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Degassing, also known as degasification, is the removal of dissolved gases from liquids, especially water or aqueous solutions. There are numerous methods for removing gases from liquids.

Gases are removed for various reasons. Chemists remove gases from solvents when the compounds they are working on are possibly air- or oxygen-sensitive (air-free technique), or when bubble formation at solid-liquid interfaces becomes a problem. The formation of gas bubbles when a liquid is frozen can also be undesirable, necessitating degassing beforehand.

Pressure reduction

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The solubility of gas obeys Henry's law, that is, the amount of a dissolved gas in a liquid is proportional to its partial pressure. Therefore, placing a solution under reduced pressure makes the dissolved gas less soluble. Sonication and stirring under reduced pressure can usually enhance the efficiency. This technique is often referred to as vacuum degasification. Specialized vacuum chambers, called vacuum degassers, are used to degas materials through pressure reduction.

Thermal regulation

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See also: Deaerator

Generally speaking, an aqueous solvent dissolves less gas at higher temperature, and vice versa for organic solvents (provided the solute and solvent do not react). Consequently, heating an aqueous solution can expel dissolved gas, whereas cooling an organic solution has the same effect. Ultrasonication and stirring during thermal regulation are also effective. This method needs no special apparatus and is easy to conduct. In some cases, however, the solvent and the solute decompose, react with each other, or evaporate at high temperature, and the rate of removal is less reproducible.

Membrane degasification

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Gas-liquid separation membranes allow gas but not liquid to pass through. Flowing a solution inside a gas-liquid separation membrane and evacuating outside makes the dissolved gas go out through the membrane. This method has the advantage of being able to prevent redissolution of the gas, so it is used to produce very pure solvents. New applications are in inkjet systems where gas in the ink forms bubbles that degrade print quality, a degassing unit is placed prior to the print head to remove gas and prevent the buildup of bubbles keeping good jetting and print quality.

The above three methods are used to remove all dissolved gases. Below are methods for more selective removal.

Ultrasonic degassing

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Ultrasonic liquid processors are a commonly used method for removing dissolved gasses and/or entrained gas bubbles from various of liquids. The advantage of this method is that that ultrasonic degassing can be done in a continuous-flow mode, which makes it suitable for commercial-scale production.[1][2][3]

Sparging by inert gas

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See also: Sparging (chemistry)

Bubbling a solution with a high-purity (typically inert) gas can pull out undesired (typically reactive) dissolved gases such as oxygen and carbon dioxide. Nitrogen, argon, helium and other inert gases are commonly used. To maximize this process called sparging, the solution is stirred vigorously and bubbled for a long time. Because helium is not very soluble in most liquids, it is particularly useful to reduce the risk of bubbles in high-performance liquid chromatography (HPLC) systems.

Addition of reductant

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If oxygen should be removed, the addition of reductants is sometimes effective. For example, especially in the field of electrochemistry, ammonium sulfite is frequently used as a reductant because it reacts with oxygen to form sulfate ions. Although this method can be applied only to oxygen and involves the risk of reduction of the solute, the dissolved oxygen is almost totally eliminated. The ketyl radical from sodium and benzophenone can also be used for removing both oxygen and water from inert solvents such as hydrocarbons and ethers; the degassed solvent can be separated by distillation. The latter method is particularly useful because a high concentration of ketyl radical generates a deep blue colour, indicating the solvent is fully degassed.

Freeze-pump-thaw cycling

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In this laboratory-scale technique, the fluid to be degassed is placed in a Schlenk flask and flash-frozen, usually with liquid nitrogen. Next a vacuum is applied, perhaps to attain a vacuum of 1 mm Hg (for illustrative purposes). The flask is sealed from the vacuum source, and the frozen solvent is allowed to thaw. Often, bubbles appears upon melting. The process is typically repeated a total of three cycles.[4] The degree of degassing is expressed by the equation (1/760)3 for the case of initial pressure being 760 mm Hg, the vacuum being 1 mm Hg, and the total number of cycles being three.[5]

Degassing wine

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Yeast uses sugar to produce alcohol and carbon dioxide. In winemaking, carbon dioxide is an undesired by-product for most wines. If the wine is bottled quickly after fermentation, it is important to degas the wine before bottling.

Wineries can skip the degassing process if they age their wines prior to bottling. Storing the wines in steel or oak barrels for months and sometimes years allows gases to be released from the wine and escape into the air through air-locks.

Oil degassing

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For separation of gaseous hydrocarbons from crude oil, see Oil refinery.

The most efficient method of industrial oil degassing is vacuum processing, which removes air and water solved in the oil.[6] This can be achieved by:

  • spraying of oil in large vacuum chambers;
  • distributing the oil into a thin layer over special surfaces (spiral rings, Raschig rings etc) in vacuum chambers.

Under vacuum, an equilibrium between the content of moisture and air (solved gases) in the liquid and gaseous phase is achieved. The equilibrium depends on the temperature and the residual pressure. The lower that pressure, the faster and more efficiently are water and gas removed.

