Scholarly article on topic 'Mass transfer of CO2 absorption in hybrid MEA-methanol solvents in packed column'

Mass transfer of CO2 absorption in hybrid MEA-methanol solvents in packed column Academic research paper on "Chemical engineering"

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{"CO2 " / monoethanolamine / methanol / "hybrid solvent" / "mass transfer"}

Abstract of research paper on Chemical engineering, author of scientific article — Teerawat Sema, Abdulaziz Naami, Phairat Usubharatana, Xiangzeng Wang, Ruimin Gao, et al.

Abstract The mass transfer performance of CO2 absorption into three solvents (i.e., 5M MEA in methanol, 5M MEA in 1:1 water-methanol volume ratio, and 5M MEA aqueous solutions) in an absorption column packed with DX structured packing at various CO2 loading, liquid flow rate, and inert gas flow rate. The mass transfer performance was evaluated in terms of volumetric overall mass transfer coefficient (KGav) and mass flux. The results showed that 5M MEA in methanol has higher mass transfer performance than those of 5M MEA in 1:1 water-methanol volume ratio and 5M MEA aqueous solutions, respectively. In addition, CO2 loading and liquid flow rate had significant effect on mass transfer performance, but inert gas flow rate has insignificant effect on mass transfer performance.

Academic research paper on topic "Mass transfer of CO2 absorption in hybrid MEA-methanol solvents in packed column"

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Energy Procedia 37 (2013) 883 - 889

GHGT-11

Mass transfer of CO2 absorption in hybrid MEA-methanol

solvents in packed column

Teerawat Semaa,b, Abdulaziz Naamib, Phairat Usubharatanaa, Xiangzeng Wangc , Ruimin Gaoa, Zhiwu Lianga,b*, Raphael Idema,b, Paitoon Tontiwachwuthikula,b*

aJoint International Center for CO2 Capture and Storage (iCCS), Department of Chemical Engineering, Hunan University,

Changsha, 410082, PR China

bInternational Test Centre for CO2 Capture (ITC), Faculty of Engineering and Applied Science, University of Regina, Regina,

Saskatchewan, S4S 0A2, Canada

_cShaanxi YanchangPetroleum (Group) Corp. Ltd., xi'an, shanxi,716000, P R China_

Abstract

The mass transfer performance of CO2 absorption into three solvents (i.e., 5 M MEA in methanol, 5 M MEA in 1:1 water-methanol volume ratio, and 5 M MEA aqueous solutions) in an absorption column packed with DX structured packing at various CO2 loading, liquid flow rate, and inert gas flow rate. The mass transfer performance was evaluated in terms of volumetric overall mass transfer coefficient (KGav) and mass flux. The results showed that 5 M MEA in methanol has higher mass transfer performance than those of 5 M MEA in 1:1 water-methanol volume ratio and 5 M MEA aqueous solutions, respectively. In addition, CO2 loading and liquid flow rate had significant effect on mass transfer performance, but inert gas flow rate has insignificant effect on mass transfer performance.

© 2013 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of GHGT

Keywords: CO2; monoethanolamine; methanol; hybrid solvent; mass transfer

1. Introduction

Absorption is one of the most commonly used processes for capturing carbon dioxide (CO2) [1-3]. The CO2 absorption into chemical solvent is driven from the difference in alkalinity between CO2 and the solvent. The disadvantages of the absorption with chemical solvent are the chemical solvent quickly becomes chemically saturated and the high energy requirement for solvent regeneration [4,5]. On the

* Corresponding author. Tel.: +86-136-1848-1627; fax: +86-731-8857-3033. E-mail address: zwliang@hnu.edu.ca and paitoon@uregina.ca.

1876-6102 © 2013 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of GHGT doi:10.1016/j.egypro.2013.05.181

other hand, the absorption with physical solvent is driven from the difference in physical solubility of CO2 into the solvent. The disadvantage of the absorption with physical solvent is the low capability of removing CO2 at low CO2 partial pressure [1,2]. To overcome these advantages, the hybrid solvent (which is the mixing of chemical and physical solvents) has been developed in order to (i) enhance solubility of CO2 in the solvent at low CO2 partial pressure; (ii) reduce energy requirement for solvent regeneration, and also (iii) maintain the capability of capturing CO2 at high CO2 partial pressure. In our previous work, the reaction kinetics and the physical solubility of CO2 into hybrid monoethanolamine (MEA)-methanol has successfully investigated in the stirred cell reactor. The results showed that the CO2 absorption performance of the hybrid MEA-methanol solution was higher than that of MEA aqueous solution [6].

