This paper presents a genera/review o f the design o f air separation plants from the initial
specifications to the final details. References to key sources for data and for design
criteria are presented in lieu of lengthy discussions.
The effect o f the product quantities and purities on the selection o f the process cycle are
discussed and genera/design guidelines and specific examples of various types of air
separation plants are given. Process cycles for large tonnange plants, unattended nitrogen
generators, small oxygen generators for sewage plants, and for all-liquid plants are
described. Unit operations used in these cycles and materials o f construction are
reviewed briefly.
Air separation plant design
D.J. Hersh and J.M. Abrardo
Air separation is a mature technology. The liquefaction of
air to produce oxygen was the first engineering application
of cryogenics and was initially used around the beginning
of this century. Today, three-quarters of a century later, it
is useful to conduct a review of air separation plant design.
This paper summarizes the considerations for process
selection, sources of design data, unit operations and
materials of construction for air plant design. Also presented
and discussed are some 'state of the art' process cycles.
Considerations for selection of basic process cycle
Air separation technology is used for the production of
oxygen, nitrogen, and the rare gases that are present in air.
Cryogenic separation is the industry's 'workhorse', but
there are a limited number of alternative methods for separation which can be competitive in certain circumstances.
Product requirements determine the selection of the process.
These include the specific product compositions and purities
of the products, their physical state, that is gas or liquid,
their delivery pressures, and the production rates. The cost
of power exerts a major influence on the process selection.
Oxygen has been produced commercially by several
methods - dissociation of water, selective adsorption,
chemical reactions, and cryogenic separation. Dissociation
of water is a power intensive process ~3 and at the present
time is not economical on a large scale. Chemical reaction
processes based on peroxide formation and decomposition,
the Brin process, have become obsolete. Knoblauch et al s4
report that oxygen from 45 to 90% purity can be produced
by adsorption on molecular sieve coke. Energy consumption
is claimed to be comparable with, or better than, cryogenic
separation for small plants (50 nm 3 h-1 ). Adsorption processes
using zeolite molecular sieves have been built for producing
oxygen at concentrations of P0 to 95%, but these units are
economical only for small product rates. ~3 Cryogenic
separation plants are presently being constructed to produce
oxygen at rates from 160 nm3h -1 to 53 000 nmah -~ .
Oxygen product purities normally range from 90% to 99.6%,
although plants can be designed for higher or lower purities.
The authors are with Air Products and Chemicals, Inc, Allentown,
Pennsylvania 18105, USA. Received 18 April 1977.
CRYOGENICS. JULY 1977
Nitrogen produced by adsorption processes is normally
limited to 99% purity, s4 As is the case for adsorption
processes for oxygen, large scale production of nitrogen by
adsorption is uneconomical. Cryogenic separation for
nitrogen yields product purities with less than 2 ppm
oxygen, on a regular commercial basis.
The rare gases, argon, neon, krypton, and xenon, are produced only by cryogenic separations since no other economical methods have been developed. Normally, these gases
are by-products of large plants which produce oxygen and
nitrogen since they are present in air in low concentrations.
When liquid products are required from the separation of
air, additional refrigeration must be supplied. In gaseous
oxygen plants, up to about 8% of the oxygen product
capacity can be produced as liquid, by increasing the expander flows. The expander flow of nitrogen-rich gas bypasses the low pressure column, where the final oxygen
separation is performed; thus increased liquid production
reduces the total recovery of oxygen. In nitrogen generators, the waste gas is expanded at a low pressure ratio but
with large flows so that normally 8 to 10% liquid can be
produced.
If larger percenta ~;s of liquid product are required,
liquefiers ,re added to the air separation units. Today, these
are usua,y r~ ~dium pressure units consiting of nitrogen
compressol~, heat exchangers, and expanders which
produce large amounts of refrigeration economically. High
pressure cycles (air compressed to 17.2 MN m -z) with
multi-level expansion were used to produce liquid but are
uneconomical today for large plants because of their higher
equipment and maintenance costs, even though they require
slightly less power per unit of liquid production.
