Preassure Swing Absorption (PSA) Units. Making Your Own Oxygen in a Time of Need and Dire Shortage.
Hello again everyone. Long time no chat. It certainly feels like the last year has been longer than it really has.
At a time when I was preparing to wind down efforts for the campaign and move the project to the backburner, I see a need has arisen to publish the below from our equipment guide. It’s geared towards hyperbaric oxygen however the oxygen is oxygen. It can be stored and used as needed in any manner of application including ventilation, and standard supplemental baric oxygen administration. This kind of low pressure production is often also safer than high pressure storage. There are a number of documents on gas handling available on the blog to read regarding safety.
I’ve been quiet lately, but today I add a section of our equipment documentation following a conversation regarding oxygen shortages in India.
I’ll not drone on but rather paste below, as it appears in our manuals, that section discussing pressure swing absorption or PSA units as they are known technically.
Better known as oxygen generators or more accurately concentrators.
Pressure Swing Absorption Oxygen Generators (PSA)
Unlike the previously discussed thermal swing absorption, pressure swing absorption (PSA) is simpler method of concentrating oxygen from the atmosphere.
PSA units are known as oxygen generators or oxygen concentrators, and work on the basis of molecular sieve filtration as opposed to distillation as is the case with oxygen liquefaction. They are used as a good alternative to purchasing bottled high-pressure oxygen. High pressure oxygen comes with its risks and special handling requirements. Low pressure generators are far safer and once the original investment is paid for, oxygen has no direct material cost to consider. Bottled oxygen, while not overly expensive, adds up when large volumes are required. Over a year it is not uncommon to spend more on cylinders of oxygen than simply buying a PSA unit. PSA concentrators are a superb option for smaller units that don’t want to be limited to buying HP gas, but are not big enough to warrant buying cryogenic oxygen. They come in larger sizes which can produce many hundreds of litres per minute which accommodate even large 20-seater chambers providing oxygen therapy up to 3 atmospheres absolute. They come in units ranging from home to commercial use and are generally rated in flow rate as producing a given number of litres per minute.
They are specialist equipment usually maintained and serviced by specialised companies who even monitor machines online these days with newer models incorporating computerisation and automated error reporting.
They operate on a molecular sieve basis and in simple terms they sieve out the nitrogen molecules to concentrate the remaining oxygen and then dispose of the waste gas outside the building.
There are a few additional considerations when safety is concerned. These are large and potentially noisy machines so appropriate PPE is advised. It is also imperative to discharge waste gas which is very low in oxygen content outside the building to the open atmosphere. The only gas that can be breathed pure is oxygen. Breathing any other pure gas will almost certainly be deadly. Oxygen content can be as low as a few percent following processing and this cannot be allowed to displace air inside a building.
Principal of operation:
As shown in the images below, a PSA unit usually consists of two vessels and a series of servo/solenoid-controlled valves and plumbing. There is usually a product storage tank attached to capture and store oxygen. This is then plumbed to its destination without the need for regulation since it is already considered low pressure with most PSA’s delivering no more than 7 or 8 bars at most. In many operations, less than 14 bars oxygen pressure is considered no higher risk than compressed air and some operators don’t even oxygen clean at this pressure. Notwithstanding that, oxygen service and cleaning is strongly advised for any pressure oxygen above 40% concentration. This does mean larger storage tanks than high pressure cylinders but is far safer when handling is concerned.
Each vessel contains a sieve bed made up of a product called Zeolite. Zeolite is such that it only allows oxygen molecules to pass through and slows the passing of larger nitrogen molecules which can then be separated. This is why it’s known as concentration rather than production. It simply concentrates existing oxygen found in air until the desired purity is achieved.
Regarding purity, technically speaking, medical oxygen is defined as 99,5% pure in most parts of the world. This is legally labelled “Medical Oxygen”. PSA units do not achieve quite this purity, but rather around 97% from more modern models and as low as 93% on older models. It is still recognised as oxygen however and is labelled “Oxygen 93”. For the purposes of most hyperbaric oxygenation, oxygen 93 is adequate. Most diving operations prefer medical oxygen or other high purity sources and even make use of welding oxygen in many operations. Welding oxygen is actually higher in purity that medical oxygen and costs about 10% of the price. Regulations and bureaucracy are all that differentiate welding and cutting oxygen from medical oxygen. Oxygen is oxygen. In the UK, welding oxygen is not permitted on medical systems. Only medical grade oxygen may be used.