Unintended degassing

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Unintended degassing can happen for various reasons, such as accidental release of methane ( CH4 ) from the seabed during human activity such as underwater exploration by the energy industry. Natural processes such as tectonic plate movement can also contribute to methane release from the ocean floor. In both cases, the volume of CH4 released can be a significant contributor to climate change.[7][8]

See also

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Degassing

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Degassing is the deliberate removal of dissolved, adsorbed, or trapped gases from liquids, solids, melts, or other materials, typically via , thermal, or mechanical methods, to enhance purity, prevent structural defects like or bubbles, and enable downstream processes. In chemistry and , it targets dissolved gases such as oxygen and in aqueous solutions to inhibit or biological growth. In and , degassing of molten metals like or aluminum expels and other volatiles, reducing inclusions and improving mechanical properties. Geologically, degassing encompasses the exsolution of volatiles from ascending in volcanoes, driving eruption styles and contributing to atmospheric gas fluxes, with excess degassing linked to intrusive formation and tectonic influences. These applications underscore degassing's role in causal processes from industrial to planetary volatile cycling, though quantification challenges persist due to varying measurement techniques across scales.

Fundamentals

Definition and Physical Principles

Degassing is the process of removing dissolved or entrained gases from liquids, such as water, molten metals, or chemical solutions, to achieve desired purity, stability, or performance characteristics. This physical separation exploits differences in phase equilibrium and mass transfer dynamics between the gas and liquid phases. The primary physical principle underlying degassing is , which states that, at constant , the of a gas in a —expressed as the concentration of dissolved gas—is directly proportional to the of that gas in the equilibrium vapor phase above the . Mathematically, this is represented as C=k⋅PC = k \cdot P, where CC is the of the dissolved gas, PP is its , and kk is the Henry's law constant specific to the gas- pair and . To drive gas removal, external conditions disrupt this equilibrium: reducing (e.g., via ) decreases , prompting dissolved gases to desorb and form bubbles; elevating generally lowers kk for most gases, further reducing as thermal energy overcomes intermolecular forces binding gas molecules to the ; or introducing mechanical agitation enhances by increasing the gas- interfacial area and sites for bubble formation. Mass transfer in degassing follows , where the of gas molecules from liquid to gas phase is proportional to the concentration gradient across the interface. In practice, methods like ultrasonic treatment leverage acoustic —rapid pressure oscillations generating microscopic vapor or gas cavities that collapse and release dissolved gases into larger bubbles for extraction—effectively supersaturating the liquid relative to ambient conditions. Membrane-based approaches rely on selective driven by differentials across a hydrophobic or selective barrier, often under , without direct liquid-gas mixing. These principles ensure efficient degassing while minimizing energy input and secondary contamination, with efficacy depending on factors like gas , liquid , and .

Gas Solubility and Driving Forces

The of gases in is fundamentally governed by , which states that at constant , the concentration of a dissolved gas (CC) is directly proportional to its (PP) above the : C=kH⋅PC = k_H \cdot P, where kHk_H is the temperature-dependent Henry's law constant specific to the gas- pair. This equilibrium relationship implies that gases dissolve exothermically in most , with kHk_H decreasing ( increasing) under higher but kHk_H increasing ( decreasing) with rising , as disrupts the solute-solvent interactions favoring dissolution. In degassing processes, the primary driving force is the deviation from this equilibrium, creating supersaturation where the actual dissolved gas concentration exceeds the solubility limit under altered conditions, prompting diffusive mass transfer of gas molecules from the liquid phase to the vapor phase per Fick's first law of diffusion. Reducing the partial pressure—such as through vacuum application—directly lowers the equilibrium solubility, establishing a concentration gradient that accelerates gas exsolution, with the rate proportional to the pressure differential. Increasing temperature further enhances this by exponentially reducing solubility (e.g., oxygen solubility in water drops from 8.3 mg/L at 25°C to 6.4 mg/L at 40°C at 1 atm), providing an additional thermodynamic impetus for bubble nucleation and release once supersaturation thresholds are met. Other influences on solubility, such as (which decreases gas solubility via the Setschenow effect) or chemical reactions forming less soluble , modulate these driving forces but are secondary to pressure and temperature manipulations in physical degassing. kinetics are enhanced by factors like liquid agitation or reduced viscosity, which minimize diffusion path lengths and resistance, though the underlying force remains the solubility disequilibrium. In practice, effective degassing requires balancing these forces to avoid incomplete removal or foaming, with quantitative models often incorporating Henry's constants calibrated for specific systems (e.g., CO₂ in has kH≈29.4k_H \approx 29.4 L·atm/mol at 25°C).

Historical Development

Early Techniques and Practices

Early degassing practices relied heavily on thermal methods, exploiting the inverse relationship between temperature and gas solubility as described by . Heating liquids to or near facilitated the expulsion of dissolved gases such as oxygen, , and through increased volatility and bubble formation. In laboratory chemistry, prolonged under atmospheric pressure—often for 15 to 30 minutes in loosely covered vessels—was a standard procedure to prepare degassed solutions, minimizing interference from bubbles in experiments like or . This approach was simple, requiring no specialized equipment beyond basic heating apparatus, and was documented in early 20th-century protocols for solvent preparation. In and systems, thermal deaeration emerged in the as boilers demanded oxygen-free feedwater to prevent . Early devices heated water to saturation temperatures, allowing gases to vent while stripping enhanced removal efficiency. By the , counter-current open feedwater heaters were introduced, combining heating with steam injection to systematically scrub dissolved gases, achieving oxygen levels below 0.005 mL/L in some designs. These systems marked a shift from ad hoc to engineered processes, though limited by energy demands and incomplete gas removal for less soluble species like . Mechanical agitation complemented thermal techniques, promoting gas release via surface renewal and sites. Stirring or liquids under heat increased interfacial area, accelerating diffusion-driven degassing in batch processes. methods, involving repeated and , were particularly effective for volatile solvents in early . In , pre-vacuum degassing of molten alloys focused on indirect gas management through alloying or slagging rather than direct removal, as dissolved and oxygen caused defects like . The foundational concept of vacuum-assisted degassing for was proposed in 1940 by Soviet metallurgists A. M. Samarin and L. M. Novik, who advocated reduced pressure to lower the partial pressure of gases and induce evolution from the melt. Initial low-vacuum trials occurred in the 1940s, but scalable industrial implementation began in 1952 at the Enakievskii Metallurgical Plant in the USSR, using ladle evacuation to achieve reductions from 10-15 ppm to under 2 ppm. These early efforts highlighted vacuum's superiority over thermal alone, though equipment limitations constrained adoption until post-war advancements.

