A disk column was used to investigate the kinetics of carbon dioxide absorption in activated methyldiethanolamine (MDEA) solutions which contain piperazine as an activator. Experiments were conducted within the temperature range 30-70 °C, MDEA concentration 1.75-4.21 kmol/m3, and piperazine concentration 0.041-0.21 kmol/m3. The kinetic data agree with a proposed mechanism which can be regarded as a rapid pseudo-first-order reversible reaction between C02 and piperazine in parallel with the reaction between C02 and MDEA. The absorption rate can be expressed as NCo2 = tfco2[-Dco,(fc2Cani + fcpCp)]1/2(Pco2 -Pco2*)> where k2 = 5.86 X 106 exp(-3984/T) and kp = 2.98x id11 exp(-6424/T).

A kinetics study of the absorption of CO 2 into a mixed aqueous solution of methyldiethanolamine (MDEA) and piperazine (PZ) was investigated by a laboratory disk column within 303-343 K and a weight ratio of MDEA to PZ 50/3-50/10 with a total amine concentration of 3.0 M. A renewal mechanism was proposed, and a kinetic model was established on the basis of the existence of piperazine carbamate. The second-order rate constant of CO 2 with PZ was expressed as k p ) 4 × 10 10 exp(-4059.4/T), which was in good agreement with that of Bishnoi and Rochelle (Chem. Eng. Sci. 2000, 55, 5531). The concentrations of various species in solution were also calculated, and the results indicated that the concentration of free PZ was very low for its special reactivity to form the protonated PZ and carbamate in the MDEA solution when encountering CO 2 .

Solubilities of CO2 in aqueous mixtures of methyldiethanolamine (MDEA) and piperazine (PZ) have been measured for temperatures and CO2 partial pressures ranging from 30 to 90 °C and 13.16 to 935.3 kPa, respectively. A modified Deshmukh−Mather thermodynamic model is used to correlate the experimental data with the average deviation of 9.8%, in which the effect of salts on Henry's constant is taken into consideration. The results also indicate that the second-order dissociation reaction for piperazine can be neglected. A simple model is also given with the average deviation of 11.6%.

Solubility data of CO2 and vapor pressure of water in 3.04−4.28 kmol/m3 aqueous N-methyldiethanolamine (MDEA) solutions were obtained at temperatures ranging from 40 to 100 °C and CO2 partial pressures ranging from 0.876 to 1013 kPa. A thermodynamic model was proposed and used for predicting CO2 solubility and water vapor pressure. An enthalpy change of absorption of CO2 in 4.28 kmol/m3 MDEA solution was estimated. The effect of piperazine (PZ) concentration on CO2 loading in MDEA solutions was determined at piperazine concentration ranging from 0 to 0.515 kmol/m3. The results show that piperazine is beneficial to the CO2 loading. The equilibrium partial pressure of piperazine in the PZ−MDEA−H2O system was measured in an Ellis Cell. Results show that the PZ−MDEA−H2O system is a typical negative deviation system, with the strength of deviation decreasing with MDEA solutions.

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