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Understanding the transition process of phase change and dehydration reaction of salt hydrate for thermal energy storage
Journal Pre-proofs Understanding the transition process of phase change and dehydration reaction of salt hydrate for thermal energy storage T.S. Yan, T.X. Li, J.X. Xu, J.W. Chao PII: DOI: Reference:
S1359-4311(19)31854-X https://doi.org/10.1016/j.applthermaleng.2019.114655 ATE 114655
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Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
21 March 2019 29 September 2019 6 November 2019
Please cite this article as: T.S. Yan, T.X. Li, J.X. Xu, J.W. Chao, Understanding the transition process of phase change and dehydration reaction of salt hydrate for thermal energy storage, Applied Thermal Engineering (2019), doi: https://doi.org/10.1016/j.applthermaleng.2019.114655
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© 2019 Published by Elsevier Ltd.
Understanding the transition process of phase change and dehydration reaction of salt hydrate for thermal energy storage T.S. Yan, T.X. Li, J.X. Xu, J.W. Chao Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai 200240, China
Abstract Salt hydrate is one kind of the most promising materials used for both latent heat and thermochemical heat storage systems. Currently, there is a lack of fundamental understanding of the correlation mechanism of salt hydrate as phase change material (PCM) and thermochemical material (TCM). The dehydrating and melting processes of salt hydrates (Na2SO4·10H2O, CH3COONa·3H2O, MgSO4·7H2O, and SrBr2·6H2O) were carefully analyzed at different heating rates. The experimental results show that the melt of salt hydrate occurs during its dehydrating process at high heating rates, and thus leads to the formation of molten hydrate. Moreover, the molten hydrate can dehydrate to lower hydrate directly with a slower dehydrating rate and higher activation energy. Furthermore, the temperature-pressure equilibrium of salt hydrate is higher than its dehydrating equilibrium line before phase change, but it is closer to the dissolving line. An equilibrium equation is developed for describing the transition process from molten hydrate salt to directly dehydrating into lower hydrate, and also used to explain the equilibrium curve after phase change. These findings of mutual effects between phase change and thermochemical dehydration can provide the understanding on the transition process of salt hydrate for thermal energy storage. Key words: salt hydrate; phase change; thermochemical; thermal energy storage
Corresponding author. Tel.: +86-21-34206335; fax: +86-21-34206335. E-mail address: [email protected] (T.X. Li). 1
Nomenclatures PCM
phase change material
TCM
thermochemical material
Dimensional variables Tm
melting temperature [K]
Tp
peak temperature [K]
P0
standard atmospheric pressure[Pa]
R
gas constant [J/(mol·K)]
A
pre-exponential factor
Ea
apparent activation energy [kJ/mol]
ΔHm
melting enthalpy [kJ/mol]
△Sm
melting entropy [J/(mol·K)]
ΔfH0
standard enthalpy of formation [kJ/mol]
ΔfS0
standard entropy of formation [J/(mol·K)]
ΔHr0
reaction enthalpy [kJ/mol]
ΔSr0
reaction entropy [J/(mol·K)]
ΔHsol0
enthalpy of the solution [kJ/mol]
φ
integral enthalpy of dilution of the solution [kJ/mol]
L
Greek symbols α
conversion rate
β
heating rate [K/min]
x
saturation solubility [wt%]
γ
dissolved ratio of salt hydrate [wt%]
2
1. Introduction Energy deficiency is increasingly severe. To reduce the energy consumption and increase the energy efficiency are more and more urgent. Among the total energy consumption, the building energy consumption can occupy a large proportion. More than 40%~60% building energy consumption should end up with heat demand, so it is of great value to increase the heat utilization efficiency. However, many renewable energies, like solar energy and low grade thermal source, are transient, fluctuant and limited by region [1]. Thermal energy storage is an effective technology to solve these problems, which can be mainly divided into three types: sensible heat storage, latent heat storage and thermochemical energy storage. Sensible heat is stored through the temperature difference of storage medium, which is simple but faces problems like low thermal storage density and large thermal loss. Latent heat and thermochemical heat storage show high thermal storage density which have gained more and more attention. Salt hydrate (MpXq·nH2O) is recognized as promising thermal storage material and widely used as phase change material (PCM) [2, 3] and thermochemical material (TCM) [4,5] in recent years. Figure 1 shows the thermal energy storage density and working temperature of typical salt hydrate as PCM or TCM [6-8]. The latent heat energy storage using salt hydrate as PCM is based on melting/crystallizing process as phase change between solid state and liquid state, and the energy storage density (phase change enthalpy) usually varies between 100~300 kJ/kg [9]. Thermochemical energy storage using salt hydrate as TCM is based on breaking/reforming bonds of water molecule and salt in crystal structure [10] as de-/rehydration reaction between solid salt and vapor, and the energy density (reaction enthalpy) is usually as high as 1000~2000 kJ/kg [11].
3
2500 Thermochemical Storage
Salt hydrate using as TCM
Thermal Energy Density（ kJ/kg）
Na2S9H2O/0.5H2O
2000 Na2SO410H2O
1500
KAl(SO4)212H2O LiClH2O
1000
CaCl2H2O MgSO47H2O/H2O MgCl2H2O/2H2O
SrBr2H2O/H2O
Na2S2O35H2O
Latent Storage
500
CH COONa3H2O Ba(OH)28H2O Na2SO410H2O 3 KAl(SO4)212H2O MgCl2H2O CaCl2H2O MgSO47H2O SrBr6H2O
0 0
50
Salt hydrate using as PCM
100 Temperature（ ℃）
150
200
Figure 1. Thermal energy storage density and working temperature of salt hydrate as PCM or TCM (Temperature by 20mbar vapor pressure) [6-8]
Unlike pure salt or other ionic solids, the water within the hydrated salt crystal leads to the complex performances of melt and dehydration. The solid hydrated salt will melt into molten hydrated salt under phase change temperature. Specifically, the molten state is usually regarded as an aqueous solution. According to melting behaviors, salt hydrate can be classified into three categories: congruent melt in which the dehydrated salt can be completely soluble in the hydration water, incongruent melt where the salt is partially soluble in the hydration water, and semi-congruent melt when the liquid and solid are in equilibrium with different compositions for the hydrate transformation to a lower hydrated salt [6,12,13]. Incongruent melt always accompanies with the formation of lower hydrate salt, which is irreversible and leads to the loss of storage capacity. During the melting process of Mg(NO3)2·6H2O, the dihydrate will be formed and leads to instability. To solve this problem, Mg(NO3)2·6H2O and extra water were mixed to prepare nanoencapuslated salt hydrate, which can realize 100+ heat uptake/release cycles [14]. El-Sebaii conducted the melting test of MgCl2·6H2O in a sealed container under the extra water principle, more than 4
1000 melt/solidification cycles were conducted with stable phase change performance [15]. As far as hydrated salt dehydrating into lower hydrated salt and vapor, it is influenced by both temperature and vapor pressure. Many salt hydrates undergo multi-step dehydration. For example, the dehydration of MgCl2·6H2O has three intermediate hydrated salts, including tetrahydrate, dihydrate and monohydrate [16]. In addition, other adverse reactions could lead to the decomposition of salt hydrate. For instance, MgCl2·6H2O will decompose into Mg(OH)Cl and HCl if dehydrating temperature is too high [17]. MgSO4·7H2O will dehydrate to MgSO4·6H2O at 25-55 ºC, while an amorphous phase is formed until converts to crystalline MgSO4 at 300 ºC [18]. When the water diffusion is limited under MgSO4·7H2O grains, an aqueous solution was produced during dehydration [19]. According to the way water released, the intermediate phase and final product, Galwey [20] described the solid-state dehydration as six different ways of water evolution type. The crystal may undergo partially melting or completely melting during dehydration process, impervious outer layer will be formed, which limits the vapor diffusion. From above, there is plenty of reports on phase change and dehydration of salt hydrate independently, but the effect of melt on dehydration or effect of dehydration on melt is seldom concerned. In this paper, we want to study the melting process of salt as PCM and dehydrating process as TCM simultaneously, the correlation of salt hydrate melting and dehydrating is concerned. Transition processes of four salt hydrates (Na2SO4·10H2O, CH3COONa·3H2O, MgSO4·7H2O, and SrBr2·6H2O) were carefully analyzed at different heating rates by TGDSC and DSC, dehydrating and melting parameters were calculated. Additionally, we designed a vacuum reactor system to study the temperature and pressure evolution during phase change process. The latent heat was analyzed based on different transition routes. This study shows the link between phase change and dehydration of salt hydrate which
5
would contribute to deepen our understanding on transition process of salt hydrate and its application for thermal energy storage.
