Method for determining the minimum eutectic temperature range of a multi-component chloride molten salt

Abstract

The present application relates to a kind of methods for determining the minimum eutectic temperature range of multicomponent chloride molten salt.The method comprises the following steps: selecting a thermodynamic model, optimizing the thermodynamic model according to the thermodynamic parameters of the molten salt system; calculating the minimum Gibbs free energy, obtaining the solid-liquid phase diagram, and obtaining the first simulated minimum eutectic point of the specified unit salt in the molten salt system with a set of preset values; determining the initial minimum eutectic temperature range, obtaining the second simulated minimum eutectic point of the specified unit salt in the molten salt system with a set of preset values according to the solid-liquid phase diagram; selecting a temperature interval higher than the second simulated minimum eutectic point by a preset temperature value as the corrected minimum eutectic temperature range; and optionally simulating the eutectic temperature in this temperature range to obtain the proportion of each unit salt, and obtaining the corresponding measured eutectic temperature by testing; comparing the simulated eutectic temperature and the measured eutectic temperature, and obtaining the final minimum eutectic temperature range of the molten salt system in combination with the solid-liquid phase diagram. The present application determines the minimum eutectic temperature range and the corresponding composition ratio of multicomponent chloride molten salt.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of chloride molten salt, and particularly relates to a method for determining a minimum eutectic temperature range of a multi-component chloride molten salt. BACKGROUND

[0002] As a traditional heat transfer and storage medium, molten salt has made great contributions to thermal energy storage due to its low price, high heat storage density and high working efficiency. In large-scale medium and high temperature heat storage technology, molten salt heat storage is relatively mature in the application of photo-thermal power generation, and can meet the high temperature use requirements of various types of photo-thermal power stations such as tower type, disc type and tank type. In order to improve the working efficiency of the heat transfer and storage system and reduce the investment cost, it is urgent to develop and design molten salt heat transfer and storage materials with excellent performance and low price.

[0003] Due to the high melting point of unit salt, different types of unit salt are usually mixed and melted to prepare eutectic salt with a low eutectic point. Initially, the preparation of molten salt by experimental means is the only design method for molten salt materials, but it has the disadvantages of long cycle, high cost and large uncertainty.

[0004] Common molten salts can be divided into oxyacid salts and halide molten salts. Among them, chloride molten salt has a wider working temperature and better high-temperature stability than mixed oxyacid salt, and has become one of the third-generation high-temperature photo-thermal power generation heat transfer and storage materials. For chloride molten salt, as the number of component types increases, the melting point of multi-component molten salt gradually decreases. However, most of the current researches are limited to the prediction of the minimum eutectic point of ternary chloride molten salt. For multi-component chloride molten salt with more than three components, there is no clear minimum eutectic point in the actual working condition, but a minimum eutectic temperature range instead. For the minimum eutectic temperature range of multi-component chloride molten salt with more than three components, a determination method is needed as a guide for the design of new multi-component chloride molten salt heat transfer and 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 lower than the first melting point; providing an aqueous hydrochloric acid solution, and mixing the aqueous hydrochloric acid solution with the first chloride component and the second chloride component under ultrasonic vibration to form a chloride solution; heating to a first temperature, and holding at the first temperature, wherein the first temperature is greater than 120°C and 10-100°C lower than the second melting point; continuing to heat to a second temperature, and holding at the second temperature; and cooling to obtain a multi-component chloride eutectic molten salt having a third melting point, wherein the third melting point is lower than the first melting point, and the second temperature is 10-200°C higher than the third melting point. This method is convenient for preparing multi-component chloride eutectic molten salts; however, it focuses on the preparation method of the molten salt and lacks consideration of the design of the molten salt material, and does not involve research on the lowest eutectic point or 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 its preparation method. The ternary chloride molten salt comprises 17.7-27.7 wt.% NaCl, 6.0-25.1 wt.% KCl, and 55.1-66.1 wt.% CaCl2. The preparation method includes obtaining a mixture ratio based on thermodynamic calculations and experimental testing; mixing NaCl, KCl, and CaCl2 according to the ratio; heating at a rate of 0.5-10 °C / min and holding at 100-350 °C for 0.5-10 h to remove any residual moisture; heating at a rate of 1-20 °C / min to 550-700 °C and holding at this temperature for 0.5-24 h; pouring out the molten salt in a molten state and allowing it to cool naturally to room temperature. This method is designed based on thermodynamic calculations and experimental testing to obtain the mixture ratio, and then preparing the chloride molten salt using a segmented heating method according to the ratio. Although this method eliminates the inefficiency of the traditional "stir-fry" method of mixing different molten salt systems to prepare molten salt, it focuses on the design of ternary chloride molten salt materials and does not take into account the minimum eutectic temperature range of ternary or higher multi-component chloride molten salts.

