High speed pressure swing adsorption liquid oxygen/liquid nitrogen generating plant

An apparatus for producing high purity oxygen and nitrogen in the liquid and gaseous phases. The apparatus is a modular, mobile system employing a low design operating pressure. The system purifies ambient air through filters and a dual immobilized fixed bed molecular sieve type pressure swing adsorber prior to the cryogenic distillation of the air. Through the incorporation of the pressure swing adsorber having a high frequency regeneration cycle, the disclosed system provides a reduced system size. The configured system employs waste gas from the distillation process to purge the pressure swing adsorber. The cryogenic distillation procedure includes a heat exchanger, a turboexpander, high and low pressure distillation columns, a subcooler and a condenser.

BACKGROUND OF THE INVENTION 
This invention relates generally to the production of liquid oxygen and 
nitrogen through cryogenic distillation of air and, in particular, to a 
compact, modular, mobile system having a low operating pressure. 
DESCRIPTION OF THE PRIOR ART 
The industrial and commercial uses of nitrogen and oxygen have created 
tremendous demands for pure oxygen and nitrogen in both liquid and gaseous 
phases. In addition, a large volume of high purity nitrogen and oxygen is 
required support. These demands for pure oxygen and nitrogen are met 
primarily through large-scale stationary production facilities. 
Unfortunately, these facilities are located a substantial distance from 
the end user, necessitating the transportation of large quantities of 
liquid oxygen and nitrogen over substantial distances. As liquid oxygen is 
highly explosive and both liquid oxygen and liquid nitrogen must be kept 
under heavy pressure at extremely low temperatures, the transportation 
process is both dangerous and expensive. 
Oxygen and nitrogen of high purity may be obtained through cryogenic 
distillation of ambient air. To obtain liquid oxygen and nitrogen of high 
purity, the ambient air must be filtered prior to the distillation 
process. Previous cryogenic systems have utilized carbon filters and 
temperature swing adsorption systems for predistillation filtering. 
Cryogenic distillation systems, employing carbon filter beds require 
shut-down periods for the replacement of the filters, and are susceptible 
to damage by vibration and shock, as would occur if the system were 
transported. Specifically, vibration causes the packed beds to disassemble 
and disintegrate. In addition, the carbon beds are subject to frequent 
fouling from hydrocarbons and sulfur base compounds. As a result, 
substantial filtering is required before the air stream enters the carbon 
beds. This additional filtering increases the size and cost of the 
cryogenic system. 
While temperature swing adsorption units in cryogenic distillation systems 
offer significant advantages over systems employing carbon filter beds, 
temperature swing adsorption units are also subject to severe limitations. 
Specifically, temperature swing adsorption units filter the air stream at 
relatively low temperatures (around 40 degrees Fahrenheit) and must be 
purged at relatively high temperatures (around 500 degrees Fahrenheit). It 
typically requires at least 3 hours to change from filtration temperature 
to regeneration temperature, to complete the regeneration and to change 
back to process temperature (one regeneration cycle). This 3 hour 
regeneration cycle permitted substantial penetration of contaminants into 
the on-line bed, thereby necessitating the use of large volume beds to 
ensure proper filtration, thereby increasing the size of the system. 
Further, in previous cryogenic systems, the filter's bed structure and bed 
packing often failed as a result of pressure swings caused by the 
transition from the on-line process status to the regenerative status. 
Finally, temperature swing adsorption units cannot properly filter the 
inlet air unless the inlet air has a low water vapor content. To achieve 
this low water vapor content, the water vapor must be condensed out of the 
inlet air by refrigeration units. These refrigeration units require 
substantial power and further increase the size of the system. 
Thus, there is needed a safe, low weight, compact mobile generator of pure 
liquid oxygen and nitrogen. 
SUMMARY OF THE INVENTION 
The present invention provides a relatively compact, modular system for the 
production of high purity oxygen and nitrogen in both the liquid and 
gaseous phase. One aspect of the invention is a system for producing 
liquid oxygen and nitrogen having an air compressor assembly, a 
coalescer/HEPA filter, a pressure swing adsorber, a warm heat exchanger, a 
main heat exchanger, a turboexpander, a nitrogen column, a condenser, a 
subcooler and an oxygen column. 
Another aspect of the present invention is the use of an ambient air 
filtration element having a regeneration cycle of less than five minutes 
and preferably, less than 35 seconds. This is significant in that this 
relatively short regeneration cycle prevents contaminants from deeply 
penetrating the bed. As a result, the filtering system can accommodate a 
higher inlet airstream water vapor contents than prior systems. This 
allows the present invention to eliminate the refrigeration units used to 
force condensation as in prior sYstems, thereby reducing the size of the 
system. Advantageously, the ambiant air filtration element incorporates 
dual beds for the filtration of the ambient air prior to the the 
distillation process, to provide for the continuous regeneration of one of 
the beds, without system down time. 
