Novel Biphasic Solvent with Tunable Phase Separation for CO2 Capture

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A novel biphasic solvent with tunable phase separation for CO2 capture: role of water content in mechanism, kinetics, and energy penalty Jiexu Ye, Chenkai Jiang, Han Chen, Yao Shen, Shihan Zhang, Lidong Wang, and Jianmeng Chen Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.9b00040 • Publication Date (Web): 20 Mar 2019 Downloaded from http://pubs.acs.org on March 21, 2019

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Environmental Science & Technology

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A novel biphasic solvent with tunable phase separation for

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CO2 capture: role of water content in mechanism, kinetics,

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and energy penalty

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Jiexu Yea, Chenkai Jianga, Han Chenb, Yao Shena, Shihan Zhanga,*, Lidong Wangc,

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Jianmeng Chena

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a

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China

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b Zhejiang University of Water Resource and Electric Power, Hangzhou, 310018, China

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c

College of Environment, Zhejiang University of Technology, Hangzhou, 310014,

School of Environmental Science and Engineering, North China Electric Power

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University, Baoding 071003, China

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ABSTRACT: The biphasic solvent-based absorption process has been regarded as a

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promising alternative to the monoethanolamine (MEA)-based process due to its high

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absorption capacity, phase separation behavior, and potential for conserving energy for

14

CO2 capture. A tradeoff between the absorption capacity and phase separation ratio is

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critical to develop an advanced biphasic solvent. Typically, water content in the

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biphasic solvent can be manipulated to tune the phase separation behavior. To explore

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the relationship between water content and phase separation behavior, an inert organic

18

solvent, 1-methyl-2-pyrrolidinone, was added as a substitute for water in a biphasic

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solvent, specifically a triethylenetetramine (TETA) and 2-(Diethylamino)ethanol

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(DEEA) blend. Moreover, the water contents-kinetics and thermodynamics

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relationships were also evaluated. Experimental results revealed that reducing the water

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content was beneficial for phase separation but adverse for adsorption capacity. Kinetic

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analysis indicated that the water content did not significantly affect the rate of CO2

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absorption at a rich loading. Furthermore, the regeneration heat decreased with the 1 ACS Paragon Plus Environment


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water content. The regeneration heat of TETA-DEEA with a water content of 20wt%

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was almost 50% less than that of MEA solution.

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analysis revealed that the water content did not affect the reaction mechanism between

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CO2 and amines.

13C

nuclear magnetic resonance

29 30

1. INTRODUCTION

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Carbon dioxide (CO2) emissions account for half of the increase in the greenhouse

32

effect.1, 2 Because fossil fuel is the most competitive option for energy demand, the

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fossil fuels-fired power plant will be the main source of CO2 emissions.3, 4 In the short

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term, a viable strategy to reduce carbon emission from power plants is carbon capture

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and sequestration.5,

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state-of-the-art but costly.7, 8 Its deployment in power plants has been estimated to result

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in cost of electricity rising by 70-100%.9, 10 The major cost is attributed to the large

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energy penalty for the solvent regeneration.11, 12 To cut down the energy consumption,

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the development of novel solvents is urgently required. 13-17

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Monoethanolamine (MEA)-based CO2 capture technology is

40 41

Biphasic solvent systems have demonstrated remarkable promise for reducing the

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amount of energy required for CO2 capture.18, 19 Typically, liquid-liquid or solid-liquid

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phase changes occur after CO2 absorption into the solvent, and the majority of the CO2

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is loaded in one phase.20 Since just the CO2-enriched phase should be regenerated,

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energy penalty can be significantly reduced.21, 22 To date, a series of investigations on

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biphasic solvents have subsequently emerged. For example, Broutin et al.22 tested the

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performance of the DMX solvent-based process with liquid-liquid phase separation.

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The amine-based blends of 2-(diethylamino)-ethanol (DEEA) and 1,4-butanediamine

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(BDA),23

N-methyl-1,3-propane-diamine

(MAPA)

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and

DEEA

,11,

19,

24,

25

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N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA) and diethylenetriamine

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(DETA),24 MEA and sulfonate,26 and 2-((2-aminoethyl) amino) ethanol (AEEA) and

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DEEA27 presented the liquid-liquid phase change as the CO2 was absorbed. Moreover,

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the kinetics and regeneration heat of the reported solvents were also evaluated. Besides

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the amine-based solvents, ionic liquid blend such as [TETAH] [Lys]-ethanol-water was

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also found to encounter phase separation.28-31

56 57

Currently, Zhang et al. provided a comprehensive overview of the development of

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biphasic solvents and their enabled processes for CO2 capture.32 All of the reported

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biphasic solvents demonstrated a lower energy penalty compared with MEA solution.

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However, for most of the reported biphasic solvents, the volumetric ratio of the lower

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phase increased with the increase in CO2 loading,20, 33 even becoming one single phase

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in some cases, depending upon the ratio of the primary amine and tertiary amine.34 For

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MAPA-DEEA blend, in presence of high concentration of the tertiary amine DEEA,

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two liquid phases existed even at its maximum loading.11 Therefore, tradeoff between

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the phase separation behavior and absorption capacity and rate is critical to develop an

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advanced biphasic solvent.35,

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become a limitation for the development of novel biphasic solvents.13,

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relationship between the composition of a solution and the absorption capacity, kinetics,

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phase separation behavior, and energy consumption of biphasic solvent requires

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investigation.

