AIR SEPARATION - GENERAL ASPECTS

An attempt is made to help an Engineer/Professional on field, to understand the basics of the Cryogenic processes that are encountered in Oil, Gas, Steel etc core sector Industries. Hope it serves the purpose intended.

1.0  GENESIS OF OXYGEN. Oxygen was discovered simultaneously by three Scientists.

In 1771, the Swedish Chemist Carl Wilhelm Scheele (1742-1786) isolated it and called it "FIRE AIR".  

In 1774, Pierre Bayen (1725-1798), a French Chemist and Pharmacist, also produced Oxygen.                                                                                                                                       But, their discoveries were not recognized immediately, because neither of them published his work. Therefore, Oxygen is said to have been officially discovered on August 1, 1774, by British Chemist Joseph Priestley (1733-1804).

These 3 Scientists used an identical method to produce Oxygen : they heated Mercuric Oxide (also known as “Mercury Oxide”), which released a ‘gas’. By studying this released gas, Priestley noted that it was slightly denser than Air; that a candle burned significantly better in this gas than in Air, and that mice were more active after breathing it. He called this gas “DEPHLOGISICATED AIR”.

In October, 1774, during one of Priestley’s visits to France, Antoine Laurent de Lavoisier (1743-1794) learnt about the discovery of this "dephlogisicated air" and decided to further experiment. He first called the gas released as “Eminently Breathable Air / Vital Air”. He understood that this gas, which kept burning flames and animals alive, also oxidized metals and turned them into Acids. Lavoisier renamed it in 1777 “Oxygene", and this term was later changed to “ Oxygen ”, from the Greek words -?ξ?? { (oxys) acid} and -γεν?? {(-genes) producer, literally begetter}. In fact, he thought that all Acids contained Oxygen.

Lavoisier’s research focused on combustion reactions involving Oxygen. For example, he proved that breathing, which consumes Oxygen, also releases Carbon Dioxide and produces “animal” heat, was based on the same principle as Carbon combustion.

In 1873, Carl von Linde succeeded in proving that the direct transfer of artificially generated cooling was more cost-effective than the natural ice method.

Oxygen is the most abundant element on the Earth’s surface. For example, Oxygen makes up by weight: - 46% of the Earth's crust (in the form of Oxides, Silicates, etc.); - 89% of the Earth's water (in the form of molecules); - 21% of the Air we breathe and - 62% of the human body (in the form of molecules). 

Nitrogen was discovered in 1772 by Daniel Rutherford who called it ” noxious Air or fixed Air”. But it was isolated only in 1786. The name Nitrogen comes from Latin Nitrogenium, where Nitrum (from Greek Nitron) means "Saltpeter" and genes means "forming". Nitrogen is a protective or inert gas with many Industrial applications to protect, inert and as a cold reserve.

The name Argon comes from the Greek αργ?ν (argos) meaning "the lazy one" in reference to its chemical inactivity.

The atmosphere contains about 0.9 % of Argon. A neutral and colorless gas like Nitrogen, it does not however exist in nature other than in the Air. It cannot sustain life, but it is highly used in certain industrial applications due to its high level of chemical inertness and the relative ease with which it can be produced.

1.1  AIR SEPARATION Out of the Air that surrounds us, we produce Oxygen, N2, Ar and Rare / Noble gases (Neon, Krypton, Xenon). Separating out these components, which occur naturally in Air, allows to develop their unique properties and use them in numerous applications.

These gases are produced in Air Separation Units that use Cryogenic Distillation.

In 1902 Georges Claude and Paul Delorme developed a process for Liquefying Air to separate the components. On seeing the First drops of liquid Air, a Company of the same name was established. Paul Delorme became its first President.

An Air Separation Unit 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 ASUs are built in medium to large size plants to produce Nitrogen, Oxygen, and Argon as gases and/ or liquid products.

Cryogenic Air separation is the preferred technology for producing very high purity Oxygen and Nitrogen. It is the most cost effective technology for large production rate plants. All plants producing liquefied industrial gas products utilize Cryogenic technology.