Another aspect of the invention is the incorporation of a rapid pressure 
swing adsorption (PSA) unit into a cryogenic distillation system, thereby 
accommodating both transportation vibrational stresses and pressure swings 
incurred during the purge cycle. 
Another aspect of the invention is the use of a stream of waste gas to 
purge the PSA beds in connection with an automatic looping system to 
ensure that a continuous supply of purge air is provided to the 
regenerative beds. In normal operation, it is desirable to use a stream of 
waste gas produced by the cryogenic distillation process to purge the 
regenerative beds. However, during the production of high quantities of 
liquid oxygen or during start-up periods, the distillation process may 
produce an insufficient quantity of waste gas to ensure sufficient purging 
of the beds. Desirably, however, an automatic looping system is provided 
to redirect a portion of the air stream exiting the PSA into the purge air 
loop. Advantageously, once the required level of waste gas reaches a level 
sufficient to purge the regenerative beds, the looping of the PSA outlet 
is discontinued. 
Another aspect of the invention is the use of a warm heat exchanger outside 
of the insulation layer and the use of a main heat exchanger enclosed 
within the insulation layer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, a schematic representation of an apparatus for 
producing high purity oxygen and nitrogen in a liquid or gaseous phase is 
shown. The plant is comprised of an air compressor assembly 50, a 
coalescer/HEPA filter 100, a pressure swing adsorber 150, a warm heat 
exchanger 250, a main heat exchanger 300, a turboexpander 350, a nitrogen 
column 400, a condenser 440, a subcooler 480 and an oxygen column 520. 
Air Compressor 
As shown in FIG. 2, the air compressor assembly 50 includes a compressor 
motor 51, an air-oil separator 52, an air aftercooler 55, an air filter 
57, an oil cooler assembly 53, a moisture seperator 56, and a compressor 
60. Desirably, the air compressor assembly 50 is Model Number SZ 75 KW-SP, 
manufactured by the Air Compressor Group of the Ingersoll-Rand Company. 
Referring to FIG. 3, the compressor 60 is a single stage, oil immersed, 
rotary lobe, screw-type compressor. The compressor 60 is directly driven 
through an integral gear box by the electric motor 51. The compressor 60 
is a two rotor, positive displacement rotary unit which compresses the air 
between an intermeshing primary rotor 62 and a secondary rotor 64. The 
primary rotor 62 includes helically inclined lobe 63. The secondary rotor 
64 includes a helically inclined groove 65. The helical lobe 63 of the 
primary rotor 62 is received within the helical groove 65 of the secondary 
rotor 64 to form a sealing engagement. Compression of the air takes place 
as the engagement point of the lobe 63 and the groove 65 travels the 
length of the helix. 
A shaft extends from the primary rotor 62 to engage the motor 51 through 
the gear box. The secondary rotor 64 is driven by its engagement with the 
main rotor 62. The oil immersion of the compressor 60 provides that the 
driving of the secondary rotor 64 is accomplished without metal contact of 
the primary rotor 62. A thin film of oil provides the interface and 
thereby the sealing surface between the primary rotor 62 and the secondary 
rotor 64. 
The discharge end of the rotors 62, 64 are received in tapered roller 
bearings. The tapered bearings prevent axial displacement of the rotors 
62, 64 toward the tapered bearings during operation of the compressor 60. 
The inlet end of the rotors 62, 64 are supported by free floating 
bearings. The retention of the inlet end of the rotors 62, 64 in the free 
floating bearings allows for thermal expansion of the rotors during 
operation of the compressor 60. The helical engagement of the rotors 62, 
64 generates a force tending to force the rotors toward the tapered 
bearings. The tapered bearings prevent this displacement while the 
floating bearings allow for thermal expansion of the rotors. 
The rotors 62, 64 and the tapered and free floating bearings are contained 
within a housing 80. The housing 80 includes an air inlet port 82 located 
at the top of the housing 80 at the end proximal to the drive shaft and an 
air outlet port 84 located proximal to the tapered bearings. An air inlet 
check valve provides for the monodirectional flow of ambient air into the 
inlet port 82, while preventing a reverse flow of air through the inlet 
port 82. Prior to entering the air inlet port 82, the ambient air passes 
through the intake air filter 57. The air filter 57 is a dual stage, 
dry-type filter equipped with a cyclone-type pre-cleaner for heavy duty 
service in dusty conditions. The air filter 57 is designed to remove 
particulate matter of 10 microns or larger, with an efficiency of 94%. 
