Solubility Measurement and Thermodynamic Properties Calculation

Article Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

pubs.acs.org/jced

Solubility Measurement and Thermodynamic Properties Calculation for Several CO2 + Ether Absorbent Systems Yun Li,*,† Yanhong You,† Weijia Huang,‡ and Jie Yang‡ †

School of Environment and Architecture, University of Shanghai for Science and Technology, Shanghai 200093, China School of Energy and Powering Engineering, University of Shanghai for Science and Technology, Shanghai 200093, China

‡

J. Chem. Eng. Data Downloaded from pubs.acs.org by MACQUARIE UNIV on 02/19/19. For personal use only.

S Supporting Information *

ABSTRACT: Six physical absorbents with the ether groups were selected for CO2 absorption: tetraethylene glycol dimethyl ether (TEGDME), diethylene glycol monohexyl ether, 2-butoxyethyl ether, triethylene glycol monobutyl ether, ethylene glycol dibutyl ether, and dipropylene glycol dimethyl ether (DPGDME). CO2 solubilities in these absorbents were measured at 273.15 and 283.15 K and 0−1.2 MPa. Henry’s constants of these CO2 + ether absorbent systems were calculated and analyzed at 273.15 K. The ether group is found more powerful than the methylene group, and the ethyl group is more effective than the hydroxyl group to improve the absorption ability of the absorbents. A lower temperature tends to facilitate the absorption process by increasing the absorption ability. Henry’s constants and mass solubilities of the ether absorbents were compared with those of the ionic liquids, common solvents, and other absorbents. TEGDME and DPGDME are potential absorbents according to the evaluation in both mole and mass fraction. The thermodynamic properties, such as entropy, enthalpy, and Gibbs free energy of solution, for CO2 + the ether absorbent systems were calculated and discussed for potential development of corresponding CO2 capture processes.

1. INTRODUCTION Excessive CO2 emissions in recent years may cause serious environmental issues such as global climate warming, permafrost thaw, sea level rise, and so on. 1,2 The absorption3/adsorption4,5/mebrane6 methods can be used to capture CO2. The absorption methods including the physical method, chemical method, and hybrid method can be easily found in huge gas treatment plants. Solvents such as methanol, N-methyl-2-pyrrolidone (NMP),7 etc. are common in the physical absorption processes. Some researchers tried to find the relationship between the CO2 absorption ability and the ether group of the physical absorbent. Based on the experimental and predictive data, Perisanu8 found that the solvents with the ether groups may show high absorption capacities for CO2 due to the theory “like dissolves like”. Gui et al.9 also concluded that the ether groups in the absorbents for CO2 may be advantageous to promote the absorption ability based on their experimental data. Thus, various researchers9−21 have paid attention to the solubility data for the CO2 + ether absorbent systems. Using the isothermal synthesis method, Gui et al.9,10 determined CO2 solubilities in six diethylene glycol ethers and two ethylene glycol ethers at 288.15−318.15 K and 0−6 MPa. Henni et al.11 selected 14 glycol ethers such as triethylene glycol monomethyl ether, tetraethylene glycol dimethyl ether (TEGDME), diethylene glycol dimethyl ether, etc. and reported Henry’s constants of CO2 in these absorbents at 298.15−333.15 K © XXXX American Chemical Society

based on the CO2 solubility data. Their results clearly indicate that TEGDME is the best solvent for CO2 removal among these 14 absorbents. Solubility data of CO2 in the ether absorbents such as dibutyl ether, diisopropyl ether, and dimethyl ether + diisopropyl ether were determined at 263.15−309.15 K and 0−3 MPa by Guo et al.,12 Zhu et al.,13 and Wu et al.,14 respectively. Miller et al.15 selected 15 volatile solvents including some ethers as CO2 absorbents and determined CO2 solubility data at 298.15 K and 0−5.3 MPa using the bubble-point method. In the authors’ previous work,16,17 six absorbents rich in ether and ester groups including diethylene glycol diethyl ether were selected and the CO2 solubilities were measured at 273.15−333.15 K and 0− 1.2 MPa. Except for the ordinary solvents, the polymers with the ether groups have become promising CO2 absorbents in recent years. Schappals et al.18 presented the experimental data of CO2 solubilities in four poly(oxymethylene) dimethyl ethers at the temperatures of 313.15 and 353.15 K and 0−4.3 MPa. The results show that the selected ethers are potential solvents for physical absorption of CO2. Miller et al.19 assessed four promising polymers with ether groups for CO2 capture based on the equilibrium data at 298.15 K and 0−6.4 MPa. Li et al.20 Received: October 18, 2018 Accepted: February 1, 2019

