Secondary oxygen purifier for molecular sieve oxygen concentrator

The secondary purifier, composed of two cylindrical adsorbent beds and valving, is used to increase the oxygen concentration of the product gas comprising oxygen and argon from a zeolite molecular sieve concentrator. The oxygen purity of the gas is increased by selectively adsorbing and exhausting the argon component by the use of beds of small particle size (16.times.40 mesh) carbon molecular sieve. In a two step cycle, during step 1 of the cycle one bed receives high pressure 30 PSIA feed gas which pressurizes the bed and establishes a product oxygen flow, and the argon component of the air is removed by preferential adsorption in the carbon molecular sieve. Simultaneously, the high pressure gas in the other bed is vented to a lower pressure usually the ambient surroundings, and this depressurization serves to desorb the argon previously adsorbed during the high pressure phase of the cycle. In step 2 of the cycle the adsorbent beds exchange roles. This constant cycling results in a continuous product stream of very high purity oxygen (up to 99.6%). The separation is conducted at a temperature of 297.degree. K. The secondary purifier does not require a regenerative purge flow for efficient operation, which minimizes the feed gas consumption.

BACKGROUND OF THE INVENTION 
The present invention relates to a secondary oxygen purifier for a 
molecular sieve oxygen concentrator. 
Molecular sieve oxygen concentrators have attracted considerable attention 
recently because they are capable of producing high purity oxygen (about 
95%) in a simple, cost-effective manner. Further, this oxygen has been 
found acceptable as a breathing gas for patients requiring oxygen therapy 
and for aircrew hypoxia protection. These concentrators operate on the 
principle of rapid pressure swing adsorption (RPSA), whereby, the pressure 
of the adsorbent beds is cycled at a typical rate of 10 sec/cycle. This 
rapid cycling improves the oxygen-nitrogen separation efficiency of the 
concentrator resulting in a significant reduction in the unit's weight and 
volume. During this cycling the nitrogen component of the air is adsorbed 
at high pressure and desorbed at low pressure to the surroundings. 
Concentrators operating on this principle are present onboard the USAF 
B1-B strategic bomber and the USN AV-8B fighter. 
The simplest oxygen concentrator is composed of two cylindrical adsorbent 
beds containing a zeolite molecular sieve, valving, and an orifice. In a 
typical two-step cycle, during step 1 of the cycle one bed receives high 
pressure (20-40 PSIG) feed air which pressurizes the bed and establishes a 
product oxygen flow, and the nitrogen component of the air is removed by 
preferential adsorption in the zeolite molecular sieve. Simultaneously, 
the high pressure gas in the other bed is vented to a lower pressure 
usually the ambient surroundings, and this depressurization serves to 
desorb the nitrogen previously adsorbed during the high pressure phase of 
the cycle. Also, a portion of the product gas from the high pressure bed 
is fed to the low pressure bed to flush the nitrogen-rich gas from that 
bed. The orifice serves to control the flow of purge gas. In step 2 of the 
cycle the adsorbent beds exchange roles. This constant cycling results in 
a continuous product stream of high purity oxygen. 
One limitation of a concentrator containing a zeolite molecular sieve is 
the maximum oxygen purity of 95% (the remainder is argon). Because the 
oxygen and argon molecules are similar in size and are nonpolar they both 
are concentrated upon passage through the beds of zeolite molecular sieve. 
U.S. patents of interest include U.S. Pat. No. 4,566,881 to Richter et al 
which discloses a process and apparatus for producing oxygen with a low 
fraction of argon from air involving a first adsorption unit comprising at 
least two adsorbers containing carbon molecular sieve which provides an 
intermediate product that is enriched with oxygen and depleted of argon by 
comparison to the supplied N.sub.2 /O.sub.2 /Ar gas mixture. Thereafter 
the intermediate product is subjected to zeolite adsorption in an 
adsorption unit. This patent discloses that when the method is carried out 
with a dry and carbon-dioxide-free air, oxygen is produced with a purity 
of 99.7 volume percent during the adsorption phase of the zeolite 
adsorption unit. This patent further discloses that the regeneration of 
the zeolite-bed adsorbers is interrupted while the first of carbon-bed 
adsorbers are regenerated by a vacuum pump which is used in common to 
regenerate the adsorbers. Similarly U.S. Pat. No. 4,190,424 to Armond et 
al discloses integrating the zeolite and carbon sieve processes to produce 
oxygen with a purity better than that which can be achieved by either of 
the known processes operated alone. The overall performance of this 
process is enhanced by the recycling as feedstock of an oxygen-rich gas 
stream from the second section to the first. A product stream with a 
proportion of oxygen as high as 99.7% is cited for one embodiment (see col 
3, line 37). In another embodiment, air is provided as feedstock to the 
zeolite sieve section as also is an oxygen rich gas stream obtained during 
a feed step in the carbon sieve section. The zeolite sieve section gives a 
product containing approximately 90% oxygen with 5% nitrogen and 5% argon 
which is passed as feedstock to the carbon sieve section (see col 3, line 
50 et seq.) U.S. Pat. Nos. 4,627,857 and 4,629,476 to Sutt, Jr. are 
directed to processes for preparation and use of carbon molecular sieves, 
with a pore size in U.S. Pat. No. 4,627,857 patent of about 3 to about 20 
Angstroms, preferably 4 to 10 Angstroms. Other patents relating to oxygen 
generators or concentrators which rely on molecular sieves include U.S. 