Operation of the Air Compressor 
Ambient air is drawn through the filter 57 and into the compressor 60 
through the air inlet port 82. The air is drawn into the space preceding 
the sealing engagement of the lobe 63 within the groove 65. As rotation of 
the rotors 62, 64 causes the contact point of lobe 63 and the groove 65 to 
progress past the inlet port 82, the ambient air is trapped between the 
rotors and the wall of the housing 80. Continued rotor rotation causes the 
air to travel down the groove 65, thereby becoming compressed. Oil is 
injected into the housing 80 during compression of the air to adsorb the 
heat of compression, lubricate the rotors 62, 64 and fluidly seal the 
contact between the rotors 62, 64 and the housing 80. 
Continued rotation of the rotors 62, 64 further reduces the volume of the 
trapped air, thereby generating more heat. The continued rotation 
displaces the compressed air towards the air outlet port 84. As the 
sealing contact between the lobe 63 and the groove 65 passes the outlet 
port 84, the compressed air and oil mixture are released through the port 
84. 
Referring to FIGS. 2 and 4, the air-oil mixture flows through an outlet 
pipe 85 and a check valve 86. The valve 86 prevents a return flow of the 
air-oil mixture into the compressor 60. After passing through the check 
valve 86, the air-oil mixture flows into the air-oil separator 52. The 
lower portion of the separator 52 includes a series of orthagonal baffles. 
The 90.degree. path changes within the baffles cause a substantial 
separation of the air and oil. The compressed airstream then travels 
upward through horizontal elements which remove the remaining oil to 
approximately 2 to 3 parts per million (ppm). 
The separated oil is then passed from the bottom of the separator 52 
through an oil cooler assembly 53. The oil cooler assembly 53 includes a 
core 75, an oil filter 54, a thermostatic mixing valve 72, oil radiator 75 
and an oil filter pressure gauge 74. The oil cooler assembly functions so 
that upon subsequent injection of the oil into the compressor 60, the oil 
temperature is between 130.degree.-160.degree. F. This temperature range 
prevents moisture condensation in the oil, while ensuring there is no 
chemical breakdown of the oil. An oil temperature within the desired range 
is maintained by the thermostatic mixing valve 72. The valve 72 determines 
the amount of oil to be passed through the oil cooler, so that the desired 
oil temperature is maintained. 
As the separated oil flows through the oil cooler 75, the cooling motor 77 
causes the fan 76 to force air across the oil cooler 75, thereby cooling 
the oil. The cooled oil then passes through the thermostatic valve 72. 
The separated oil is then filtered through the oil filter 54 to remove any 
particulate matter which may have entered the system entrained in the 
ambient air. The filtering of the separated oil removes the suspended 
particulate matter which, if retained in the oil, could damage the rotors 
62, 64 upon recirculation. 
The oil filter 54 is a hydraulic-type full-flow filter with a single 
replaceable element. The filter 54, rated at 10 microns, is disposed 
downstream of the oil cooler assembly 53 and the thermostatic control 
valve 72. An oil filter pressure gauge 74 measurers the oil pressure 
differential across the filter 54 A 15 psi differential across a 10 micron 
rated filter indicates the limit of the filter efficiency, thereby 
indicating a replacement of the filter 54 is necessary. 
As shown in FIGS. 1 and 2, the compressed air, at approximately 150 psig, 
then passes from the separator 52 to the aftercooler 55. The aftercooler 
55 is an air cooled heat exchanger which reduces the temperature of the 
compressed air. This reduction in temperature causes a substantial portion 
of the water vapor within the air to condense. The condensate is removed 
through a moisture separator 56. 
Coalescing/HEPA Filter 
As the airstream leaves the moisture separator 56, the air has a relative 
humidity of approximately 75%. Referring to FIG. 1, the airstream then 
passes into a coalescing filter 100, such as Pall Coalescing HEPA-grade 
filter Model Number PC535001G24, manufactured by Pneumatic Products 
Corporation, a Division of Pall Safety Atmospheres, Inc. The filter 100 is 
a high area, pleated cartridge encased within an outer cylinder. The 
cylinder is non-woven fluorocarbon media. The air flow within the filter 
100 is directed radially outward from the inside. The tortuosity and 
controlled pore size of the cylinder traps fine aerosols by impingement 
and blocking. The particles subsequently evaporate and pass downstream as 
a gas or agglomerate and drain by gravity to a sump. The media of the 
cylinder is hydrophobic and oleophobic treated to reduce pressure drop to 
approximately 2 psig under saturated airstream conditions. 
The airstream, now free of entrained liquid, passes through the outer 
HEPA-grade media. The media is capable of removing 99.997% of 3 micron 
mean particulate diameter matter and 99.999% of particulate matter in the 
0.6 to 1.7 micron size. The airstream then exits the filter 100 and passes 
to the pressure swing adsorber 150. 