A

DOI: 10.1021/acs.jced.8b00936 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Table 1. Molecular Structure, CAS Registry Number, IUPAC Systematic Name, Purity, and Source of the Used Chemicals

a

The purity was stated by the supplier.

Additionally, the solubility data and the thermodynamic properties based on the solubility data may be of valuable for the NIST database and are very essential for designing an absorption process. In this work, the solubility data for CO2 + the ether absorbent systems were determined at 273.15 and 283.15 K, and 0−1.2 MPa. The absorption ability and thermodynamic properties for these systems were assessed.

determined CO2 solubilities in the liquid polyethylene glycols with ether groups at 303.15−333.15 K and 0−1.2 MPa. In the authors’ previous work,21 CO2 solubilities in three kinds of polymers with ether groups were measured at 273.15−303.15 K and 0−1.2 MPa. In addition, various solubility data of CO2 in other physical solvents with ester, hydroxyl, carbonyl groups and so on, were determined in the literature.22−32 In this paper, six ether absorbents including TEGDME, diethylene glycol monohexyl ether (DEGMHE), 2-butoxyethyl ether (BEE), triethylene glycol monobutyl ether (TEGMBE), ethylene glycol dibutyl ether (EGDBE), and dipropylene glycol dimethyl ether (DPGDME) were selected to investigate the CO2 absorption performance. CO2 solubilities in DEGMHE and BEE at 333.15−413.15 K and 1−20 MPa were measured by Lin et al.33 The solubility data for the CO2 + TEGDME system at 333.15 K and transcritical pressure range of 1−7.5 MPa were measured by Kodama et al.34 Sweeney et al.35 obtained Henry’s constants for the CO2 + TEGDME system at 298.15 and 323.15 K, while Wang et al.10 reported Henry’s constants for CO2 in BEE at 288.15−318.15 K. Sciamanna et al.36 measured CO2 solubilities in TEGDME and TEGMBE, and reported Henry’s constants at 298.15 K. Furthermore, Henry’s constants for CO2 in the above two absorbents were determined at 298.15−333.15 K by Henni et al.11,37 It is found that the current researchers11,33−37 mainly focus on the CO2 absorption ability of these ether absorbents at temperatures above the room temperature. The selected absorbents should have higher CO2 absorption ability at lower temperatures, since lowering the absorption temperature is of tremendous benefit to CO2 solubility growth in the previous work.16,17,21 Therefore, the relatively low temperatures of 273.15 and 283.15 K were selected for the CO2 solubility measurement in this paper, due to the temperature limitation of our device. There is no available data about the CO2 solubilities in the selected absorbents at the above two temperatures in the literatures, according to the comprehensive searches in the National Institute of Standards and Technology (NIST) Standard Reference Database. Since the CO2 solubility is an important criteria for the absorption ability of the absorbent, the solubility measurement is necessary for the initial screening of the potential absorbents.

2. EXPERIMENT 2.1. Materials. The gas and the absorbents were purchased and used without further purification. The molecular structure, CAS registry number, IUPAC systematic name, and the manufactures of the materials used in this paper are provided in Table 1, as well as the purity that was stated by the supplier. 2.2. Apparatus. In the authors’ previous work, the device with the isothermal synthetic method was used for the solubility measurement for CO2 + physical16,17,21,30−32/ hybrid38 absorbents systems. The principle of the isothermal synthetic method is to add the known amount of the gas and the liquid and record the values of the temperatures and pressures at the vapor−liquid equilibrium state without any sampling of the liquid and the vapor during the experimental procedures. The method used in this paper refers to the pressure determination as the temperature of the system is maintained at a constant level and the calculation of CO2 solubility in the absorbent according to the mass balance of CO2. The device mainly consists of a gas chamber, an equilibrium cell, a temperature control system, two pressure sensors, and a vacuum system. Initially, the vacuum system was used to remove the air from the two containers. Then, a certain amount of the ether was put into the equilibrium cell. The pressures of the containers were determined by the pressure sensors after the temperature of the containers was maintained at the specified value. Some CO2 was injected from the chamber into the cell. The absorption equilibrium was obtained as the pressure of the cell decreased a steady value. Just like the works in the literature,9,20 each data point of CO2 solubility was determined once in this work. CO2 solubilities (x1) in the ether absorbents based on the mole fraction can be calculated by the following equations. B