2. Materials and Methods 2.1 Materials and Characterizations Four typical salt hydrates, Na2SO4·10H2O (99%, J&K China Chemical Ltd.), CH3COONa·3H2O (99.5%, Energy Chemical Co.), MgSO4·7H2O (99.5%, Shanghai Macklin Biochemical Co., Ltd), SrBr2·6H2O (99%, Energy Chemical Co.), were selected to investigate transition processes of phase change and dehydration. Differential Scanning Calorimetry (DSC8000, Pekin-Elmer) was used to determine phase change enthalpy (ΔHm) and melting temperature (Tm) of salt hydrate. Sample mass with about 5~10 mg was sealed in aluminum dish. Different heating rates (0.5K/min, 1K/min, 2K/min, 5K/min, 10K/min) were tested. Simultaneous thermal analysis (Netzsch STA 449 F3) was used to determine dehydration characteristics of salt hydrate, including reaction temperature, mass loss, reaction enthalpy, etc. About 20 mg salt hydrate sample was placed inside an aluminum dish and heated from room temperature to 300 ºC at different heat rates (0.2K/min, 1K/min, 2K/min, 5K/min, 10K/min). Nitrogen was used as a purge gas with 20mL/min to remove the moisture from the crucible.
2.2 transition experimental setup for salt hydrate A vacuum experimental device system was designed to test temperature and pressure change during transition process as shown in figure 2. The system is mainly composed of a vacuum reactor, a liquid reservoir, thermostatic baths, a vacuum pump. The temperature
6
sensor, PT100 with an accuracy of 0.15 ºC and pressure sensor with an accuracy of 1 mbar were used to measure temperature and pressure evolution.
Figure 2. Experimental setup for testing the transition process of salt hydrate
Dynamic melting test, test procedures including 1) preparation step: salt hydrates with thickness about 1~2 cm and mass about 200~300g, were sealed at the vacuum reactor. Air within the reactor will degas to less than 1mbar by vacuum pump. Thermostatic bath (oil bath) is used to adjust the salt at initial temperature. 2) heating step: set the thermostatic bath at end temperature to heat the salt to melt. Typically, for Na2SO4·10H2O, temperature increases from 20ºC to 50ºC, MgSO4·7H2O from 30ºC to 70ºC, CH3COONa·3H2O from 30ºC to 80ºC, SrBr2·6H2O from 50ºC to 100ºC. Salt temperature and vapor pressure were recorded. Multi-step equilibrium test: test procedures are similar to dynamic melting test. However, the heating temperature increases step by step, salt temperature and vapor pressure will keep enough time to reach equilibrium at each steps.
7
3. Results and discussions 3.1. Dehydrating and melting process of salt hydrates To analysis the dehydrating process, salt hydrate was heated in an open environment to ensure fast vapor exchange. Dehydrating curves, including mass loss line(TG), mass loss rate line(DTG), heat flow line(DSC), are shown in figure 3, figure 4, and figures S1-S4. As for the melting process, salt hydrate was sealed in a close environment to avoid vapor evaporation. Melting process at different heating rates, shown in figures S5-S8, were conducted to compare with the dehydrating process and calculate the activation energy of dynamic transition process. From figure 3 and figure 4, it can be seen that melt can influence the dehydrating process as the heating rate increases. For sodium sulfate decahydrate, melting heat peak will appear at all tested heating rates (see figure 3a and figure S1). Only part of hydrate melt during dehydration as the melting heat ranges from 150 J/g to 200 J/g in STA, in comparison with 248J/g in DSC. The additional melt won’t influence the sequential dehydrating process seen from the comparison of mass loss rate and heat flow curves in figure S1. However, for sodium acetate trihydrate, dehydration is divided into two steps when the heating rate is larger than 1K/min. As mass loss rate (DTG) curve and heat flow curve shown in figure S2, melting heat will lead heat flow deviates from DTG curve. After molten hydrate formed, the subsequent dehydration shows lower dehydrating rate. With the heating rate increasing, the molten hydrate proportion would increase from 32% (mass loss ratio compare to total mass loss) at 2K/min to 92% at 10K/min.
8
(b)
(a) 100
i Na2SO410H2O(s)Na2SO4(s)
-9
Mass(%)
80 -10
60 40
2
ln(/Tp)
i
-50
Na2SO410H2O 0.2K/min
-13
Na2SO410H2O 10K/min
20
40
60
80
100
120
Na2SO410H2O(s)Na2SO410H2O(l)(STA) Na2SO410H2O(s)Na2SO410H2O(l)(DSC)
0.0026
140
Temp(C)
0.0028
0.0030
0.0032
1/Tp
(d) -9
i CH3COONa3H2O(s)CH3COONa(s)
100
ii CH3COONa3H2O(l)CH3COONa(s)
80
-10
60 i
20 40
ii
0
-12
-20 CH3COONa3H2O 0.2K/min
-40
CH3COONa3H2O(s)CH3COONa(s)
CH3COONa3H2O 1K/min
-60
CH3COONa3H2O 2K/min
-13
CH3COONa3H2O 5K/min
-80 -100
-11
2
Mass(%)
Na2SO410H2O(s)Na2SO4(s)
Na2SO410H2O 5K/min
Melting
(c)
Heat flow(J/g/K)
-12
Na2SO410H2O 2K/min
-150 -200
-11
Na2SO410H2O 1K/min
-100
ln(/Tp)
Heat (J/g/K)
0
CH3COONa3H2O 10K/min
20
40
60
80
100 120 140 160 180 200
0.0024
Temp(C)
CH3COONa3H2O(l)CH3COONa(s) CH3COONa3H2O(s)CH3COONa3H2O(l) (DSC)
0.0026
0.0028
0.0030
1/Tp
Figure 3. TG-DSC curves of (a) Na2SO4·10H2O and (c) CH3COONa·3H2O vary with the change of heating rate β(K/min). Kinetic results of thermal dehydration or melting steps for (b) Na2SO4·10H2O and (d) CH3COONa·3H2O
For magnesium sulfate heptahydrate shown in figure 4a and figure S3, the dehydrating process can be divided into three steps. The first step was dehydration from MgSO4·7H2O to MgSO4·6H2O by losing 1 mol water at temperature range of 30~50 °C, the second step was dehydration from MgSO4·6H2O to MgSO4·xH2O (x < 0.1) at temperature range of 50~260 °C, and the final step (260~280 ºC) completely lost all the water to become MgSO4. Results are in agreement with previous reports in literature [18]. When heating rate is larger than 1K/min, a melting peak appears at 45~65 °C which is similar to sodium sulfate decahydrate. With the heating rate increasing from 2 K/min to 10 K/min, the proportion of
9
0.0032
melt will increase from 2% to 70%. When the heating rate reaches 10 K/min, the multi-step dehydration process becomes indistinguishable.
(b)
(a)
-9 i MgSO47H2O(s)MgSO46H2O(s)
100
Mass(%)
ii MgSO46H2O(s)MgSO4(s)xH2O(s)
-10
80 60
-11
i
2
20 40
ln(/Tp)
ii
Heat(J/g/K)
0 -20
MgSO47H2O 0.2K/min
-40
MgSO47H2O 2K/min
MgSO47H2O 1K/min
-12
-13
MgSO47H2O 10K/min
50
100
150
200
250
300
Temp(C)
(c)
MgSO46H2OMgSO4(s) MgSO47H2O(s)MgSO47H2O(l)(STA)
MgSO47H2O 5K/min
-60 Melting
MgSO47H2O(s)MgSO46H2O(s)
-14 0.0026
MgSO47H2O(s)MgSO47H2O(l)(DSC)
0.0028
0.0030
0.0032
1/Tp
(d)
110 i SrBr26H2O(s)SrBr2H2O(s)
100
Mass(%)
-9.5
ii SrBr2H2O(s)SrBr2(s)
90 80
-10.0
60
2
ii
i
10 50
ln(/Tp)
70
Heat(J/g/K)
0 -10
-11.0
SrBr26H2O 1K/min
-20
SrBr26H2O(s)SrBr2H2O(s) 1~5K/min
SrBr26H2O 2K/min
-11.5
SrBr26H2O 5K/min
-30
SrBr26H2O 8K/min
-40 -50
-10.5
100
150
200
250
SrBr26H2O(s) SrBr2H2O(s) 2nd stage 8~12K/min SrBr26H2O(s) SrBr26H2O(l) (DSC)
SrBr26H2O 10K/min
50
SrBr26H2O(s) SrBr2H2O(s) 1st stage 8~12K/min
300
-12.0 0.0022
0.0024
0.0026
0.0028
1/Tp
Temp(C)
Figure 4. TG-DSC curves of (a) MgSO4·7H2O and (c) SrBr2·6H2O vary with the change of heating rate β(K/min). Kinetic results of thermal dehydration or melting steps for (b) MgSO4·7H2O and (b) SrBr2·6H2O
While for strontium bromide hexahydrate, two-step dehydration of SrBr2·6H2O occurs at low heating rate (1~5K/min). The first step is dehydrating five water from hexahydrate to monohydrate at 40~120 ºC. The second step is dehydrating from monohydrate to anhydrate at 150~250 ºC. When heating rate is higher than 8K/min, the melting effect can be observed. Seen from figure 4c and figure S4, melt leads to two mass loss peak occurring at hexahydrate dehydrating to monohydrate step. Compare mass loss 10
rate curve with heat flow curve, the first peak of heat flow is composed of dehydration and melt. The mass loss proportion of 1st peak and 2nd peak decreases with heating rate increasing, which means no stable intermediate product formed but the melt interrupts the dehydrating process. Transition parameters are summarized in table 1. The dehydrating process or melting process can be described by Arrhenius equation. The activation energy is used to evaluate the energy required to initiate reaction, which can be calculated by Kissinger equation [21, 22]. a d A e RT f ( ) dt E
d(ln
(1)
) Tp 2 E a 1 R d( ) Tp
(2)
Compare melting parameters in STA and DSC, for Na2SO4· 10H2O and MgSO4· 7H2O, only part of salt hydrate melt in STA, and activation energies are lower than melt in DSC. In addition, when melt happens, the activation energy of sequential dehydration is higher than the original dehydration. It is consistent with higher dehydrating temperature and lower dehydrating rate for molten hydrate directly dehydrating into lower hydrate shown in figure 3 and figure 4.