Mid-20th Century Advances

In the field of , vacuum degassing techniques advanced significantly during the to address inclusions and gas-related defects in molten . Batch vacuum degassing systems, such as stream degassing, were refined in the United States and , with early installations enabling the treatment of steel under reduced pressure to remove and other dissolved gases. By the late , the Dortmund-Hörder (DH) process emerged in , involving the circulation of molten through a via snorkel tubes, which improved efficiency over prior ladle methods and reduced processing times to under 30 minutes for heats up to 100 tons. The Ruhrstahl-Heraeus (RH) process, also developed in around 1958, represented a key innovation by using gas to lift and recirculate between a ladle and a vessel, achieving levels below 2 ppm and enhancing cleanliness for high-value applications like automotive and components. These vacuum methods supplanted earlier open-stream techniques, as they minimized oxidation and allowed precise control of alloying elements, with initial industrial trials demonstrating yield improvements of 1-2% in production. Ultrasonic degassing saw parallel progress, particularly for non-ferrous metals. Pioneering work in the explored sonic vibrations for removing gases from aluminum-magnesium alloys without direct contact, leveraging to nucleate and expel bubbles. By 1959, the Soviet-developed UZD-200 system enabled ultrasonic treatment of up to 250 kg melts in ladles, using frequencies around 20 kHz to achieve gas reductions of 50-90% in aluminum and magnesium alloys, paving the way for defect-free castings. These mid-century developments, driven by post-war industrial demands for purer metals, laid foundational principles for modern secondary , emphasizing reduced pressure and acoustic energy to enhance gas removal kinetics over traditional thermal or chemical approaches.

Recent Innovations Since 2000

Since 2000, degasification has gained prominence as an efficient gas-liquid separation technique, leveraging hydrophobic to remove dissolved gases like CO2 and O2 from and other liquids with reduced compared to traditional packed towers. Advances include the development of antifouling hydrophobic that minimize scaling and , enhancing longevity in processes where non-condensable gases are stripped under or sweep gas conditions. By 2023, these systems demonstrated up to 99% removal efficiency for specific gases in industrial-scale applications, driven by improved module designs and material innovations like and PVDF composites. Ultrasonic degassing has seen refinements in , particularly for aluminum and magnesium alloys, where cavitation-induced bubble accelerates removal and grain refinement. Post-2000 studies established optimal frequencies between 200-1000 kHz for maximal degassing at , with efficiency scaling nonlinearly with power input up to 1 kW, reducing by 50-70% in castings. In aluminum composites, intermittent ultrasonic vibration schemes achieved relative densities exceeding 99.5% by 2024, outperforming continuous methods through controlled acoustic streaming that minimizes entrapment. Vacuum degassing technologies advanced in steelmaking with the proliferation of Ruhrstahl-Heraeus (RH) and ladle vacuum systems, enabling treatment of melts up to 330 tons since 2000, which lowered hydrogen levels to below 1 ppm for ultralow-carbon steels. Claw vacuum pumps, such as MINK series integrated in extrusion lines by 2020, provided contactless operation with maintenance intervals extended to 20,000 hours, facilitating inline degassing of polymer melts at rates over 1000 kg/h while reducing dissolved volatiles by 90%. In sulfur recovery, the ICOn system, commercialized post-2010, employs catalytic conversion under vacuum to cut H2S emissions from 1000 ppm to under 10 ppm in elemental sulfur products. Hybrid innovations include the SNIF Sheer NEO system introduced around 2015 for , which combines injection with electromagnetic stirring to boost inclusion removal by 40% over prior generations through redesigned geometries. For beverages, alternating high-vacuum cycles down to -800 mbar in degassing units, refined by BMM since the early 2000s, preserved flavor volatiles while extracting 95% of entrained CO2 within 24 hours post-roast. These developments emphasize , with peer-reviewed validations confirming causal links between parameters and gas removal kinetics via deviations under dynamic conditions.