Pressure Swing Adsorber 
The system design ensures the filtration of aerosols prior to the airstream 
entering the pressure swing adsorber 150. Aerosols can penetrate the 
packed beds of the pressure swing adsorber 150 and adversely effect the 
useful life and efficiency of the pressure swing adsorber. 
The pressure swing adsorber 150 is a molecular sieve bed having immobilized 
beads of 13X type molecular sieve manufactured by Pall Safety Atmospheres, 
Inc. The molecular sieve-type beads of the adsorber 150 are coated and 
bonded (immobilized) by a proprietary process owned by Pall Safety 
Atmospheres, Inc. The adsorber functions to remove chemical impurities, 
water and carbon dioxide vapor to less than 1 ppm. In addition, the 
pressure swing adsorber 150 removes common pollutants found in the 
atmosphere, such as carbon monoxide, methane, ethane, nitrous oxides and 
oil vapors. 
As shown in FIG. 3, the pressure swing adsorber 150 of the present 
invention employs a dual bed packed system which allows for continuous 
regeneration. The adsorber 150 includes two immobilized molecular 
sieve-type, bonded regenerable packed cylindrical beds 160 and 170. 
Referring to FIG. 3a, the pressure swing adsorber 150 includes a 
microprocessor control 200 based control similar to those found in 
temperature swing adsorption units to direct inlet air flow to one of the 
beds 160, 170. The microprocessor control 200 cycles between the off-line 
purge and on-line process status of the beds in 30 to 60 second cycles. 
Therefore, as one bed is on-line processing the inlet airstream, the 
second bed is off-line being purged and regenerated. The concurrent 
regeneration of the off-line bed is accomplished through a flow of waste 
gas from the distillation process, as discussed infra. The regeneration of 
the off-line bed allows the present invention to operate continuously 
without shut down during periods of bed regeneration. In addition to 
removing the necessity of a refrigeration unit and a heated purge air 
unit, the pressure swing adsorber 150 provides dried, purified air for the 
distillation process, thereby allowing the present invention to employ a 
simple, modular cryogenic distillation process of a reduced size and 
complexity. 
As shown in FIG. 3a, the pressure swing adsorber 150 also includes an 
automatic loop control 220 which functions to ensure that a sufficient 
quantity of purge gas is available purge the off-line bed. Under normal 
operating conditions the automatic loop control 220 uses the waste gas 
produced in the cryogenic distillation process. However, during startup 
conditions and the production of large quantities of liquid oxygyen, the 
automatic loop control 220 employs a portion of the airstream exiting the 
on-line bed to purge the off-line bed. When the cryogenic distillation 
process yields sufficient waste gas to purge the off-line bed, the 
automatic loop control 220 directs the airstream from the on-line bed to 
the distillation process. 
Warm Heat Exchanger 
Referring to FIG. 1, the dried, purified inlet airstream, exits the 
adsorber and passes through a filter 180 which removes any particular 
matter produced by the pressure swing adsorber 150. The inlet airstream 
then passes through a conduit 240 and passes into a warm heat exchanger 
250. The heat exchanger 250 provides for the conductive and radiative heat 
transfer between the inlet airstream, the waste gas flow and the 
turboexpander exhaust. The inlet airstream is cooled from its temperature 
of approximately 15.degree. F. above ambient air temperature to 
approximately 50.degree. F. as the airstream exits the warm heat exchanger 
250. 
The warm heat exchanger 250, the main heat exchanger 300, the subcooler 480 
and the condenser 440 are brazed aluminum plate-fin type heat exchangers 
as well known in the art. Referring to FIG. 5, the heat exchange surfaces 
are obtained by stacking alternate layers of corrugated fins 310 between 
flat aluminum separator plates 320. The plates 320 provide the primary 
heat exchanging surfaces of the heat exchanger. Therefore, the thickness 
of the plates 320 may be varied to accommodate the designed operating 
pressure. The fins 310, being disposed between the plates 320, provide the 
secondary heat exchanging surface. The fins 310 may be of a quantity, 
shape, spacing and size to accommodate the desired design operating 
pressure, heat exchange rates, pressure drop, fluid properties and fluid 
flow rate. A given configuration of the fin 310 disposed between the two 
separator plates 320 is fluidly sealed at the edges by solid aluminum side 
bars 330. A vertical configuration of layered plates 320, fins 310 and 
side bars 330 is bonded together by a brazing process to yield an integral 
rigid structure having a series of flow passages. 
As shown in FIGS. 6-8, the passages defined by the fins 310, plates 320 and 
side bars 330 define adjacent layers which may exhibit perpendicular flow 
paths, parallel flow paths or a combination thereof, as determined by the 
number of flows through the heat exchanger. 
