Method for determining lowest eutectic temperature range of multi-element chloride molten salt

Abstract

The invention relates to a method for determining the lowest eutectic temperature range of multi-element chloride molten salt. Comprising the following steps: selecting a thermodynamic model, and obtaining an optimized thermodynamic model according to thermodynamic parameters of a molten salt system; calculating the minimum Gibbs free energy to obtain a solid-liquid phase diagram so as to obtain a first simulated lowest eutectic point in which the proportion of specified unit salt in the molten salt system is a group of preset values; determining an initial lowest eutectic temperature range, and obtaining a second simulated lowest eutectic point with the proportion of specified unit salt as two groups of preset values in the molten salt system according to the solid-liquid phase diagram; selecting a temperature interval with a preset temperature value higher than the second simulated lowest eutectic point as a corrected lowest eutectic temperature range; optionally simulating the eutectic temperature in the temperature range to obtain the proportion of each unit salt, and testing to obtain the corresponding actually measured eutectic temperature; and comparing the simulated eutectic temperature with the actually measured eutectic temperature, and combining the solid-liquid phase diagram to obtain the final lowest eutectic temperature range of the molten salt system. According to the method, the lowest eutectic melting temperature range and the corresponding composition proportion of the multi-element chloride molten salt are determined.

Description

Technical Field

[0001] The invention relates to the technical field of chloride molten salts, and in particular to a method for determining the lowest eutectic temperature range of a polynary chloride molten salt. Background Art

[0002] As a traditional heat transfer and heat storage medium, molten salt has made great contributions to thermal energy storage due to its advantages such as low price, high heat storage density and high working efficiency. Among large-scale medium and high temperature heat storage technologies, the application of molten salt heat storage in the field of solar thermal power generation is relatively mature and can meet the high temperature use requirements of various types of solar thermal power stations such as tower, dish and trough. In order to improve the working efficiency of the heat transfer and heat storage system and reduce the investment cost, the development and design of molten salt heat transfer and heat storage materials with excellent performance and low price has become a problem that needs to be solved urgently.

[0003] Since the melting point of unit salt is relatively high, different types of unit salts are usually mixed and melted by adjusting the ratio to prepare eutectic salt with a low eutectic point. Initially, the only method of molten salt material design was to prepare molten salt by experimental means, but it had the disadvantages of long cycle, high cost and great uncertainty.

[0004] Common molten salts can be divided into two categories: oxygen-containing acid salts and halide molten salts. Among them, chloride molten salts have a wider operating temperature range and better high-temperature stability than mixed oxygen-containing acid salts, and have become one of the third-generation high-temperature solar thermal power generation heat transfer and heat storage materials. For chloride molten salts, as the number of components increases, the melting point of multi-component molten salts gradually decreases. However, most current research remains on the prediction of the lowest eutectic point of ternary chloride molten salts. For multi-component chloride molten salts with more than three components, there is no clear lowest eutectic point in actual working conditions. Instead, there is a minimum eutectic temperature range. For the lowest eutectic temperature range of multi-component chloride molten salts with more than three components, a determination method is needed as a guiding basis for the design of new multi-component chloride molten salt heat transfer and heat storage materials.

[0005] CN 109097001 A discloses a method for preparing a multi-component chloride eutectic molten salt, comprising: providing a first chloride component having a first melting point and a second chloride component having a second melting point, wherein the second melting point is less than the first melting point; providing a hydrochloric acid aqueous solution, mixing the hydrochloric acid aqueous solution with the first chloride component and the second chloride component under ultrasonic vibration to form a chloride solution; heating to a first temperature, keeping the temperature at the first temperature, wherein the first temperature is greater than 120°C and is 10-100°C lower than the second melting point; continuing to heat to a second temperature, keeping the temperature at the second temperature; cooling to obtain a multi-component chloride eutectic molten salt having a third melting point, wherein the third melting point is less than the first melting point, and the second temperature is 10-200°C higher than the third melting point. The method is convenient for preparing a multi-component chloride eutectic molten salt, but the method focuses on the preparation method of the molten salt, lacks the design of the molten salt material, and does not involve the study of the lowest eutectic point or the lowest eutectic temperature range of the multi-component chloride eutectic molten salt.

[0006] CN 113372886 A discloses a ternary chloride molten salt with high temperature thermal stability and a preparation method thereof. The ternary chloride molten salt comprises 17.7-27.7wt.% NaCl, 6.0-25.1wt.% KCl and 55.1-66.1wt.% CaCl2. The preparation method of the ternary chloride molten salt comprises obtaining a proportion based on thermodynamic calculation and experimental test; mixing NaCl, KCl and CaCl2 according to the proportion; heating at a heating rate of 0.5-10℃ / min, and keeping the temperature at 100-350℃ for 0.5-10h to remove the moisture that may remain in the molten salt; heating to 550-700℃ at a heating rate of 1-20℃ / min, keeping the temperature at this temperature for 0.5-24h, pouring out the molten salt in a molten state, and cooling naturally to room temperature. The method is designed to obtain a proportion based on thermodynamic calculation and experimental test, and prepare the chloride molten salt by a mixed segmented heating method according to the proportion. Although this method abandons the inefficiency of the traditional "cooking" method of mixing different molten salt systems to prepare molten salt, this method focuses on the design of ternary chloride molten salt materials and does not take into account the lowest eutectic temperature range of multi-component chloride molten salts with more than ternary components.

[0007] Therefore, it is urgent to study a method for determining the lowest eutectic temperature range of multi-component chloride molten salts, so as to provide a reliable reference for the design of new multi-component chloride molten salt heat transfer and storage materials, and also provide guidance for the optimal design of heat transfer and storage systems. Summary of the invention

[0008] The purpose of the present invention is to provide a method for determining the lowest eutectic temperature range of a polycyclic chloride molten salt, so as to determine the lowest eutectic temperature range and the corresponding composition ratio of the polycyclic chloride molten salt, provide a reliable reference for the design of a new type of multi-component chloride molten salt heat transfer and heat storage material, and also provide a guiding basis for the optimal design of a heat transfer and heat storage system.

