Absorption of CO2 in the primary amine MEA has been studied extensively [13-22]. All the literatures reported a first order dependence of reaction rate of CO2 with MEA. Blauwhoff et al. [14] studied a large number of rate data available in the literature for CO2 and concluded that the rate expression of Hikita et al. [18] fits the data well over the temperature range 278 – 353 K. More recently, Barth et al. [15] made kinetic studies, and found that their results compared very well with the previous literature data. Barth et al. 15] also studied the reaction rate of CO2 with diglycolamine (DGA®) at temperatures of 293 and 298 K. Interestingly, they found the rates to be indistinguishably similar to MEA.
DEA, a secondary alkanolamine, is a widely popular solvent in the gas treating industry because like MEA it has a relatively high reactivity towards CO2. On the other hand, DEA is relatively less corrosive than MEA and has a lower exothermic heat of reaction. Literature covering absorption of CO2 in DEA is also extensive [14, 15, 17-19, 21, 23-31]. However, there is a general disagreement with the order of the reaction with respect to DEA as reported by several researchers. The reason for this discrepancy is most likely due to the assumption of simplified mechanism for the CO2-DEA system. Blauwhoff et al. [14] found that the zwitterion mechanism resolved much of the discrepancies in the literatures. The zwitterion mechanism, which was originally proposed by Caplow [32] and reintroduced by Danckwerts [16], are the most widely accepted mechanism for primary and secondary amine reactions with CO2 [14, 30, 31, 33–36]. There are two limiting cases in the zwitterion mechanism.
When the zwitterion formation reaction is rate determining, the reaction rate appears to be first order in both the amine and CO2 concentrations. In the case of MEA, a primary alkanolamine, the formation of zwitterion has been shown to be the rate-determining step [14, 31, 34, 37]. On the other hand, when the zwitterion deprotonation reactions are rate limiting, the overall reaction rate appears to have a fractional order between 1 and 2 in the amine concentration. Several authors have reported rate coefficients for this limiting case of the zwitterion mechanism for DEA and DIPA [14, 31, 33]. Fractional orders are usually observed only for reactions between CO2 and secondary amines [30, 34, 37]. Rinker et al. [28] studied the kinetics of the reaction of CO2 and DEA over the temperature range of 293 to 343 K using a laminar-liquid jet absorber. A rigorous numerical mass transfer model based on penetration theory in which all chemical reactions are considered to be reversible was used to estimate kinetic rate coefficients from their experimental absorption data. The kinetic data found by Rinker et al. [28] were claimed to be consistent with the zwitterion mechanism.
Although absorption of CO2 in MEA has been studied extensively so far, researchers worldwide are still working with MEA to study different modeling aspects and different gas- liquid contactors. Freguia and Rochelle [38] studied and modeled a process for CO2 removal from flue gases using 30 wt. % MEA in water with RateFrac which consisted of an absorber, a stripper, and a cross heat exchanger. They described how the design variables affect each other at the level of the whole process. They built a model using Aspen Plus™ to analyze the
effect of several process variables on energy requirements and to find operating conditions that allowed CO2 removal with less energy. For that they performed sensitivity analyses on process variables to find operating conditions at low steam requirement. From an overall optimization they found that many variables strongly affected the process performance, but, there were no economical ways to reduce the steam requirements by more than 10%. They also reported that the power plant lost work was affected by varying stripper pressure, but not significantly, so any convenient pressure could be chosen to operate the stripper. Migita et al.
[39] examined the performance of a noble gas absorber ‘wetted-wire column’ for the absorption of CO2 into aqueous solutions of 15 wt. % and 30 wt. % MEA. The gas–liquid contact device was equipped with 109 built-in vertically oriented wires. They confirmed that the wetted-wire column had an absorption performance quite comparable to conventional packed-bed columns. They also reported a pressure loss smaller by one to two orders than that in packed-bed columns of the same height. Akanksha et al. [40] experimentally analyzed the absorption of CO2 by MEA using a continuous film contactor. They proposed a numerical scheme simulating the results based on momentum, mass and heat balance to provide a mechanistic interpretation of the experimental results, and a means to predict the gas- absorption performance at arbitrary adjustments of operational parameters such as reactants (gas and liquid) concentration, flow rate of the absorbent, and flow rate of the gas mixture.
Jassim et al. [41] measured the absorption and desorption of carbon dioxide in aqueous solutions of MEA using a rotating packed bed. Their comparison with conventional columns showed the advantages of using rotating packed beds in terms of saving size and space and efficient operation.
Ma’mun et al. [42] studied the absorption of CO2 in 2-((2-Aminoethyl)amino)ethanol (AEEA), a diamine containg primary and secondary amino groups. The reaction kinetics between CO2 and aqueous solutions of AEEA were measured in the range of temperatures of 305 to 322 K with concentrations of AEEA ranging from 1.19 - 3.46 kmol m-3, using a string- of-disks contactor. The results were interpreted using the single-step-termolecular mechanism approach.