Journal of Chemical & Engineering Data nether =

x1 =

ρether Vether Mether

nCO2 nether + nCO2

nCO2 =

Article

p zy V − Vether ij pE2 p yz VG jij pG1 jj − G2 zzz − E − E1 zzz jj j j j z RT k ZG1 ZG2 { RT Z E1 z{ k Z E2

temperature and pressure are the key factors for the absorption and desorption processes.

(1)

Table 2. CO2 Solubility (x1) in Tetraethylene Glycol Dimethyl Ether under Different Temperatures (T) and Different Partial Pressures of CO2 (pCO2)a

(2)

(3)

where nCO2 and nether refer to the moles of CO2 absorption capacity and the ether that put into the equilibrium cell; VG, VE, and Vether refer to the volumes of the gas chamber, the equilibrium cell, and the ether injected; T refers to the temperature; pG1 and pG2 are the initial and final pressures of the gas chamber; ZG1 and ZG2 are the compressibility factors corresponding to the initial and final pressures in the gas chamber; pE1 and pE2 are the initial and final pressures of the equilibrium cell; and ZE1 and ZE2 are the compressibility factors corresponding to the initial and final pressures in the equilibrium cell. Since the density data and the vapor pressure data were necessary for the calculation of CO2 solubility and the partial pressure of CO2(p(CO2)), these data for all the selected absorbents at 273.15 and 283.15 K were obtained as follows. The density of TEGDME can be obtained directly from Kodama’s work,34 while those of DEGMHE,39 BEE,40 TEGMBE,41 and DPGDME42 can be estimated by the extrapolations of the fitting curve in the literature. The density data of EGDBE were determined and are shown in the Supporting Information. The vapor pressures of TEGDME,43 DEGMHE,44 and BEE44 at the selected temperatures were estimated as zero by the extrapolations of the fitting curve in the literatures. In addition, the vapor pressures of TEGMBE, EGDBE, and DPGDME were considered as zero as they were found to be below the detection limit (0.2 kPa) of the determination device. The device with the boiling point method mainly contains a three-necked round-bottomed flask, a condenser, a temperature sensor, a U-tube mercury manometer, an oil bath, and a vacuum pump. Details of the device, the procedures, and the related validation experiments can be found in the previous works45−47 and are not repeated at length here. Similar to the studies of potential CO2 absorbents in the literature,9,16,20 only the mole fraction of CO2 in the liquid phase was determined and the absorption ability was focused on in this paper, since the compositions of the vapor phase cannot be obtained with the above determination method. In addition, at the selected temperatures, it appears that almost all of the vapor phase is CO2 as the vapor pressures of the selected absorbents are zero. In the previous work,16 CO2 solubilities in water at 273.15 K and 0−1 MPa were determined with this device and compared with the data in the literature,48 showing the accuracy and reliability of the device.

T/K

pCO2b/ MPa

x1/ mol·mol−1

T/K

pCO2b/ MPa

x1/ mol·mol−1

273.15 273.15 273.15 273.15 273.15 273.15 273.15 273.15

0.1566 0.3012 0.4261 0.5720 0.7209 0.9127 1.0631 1.1815

0.0721 0.1345 0.1920 0.2640 0.3352 0.4106 0.4841 0.5409

283.15 283.15 283.15 283.15 283.15 283.15 283.15 283.15

0.1405 0.2859 0.4062 0.5457 0.6808 0.8303 0.9782 1.1267

0.0530 0.1130 0.1538 0.2160 0.2665 0.3210 0.3775 0.4389

a Standard uncertainties u are u(pCO2) = 0.0002 MPa and u(T) = 0.03 K, and the relative standard uncertainty of CO2 solubility is ur(x1) = 0.0283. CO2 solubility (x1) refers to the mole fraction of CO2 in the liquid phase. bThe total pressure (ptot) = the partial pressure of CO2 (pCO2)