[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 novel multi-component chloride molten salt heat transfer and heat storage materials, and also to provide guidance for the optimized design of heat transfer and heat storage systems. Summary of the Invention

[0008] The purpose of this invention is to provide a method for determining the lowest eutectic temperature range of multi-component chloride molten salts, so as to determine the lowest eutectic temperature range and corresponding composition ratio of multi-component chloride molten salts, provide a reliable reference for the design of novel multi-component chloride molten salt heat transfer and heat storage materials, and also provide guidance for the optimized design of heat transfer and heat storage systems.

[0009] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0010] A method for determining the lowest eutectic temperature range of a multi-component chloride molten salt includes the following steps:

[0011] S1. Select a thermodynamic model based on the multi-component chloride molten salt system, and optimize the phase equilibrium parameters of the thermodynamic model based on the thermodynamic parameters of each unit salt in the multi-component chloride molten salt system to obtain the optimized thermodynamic model.

[0012] S2. Based on the optimized thermodynamic model, calculate the minimum Gibbs free energy of the multi-component chloride molten salt system, obtain the solid-liquid phase diagram of the multi-component chloride molten salt system, and based on the solid-liquid phase diagram, obtain the first simulated minimum eutectic point when the proportion of the specified unit salt in the multi-component chloride molten salt system is the first set of preset values.

[0013] S3. Based on the first simulated minimum eutectic point and the first set of preset values, determine the initial minimum eutectic temperature range and the initial proportion range corresponding to the specified unit salt. Based on the solid-liquid phase diagram, obtain the second simulated minimum eutectic point when the proportion of the specified unit salt in the multi-component chloride molten salt system is the second set of preset values, wherein the second set of preset values ​​is located within the initial proportion range.

[0014] S4. Based on the second simulated minimum eutectic point and the second set of preset values, select the proportion of the specified unit salt corresponding to the preset temperature value higher than the second simulated minimum eutectic point as the correction proportion range, and use the temperature range corresponding to the correction proportion range as the correction minimum eutectic temperature range.

[0015] S5. Within the modified minimum eutectic temperature range, arbitrarily select two simulated eutectic temperature points, obtain the proportion of each unit salt in the multi-component chloride 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. By comparing the two simulated eutectic temperature points and the two measured eutectic temperature points respectively, and combining them with the solid-liquid phase diagram, the final lowest eutectic temperature range of the multi-component chloride molten salt system is obtained.

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

[0018] Preferably, in S1, the thermodynamic model is a modified chemical solution-like thermodynamic model selected based on the strong interaction forces between ions in the multi-component chloride molten salt system.

[0019] Preferably, S1 specifically includes:

[0020] A thermodynamic model was established based on the multi-component chloride molten salt system. The enthalpy, entropy, and specific heat capacity of each unit salt in the solid, liquid, and gas phases of the multi-component chloride molten salt system were 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 multi-component chloride molten salt system is closest to the experimental data in the literature, thus obtaining the optimized thermodynamic model.

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

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

[0023] Preferably, step S5 specifically includes:

[0024] 1) Select any two simulated eutectic temperature points within the modified minimum eutectic temperature range, and obtain the proportion of each unit salt in the multi-component chloride molten salt system corresponding to the two simulated eutectic temperature points;

[0025] 2) Weigh the corresponding unit salts according to the proportion of each unit salt in the multi-component chloride molten salt system, mix and melt each unit salt using the static melting method, and after cooling, obtain the heating curve of the multi-component chloride molten salt by DSC test, and obtain two measured eutectic temperature points corresponding to the two simulated eutectic temperature points through the heating curve.

[0026] Preferably, the corresponding unit salts are weighed according to the proportion of each unit salt in the multi-component chloride molten salt system, and the unit salts are mixed and melted using a static melting method, followed by cooling. Specifically, this includes:

[0027] According to the proportion of each unit salt in the multi-component chloride molten salt system, weigh the corresponding unit salt, dry each unit salt at 393K for 24h, then mix each unit salt using the static melting method and heat to the target temperature, hold for 10h, cool and solidify, and grind into powdered multi-component chloride molten salt; wherein, the heating rate is 5K / min.

[0028] Preferably, the heating curve of the multi-component chloride molten salt is obtained by DSC testing, and two measured eutectic temperatures corresponding to two simulated eutectic temperatures are obtained from the heating curve, specifically including:

[0029] 1) Place the powdered multi-chloride molten salt sample in the platinum crucible of the DSC test equipment, and use argon as the purge gas and protective gas. The flow rate of argon is 30-50 mL / min.