The complexity of the Cryogenic Air separation process, the physical sizes of equipment, and the energy required to operate the process all vary with the number of gaseous and liquid products, required product purities, and required delivery pressures.

Nitrogen-only production plants are less complex and require less power to operate than an Oxygen -only plant making the same amount of product. Co-production of both products, when both are needed, increases capital and energy efficiency. Making these products in liquid form requires additional equipment and more than doubles the amount of power required per unit of delivered product.

Ar production is economical only as a co-product with Oxygen. Its production at high purity adds to the physical size and complexity of the plant.

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 Ar used for Semiconductor device fabrication requires Cryogenic distillation. Similarly, the only viable sources of the rare gases Neon, Krypton, and Xenon is the distillation of Air using at least two distillation columns.

1.2   AIR COMPOSITION Dry Air is relatively uniform in composition, with primary constituents as shown below. Ambient Air, may have up to about 5% vol 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: Nitrogen : 78.08%vol -- 75.47%wt -- 780,805 vpm; Oxygen : 20.95%vol -- 23.20%wt -- 209,450 vpm; Argon : 0.93%vol -- 1.28%wt --9,340 vp,; and Carbon dioxide : 0.039%vol -- 0.0606%wt -- 390 vpm.

2.0 CRYOGENIC AIR LIQUEFACTION PROCESS There are numerous variations in Air separation cycles used to produce industrial gas products. Design variations arise from differences in user requirements. Process cycles are somewhat different depending upon how many products are desired (either Nitrogen or Oxygen; both Oxygen and Nitrogen; or Nitrogen, Oxygen and Argon); the required product purities; the gaseous product delivery pressures desired; and whether one or more products will be produced and stored in liquid form.

All Cryogenic Air separation processes consist of similar steps. Variations in selected process configuration and pressure levels reflect the desired product mix (or mixes) and the priorities / evaluation criteria of the user. Some process cycles minimize capital cost, some minimize energy usage, some maximize product recovery, and some allow maximum operating flexibility.

The Cryogenic Air separation flow diagram shown below illustrates (in a generic fashion) many of the important steps in producing Nitrogen, Oxygen and Argon as both gas and liquid products. It does not represent any particular plant.

Pure 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 gases. The 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 ASU requires a refrigeration cycle that operates by means of the Joule–Thomson 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 THREE main steps:

(A)    The first process step in any ASP is filtering off dust and foreign matter, compressing, and cooling the incoming Air.

In most cases the Air is compressed to somewhere between 5 and 10 bar g, depending upon the intended product mix –gas/liquid, and desired product pressures. 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. The compressed Air is cooled, and much of the Water vapor in the incoming Air is condensed and removed, as the Air passes through a series of interstage coolers plus an aftercooler following the final stage of compression.

Because the final temperature of the compressed Air is limited by the temperature of the available cooling medium, which in almost all cases is limited by the wet or dry bulb temperature of the Air, the temperature of the compressed Air is sometimes well above optimum for maximizing the efficiency of downstream unit operations.

(B)    The next major step is removal of impurities, in particular, but not limited to, residual Water vapor plus Carbon dioxide.

These components of Air must be removed to meet product quality specifications. In addition, they must be removed prior to the Air entering the distillation portion of the plant; because very low temperatures would cause the Water and Carbon dioxide to freeze and deposit on the surfaces within the process equipment.

There are two basic approaches to removing the Water vapor and Carbon dioxide- "Molecular Sieve units" and "Reversing Exchangers".

Most new ASPs employ a “Molecular Sieve” "pre-purification unit" (PPU) to remove Carbon dioxide and Water from the incoming Air by adsorbing these molecules onto the surface of "Molecular Sieve" at near-ambient temperature.

