Selection of new absorbents for carbon dioxide capture

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Abstract

This work focuses on selecting new absorbents for CO2 capture. Absorption of CO2 was studied at 40 °C using both single and mixed amine-based absorbents. The experimental results show that most absorbents tested have a poorer performance than MEA, but that aqueous AEEA might be a possible contender. In addition to the absorption measurements, the VLE of CO2 in the selected absorbent, the aqueous 2.9 M AEEA, were studied at 40 and 120 °C. The equilibrium partial pressures of CO2 in the aqueous 2.9 M AEEA at the temperature of removal (40 °C) and that of regeneration (120 °C) are lower than for aqueous 5.0 M MEA, but the maximum net cyclic capacity is somewhat higher.

Introduction

Removal of acidic gases, e.g., carbon dioxide (CO2), is an important industrial operation. Carbon dioxide is produced in large quantities by many important industries such as fossil fuel fired power plants, steel production, chemical and petrochemical manufacturing, cement production and natural gas purification. The reasons for CO2 removal are traditionally technical and economical concerns. Carbon dioxide present in natural gas will reduce the heating value of the gas, and as an acidic component, it has the potential to cause corrosion in pipes and process equipment and also to cause catalyst poisoning in ammonia synthesis [1]. Natural gas pipe lines usually permit from 1% to 2% CO2 and sometimes as high as 5% [2]. In the past decades, CO2 removal from flue gas streams started as a potentially economic source of CO2, mainly for enhanced oil recovery (EOR) operations. Moreover, CO2 was also produced for other industrial applications such as carbonation of brine, welding as an inert gas, food and beverage carbonation, dry ice, urea production and soda ash industry [3], [4]. However, environmental concerns, such as global climate change, are now focused as one of the most important and challenging environmental issues facing the world community and have motivated intensive research on CO2 capture and sequestration. Carbon dioxide, as one of the greenhouse gases (GHG), is currently responsible for over 60% of the enhanced greenhouse effect, methane (CH4) contributes 20% and the remaining 20% is caused by nitrous oxide (N2O), a number of industrial gases and ozone. Scientific evidence now strongly suggests that increased levels of GHG may lead to higher temperature and cause climate change on a global scale. Various climate models estimate that the global average temperature may rise by about 1.4–5.8 °C by the year 2100 [5].

A wide range of technologies currently exists for separation and capture of CO2 from gas streams as given in Ref. [3]. Such systems have been used in the chemical industry and in the production of technical gases for industrial and laboratory use [6]. Absorption with amine-based absorbents is the most common technology for CO2 removal today. It is a process with considerable inherent problems, particularly when used on large gas flows, e.g., exhaust resulting from fossil fuel fired power stations. The processes are bulky, leading to large investment costs and high energy consumption, and the absorbents in use today are not stable and form degradation products that need to be handled.

A chemical that is to be used as a new commercial absorbent for removal of CO2 will require both a high net cyclic capacity and high reaction/absorption rate for CO2, as well as high chemical stability, low vapor pressure and low corrosiveness. Aqueous solutions of alkanolamines are the most commonly used chemical absorbents for the removal of acidic gases (CO2 and H2S) from natural, refinery and synthesis gas streams. Among them, aqueous monoethanolamine (MEA) as a primary amine has been used extensively for this purpose, especially for removal of CO2. It has several advantages over other commercial alkanolamines, such as high reactivity, low solvent cost, low molecular weight and, thus, high absorbing capacity on a mass basis and reasonable thermal stability and thermal degradation rate. The disadvantages of MEA include high enthalpy of reaction with CO2, leading to higher desorber energy consumption, the formation of a stable carbamate and also the formation of degradation products with COS or oxygen bearing gases, inability to remove mercaptans, vaporization losses because of high vapor pressure and more corrosive than many other alkanolamines and, thus, needs corrosion inhibitors when used in higher concentration [7], [8], [9], [10]. Because of its wide use and advantages compared to other alkanolamines, MEA is set as a base case in this work. The parameters evaluated here are the absorption rate and the cyclic capacity.

This work focuses on selecting new absorbents for CO2 capture using a screening method giving the absorption rate as function of loading, a molar ratio between CO2 absorbed and the absorbent used. The vapor–liquid equilibria (VLE) of CO2 in a selected absorbent at temperature of removal (40 °C) and that of regeneration (120 °C) were also studied. The objective of the work described here is to select new and more acceptable absorbents or absorbent mixtures whose absorption rate and net cyclic capacity are higher than the existing ones, thereby reducing the energy consumption of the removal process.

Section snippets

Materials

The CO2 (min. 99.99 mol.%) and N2 (min. 99.6 and 99.999 mol.%) gases used were obtained from AGA Gas GmbH. The alkanolamines were obtained from Acros Organics and used without further purification. Those selected were monoethanolamine (MEA) – [H2N(CH2)2OH], 2-(butylamino)ethanol (BEA) – [CH3(CH2)3NH(CH2)2OH], N-methyldiethanolamine (MDEA) – [CH3N(CH2CH2OH)2], 2-(methylamino)ethanol (MMEA) – [CH3NH(CH2)2OH], 2-(ethylamino)ethanol (EMEA) – [CH3CH2NH(CH2)2OH], 2-(2-aminoethyl-amino)ethanol (AEEA) –

Screening tests

Absorption rates of CO2 in amine based absorbents and their mixtures were measured at 40 °C. To evaluate the absorption rate of CO2 in single amine based absorbents, BEA, MDEA, EMEA, AEEA, and MMEA, a constant mass basis, 30 mass%, was used for all absorbents. This implies that the molar concentrations (M) were not the same, and generally lower than that of MEA. However, the optimal operational concentrations for these absorbents are not known a priori so a comparison based on mass fraction might

Conclusions

An apparatus for rapid screening of CO2 absorption chemicals has been developed, and a range of absorbents was tested. In general, the main absorption characteristics of absorbents for CO2 removal are the absorption rate and the absorption capacity of CO2. The experimental results show that AEEA seems to be a potentially good absorbent for capturing CO2 from low pressure gases according to the above criteria. It offers a high absorption rate combined with high absorption capacity compared to

Acknowledgements

This research was supported financially by the Norwegian Research Council “Klimatek Programme” through the SINTEF Project 661292.01.

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Present address: Department of Chemical Engineering, Gadjah Mada University, Jl. Grafika 2 Jogjakarta, Indonesia 55281.

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