Design and thermal stability study of LiNO3-NaNO3-KNO3 ternary molten salt system

doi:10.11949/j.issn.0438-1157.20181406

Abstract

Abstract:

According to the binary molten salt system phase diagrams, the phase diagram of LiNO₃-NaNO₃-KNO₃ ternary molten salt system was calculated with the CIS model, it turned out that, the lowest eutectic melting point was 117.7℃ and the corresponding composition was x(LiNO₃) = 0.375，x(NaNO₃) = 0.075 and x(KNO₃) = 0.550. The eutectic mixture was prepared based on the predicted eutectic composition, and a experiment test was carried out by the thermal gravimetric analysis (TG) and differential scanning calorimeter (DSC). The DSC test showed that the experimental melting point was 117.7℃, which was in good agreement with the result of this calculation. The TG test results show that the ternary molten salt system is stable when the temperature is below 587.2°C, and its working temperature range is 118.3—587.2°C. The system is quite suitable as a material for high-temperature heat transfer and heat storage in solar thermal power generation.

Key words: solar energy, ternary molten salt, phase equilibria, moldling, latent heat, thermal stability

摘要：

采用共形离子溶液模型(conformalionic solution model, CIS) 在二元熔盐体系相图的基础上，对三元熔盐体系LiNO₃-NaNO₃-KNO₃进行了相图计算，得到该三元体系最低共熔点为117.7℃，相应的摩尔分数组成分别为x(LiNO₃) = 0.375，x(NaNO₃) = 0.075，x(KNO₃) = 0.550。按照热力学最低共熔点计算结果，采用熔融法制备了三元硝酸熔盐，通过DSC和TG实验测定其最低共熔点为118.3℃，这与计算得到的结果(117.7℃)基本一致。TG测试结果表明当温度低于587.2℃时，该三元熔盐体系较为稳定，其工作温度范围为118.3~587.2℃，该三元硝酸熔盐适合在太阳能热发电中作为高温传热蓄热材料使用。

关键词: 太阳能, 三元熔盐, 相平衡, 模型, 潜热, 热稳定性

CLC Number:

TK 02

Baicen ZHAO, Jing DING, Xiaolan WEI, Bin LIU, Jianfeng LU, Weilong WANG. Design and thermal stability study of LiNO₃-NaNO₃-KNO₃ ternary molten salt system[J]. CIESC Journal, 2019, 70(6): 2083-2091.

赵柏岑, 丁静, 魏小兰, 刘彬, 陆建峰, 王维龙. LiNO₃-NaNO₃-KNO₃三元熔盐材料的设计及热稳定性研究[J]. 化工学报, 2019, 70(6): 2083-2091.

Figures/Tables 15

Table 1 Physical parameters for single salt

Component	$T f / ℃$	$Δ H f / (J / m o l)$	$Δ c p / (J / (m o l · K))$
LiNO₃	255	24615.15	0.281
NaNO₃	305.85	15502	0.879
KNO₃	334.45	9623	2.887

Table 1 Physical parameters for single salt

Component	$T f / ℃$	$Δ H f / (J / m o l)$	$Δ c p / (J / (m o l · K))$
LiNO₃	255	24615.15	0.281
NaNO₃	305.85	15502	0.879
KNO₃	334.45	9623	2.887

Table 2 Eutectic temperature, composition in phase diagrams, calculated composition and interaction parameter λ

Component	最低共熔点/℃	实验相图		计算结果		相关系数λ
Component	最低共熔点/℃	x _A/mol	x _B/mol	x _A/mol	x _B/mol	相关系数λ
LiNO₃-NaNO₃	195	0.54	0.46	0.509562	0.490438	?711.962
LiNO₃-KNO₃	125	0.42	0.58	0.407864	0.592136	?8799.22
NaNO₃-KNO₃	223	0.49	0.51	0.448873	0.551127	3593.36

Fig.1 Phase disgram of ternary system

Table 3 Predicted low eutectic melting point temperature and composition

Component	x _A/mol	x _B/mol	x _C/mol	T _f/℃
LiNO₃-NaNO₃-KNO₃	0.375	0.075	0.550	117.7

Fig 2 DSC plot for single salt

Table 4 Experimental results of predicted tenary eutectic

Component	x _A/mol	x _B/mol	x _C/mol	T _f/℃
1	0.375	0.075	0.550	118.3
2	0.355	0.088	0.557	125.5
3	0.399	0.091	0.510	123.6
4	0.385	0.055	0.560	123.1
5	0.370	0.079	0.551	121.3
6	0.386	0.082	0.532	122.7
7	0.383	0.080	0.537	127.8
8	0.391	0.086	0.523	126
9	0.388	0.102	0.51	124.8
10	0.328	0.205	0.467	124.7
11	0.375	0.182	0.443	125.6
12	0.344	0.186	0.470	121.9
13	0.455	0.177	0.368	123.3
14	0.397	0.251	0.388	121.1
15	0.359	0.131	0.51	122.1
16	0.370	0.089	0.541	121.8
17	0.475	0.059	0.466	127.9
18	0.407	0.040	0.553	127.9
19	0.455	0.150	0.395	122.9
20	0.220	0.191	0.589	128.9

Table 5 Experimental results of references

Component	x _A/mol	x _B/mol	x _C/mol	T _f/℃	差值,ΔT _f/℃
本文实验值	0.375	0.075	0.550	118.3	0
A ^[21]	0.388	0.102	0.510	124.17	5.87
B ^[22]	0.328	0.205	0.467	116	-2.3
C ^[6]	0.375	0.182	0.443	120	1.7
D ^[23]	0.344	0.186	0.470	120	1.7
E ^[24]	0.371	0.181	0.448	120	1.7
F ^[25]	0.455	0.177	0.368	120	1.7

Fig 3 DSC plot for sample 1—5

Fig.4 DSC plot for sample 6—10

Fig.5 DSC plot for sample 11—15

Fig.6 DSC plot for sample 16—20

Fig.7 DSC plot for LiNO3-NaNO3-KNO3

Fig 8 TG plot for LiNO3-NaNO3-KNO3

Fig.9 Short-term mass loss curves of molten salt under isothermal conditions

Fig.10 Long-term mass loss curves of molten salt under isothermal conditions

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Design and thermal stability study of LiNO₃-NaNO₃-KNO₃ ternary molten salt system

LiNO₃-NaNO₃-KNO₃三元熔盐材料的设计及热稳定性研究

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References 31

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