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DEVELOPMENT OF CRYOGENIC AIR SEPARATION PLANTS

S.N. Sapali, College of Engineering, Pune (India)

The industrial gases like oxygen, nitrogen and argon are produced in gaseous or in liquid formin a wide variety of purities by separating air using a cryogenic technology. Each of the gases

produced may have a variety of uses and to meet these uses, different gas qualities are often

required. A few applications of these gases have been discussed in the following sections.

Applications of Nitrogen

Air is the main source of nitrogen. Nitrogen in large scale in its purest form can be

economically produced by cryogenic technology that finds wide spread applications.

The quality of nitrogen product required differs significantly from one industry to another. The

electronics industry demands the highest purity of 1 ppb O2, dust level 1/ std ft3

and moisture level

at 0.1 ppm. The US Semiconductor Industry Associations National Technology for

Semiconductors forecasts that the N2 purity requirements at a point of use (POU) as 0.1 to 1 ppb O2

in 2001, and 0.01 to 0.1 ppb in the year 2010 [Ha 96, Agr 96]. The purity of 2 to 100 ppm O 2 is

sufficient for aluminum, rubber, glass, textile, chemicals and steel industries [Che 89]. The purity

of 10 ppm at a pressure of 300 to 400 bara (30 to 40 Mpa) will meet its requirements in petroleum

industry. However the product pressure at POU depend on the specific applications.

Liquid nitrogen (LIN) is a useful source of cold and finds diverse applications. Some ofthese applications are mentioned here. (1) Using liquid nitrogen food can be frozen in few seconds

thus preserving much of its original taste, color and texture. It is reported that weight losses can be

reduced considerably when food is frozen cryogenically rather than by any other means. The purity

of LIN ranging from 95 to 98% is sufficient for freezing food (2) Cryosurgery is a technique that

destroys cancer cells by freezing. It has been used at some top medical centers for tumors of theprostate, liver, lung, breast and brain as well as for cataracts, gynecological problems and other

diseases. (3) LIN is used in storing biological specimens, especially bull semen for the cattleindustry. (4) In ground freezing, LIN enables tunneling and excavation operations to be performed

in wet and unstable soils. (5) In heat treatment of metals, LIN supposedly transforms metallurgical

properties, which improve the wear resistances of carbon tool steels. (6) The automobile tires have

been one of the most difficult items to recycle or even worse to discard. Cryogenic provides the

necessary technology for the effective recovery, separation, and reuse of all materials used in the

tire. In fact, the use of LIN is the only known way to recover the rubber from the steel radial tire.

(7) Mechanical breakdown of solids into smaller particles is known as grinding. Cryogenicgrinding is a method of powdering materials at sub-zero temperatures .The materials are frozen

with LIN when they are ground.

Applications of Oxygen

Oxygen was one of the first atmospheric gases liquefied by Cailletet and Pictet in 1877.Later Polish scientists Olzewski and Wroblewski, at Cracow in 1883 produced stable liquid oxygen

in U tube whose properties could be studied. Therefore, oxygen began its useful life in industry

early in the twentieth century.

A huge quantity of oxygen is consumed in steel making industry following LD or BOF

process. These process need 99.5% (conventional standard grade) purity of oxygen to accelerate

the oxidation and conversion of iron to steel . The daily consumption amounts to several thousand

tons and all the modern steel plants, therefore, have tonnage oxygen plants. One of the most


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common uses of oxygen is in the fabrication and cutting of metals using oxy-acetylene torch.Another major use of oxygen is in the field of medical profession to treat patients in breathing.

Oxygen is used in the preparation of chemicals. For example, manufacturing of ethylene oxide

requires 40% oxygen while acetylene consumes almost 20%. Titanium dioxide, propylene oxide,

and vinyl acetate need 10 15 percent O2 for their manufacture.

In glass manufacture, oxygen is added to enrich the combustion air in glass-meltingfurnaces. Jet aircraft for high altitude missions are equipped with oxygen systems for breathing

atmosphere. Coal gasification is also one of the large consumers of gaseous oxygen.

