SEPARATION OF OXYGEN AND NITROGEN FROM ATMOSPHERIC AIRAIR COMPOSITION Dry Air is relatively uniform in composition, with primary constituents as shown below. Ambient Air, may have up to about 5% (by volume) Water content and may contain a number of other gases (usually in trace amounts) that are removed at one or more points in the Air separation and product purification system. Primary components of Dry Air

An air separation plant separates atmospheric air into its primary components, typically nitrogen and oxygen, and sometimes also argon and other rare inert gases.The most common method for air separation is cryogenic distillation. Cryogenic air separation units (ASUs) are built to provide nitrogen or oxygen and often co-produce argon. Other methods such as Membrane, pressure swing adsorption (PSA) and Vacuum Pressure Swing Adsorption (VPSA), are commercially used to separate a single component from ordinary air. High purity oxygen, nitrogen, and argon used for Semiconductor device fabrication requires cryogenic distillation. 13. REFERENCES 1.^ NASA Earth Fact Sheet, (updated November 2007) 2.^ Linde Group, Corporate History 3.^ Latimer, Chemical Engineering Progress (1967) Vol. 63 (2) pp. 35-59 4.^ R. Thorogood, "Developments in Air separation", Gas Separation & Purification (1991) Vol. 5 June pp. 83-94 [1] 5.^ R. Agrawal, "Synthesis of Distillation Column Configurations for a Multicomponent Separation", Industrial Engineering Chemistry Research (1996) Vol. 35 pp. 1059-1071 [2] 6.^ W.F. Castle, "Air Separation and liquefaction: recent developments and prospects for the beginning of the new millennium", International Journal of Refrigeration (2002) Vol. 25, pp. 158-172 [3] 7.^ Particulate matter from forest fires caused an explosion in the ASU of a Gas to Liquid plant, see [4] 8.^ Computer Chemical Engineering (2006) Vol. 30 pp. 1436-1446 9.^ Higman, Christopher; van der Burgt, Maarten (ed) Gasification (2nd Edition) Elsevier 2008 ISBN 978-0-7506-8528-3 p. 324 10. Wikipedia. 11. LAir Liquide India Ltd, New Delhi.

Cryogenic liquification process

Distillation column in a cryogenic air separation plantPure gases can be separated from air by first cooling it until it liquefies, then selectively distilling the components at their various boiling temperatures. The process can produce high purity gases but is energy-intensive. This process was pioneered by Dr. Carl von Linde in the early 20th century and is still used today to produce high purity gasesThe cryogenic separation process requires a very tight integration of heat exchangers and separation columns to obtain a good efficiency and all the energy for refrigeration is provided by the compression of the air at the inlet of the unit.To achieve the low distillation temperatures an air separation unit requires a refrigeration cycle that operates by means of the JouleThomson effect, and the cold equipment has to be kept within an insulated enclosure (commonly called a "cold box"). The cooling of the gases requires a large amount of energy to make this refrigeration cycle work and is delivered by an air compressor. Modern ASUs use expansion turbines for cooling; the output of the expander helps drive the air compressor, for improved efficiency. The process consists of the following main steps:1. Before compression the air is pre-filtered of dust.2. Air is compressed where the final delivery pressure is determined by recoveries and the fluid state (gas or liquid) of the products. Typical pressures range between 5 and 10 bar gauge. The air stream may also be compressed to different pressures to enhance the efficiency of the ASU. During compression water is condensed out in inter-stage coolers.3. The process air is generally passed through a molecular sieve bed, which removes any remaining water vapour, as well as carbon dioxide, which would freeze and plug the cryogenic equipment. Molecular sieves are often designed to remove any gaseous hydrocarbons from the air, since these can be a problem in the subsequent air distillation that could lead to explosions.[6] The molecular sieves bed must be regenerated. This is done by installing multiple units operating in alternating mode and using the dry co-produced waste gas to desorb the water.4. Process air is passed through an integrated heat exchanger (usually a plate fin heat exchanger) and cooled against product (and waste) cryogenic streams. Part of the air liquefies to form a liquid that is enriched in oxygen. The remaining gas is richer in nitrogen and is distilled to almost pure nitrogen (typically < 1ppm) in a high pressure (HP) distillation column. The condenser of this column requires refrigeration which is obtained from expanding the more oxygen rich stream further across a valve or through an Expander, (a reverse compressor).5. Alternatively the condenser may be cooled by interchanging heat with a reboiler in a low pressure (LP) distillation column (operating at 1.2-1.3 bar abs.) when the ASU is producing pure oxygen. To minimize the compression cost the combined condenser/reboiler of the HP/LP columns must operate with a temperature difference of only 1-2 kelvin, requiring plate fin brazed aluminium heat exchangers. Typical oxygen purities range in from 97.5% to 99.5% and influences the maximum recovery of oxygen. The refrigeration required for producing liquid products is obtained using the JT effect in an expander which feeds compressed air directly to the low pressure column. Hence, a certain part of the air is not to be separated and must leave the low pressure column as a waste stream from its upper section.6. Because the boiling point of argon (87.3 K at standard conditions) lies between that of oxygen (90.2 K) and nitrogen (77.4 K), argon builds up in the lower section of the low pressure column. When argon is produced, a vapor side draw is taken from the low pressure column where the argon concentration is highest. It is sent to another column rectifying the argon to the desired purity from which liquid is returned to the same location in the LP column. Use of modern structured packings which have very low pressure drops enable argon purities of less than 1 ppm. Though argon is present in less to 1% of the incoming, the air argon column requires a significant amount of energy due to the high reflux ratio required (about 30) in the argon column. Cooling of the argon column can be supplied from cold expanded rich liquid or by liquid nitrogen.7. Finally the products produced in gas form are warmed against the incoming air to ambient temperatures. This requires a carefully crafted heat integration that must allow for robustness against disturbances (due to switch over of the molecular sieve beds[7]). It may also require additional external refrigeration during start-up.The separated products are sometimes supplied by pipeline to large industrial users near the production plant. Long distance transportation of products is by shipping liquid product for large quantities or as dewar flasks or gas cylinders for small quantities.

