Available online at www.sciencedirect.com
ScienceDirect Energy Procedia 75 (2015) 2305 – 2310
The 7th International Conference on Applied Energy – ICAE2015
Sensitivity study of thermo-physical properties of gas phase on absorber design for CO2 capture using monoethanolamine Worrada Nookueaa,*, Yuting Tanb, Hailong Lia, Eva Thorina, Jinyue Yana,b,* b
a School of Business, Society and Engineering, Mälardalen University, SE 721 23 Västerås, Sweden School of Chemical Science and Engineering, Royal Institute of Technology, SE 100 44 Stockholm, Sweden
Abstract Absorption of CO2 with aqueous amines in post-combustion capture is characterized as mass transfer process with chemical reaction. Hydrodynamics and mass transfer in gas and liquid phases in a packed column have significant influences on absorber design especially for the design of packing height. In this paper, the sensitivity study has been conducted to investigate the impacts of gas phase density, viscosity and diffusivity on the hydrodynamics and mass transfer and further the total packing height of a countercurrent flow with random packing column, using reactive absorption process and integral rate-based models. Results show that density and diffusivity have opposite effect to viscosity. Amongst various properties, diffusivity has the most significant effect on the packing height compared to density and viscosity. Overestimation of diffusivity of 5% may result in decrease of 3.2% of packing height. Moreover, developing more accurate diffusivity model should be prioritized for more accurate absorber design. © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license © 2015 The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or peer-review under responsibility of ICAE Peer-review under responsibility of Applied Energy Innovation Institute Keywords: Post-combustion capture; Thermo-physical properties; Chemical absorption; Absorber design; Property impact
1. Introduction Post-combustion capture by chemical absorption is currently considered one of the most viable options for CO 2 capture in power plants. Chemical absorptions using aqueous amines can be used to capture CO2 from the gas stream with low CO2 concentration due to its high reaction rate with CO2 [1,2]. A wide variety of amines have been proposed and studied for CO2 capture by chemical absorption such as monoethanolamine (MEA), diethanolamine (DEA) and Nmethyldiethanolamine (MDEA) [3,4]. There are many studies concerning the process operation parameters, such as absorber inlet temperature, amines concentration, CO2 loading and stripper operating pressure [5 - 7]. In addition, studies focusing on modelling the key properties required by the process design, simulation and optimization were also conducted [8-13]. Paper [14] compared the results of temperature profile in the packed column absorber from rate-based absorption model using different liquid density and heat capacity correlations. Studies [15,16] evaluated different correlations for liquid properties and mass transfer coefficients in a rate-based absorption model. Predicting the hydrodynamics and mass transfer accurately plays an important role in the absorber design. Several thermo-physical properties are involved in the calculations of hydrodynamics and mass transfer, such as density, viscosity, diffusivity and surface tension. However, the impact of the fluid thermo-physical properties on the design parameters of the absorber, such as column diameter and packing height, has rarely been studied. The aim of this paper is to assess the influences of density, viscosity and diffusivity of the gas phase on the column design and identify the key properties. Moreover, the impacts of property model deviations on the design packing height were also evaluated.
* Corresponding authors. Tel.: +46-(0)2-110-1630 and +46-(0)8-790-6528 E-mail address:
[email protected] and
[email protected]
1876-6102 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of Applied Energy Innovation Institute doi:10.1016/j.egypro.2015.07.426
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2. Model formulation Absorption of CO2 with aqueous amines is characterized as mass transfer enhanced by chemical reaction. CO2 reacts either directly or through an acid-base buffer mechanism with amines to form non-volatile ionic species [17]. The twofilm mass transfer theory can be used to describe the transport of CO2 from gas to liquid phase [18]. The approach used in this study is a combination of the reactive absorption process model, which consists of a reaction model based on two-film model for fast reaction and a gas liquid contactor model based on the mass and energy balances [15], and the integral rate based model for packing height estimation [19]. The input data is listed in Table 1 [20]. Table 1. Input summary for the calculation [20] Flue gas Composition (mol %)
CO2 H2O N2 O2
Absorber inlet temperature Absorber inlet pressure Mole flow
11.77% 7.77% 75.67% 4.86% 328.15 K 111.325 kPa 10,000 kmol/hr
CO2 removal Lean solvent temperature Rich solvent mole flow Rich loading
85% 313.15 K 47,080 kmol/hr 0.413
Diameter Packing type
7m Cascade Mini Rings #2
Solvent
Absorber
2.1 Reaction model The overall reactions represent the reaction mechanism for CO 2-MEA system consisting of carbamate formation, bicarbonate formation and reversion of carbamate to bicarbonate as shown in reactions (A), (B), and (C), respectively. CO2 + 2RNH2 → RNHCOO− + RNH3+
(A)
CO2 + RNH2 + H2O → HCO3− + RNH3+
(B)
RNHCOO− + CO2 + 2H2O → HCO3− + 2RNH3+
(C)
Since the absorption at CO2/MEA mole ratio less than 0.5, the rate of bicarbonate formation is insignificant compared to the formation of carbamate. Therefore, the overall reactions can be expressed by reaction (A) and considered to be approximately irreversible [15,18]. The overall reaction rate is second-order with respect to CO2 and amine concentrations. Hikita et al. [21] purposed the correlation for the second-order rate constant,݇ଶ ሺ݉ଷ Ȁ݇݉ݏ݈ሻ, depends on the temperature, ܶ ሺܭሻ, of the liquid solution: ሺ݇ଶ ሻ ൌ ͳͲǤͻͻ െ
ଶଵହଶ ்ಽ
(1)
For the reaction model, the overall rate of absorption of CO 2 in aqueous amine based on the two-film model for a fast second order reaction can be expressed as equation (2) [15]: ݎைమ ൌ
ೀమ ሺଵȀಸ ሻାሺுೀమ Ȁಽ ாሻ
(2)
where ܽ is the gas-liquid interfacial area per unit volume of packingሺ݉ଶ Ȁ݉ଷ ሻ, ܧis the enhancement factor, ܪைమ is the Henry’s law constantሺ݈݇݉Ȁ݉ଷ ݇ܲܽ), ݇ீ is the gas film mass transfer coefficientsሺ݈݇݉Ȁ݉ଶ )ܽܲ݇ݏ, ݇ is the liquid film mass transfer coefficientsሺ݉Τ ݏሻand ைమ is the partial pressure of CO2 in bulk gas phaseሺ݇ܲܽሻ. The correlations of mass transfer and effective interfacial area for random and structured packing were collected in [22]. The correlations purposed by Onda et al. [23] are used to calculate the gas and liquid film mass transfer coefficient and the effective interfacial area for random packing:
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ఘಸ ௨ಸ
݇ீ ൌ ܿ ൬
ஜಸ
Ǥ
൰
݇ ൌ ͲǤͲͲͷͳ ቀ
ቀ
ஜಸ ఘಸ ಲಸ
ఘಽ ௨ಽ ଶΤଷ ஜಽ
ቁ
ቁ
ቀ
ଵ Τଷ
ቀ
ஜಽ ఘಽ ಲಽ
ఙ
ൌ ͳ െ െͳǤͶͷ ቀ ቁ
Ǥହ
ఙಽ
ಲಸ
ቁ
ோ்ಸ ିǤହ
ቁ ሺܽ ݀ ሻିଶǤ
ቀ
ஜಽ ଵȀଷ ఘಽ
ቁ
(3) Ǥସ
൫ܽ ݀ ൯
(4)
ሺܴ݁ ሻǤଵ ሺݎܨ ሻିǤହ ሺܹ݁ ሻǤଶ ൨
(5)
where ܽ is the packing specific surface areaሺ݉ଶ Ȁ݉ଷ ሻ, ܽ is the effective interfacial area ሺͳȀ݉ሻ, ܿ is dimensionless constant which depends on the packing size, ܦீ and ܦ are the diffusivities of CO2 in gas and liquid phase ሺ݉ଶ Ȁݏሻ, R is the universal gas constant ሺܬȀ݈݉ܭሻ, ܶீ is the gas phase temperature ሺܭሻ, ீݑand ݑ are the superficial velocity of gas and liquidሺ݉Ȁݏሻ, ߪ is the critical surface tension of packing material ሺܰȀ݉ሻ, ߪ is the surface tension of liquid ሺܰȀ݉ሻ, Ɋீ and Ɋ are the viscosity of gas and liquid ሺ݇݃Ȁ݉ݏሻ, ߩீ and ߩ are the density of gas and liquid ሺ݇݃Ȁ݉ଷ ሻ, ܴ݁ ,ݎܨ and ܹ݁ are Reynolds, Froude and Weber number respectively: ܴ݁ ൌ