11
Table 1 Transition parameters of dehydrating and melting processes at different heating rates β(K/min). Heating rate β
Mass loss Δm
Dehydrating heat or melting heat
Peak temperature Tp
Activation energy
(K/min)
(%)
(kJ/kg)
(ºC)
(kJ/mol)
1. Na2SO4·10H2O(s)→Na2SO4(s)+10H2O(g)
0.2/1/2/5/10
53.9
1764.0
45.6/60.9/70.2/77.0/99.2
67.9
2. Na2SO4·10H2O(s)→Na2SO4·10H2O(l) by STA
0.2/1/2/5/10
-
160.3/202.8/199.0/153.7/150.9
35.9/39.8/42.7/46.0/49.9
224.3
3.Na2SO4·10H2O(s)→Na2SO4·10H2O(l) by DSC
1/2/5/10
-
248.0
34.3/35.4/37.4/38.9
386.4
1. CH3COONa·3H2O(s)→CH3COONa(s)+3H2O(g)
0.2/1/2/5/10
40.3
1334.0
44.8/67.0/71.2/81.5/80.1
75.3
2. CH3COONa·3H2O(l)→CH3COONa(s)+3H2O(g)-2nd
2/5/10
13.2/26.4/37.1
-
67.1/70.2/65.1
97.9
3. CH3COONa·3H2O(s)→CH3COONa·3H2O(l) by DSC
1/2/5/10
-
249.7
58.6/59.3/63.4/63.8
332.1
1.MgSO4·7H2O(s)→MgSO4·6H2O(s)+H2O(g)
1/2/5/10
5.9/4.5/1.5/-
229.0/182.5/133.6/99.1
45.0/47.9/49.4/51.6
303.4
2. MgSO4·7H2O(s)→MgSO4·7H2O(l) by STA
1/2/5/10
-
1.7/16.3/23.6/49.3
51.4/52.9/55.9/57.9
88.4
3.MgSO4·6H2O(s)→MgSO4·xH2O(s)+(6-x)H2O(g)
1/2/5/10
43.2/45.5/47.3/52.3
1638.3/1510.5/1323.4/1305.9
57.5/78.1/85.6/95.4/97.1
30.3
4. MgSO4·7H2O(s)→MgSO4·7H2O(l) by DSC
0.5/1/2/5/10
-
69.4
50.1/51.0/51.7/54.2/56.4
398.4
1.SrBr2·6H2O(s)→SrBr2·H2O(s)+5H2O(g)
1/2/5
25.2
791.0
80.5/91.1/108.1
59
1-1 SrBr2·6H2O(s)→SrBr2·H2O(s)+5H2O(g) 1st stage
8/10/12
20.95/18.62/15.86
123/126.1/128.1
97.5
154.4/156.9/158.8
134.1
89.4/89.1/89.2/91.3/93.6
502.2
Transition process Na2SO4·10H2O
CH3COONa·3H2O
MgSO4·7H2O
SrBr2·6H2O
667.0 1-2 SrBr2·6H2O(s)→SrBr2·H2O(s)+5H2O(g) 2nd stage
8/10/12
4.62/6.92/9.34
2. SrBr2·6H2O(s)→SrBr2·6H2O(l) by DSC
0.5/1/2/5/10
-
145.7
12
3.2 Transition route of salt hydrates From above results, when heating temperature is higher than phase change temperature, melt will happen and alter the dehydrating process. Molten salt hydrate directly dehydrating to lower hydrate could be described as follows: M p X q nH 2 O(l ) M p X q mH 2 O( s ) (n m) H 2 O( g ) H r0', n m f H 0 ( M p X q mH 2 O, s ) f H 0 ( M p X q nH 2 O, l ) (n m) f H 0 ( H 2 O, g )
f H 0 (M p X q nH 2 O, l ) f H 0 (M p X q nH 2 O, s) H m (M p X q nH 2O)
(3) (4)
S r0', n m f S 0 ( M p X q mH 2 O, s ) f S 0 ( M p X q nH 2 O, l ) (n m) f S 0 ( H 2 O, g ) (5) f S 0 ( M p X q nH 2 O, l ) f S 0 ( M p X q nH 2 O, s )
ln
H m ( M p X q nH 2 O ) Tm
Gr' , mn H r0', mn Sr0', mn p RT RT R p0
(6)
(7)
So, the dehydrating equilibrium line can be corrected based on thermal physical parameters [7,23-25] and DSC results (see in table 1) The temperature and pressure evolution during melting process was conducted to analysis its transition route. Figure 5 and figure S9 illustrate the temperature and pressure route of MgSO4·7H2O during melting process. The vapor pressure will increase with the salt temperature before melt, then a plateau (50.5ºC, 9.5kPa) was found when reaching phase change temperature. After completely melt, salt temperature and vapor pressure continually increases. It should be noted that the P-T curve of MgSO4·7H2O phase change process is above the solid-gas equilibrium line when TTm was close to a new proposed equilibrium line of MgSO4·7H2O (molten state) and MgSO4·6H2O (solid state), which can be seen in figure 5-6, while this behavior has not gained much attention by other researchers. 13
7(l)->(s)
water
Pressure(kPa)
10
O( H2 6
s) O(
H2
7
M
s)
SO 4
Mg
O4 gS
7(s)->(s)
1 20
30
40
50
Temp(
℃
60
)
70
80
Figure 5. Melting process of MgSO4·7H2O under phase diagram, black dashed line stands for the dynamic phase change route, the black rectangle scatter is the equilibrium point gained from figure S9, 7(s)->6(s) means the solid state heptahydrate dehydrating into hexahydrate, 7(l) means the molten state of heptahydrate. 100 6(l)->1(s)
Pressure(kPa)
Water
10
Sr
6H Br 2
O(
s)
2
SrBr2H2O(s) 6(s)->1(s) 1 40
60
80
Temp(
℃
100
120
)
Figure 6. Melting process of SrBr2·6H2O under phase diagram, black dashed line stands for the dynamic phase change route, the black rectangle scatter is the equilibrium point gained from figure S10, 6(s)->1(s) means the solid state hexahydrate dehydrating into monohydrate, 6(l) means the molten of state hexahydrate.
The dynamic melting process and multi-step equilibrium procedure of SrBr2·6H2O are 14
shown in figure S10, and the P-T line is drawn in figure 6. The salt temperature keeps stable at 89.6 ºC, while vapor pressure is 17.6 kPa, particularly, the vapor pressure starts to increase before salt temperature begins to increase. Similar to MgSO4·7H2O, the equilibrium P-T curve of SrBr2·6H2O before melt won’t follow the crystallization line, which is closer to the solubility line. After completely melt, the molten salt hydrate dehydrating into lower hydrate can explain the temperature and pressure relation. The same experimental phenomenon was also found for Na2SO4·10H2O and CH3COONa·3H2O shown in figure S11-S14.
3.3 Latent heat evaluation for salt hydrate As previously mentioned, the equilibrium state before melt of salt hydrate is above the crystalline line but close to the solubility line. Generally, salt hydrate melt is transition from crystalline phase to liquid phase which undergoes no chemical change. However, the dehydrating process accompanying with salt dissolving is suggested here to evaluate the melting process. Two-step melting equations are described as follow, M p X q nH 2 O( s) M p X q mH 2 O( s ) (n m) H 2 O(l ) M p X q mH 2 O(s) (n m) H 2 O(l ) pM q (aq) qX p (aq) nH 2 O(l ) H r0, n m f H 0 ( M p X q mH 2 O, s ) f H 0 ( M p X q nH 2 O, s ) (n m)+ f H 0 ( H 2 O, l )
(8)
0 H sol { p f H 0 (M q , aq) q f H 0 ( X p , aq) m f H 0 ( H 2 O, l )}- f H 0 (M p X q mH 2 O, s) (9)
H sol H 0 sol L
(10)
H m H Sol H r0, nm
(11)
M MP Xq
18x(n m) x(M M P X q 18m)
(12)
As the phase change point is the equilibrium point of both dehydrating and dissolving processes, so it is possibility to realize phase transition from solid phase to highly concentrated aqueous phase. According to equations (8) to (12), the phase change enthalpy 15
was calculated to evaluate the validity of the dehydrating-dissolving path as listed in Table 2. It can be seen that the calculated melting heat of Na2SO4·10H2O was only 3% compared to experimental results. It indicates the possibility of Na2SO4·10H2O undergoes dehydrating-dissolving to accomplish melting and results in an aqueous solution. In addition, due to high content and weak linked water molecule within sodium sulfate decahydrate crystal, molten hydrate state can be considered to concentrated salt aqueous, thus leading the unstable for phase change cycle. However, calculated melting heats of MgSO4·7H2O, CH3COONa·3H2O and SrBr2·6H2O differ widely, which indicates the dehydratingdissolving route is discrepant for them. Thus, the aqueous solution was not the final melting product.