Normally, gaseous products are required at pressures above
atmospheric and may be produced at the required pressure
or may be further compressed, depending on the use pressure
In order to produce oxygen at elevated pressures directly
from the low pressure distillation column, higher than
normal discharge pressures are required for the main air
compressor. The additional compression of the large air
flow is usually uneconomical when compared with compressing the smaller oxygen flow. Nitrogen can be produced
directly from the cold box at high pressure column
383
Liquid
nitrogen
product
f
Check
valves
Superheater
h
Main
exchanger
Inlet
filter
1~
Guard
absorb
Gaseous
oxygen
product
LOW ~
ressure I
olumn~
.....
I Reboiler
Heal ~ ~ j n d e n s e r
puml
i
/
I
J
JJ
L//
///
Hydrocarbon
I absorber
/ / Product nitru.jen,
waste nitrogen
subcooler
J
"--'~_
\
i
Afterc:;ler
Aftercooler
separator
Expander
Air
H20
compressor
Liquid
oxygen
product
Fig. 1 Oxygengenerator
pressures, equivalent to the main air compressor discharge
pressure, without incurring additional power costs, if only
nitrogen is being produced. Pressures up to about
1.0 MN m -2 can be attained in this manner, with higher
pressures requiring booster compressors.
Oxygen plants can also produce nitrogen at various purities
at rates from one half to equal that of the oxygen capacity
depending on plant size, with little additional cost. Higher
nitrogen production rates require additional trays in the
nitrogen-rich section of the low pressure distillation column.
Dry gas product rates which add up to more than 50 percent
of the air flow are limited by the water and carbon dioxide
clean-up in the reversing exchangers or regenerators. High
air pressures can be used to allow an increase in the total
dry gas production. A molecular sieve clean-up system for
water and carbon dioxide enables about 80% of the air to
be recovered as dry gas product.
The basic cycle must be optimized to obtain the most
economic method for producing the desired products.
Balancing capital costs against power costs determines
whether more heat exchangers (higher capital) should be
installed to reduce the pressure drops and thus decrease
the power costs. Recoveries of products can also be
optimized by the distillation column design, air flows,
and main air compressor discharge pressures. Hvizdos ss
provides an analytical method of carrying out this
optimization procedure.
384
Basic design data
Basic thermodynamic data are necessary to determine the
material and energy requirement of the process. Transport
properties and mass and energy transfer relationships must
be known to design the various pieces of process equipment.
Fortunately, for air and its major components, extensive
tables of data and correlations for these data have been
prepared and are available in the literature.
For vapour-liquid equilibria, data for the Ar-N2 1 , N2-O2 7 ,
and Ar-O22'3'4's binary mixtures are available and have
been reviewed. Latimer 6 presented equations and graphs
for the vapour-liquid equilibria of the N2-Ar-O2 system,
based on published binary and ternary data tempered with
actual air separation plant performance data. Vapour-liquid
equilibria properties of the N2-02-Ar system along thirteen
isotherms at pressure levels from 0.1 to 2.6 MN m -2 have
been reported by Wilson, Silverberg, and Zellner. 7 This
reference contains both binary and ternary data and presents
1962 data points.
Extensive tables of thermodynamic properties and transport
properties for air systems s have been prepared and tables
have also been prepared for oxygen, 9 nitrogen,lO and
argon. 11 Reference 12 provides a source for additional
references on the solid phases of oxygen, nitrogen, and
argon and thermophysical data for the more exotic rare
gases such as krypton and xenon.
CRYOGENICS. JULY 1977
As an alternative to the use of tables of equilibrium data
and thermodynamic data, computer-based systems for the
prediction of these properties have been developed, using
correlations reported in the literature. 6'7 Computer-based
systems are useful tools for rapidly evaluating a number of
alternate process designs.
