Using Molten Salts as a Heat Transfer Fluid and Thermal-Storage Medium
One of the most commonly used molten salts is a eutectic blend of sodium nitrate and potassium nitrate. Why use molten salts? They extend the upper range of the temperature limits in which a liquid heat transfer media can be used.
Many different types of heat transfer fluids can be used in process heating applications. How do engineers identify the type of heat transfer fluid that is the best for their system?
Design parameters such as heat transfer efficiency, pumping power and operational life must be taken into account. Each will factor into the performance of the fluid and operating expenses. Also, safety parameters such as pressure buildup, flashpoint, flammability and toxicity cannot be overlooked. With these concerns in mind, an engineer can pick from a selection of water-based heat transfer fluids, oil-based heat transfer fluids and even high temperature molten salts.
Water-based fluids generally will perform the best due to their excellent thermophysical properties. Mineral oils and synthetic oils generally are used at slightly higher temperatures — in the range of 212 to 662°F (100 to 350°C) — where the oil will not degrade as readily as a glycol, and the system pressure will be kept low. However, above 662°F, the options for using a hydrocarbon heat transfer fluid become limited. Natural oils or mineral oils degrade near this temperature and may need to be replaced regularly.
Synthetic hydrocarbon heat transfer fluids such as diphenyl/diphenyl oxide can be used at temperatures up to 734°F (390°C). However, these synthetic fluids can be expensive and will undergo some degree of degradation over time. This can lead to fouling of heat exchange equipment and loss of efficiency. In addition, the degradation byproducts can be toxic.
For operating temperatures higher than 734°F (390°C) that require a liquid heat transfer media, a molten salt fluid must be used.
What is a Molten Salt?
Whenever most people think of salt, they think of common table salt spread on food. In theory, you could use table salt — sodium chloride — as a heat transfer medium, but you would end up with corrosion problems due to the aggressive chloride ion. There are many types of salt such as chlorides, fluorides, bromides, nitrates and organic salts. The molten salts most commonly used in heat transfer applications are the nitrate salts due to their low melting point, high operating temperature, thermophysical properties, low toxicity, low vapor pressure and corrosion performance.
The most common nitrate molten salts generally are a eutectic blend of sodium nitrate and potassium nitrate. Individual salts can be used, but by combining the two different salts together, the melting point is reduced. This allows for lower minimum operating temperatures and thus reduces the chance of freezeup. Sodium nitrate has a melting point of 584°F (307°C), and potassium nitrate has a melting point of 631°F (333°C). By mixing the two together, however, you can create a salt blend with a melting point of 431°F (222°C). This significantly expands the operational flexibility of the salt used in high temperature applications.
What Is the Temperature Range at Which a Molten Salt Can Operate?
Many different salt blends can be created to achieve certain operating temperatures. The sodium nitrate/potassium nitrate salt generally is used from 545 to 1049°F (285 to 565°C) to provide enough safety factor above the freezing point. This salt will begin to experience degradation when it reaches temperatures above 1049°F (565°C). Above that point, it will slowly degrade into nitrogen and nitrogen oxides. An advantage of molten salts over hydrocarbon fluids is that they experience almost negligible vapor pressure even near the peak operating temperatures (figure 1). This significantly reduces the need for high pressure piping and equipment.
The ternary lithium nitrate/sodium nitrate/potassium nitrate mixture is another option, with the eutectic melting temperature occurring at 266°F (130°C). The disadvantage of this mixture — compared to the binary mixture — is that it can only be used to a maximum temperature near 932°F (500°C). Above 932°F, the lithium nitrate begins to degrade. Also, with the advent of lithium-ion batteries, lithium nitrate has become expensive compared to sodium or potassium salts.
When using a molten salt, it is recommended that the fluid temperature be at least 36 to 54°F (20 to 30°C) above the melting point. This reduces the risk of freezeups where there may be cold spots. Also, it is recommended to keep the maximum operating temperature of the salt at least 36°F (20°C) below the degradation temperature. Generally, nitrate salts provide a balance of wide operating range, thermal stability, corrosion performance and cost.
