Oxygen (O2) is a colorless, odorless, tasteless gas essential to living organisms, being taken up by animals, which convert it to carbon dioxide; plants, in turn, utilize carbon dioxide as a source of carbon and return the oxygen to the atmosphere. Oxygen forms compounds by reaction with practically any other element, as well as by reactions that displace elements from their combinations with each other; in many cases, these processes are accompanied by the evolution of heat and light and in such cases are called combustions. Its most important compound is water [ [i]].
Oxygen is the most abundant element on the earth’s surface: it occurs both as the free element and combined in innumerable compounds and comprises 23% of the atmosphere by weight [[ii]]. Oxygen makes up 21% of the atmosphere by volume. This is halfway between 17% (below which breathing for unacclimated people becomes difficult) and 25% (above which many organic compounds are highly flammable). The element and its compounds make up 49.2% by mass of the Earth’s crust, and about two-thirds of the human body. There are two key methods used to obtain oxygen gas. The first is by the distillation of liquid air. The second is to pass clean, dry air through a zeolite that absorbs nitrogen and leaves oxygen. A newer method, which gives oxygen of a higher purity, is to pass air over a partially permeable ceramic membrane. In the laboratory it can be prepared by the electrolysis of water or by adding a Manganese (IV) oxide catalyst to aqueous hydrogen peroxide [[iii]].
Oxygen was discovered about 1772 by a Swedish chemist, Carl Wilhelm Scheele, who obtained it by heating potassium nitrate, mercuric oxide, and many other substances. An English chemist, Joseph Priestley, independently discovered oxygen in 1774 by the thermal decomposition of mercuric oxide and published his findings the same year, three years before Scheele published. In 1775–80, French chemist Antoine-Laurent Lavoisier, with remarkable insight, interpreted the role of oxygen in respiration as well as combustion, discarding the phlogiston theory, which had been accepted up to that time; he noted its tendency to form acids by combining with many different substances and accordingly named the element oxygen (oxygène) from the Greek words for “acid former”[[iv]].
- Atomic Properties
|Atomic Weight||15.9994 amu|
|Covalent radius||60 (48) pm|
|van der Waals radius||152 pm|
|Oxidation States||−1, −2, +2 (in compounds with fluorine)|
|Electron configuration||[He] 2s22p4|
|e-1s per energy level||2, 6|
Spectral lines of oxygen
- Physical Properties
Oxygen has four known allotropic forms:
1. Diatomic (O2) ordinarily found in air
2. Triatomic (O3), known as ozone (→Ozone)
3. The unstable, tetratomic form (O4) referred to as Oxozone
4. Octatomic (O8) known as ε oxygen or red oxygen [[vi]]
Many biological processes and technical applications of oxygen are based on and at the same time limited by its aqueous solubility behavior (e.g., wastewater treatment, homogeneous and heterogeneous liquid phase reactions). Oxygen is sparingly soluble in water. Its solubility is dependent on pressure and temperature and may be approximated by Henry’s law at low pressures. At 0°C, the Henry’s constant (absorption coefficient) for pure oxygen in water is 692 mg L-1 MPa-1[vii]. Because air contains roughly only 21% oxygen, the equilibrium solubility of oxygen in water in contact with air at 0°C and at 100 kPa is 14.5 mg/L.
At elevated pressures diatomic oxygen and water form a solid clathrate hydrate phase (0°C, 1.22 × 104 kPa)[viii]Ozone has also been reported to be embedded in clathrate phases in the presence of water and an additional helper gas [[ix]].
