Thermodynamic and cycle model for MEA-based chemical CO2 absorption

Available online at www.sciencedirect.com Available online at www.sciencedirect.com

ScienceDirect ScienceDirect

Energy Procedia 00 (2018) 000–000 Available online www.sciencedirect.com Available online atatwww.sciencedirect.com Energy Procedia 00 (2018) 000–000

ScienceDirect ScienceDirect

www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia

Energy Procedia 158 Energy Procedia 00(2019) (2017)4941–4946 000–000 www.elsevier.com/locate/procedia

10th International Conference on Applied Energy (ICAE2018), 22-25 August 2018, Hong Kong, 10th International Conference on Applied Energy China(ICAE2018), 22-25 August 2018, Hong Kong, China

Thermodynamic and cycle model for MEA-based chemical CO2 The 15th International on District Heating andchemical Cooling Thermodynamic and cycleSymposium model for MEA-based CO2 absorption absorption Assessing the feasibility of using the heat demand-outdoor a,b a Junyao Wanga,b, Shuai Denga, Taiwei Sunaa, Yaofeng Xuaa, Kaixiang Liaa, Jun Zhaoaa* Junyao Wang ,function Shuai Dengfor , Taiwei Sun , Yaofeng Xu , heat Kaixiang Li , Junforecast Zhao * temperature a long-term district demand Key Laboratory of Efficient Utilization of Low and Medium Grade Energy (Tianjin University), Ministry of Education of China, Tianjin 300072,

a

b China. Utilization School of of environmental science and Energy engineering, Tianjin University, Tianjin, 300072,China Key Laboratory of Efficient Low and Medium Grade (Tianjin University), Ministry of Education of China, Tianjin 300072, a,b,c a a b c c China. bSchool of environmental science and engineering, Tianjin University, Tianjin, 300072,China

a

I. Andrić

a

*, A. Pina , P. Ferrão , J. Fournier ., B. Lacarrière , O. Le Corre

IN+ Center for Innovation, Technology and Policy Research - Instituto Superior Técnico, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal

b Veolia Recherche & Innovation, 291 Avenue Dreyfous Daniel, 78520 Limay, France Abstract c Département Systèmes Énergétiques et Environnement - IMT Atlantique, 4 rue Alfred Kastler, 44300 Nantes, France Abstract Low energy consumption is one of the most critical factors for an ideal carbon capture solvent. Numbers of studies have been Low energy consumption one of the most critical factors for an ideal carbonconventional capture solvent. Numbers of studies have been carried out focusing on theis development of novel chemical solvent replacing monoethanolamine (MEA) solvent. carried outthere focusing the development of novel chemical solvent replacing monoethanolamine However, is no on complete research framework to evaluate and explain theconventional energy performance for different(MEA) carbon solvent. capture Abstractthere However, is no researchperspective. framework to evaluate explain the energy performance for different carbon technologies from thecomplete thermodynamic This study and presents a comprehensive thermodynamic analysis on a capture typical technologies from the absorption thermodynamic This study presents a comprehensive thermodynamic analysis on a typical MEA-based chemical (CA) perspective. process. A MEA-based CA cycle is established based on the vapor-liquid equilibrium District of heating networks are commonly addressed in the literature one of the mostinto effective solutions forCO decreasing the MEA-based chemical absorption (CA) process. A efficiency MEA-based CA cycle is established based on the vapor-liquid property MEA-CO Further energy analysis isasconducted taking account the heat of absorption 2-H 2O system. 2equilibrium greenhouse gas emissions from the building sector. These systems require high investments which are returned through the heat -H O system. Further energy efficiency analysis is conducted taking into account the heat of CO absorption property of MEA-CO 2 2 as well as liquid heat2 capacity. Results show that among the three parts of energy duty of absorption heat (𝑄𝑄𝑎𝑎𝑎𝑎𝑎𝑎 ), sensible heat Due to the changed and building renovation policies, heat in the could decrease, heat assales. well),as liquid heat capacity.climate Results show among the three parts energy duty ofdemand absorption heatfuture (𝑄𝑄range (𝑄𝑄 and water evaporation heat ( 𝑄𝑄conditions ), 𝑄𝑄that quite stable along theofresearched desorption temperature between 373 𝑎𝑎𝑎𝑎𝑎𝑎 ), sensible 𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 𝑣𝑣𝑣𝑣𝑣𝑣 𝑎𝑎𝑎𝑎𝑎𝑎 is prolonging the investment period. ), and water evaporation ( 𝑄𝑄𝑣𝑣𝑣𝑣𝑣𝑣 ), 𝑄𝑄opposite stable trend along with the researched desorption temperature between idea 373 (𝑄𝑄 to 293K. However, the 𝑄𝑄𝑣𝑣𝑣𝑣𝑣𝑣return andheat 𝑄𝑄 show variation the increase of temperature. For anrange approximate 𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 𝑎𝑎𝑎𝑎𝑎𝑎 is quite 𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 main scope of the this is to𝑄𝑄assess the feasibility of using trend the demand – outdoor temperature function for heat demand toThe 293K. However, 𝑄𝑄paper and show opposite with the increase of temperature. an approximate idea cycle, 𝑄𝑄𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 is always the dominate factor leading to thevariation increasing ofheat total energy duty with the growth ofFor temperature. However, 𝑣𝑣𝑣𝑣𝑣𝑣 𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 forecast. The district of Alvalade, located in Lisbon (Portugal), was used as a case study. The district is consisted of 665 𝑄𝑄 is always the dominate factor leading to the increasing of total energy duty with the growth of temperature. However, cycle, for a real𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 cycle, 𝑄𝑄𝑣𝑣𝑣𝑣𝑣𝑣 is becoming increasingly important with the increase of the partial pressure ratio for rich solvent. buildings that 𝑄𝑄 vary in both construction period and typology. Three weather scenarios (low, medium, high) and three district for a real cycle, 𝑣𝑣𝑣𝑣𝑣𝑣 is becoming increasingly important with the increase of the partial pressure ratio for rich solvent. renovation scenarios were developed (shallow, intermediate, deep). To estimate the error, obtained heat demand values were Copyright © 2018 Elsevier Ltd. All rights reserved. ©compared 2019 The Authors. Published by Elsevier Ltd. results fromLtd. a dynamic heat demand model, previously developed and validated by the authors. Copyright ©with 2018 Elsevier rights reserved. Selection and peer-review underAll responsibility of the scientific committee of the 10th International Conference on Applied Energy This isresults an open accessthat article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) The showed when only weather change is considered, the margin of error could be acceptable applications th Selection and peer-review under responsibility of the scientific committee of the 10 International Conferencefor onsome Applied Energy (ICAE2018). Peer-review responsibility of lower the scientific committee of ICAE2018 – Theconsidered). 10th International Conference on Appliedrenovation Energy. (the error inunder annual demand was than 20% for all weather scenarios However, after introducing (ICAE2018). scenarios,MEA; the error value increased up toThomodybamic; 59.5% (depending on the weather and renovation scenarios combination considered). Keywords: Chemical absorption; Cycle; Heat duty. The value of slope coefficient increased on average within the range of 3.8% up to 8% per decade, that corresponds to the Keywords: MEA; Chemical absorption; Cycle; Thomodybamic; Heat duty. decrease in the number of heating hours of 22-139h during the heating season (depending on the combination of weather and renovation scenarios considered). On the other hand, function intercept increased for 7.8-12.7% per decade (depending on the coupled scenarios). The values suggested could be used to modify the function parameters for the scenarios considered, and improve the accuracy of heat demand estimations.

© 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. * Corresponding author. Tel.: +86 22 27890051

E-mail address:author. [email protected] * Corresponding Tel.: +86 22 27890051 Keywords: Heat demand; Forecast; Climate change E-mail address: [email protected] 1876-6102 Copyright © 2018 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility the scientific 1876-6102 Copyright © 2018 Elsevier Ltd. All of rights reserved. committee of the 10th International Conference on Applied Energy (ICAE2018). Selection and peer-review under responsibility of the scientific committee of the 10th International Conference on Applied Energy (ICAE2018). 1876-6102 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the Scientific Committee of The 15th International Symposium on District Heating and Cooling. 1876-6102 © 2019 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 the scientific committee of ICAE2018 – The 10th International Conference on Applied Energy. 10.1016/j.egypro.2019.01.695

Junyao Wang et al. / Energy Procedia 158 (2019) 4941–4946 Author name / Energy Procedia 00 (2018) 000–000