[0009] In order to achieve the above object, the technical solution adopted by the present invention is as follows:

[0010] A method for determining the lowest eutectic temperature range of a polyvalent chloride molten salt comprises the following steps:

[0011] S1. Selecting a thermodynamic model according to a polycyclic chloride molten salt system, and optimizing phase equilibrium parameters of the thermodynamic model according to thermodynamic parameters of each unit salt in the polycyclic chloride molten salt system to obtain an optimized thermodynamic model;

[0012] S2. Calculate the minimum Gibbs free energy of the polychloride molten salt system according to the optimized thermodynamic model, obtain the solid-liquid phase diagram of the polychloride molten salt system, and obtain the first simulated lowest eutectic point when the proportion of the specified unit salt in the polychloride molten salt system is the first set of preset values ​​according to the solid-liquid phase diagram;

[0013] S3, according to the first simulated lowest eutectic point and the first set of preset values, determining the initial lowest eutectic temperature range and the initial proportion interval corresponding to the specified unit salt, and according to the solid-liquid phase diagram, obtaining the second simulated lowest eutectic point when the proportion of the specified unit salt in the polychloride molten salt system is the second set of preset values, wherein the second set of preset values ​​is within the initial proportion interval;

[0014] S4, according to the second simulated lowest eutectic point and the second set of preset values, selecting the proportion of the designated unit salt corresponding to the preset temperature value higher than the second simulated lowest eutectic point as the modified proportion interval, and taking the temperature interval corresponding to the modified proportion interval as the modified lowest eutectic temperature range;

[0015] S5. Randomly select two simulated eutectic temperature points within the corrected lowest eutectic temperature range, obtain the proportions of each unit salt in the polychloride molten salt system corresponding to the two simulated eutectic temperature points, and obtain the corresponding two measured eutectic temperature points through DSC testing;

[0016] S6. Compare the two simulated eutectic temperature points and the two measured eutectic temperature points respectively, and combine the solid-liquid phase diagram to obtain the final lowest eutectic temperature range of the polyvalent chloride molten salt system.

[0017] Preferably, in S1, the thermodynamic parameters of each unit salt include enthalpy, entropy and specific heat capacity of each unit salt in solid phase, liquid phase and gas phase respectively; the phase equilibrium parameters include molar non-configurational entropy change and molar enthalpy change.

[0018] Preferably, in S1, the thermodynamic model is a chemical solution-like thermodynamic model selected and modified according to the strong interaction characteristics between ions in the polychloride molten salt system.

[0019] Preferably, the S1 specifically includes:

[0020] A thermodynamic model is established based on the polycyclic chloride molten salt system. The enthalpy, entropy and specific heat capacity of each unit salt in the polycyclic chloride molten salt system in the solid phase, liquid phase and gas phase are obtained from the molten salt phase diagram database, and used to optimize the phase equilibrium parameters of the thermodynamic model so that the activity value of the polycyclic chloride molten salt system is closest to the experimental data in the literature, thereby obtaining an optimized thermodynamic model.

[0021] Preferably, the difference between adjacent preset values ​​in the second set of preset values ​​is smaller than the difference between adjacent preset values ​​in the first set of preset values.

[0022] Preferably, in S4, the preset temperature value is 9-11K.

[0023] Preferably, the S5 specifically includes:

[0024] 1) arbitrarily selecting two simulated eutectic temperature points within the modified lowest eutectic temperature range, and obtaining the proportions of each unit salt in the polychloride molten salt system corresponding to the two simulated eutectic temperature points;

[0025] 2) According to the proportion of each unit salt in the polychloride molten salt system, the corresponding unit salt is weighed, and the unit salts are mixed and melted by a static melting method. After cooling, a temperature rise curve of the polychloride molten salt is obtained by DSC testing, and two measured eutectic temperature points corresponding to the two simulated eutectic temperature points are obtained through the temperature rise curve.

[0026] Preferably, the corresponding unit salts are weighed according to the proportion of each unit salt in the polychloride molten salt system, and the unit salts are mixed and melted by a static melting method, and cooled, which specifically includes:

[0027] The corresponding unit salts are weighed according to the proportions of each unit salt in the polychloride molten salt system, and each weighed unit salt is dried at 393K for 24 hours. Then, the unit salts are mixed and heated to the target temperature using a static melting method, and kept warm for 10 hours. After cooling and solidification, the polychloride molten salt is ground into a powdery state. The heating rate is 5K / min.

[0028] Preferably, a temperature rise curve of the polyvalent chloride molten salt is obtained by DSC testing, and two measured eutectic temperature points corresponding to the two simulated eutectic temperature points are obtained by the temperature rise curve, specifically including:

[0029] 1) placing a powdered polychloride molten salt sample in a platinum crucible of a DSC testing device, using argon as a purge gas and a protective gas, with an argon flow rate of 30-50 mL / min;

[0030] 2) The polychloride molten salt sample was heated from room temperature to 973 K at a rate of 25 K / min and then kept at this temperature for 1 minute;

[0031] 3) Cool down to 623K at a rate of 25K / min and keep the temperature constant for 3 minutes, then heat up to 973K at a rate of 10K / min and keep the temperature constant for 1 minute to obtain the first DSC heat flow curve;

[0032] 4) Repeat step 3) to obtain a second DSC heat flow curve;

[0033] 5) According to the first DSC heat flow curve and the second DSC heat flow curve, two measured eutectic temperature points corresponding to the two simulated eutectic temperature points are obtained.

[0034] Preferably, S6 specifically includes: respectively comparing two simulated eutectic temperature points and two measured eutectic temperature points so that the relative error is within 5%, and combining the solid-liquid phase diagram to obtain the final lowest eutectic temperature range of the polychloride molten salt system.

