An oxygen concentrator is a device that concentrates the oxygen from a gas supply (typically ambient air) by selectively removing nitrogen to supply an oxygen-enriched product gas stream. They are used in industry, to provide supplemental oxygen at high altitudes, and as medical devices for oxygen therapy.
Oxygen concentrators are widely used for oxygen provision in healthcare applications, especially where liquid or pressurized oxygen is too dangerous or inconvenient, such as in homes or portable clinics, and can also provide an economical source of oxygen in industrial processes, where they are also known as oxygen gas generators or oxygen generation plants. Two methods in common use are pressure swing adsorption and membrane gas separation.
Pressure swing adsorption (PSA) oxygen concentrators use a molecular sieve to adsorb gases and operate on the principle of rapid pressure swing adsorption of atmospheric nitrogen onto zeolite minerals at high pressure. This type of adsorption system functions as a nitrogen scrubber, allowing the other atmospheric gases to pass through, leaving oxygen as the primary gas remaining. PSA technology is a reliable and economical technique for small to mid-scale oxygen generation. Cryogenic separation is more suitable at higher volumes.
Gas separation across a membrane is a pressure-driven process, where the driving force is the difference in pressure between inlet of raw material and outlet of product. The membrane used in the process is generally non-porous, minimizing gas leakage through the membrane. The performance of the membrane depends on permeability and selectivity. Permeability is affected by the penetrant size. Larger gas molecules have a lower diffusion coefficient. The membrane gas separation equipment typically pumps gas into the membrane module and the targeted gases are separated based on the difference in diffusivity and solubility. For example, oxygen will be separated from the ambient air and collected at the upstream side, and nitrogen at the downstream side. As of 2016, membrane technology was reported as capable of producing 10 to 25 tonnes of 25 to 40% oxygen per day.
History Home medical oxygen concentrators were invented in the early 1970s, with the manufacturing output of these devices increasing in the late 1970s. Union Carbide Corporation and Bendix Corporation were both early manufacturers. Before that era, home medical oxygen therapy required the use of heavy high-pressure oxygen cylinders or small cryogenic liquid oxygen systems. Both of these delivery systems required frequent home visits by suppliers to replenish oxygen supplies. In the United States, Medicare switched from fee-for-service payment to a flat monthly rate for home oxygen therapy in the mid-1980s, causing the durable medical equipment (DME) industry to rapidly embrace concentrators as a way to control costs. This reimbursement change dramatically decreased the number of primary high pressure and liquid oxygen delivery systems in use in homes in the United States at that time. Oxygen concentrators became the preferred and most common means of delivering home oxygen. The number of manufacturers entering the oxygen concentrator market increased significantly as a result of this change. Union Carbide Corporation invented the molecular sieve in the 1950s, which made these devices possible. It also invented the first cryogenic liquid home medical oxygen systems in the 1960s.
How oxygen concentrators work
Oxygen concentrators using pressure swing adsorption (PSA) technology are used widely for oxygen provision in healthcare applications, especially where liquid or pressurized oxygen is too dangerous or inconvenient, such as in homes or portable clinics. For other purposes, there are also concentrators based on nitrogen separation membrane technology.
An oxygen concentrator removes nitrogen from ambient air, leaving an oxygen-enriched gas for use by people requiring medical oxygen due to low oxygen levels in their blood. Oxygen concentrators provide an economical source of oxygen in industrial processes, where they are also known as oxygen gas generators or oxygen generation plants.
Pressure swing adsorption
These oxygen concentrators utilize a molecular sieve to adsorb gases and operate on the principle of rapid pressure swing adsorption of atmospheric nitrogen onto zeolite minerals at high pressure. This type of adsorption system is therefore functionally a nitrogen scrubber, allowing the other atmospheric gases to pass through, leaving oxygen as the primary gas remaining. PSA technology is a reliable and economical technique for small- to mid-scale oxygen generation. Cryogenic separation is more suitable at higher volumes, and external delivery is generally more suitable for small volumes.
At high pressure, the porous zeolite adsorbs large quantities of nitrogen because of its large surface area and chemical characteristics. The oxygen concentrator compresses air and passes it over zeolite, causing the zeolite to adsorb the nitrogen from the air. It then collects the remaining gas, which is mostly oxygen, and the nitrogen desorbs from the zeolite under the reduced pressure to be vented.