Table 3. CO2 Solubility (x1) in Diethylene Glycol Monohexyl Ether under Different Temperatures (T) and Partial Pressures of CO2 (pCO2)a T/K

pCO2b/ MPa

x1/ mol·mol−1

T/K

pCO2b/ MPa

x1/ mol·mol−1

273.15 273.15 273.15 273.15 273.15 273.15 273.15 273.15

0.1282 0.2830 0.4292 0.5775 0.7239 0.8754 1.0209 1.1694

0.0378 0.0864 0.1282 0.1731 0.2178 0.2639 0.3013 0.3466

283.15 283.15 283.15 283.15 283.15 283.15 283.15 283.15

0.1301 0.2749 0.4092 0.5677 0.7077 0.8531 0.9985 1.1459

0.0310 0.0677 0.0994 0.1395 0.1748 0.2050 0.2392 0.2789

a

Standard uncertainties u are u(pCO2) = 0.0002 MPa and u(T) = 0.03 K, and the relative standard uncertainty of CO2 solubility is ur(x1) = 0.0283. CO2 solubility (x1) refers to the mole fraction of CO2 in the liquid phase. bThe total pressure (ptot) = the partial pressure of CO2 (pCO2)

Table 4. CO2 Solubility (x1) in 2-Butoxyethyl Ether under Different Temperatures (T) and Partial Pressures of CO2 (pCO2)a

3. RESULTS AND DISCUSSION 3.1. Solubilities. CO 2 solubilities in the selected absorbents were determined at 273.15 and 283.15 K and 0− 1.2 MPa. The results indicate that the absorbents show higher absorption abilities at lower temperatures and higher pressures, which is consistent with the characteristic of classic physical absorption, as shown in Tables 2−7. Thus, the proper

T/K

pCO2b/ MPa

x1/ mol·mol−1

T/K

pCO2b/ MPa

x1/ mol·mol−1

273.15 273.15 273.15 273.15 273.15 273.15 273.15 273.15

0.1228 0.2910 0.4150 0.5586 0.7025 0.8569 1.0007 1.1387

0.0488 0.1202 0.1711 0.2256 0.2839 0.3462 0.4068 0.4683

283.15 283.15 283.15 283.15 283.15 283.15 283.15 283.15

0.1462 0.2955 0.4302 0.6600 0.7419 0.9140 1.0514 1.1738

0.0457 0.0929 0.1390 0.2077 0.2399 0.2919 0.3327 0.3768

a

Standard uncertainties u are u(pCO2) = 0.0002 MPa and u(T) = 0.03 K, and the relative standard uncertainty of CO2 solubility is ur(x1) = 0.0283. CO2 solubility (x1) refers to the mole fraction of CO2 in the liquid phase. bThe total pressure (ptot) = the partial pressure of CO2 (pCO2). C



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Table 5. CO2 Solubility (x1) in Triethylene Glycol Monobutyl Ether under Different Temperatures (T) and Partial Pressures of CO2 (pCO2)a T/K

pCO2b/ MPa

x1/ mol·mol−1

T/K

pCO2b/ MPa

x1/ mol·mol−1

273.15 273.15 273.15 273.15 273.15 273.15 273.15 273.15

0.1227 0.2717 0.4080 0.5638 0.7095 0.8525 0.9885 1.1389

0.0405 0.0910 0.1373 0.1904 0.2391 0.2818 0.3333 0.3775

283.15 283.15 283.15 283.15 283.15 283.15 283.15 283.15

0.1309 0.2879 0.4303 0.5815 0.7353 0.8880 1.0226 1.1684

0.0347 0.0747 0.1130 0.1519 0.1979 0.2345 0.2744 0.3122

Figure 1. CO2 solubilities (x1) in TEGDME at partial pressures of CO2 (pCO2). ■, 273.15 K; ●, 283.15 K; , linear fit.