In the preferred embodiment, the warm heat exchanger 250 is of a 
rectangular configuration having a length of approximately 58 inches, a 
width of approximately 17 inches and a depth of approximately 4.2 inches. 
As the airstream exits the warm heat exchanger 250, the airstream passes 
through an insulating layer which forms a cold box 280 surrounding the 
cryogenic distillation encloses the main heat exchanger 300, the condenser 
440, the nitrogen column 400, the oxygen column 520 and the subcooler 480 
within the insulating layer, thereby forming an area which may be 
preferably packed with Perlite. By precooling the inlet airstream through 
the warm heat exchanger 250, the main heat exchanger 300, disposed within 
the cold box 280, may be of a reduced size. The size reduction of the main 
heat exchanger 300 permits the cold box to also be of a reduced size. 
Main Heat Exchanger 
The main heat exchanger 300 is of a similar construction to the warm heat 
exchanger, as discussion infra. The main heat exchanger 300 of the 
preferred embodiment is approximately 79 inches high, 11 inches wide and 8 
inches deep. The main heat exchanger 300 provides for the conductive and 
radiative heat transfer between the inlet airstream, the waste gas flow 
and the turboexpander discharge. The inlet air is cooled to cryogenic 
temperatures in the main heat exchanger 300, partially liquefying the 
inlet airstream. 
Turboexpander 
Prior to the inlet airstream exiting the main heat exchanger 300, 
approximately 75% of the inlet airstream is diverted through the 
turboexpander 350. The basic turboexpander is manufactured by Aerodyne 
Dallas as Model No. 9300. However, as shown in FIGS. 9 and 10, the unit is 
modified to include a fiberglass based, with stainless steel reenforced 
thermal barrier 384 between a turbine wheel 354 and a compressor wheel 
358. In addition, the turboexpander is also modified to incorporate a 
fiberglass rim 374 disposed proximal to a turbine wheel 354. The 
turboexpander 350 is an energy removal device which provides the primary 
means of refrigeration in the main heat exchanger 300. 
Referring to FIG. 9, the turboexpander 350 includes a housing 352 which 
supports the turbine wheel 354. The turbine wheel 354 includes a plurality 
of radially extending vanes 355. The housing 352 also contains a nozzle 
ring 356 which directs the airstream against the vanes 355. A compressor 
wheel 358 is directly coupled to the turbine wheel 354 by a shaft 360. The 
shaft 360 is a cantilever design supported by the bearings 362 which are 
disposed on the warm side of the turboexpander 350, proximal to the 
compressor wheel 358. 
Under design operating conditions the shaft 360 exhibits a deflection of 
less than 0.001 inches. To accommodate any excess deflection of the shaft 
360, the housing 352 includes a fiberglass funnel rim 374 disposed 
proximal to the turbine vanes 355. Upon excess deflection of the shaft 360 
and hence blades 355, the blades 355 engage and abrade the fiberglass rim 
374. Since the blades 355 engage the fiberglass rim 374 rather than a 
metal surface, the turboexpander blades 355 are capable of contacting the 
rim under abnormal shaft deflection without destroying the turbine wheel 
354. 
Lubrication of the bearings 362 is accomplished through a passive 
lubrication system. The cantilever design of the shaft 360 permits the 
bearings 362 to be disposed on the compressor side of the turboexpander 
350, away from the turbine wheel 354. In addition to exposing the bearings 
to non-cryogenic temperatures, the positioning of the bearings 362 
prevents oil from entering the process flow system as the flow passes 
through the turbine wheel 354. The oil lubrication system 366, including 
the wick 368 and reservoir 370, is a passive system employing the 
capillary action of the oil to generate an oil flow from the reservoir 370 
through the wick 368. The lubrication system 366 therefore does not 
require either a pressurized oil feed or a buffer of inert gas on the 
seals to preVent oil from entering the process air stream. 
Because the bearings 362 of the turboexpander 350 are disposed outside of 
the expanded and supercooled airstream the bearings 362 are therefore 
exposed to substantially ambient air temperatures. Because the bearings 
362 do not operate under cryogenic temperatures, the bearings are housed 
in a reliable passive lubrication system 366 providing an increased 
operating life. 
Operation of the Turboexpander 
Referring to FIGS. 9 and 10, pressurized air from the inlet airstream 
enters the turboexpander 350 and is directed by the nozzle ring 356 to 
impinge upon the turbine vanes 355. The force of the airstream on the 
turbine vanes 355 causes the turbine wheel 354 to rotate. The airstream 
then travels radially inward toward the center of the rotating wheel 354 
where the airstream further expands and is redirected in a 90.degree. 
direction change. As the air travels inward, it expands from its inlet 
pressure of approximately 150 psig. As the exhaust airstream exits the 
turboexpander 354, the airstream exhibits a pressure of approximately 2 
psig. The expansion of the air from approximately 150 psig to 2 psig 
creates a cryogenic air flow which is employed to cool the remaining 
process stream in the main heat exchanger 300. 
