Subscriber access provided by UNIV OF LOUISIANA
Environmental and Carbon Dioxide Issues
Efficient Removal of Sulfuric Acid from Sodium Lactate Aqueous Solution Based on Common-Ion Effect for the Absorption of SO2 of Flue Gas Zhangjin Wu, Yucui Hou, Weize Wu, Shuhang Ren, and Kai Zhang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b03829 • Publication Date (Web): 22 Apr 2019 Downloaded from http://pubs.acs.org on April 28, 2019
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After
is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Subscriber access provided by UNIV OF LOUISIANA
a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
Energy & Fuels
Absorption of SO2 from flue gas by NaLa(aq) SO2 contained flue gas
SO2 Distillation tower
Scrubber NaLa(aq)
Desorption tower
SO2 removed flue gas
Absorption tower
Feed container
NaOH or Na2CO3
Mixer
Water purifier
Split vavle
Filter
Na2SO4
Removal of byproduct (H2SO4) from SO2 oxidation: H2SO4+2NaOH+Na+→Na2SO4↓+2H2O+Na+ ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
1
Efficient Removal of Sulfuric Acid from Sodium Lactate
2
Aqueous Solution Based on Common-Ion Effect for the
3
Absorption of SO2 of Flue Gas
4
Zhangjin Wu a, Yucui Hou b, Weize Wu a,*, Shuhang Ren a,*, Kai Zhang a
5
a
6
Chemical Technology, Beijing, 100029, China
7
b
Department of Chemistry, Taiyuan Normal University, Shanxi, 030619, China
*
Corresponding authors. E-mail:
[email protected](W. W.);
[email protected](S. R.)
State Key Laboratory of Chemical Resource Engineering, Beijing University of
8
1
ACS Paragon Plus Environment
Page 2 of 29
Page 3 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
1
ABSTRACT:
2
Sulfur dioxide (SO2) is the main component of air pollution. Recently, sodium lactate
3
(NaLa) aqueous solution (aq) was demonstrated as a highly efficient, renewable and
4
stable absorbent for SO2 absorption from flue gas. However, sulfuric acid (H2SO4)
5
accumulated during long-term SO2 absorption and desorption cycles. In order to
6
remove H2SO4 from NaLa (aq) efficiently, a method based on the common-ion effect
7
was proposed in this work. The results indicated the residual concentration of sulfate
8
anion (SO42–) decreased with decreasing the water content. The residual SO2 in NaLa
9
(aq) did not influence the removal of H2SO4. After the removal, the content of residual
10
SO42– was 0.47 wt% and the removal rate of H2SO4 was 95.6% with a water content of
11
34 wt% at 20 oC. Compared to other reported methods, our method to remove H2SO4
12
in absorbents did not introduce other ions, and block absorption tower. It was noted that
13
the evaporation of water to decrease the water content was combined with the
14
regeneration of NaLa (aq), without any extra evaporation energy. NaLa (aq) was not
15
changed during the removal process. Moreover, the SO2 absorption capacity in H2SO4-
16
removed NaLa (aq) was comparable with the capacity of virgin NaLa (aq), and no
17
obvious change could be found after five cycles of H2SO4 removal. The mechanism of
18
removing H2SO4 from NaLa (aq) studied with FI-IR indicated that H2SO4 was removed
19
in the form of Na2SO4 without crystal water due to the common-ion effect. Furthermore,
20
a new process for SO2 absorption with H2SO4 removal from NaLa (aq) was proposed.
21
2
ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
1
1. INTRODUCTION
2
The emission of sulfur dioxide (SO2) in flue gas has caused serious air pollution,
3
which influences the environment and human health. Therefore, SO2 capture has been
4
drawing much attention all over the world. The most important method to control the
5
emission of SO2 is flue gas desulfurization (FGD). There are many traditional methods
6
of FGD, basically based on chemical absorption, such as using limestone,1 sodium
7
hydroxide,2 magnesium hydroxide3 and ammonia4 as absorbents. However, those
8
traditional methods have many drawbacks. For instance, SO2 cannot be recovered and
9
absorbents cannot be recycled, and there are solid wastes produced. In addition, the
10
volatility of absorbents causes secondary contamination. Therefore, recyclable and
11
non-volatile absorbents are in urgent demand.
12
Recently, ionic liquids (ILs), especially functional ILs,5-7 were designed as
13
absorbents for SO2 capture and recovery, such as ILs based on quaternary
14
phosphonium,8 imidazolium,9 guanidinium10 and hydroxylammonium11. Those ILs can
15
reversibly absorb SO2 from flue gas, and their vapor pressures are extremely low. In
16
addition, based on the theory of SO2 capture by ILs, sodium lactate (NaLa) aqueous
17
solution (aq) was designed as an efficient absorbent for the capture of SO2 in flue gas.12
18
Due to the introduction of sodium cation, compared to those of reported ILs, NaLa (aq)
19
has the advantages of low molecular weight, low cost, extremely low volatility, high
20
solubility in water, and long-term stability.