Sources of heat transfer data are available in the
literature 13'14'15 for conventional heat transfer. Cryogenic
processing requires the use of specialized compact heat
exchangers of the plate and fin matrix type, and of finned
tube exchangers. Kays and London ~6 and Kern and Kraus ~7
discuss heat transfer in these types of exchangers and more
detailed heat transfer design factors can be obtained from
various manufacturers. Mass transfer rates for the various
unit operations are discussed by Treybal. la
Specific plants and e q u i p m e n t design considerations
The experienced process designer usually determines the
most efficient process to achieve specific product requirements, based on his knowledge of the effect of various
parameters on process cycle selection. He uses basic design
data to determine the mass and energy requirements for the
cycle. The following examples illustrate the development of
process cycles for specified products.
replace storage tank boil-off and to provide back-up oxygen
for peak demand periods and downtime. Standard size
plants produce from 450 nm3h -~ to 4000 nmah -~ of oxygen
at a pressure of 0.12 MN m -2 , while larger plants have
also been designed. The required air feed pressure ranges
from 0.55 to 0.65 M Nm -2 depending on product purity and
cold box piping design. A more detailed discussion of the low
pressure oxygen cycle is presented by McAuley. 5a
Nitrogen generators
High purity nitrogen for a variety of uses such as for inert
atmospheres or for pressuring reservoirs (secondary and
tertiary oil recovery) is produced by cryogenic generators.
Nitrogen generators can be designed for high or low recovery,
depending on the quantity of nitrogen required and power
versus capital costs. (See Figs 2 and 3 respectively for process
flowsheets.) Standard size low recovery-single column
generators produce from 530 nm3h -1 to about 4500 nm3h -~
of high purity nitrogen at pressures between 0.55 to as high
as 0.9 M Nm -2. Required air feed pressure ranges from
about 0.6 to 1.0 M Nm -2 . High recovery-double column
generators produce large volumes of nitrogen (greater than
about 6600 nm3h -1 ) at purities of 2 ppm oxygen. Product
pressure is about 0.55 MN m -2 with air feed pressures up
to 1.0 M Nm -2 . Both types of plants are capable of producing liquid nitrogen for back-up purposes.
Oxygen generators
Low pressure oxygen generators are widely used in municipal
sewage treatment facilities to provide oxygen to the activated
sludge aeration basins. These plants produce gaseous oxygen
product of 90 to 98% purity (see Fig. 1 for process flowsheet).
A typical purity is 95%. Liquid oxygen is also produced to
Tonnage air plants
Large tonnage air plants produce oxygen, nitrogen, argon,
and, if desired, neon, krypton, and xenon. Fig. 4 contains
a simplified process flowsheet for a tonnage plant without
krypton, xenon, or neon recovery. Reference 51 shows
Liquid
nitrogen
product
Check
valves
-J~r-- ~
i
~---
Low
,,
/pressure I
- - --
J
/i / f/
L.column "~J
"-1..
Main
exchanger
,n,.
r;-""~-1
nitrogen
product
t ~
Guard
//
"ligh
)ressure
• :olumn
J Hydrocarbon
absorber
// Productnitrogen/
T subc~ler
waste nitrogen
=
Aft~ler
v
separator
Air
H20
compressor
CRYOGENICS
Reboiler
I,r'-,. 4-- --condenser
J
t
Expander
Fig2. Higrecovery-doubl
h
column
enitrogen
generator
. JULY
1977
385
Safety
purge
Check
valve
/ ~
eboiler-condenser
I I Li¢uefier
¢o, )ler
Liquid
=- nitrogen
product
Main
exchanger
!