4942 2

1. Introduction Amine-based chemical absorption is widely regarded as the most mature technology for post-combustion carbon capture process which has been commercially operated at large scale [1]. However, the main bottlenecks for this technology are still lies in how to reduce the energy duty. Therefore, numbers of studies have been focused on developing advanced absorbent including blended amine solutions, ionic liquid amine-blends, phase change solvent such as chilled ammonia and etc.[2]. However, in terms of energy efficiency analysis, there is no developed research framework to provide a clear pathway for various carbon capture technologies towards energy savings. Currently, there are mainly three research approaches to access the performance of energy efficiency for CO2 carbon capture process: (1) the separation model [3] to calculate the minimum separation work; (2) the process model to simulate the operation process, which is generally based on commercial software such as Aspen Plus, Pro II etc.; and (3) the life cycle energy and emergy assessment to evaluate energy efficiency through a life cycle approach. Although they are effective methods to analysis the energy performance for carbon capture technology comparatively, they cannot provide a genetic and fair energy efficiency analysis from the thermodynamic perspective by considering the basic thermodynamic properties, the operation conditions and the energy source of different carbon capture technologies. To solve the mentioned knowledge gap, Zhao et al [4] proposed the carbon pump model to analyze carbon capture process thermodynamically. However, the detailed thermodynamic cycle description and thermodynamic properties are not further developed for chemical based carbon capture process. Therefore, this study firstly established a typical MEA-based chemical absorption thermodynamic cycle. Base on the thermodynamic properties of vapor-liquid equilibrium, enthalpy of absorption and heat capacity, the energy consumption and distribution as well as energy efficiency is evaluated. 2. Thermodynamic research frameworks 2.1 Thermodynamic properties 2.1.1 Vapor-liquid equilibrium (VLE) Knowledge of VLE is the basis for thermodynamic analysis. Empirical models from Ref. [5] is applied to describe the vapor-liquid equilibrium curve for CO2-MEA-H2O system of 30wt % MEA. The proposed correlation of CO2 partial pressure with CO2 loading in solutions and temperature is given by Eq.1. The pure water vapor and MEA pressure can be expressed as Eq. 2 [6] and Eq. 3 [7]. ln 𝑃𝑃𝐶𝐶𝐶𝐶2 = 39.3 −

𝑃𝑃∗𝐻𝐻2 𝑂𝑂 (pa)

12155

= exp⁡(73.649 −

𝑇𝑇

𝛼𝛼

− 19.0𝛼𝛼2 + 1105 + 12800

𝑇𝑇 7258.2

∗ 𝑃𝑃𝑀𝑀𝑀𝑀𝑀𝑀 (pa) = exp⁡(92.624 −

𝑇𝑇

𝛼𝛼2 𝑇𝑇

(1) 2

− 7.3037𝑙𝑙𝑙𝑙𝑙𝑙 + 4.1653𝐸𝐸(−6)𝑇𝑇

10367 𝑇𝑇

− 9.4699𝑙𝑙𝑙𝑙𝑙𝑙 + 1.9(−18)𝑇𝑇6

(2) (3)

Where T is the Temperature in K; and α is the CO2 loading (mol CO2/mol MEA)

2.1.2 Enthalpy of CO2 absorption (∆Habs ) Enthalpy of CO2 absorption is directly related to the energy requirements for solvent regeneration. The ∆Habs is calculated through a piecewise function as in Eq. 4 [7]. ⁡−∆𝐻𝐻𝑎𝑎𝑎𝑎𝑎𝑎 = -13.67+0.308T ⁡−∆𝐻𝐻𝑎𝑎𝑎𝑎𝑎𝑎 = -127.13+246.65α +1.02T-1.54αT ⁡⁡⁡⁡⁡⁡⁡⁡⁡⁡⁡⁡⁡⁡⁡⁡⁡⁡⁡⁡⁡⁡⁡⁡⁡⁡⁡⁡⁡⁡⁡⁡⁡⁡−∆𝐻𝐻𝑎𝑎𝑎𝑎𝑎𝑎 = 35.66 Where ∆𝐻𝐻𝑎𝑎𝑎𝑎𝑎𝑎 ( kJ/mol CO2)is the enthalpy of CO2 absorption.