Pat. Nos. 4,681,602 to Glenn et al, 4,681,099 to Sato et al, 4,661,124 to 
Hamlin et al, 4,648,888 and 4,561,527 to Rowland, 4,614,525 to Reiss, and 
4,272,265 to Snyder; and of these Glenn et al, Hamline et al and Snyder 
cite aircraft applicability. 
SUMMARY OF THE INVENTION 
An objective of the invention is to increase the oxygen concentration of 
the product gas from a zeolite molecular sieve oxygen concentrator. 
The oxygen purity of the gas is increased by selectively adsorbing and 
exhausting the argon component by the use of beds of small particle size 
(16.times.40 mesh) carbon molecular sieve. The mesh size corresponds to a 
particle size of 1.2 to 0.42 mm. This invention will improve the present 
molecular sieve oxygen concentrators for field hospitals and portable 
oxygen therapy. The two adsorbent beds are operatively connected to six 
solenoid actuated valves, one manual valve and a programmable solenoid 
actuator. The system according to the invention does not require a 
regenerative purge flow for efficient operation. This feature minimizes 
the feed gas consumption. 
The primary use of the invention would be as a secondary oxygen purifier 
for molecular sieve oxygen concentrators supplying oxygen therapy.

DETAILED DESCRIPTION 
As shown in FIG. 1, the simplest typical prior art oxygen concentrator is 
composed of two cylindrical adsorbent beds containing a zeolite molecular 
sieve, valving (not shown), and an orifice 10; and operates in a two-step 
cycle. During step 1 of the cycle bed A receives high pressure (20-40 
PSIG) feed air which pressurizes the bed and establishes a product oxygen 
flow. The nitrogen component of the air is removed by preferential 
adsorption in the zeolite molecular sieve. Simultaneously, the high 
pressure gas in bed B is vented to a lower pressure usually the ambient 
surroundings. This depressurization of bed B serves to desorb the nitrogen 
previously adsorbed during the high pressure phase of the cycle. Also, a 
portion of the product gas from bed A is fed to bed B in countercurrent 
fashion to flush the nitrogenrich gas from the bed. The orifice serves to 
control the flow of purge gas. In step 2 of the cycle the adsorbent beds 
exchange roles. This constant cycling results in a continuous product 
stream of high purity oxygen (up to 95%). 
A schematic of a miniaturized version of the apparatus for practicing the 
invention is shown in FIG. 2. The apparatus is composed of two adsorbent 
beds B1 and B2 containing about 100 grams of 16.times.40 mesh carbon 
molecular sieve, one manual valve V1, six controlled valves V2-V7, and a 
programmable solenoid actuator unit 300. The mesh size corresponds to a 
particle size of 1.2 mm to 0.42 mm. Earlier experiments with 2.67 mm 
diameter pellets for the carbon molecular sieve or at an operational 
temperature of 203.degree. K. gave unsatisfactory results. 
The piping and valves comprise a feed gas line 210 connected to valves V5 
and V6, exhaust lines 211 and 221 connected respectively to valves V4 and 
V7, a line 212 from the valves V4 and V5 to the bed B1, a line 222 from 
the valves V6 and V7 to the bed B2, a line 214 from bed B1 to valve V2, a 
line 224 from bed B2 to valve V3, a line 216 from valve V2 to valve V1, a 
line 226 from valve V3 to valve V1, and a line 220 from valve V1 for the 
product gas output. 
A full scale model of the apparatus would require a greater quantity of 
carbon molecular sieve. A schematic electrical diagram of the programmable 
solenoid actuator is shown in FIG. 3. The adsorbent beds were constructed 
of one-ich stainless steel tubing 34.3 cm in length with an outer diameter 
of 2.54 cm and an inner diameter of 2.36 cm. The apparatus was operated at 
an optimum cycle time of 5.0 seconds, temperature of 297.degree. K., an 
inlet pressure of 30 PSIA, and an exhaust pressure of 14.7 PSIA, with a 
feed gas composition of approximately 95% oxygen and 5% argon. The 
apparatus did not have a purge flow for regeneration of the adsorbent. 