Table 2. Thermal evaluation for the dehydrating-dissolving route [7,25-27] ΔHr0
ΔHsol
Dissolve
ΔHm,cal
ΔHm,exp
Error
kJ/mol
kJ/mol
ratio,γ
kJ/mol
kJ/mol
%
Na2SO4·10H2O
81.4
-8.795
0.56
76.5
79.27
3
MgSO4·7H2O
15.92
-13.19
0.13
14.18
16.45
14
CH3COONa·3H2O
37.02
-17.30
0.94
20.75
37.16
44
SrBr2·6H2O
69.24
-42.22
0.85
33.48
49.80
33
4. Conclusions Four salt hydrates (Na2SO4·10H2O, CH3COONa·3H2O, MgSO4·7H2O, and SrBr2·6H2O) were used as typical examples to investigate the relation of dehydrating process and phase change melting process. The influence of heating rate on the dehydrating of salt hydrate was carefully studied. High heating rate during dehydrating process leads to the formation of molten hydrate, and the molten hydrate shows a lower dehydrating rate and higher activation energy when it further directly dehydrates to lower hydrate. The equilibrium curve of molten hydrate salt for directly dehydrating into lower hydrate is 16
determined by its melting enthalpy and entropy. The temperature-pressure equilibrium of salt hydrate before melt is above its dehydrating equilibrium line but close to the dissolving line. Dehydrating-dissolving process is suggested to describe the phase change of Na2SO4·10H2O. The mutual effect of melt and dehydration has a crucial influence on the equilibrium state and the thermal decomposition kinetics of salt hydrate, which are essential for the design and operation of thermochemical storage system. However, complex composition and crystal structure of salt hydrate during phase change and dehydration need further research to fully understand the transition process of salt hydrate for thermal energy storage.
Acknowledgments This work was supported by the National Key R&D Program of China under the contract No.2018YFE0100300 and the National Natural Science Foundation of China under the contract No.51876117.
17
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21
Supplementary material
100
60 i
Heat (J/g/K)
40 20
2
0
0
-20
-2
-40
-4
-60
-6
-80
Melting
-100
40
DTG(%/min)
Mass/%
i Na2SO410H2O(s)Na2SO4(s)
80
-8 60
80
100
Temp(C)
Figure S1 TG-DTG-DSC curves of Na2SO4·10H2O with heating rate at 2K/min. At 35~50 ºC, an obvious heat peak but restricted mass loss peak indicates melt happens.
i CH3COONa3H2O(s)CH3COONa(s)
100
60
Heat flow(J/g/K)
i
ii
20 40
1
0
0 -1
-20 -2
-40 -60
-3
DTG(%/min)
Mass(%)
ii CH3COONa3H2O(l)CH3COONa(s)
80
-4
40
60
80
100
120
140
Temp(C)
Figure S2 TG-DTG-DSC curves of CH3COONa·3H2O with heating rate at 2K/min. The dehydration process can be divided into two stage due to the melting process.
22
Mass(%)
100
i MgSO47H2O(s)MgSO46H2O(s) ii MgSO46H2O(s)MgSO4(s)
80 60 ii
i
20 40
Heat(J/g/K)
-1
-20 Melting
-40
-2
DTG(%/min)
0
0
-60 -3
40
60
80
100
120
140
Temp(C)
Figure S3 TG-DTG-DSC curves of MgSO4·7H2O with heating rate at 2K/min. At 50~60 ºC, an obvious heat peak but restricted mass loss peak indicates melt occurs 110 i SrBr26H2O(s)SrBr2H2O(s)
Mass(%)
100
ii SrBr2H2O(s)SrBr2(s)
90 80 70 i
60
ii
5 50
-2
-5 2nd peak
-10 -15
-4
1st peak Melting
50
100
DTG(%/min)
Heat(J/g/K)
0
0
-6
150
200
250
300
Temp(C)
Figure S4 TG-DTG-DSC curves of SrBr2·6H2O with heating rate at 10K/min. The mass loss curve can be divided into three peaks due to the melting process, the first heat peak is composed of melting heat and dehydrating heat.
23
Na2SO410H2O(s)Na2SO410H2O(l)
Heat flow(mJ/K)
10K/min 5K/min 2K/min
1K/min 0.5K/min
25
30
35
40
45
Temp(C)
Figure S5 DSC curves of Na2SO4·10H2O with different heating rates
Heat flow(mJ/K)
CH3COONa3H2O(s)CH3COONa3H2O(l)
10K/min 5K/min 2K/min 1K/min
45
50
55
60
65
70
Temp(C)
Figure S6 DSC curves of CH3COONa·3H2O with different heating rates
24
MgSO47H2O(s)MgSO47H2O(l)
10K/min
Heat flow(mJ/K)
5K/min
2K/min
1K/min
0.5K/min
40
45
50
55
60
65
70
Temp(C)
Figure S7 DSC curves of MgSO4·7H2O with different heating rates
SrBr26H2O(s)SrBr26H2O(l)
Heat flow(mJ/K)
10K/min
5K/min 2K/min
1K/min
0.5K/min
80
85
90
95
100
Temp(C)
Figure S8 DSC curves of SrBr2·6H2O with different heating rates
25
(b)
Tsalt(C)
70
80
25
70
20
60
20
50
15
40
10
5
30
5
0 60
20
60 15 50 10 40
30 0
20
40
Tsalt(C)
Tsalt Pvapor
30
Pvapor(kPa)
80
30 Tsalt Pvapor
25
Pvapor(kPa)
(a)
0 0
50
100
t (min)
150
200
250
300
t (min)
Figure S9 Salt temperature and vapor pressure evaluation of MgSO4·7H2O during (a) a dynamic melting process and (b) multi heating steps under equilibrium condition
(b) 30 Tsalt Pvapor
100
120
50 Tsalt Pvapor
110
25
40
100
15 10
60
90 Tsalt(C)
80
Pvapor(kPa)
Tsalt(C)
20
30
80 20
70 60
5 40 0
20
40
60
80
0 100
t (min)
10
50 40 0
100
200
300
400
t (min)
Figure S10 Salt temperature and vapor pressure evaluation of SrBr2·6H2O during (a) a dynamic melting process and (b) multi heating steps under equilibrium condition 26
500
0 600
Pvapor(kPa)
(a)
Na2SO4 (s)->Na2SO4(aq)
10
Pressure(kPa)
Water
Na2SO410H2O(s) N
O aS
4(s
)
2
10(s)->0(s) 1 10
20
30
Temp(
℃
40
)
50
60
Figure S11 Melting process of Na2SO4·10H2O under phase diagram, black dashed line stands for the dynamic phase change route, the black rectangle scatter is the equilibrium point gained from figure S12, 10(s)->0(s) means the solid state decahydrate dehydrating into anhydrate.
(a)
(b) 20 Tsalt Pvapor
50
14
Tsalt Pvapor
50
12
15
30 5
20
Tsalt(C)
10
Pvapor(kPa)
Tsalt(C)
40 40
10 8
30
6 4
20
2 10
0 0
100
200
300
400
500
10 0
t (min)
20
40
60
t (min)
Figure S12 Salt temperature and vapor pressure evaluation of Na2SO4·10H2O during (a) a dynamic melting process and (b)multi heating steps under equilibrium condition
27
0 80
Pvapor(kPa)
60
3(l)->0(s) Water
Pressure(kPa)
10
a
CO
ON
O( H2 3
s)
a(s
)
N OO
C CH 3
CH 3 3(s)->0(s) 1 20
40
Temp(
℃
)
60
80
Figure S13. Melting process of CH3COONa·3H2O, black dashed line stands for the dynamic phase change route, the black rectangle scatter is the equilibrium point gained from figure S14, 3(s)->0(s) means the solid state trihydrate dehydrating into anhydrate, 3(l) means the liquid state trihydrate.
