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 .
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.
An experimental process of separating a mixed gas of NH 3 /CO 2 ) 2 (mole ratio) contains a first section, which is a two-step series bubble absorber for selective absorption of NH 3 under 80-140 m/s inlet velocity, and a second section, which is a free NH 3 recovery of the obtained aqueous solution followed by desorption of the solution. In the two-step series absorber, the total absorptivity of NH 3 reaches 96.7% and the total absorptivity of CO 2 is only 33.1%, so the exit content of CO 2 for the second step absorber reaches 90.9% (by mole). Free NH 3 of the solution is recovered to liquid NH 3 with a purity of 99.9% under 1.7-1.9 MPa, and the remaining solution is then desorbed to NH 3 and CO 2 , which is returned to the front first absorber for continuing separation. Then the discharged liquid contained 0.014% NH 3 and 0.006% CO 2 ; it can be returned to the absorption system to be used. From this experimental condition, the heat consumption for the separating process is estimated at 3.4 kg of steam of 1 MPa/kg of NH 3 .
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