2.1.3 Liquid heat Capacity (Cp)

(α ≤ 0.46) (0.46 ≤ α ≤ 0.66) (0.66≤ α)

(4)



Junyao Wang et al. / Energy Procedia 158 (2019) 4941–4946 Author name / Energy Procedia 00 (2018) 000–000

4943 3

Specific heat capacity of MEA-CO2-H2O is a critical thermodynamic property determining the requirement of sensible heat in the solvent regeneration process. As it is assumed that CO2 is not stripped out during pre-heating process, Cp is expressed as a function of temperature at constant CO2 loading around 0.5 mol CO2/mol MEA. A simple temperature dependent correlation was developed based on Apsen Plus as Eq. 5. (40⁡≤ T ≤ 140) Cp = 1E(−7)T 3 − 1𝐸𝐸(−5)𝑇𝑇 2 + 0.0022𝑇𝑇 + 3.0205 Where Cp (kJ/kg K) is the specific heat capacity of CO2 loading solvent of 30%wt MEA solution.

(5)

2.2 Description of monoethanolamine (MEA)-based chemical absorption (CA) cycle 2.2.1 Process description A general chemical-based CO2 process is shown in Fig.1. The flue gas mainly contains N2 and CO2 is cooled to the requirement temperature around 313K and enters the absorption tower. The lean solvent is brought into contact with flue gas in the absorber tower. Then the rich solvent leaves at the bottom of the absorber and is pumped to the stripper tower after being preheated. In stripper tower, the rich solvent is regenerated at round 393K and the lean solvent from the base of the tower is cooled and cycled back to the absorber. 2.2.2 Cycle description The demonstration of CA process is based on several key assumptions: (1) No pressure drop; (2) The vapor and liquid are in thermodynamic equilibrium and there is no mass transfer driving force during the absorption and desorption process; (3) Uniform temperature during the absorption and desorption process; (3) The amount of N 2 absorbed is negligible; (4) No heat loss during heat exchange process; and (5) the factors having influencing on the process are ignored including the geometric dimension of absorber and stripper, velocity of flue gas etc. The schematic description of CA cycle is illustrated in Fig.2, which includes four steps of absorption, preheating, desorption and precooling. The details of each step for CA are demonstrated as following: Step 1-2 is the adiabatic CO2 absorption process when the lean solvent contacts the flue gas and captures CO 2 in the absorber through an adiabatic process. In this step, the temperature of the lean solvent increased due to the heat generation of CO2 absorption process. Step 2-3 is the preheating step, in which process the rich solvent is preheated to the desorption temperature (373K to 403K). The temperature of the rich solvent as well as pressure increases during this stage while there is no CO2 desorbed in this stage without introducing vapor phase. Step 3-4 is the isothermal desorption step. In this step, water vapor is generated with continuous heating and pressure decrease and CO2 is released from liquid rich solvent to the vapor phase. Step 4-1 is the precooling step. The lean solvent is cooled to 313K and a new cycle is started. CO2

N2

Lean solvent Rich solvent N2, CO2

Absorption

Desorption

Fig.1. Process schematic for CA process

Fig.2. Demonstration of typical MEA-based CA cycle in the CO2 absorption diagram. Solid lines: empirical model.[7] ; Solid square: experimental data T=313K [8]; Hollow square: experimental data T=393K [9]

4944 4

Junyao Wang et al. / Energy Procedia 158 (2019) 4941–4946 Author name / Energy Procedia 00 (2018) 000–000