[0035] Beneficial effects of the present invention:

[0036] The invention discloses a method for determining the lowest eutectic temperature range of a polychloride molten salt. The method comprises the following steps: optimizing the phase equilibrium parameters of a thermodynamic model according to the thermodynamic parameters of each unit salt in the polychloride molten salt system to obtain an optimized thermodynamic model; obtaining a solid-liquid phase diagram of the polychloride molten salt system by optimizing the thermodynamic model; then setting the component ratio of a certain unit salt of the polychloride molten salt system as a set of fixed values, and calculating the lowest eutectic point of the polychloride molten salt system when the component ratio of a certain unit salt is fixed by means of FactSage software; selecting an initial lowest eutectic temperature range by means of the lowest eutectic point and a set of fixed values, and then setting a set of fixed values ​​as the component ratio of a certain unit salt, and calculating the lowest eutectic point of the polychloride molten salt system when the component ratio of a certain unit salt is fixed by means of FactSage software, and selecting a revised lowest eutectic temperature range; and randomly calculating the lowest eutectic point of the polychloride molten salt system when the component ratio of a certain unit salt is fixed in the revised lowest eutectic temperature range. Two temperature points are selected, and the composition ratio of each unit salt of the polycyclic chloride molten salt system at the temperature points is obtained; a molten salt sample of the polycyclic chloride molten salt system is prepared by a static melting method, and the melting point of the molten salt sample is measured by a DSC testing device, and the simulation value and the experimental value are comprehensively compared, and the error range is considered to finally determine the lowest eutectic temperature range of the polycyclic chloride molten salt system. The method of the present invention combines the solid-liquid phase diagram calculation method with experimental means to determine the lowest eutectic temperature range and the corresponding composition ratio of the polycyclic chloride molten salt system, which can not only ensure the accuracy of the lowest eutectic temperature range of the polycyclic chloride molten salt system, but also reduce the research and development cost of molten salt materials, provide a reliable reference for the design of new multi-component chloride molten salt heat transfer and heat storage materials, and also provide a guiding basis for the optimal design of heat transfer and heat storage systems, and has promotion and application value in the technical field of polycyclic chloride molten salt systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 A flow chart of a method for determining the lowest eutectic temperature range of a polyvalent chloride molten salt of the present invention;

[0038] Figure 2 This is a graph showing the change in the lowest eutectic point of the quaternary chloride molten salt KCl-NaCl-CaCl2-MgCl2 when the proportion of MgCl2 is fixed;

[0039] Figure 3 This is the solid-liquid phase diagram of KCl-NaCl-CaCl2-MgCl2 when MgCl2 is 0.33mol%;

[0040] Figure 4 This is the solid-liquid phase diagram of KCl-NaCl-CaCl2-MgCl2 when MgCl2 is 0.36mol%;

[0041] Figure 5The DSC heat flow curve of KCl-NaCl-CaCl2-MgCl2 (0.2143mol%-0.2924mol%-0.1633mol%-0.33mol%) is shown;

[0042] Figure 6 The DSC heat flow curve of KCl-NaCl-CaCl2-MgCl2 (0.23mol%-0.2672mol%-0.1428mol%-0.36mol%) is shown;

[0043] Figure 7 It is the change of the lowest eutectic point of the quaternary chloride molten salt KCl-NaCl-CaCl2-LiCl when the proportion of LiCl is fixed;

[0044] Figure 8 This is the solid-liquid phase diagram of KCl-NaCl-CaCl2-LiCl when LiCl is 0.46 mol%;

[0045] Fig. 9 This is the solid-liquid phase diagram of KCl-NaCl-CaCl2-LiCl when LiCl is 0.50mol%;

[0046] Fig.10 The DSC heat flow curve of KCl-NaCl-CaCl2-LiCl (0.3891mol%-0.1073mol%-0.0436mol%-0.46mol%) is shown;

[0047] Fig.11 This is the DSC heat flow curve of KCl-NaCl-CaCl2-LiCl (0.3956mol%-00639mol%-0.0405mol%-0.50mol%). DETAILED DESCRIPTION

[0048] The following will describe the embodiments of the present invention with reference to the accompanying drawings and preferred embodiments. Those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present invention. It should be understood that the preferred embodiments are only for illustrating the present invention, not for limiting the scope of protection of the present invention.

[0049] It should be noted that the illustrations provided in the following embodiments are only schematic illustrations of the basic concept of the present invention, and thus the drawings only show components related to the present invention rather than being drawn according to the number, shape and size of components in actual implementation. In actual implementation, the type, quantity and proportion of each component may be changed arbitrarily, and the component layout may also be more complicated.

[0050] Example 1

[0051] like Figure 1 As shown, a method for determining the lowest eutectic temperature range of a polyvalent chloride molten salt, in this embodiment, the polyvalent chloride molten salt is a quaternary chloride molten salt system of KCl-NaCl-CaCl2-MgCl2, and the determination method comprises the following steps:

[0052] S1. Selecting a thermodynamic model according to a polycyclic chloride molten salt system, and optimizing the phase equilibrium parameters of the thermodynamic model according to the thermodynamic parameters of each unit salt in the polycyclic chloride molten salt system to obtain an optimized thermodynamic model, specifically comprising:

[0053] S11. According to the strong interaction characteristics between ions in the KCl-NaCl-CaCl2-MgCl2 quaternary chloride molten salt system, a modified quasi-chemical solution thermodynamic model is selected;

[0054] S12. Obtain the enthalpy, entropy and specific heat capacity thermodynamic parameters of pure KCl salt, pure NaCl salt, pure CaCl2 salt and pure MgCl2 salt in the solid phase, liquid phase and gas phase respectively from the molten salt phase diagram database, and use FactSage phase diagram software to optimize the molar non-configurational entropy change and molar enthalpy change phase equilibrium parameters until the activity value of the KCl-NaCl-CaCl2-MgCl2 quaternary chloride molten salt system is closest to the experimental data in the literature, and obtain the optimized thermodynamic model;

