Degradation of aqueous methyldiethanolamine by temperature and oxygen cycling

The primary degradation products identified in aqueous 7 molal (m) methyldiethanolamine (MDEA) loaded to 0.1 mol CO2/mol alkalinity and degraded in the Integrated Solvent Degradation Apparatus (ISDA) were diethanolamine (DEA), bicine and formate. DEA and bicine represented 43 and 9 % of the carbon loss, respectively, after the solvent was cycled for 167 hours. An MDEA loss of 8.8 mM/hr and a formate production of 0.6 mM/hr were measured when 7 m MDEA cycled from 55 to 120 °C in the ISDA in its initial design, which allowed bubble entrainment. When bubble entrainment was minimized through coalescence and removal, the MDEA loss and formate production were cut in half (4.6 mM/hr and 0.31 mM/hr). When dissolved oxygen was stripped from cycled MDEA with N2 gas at 2 L/min, the MDEA loss was negligible, and the formate production was reduced to 0.05 mM/hr. Designers of CO2 scrubbing systems for post-combustion capture should minimize dissolved and entrained oxygen carryover into the stripper. Stripping of entrained and dissolved oxygen before the stripper is recommended to avoid high temperature oxidation. An oxygen solubility limit exists at a thermal reactor temperature of 120 °C which can be expressed as an apparent upper limit of dissolved O2 available to degrade ∼1.3 mM MDEA/pass. The oxidative degradation model compensates for the complete consumption of dissolved oxygen in the thermal reactor at higher temperatures (>100 °C). The model assumes that all oxidative degradation is occurring in the thermal reactor under plug-flow reactor (PFR) behavior, and compensates for complete oxygen consumption with a stoichiometric factor (S). The regressed values of S, Ea, and ko for formate were 0.1, 107 kJ/mol, and 2.4 1/hr, respectively. The predicted rates of formate production approximately match the measured rates over the entire measured temperature range of 55 to 120 °C. Oxidation Inhibitor A is ineffective over the temperature range of 90 to 120 °C in cycled solvents