In the present work, the mass transfer performance of hybrid MEA-methanol solvent for capturing CO2 was investigated in terms of mass flux and volumetric overall mass transfer coefficient. Three different solvents (i.e., 5 M MEA in methanol, 5 M MEA in 1:1 water-methanol volume ratio, and 5 M MEA aqueous solutions) were tested in a lab-scale absorber packed with high efficiency DX structured packing under atmospheric pressure, using a premixed 15% CO2 balanced with N2 as a feed gas.

2. Materials and methods

2.1 Chemicals

MEA with a purity of >99% was purchased from Sigma-Aldrich Ltd., Oakville, Ontario, Canada. Methanol with a purity of >99% was supplied from Fisher Scientific, Fair Lawn, New Jersey, USA. The premixed 15% CO2 balanced with nitrogen (N2) was also obtained from Praxair Inc, Canada. All materials in this study were used as received without further purification.

2.2 Mass transfer in packed column

In the case of absorption, the mass transfer occurs when a component, A, in a gas phase transfers across a gas-liquid interface into a liquid phase. Based on film theory, at a steady-state condition, combining mass flux and material balance equations, overall mass transfer coefficient (KGav) can be defined as presented in Equation 1.

The variables presented in Equation 1 can be obtained from the absorption experiments. The inert gas flow rate (G) is an operating condition of the experiment, which is conducted at atmospheric pressure (P). The concentration of A, which is CO2, in the gas phase (yA,G) can be measured along the height of the packed column using an infrared CO2 gas analyzer. The measured CO2 concentration in terms of mole fraction of gas A in the gas phase (yA,G) were converted into mole ratio of gas A in the gas phase (YA,G); then, plotted against the height of the column (Z) to obtain the solute mole ratio concentration gradient (dYA,G/dZ). The details description on the determination of KGav can be found in our previous works [5,7,8]. The bench-scale absorption unit consists of a glass absorption column (3.4 x 10-2 m in diameter and 0.4 m in height), packed with stainless steel structured packing (Sulzer DX). The column was designed for a counter-current mode of operation in which a liquid absorbent was introduced from the top of the column while a 15% CO2 balance with N2 was fed at the bottom of the column. Also, a condenser using cooling water was assembled at the top of the column in order to minimize solvent lost due to vaporization. The experimental set up for the absorption column is presented in Fig 1.

Fig 1. Experimental schematic diagram for CO2 absorption in packed column

3. Results and discussion

In the present work, the performance of CO2 absorption into three solvents (i.e., 5 M MEA in methanol, 5 M MEA in 1:1 water-methanol volume ratio, and 5 M MEA aqueous solutions) was investigated in packed column in terms of overall mass transfer coefficient (KGav) and mass flux of CO2 absorption over ranges of methanol composition, CO2 loading, liquid flow rate, and inert gas flow rate.

3.1 Effect of methanol composition

It was observed that the mass transfer performance (in terms of KGav and mass flux) of 5 M MEA in methanol was higher than those of 5M MEA in1:1 water-methanol volume ratio and 5 M MEA aqueous solutions, respectively, as presented in Figs 2 and 3. It can be implied from this observation that methanol can enhance the mass transfer of CO2 absorption. This is because of methanol has higher physical solubility and physical diffusivity of CO2 than water [6,9]. The higher methanol composition resulting in the higher physical solubility and physical diffusivity of CO2; thus, the mass transfer performance of CO2 absorption was then found to be increased.

3.2 Effect of CO2 loading

CO2 loading is considered as one of the very most important parameters on CO2 absorption process because CO2 loading represents the amount of active absorbent in the solution in term of mole of CO2 dissolved in the solution per mole of absorbent in the solution. In the present work, CO2 loading was varied from 0.05-0.3 mol CO2/mol amine in order to investigate the effect of CO2 loading on mass transfer performance in packed column. Fig 2 shows that the mass transfer performance in terms of KGav decreases as CO2 loading increases due to the fact that as CO2 loading increases, the amount of active amine decreases, causing the KGav to decrease.