Applications of Argon

Usually argon is obtained from air which contains 0.0093% of argon by volume. It ishighly inert and finds its applications over a wide range of conditions, both at cryogenic and at high

temperatures. Argon is relatively expensive and its use is limited to applications where its highly

inert properties are essential. The largest consumer of argon in worldwide basis is the argon oxygen

decarburisation process for producing low carbon stainless steels [Dow 97]. MIG welding

developed by Airco in the 1940s and TIG welding represents large markets for argon. The lightbulb industry uses argon to fill light bulbs. This gives longer life to the filament because the argon

does not react, even at high temperatures.

Separation of Air into Components

Dry air is a mixture of component gases, consisting principally of nitrogen, oxygen and

argon with traces of other gases. The traditional process for separation of mixtures of light gases or

vapors involves cryogenic distillation. The liquefaction and distillation of air, termed cryogenic

distillation, is the most common and mature of the processes available to separate air. There are

other methods for separating air components, such as pressure swing adsorption (PSA), vacuum

swing adsorption (VSA), and membrane separation.

The purity levels achievable through membrane separation and PSA are much lower thanthat which can be achieved with cryogenic techniques [CLS 93, Jon 90, Sam 93, Sch 48, Sch 96

and GL 96]. Since air contains 0.93% argon and its boiling point lies between those of nitrogen and

oxygen, it remains, if not separated, as an impurity in either N2 or O2 product or in both. Argon

separation from N2 or O2 is necessary on two counts: one, because argon is a valuable product and

has a price in the market and two, because argon separation (removal) alone can ensure thatproduct O2 and N2 achieve their desired ppm or ppb purity level. When the product purities are in

ppm or ppb level, then the valuable product argon must be separated. Separation of argon is

possible only through cryogenic distillation.

The cryogenic distillation has the following advantages:

1. It is the only method to produce a large quantity of industrial gases economically, though

initial investment is high.

2. High purity products with impurities as low as fraction of ppb can be produced.

3. Both gaseous and liquid products can be obtained.

4. Transportation of products in liquid form is easy.5. Argon recovery is possible.

The choice of a particular method of separation, which may suit a particular application,

depends not only on the purity and the quantity desired but also on the investment required. It is

well known that cryogenic distillation is more economical for a facility that requires more than 100tons/day of oxygen. If oxygen purity above 99.5% is required, cryogenic distillation is the only

solution.


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Cryogenic Air Separation Plant

Most of the equipment involved in air separation plant are dedicated to the liquefaction

processes which involves compressors, equipment for separation of moisture, CO2, hydrocarbons,

heat exchangers, expansion devices, cryogenic pumps etc.

The compression block consists of a multistage centrifugal compressor with

intercoolers and after-coolers, is used to compress air to the pressure required in the high-pressurecolumn (HP column).

Air is then processed through a purification block, which consists of a

refrigeration/cooling system for air to be cooled to a suitable temperature for the H 2O, CO2 and

hydrocarbons to be adsorbed in the molecular sieve adsorbers.

Air further passes through the heat exchanger block, consisting of plate-fin heat

exchangers, where it is cooled to a temperature at which it is partially liquefied, using the cold from

the return product and waste streams.

Purge gases

Q (loss)

Partially liquefied air is sent to the double column system integrated with crude argon

column (distillation block), where it is separated into O2, N2 and crude argon fractions. Product

O2 is taken from the bottom of the low-pressure column (LP column). While part of it may be taken

as liquid product, the remaining is passed through main heat exchanger (in the heat exchanger

block) and collected as gaseous O2 product after the recovery of its cold. Liquid nitrogen is drawn

from the top of the high-pressure column. Gaseous N2 is drawn from the top of the pure nitrogen

column, which is placed on the top of the low pressure column. The non-condensable gases are

purged from the top of the high pressure column. The crude argon, normally with 97.5% argon

content, is drawn from the top of the crude argon column and is processed in purification unit toachieve the required purity level.

Recycle Compressor and liquid production block is responsible for creating the liquid

which is needed for distillation, to overcome heat inleak and to produce liquid cryogens. Product

gases or waste gases are pressurized to the required pressure in the recycle compressor and boostercompressor, cooled in the refrigeration units and/or heat exchagers and are further expanded in the

turbine to produce refrigeration, so that product gases can be liquefied. The output of the turbine is

used to drive the booster compressor.