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

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

[0032] 4) Repeat step 3) to obtain the second DSC heat flux curve;

[0033] 5) Based on the first DSC heat flow curve and the second DSC heat flow curve, obtain two measured eutectic temperatures corresponding to the two simulated eutectic temperatures.

[0034] Preferably, step S6 specifically includes: comparing two simulated eutectic temperature points and two measured eutectic temperature points respectively, 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 multi-component chloride molten salt system.

[0035] The beneficial effects of this invention are:

[0036] This invention discloses a method for determining the lowest eutectic temperature range of a multi-component chloride molten salt. The method involves optimizing the phase equilibrium parameters of a thermodynamic model based on the thermodynamic parameters of each unit salt in the multi-component chloride molten salt system to obtain an optimized thermodynamic model. A solid-liquid phase diagram of the multi-component chloride molten salt system is obtained through the optimized thermodynamic model. Then, by setting a fixed set of component proportions for a certain unit salt in the multi-component chloride molten salt system, the lowest eutectic point of the multi-component chloride molten salt system when the component proportions of the certain unit salt are fixed is calculated using FactSage software. After selecting an initial lowest eutectic temperature range using the lowest eutectic point and a set of fixed values, another set of fixed values ​​is set as the component proportion of a certain unit salt. The lowest eutectic point of the multi-component chloride molten salt system when the component proportions of the certain unit salt are fixed is calculated again using FactSage software, and a modified lowest eutectic temperature range is selected. Random selection is then performed within the modified lowest eutectic temperature range. Two temperature points were selected, and the composition ratio of each unit salt in the multi-component chloride molten salt system at these temperatures was obtained. Molten salt samples of the multi-component chloride molten salt system were prepared using a static melting method, and the melting point of the molten salt samples was measured using a DSC testing device. By comprehensively comparing simulated and experimental values ​​and considering the error range, the lowest eutectic temperature range of the multi-component chloride molten salt system was finally determined. This invention combines solid-liquid phase diagram calculation methods with experimental means to determine the lowest eutectic temperature range and corresponding composition ratio of the multi-component chloride molten salt system. This method can ensure the accuracy of the lowest eutectic temperature range of the multi-component chloride molten salt system and reduce the research and development cost of molten salt materials. It provides a reliable reference for the design of novel multi-component chloride molten salt heat transfer and heat storage materials, and also provides guidance for the optimized design of heat transfer and heat storage systems. It has promotional and application value in the field of multi-component chloride molten salt system technology. Attached Figure Description

[0037] Figure 1 This is a flowchart of the method for determining the lowest eutectic temperature range of multi-component chloride molten salts according to the present invention;

[0038] Figure 2 The graph shows the variation of the lowest eutectic point of the quaternary chloride molten salt KCl-NaCl-CaCl2-MgCl2 with a fixed proportion of MgCl2.

[0039] Figure 3 The solid-liquid phase diagram of KCl-NaCl-CaCl2-MgCl2 at 0.33 mol% is shown.

[0040] Figure 4 The solid-liquid phase diagram of KCl-NaCl-CaCl2-MgCl2 at 0.36 mol% is shown.

[0041] Figure 5DSC heat flow curves for KCl-NaCl-CaCl2-MgCl2 (0.2143 mol% - 0.2924 mol% - 0.1633 mol% - 0.33 mol%);

[0042] Figure 6 DSC heat flow curves for KCl-NaCl-CaCl2-MgCl2 (0.23 mol% - 0.2672 mol% - 0.1428 mol% - 0.36 mol%);

[0043] Figure 7 The variation of the lowest eutectic point of the KCl-NaCl-CaCl2-LiCl quaternary chloride molten salt with a fixed proportion of LiCl;

[0044] Figure 8 The solid-liquid phase diagram of KCl-NaCl-CaCl2-LiCl at 0.46 mol% is shown.

[0045] Figure 9 The solid-liquid phase diagram of KCl-NaCl-CaCl2-LiCl at 0.50 mol% is shown.

[0046] Figure 10 DSC heat flow curves for KCl-NaCl-CaCl2-LiCl (0.3891 mol% - 0.1073 mol% - 0.0436 mol% - 0.46 mol%);

[0047] Figure 11 DSC heat flow curves for KCl-NaCl-CaCl2-LiCl (0.3956 mol% - 0.0639 mol% - 0.0405 mol% - 0.50 mol%). Detailed Implementation

[0048] The embodiments of the present invention will be described below 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 content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed 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 and 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 representations of the basic concept of the present invention. Therefore, the drawings only show the components related to the present invention and are not drawn according to the actual number, shape and size of the components in the actual implementation. In the actual implementation, the form, quantity and proportion of each component can be arbitrarily changed, and the layout of the components may also be more complex.