[0055] S2. According to the optimized thermodynamic model, the minimum Gibbs free energy of the polychloride molten salt system is calculated to obtain the solid-liquid phase diagram of the polychloride molten salt system, and according to the solid-liquid phase diagram, the first simulated lowest eutectic point when the proportion of the specified unit salt in the polychloride molten salt system is the first set of preset values ​​is obtained, specifically including:

[0056] S21. According to the optimized thermodynamic model, the minimum Gibbs free energy of the KCl-NaCl-CaCl2-MgCl2 quaternary chloride molten salt system is calculated to obtain the solid-liquid phase diagram of the KCl-NaCl-CaCl2-MgCl2 quaternary chloride molten salt system;

[0057] S22. Set the proportion of MgCl2 in the KCl-NaCl-CaCl2-MgCl2 quaternary chloride molten salt system to the first set of preset values. In this embodiment, the first set of preset values ​​is 0.05mol%, 0.10mol%, 0.15mol%, 0.20mol%, 0.25mol%, 0.30mol%, 0.35mol%, 0.40mol%, 0.45mol%, 0.50mol%, 0.55mol%, 0.60mol%, 0.65mol%, 0.70mol%, 0.75mol%, 0.80mol%, 0.85mol%, 0.90mol%, 0.95mol%;

[0058] S23. According to the solid-liquid phase diagram in S21, a first simulated lowest eutectic point is obtained when the proportion of MgCl2 is the first set of preset values. In this embodiment, a lowest eutectic point variation diagram of the KCl-NaCl-CaCl2-MgCl2 quaternary chloride molten salt system when the proportion of MgCl2 is the first set of preset values ​​is obtained through the solid-liquid phase diagram, such as Figure 2 As shown, from Figure 2 It is known that when the proportion of MgCl2 is between 0.30mol% and 0.45mol%, the lowest eutectic point appears, which is the first simulated lowest eutectic point;

[0059] In this embodiment, percentage refers to molar mass percentage;

[0060] S3. According to the first simulated lowest eutectic point and the first set of preset values, determine the initial lowest eutectic temperature range and the initial proportion interval corresponding to the specified unit salt, and according to the solid-liquid phase diagram, obtain the second simulated lowest eutectic point when the proportion of the specified unit salt in the polychloride molten salt system is the second set of preset values, wherein the second set of preset values ​​is within the initial proportion interval, specifically including:

[0061] S31. When the proportion of MgCl2 is between 0.30mol% and 0.45mol%, the lowest eutectic point appears, and when the proportion of MgCl2 exceeds 0.50mol%, the lowest eutectic point no longer exists. Therefore, the eutectic temperatures corresponding to the proportions of MgCl2 of 0.30mol% and 0.45mol% are selected as the initial lowest eutectic temperature range, that is, the initial lowest eutectic temperature range is 650.94K to 652.49K, and 0.30mol% to 0.45mol% is used as the corresponding initial proportion interval;

[0062] S32. Select a second set of preset values ​​within the initial proportion interval of 0.30 mol% to 0.45 mol%. In this embodiment, the second set of preset values ​​is 0.305 mol%, 0.310 mol%, 0.315 mol%, 0.320 mol%, 0.325 mol%, 0.330 mol%, 0.335 mol%, 0.340 mol%, 0.345 mol%, 0.355 mol%, 0.360 mol%, 0.365 mol%, 0.370 mol%, 0.375 mol%, 0.380 mol%, 0.385 mol%, 0.390 mol%, 0.395 mol%, 0.405 mol%, 0.410 mol%, 0.415 mol%, 0.420 mol%, 0.425 mol%, 0.430 mol%, 0.435 mol%, 0.440 mol%, 0.445 mol%.

[0063] S33, according to the solid-liquid phase diagram in S21, obtain the second simulated lowest eutectic point when the proportion of MgCl2 is the second set of preset values. In this embodiment, the lowest eutectic point change diagram of the KCl-NaCl-CaCl2-MgCl2 quaternary chloride molten salt system when the proportion of MgCl2 is the second set of preset values ​​is obtained through the solid-liquid phase diagram, such as Figure 2 As shown, from Figure 2 It is known that the lowest eutectic point of 635.28K appears when the proportion of MgCl2 is 0.360mol%, which is the second simulated lowest eutectic point;

[0064] S4, according to the second simulated lowest eutectic point and the second set of preset values, selecting the proportion of the corresponding designated unit salt with a preset temperature value higher than the second simulated lowest eutectic point as the modified proportion interval, and taking the temperature interval corresponding to the modified proportion interval as the modified lowest eutectic temperature range, specifically including:

[0065] Since the lowest eutectic point of the second simulation is 635.28K, the proportion of the designated unit salt corresponding to about 10K higher than 635.28K is 0.33mol% to 0.425mol%, that is, the corrected proportion interval, and the temperature range of 644.29K to 646.55K corresponding to the corrected proportion interval of 0.33mol% to 0.425mol% is used as the corrected lowest eutectic temperature range;

[0066] Among them, since there is no clear lowest eutectic point for multi-component chloride molten salts with more than ternary components in actual working conditions, and the temperature around 10K near the lowest eutectic point is within the error range, the temperature range corresponding to the correction ratio range corresponding to about 10K higher than 635.28K is selected as the corrected lowest eutectic temperature range, that is, in this embodiment, the corrected lowest eutectic temperature range is 644.29K~646.55K;

[0067] S5. Randomly select two simulated eutectic temperature points within the corrected minimum eutectic temperature range, obtain the proportions of each unit salt in the polyvalent chloride molten salt system corresponding to the two simulated eutectic temperature points, and obtain the corresponding two measured eutectic temperature points through DSC testing, specifically including:

[0068] S51. Arbitrarily select two simulated eutectic temperature points within the modified minimum eutectic temperature range. In this embodiment, 644.29K and 635.28K are selected as simulated eutectic temperature points, and the corresponding MgCl2 accounts for 0.33mol% and 0.36mol%. At this ratio of MgCl2, the solid-liquid phase diagrams of the quaternary chloride molten salt of KCl-NaCl-CaCl2-MgCl2 are as follows: Figure 3 and Figure 4 As shown, the molar mass ratios of each unit salt corresponding to the two simulated eutectic temperature points of the KCl-NaCl-CaCl2-MgCl2 quaternary chloride molten salt are shown in Table 1;

[0069] S52. Prepare molten salt samples according to the composition ratio of each unit salt of the two groups of KCl-NaCl-CaCl2-MgCl2 quaternary chloride molten salts obtained. The unit salt reagents used need to be vacuum dried at 393K for 24 hours in advance; because the calculation result of the molar mass ratio of the components corresponding to the simulated eutectic temperature point is retained to four decimal places, the actual sample weighing is limited by the accuracy of the electronic balance, so the ratio is approximately retained to two decimal places and then converted into a mass ratio, and the weighing is completed in an argon atmosphere glove box; each unit molten salt sample is placed in a corundum crucible and stirred to mix evenly, and sent into a muffle furnace to heat the molten salt sample to 800K at a heating rate of 5K / min, and kept warm for 10 hours to ensure that the molten salt is fully melted; finally, pour out the fired molten salt sample in liquid form, grind it into powder after solidification, and vacuum seal it for storage after natural cooling;

[0070] S53, take 8-10mg of powdered polychloride molten salt sample and put it into the platinum crucible of DSC testing equipment, ensure that the purge gas and the protective gas are both high-purity argon gas with a flow rate of 30-50mL / min; heat the polychloride molten salt sample from room temperature to 973K at a rate of 25K / min and keep the temperature constant for 1 minute; then repeat the following steps twice: cool down to 623K at a rate of 25K / min and keep the temperature constant for 3 minutes, then heat up to 973K at a rate of 10K / min and keep the temperature constant for 1 minute, and obtain two DSC heat flow curves as experimental results;

[0071] In this embodiment, KCl-NaCl-CaCl2-MgCl2 was obtained by DSC test.

[0072] (0.2143mol%-0.2924mol%-0.1633mol%-0.33mol%) and KCl-NaCl-CaCl2-MgCl2

[0073] The DSC heat flow curves of (0.23mol%-0.2672mol%-0.1428mol%-0.36mol%) are as follows Figure 5 and Figure 6 As shown;

[0074] The melting point is the temperature corresponding to the peak position of the melting endothermic peak. Figure 5 Learned

[0075] The melting point of KCl-NaCl-CaCl2-MgCl2 (0.2143mol%-0.2924mol%-0.1633mol%-0.33mol%) is 657.45K. Figure 6 It is known that KCl-NaCl-CaCl2-MgCl2

[0076] The melting point of (0.23mol%-0.2672mol%-0.1428mol%-0.36mol%) is 655.65K;

[0077] S6. Comparing the two simulated eutectic temperature points and the two measured eutectic temperature points respectively, and combining the solid-liquid phase diagram, obtaining the final lowest eutectic temperature range of the polyvalent chloride molten salt system, specifically comprising:

[0078] By comparing the simulated eutectic temperature point of 644.29K with the measured eutectic temperature point of 657.45K, the calculated relative error is 2.04%; by comparing the simulated eutectic temperature point of 635.28K with the measured eutectic temperature point of 655.65K, the calculated relative error is 3.21%; therefore, it is known that the relative errors are all within 5%. It is not difficult to see that the melting points of the KCl-NaCl-CaCl2-MgCl2 quaternary chloride molten salt at the above two composition ratios are very close. The results can well reflect the existence of the lowest eutectic temperature range. Therefore, it is finally determined that the lowest eutectic temperature range of the KCl-NaCl-CaCl2-MgCl2 quaternary chloride molten salt system is 650K~660K.

[0079] Table 1 shows the simulated eutectic temperature points of the quaternary chloride molten salt system and the molar mass ratio of each unit salt

[0080]

[0081] Example 2

[0082] like Figure 1As shown, a method for determining the lowest eutectic temperature range of a polyvalent chloride molten salt, in this embodiment, the polyvalent chloride molten salt is a quaternary chloride molten salt system of KCl-NaCl-CaCl2-LiCl, and the determination method comprises the following steps:

[0083] S1. Selecting a thermodynamic model according to a polycyclic chloride molten salt system, and optimizing the phase equilibrium parameters of the thermodynamic model according to the thermodynamic parameters of each unit salt in the polycyclic chloride molten salt system to obtain an optimized thermodynamic model, specifically comprising:

[0084] S11. According to the strong interaction characteristics between ions in the KCl-NaCl-CaCl2-LiCl quaternary chloride molten salt system, a modified chemical solution thermodynamic model is selected;

[0085] S12. Obtain the enthalpy, entropy and specific heat capacity thermodynamic parameters of pure KCl salt, pure NaCl salt, pure CaCl2 salt and pure LiCl salt in the solid phase, liquid phase and gas phase respectively from the molten salt phase diagram database, and use FactSage phase diagram software to optimize the molar non-configurational entropy change and molar enthalpy change phase equilibrium parameters until the activity value of the KCl-NaCl-CaCl2-LiCl quaternary chloride molten salt system is closest to the experimental data in the literature, and obtain the optimized thermodynamic model;

[0086] S2. According to the optimized thermodynamic model, the minimum Gibbs free energy of the polychloride molten salt system is calculated to obtain the solid-liquid phase diagram of the polychloride molten salt system, and according to the solid-liquid phase diagram, the first simulated lowest eutectic point when the proportion of the specified unit salt in the polychloride molten salt system is the first set of preset values ​​is obtained, specifically including:

[0087] S21. Calculate the minimum Gibbs free energy of the KCl-NaCl-CaCl2-LiCl quaternary chloride molten salt system according to the optimized thermodynamic model, and obtain the solid-liquid phase diagram of the KCl-NaCl-CaCl2-LiCl quaternary chloride molten salt system;

[0088] S22, setting the proportion of LiCl in the KCl-NaCl-CaCl2-LiCl quaternary chloride molten salt system to the first set of preset values. In this embodiment, the first set of preset values ​​is 0.05mol%, 0.10mol%, 0.15mol%, 0.20mol%, 0.25mol%, 0.30mol%, 0.35mol%, 0.40mol%, 0.45mol%, 0.50mol%, 0.55mol%, 0.60mol%, 0.65mol%, 0.70mol%, 0.75mol%, 0.80mol%, 0.85mol%, 0.90mol%, 0.95mol%;

[0089] S23. According to the solid-liquid phase diagram in S21, a first simulated lowest eutectic point is obtained when the proportion of LiCl is the first set of preset values. In this embodiment, a lowest eutectic point variation diagram of the KCl-NaCl-CaCl2-LiCl quaternary chloride molten salt system when the proportion of LiCl is the first set of preset values ​​is obtained through the solid-liquid phase diagram, such as Figure 7 As shown, from Figure 7 It is known that when the proportion of LiCl is between 0.45mol% and 0.55mol%, the lowest eutectic point appears, which is the first simulated lowest eutectic point;

[0090] In this embodiment, percentage refers to molar mass percentage;

[0091] S3. According to the first simulated lowest eutectic point and the first set of preset values, determine the initial lowest eutectic temperature range and the initial proportion interval corresponding to the specified unit salt, and according to the solid-liquid phase diagram, obtain the second simulated lowest eutectic point when the proportion of the specified unit salt in the polychloride molten salt system is the second set of preset values, wherein the second set of preset values ​​is within the initial proportion interval, specifically including:

[0092] S31. When the proportion of LiCl is between 0.45mol% and 0.55mol%, the lowest eutectic point appears, and when the proportion of MgCl2 exceeds 0.50mol%, the lowest eutectic point no longer exists. Therefore, the eutectic temperatures corresponding to the proportions of LiCl of 0.45mol% and 0.55mol% are selected as the initial lowest eutectic temperature range, that is, the initial lowest eutectic temperature range is 628.95K~NONE, and 0.45mol%~0.55mol% is used as the corresponding initial proportion interval;

[0093] S32, selecting a second set of preset values ​​within the initial proportion interval of 0.45 mol% to 0.55 mol%. In this embodiment, the second set of preset values ​​is 0.451 mol%, 0.452 mol%, 0.453 mol%, ... 0.547 mol%, 0.548 mol%, 0.549 mol%, that is, the difference between adjacent preset values ​​is 0.001 mol%;

[0094] S33, according to the solid-liquid phase diagram in S21, obtain the second simulated lowest eutectic point when the proportion of LiCl is the second set of preset values. In this embodiment, the lowest eutectic point change diagram of the KCl-NaCl-CaCl2-LiCl quaternary chloride molten salt system when the proportion of LiCl is the second set of preset values ​​is obtained through the solid-liquid phase diagram, such as Figure 7 As shown, from Figure 7 It is known that when the proportion of LiCl is 0.500mol%, the lowest eutectic point is 610.92K, which is the second simulated lowest eutectic point;

[0095] S4, according to the second simulated lowest eutectic point and the second set of preset values, selecting the proportion of the corresponding designated unit salt with a preset temperature value higher than the second simulated lowest eutectic point as the modified proportion interval, and taking the temperature interval corresponding to the modified proportion interval as the modified lowest eutectic temperature range, specifically including:

[0096] Since the lowest eutectic point of the second simulation is 610.92K, the left endpoint of the proportion of the designated unit salt corresponding to about 10K higher than 610.92K is 0.46mol%, that is, the left endpoint of the modified proportion interval. Since the lowest eutectic point no longer exists after the proportion of LiCl exceeds 0.528mol%, 0.528mol% is selected as the right interval endpoint of the modified proportion area, and the temperature range of 623.88K to 613.90K corresponding to the modified proportion interval of 0.46mol% to 0.528mol% is used as the modified lowest eutectic temperature range;

[0097] Among them, since there is no clear lowest eutectic point for multi-component chloride molten salts with more than ternary components in actual working conditions, and the temperature around 10K near the lowest eutectic point is within the error range, the temperature range corresponding to the correction ratio range corresponding to about 10K higher than 610.92K is selected as the corrected lowest eutectic temperature range, that is, in this embodiment, the corrected lowest eutectic temperature range is 623.88K~613.90K;

[0098] S5. Randomly select two simulated eutectic temperature points within the corrected minimum eutectic temperature range, obtain the proportions of each unit salt in the polyvalent chloride molten salt system corresponding to the two simulated eutectic temperature points, and obtain the corresponding two measured eutectic temperature points through DSC testing, specifically including:

[0099] S51. Arbitrarily select two simulated eutectic temperature points within the modified minimum eutectic temperature range. In this embodiment, 623.88K and 610.92K are selected as simulated eutectic temperature points, and the corresponding LiCl accounts for 0.46mol% and 0.50mol%. At this ratio of LiCl, the solid-liquid phase diagrams of the quaternary chloride molten salt of KCl-NaCl-CaCl2-LiCl are as follows: Figure 8 and Fig. 9 As shown, the molar mass ratios of each unit salt corresponding to the two simulated eutectic temperature points of the KCl-NaCl-CaCl2-LiCl quaternary chloride molten salt are shown in Table 2;

