Gas blending

Last updated August 09, 2024

Gas blending is the process of mixing gases for a specific purpose where the composition of the resulting mixture is specified and controlled. A wide range of applications include scientific and industrial processes, food production and storage and breathing gases.

Applications
Shielding gases for welding
Modified atmosphere packaging in the food industry
Gas mixtures for brewing
Breathing gas mixtures for diving
Medical gas mixtures
Chemical production processes
Controlled atmosphere manufacture and storage
Customized gas mixtures for analytical applications
Methods
Batch methods
Continuous processes
Gas analysis
References
See also

Gas mixtures are usually specified in terms of molar gas fraction (which is closely approximated by volumetric gas fraction for many permanent gases): by percentage, parts per thousand or parts per million. Volumetric gas fraction converts trivially to partial pressure ratio, following Dalton's law of partial pressures. Partial pressure blending at constant temperature is computationally simple, and pressure measurement is relatively inexpensive, but maintaining constant temperature during pressure changes requires significant delays for temperature equalization. Blending by mass fraction is unaffected by temperature variation during the process, but requires accurate measurement of mass or weight, and calculation of constituent masses from the specified molar ratio. Both partial pressure and mass fraction blending are used in practice.

Applications

Shielding gases for welding

Shielding gases are inert or semi-inert gases used in gas metal arc welding and gas tungsten arc welding to protect the weld area from oxygen and water vapour, which can reduce the quality of the weld or make the welding more difficult.

Gas metal arc welding (GMAW), or metal inert gas (MIG) welding, is a process that uses a continuous wire feed as a consumable electrode and an inert or semi-inert gas mixture to protect the weld from contamination.^[1] Gas tungsten arc welding (GTAW), or tungsten inert gas (TIG) welding, is a manual welding process that uses a nonconsumable tungsten electrode, an inert or semi-inert gas mixture, and a separate filler material.^[2]

Modified atmosphere packaging in the food industry

Modified atmosphere packaging preserves fresh produce to improve delivered quality of the product and extend its life. The gas composition used to pack food products depends on the product. A high oxygen content helps to retain the red colour of meat, while low oxygen reduces mould growth in bread and vegetables.^[3]

Gas mixtures for brewing

Sparging: An inert gas such as nitrogen is bubbled through the wine, which removes the dissolved oxygen. Carbon dioxide is also removed and to ensure that an appropriate amount of carbon dioxide remains, a mixture of nitrogen and carbon dioxide may be used for the sparging gas.^[3]
Purging and blanketing: The removal of oxygen from the headspace above the wine in a container by flushing with a similar gas mixture to that used for sparging is called purging, and if it is left there it is called blanketing or inerting.^[3]

Breathing gas mixtures for diving

A breathing gas is a mixture of gaseous chemical elements and compounds used for respiration. The essential component for any breathing gas is a partial pressure of oxygen of between roughly 0.16 and 1.60 bar at the ambient pressure. The oxygen is usually the only metabolically active component unless the gas is an anaesthetic mixture. Some of the oxygen in the breathing gas is consumed by the metabolic processes, and the inert components are unchanged, and serve mainly to dilute the oxygen to an appropriate concentration, and are therefore also known as diluent gases.

Scuba diving

Gas blending for scuba diving is the filling of diving cylinders with non-air breathing gases such as nitrox, trimix and heliox. Use of these gases is generally intended to improve overall safety of the planned dive, by reducing the risk of decompression sickness and/or nitrogen narcosis, and may improve ease of breathing.

Surface supplied and saturation diving

Gas blending for surface supplied and saturation diving may include the filling of bulk storage cylinders and bailout cylinders with breathing gases, but it also involves the mixing of breathing gases at lower pressure which are supplied directly to the diver or to the hyperbaric life-support system. Part of the operation of the life-support system is the replenishment of oxygen used by the occupants, and removal of the carbon dioxide waste product by the gas conditioning unit. This entails monitoring of the composition of the chamber gas and periodic addition of oxygen to the chamber gas at the internal pressure of the chamber.