80
(b)
20
70
25 80
Tsalt Pvapor
15
Tsalt Pvapor
20
70
50 40
5
Tsalt(C)
10
Pvapor(kPa)
Tsalt(C)
60 60
15
50
10
40
30
5
30 0 0
20
40
60
80
0
t (min)
100
200
300
400
500
t (min)
Figure S14 Salt temperature and vapor pressure evaluation of CH3COONa·10H2O during a dynamic melting process and multi heating steps under equilibrium condition
28
0 600
Pvapor(kPa)
(a)
Highlights
1. The correlation of phase change and dehydration reaction of salt hydrates are analyzed
2. Phase change has an influence on the dehydrating process at high heating rate.
3. The equilibrium route between temperature and pressure during melting is revealed.
4. Dehydrating-dissolving melting of salt hydrate is discussed.
29
S1359-4311(19)31854-X https://doi.org/10.1016/j.applthermaleng.2019.114655 ATE 114655
To appear in:
Applied Thermal Engineering
Received Date: Revised Date: Accepted Date:
21 March 2019 29 September 2019 6 November 2019
Please cite this article as: T.S. Yan, T.X. Li, J.X. Xu, J.W. Chao, Understanding the transition process of phase change and dehydration reaction of salt hydrate for thermal energy storage, Applied Thermal Engineering (2019), doi: https://doi.org/10.1016/j.applthermaleng.2019.114655
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© 2019 Published by Elsevier Ltd.
Understanding the transition process of phase change and dehydration reaction of salt hydrate for thermal energy storage T.S. Yan, T.X. Li, J.X. Xu, J.W. Chao Institute of Refrigeration and Cryogenics, Shanghai Jiao Tong University, Shanghai 200240, China
Abstract Salt hydrate is one kind of the most promising materials used for both latent heat and thermochemical heat storage systems. Currently, there is a lack of fundamental understanding of the correlation mechanism of salt hydrate as phase change material (PCM) and thermochemical material (TCM). The dehydrating and melting processes of salt hydrates (Na2SO4·10H2O, CH3COONa·3H2O, MgSO4·7H2O, and SrBr2·6H2O) were carefully analyzed at different heating rates. The experimental results show that the melt of salt hydrate occurs during its dehydrating process at high heating rates, and thus leads to the formation of molten hydrate. Moreover, the molten hydrate can dehydrate to lower hydrate directly with a slower dehydrating rate and higher activation energy. Furthermore, the temperature-pressure equilibrium of salt hydrate is higher than its dehydrating equilibrium line before phase change, but it is closer to the dissolving line. An equilibrium equation is developed for describing the transition process from molten hydrate salt to directly dehydrating into lower hydrate, and also used to explain the equilibrium curve after phase change. These findings of mutual effects between phase change and thermochemical dehydration can provide the understanding on the transition process of salt hydrate for thermal energy storage. Key words: salt hydrate; phase change; thermochemical; thermal energy storage
Corresponding author. Tel.: +86-21-34206335; fax: +86-21-34206335. E-mail address: [email protected] (T.X. Li). 1
Nomenclatures PCM
phase change material
TCM
thermochemical material
Dimensional variables Tm
melting temperature [K]
Tp
peak temperature [K]
P0
standard atmospheric pressure[Pa]
R
gas constant [J/(mol·K)]
A
pre-exponential factor
Ea
apparent activation energy [kJ/mol]
ΔHm
melting enthalpy [kJ/mol]
△Sm
melting entropy [J/(mol·K)]
ΔfH0
standard enthalpy of formation [kJ/mol]
ΔfS0
standard entropy of formation [J/(mol·K)]
ΔHr0
reaction enthalpy [kJ/mol]
ΔSr0
reaction entropy [J/(mol·K)]
ΔHsol0
enthalpy of the solution [kJ/mol]
φ
integral enthalpy of dilution of the solution [kJ/mol]
L
Greek symbols α
conversion rate
β
heating rate [K/min]
x
saturation solubility [wt%]
γ
dissolved ratio of salt hydrate [wt%]
2
1. Introduction Energy deficiency is increasingly severe. To reduce the energy consumption and increase the energy efficiency are more and more urgent. Among the total energy consumption, the building energy consumption can occupy a large proportion. More than 40%~60% building energy consumption should end up with heat demand, so it is of great value to increase the heat utilization efficiency. However, many renewable energies, like solar energy and low grade thermal source, are transient, fluctuant and limited by region [1]. Thermal energy storage is an effective technology to solve these problems, which can be mainly divided into three types: sensible heat storage, latent heat storage and thermochemical energy storage. Sensible heat is stored through the temperature difference of storage medium, which is simple but faces problems like low thermal storage density and large thermal loss. Latent heat and thermochemical heat storage show high thermal storage density which have gained more and more attention. Salt hydrate (MpXq·nH2O) is recognized as promising thermal storage material and widely used as phase change material (PCM) [2, 3] and thermochemical material (TCM) [4,5] in recent years. Figure 1 shows the thermal energy storage density and working temperature of typical salt hydrate as PCM or TCM [6-8]. The latent heat energy storage using salt hydrate as PCM is based on melting/crystallizing process as phase change between solid state and liquid state, and the energy storage density (phase change enthalpy) usually varies between 100~300 kJ/kg [9]. Thermochemical energy storage using salt hydrate as TCM is based on breaking/reforming bonds of water molecule and salt in crystal structure [10] as de-/rehydration reaction between solid salt and vapor, and the energy density (reaction enthalpy) is usually as high as 1000~2000 kJ/kg [11].
3
2500 Thermochemical Storage
Salt hydrate using as TCM
Thermal Energy Density（ kJ/kg）
Na2S9H2O/0.5H2O
2000 Na2SO410H2O
1500
KAl(SO4)212H2O LiClH2O
1000
CaCl2H2O MgSO47H2O/H2O MgCl2H2O/2H2O
SrBr2H2O/H2O
Na2S2O35H2O
Latent Storage
500
CH COONa3H2O Ba(OH)28H2O Na2SO410H2O 3 KAl(SO4)212H2O MgCl2H2O CaCl2H2O MgSO47H2O SrBr6H2O
0 0
50
Salt hydrate using as PCM
100 Temperature（ ℃）
150
200
Figure 1. Thermal energy storage density and working temperature of salt hydrate as PCM or TCM (Temperature by 20mbar vapor pressure) [6-8]
Unlike pure salt or other ionic solids, the water within the hydrated salt crystal leads to the complex performances of melt and dehydration. The solid hydrated salt will melt into molten hydrated salt under phase change temperature. Specifically, the molten state is usually regarded as an aqueous solution. According to melting behaviors, salt hydrate can be classified into three categories: congruent melt in which the dehydrated salt can be completely soluble in the hydration water, incongruent melt where the salt is partially soluble in the hydration water, and semi-congruent melt when the liquid and solid are in equilibrium with different compositions for the hydrate transformation to a lower hydrated salt [6,12,13]. Incongruent melt always accompanies with the formation of lower hydrate salt, which is irreversible and leads to the loss of storage capacity. During the melting process of Mg(NO3)2·6H2O, the dihydrate will be formed and leads to instability. To solve this problem, Mg(NO3)2·6H2O and extra water were mixed to prepare nanoencapuslated salt hydrate, which can realize 100+ heat uptake/release cycles [14]. El-Sebaii conducted the melting test of MgCl2·6H2O in a sealed container under the extra water principle, more than 4
1000 melt/solidification cycles were conducted with stable phase change performance [15]. As far as hydrated salt dehydrating into lower hydrated salt and vapor, it is influenced by both temperature and vapor pressure. Many salt hydrates undergo multi-step dehydration. For example, the dehydration of MgCl2·6H2O has three intermediate hydrated salts, including tetrahydrate, dihydrate and monohydrate [16]. In addition, other adverse reactions could lead to the decomposition of salt hydrate. For instance, MgCl2·6H2O will decompose into Mg(OH)Cl and HCl if dehydrating temperature is too high [17]. MgSO4·7H2O will dehydrate to MgSO4·6H2O at 25-55 ºC, while an amorphous phase is formed until converts to crystalline MgSO4 at 300 ºC [18]. When the water diffusion is limited under MgSO4·7H2O grains, an aqueous solution was produced during dehydration [19]. According to the way water released, the intermediate phase and final product, Galwey [20] described the solid-state dehydration as six different ways of water evolution type. The crystal may undergo partially melting or completely melting during dehydration process, impervious outer layer will be formed, which limits the vapor diffusion. From above, there is plenty of reports on phase change and dehydration of salt hydrate independently, but the effect of melt on dehydration or effect of dehydration on melt is seldom concerned. In this paper, we want to study the melting process of salt as PCM and dehydrating process as TCM simultaneously, the correlation of salt hydrate melting and dehydrating is concerned. Transition processes of four salt hydrates (Na2SO4·10H2O, CH3COONa·3H2O, MgSO4·7H2O, and SrBr2·6H2O) were carefully analyzed at different heating rates by TGDSC and DSC, dehydrating and melting parameters were calculated. Additionally, we designed a vacuum reactor system to study the temperature and pressure evolution during phase change process. The latent heat was analyzed based on different transition routes. This study shows the link between phase change and dehydration of salt hydrate which
5
would contribute to deepen our understanding on transition process of salt hydrate and its application for thermal energy storage.