Gaseous
nitrogen <
product
i
pressure I
;'°m\l
I
l Hydrocarbon
absorber
L
Switch
Air
compressor
"t
[ I
Inlet
filter
Vent
Expander
Aftercooler
/
H20
Aftercooler
~tj separator
y
Fig. 3 Low recovery-single column nitrogen generator
several flowsheets for krypton recovery while reference 20
contains flowsheets for neon and xenon recovery. Oxygen
and nitrogen production rates and purities are very flexible
with the oxygen production normally setting the required
air flow. Portions of the products can be taken off as
liquids up to the expander limitations; additional liquid can
be produced by adding liquefiers. Argon recovery is maximized to produce the valuable product and to increase the
oxygen recovery. Crude argon is produced in the air plant
and then purified in a combination deoxo-cryogenic process, sl
A typical switching air separation plant producing 26 500
nmSh -t of oxygen at 99.6 percent purity can produce up
to 33 000 nmSh -1 of 2 ppm nitrogen and 800 nmSh -~ of
crude argon (96-98% argon, remainder oxygen). All products are at low pressure. The required air pressure to the
cold box depends on optimization of capital versus power
costs and normally ranges from about 0.6 to 0.8 MN m -2 ,
Liquid plants
Liquid plants are essentially small tonnage air separation
units producing all their products as liquid with the aid of
one or more liquefiers. These liquefiers are completely
integrated with the air plant as shown in Fig. 5.
The preceeding process cycles can be conveniently divided
into the basic operations of (1) front end clean-up,
(2) distillation, (3) heat exchange, (4) refrigeration, and
(5) compression.
Front end clean-up. Carbon dioxide and water vapour, which
freeze in the cryogenic processing areas must be removed
from the incoming air. There are a number of ways for
removing these impurities such as partial condensation,
adsoprtion, and freeze-out on heat exchange surfaces. If the
process cycle is such that the air is compressed to pressures
of 13 to 17 M Nm -2, most of the water can be removed by
cooling, with supplemental chilling, to 277 K. The remaining
water can then be adsorbed on absorbent beds 19 usually
386
alumina or molecular sieve. Alternatively, the water can be
frozen-out on heat exchange surfaces such as in stone-filled
regenerators 2° or switching heat exchangers. 21 Carbon
dioxide can be removed in the warm processing area by
scrubbing with caustic soda or by simultaneous adsoprtion
of the water and carbon dioxide on molecular sieves. Heat
exchange surfaces such as in regenerators and in switching
exchangers can be used to freeze-out the carbon dioxide.
The removal of the water and carbon dioxide by freeze-out
in regenerators or switching exchangers, is the preferred
method for clean-up, followed by adsorption on molecular
sieves. The choice of the processing method depends on a
number of factors including the following:
1. Quantities of pure product.
2. Relative energy consumption.
3. Relative costs.
4. Space requirements.
And some less quantifiable considerations such as:
1. Sensitivity to misoperation.
2. Corrosion resistance.
3. Equipment life.
4. Ease of repair.
Each user's capabilities and experiences dictates his approach
to selection of the best scheme for clean-up. The literature
provides some guideline for the selection of the various
alternatives.22,2~
If regenerators or switching exchangers are used, attention
must be paid to the amount of reversing stream flow and to
the temperature difference between the high pressure air and
the low pressure reversing streams. Lobo and Skaperdas 24
describe the operation of a reversing heat exchanger and
present methods of calculating the maximum tolerable
temperature differences. Springmann 22'2s provides similar
information covering a wider range of operation of reversing
CRYOGENICS. JULY 1977
exchangers and also discusses the maximum allowable
temperature differences in regenerators.
The effects of entrainment on distillation columns in
cryogenic service have been discussed by Chatterjee. 39
Distillation: Cryogenic distillation uses the same basic separation concepts used in distillation processes in the chemical
industry. Many excellent texts are available. 3a-3s'a'ls
Latimer ~9 provides a general review of cryogenic distillation
After the required products, purities, and rates are specified, the number of theoretical stages required for the separation of air can be calculated using the accurate thermodynamic data which are available for air. Procedures for
determining the tray efficiencies and hence the actual tray
counts have been documented by Jeromin et al, 4° Sherwood
and Jenny, 36 and Colbum. a7 The tray design must provide
good vapour-liquid contact for good tray efficiencies. Tray
hydraulics and design have been described by Fair, 41
Pavlov, 42 and Azbel. 43
There are special design considerations in air separation
distillation columns due to the requirements for a large
number of theoretical stages (as many as 150 stages for
separations including argon) and the need to keep refrigeration losses to the ambient at a minimum. Tray spacing is
usually between 100 and 150 mm although some plants
have been built with larger tray spacings. Vapour-liquid
disengagement between stages becomes a major factor at
these spacings since entrainment reduces the efficiency
and pushes the hydraulic capacity towards the flood point.