The gas mixing unit is part of the life support equipment of a saturation system, along with other components which may include bulk gas storage, compressors, helium recovery unit, bell and diver hot water supply, gas conditioning unit and emergency power supply^[4]

Medical gas mixtures

The anesthetic machine is used to blend breathing gas for patients under anesthesia during surgery. The gas mixing and delivery system lets the anesthetist control oxygen fraction, nitrous oxide concentration and the concentration of volatile anesthetic agents.^[5] The machine is usually supplied with oxygen (O₂) and nitrous oxide (N₂O) from low pressure lines and high pressure reserve cylinders, and the metered gas is mixed at ambient pressure, after which additional anesthetic agents may be added by a vaporizer, and the gas may be humidified. Air is used as a diluent to decrease oxygen concentration. In special cases other gases may also be added to the mixture. These may include carbon dioxide (CO₂), used to stimulate respiration, and helium (He) to reduce resistance to flow or to enhance heat transfer.^[6]

Gas mixing systems may be mechanical, using conventional rotameter banks, or electronic, using proportional solenoids or pulsed injectors, and control may be manual or automatic.^[5]

Chemical production processes

Providing reactive gaseous materials for chemical production processes in the required ratio

Controlled atmosphere manufacture and storage

Protective gas mixtures may be used to exclude air or other gases from the surface of sensitive materials during processing. Examples include melting of reactive metals such as magnesium, and heat treatment of steels.

Customized gas mixtures for analytical applications

Calibration gases:

Span gases are used for testing and calibrating gas detection equipment by exposing the sensor to a known concentration of a contaminant. The gases are used as a reference point to ensure correct readings after calibration and have very accurate composition, with a content of the gas to be detected close to the set value for the detector.
Zero gas is normally a gas free of the component to be measured, and as similar as practicable to the composition of the gas to be monitored, used to calibrate the zero point of the sensor.

Calibration gas mixtures are generally produced in batches by gravimetric or volumetric methods.

The gravimetric method uses sensitive and accurately calibrated scales to weigh the amounts of gases added into the cylinder. Precise measurement is required as inaccuracy or impurities can result in incorrect calibration. The container for calibration gas must be as close to perfectly clean as practicable. The cylinders may be cleaned by purging with high purity nitrogen, the vacuumed. For particularly critical mixtures the cylinder may be heated while being vacuumed to facilitate removal of any impurities adhering to the walls.^[7]

After filling, the gas mixture must be thoroughly mixed to ensure that all components are evenly distributed throughout the container to prevent possible variations on composition within the container. This is commonly done by rolling the container horizontally for 2 to 4 hours.^[7]

Methods

Several methods are available for gas blending. These may be distinguished as batch methods and continuous processes.

Batch methods

Batch gas blending requires the appropriate amounts of the constituent gases to be measured and mixed together until the mixture is homogeneous. The amounts are based on the mole (or molar) fractions, but measured either by volume or by mass. Volume measurement may be done indirectly by partial pressure, as the gases are often sequentially decanted into the same container for mixing, and therefore occupy the same volume. Weight measurement is generally used as a proxy for mass measurement as acceleration can usually be considered constant.

The mole fraction is also called the amount fraction, and is the number of molecules of a constituent divided by the total number of all molecules in the mixture. For example, a 50% oxygen, 50% helium mixture will contain approximately the same number of molecules of oxygen and helium. As both oxygen and helium approximate ideal gases at pressures below 200 bar, each will occupy the same volume at the same pressure and temperature, so they can be measured by volume at the same pressure, then mixed, or by partial pressure when decanted into the same container.

The mass fraction can be calculated from the molar fraction by multiplying the molar fraction by the molecular mass for each constituent, to find a constituent mass, and comparing it to the summed masses of all the constituents. The actual mass of each constituent needed for a mixture is calculated by multiplying the mass fraction by the desired mass of the mixture.

Partial pressure blending

Also known as volumetric blending. This must be done at constant temperature for best accuracy, though it is possible to compensate for temperature changes in proportion to the accuracy of the temperature measured before and after each gas is added to the mixture.

Partial pressure blending is commonly used for breathing gases for diving. The accuracy required for this application can be achieved by using a pressure gauge which reads accurately to 0.5 bar, and allowing the temperature to equilibrate after each gas is added.