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In the adsorption block, the hydrocarbons are removed from liquid oxygen and oxygen-rich bottom product of HP column by passing through the MS adsorbers

Though the raw material is available absolutely free in nature, cryogenic air separation plants are

energy intensive. Under the growing crunch of energy crisis, the air separation industry has no

option but to seek improvement through optimized design and operation.

History of Evolution of Air Separation Industry

The genesis of cryogenics can be traced to the year 1877 when oxygen was liquefied by the

scientists Cailletet and Pictet of the French Academy of Sciences and Geneva respectively. These

events proved to be a landmark advancement in low temperature physics. To obtain a mist of

oxygen, Cailletet compressed oxygen to about 300 bara (30 MPa) and cooled to 29O

C. Uponreleasing the pressure of O2, he observed a mist of O2, which quickly disappeared. Pictet liquefied

O2 by compressing it to about 320 bara (32 MPa) pressure and cooling through cascade process.

Later Polish scientists Olzewski and Wroblewski at Cracow in 1883 produced stable liquid oxygen

in U tube whose properties could be studied.

Within a span of six years from 1877 to 1883, not only were the constituents of

atmospheric air liquefied, but more significantly, the basic processes for gas liquefaction and

refrigeration were also demonstrated. After the liquefaction of air and other gases, it was the turn of

the storage system to be developed. It became necessary to provide good insulation to prevent or to

reduce the ingress of heat from the surroundings. In 1892, James Dewar became the first scientist

to develop a vessel with vacuum jacket for cryogenic fluid storage and this solved the problem of

storing liquefied gases. It was a double walled glass vessel with the inner surface silvered that

resulted in a reduction of evaporation rate of the stored fluid by a factor of over 20 to that of a bare

container.

Eighteen years after the first liquefaction of oxygen took place, the scientist Carl von Linde

of Germany, in 1895, used the Joule-Thompson isenthalpic expansion effect for large-scale

liquefaction of air. By 1898, Charles Tripler, an engineer in New York, had constructed a similar

but much larger air liquefier driven by a 75 kW steam engine, which produced literally gallons of

liquid air per hour [Scu 90].In 1902 George Claude, a French engineer, developed a practical system for air

liquefaction in which a large portion of cooling effect of the system was obtained through the use

of an expansion engine. Claude first used reciprocating engines with leather seals. During the same

year Claude established L'Air Liquide to develop and produce his liquefaction systems.

Peter Kapitza (1939) of USSR produced an experimental air liquefier operating at 6 bara

(600 kPa) pressure using an expansion turbine which produced 15,000 Nm3/h gaseous oxygen. He

succeeded in liquefying helium by employing an expansion engine without the use of liquidnitrogen pre-cooling. Ruhemann (1930) developed a miniature Joule-Thompson liquefier for

hydrogen and helium. Later in 1932, Simon invented the single expansion helium liquefier based

on the concept of Cailletet.

Linde (1896) developed a rectification device called Linde single column, an apparatus forfractional distillation, which was fully made operative in 1902. In 1910, he developed a double

column system to produce both oxygen and nitrogen simultaneously.

A new process for the production of oxygen was developed in this period through the

electrolysis of water. This however was unable to match the existing processes due to the high cost

of power required and the lack of demand for the hydrogen by-product at that time. During the

period of World War I from 1914 18, the production of oxygen was greatly increased due to

demands particularly in the welding and allied activities. At the end of war, oxygen production


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turned into a firmly established business. Oxygen was distributed under pressure in steel cylinders.As the demand for oxygen increased, it became increasingly difficult to transport the heavy

metallic containers from the place of production to the places of use. In 1920 the development of

the Heylandt process in Germany was a significant step forward for producing liquid oxygen.

Around 1925, another important invention took place in the field of steel production from

iron ore. It was the LD process that needed a large quantity of gaseous oxygen to oxidize theimpurities of the ore to recover steel. To meet the demand of gaseous oxygen, German Company,

Linde developed its Linde-Frnkl process system.

As the air separation plants grew in size, the opportunity for argon separation along with

N2 and O2 arose. The classical method of argon recovery was introduced in the early 1930s. By this

time the air separation plant was equipped with three or more columns.