21
During the long-term absorption of SO2 from simulated flue gas by NaLa (aq), it
22
was found that a small part of the absorbed SO2 was oxidized to sulfuric acid (H2SO4),
23
the same as other reported ILs.13 As we know, the real flue gas consists of N2, CO2,
24
SO2, NOX, H2O, ash, O2 and other compounds. In the presence of O2, a part of the
25
absorbed SO2 is oxidized to H2SO4 under the catalysis of ashes, and it can be 3
ACS Paragon Plus Environment
Page 4 of 29
Page 5 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
1
accumulated. According to the absorption mechanism proposed,14 the oxidized SO2 (or
2
SO3) is unable to be recovered and the generated H2SO4 can reduce the SO2 absorption
3
capacity of NaLa (aq). In other word, it influences the absorption and desorption of
4
SO2. Thus, the oxidation of absorbed SO2 can be a serious problem during the SO2
5
absorption process. Only with this problem solved will NaLa (aq) be an available
6
absorbent for the application of SO2 capture of flue gas.
7
Several methods were reported about the removal of sulfate anion (SO42−) in
8
aqueous solution. Korngold et al.15 used a weak anion exchanger to remove SO42− from
9
brackish water, which was regenerated with a potassium chloride solution. Moreover,
10
the potassium sulfate was removed from the generation solution by adding solid
11
potassium chloride, which reduced the solubility of potassium sulfate. However, this
12
method introduced chloride into the system, which was not suitable for NaLa (aq).
13
Mohebbi et al.16 reported a nanofiltration process in SO42− removal from artificial and
14
industrial wastewater, which could deal with wastewater with 400−500 ppm SO42−. The
15
retention ratio of SO42– could be about 99%. Nevertheless, this process is not available
16
in NaLa (aq), in which the size of SO42– is in the middle between Na+ and La–. As a
17
result, SO42– is unable to be separated solely and the ash of flue gas can easily block the
18
membrane. Cattoir et al.17 investigated the recovery of H2SO4 from decontamination
19
effluents by means of electro–electrodialysis, which was a stable and efficient process
20
to deal with SO42–. In spite of the low cost and efficiency of the process, the same
21
drawback with nanofiltration process is that the ash contained in NaLa (aq) can easily
22
block the used membranes in the process. Hlabela et al.18 reported a process that used
23
barium carbonate to remove SO42– from mine water, which could reduce SO42– to less
24
than 200 mg/L. The removal rate of SO42– was very high. However, the cost of barium
25
carbonate was high, and the introduction of Ba2+ was inevitable. In addition, if barium 4
ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
1
carbonate is used to remove H2SO4 in NaLa (aq), the H2SO4-removed absorbent can be
2
saturated solution of BaSO4. When it returns to the absorption tower, the generated
3
H2SO4 can form precipitation in the absorbent, which may block the tower. Benatti et
4
al.19 reported that adding CaCl2 to an aqueous solution with SO42– was able to react
5
with SO42– to form precipitation, which could reduce SO42–. Nevertheless, the removal
6
rate of SO42– was low and the process introduced Cl– and Ca2+ into the system that
7
might influence the absorption and desorption of SO2 in NaLa (aq). In addition, we
8
found that the generated CaSO4 inducted NaLa (aq) forming a gel that could not be
9
filtered in NaLa (aq). Therefore, an available method to remove H2SO4 in NaLa (aq) is
10
expected.
11
As discussed above, the concentration of Na+ in the NaLa solution is much high.
12
According to the common-ion effect of Na+, Na2SO4 should have a very low solubility.
13
Thus, H2SO4 in the aqueous solution is turned into Na2SO4 by adding NaOH of a
14
stoichiometric amount and then it precipitates due to the extremely low solubility and
15
can be removed by filtration. Our results indicate that by simply adding equivalent
16
NaOH, H2SO4 can be removed from NaLa (aq) in solid Na2SO4 without crystal water.
17
Importantly, this method can avoid the problems mentioned above and is an efficient
18
method.
19
2. EXPERIMENTAL SECTION
20
2.1. Materials. SO2 (99.95%) and N2 (99.999%) were delivered by Beijing Haipu
21
Gases Co., Ltd., Beijing, China. The simulated flue gas with different SO2
22
concentrations was obtained by mixing SO2 and N2 in a high-pressure gas cylinder.
23
NaOH (97%), Na2SO4 (99%) and lactic acid (80–85% in aqueous solution) were bought
24
from Aladdin Co., Ltd., Shanghai, China. H2SO4 (96–98%) was purchased from Beijing
25
Modern Oriental Fine Chemicals Co., Ltd, Beijing, China. Sodium lactate (NaLa) was 5
ACS Paragon Plus Environment
Page 6 of 29
Page 7 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
1
obtained by neutralization between NaOH and lactic acid with equal mole. The water
2
content was determined by Karl Fischer titration (ZDY-502, Shanghai INESA
3
Scientific Instrument Co., Ltd, Shanghai, China).