Mass fraction blending

Also known as gravimetric blending. This is relatively unaffected by temperature, and accuracy depends on the accuracy of mass measurement of the constituents.

Mass fraction blending is used where great accuracy of the mixture is critical, such as in calibration gases. The method is not suited to moving platforms where the accelerations can cause inaccurate measurement, and therefore is unsuitable for mixing diving gases on vessels.

Continuous processes

Additive

Constant flow blending – a controlled flow of the constituent gases is mixed to form the product. Blending may occur at ambient pressure or at a pressure setting above ambient but lower than supply gas pressures.
- Constant mass flow supply: Precision mass flow controllers are used to control the flow rate of each gas for blending. Mass flow meters may be installed on the outputs of the mass flow controllers to monitor the output. The gases may be passed through a static mixer to ensure homogeneous output.

Continuous gas blending is used for some surface supplied diving applications, and for many chemical processes using reactive gas mixtures, particularly where there may be a need to alter the mixture during the operation or process.

Subtractive

These processes start with a mixture of gases, usually air, and reduce the concentration of one or more of the constituents. These processes can be used for the production of Nitrox for scuba diving and deoxygenated air for blanketing purposes.

Pressure swing adsorption – Selective adsorption of gas on a medium which is reversible and proportional to pressure. Gas is loaded onto the medium during the high pressure phase and is released during the low pressure phase.
Membrane gas separation – Gas is forced through a semi-permeable membrane by a pressure difference. Some of the constituent gases pass through the membrane more easily than the others, and the output from the low pressure side is enriched with the gases which pass through more easily. Gases which are slower to pass through the membrane accumulate on the high pressure side and are continuously discharged to retain a steady concentration. The process may be repeated in several stages to increase concentrations.

Gas analysis

Gas mixtures must generally be analysed either in process or after blending for quality control. This is particularly important for breathing gas mixtures where errors can affect the health and safety of the end user.

Oxygen content is relatively simple to monitor using electro-galvanic cells and these are routinely used in the underwater diving industry for this purpose, though other methods may be more accurate and reliable.

Related Research Articles

Nitrox refers to any gas mixture composed of nitrogen and oxygen. This includes atmospheric air, which is approximately 78% nitrogen, 21% oxygen, and 1% other gases, primarily argon. In the usual application, underwater diving, nitrox is normally distinguished from air and handled differently. The most common use of nitrox mixtures containing oxygen in higher proportions than atmospheric air is in scuba diving, where the reduced partial pressure of nitrogen is advantageous in reducing nitrogen uptake in the body's tissues, thereby extending the practicable underwater dive time by reducing the decompression requirement, or reducing the risk of decompression sickness.

Trimix is a breathing gas consisting of oxygen, helium and nitrogen and is used in deep commercial diving, during the deep phase of dives carried out using technical diving techniques, and in advanced recreational diving.

Heliox is a breathing gas mixture of helium (He) and oxygen (O₂). It is used as a medical treatment for patients with difficulty breathing because this mixture generates less resistance than atmospheric air when passing through the airways of the lungs, and thus requires less effort by a patient to breathe in and out of the lungs. It is also used as a breathing gas diluent for deep ambient pressure diving as it is not narcotic at high pressure, and for its low work of breathing.

In a mixture of gases, each constituent gas has a partial pressure which is the notional pressure of that constituent gas as if it alone occupied the entire volume of the original mixture at the same temperature. The total pressure of an ideal gas mixture is the sum of the partial pressures of the gases in the mixture.

A rebreather is a breathing apparatus that absorbs the carbon dioxide of a user's exhaled breath to permit the rebreathing (recycling) of the substantially unused oxygen content, and unused inert content when present, of each breath. Oxygen is added to replenish the amount metabolised by the user. This differs from open-circuit breathing apparatus, where the exhaled gas is discharged directly into the environment. The purpose is to extend the breathing endurance of a limited gas supply, while also eliminating the bubbles otherwise produced by an open circuit system. The latter advantage over other systems is useful for covert military operations by frogmen, as well as for undisturbed observation of underwater wildlife. A rebreather is generally understood to be a portable apparatus carried by the user. The same technology on a vehicle or non-mobile installation is more likely to be referred to as a life-support system.

