About nitrox fabrication (August 2022)

The development of the substitute energies to hydrocarbons described in the previous food for thought topics has resulted in many dives being performed at shallow depths where “nitrox” can replace air to increase productivity and safety. For example, using the COMEX table MT 92 and a mix of 40% oxygen, the equivalent depth allows a no-decompression dive of 165 minutes at 18 m instead of 50 minutes with air. However, numerous companies do not apply these procedures despite their advantage. One of the reasons is linked to the fact that they are more complex to implement than air diving, and another one is the price of oxygen. The complexity of the implementation of nitrox is often linked to personnel formation and is not discussed in this article. However, difficulties related to the price of the oxygen can be partially solved by fabricating nitrox mixes using specific membrane systems or fabricating oxygen that can be used to compose nitrox by using oxygen generators. Manufacturers say these devices can save up to 50% of the price of oxygen classically manufactured by cryogenic separation. Let’s start describing these machines with the fabrication of nitrox using membrane systems: 1 - Description Membrane systems selectively separate nitrogen and oxygen, from the air. They have the advantages of fabricating nitrox at a low cost, reducing logistical problems, and being easily transportable. They can be classified into two main categories, which are not based on the same principle of work. “Hollow fiber membranes” consist of hollow tubes made of fiber manufactured by a co-extrusion-like process. The permeance of gasses across the polymeric membrane is based on the solubility of the gas in the polymer and the rate of gas diffusion across the membrane. For these reasons, polymers are selected for the membranes that are conducive to high permeance efficiency, light- weight, and reliability. The cross-section of a typical fiber has an outside diameter of 140- 180 microns and an inside diameter of approximately 100 to 140 microns. The majority of the fiber wall thickness is a porous sponge-like material that makes up the fiber core. The purpose of the core is merely to support an outer boundary layer of a thickness of approximately 2 microns, called the sheath, where gas separation occurs. The sheath and the outer skin of this layer, measured in angstroms, determine the performance of the membrane. These fibers are assembled in bundles to form the air separation modules. The air is supplied at one end of each fiber and moves to the other end. During this process, oxygen is absorbed through the polymer walls of the fiber due to the pressure difference. As a result, the gas that exits the downstream end of the hollow fiber is decreased in oxygen concentration. Therefore, oxygen-enriched air with oxygen concentration up to 40% is produced . The advantages of this technology include the absence of moving parts, the low weight, the inexpensive nature of the materials of construction, and the lack of any substantial time lag in system start-up. The air inlet pressure is usually between 13 & 14 bar. This inlet air should be filtered and dried to limit the particles size to 0.01µm, and having a maximum oil vapour content below 0.01 mg/m³. Several companies produce modules that are designed to be compiled with adequate filtration systems and compressors. It is, for example, the case of Parker, L’Air Liquide, Gereron, and others. The scheme below shows how a nitrox production unit using these modules should be organized. Note that the NASA study “Onboard oxygen generation system” says that that multi staging can improve the concentration of the oxygen provided by such membrane systems. The second category of membrane separation systems is called “ceramic membranes”. The principle of the ceramic oxygen permeation process uses the catalytic properties of specialized ceramic materials to transfer the oxygen in the form of ions instead of molecules, so the ions of other gas molecules cannot pass through the membrane. As a result, the oxygen concentration can reach 99.5% or even higher. The system is based on a membrane where one side is the cathode, and the other is the anode that is separated from the cathode by an electrolyte. Oxygen molecules' absorption starts at the membrane's cathode side, where they are dissociated into oxygen ions. These oxygen ions migrate to the anode side through the membrane and then recombine into oxygen molecules. The compensation of electric charges during the process is achieved through reverse-direction migration of the electrons in an external electric circuit. Note that the operating temperature of ceramic membranes is between 600 and 900 °C. Like hollow fiber membranes, ceramic membrane systems do not use moving parts. In addition, they have the advantage of not being sensitive to water vapour and other contaminants. Nevertheless, these devices need to be energized by an electric circuit to work. These systems are fabricated mostly for aerospace and defense industries. 2 - Design and operating procedures of a system Like for other types of equipment, it is essential to study an existing system's design and operating procedures to understand better how such equipment can be adapted to commercial diving activities. The device taken as a reference is the "230n3 Series" created by "Nuvair" (https://www.nuvair.com/), a company based in Oxnard, California, USA. This system, which is among the most sold, can supply nitrox with an oxygen concentration of up to 40%. 