[0050] Example 1

[0051] like Figure 1 As shown, a method for determining the lowest eutectic temperature range of a multi-component chloride molten salt is described. In this embodiment, the multi-component chloride molten salt is a KCl-NaCl-CaCl2-MgCl2 quaternary chloride molten salt system. The determination method includes the following steps:

[0052] S1. Select a thermodynamic model based on the multi-component chloride molten salt system, and optimize the phase equilibrium parameters of the thermodynamic model based on the thermodynamic parameters of each unit salt in the multi-component chloride molten salt system to obtain the optimized thermodynamic model, specifically including:

[0053] S11. Based on the strong interionic interaction characteristics of 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, liquid, and gas phases, respectively, from the molten salt phase diagram database. Use FactSage phase diagram software to optimize the phase equilibrium parameters of molar nonconfiguration entropy change and molar enthalpy change 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. Based on the optimized thermodynamic model, calculate the minimum Gibbs free energy of the multi-component chloride molten salt system, obtain the solid-liquid phase diagram of the multi-component chloride molten salt system, and based on the solid-liquid phase diagram, obtain the first simulated minimum eutectic point when the proportion of a specified unit salt in the multi-component chloride molten salt system is the first set of preset values, specifically including:

[0056] S21. Based on the optimized thermodynamic model, calculate the minimum Gibbs free energy of the KCl-NaCl-CaCl2-MgCl2 quaternary chloride molten salt system and obtain the solid-liquid phase diagram of the KCl-NaCl-CaCl2-MgCl2 quaternary chloride molten salt system.

[0057] S22. Set the percentage 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 ​​are 0.05 mol%, 0.10 mol%, 0.15 mol%, 0.20 mol%, 0.25 mol%, 0.30 mol%, 0.35 mol%, 0.40 mol%, 0.45 mol%, 0.50 mol%, 0.55 mol%, 0.60 mol%, 0.65 mol%, 0.70 mol%, 0.75 mol%, 0.80 mol%, 0.85 mol%, 0.90 mol%, and 0.95 mol%.

[0058] S23. Based on the solid-liquid phase diagram in S21, obtain the first simulated minimum eutectic point when the proportion of MgCl2 is the first set of preset values. In this embodiment, the minimum 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, as shown below. Figure 2 As shown, from Figure 2 It was found that when the proportion of MgCl2 was between 0.30 mol% and 0.45 mol%, the lowest eutectic point appeared, which is the first simulated lowest eutectic point;

[0059] In this embodiment, the percentage represents the molar mass percentage;

[0060] S3. Based on the first simulated minimum eutectic point and the first set of preset values, determine the initial minimum eutectic temperature range and the initial proportion range corresponding to the specified unit salt. Based on the solid-liquid phase diagram, obtain the second simulated minimum eutectic point when the proportion of the specified unit salt in the multi-component chloride molten salt system is the second set of preset values. The second set of preset values ​​is located within the initial proportion range and specifically includes:

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

[0062] S32. Select a second set of preset values ​​within the initial percentage range of 0.30 mol% to 0.45 mol%. In this embodiment, the second set of preset values ​​are 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%, and 0.445 mol%.

[0063] S33. Based on the solid-liquid phase diagram in S21, obtain the second simulated minimum eutectic point when the proportion of MgCl2 is the second set of preset values. In this embodiment, the minimum eutectic point variation 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, as shown below. Figure 2 As shown, from Figure 2 It was found that the lowest eutectic point of 635.28 K, which is the second simulated lowest eutectic point, occurs when the proportion of MgCl2 is 0.360 mol%.

[0064] S4. Based on the second simulated minimum eutectic point and the second set of preset values, select the proportion of the specified unit salt corresponding to the preset temperature value higher than the second simulated minimum eutectic point as the correction proportion range, and use the temperature range corresponding to the correction proportion range as the correction minimum eutectic temperature range, specifically including:

[0065] Since the second simulation's lowest eutectic point is 635.28 K, the proportion of the specified unit salt corresponding to a value about 10 K higher than 635.28 K is 0.33 mol% to 0.425 mol%, which is the corrected proportion range. The temperature range of 644.29 K to 646.55 K corresponding to the corrected proportion range of 0.33 mol% to 0.425 mol% is taken as the corrected lowest eutectic temperature range.

