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How Does Cryogenic Distillation Work?

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What Is The Cryogenic Distillation Process?

Cryogenic distillation (also known as Low-Temperature Rectification) is a separation process that works by liquifying the gas mixture at very low temperatures and then selectively distilling the specific gas component at its boiling point.

Though the process yields products of high purity, it is energy-intensive due to purification requirements (removal of water, hydrocarbons, CO2) and refrigeration requirements (bringing the temperature down to -200°C) [1].  The process is used primarily for the large-scale/industrial manufacturing of high purity oxygen and nitrogen (gaseous or liquid) products, or argon if required.

The process flow of the cryogenic distillation of air is as follows:

Cryogenic Distillation Process
Cryogenic Distillation Process Flow Diagram

How Does Cryogenic Air Separation Work?

FILTRATION PRE-TREATMENT

  • For the pre-treatment, air passes through a series of filtration devices to remove dust particles and moisture. Then, it also passes through a series of compressors to remove the remaining water vapor.

 

COOLING DOWN OF COMPRESSED AIR

  • The compressor compresses the air to between 5-8 bars (depending on the desired product pressure). From Charles’ Law, temperature increases when air is compressed. Heat removal, via heat exchangers or other refrigeration means, is necessary to reach optimum temperatures for the separation of air components.
  • The heat exchanger consists of coiled tubes covering the discharge pipe from which the warm compressed air flows. The coiled tubes with the cooling liquid (can be water, liquid nitrogen, or any other deep-freezing liquid), cools down the compressed warm air to negative temperature.

 

PURIFICATION OF AIR (REMOVAL OF CO2)

  • Cooled compressed air passes through a separation unit which causes the air to partially expand. This expansion results in a temperature drop that solidifies carbon dioxide (freezing pt. -79°C). The airstream passes through molecular sieves to remove the solidified CO2 along with the remaining water vapor.
  • Removal of CO2 and water vapor is essential to the process since lower temperatures will cause these to freeze and clog the equipment.

 

BRINGING AIR TO CRYOGENIC TEMPERATURE

  • After the removal of CO2, expanded air (which now consists of oxygen, nitrogen, and argon) further expands through a nozzle, forming a jet. The expansion causes the air temperature to drop from -80°C to -200°C. Through a process called liquefaction, air condenses and becomes liquid at cryogenic temperature.

 

DISTILLATION OF AIR

  • By slowly warming up the liquid air, distillation separates air into its different component gases. Nitrogen is separated first (boiling point -196°C), then Argon (boiling point -186°C), and Oxygen last (boiling point -183°C). Each product stream is collected separately and may employ multiple distillation columns.

 

Industrial Applications of Cryogenic Distillation

Industrial Production of Oxygen

The process of separating oxygen from atmospheric air is as follows:

Cryogenic Distillation Process
Process Flow Diagram of Cryogenic Distillation of Oxygen from Air (by Up To Speed)

Through cryogenic air separation, oxygen is collected at the bottom of the distillation column through a pipe connected to a reservoir. This pure oxygen (at least 99% purity) is then pressurized according to the requirements for its intended purpose.

Sweetening of Natural Gas: Separation of Nitrogen From Natural Gas

Cryogenic distillation is used in removing nitrogen from natural gas as a concentration of more than 4% poses the danger of vapor lock or combustion [2]. Nitrogen also dilutes the heating value of natural gas (decreased BTU) and therefore a lower commercial value. High-Nitrogen natural gas is essentially stranded as it is not feasible for transport through the pipelines to the market.

During cryogenic distillation, nitrogen gas separates from natural gas by utilizing their boiling point difference. Methane (boiling point -161.5°C) liquefies before nitrogen, allowing the two to be separated and recovered efficiently. 

Prior to employing cryogenic distillation, impurities such as H2O, CO2, aromatics, and C3+ hydrocarbons have to be removed to avoid damage to equipment. As such, it is not economically viable to use cryogenic distillation for flow rates below 50-100 MMSCFD [3].

The video below shows how liquefied natural gas is processed.

Advantages & Disadvantages of Cryogenic Distillation

In summary, below are the advantages and disadvantages of cryogenic distillation. Though it has its downsides, it is still the most cost-effective separation method for homogeneous gas mixtures.

FAQs

1. When was cryogenic distillation invented?

Cryogenic distillation was discovered by Carl Von Linde in 1985 but was first applied in industries only in 1905. Because it has been used industrially for more than 75 years, the technology is well recognized for its reliability and can be designed for high capacity (up to 5,000 tons per day) [4].

2. Is cryogenic distillation expensive?

Cryogenic distillation is the most efficient approach in separating individual gases from air or from gaseous feeds as compared to other methods such as membrane separation and pressure swing adsorption. However, due to multiple requirements for purification and refrigeration, it can be energy and cost-intensive.

3. Which material is suitable for a cryogenic plant?

  • Cryogenic plants require materials that could withstand extreme cold since certain materials abruptly lose ductility when a certain threshold is reached. 
  • Almost all aluminum alloys (except series 7075-T6 and 7178-T6), titanium alloys 13V-11Cr-8Al or 8Mn, copper and nickel alloys can be used at -45°C. 
  • Low alloy, quenched and tempered steels, ferritic nickel steels, and low carbon martensitic steels can be used at -75°C with sufficient reliability. 
  • Low carbon, 3.5% nickel steels, and many aluminum, nickel, and titanium alloys are suitable for temperatures of -100°C. 
  • Austenitic stainless steels and maraging steels with nickel content between 20-25% and the addition of cobalt, molybdenum, titanium, aluminum, and niobium are suitable for use up to -196°C. Among steels, only high alloy austenitic stainless steels are suitable for temperatures below -196°C [5]. 

References

  1. Curtis, A.B. & Wess, M. “Commercialization of Nitrogen-Rich Natural Gas.” 2 May 2008
  2. Dey, Anup Kumar. “Cryogenic Air Separation Process: A Brief Introduction.” https://whatispiping.com/cryogenic-air-separation-process/ . Accessed 17 May 2022
  3. GAS RNG Systems. “Nitrogen Rejection.” 2020. gasrng.com/products/nitrogen-rejection/ . Accessed 19 May 2022
  4. Gasparini Industries, “Metals and Materials for Low Temperatures and Cryogenic Applications.” 14 January 2019. https://www.gasparini.com/en/blog/metals-and-materials-for-low-temperatures/ . Accessed 25 May 2022
  5. Ghasem, Nayef. “Advances in Carbon Capture.” 2020
  6. JALON. “101 Process Guide to Cryogenic Air Distillation.”  https://www.jalonzeolite.com/101-process-guide-to-cryogenic-air-separation/ . Accessed 18 May 2022
  7. Li, Zhikao. “Separation of Nitrogen from Natural Gas: Conventional and Emerging Technologies.” 2018. https://api.research-repository.uwa.edu . Accessed 20 May 2022
  8. Membrane Technology and Research, Inc. “Nitrogen Removal From Natural Gas.” 1995-1996. https://digital.library.unt.edu . Accessed 19 May 2022
  9. National Energy Technology Laboratory. “3.1 Commercial Technologies for Oxygen Production.” https://www.netl.doe.gov/research/Coal/energy-systems/gasification/gasifipedia/commercial-oxygen. Accessed 21 May 2022
  10. Raz. “Cryogenic Distillation Process (How Oxygen is Produced?)”. 2021. https://www.uptothespeed.com/2021/12/Oxygen-production.html . Accessed 18 May 2022

4 Responses

  1. Great article guys! Used cryogenic distillation as a unit operation in my final design project in school. Would have been great to have this information then.

    Reply

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