métodos no criogéncos

8/6/2019 mtodos no criogncos

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1. Introduction

Methods of air separation include both cryogenic and

non-cryogenic systems. The cryogenic systems have the

capability to deliver the largest capacities for products

and for very high purities. Non-cryogenic systems are

employed at the lower end of the production scale and

generally for lower product purities. The latter systems

Contents

1. Introduction ................................................................................................ ........................................................... 159

2. Recent developments in cryogenic air separation .................................................................................................. 160

2.1. Tonnage plants .............................................................................................................................................. 160

2.2. Medium capacity plant.................................................................................................................................. 1642.3. Plants for production of liquid................................................................................................ ...................... 165

3. Developments in pressure swing adsorption .......................................................................................................... 166

3.1. Nitrogen PSA ................................................................................................................................................ 166

3.2. Oxygen PSA/VPSA (vacuum pressure swing adsorption) ............................................................................. 167

3.3. Membrane process................................................................................................ ......................................... 167

4. Other methods of air separation ............................................................................................................................ 167

5. Process integration ................................................................................................................................................. 168

6. Capital costs................................................................................................ ........................................................... 169

7. Technology................................................................................................ ............................................................. 170

7.1. For cryogenic plant ....................................................................................................................................... 170

7.2. For PSA plants for nitrogen and PSA and VPSA for oxygen ...................................................................... 170

7.3. For membrane processes ................................................................................................ ............................... 170

7.4. In addition, for all processes ......................................................................................................................... 170

References ................................................................................................................................................................... 171

Nomenclature

tonnes/day metric tonnes per day

% (purity) gaseous volume composition percen-

tage

% (on-stream) percentage of available production

time per annum

vppm gaseous volume parts per million

vppb gaseous volume parts per billion

(109)

bar.g bar, pressure above atmospheric

kWh/Nm3 Specic Power for production, kilo-

watt hours per normal cubic metre

Nm3

Normal cubic metre: gaseous volumeat C, 1.013 bar absolute pressure, dry

Terminology

LP column low pressure distillation column

(operating pressure about 0.4 bar g)

HP column high pressure distillation column

(operating pressure about 5.0 bar g)PL poor liquid liquid nitrogen from

top of HP column (poor in oxygen)

PSA pressure swing adsorption regen-

eration by pressure variation in

adsorbers

RL rich liquid liquid from base of

HP column, `rich' in oxygen (38

40%)

VPSA vacuum pressure swing adsorption,

as PSA but additional regeneration

of adsorbers by vacuum

EOR enhanced oil recovery techniquesfor extracting extra oil from wells i.e.

more than that available from the

natural ow of oil using reservoir

pressure. In this paper, the use of

high pressure nitrogen

W.F. Castle / International Journal of Refrigeration 25 (2002) 158172 159


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are based on pressure-swing adsorption (PSA) and mem-

brane technologies. Other systems exist, based on chemi-

cal processes, but currently do not compete with the three

processes mentioned.

Each of these three systems, cryogenic, PSA and

membrane, are considered in the paper and recent

developments and potential improvements noted.Table 1 shows the typical composition of dry air. The

main constituents are oxygen, nitrogen and argon. Other

components, such as the rare gases can be extracted by the

cryogenic route and are used commercially.

2. Recent developments in cryogenic air separation

2.1. Tonnage plants

Plants producing 100 tonnes per day or more of pro-

duct are termed `tonnage' plants. These currently range

to over 3000 tonnes per day of oxygen and, alternately,to 10,000 tonnes per day of nitrogen.

A typical process for tonnage cryogenic air separation

is shown as a simplied diagram in Fig. 1. This illus-

trates a low pressure cycle for production of oxygen,

nitrogen and argon. These are the main products of

large-scale air separation.

Cryogenic separation of air involves the following

steps, as illustrated:

. Air compression and re-cooling.

. Removal of water and carbon dioxide from the air

feed by use of molecular sieve adsorbers.. Heat exchange to cool air for liquefaction and to

re-warm gaseous products of air separation.

. Refrigeration to allow liquefaction of air. This is

achieved by work expansion of air taken from a

mid-point of the heat exchanger. This air has

undergone further compression by a booster com-

pressor directly coupled to the expander. Thus it

uses the work generated in the expander.