During operation, valve V1 is partially open, and the adsorbent beds B1 and 
B2 are alternately cycled through steps of adsorption and desorption. In 
the first half-cycle of operation, valves V2, V5, and V7 are activated 
open for a period of 2.5 seconds, while the valves V3, V4, and V6 are 
closed. High pressure feed gas enters bed B1 from line 210 via valve V5 
and 212 to pressurize the bed and initiate the flow of product gas at the 
outlet port 220 via line 214, valve V2, line 216 and valve V1. As the feed 
gas passes through the adsorbent bed, argon is preferentially adsorbed and 
oxygen in the feed gas is concentrated. Simultaneously, bed B2 is 
depressurized to the ambient surroundings via line 222 and valve V7 and 
the argon adsorbed during the previous cycle is desorbed and exhausted 
from the apparatus. During the second half-cycle, valves V3, V4, and V5 
are energized open for a period of 2.5 seconds, while the valves V2, V5, 
and V7 are closed. During this phase of the cycle bed B2 is pressurized 
from line 210 via valve V6 and line 222 and produces product gas from its 
outlet via line 224, valve V3, line 226 and valve V1; while bed B1 is 
depressurized via line 212 and valve V4. By repeating these steps of 
adsorption and desorption, a continuous stream of very high purity oxygen 
is produced. Additionally, it should be noted that a purge is not required 
for regeneration of the adsorbent beds during the depressurization phase 
of the cycle, as does the zeolite molecular sieve oxygen concentrator. 
This feature improves the efficiency by reducing the feed gas consumption. 
The programmable solenoid actuator unit 300 provides the timing for 
controlling the operation of the valves V2-V7. The unit 300 is supplied 
with 115 volt AC power via a line 310. There are four female output 
receptacles, comprising a pair 1A and 2A in parallel, and another pair 1B 
and 2B in parallel. The AC power from line 310 is connected to the 
receptacles 1A and 2A during the first half-cycle of the bed operation, 
and to the receptacles 1B and 2B during the other half-cycle. There is a 
switch 312 for turning on the power, and a neon lamp 314 for indicating 
power on. "Programmable" refers to the timing being adjustable, as 
controlled by a thumbwheel switch 316 and potentiometer with a control 
318. The unit 300 may be any apparatus which provides for programming of 
the operation of the valve V2-V7 in equal half cycles, with an adjustable 
cycle time. 
In one embodiment of the secondary oxygen purifier comprising beds B1 and 
B2, the valves V1-V7 were solenoid actuated valves (Airmatic Model 
#20316). Power for operating the solenoids was supplied via electrical 
wiring with valves V2, V5 and V7 connected to the receptacles 1A and/or 
2A, and the valves V3, V4 and V6 connected to the receptables 1B and/or 
2B. The oxygen concentration of the product gas at the outlet 220 was 
98.5%, with gas at the inlet 210 having approximately 95% oxygen and 5% 
argon. Originally purge flow was controlled using a Whitney #SS-OVM2 
valve, but it was determined that the purge flow was not required. 
In an improved embodiment, the solenoid actuated valves were replaced by 
air operated valves (Whitney Model #SS-92M4-NC) for the valves V2-V7 in 
FIG. 2. These are normally closed valves which are actuated open upon 
receiving an air pressure signal. Compressed air for operation of the 
valves V2-V7 is supplied via a solenoid operated valve V8 (Numatic Model 
MK-7 #11SAD4410). The solenoid is connected to receptacle 1A or 2A of the 
actuator 300. During one half cycle, the valve V8 is energized to supply 
compressed air at 75 PSIA from a line 230 to an air line A to actuate the 
valves V2, V5 and V7; and during the alternate half cycles, when the valve 
V8 is not energized, air from line 230 is supplied from line 230 and valve 
V8 via an air line B to actuate the valves V3, V4 and V6. The manual valve 
V1 may be Whitney Model #SS-21RS4-A. This improvement has resulted in a 
further increase in the oxygen concentration of the product gas at line 
220. 
In one test, bed B1 had 115.8 grams and bed B2 had 116.1 grams, for a total 
of 231.1 grams of the carbon molecular sieve, 16.times.40 mesh. The data 
were taken (with purge piping removed) with inlet gas calibrated at 94.73% 
O.sub.2, 5.00% Ar and 0.27% N.sub.2 at an inlet pressure of 30 PSIA 
(lbs./sq. inch abs.) and a temperature of 297 K. The product flow was 100 
SCCM (Standard Cubic Centimeters Minute), and the cycle time was 5.00 
seconds (2.5 seconds for each half cycle). The product gas at line 220 was 
99.65% O.sub.2, 0.25% Ar and 0.10% N.sub.2. 
A suitable programmable solenoid actuator unit 300 (originally designed by 
George Rex) is shown by a functional block and schematic diagram in FIG. 