Consequently, the compressed Air is often cooled to a somewhat lower temperature in a mechanical refrigeration system. In addition to lowering and stabilizing the inlet temperature to downstream compression and heat exchange systems, which enhances the efficiency and stability of the overall Air separation process, reducing the compressed Air temperature allows removal of additional Water vapor by condensation, reducing the Water-removal load in Molecular Sieve pre-purification equipment. with a mechanical refrigeration system or, In some cases, cooling may be accomplished with a direct contact aftercooler system (DCAC) instead of mechanical refrigeration. DCAC systems utilize cool, dry waste gas to chill a a circulating cooling Water stream in a "chill tower", and then use the chilled Water stream to cool the compressed Air in a second tower.

The process Air is generally passed through a Molecular Sieve bed, which removes any remaining Water vapor, 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 cause a problem in the subsequent Air distillation that could lead to explosions even.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. Molecular Sieve pre-purification is the natural choice when a high ratio of Nitrogen recovery is desired.

The other approach is to use “reversing” Heat Exchangers (RHE) to remove Water and Carbon dioxide. RHEs can be more cost effective for smaller production rate Nitrogen or Oxygen plants. In plants utilizing RHEs, the cool-down of the compressed Air feed is done in two sets of brazed Aluminum HEs.

In the "warm end" HEs, the incoming Air is cooled to a low enough temperature that the Water vapor and Carbon dioxide freeze out onto the walls of the HE Air passages. At frequent intervals, a set of valves reverse the duty of the Air and waste gas passages. After a passage in the HE is switched from incoming Air cooling to waste gas warming service, the very dry, partially-warmed waste gas evaporates the Water and sublimes the Carbon dioxide ices that were deposited during the last Air cooling period. These gases return to the atmosphere, and after they have been fully removed, the passage is returned to incoming Air cooling service.

When REs are used, cold absorption units are installed to remove any Hydrocarbons which make their way into the distillation system. (When a Molecular Sieve "front end" is used, Hydrocarbons are removed along with Water vapor and Carbon dioxide in the PPU.)

(C)    The next step is additional heat transfer against product and waste gas streams to bring the Air feed to Cryogenic temperature (approximately -185°C).

This cooling is done in Brazed Aluminum HEs which allow the exchange of heat between the incoming warm Air feed and cold product and waste gas streams exiting the separation process. The exiting gas streams are warmed to close-to-ambient Air temperature. Recovering frigories from the gaseous product streams and waste stream minimizes the amount of refrigeration that must be produced by the plant.

Typical Oxygen purities range from 97.5% to 99.5% and influence the maximum recovery of Oxygen. The refrigeration required for producing liquid products is obtained using the Joule-Thomson 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.

Process Air is passed through an integrated HE (usually a plate fin Brazed Aluminum 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 < 1 ppm) in a medium / high pressure (MP / 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).

Alternatively, the Condenser may be cooled by interchanging heat with a Reboiler in a low pressure (LP) distillation column (operating at 1.2 to1.3 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 to 20°Kelvin, requiring plate fin brazed aluminum HEs.

3.0  NITROGEN VS OXYGEN PLANTS NITROGEN plants may have only one column, although many have two Nitrogen leaves the top of each distillation column; Oxygen leaves from the bottom. Impure Oxygen produced in the initial (higher pressure) column is further purified in the second, lower pressure column.

4.0  ARGON RECOVERY AND PURIFICATION Argon, Ar, has a boiling point similar to that of Oxygen and will preferentially stay with the Oxygen product. If high purity Oxygen is required, Ar must be removed from the distillation system. Because the boiling point of Ar (87.3°K at standard conditions) lies between that of Oxygen (90.2°K) and Nitrogen (77.4°K), Ar builds up in the lower section of the low pressure column. When Ar is produced, a vapor side draw is taken from the low pressure column where the Ar concentration is highest. It is sent to another column rectifying the Ar 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 Ar purities of less than 1 ppm. Though Ar is present in less than 1% of the incoming Air, Ar column requires a significant amount of energy due to the high reflux ratio required (about 30) in the Ar column. Cooling of the Ar column can be supplied from cold expanded rich liquid or by Liquid Nitrogen.