Physical Degassing Methods

Vacuum and Pressure Reduction

Vacuum and pressure reduction degassing operates on , which states that the solubility of a gas in a is directly proportional to the of that gas above the at constant . Reducing the ambient pressure lowers the , rendering dissolved gases supersaturated and promoting their to the liquid-gas interface for removal. This physical process avoids chemical additives, relying instead on driven by concentration gradients at the . In vacuum degassing, the material—typically a molten metal or —is exposed to pressures as low as 0.1–1 mbar in sealed chambers equipped with pumps and sometimes stirring mechanisms to enhance gas evolution. For instance, in aluminum processing, vacuum treatment reduces content by nucleating and growing bubbles that rise and burst, expelling gas; efficiencies can exceed 90% under optimized conditions with rotational impellers. The rate-limiting step is often through the boundary layer, influenced by , , and surface area, rather than bulk phase equilibrium. Pressure reduction without full vacuum, such as in staged decompression systems, applies milder reductions (e.g., from atmospheric to 100–500 mbar) in pipelines or vessels to release volatile hydrocarbons or dissolved air, common in storage and transport. This method minimizes equipment complexity but achieves lower degassing rates compared to deep , as partial gradients are smaller; it is frequently combined with monitoring for lower explosive limits to ensure during operations. In steel metallurgy, vacuum degassing treatments like ladle vacuum degassing (VD) or Ruhrstahl-Heraeus (RH) processes reduce from ~10 ppm to below 2 ppm and remove inclusions by exposing ladles of 100–300 tons of melt to for 10–30 minutes. These techniques improve and resistance by mitigating hydrogen-induced cracking, with stirring often augmenting bubble formation and circulation. Limitations include energy-intensive pumping and potential for recontamination if vacuum seals fail, necessitating high-integrity systems.

Thermal Regulation

Thermal regulation in degassing leverages the inverse relationship between temperature and gas in liquids or melts, as governed by , which states that the of a gas is proportional to its above the liquid, and empirical observations like Winkler's law indicating reduced with elevated temperatures for gases such as oxygen and in . By heating the medium to specific thresholds, dissolved gases become less soluble and migrate to the vapor phase for removal, often without additional mechanical or chemical aids. This method is particularly effective for non-reactive gases in aqueous solutions, where of oxygen drops from 11.2 mg/L at 10°C to 0.12 mg/L at 100°C under atmospheric conditions. In , thermal degassers operate by heating demineralized to approximately 104°C in a controlled environment, typically using direct injection or indirect heating in tray, spray, or hybrid columns to strip gases like O₂ and CO₂, which are then vented overhead. Spray degassers, the most common variant, atomize into a countercurrent flow, achieving residual oxygen levels below 7 ppb, while mixed spray-tray systems can reach under 3 ppb at the cost of higher complexity. Pressure is maintained slightly above atmospheric (e.g., 0.1-0.3 bar) to prevent excessive , with automated controls including sensors, transmitters, level indicators, and flowmeters ensuring stable operation and preventing recontamination. This process meets standards like UNE-EN 12952-12:2004 for , reducing oxygen to less than 0.02 ppm (20 ppb) to mitigate in high-pressure systems. In metallurgical applications, thermal regulation supports degassing by maintaining melt temperatures that optimize fluidity and gas rates, such as 1600–1650°C in degassing of to promote and exsolution without solidification. For aluminum alloys, however, elevated temperatures prolong required degassing times due to increased absorption potential from atmospheric exposure, necessitating precise control to balance reduction against re-entrainment risks during rotary or flux-assisted processes. Overall, thermal methods excel in scalability for large-volume but require energy input for heating, with efficiencies enhanced by integration with or stripping to achieve sub-ppm gas residuals in corrosion-sensitive applications.

Ultrasonic Degassing

Ultrasonic degassing employs high-intensity waves, typically in the range of 20-40 kHz, to remove dissolved gases and entrained bubbles from liquids or molten materials through the generation of acoustic . The process induces the formation, growth, and violent implosion of cavitation bubbles, which create localized high-pressure and high-temperature conditions, microjets, and shock waves that facilitate the detachment and coalescence of gas bubbles for subsequent release at the liquid surface. In metallic melts, where bubbles are scarce, cavitation plays a critical role by nucleating sites for gas diffusion from the supersaturated solution into the bubbles, enabling efficient removal without reliance on chemical additives or injection. The mechanism relies on the diffusion of dissolved gas molecules into cavitation voids, followed by bubble migration to the surface driven by buoyancy and acoustic forces; this contrasts with passive methods by accelerating mass transfer rates up to several orders of magnitude. Experimental studies on aluminum alloys demonstrate that ultrasonic treatment at amplitudes of 20-50 μm can reduce hydrogen content from 0.5-1.0 ml/100g Al to below 0.2 ml/100g Al within 2-5 minutes of exposure, significantly outperforming traditional rotary degassing in speed and uniformity. Optimal efficacy requires precise control of ultrasound power density (typically 10-100 W/cm²) and treatment duration to avoid excessive cavitation erosion on equipment or incomplete degassing due to bubble shielding in viscous media. In , ultrasonic degassing is applied to aluminum and magnesium alloys during to minimize , enhance mechanical properties such as tensile strength by 10-20%, and improve microstructural homogeneity by promoting grain refinement alongside gas removal. For instance, in aluminum melt processing, immersion probes or bath setups deliver ultrasound directly into the melt, reducing dissolved —a primary cause of pinhole defects in castings—and enabling denser products suitable for and automotive components. Beyond metals, the technique degasses viscous liquids like polymers or oils and is used in refining to lower refining temperatures by 50-100°C, thereby cutting . Advantages include rapid processing times (often under 5 minutes versus hours for methods), chemical-free operation that preserves melt purity, and scalability for continuous in-line systems, which minimize oxidation and formation compared to gas sparging. Limitations encompass equipment wear from intense , challenges in treating large volumes due to in depth (effective penetration limited to 10-20 cm in dense melts), and dependency on melt and composition, where high can reduce bubble mobility and efficiency. Despite these, advancements in design since the 2010s have expanded commercial viability, particularly in foundries seeking to optimize yield and .

