Encyclopedia of Membranes

Living Edition
| Editors: Enrico Drioli, Lidietta Giorno

Gas Separation

  • Mariolino CartaEmail author
Living reference work entry
DOI: https://doi.org/10.1007/978-3-642-40872-4_261-1
Gas separation is a widely used technique in which the objective is the separation of one or more gases from a mixture. It is becoming crucial for several industrial processes such as the treatment of fumes from coal-fired plants, in particular, aiming for the removal of CO2 to reduce the greenhouse effect. Growing interest is also given to other applications such as the separation and purification of commercially important gases such as H2, CH4, and O2 from natural gas. The most common methods to perform gas separation are:
  1. 1.

    Separation with solvent/sorbents

     
  2. 2.

    Separation by cryogenic distillation

     
  3. 3.

    Separation with membranes

     

Separation with Solvent/Sorbents

The separation with solvent/sorbents is based on the affinity of the gas toward a specific sorbent such as zeolites, alumina, or activated carbon or a solvent, for instance, MEA (methanolamine). The most illustrative example of this technology is represented by the pressure swing adsorption (PSA) method. The separation occurs when the gas mixture comes in contact with the sorbent/solvent in a vessel which is then pressurized. The gas with the highest affinity for the adsorbent is “trapped,” whereas the others pass through the system. The regeneration of the vessel is carried out by returning it to atmospheric pressure or by increasing the temperature, with release of the “trapped” gas. The main advantage of this technique is the high purity of the separated gas; the disadvantage consists in the high energy required for running the system, especially for the regeneration of the vessel (Fig. 1).
Fig. 1

Schematic representation of pressure swing adsorption method (PSA)

Separation by Cryogenic Distillation

Cryogenic distillation is based on the fact that in a mixture of gases, they all have different boiling points and they could be separated by increasing/decreasing the temperature and pressure of the system in which they are stored, so that they can be divided into their single components. The gas mixture is cooled down to a low temperature (typically < −50 °C). Once in the liquid form, the components of the gas can be directed in a distillation column, and through a series of compression, cooling, and expansion steps, they can be distributed to different channels, depending on their boiling points (Fig. 2).
Fig. 2

Schematic representation of cryogenic distillation method

It is a widely used technique for streams that already have a high concentration of desired gas (typically >90 %), but it is not very appropriate for more dilute gas streams.

The main advantage of the cryogenic gas separation is that it enables direct production of liquid gas, which is often very useful for certain transport options, such as transport by ship.

A major disadvantage is connected with the high amount of energy required for the refrigeration especially for dilute gas streams.

Separation with Membranes

Separation of gases with membranes relies on the different affinities of one or more gases toward the membrane material, causing one gas to permeate faster (or slower) than others. It is one of the fastest growing field for gas separation techniques, especially due to the high variety of materials which the membrane could be composed of, including microporous organic polymers, zeolites, ceramic, and metal-containing materials (for a more in-depth reading, see Yampolskii and Freeman (Yampolskii et al. 2010)).

The gas mixture is directed into a vessel and put in contact to the membrane material which is at the interface with another vessel (Fig. 3). The mixture is allowed to diffuse into the second vessel under a pressure gradient which promotes the mass transport through the membrane separating the retentate (slower gas) from the permeate (faster gas).
Fig. 3

Schematic representation of membranes for gas separation

The use of membranes for gas separation offers several benefits, probably the most valuable is the high cost efficiency (both for the mechanical simplicity of the system and for low-energy regeneration). In fact, they do not require thermal regeneration, a phase change, or active moving parts in their operation.

Probably the greatest limitation of membranes for gas separation is derived from their trade-off relationship between permeability and selectivity for a required gas component. This means that high permeable membranes have low selectivity, requiring several run for a good separation, and highly selective membranes have low permeability, meaning long operational times. This trade-off was well addressed by Robeson in two well-known articles (Robeson 1991, 2008) in which he studied the gas separation performance of several membrane-forming materials in terms of permeability of a particular species (P A) and selectivity toward one component of a gas pair (α A/B = P A/P B), organizing the data in double logarithmic plots for a series of commercially selected important gas pairs such as H2/CH4, H2/CO2, and O2/N2. He confirmed that highly selective membranes generally exhibit low permeability and vice versa. The most important outcome of this study is represented by the so-called Robeson upper bound, an empirical line which is drawn for every gas pair plot that is meant to define how good a material for gas separation is. In Fig. 4, there is a typical example (Carta et al. 2013) in which the red line represents the 2008 upper bound for the gas pair O2/N2. Supposedly, if we plot the selectivity α A/B versus permeability P A for a new membrane and the data point fall close or go over the upper bound, it is widely accepted that the material has an excellent compromise between P (rate of separation) and α (goodness of separation).
Fig. 4

An example of Robeson plot, in this case O2/N2 (Carta et al. 2013). The black line represents the 1991 (Robeson 1991) upper bound, whereas the red line is the current (2008) upper bound (Robeson 2008)

References

  1. Carta M, Malpass-Evans R, Croad M, Rogan Y, Jansen JC, Bernardo P, Bazzarelli F, McKeown NB (2013) An efficient polymer molecular sieve for membrane gas separations. Science 339(6117):303–307CrossRefGoogle Scholar
  2. Robeson LM (1991) Correlation of separation factor versus permeability for polymeric membranes. J Membr Sci 62(2):165–185CrossRefGoogle Scholar
  3. Yampolskii Y, Freeman B (eds) (2010) Membrane gas separation. Wiley, Chichester, UK, 370 ppGoogle Scholar
  4. Robeson LM (2008) The upper bound revisited. J Membr Sci 320(1+2):390–400CrossRefGoogle Scholar

Copyright information

© Springer-Verlag Berlin Heidelberg 2015

Authors and Affiliations

  1. 1.School of ChemistryThe University of EdinburghEdinburghUK