolamine (MDEA ) /2 m piperazine (PZ ) presents an attractive option to industry standard solvents including monoethanolamine (MEA ) for carbon dioxide (CO₂ ) capture in coal -fired power plant flue gas scrubbing applications . The solvent was tested under thermal and oxidizing conditions , including temperature cycling in the Integrated Solvent Degradation Apparatus (ISDA ) , to measure rates of degradation for comparison to other solvents . Unloaded 7 m MDEA /2 m PZ was generally thermally stable up to 150 °C , exhibiting very low loss rates . However , at a loading of 0 .25 mol CO2 /mol alkalinity , loss rates of 0 .17 ± 0 .21 and 0 .24 ± 0 .06 mM /hr , respectively , for MDEA and PZ were measured . No amine loss was observed in the unloaded blend . Thermal degradation was modeled as first -order in [MDEAH⁺] , and a universal Ea for amine loss was estimated at 104 kJ /mol . An oxidative degradation model for 7 m MDEA was developed based on the ISDA data . From the model , the rate of amine loss in 7 m MDEA /2 m PZ was estimated at 1 .3 X 10⁵ kg /yr , based on a 500 MW power plant and 90 % CO₂ capture . In terms of amine loss , the solvent can be ranked with other cycled solvents from greatest to least as follows : 7 m MDEA >7 m MDEA /2 m PZ >8 m PZ . Thermal degradation pathways and mechanisms for 7 m MDEA /2 m PZ include SN2 substitution reactions to form diethanolamine (DEA ) , methylaminoethanol (MAE ) , 1 -methylpiperazine (1 -MPZ ) , and 1 ,4 -dimethylpiperazine (1 ,4 -DMPZ ) . The formation of the amino acids bicine and hydroxyethyl sarcosine (HES ) has been directly tied to the formation of DEA and MAE , respectively , through oxidation . As a result of the construction and operation of the ISDA for cycling of solvents from an oxidative reactor to a thermal reactor , several practical findings related to solvent degradation were made . The ISDA results demonstrated that increasing dissolved oxygen in solvents leaving the absorber will increase the rate of oxidation . A simple N2 gas stripping method was tested and resulted in a reduction to 1 /5th the high temperature oxidation rate associated with dissolved oxygen present in the higher temperature regions of an absorber /stripper system . The ISDA experiments also demonstrated the need to minimize entrained gas bubbles in absorber /stripper systems to control oxidation . When the ISDA was modified to intercept entrained gas bubbles , the oxidation rate was reduced 2 to 3X . |