4
2.2. SO2 Absorption and Desorption Experiment. The schematic diagram
5
of the experimental equipment is shown in Figure 1. It consists of a gas cylinder, a
6
rotameter, a water glass tube, an absorbent glass tube, a water (or oil) bath, a
7
temperature controller, and an absorption device of tail gas.
8
The absorbents with 30 wt% NaLa were prepared by mixing NaLa and water, and
9
the absorbents with different H2SO4 contents were prepared by mixing NaLa, water and
10
H2SO4. The SO2 absorption and desorption experiments were carried out using the
11
absorbent glass tube with a length of 100 mm and an inner diameter of 15 mm under
12
the ambient pressure. Generally, 2.00 g water and 2.00 g absorbent were put into the
13
water glass tube and the absorbent glass tube, respectively. Then, the two glass tubes
14
were soaked in the water bath of 40 oC. A simulated flue gas of 100 cm3/min was
15
bubbled through water in the water glass tube before the absorption to make up the
16
water evaporated from the absorption tube. The desorption experiments were carried
17
out in an oil bath of 100 oC, where 100 cm3/min N2 was bubbled through the absorbent
18
glass tube. The temperatures of the water bath and the oil bath were maintained within
19
±0.5 °C. The concentrations of SO2 in absorbents were determined by an iodine titration
20
method (HJ/T 56–2000, a standard method of the State Environmental Protection
21
Administration of China). Moreover, the water contents in the samples were determined
22
by Karl Fischer titration.
23
2.3. Removal of H2SO4 in NaLa aqueous solution. In this work, the
24
solubility of Na2SO4 in NaLa (aq) was measured using a dynamic method.20 Briefly, a
25
crystallizer of glass was used to measure the solubility, and its temperature was 6
ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
1
controlled by an external circulation water bath. A certain amount of NaLa (aq) was
2
added into the crystallizer of glass and heated to a constant temperature. Then Na2SO4
3
was gradually added into the crystallizer of glass with an increment of 0.010 g, which
4
was stirred until Na2SO4 was not dissolved completely, indicating the solubility of
5
Na2SO4 in NaLa (aq). The temperature or concentration of NaLa (aq) was changed and
6
the above process was repeated for other solubility of Na2SO4 in NaLa (aq).
7
The schematic diagram of the experimental equipment for removing H2SO4 in
8
NaLa (aq) is shown in Figure 2. It consists of a gas cylinder, a rotameter, a glass tube,
9
an oil bath, and a temperature controller. The method to reduce water was reported in
10
our previous work.21
11
The absorbents with different SO2 contents were prepared, and 𝑤𝑠𝑜2 represented
12
the mass fraction of SO2 in NaLa (aq). The experiments of the removal of H2SO4 in
13
NaLa (aq) were carried out in the glass tube with a length of 100 mm and an inner
14
diameter of 30 mm under the ambient pressure. Generally, 10.00 g 30 wt% NaLa (aq)
15
with 5wt% H2SO4 was charged into the glass tube, 0.4082 g NaOH was added into the
16
glass tube too, and then the glass tube was soaked in the oil bath of 100 oC. N2 of 100
17
cm3/min was bubbled through the glass tube. Consequently, a precipitate generated was
18
separated by filtration at 20 oC from the solution. At the same time, the water content
19
and H2SO4 content were determined. After that, the H2SO4-removed solution was
20
diluted to 30 wt% by adding water. In addition, five cycles of experiments for removal
21
of H2SO4 in NaLa (aq) with the same absorbent was carried out. In this way, the same
22
weight ratio H2SO4 was added into the H2SO4-removed absorbent, and then H2SO4 was
23
removed from the absorbent again for five cycles. The temperature of the oil bath was
24
maintained within ±0.5 °C. The concentrations of H2SO4 were determined by a weight
25
method (GB/T 13025.8−2012. A standard method of General Administration of Quality 7
ACS Paragon Plus Environment
Page 8 of 29
Page 9 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
1
Supervision, Inspection and Quarantine of the People’s Republic of China.), and the
2
water content was determined by Karl Fischer tiltrotor.
3 4
The removal rate of H2SO4, Re, was calculated using equation (1).
𝑅𝑒 =
𝐼𝑛𝑖𝑡𝑖𝑎𝑙 𝑚𝑎𝑠𝑠 ― 𝑅𝑒𝑠𝑖𝑑𝑢𝑎𝑙 𝑚𝑎𝑠𝑠 𝐼𝑛𝑖𝑡𝑖𝑎𝑙 𝑚𝑎𝑠𝑠
(1)
× 100%
5
where initial mass and residual mass stand for the masses of [SO42 ― ] in g before and
6
after removing, respectively.