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Furthermore, the intensity of solvent screening has 23, 34

The

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Typically, the water content in a biphasic solvent can tune the phase separation behavior

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as a result of its effect on the ionic strength of the CO2-enriched phase, and thus alter

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the salt-out effect.37 However, water content has also been reported to affect the kinetics 3 ACS Paragon Plus Environment


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of CO2 absorption into an amine-based solvent.38, 39 According to Hwang’s work, the

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presence of water improves the CO2 absorption rate of primary and secondary amines

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because water transfer protons from zwitterion to unreacted amine molecules based on

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zwitterion theory.38 Moreover, water directly participates in the reaction between CO2

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and tertiary amine as follows:

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𝐶𝑂2 + 𝑅1𝑅2𝑅3𝑁 + 𝐻2𝑂↔𝑅1𝑅2𝑅3𝑁𝐻 + + 𝐻𝐶𝑂3―

(1)

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However, limited information is available regarding the effects of water content on the

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performance of biphasic solvent for CO2 capture. As a result, the relationship between

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water content and the absorption capacity, kinetics, phase separation behavior, and

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energy consumption of biphasic solvents must be investigated.

85 86

In this work, a triethylenetetramine (TETA)-DEEA blend was used as the biphasic

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solvent because of its high absorption capacity and absorption rate, as reported in our

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previous work.21 1-Methyl-2-pyrrolidinone (NMP), an inert organic solvent, was used

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to regulate the water content in TETA-DEEA solution because it is inert for CO2

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absorption and chemically stable.40,

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separation behavior of CO2 absorption into TETA-DEEA solution was carried out.42

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Moreover, the effects of different water contents on the kinetic and thermodynamic

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properties of CO2 absorption into the TETA-DEEA solution were also determined. The

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impact of NMP on the absorption mechanism between TETA-DEEA and CO2 was

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investigated through

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research advances the development of a promising biphasic solvent with tunable phase

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separation behavior for CO2 capture.

13C

41

The effect of the water content on the phase

nuclear magnetic resonance (NMR) analysis.43,

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This

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2. MATERIALS AND METHODS

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2.1. Chemicals

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TETA with a purity over 68%, MEA with a purity over 99%, and NMP with a purity

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over 99% were provided by Aladdin Industrial Corporation, China. DEEA with a purity

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over 99% was supplied by Macklin, China. CO2 and N2 with a purity over 99.99vol%

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were obtained from Jingong Gas Co., China.

105 106

2.2. Experimental Procedure

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The CO2 absorption into TETA-DEEA blend with different water contents (20, 25, 30,

108

and 46wt%) were investigated in a bubbling reactor with a volume of 50 mL. The

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solvent with a water content of 46wt% was the neat TETA-DEEA blend without the

110

addition of NMP. 13vol% of the CO2 equilibrated with N2 was introduced into 30 mL

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of the tested solvent at 200 mL min-1, 40 °C, and 1atm. The total concentration of amine

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in the tested solvent was 4 M and the ratio of TETA to DEEA was 1:3. The data of

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Vapor-liquid equilibrium (VLE) were measured under various temperatures (40-60 °C)

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and CO2 partial pressure (0.09-13 kPa). After equilibrium was reached, as indicated by

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the same CO2 concentrations in the inlet and outlet, the two liquid phases, if available,

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were collected, and the CO2 loadings were determined using a Chittick apparatus.45 The

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samples (0.5 mL) collected from the upper and lower phase (if formed) were titrated by

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2M hydrochloric acid. The released CO2 was collected and determined by a graduated

119

measuring tube. The CO2 concentration was calculated based on the measured CO2

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amount and the volume of the sampled solvent. The total absorbed CO2 amount was the

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sum of amount loaded in two-liquid phases (if formed). The CO2 loading of the solvent

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was determined based on the basis of the total amine content in both phases.

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A double stirred cell reactor (DSCR, 500 mL), reported in the previous work,34 was

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employed to determine the CO2 absorption rates. The internal diameter and internal

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cross-section area of the DSCR were 6 cm and 28.3 cm2, respectively. Two different

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concentrations (1.5 and 15vol%) of N2-balanced CO2 were introduced into 250 mL

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CO2-lean and CO2-rich TETA-DEEA solutions at a gas flow rate of 1 L min-1 at various

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temperatures (40, 50, and 60 °C) and water contents (20, 25, 30, and 46wt%). The

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agitation rates in the gas phase (250 rpm) and liquid phase (300 rpm) were controlled

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to achieve homogeneous mixing and smooth gas-liquid interface. A soap-ﬁlm

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ﬂowmeter (see Figure S1) was used to determine the absorption rate. As the gas was

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introduced from the bottom into the graduated glass tube, a smooth soap bubble formed