. Distillation to separate, mainly the oxygen, nitro-

gen and argon in the air. A double column system

is generally used, the high pressure (HP) column

Fig. 1. Simplied owsheet of low pressure air separation plant.

Fig. 1. Schema simplie d'une installation de separation d'air a basse pression.

Table 1

Typical composition of dry air at sea levela

Tableau 1

Composition typique de l'air sec

Component Fraction

of air

Componentb Fraction

of air

Nitrogen 78.09% Methane 12 vppmc

Oxygen 20.95% Acetylene


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providing two reux streams for the low pressure

column. The two reux streams are: poor liquid

(PL) comprising liquid nitrogen, and rich liquid

(RL) containing about 38-40% oxygen. The main

condenser-reboiler condenses nitrogen in the HP

column, while vaporising, by heat exchange, liquid

oxygen in the base of the LP (low pressure) column.Argon is produced by drawing o a feed stream

from the LP column containing about 1012%

argon and separating it from oxygen and some

nitrogen. This separation requires a large number

of theoretical stages and is accomplished by using

low pressure drop structure packing, rather than

trays, to produce high purity argon [1].

. Oxygen and nitrogen products can be compressed

externally, or, pumped internally, to the required

pressure. The process can provide a fraction of

liquid as well as gaseous products, or alternately,

totally liquid products.

The low pressure tonnage oxygen cycle and similar

cycles producing oxygen at just above atmospheric

pressure have been developed over the years to improve

the power consumption.

Other process cycles are used, depending on the scale of

production, the application and products required [24].

A number of developments have led to this continual

improvement. In the past ve years power consumption

has been reduced by about 6%. It would seem that we

should expect further reduction in specic power to

0.28-0.3 kWh/Nm3 oxygen by 2010! (see Fig. 2).The contributions to reduced power consumption are

touched upon below.

The major consumer of power for the low pressure

plant in separating air is the air compressor. Com-

pressors (and expander) developments have used three-

dimensional machining, integrated with computational

uid dynamics, to form wheels and vanes giving opti-

mum eciency for their duty [810].

Development of active magnetic bearings, has further

increased overall eciency of the expansion turbines by

reducing friction losses [11].

Air purication has beneted from improved adsor-bents, allied with substantial work to optimise the engi-

neering and design of the equipment to reduce capital

and operating costs.

Large, vacuum-brazed heat exchangers provide ecient

heat transfer, conserving refrigeration and permitting

Fig. 2. Progress in reduction of specic power to produce low pressure gaseous oxygen.

Fig. 2. Production d'oxygene gazeux a basse pression : progres en matiere de consommation d'energie.



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ecient multi-stream operation. As vacuum braze facil-

ities are replaced it can be expected that new facilities

will be provided to provide still larger cores, giving a

potential for more overall cost reduction.

Downow condenser-reboilers can be used to con-

tribute to lower power consumption. The conventional

arrangement of condenser/reboiler is of heat exchangercores submerged in the liquid oxygen bath in the LP

column sump. The downow arrangement, with vir-

tually no hydraulic head to increase the temperature of

the liquid oxygen, can operate with temperature dier-

ences as low as 0.4 K, compared to 1.01.5 K, an

advantage of some 34% less power. Both arrangements

need very careful attention to purication to remove

hydrocarbons from the liquid oxygen, but the downow

arrangement needs even greater care.

Refrigeration circuits using high-pressure air to max-

imise cooling capacity together with pumping of liquid

oxygen to give product at user pressure (evaporating itin the heat exchangers) are current features [1214].

These processes are readily integrated with liquefaction

cycles using air as a working uid. It is possible to pro-

vide for large proportions of liquid product with such

plants. Fig. 3 illustrates a simplied owsheet of a liquid

pump plant.

The inlet air pressure of the expansion turbines is boos-

ted by the additional compressor so that a large reserve of

refrigeration can be used for additional liquid oxygen,

nitrogen and argon products as well as that required to

provide for the pressurised oxygen product. Such circuits

are employed in major installations for example in co-

generation schemes and gasication processes.Complex process schemes to improve overall recovery

of products or to reduce power consumption, or both,

are additional features of large plant.

In distillation a signicant advance has been the use

of low pressure-drop, high eciency structured packing

instead of the earlier use of sieve trays [6,15].