[0100] S52. Prepare molten salt samples according to the composition ratio of each unit salt of the two groups of KCl-NaCl-CaCl2-LiCl quaternary chloride molten salts obtained. The unit salt reagents used need to be vacuum dried at 393K for 24 hours in advance; because the calculation result of the molar mass ratio of the components corresponding to the simulated eutectic temperature point is retained to four decimal places, due to the accuracy limitation of the electronic balance in the actual weighing, the ratio is approximately retained to two decimal places and then converted into a mass ratio, and the weighing is completed in an argon atmosphere glove box; each unit molten salt sample is placed in a corundum crucible and stirred to mix evenly, and sent into a muffle furnace to heat the molten salt sample to 800K at a heating rate of 5K / min, and kept warm for 10 hours to ensure that the molten salt is fully melted; finally, pour out the fired molten salt sample in liquid form, grind it into powder after solidification, and vacuum seal it for storage after natural cooling;

[0101] S53, take 8-10mg of powdered polychloride molten salt sample and put it into the platinum crucible of DSC testing equipment, ensure that the purge gas and the protective gas are both high-purity argon gas with a flow rate of 30-50mL / min; heat the polychloride molten salt sample from room temperature to 973K at a rate of 25K / min and keep the temperature constant for 1 minute; then repeat the following steps twice: cool down to 623K at a rate of 25K / min and keep the temperature constant for 3 minutes, then heat up to 973K at a rate of 10K / min and keep the temperature constant for 1 minute, and obtain two DSC heat flow curves as experimental results;

[0102] In this embodiment, KCl-NaCl-CaCl2-LiCl was obtained by DSC test.

[0103] (0.3891mol%-0.1073mol%-0.0436mol%-0.46mol%) and KCl-NaCl-CaCl2-LiCl

[0104] The DSC heat flow curves of (0.3956mol%-0.0639mol%-0.0405mol%-0.50mol%) are as follows Fig.10 and Fig.11 As shown;

[0105] The melting point is the temperature corresponding to the peak position of the melting endothermic peak. Fig.10 It is known that the melting point of KCl-NaCl-CaCl2-LiCl (0.3891mol%-0.1073mol%-0.0436mol%-0.46mol%) is 631.37K. Fig.11 It is known that the melting point of KCl-NaCl-CaCl2-LiCl (0.3956mol%-0.0639mol%-0.0405mol%-0.50mol%) is 628.69K;

[0106] S6. Comparing the two simulated eutectic temperature points and the two measured eutectic temperature points respectively, and combining the solid-liquid phase diagram, obtaining the final lowest eutectic temperature range of the polyvalent chloride molten salt system, specifically comprising:

[0107] By comparing the simulated eutectic temperature point of 623.88K with the measured eutectic temperature point of 631.37K, the calculated relative error is 1.20%; by comparing the simulated eutectic temperature point of 610.92K with the measured eutectic temperature point of 628.69K, the calculated relative error is 2.91%; therefore, it is known that the relative errors are all within 5%. It is not difficult to see that the melting points of the KCl-NaCl-CaCl2-LiCl quaternary chloride molten salt at the above two composition ratios are very close, and the results can well reflect the existence of the lowest eutectic temperature range. Therefore, it is finally determined that the lowest eutectic temperature range of the KCl-NaCl-CaCl2-LiCl quaternary chloride molten salt system is 620K~630K.

[0108] Table 2 shows the simulated eutectic temperature points of the quaternary chloride molten salt system and the molar mass ratio of each unit salt

[0109]

[0110]

[0111] In summary, a method for determining the lowest eutectic temperature range of a polycyclic chloride molten salt of the present invention optimizes the phase equilibrium parameters of a thermodynamic model according to the thermodynamic parameters of each unit salt in the polycyclic chloride molten salt system to obtain an optimized thermodynamic model; a solid-liquid phase diagram of the polycyclic chloride molten salt system is obtained by optimizing the thermodynamic model; then, by setting the component ratio of a certain unit salt of the polycyclic chloride molten salt system to a set of fixed values, the lowest eutectic point of the polycyclic chloride molten salt system when the component ratio of a certain unit salt is fixed is calculated with the help of FactSage software; after selecting an initial lowest eutectic temperature range through the lowest eutectic point and a set of fixed values, a set of fixed values ​​is set as the component ratio of a certain unit salt, the lowest eutectic point of the polycyclic chloride molten salt system when the component ratio of a certain unit salt is fixed is calculated again with the help of FactSage software, and a revised lowest eutectic temperature range is selected; in the revised lowest eutectic Two temperature points are randomly selected in the melting temperature domain, and the composition ratio of each unit salt of the polycyclic chloride molten salt system at the temperature point is obtained; a molten salt sample of the polycyclic chloride molten salt system is prepared by a static melting method, and the melting point of the molten salt sample is measured by a DSC testing device, and the simulation value and the experimental value are comprehensively compared, and the error range is considered to finally determine the lowest eutectic temperature domain of the polycyclic chloride molten salt system. The method of the present invention determines the lowest eutectic temperature domain and the corresponding composition ratio of the polycyclic chloride molten salt system by combining calculation and experimental means, which can not only ensure the accuracy of the lowest eutectic temperature domain of the polycyclic chloride molten salt system, but also reduce the research and development cost of molten salt materials, provide a reliable reference for the design of new multi-component chloride molten salt heat transfer and heat storage materials, but also provide a guiding basis for the optimal design of heat transfer and heat storage systems, and has promotion and application value in the technical field of polycyclic chloride molten salt systems.

[0112] The above embodiments are only preferred embodiments for fully illustrating the present invention, and the protection scope of the present invention is not limited thereto. Any equivalent substitution or change made by a person skilled in the art based on the present invention is within the protection scope of the present invention.