2. Materials and Methods 2.1 Materials and Characterizations Four typical salt hydrates, Na2SO4·10H2O (99%, J&K China Chemical Ltd.), CH3COONa·3H2O (99.5%, Energy Chemical Co.), MgSO4·7H2O (99.5%, Shanghai Macklin Biochemical Co., Ltd), SrBr2·6H2O (99%, Energy Chemical Co.), were selected to investigate transition processes of phase change and dehydration. Differential Scanning Calorimetry (DSC8000, Pekin-Elmer) was used to determine phase change enthalpy (ΔHm) and melting temperature (Tm) of salt hydrate. Sample mass with about 5~10 mg was sealed in aluminum dish. Different heating rates (0.5K/min, 1K/min, 2K/min, 5K/min, 10K/min) were tested. Simultaneous thermal analysis (Netzsch STA 449 F3) was used to determine dehydration characteristics of salt hydrate, including reaction temperature, mass loss, reaction enthalpy, etc. About 20 mg salt hydrate sample was placed inside an aluminum dish and heated from room temperature to 300 ºC at different heat rates (0.2K/min, 1K/min, 2K/min, 5K/min, 10K/min). Nitrogen was used as a purge gas with 20mL/min to remove the moisture from the crucible.
2.2 transition experimental setup for salt hydrate A vacuum experimental device system was designed to test temperature and pressure change during transition process as shown in figure 2. The system is mainly composed of a vacuum reactor, a liquid reservoir, thermostatic baths, a vacuum pump. The temperature
6
sensor, PT100 with an accuracy of 0.15 ºC and pressure sensor with an accuracy of 1 mbar were used to measure temperature and pressure evolution.
Figure 2. Experimental setup for testing the transition process of salt hydrate
Dynamic melting test, test procedures including 1) preparation step: salt hydrates with thickness about 1~2 cm and mass about 200~300g, were sealed at the vacuum reactor. Air within the reactor will degas to less than 1mbar by vacuum pump. Thermostatic bath (oil bath) is used to adjust the salt at initial temperature. 2) heating step: set the thermostatic bath at end temperature to heat the salt to melt. Typically, for Na2SO4·10H2O, temperature increases from 20ºC to 50ºC, MgSO4·7H2O from 30ºC to 70ºC, CH3COONa·3H2O from 30ºC to 80ºC, SrBr2·6H2O from 50ºC to 100ºC. Salt temperature and vapor pressure were recorded. Multi-step equilibrium test: test procedures are similar to dynamic melting test. However, the heating temperature increases step by step, salt temperature and vapor pressure will keep enough time to reach equilibrium at each steps.
7
3. Results and discussions 3.1. Dehydrating and melting process of salt hydrates To analysis the dehydrating process, salt hydrate was heated in an open environment to ensure fast vapor exchange. Dehydrating curves, including mass loss line(TG), mass loss rate line(DTG), heat flow line(DSC), are shown in figure 3, figure 4, and figures S1-S4. As for the melting process, salt hydrate was sealed in a close environment to avoid vapor evaporation. Melting process at different heating rates, shown in figures S5-S8, were conducted to compare with the dehydrating process and calculate the activation energy of dynamic transition process. From figure 3 and figure 4, it can be seen that melt can influence the dehydrating process as the heating rate increases. For sodium sulfate decahydrate, melting heat peak will appear at all tested heating rates (see figure 3a and figure S1). Only part of hydrate melt during dehydration as the melting heat ranges from 150 J/g to 200 J/g in STA, in comparison with 248J/g in DSC. The additional melt won’t influence the sequential dehydrating process seen from the comparison of mass loss rate and heat flow curves in figure S1. However, for sodium acetate trihydrate, dehydration is divided into two steps when the heating rate is larger than 1K/min. As mass loss rate (DTG) curve and heat flow curve shown in figure S2, melting heat will lead heat flow deviates from DTG curve. After molten hydrate formed, the subsequent dehydration shows lower dehydrating rate. With the heating rate increasing, the molten hydrate proportion would increase from 32% (mass loss ratio compare to total mass loss) at 2K/min to 92% at 10K/min.
8
(b)
(a) 100
i Na2SO410H2O(s)Na2SO4(s)
-9
Mass(%)
80 -10
60 40
2
ln(/Tp)
i
-50
Na2SO410H2O 0.2K/min
-13
Na2SO410H2O 10K/min
20
40
60
80
100
120
Na2SO410H2O(s)Na2SO410H2O(l)(STA) Na2SO410H2O(s)Na2SO410H2O(l)(DSC)
0.0026
140
Temp(C)
0.0028
0.0030
0.0032
1/Tp
(d) -9
i CH3COONa3H2O(s)CH3COONa(s)
100
ii CH3COONa3H2O(l)CH3COONa(s)
80
-10
60 i
20 40
ii
0
-12
-20 CH3COONa3H2O 0.2K/min
-40
CH3COONa3H2O(s)CH3COONa(s)
CH3COONa3H2O 1K/min
-60
CH3COONa3H2O 2K/min
-13
CH3COONa3H2O 5K/min
-80 -100
-11
2
Mass(%)
Na2SO410H2O(s)Na2SO4(s)
Na2SO410H2O 5K/min
Melting
(c)
Heat flow(J/g/K)
-12
Na2SO410H2O 2K/min
-150 -200
-11
Na2SO410H2O 1K/min
-100
ln(/Tp)
Heat (J/g/K)
0
CH3COONa3H2O 10K/min
20
40
60
80
100 120 140 160 180 200
0.0024
Temp(C)
CH3COONa3H2O(l)CH3COONa(s) CH3COONa3H2O(s)CH3COONa3H2O(l) (DSC)
0.0026
0.0028
0.0030
1/Tp
Figure 3. TG-DSC curves of (a) Na2SO4·10H2O and (c) CH3COONa·3H2O vary with the change of heating rate β(K/min). Kinetic results of thermal dehydration or melting steps for (b) Na2SO4·10H2O and (d) CH3COONa·3H2O
For magnesium sulfate heptahydrate shown in figure 4a and figure S3, the dehydrating process can be divided into three steps. The first step was dehydration from MgSO4·7H2O to MgSO4·6H2O by losing 1 mol water at temperature range of 30~50 °C, the second step was dehydration from MgSO4·6H2O to MgSO4·xH2O (x < 0.1) at temperature range of 50~260 °C, and the final step (260~280 ºC) completely lost all the water to become MgSO4. Results are in agreement with previous reports in literature [18]. When heating rate is larger than 1K/min, a melting peak appears at 45~65 °C which is similar to sodium sulfate decahydrate. With the heating rate increasing from 2 K/min to 10 K/min, the proportion of
9
0.0032
melt will increase from 2% to 70%. When the heating rate reaches 10 K/min, the multi-step dehydration process becomes indistinguishable.
(b)
(a)
-9 i MgSO47H2O(s)MgSO46H2O(s)
100
Mass(%)
ii MgSO46H2O(s)MgSO4(s)xH2O(s)
-10
80 60
-11
i
2
20 40
ln(/Tp)
ii
Heat(J/g/K)
0 -20
MgSO47H2O 0.2K/min
-40
MgSO47H2O 2K/min
MgSO47H2O 1K/min
-12
-13
MgSO47H2O 10K/min
50
100
150
200
250
300
Temp(C)
(c)
MgSO46H2OMgSO4(s) MgSO47H2O(s)MgSO47H2O(l)(STA)
MgSO47H2O 5K/min
-60 Melting
MgSO47H2O(s)MgSO46H2O(s)
-14 0.0026
MgSO47H2O(s)MgSO47H2O(l)(DSC)
0.0028
0.0030
0.0032
1/Tp
(d)
110 i SrBr26H2O(s)SrBr2H2O(s)
100
Mass(%)
-9.5
ii SrBr2H2O(s)SrBr2(s)
90 80
-10.0
60
2
ii
i
10 50
ln(/Tp)
70
Heat(J/g/K)
0 -10
-11.0
SrBr26H2O 1K/min
-20
SrBr26H2O(s)SrBr2H2O(s) 1~5K/min
SrBr26H2O 2K/min
-11.5
SrBr26H2O 5K/min
-30
SrBr26H2O 8K/min
-40 -50
-10.5
100
150
200
250
SrBr26H2O(s) SrBr2H2O(s) 2nd stage 8~12K/min SrBr26H2O(s) SrBr26H2O(l) (DSC)
SrBr26H2O 10K/min
50
SrBr26H2O(s) SrBr2H2O(s) 1st stage 8~12K/min
300
-12.0 0.0022
0.0024
0.0026
0.0028
1/Tp
Temp(C)
Figure 4. TG-DSC curves of (a) MgSO4·7H2O and (c) SrBr2·6H2O vary with the change of heating rate β(K/min). Kinetic results of thermal dehydration or melting steps for (b) MgSO4·7H2O and (b) SrBr2·6H2O
While for strontium bromide hexahydrate, two-step dehydration of SrBr2·6H2O occurs at low heating rate (1~5K/min). The first step is dehydrating five water from hexahydrate to monohydrate at 40~120 ºC. The second step is dehydrating from monohydrate to anhydrate at 150~250 ºC. When heating rate is higher than 8K/min, the melting effect can be observed. Seen from figure 4c and figure S4, melt leads to two mass loss peak occurring at hexahydrate dehydrating to monohydrate step. Compare mass loss 10
rate curve with heat flow curve, the first peak of heat flow is composed of dehydration and melt. The mass loss proportion of 1st peak and 2nd peak decreases with heating rate increasing, which means no stable intermediate product formed but the melt interrupts the dehydrating process. Transition parameters are summarized in table 1. The dehydrating process or melting process can be described by Arrhenius equation. The activation energy is used to evaluate the energy required to initiate reaction, which can be calculated by Kissinger equation [21, 22]. a d A e RT f ( ) dt E
d(ln
(1)
) Tp 2 E a 1 R d( ) Tp
(2)
Compare melting parameters in STA and DSC, for Na2SO4· 10H2O and MgSO4· 7H2O, only part of salt hydrate melt in STA, and activation energies are lower than melt in DSC. In addition, when melt happens, the activation energy of sequential dehydration is higher than the original dehydration. It is consistent with higher dehydrating temperature and lower dehydrating rate for molten hydrate directly dehydrating into lower hydrate shown in figure 3 and figure 4.