1. The Membrane Systems require a source of clean, pressurized, and heated air for separation. The two most common sources are a Low Pressure Compressor (LP Supply) or High Pressure air storage tanks (HP Supply). 2. The air must be properly filtered to be “oxygen compatible” quality prior to entering the membrane system so it will not damage or plug the membrane fibbers. Standard systems are rated for maximum supply pressures of 17 bar (250 psi) for LP Supply and 345 bar (5000 psi) or sometimes more for HP Supply. 3. An “input pressure regulator” reduces these pressures to acceptable levels for the membrane. 4. The air is then heated to a temperature that provides stability over a wide range of ambient conditions and is optimal for membrane permeation. 5. The heated air enters the membrane, which is made up of thousands of miniature hollow fibers. The walls of these fibers are semi-permeable and designed for different gases to move through them (or permeate) at different speeds. The resulting gas mixture is known as the “permeate”. 6. As air flows through the hollow fibers, both oxygen and nitrogen permeate through the fiber walls. The oxygen permeates faster than the nitrogen, which produces permeate with an oxygen content greater than air. The gas that reaches the end of the hollow fibers without permeating is almost entirely nitrogen and is discharged. The flow rate of this discharge is set by the factory via a fixed orifice to allow the membrane to operate at maximum volume and efficiency. The resulting permeate contains approximately 40% O2 and is constant under all operating conditions. 7. The permeate is a concentrated mixture that must be diluted with additional air prior to entering the nitrox compressor. It exits the membrane at ambient to slightly negative pressure and travels into the “mixing tube”, where it mixes homogeneously with filtered outside air. The amount of dilution, and thus final % O2 , is obtained by adjusting the “input pressure regulator”: As pressure is increased, permeate flow increases, air flow decreases, and a higher % O2 Nitrox is produced. As pressure is decreased, permeate flow decreases, air flow increases, and a lower % O2 Nitrox is produced. This relationship between permeate flow and air flow exists because the total of these two flow rates will always equal the intake flow rate demanded by the Nitrox Compressor. 8. The resulting nitrox mixture is analysed for approximate % O2 before entering the nitrox compressor and again prior to use for precise % O2 . 9. The input pressure that correlates to a specific nitrox % O2 is repeatable. If nitrox with 36% O2 is produced when the input pressure is at 9 bar (125 psi), then adjusting the Regulator to the same pressure during the next use will produce the same gas mixture. Identification of the components of a system supplied with low pressure: Identification of the components of a system supplied with high pressure: Overall view of the components of a system supplied with high and low pressure: As said previously, the higher the % of O2 desired in the final product, the greater the volume of supply air, and the higher the input pressure required, as shown in the example below for a 10 cfm (283 L/min) membrane system: The air supplied should comply with EN 12021 or CGA G-7.1-1997 grade D or E. Manufacturers of nitrox membrane systems also sell complete ensembles that include the compressor, such as the machine below designed by Nuvair that can produce up to 481 litres/min of nitrox 40 % (max. pressure: 250 bar). 3 - Where to install the machine? Like all air compressors used on worksites, the machine's air intake should be placed at height and away from any potential sources of dangerous gas. In addition, the area where it is installed should be cleared of pollutants that may ignite oxygen such as oil and grease. Thus, a risk assessment regarding its surrounding should be done. Also, nitrox mixtures used for diving usually have proportions of oxygen above 25%, so they must be handled as pure oxygen. The document IMCA D 022 chapter 9/point 9.6 recommends not to pump oxygen. For this reason, many companies transfer pure oxygen and nitrox mixes by decanting only during operations at sea, and some others buy mixes fabricated in factories. However, a lot of companies have ceased to apply this guideline and argue that, if relevant precautions are implemented, it is no riskier to pump nitrox mixes offshore than implementing other operations that are commonly done, such as fuel bunkering or the gas containers transfer by crane. We must admit that accidents with these machines are rare, as we have not found a recent paper regarding such events. For this reason, we can consider that pumping nitrox mixes is possible if a risk assessment has been undertaken to ensure the desirability of doing it, that the fire surveillance and fire fighting systems are sufficient, and that this operation is performed in an isolated ventilated part of the deck where a fire can be easily and quickly contained. In addition to extinguishers and fire lances in immediate proximity to the room, the fire fighting systems should include a deluge or a water mist system with a fire alarm system linked to the vessel's bridge, Of course, the vessel owner, the client, and the state representative can reject such a procedure. Thanks to Erick Estrada & Ron Case from NUVAIRr for their kindness and efficient support.