Development of Air Separation Cycles

There has been a continuous developments of air separation cycles in terms of percentage

of recovery, power consumption, purity, ease of maintenance, cost of manufacturing etc. While the

earlier developments are spectacular in many respects, the later day improvements are found to berather gradual. The plants are categorized broadly into 19 categories and hopefully the description

below would present a chronicle for air separation plants that is logical and understandable. Theplants have generally been given two adjectives: 1) gas, liquid or both and 2) high-pressure,

medium pressure or low pressure. The first adjective refers to the state of product (liquid or gas)

that the plant is producing. The second refers to the highest pressure in the plant that is exerted

either on the main air flow or recycle air /nitrogen flow. High pressure usually refers to somewhere

between 130 bara to 300 bara (13 MPa to 30 MPa). Medium pressure refers to 30 bara to 60 bara (3

MPa to 6 MPa), while the low pressure from 6 bara to 12 bara (600 kPa to 1200 kPa). The

categories of air separation plants are given below:

1. Linde single column producing gaseous oxygen

2. Linde double column producing gaseous oxygen

3. Claude single column producing gaseous oxygen

4. Double column system producing gaseous oxygen using expansion engine

5. Heylandt high pressure system producing liquid oxygen using room temperature

expansion engine

6. Linde-Frnkl system using reversing regenerator producing gaseous oxygen and

nitrogen7. Rescol cycle producing gaseous oxygen using turbine

8. Reversing regenerator system producing compressed gaseous oxygen and nitrogen

9. Pumped liquid oxygen cycle with split air feed producing compressed oxygen

10. Single column gaseous nitrogen production

11. Argon separation system12. Waste-nitrogen-recycled high-pressure liquid plant

13. Nitrogen-from-HP-column-recycled medium pressure liquid plant

14. Nitrogen-from-HP- and LP-column-recycled medium pressure liquid plant

15. Air-recycled medium pressure liquid plant16. Air-recycled low-pressure gas plant17. HP-column-nitrogen-recycled low-pressure gas plant

18. LP-column-nitrogen-recycled low-pressure liquid plant

19. Low-pressure gas plant with structured packing

It is not possible to explain each one of above listed systems however few are covered in

the following text.


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Linde Single-Column System

The Linde single column system, as shown in Fig 2, consists of a concentric tube type heat

exchanger, a distillation column and a JT valve. The feed air is cooled against product gas in theheat exchanger and is further cooled against the liquid in the kettle. This serves two purposes: (1)

air, which is cooled to lower temperature, yields a greater amount of liquid upon expansion and (2)

it supplies heat to the kettle and generates vapor required for the distillation.The air, free from water vapor and CO2 is compressed to a pressure in the range of 3060 bara

(3-6 MPa) to get O2 product in the gaseous form. The pressure needed to obtain the O2 product in

the liquid form is about 200 bara (20 MPa). This cycle possesses the following disadvantages:

1. Only pure oxygen can be produced.

2. A large quantity of oxygen is wasted in the impure nitrogen exhaust gas.

3.

WN2

GO

JT valve

Main HE

LOX

Fig 2: Linde single column

Air

One column can produce only one pure product: oxygen or nitrogen. Two columns are

needed to separate oxygen and nitrogen economically. Further, production of argon requires

separate columns. The boiling point of argon lies between those of N2 and O2 and hence it is

distributed between those two products. The double-column system, as shown in Fig 3, is rather aspecial one to air separation system. This is because, the composition of air and the properties of its

constituents are such that the two columns can be thermally 'linked' through the condenser-reboilerto provide the necessary reflux at the top of the HP column and 'boil up' at the bottom of LP

column. This solution provided by Carl von Linde in 1910 marked the beginning of the economical

production of atmospheric gases and their consequent widespread industrial use.

Linde Double-Column System

The LP column operating at near atmospheric pressure is placed over the HP column, which

operates at a pressure of about 5 bara to 6 bara (500 to 600 kPa). This difference of pressures in the

columns provides the much needed temperature difference across the condenser-reboiler. Theboiling liquid oxygen in the base of the LP column provides the cooling effect to condense the

vapor N2 in the HP column.Depending on the number of stages in LP column, any purity of either or both the products

can be practically achieved. When extremely high purity products are desired, the argon component

must also be separated.