An oxygen concentrator has an air compressor, two cylinders filled with zeolite pellets, a pressure-equalizing reservoir, and some valves and tubes. In the first half-cycle, which lasts about 3 seconds, the first cylinder receives air from the compressor. During that time, the pressure in the first cylinder rises from atmospheric to about 2.5 times normal atmospheric pressure (typically 20 psi/138 kPa gauge, or 2.36 atmospheres absolute) and the zeolite becomes saturated with nitrogen. As the first cylinder reaches near pure oxygen (there are small amounts of argon, CO2, water vapour, and other minor atmospheric components) in the first half-cycle, a valve opens and the oxygen-enriched gas flows to the pressure-equalizing reservoir, which connects to the patient's oxygen hose. At the end of the first half of the cycle, there is another valve position change so that the air from the compressor is directed to the second cylinder. The pressure in the first cylinder drops as the enriched oxygen moves into the reservoir, allowing the nitrogen to be desorbed back into gas. Partway through the second half of the cycle, there is another valve position change to vent the gas in the first cylinder back into the ambient atmosphere, keeping the concentration of oxygen in the pressure-equalizing reservoir from falling below about 90%. The pressure in the hose delivering oxygen from the equalizing reservoir is kept steady by a pressure-reducing valve.
Older units cycled for a period of about 20 seconds and supplied up to 5 litres per minute of 90+% oxygen. Since about 1999, units capable of supplying up to 10 L/min have been available.
Earlier oxygen concentrators employed two-bed molecular sieves; more recent designs incorporate multi-bed molecular sieves to allow continuous oxygen production during bed regeneration. Redundancy is provided by staggering 10 L/min molecular sieves. With this, over 960 L/min can be produced. The ramp-up time — the elapsed time until a multi-bed concentrator is producing oxygen at >90% concentration — is often less than 2 minutes, much faster than simple two-bed concentrators. The faster ramp-up improves sustainability for use in modern or emergency settings. The option to fill standard oxygen cylinders (e.g., 50 L at 200 bar = 10,000 L each) with high-pressure boosters, to ensure automatic failover to previously filled reserve cylinders and to ensure the oxygen supply chain, e.g., in case of power failure, is given with those systems.
Membrane separation In membrane gas separation, membranes act as a permeable barrier, which different compounds move across at different rates or do not cross at all.
Gas mixtures can be effectively separated by synthetic membranes made from polymers such as polyamide or cellulose acetate, or from ceramic materials.
While polymeric membranes are economical and technologically useful, their performance is constrained by the Robeson limit (permeability must be sacrificed for selectivity and vice versa). This limit affects polymeric membrane use for CO2 separation from flue gas streams, since mass transport becomes limiting and CO2 separation becomes very expensive due to low permeabilities. Membrane materials have expanded into the realm of silica, zeolites, metal-organic frameworks, and perovskites, due to their strong thermal and chemical resistance as well as high tunability (ability to be modified and functionalized), leading to increased permeability and selectivity. Membranes can be used for separating gas mixtures, where they act as a permeable barrier through which different compounds move across at different rates or do not move at all. The membranes can be nanoporous, polymer, etc., and the gas molecules penetrate according to their size, diffusivity, and solubility.
Gas separation across a membrane is a pressure-driven process, driven by the difference in pressure between the inlet of the raw material and the outlet of product. The membrane used in the process is generally a non-porous layer, so there will not be a severe leakage of gas through the membrane. The performance of the membrane depends on permeability and selectivity. Permeability is affected by the penetrant size. Larger gas molecules have a lower diffusion coefficient. The polymer chain flexibility and free volume in the polymer of the membrane material influence the diffusion coefficient, as the space within the permeable membrane must be large enough for the gas molecules to diffuse across. The solubility is expressed as the ratio of the concentration of the gas in the polymer to the pressure of the gas in contact with it. Permeability is the ability of the membrane to allow the permeating gas to diffuse through the material of the membrane as a consequence of the pressure difference over the membrane, and can be measured in terms of the permeate flow rate, membrane thickness and area, and the pressure difference across the membrane. The selectivity of a membrane is a measure of the ratio of the permeabilities of the different gases for the membrane. It can be calculated as the ratio of permeability of two gases in binary separation.
The membrane gas separation equipment typically pumps gas into the membrane module, and the targeted gases are separated based on the difference in diffusivity and solubility. For example, oxygen will be separated from the ambient air and collected at the upstream side and nitrogen at the downstream side. As of 2016, membrane technology was reported as capable of producing 10 to 25 tonnes of 25% to 40% oxygen per day.
Applications Medical oxygen concentrators are used in hospitals or at home to concentrate oxygen for patients. They can be used in various industries, including medical, pharmaceutical production, water treatment, and glass manufacture.
PSA generators are particularly useful in remote or inaccessible parts of the world or mobile medical facilities (military hospitals, disaster facilities).
Portable oxygen concentrators
Since the early 2000s, many companies have produced portable oxygen concentrators. Typically, these devices produce the equivalent of one to five liters per minute of continuous oxygen flow and they use some version of pulse flow or "demand flow" to deliver oxygen only when the patient is inhaling. They can also provide pulses of oxygen either to provide higher intermittent flows or to reduce power consumption.
Research into oxygen concentration is ongoing, and modern techniques suggest that the amount of adsorbent required by medical oxygen concentrators can be potentially "reduced by a factor of three while offering ~10–20% higher oxygen recovery compared to a typical commercial unit."