A breathing gas is a mixture of gaseous chemical elements and compounds used for respiration. Air is the most common and only natural breathing gas, but other mixtures of gases, or pure oxygen, are also used in breathing equipment and enclosed habitats. Oxygen is the essential component for any breathing gas. Breathing gases for hyperbaric use have been developed to improve on the performance of ordinary air by reducing the risk of decompression sickness, reducing the duration of decompression, reducing nitrogen narcosis or allowing safer deep diving.

Diving physics, or the physics of underwater diving is the basic aspects of physics which describe the effects of the underwater environment on the underwater diver and their equipment, and the effects of blending, compressing, and storing breathing gas mixtures, and supplying them for use at ambient pressure. These effects are mostly consequences of immersion in water, the hydrostatic pressure of depth and the effects of pressure and temperature on breathing gases. An understanding of the physics behind is useful when considering the physiological effects of diving, breathing gas planning and management, diver buoyancy control and trim, and the hazards and risks of diving.

Saturation diving is diving for periods long enough to bring all tissues into equilibrium with the partial pressures of the inert components of the breathing gas used. It is a diving mode that reduces the number of decompressions divers working at great depths must undergo by only decompressing divers once at the end of the diving operation, which may last days to weeks, having them remain under pressure for the whole period. A diver breathing pressurized gas accumulates dissolved inert gas used in the breathing mixture to dilute the oxygen to a non-toxic level in the tissues, which can cause potentially fatal decompression sickness if permitted to come out of solution within the body tissues; hence, returning to the surface safely requires lengthy decompression so that the inert gases can be eliminated via the lungs. Once the dissolved gases in a diver's tissues reach the saturation point, however, decompression time does not increase with further exposure, as no more inert gas is accumulated.

An electro-galvanic fuel cell is an electrochemical device which consumes a fuel to produce an electrical output by a chemical reaction. One form of electro-galvanic fuel cell based on the oxidation of lead is commonly used to measure the concentration of oxygen gas in underwater diving and medical breathing gases.

Gas blending for scuba diving is the filling of diving cylinders with non-air breathing gases such as nitrox, trimix and heliox. Use of these gases is generally intended to improve overall safety of the planned dive, by reducing the risk of decompression sickness and/or nitrogen narcosis, and may improve ease of breathing.

In physiology, isobaric counterdiffusion (ICD) is the diffusion of different gases into and out of tissues while under a constant ambient pressure, after a change of gas composition, and the physiological effects of this phenomenon. The term inert gas counterdiffusion is sometimes used as a synonym, but can also be applied to situations where the ambient pressure changes. It has relevance in mixed gas diving and anesthesiology.

Equivalent narcotic depth (END) (historically also equivalent nitrogen depth) is used in technical diving as a way of estimating the narcotic effect of a breathing gas mixture, such as nitrox, heliox or trimix. The method is used, for a given breathing gas mix and dive depth, to calculate the equivalent depth which would produce about the same narcotic effect when breathing air.

A Helium analyzer is an instrument used to identify the presence and concentration of helium in a mixture of gases. In Technical diving where breathing gas mixtures known as Trimix comprising oxygen, helium and nitrogen are used, it is necessary to know the fraction of helium in the mixture to reliably calculate decompression schedules for dives using that mixture.

Scuba gas planning is the aspect of dive planning and of gas management which deals with the calculation or estimation of the amounts and mixtures of gases to be used for a planned dive. It may assume that the dive profile, including decompression, is known, but the process may be iterative, involving changes to the dive profile as a consequence of the gas requirement calculation, or changes to the gas mixtures chosen. Use of calculated reserves based on planned dive profile and estimated gas consumption rates rather than an arbitrary pressure is sometimes referred to as rock bottom gas management. The purpose of gas planning is to ensure that for all reasonably foreseeable contingencies, the divers of a team have sufficient breathing gas to safely return to a place where more breathing gas is available. In almost all cases this will be the surface.