The expected operating range of the distillation columns is
also an important aspect of these designs. If a large range
of product rates is required, the column must be designed
to provide the required separation as efficiently as possible.
Sieve trays with small diameter holes can be expected to
operate with reasonable efficiencies at turndown to 60%
of the design capacity. Operation at lower turndown rates
can be obtained if bubble cap trays are used. It is important
to design the plant for the smallest operating range
required since with large turndown capability of the
column, pressure drops at high loadings are larger in order
to avoid weeping at the lower operating rates.
Recent developments toward improving vapour-liquid
contact and tray efficiencies have been reported by Winter
and Utti, 46 Haselden,'~ and Smith and Delnicki. a~
Conventional type sieve or bubble-cap trays are the
usual means for obtaining the separation. Haselden47
reports that 'overflow packing' has distinct advantages
under certain conditions and the use of dephlegmators
(wetted-wall columns) is also described in the literature, '~
but these special separating devices are not in commercial
use.
Heat exchange: Ambient temperature heat transfer is a
vital part of air separation plant design and is well documented and described in a number of references.13'14'1s
Fig. 4 Tonnage air separation plant
Crude
llquld argon
Liquid
nitrogen
II Crud.
argon ~ r
F/
I I
.
Ic°"*nse r L .....
U"
I
I
,
To liquefier
Check
valves
4
rl
I
~eeous
,!! ii
-
G(meous nitrogen
oxygen
HP LP
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filter
TT
Switch
valve
Air-Aftercooler
compressor [
CRYOGENICS. JULY 1977
separator
"Pr!~ure"7
.col0mn.~J
J TSuperheoter
Main
exchanger
J
7,,-
~rbon
L.
I
I
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. J j condenser
J/,
rber
/,
I //
!J
//
High
,'
Subcooler
,/
Vent
-J
~
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L
Liquid
oxygen
387
Discussions on heat transfer will be limited to the cryogenic
area. Two major over-riding considerations influence the
design of cryogenic heat exchange equipment. First,
because of the lower operating temperatures, temperature
differences between the streams exchanging heat must be
minimized in order to achieve high thermodynamic
process efficiency. Second, inefficiencies due to the transfer
of external heat into the cryogenic exchanger must be
minimized. These requirements have led to 'compact'
heat exchangers containing a large amount of heat transfer
surface per unit of heat exchanger volume. Exchangers
manufactured today contain as much as 125 m 2 of heat
transfer surface per cubic foot of exchanger volume.
Many varieties of compact heat exchanger surfaces are
available. 16 Regenerators, which have a mass of material,
usually quartz pebbles, to alternately store heat and refrigeration, are designed also for high heat transfer surface per
unit volume.
rs
Cold
expander
Most of the heat exchange requirements are for cooling the
incoming air and for warming the products and/or waste
gases. In many plants, the removal of carbon dioxide and
water by alternate freezing and vaporization is also an
important aspect of the heat transfer requirements. Methods
suitable for 'hand' calculations of regenerators and plate-andfm heat exchangers are available for determining surface area
cle
rotor
HP g~
nitro
Fig, 5
All-liquid air separation plant
Crude
liquid argon
Liquid
nitrogen
: Crude
argon ~ .
condenser-"
To liquefier
I L.--~
Check
valves_j
! .It
,
exchanger
.a,.
I
I
Gaseous
oxygen
Gaseous
nitrogen
HP LP
~J
!!'