For applications that need a liquid heat transfer media above 1112°F (600°C), the options are limited to chloride and fluoride salts. These salts are more stable than the nitrate salts. Chlorides and fluorides can operate to slightly higher temperatures — near 1652°F (900°C). These salts also have higher melting points of 572 to 932°F (300 to 500°C), which increases the risk of the salt freezing. In addition, chlorides and fluorides are extremely corrosive, especially at high temperatures. As a result, they require expensive alloy materials of construction.
One advantage of the nitrate salts is corrosion performance (figure 2). Nitrate salts are compatible with stainless steels up to 1049°F (565°C) and even carbon steel if the operating temperature is kept below 752°F (400°C). This makes them advantageous to use with common materials of construction to keep costs low. It is not advised to use yellow metals such as copper or brass with molten salts because they are not as compatible as steel at higher temperatures.
Molten Salts and Conventional Heat Transfer Fluids
Oil-based heat transfer fluids are relatively easy to use because they are liquid at room temperature. Synthetic fluids generally have maximum operating temperatures that limit them to 662°F (350°C) before undergoing degradation. Even below this temperature, oils can degrade and may need to be replaced after several years of use. An additional consequence of this degradation is that oil will eventually break down into carbon or polymerize. This can lead to fouling of the heat exchange equipment or sensitive electronics.
Molten salts can be used at higher temperatures compared to synthetic oils or silicone fluids, possess good heat transfer properties and are thermally stable (table 1). The main disadvantage of molten salts is that they will freeze at temperatures (248 to 428°F [120 to 220°C]) well above room temperature. Potentially, this can create significant risks for systems that are not designed with this in mind.
Applications that use molten salts require heat tracing in all piping and equipment to prevent the salt from freezing. Also, salts expand when they re-melt. If the salt is not heated uniformly during the re-melting process, it can damage piping, valves and other equipment. As a result, when using a molten salt, it is essential to take the necessary precautions to prevent the salt from freezing during operation.
Design Considerations for Molten Salts
When designing a high temperature system that requires some form of heat transfer media, it is important to balance the pros and cons of using a synthetic oil or molten salt. Synthetic oils are generally easier to use because they can be charged into a system and circulated readily. Molten salts are more challenging to use due to the melting and freezing. With these concerns in mind, as long as the system is designed with proper heat trace, molten salts will provide excellent heat transfer and minimal degradation throughout the lifetime of your system.
Nitrate salts usually are manufactured in prill form (small beads) and may arrive in 2,000-lb super sacks or smaller packages. For smaller systems, the salt can be added directly after opening the container, ensuring the salt is spread out uniformly for better heat distribution. As the salt melts, its volume from crystalline prill form to liquid form will reduce by about 50 percent (due to the air spaces between the prills). If the melting tank is large enough, the salt can be added all at once. If there are some space restrictions, the salt can be melted and added in several increments. For much larger systems, the salt may be emptied and loaded onto conveyor systems, where it is deposited inside the large tank as the salt melts.
Nitrate molten salts are stable and have low toxicity, but they are oxidizers. In the presence of fire or open flames, they can break down and liberate oxygen, providing a fuel source for the flame. Molten nitrate salts should be kept away from open flame, sparks and other sources of ignition. If proper precautions are taken, the salts should not pose any hazards.
Molten salts are used in applications ranging from high temperature circulating systems to thermal storage and phase-change systems. Molten salts often are used in concentrated solar power plants (CSP), where the salt can store heat obtained from sunlight for long periods of time. Some of these CSP systems flow the salt directly through the receiver tubing, where sunlight hits a parabolic mirror and concentrates the sunlight to a tube in which the salt flows. Other systems use a combination of a high temperature synthetic fluid with the salt storage. The synthetic fluid flows through the receiver tubing and transfers the heat into the molten salt storage tanks. This takes advantage of minimizing the risk of freezeup and also utilizing the salt’s excellent heat transfer properties.
Other systems may use molten salts for preheating natural gas lines or keeping reactors hot by using the molten salt in the jacket around a tank. Salts also have relatively high latent heat capacity — similar to many of the phase-change waxes that are used today — and are sometimes used in phase-change applications. When salts melt, they absorb large amounts of heat, and when they freeze, they release large amounts of heat. By utilizing this phenomenon, it is possible to create advanced thermal storage or temperature-regulating systems, depending on the process.