Crude Ar  The Ar which is removed is usually processed in an additional "side-draw" crude Ar distillation column that is integrated with the low pressure column. Crude Ar may be vented, further processed on site, or collected as liquid and shipped to a remote "Ar refinery". The choice depends upon the quantity of Ar available and economic analysis of the various alternatives.

Pure Ar is typically produced from crude Ar by a multi-step process. The traditional approach is removal of the 2 – 3% Oxygen present in the crude Ar in a “de-oxo” unit. These small units chemically combine the Oxygen with Hydrogen in a catalyst-containing vessel. The resultant Water is easily removed (after cooling) in a Molecular Sieve drier. The Oxygen-free Ar stream is further processed in a "pure Ar" distillation column to remove residual Nitrogen and unreacted Hydrogen.

Advances in packed-column distillation technology have created a second Ar production option, totally Cryogenic Ar recovery that uses a very tall (but small diameter) distillation column to make the difficult Ar/ Oxygen separation. The amount of Ar that can be produced by a plant is limited by the amount of Oxygen processed in the distillation system; plus a number of other variables that affect the recovery percentage. These include the amount of Oxygen produced as liquid and the steadiness of plant operating conditions. Due to the naturally-occurring ratio of gases in Air, Ar production cannot exceed 4.4% of the Oxygen feed rate by volume or 5.5% by weight.

ASPs use a refrigeration cycle that is similar, in principle, to that used in home and automobile Air conditioning systems. One or more elevated pressure streams (which may be Nitrogen, waste gas, feed gas, or product gas, depending upon the type of plant) are reduced in pressure, which chills the stream.

To maximize chilling and plant energy efficiency, the pressure reduction (or expansion) takes place inside an Expander (a form of Turbine). Removing energy from the gas stream reduces its temperature more than would be the case with simple expansion across a valve. The energy produced by the expander is put to use to drive a process compressor, an electrical generator, or other energy-consuming device such as an oil pump or Air blower.

The cold gaseous products and waste streams that emerge from the Air separation columns are routed back through the front end HEs. As they are warmed to near-ambient temperature, they chill the incoming Air. As noted previously, the heat exchange between feed and product streams minimizes the net refrigeration load on the plant and, therefore, energy consumption.

Refrigeration is produced at Cryogenic temperature levels to compensate for heat leak into the cold equipment and for imperfect heat exchange between incoming and outgoing gaseous streams.

5.0  PRODUCTS COMPRESSION AND ALTERNATIVES 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[8]). It may also require additional external refrigeration during start-up.

Gaseous products typically exit the cold box (the insulated vessel containing the distillation columns and other equipment operating at very low temperatures) at relatively low pressures, often just over 1 Atm abs.

In general, the lower the delivery pressure, the higher the efficiency of the separation and purification process.

When products will be used at relatively low gauge pressure (up to several atmospheres) plants can be designed and operated to produce product at the required pressure. In many cases, however, it is more cost effective to produce the product at low pressure and compress the product gas to the required delivery pressure(s).

If GOx is required at moderate pressure, a process option is to use a "LOX boil" or "pumped LOX" cycle. These process cycles vaporize LOx at just above delivery pressure, against incoming Air which has been boosted in pressure to allow it to partially condense against the vaporizing LOx. These cycles have appeal because they effectively substitute additional stages of Air compression and a Cryogenic pump for an Oxygen compressor; which can result in a more compact and less expensive plant.

“Pumped LOX” systems are most applicable when there is fairly constant product demand. The heat for vaporizing and warming the vaporized LOX is drawn from the Air feed, which is partially condensed and sent to the distillation system, rapid changes in Oxygen demand will negatively affect plant performance, as each sudden change will tend to “bounce” the distillation columns.

The portions of the Cryogenic Air separation process that operate at very low temperatures, i.e., the distillation columns, HEs and cold interconnecting piping, must be well insulated. These items are located inside sealed (and Nitrogen purged) “cold boxes”, which are relatively tall structures that may be either rectangular or round in cross section. Cold boxes are "packed" with rock wool or perlite to provide insulation and minimize convection currents.