Scuba gas management is the aspect of scuba diving which includes the gas planning, blending, filling, analysing, marking, storage, and transportation of gas cylinders for a dive, the monitoring and switching of breathing gases during a dive, efficient and correct use of the gas, and the provision of emergency gas to another member of the dive team. The primary aim is to ensure that everyone has enough to breathe of a gas suitable for the current depth at all times, and is aware of the gas mixture in use and its effect on decompression obligations, nitrogen narcosis, and oxygen toxicity risk. Some of these functions may be delegated to others, such as the filling of cylinders, or transportation to the dive site, but others are the direct responsibility of the diver using the gas.

Rebreather diving is underwater diving using diving rebreathers, a class of underwater breathing apparatus which recirculate the breathing gas exhaled by the diver after replacing the oxygen used and removing the carbon dioxide metabolic product. Rebreather diving is practiced by recreational, military and scientific divers in applications where it has advantages over open circuit scuba, and surface supply of breathing gas is impracticable. The main advantages of rebreather diving are extended gas endurance, low noise levels, and lack of bubbles.

The physiology of decompression is the aspect of physiology which is affected by exposure to large changes in ambient pressure. It involves a complex interaction of gas solubility, partial pressures and concentration gradients, diffusion, bulk transport and bubble mechanics in living tissues. Gas is breathed at ambient pressure, and some of this gas dissolves into the blood and other fluids. Inert gas continues to be taken up until the gas dissolved in the tissues is in a state of equilibrium with the gas in the lungs, or the ambient pressure is reduced until the inert gases dissolved in the tissues are at a higher concentration than the equilibrium state, and start diffusing out again.

Diving support equipment is the equipment used to facilitate a diving operation. It is either not taken into the water during the dive, such as the gas panel and compressor, or is not integral to the actual diving, being there to make the dive easier or safer, such as a surface decompression chamber. Some equipment, like a diving stage, is not easily categorised as diving or support equipment, and may be considered as either.

A Diving rebreather is an underwater breathing apparatus that absorbs the carbon dioxide of a diver's exhaled breath to permit the rebreathing (recycling) of the substantially unused oxygen content, and unused inert content when present, of each breath. Oxygen is added to replenish the amount metabolised by the diver. This differs from open-circuit breathing apparatus, where the exhaled gas is discharged directly into the environment. The purpose is to extend the breathing endurance of a limited gas supply, and, for covert military use by frogmen or observation of underwater life, to eliminate the bubbles produced by an open circuit system. A diving rebreather is generally understood to be a portable unit carried by the user, and is therefore a type of self-contained underwater breathing apparatus (scuba). A semi-closed rebreather carried by the diver may also be known as a gas extender. The same technology on a submersible or surface installation is more likely to be referred to as a life-support system.

References

↑ Lincoln Electric, p. 5.4-3
↑ Weman, p. 31
1 2 3 "Gas mixing for shielding gas, modified atmosphere packaging and purging applications - Dansensor". www.gasmixing.com. Archived from the original on 2014-03-27.
↑ Bevan, John, ed. (2005). "Section 5.4". The Professional Divers's Handbook (second ed.). Alverstoke, GOSPORT, Hampshire: Submex Ltd. p. 242. ISBN 978-0950824260.
1 2 The Anaesthetic Machine - Gas mixing systems http://www.anaesthesia.med.usyd.edu.au/resources/lectures/gas_supplies_clt/gasmixing.html
↑ Bronzino, Joseph D. (2000-02-15). The Biomedical Engineering Handbook 1. Springer Science & Business Media. ISBN 978-3-540-66351-5.
1 2 Staff, Wilhelmsen Ships Service: "Span gases" http://www.wilhelmsen.com/services/maritime/companies/buss/BUSS_Pressroom/Documents/Span%20Gases.pdf

Lincoln Electric (1994). The Procedure Handbook of Arc Welding. Cleveland: Lincoln Electric. ISBN 99949-25-82-2.
Weman, Klas (2003). Welding processes handbook. New York, NY: CRC Press LLC. ISBN 0-8493-1773-8.