..!e - - v--
Low
"pressure
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absorber
.colu~mn-..
j
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condenser
H
it
IJ
m
)
¢ ~
/
r
!
/
/
/
I
I
filter
JL
_j--
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argon
column
i
"High/
,.pressure..
column
--
Guard--~
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~,
Subcooler
¢
1
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valve
Vent
,I
v
Liquid
oxygen
388
CRYOGENICS.
J U L Y 1977
requirements. 16'~7'2° Equations representing more rigorous
approaches which are suitable for computer solution are
available for both types of exchangers. 2s'26 Major design
considerations, include effects of longitudinal conduction,
mass flow rates, and mechanical considerations and mass
balances for clean-up considerations. 22-24'27'2s
Although the temperature differences and mass flows are
important for clean-up calculations, the heat capacities of
the freezing and vaporizing water and carbon dioxide are
usually ignored, except in the most detailed calculations.
Calculations are usually made for steady state operation
although designers recognize that 'switching' plants are
always operating under transient conditions.
The reboiler-condenser of the double column system is a
critically important area requiring careful heat transfer
calculations. Larger reboiler-condensers are usually fabricated as a thermosyphon reboiler, with boiling oxygen and
condensing nitrogen. The heat exchanger can be of the shell
and tube, 29 or plate-and-fin types or enhanced surface
configurations. 3°'al Data for heat transfer and pressure
drop in vertical tube and plate-and-fin type exchangers
have been reported. 32 An important aspect of the reboiler
design is the necessity for designing and operating to avoid
'dry' boiling of oxygen, usually by total submergence of
the reboiler or by high liquid levels, in order to operate
safely. The design must simultaneously avoid excessive
pressures on the boiling oxygen in order to obtain efficient
operation. Any condensing surface of the pure nitrogen,
and associated piping must be designed to prevent flooding.
The heat transfer associated with the subcooling of liquids
and the superheating of gases is usually carried out most
effectively in the plate-and-fin type exchanger in larger
plants. Smaller plants use other, more conventional types
of extended surface heat exchange. In addition to heat
transfer and pressure drop, elevation is a major design
consideration since many of the fluids in the cold part of
the plant are liquids close to their boiling points or vapours
close to their condensation temperatures. Proper elevations
must be maintained to avoid undesirable conditions of
flashing and slugging flow.
Refrigeration: Refrigeration required to maintain the plant
heat balance and to produce liquid products is normally
supplied by reciprocating expansion engines or turboexpanders. Some refrigeration is provided by JouleThompson expansion, and in some plants supplementary
refrigeration with freon is provided with conventional
vapour-recompression refrigerators, at temperatures from
233 K to 277 K.
Reciprocating expanders or turbo-expanders usually have
isentropic efficiencies between 70 and 82%. The expanders
are usually loaded with blowers or generators to remove
the work of expansion. Generator loaded expanders can
produce useable electric power for other equipment in the
plant; however, they are usually only economic in larger
plants which require large amounts of refrigeration.
Blower loaded expanders are used in some cycles to provide supplementary compression to a process stream. In
small plants, the expansion work may be wasted. More
detailed discussions of expanders can be found in references
49, 50, and 20. Reference 51 contains working drawings of
various expanders.
C R Y O G E N I C S . J U L Y 1977
Some high pressure cycles use a combination of JouleThompson refrigeration and supplementary refrigeration
by heat exchange with a low temperature refrigerant.
Several liquid plants have been built which use refrigeration
available from LNG. Conventional commercial refrigerators
are used to precool the air feed to about 275 K in front end
molecular sieve clean-up systems, to reduce water loadings.
Compression: Gas compression is described in great detail
in the literature, such as in reference 52. In air separation,
compression equipment is used to compress the air to the
high pressure column pressure, to boost product streams to
their use pressures, and to provide Joule-Thompson
refrigeration.
The main air compressor is usually sized to supply the
minimum air to obtain the required quantities of products.