Depending on plant type and capacity, cold boxes may measure 2 to 4 m on a side and have a height of 15 to 60 m. They may be totally shop fabricated for rapid field erection, or the distillation columns, HEs, and their interconnecting manifolds may shop fabricated for field assembly and erection. This is done when a shop fabricated box would be too large or heavy to ship to the site.

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.

6.0  LIN-ASSIST PLANTS LIN assist plants are a special kind of Cryogenic plant that can cost-effectively produce gaseous Nitrogen at relatively low production rates. They differ from "normal" Cryogenic plants in that they do not have their own mechanical refrigeration system. They effectively "import" the refrigeration required for on-site Nitrogen production from a remote high-volume, high efficiency merchant liquid plant. They accomplish this by continuously injecting a small amount of LiN into the distillation process, where the "imported" LIN provides reflux for distillation, then vaporizes and mixes with the locally-produced gaseous Nitrogen, becoming part of the final product stream. Use of LIN-assist instead of a mechanical refrigeration system simplifies the plant design, makes the system somewhat more compact, reduces capital cost and can, under the right conditions, provide better overall economics than either an all-bulk-liquid supply or a new Cryogenic Nitrogen plant with a standard internal refrigeration cycle

7.0  LIQUEFIERS When a large percentage of plant production must be produced as liquid product(s), a supplemental refrigeration unit must be added to (or integrated into) a basic ASP. These units are called Liquefiers and most use Nitrogen as the primary working fluid. The required liquefier capacity is determined by considering the anticipated average daily demand for bulk liquid products and the need to produce some additional liquid to back up on-site gas customers served out of the same ASP. Liquefier capacity may range from a small fraction of the ASP capacity up to the plant's maximum production capacity for Oxygen plus Nitrogen and Ar.

The basic process cycle used in liquefiers has been unchanged for decades. The basic difference between newer and older liquefiers is that the maximum operating pressure rating of Cryogenic HEs has increased as Cryogenic HE manufacturing technology has improved. A typical new liquefier can be more energy efficient than one built thirty years ago if it employs higher peak cycle pressures and higher efficiency expanders.

8.0 STAND-ALONE LIQUEFIER A classic "stand alone" liquefier takes in near-ambient-temperature-and-pressure Nitrogen, compresses it, cools it, then expands the high pressure stream to produce refrigeration. In some liquefier systems a second refrigeration system using an environmentally-friendly form of refrigerant provides some of the higher temperature duty.

A stand-alone liquefier cycle produces only LiN. If it is desired to produce LOx, and both the ASU and liquefier are new units, a portion of the LiN production will typically be sent to the ASU to provide the refrigeration which is needed to allow withdraw the desired amount of LOx from the cold box.

If the liquefier is being added to an existing ASU, the ASU may not have been designed to allow high rates of LOx withdrawal. In that case, one solution is to add extra HE circuit to liquefy GOx while vaporizing LiN.

9.0  INTEGRATED LIQUEFIERS In highly integrated Air separation and liquefaction plants, most if not all of the refrigeration for both Air separation and product liquefaction is produced in the liquefier section. Refrigeration is transferred to the Air separation section of the plant through HEs and injection of LiN as distillation column reflux. Highly integrated merchant liquid production plants are less expensive to build and more thermodynamically efficient.

They can be very flexible in the sense of allowing production of varying mixes of LiN and LOx. On the other hand, they have a potential disadvantage - the liquefier cannot be shut down independent of the ASU.

When a totally new ASP is designed, an important question to address is whether the ASU and NLU (Nitrogen Liquefier Unit) will typically operate in tandem, or whether independent operation may be desirable. Bulk liquid only plants are good candidates for close integration with the Air separation process cycle. "Piggyback" plants with substantial pipelined gas demand may want the ability to operate independently of the liquefier.