2.3 Thermodynamic analysis of CA process 2.3.1 The minimum separation work (Wmin) Wmin for carbon capture system is the minimum work required to separate CO 2 from idea gas mixtures through a reversible process without chemical reactions at constant pressure and temperature [10]. Wmin indicates the level of gas separation theoretically, which only depends on three parameters including the separation temperature, the initial concentration of CO2 and the CO2 recovery ratio. For the typical N2/CO2 separation process, it can be expressed as Eq. 6 [10]. 𝑊𝑊𝑚𝑚𝑚𝑚𝑚𝑚 = 𝑛𝑛𝐶𝐶𝐶𝐶2 𝐺𝐺𝐶𝐶𝐶𝐶2 + 𝑛𝑛𝑊𝑊 𝐺𝐺𝑊𝑊 − 𝑛𝑛𝑀𝑀 𝐺𝐺𝑀𝑀 (6) = 𝑅𝑅𝑅𝑅{⁡𝑛𝑛𝐶𝐶𝐶𝐶2 ∑(𝑦𝑦𝐶𝐶𝐶𝐶2 ,𝑘𝑘⁡ 𝑙𝑙𝑙𝑙𝑦𝑦𝐶𝐶𝐶𝐶2 ,𝑘𝑘⁡ ) + ⁡𝑛𝑛𝑤𝑤 ∑(𝑦𝑦𝑤𝑤,𝑘𝑘⁡ 𝑙𝑙𝑙𝑙 𝑦𝑦𝑤𝑤,𝑘𝑘 ) − ⁡𝑛𝑛𝑀𝑀 ∑(𝑦𝑦𝑀𝑀,𝑘𝑘⁡ 𝑙𝑙𝑙𝑙 𝑦𝑦𝑀𝑀,𝑘𝑘 )} Where ⁡𝑛𝑛𝐶𝐶𝐶𝐶2 , 𝑛𝑛𝑊𝑊 and 𝑛𝑛𝑀𝑀 denotes the moles of CO2, waste gas and gas mixture; G is the Gibbs free energy; and y is the mole fraction of gas. 2.3.2 The second low efficiency (Eff2nd) The second law efficiency is the ratio of the minimum separation work to the actual energy consumption (Wac), which is defined as Eq. 7. 𝑊𝑊 𝑊𝑊𝑚𝑚𝑚𝑚𝑚𝑚 (7) Eff2𝑛𝑛𝑛𝑛 = 𝑚𝑚𝑚𝑚𝑚𝑚 = 𝑇𝑇0 𝑇𝑇0 𝑊𝑊𝑎𝑎𝑎𝑎

𝑊𝑊𝑠𝑠 +𝑄𝑄𝐻𝐻 (1−

𝑇𝑇𝐻𝐻

)−𝑄𝑄𝐿𝐿 (1−

𝑇𝑇𝐿𝐿

)

Where Ws is the shaft work; 𝑄𝑄𝐻𝐻 is the required heat source and 𝑄𝑄𝐿𝐿 is the required cooling source; TH is the temperature of heat source; TL is the temperature of the cooling source; and T0 is the environment temperature. It is assumed the temperature of the cooling source is equal to the environment temperature and Ws of solvent pump is neglected for an ideal cycle. Therefore, the thermodynamic efficiency is mainly determined by 𝑄𝑄𝐻𝐻 . For the typical MEA-based CO2 capture cycle, 𝑄𝑄𝐻𝐻 can be expressed as the total energy consumption of three parts: the sensible heat (qsens) to raise the solvent from temperature downstream the rich-solvent heat exchanger to the regeneration temperature; the heat of water evaporation (qvap) to produce the stripping steam; and the absorption heat (qabs ) [11]. Therefore, 𝑄𝑄𝐻𝐻 can be calculated by Eq. 8 to Eq.13. 𝛼𝛼4 (8) 𝑄𝑄𝑎𝑎𝑎𝑎𝑎𝑎 = 𝐺𝐺𝐶𝐶𝑂𝑂2 ∫𝛼𝛼3 ∆𝐻𝐻𝑎𝑎𝑎𝑎𝑎𝑎 (𝑇𝑇)𝑑𝑑𝑑𝑑 𝐺𝐺𝐶𝐶𝑂𝑂2 = 𝜂𝜂𝜂𝜂𝐶𝐶𝑂𝑂2 𝑉𝑉 (9) Where 𝐺𝐺𝐶𝐶𝑂𝑂2 (mol/s) is the molar flow rate of captured CO2; 𝑋𝑋𝐶𝐶𝑂𝑂2 is the molar fraction of CO2 in the mixture gas; 𝑉𝑉 is the molar flow rate of mixture gas; and 𝜂𝜂 is the CO2 capture rate. 𝑇𝑇

𝑄𝑄𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 = ∫𝑇𝑇 3 𝑚𝑚𝐿𝐿 𝐶𝐶𝑃𝑃 𝑑𝑑𝑑𝑑 2

𝑚𝑚𝐿𝐿 =

𝜂𝜂𝜂𝜂𝐶𝐶𝑂𝑂2

∆𝛼𝛼∙𝑥𝑥𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠

V

(10) (11)