2. Non-cryogenic industrial gas processes2.1. AdsorptionAdsorption processes are based on the ability of some natural and synthetic materialsto preferentially adsorb nitrogen. In the case of zeolites, non-uniform electric fields existin the void spaces of the material, causing preferential adsorption of molecules, whichare more polarizable as those that have greater electrostatic quadrapolar moments. Thus,in air separation, nitrogen molecules are more strongly adsorbed than oxygen or argonmolecules. As air is passed through a bed of zeolitic material, nitrogen is retained and anoxygen-rich stream exits the bed. Carbon molecular sieves have pore sizes on the sameorder of magnitude as the size of air molecules. Since oxygen molecules are slightlysmaller than nitrogen molecules, they diffuse more quickly into the cavities of theadsorbent. Thus, carbon molecular sieves are selective for oxygen and zeolites areselective for nitrogen.Zeolites are typically used in adsorption-based processes for oxygen production. Atypical flowsheet is shown in Fig. 1. Pressurized air enters a vessel containing theadsorbent. Nitrogen is adsorbed and an oxygen-rich effluent stream is produced until thebed has been saturated with nitrogen. At this point, the feed air is switched to a freshvessel and regeneration of the first bed can begin. Regeneration can be accomplished byheating the bed or by reducing the pressure in the bed, which reduces the equilibriumnitrogen holding capacity of the adsorbent. Heat addition is commonly referred to astemperature swing adsorption TSA., and pressure reduction as pressure or vacuumswing adsorption PSA or VSA.. The faster cycle time and simplified operationassociated with pressure reduction usually makes it the process of choice for airseparation.Variations in the process that effect operating efficiency include separate pretreatmentof the air to remove water and carbon dioxide, multiple beds to permit pressure energyrecovery during bed switching, and vacuum operation during depressurization. Optimizationof the system is based on product flow, purity and pressure, energy cost andexpected operating life. Oxygen purity is typically 9395 vol.%.