The purity obtainable in the argon column with dis-

tillation trays and their relatively high pressure drop is

limited. The pressure at the top of the argon column is

xed by the need to provide for ow through the plant

to atmosphere. The saturation temperature of the

expanded rich liquid from the HP (high pressure) col-umn in the argon column condenser and the tempera-

ture dierence across condenser then xes the pressure

at the top of the argon column. The system thus limits

the pressure drop available for distillation in the argon

column. With conventional trays only about 4050 the-

oretical stages of separation can be achieved. With the

Fig. 3. Simplied owsheet of a liquid pump plant.

Fig. 3. Schema simplie d'une installation a pompe.

162 W.F. Castle / International Journal of Refrigeration 25 (2002) 158172


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use of structured packing as many as 150220 theore-

tical stages of separation can be provided within this

limitation. Thus a very high purity of argon containing

only 1 or 2 vppm of oxygen can be produced cryogeni-

cally without resorting to catalytic combustion with

hydrogen to remove the oxygen.

Structured packing [15] is a form of liquid-vapourcontact device having low pressure drop and high e-

ciency. The packing consists of individual, corrugated,

and perforated metal sheets coiled alternately so that the

adjacent corrugations are at 90 to each other and thus

create intersecting open channels. [Structured packing is

increasingly being used in the LP (low pressure) column

for distillation rather than trays, since the low pressure

drop characteristic has advantages in that application].

Modern plants have a high recovery of products from

the air, together with a need to avoid variation in the

usually very high product purity.

Computer simulation of processes has enabled design-ers to enhance eciency and application of process cycles

by better integration of the specic unit operations.

Also computer control of plant has provided a means of

ensuring the yield of air separation products at the ever

more demanding recoveries and purity requirements nee-

ded for present day, and future, supply. Computer control

systems are widely used to ensure maximum stability and

purity control during both steady state and dynamic

conditions [16,17]. The systems can also be used to

control the plants remotely.

Double column systems have been used for nitrogen

and have been applied to produce large quantities of

nitrogen for enhanced oil recovery [57]. With such a

cycle nitrogen is produced at 70% yield from the air and

two product streams at 8 and 15 barg. For applicationsto enhanced oil recovery, (EOR) these are further com-

pressed externally to pressures of 275400 barg for

injection into the well.

A Mexican project [7] uses four separate air separa-

tion plants. Each plant is the largest size yet built, and

produces over 10,000 tonnes/day of very high-pressure

nitrogen, used to improve the recovery of oil from

reservoirs. The air processed is equivalent to a plant

producing 3400 tonnes per day of oxygen. The progress

in plant capacity over the years is illustrated in Fig. 4.

Interestingly the curve, using data from dierent sour-

ces, shows a reasonably smooth progression. Extrapolat-ing the curve, it would seem that by 2010 the maximum

capacity for a single stream plant will be around 4000

4500 tonnes per day oxygen product.

Reliability and safety of cryogenic air separation

plants are well-proven, on-stream reliability being in

excess of 99.5%. Their reliability is usually greater than

that of chemical or metallurgical process units supplied

by the oxygen plants. Major maintenance can usually be

Fig. 4. The largest capacity of oxygen plant versus time.

Fig. 4. Capacite maximale des installations en fonction de l'epoque de la construction.



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scheduled for intervals of four years. Reference can be

made to safe design and operating practices for air

separation recommended by the British Cryoengineering

Society in their Cryogenic Safety Manual [18].

In summary, then, it would seem that we should expect

to see reduced specic power consumption, still larger

plant for tonnage application, and a consolidation of theuse of liquid pump plants for large projects. This latter

feature largely avoids the need for an expensive gaseous

oxygen compressor, albeit at a penalty of a higher

overall power consumption.

2.2. Medium capacity plant

Similar plant cycles to those described above are used

if single products, e.g. oxygen or nitrogen alone are

required [24,19]. For nitrogen, single column plants are

employed (Fig. 5). In these the air is compressed, cooled

and enters the column which separates the air intonitrogen and a liquid rich in oxygen. The latter is

expanded into a condenser reboiler to liquefy some of

the nitrogen so as to provide reux in the column. This

vaporised rich liquid is then further expanded in an

expander to provide refrigeration for the process. The

expander is coupled to a cold compressor which recom-

presses some of the vapour. The recompressed vapour is

returned to the bottom of the column, further increasing

reboil and reux in the column and hence yield of

nitrogen to about 6065% of processed air.The liquid pump plant for medium oxygen capacities

(Fig. 6) is similar in principle to that used for tonnage

application, but usually limited to one expansion turbine.