[0066] Since there is no clear minimum eutectic point for multi-component chloride molten salts with ternary or higher components in actual working conditions, and the temperature around 10K near the minimum eutectic point is within the error range, the temperature range corresponding to the correction ratio range that is about 10K higher than 635.28K is selected as the correction minimum eutectic temperature range. That is, in this embodiment, the correction minimum eutectic temperature range is 644.29K to 646.55K.

[0067] S5. Within the corrected minimum eutectic temperature range, arbitrarily select two simulated eutectic temperature points, obtain the proportion of each unit salt in the multi-component 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. Within the modified minimum eutectic temperature range, two simulated eutectic temperature points are randomly selected. In this embodiment, 644.29K and 635.28K are selected as simulated eutectic temperature points, corresponding to MgCl2 proportions of 0.33mol% and 0.36mol%. Under these proportions, the solid-liquid phase diagrams of the KCl-NaCl-CaCl2-MgCl2 quaternary chloride molten salt are as follows: Figure 3 and Figure 4 As shown in Table 1, the molar mass percentages of each unit salt corresponding to the KCl-NaCl-CaCl2-MgCl2 quaternary chloride molten salt at two simulated eutectic temperature points are obtained.

[0069] S52. Based on the composition ratio of each unit salt in the two groups of KCl-NaCl-CaCl2-MgCl2 quaternary chloride molten salts, prepare molten salt samples. The unit salt reagents used need to be vacuum dried at 393K for 24 hours in advance. Because the calculated molar mass ratio of the components corresponding to the simulated eutectic temperature point is retained to four decimal places, due to the limitation of the accuracy of the electronic balance in actual sample weighing, the ratio is approximately retained to two decimal places and then converted into a mass ratio. After weighing in an argon atmosphere glove box, place each unit molten salt sample in a corundum crucible and stir thoroughly to mix evenly. Then, put it into a muffle furnace and heat the molten salt sample to 800K at a heating rate of 5K / min. Hold it at this temperature for 10 hours to ensure that the molten salt is fully melted. Finally, pour out the molten salt sample in liquid form, wait for it to solidify, crush and grind it into powder, and store it in a vacuum seal after natural cooling for later use.

[0070] S53. Take 8-10 mg of powdered multi-chloride molten salt sample and place it in the platinum crucible of the DSC test equipment. Ensure that the purge gas and protective gas are both high-purity argon gas with a flow rate of 30-50 mL / min. Heat the multi-chloride molten salt sample from room temperature to 973 K at a rate of 25 K / min and hold it at that temperature for 1 minute. Then repeat the following steps twice: cool down to 623 K at a rate of 25 K / min and hold it at that temperature for 3 minutes, then heat up to 973 K at a rate of 10 K / min and hold it at that temperature for 1 minute. Obtain two DSC heat flux curves as experimental results.

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

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

[0073] The DSC heat flux curves for (0.23 mol% - 0.2672 mol% - 0.1428 mol% - 0.36 mol%) are shown below. Figure 5 and Figure 6 As shown;

[0074] The melting point is the temperature corresponding to the onset of the endothermic melting peak. Figure 5 I learned from

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

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

[0077] S6. By comparing the two simulated eutectic temperature points and the two measured eutectic temperature points respectively, and combining them with the solid-liquid phase diagram, the final lowest eutectic temperature range of the multi-component chloride molten salt system is obtained, specifically including:

[0078] Comparing the simulated eutectic temperature of 644.29 K with the measured eutectic temperature of 657.45 K, the relative error was calculated to be 2.04%; comparing the simulated eutectic temperature of 635.28 K with the measured eutectic temperature of 655.65 K, the relative error was calculated to be 3.21%. Therefore, it can be seen 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 are very close in the two composition ratios mentioned above. The results can well reflect the existence of the lowest eutectic temperature range. Therefore, the lowest eutectic temperature range of the KCl-NaCl-CaCl2-MgCl2 quaternary chloride molten salt system is finally determined to be 650 K to 660 K.

[0079] Table 1 shows the simulated eutectic temperatures and the molar mass percentage of each unit salt in the quaternary chloride molten salt system.