The pressure of this air is normally dictated by the pressure
drop from the low pressure column through the waste
circuits, the pressure drop in the low pressure column, the
pressure required to condense nitrogen in the reboilercondenser against low pressure boiling oxygen, the pressure
drops in the high pressure column, and finally the pressure
drop through the heat exchanger and piping to the main
air compressor. Cycles which produce high pressure products,
or require air at higher pressures for clean-up or for
additional refrigeration, impose additional pressure requirements on the main air compressor. The operating range of
compressors should match the turndown capabilities of the
distillation section. Occasionally, multiple compressors
such as two 60% machines, are used where operation over
a wide range of plant capacities is required.
Oxygen compressors must be designed specifically to meet
safety requirements. It is imperative that the system be
maintained in a rigorously clean manner and to have only
oxygen compatible materials in direct contact with the
oxygen. Seal designs are critical. It is advisable to provide
personnel protection barriers around compressors since
mechanical failures can lead to fires. Existing centrifugal
oxygen compressors operate at discharge pressures up to
about 7 MN m -2 . Reciprocating compressors are usually
used for higher pressure. An alternative method for obtaining oxygen at high pressure, without gaseous compression
equipment, is the use of liquid vaporization cycles in which
liquid oxygen is pumped to high pressure and then vaporized
in a heat exchanger.
Materials of construction
Selection of the proper materials of construction for the
cryogenic section of an air plant is important in order that
a safe, economical design can be achieved. Under normal
operating conditions, parts of the air separation plant will
be operating at temperatures around 90 K. All metals become
stronger at these temperatures, but some become more
brittle. The problem of brittle fracture is of prime importance in selecting the proper material of construction.
Typical material used in air separation plants are copper,
alurninium alloys, austenitic stainless steel, and 9% nickel
steel. After eliminating metals and alloys subject to
brittle failure, the choice is determined by economic considerations. Stress values and relative economics are presented
in reference 56.
When air separation plants are placed on stream from warm
conditions, residual moisture must be removed from the
389
system in order to avoid subsequent freeze-up problems.
This could be accomplished easily by forcing air or nitrogen
through the plant at a temperature of about 395 K.
However, code requirements for many aluminium alloys
limit the maximum working temperature to 338 K, and
this is the normal air temperature used for defrost. The
piping and supports of the cryogenic system must be
designed to operate over a range of temperatures from 340
to 90 K. Expansion joints, expansion loops, and bellows
are used to provide the necessary flexibility to operate over
the 250 K thermal expansion range. 'Transition' fittings are
available which enable aluminium to steel connexions to be
made by welding and brazing, provided proper design is
used to eliminate misalignments and other stress raisers.
Flange joints are avoided, if possible. Copper tubing, which
is used for smaller flow requirements, is usually silver-brazed.
The materials of construction in the oxygen rich part of the
air plant are critical since all industrial metals will b u m in
pure oxygen. However, some metals and alloys, principally
non-ferrous copper containing alloys, are less susceptible to
ignition. Ignition energy can be generated as a result of
impact of particles, or rupture of materials. Data on various
ignition parameters are available for both metals and nonmetals. 57 All these parameters must be considered when
selecting a material for oxygen service.
References
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
390
Fastovskii, V.G., Petrovskii, Y.V. Zhur Fiz Khim 30 No 1
(1956) 76
Bu~bo,F., lshkin, 1. Phy Zeit Der Sol J 16 (1936) 271
Din, F. Proc IIR (1953)
Fastovskii, V.G., Petrovskii, Y.V. Zhur Fiz Khim 29, No 7
(1955) 1311
Sagenkahn, M.L., Fink, H.L. ORSD No 4493, Contract
ORMAR-685, Penn State College (December 1944)
Latimer, R.E.AIChEJ3 (1957) 75
Wilson,G.M., Silverbetg, P.M., Zellner, M.G. International
Advances in Cryogenic Engineering (Plenium Press, New
York, 1965) 192
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. JULY
1977