The cool, expanded turboexpander exhaust airstream exits the turboexpander 
350 at approximately -296.degree. F. The exhaust airstream is then passed 
through the main heat exchanger 300. The exhaust airstream then passes 
through the warm heat exchanger 250, where it precools the inlet 
airstream. 
The work done by the expanding air on the turbine wheel 354 causes the 
compression wheel 358 to rotate. The exhaust airstream is drawn from the 
warm heat exchanger 250 into the vacuum generated by the compressor wheel 
358. The compression of the exhaust airstream provides resistance to the 
turbine wheel 354, so that the rotational speed of the turbine wheel 354 
and hence expansion of the inlet airstream and its temperature may be 
controlled. 
The design of the turboexpander 354 prevents the exhaust airstream which is 
being compressed by the compressor wheel 358, from entering and thereby 
contaminating the expanding inlet airstream. The higher pressure of the 
expanding inlet airstream interfaces with the compressing exhaust 
airstream, thereby creating a pressure barrier which prevents the exhaust 
gas from contaminating the expanding inlet airstream. 
Referring to FIG. 1, the remaining 25% of inlet airstream in the main heat 
exchanger 300, having been cooled by the turboexpander exhaust airstream 
and the waste approximately 150 psig and -265.degree. F. The low 
temperature of the pressurized inlet airstream creates partial 
condensation of the airstream. 
Nitrogen Column 
As shown in FIG. 1, the inlet airstream then passes through an air inlet 
expansion valve 390. The valve 390 permits a reduction of the inlet 
airstream pressure to approximately 85 psia, which results in a reduction 
of the temperature to approximately -280.degree. F. The passage of the 
inlet stream through the expansion valve 390 causes more condensate to 
form as the inlet airstream then enters the nitrogen column 400. 
Referring to FIG. 11, the nitrogen column 400 and the oxygen column 520 are 
distillation columns having a cylindrical configuration disposed in a 
vertical orientation. The columns 400, 520 are packed with 0.24 Pro-Pak 
inch Protruded Metal Distillation Packing manufactured by Scientific 
Development Company of State College, Pa. The packing provides a wetting 
surface upon which the condensate may accumulate and provide an increased 
exposure time to vapor within the column. 
The nitrogen column 400 of the preferred embodiment has a cylindrical 
configuration with a height of approximately 60 inches and a diameter of 
approximately 7 inches. The nitrogen column 400 includes an inlet port 402 
approximately 12.5 inches from the bottom of the column 400, a liquid 
oxygen port 404 approximately 3.5 inches from the bottom of the column 
400, a vapor discharge port 406 located at the top of the column 400, and 
a reflux inlet 408 approximately 6 inches from the top of the column 400. 
Operation of the Nitrogen Column 
The inlet airstream entering the nitrogen column 400 through the inlet port 
402 at approximately 85 psia and -280.degree. F. and includes an 
oxygen-rich condensate. The condensate collects in the bottom of the 
nitrogen column 400 and is subsequently transferred to the oxygen column 
520 as feed stock. The vapor entering the nitrogen column 400 has a low 
oxygen content and rises to the top of the column 400 through the packing. 
At the vapor rises through the packing to the top of the column 400, the 
vapor passes through the packing and releases oxygen to the liquid on the 
packing, as the liquid of the packing releases nitrogen to the vapor. 
Likewise, as the liquid falls, it releases nitrogen to the vapor, as the 
vapor releases oxygen to the liquid. The vapor thereby increases in 
nitrogen concentration as the condensate increases in oxygen 
concentration. The vapor at the top of the column 400 is of 99.5% purity 
at a pressure of approximately 85 psia at a temperature of approximately 
-287.degree. F. The oxygen condensate at the bottom of the column 400 is 
at a pressure of approximately 85.5 psia at approximately -280.degree. F. 
The oxygen-rich condensate passes to the oxygen column 520 as a feed 
stock. 
Oxygen Column 
Referring to FIG. 12, the oxygen column 520 is a packed distillation column 
similar to the nitrogen column 400. The oxygen column is packed with 0.24 
inch Pro-Pak Protruded Metal Distillation Packing manufactured by 
Scientific Development Company. In the preferred embodiment, the oxygen 
column 520 is a vertically oriented cylinder approximately 75 inches high 
having a diameter of approximately 8.5 inches. The oxygen column 520 
includes a condensate inlet port 524 approximately 19 inches from the top 
of the column 520, a liquid oxygen discharge port 526 approximately 2.7 
inches from the bottom of the column 520, a two phase inlet port 528 
approximately 9.5 inches from the bottom of the column 520, a reflux inlet 
port 530 approximately 5 inches below the top of the column 520, and a 
waste gas outlet port 532 at the top of the oxygen column 520. 