Membrane Degasification

Membrane degasification employs hydrophobic microporous to selectively remove dissolved gases from , primarily through a pressure-driven without direct gas- contact beyond the interface. The flows along one side of the , while a or inert sweep gas is applied to the opposite side, establishing a partial pressure gradient that compels gases like (CO₂), oxygen (O₂), or (H₂S) to desorb from the , permeate through the pores, and exit into the low-pressure zone. This method relies on , where the solubility of a gas in a is proportional to its above the ; reducing the gas on the permeate side shifts equilibrium toward degassing. Unlike traditional stripping towers, the prevents entrainment or flooding, enabling operation at near-atmospheric pressures and minimizing use for gas handling. Common configurations utilize hollow-fiber membrane contactors, where thousands of fibers provide a high —often exceeding 1,000 m²/m³—facilitating efficient coefficients on the order of 10⁻⁵ to 10⁻⁴ m/s for CO₂ removal. Hydrophobic materials such as (PP), (PTFE), or (PVDF) are selected for their low , ensuring pores (typically 0.01–0.5 µm) remain gas-filled and non-wetted by the aqueous phase under operational transmembrane pressures below the liquid entry pressure (e.g., <0.5 bar for PP). Vacuum degasification targets can achieve CO₂ levels below 2 mg/L or O₂ below 5 ppb in treated water, depending on residence time and vacuum level (e.g., 50–100 mbar), with sweep gas modes enhancing selectivity for specific gases. The process operates continuously at ambient temperatures, avoiding thermal degradation risks associated with heating-based methods. Advantages include a compact footprint (up to 80% smaller than packed columns), reduced energy consumption (typically 0.1–0.5 kWh/m³ treated), and absence of chemical additives, making it suitable for ultrapure water production in boiler feed or semiconductor rinsing. It outperforms conventional vacuum degassing by avoiding foaming or aerosol formation and provides higher removal efficiencies without excessive gas recirculation. However, membrane fouling from particulates or organics necessitates prefiltration, and module replacement intervals (3–10 years) can elevate capital costs, though operational savings often offset this. Emerging polymeric membranes with enhanced permeability, such as those incorporating polydimethylsiloxane composites, continue to improve flux rates for industrial scalability. Applications span water treatment for desalination pretreatment and corrosion prevention in power generation, where dissolved O₂ reduction mitigates pitting in carbon steel piping.

Chemical and Hybrid Degassing Methods

Inert Gas Sparging

Inert gas sparging involves the injection of non-reactive gases, such as nitrogen, argon, or helium, into a liquid medium through nozzles, lances, or porous plugs to facilitate the removal of dissolved reactive gases like oxygen or hydrogen via mass transfer across the bubble-liquid interface. The process creates a high surface area for diffusion, where dissolved gases partition into the rising inert gas bubbles and are expelled from the system, effectively reducing gas solubility without introducing chemical reactants. This method is particularly suited for oxygen-sensitive applications, as the inert gas displaces rather than chemically neutralizes the target gases, though it may leave trace amounts of the sparging gas dissolved in the liquid. In laboratory and chemical processing contexts, sparging is performed by bubbling the gas through sealed flasks or vessels at controlled flow rates, typically 500–1000 cc/min for 100-cc samples of water or organic solvents, achieving 95–98% removal of dissolved oxygen in 15–30 seconds. Argon is often preferred over nitrogen for its higher density, which enhances purging efficiency in preventing re-entry of atmospheric oxygen, and helium for its minimal solubility, allowing near-complete gas elimination except for itself. For volatile solvents like diethyl ether, cooling in an ice bath during sparging minimizes evaporative losses, with durations scaled to 1 minute per 5 mL of solvent for effective deoxygenation comparable to vacuum cycling but inferior to freeze-pump-thaw methods that evacuate all gases. In metallurgical applications, such as aluminum casting, inert gas sparging targets removal from molten metal to prevent porosity defects, with argon or high-purity (>99.99%) bubbled through the melt via immersed lances, promoting into bubbles based on experimentally determined coefficients at temperatures around 700°C. The technique outperforms static methods by enhancing circulation and gas-liquid contact but requires careful control to avoid excessive or metal oxidation if reacts under certain conditions. Hybrid variants may incorporate trace like for simultaneous fluxing of inclusions, though pure inert sparging prioritizes gas removal alone. Advantages include scalability for large volumes, as seen in where sparging handles high throughput without equipment complexity, and rapid kinetics due to bubble-induced mixing. Limitations encompass potential incomplete removal if sparging gas solubility is high (e.g., in at 0.000014 under standard conditions) and costs for gas compression, with further improvable via ultrasonics to boost rates.