Adsorption based air separation process

Polymeric membranesMembrane processes using polymeric materials are based on the difference in rates ofdiffusion of oxygen and nitrogen through a membrane which separates high-pressureand low-pressure process streams. Flux and selectivity are the two properties thatdetermine the economics of membrane systems, and both are functions of the specificmembrane material. Flux determines the membrane surface area, and is a function of thepressure difference divided by the membrane thickness. A constant of proportionalitythat varies with the type of membrane is called the permeability. Selectivity is the ratioof the permeabilities of the gases to be separated. Due to the smaller size of the oxygenmolecule, most membrane materials are more permeable to oxygen than to nitrogen.Membrane systems are usually limited to the production of oxygen enriched air2550% oxygen.. Active or facilitated transport membranes, which incorporate anoxygen-complexing agent to increase oxygen selectivity, are a potential means toincrease the oxygen purity from membrane systems, assuming oxygen compatiblemembrane materials are also available.A typical membrane system is shown in Fig. 3. A major benefit of membraneseparation is the simple, continuous nature of the process and operation at near ambientconditions. An air blower supplies enough head pressure to overcome pressure dropthrough the filters, membrane tubes and piping. Membrane materials are usuallyassembled into cylindrical modules that are manifolded together to provide the requiredA.R. Smith, J. KlosekrFuel Processing Technology 70 (2001) 115134 119Fig. 3. Polymeric membrane air separation process.production capacity. Oxygen permeates through a fiber hollow fiber type. or throughsheets spiral wound type. and is withdrawn as product. A vacuum pump typicallymaintains the pressure difference across the membrane and delivers oxygen at therequired pressure. Carbon dioxide and water usually appear in the oxygen enriched airproduct, since they are more permeable than oxygen for most membrane materials.As with adsorption systems, capital is essentially a linear function of production rateand product backup is typically not available without a separate liquid oxygen storagetank and delivery support system. Membrane systems readily fit applications up to 20tonsrday, where air enrichment purities with water and carbon dioxide contaminants canbe tolerated. This technology is newer than adsorption or cryogenics and improvementsin materials could make membranes attractive for somewhat larger oxygen requirements.The fast start-up time, due to the near ambient operation, is especially attractive foroxygen-use systems than exhibit discontinuous usage patterns. The passive nature of theprocess is also appealing.

2.4. Ion transport membrane (ITM)ITMs are solid inorganic oxide ceramic materials that produce oxygen by the passageof oxygen ions through the ceramic crystal structure. These systems operate at hightemperatures, generally over 11008F. Oxygen molecules are converted to oxygen ions atthe surface of the membrane and transported through the membrane by an appliedelectric voltage or oxygen partial pressure difference, then reform oxygen moleculesafter passing through the membrane material. Membrane materials can be fabricated intoflat sheets or tubes.For large energy conversion processes the pressure difference transport driving forceis the method of choice. Membranes, which operate by a pressure difference, ar referred to as mixed conducting membranes since they conduct both oxygen ions andelectrons. The oxygen ions travel through the ITM at very high flow rates and producenearly pure oxygen on the permeate side of the membrane. The oxygen can be separatedas a pure product, or another gas can be used to sweep on the permeate side of themembrane to produce a lower purity product. If a reactive sweep gas is used, anoxidative product can be produced directly, e.g. natural gas methane sweep to makesynthesis gas for gas-to-liquid GTL. conversion.

Ion transport membrane air separation process

A simple schematic of an ITM oxygen separation process is depicted in Fig. 4. Air iscompressed and then heated to operating temperature by exchange against the hotprocess streams non-permeate and oxygen product. and then auxiliary heat addition. Ingeneral, the heating of air can be done by either indirect heat exchange andror directfiring of fuel. The oxygen stream is compressed to delivery pressure for use in IGCC orother applications. The pressurized nitrogen enriched non-permeate stream is usedelsewhere in balance of the energy conversion process, for instance, expanded in anintegrated gas turbine cycle to generat e electric power.

Comparisons of process alternativesAdsorption and polymeric membrane processes will continue to improve in both costand energy efficiency through ongoing research and development of adsorbents andmembrane materials. Neither technology is expected to challenge cryogenics for largetonnage production of oxygen, especially at high purities. Both adsorption and membranesystems produce byproduct nitrogen containing significant amounts of oxygen. Ifhigh purity nitrogen is required, an add-on deoxo or other purification system must beemployed to upgrade the quality of the nitrogen. Neither process has the ability todirectly produce argon or rare gases. Production of liquid oxygen or nitrogen for systemback-up requires an add-on cryogenic unit or delivery of product from a merchantfacility. On the other hand, adsorption and membrane processes are less complex andmore passive than cryogenic technology.Chemical processes offer the potential for continuous operation and economies ofscale through large production output from single trains, but to date have not been ableto overcome material corrosion problems. ITM technology is currently foreseen as thebest candidate to challenge cryogenics for the production of high purity, tonnagequantities of oxygen. As with the other non-cryogenic processes, ITM has limitations inregard to production of pure byproducts and liquids for storage and backup. ITM is alsoat an embryonic state of development, so the potential for improvement is large.Table 1 compares the merits of the various technologies based on the followingcategories. Status is the degree to which the technology has been commercialized,varying from mature for cryogenics through developing for ITM. Economic Range isthe typical production range where the technology is currently economically feasible