Heylandt High-pressure System Producing Liquid Oxygen Using Room Temperature

Expansion Engine

The flow sheet of a typical Heylandt plant is shown in Fig 4. Liquid O2 and Liquid N2 are

produced from this plant. This process uses a high pressure of the order of 100 to 200 bara (20


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MPa) and a high pressure expansion engine operating on the Claude principle with part of the airinput expanded in the engine. Because of the high working pressure, it is possible to produce the

Fig 3: Linde double column system

LP column

GO

LOX

Reboiler/condenser

HP column

Boiler

Rich liquid

LIN

GO

GAN

Main HE

Air

whole O2 product in the liquid form having purity of about 99.5%. Although the process used more

expensive high-pressure equipment such as reciprocating compressors, it was a reliable process to

produce large quantity of liquid oxygen at that time.

The reciprocating expansion engine is required to operate between 250 K at the inlet and113 K at the outlet (little above saturation temperature of air). Although low freezing point oil was

used for the purpose of lubrication, it did not remain in liquid phase due to low operatingtemperatures. In spite of using good quality oil filters on the discharge side of the engine, the oil

carry-over problem was quite serious [Jen 54].

LP column

Fig 4: Heylandt cycle

LOX

LIN

Compressor

Feed air

Filter

GN2

HP column

Main HE

NH3Forecooler

Precooler

Expander


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Gardner et al. [GS 57] have carried out the detailed thermodynamic analyses of theHeylandt cycle and stated that it is the most efficient process cycle to produce liquid oxygen having

an overall thermodynamic efficiency of 23%. The maximum irreversibilities observed are in the

compressor and in the expansion engine. The major part of the compressor power is being utilized

to produce the refrigeration for getting the oxygen in liquid form. Overall power consumption is

about 1.24 kWh/Nm3

of liquid oxygen (measured as gas) [Gar 86].

The increased availability of relatively low-cost oxygen encouraged its applications in

industries. There was a large demand for the oxygen gas in the decarburisation of steel, in an

electric arc and open-hearth furnaces (almost not used in these days) and also in the bottom-blown

Bessemer process of steel production in the steel industry. The steel-making process needs gaseous

oxygen to accelerate the oxidation and conversion of iron to steel. To meet the demand, in 1930s,

Linde Company developed a low-cost gaseous oxygen production known as LindeFrnkl process.

Linde-Frnkl System Using Reversing Regenerator Producing Gaseous O2 and N2

Linde-Frankl process for gaseous oxygen production is shown in Fig5.Air is compressed

to 56 bara (500600 kPa) pressure in the centrifugal compressor and water and CO2 are removed

in the regenerators. The regenerators were used for the first time in this cycle [Sch 48]. Moisture

and CO2 are deposited on the packing during the air on cycle and these are revaporised with thereturn low-pressure products during the air 'off' cycle, which generally takes two to four minutes.

For complete resublimation of CO2 and moisture, a small excess flow of return gas is needed. This

greatly reduced purification costs, which led to an overall reduction in production costs of gaseous

oxygen.

HP column

Fig 5: Linde-Fr nkl cycle with turbineRL

Feed air

Air

N2 gasout

NH3 HE

HE

Expander

Gaseous air

LN2

GO

N2 gas outO2 gas out

Regenerators

Condenser/reboiler

As to the question of cold production, the isenthalpic expansion (Joule-Thomson effect) which is

used to a large extent is thermodynamically inefficient and has to be abandoned as the sole source

of cold is in favor of engines with production of external work. To meet the refrigeration demand,

nitrogen vapor from the top of HP column is expanded in the turbine as shown in Fig 5. Haselden

[Has 58] carried out the thermodynamic analyses of Linde-Frankl cycle with the Lachmann air


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arrangement, and further recommended a modified Linde-Frnkl cycle which can operate with theair compressed to only 4.0 bara (400 kPa) pressure, producing oxygen at a purity of 90%. The

cycle is discussed in detail in the references Bar 85 and Isa 89.

The improvements in cryogenic air separation plant during the period 1930 1960

occurred from the vastly improved components though the basic processes remained the same.