Being able to operate the ASU without also operating the liquefier can be advantageous:

When liquid inventories are at high levels but a pipeline-supplied GOx customer continues to require a large amount of product, or when total liquid demand is consistently less than the full plant capacity. In this case, plants with independent liquefiers may be operated in what is commonly called a "campaign" mode - where periods of full capacity operation of the liquefier are alternated with periods when the liquefier is idle.

10.0  CAMPAIGN MODE OF OPERATION Campaign operations take advantage of the fact that liquefiers are most energy efficient when operating near full capacity and that shutdown and startup of an independent liquefier system can be done relatively easily and with little adverse impact on ASP operation. When the efficiency savings available with campaign operation are coupled with production run timing that takes advantage of lower-cost power periods (nights, weekends, etc.), significant operating cost savings can be achieved versus constant operation at reduced liquid production rates.

11.0  NON-CRYOGENIC PROCESSES Pressure swing adsorption, PSA provides separation of Oxygen or Nitrogen from Air without liquefaction. The process operates around ambient temperature; a zeolite (molecular sponge) is exposed to high pressure Air, then the Air is released and an adsorbed film of the desired gas is released. The size of compressor is much reduced over a liquefaction plant, and portable units can be made to provide Oxygen-enriched Air for medical purposes.

Vacuum swing adsorption is a similar process, but the product gas is evolved from the zeolite at sub-atmospheric pressure.

Membrane technologies can provide alternate, lower-energy approaches to Air separation. For example, a number of approaches are being explored for Oxygen generation.

Polymeric-membranes operating at ambient or warm temperatures, for example, may be able to produce Oxygen-enriched Air (25-50% Oxygen).

Ceramic membranes can provide high-purity Oxygen (90% or more) but require higher temperatures (800-900 deg C) to operate. These ceramic membranes include Ion Transport Membranes (ITM) and Oxygen Transport Membranes (OTM). Air Products and Chemicals Inc and Praxair are developing flat ITM and tubular OTM systems, respectively.     

12.0 APPLICATIONS Large amounts of Oxygen are required for coal gasification projects; Cryogenic plants producing 3000 tons/day are found in some projects. In steelmaking Oxygen is required for the basic Oxygen steelmaking. Large amounts of Nitrogen with low Oxygen impurities are used for inerting storage tanks of ships and tanks for petroleum products, or for protecting edible oil products from oxidation.

13.0  MANUFACTURERS Few key manufacturers of the Air Separation Units are: M/s Taiyo Nippon Sanso; L’Air Liquide; Yingde Gases Group Company Limited, Enerflex Ltd; Messer Group; Air Products; Praxair Inc; Linde Group; Hitachi; Universal Industrial Gases Inc.

14.0  SAFETY Needless to stress that Safety is of prime importance while handling Cryogenic plants right from fabrication thru Installation and Operation. Oil or its traces are not to be present in the piping, equipments of the ASU during fabrication, Installation and Testing. Oil, fire and Hydrocarbons are the most dangerous elements in an operating plant. In a large ASU during fabrication instead of a Non-lubricated Air Compressor a Lubricated compressor was deployed for pipelines and ASU testing etc. During a maintenance period on one night an Operative found the Lube Oil level in the compressor had fallen critically low, hence, it was topped up twice without ascertaining where the oil was going. The Duplex oil filters were not checked or switched during the process, little noticing one running filter was choked. Oil entered till Aluminium exchangers, piping etc before it was notied and the Air Compressor was stopped. This folly resulted in an elaborate cleaning of the affected parts attendant with many weeks of lost production and of course huge cost.

15.0 REFERENCE 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; 5. R. Agrawal, "Synthesis of Distillation Column Configurations for a Multicomponent Separation", Industrial Engineering Chemistry Research (1996) Vol. 35 pp. 1059-1071; 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; 7. Particulate matter from forest fires caused an explosion in the ASU of a Gas to Liquid plant; 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 Encyclopaedia; 11. L’Air Liquide India Ltd, New Delhi, and 12. Universal Industrial Gases Inc, Pennsylvania, USA.

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