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Molten salts: Why they’re a great heat transfer medium

9 minutes for read

Molten salts are a novel heat transfer system whose potential is just beginning to be realised. 

They’re safe, stable, and efficient for higher temperature systems. Molten salts are also environmentally friendly and pose few health hazards. In this article, you can find out what molten salts are, how to use them, and why circular manufacturing companies like Contec use molten salts in their waste tire pyrolysis process.

What are molten salts?

Molten salts are a phase change material used to store thermal energy.

Phase change materials are solid at room temperatures and atmospheric pressure and turn into fluids when heated. Molten salts store the energy applied to convert them into liquids as latent heat, which they can transfer to other materials. Heat transfer, therefore, occurs in two directions. When heat is applied, the salts melt, and when heat is removed, the liquid solidifies again.

As phase change materials, molten salts have a higher latent heat capacity than conventional materials, and minimal temperature changes are needed to increase their heat capacity.

Molten salts are composed of nitrates, nitrites, carbonates, chlorides, and fluorides. Each of them has unique properties that make them useful for varying applications. But the common feature of all molten salts is their thermal stability at high temperatures.

What are molten salts used for?

Many molten salts remain liquids at 250 to 1000°C and have a low vapour pressure.

This property makes them suitable for applications where liquids at very high temperatures are necessary for heat storage or transfer.

Molten salts heated beyond their liquid temperature range degrade into gaseous components. Combining different salts can lower the melting points of the salts and increase the temperature range where they remain as liquids. Depending on the temperatures required and the applications, different mixtures of salts are used.

The common molten salts you find used as a heat transfer medium are a mixture of two salts—60 per cent sodium nitrate and 40 per cent potassium nitrate, which melt when heated at 220°C. They remain as liquids in the temperature range of 220-600°C and decompose into nitrogen and nitrogen oxides at temperatures over 600°C.

The most widespread use of molten salts is to store thermal energy in solar power plants. During the day, the excess solar heat not used for making electricity is sent to molten salts for storage. Solar plants use the heat stored in molten salts to produce steam and generate electricity overnight. Molten salt systems can increase the capacity of solar plants from 25 per cent to 70 per cent.

Molten salts are used as heat transfer systems, for process heating of waste tires, and in pyrolysis for material recovery. Molten salts are also commonly used for heating and quenching steel.

Using molten salts as a heat transfer fluid

Water and synthetic oils are common fluids currently used as heat transfer mediums. Some aspects to consider when choosing a material for high heat transfer are heat transfer efficiency, operational life, and pumping power. Vapour pressure buildup, flammability, and toxicity of materials are also crucial parameters as they can affect plant safety.

Considering all these selection criteria, molten salts can replace water and synthetic oils as heat transfer mediums.

Water is widely used for heat transfer because it has high thermal conductivity, density, latent energy, and moderate viscosity. But water can only transfer heat at temperatures less than 100°C because it produces high vapour pressure at its boiling point. 

Synthetic or thermal oils remain as liquids up to 300°C. While these are high temperatures, it’s not enough for the process heating of materials like waste tires. Moreover, the oils have low density and chemical stability; they’re also flammable and can cause high-pressure buildup, a safety risk.

Molten salts make good heat storage and transfer mediums because they have low viscosity, high thermal and electrical conductivity, and good chemical and thermal stability. Since they have a low vapour pressure, they’re also suitable for heat transfer where people want to avoid pressure buildup and reduce the need to use heavy piping.

Molten salts are chemically stable and environmentally friendly as they pose no safety risk. They’re also not toxic when spilled in small quantities.

However, there are a few disadvantages of molten salts.

Many molten salts are corrosive; among them, nitrates are the least corrosive. Molten salts freeze at the solidification temperature, higher than atmospheric temperatures. For the standard molten salt mixture of sodium nitrate and potassium nitrate, this temperature is as high as 220 to 240°C. Freezing occurs due to the development of cold spots because of uneven heating or on winter evenings. The resultant salt expansion damages piping and equipment.

Common types of molten salt systems

The three methods of using molten salts for heat transfer are salt baths, circulated molten salts, and direct heating.

  • Salt baths: The molten salts are in an open vessel, and heat transfer occurs through natural convection. Steel heating and quenching use this system, where objects to be heated is in contact with the molten salt mass.
  • Circulating molten salts: Molten salt systems are kept in circulation in a closed-loop as a heat medium for process heating or heat exchange. First, the salts are melted in a salt tank with the help of electricity or fuel. Specially designed pumps keep the molten salts circulating in the loop and return them to the salt tank after the process is over or when the salts need reheating. Solar power plants and pyrolysis of waste tires use this method.
  • Direct heating: Metal assemblies use molten salts for direct heating.

Molten salts at Contec

Industrial applications of molten salt outside energy storage for solar power are just beginning. So is the application of pyrolysis to treat and recycle end-of-life tires to recover economic components, such as recovered Carbon Black, steel, oil, and gas.

Contec is the only recovered Carbon Black producer that has integrated molten salts as a heat transfer medium in its pyrolysis process.

Contec uses molten salts to achieve even heating of waste tire materials to recover consistent quality products. The efficient uptake and release of heat by molten salts reduce Contec’s energy consumption. 

Molten salts are also more economical and can be recycled and used for heat transfer for many years.

In combination with innovative plant design, engineering, and other safety protocols, molten salts have made the Contec pyrolysis process for tire recycling safer, environmentally friendly, and circular. Get in touch to learn more about our sustainable solutions.

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