11
Table 1 Transition parameters of dehydrating and melting processes at different heating rates β(K/min). Heating rate β
Mass loss Δm
Dehydrating heat or melting heat
Peak temperature Tp
Activation energy
(K/min)
(%)
(kJ/kg)
(ºC)
(kJ/mol)
1. Na2SO4·10H2O(s)→Na2SO4(s)+10H2O(g)
0.2/1/2/5/10
53.9
1764.0
45.6/60.9/70.2/77.0/99.2
67.9
2. Na2SO4·10H2O(s)→Na2SO4·10H2O(l) by STA
0.2/1/2/5/10
-
160.3/202.8/199.0/153.7/150.9
35.9/39.8/42.7/46.0/49.9
224.3
3.Na2SO4·10H2O(s)→Na2SO4·10H2O(l) by DSC
1/2/5/10
-
248.0
34.3/35.4/37.4/38.9
386.4
1. CH3COONa·3H2O(s)→CH3COONa(s)+3H2O(g)
0.2/1/2/5/10
40.3
1334.0
44.8/67.0/71.2/81.5/80.1
75.3
2. CH3COONa·3H2O(l)→CH3COONa(s)+3H2O(g)-2nd
2/5/10
13.2/26.4/37.1
-
67.1/70.2/65.1
97.9
3. CH3COONa·3H2O(s)→CH3COONa·3H2O(l) by DSC
1/2/5/10
-
249.7
58.6/59.3/63.4/63.8
332.1
1.MgSO4·7H2O(s)→MgSO4·6H2O(s)+H2O(g)
1/2/5/10
5.9/4.5/1.5/-
229.0/182.5/133.6/99.1
45.0/47.9/49.4/51.6
303.4
2. MgSO4·7H2O(s)→MgSO4·7H2O(l) by STA
1/2/5/10
-
1.7/16.3/23.6/49.3
51.4/52.9/55.9/57.9
88.4
3.MgSO4·6H2O(s)→MgSO4·xH2O(s)+(6-x)H2O(g)
1/2/5/10
43.2/45.5/47.3/52.3
1638.3/1510.5/1323.4/1305.9
57.5/78.1/85.6/95.4/97.1
30.3
4. MgSO4·7H2O(s)→MgSO4·7H2O(l) by DSC
0.5/1/2/5/10
-
69.4
50.1/51.0/51.7/54.2/56.4
398.4
1.SrBr2·6H2O(s)→SrBr2·H2O(s)+5H2O(g)
1/2/5
25.2
791.0
80.5/91.1/108.1
59
1-1 SrBr2·6H2O(s)→SrBr2·H2O(s)+5H2O(g) 1st stage
8/10/12
20.95/18.62/15.86
123/126.1/128.1
97.5
154.4/156.9/158.8
134.1
89.4/89.1/89.2/91.3/93.6
502.2
Transition process Na2SO4·10H2O
CH3COONa·3H2O
MgSO4·7H2O
SrBr2·6H2O
667.0 1-2 SrBr2·6H2O(s)→SrBr2·H2O(s)+5H2O(g) 2nd stage
8/10/12
4.62/6.92/9.34
2. SrBr2·6H2O(s)→SrBr2·6H2O(l) by DSC
0.5/1/2/5/10
-
145.7
12
3.2 Transition route of salt hydrates From above results, when heating temperature is higher than phase change temperature, melt will happen and alter the dehydrating process. Molten salt hydrate directly dehydrating to lower hydrate could be described as follows: M p X q nH 2 O(l ) M p X q mH 2 O( s ) (n m) H 2 O( g ) H r0', n m f H 0 ( M p X q mH 2 O, s ) f H 0 ( M p X q nH 2 O, l ) (n m) f H 0 ( H 2 O, g )
f H 0 (M p X q nH 2 O, l ) f H 0 (M p X q nH 2 O, s) H m (M p X q nH 2O)
(3) (4)
S r0', n m f S 0 ( M p X q mH 2 O, s ) f S 0 ( M p X q nH 2 O, l ) (n m) f S 0 ( H 2 O, g ) (5) f S 0 ( M p X q nH 2 O, l ) f S 0 ( M p X q nH 2 O, s )
ln
H m ( M p X q nH 2 O ) Tm
Gr' , mn H r0', mn Sr0', mn p RT RT R p0
(6)
(7)
So, the dehydrating equilibrium line can be corrected based on thermal physical parameters [7,23-25] and DSC results (see in table 1) The temperature and pressure evolution during melting process was conducted to analysis its transition route. Figure 5 and figure S9 illustrate the temperature and pressure route of MgSO4·7H2O during melting process. The vapor pressure will increase with the salt temperature before melt, then a plateau (50.5ºC, 9.5kPa) was found when reaching phase change temperature. After completely melt, salt temperature and vapor pressure continually increases. It should be noted that the P-T curve of MgSO4·7H2O phase change process is above the solid-gas equilibrium line when T
7(l)->(s)
water
Pressure(kPa)
10
O( H2 6
s) O(
H2
7
M
s)
SO 4
Mg
O4 gS
7(s)->(s)
1 20
30
40
50
Temp(
℃
60
)
70
80
Figure 5. Melting process of MgSO4·7H2O under phase diagram, black dashed line stands for the dynamic phase change route, the black rectangle scatter is the equilibrium point gained from figure S9, 7(s)->6(s) means the solid state heptahydrate dehydrating into hexahydrate, 7(l) means the molten state of heptahydrate. 100 6(l)->1(s)
Pressure(kPa)
Water
10
Sr
6H Br 2
O(
s)
2
SrBr2H2O(s) 6(s)->1(s) 1 40
60
80
Temp(
℃
100
120
)
Figure 6. Melting process of SrBr2·6H2O under phase diagram, black dashed line stands for the dynamic phase change route, the black rectangle scatter is the equilibrium point gained from figure S10, 6(s)->1(s) means the solid state hexahydrate dehydrating into monohydrate, 6(l) means the molten of state hexahydrate.
The dynamic melting process and multi-step equilibrium procedure of SrBr2·6H2O are 14
shown in figure S10, and the P-T line is drawn in figure 6. The salt temperature keeps stable at 89.6 ºC, while vapor pressure is 17.6 kPa, particularly, the vapor pressure starts to increase before salt temperature begins to increase. Similar to MgSO4·7H2O, the equilibrium P-T curve of SrBr2·6H2O before melt won’t follow the crystallization line, which is closer to the solubility line. After completely melt, the molten salt hydrate dehydrating into lower hydrate can explain the temperature and pressure relation. The same experimental phenomenon was also found for Na2SO4·10H2O and CH3COONa·3H2O shown in figure S11-S14.
3.3 Latent heat evaluation for salt hydrate As previously mentioned, the equilibrium state before melt of salt hydrate is above the crystalline line but close to the solubility line. Generally, salt hydrate melt is transition from crystalline phase to liquid phase which undergoes no chemical change. However, the dehydrating process accompanying with salt dissolving is suggested here to evaluate the melting process. Two-step melting equations are described as follow, M p X q nH 2 O( s) M p X q mH 2 O( s ) (n m) H 2 O(l ) M p X q mH 2 O(s) (n m) H 2 O(l ) pM q (aq) qX p (aq) nH 2 O(l ) H r0, n m f H 0 ( M p X q mH 2 O, s ) f H 0 ( M p X q nH 2 O, s ) (n m)+ f H 0 ( H 2 O, l )
(8)
0 H sol { p f H 0 (M q , aq) q f H 0 ( X p , aq) m f H 0 ( H 2 O, l )}- f H 0 (M p X q mH 2 O, s) (9)
H sol H 0 sol L
(10)
H m H Sol H r0, nm
(11)
M MP Xq
18x(n m) x(M M P X q 18m)
(12)
As the phase change point is the equilibrium point of both dehydrating and dissolving processes, so it is possibility to realize phase transition from solid phase to highly concentrated aqueous phase. According to equations (8) to (12), the phase change enthalpy 15
was calculated to evaluate the validity of the dehydrating-dissolving path as listed in Table 2. It can be seen that the calculated melting heat of Na2SO4·10H2O was only 3% compared to experimental results. It indicates the possibility of Na2SO4·10H2O undergoes dehydrating-dissolving to accomplish melting and results in an aqueous solution. In addition, due to high content and weak linked water molecule within sodium sulfate decahydrate crystal, molten hydrate state can be considered to concentrated salt aqueous, thus leading the unstable for phase change cycle. However, calculated melting heats of MgSO4·7H2O, CH3COONa·3H2O and SrBr2·6H2O differ widely, which indicates the dehydratingdissolving route is discrepant for them. Thus, the aqueous solution was not the final melting product.