Fig 2. Effect of methanol composition and CO2 loading on KGav at 298 K, liquid flow rate of 4.71 m /m hr, and inert gas flow rate of 31.5 kmol/m2 hr

30 40 50 eo Liquid fJowrate fmUmin)

Fig 3. Effect of solvent composition and liquid flow rate on mass flux at 298 K, inert gas flow rate of 10 L/min with unloaded solvents

3.3 Effect of liquid flow rate

Liquid flow rate also has a significant impact on the CO2 removal efficiency in the DX structured packed column. As shown in Figs 3-5 that the mass transfer performance in terms of KGav and mass flux increase as liquid flow rate increases over a liquid flow rate range of 12-98 ml/min. Because of the increasing of liquid flow rate leads to a greater degree of wetted packing surface and increasing of mass transfer performance. However, at too high liquid flow rate, the bubbles will be produced, which directly affects the active surface area between CO2 and absorbent. Thus, the mass transfer performance will be decreased. In the present work, no bubbles were created in the absorption column. In addition, the increasing of liquid flow rate leads to higher circulation rate; thus, it might not lead to optimum operating conditions.

Liquid flow rate (mL/min]

Fig 4. Effect of liquid flow rate and inert gas flow rate on KGav at 298 K for unloaded 5M MEA in 1:1 water-methanol solvent

♦ G'5 L/rnin

■ G L'min

*G = i L'min ii

1Ü UVriin 0 # + ■

X * ■ *

Liquid flew rate im Urn In)

Fig 5. Effect of liquid flow rate and inert gas flow rate on mass flux at 298 K for unloaded 5M MEA in 1:1 water-methanol solvent

3.4 Effect of inert gas flow rate

The effect of inert gas flow rate on mass transfer performance (i.e, KGav and mass flux) can be found in Figs 4 and 5. It was found that the inert gas flow rate has no significant effect on mass transfer performance since both KGav and mass flux were found to be rarely changed as inert gas flow rate was varied. This is because the CO2 absorption into reactive amine solution is considered as liquid film control process (Fu). This observation corresponds well with the works of Aroonwilas and Tontiwachwuthikul [10], deMintigny et al. [11] and Fu et al. [8], who studied the mass transfer performance of CO2 absorption into aqueous solutions of MEA, AMP, and DETA.

4. Conclusions

In the present work, the mass transfer experiment of absorption of CO2 into three solvents (i.e., 5 M MEA in methanol, 5 M MEA in 1:1 water-methanol volume ratio, and 5 M MEA aqueous solutions) were investigated in a laboratory-scale absorption column packed with DX structured packing. The absorption performance was evaluated in terms of volumetric overall mass transfer coefficient (KGav) and mass flux. The results showed that the mass transfer performance of 5 M MEA in methanol was higher than those of 5M MEA in1:1 water-methanol volume ratio and 5 M MEA aqueous solutions, respectively. In addition, it was observed that CO2 loading and liquid flow rate directly affect the mass transfer performance in that the mass transfer performance increases as liquid flow rate increases but decreases as CO2 loading increases. However, the inert gas flow rate has insignificant effect on mass transfer performance.

Acknowledgements

We acknowledge the research support over the past many years of the Industrial Research Consortium - Future Cap Phase II of the International Test Centre for CO2 Capture (ITC) at the University of Regina. We also acknowledge the research support from the followings organizations: Natural Sciences and Engineering Research Council of Canada (NSERC), Canada Foundation for Innovation (CFI), Saskatchewan Ministry of Energy & Resources, Western Economic Diversification, Saskatchewan Power Corporation, Alberta Energy Research Institute (AERI) and Research Institute of Innovative and Technology for the Earth (RITE). In addition, we acknowledge the financial support from National Natural Science Foundation of China (NSFC No. 21276068), the Ministry of Science and Technology of the P R China (MOST No. 2012BAC26B01), the Ministry of Hunan Provincial Science and Technology (No. 2010SK2001), China's State "Project 985" in Hunan University Novel Technology Research & Development for CO2 Capture and National Students Innovation Training (SIT).

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