a Standard uncertainties u are u(pCO2) = 0.0002 MPa and u(T) = 0.03 K, and the relative standard uncertainty of CO2 solubility is ur(x1) = 0.0283. CO2 solubility (x1) refers to the mole fraction of CO2 in the liquid phase. bThe total pressure (ptot) = the partial pressure of CO2 (pCO2)

Table 6. CO2 Solubility (x1) in Ethylene Glycol Dibutyl Ether under Different Temperatures (T) and Partial Pressures of CO2 (pCO2)a T/K

pCO2b/ MPa

x1/ mol·mol−1

T/K

pCO2b/ MPa

x1/ mol·mol−1

273.15 273.15 273.15 273.15 273.15 273.15 273.15 273.15

0.1306 0.2840 0.4303 0.5643 0.7131 0.8621 1.0040 1.1489

0.0460 0.0989 0.1464 0.1926 0.2479 0.3003 0.3502 0.3902

283.15 283.15 283.15 283.15 283.15 283.15 283.15 283.15

0.1370 0.2784 0.4204 0.5838 0.7574 0.8688 1.0120 1.1570

0.0383 0.0814 0.1211 0.1660 0.2169 0.2455 0.2886 0.3229

Figure 2. CO2 solubilities (x1) in DEGMHE at partial pressures of CO2 (pCO2). ■, 273.15 K; ●, 283.15 K; , linear fit.

a

Standard uncertainties u are u(pCO2) = 0.0002 MPa and u(T) = 0.03 K, and the relative standard uncertainty of CO2 solubility is ur(x1) = 0.0283. CO2 solubility (x1) refers to the mole fraction of CO2 in the liquid phase. bThe total pressure (ptot) = the partial pressure of CO2 (pCO2) Figure 3. CO2 solubilities (x1) in BEE at partial pressures of CO2 (pCO2). ■, 273.15 K; ●, 283.15 K; , linear fit.

Table 7. CO2 Solubility (x1) in Dipropylene Glycol Dimethyl Ether under Different Temperatures (T) and Partial Pressures of CO2 (pCO2)a T/K

pCO2b/ MPa

x1/ mol·mol−1

T/K

pCO2b/ MPa

x1/ mol·mol−1

273.15 273.15 273.15 273.15 273.15 273.15 273.15 273.15

0.1140 0.2781 0.4044 0.5452 0.6897 0.8406 0.9873 1.1281

0.0471 0.1149 0.1741 0.2313 0.2889 0.3524 0.4180 0.4714

283.15 283.15 283.15 283.15 283.15 283.15 283.15 283.15

0.1328 0.2748 0.4158 0.5595 0.7309 0.8707 1.0185 1.1702

0.0454 0.0909 0.1410 0.1891 0.2474 0.2931 0.3385 0.3880

Figure 4. CO2 solubilities (x1) in TEGMBE at partial pressures of CO2 (pCO2). ■, 273.15 K; ●, 283.15 K; , linear fit.

a

Standard uncertainties u are u(pCO2) = 0.0002 MPa and u(T) = 0.03 K, and the relative standard uncertainty of CO2 solubility is ur(x1) = 0.0283. bThe total pressure (ptot) = the partial pressure of CO2 (pCO2)

mole fraction of CO2 in the liquid phase, Henry’s constants of the selected absorbents can be adopted as the criteria of the absorption ability for per mole absorbents. The order of absorbents for Henry’s constants of these absorbents at the constant temperature is TEGDME < DPGDME < BEE < EGDBE < TEGMBE < DEGMHE. As far as the selected absorbents concerned in this paper, the absorbents with the ether groups only, i.e., TEGDME,

3.2. Henry’s Constant. Henry’s law49 means that CO2 solubilities increase linearly with the increasing of gas pressure. The law was obviously observed for CO2 + the ether absorbent systems at the pressure and temperature ranges studied in this paper, as shown in Figures 1−6. In addition, Henry’s constants for these systems can be calculated from the slope of the pCO2− x1 lines for x1 → 0,11 as listed in Table 8. Since x1 refers to the D



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Figure 5. CO2 solubilities (x1) in EGDBE at partial pressures of CO2 (pCO2). ■, 273.15 K; ●, 283.15 K; , linear fit.