Table 2. Thermal evaluation for the dehydrating-dissolving route [7,25-27] ΔHr0
ΔHsol
Dissolve
ΔHm,cal
ΔHm,exp
Error
kJ/mol
kJ/mol
ratio,γ
kJ/mol
kJ/mol
%
Na2SO4·10H2O
81.4
-8.795
0.56
76.5
79.27
3
MgSO4·7H2O
15.92
-13.19
0.13
14.18
16.45
14
CH3COONa·3H2O
37.02
-17.30
0.94
20.75
37.16
44
SrBr2·6H2O
69.24
-42.22
0.85
33.48
49.80
33
4. Conclusions Four salt hydrates (Na2SO4·10H2O, CH3COONa·3H2O, MgSO4·7H2O, and SrBr2·6H2O) were used as typical examples to investigate the relation of dehydrating process and phase change melting process. The influence of heating rate on the dehydrating of salt hydrate was carefully studied. High heating rate during dehydrating process leads to the formation of molten hydrate, and the molten hydrate shows a lower dehydrating rate and higher activation energy when it further directly dehydrates to lower hydrate. The equilibrium curve of molten hydrate salt for directly dehydrating into lower hydrate is 16
determined by its melting enthalpy and entropy. The temperature-pressure equilibrium of salt hydrate before melt is above its dehydrating equilibrium line but close to the dissolving line. Dehydrating-dissolving process is suggested to describe the phase change of Na2SO4·10H2O. The mutual effect of melt and dehydration has a crucial influence on the equilibrium state and the thermal decomposition kinetics of salt hydrate, which are essential for the design and operation of thermochemical storage system. However, complex composition and crystal structure of salt hydrate during phase change and dehydration need further research to fully understand the transition process of salt hydrate for thermal energy storage.
Acknowledgments This work was supported by the National Key R&D Program of China under the contract No.2018YFE0100300 and the National Natural Science Foundation of China under the contract No.51876117.
17
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21
Supplementary material
100
60 i
Heat (J/g/K)
40 20
2
0
0
-20
-2
-40
-4
-60
-6
-80
Melting
-100
40
DTG(%/min)
Mass/%
i Na2SO410H2O(s)Na2SO4(s)
80
-8 60
80
100
Temp(C)
Figure S1 TG-DTG-DSC curves of Na2SO4·10H2O with heating rate at 2K/min. At 35~50 ºC, an obvious heat peak but restricted mass loss peak indicates melt happens.
i CH3COONa3H2O(s)CH3COONa(s)
100
60
Heat flow(J/g/K)
i
ii
20 40
1
0
0 -1
-20 -2
-40 -60
-3
DTG(%/min)
Mass(%)
ii CH3COONa3H2O(l)CH3COONa(s)
80
-4
40
60
80
100
120
140
Temp(C)
Figure S2 TG-DTG-DSC curves of CH3COONa·3H2O with heating rate at 2K/min. The dehydration process can be divided into two stage due to the melting process.
22
Mass(%)
100
i MgSO47H2O(s)MgSO46H2O(s) ii MgSO46H2O(s)MgSO4(s)
80 60 ii
i
20 40
Heat(J/g/K)
-1
-20 Melting
-40
-2
DTG(%/min)
0
0
-60 -3
40
60
80
100
120
140
Temp(C)
Figure S3 TG-DTG-DSC curves of MgSO4·7H2O with heating rate at 2K/min. At 50~60 ºC, an obvious heat peak but restricted mass loss peak indicates melt occurs 110 i SrBr26H2O(s)SrBr2H2O(s)
Mass(%)
100
ii SrBr2H2O(s)SrBr2(s)
90 80 70 i
60
ii
5 50
-2
-5 2nd peak
-10 -15
-4
1st peak Melting
50
100
DTG(%/min)
Heat(J/g/K)
0
0
-6
150
200
250
300
Temp(C)
Figure S4 TG-DTG-DSC curves of SrBr2·6H2O with heating rate at 10K/min. The mass loss curve can be divided into three peaks due to the melting process, the first heat peak is composed of melting heat and dehydrating heat.
23
Na2SO410H2O(s)Na2SO410H2O(l)
Heat flow(mJ/K)
10K/min 5K/min 2K/min
1K/min 0.5K/min
25
30
35
40
45
Temp(C)
Figure S5 DSC curves of Na2SO4·10H2O with different heating rates
Heat flow(mJ/K)
CH3COONa3H2O(s)CH3COONa3H2O(l)
10K/min 5K/min 2K/min 1K/min
45
50
55
60
65
70
Temp(C)
Figure S6 DSC curves of CH3COONa·3H2O with different heating rates
24
MgSO47H2O(s)MgSO47H2O(l)
10K/min
Heat flow(mJ/K)
5K/min
2K/min
1K/min
0.5K/min
40
45
50
55
60
65
70
Temp(C)
Figure S7 DSC curves of MgSO4·7H2O with different heating rates
SrBr26H2O(s)SrBr26H2O(l)
Heat flow(mJ/K)
10K/min
5K/min 2K/min
1K/min
0.5K/min
80
85
90
95
100
Temp(C)
Figure S8 DSC curves of SrBr2·6H2O with different heating rates
25
(b)
Tsalt(C)
70
80
25
70
20
60
20
50
15
40
10
5
30
5
0 60
20
60 15 50 10 40
30 0
20
40
Tsalt(C)
Tsalt Pvapor
30
Pvapor(kPa)
80
30 Tsalt Pvapor
25
Pvapor(kPa)
(a)
0 0
50
100
t (min)
150
200
250
300
t (min)
Figure S9 Salt temperature and vapor pressure evaluation of MgSO4·7H2O during (a) a dynamic melting process and (b) multi heating steps under equilibrium condition
(b) 30 Tsalt Pvapor
100
120
50 Tsalt Pvapor
110
25
40
100
15 10
60
90 Tsalt(C)
80
Pvapor(kPa)
Tsalt(C)
20
30
80 20
70 60
5 40 0
20
40
60
80
0 100
t (min)
10
50 40 0
100
200
300
400
t (min)
Figure S10 Salt temperature and vapor pressure evaluation of SrBr2·6H2O during (a) a dynamic melting process and (b) multi heating steps under equilibrium condition 26
500
0 600
Pvapor(kPa)
(a)
Na2SO4 (s)->Na2SO4(aq)
10
Pressure(kPa)
Water
Na2SO410H2O(s) N
O aS
4(s
)
2
10(s)->0(s) 1 10
20
30
Temp(
℃
40
)
50
60
Figure S11 Melting process of Na2SO4·10H2O under phase diagram, black dashed line stands for the dynamic phase change route, the black rectangle scatter is the equilibrium point gained from figure S12, 10(s)->0(s) means the solid state decahydrate dehydrating into anhydrate.
(a)
(b) 20 Tsalt Pvapor
50
14
Tsalt Pvapor
50
12
15
30 5
20
Tsalt(C)
10
Pvapor(kPa)
Tsalt(C)
40 40
10 8
30
6 4
20
2 10
0 0
100
200
300
400
500
10 0
t (min)
20
40
60
t (min)
Figure S12 Salt temperature and vapor pressure evaluation of Na2SO4·10H2O during (a) a dynamic melting process and (b)multi heating steps under equilibrium condition
27
0 80
Pvapor(kPa)
60
3(l)->0(s) Water
Pressure(kPa)
10
a
CO
ON
O( H2 3
s)
a(s
)
N OO
C CH 3
CH 3 3(s)->0(s) 1 20
40
Temp(
℃
)
60
80
Figure S13. Melting process of CH3COONa·3H2O, black dashed line stands for the dynamic phase change route, the black rectangle scatter is the equilibrium point gained from figure S14, 3(s)->0(s) means the solid state trihydrate dehydrating into anhydrate, 3(l) means the liquid state trihydrate.
80
(b)
20
70
25 80
Tsalt Pvapor
15
Tsalt Pvapor
20
70
50 40
5
Tsalt(C)
10
Pvapor(kPa)
Tsalt(C)
60 60
15
50
10
40
30
5
30 0 0
20
40
60
80
0
t (min)
100
200
300
400
500
t (min)
Figure S14 Salt temperature and vapor pressure evaluation of CH3COONa·10H2O during a dynamic melting process and multi heating steps under equilibrium condition
28
0 600
Pvapor(kPa)
(a)
Highlights
1. The correlation of phase change and dehydration reaction of salt hydrates are analyzed
2. Phase change has an influence on the dehydrating process at high heating rate.
3. The equilibrium route between temperature and pressure during melting is revealed.
4. Dehydrating-dissolving melting of salt hydrate is discussed.
29