[0080]

[0081] Example 2

[0082] like Figure 1As shown, a method for determining the lowest eutectic temperature range of a multi-component chloride molten salt is described. In this embodiment, the multi-component chloride molten salt is a KCl-NaCl-CaCl2-LiCl quaternary chloride molten salt system. The determination method includes the following steps:

[0083] S1. Select a thermodynamic model based on the multi-component chloride molten salt system, and optimize the phase equilibrium parameters of the thermodynamic model based on the thermodynamic parameters of each unit salt in the multi-component chloride molten salt system to obtain the optimized thermodynamic model, specifically including:

[0084] S11. Based on the strong interionic interaction characteristics of the KCl-NaCl-CaCl2-LiCl quaternary chloride molten salt system, a modified quasi-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, liquid, and gas phases, respectively, from the molten salt phase diagram database. Use FactSage phase diagram software to optimize the phase equilibrium parameters of molar nonconfiguration entropy change and molar enthalpy change 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. Based on the optimized thermodynamic model, calculate the minimum Gibbs free energy of the multi-component chloride molten salt system, obtain the solid-liquid phase diagram of the multi-component chloride molten salt system, and based on the solid-liquid phase diagram, obtain the first simulated minimum eutectic point when the proportion of a specified unit salt in the multi-component chloride molten salt system is the first set of preset values, specifically including:

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

[0088] S22. Set 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 ​​are 0.05 mol%, 0.10 mol%, 0.15 mol%, 0.20 mol%, 0.25 mol%, 0.30 mol%, 0.35 mol%, 0.40 mol%, 0.45 mol%, 0.50 mol%, 0.55 mol%, 0.60 mol%, 0.65 mol%, 0.70 mol%, 0.75 mol%, 0.80 mol%, 0.85 mol%, 0.90 mol%, and 0.95 mol%.

[0089] S23. Based on the solid-liquid phase diagram in S21, obtain the first simulated minimum eutectic point when the proportion of LiCl is the first set of preset values. In this embodiment, the minimum 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, as shown below. Figure 7 As shown, from Figure 7 It was found that the lowest eutectic point, i.e. the first simulated lowest eutectic point, appeared when the proportion of LiCl was between 0.45 mol% and 0.55 mol%.

[0090] In this embodiment, the percentage represents the molar mass percentage;

[0091] S3. Based on the first simulated minimum eutectic point and the first set of preset values, determine the initial minimum eutectic temperature range and the initial proportion range corresponding to the specified unit salt. Based on the solid-liquid phase diagram, obtain the second simulated minimum eutectic point when the proportion of the specified unit salt in the multi-component chloride molten salt system is the second set of preset values. The second set of preset values ​​is located within the initial proportion range and specifically includes:

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

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

[0094] S33. Based on the solid-liquid phase diagram in S21, obtain the second simulated minimum eutectic point when the proportion of LiCl is the second set of preset values. In this embodiment, the minimum eutectic point variation 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, as shown below. Figure 7 As shown, from Figure 7 It was found that the lowest eutectic point of 610.92 K, which is the second simulated lowest eutectic point, occurs when the proportion of LiCl is 0.500 mol%.

[0095] S4. Based on the second simulated minimum eutectic point and the second set of preset values, select the proportion of the specified unit salt corresponding to the preset temperature value higher than the second simulated minimum eutectic point as the correction proportion range, and use the temperature range corresponding to the correction proportion range as the correction minimum eutectic temperature range, specifically including:

[0096] Since the second simulation's lowest eutectic point is 610.92 K, the left endpoint of the proportion of the specified unit salt corresponding to a value about 10 K higher than 610.92 K is 0.46 mol%, which is the left endpoint of the corrected proportion range. Since there is no lowest eutectic point after the proportion of LiCl exceeds 0.528 mol%, 0.528 mol% is selected as the right endpoint of the corrected proportion range. The temperature range of 623.88 K to 613.90 K corresponding to the corrected proportion range of 0.46 mol% to 0.528 mol% is used as the corrected lowest eutectic temperature range.

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

[0098] S5. Within the corrected minimum eutectic temperature range, arbitrarily select two simulated eutectic temperature points, obtain the proportion of each unit salt in the multi-component 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. Within the modified minimum eutectic temperature range, two simulated eutectic temperature points are randomly selected. In this embodiment, 623.88K and 610.92K are selected as simulated eutectic temperature points, corresponding to LiCl proportions of 0.46 mol% and 0.50 mol% respectively. The solid-liquid phase diagrams of the KCl-NaCl-CaCl2-LiCl quaternary chloride molten salt at these proportions are shown below. Figure 8 and Figure 9 As shown in Table 2, the molar mass percentages of each unit salt corresponding to the KCl-NaCl-CaCl2-LiCl quaternary chloride molten salt at two simulated eutectic temperature points are obtained.