Oxygen product pressures for such plant are between 0.7

and 16 barg. and purities of 9598% although higher

purities are possible. Should an argon product be required

then the oxygen purity is above 99.5%.

Current developments for these plants are similar to

those for tonnage plants. Structured packing is being

used for distillation columns, for example.

For air purication the molecular sieve adsorbers can

operate on a temperature swing basis, the beds beingregenerated by hot nitrogen and recooled cyclically.

Pressure swing adsorption to remove water and carbon

dioxide is increasingly being used for this range of

Fig. 5. Nitrogen generator with recycle. (1) Main heat exchanger, (2) expander/cold compressor, (3) distillation column, (4) subcooler

heat exchanger.

Fig. 5. Generateur d'azote avec recyclage. (1) Echangeur de chaleur principal, (2) Module de detendeur/compresseur, (3) colonne de

distillation, (4) echangeur de chaleur sous-refroidisseur.



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plants. This brings a certain simplication to the pre-

purication step compared to the temperature regen-

eration system. It is likely that this will be a trend for

future plants in the lower capacity range.

Where argon is produced from oxygen plants the bal-

ance between all-cryogenic process mentioned above and

removal of oxygen by catalytic combustion with hydrogenis dierent at the lower scale. The capital cost of the two

systems has to be compared and account taken of the cost

of hydrogen consumption. Especially for smaller capa-

cities this review is likely to favour the catalytic route.

With nitrogen generators producing ultra high purity

nitrogen the product specication already calls for purities

in the range of 12 vppb for microchip production. It is

very dicult, but perhaps not totally impossible (!) to see

a need for further reduction in impurity levels. Nitrogen

generators of 200 tonnes per day producing at purities

of 12 vppb have been built. This size is not a limit, but

is set by the demand of the microchip facility.

2.3. Plants for production of liquid

If large fractions of liquid product are needed then an

integrated refrigeration circuit can be incorporated, as

mentioned above, by an air refrigeration cycle in the

compressed air stream before the column system.

In Fig. 3 the details have been simplied to emphasise

the refrigeration cycle. Two expansion turbines are

employed, each expanding air from the booster com-

pressor to provide the refrigeration to enable liquid oxy-

gen and nitrogen to be withdrawn from the plant. Thepumped liquid oxygen may be part liquid product and

part pumped to the main heat exchanger to be vaporised

as pressurised gaseous oxygen product.

Mention has already been made of the use of external

liqueers for low-pressure tonnage plants which are

required to produce extra liquids. Such a liqueer

greatly resembles the essential parts of the air liqueer in

Fig. 3. This comprises the booster compressor, expan-

sion turbines and heat exchanger but uses nitrogen, not

air, for the working uid. An additional valve expansion

of nitrogen from the delivery of the booster compressor

is cooled through the main heat exchanger, to form theliquid nitrogen (LN) product after expansion. Alter-

nately it is possible to inject the LN into the top of the

HP column to provide refrigeration to compensate for

removal of liquid oxygen from the condenser-reboiler in

the LP column.

Fig. 6. Medium capacity liquid pump plant. (1) Main air compressor, (2) booster air compressor, (3) main heat exchanger, (4)

expander, (5) liquid pump, (6) HP column, (7) LP column.

Fig. 6. Circuit de liquide a pompe, de capacite moyenne. (1) Compresseur d'air principal, (2) precompresseur d'air, (3) echangeur de

chaleur principal, (4) turbine de detente, (5) pompe, (6) colonne haute pression, (7) colonne basse pression.



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Vapour-recompression cycles using Freons have been

used in air separation, for precooling air before pur-

ication and for low temperature refrigeration in lique-

ers. With the phasing out of Freons, alternate systems

have been proscribed for replacement [2021]. In any

event, even apart from the global warming aspect, large

capacity, vapour recompression refrigeration units hadbecome relatively expensive, another reason for seeking

alternatives.

