What are cryogenic liquids and how are they obtained?

Cryogenic liquids are gases that have been intensely cooled, which have a boiling point close to -100º or lower. These gases must be cooled below ambient temperature.

Although different cryogenic gases have specific properties and turn into liquids under different temperature conditions, they all have one thing in common: small amounts of their liquid can expand into large volumes of gas (which also remains extremely cold).

Classification of cryogenic fluids

Regardless of the individual characteristics of cryogenic fluids, almost all of them can be grouped into one of three main groups for classification:

  • Inert gases: These gases do not exhibit any significant chemical reaction. They don’t burn or allow combustion. They include nitrogen, helium, neon, argon and krypton.
  • Flammable gases: The gas produced by these cryogenic liquids can burn in the air. The most common examples are hydrogen, methane and liquefied natural gas.
  • Oxygen: Many of the materials considered ‘non-combustible’ can burn in the presence of liquid oxygen. Organic materials can react explosively with liquid oxygen. The hazards and precautions associated with handling liquid oxygen must therefore be considered separately. 

The most commonly used cryogenic liquids

Cryogenic fluid applications are common in large industries with different needs such as medicine, tool manufacturing, aerospace, nuclear research and laboratory work. 

Both cryogenic liquids have common properties, such as their ability to cool and freeze materials and the ability to be stored at low temperatures. However, they also have significant differences in their boiling points, densities and specific applications.

Liquid nitrogen

One of the most common freezing liquids is nitrogen in its pure state, is a colorless and odorless gas at a temperature of -196º or lower. Its uses are wide and varied; examples include its use in the aeronautics sector to create very lightweight and highly-resistant materials, or its use in the automotive sector to weld parts, frames and other essential components in car manufacturing. 

In the healthcare sector, it is used to freeze and transport blood and other biological samples without damaging the tissue structure, as well as for freezing eggs and destroying diseased tissue in dermatology. Other applications range from food and gastronomy (catering) to metalworking and welding.

Liquid helium

Helium, which has the lowest known boiling point, is found in liquid state at a temperature of -269º. Helium is widely used in the aerospace and aviation industry as a pressurising agent for ground and flight fluid systems. It is also used in the automotive sector to test critical components and in diving, together with oxygen, to enable divers to dive deeper by reducing breathing resistance at depth.

The electronics industry also uses helium for manufacturing semiconductors and other components, cooling parts rapidly to improve their performance. In the hospital industry, it is used to capture high-resolution images in magnetic resonance imaging. 

Freezing liquids such as liquid oxygen or liquid carbon dioxide are also extensively used for industrial, medical and food purposes. The CO2 liquid can be used in indoor crops to increase yields, while liquid oxygen increases productivity and efficiency in industrial processes.

You might be interested in: Everything you should know about storing liquefied natural gas (LNG)

How to store cryogenic liquids

Cryogenic liquids that are stored are compressed gases and are kept at extremely low temperatures. Gas decompression would cause such a rapid expansion that the container would rupture and possibly explode.

The containers and vessels for cryogenic fluids must therefore be built specifically for this purpose and follow the highest quality and safety standards.

Cryogenic vessels: storage types

The very low temperatures in these fluids cause most solids to be damaged and become brittle, so materials such as carbon steel and all types of plastics and rubber are ruled out for direct contact with the substance.

Pressure vessels for storing these fluids are built in a variety of sizes, depending on consumption needs. From laboratory flasks to large capacity Dewars or even cryogenic tanks.  Everything will depend on the storage needs and particular characteristics of each liquid.

Materials for storage containers

Although cryogenic vessels have different shape and design characteristics depending on the substance they contain, industrial vessels are made of a structure divided into three main parts:

  • An inner container made of stainless steel  austenitic/aluminium and designed to withstand very low temperatures.
  • An outer vessel, usually made of carbon steel.
  • A layer of insulation between the two containers, usually consisting of a combination of high vacuum and insulating material.

In addition to the two insulated vessels, larger-capacity vessels have another key component in their correct operation: a pressure regulation system designed to ensure that the pressure is always constant and appropriate to the user’s needs.

Highly stringent safety requirements and protocols must be followed for the design and calculation of the tanks, and an internationally recognised design code (by country) is used to ensure that the tanks are manufactured to the millimetre.

Read more: Uses and curiosities of liquid ethylene

Hazards when working with cryogenic fluids

Operations with cryogenic liquids are extremely delicate; the slightest mistake can result in a serious accident. Seemingly harmless actions can therefore trigger a disaster.

Main hazards of working with cryogenic liquids

One of the biggest risks when working with cryogenic fluids is the extremely high speed at which they can evaporate (700I); they can cause explosion and asphyxiation in the blink of an eye. 

The vapour emitted by these gases can cool faster than the liquid itself, which means that contact with the liquid can cause bronchial and eye frostbite

Direct contact with the liquid can also cause first-degree frostbite burns, which is why it is so important to use appropriate protective equipment (eyes, hands and clothing) and to always follow the relevant safety measures, taking special care with the eyes.

In addition, it is important to ensure that the work environment is well ventilated. If you are working in an enclosed space, you should have an correct ventilation system and ensure an emergency exit available.

Instructions to be followed when working with cryogenic liquids

Insulated containers (Dewars) used for transporting and storing cryogenic liquids must be handled specifically according to the marked guidelines. Closing or hermetically sealing them, for example, will result in a high risk of explosion due to oxygen condensation.

Those working with such fluids should do so in open or at least sufficiently ventilated areas to prevent oxygen deprivation and the risk of dizziness. Special care should be taken when transferring, which should never take place in enclosed spaces and strict measures must be followed such as using an air transfer, having an eye wash and shower nearby and keeping everything free of grease.

Cryospain: Reliable cryogenic solutions

Are you looking for a reliable technology partner to provide you with the most innovative and advanced cryogenic engineering solutions on the market? Contact us to receive specialised advice from highly qualified technicians who can guide you in your new challenges and projects.

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