[0100] S52. Prepare molten salt samples according to the composition ratio of each unit salt in the two groups of KCl-NaCl-CaCl2-LiCl quaternary chloride molten salts. The unit salt reagents used need to be vacuum dried at 393K for 24 hours in advance. Since the calculation results of the molar mass ratio of the components corresponding to the simulated eutectic temperature point are retained to four decimal places, the actual sample weighing is limited by the accuracy of the electronic balance. Therefore, the ratio is approximately retained to two decimal places and then converted into a mass ratio. After weighing in an argon atmosphere glove box, each unit molten salt sample is placed in a corundum crucible and stirred thoroughly to ensure uniform mixing. The sample is then placed in a muffle furnace and heated to 800K at a heating rate of 5K / min. The temperature is maintained for 10 hours to ensure that the molten salt is fully melted. Finally, the molten salt sample is poured out in liquid form. After solidification, it is crushed and ground into powder. After natural cooling, it is vacuum sealed and stored for later use.

[0101] S53. Take 8-10 mg of powdered multi-chloride molten salt sample and place it in the platinum crucible of the DSC test equipment. Ensure that the purge gas and protective gas are both high-purity argon gas with a flow rate of 30-50 mL / min. Heat the multi-chloride molten salt sample from room temperature to 973 K at a rate of 25 K / min and hold it at that temperature for 1 minute. Then repeat the following steps twice: cool down to 623 K at a rate of 25 K / min and hold it at that temperature for 3 minutes, then heat up to 973 K at a rate of 10 K / min and hold it at that temperature for 1 minute. Obtain two DSC heat flux curves as experimental results.

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

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

[0104] The DSC heat flux curves for (0.3956 mol% - 0.0639 mol% - 0.0405 mol% - 0.50 mol%) are shown below. Figure 10 and Figure 11 As shown;

[0105] The melting point is the temperature corresponding to the onset of the endothermic melting peak. Figure 10 The melting point of KCl-NaCl-CaCl2-LiCl (0.3891 mol% - 0.1073 mol% - 0.0436 mol% - 0.46 mol%) is 631.37 K. Figure 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. By comparing the two simulated eutectic temperature points and the two measured eutectic temperature points respectively, and combining them with the solid-liquid phase diagram, the final lowest eutectic temperature range of the multi-component chloride molten salt system is obtained, specifically including:

[0107] Comparing the simulated eutectic temperature of 623.88 K with the measured eutectic temperature of 631.37 K, the relative error was calculated to be 1.20%; comparing the simulated eutectic temperature of 610.92 K with the measured eutectic temperature of 628.69 K, the relative error was calculated to be 2.91%. Therefore, it can be seen 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 are very close in the two composition ratios mentioned above. The results can well reflect the existence of the lowest eutectic temperature range. Therefore, the lowest eutectic temperature range of the KCl-NaCl-CaCl2-LiCl quaternary chloride molten salt system is finally determined to be 620 K to 630 K.

[0108] Table 2 shows the simulated eutectic temperatures and the molar mass percentage of each unit salt in the quaternary chloride molten salt system.

[0109]

[0110]

[0111] In summary, the method for determining the lowest eutectic temperature range of a multi-component chloride molten salt according to the present invention involves optimizing the phase equilibrium parameters of a thermodynamic model based on the thermodynamic parameters of each unit salt in the multi-component chloride molten salt system to obtain an optimized thermodynamic model; obtaining the solid-liquid phase diagram of the multi-component chloride molten salt system through the optimized thermodynamic model; then, by setting a set of fixed values ​​for the component proportion of a certain unit salt in the multi-component chloride molten salt system, calculating the lowest eutectic point of the multi-component chloride molten salt system when the component proportion of the certain unit salt is fixed using FactSage software; selecting an initial lowest eutectic temperature range using the lowest eutectic point and a set of fixed values; then setting another set of fixed values ​​as the component proportion of a certain unit salt, calculating the lowest eutectic point of the multi-component chloride molten salt system again when the component proportion of the certain unit salt is fixed using FactSage software, and selecting a modified lowest eutectic temperature range; and finally, determining the modified lowest eutectic temperature range. Two temperature points are randomly selected within the melting temperature range, and the composition ratio of each unit salt in the multi-component chloride molten salt system at these temperatures is obtained. Molten salt samples of the multi-component chloride molten salt system are prepared using a static melting method, and the melting point of the molten salt samples is measured using a DSC testing device. By comprehensively comparing simulated and experimental values ​​and considering the error range, the lowest eutectic temperature range of the multi-component chloride molten salt system is finally determined. This invention combines calculation and experimental methods to determine the lowest eutectic temperature range and corresponding composition ratio of the multi-component chloride molten salt system. This method can ensure the accuracy of the lowest eutectic temperature range of the multi-component chloride molten salt system and reduce the research and development cost of molten salt materials. It provides a reliable reference for the design of novel multi-component chloride molten salt heat transfer and heat storage materials, and also provides guidance for the optimized design of heat transfer and heat storage systems. It has promotional and application value in the field of multi-component chloride molten salt system technology.