3. Developments in pressure swing adsorption

Recent developments in pressure swing adsorption,

and non-cryogenic methods of air separation have been

reviewed by Nataro [22], so this will be a brief summary

of the major points. Yang [23] has also documented and

reviewed adsorption processes, including pressure swing

adsorption.

3.1. Nitrogen PSA

For nitrogen PSA systems, (see Fig. 7) air is compressed,

re-cooled and enters one of two beds of adsorbent. The

adsorbent normally used is carbon molecular sieve (CMS).

Oxygen (see Fig. 8) is more strongly adsorbed than nitro-

gen, so the nitrogen passes through the bed and is collected

as product. The surge drum illustrated is to smooth out

pressure and composition uctuations. As the bed of

adsorbent is used it becomes saturated with oxygen and

Fig. 7. Diagram of a nitrogen PSA plant. (1)-Air compressor, (2a) and (2b) PSA adsorber vessels, (3) product surge drum.

Fig. 7. Schema d'une installation de production d'azote PSA. (1) Compresseur d'air, (2a) et (2b) recipients d'adsorption PSA, (3) bouteille

de securite.

Fig. 8. Diagram of a VPSA oxygen plant. (1)-Air compressor, (2a) and (2b) PSA adsorber vessels, (3) product surge drum, (4) vacuum

pump.

Fig. 8. Schema d'une installation de production d'oxygene VPSA. (1) compresseur d'air, (2a) et (2b) recipients d'adsorption, (3) bouteille

de securite, (4) pompe a vide.



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the other bed is pressurised to take the ow, while the

rst bed is depressurised to remove most of the adsor-

bate and await its turn for re-pressurisation and reuse.

Purities available from the nitrogen PSA process are

9899.8%, the yield of nitrogen from air being lower the

higher the purity required [24]. Higher purities can be

obtained by use of a further purier unit e.g. burningthe oxygen catalytically with hydrogen and dealing with

or tolerating any excess hydrogen. Over 12 years (1984

1996) nitrogen yields were improved from 34.4 to 41.7%

for nitrogen with 2% oxygen and from 13.5 to 25% for

nitrogen containing 0.1% oxygen. During the same

period signicant improvement in sieve capacity has

been achieved, 80% for nitrogen at 0.1% oxygen, 34%

for 1% oxygen impurity. Progress in such improvement

should continue, even if not at the same rate. These

developments lead to both capital cost and specic

power reductions.

3.2. Oxygen PSA/VPSA (vacuum pressure swing

adsorption)

PSA and VPSA for oxygen operate in a similar way

overall to the nitrogen process [24]. The dierence is in

the adsorbent, usually zeolite molecular sieve, and in

that nitrogen is adsorbed more strongly than oxygen.

This means that oxygen product passes through the bed

and nitrogen is adsorbed. Regeneration is necessary

when the bed reaches saturation and the air stream is

switched to another bed. In the case of PSA the satu-

rated bed is depressurised to atmospheric pressure to

regenerate it and repressurised ready for re-use. For

VPSA (Vacuum Pressure Swing Adsorption) the bed is

additionally subjected to a vacuum stage, to provide

more complete removal of adsorbate.

The need to employ adsorber beds with an appro-

priate ratio of diameter to length to avoid maldistribu-

tion means that three or four beds may be used in a PSA

or VPSA unit, rather than just two, to cater for higher

capacities. This also means that PSA/VPSA systems do

not have such an advantageous scale-up characteristic

as the cryogenic process, since at the limit, extra beds

have to be added to obtain higher output.

Adsorbent performance is the biggest factor in overall

capital and operating costs of PSA/VPSA plants. It hasbeen estimated [22] that adsorbent performance and

improvement in design and selection of process equipment

has reduced cost of production to nearly 50% in 10 years.

A signicant reduction can be expected with the use of

lithium zeolites (Li-X) in place of sodium zeolites (Na-

X). Baksh et al. [2526] have shown that Li-X has a

potential to make signicant savings in energy over Na-X.

Such performance will also lead to lower capital costs by

reduction of adsorbent bed and vessel size for a given

output or alternately an increase in output for a given size.

Further cost reduction is possible in the mechanical

design and fabrication of the plants themselves to

reduce costs of the whole assembly and packaging

[2728].