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and rose gradually. The time elapsed between the displacement of the soap film was

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recorded at the same distance. Based on the recorded time and the occupied gas volume

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in the glass tube, the gas flow rate can be determined. Therefore, the absorption rate of

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CO2 can be determined as:46

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𝑁=―

𝑇0(𝑄𝑜𝑢𝑡 ― 𝑄𝑖𝑛) 𝑉𝑀,0𝑇𝑅𝐴

#(2)

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where N (kmol m-2 s-1) is the absorption rate; Qin and Qout (m3 s-1) are the gas flow rates

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at the inlet and outlet, respectively; TR (K) and T0 (K) are the room and standard

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temperatures, respectively; VM,0 (m3 kmol-1) is the gaseous molar volume at T0 and 1atm,

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and A (m2) is the gas-liquid interface area. Moreover, the details of the determination

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method of the mass transfer resistance, rate constant, and enhancement factor can be

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found in the Supporting Information.

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2.3. Thermodynamic analysis

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The estimation of regeneration heat (Qregen) uses the method reported in the literature.47 6 ACS Paragon Plus Environment

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Qregen is composed of reaction heat (Qrxn), latent heat (Qlatent) and sensible heat (Qsens),

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which can be calculated as follows:47, 48

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Q𝑟𝑒𝑔𝑒𝑛 = 𝑄𝑟𝑥𝑛 + 𝑄𝑠𝑒𝑛𝑠 + 𝑄𝑙𝑎𝑡𝑒𝑛𝑡#(3)#

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𝑄𝑟𝑥𝑛 = ― m𝑎𝑚

∫

𝑟𝑖𝑐ℎ

∆ℎ𝑟𝑥𝑛(𝛼)𝑑𝛼#(4)# 𝑙𝑒𝑎𝑛

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𝑄𝑠𝑒𝑛𝑠 = m𝑎𝑚(𝑐𝑎𝑚 + 𝑟𝑊𝑐𝑊 + 𝛼𝑙𝑒𝑎𝑛𝑐𝐶𝑂2)(𝑇𝑏𝑜𝑡 ― 𝑇𝑡𝑜𝑝)#(5)#

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𝑄𝑙𝑎𝑡𝑒𝑛𝑡 = m𝑊𝜆#(6)#

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where m𝑎𝑚 is the amine amount in the recycled solvent to capture one ton CO2, kmol;

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𝛼 is the CO2 loading, kmol kmol-1; ∆ℎ𝑟𝑥𝑛(𝛼) is the enthalpy of the CO2 absorption at

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various CO2 loading, kJ kmol-1 ; 𝑟𝑊 is the ratio in molarity between water and amines

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in the fresh solution; 𝑐𝑎𝑚, 𝑐𝑊, and 𝑐𝐶𝑂2 are the heat capacity of amines, water, and

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CO2, respectively, kJ kmol-1 K-1 ; 𝛼𝑙𝑒𝑎𝑛 is the CO2 lean loading, kmol kmol-1; m𝑊 is

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the amount of stripped water vapor to produce one ton CO2, kmol; and 𝜆 is the latent

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heat of water, kJ kmol-1. In this work, we assumed that the heat capacity of the solvent

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is equal to the water.

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Based on the VLE data, Gibbs-Helmholtz equation was used to determine the CO2

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absorption enthalpy:47

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―

∆ℎ𝑟𝑥𝑛 𝑅

=

[ ()] ∗ ∂𝑙𝑛𝑃𝐶𝑂 2

1 ∂ 𝑇

#(7)#

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∗ where 𝑃𝐶𝑂 is the equilibrium partial pressure of CO249, kPa, which can be obtained 2

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from the measured VLE curve. The m𝑊 was estimated by:

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m𝑊 =

𝑀𝑃𝑤(𝑇𝑡𝑜𝑝)

#(8)# ∗ ( ) 𝑃𝐶𝑂 𝑇 ,𝛼 𝑡𝑜𝑝 𝑡𝑜𝑝 2 7

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∗ ( 𝑃𝐶𝑂 𝑇𝑡𝑜𝑝,𝛼𝑡𝑜𝑝) = 𝑃𝑡𝑜𝑡𝑎𝑙 ― 𝑃𝑊(𝑇𝑡𝑜𝑝)#(9)# 2

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∗ ( 𝑃𝑡𝑜𝑡𝑎𝑙 = 𝑃𝐶𝑂 𝑇𝑏𝑜𝑡,𝛼𝑏𝑜𝑡) + 𝑃𝑊(𝑇𝑏𝑜𝑡)#(10)# 2

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(

∗ ( ∗ ( 𝑃𝐶𝑂 𝑇𝑏𝑜𝑡,𝛼𝑏𝑜𝑡) = 𝑃𝐶𝑂 313,𝛼𝑏𝑜𝑡)exp ― 2 2

∆ℎ𝑟𝑥𝑛 1 1 ― 𝑅 𝑇𝑏𝑜𝑡 313

(

Page 8 of 35

))

#(11)#

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Where 𝑀 is the value of CO2 in molarity for one ton CO2, kmol; 𝑃𝑤 is the steam

173

partial pressure, kPa; 𝑃𝑡𝑜𝑡𝑎𝑙 is the total pressure in the striping column, kPa; 𝛼𝑏𝑜𝑡 and

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𝛼𝑡𝑜𝑝 are the CO2 loadings in the solvent at the bottom and top of stripping column,

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kmol kmol-1. The corresponding VLE data at the stripping temperature (e.g., 120 oC)

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was estimated by Gibbs-Helmholtz equation. Typically, the total pressure in the

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tripping column is considered to be the pressure at the bottom of stripping column.48, 50

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2.4. 13C NMR Analysis.