Improvements were made by better pre-purification units to remove water vapor, carbon dioxide,

and hydrocarbons from the feed air, more efficient heat exchangers which allow lower temperatureapproaches, more efficient compressors and expanders and better distillation column design having

more efficient contact between liquid and vapor and tighter tray spacing. Immediately after the

Second World War, considerable efforts were made to reduce the production cost by making large

liquid and gaseous oxygen plants [Spr 85, SGE 96]. To achieve this, the improvements in the

process cycle were also needed. As the compressor consumes more than 50% of the total energy

required for separation, the improvement could not be so easy in this component. As stated earlier,

addition of the expansion engine made the cold production process more efficient. Schuftan [Sch

48] reached a conclusion that large improvements are possible by making the rectification more

efficient and a first step in this direction was the adoption of the Lachmann principle, together with

the development of more efficient trays. Effort was necessary in improving the thermodynamic

irreversibility and efficiency of separation. Schuftan [Sch 48] devised a cycle known as Rescol

cycle shown in Fig 2.2.7. He also reported its analyses and compared it with other cycles such asLindeFrnkl process with Lachmann air and reciprocating expansion engine and Kellogg cycle.

Reversing exchangers of brazed aluminum plate and fin type, which removes carbon

dioxide and moisture from the air and also exchange heat between the incoming air and the

separation streams, were first used around 1955.

Reversing Regenerator System Producing Compressed Gaseous Oxygen and Nitrogen

The regenerators in an air separation plant perform purification function as well as heat

exchange. Water vapor from the intake air stream is deposited, mainly as snow, in the warmer

section of the regenerator and CO2 is deposited in a similar way towards the cold end. On

switching, the cold stream removes (purge) some or all of the deposits depending on pressure,

flow rate, and temperature. Since the pressures and flow rates are normally fixed by process, thetemperatures need to be modified by a Trumpler stream, a second cold stream operating only inthe cold section of the regenerator (Fig 6).

Gaseous oxygen at high pressure is needed in many applications. For example, large

quantity of oxygen is distributed to steel works through pipeline at 40 bara (4 MPa). The simplest

scheme to produce oxygen gas at high pressure is shown in Fig 6. In this scheme product oxygen at

room temperature and at ambient pressure is raised to 30 bara to 40 bara (3 MPa to 4 MPa) in a

multi-stage centrifugal compressor. For the pressure of the order of 70100 bara (710 MPa) one

needs to use the reciprocating compressors. The choice of compressors is dependent upon thevolumetric flow and for small flow rates reciprocating types are preferred.

Pumped Liquid Oxygen Cycle with Split Feed Air Producing Compressed Oxygen

Many problems were encountered in 1950s and 1960s with gaseous oxygen compressors,

particularly due to fire hazard and explosions. This led to the development of a cycle in which

liquid oxygen is pumped to the required pressure and heated to ambient temperature against feed

air or recycled nitrogen. The term 'split' denotes that separate fractions of the air are compressed to

different pressures. About of the feed air is compressed to the HP column pressure and cooledand purified in the conventional reversing heat exchanger. The rest 1/4 of feed air is compressed to

high pressure and cooled in the high-pressure heat exchanger against oxygen stream and N2 stream


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before being purified in MS adsorbers. The process consumes 8% more power and requires morecapital investment than low-pressure plant with O2 compression.

HP column

Cooler

Cooler

Expander

Feed air

LP column

HCA

RR

WN2GAN

O2 C

C

Legend:C = compressor, O2 C = oxygen compressor, HCA = hydrocarbon absorber, HP = high pressure, N2 C =nitrogen compressor, LP = low pressure, RR = reversing regenerator, GAN = gaseous nitrogen, WN2 = wastenitrogen, TS = Trumpler stream

TS

Cooler

N2 C

GO

GO

Low-pressure Gas Plant with Structured Packing

There have been large improvements in the process equipment, which are primarilyin the areas of compressor efficiency, heat exchanger efficiency, quality of molecular sieve

and turbine efficiency. But the most impressive development of the 80's decade has been

the development of structured packing by Sulzer for the distillation columns [SDR 90].The reduction of price, easier availability and enhancement of performance-related

knowledge of the designers have lead to the widespread use of structured packing in the

decade 1990, which has resulted in the elimination of H2 dosification for the production ofpure argon. With the use of structured packing, argon can now be produced at 5 ppm

impurity without going through the expenditure and hazards of using H2. Fig 7 shows a

primarily gaseous plant, which uses structured packing for the column.