These improvements will increase the scale at which

VPSA will breakeven in product cost with the cryogenic

route for the same product purity. While this is currently

about 250 tonnes/day for 93% oxygen, it could be expec-

ted to rise to 350 tonnes/day within a decade or less.

3.3. Membrane process

Relatively impure nitrogen can be produced using

membrane processes. In such a process compressed air is

passed to a membrane unit comprising a tube bundle,

similar to a shell-and-tube heat exchanger, but with much

smaller tubes sealed into the tube plates. The tubes, with a

diameter usually less than 0.2 mm, are formed from semi-

permeable material supported on a non-selective mem-

brane sheath. Great care has to be taken to provide

uniform membrane elements, otherwise maldistribution

can occur and limit product yield and/or purity.Materials such as polysulphone or acetate membranes

make it possible to permeate oxygen some ve times

that of nitrogen. Membrane units capable of producing

nearly 600 tonnes per day nitrogen, with a purity range

of 9099% have been built and operated.

A simplied diagram of a nitrogen membrane process

is shown in Fig. 9.

Air is compressed, passed through one of two lter

units to remove particulate matter that can block mem-

brane material, and then to the membrane unit as

described above. Gases that permeate more quickly than

nitrogen, such as oxygen, carbon dioxide and water

vapour pass through the membrane and nitrogen pro-

duct can be obtained from the outlet of the unit.

Yield of nitrogen from the process is dependent on the

product purity required. As an example, only 50% of the

inlet air will be recovered for nitrogen at 99% purity.

Water and carbon dioxide in the compressed air, permeate

much faster than nitrogen and exit with the oxygen.

4. Other methods of air separation

Chemical methods of separating oxygen from air have

been used historically. For completeness a brief reviewof these is worthwhile, since it might be foolhardy to

rule out such processes being further developed!

1. Du Motay and Marechal in 1866 used thermal

cycling of alkaline manganates, but commercial opera-

tion was not long-lasting [29]. Boussingault used barium

oxide, at 540C to `absorb' oxygen and 930C to `des-

orb'. This process was improved by the Brin's and still

further by Murray. The process developed by Murray

held the barium oxide at 600C and absorbing oxygen at

a pressure of 1.7 bar. Then oxygen was obtained by sub-

jecting the oxide to a vacuum at the same temperature.

This appears to be an early example of a VPSA process!



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2. A more recent process is called `MOLTOX', [30

32] which uses alkali metal nitrates and nitrites to pro-

duce oxygen. The process is a temperature swing

[between 540 and 620C] absorption separation of oxy-

gen from air and is performed with an oxygen acceptor

of alkali metal nitrate and nitrite. While there may be

specic application for the process, it has not, to the

author's knowledge, been commercially applied.

3. Another still more recent process is that of Phillips

and Taylor [33]. This applies an electrical current to a

ceramic membrane, allowing the diusion of oxygen

through the membrane for collection as product. This

application is not intended for full-scale production,

being aimed at supply of oxygen for aircrew. Never-

theless it could be possible that further development

might increase the scale at which the process could be

applied economically for less specialised applications.4. Balanchandran and co-workers have described

another type of membrane process [34]. The process has

a membrane using mixed-conductivity oxides as oxygen-

permeating membranes operating without electrodes or

external electrical circuits. The authors claim that it is

possible to apply the process to syngas generation by

partial oxidation of methane with the oxygen permeate

and consider that a commercial application is feasible.

5. For most of the above processes, the actual equip-

ment needs further development to allow use for rea-

sonably large outputs to achieve economic application.

Some, at least, produce oxygen at a relatively high

specic power and further development for reduction of

this is also needed to allow commercial use.

5. Process integration

The air separation plant supplying user processes can

be integrated with those processes to improve the over-

all power consumption. This can go further than merely

using the same source of power, for instance. In many

instances the oxygen product supplies a process which

itself produces excess energy which can be used to generate

steam. This in turn can be used to drive steam turbines to

power the air compressor of the air separation process.

Alternatively a fuel gas may be produced which, if in

excess of that used by the supplied process itself, can be

used to power a gas turbine-drive to a compressor for theair separation process. Gasication, steel making, partial

oxidation, co-generation of power and similar processes

can all be viewed to determine the potential advantage

of integration. Rathbone [35], Jahnke et al. [36], and

Bush and Lavin [37], cover many of the factors for the

respective integrated processes.