Figure 7. Henry’s constant of CO2 in TEGDME. ■, Henni;11 □, Sweeney;35 ○, Sciamanna;36 ●, this work.

Figure 6. CO2 solubilities (x1) in DPGDME at partial pressures of CO2 (pCO2). ■, 273.15 K; ●, 283.15 K; , linear fit.

Figure 8. Henry’s constant of CO2 in DEGMHE. ■, Lin;33 ●, this work.

Table 8. Henry’s Constants (Hx) of CO2 + the Selected Absorbents Systems at Different Temperatures (T) Hx/MPa absorbent

T = 273.15 K

T = 283.15 K

TEGDME DEGMHE BEE TEGMBE EGDBE DPGDME

2.172 3.392 2.516 3.030 2.839 2.420

2.650 4.197 3.199 3.772 3.577 2.925

Figure 9. Henry’s constant of CO2 in BEE. ■, Lin;33 □, Wang;10 ●, this work.

DPGDME, BEE, and EGDBE, show better CO2 absorption ability than the absorbents with the ether groups and a hydroxyl group, i.e., TEGMBE and DEGMHE. Furthermore, TEGDME, which contains the most ether groups, is the best among all the absorbents; whereas DEGMHE, which contains the least ether groups and a hydroxyl group, is the worst. The results may be explained according to Gui’s research9 and the study in the previous work.17 The hydroxyl group is found to be phobic-CO2 and unfavorable for CO2 absorption.9 Instead, adding the ether group to the molecular structure of the absorbent is effective for improving CO2 absorption ability.9,17 Similarly, differences between CO2 solubilities in TEGMBE and DEGMHE may be due to the increase of the ether group. In addition, if the ether group in TEGDME is replaced with a methylene group or the hydroxyl group in TEGMBE is replaced with an ethyl group, the absorbent will turn to be BEE. Then the above-mentioned order shows that the ether group is more advantageous than the methylene group and that the ethyl group is more effective than the hydroxyl group. The effective comparisons with available literature data10,11,33,35,36 are advantageous to assess the data consistency between this paper and the literature, which is also the journal requirement. The detailed comparisons are shown in Figures 7−10. The deviations among the data in the literature are

Figure 10. Henry’s constant of CO2 in TEGMBE. ■, Henni;11 ○, Sciamanna;36 ●, this work.

listed in Table 9. The data of the CO2 + TEGDME system in this work is found to be consistent with that determined by Sweeney et al.35 and Sciamanna et al.36 and inconsistent with that of Henni et al.,11 as shown in Figure 7. The data determined by Henni et al.,11 Sweeney et al.,35 and Sciamanna et al.36 show no significant difference at 298.15 K, while there are obvious differences between Henni’s11 data and SweeE



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Table 9. Henry’s Constants (Hx) Comparison at Different Temperatures (T) absorbent TEGDME

TEGMBE

T/K 298.15 298.15 313.15 323.15 298.15

Hx

H′x 36

3.002 3.1435 4.11a,35 4.7435 3.70836

11

3 311 4.311 5.4a,11 5.311

Table 10. Solubility Comparison with Other Solvents at 273.15 Ka

102|Hx − H′x|/H′x

ionic liquids

0.07 4.67 4.42 12.22 30.04

HmimPF650

BmimBF450

BmimTf2N51

3.06 2.44

4.07 2.52 common solvents

1.86 3.03

Hx/MPa S/cm3(STP)·g−1·atm−1

2-methyl-1-propanol52

The value was estimated by the extrapolations of the fitting curve in the literature. a


9.39 3.29

ij H yz Δsol G∞ = RT lnjjj 0x zzz jp z k {

É ÄÅ ∞ ÅÅ i H yÑÑÑ ∂ ijj Δsol G yzz jj x zzÑÑ 2 ∂ Å Å Δsol H = −T Ålnjj zzÑ j z = −RT ∂T jk T z{ ∂T ÅÅÅÅÇ jk p0 z{ÑÑÑÑÖ ∞