7 8 9
The solubility product constants, Ksp, of Na2SO4 in NaLa (aq) were calculated using equation (2). 2
𝐾sp(Na2SO4) = [Na + ] × [SO42 ― ]
(2)
10
where Ksp is the of solubility product constants of Na2SO4 in mol3/dm9, [Na + ]
11
represents the molar concentration of Na+ in mol/dm3 and
12
molar concentration of SO42− in mol/dm3.
[SO42 ― ]
represents the
13
2.4. Analyses of FT-IR, XRD and 1H NMR. 1H NMR spectra of original and
14
H2SO4-removed absorbents were performed on a 400 MHz spectrometer (Bruker
15
Avance III, Switzerland). FT-IR spectra of the crystalline samples were recorded on a
16
Fourier transform spectrometer (Nicolet 6700, USA) with wavenumbers from 400 to
17
4000 cm−1. XRD spectra of the crystalline were conducted on an X-ray diffractometer
18
(Bruke D8 Advance) with scanning intervals from 10 o to 80 o.
19
3. RESULTS AND DISCUSSION
20
3.1. Effect of H2SO4 on SO2 capacity of NaLa (aq). The effect of H2SO4 on
21
SO2 absorption by 30 wt% NaLa (aq) was investigated at 𝑐𝑆𝑂2= 2.1 vol% and 40 oC,
22
and the results are showed in Figure 3. Similar to ethanolamine lactate reported,22
23
H2SO4 has a negative influence on the SO2 capacity of NaLa (aq). The SO2 capacity
24
decreases with increasing the content of H2SO4, owing to that H2SO4 can react with 8
ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
1
NaLa, which turns NaLa into Na2SO4 and lactic acid that cannot absorb SO2.14 In
2
addition, when the H2SO4 content reaches 4.05 wt%, the SO2 absorption capacity of
3
NaLa (aq) is a half of that of the fresh absorbent. The absorbent requires removing
4
H2SO4 as its absorption capacity of SO2 decreases to 80% of the fresh absorbent with a
5
H2SO4 content of about 2 wt%, corresponding to 0.076 molar ratio of SO42− to NaLa.
6
3.2. Effect of Water on the Removal of H2SO4. Firstly, the solubility of
7
Na2SO4 in NaLa (aq) was measured using a dynamic method.20 The results are shown
8
in Figure 4. The solubility of Na2SO4 in NaLa (aq) increases with increasing
9
temperature from 15 oC to 40 oC. When the temperature is higher than 40 oC, the
10
solubility decreases to a small extent. Additionally, the solubility of Na2SO4 decreases
11
rapidly as the concentration of NaLa increases. The results indicate that H2SO4 in the
12
absorbent can be removed by adding NaOH to neutralize H2SO4 to Na2SO4 and
13
increasing the concentration of NaLa.
14
Then, the effect of water content on the removal of H2SO4 was studied at 20 oC
15
with an initial H2SO4 content of 5 wt% and an initial NaLa content of 30 wt%, and the
16
results are shown in Figure 5. The experiment demonstrated that Na2SO4 crystallized
17
as the water content decreased, and could be separated. As shown in Figure 5, the
18
residual content of SO42– is 0.47 wt% when the water content decreases to 34 wt% at
19
20 oC. After calculation, the removal rate of H2SO4 is 95.6%, which satisfies the need
20
of absorbents. On this condition, the solubility product constants, Ksp, of Na2SO4 in
21
NaLa (aq) are shown in Table 1. Compared with that in the literature 23, the value of
22
Ksp in this work at 20 oC is between those at 15 oC and 35 oC. Moreover, the value of
23
Ksp in this work is between those reported in the literature 24 and 25 at the same
24
temperature of 20 oC. Considering the difference of temperature and solution system,
25
the deviation of Ksp of Na2SO4 is reasonable. 9
ACS Paragon Plus Environment
Page 10 of 29
Page 11 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
1
The Na+ content increases and the free water reduces with decreasing water
2
content, which results in the diminution of Na2SO4 solubility. This is called the
3
common-ion effect. There is Na+ in a high concentration in the NaLa (aq). Thus, the
4
solubility of Na2SO4 in the NaLa (aq) is much low due to the common-ion effect. When
5
the water content is reduced, the Na+ concentration is improved. As a result, the
6
solubility of Na2SO4 decreases further until it is low enough for the removal of H2SO4.