In processes where a gas turbine will be used to drive an

air compressor the air is delivered at a higher pressure than

normal for stand alone air separation plants. The double

column system then operates also at higher pressures. As

separation of the constituent gases, oxygen, nitrogen and

argon is more dicult at higher pressures, due to lower

Fig. 9. Simplied diagram membrane system.

Fig. 9. Schema simplie systeme a membrane.



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volatility of the components, extra circuitry has to be

introduced if eciency is to be maintained. Examples are

use of a ash separator as an addition to the double

column system, an intermediate column added to the

double column and multiple condenser/reboilers [38,39].

6. Capital costs

Capital costs for air separation plant are usually

equally important in setting operating costs as the

power consumption of the plant. The capital charges to

recover the investment and provide a return on invest-

ment comprise nearly fty per cent of the total operat-

ing costs, the remainder being power costs, at about the

same proportion, and a small cost for operating labour.

Because of the proprietorial nature of cost information

and the variability of total scope of the installations it is

not realistically feasible to track capital cost trends frompublished or readily available information. The picture

is further complicated by the change in the production

of higher yield of products from a given air feed and in

producing higher product purities. Nevertheless there is

a general belief that, in real terms the capital cost per

tonne of product has reduced over the years.

Capital costs, including fabrication, of the producing

plant, itself do not constitute the total investment. Total

investment includes:

. construction on site,

. costs of services,

. provision of power, cooling water,

. site access, roadways,

. land for the plant site,

. pipeline facilities for delivering gaseous products

to large consumers,

. liquid storage tanks and vaporisers for security

back-up to vaporise product in the event of power

failure,

and,

. if liquid product is to be exported, tanker vehicles

also.

In order to control capital costs and subsequent

capital charges to the project, the total scope has to be

examined very carefully to avoid duplication of facil-

ities, over-specication of equipment e.g. to meet

extreme conditions that may not be met frequently, too

generous land usage, etc.

. For the plant equipment itself the important fac-

tor is to ensure that the overall design is carefully

integrated and opportunity is taken to match

machinery performance with process cycle needs

without unnecessary complexity. For example:

. It is possible to employ a multistage compressor

with six stages coupled around a bull gear to

drive, say, a four-stage air compressor, a booster

compressor, and accept a shaft from an expander-

turbine to recover its power directly.

. Availability of even larger matrix heat exchangers

would help reduce costs, especially for large plantusing multiple cores. Larger and fewer cores

means less manifolding and lower cost, and also

allows a more compact arrangement into a smaller

cold box- reducing costs again.

. A higher density, high eciency structured pack-

ing which might be made at an economic cost

would not only reduce distillation column sizes

but also allow them to be contained in a smaller

cold box with further cost savings.

Other examples can be cited given more space than is

available in this paper. Process and Engineering eort toreduce costs, and at the same time even improving perfor-

mance, have to be exerted continuously, but care should be

taken as to how and when the changes are introduced.

Another aspect of air separation plant costs is that of

construction. For `packaged' plant or smaller to

medium sized plant site construction can absorb 15

20% of total installed cost. For larger plant the gure is

more likely to be 3040%. The practice with smaller/

medium sized plant is to pre-package the plant, some-

times, if dimensions permit, packing the whole unit in

an ISO dimensioned shipping container module or

modules. The practice is relatively easy with PSA, VPSA

and membrane plants. This minimises on-site cost while

allowing assembly to be made under controlled condi-

tions in the workshop, where quality control is much

more readily maintained than most construction sites.

For cryogenic plants cold boxes, certainly up to 400

tonnes/day oxygen capacity have been pre-packaged.

On occasion it has been used for larger plants where site

conditions or skilled labour is dicult to obtain. An

example is a 1500 tonnes/day oxygen plant described by

Castle [40] in which the pre-package comprised eight

modules. Special attention has to be paid to transport

methods and dimensional limits a need for a special

transport survey!For smaller and medium sized plants the direct Engi-

neering cost can be about 10% of the total of an indivi-

dual design. This proportion can be reduced, and cost of

engineering of each plant thereby also lowered by pro-

ducing `standard' designs. A number of options have to

be considered for the standard design in order to cope

with diering customer requirements and duties. A bal-

ance has to be made between extra hardware cost to

make the standard design exible enough and the engi-

neering costs saved by spreading them over several plant

options a challenge to marketing departments to

dene potential sales!