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The NMR spectra of TETA-DEEA blends with various loadings of CO2 at three water

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contents of 20, 30, and 46wt% were measured by a 600 MHz DD2 spectrometer with

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one probe head (Agilent) at a frequency of 150.83 MHz. The 13C NMR spectra were

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recorded by applying a delay time of 2 s, acquisition time of 0.8651 s, and 1D sequence

184

of 512 scans at 25 °C. The obtained spectra were analyzed by ACD-LAB 6.0 and

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MestReNova. The details of the NMR technology description can be found in the

186

literature.29, 34, 39

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3. RESULTS AND DISCUSSION

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3.1. Water content-absorption capacity and phase separation behavior

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relationship

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Figure 1 presents the relationship between water content and CO2 absorption capacity.

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The experimental results revealed that the absorption capacity of TETA-DEEA 8 ACS Paragon Plus Environment

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declined as the water content decreased, implying that water acted as a reactive

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component and that it plays a crucial role in CO2 absorption. As shown in Figure 1, the

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absorption capacity decreased from 0.93 to 0.58 mol mol-1 as the water content in the

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TETA-DEEA solvent decreased from the 46 to 20wt%. However, compared with the

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MEA solution, all the tested solvents achieved a higher absorption capacity.

198 199

Figure 2 displays the process adopted for measuring the phase separation behavior of

200

the blends with different water contents. Notably, only one solvent (Figure 2a, water

201

content = 46 wt%) transformed from one single phase to two-liquid phases with the

202

increase of CO2 loading; and it subsequently reconverted into one single phase at a CO2

203

loading of 0.97 mol mol-1. The other three tested solvents exhibited the same

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phenomenon of maintained biphasic status following the occurrence of liquid-liquid

205

phase separation. Furthermore, the volumetric ratio of the lower phase declined as the

206

water content in the biphasic solvent was decreased. For example, at a CO2 loading of

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~0.5 mol mol-1, the lower phase ratio of the 20wt% water content solvent was 42%,

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which was decreased by 43% compared with that of the 46wt% water content solvent

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(Figure 2). The similar phenomena were also observed under the other tested loadings.

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These results indicated that the phase separation behavior could be tuned by varying

211

the water content in the biphasic solvent. The solvent polarity was expected to be lower

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in the case of low water content in the solvent, meaning that the salt-out effect would

213

be more significant. In addition, more DEEA should remain in the solvent with the

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decrease in water content because the occurrence of the direct reaction between tertiary

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amine and CO2 in presence of the water decreased, leading to an increase in the ratio of

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the upper phase.



218

3.2. Water content-kinetics relationship

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The CO2 absorption rates into the tested solutions with the various water contents were

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determined at both lean and rich loadings to simulate the reaction occurred at the top

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and bottom of the absorption column, respectively. The selected lean loading was 0.25

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mol mol-1 and the selected rich one was 90% of the maximum loading. The measured

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CO2 absorption rate into MEA solution was comparable with the literature data,8, 51

224

confirming that the employment of the DSCR for the kinetic measurement is relevant.

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As indicated in Figure 3, the CO2 absorption rate into the lean solution increased with

226

the increase in temperature, which was contrary to the result of the rich-loading solution.

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Notably, at the same temperature, the CO2 absorption rate into TETA-DEEA decreased

228

with the decrease of the water content. For instance, at 40 oC, the rates of CO2

229

absorption into the 20wt% water content solvent at lean and rich loading were 55% and

230

26% lower than those into the 46wt% water content solvent, respectively, indicating

231

that water may play a role in CO2 absorption.12, 39, 46 The CO2 absorption rates of the

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CO2-lean solution with all water contents were comparable with benchmark MEA.25, 51

233 234

The data of the gas-side, liquid-side, and total mass transfer resistance are presented in

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Table 1. As shown in Table 1, for the lean solution, compared with the mass transfer

236

resistance at the liquid side, the gas-side resistance was greater under various water

237

contents and temperatures. Regarding the rich solution, the liquid-side resistance was

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greater at 40 °C, and with the increase in temperature from 40 to 60 °C, the gas-side

239

resistance became more significantly increased compared with the liquid-side

240

resistance at 60 °C, even exceeding 50% of the total mass transfer resistance, thus

241

occupying a dominant position because of the high reaction rate between CO2 and

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amine at high temperatures. For both the lean and rich solutions, the liquid-side 10 ACS Paragon Plus Environment

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resistance increased as the water content decreased (Table 1). For example, at 50 °C in

244

the lean solution (0.25 mol mol-1), with the decrease in water content from 46 to 20wt%,

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the percentage of liquid-side resistance increased from 3.0% to 7.9%, and the liquid-

246

side resistance increased from 50.9% to 76.4% in the rich solution, confirming that the

247

reaction rate between CO2 and amine decreased under the condition of low water

248

content. In general, the liquid-side resistance increased with water content at low and

249

high loadings and decreased with increasing temperature.