Reductant Addition

Reductant addition is a chemical degassing technique that employs reducing agents to react with dissolved gases, primarily oxygen, in liquids or melts, converting them into separable compounds such as oxides or sulfates. This method lowers gas solubility by direct chemical reduction rather than relying solely on physical processes like or heating. Common reductants include for aqueous systems, sodium-benzophenone ketyl for organic solvents, and elements like aluminum, , or calcium for molten metals. In , particularly for , (Na₂SO₃) serves as an through the reaction 2Na₂SO₃ + O₂ → 2Na₂SO₄, effectively reducing dissolved oxygen levels to below 0.005 ppm under optimal conditions. Catalyzed variants, incorporating or , accelerate the reaction rate, which is otherwise pH-dependent and inhibited by chelants or feedwater contaminants. This approach prevents in systems but introduces ions, necessitating monitoring to avoid scaling or environmental discharge issues. For organic solvent preparation in laboratory settings, sodium metal combined with benzophenone forms the ketyl radical (Ph₂C•O⁻Na⁺), which reduces oxygen to and indicates via a persistent deep blue-purple color upon completion. Solvents like (THF) or are refluxed over this mixture, distilling the degassed product; the method simultaneously removes trace water by forming insoluble . It is favored for air-sensitive reactions but limited to non-reactive solvents, as ketyl radicals are incompatible with protic or halogenated compounds. In metallurgical processes, such as steelmaking or superalloy refining, aluminum and silicon act as reductants for deoxidation: 4Al + 3[O] → 2Al₂O₃ and 2Si + [O] → 2SiO (further oxidized to SiO₂), where [O] denotes dissolved oxygen. These form slag inclusions that can be floated or filtered, reducing oxygen content to support vacuum degassing hybrids. Calcium addition, often via wire injection, provides stronger reduction (Ca + [O] → CaO), enhancing removal of oxygen, nitrogen, and hydrogen in vacuum induction melting, with studies showing up to 90% oxygen reduction in IN713LC superalloy after 30 minutes of refining. However, excess reductants risk reoxidation or undesirable inclusions, requiring precise stoichiometry based on melt oxygen activity. This method integrates with ladle metallurgy but demands high-purity inputs to minimize impurities from reductant sources.

Freeze-Pump-Thaw Cycling

Freeze-pump-thaw cycling is a physical degassing technique employed in chemical laboratories to eliminate dissolved gases, such as oxygen or nitrogen, from solvents, solutions, or liquid reagents, thereby enabling air-free conditions for sensitive reactions. The method leverages the phase change of the liquid to immobilize it, allowing vacuum evacuation of gases from the headspace without significant solvent evaporation. It is most commonly applied on small scales, typically volumes under 50 mL, to minimize risks associated with thermal expansion. The procedure begins with transferring the liquid into a Schlenk flask or sealed tube, filling no more than 50% of the vessel volume to accommodate expansion, and closing the stopcock. The flask is then immersed in a cryogenic bath, such as at -196°C or a dry ice-acetone slurry at approximately -78°C, to rapidly freeze the contents solid. Once frozen, the stopcock is opened to a high-vacuum line (typically <10^{-3} ), and the system is evacuated for 2-3 minutes to remove gases desorbed from the solid-liquid interface and headspace. The vacuum is then closed, and the sample is allowed to thaw gradually at or under controlled warming, promoting the release of trapped gases into the headspace for subsequent evacuation. This cycle is repeated 3-5 times, with thawing performed from the top downward to avoid pressure buildup and potential vessel fracture. Effectiveness stems from the repeated disruption of gas : freezing expels dissolved gases as bubbles that migrate to the surface, while removal prevents re-dissolution during thawing. Studies and protocols indicate it achieves near-complete degassing for non-volatile solvents, outperforming single-cycle degassing by reducing oxygen levels to below 1 ppm after multiple iterations. However, efficacy diminishes for highly volatile or low-boiling-point liquids, where partial can occur despite sealing. In practice, the technique integrates with setups for inert atmosphere handling, making it suitable for organometallic synthesis or reactions intolerant to oxidative impurities. Advantages include minimal loss compared to gas sparging—ideal for costly or hazardous reagents—and no introduction of foreign gases, ensuring purity. Drawbacks encompass time intensity (each cycle taking 10-20 minutes), equipment requirements like vacuum manifolds, and safety hazards from implosions if vessels are overfilled or thawed unevenly. For larger scales, alternatives like ultrasonic or membrane degassing are preferred due to scalability limitations.

Industrial Applications

Metallurgy and Steelmaking

In steelmaking, degassing is essential to remove dissolved gases such as , , and oxygen from molten , which can otherwise cause defects like flaking, embrittlement, and inclusions that compromise structural integrity and mechanical properties. , primarily introduced from in or lime, leads to delayed cracking upon solidification, while from ferroalloys or promotes ; oxygen contributes to non-metallic inclusions. These processes occur during secondary in ladles after primary melting in furnaces or converters, enabling production of high-quality for applications requiring uniformity and cleanliness. Vacuum degassing, a core technique, involves exposing the molten to reduced pressures (typically 0.5–10 mbar) in a sealed chamber, decreasing gas and promoting across the gas-metal interface. Common variants include tank degassing (VD), where in a ladle is treated statically; circulation types like Ruhrstahl-Heraeus (RH), processing up to 110 tons in 20 minutes via dual snorkels for enhanced surface renewal; and Dortmund-Hörder (DH), using a single snorkel for 10–15% circulation per cycle. levels are routinely lowered to below 2 ppm (as low as 1.5 ppm), while removal is less efficient at under 20%, often supplemented by stirring to increase . Benefits include improved desulfurization (up to 80% in some systems), alloy homogeneity, and reduced inclusions, with treatment times of 20–30 minutes; the process originated industrially in the following early proposals in the . For stainless and alloy steels, vacuum oxygen decarburization (VOD) integrates oxygen lancing under vacuum (100–250 mm Hg) to simultaneously decarburize to below 0.005% carbon and degas, protecting elements like chromium from oxidation. Argon oxygen decarburization (AOD), conducted in a converter, employs controlled argon-oxygen mixtures (argon diluting oxygen to minimize chromium loss) blown through submerged tuyeres, generating CO bubbles that stir the bath and facilitate hydrogen and nitrogen expulsion via inert gas dilution and flotation. The process reduces carbon from 1.5–2.5% to under 0.05% in 20–35 minutes, with gas flows of 1–3 Nm³/min per ton, enabling sulfur levels below 0.005% and nitrogen control in grades like 304 stainless (solubility around 0.23%). AOD's steps—decarburization, reduction with ferrosilicon for chromium recovery, and desulfurization—enhance metallic yields and raw material flexibility, making it dominant for producing low-carbon stainless steels since its development in the 1960s. Inert gas sparging, often argon bubbling in ladles, complements vacuum methods by inducing circulation to accelerate and removal without vacuum equipment, reducing oxygen activity and inclusions through flotation. These techniques collectively enable steel with exceptional and resistance, critical for , automotive, and structural components, though control remains challenging due to its lower diffusivity compared to .

