Amine blends using concentrated piperazine

Blends using concentrated (25-35 wt %) piperazine (PZ) were characterized as solvents for CO₂ capture at typical coal flue gas conditions. The new blends are 6 m PZ/2 m hexamethylenediamine (HMDA), 6 m PZ/2 m diaminobutane (DAB), 6 m PZ/2 m bis(aminoethyl)ether (BAE), 5 m PZ/2 m aminoethylpiperazine (AEP), and 5 m PZ/2.3 m 2-amino 2-methyl-propanol (AMP). The CO₂ absorption rate of the blends was measured using a wetted wall column (WWC). The CO₂ vapor liquid equilibrium was measured at 20-160 °C. Amine vapor pressure measurements are reported to show potential volatility at practical conditions. The rate of thermal degradation was measured from 135 to 175 °C. Oxidative degradation was measured in two semi-batch experiments with different O2 rate at absorber conditions. Advanced parameters are introduced to demonstrate the overall rate and energy performance of the solvents in a real process. The performance of 7 m MEA, 8 m PZ, and six other competitive PZ blends are evaluated based on previous results and presented as basis of comparison. All of the PZ blends have better solubility window than 8 m PZ, with no precipitation at rich loading. The absorption rate of the concentrated PZ blends is similar to that of 8 m PZ and 1.5-2 times higher than 7 m MEA; the solvent capacity is about 20% lower than 8 m PZ and 15% higher than 7 m MEA. Among all of the PZ blends, 5 m PZ/5 m MDEA has the best combination of rate and capacity. Blends using HMDA, AEP, BAE, AMP, and MEA have a high heat of CO₂ absorption. Blends using MEA, MDEA, and AMP are not thermally stable, while other blends have good thermal stability. The combination of high H_abs and thermal stability leads to good overall energy performance, which is observed for 6 m PZ/2 m HMDA, 6 m PZ/2 m BAE and 5 m PZ/2 m AEP. AMP has relatively high volatility, whereas BAE and AEP are expected to have low volatilities. The only drawback of 6 m PZ/2 m BAE is its high oxidation rate. 6 m PZ/2 m HMDA and 6 m PZ/2 m DAB have good oxidative stability. PZ blends using low viscosity amines will have high absorption rate, and tertiary and hindered amines will contribute to high capacity. Amines with high pKa will improve the blend H_abs. For thermal stability, alkanolamines should not be used together with PZ. Highly viscous blends such as 6 m PZ/2 m HMDA are expected to have 10-20% higher cost associated with cross-exchanger design and operation than solvents with low viscosity.