Solubility of N2O and CO2 in non-aqueous systems of monoethanolamine and glycol ethers: Measurements and model representation

doi:10.1016/j.jct.2019.05.017

The Journal of Chemical Thermodynamics

Volume 137, October 2019, Pages 76-85

https://doi.org/10.1016/j.jct.2019.05.017 Get rights and content

Highlights

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Solubility of N₂O into EGME, EGEE and MEA-glycol ether solutions was measured.
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Solubility of CO₂ in 5.0 M MEA-glycol ether solutions was obtained by the N₂O analogy.
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A mathematical model was proposed to interpret the VLE data of CO₂-MEA-glycol ether systems.
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The AADs are within 0.8% for Henry’s constant and 10% for CO₂ solubility, respectively.

Abstract

Non-aqueous mixtures of monoethanolamine (MEA) with glycol ethers as low viscosity absorbents have potential advantages for CO₂ capture. In this work, physical solubility of N₂O and CO₂ in glycol ethers (2-methoxyethanol and 2-ethoxyethanol) and their blends with MEA was measured and correlated at temperatures ranging from (293.15 to 333.15) K. Physical solubility of CO₂ in the blends was also estimated by N₂O/CO₂ analogy method. A mathematic model based on the reaction mechanism was proposed to interpret the vapour-liquid equilibrium data for the systems of CO₂/MEA/glycol ether solution. Results show that glycol ethers have greater ability to dissolve CO₂ than glycols and water. The apparent chemical equilibrium constant was obtained and correlated as a function of temperature by non-linear regression. The proposed model could reasonably fit the experimental data with an average absolute deviation within 10%. The heat of absorption of CO₂ in MEA/glycol ether systems was also calculated.

Introduction

Global warming is considered as a vital environmental issue today, and it has been widely accepted that carbon dioxide (CO₂) emission is one of major contributors to global climate change [1]. At present, chemical absorption using aqueous monoethanolamine (MEA) is the most mature technology for CO₂ capture and has been applied in many industrial processes [2]. However, high energy consumption for regeneration resulting in high capture cost is currently the critical limitation to prevent the implementation of this technology globally. The use of water as a stripping agent to maintain off-gassing can contribute roughly (40–60) % of energy consumption in regeneration process [3]. Large amounts of energy are unavoidably used for heating the CO₂ absorbed solution and consumed for water vapour evaporation. Using non-aqueous blended systems by replacing water with organic solvents can be beneficial with regard to sensible heat and heat of vaporisation, which seems to be an attractive method to address the problems.

In recent years, there are some research groups who have proposed non-aqueous absorbents and investigated the absorption and desorption performance, reaction mechanism and reaction kinetics for CO₂ capture. Organics are commonly used as non-aqueous diluents as components. Specifically, these non-aqueous systems include the use of alcohols [4], [5], [6], [7], [8], [9], [10], glycols [11], [12], [13], [14], [15], ketone [16], dimethyl sulphoxide [17], dimethyl formamide [18], ionic liquids [19], [20] and ethers [21]. Very recently, Guo et al. [22], [23] have proposed amine-based absorbents in glycol ethers as non-aqueous absorbents for CO₂ removal from biogas upgrading and natural gas processing. Compared with water and volatile or high viscosity organics, glycol ethers such as 2-methoxyethanol (EGME) and 2-ethoxyethanol (EGEE) have several advantages such as low volatility and low viscosity and low specific heat capacity. Physicochemical properties of MEA- EGME blend such as density, viscosity and CO₂ solubility were investigated in our previous works [22]. However, physical solubility of CO₂ in MEA-glycol ether systems has not been available yet in the open literature. Physical solubility is an essential parameter for interpreting the reaction kinetics as well as a rigorous thermodynamics model for process modelling and industrial design using this prospective system. Unfortunately, physical solubility of CO₂ in the system of MEA and glycol ether cannot be measured directly because of the reactive nature between CO₂ and amines. N₂O is a nonreactive gas in amine solutions. Accordingly, the method of N₂O/CO₂ analogy is widely used for aqueous and non-aqueous systems due to their similarities in molecular volume, configuration and electronic structure [24], [25], [26].

The objective of this work is to obtain the Henry’s constant of CO₂ in MEA-glycol ether systems via N₂O analogy. Additionally, a model based on the reaction mechanism was proposed to interpret the vapour-liquid equilibrium data for the systems of CO₂/MEA/glycol ether solution. Therefore, we measured the N₂O physical solubility in pure solvents (EGME, EGEE), 31.0 mass % MEA-EGME and 31.8 mass % MEA-EGEE over the temperature range of (293.15–333.15) K. CO₂ solubility in pure solvents (EGME, EGEE) was also investigated and compared with the literature.