Operation of the Oxygen Column 
The oxygen rich condensate from the bottom of the nitrogen column 400 exits 
through the liquid oxygen port 404 and passes through an expansion valve 
430. The expansion valve 430 causes the pressure to drop from 
approximately 85 psia to approximately 21.7 psia. This further expansion 
results in a lowering of the condensate temperature to approximately 
-308.degree. F. The condensate then enters the oxygen column 520 through 
the condensate inlet port 524. As the condensate enters the column 520, it 
begins to descend through the column packing. As the condensate descends, 
the condensate saturates the packing and releases nitrogen to the vapor 
rising through the packing. Likewise, as the vapor rises, it releases 
oxygen to the condensate, as the condensate releases nitrogen to the 
vapor. The vapor rises up through the column 520 and is discharged through 
the waste gas outlet port 532. The waste gas exiting through the waste gas 
outlet port 532 has a pressure of approximately 21.7 psia, a temperature 
of approximately -316.degree. F. and is substantially comprised of 
nitrogen. 
As the nitrogen evaporates from the descending condensate, the condensate 
becomes a higher purity of oxygen. When the condensate has descended 
through the packing to the bottom of the oxygen column 520, any remaining 
nitrogen has evaporated, thereby leaving liquid oxygen of 99.5% purity in 
the bottom of the column 520. The liquid oxygen at the bottom of the 
column 520 has a pressure of approximately 22.3 psia and a temperature of 
approximately -291.degree. F. 
The liquid oxygen may be withdrawn from the bottom of the oxygen column 520 
through the liquid oxygen port 526 by a circulation pump 540. The pump 540 
is an air operated, magnetically coupled unit. The drive motor and the 
impeller shaft are effectively decoupled by the encasement of the impeller 
shaft in a separate casing, thereby reducing the risk of explosions. 
Liquid oxygen withdrawn by the pump 540 may be directed toward the 
subcooler 480 and the condenser 440, or solely to the condenser. If the 
liquid oxygen is directed to the condenser 440, the flow bypasses the 
subcooler 480 and flows directly to the condenser 440. 
Condenser 
Referring to FIG. 1, the condenser 440 is a constant temperature and 
constant pressure heat exchanger well known in the art. The configuration 
of the condenser 440 is similar to the main heat exchanger 300, the warm 
heat exchanger 250, and the subcooler 480. The condenser 440 permits the 
radiative and conductive heat transfer between the liquid oxygen and the 
nitrogen vapor from the nitrogen column 400. 
In the preferred embodiment, the condenser 440 is of a substantially 
rectangular configuration approximately 11.8 inches high, 9.2 inches wide 
and 20.8 inches long. The condenser includes a nitrogen vapor inlet 442, a 
liquid oxygen inlet 444, a liquid nitrogen outlet 446, and a two phase 
oxygen outlet 448. 
Operation of the Condenser 
As stated supra, the condenser 440 provides a constant temperature constant 
pressure conductive and radiative heat transfer between the nitrogen vapor 
from the nitrogen column 400 and liquid oxygen. The nitrogen vapor enters 
the condenser 440 through the nitrogen vapor inlet 442 at approximately 85 
psia and -287.degree. F. The liquid oxygen enters the condenser 440 
through the liquid oxygen inlet 444 at approximately 22.3 psia and 
-291.degree. F. The nitrogen vapor is thermally exposed to the colder 
liquid oxygen, thereby causing the nitrogen to condense, forming liquid 
nitrogen. The heat of condensation from the condensing nitrogen causes 
some of the liquid oxygen to vaporize thereby creating a two-phase oxygen 
mixture. 
Usage of Liquid Nitrogen 
The liquid nitrogen exits the condenser 440 through the liquid nitrogen 
outlet 446 and passes to a valve 548 which directs the liquid nitrogen to 
either the subcooler 480 or the nitrogen column 400. By means of a valve 
548, the liquid nitrogen from the condenser 440 may be returned to the 
nitrogen column 400 through the regeneration port 408 and additionally may 
pass through the subcooler 480 to be collected in a storage tank 410 or be 
redistilled through the oxygen column 520. 
Liquid nitrogen which is returned to the nitrogen column 400, through the 
regeneration port 408 acts as a cold cap and reflux for the calcium 400. 
The liquid nitrogen entering the nitrogen column 400 through the 
regeneration port 408 has a pressure of approximately 84.9 psia be at 
approximately -287.degree. F. As the liquid nitrogen descends through the 
packing nitrogen column 400, the nitrogen evaporates and cools the column. 