ఘಽ ௨ಽ ஜಽ
, ݎܨ ൌ
௨ಽమ
and ܹ݁ ൌ
ఘಽ ௨ಽమ
(6), (7), (8)
ఙಽ
where ݃ is the gravitational constantሺ݉Ȁ ݏଶ ሻ. The applicable ranges of the effective interfacial area correlations areͲǤͲͶ ൏ ܴ݁ ܮ൏ ͷͲͲ,ͷ ൈ ͳͲ ൏ ݎܨ ൏ ͳǤͺ ൈ ͳͲ , ͳǤʹ ൈ ͳͲ ൏ ܹ݁ ൏ ͲǤʹʹ andͲǤ͵ ൏ ߪ Τߪ ൏ ʹ. Since mass transfer is enhanced by chemical reaction, the enhancement factor,ܧ, needs to be considered. The explicit relation for enhancement factor developed by Wellek et al. [24] in term of Hatta modulus,ܯ, and an instantaneous reaction, ܧஶ , was used in this work: െͻ
ܧൌͳ
െʹ
െͺ
ଵ
(9)
ሾሺଵȀሺாಮ ିଵሻሻభǤయఱ ାሺଵȀሺாభ ିଵሻሻభǤయఱ ሿభȀభǤయఱ
ܧஶ ൌ ͳ ቀ
ಳಽಳಽ ಲಽ ಲ
ቁ, ܧଵ ൌ
ξெ ௧ξெ
and ܯଶ ൌ
ಲಽ మ ಳಽ ಽ మ
(10), (11), (12)
where ܥ is the CO2 concentration at the gas-liquid interface ሺ݈݇݉Ȁ݉ଷ ሻ, ܥ is the MEA concentration in bulk liquid ሺ݈݇݉Ȁ݉ଷ ሻ, and ܦ is the diffusivity of MEA in liquid ሺ݉ଶ Ȁݏሻ. Thermo-physical properties of gas and liquid phases at different temperatures and compositions are necessary for the absorption rate calculation. The gas phase properties at different temperatures and compositions were retrieved from a transport property database [25]. The density and viscosity of liquid phase were obtained from the empirical correlations purposed by Weiland et al. [10]. The semi-empirical correlations from Versteeg and van Swaaij [26] were used to obtain the diffusivity of CO2 in water and CO2 loaded solution. The surface tension of CO 2 loaded solution was determined by the empirical correlation purposed by Jayarathna et al. [11]. The semi-empirical correlation purposed by Penttilä et al. [27] was used to obtain the value of the Henry’s law constant. 2.2 Design algorithm To calculate the mass and energy balances, the column is divided into small segments. As shown in Fig 1, the MEA concentration and the temperature of liquid phase in each segment was calculated. Besides the assumptions made in [15] except the constant flow rate of gas and liquid phases, it was further assumed that the heat generated in liquid phase due to the exothermic chemical reaction is dissipated in the liquid and consequently only resulting in the rise of liquid temperature. Therefore, the gas phase temperature is constant along the columnሺܶீǡ ൌ ܶீǡାଵ ሻ. The heat of reaction for absorption of CO2 in alkanolamine solutions was taken from Kohl and Risenfield [28]. The operation was assumed to be adiabatic. Both phases move in a plug flow manner and the operation is at constant pressure. The integral rate-based model for calculating the height of packing ( ο )ݖwas taken from [19]: ο ݖൌ
ௗ௬ೀమ ǡಸ ௬భ ீೞ ௌ ௬మ ఉሺ௬ೀమ ǡಸ ି௬ೀమ ǡಽ ሻሺଵି௬ೀమ ǡಸሻమ
ߚൌቀ
ଵ ಸ
ଵ ಽ ுா
ିଵ
ቁ
(13) (14)
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Fig 1. Gas and liquid mass transfer occurring in a control volume based on the two-film theory of mass transfer (adapted from [15] and [29])
where ܩ௦ is the molar flow rate of inerts ሺ݈݇݉Ȁݏሻ, ்ܲ is the total pressure ሺ݇ܲܽሻ, ܵ is the cross-sectional area of the column ሺ݉ଶ ሻ, ݕைమ ǡ and ݕைమ ǡீ are the mole fraction of CO2 in the bulk liquid and in the gas phase, respectively, and ߚ is the variable defined by equation (14). Since the absorption is accompanied by the chemical reaction, the absorption rate varies along the packing height due to the change in the enhancement factor, therefore the value of ߚ is not constant. In this work, equation (15) was used for the calculation of packing height since the overall reaction can be expressed by reaction (A) and considered to be approximately irreversible. The effect of back pressure was not considered: ο ݖൌ
ௗ௬ೀమ ǡಸ ௬భ ீೞ ௌ ఉ ௬మ ௬ೀమ ǡಸ ሺଵି௬ೀమ ǡಸ ሻమ
(15)
The flowchart for packing height calculation is presented in Fig 2. The calculated liquid temperature and concentration profiles from the reactive absorption process model and the integral rate based model are validated with the absorber profile provided by [20].