Where 𝑚𝑚𝐿𝐿 is the molar flow of solution; ∆𝛼𝛼 is the CO2 loading difference between the lean and rich solvent; and 𝑥𝑥𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 is the molar fraction of the solvent in the solution. 𝑃𝑃𝐻𝐻 𝑂𝑂 (12) 𝑄𝑄𝑣𝑣𝑣𝑣𝑣𝑣 = 𝐻𝐻𝑣𝑣𝑣𝑣𝑣𝑣 𝐺𝐺𝐶𝐶𝑂𝑂2 2 𝑃𝑃𝐶𝐶𝐶𝐶2

Where 𝐻𝐻𝑣𝑣𝑣𝑣𝑣𝑣 is the heat of water evaporation; 𝑃𝑃𝐻𝐻2𝑂𝑂 is the partial pressure water vapor; and 𝑃𝑃𝐶𝐶𝐶𝐶2 is the partial pressure of CO2 at point 3. The total energy duty could be calculated as Eq.13: (13) 𝑄𝑄𝐻𝐻 = 𝑄𝑄𝑎𝑎𝑎𝑎𝑎𝑎 + 𝑄𝑄𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 +𝑄𝑄𝑣𝑣𝑣𝑣𝑣𝑣 3. Results and discussion

Results of the thermodynamic analysis for a typical MEA-based CA cycle are discussed in the following section. The operation parameters for base case is listed in Table 1.



Junyao Wang et al. / Energy Procedia 158 (2019) 4941–4946 Author name / Energy Procedia 00 (2018) 000–000

4945 5

Table 1. Operation Parameters for base case Item

Parameter

Item

Parameter

Solvent

30wt% MEA

CO2 capture rate

90%

Absorption T

313K

CO2 concentration in mixture gas

10%

The logarithmic mean temperature difference for heat exchanger

5K

Heat of water evaporation

41kJ/kg

Partial pressure ratio for lean solvent*

150

Partial pressure ratio for rich solvent

1

* The partial pressure ratio between the CO2 in sweet gas and equilibrium CO2 partial of lean solvent 3.1 Effect of desorption temperature Fig.3 and Fig.4 show the variation of heat duty and second-law efficiency of a typical MEA-based CA cycle with the change of desorption temperature. It is obvious that the 𝑄𝑄𝑎𝑎𝑎𝑎𝑎𝑎 accounts for the most major of total energy duty from 65% to 76% over the temperature stabilizing at around 2200KJ/kg. However, sensible heat shows significant upward trend with the growth of temperature, which is absolutely true as the temperature difference in Eq.10 increases. However, it should be noted that, although making up the least contribution, 𝑄𝑄𝑣𝑣𝑣𝑣𝑣𝑣 does show a notable decline trend with the increasing of desorption temperature because less stripping stream is needed along with temperature growth. The minimum separation work keeps constant at around 7.8 KJ/mol as it only depends on the separation temperature, capture rate as well as the CO2 concentration in the mixture gas. The second law efficiency, in contrast, presents significant decrease trend with temperature rising as both 𝑄𝑄𝐻𝐻 and 𝑇𝑇𝐻𝐻 increased in Eq. 7. 3.2 Effect of partial pressure ratio for rich solvent Under the base case condition of desorption, the CO2 partial pressure of the end of preheating step is approximately 1393 kPa at equilibrium status. However, in a real process, there is driving force over the stripper height. Therefore, it is assumed that a partial pressure ratio for rich solvent is 10, which indicates that the partial pressure of CO2 is 10 𝑃𝑃𝐻𝐻 𝑂𝑂 times less than that of the equilibrium status. Fig. 5 and 6 indicates the 2 in Eq.12 at the partial pressure ratio of 10 𝑃𝑃𝐶𝐶𝐶𝐶2

for rich solvent as well as corresponding heat duty. It is clear that, the sensible heat and water vaporization heat show opposite trend with increasing of temperature when the partial pressure ratio for rich solvent is 10. When the temperature is relatively low, the 𝑄𝑄𝑣𝑣𝑣𝑣𝑣𝑣 is higher than 𝑄𝑄𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 , which is the predominant factor affecting the total energy duty. However, with the rising of temperature, 𝑄𝑄𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 increases considerably and the energy duty reaches the bottom at around 383K. In terms of the second law efficiency, it is noted that although with heat duty fluctuation over the temperature range, the Eff2nd shows decline trend continuously due to the increase of TH. 0.40

8.0

Qabs

Qsens

3500

7.8

0.35

Wmin (kJ/mol)