Purpose and potential applications: Membrane gas separation systems used to extract the oxygen, such as the nitrox membrane system Nuvair described in the previous post (see in discussion “August 2022”) or models from other manufacturers, provide nitrox mixes limited to approximately 40% oxygen. However, it is possible to obtain nearly pure oxygen with technologies such as Pressure Swing Absorption (PSA) oxygen generators. In recent decades, this technology has improved to become efficient, reliable, transportable, and financially accessible. Like the nitrox membrane system described in the previous section, these apparatus extract oxygen from the natural air. The purity obtained ranks from 90 to 99.5%, depending on the equipment. Note that 90% is the minimum required by standardization organizations. Pressure Swing Adsorption oxygen generators are primarily used for medical support, particularly for mobile hospitals and those installed in isolated areas. They are also increasingly installed to reduce the cost of therapeutic gasses in hospitals established in towns and provide oxygen treatment for individuals at home. However, the study of their working process proves that they can be employed for other applications, such as the production of nitrox mixes. For remembering, 99.5% is the minimum recommended oxygen purity of the European standard EN 12021, US Navy, and others. For this reason, oxygen not complying with this minimum must not be used for decompression and therapeutic treatments, as the tables have been calculated according to this minimum oxygen purity. Thus, except if the oxygen produced conforms with the above, these apparatus cannot be used to supply pure oxygen. However, nothing prevents us from using oxygen extracted from the air with less than 99.5% purity for nitrox mixes, considering that the remaining 10 to 1% of impurities of the oxygen produced by these apparatus are nitrogen and argon. As a result, oxygen with more than 99.5% purity can be kept for decompression and medical treatment, and nitrox can be produced with oxygen extracted from the air without affecting these reserves. Note that most machines currently available on the market produce oxygen with a purity between 93 & 97%. However, a few manufacturers are able to sell machines producing oxygen with a minimum purity of 99.5%. Description: Molecular sieves used with “Pressure Swing Adsorption (PSA)” systems are crystalline synthetic or naturally occurring zeolites (aluminosilicate minerals with microporous structure) with pores of precise and uniform size that have the capacity to absorb and separate gasses and liquids. The absorption and separation of the molecules are based on the size of the molecules, so only small enough ones can enter the pores, and it is also based on their electric charge (electro-static fields). Molecular sieves are classified according to their chemical formula and pore sizes. They are used for applications such as drying gases, absorbing undesirable gasses such as ammonia, methanol, ethanol, carbon dioxide, hydrogen sulfide, and fabricating gasses such as oxygen, nitrogen, or hydrogen. Molecular sieves type 13X are commonly used to separate nitrogen from the air to produce oxygen. They are the sodium form of the aluminosilicate molecular sieves with pore diameters of approximately 10 angstroms, an external diameter between 0.4 to 2.5 mm, and a light grey colour. They can be thermally regenerated at temperatures from 180 °C to 300 °C. Another regeneration method consists of gradually reducing the applied pressure. Zeolites of Pressure Swing Adsorption systems have the inconvenience of increasing the proportion of argon in the oxygen delivered. As an example, for a system delivering a mix with 93% oxygen purity, the ratio of argon is 4% instead of 1% in the atmosphere. For this reason, NASA studies on onboard oxygen generation systems recommend using a 2nd bed made of carbon to absorb the excess argon. Most systems use two zeolites towers so that one unit is in use when the molecular sieves in the second unit are regenerated. The basic scheme of the systems commonly used is similar to the one displayed below.

