American Institute of Chemical Engineers (AIChE) Annual Meeting
The research of coordinative and competitive relationship of CO2 absorption into MEA and DEA in blended aqueous amines
Moxia Li, Helei Liu, Zhiwu Liang *, Raphael Idem, Paitoon Tontiwachwuthikul
Joint International Center for CO2 Capture and Storage (iCCS), Hunan Provincial Key Laboratory for Cost-effective Utilization of Fossil Fuel Aimed at Reducing CO2 Emissions, College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, PR China
Keywords: Blended amines; CO2 absorption; NMR; Coordinative and competitive
* Author for correspondence: Dr. Zhiwu Liang - Email: email@example.com
Tel: +86-13618481627 and +86-73188573033
The extensive usage of fossil fuels by mankind is resulting in increased levels of CO2 in the atmosphere, which causes not only the greenhouse effect, but also leads to global warming. In order to decrease the emission of CO2 by human activities, it is urgent that CO2 capture technology and resource utilization technology be rapidly developed. Meanwhile, CO2 that is captured can also be used to provide economic benefit to many processes in industrial production (e.g. Enhanced Oil Recovery and Carbonated Beverage).1
The technology of chemical absorption with amine solvents, proposed in the 1930s, is widely studied and has become one of the most important industrial methods for CO2 capture due to its fast absorption rate, easy desorption, and low corrosion.2 Monoethanolamine (MEA) and diethanolamine (DEA) are the most common commercial solvents for CO2 capture. However, there are some disadvantages with these amine solvents. Even though MEA has a very fast CO2 absorption rate, the cost of handling, the corrosivity, and the degradation rate of MEA are high. MEA, a primary amine, is sometimes substituted by diethanolamine (DEA), a secondary amine, which has a lower reaction enthalpy and is less corrosive. Some researchers have proposed mixed solvents containing the advantages of different amines as the solvent for CO2 absorption.3
However, the process of the mixed amine reacting with CO2 is very complex due to a coordinative and competitive relationship between the different types of amines as they react with CO2.4,5 A better understanding of the reaction mechanisms for the mixed amine reacting with CO2 plays a vital role in the design of processes for mixed amine absorption of CO2. In this work, the Nuclear Magnetic Resonance (NMR) technology was employed to present the process of the reaction of a mixed solvent (MEA and DEA) with CO2. The process of the reaction with 2M MEA:DEA (1:1) was represented in terms of concentrations of MEACOO-, DEACOO-, HCO3-, CO32- in the CO2 loading range of 0-0.7 mol CO2/mol amine at the temperature of 298K. The concentrations of MEACOO-, DEACOO-, HCO3-, CO32- are calculated6 as follows:
Where Ri (i=0, 1, 2) represents the ratio of the integral of the peak area for different carbon, [ ] is concentration of species in this system, [CO2]0 represents initial CO2 concentration of the mixed amine solution, 汛 represents the chemical shift of HCO3-/CO32- in the mixed amine solution, 168.84 and 161.23 are the chemical shifts of pure CO32- in Na2CO3 solution, and HCO3- in NaHCO3 solution, respectively.
From Figure 1, it can be seen that MEA/MEAH+ have two different carbons, which show different chemical shifts. However, MEACOO- would show three different chemical shifts from MEA due to the existence of the -COO- group. From Figure 2, DEA/DEAH+ has four carbons of two different bonds, which are different from carbons of MEA. Also, DEACOO- has three different carbons because of -COO- group. In this present work, the aqueous blend of MEA and DEA reacting with CO2 has been studied with CO2 loading from 0 to 0.7 mol CO2/mol amine using NMR technology at 298K. From Figure 3 and Figure 4, it can be seen that MEACOO- occurred as soon as CO2 was absorbed by the solution. Meanwhile, DEACOO-, and HCO3- and CO32- have not yet appeared. The concentration of MEACOO- increases with CO2 loading. This means that CO2 reacts only with MEA at the low CO2 loading because MEA is a stronger base compared to DEA and has a stronger ability to react with CO2 The chemical shift of DEACOO- can be seen at the CO2 loading of 0.17 mol CO2/mol amine, which means that DEA has now begun to react with CO2. In order to develop the ion speciation plots, the concentrations of MEACOO-, DEACOO-, HCO3-, CO32- are calculated using equations 1-8, as shown in Figure 4. In Figure 4, it can be found that both MEA and DEA reacted with CO2 indicating that MEA and DEA have a coordinative relationship in reacting with CO2. It can be clearly seen that MEA reacts with CO2 first. When the CO2 loading reaches a certain level (about 0.17 mol CO2/mol amine in this work), DEA begins to react with CO2. This shows that MEA and DEA in the mixed solvent system react with CO2 in sequence. Above 0.17 mol CO2/mol amine, MEA and DEA compete with each other in the process of reacting with CO2.
To conclude, MEA and DEA have both coordinative and competitive relationships when mixed as CO2 absorbents. This investigation provides some useful information to understand the reaction mechanism of CO2 being absorbed into mixed amines.
Figure 1. The structure of MEA /MEAH+ and MEACOO-
Figure 2. The structure of DEA /DEAH+ and DEACOO-
Figure 3. 13CNMR spectrum of MEA-DEA-CO2-H2O, as a function of the CO2 loading at the temperature of 298K.
Figure 4. Ion speciation plots of MEA-DEA-CO2-H2O system at the temperature of 298K.
Acknowledgment: The financial supports from the National Natural Science Foundation of China (Nos. 21476064, 21376067 and U1362112), National Key Technology R&D Program (Nos. 2012BAC26B01 and2014BAC18B04), Innovative Research Team Development Plan-Ministry of Education of China (No. IRT1238), Shanxi Yanchang Petroleum (Group) Co., LTD, Specialized Research Fund for the Doctoral Program of Higher Education (No. 20130161110025),and China's State "Project 985" in Hunan University-Novel Technology Research and Development for CO2 Capture as well as Hunan University to the Joint International Center for CO2 Capture and Storage (iCCS) is gratefully acknowledged.
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