3. It includes a timer 320, a settable counter 330 and a flip flop 340, 
which may use CMOS MSI integrated circuit (IC) devices. The AC supply line 
310 has a hot lead connected via the switch 312 to lead H, a neutral lead 
N, and a ground lead G connected to chassis ground. A neon light 314 in 
series with a 47K-ohm resistor between leads H and N indicates when the 
power is on. A 15-volt direct current power supply 350 (type 15E10) has 
its input connected to the AC leads H and N, and its output has + and - 
terminals connected respectively via leads V.sub.DD and V.sub.SS to the 
electronic devices. 
The timer 320 may be a type ICM7555 IC device configured with an RC circuit 
as a free-running square-wave oscillator operating at a frequency of 
approximately two hertz. Lead V.sub.DD is connected to pins 4 and 8 and 
via a 10K-ohm resistor to pin 7. Lead V.sub.SS is connected to pin 1 and 
via a 47-microfarad capacitor to pins 2 and 6. Pins 2 and 6 are also 
connected via a 1M potentiometer to the output pin 3. The slider 318 of 
the potentiometer is used to vary the frequency. The output pin 3 is 
connected to a resistive voltage divider comprising a 10K-ohm resistor in 
series with a 1M-ohm resistor, with the junction of the resistors 
connected to the clock input of the settable counter module 330. 
The counter module 330 is a Unimax Counter/Timer Decade Thumbwheel Switch 
model SR/SF-179, which has an MM74C90 4-bit decade counter CMOS IC device 
implemented on the switch, as well as a 47K-ohm pull-up resistor from an 
an output terminal 1 to terminal 8, and a 10K-ohm resistor between 
terminals 8 and 9. The device 330 has 12 terminals (only those connected 
being shown) connected to the circuit via a 14-pin ribbon wire connector 
331. Terminals 8 and 2 are connected to the power leads V.sub.DD and 
V.sub.SS, terminal 11 is the clock input, terminal 9 is a reset input 
connected via a 3M-ohm resistor to the output terminal 1. The module 
includes the thumbwheel switch 316, which has ten positions 0-9, and is 
shown in position 2. In operation, the module 330 counts the input clock 
pulses from terminal 11, and for every N input pulses, the output at 
terminal 1 supplies one output pulse, where N is the digit dialed on the 
thumbwheel switch. If the thumbwheel switch is set to the digit 0, the 
output remains at a high voltage level continuously and the circuit 
remains in a static state with no switching at the output occurring. All 
other selections of the thumbwheel switch (digits 1 through 9) will 
produce output pulses from the counter/timer module at the appropriate 
times. The output is also supplied back to the reset terminal 9 to reset 
the counter to the starting state after a count cycle has been completed. 
The flip-flop 340 is one of the units of a dual D IC device type CD4013, 
connected to perform as a toggle (pin 2 connected to pin 5). Each time an 
output pulse from terminal 1 of the counter module 330 is applied to the 
clock input at pin 3 of the flip-flop 340, the signals at the two outputs 
at pins 1 and 2 will change states. The Q output is at pin 1, and the 
complement of Q at pin 2. Relay driver transistors 341 and 342 (type 
2N956) have their base inputs connected respectively to the outputs at 
pins 1 and 2 of the flip-flop 340, and their collector electrodes 
connected via the windings of relays 343 and 344 and respective 750-ohm 
resistors to the power supply lead V.sub.DD, the emitters of the 
transistors being connected to the common lead V.sub.SS. When the 
flip-flop 340 is in its "set" state, the transistor 341 is energized to 
operate the relay 344, which at its contacts 344C connects the AC power 
lead N to the outlet connectors 1A and 2A; and when the flip-flop 340 is 
in its "reset" state, the transistor 342 is energized to operate the relay 
343, which at its contacts 343C connects the AC power lead N to the outlet 
connectors 1B and 2B. The hot lead H is directly connected to all four of 
the connectors 1A, 1B, 2A and 2B. During operation one relay is on when 
the other is off. 
The timing for a typical setting of 2 hertz for the timer 320 and the 
thumbwheel switch set at "2", is a pulse produced by the timer 320 each 
0.5 sec., with the counter module 330 producing an output pulse for every 
two oscillator pulses, or one output pulse per second. Then each output 
cycle from the solenoid actuator 300 will be two seconds, with one second 
of power on at connectors 1A and 2A, and one second of power on at 
connectors 1B and 2B. Likewise, for a thumbwheel digit selection of "3", a 
counter output pulse will be produced every 1.5 seconds (0.5 
seconds.times.3) for a total cycle of 3 seconds. 
It is understood that certain modifications to the invention as described 
may be made, as might occur to one with skill in the field of the 
invention, within the scope of the appended claims. Therefore, all 
embodiments contemplated hereunder which achieve the objects of the 
present invention have not been shown in complete detail. Other 
embodiments may be developed without departing from the scope of the 
appended claims.