User:Joyce Kathryn Nicholson

SEPERATION PROCESS OF GASES IN A GAS by joyce kathryn nicholson

Gases in Gases are a pure substances in the entire atmosphere mostly made up of Nitrogen and Oxygen. It cannot be separated into simpler substances by physical or mechanical means such as sifting, filtering, crystallization, distillation, etc. e.g., distilling pure water (H2O) does not separate water into hydrogen and oxygen, it only produces water vapour.

There are three ways to separate gases from gases a) separation with sorbents/solvents b) separation with membrane c) separation by cryogenic distillation.

A)	Sorbents removal: For removal of hydrogen sulphide and CO2 from gas streams. It is the most well-established of the techniques needed for CO2 capture although practical experience is mainly in gas streams which are chemically dropping, the opposite of the oxidising environment of a flue gas stream. There are several facilities in which amines are used to capture CO2 from flue gas streams today, one example being the Warrior Run coal fired power station in the USA where 150 t/d of CO2 is captured.

B) Solvents removal: In temperature swing adsorption (TSA), the adsorbent is regenerated by raising its temperature. PSA and TSA are commercially practiced methods of gas separation and are used to some extent in hydrogen production and in removal of CO2 from natural gas. Adsorption is not yet considered attractive for large-scale separation of CO2 from flue gas because the capacity and CO2 selectivity of available adsorbents is low. However, it may be successful in combination with another capture technology.

Separation with membrane The separation process includes the vapours and gases are separated due to their different solubility and diffusivity in the polymers. The permeability or permeation coefficient, of such nonporous membranes can generally be expressed as the solubility, of the gas in the membrane polymer multiplied by the diffusivity, D, of the gas in the polymer; in such cases, permeation is said to occur by a "solution-diffusion" model. Polymers in the glassy state are generally more effective for separation, and predominantly differentiate gases based on their different diffusivities. Small molecules of penetrants move among polymer chains according to the formation of local gaps by thermal motion of polymer segments.

And finally the separation process by cryogenic distillation. Cryogenic separation is widely used commercially for purification of CO2 from streams that already have high CO2 concentration. Need for relatively high concentration of CO2 for cryogenic unit is important because in order to minimize CO2 loss from the top of the column it would be necessary to operate as close to the triple-point temperature as possible, but the minimum partial pressure of CO2 achievable in the vent gas would be 5.18 bar abs. this meant that as the concentration of other component in the CO2/gas mixture increases the pressure of the stripping column would have to be increased in order to achieve a certain CO2 recovery from process. At 75% feed purity and 90% recovery, the column pressure would be about 26 bar pressure, for 95% recovery it would be 46 bar.Cryogenic systems are a low temperature physical approach to separation, in which the CO2 is separated directly by phase change. This method is advantageous with respect to direct production of liquid CO2 or pure CO2 gas stream in high pressure which would be liquefied more easily. There are some difficulties for applying this method as well. For dilute CO2 stream, the refrigeration energy is high. Water has to be removed before the cryogenic cooling step to avoid blockage from freezing.

Bibliography

Websites used: www.asynsteel.com/markets/...gases/cryogenic-air-sepration.html www.sciencedaily.com/releases/2013/01/130117142514.htm Book used Membrane engineering of treatment of gases From Parramatta city council library