7
3.3. Effect of absorbed SO2 on the removal of H2SO4. Due to the use of
8
NaLa (aq) to absorb SO2, the influence of absorbed SO2 on the removal of H2SO4 was
9
also investigated at a NaLa content of 30 wt% and an H2SO4 content of 5 wt%. The
10
water contents of the absorption solution were reduced to 35 wt% (called a final water
11
content), and the H2SO4 content was determined at 20 oC. As shown in Figure 6,
12
absorbed SO2 has almost no effect on the process of removing H2SO4. Absorbed SO2
13
becomes sodium sulfite (Na2SO3) by reacting with NaLa. According to the experiment
14
result, the solubility product constant Ksp of Na2SO3 at 20 oC is 28.69, which is much
15
higher than 1.40 of Na2SO4. Theoretically, generated Na2SO3 can dissolve in NaLa (aq).
16
Thus, absorbed SO2 will not affect the removal of H2SO4 due to the same Na+
17
concentration. In addition, it was observed that there was no precipitate existing when
18
30 wt% NaLa (aq) with 5 wt% H2SO4 was saturated with absorbed SO2. Therefore,
19
absorbed SO2 does not affect the removal of H2SO4.
20
3.4. Effect of the H2SO4 removal process on NaLa. The effect of the H2SO4
21
removal process in NaLa (aq) on the stability of NaLa was studied. As shown in Figure
22
7, the 1H NMR spectra of NaLa (aq) before and after removing H2SO4 indicate that
23
there is no any change on NaLa before and after the H2SO4 removal process. It means
24
that the H2SO4 removal process has no effect on NaLa.
25
The SO2 absorption capacity of H2SO4-removed NaLa (aq) was also investigated, 10
ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
1
which was carried out by five cycles of the H2SO4 removal experiment, as shown in
2
Figure 8. The results indicate that there is a negligible change of SO2 absorption
3
capacity in NaLa (aq), and the content of residual SO42− is less than 0.5% in all five
4
cycles. In addition, compared to fresh NaLa (aq), the SO2 capacity of the H2SO4-
5
removed NaLa (aq) is about 97%. It means that the NaLa (aq) still has the highly
6
absorptive performance for SO2.
7
3.5. Mechanism of H2SO4 removal. The solubility product constant (Ksp) of
8
Na2SO4 was calculated by equation (2). As we know, Ksp is a function of temperature.
9
When the temperature is constant, Ksp is a certain value. According to equation (2), the
10
concentration of SO42− decreases drastically when the concentration of Na+ increases.
11
This is the principle of H2SO4-removal, which is known as the common-ion effect. The
12
addition of NaOH to neutralize H2SO4 and the decrease of water from NaLa (aq) result
13
in the increase of Na+ content in NaLa (aq), and then H2SO4 is removed from NaLa (aq)
14
as Na2SO4 crystalline.
15
Additionally, the crystalline was analyzed by FTIR and XRD. As for an inorganic
16
compound, there are absorption peaks caused by the lattice vibration of anion and
17
crystal water in the mid-infrared area. The cations just influence the position of them.
18
As shown in Figure 9, there are only absorption peaks at 1128 cm–1, 616 cm–1 and 641
19
cm–1, which can be assigned to SO42–. It indicates that the Na2SO4 crystalline does not
20
have crystal water. Meanwhile, as shown in Figure 10, compared to Na2SO4 standard
21
card (PDF # 36-0397), it can be found that the characteristic peaks of 111, 131, 113,
22
220, 222 and 153, etc. in the spectrum of the sample indicate that the crystalline was
23
Na2SO4. It can be explained by the common-ion effect. When the concentration of Na+
24
is high in NaLa solution at a decreased water content, SO42– is surrounded by Na+
25
because of electrostatic interaction, which can prevent hydration between SO42– and 11
ACS Paragon Plus Environment
Page 12 of 29
Page 13 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
1
H2O. Consequently, Na2SO4 is crystallized without crystal water. As proved, SO42– is
2
surrounded by Na+ in a high concentration, which forms a large space resistance for
3
water to solvate SO42–. Therefore, with decreasing free water, the solubility of Na2SO4
4
is reduced further. Hence, H2SO4 can be removed from NaLa (aq) in the form of Na2SO4
5
without crystal water.
6
3.6. Process of the technology for SO2 absorption with H2SO4 removal.
7
A possible process of technology for SO2 absorption with the removal of H2SO4 is
8
proposed and shown in Figure 11, which is based on the common-ion effect. It mainly
9
contains the process of pre-purification of flue gas, SO2 absorption, SO2 desorption,
10
SO2–H2O separation and H2SO4 removal. It is noted that the evaporation of water to
11
decrease water content is combined with the regeneration of NaLa (aq), without any
12
extra evaporation energy. Moreover, a split valve (9) is controllable, which is decided
13
by the H2SO4 content in NaLa (aq). After regeneration, a certain amount of NaOH is
14
added into NaLa (aq) in the feed container 10, and then generated Na2SO4 is separated
15
from filter 11. Compared with the process using BaCl2 or BaCO3, the proposed
16
technology is low-cost and environmentally friendly, and no solid waste is produced.