Section snippets

Chemicals and materials

Monoethanolamine (MEA, o.9912 GC mass fraction purity, CAS No.141-43-5), 2-methoxyethanol (EGME, 0.9989 GC mass fraction purity, CAS No.109-86-4) and 2-ethoxyethanol (EGEE, 0.9918 GC mass fraction purity, CAS No.110-80-5) were purchased from Aladdin reagent, China. All the reagents were used without further purification. The water content in the studied solvents was determined by Automatic Karl Fischer moisture titrator (Shanghai Anting Electronic Instrument, ZSD-2). The MEA-glycol ether

Modelling of vapour-liquid equilibrium in CO₂-MEA-glycol ether solution

In anhydrous solutions, reaction mechanisms between CO₂ and amines were proposed in recent literature [6], [10], [14], [17], [23]. CO₂ can react with the primary amines forming unstable carbamic acids or zwitterions as intermediates in the liquid phase. Once formed, the intermediates may react with an excess of amine to form the corresponding ionic couples amine carbamate (MEACOO⁻) and protonated amine (MEAH⁺) that have limited solubility in glycol ethers. Glycol ethers are considered as

Reliability test of the experimental method

To validate the reliability and accuracy of the physical solubility method, Henry’s law constants of N₂O and CO₂ into MEA, EGME and EGEE were measured at near atmospheric pressure, and compared with the published data [29], [30], [31], [32], [33], [34], [35]. The scattered literature data are presented in Supporting Information (SI) Tables S1 and S2. The measured data are also listed in these Tables and the graphic comparison of the results is shown in Fig. 2. It was noted that the units of

Conclusions

The solubility of N₂O into EGME, EGEE and MEA-glycol ether solutions was measured over the temperature range from 293.15 K to 333.15 K. Solubility of CO₂ in 5.0 M MEA-glycol ether solutions was obtained by the N₂O analogy method. The solubility data with respect to temperature were correlated by an exponential model. The correlations give good agreement with the measured with AADs within 0.8%. A mathematical model based on absorption mechanism was proposed to correlate the vapour liquid

Acknowledgements

The authors would like to acknowledge Key Program of Hebei Provincial Natural Science Foundation (Grant No. B2018208154) and Training Program for Talent Engineers of Hebei Province (Grant No. A2017002022) for financial support.

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Nonaqueous secondary alkanolamine-based systems, replacing conventional water with 2-alkoxyethanols as cosolvents, have recently attracted much attention for an energy-efficient CO₂ capture process. In this work, physical solubility of N₂O and CO₂ in 2-(methylamino)ethanol (MAE), 2-methoxyethanol (2-ME) and their blends was measured and correlated at temperatures ranging from (283.2 to 333.2) K. Vapor-liquid equilibrium (VLE) data for the CO₂ + MAE + 2-ME system at 7.8, 19.4 and 39.1 mass% MAE were also measured in a stirred equilibrium cell at temperatures of (313.2–393.2) K and CO₂ partial pressures up to 270 kPa. VLE data were represented by a modified Kent-Eisenberg model with an AARD of 12.6%. The apparent equilibrium constants considering the liquid nonideality were well correlated as a function of temperature, concentration of carbamate product and CO₂ loading.
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Solubility of N2O and CO2 in non-aqueous systems of monoethanolamine and glycol ethers: Measurements and model representation

Highlights

Abstract

Introduction

Section snippets

Chemicals and materials

Modelling of vapour-liquid equilibrium in CO2-MEA-glycol ether solution

Reliability test of the experimental method

Conclusions

Acknowledgements

Int. J. Greenh. Gas Control

Int. J. Greenh. Gas Control

Int. J. Greenh. Gas Control.

Sep. Purif. Technol.

Tetrahedron

J. Environ. Chem. Eng.

Appl. Energy

Appl. Energy

J. Chem. Thermodyn.

Appl. Energy

J. Chem. Thermodyn.

Int. J. Greenh. Gas Control

Chin. J. Chem. Eng.

Chem. Eng. J.

Fluid Phase Equilib.

Fluid Phase Equilib.

J. Chem. Thermodyn.

J. Chem. Thermodyn.

J. Mol. Liq.

Fluid Phase Equilib.

J. Mol. Liq.

Fluid Phase Equilib.

An overview of CO2 capture technologies

Energy Environ. Sci.

Introduction: carbon capture and separation

Chem. Rev.

Solubility of N₂O and CO₂ in non-aqueous systems of monoethanolamine and glycol ethers: Measurements and model representation

Modelling of vapour-liquid equilibrium in CO₂-MEA-glycol ether solution

An overview of CO₂ capture technologies