250 251

3.3. Water content-Thermodynamics relationship

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Figure 4 presents the VLE data of MEA solution and the tested biphasic solvents. The

253

VLE curve of the MEA has the shape of an inverse S (Figure 4), and the measured MEA

254

data is in good agreement with the data reported in the literature, thus confirming the

255

relevance of the employed method for VLE data measurement.52-55 As expected, with

256

the increase in temperature, the CO2 partial pressure in equilibrium gradually increased

257

under the same CO2 loadings (Figure 4).

258 259

Figure 5 displays the relationships between water content and regeneration heat, latent

260

heat, sensible heat, and reaction heat. A preliminary analysis revealed that the

261

regeneration heat gradually decreased with lower water content. For example, at a lean

262

loading of 0.25 mol mol-1, lowering the water content from 46 to 20wt% resulted in a

263

decrease in the regeneration heat from 2.98 to 2.15 GJ/t CO2. Conversely, as the lean

264

loading increased, the regeneration heat exhibited a decreasing tendency. Figure 5b

265

reveals that the latent heat significantly decreased in accordance with water. This was

266

because of that the water vapor pressure prevailed in the gas phase decreased in

267

accordance with the water content in the solution. Notably, the sensible heat at a water 11 ACS Paragon Plus Environment


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content of 20wt% was higher than that at a water content of 46wt% (Figure 5c). This is

269

attributable to the low absorption capacity at a water content of 20wt% and thus the

270

greater amount of solution requires to capture CO2, resulting in the increase of sensible

271

heat. As shown in Figure 5d, the reaction heat decreased with the decrease in water

272

content, indicating that the water participated in the CO2 absorption reaction. Moreover,

273

the regeneration heat of TETA-DEEA with the different water contents were much

274

lower than that of MEA. The regeneration heat of the biphasic solvent with a water

275

content of 20wt% was almost 50% less than that of the 5 M MEA solvent. Overall, the

276

low water content was demonstrated to be beneficial for reducing the energy

277

consumption of the biphasic solvent to capture CO2.

278 279

3.4. Effect of water content on the absorption mechanism

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NMR refers to a non-invasive analytical method for analyzing the entire process of CO2

281

absorption into a solvent. 13C NMR analysis can be used to evaluate the distribution of

282

substances in an amine-CO2-H2O system. Moreover, with this method, all information

283

regarding C-containing materials other than H2O, H3O+ and OH- can be obtained

284

directly. Relevant studies have applied NMR to analyze the mechanism of the reaction

285

between amines and CO2.56, 57 In this work, the fresh solvent as well as the lower and

286

upper phases of TETA-DEEA blend with lean and rich CO2 loadings under three

287

different water contents (20, 30, and 46wt%) were sampled for 13C NMR analysis to

288

clarify the effect of water content on the CO2 absorption mechanism.

289 290

As displayed in Figure 6, the characteristic peaks corresponding to TETA were detected

291

at the signals of 42.5, 53.4 and 50.2 ppm for all the tested solutions. The detection of

292

the signals at 11.3, 47.7, 55.3 and 59.2 was assigned to DEEA based on the data reported 12 ACS Paragon Plus Environment

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by Ciftja et al..58 Moreover, the characteristic peaks corresponding to NMP were

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detected at signals of 19.8, 32, 33.2, 52.8, and 180.4 ppm for the solvent with water

295

contents of 20 and 30wt%. Upon CO2 absorption, all the tested solvents were separated

296

into two liquid phases. TETA was prevalent in the lower phase, and DEEA was mainly

297

observed in the upper phase. NMP was detected in both of the two phases. With the

298

increase in CO2 loading for each water content, the values of the characteristic peaks

299

for TETA were considerably decreased in the lower phase. Since TETA contains two

300

primary and two secondary amine groups, multiple amine group participated into the

301

CO2 absorption with the increase in CO2 loading, and thus four types of carbamates

302

were generated as shown in the Figure S2. Following further increase in CO2 loading

303

from lean to rich, characteristic peaks corresponding to CO32- and HCO3- were detected

304

at the signals of 158 and 161 ppm in the lower phase, which were generated due to the

305

reaction between CO2 and DEEA, proving that tertiary amine was involved in the

306

reaction. Moreover, the characteristic peaks corresponding to DEEA were also

307

observed in the lower phase, confirming that the tertiary amine DEEA shifted from the

308

upper to lower phase. Typically, the reaction between amine and CO2 follows the order

309

of primary, secondary, and tertiary amine. Notably, in the lower phase of the rich

310

solution, the characteristic peaks of TETA were also detected, which indicates that

311

TETA may be regenerated by DEEA.