17
Above all, the H2SO4-removed NaLa (aq) is an unsaturated solution of Na2SO4, which
18
cannot result in blocking the absorption tower as H2SO4 is newly produced.
19
4. CONCLUSIONS
20
In summary, a method to efficiently remove H2SO4 in NaLa (aq) based on the
21
common-ion effect has been proposed. The residual content of SO42– can be less than
22
0.5 wt% in H2SO4-removed NaLa (aq) when the removal rate of H2SO4 is about 95%.
23
The SO2 absorption capacity of H2SO4-removed absorbent is 97% of original absorbent
24
after five cycles. Water content plays a crucial role in the removal of H2SO4 according
25
to the common-ion effect. SO2 has no effect on the H2SO4-removed process, and the 12
ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
1
process of removing H2SO4 has a negligible influence on the absorptive performance
2
of NaLa (aq) for SO2. Finally, a process for SO2 absorption in NaLa (aq) with the
3
removal of H2SO4 was proposed.
4
5
Corresponding Authors
6
* E-mail:
[email protected] (W. W.);
[email protected] (S. R.)
7
ORCID
8
Weize Wu: 0000-0002-0843-3359
9
Shuhang Ren: 0000-0003-3253-8852
AUTHOR INFORMATION
10
Notes
11
The authors declare no competing financial interest.
12
13
The authors thank Professors Zhenyu Liu and Qingya Liu for their helpful discussion
14
and suggestions, and also thank the long-term subsidy mechanism from the Ministry of
15
Finance and the Ministry of Education of PRC (BUCT). The project is financially
16
supported by the National Natural Science Foundation of China (21176020 and
17
21306007).
ACKNOWLEDGMENTS
18 19
REFERENCES
20
1.
Atkins, R. S., Commercialization of Lime-Limestone Flue Gas Scrubbing Technology. Sulfur Removal and Recovery 1975, 84, (139), 120-129.
21 22
2.
Wang, Z. K.; Peng, Y.; Ren, X. C.; Gui, S. Y.; Zhang, G. K., Absorption of Sulfur
23
Dioxide with Sodium Hydroxide Solution in Spray Columns. Ind. Eng. Chem. Res.
24
2015, 54, (35), 8670-8677. 13
ACS Paragon Plus Environment
Page 14 of 29
Page 15 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
1
3.
Sada, E.; Kumazawa, H.; Sawada, Y.; Kondo, T., Simultaneous Absorption of
2
Dilute Nitric Oxide and Sulfur Dioxide into Aqueous Slurries of Magnesium
3
Hydroxide with Added Iron(II)-EDTA Chelate. Ind. Eng. Chem. Process Des. Dev.
4
1982, 21, (4), 771-774.
5
4.
Yang, J. T.; Gao, H. Y.; Hu, G. X.; Wang, S. Y.; Zhang, Y., Novel Process of
6
Removal of Sulfur Dioxide by Aqueous Ammonia–Fulvic Acid Solution with
7
Ammonia Escape Inhibition. Energy Fuels 2016, 30, (4), 3205-3218.
8
5.
Jung, K. D., Ether-Functionalized Ionic Liquids as Highly Efficient SO2
9
Absorbents. Energ. Environ. Sci. 2011, 4, (5), 1802-1806.
10 11
Hong, S. Y.; Im, J.; Palgunadi, J.; Lee, S. D.; Lee, J. S.; Kim, H. S.; Cheong, M.;
6.
Shang, Y.; Li, H. P.; Zhang, S. J.; Xu, H.; Wang, Z. X.; Zhang, L.; Zhang, J.,
12
Guanidinium-based Ionic Liquids for Sulfur Dioxide Sorption. Chem. Eng. J. 2011,
13
175, (1), 324-329.
14
7.
Absorption of SO2. Green Energ. Environ. 2018, 3, (3), 179-190.
15 16
Ren, S. H.; Hou, Y. C.; Zhang, K.; Wu, W. Z., Ionic liquids: Functionalization and
8.
Qu, G. F.; Zhang, J.; Li, J. Y.; Ning, P., SO2 Absorption/Desorption
17
Characteristics of Two Novel Phosphate Ionic Liquids. Sep. Sci. Technol. 2013,
18
48, (18), 2876-2879.
19
9.
Wang, C. M.; Cui, G. K.; Luo, X. Y.; Xu, Y. J.; Li, H. R.; Dai, S., Highly Efficient
20
and Reversible SO2 Capture by Tunable Azole-Based Ionic Liquids through
21
Multiple-Site Chemical Absorption. J. Am. Chem. Soc. 2011, 133, (31), 11916-
22
11919.