Selective introduction of the liquid nitrogen into the nitrogen column 400 
allows for thermal control of the column and hence production of liquid 
nitrogen. 
Additionally, the valve 548 may direct the liquid nitrogen through the 
subcooler 480 after which it may be directed to either the storage tank 
410 or the oxygen column 520 by means of a valve 542. The liquid nitrogen 
directed to the oxygen column 520 enters the column 520 through a 
regeneration inlet 530 at approximately 21.7 psia and approximately 
-315.degree. F. The flow of liquid nitrogen into the oxygen column 520 
serves to control the oxygen purity and column pressure during the 
production of liquid oxygen. As the liquid nitrogen enters the oxygen 
column 520 the liquid descends through the packing. The evaporating 
nitrogen serves to cool the oxygen column 520 thereby regulating the 
production of liquid oxygen. 
Subcooler 
Referring to FIG. 1, upon direction of the liquid nitrogen to the subcooler 
480 through the valve 548, the liquid nitrogen is exposed to radiative and 
conductive heat transfer with the waste gas and liquid oxygen flow 
streams. 
The subcooler 480 is of the same design as the main heat exchanger 300, the 
warm heat exchanger 250, and the condenser 440 described supra. In the 
preferred embodiment, the subcooler is of an elongated rectangular 
configuration approximately 50.2 inches high, 3.5 inches deep and 4.5 
inches wide. Disposed proximal to the bottom of the subcooler 480 is a 
liquid nitrogen inlet 482, a liquid oxygen inlet 484, and a waste gas 
outlet 496. Disposed proximal to the top of the subcooler 480 is a waste 
gas inlet 486, a liquid nitrogen outlet 492, and a liquid oxygen outlet 
494. 
Because the storage tanks 410, 510 are desirably at a lower pressure than 
the corresponding column 400, 520, the liquid oxygen and nitrogen must be 
subcooled to remain in a liquid phase. The subcooler 480 thereby subcools 
the liquid oxygen and liquid nitrogen below their condensing temperatures, 
which allows for transfer of the fluids to storage tanks without incurring 
vaporization of the liquid oxygen and nitrogen. 
Operation of the Subcooler 
Waste gas exiting the oxygen column 520 through the port 532, and passes 
into the subcooler 480 through the waste gas inlet port 486. The waste gas 
vapor enters the subcooler 480 at approximately 21.7 , psia and 
approximately -316.degree. F. The waste gas vapor cools the liquid oxygen 
and liquid nitrogen streams to a temperature below their respective 
boiling temperatures. Supercooling of the liquid oxygen and liquid 
nitrogen streams is necessary to ensure that upon passing from the 
subcooler 480 to the respective storage tanks 510, 410 no vaporization of 
the liquid streams occurs. 
Use of Waste Gas to Regenerate Beds 
The waste gas exits the subcooler 480 through the waste gas outlet 496 and 
passes through the main heat exchanger 300, the warm heat exchanger 250, 
and finally to the pressure swing adsorber 150. The waste gas is used to 
purge and regenerate the beds 160, 170 of the adsorber 150. Specifically, 
the microprocessor control 200 alternates the flow of inlet air from one 
bed to another in cycles of 30-60 seconds. As the inlet air has a pressure 
of approximately 150 psig, the pressure within the on-line bed is also 
approximately 150 psig. When the bed is switched off-line by the 
microprocessor control 200, the bed is rapidly decompressed to 
approximately 3-7 psig through the release of pressure through the upsteam 
end of the bed. Waste gas is then allowed to flow from the downstream end 
of the bed to the uptream end of the bed to purge the bed. 
Although this flow of waste gas from the oxygen column 520 is typically 
sufficient to purge the beds 160, 170 during normal operation, during the 
production of high quantities of liquid oxygen or during start-up periods, 
the distillation process may produce an insufficient quantity of waste 
nitrogen to ensure sufficient purging of the beds. Desirably, however, the 
automatic loop control 220 is provided to redirect a portion of the air 
stream exiting the PSA into the purge air loop, thereby creating an 
automatic looping system. Once the flow of waste gas reaches a level 
sufficient to purge the regenerative beds, the looping of the air stream 
to the PSA outlet is discontinued. 
Production of Gaseous Oxygen and Nitrogen 
The present invention produces gaseous oxygen and nitrogen through the 
vaporization of the liquid oxygen and nitrogen. Referring to FIG. 1, the 
liquid product is withdrawn from the respective storage container 410, 
510, and passed through a pump 570. As the liquid discharges from the pump 
570, the liquid is warmed by an electric vaporizer 580 which vaporizes the 
liquid to produce the gaseous phase of oxygen or nitrogen, wherein the 
produced gases have the same 99.5% purity as the liquid phase.