DEGDEE

2.97 6.62


8.33 8.49 CA

2.93 4.87 this work

TEGDME Hx/MPa S/cm3(STP)·g−1·atm−1

methanol54

3.80 4.91 our previous work16

EGMEA

ney’s35 data as the temperature exceeds 313.15 K, as shown in Figure 7 and Table 9. Good agreement between Lin’s33 data and the data in this work is observed for BEE, whereas there are large deviations between the data for DEGMHE, as shown in Figures 8 and 9. Henni’s11 data and the data in this work for TEGMBE show no significant difference, as shown in Figure 10. However, Wang’s10 data and Lin’s33 data for BEE are conflicting with each other in Figure 9, as well as Henni’s11 data and Sciamanna’s36 data for TEGMBE in Figure 10 and Table 9. Additionally, as expected, the selected absorbents show higher CO2 absorption ability at 273.15 and 283.15 K than those at higher temperatures in the literatures.10,11,33,35,36 For example, CO2 solubility in TEGDME at 273.15 K is about 3 and 1.4 times that at 333.15 and 298.15 K, respectively. Two parameters, Henry’s constant (Hx) and mass solubility (S), were used to assess the absorption ability for per mole and gram absorbent, respectively. Thus, the parameters at 273.15 K for the ether absorbents, ionic liquids, 50,51 common solvents,52−54 and absorbents studied in previous work16 were compared and are shown in Table 10. The absorption ability of per gram absorbent may be dominated by Hx and the molecular weight of the absorbent, which can be verified by Table 10. For example, BmimTf2N may be considered as a potential absorbent based on the mole fraction; however, it is not that prospective based on the mass fraction due to the high molecular weight. Instead, methanol shows outstanding absorption ability based on the mass fraction for the low molecular weight of 32. The comparison results show that the absorption ability of TEGDME in this work is higher than those of the other absorbents (except BmimTf2N) in Table 10, based on the mole fraction. DPGDME performs with a relatively higher absorption ability than the other absorbents (except methanol and EGMEA) in Table 10, based the on mass fraction. 3.3. Thermodynamic Properties. Essential thermodynamic properties such as ΔsolG∞, ΔsolH∞, and ΔsolS∞ are also necessary for the design of the gas absorption process and can be derived by the following equations:55,56

DMCH53

2.81 4.69

DEGMHE

BEE

2.17 4.87

3.39 3.59 this work

2.52 4.25

TEGMBE

EGDBE

DPGDME

3.03 3.70

2.84 4.63

2.42 5.89

a S refers to the CO2 volume (STP) absorbed by per gram absorbent. STP: standard temperature and pressure. It refers to the temperature of 273.15 K and the absolute pressure of 100 kPa. HmimPF6: 1-hexyl3-methylimidazolium hexafluorophosphate. BmimBF4: 1-butyl-3methylimidazolium tetrafluoroborate. BmimTf2N: 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. DMCH: 2,6-dimethyl-cyclohexanone. EGMEA: ethylene glycol methyl ether acetate. DEGDEE: diethylene glycol diethyl ether. CA: carbitol acetate.

ÄÅ É ÅÅ i H yÑÑÑ (Δsol H ∞ − Δsol G∞) j z x 2 ∂ Å ÅÅlnjj zzÑÑÑ Δsol S = = −RT j z T ∂T ÅÅÅÅÇ jk p0 z{ÑÑÑÑÖ ∞

ji H zy − RT lnjjj 0x zzz jp z k {

(6)

where p0 refers to the standard state of the gas and R refers to gas constant. The linear equation for the Hx−T relationship was used in this paper and the previous works,16,32 since the temperature interval was as relatively small as 10 K. Then, the values of ΔsolG∞, ΔsolH∞, and ΔsolS∞ for the selected systems were calculated at 278.15 K in Table 11 and found to be consistent with those for other systems of physical absorption,20,26 indicating that Henry’s constants at the two temperatures can be used for the estimation of the thermodynamic properties. It Table 11. Gibbs Free Energy (ΔsolG∞), Enthalpy (ΔsolH∞), and Entropy (ΔsolS∞) of Solution for CO2 in Physical Absorbents at 278.15 K

(4)