Note that the air from the compressor should be oxygen compatible. The following specifications are commonly asked:

Oil free screw air compressors are known to deliver high flow of low pressure air, and are often preferred to supply the installations. Also, most installations are provided with a low pressure storage tank. As an example of machine that can be adapted for the production of nitrox, the Pressure Swing Adsorption (PSA) Oxygen generator model NZO-30 PSA below, designed by Hanghou Nuzhuo Technology co, Ltd (https://www.hznuzhuo.com/), can deliver 30 m3/hr of oxygen with 95% purity, so 3 cylinders 50 litres/200 bar/hour. It is made of the following components:

1 - Screw compressor 7.5 m3/min 2 - Air purification: Dryer 6 m3/min 3- Air purification: Filtration 4 - Air buffer tank 5 - Absorption tower “A” 6 - Absorption tower “B” 7 - Oxygen buffer tank 8 - Oxygen compressor 30 m3/h 9 - Controller 10 - Valves

11 - Flow meter 12 - Manifold for filling

This equipment was initially designed to be installed in ventilated rooms with a surface of at least 30 m² and a height not less than 4 m. However, Hanghou Nuzhuo Technology provides the photo above that shows that this machine can be installed in a 20 feet container, provided that adequate ventilation and fire fighting systems are in place. The manufacturer recommends to keep the machine at an ambient temperature of not less than 5 ºC and not more than 49 ºC. Low noise fans directing the hot air to the outside of the room are suggested for this purpose and to avoid the accumulation of oxygen in the room. Note that the heat and gas accumulation problems that may arise due to lack of space in offshore containers can be efficiently compensated by providing large top and bottom openings, ensuring adequate air circulation. Usually, such openings are provided with louvers so they can be kept open when it rains. Waterproof external shutters are typically provided to close these openings when the machine is not used and transferred to another place. Note that the controller allows managing the oxygen production and sieve regeneration automatically. This machine is the less powerful of the range sold by the manufacturer taken in reference. It allows providing the necessary oxygen (95% purity) for 24 hours diving operation at 18 m using a mix 50/50 and 1 diver in the water within less than two hours of compression. However, many diving operations do not need so powerful machines, and the space available on the surface support may not be sufficient to accommodate a container like the one above. Some manufacturers specialize in less powerful and more transportable machines initially designed for small hospitals and individuals. It is the case of the models developed by OXUS (https://www.oxus.co.kr/en/), a company based in Korea that sells a range of machines installed in cabinets mounted on castors. These cabinets accommodate several small appliances that can be switched on and off, depending on the volume of oxygen to fabricate. For example, the model RAK-U06M2E below is composed of an ensemble of six small oxygen generators, each of which can work independently and two control units. The size of the cabinet (W x D x H) taken in reference is 960 x 600 x 1,550 mm. Optional oxygen storage tanks are provided on the top of the cabinet. However, the system is designed to supply hospital circuits at the outlet pressure delivered (4 - 5 bar), and an HP compressor is to be added to the machine to fill HP gas cylinders. OXUS provides a high pressure booster that can compress the cylinders to a maximum pressure of 150 bar for this purpose.

This system can fabricate 3600 l of oxygen per hour, so 89400 l per day. Note that smaller units are also available on the market. For example, the manufacturer of the model described above sells a machine designed to provide 43200 litres per 24 hours and another tiny unit designed for 7200 litres per day. Even though this last machine is unsuitable for the fabrication of oxygen for the diving team, it can be used in the onboard hospital for oxygen supports other than hyperbaric treatments. Implement the machine: Using such machines on worksites is a new idea. However, considering that they are successfully used for hospitals in isolated areas, there is no reason for not implementing them, provided that the precautions indicated by the manufacturers and above are implemented. These precautions include the position of the air inlet of the compressor and the elements already discussed for the implementation of membrane systems for nitrox fabrication. Thus, a risk assessment should be done to ensure that the machine's surroundings are safe, and that relevant firefighting systems are ready for use. Of course, the vessel owner, the client, and the state representative can reject using such a machine onboard the ship. In addition, using the oxygen produced by such a machine to fabricate various nitrox implies that the personnel implementing it and then performing mixing of the oxygen with air to make the desired nitrox must have a relevant formation.