Oil and Petrochemical Processing

In oil production, degassing stabilizes crude by extracting dissolved gases and at the or stabilization units, mitigating in pipelines and storage, while enhancing transport safety and product quality. Associated liquids (NGLs), which emerge as byproducts during extraction, undergo degassing to separate lighter hydrocarbons and prevent pipeline blockages from gas accumulation at high points. Self-acting float-controlled bleeding valves, constructed from -resistant , facilitate this by continuously discharging gases without external power, operating effectively up to 130°C and with fluid densities as low as 640 kg/m³. In operations, degassing targets volatile organic compounds and entrained vapors from streams to avoid downstream damage and ensure compliance with thresholds. processing employs degassing primarily during equipment shutdowns and turnarounds to eliminate hazardous residues like hydrocarbons, , mercaptans, and pyrophoric iron sulfides from reactors, vessels, and piping before maintenance. Chemical methods accelerate this by circulating or injecting —such as hydrocarbon encapsulators (e.g., C) or H2S neutralizers (e.g., Sourguard)—often augmented with to elevate system temperatures to approximately 90°C, promoting gas release and emulsification while reducing lower explosive limit (LEL) concentrations below ignition thresholds. These interventions, sometimes preceded by decontamination to clear heavy deposits, minimize worker exposure to flammables and toxics, shorten downtime to as little as 12 hours, and curb environmental releases of volatile emissions. Vacuum-assisted or techniques may supplement chemical approaches in high-precision applications, though steam-chemical hybrids predominate for their efficacy against persistent sulfides.

Water Treatment and Purification

Degassing in water treatment removes dissolved gases such as oxygen, , and from water supplies, mitigating in and equipment, reducing acidity to stabilize , and eliminating odors or tastes that affect palatability. In industrial and municipal contexts, untreated dissolved oxygen levels exceeding 0.1 mg/L can accelerate in metal surfaces, while excess lowers below 7, promoting further degradation and scaling when combined with hardness ions. These processes enhance for downstream uses, including operations and potable distribution, by targeting gas solubilities governed by , where lower partial pressures or temperatures facilitate stripping. A primary application occurs in preparation, where deaeration targets oxygen removal to levels below 7 , preventing oxidative in high-pressure systems that operate above 100°C. Pressure deaerators, often operating at 5-15 psig and 105-120°C, scrub gases via contact, simultaneously heating feedwater to minimize upon entry and removing to curb acidic condensate formation. This is essential in power generation and , where untreated feedwater costs industries billions annually in and downtime, as evidenced by U.S. Department of Energy assessments of system efficiencies. In municipal treatment, decarbonation addresses ingress from sources like , which can elevate levels to 50-100 mg/L, necessitating post-ion-exchange degassing to neutralize acidity and reduce chemical dosing for adjustment. Towers or packed columns strip CO2 via countercurrent air or flow, achieving reductions over 90% and improving chlorine stability while curbing pipe leaching of metals like lead. degassing similarly targets odor thresholds below 0.05 mg/L in well waters, using similar stripping to prevent and consumer complaints in distribution networks. Industrial purification for pharmaceuticals, semiconductors, and demands ultra-low gas content, with or systems reducing oxygen to sub-ppb traces for corrosion-free loops. These applications, integrated after softening or , ensure compliance with standards like those from the International Purity for , minimizing defects in sensitive processes such as microchip fabrication where gas-induced bubbles compromise yields.

Food and Beverage Processing

Degassing in food and beverage processing removes dissolved gases, particularly oxygen (O₂) and (CO₂), from liquids to mitigate oxidation, preserve flavor profiles, and prolong . This process prevents enzymatic , vitamin degradation, and microbial proliferation in products such as juices, , and bases. Vacuum degassing predominates, applying reduced to liberate gases, often combined with agitation or stripping agents like CO₂ for enhanced efficiency. In soft drinks and production, degassing targets oxygen removal from source water via chambers with continuous pumping, averting flavor oxidation and maintaining beverage clarity. Single-stage spray degassing suffices for oxygen levels adequate in this sector, typically integrating flowmeters and sensors for process control. Breweries employ degassing for process water used in kieselguhr filtration, pipe rinsing, and high-gravity blending, achieving residual oxygen below 0.02 ppm to minimize flavor alteration and oxygen ingress. Multi-stage spray or column methods, sometimes with CO₂ stripping, outperform single-stage approaches required for compared to soft drinks. degassing via hollow fibers under also attains <0.02 ppm for smaller-scale operations. For juices, purees, and , degassers operate at 60–65°C and 0.08 MPa , eliminating suspended gases to curb foaming during filling and in , with capacities reaching 15,000 L/h. These systems incorporate aroma recovery condensers to retain volatile flavors in fruit-based products like or , supporting densities from 0.5–2.5 g/cm³ and particle sizes up to 1000 µm. In wine and dealcoholized beer production, two-stage vacuum column degassing reduces oxygen to <10 ppb without heating or stripping gases, integrating with pasteurization to safeguard product integrity against oxidative off-flavors.

