Mass transfer and kinetic characteristics for CO₂ absorption in microstructured reactors using an aqueous mixed amine

Microreactors are frequently used for the process intensification of CO₂ absorption, but most CO₂ absorption studies in microreactors are restricted to a low flow rate ratio of feed gas to absorbent because of insufficient CO₂ loading of conventional amines. Herein, we investigated mass transfer and kinetic characteristics of CO₂ absorption using an aqueous mixed amine absorbent composed of 25 wt.% N-methyldiethanolamine (MDEA) and 5 wt.% hexamethylenediamine (HMDA) at the gas to liquid flow rate ratio range from 88.9 to 666.7 in four microstructured reactors with metal foams as packing materials in the packed-bed section. The CO₂ loading of this amine absorbent reached 0.32 mol of CO₂ per mol of MDEA at the gas to liquid flow rate ratio of 400, which is significantly higher than the conventional amines. Regeneration efficiency of this mixed amine absorbent was remarkable in three successive regenerated cycles at the gas to liquid flow rate ratios of 181.8 and 222.2. The second-order rate constants of HMDA were higher than the conventional amines, reflecting its high ability to reduce energy barrier and accelerate the CO₂ absorption rate. The overall volumetric mass transfer coefficient and absorption efficiency were raised to 13.5 s⁻¹ and 97% at the gas to liquid flow rate ratios of 153.8 and 88.9, which are obviously higher than those reported for other microreactors. Optimization of overall reaction rate constant and mass transfer coefficient was executed through multi-objective optimization using genetic algorithm and validated through experimentation. Finally, the comparison of the mass transfer characteristics among various microreactors indicated that the strategy of combining this remarkable mixed amine absorbent with microstructured reactors with metal foams packing has strong process intensification potential for CO₂ absorption at high gas to liquid flow rate ratios.