2

(5) F

physical absorbent

ΔsolG∞/ kJ·mol−1

ΔsolH∞/ kJ·mol−1

ΔsolS∞/ J·mol−1·K−1

TEGDME DEGMHE BEE TEGMBE EGDBE DPGDME

7.360 8.409 7.753 8.156 8.020 7.598

−12.84 −13.75 −15.54 −14.15 −14.94 −12.23

−72.62 −79.68 −83.73 −80.20 −82.55 −72.28



Article

DPGDME system are the highest and the lowest, respectively. The comparison of the ΔsolS∞ values means that the solution after gas absorption is the most chaotic for the CO2 + BEE system, whereas it is the least chaotic for the CO2 + DPGDME system.

is worth noting that more accurate values of the thermodynamic properties should be obtained based on Henry’s constants at more temperatures. The result shows that the values of ΔsolG∞ for the selected systems were dominated by the values of ΔsolS∞ since the effect of ΔsolS∞ on ΔsolG∞ was stronger than that of ΔsolH∞. According to eq 4, the values of ΔsolG∞ decrease with the decrease of the temperature, which means that the driving force required is less to reach the absorption equilibrium and the absorption process is more likely to accomplish at lower temperatures. The absorption processes for all the selected absorbents are exothermal, verified by the negative values of ΔsolH∞ and the experimental processes. In addition, the absorption processes for these absorbents are entropy decreasing, verified by the negative values of ΔsolS∞ or the reduction in the number of the gas (CO2) molecules during the absorption process. There is a little difference among the values of ΔsolG∞ for the ether absorbents at constant temperature, indicating that the driving force required to reach the gas−liquid equilibrium is basically the same. Moreover, the value of ΔsolG∞ for the CO2 + TEGDME system is the lowest, and that for the CO2 + DEGMHE system is the highest, showing that the absorption process for the CO2 + TEGDME system is most likely to happen while that for the CO2 + DEGMHE system is most unlikely to occur. The absolute value of ΔsolH∞ for the CO2 + BEE system is the highest and that for the CO2 + DPGDME system is the lowest, showing that the generated heat for the CO2 + BEE system is the highest and that for the CO2 + DPGDME system is the lowest. Similarly, the absolute value of ΔsolS∞ for the CO2 + BEE system is the highest and that for the CO2 + DPGDME system is the lowest, which means that the solution after gas absorption is the most chaotic for the CO2 + BEE system; whereas that is the least chaotic for the CO2 + DPGDME system.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.8b00936.

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Densities of ethylene glycol dibutyl ether at 273.15 and 283.15 K (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], [email protected] (Y. Li). ORCID

Yun Li: 0000-0002-7400-4237 Weijia Huang: 0000-0002-4740-2950 Funding

This work is supported by the National Natural Science Foundation of China (21506124, 51741605). Notes

The authors declare no competing financial interest.

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REFERENCES

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4. CONCLUSION CO2 solubilties in six ether absorbents, TEGDME, DEGMHE, BEE, TEGMBE, EGDBE, and DPGDME, were determined at 273.15 and 283.15 K and 0−1.2 MPa. Based on the solubility data, Henry’s constants of CO2 in the surveyed absorbents were calculated and compared. It is found that the ether group is more powerful than the methylene group and the ethyl group is more effective than the hydroxyl group to improve the absorption ability of the absorbents. The decrease of the temperature may be extremely valuable for the enhancement of the absorption ability as expected and make the absorption process more likely to occur. Henry’s constant and mass solubility were selected to assess the absorption ability for per mole and gram absorbent. The parameters of the ether absorbents were compared with those of the ionic liquids, common solvents, and absorbents studied in the previous works. The results show that TEGDME and DPGDME are relatively potential absorbents based on per mole and gram absorbent, respectively. The thermodynamic properties such as ΔsolG∞, ΔsolH∞, and ΔsolS∞ were calculated at 278.15 K based on Henry’s constants and compared for the surveyed absorbents. The comparison of ΔsolG∞ values shows that the absorption process for the CO2 + TEGDME system is most likely, and the CO2 + DEGMHE system is most unlikely to happen among all the surveyed absorbents. The comparison of ΔsolH∞ values indicates that the generated heat for the CO2 + BEE system and the CO2 + G



Article

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Solubility Measurement and Thermodynamic Properties Calculation

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