Dr. S S Verma
Oxygen has been the only element to support life through respiration and was first used in the medical field in 1810. However, it took about 150 years for the gas to be used throughout medicine. In the early to mid of 20th century oxygen therapy became rational and scientific. It is no exaggeration to say that today modern medicine could not be practiced without the support of oxygen supply. Oxygen cylinders in different applications in general and in hospitals in particular are well known to masses. But due to the importance and enormous need of oxygen (known as medical oxygen) support to the COVID-19 patient as shortness of breath is one of the most common symptoms of severe COVID-19 disease. One of the ways through which medical oxygen can be provided to patient is Liquid Medical Oxygen (LMO). In fact, many persons suffering from the disease were in dire need of medical oxygen. With the growing demand of medical oxygen recently, the oxygen concentrator has emerged the most sought after device. Though medical oxygen is generally sourced from industrial oxygen units and supplied via cylinders to its users but oxygen concentrators are the devices that can be operated with ease at home. Thus, oxygen concentrator has been a buzz word with people during COVID-19 pandemic. It generates a curiosity in common reader to know about oxygen concentrator and its working. Thanks to such good technology based machines which have made it much easier during these tough times to increase the health infrastructure and meet most requirements of the economy.
Generally, oxygen concentrators describe smaller home medical oxygen systems whereas oxygen generators describe all other size plants. Smaller oxygen concentrator cannot produce higher pressure like 65 PSI or 4.48 bar, whereas oxygen generator can work on this kind of pressure. From scientific point of view, an oxygen concentrator is a device that concentrates the oxygen from a gas supply (typically ambient air: a combination of nitrogen, oxygen and other gases in small fractions) by selectively removing nitrogen and to supply an oxygen-enriched product gas stream. These devices are powered by plugging into an electrical outlet or by using a battery. If the oxygen concentrator is a battery operated, then it will need to be charged by plugging it into an electrical outlet. Most concentrators also come with an adapter making their use possible while on journey. An oxygen concentrator takes in air and separates the oxygen and delivers air that is up to 95% oxygen into a person via a nasal cannula. Respiratory infections causing oxygen saturation levels to dip below 90%, use of an oxygen concentrator helps to maintain the oxygen level which eases the burden on the lungs. However, in cases of severe respiratory distress, it may be necessary to provide oxygen that is almost 99% pure and oxygen concentrator is not up to that job.
There are various methods to produce liquid medical oxygen but the most common production method is the separation of oxygen in Air Separation Units (ASUs) those help to separate large volumes of gases. To produce pure oxygen from atmospheric air, ASUs use a well known method known as the ‘Fractional Distillation Method’.
Fractional Distillation Method: In this method, the various gases constituting atmospheric air are separated into different components after cooling them into a liquid state. Liquid oxygen is then extracted from it. Atmospheric air is first cooled to -181 degree Celsius, the point of oxygen liquefaction. As the boiling point of Nitrogen is -196 degree Celsius so it still remains in a gaseous state at a temperature of -181 degree Celsius. But Argon has a boiling point similar to that of oxygen (-186 degree Celsius) and hence a significant amount of Argon liquefies along with Oxygen. The mixture of oxygen and argon thus produced is drained, decompressed and passed through a second low-pressure distillation vessel in order to separate the two gases and then purified liquid oxygen is obtained using cryogenic containers.
Pressure Swing Adsorption Technique: This is another method through which LMO can be produced non-cryogenically through selective absorption. This method makes use of the property exhibited by some materials to attract specific gases to the solid surfaces under high pressure. The higher is the pressure, the more will be the adsorption of gas. If air is passed under pressure through a vessel containing an adsorbent bed of zeolites that attracts nitrogen more strongly than oxygen, a part or all of the nitrogen will stay in the bed, and the gas exiting the vessel will be richer in oxygen, relative to the mixture entering the vessel. Wonder materials known as Zeolites are microporous crystalline solids with well-defined structures. Zeolites generally contain silicon, aluminium and oxygen in their framework whereas cations, water and/or other molecules within their pores. Many such materials occur naturally as minerals, and are extensively mined in many parts of the world, however, industrially important zeolites are also produced synthetically.
There are also pulse and continuous flow concentrators. The continuous flow oxygen concentrators deliver oxygen at a constant rate whereas pulse type concentrators make use of a sensor to deliver a puff of oxygen when a user is about to inhale. Oxygen concentrators come with a variety of specifications and with varying oxygen outputs. For device with a 5L-10 L, the cost of can range from Rs 40,000-Rs 90,000. The main components and working steps of an oxygen concentrator in brief are given as:
Number of filters to filter out impurities present in air.
Two molecular sieve beds of Zeolite (Micro porous Aluminosilicate mineral) having an ability to trap Nitrogen.
The air is pushed into the machine and forwarded to the molecular sieve beds by Air Compressor.
Switch Valve switches the output of compressor between the two molecular sieve beds.
Oxygen outlet is an opening that gives out oxygen to the patient.
Flow meter to set the flow in Litres Per Minute (LPM)
Ambient air (i.e., room air) is drawn into the machine by a compressor to pass through a series of filters.
The molecular sieve beds are porous and thus having large surface area due to which they adsorb large amount of nitrogen. Air is compressed into the 1st molecular sieve bed where all the nitrogen is adsorbed.
The compressor keeps on compressing air into the 1st molecular sieve bed till it gets saturated (filled) by Nitrogen which usually gets saturated at pressure of 20 psi.
Just before 1st molecular sieve bed gets saturated, the switch valve comes into action and output of the air compressor is immediately switched to 2nd sieve bed.
While this sieve bed gets saturated by Nitrogen, the Nitrogen that was trapped in the 1st sieve bed is vented out. The little Nitrogen that is left in the sieve bed after discharging is removed by back-flushing of Oxygen from the other sieve bed.
The switch valve again switches the output of air compressor back to the 1st sieve bed as soon as the 2nd sieve bed approaches saturation.
This process keeps on repeating to ensure continuous flow of Oxygen.
The process of switching the sieve beds is known as Pressure Swing Adsorption (PSA).
Dr. S S Verma