312 313

Compared with the NMR spectra of the TETA-DEEA solution in the absence of NMP

314

(46wt% water content) under various CO2 loadings, the corresponding NMR spectra of

315

the TETA-DEEA solution with NMP (20 and 30wt% water content) were similar,

316

suggesting that the addition of inert organic compound, such as NMP, does not affect

317

the reaction mechanism between CO2 and TETA-DEEA blend.59, 13 ACS Paragon Plus Environment

60

Moreover, the


318

intensities of the NMP characteristic peaks kept the same in the upper and lower phases,

319

indicating that the distribution of NMP was dynamically balanced rather than

320

transferring from one phase to another during the CO2 absorption. In addition, no

321

carbamate generation as a result of the CO2 and NMP reaction was detected, confirming

322

that NMP did not participate in CO2 absorption. With CO2 absorption, the pH value of

323

the solution decreased. As a result, a slight chemical shift of the characteristic peaks of

324

the carbon in DEEA, TETA, and NMP was observed (see Table S1).

325 326

According to NMR analysis, the mechanism of CO2 reaction with the TETA-DEEA

327

under the various water contents can be proposed (Figure 7). Overall, the water content

328

did not affect the CO2 absorption mechanism. Because NMP was not involved in CO2

329

absorption, the main reaction can be described as follows:

330

𝑇𝐸𝑇𝐴 + 𝐶𝑂2↔[𝑇𝐸𝑇𝐴] + 𝐶𝑂𝑂 ― #(𝑅1)#

331

[𝑇𝐸𝑇𝐴] + 𝐶𝑂𝑂 ― + 𝑇𝐸𝑇𝐴↔[𝑇𝐸𝑇𝐴]𝐻 + + [𝑇𝐸𝑇𝐴]𝐶𝑂𝑂 ― #(𝑅2)#

332

[𝑇𝐸𝑇𝐴] + 𝐶𝑂𝑂 ― + 𝐷𝐸𝐸𝐴↔[𝐷𝐸𝐸𝐴]𝐻 + + [𝑇𝐸𝑇𝐴]𝐶𝑂𝑂 ― #(𝑅3)#

333

[𝑇𝐸𝑇𝐴]𝐻 + + 𝐷𝐸𝐸𝐴↔[𝑇𝐸𝑇𝐴]𝐻 + + 𝑇𝐸𝑇𝐴#(𝑅4)#

334

As indicated in Figure 7, CO2 first reacts with the primary amine group in TETA, and

335

then reacts with its secondary amine group. With the consumption of TETA, the tertiary

336

amine, DEEA, transfers from upper to lower phase to regenerate TETA, leading to

337

sustained CO2 absorption. Meanwhile, the shift of DEEA from upper to lower phase

338

also resulted in the increased volume in the lower phase.

339 340

In summary, this study demonstrated that the variation of water content in biphasic

341

solvents can be applied to tune phase separation behavior. However, absorption

342

capacity and rates decreased with reduced water content in the solvent, indicating that 14 ACS Paragon Plus Environment

Page 14 of 35

Page 15 of 35


343

water plays a role in CO2 absorption. Moreover, lowering the water content in the

344

biphasic solvent was beneficial for reducing the energy penalty of CO2 capture caused

345

by decreases in latent and reaction heat. 13C NMR analysis revealed that the presence

346

of the inert organic compound did not affect the absorption mechanism. The CO2

347

absorption into the TETA-DEEA blend was initiated by the reaction between TETA

348

and CO2, and DEEA acted as the regenerating agent for the protonated TETA to sustain

349

CO2 absorption.

350 351

AUTHOR INFORMATION

352

Corresponding Author

353

*(S.H.Z.) Tel: (+86)57188320853; E-mail: [email protected]

354

Note

355

The authors declare no competing financial interest.

356 357

ACKNOWLEDGMENTS

358

We appreciate the financial support from National Natural Science Foundation of China

359

(Nos. 21606204, 21876157), Program for Changjiang Scholars and Innovative

360

Research Team in University (IRT13096), and Zhejiang University of Technology

361

Initial Research Foundation (No. 2017129000729).

362 363

SUPPORTING INFORMATION

364

The additional table provide information regarding the chemical shift of carbon in NMP,

365

DEEA, and TETA. The additional figures depict the soap-film flowmeter, potential

366

TETA-based carbamates, the double-stirred cell reactor, and bubbling reactor.