23
10. Wu, W. Z.; Han, B. X.; Gao, H. X.; Liu, Z. M.; Jiang, T.; Huang, J.,
24
Desulfurization of Flue Gas: SO2 Absorption by an Ionic Liquid. Angew. Chem.,
25
Int. Ed. 2004, 43, (18), 2415-2417. 14
ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
1
11. Yuan, X. L.; Zhang, S. J.; Lu, X. M., Hydroxyl Ammonium Ionic Liquids:
2
Synthesis, Properties, and Solubility of SO2. J. Chem. Eng. Data 2007, 2, (52),
3
596-599.
4
12. Zhang, K.; Ren, S. H.; Hou, Y. C.; Wu, W. Z.; Bao, Y. Y., Sodium Lactate
5
Aqueous Solution, a Green and Stable Absorbent for Desulfurization of Flue Gas.
6
Ind. Eng. Chem. Res. 2017, 56, (46), 13844-13849.
7
13. Ren, S. H.; Hou, Y. C.; Wu, W. Z.; Jin, M. J., Oxidation of SO2 Absorbed by an
8
Ionic Liquid During Desulfurization of Simulated Flue Gases. Ind. Eng. Chem.
9
Res. 2014, 50, (2), 998-1002.
10
14. Ren, S. H.; Hou, Y. C.; Tian, S. D.; Chen, X. M.; Wu, W. Z., What Are Functional
11
Ionic Liquids for the Absorption of Acidic Gases? J. Phys. Chem. B 2013, 117,
12
(8), 2482-2486.
13
15. Korngold, E.; Bejerano, T. T., Sulfate Removal from Brackish Water Using an
14
Anion Exchanger with Recycled Regeneration. Desalin. Water Treat. 2015, 57,
15
(31), 14364-14368.
16
16. Mohebbi, A.; Hangi, A.; Mirzaei, M.; Kaydani, H., Evaluation and Comparison of
17
Sulfate Anions Removal from Artificial and Industrial Wastewaters by
18
Nanofiltration Process in A Laboratory Scale. J. Memb. Separ. Technol. 2015, 4,
19
(2), 40-52.
20
17. Cattoir, S.; Smets, D.; Rahier, A., The Use of Electro-Electrodialysis for the
21
Removal of Sulphuric Acid from Decontamination Effluents. Desalination 1999,
22
121, (121), 123-130.
23 24 25
18. Hlabela, P.; Maree, J.; Bruinsma, D., Barium Carbonate Process for Sulphate and Metal Removal from Mine Water. Mine. Water. Environ. 2007, 26, (1), 14-22. 19. Benatti, C. T.; Tavares, C. R.; Lenzi, E., Sulfate Removal from Waste Chemicals 15
ACS Paragon Plus Environment
Page 16 of 29
Page 17 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
1 2 3 4 5
by Precipitation. J. Environ. Manage. 2009, 90, (1), 504-511. 20. Clynne, C. A. P., II, R. W.; Haas, Jr. J. L. , Solubility of NaCl in Aqueous Electrolyte Solutions from 10 to 100 °C. J. Chem. Eng. Data 1981, 26, 396-398. 21. Ren, S. H.; Hou, Y. C.; Wu, W. Z.; Liu, W. N., Purification of Ionic Liquids: Sweeping Solvents by Nitrogen. J. Chem. Eng. Data 2010, 55, (11), 5074-5077.
6
22. Ren, S. H.; Hou, Y. C.; Tian, S. D.; Wu, W. Z.; Liu, W. N., Deactivation and
7
Regeneration of an Ionic Liquid during Desulfurization of Simulated Flue Gas.
8
Ind. Eng. Chem. Res. 2014, 51, (8), 3425-3429.
9
23. Guo Y. F. , L. Y. H., Wang Q. , Lin C. X. , Wang S. Q. , Deng T. L., Phase
10
Equilibria and Phase Diagrams for the Aqueous Ternary System (Na2SO4 +
11
Li2SO4 + H2O) at (288 and 308) K. J. Chem. Eng. Data 2013, 58, 2763-2767.
12
24. David R. Lide, e., CRC Handbook of Chemistry and Physics, Internet Version
13
2005. CRC Press: Boca Raton, FL, 2005.
14
25. Liu, G. Q., Ma, L. X., Liu, J., Physical Property Data Handbook of Chemistry and
15
Chemical Engineering. Inorganic Volume. Chemical Industry Press: Beijing, 2002.
16
16
ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 18 of 29
1
Table 1. The solubility product constant (Ksp) of Na2SO4
2
Item Temperature/oC
This work 20
Ref23
Ref23
Ref24
Ref25
15
35
20
20
Density/g∙cm−3
1.3454
1.0049
1.3287
1.1301
1.082
Mass fraction/%
0.695
11.74
33.06
13.89
9.260
Ksp/mol3∙dm−9
4.189
2.294
118.4
5.402
1.403
3
17
ACS Paragon Plus Environment
Page 19 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
1
2 3
Figure 1. Schematic diagram of the experimental system for absorption and desorption
4
of SO2. (1) SO2-contained N2 or N2 gas cylinder; (2) pressure reducing valve; (3)
5
rotameter; (4) glass tube with water; (5) glass tube with absorbent; (6) water or oil bath;
6
(7) temperature controller; (8) absorption device of tail gas.