Fig 2. Flowchart for packing height calculation (adapted from [19])
3. Results and discussions In order to assess the impact of properties on the column design, sensitivity studies have been conducted regarding the properties of gas phase density, viscosity and diffusivity, which were varied from -20 to 20% of the initial values. The results of variation on the total packing height are demonstrated in Fig 3(a). The impacts of the properties on the packing height are different. The initial packing height is 14.20 m. When density, viscosity and diffusivity vary +5%, the height changes to 13.95, 14.46 and 13.75 m, respectively, which correspond to changes of -1.8%, +1.8% and -3.2%. It is clear that comparatively, the diffusivity of CO 2 in flue gas has more significant impact than density and viscosity. By re-arranging equation (3), ݇ீ is directly proportional to the density and diffusivity to the 0.37 and 0.67 power,
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respectively; whereas, it is inversely proportional to the viscosity to the 0.37 power. Since ߚ is directly proportional to ݇ீ and the packing height is inversely proportional toߚ, the packing height is directly proportional to the viscosity, but inversely proportional to density and diffusivity. In order to have the right design, it is more important to calculate the diffusivity of CO2 in gas phase correctly. For different properties, the property models give different accuracies. The maximum deviation of the properties models of CO2 mixtures is also illustrated in Fig 3(b). According to [30-35], for predicting the gas volume, viscosity and diffusivity, the maximum modelling deviations are ±14%, ±8% and ±21% respectively; correspondingly, the maximum deviations lead to the variation of height of ±5.2%, ±2.9% and < ±16.1%. Therefore, from the perspective of property modelling, developing more accurate diffusivity model should be prioritized. 17
20 D ensity
D iffusivity
15
14
13
V iscosity
(b) Packing height deviation [%]
16 Total packing height [m]
D ensity
V iscosity
(a)
D iffusivity
10
0
DevDensity = ± 14 %
-10
DevViscosity = ± 8 % DevDiffusivity = ± 21%
12
-20
-10
0
10
Percentage of variation [%]
20
-20
-20
-10
0
10
20
Percentage of variation [%]
Fig 3. Sensitivity results of density, viscosity and diffusivity of gas phase on packing height calculation: (a) Total packing height; (b) Percentage of deviation
The current work only considered the properties of the gas phase in the design of packing height. The liquid phase properties also need to be evaluated in the future work, which may be more complicate. Changing liquid properties affects more design parameters than gas phase properties, such as effective interfacial area and enhancement factor. Additionally, the impacts of heat capacity and thermal conductivity are also expected to have impacts on the column design when a more rigorous energy balance is needed. Column diameter is another important design parameter apart from the height. The impacts of gas and liquid phase properties on the column diameter are also needed to be evaluated, since the diameter can be either scaled with the superficial gas velocity or calculated from the flooding constraint. 4. Conclusions This paper studies the impact of the thermo-physical property on the design of an absorber for CO 2 capture. Results show that the density and diffusivity have opposite effect to viscosity; and amongst the three investigated properties, gas phase density, viscosity and diffusivity of the gas mixture, diffusivity was found to have the most significant effect on the packing height. 5% overestimation of diffusivity may result in decrease of 3.2% of column height. Moreover, as the model of diffusivity has larger maximum deviation than the models of density and viscosity, developing more accurate diffusivity model should be prioritized. Additionally, for the column design with a more rigorous energy balance, the impacts of the heat capacity and thermal conductivity are expected to have impacts on the design. Acknowledgements This study is supported by the Swedish Energy Agency and Swedish Research Council. References [1] Nuchitprasittichai A, Cremaschi S. Sensitivity of amine-based CO2 capture cost: The influences of CO2 concentration in flue gas and utility cost fluctuations. Int J Greenh Gas Control 2013;13(0):34-43.
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Biography Worrada Nookuea is currently a Ph.D. student in the School of Business, Society and Engineering, Mälardalen University. Her research interest is in the separation processes of Carbon Capture and Storage (CCS).