Page 16 of 35

368

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Page 24 of 35

Table 1. Mass transfer resistances of CO2 absorption into TETA-DEEA with various water contents. Water content (wt%)

T(K) 313

46

323 333 313

30

323 333 313

25

323 333 313

20

323 333

CO2 loading (mol mol-1)

𝑷𝑪𝑶𝟐 (kPa)

𝑪𝒂𝒎𝒊𝒏𝒆 (kmol m-3)

𝑫𝑪𝑶𝟐,𝒂𝒎𝒊𝒏𝒆 (m2 s-1)

0.25 0.75 0.25 0.75 0.25 0.75 0.25 0.70 0.25 0.70 0.25 0.70 0.25 0.65 0.25 0.65 0.25 0.65 0.25 0.55 0.25 0.55 0.25 0.55

1.3 13 1.3 13 1.3 13 1.3 13 1.3 13 1.3 13 1.3 13 1.3 13 1.3 13 1.3 13 1.3 13 1.3 13

3.5 2.0 3.5 2.0 3.5 2.0 3.5 2.2 3.5 2.2 3.5 2.2 3.5 2.4 3.5 2.4 3.5 2.4 3.5 2.8 3.5 2.8 3.5 2.8

3.99E-10 8.62E-11 4.75E-10 1.32E-10 6.72E-10 1.92E-10 3.42E-10 4.51E-11 4.98E-10 7.72E-11 6.05E-10 1.10E-10 2.89E-10 1.60E-11 4.01E-10 3.02E-11 5.21E-10 5.45E-11 2.48E-10 1.29E-11 3.63E-10 1.76E-11 4.98E-10 2.13E-11

𝒌𝑮 (m

s-1)


564


𝒌𝑳 (m

s-1)


Total resistance (1/𝒌𝑳)

Gas-side resistance (%)

Liquid-side resistance (%)

1092 1161 1405 1251 1983 1417 1067 1218 1484 995 1916 1146 1011 1020 1350 739 1782 863 948 1217 1288 998 1755 709

94.4 35.1 97.0 49.1 98.4 62.8 89.5 23.9 94.4 46.7 96.7 58.2 86.6 16.7 92.9 38.5 95.9 53.6 85.4 13.6 92.1 23.6 95.6 43.8

5.6 64.9 3.0 50.9 1.6 37.2 10.5 76.1 5.6 53.3 3.3 41.8 13.4 83.3 7.1 61.5 4.1 46.4 14.6 86.4 7.9 76.4 4.4 56.2

Page 25 of 35


565

Figure captions:

566

Figure 1. Relationship between water content and CO2 absorption capacity. (total

567

amine concentration of the blends: 4 M, TETA:DEEA=1:3, 40 °C, 13vol% CO2, and

568

200 mL min−1).

569

Figure 2. Phase separation behaviors of four tested TETA-DEEA blends with different

570

water contents. (total amine concentration of the blends: 4 M, TETA:DEEA=1:3, 40 °C,

571

13vol% CO2, and 200 mL min−1).

572

Figure 3. Comparison of the absorption rates of TETA-DEEA solutions with different

573

water content (46, 30, 25 and 20wt%) and MEA under simulated conditions

574

corresponding to the top and bottom of an absorber. (a) Lean solutions measured under

575

CO2 partial pressure of 1.30 kPa at a 0.25 mol mol-1 CO2 loading; (b) Rich solutions

576

measured under CO2 partial pressure of 13.0 kPa at 90% of the CO2 absorption capacity.

577

Figure 4. VLE data of the four tested solvents with different water contents at 40, 50,

578

and 60 °C (a) 46wt%, (b) 30wt%, (c) 25wt%, and (d) 20wt%. MEA 40°C lit. and MEA

579

60°C lit. represent VLE data of the MEA solution measured at 40 and 60 °C, as reported

580

in the literature, respectively.52-55

581

Figure 5. Regeneration heat estimation of TETA-DEEA at lean loadings of 0.25 and

582

0.40 mol mol-1 with different water contents. (a) Regeneration heat, (b) Latent heat, (c)

583

Sensible heat, and (d) Reaction heat. (Treboiler=393K, ΔT=10°C, normalized rich

584

loading=90% of the maximum CO2 loading). The MEA data were obtained from Kim

585

et al.47

586

Figure 6. 13C NMR spectra of TETA-DEEA solvent with various water contents: (a)

587

46wt%, (b) 30wt%, and (c) 20wt%.

588

Figure 7. Schematic diagram of the CO2 absorption mechanism in the TETA-DEEA

589

solution with NMP under low water content and various CO2 loadings. 25 ACS Paragon Plus Environment


Page 26 of 35

CO2 loading (mol/mol)

1.0 0.8 0.6 0.4 0.2 0.0

46%

30%

25%

20%

MEA

Water content (wt%) 590 591

Figure 1. Relationship between water content and CO2 absorption capacity. (total

592

amine concentration of the blends: 4 M, TETA:DEEA=1:3, 40 °C, 13vol% CO2, and

593

200 mL min−1).