Claims

1. A method for determining the lowest eutectic temperature range of a polyvalent chloride molten salt, characterized in that: The following steps are involved: S1. Selecting a thermodynamic model according to a polycyclic chloride molten salt system, and optimizing phase equilibrium parameters of the thermodynamic model according to thermodynamic parameters of each unit salt in the polycyclic chloride molten salt system to obtain an optimized thermodynamic model; S2. Calculate the minimum Gibbs free energy of the polychloride molten salt system according to the optimized thermodynamic model, obtain the solid-liquid phase diagram of the polychloride molten salt system, and obtain the first simulated lowest eutectic point when the proportion of the specified unit salt in the polychloride molten salt system is the first set of preset values ​​according to the solid-liquid phase diagram; S3, according to the first simulated lowest eutectic point and the first set of preset values, determining the initial lowest eutectic temperature range and the initial proportion interval corresponding to the specified unit salt, and according to the solid-liquid phase diagram, obtaining the second simulated lowest eutectic point when the proportion of the specified unit salt in the polychloride molten salt system is the second set of preset values, wherein the second set of preset values ​​is within the initial proportion interval; S4, according to the second simulated lowest eutectic point and the second set of preset values, selecting the proportion of the designated unit salt corresponding to the preset temperature value higher than the second simulated lowest eutectic point as the modified proportion interval, and taking the temperature interval corresponding to the modified proportion interval as the modified lowest eutectic temperature range; S5. Randomly select two simulated eutectic temperature points within the corrected lowest eutectic temperature range, obtain the proportions of each unit salt in the polychloride molten salt system corresponding to the two simulated eutectic temperature points, and obtain the corresponding two measured eutectic temperature points through DSC testing; S6. Compare the two simulated eutectic temperature points and the two measured eutectic temperature points respectively, and combine the solid-liquid phase diagram to obtain the final lowest eutectic temperature range of the polyvalent chloride molten salt system.

2. The method for determining the lowest eutectic temperature range of a polyvalent chloride molten salt according to claim 1, characterized in that: In S1, the thermodynamic parameters of each unit salt include enthalpy, entropy and specific heat capacity of each unit salt in solid phase, liquid phase and gas phase respectively; the phase equilibrium parameters include molar non-configurational entropy change and molar enthalpy change.

3. The method for determining the lowest eutectic temperature range of a polyvalent chloride molten salt according to claim 1, characterized in that: In S1, the thermodynamic model is a chemical solution-like thermodynamic model selected and modified according to the strong interaction characteristics between ions in the polychloride molten salt system.

4. The method for determining the lowest eutectic temperature range of a polyvalent chloride molten salt according to claim 1, characterized in that: Said S1 specifically includes: A thermodynamic model is established based on the polycyclic chloride molten salt system. The enthalpy, entropy and specific heat capacity of each unit salt in the polycyclic chloride molten salt system in the solid phase, liquid phase and gas phase are obtained from the molten salt phase diagram database, and used to optimize the phase equilibrium parameters of the thermodynamic model so that the activity value of the polycyclic chloride molten salt system is closest to the experimental data in the literature, thereby obtaining an optimized thermodynamic model.

5. The method for determining the lowest eutectic temperature range of a polyvalent chloride molten salt according to claim 1, characterized in that: The difference between adjacent preset values ​​in the second set of preset values ​​is smaller than the difference between adjacent preset values ​​in the first set of preset values.

6. The method for determining the lowest eutectic temperature range of a polyvalent chloride molten salt according to claim 1, characterized in that: In S4, the preset temperature value is 9~11K.

7. The method for determining the lowest eutectic temperature range of a polyvalent chloride molten salt according to claim 1, characterized in that: The S5 specifically includes: 1) arbitrarily selecting two simulated eutectic temperature points within the modified lowest eutectic temperature range, and obtaining the proportions of each unit salt in the polychloride molten salt system corresponding to the two simulated eutectic temperature points; 2) According to the proportion of each unit salt in the polychloride molten salt system, the corresponding unit salt is weighed, and the unit salts are mixed and melted by a static melting method. After cooling, a temperature rise curve of the polychloride molten salt is obtained by DSC testing, and two measured eutectic temperature points corresponding to the two simulated eutectic temperature points are obtained through the temperature rise curve.

8. The method for determining the lowest eutectic temperature range of polyvalent chloride molten salt according to claim 7, characterized in that: Weighing corresponding unit salts according to the proportion of each unit salt in the polychloride molten salt system, mixing and melting each unit salt by a static melting method, and cooling, specifically comprising: The corresponding unit salts are weighed according to the proportions of each unit salt in the polychloride molten salt system, and each weighed unit salt is dried at 393K for 24 hours. Then, the unit salts are mixed and heated to the target temperature using a static melting method, and kept warm for 10 hours. After cooling and solidification, the polychloride molten salt is ground into a powdery state. The heating rate is 5K / min.

9. The method for determining the lowest eutectic temperature range of polyvalent chloride molten salt according to claim 8, characterized in that: The temperature rise curve of the polychloride molten salt is obtained by DSC test, and two measured eutectic temperature points corresponding to the two simulated eutectic temperature points are obtained by the temperature rise curve, specifically including: 1) Place the powdered polychloride molten salt sample in a platinum crucible of the DSC testing equipment, use argon as the purge gas and protective gas, and the argon flow rate is 30-50mL / min; 2) The polychloride molten salt sample was heated from room temperature to 973 K at a rate of 25 K / min and then kept at this temperature for 1 minute; 3) Cool down to 623K at a rate of 25K / min and keep the temperature constant for 3 minutes, then heat up to 973K at a rate of 10K / min and keep the temperature constant for 1 minute to obtain the first DSC heat flow curve; 4) Repeat step 3) to obtain the second DSC heat flow curve; 5) According to the first DSC heat flow curve and the second DSC heat flow curve, two measured eutectic temperature points corresponding to the two simulated eutectic temperature points are obtained.

10. The method for determining the lowest eutectic temperature range of polyvalent chloride molten salt according to claim 1, characterized in that: The S6 specifically includes: respectively comparing two simulated eutectic temperature points and two measured eutectic temperature points so that the relative error is within 5%, and combining the solid-liquid phase diagram to obtain the final lowest eutectic temperature range of the polychloride molten salt system.

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