Unintended Degassing and Risks

Unintended degassing in industrial processes occurs when dissolved or entrained gases are suddenly released due to changes in pressure, temperature, or chemical reactions, often resulting in pressure surges, equipment ruptures, or hazardous releases. These incidents can arise from inadequate process controls, material incompatibilities, or procedural errors, leading to explosions, toxic exposures, or fires. Such events underscore the risks in handling reactive substances or supersaturated solutions where gas evolution is not anticipated or mitigated. On May 30, 2007, at a special center in , a road tanker containing incompatible wastes—30% mixed with 5% acid resins from a transfer error on May 29—underwent exothermic , generating gases that caused a sudden surge. The manhole lid ruptured, propelling the tanker 15 meters and ejecting material; cooling was applied for 30 minutes to control the reaction. One employee sustained partial foot burns, prompting evacuation of operations staff, while administrative personnel remained in offices; the incident highlighted failures in waste acceptance protocols, including lack of temperature monitoring and venting. In a 1993 incident at a Dutch caprolactam production plant, the degassing line connected to filter B of a gas/slurry loop reactor (Hyam process) ruptured on July 23 due to an internal detonation, likely from methane accumulation or ignition during routine degassing operations. The explosion damaged the line, but specific injury or further release details were not publicly detailed in investigations; analysis attributed it to uncontrolled gas buildup in the slurry filtration and degassing system, emphasizing the need for detonation arrestors and inerting in polymer reactor degassing. Degassing-related toxic releases have also occurred during handling of corrosive chemicals; for instance, a test on a damaged (PCl3) iso-container led to a large cloud of PCl3 and gas, necessitating response and highlighting risks of incomplete neutralization or pressure relief failures in container degassing procedures. These cases illustrate how unintended degassing amplifies hazards in chemical and waste processing, where empirical monitoring of gas and reaction kinetics is critical to prevent cascading failures.

Environmental Releases and Impacts

Unintended degassing in petrochemical and oil processing often results in fugitive emissions of volatile organic compounds (VOCs), methane, and hydrogen sulfide (H2S) during tank or vessel maintenance when vapor control systems fail or are bypassed. Traditional atmospheric venting methods, used historically for tanker degassing, release these hydrocarbon vapors directly into the air, contributing to localized air pollution. In regions like the Netherlands, emissions from degassing inland tank vessels total approximately 1.79 kilotons of VOCs annually, equivalent to 1.2% of national VOC emissions, with potential higher contributions from toxic components. In the oil and gas sector, unintended degassing-related venting and leaks from storage tanks and equipment release , a with a 28-36 times that of CO2 over a 100-year period. The U.S. Environmental Protection Agency identifies equipment venting—often tied to degassing operations—as a key source of such emissions, alongside unintentional leaks, with global oil and gas outputs estimated at 82.5 million metric tons per year. These releases amplify atmospheric concentrations, accelerating short-term forcing and representing lost commercial value exceeding $1 billion annually in the U.S. alone from major operations. Environmental impacts include enhanced tropospheric ozone formation from VOCs via photochemical reactions, leading to smog and respiratory health risks in downwind areas; methane's contribution to radiative forcing, exacerbating global warming; and H2S's role in acid deposition, which harms vegetation and aquatic ecosystems. In metallurgy, such as steelmaking, unintended off-gas releases from degassing—containing hydrogen, nitrogen, and oxygen—can occur if emission controls are inadequate, though vacuum processes generally reduce overall chemical emissions compared to alternatives. Regulatory frameworks, like those from the Texas Commission on Environmental Quality, mandate vapor recovery or combustion to mitigate these risks during degassing.

Health and Safety Considerations

Degassing operations, particularly in industrial settings like oil processing and , expose workers to toxic gases such as (H2S) and volatile organic compounds (VOCs), which can lead to acute respiratory distress, neurological effects, or fatality at concentrations above 500 ppm for H2S. In vessel degassing, incomplete removal of flammable vapors risks explosions if lower explosive limit (LEL) levels are not monitored and scavenged, as evidenced by incidents involving releases during tank openings. In metallurgical vacuum degassing of , primary hazards include burns from molten metal at temperatures exceeding 1,500°C, inhalation of dust-laden off-gases containing (CO) and hydrogen (H2), and potential system failures leading to sudden pressure changes or leaks that could ignite flammable byproducts. Noise levels often surpass 85 dB, contributing to without proper protection, while hot, dust-laden off-gases require cooling and post-combustion to mitigate release risks. degassing via methods reduces chemical exposure compared to traditional techniques but still necessitates ventilation to prevent asphyxiation from displaced oxygen. Mitigation strategies, as outlined in OSHA and NIOSH guidelines, emphasize continuous gas monitoring, (PPE) including respirators and flame-retardant clothing, and adherence to entry protocols under 29 CFR 1910.146 to address risks during manual gauging or vessel access. Operator training on emergency response, such as immediate for H2S exposure, and routing vapors to thermal oxidizers during tank degassing have reduced incidents, though underreporting persists in high-risk sectors. In food and beverage processing, degassing poses lower toxicity risks but requires safeguards against oxygen displacement leading to hypoxic environments.

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