594 595


Page 27 of 35


(a) 46 wt% water content

100

12% 36%

80

Percent (%)

Upper phase Lower phase

80

60 100%

88%

40 64%

100%

74%

20 0

0

0.38

0.51

0.82

(c) 25 wt% water content

54%

100

60 100% 40 20 0

33%

0

0.22

40%

0.31

39%

50%

53%

58%

61%

0.2

0.35

0.61

0.79

0

46%

50%

0.55

0.72


(d) 20 wt% water content

80

50%

Percent (%)

Percent (%)

67%

60%

42%

CO2 loading (mol/mol) Upper phase Lower phase

80

47%

100%

0

0.97

50%

40


100


60

20

596

597

(b) 30 wt% water content

26%

Percent (%)

100

65%

61%

58%

55%

35%

39%

42%

45%

0.3

0.36

0.48

0.63

60 100% 40 20 0

0



598

Figure 2. Phase separation behaviors of four tested TETA-DEEA blends with different

599

water contents. (total amine concentration of the blends: 4 M, TETA:DEEA= 1:3, 40 °C,

600

13vol% CO2, and 200 mL min−1)

601


500

(a)

Absorption rate ×108 ℃ kmol·m-2·s-1℃

Absorption rate ×108 ℃ kmol·m-2·s-1℃


40℃ 50℃ 60℃

400 300 200 100 0

46

30

25

20

MEA

500

Page 28 of 35

(b)

40℃ 50℃ 60℃

400 300 200 100 0

46

Water content (wt%)

30

25

20

MEA

Water content (wt%)

602 603

Figure 3. Comparison of the absorption rates of TETA-DEEA solutions with different

604

water content (46, 30, 25 and 20wt%) and MEA under simulated conditions

605

corresponding to the top and bottom of an absorber. (a) Lean solutions measured under

606

CO2 partial pressure of 1.30 kPa at a 0.25 mol mol-1 CO2 loading; (b) Rich solutions

607

measured under CO2 partial pressure of 13.0 kPa at 90% of the CO2 absorption capacity.

608


Page 29 of 35


609

(b) 40℃ 50℃ 50℃ MEA 40℃ MEA 40℃ lit. (Zhang et. al.) MEA 40℃ lit. (Idris et. al.) MEA 60℃ MEA 60℃ lit. (Pellegrini et. al.) MEA 60℃ lit. (Aronu et. al.)

1

0.1

0.01

0.001

CO2 partial pressure(kPa)


(a) 10

10

1

0.1

0.01

0.001

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2

-1


CO2 loading (mol mol )

610

10

1

0.1

0.01

0.001

(d)

40℃ 50℃ 50℃ MEA 40℃ MEA 40℃ lit. (Zhang et. al.) MEA 40℃ lit. (Idris et. al.) MEA 60℃ MEA 60℃ lit. (Pellegrini et. al.) MEA 60℃ lit. (Aronu et. al.)

10



(c)

611


1

0.1

0.01

0.001

0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2


0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2



612

Figure 4. VLE data of the four tested solvents with different water contents at 40, 50,

613

and 60 °C (a) 46wt%, (b) 30wt%, (c) 25wt%, and (d) 20wt%. MEA 40°C lit. and MEA

614

60°C lit. represent VLE data of the MEA solution measured at 40 and 60 °C, as reported

615

in the literature, respectively.52-55

616



4.0

(a)

0.25 mol·mol-1 0.40 mol·mol-1

3.80

1.2

3.5 2.98

3.0

2.67

2.73 2.49

2.5

2.26

2.08

2.0

2.14 1.89

1.5 1.0 0.5 0.0

MEA lit.

46

30

25

(b) 1.10

Latent heat(GJ/t CO2)

Regeneration heat(GJ/t CO2)

4.5

0.99 0.83

0.8

0.41

0.4

0.29

0.2

0.19

MEA lit.

(c)


0.90

0.8 0.6 0.51 0.45

0.4

0.38

0.37

0.32 0.26

0.24

46

30

25

20

Water content (wt%)

Reaction heat(GJ/t CO2)

Sensible heat(GJ/t CO2)

1.0

0.68

0.63

0.6

Water content (wt%)

617


1.06

1.0

0.0

20

Page 30 of 35

0.28

0.2

2.0

(d)

1.8

1.80

0.25 mol·mol-1 0.40 mol·mol-1 1.59 1.59

1.6

1.47 1.47

1.4

1.42 1.42 1.19 1.19

1.2 1.0 0.8 0.6 0.4 0.2

0.0

MEA lit.

46

30

25

0.0

20

MEA lit.

Water content (wt%)

618

46

30

25

20

Water content (wt%)

619

Figure 5. Regeneration heat estimation of TETA-DEEA at lean loadings of 0.25 and

620

0.40 mol mol-1 with different water contents. (a) Regeneration heat, (b) Latent heat, (c)

621

Sensible heat, and (d) Reaction heat. (Treboiler =393K, ΔT=10°C, normalized rich

622

loading=90% of the maximum CO2 loading). The MEA data were obtained from Kim

623

et al.47


Page 31 of 35


624



625


Page 32 of 35

Page 33 of 35


626 627

Figure 6. 13C NMR spectra of TETA-DEEA solvent with various water contents: (a) 46wt%, (b) 30wt%, and (c) 20wt%. 33 ACS Paragon Plus Environment


628 629

Figure 7. Schematic diagram of the CO2 absorption mechanism in the TETA-DEEA

630

solution with NMP under low water content and various CO2 loadings.

631


Page 34 of 35

Page 35 of 35


632 633

TOC art:

634


Novel Biphasic Solvent with Tunable Phase Separation for CO2 Capture

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