93% oxygen would actually be largely ok for most treatment tables as well, however it’s use is normally limited to elective hyperbaric oxygen therapy.
Most HBOT treatments are based on final target partial pressure of oxygen, and this can easily be achieved with ‘oxygen 93’ by simply increasing chamber pressure slightly. The ambient pressure increase is insignificant in other terms.
100% oxygen at 2 ata has a partial pressure of 2ata.
93% oxygen would need to be pressurised to not much more at 2,15 ata to achieve the same partial pressure.
The chamber pressure would need to be 2,15 ata rather than 2 ata which is an insignificant difference in most other terms.
2,15 ATA X 0.93ppO2 = 1.9995 ATA – Rounded = 2 ATA
For chronic cases of decompression illness however, the added 7% nitrogen would complicate the overall calculation of the treatment table. Less nitrogen would scrub from a patient’s blood and tissue making treatment tables less accurate and predictable, and appropriate extensions would be needed if only ‘oxygen 93’ was available. The added 7% N2 would reduce the differential gradient in N2 gas pressure and tissue tension, reducing outgassing accordingly. It would still be better than taking a non-intervention approach though. ‘Oxygen 93’ can be used if extensions to the various table sections are considered and no higher purity is available without acceptable delay to treatment. Undue delay can cause DCI to worsen, and this decision would depend very much on each individual case. Treatment is often permitted in law if such delay wold cause a patient’s condition to worsen.
Most modern PSA’s are computer controlled and have become considerably more efficient in recent years producing more oxygen from less air so to speak. This means that the size of units has decreased considerably in recent years. Modern units are simple on and off turnkey operations, with any settings being handled by the manufacturer and are factory set.
PSA’s like being worked hard and they should be allowed to run uninterrupted. Periods of inactivity are generally followed by a period of lower purity and they must be allowed to run for days rather hours following lengthy periods of shut down. They produce the best purity when running continuously. Any running up of a unit following a time of inactivity must be accompanied with discarding product outside the building until the desired purity is achieved. Simply bleed off the oxygen product to atmosphere until desired purity is achieved.
Most PSA’s have built in analysers to ensure purity; however, portable analysers can also be used on sample lines provided the flow is correct over the fuel cell. If no analyser is provided it is also possible to attach a sample line to the chamber analyser to test purity when the chamber is not in use. A simple enough workaround.
Outside of that operators need not be technicians of these PSA units. They most often take care of themselves, and online error reporting via the internet helps remote technicians detect and identify faults which are easily remedied with remote instruction or a technician visit.
PSA’s operate on compressed air in the LP range and usually draw a compressed and scrubbed air line from the main compressor stored in a dedicated receiver. The air must be scrubbed to an oxygen standard. It’s not uncommon to scrub air 3 or 4 times before allowing it to flow through the PSA. Cooler air produces higher purity and it is also common to refrigerate warm air in warm climates prior to entering the sieve beds. Zeolite doesn’t like moisture and air must be adequately dried and condensate removed to avoid damaging the machine. Zeolite is extremely expensive and constitutes the majority of the cost of a new machine. Replacing the Zeolite is possible; however, it costs almost the same as a new machine. Take care with air quality before it enters the concentrator and sieve bed.
Inlet pressures vary from make to make, and many different brands provide equal quality. This can range from 4 bars to 9 bars but seldomly exceeds 10 bars inlet. Outlet is usually regulated within the concentration process down to around 6 or 8 bars. Some as low as 4 bars outlet. Provided the oxygen is at a high enough pressure to enter the chamber and high enough for proper function of BIBS, no higher pressure is required. 8 bars are usually more than enough considering usually no treatment beyond 4 ata uses pure oxygen. That’s 4 for the chamber, and another 4 ata over ambient to adequately operate BIBS mask regulators.
Most leading gas suppliers also make oxygen concentrators including: The British Oxygen Company (BOC), Inmatec who are based in Germany, Air Products have the Prism onsite gas generation products, Omega Air and so on. Due diligence is advised when sourcing a generator as these are expensive items. After sales service should be considered and availability of spares and service technicians. They are not quite as user serviceable as other items of equipment. They are brilliant additions however to hyperbaric unit. Oxygen on tap. Below are some images illustrating large, small and home use oxygen concentrators.