13/15

For a large tonnage plant there is greater diculty in

using standard plant concepts competitively. Some

modular parts of a design might well be usable on a sub-

sequent unit, avoiding some design costs, or a plant might

be sold to suit an existing design. However the bulk of the

design is likely to be changed to meet customer require-

ments for product mix and purities and to supply a plantwith competitive performance for the specic project.

Fortunately the use of process simulation techniques and

CAD for mechanical design enables changes to design to

be made eciently. Care must be taken to ensure that

such changes are monitored and reviewed.

Most important with large plant is to maintain com-

petitive advantage by reviewing technology and

improving costs and performance. This usually lessens

the chance of using totally standard designs if the cor-

poration wishes to stay ahead!

7. Technology

It seems signicant that, from a quick review of the

US Patent database, 429 patents have been granted

from 1990 to mid-1999 for air separation processes.

[These cover just the air separation process steps

overall cycles and unit operations purication, heat

exchange, etc., not product applications.] Of those

patents 4045% are for cryogenic processes. This is for

the cryogenic process that many thought was mature

two decades or more ago. To the contrary there seems

to be plenty of room for innovation still! (But there are

probably more patents in other territories that may not

be covered by corresponding patents in the US).

Although the review was taken from the US patent

database, many international companies apply for cover

in the US as well as their own territory (e.g. Europe,

Japan) so that the gures should be representative of the

amount of work being patented in air separation pro-

cesses alone.

The gure does emphasise the need to obtain patent

cover for any promising new technology in air separa-

tion. Also it demonstrates the value of patent searches

to ensure that any process activity or unit processes used

in air separation plant does not use and infringe existingpatents by others. Of course the use of known prior art

features is a suitable tactic, but this might mean using or

selling less competitive plant or processes.

Possible advances in technology for each of the pro-

cesses have been mentioned in the respective sections

above, but summaries of some of the main points are

given below. It can be expected that:

7.1. For cryogenic plant

. These will be used on an even larger scale (say

4500 tonnes/day oxygen or 14,000 tonnes/day

nitrogen for EOR),with specic power performance

for LP-type plants around 0.280.29 kWh/Nm3.

. That distillation will be carried out with wider

application of structured packing, probably with

higher packing density and specic surface area

costs of such should be lower if this is to come

into use.. Greater use of downow condenser/reboilers,

with careful consideration of safe operation to

prevent hazardous hydrocarbon accumulation.

. Use of PSA adsorption techniques to larger scale

air separation plants for feed air pre-purication.

. More complex cycles (e.g. using RL ash separa-

tors, or mixing columns, and liquid pump com-

pression of product) operating with higher column

pressures to match integrated user processes and

to provide substantial overall power and capital

cost savings.

7.2. For PSA plants for nitrogen and PSA and VPSA

for oxygen

. Improved performance of adsorbent materials,

giving both lower power and capital.

. Continued engineering and selection of hardware

to reduce these costs further.

7.3. For membrane processes

. More development of semi-permeable materials toprovide greater selectivity and faster permeability.

. Production of such materials to obtain improved

uniformity of membrane systems to allow reduc-

tion of maldistribution and increased output.

7.4. In addition, for all processes

. Continuous work to reduce capital costs of plant,

not only for the process equipment itself, but to

reduce on-site construction and installation costs.

. An essential part of plant development is to

monitor plant performance so as to feed back

results to the designers. This enables them to

upgrade plant specications for capacity and/or

purities and thus contribute to improved compe-

titiveness.

. The drive to reduce costs and economics of pro-

cesses leads to changes in the market place. For

example, where supplies to certain customers have

been by merchant liquid deliveries, some can be

supplied from non-cryogenic installations, releas-

ing the liquid capacity for customers with larger

demands or more stringent purity requirements.



14/15

This also helps to reduce overall investment for

supply to the market.

All the above developments will, of course be aimed

at driving down the cost of products and hence leading

to wider, economic applications

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