3000

Q (kJ/kgCO2)

2500 2000 1500 1000

7.6 0.30 7.4

Eff2nd Wmin

7.2

Eff2nd (%)

Qvap

0.25

500 0

100

105

110

115

120

125

T (℃)

Fig.3. Heat duty variation of MEA-based CA cycle with desorption temperature

7.0

100

105

110

115

120

125

0.20

T (℃)

Fig.4. Wmin and Eff2nd variation of MEA-based CA cycle with desorption temperature

Junyao Wang et al. /Procedia Energy Procedia 158 (2019) 4941–4946 Author name / Energy 00 (2018) 000–000

Qvap

4500

Qabs

1.8

Qsens

4000

Eff2nd

0.24

1.6

3500 3000

PH2O/PCO2

Q (kJ/kgCO2)

0.26

PH2O/PCO2

1.7

2500 2000

1.5

0.22

1.4 0.20

Eff2nd (%)

64946

1.3

1500

0.18

1.2

1000 1.1

500

1.0

100

105

110

115

120

125

0.16 100

110

115

120

125

T (℃)

T (℃)

Fig.5. Heat duty variation of MEA-based CA cycle with desorption temperature at partial pressure ratio of 10 for rich solvent

105

Fig.6.

𝑃𝑃𝐻𝐻2 𝑂𝑂 𝑃𝑃𝐶𝐶𝑂𝑂2

and Eff2nd variation of MEA-based CA cycle with

desorption temperature at partial pressure ratio of 10 for rich solvent

4. Conclusion A MEA-based chemical absorption thermodynamic cycle is established based on the CO2 absorption diagram. Key cyclic parameters including desorption temperature and the partial pressure ratio for rich solvent are analyzed. Results show that in a relative ideal cycle with partial pressure ratio for rich solvent at 1, the energy duty shows an upward trend with the rising of desorption temperature as sensible heat increases significantly and dominated the total energy consumption. However, when the partial pressure ratio for rich solvent is set at 10 which is close to the real cycle, the effect of partial pressure ratio between water vapor and CO2 is enlarged and there is a balance temperature shows the minimum energy duty. It is predicted when the he partial pressure ratio for rich solvent is large enough, the water evaporation heat will dominate the tendency of heat duty. Acknowledgements The authors are grateful for the support provided by the China National Natural Science Funds under Grant No. 51506149; The Research Plan of Application Foundation and Advanced Technology of Tianjin City under Grant No. 15JCQNJC06700; and The Research Plan of Key Laboratory of Solar Energy Jiangsu Province (Southeast University). References: [1] Zhiwu, et al., Review on current advances, future challenges and consideration issues for post-combustion CO2 capture using amine-based absorbents. Chinese Journal of Chemical Engineering, 2016. 24(2): p. 278-288. [2] Liang, Z., et al., Recent progress and new developments in post-combustion carbon-capture technology with amine based solvents. International Journal of Greenhouse Gas Control, 2015. 40: p. 26-54. [3] Wilcox, J., Carbon Capture. 2012: Springer New York. 219-229. [4] Zhao, R., et al., Carbon pump: Fundamental theory and applications. Energy, 2017. 119: p. 1131-1143. [5] Xu and Qing, Thermodynamics of CO₂ loaded aqueous amines. 2011. [6] DIPPR Provo, U.B.D., Thermophysical Properties Laboratory. 1998. [7] Xu and Qing, Thermodynamics of CO₂ loaded aqueous amines. 2011. [8] Jou, F.Y., A.E. Mather and F.D. Otto, The solubility of CO2 in a 30 mass percent monoethanolamine solution. Canadian Journal of Chemical Engineering, 1995. 73(1): p. 140-147. [9] Aronu, U.E., et al., Solubility of CO 2 in 15, 30, 45 and 60 mass% MEA from 40 to 120 °C and model representation using the extended UNIQUAC framework. Chemical Engineering Science, 2011. 66(24): p. 6393-6406. [10] Zhao, R., et al., Carbon pump: Fundamental theory and applications. Energy, 2017. 119: p. 1131-1143. [11] Oexmann, J. and A. Kather, Minimising the regeneration heat duty of post-combustion CO2 capture by wet chemical absorption: The misguided focus on low heat of absorption solvents. International Journal of Greenhouse Gas Control, 2010. 4(1): p. 36-43.