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H2O

PL

PL

GORLL

LOX

L Ar (5 ppm)

Air

RLV

HP column

LPC

P

ureArcol

Fig 7: A low-pressure modern gaseous oxygen plant (1995)

MPN2WN2PN2

C Ar Col - IIPump

Purge

C Ar Col - 1

REFERENCES

[Agr 96] Agrawal, R. Production of Ultrahigh Purity Nitrogen Free of Light Impurities,

proceedings of the Munich Meeting on Air Separation Technology, Munich, pp.25

29, October 1011, 1996.

[Bar 85] Barron, R.F. Cryogenic Systems, Oxford University Press, New York, 1985.[CC 89] Chen, G.K. and Chaung, K.T. Recent Developments in Distillation, Hydrocarbon

Processing, pp.37 45, February 1989.[Cho 99] Chowdhury, K. Advances in Cryogenic Air Separation, presented in 21

stNational Seminar

on Industrial Gases, held at Bangalore, pp. 2930, January, 1999.

[CLS 93] Campbell, M.J., Lagree, D.A. and Smolarek, J. Advances in Oxygen Production

by Pressure Swing Adsorption, proceedings of AICHE Symposium Series, Vol. 89,

edited by E.L. Garden and L.A. Wenzel, New York, pp.104 108, 1993.

[Dow 97] Downie, N.A. Industrial gases, Blackie Academic & Professional, pp.371 381,

New York, 1997.

[Fai 93] Fair, J.R. How to Design Baffle Tray Columns, Hydrocarbon Processing, pp.75 80, May 1993.

[Gar 86] Gardener, J. One Hundred Years of Commercial Oxygen Production, a BOC

article, 1986.

[GL 96] Grahl, M. and Leitgeb, P. Oxygen Production by Pressure Swing Adsorption,

proceedings of the Munich Meeting on Air Separation Technology, pp 135 146,

October 10 11, 1996.

[GP 86] Grenier, M. and Petit, P. Cryogenic Air Separation: The Last Twenty Years,

Advances In Cryogenic Engineering, edited by R.W. Fast, Plenum Press, Illinois,

Vol. 31, pp. 1063 1070, 1986.


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[GS 57] Gardner, J.B. and Smith, K.C. Power Consumption and Thermodynamic

Reversibility in Low Temperature Refrigeration and Separation Processes, Advances

in Cryogenic Engineering, edited by K.D. Timmerhaus, Plenum Press, Colorado,

Vol. 3, pp. 32 46, 1957.

[Ha 96] Ha, B. Cryogenic Processes for Ultra High Purity Gases, proceedings of the Munich

Meeting on Air Separation Technology, Munich, pp.13

23, October 10

11, 1996.[Has 58] Haselden, G.G. An Approach to Minimum Power Consumption in Low

Temperature Gas Separation, Trans. Instn. Chem. Engrs, Vol. 36, pp. 123 132,

1958.

[Isa 89] Isalski, W.H. Separation of Gases, Clarendon Press, Oxford, 1989.

[Jen 54] Jensen, J.E. Performance of an Air Expansion Engine, Advances in Cryogenic

Engineering, edited by K.D. Timmerhaus, Plenum Press, Colorado, Vol. 1, pp.105

110, 1954.

[Jon 90] Jonas, L.A. Latest Developments in the Field of Air Separation, Gas Separation

Technology, edited by E.F. Vansant and R. Dewolfs, Elsevier Science Publishers

B.V., Amsterdam, pp.161 180, 1990.

[Sam 93] Samy S. J. A. Steady State and Dynamic Simulation of Cryogenic Distillation

Columns, a Ph.D. thesis, IIT Kharagpur, 1993.[SC 01] Sapali, S.N. and Chowdhury, K. Cryogenic Air Separation Plants: Parametric

Evaluation and Exergy Analyses, paper presented in 23rd

National seminar on

industrial gases, held at Goa, January 1920, 2001,

[Sch 48] Schuftan, P.M. Modern Gaseous Oxygen Production Methods, Trans. Inst. Chem.

Engg., pp. 30 36, 1948.

[Sch 96] Schulte-Schulze Berndt, A. Nitrogen Production based on Pressure Swing

Adsorption, proceedings of the Munich Meeting on Air Separation Technology,

Munich, pp. 185 214, October 10-11, 1996.

[Scu 90] Scurlock, R.G. A Matter of Degrees: A Brief History of Cryogenics, Cryogenics,

Vol. 30, pp. 483 500, June 1990.

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