Chakraborty et al. [47] studied the absorption of CO2 in AMP in a pressure decrease cell (PDC) and continuous flow cell (CFC) at 315 K. On the basis of 13C NMR spectra of liquid samples at equilibrium, they have concluded the reaction to be amine catalyzed hydration of CO2, since they did not observe any carbamate peak in the spectrum. The rate data was found out by them from the transient part of the absorption experiment in the PDC. For the CO2- AMP reaction they have reported the order with respect to both CO2 and AMP as unity. Yih and Shen [48] investigated the kinetics of CO2-AMP system by gas absorption in a wetted wall column at 313 K. They analyzed their results using the methodology of “gas absorption with fast pseudo-first-order reaction”. They found out the order with respect to both CO2 and AMP for the CO2-AMP reaction to be unity. The second order reaction rate constant was reported by them as 1270 m3 kmol-1 s-1 at 313 K. They concluded that the carbamate reaction might still have significant effect on the overall CO2-AMP reaction even though the value of the carbamate stability constant as reported by Sartori et al. [11] was very low.
Bosch et al. [49] analyzed the results of Chakraborty et al. [47] based on the numerical method developed by Versteeg et al. [50] for parallel reversible reactions. They demonstrated that the absorption of CO2 in hindered amines could be explained in terms of the established mechanism for the reaction of CO2 with conventional alkanolamines. Bosch et al. [51] also studied the absorption of CO2 into aqueous solutions of AMP under reaction controlled conditions in a stirred vessel at 298 K. They have observed that the CO2-AMP reaction could be described according to the generally accepted zwitterion mechanism.
On the basis of the results, of Bosch et al. [51], Alper [52] speculated that the CO2-AMP reaction proceeded according to the accepted zwitterion mechanism to form carbamate ion, with the zwitterion formation reaction being possibly the rate controlling step. The carbamate ion was then hydrolyzed into bicarbonate ion so that the final reaction mixture had no or little carbamate ion. This reconciled the findings of 13C NMR spectra of CO2-AMP equilibrium mixtures reported by Chakraborty et al. [47]. To avoid the confusion of mass transfer in gas absorption, Alper [52] used the stopped flow technique to study the kinetics of the homogeneous reaction between aqueous solutions of CO2 and AMP. On the basis of the results Alper [52] proposed a correlation for the second order reaction rate constant
2 AMP CO −
k as a function of temperature. The corresponding value of activation energy was
found to be 41.7 kJ mol-1. Alper’s [52] predicted value of 1165 m3 kmol-1 s-1 for AMP
CO2−
k at
313 K agrees well with 1270 m3 kmol-1 s-1 reported by Yih and Shen [48] at the same temperature. However, the AMP
CO2−
k value of 520 m3 kmol-1 s-1 predicted by Alper [52] from the correlation at 298 K, is somewhat smaller than 1048 m3 kmol-1 s-1 at 298 K reported by Sharma [45]. Saha [22] and Saha et al. [53] investigated the kinetics of CO2-AMP system by gas absorption in a wetted wall column. The reaction of CO2 with AMP was found out to be first order with respect to both CO2 and AMP. The values of the second order rate constants for the CO2-AMP reaction were determined as 437, 681, 1183 and 1636 m3 kmol-1 s-1 at 294, 301.5, 311.5 and 318 K, respectively. An activation energy of 43 kJ mol-1 was obtained for the CO2-AMP reaction.
Camacho et al. [54] analyzed the process of CO2 absorption at high partial pressures, in aqueous solutions of AMP, with respect to the thermal effects involved. They carried out experiments in a stirred tank reactor with known interfacial area. They reported that the absorption process of pure CO2 into aqueous solutions of AMP took place in the instantaneous nonisothermal regime at low concentrations, whereas at high concentrations the regime might be fast. In the experiments at low amine concentrations, they proposed an equation that enabled the evaluation of the rise in temperature in the gas–liquid interface. At high concentrations, they determined a reaction order of one with respect to the amine and developed an expression for the kinetic constant valid throughout the entire range of temperatures and concentrations assayed.