[0112] The above embodiments are merely preferred embodiments provided to fully illustrate the present invention, and the scope of protection of the present invention is not limited thereto. Equivalent substitutions or modifications made by those skilled in the art based on the present invention are all within the scope of protection of the present invention.

Claims

1. A method for determining the lowest eutectic temperature range of a multi-component chloride molten salt, characterized in that, Includes the following steps: S1. Select a thermodynamic model based on the multi-component chloride molten salt system, and optimize the phase equilibrium parameters of the thermodynamic model based on the thermodynamic parameters of each unit salt in the multi-component chloride molten salt system to obtain the optimized thermodynamic model. S2. Based on the optimized thermodynamic model, calculate the minimum Gibbs free energy of the multi-component chloride molten salt system, obtain the solid-liquid phase diagram of the multi-component chloride molten salt system, and based on the solid-liquid phase diagram, obtain the first simulated minimum eutectic point when the proportion of the specified unit salt in the multi-component chloride molten salt system is the first set of preset values. S3. Based on the first simulated minimum eutectic point and the first set of preset values, determine the initial minimum eutectic temperature range and the initial proportion range corresponding to the specified unit salt. Based on the solid-liquid phase diagram, obtain the second simulated minimum eutectic point when the proportion of the specified unit salt in the multi-component chloride molten salt system is the second set of preset values, wherein the second set of preset values ​​is located within the initial proportion range. S4. Based on the second simulated minimum eutectic point and the second set of preset values, select the proportion of the specified unit salt corresponding to the preset temperature value higher than the second simulated minimum eutectic point as the correction proportion range, and use the temperature range corresponding to the correction proportion range as the correction minimum eutectic temperature range. S5. Within the modified minimum eutectic temperature range, arbitrarily select two simulated eutectic temperature points, obtain the proportion of each unit salt in the multi-component chloride molten salt system corresponding to the two simulated eutectic temperature points, and obtain the corresponding two measured eutectic temperature points through DSC testing. S6. By comparing the two simulated eutectic temperature points and the two measured eutectic temperature points respectively, and combining them with the solid-liquid phase diagram, the final lowest eutectic temperature range of the multi-component chloride molten salt system is obtained.

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

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

4. The method for determining the lowest eutectic temperature range of a multi-component chloride molten salt according to claim 1, characterized in that, Specifically, S1 includes: A thermodynamic model was established based on the multi-component chloride molten salt system. The enthalpy, entropy, and specific heat capacity of each unit salt in the solid, liquid, and gas phases of the multi-component chloride molten salt system were 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 multi-component chloride molten salt system is closest to the experimental data in the literature, thus obtaining the optimized thermodynamic model.

5. The method for determining the lowest eutectic temperature range of a multi-component chloride molten salt according to claim 1, characterized in that, The difference between adjacent preset values ​​in the second set of preset values ​​is less 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 multi-component chloride molten salt according to claim 1, characterized in that, In step S4, the preset temperature value is 9~11K.

7. The method for determining the lowest eutectic temperature range of a multi-component chloride molten salt according to claim 1, characterized in that, Specifically, S5 includes: 1) Select any two simulated eutectic temperature points within the modified minimum eutectic temperature range, and obtain the proportion of each unit salt in the multi-component chloride molten salt system corresponding to the two simulated eutectic temperature points; 2) Weigh the corresponding unit salts according to the proportion of each unit salt in the multi-component chloride molten salt system, mix and melt each unit salt using the static melting method, and after cooling, obtain the heating curve of the multi-component chloride molten salt by DSC test, and obtain two measured eutectic temperature points corresponding to the two simulated eutectic temperature points through the heating curve.

8. The method for determining the lowest eutectic temperature range of a multi-component chloride molten salt according to claim 7, characterized in that, According to the proportion of each unit salt in the multi-component chloride molten salt system, the corresponding unit salts are weighed, and the unit salts are mixed and melted using a static melting method, followed by cooling. Specifically, this includes: According to the proportion of each unit salt in the multi-component chloride molten salt system, weigh the corresponding unit salt, dry each unit salt at 393K for 24h, then mix each unit salt using the static melting method and heat to the target temperature, hold for 10h, cool and solidify, and grind into powdered multi-component chloride molten salt; wherein, the heating rate is 5K / min.

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

10. The method for determining the lowest eutectic temperature range of a multi-component chloride molten salt according to claim 1, characterized in that, S6 specifically includes: comparing two simulated eutectic temperature points and two measured eutectic temperature points respectively, 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 multi-component chloride molten salt system.

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