7
18
ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
1
2 3
Figure 2. Schematic diagram of the experimental system for removing H2SO4 in NaLa
4
(aq). (1) N2 gas cylinder; (2) pressure reducing valve; (3) rotameter; (4) glass tube; (5)
5
oil bath; (6) temperature controller.
6
19
ACS Paragon Plus Environment
Page 20 of 29
Page 21 of 29
1
0.28
SO2 capacity (g/g NaLa)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
0.24 0.20 0.16 0.12 0.08 0.04 0.00
0
2
4
6
8
10
H2SO4 content (wt%)
2 3
Figure 3. The influence of H2SO4 on SO2 capacity of NaLa (aq) at 40 oC with an SO2
4
concentration of 2.1%.
5
20
ACS Paragon Plus Environment
Energy & Fuels
1
12
Solubility of Na2SO4 (g/100g)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 22 of 29
10
30 wt% NaLa 40 wt% NaLa 50 wt% NaLa
8 6 4 2 0 10
20
30
40
50
o
Temperature ( C)
2 3
Figure 4. The solubility of Na2SO4 in NaLa (aq) with different concentrations as a
4
function of temperature.
5
21
ACS Paragon Plus Environment
Page 23 of 29
1
SO4
2.5
2-
2-
0.10 content in mass fraction
Molar ratio of SO4
2-
to NaLa
0.08
2.0 0.06 1.5 0.04
1.0
0.02
0.5 0.0 30
2
35
40
45
50
55
molar ratio of SO42- to NaLa
3.0
SO4 content (wt%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
0.00 60
water content (wt%)
3
Figure 5. The influence of water content on the removal of H2SO4 at 20 oC with an
4
initial H2SO4 content of 5 wt% and an initial NaLa content of 30 wt%.
5
22
ACS Paragon Plus Environment
Energy & Fuels
1
5 4 3 2
2-
SO4 content (wt%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 24 of 29
1 0
2
0.0
0.5
1.0
1.5
2.0
SO2 content in NaLa(aq) (wt%)
3
Figure 6. The influence of SO2 content on the removal of H2SO4 at 20 oC with a final
4
water content of 35 wt% and an initial NaLa content of 30 wt%.
5
23
ACS Paragon Plus Environment
Page 25 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
1
1.26 H2SO4-removal NaLa
4.06
1.26 Original NaLa
4.05
-CH3
-CH-
5
2 3
4
3
2
1
0
(ppm)
Figure 7. 1H NMR spectra of NaLa (aq) before and after the H2SO4 removal.
4
24
ACS Paragon Plus Environment
Energy & Fuels
1
0.30
Absorption 2SO4 content
4
0.20 3
0.15 2
0.10 1
0.05 0.00
1
2
3
4
5
2-
SO2 Capacity (g/g IL)
0.25
5
SO4 content (wt%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 26 of 29
0
Cycles 2 3
Figure 8. Five cycles of the H2SO4-removed experiment in NaLa (aq) (𝑤𝑁𝑎𝐿𝑎 = 30 wt
4
%). In each cycle, SO2 (2.1 vol%) was absorbed at 40 °C and the SO42− content was
5
determined at 20 oC.
6
25
ACS Paragon Plus Environment
Page 27 of 29
1
1128
Abosorbance (a.u.)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
618 641
4000 3500 3000 2500 2000 1500 1000 -1
2 3
Wavenumber (cm )
Figure 9. FTIR spectrum of Na2SO4 crystalline separated
4
26
ACS Paragon Plus Environment
500
Energy & Fuels
1
Na2SO4 sample Na2SO4 PDF#36-0397
Intensity (a.u.)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 28 of 29
111
113 220 131 222 022 040
10
20
30
153 313 333
40
50
60
440
70
80
2 (°)
2 3
Figure 10. XRD spectra of Na2SO4 crystalline separated and standard Na2SO4
4
crystalline
5
27
ACS Paragon Plus Environment
Page 29 of 29 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
1 SO2 8
SO2 contained flue gas SO2 -removed flue gas
4 1
6
NaOH
3
7 10
5
9
2
11
Na2SO4
2 3
Figure 11. Flowchart of the process of the technology for removing H2SO4: (1)
4
scrubber; (2) water purifier; (3) heat exchanger; (4) absorption tower; (5) mixer; (6)
5
desorption tower; (7) heat exchanger with extra steam; (8) distillation tower; (9) split
6
valve; (10) feed container; (11) filter.
7
28
ACS Paragon Plus Environment