Gabrielsen et al. [55] proposed an explicit model for CO2 solubility in an aqueous solution of AMP and developed an expression for the heat of absorption of CO2 as a function of loading and temperature. A rate-based steady-state model for CO2 absorption into an AMP solution was proposed, using both the proposed expression for the CO2 solubility and the expression for the heat of absorption along with an expression for the enhancement factor and physicochemical data from the literature. They successfully applied the proposed model to absorption of CO2 into an AMP solution in a packed tower and validated against pilot-plant data from the literature. Aboudheir et al. [56] developed a rigorous computer model for the simulation of the absorption of CO2 in aqueous AMP solutions in a packed absorption column
also taking into account the heat effects. Their model predicted the concentration and the temperature profiles along the packed column for the CO2-AMP system. They compared those profiles with the experimental data obtained from two pilot-plant studies.
Zhang et al. [57] developed a rigorous model for the absorption of CO2 into aqueous solutions of AMP at a temperature of 303 K using a double stirred-cell absorber with a planar gas-liquid interface. It was demonstrated that the kinetics region of absorption of CO2 into aqueous AMP was the fast pseudo-first order reaction regime. They used mass transfer- reaction kinetics equilibrium model according to the film theory to represent CO2 absorption into aqueous AMP. The proposed model can handle the prediction much more effectively when CO2 loading is much smaller.
Xu et al. [58] determined the kinetics for the reaction between CO2 and AMP from measurements of the rate of absorption of CO2 into both aqueous and nonaqueous (1- propanol) AMP solutions using a stirred-cell reactor for temperatures from 288 to 318 K and over the concentration range of 0.25–3.5 kmol m−3 of AMP.. The zwitterion mechanism was found to be suitable for modelling the absorption of CO2 into the aqueous and organic (1- propanol) solutions of AMP. The order of reaction in amine was reported to be greater than one for both cases.
There is very little information available regarding the absorption of CO2 in aqueous solution of 2-PE. Shen et al. [59] studied the kinetics of absorption of CO2 into aqueous solution of 2-PE at 303 K within the amine concentration range of 0.2-1.0 kmol m-3 using a wetted-wall column. They found a second order forward rate constant of 195 m3 kmol-1 s-1 at 303 K. Xu et al. [60]explored the kinetics of absorption of CO2 into aqueous solutions of 2-PE at temperature range of 283 – 313 K within the amine concentration range 0.25-2.5 kmol m-3 using a stirrer cell absorber. They reported a much higher second order forward rate constant of 1468 m3 kmol-1 s-1 than that found by Shen et al. [59]. Both of them described the reaction according to zwitterionic mechanism.
Recently, many studies have been made on identifying new sterically hindered amines to reduce the total capital and operating costs in the CO2 absorption process. Mimura et al. [61]
investigated the kinetics of the reaction of CO2 with secondary sterically hindered amines viz., methylaminoethanol (MAE), n-butylaminoethanol (BAE) and ethylaminoethanol (EAE) (also known as N-ethylethanolamine (EEA)), for CO2 recovery from power plant flue gases using a stirred tank absorber with a plane unbroken gas-liquid interface at 298 K. They reported a comparable second order rate constant for EEA with MEA at 298 K. Baek and Yoon [62]
have proposed 2-amino-2-methyl-1,3-propanediol (AMPD), a primary hindered amine, as a potential absorbent in the solubility study of CO2 in amine solutions. Yoon et al. [63, 64]
studied the kinetics of absorption of CO2 into aqueous solution of 2-amino-2-ethyl-1,3- propanediol (AEPD) and AMPD, which are primary hindered amines, within the amine concentration range of 5 – 25 mass% using a wetted-wall column. The reactions of CO2 in both of these two solvents were described by zwitterionic mechanism.