Table 2: Oxygen Physical Properties [[x]]
|Melting Point||−218.4 °C (−361.1 °F)|
|Boiling Point||−183.0 °C (−297.4 °F)|
|Molar Volume||17.36 x 10-6 m3/mol|
|Electron configuration||[He] 2s22p4|
|Density (1 atm, 0 °C)||1.429 g/liter|
|State at 20°C||Gas|
|Heat of vaporization||3.4099 kj/mol|
|Heat of fusion||0.22259 kJ/mol|
|Vapor pressure||__ Pa at __ K|
|Speed of sound||317.5 m/s at 293 K|
|Specific volume (25°C, 101 kPa)||0.76407 m3/kg|
|bp (101 kPa)||-182.96°C|
|mp (101 kPa)||– 218.35°C|
|Triple point temperature/pressure||-218.79°C/ 14.8 kPa|
|Density, gas (21.1°C, 101 kPa)||1.3088 kg/m3|
|Relative density, gas (25°C, 101 kPa, air = 1)||1.1054|
|Density, liquid (-182.96°C)||1141.2 kg/m3|
|Density, gas (-182.96°C)||4.4671 kg/m3|
|Critical pressure||5043 kPa|
|Critical density||436.14 kg/m3|
|Latent heat of vaporization (-182.96°C)||213.06 kJ/kg|
|Latent heat of fusion (-218.35°C)||13.86 kJ/kg|
|Isobaric specific heat capacity, gas (25°C, 101 kPa) Cp||0.9196 kJ kg-1 K-1|
|Isochoric specific heat capacity, gas (25°C,101 kPa) Cv||0.6585 kJ kg-1 K-1|
|Viscosity, gas (25°C, 101 kPa)||20.56 × 10-6 Pa s|
|Thermal conductivity, gas (25°C, 101 kPa)||26.34 × 10-3 W m-1 K-1|
|Surface tension (-182.96°C)||13.15 × 10-3 N/m|
|Viscosity, saturated liquid (-182.96°C)||194.67 × 10-6 Pa s|
|Thermal conductivity, saturated liquid (-182.96°C)||150.78 × 10-3 W m-1 K-1|
|Van der Waals diameter||1.52 × 10-10 m|
In total eight different crystal structures of solid Oxygen have been identified. In the diatomic form, two unpaired electrons lie in antibonding orbitals resulting to its paramagnetic behavior. The thirteen isotopes of Oxygen are listed in Table 4 [[xi]].
Table 3: Oxygen isotopes [[xii]]
|Mass number||Natural abundance, %||Half-life, s|
|12||< 6 × 10-22|
- Chemical Properties
Most of the chemical reactions with oxygen are redox reactions where the oxygen receives two electrons during the formation of an oxide. In many cases such redox reactions proceed exothermally due to the release of energy after formation of new bonds or a stable lattice. Well-known and dreaded explosive events are reaction of an oxyhydrogen gas mixture, hydrocarbon–oxygen mixture, and reactions with flammable finely dispersed dust with oxygen (coal dust, flour). An explosive reaction of a fuel–oxygen mixture can be avoided outside of the explosion range, which is individual for every fuel. However, such explosions are kinetically inhibited and need an ignition source (for instance high temperature of a spark) to overcome the activation barrier [[xiii]].
Table 4: Oxygen Chemical Properties [[xiv]]
|Chemical Formula||O Oxygen gas (O2) Ozone (O3)|
|Flammability||Does not burn|
|Combustion||Supports combustion but does not burn|
|Compounds||Occurs in many compounds, including water, carbon dioxide, and iron ore|
|Oxidation||The common reaction in which it unites with another substance is called oxidation Oxides of some metals form peroxides by the addition of oxygen|
4-1- Chemical properties of Dioxygen (O2)
Molecular oxygen, O2, is unique among gaseous diatomic species with an even number of electrons in being paramagnetic. This property, first observed by M. Faraday in 1848, receives a satisfying explanation in terms of molecular [[xv]].
orbital theory Oxygen is an extremely reactive gas which vigorously oxidizes many elements directly, either at room temperature or above. Despite the high bond dissociation energy of 02 (493.4 kJ mol – 1) these reactions are frequently highly exothermic and, once initiated, can continue spontaneously (combustion) or even explosively. Familiar examples are its reactions with carbon (charcoal) and hydrogen.
Some elements do not combine with oxygen directly, e.g. certain refractory or noble metals such as W, Pt, Au and the noble gases, though oxo compounds of all elements are known except for He, Ne, Ar and possibly Kr. This great range of compounds was one of the reasons why Mendeleev chose oxides to exemplify his periodic law and why oxygen was chosen as the standard element for the atomic weight scale in the early days when atomic weights were determined mainly by chemical stoichiometry.
- COMMERCIAL TECHNOLOGIES FOR OXYGEN PRODUCTION
Gasification processes require an oxidant, most commonly oxygen; less frequently air or just steam may suffice as the gasification agent depending on the process. Oxygen-blown systems have the advantage of minimizing the size of the gasification reactor and its auxiliary process systems. However, the oxygen for the process must be separated from the atmosphere. Commercial large-scale air separation plants are based on cryogenic distillation technology, capable of supplying oxygen at high purity1 and pressure. This technology is well understood, having been in practice for over 75 years. Cryogenic air separation is recognized for its reliability, and it can be designed for high capacity (up to 5,000 tons per day) [[xvi]].
5-1- Oxygen Production from Cryogenic Air Separation Plant [[xvii]]
Cryogenic air separation technology is because the different constituent gasses of air all have different boiling points and by manipulating the immediate environment in terms of temperature and pressure, the air can be separated into its components. Various processes are needed in a cryogenic air separation plant, of which the fundamental ones are air compression, air purification, heat exchanging, distillation, and product compression.
Figure 1: Processes involved in cryogenic air separation
Each process of figure has a certain function and this function is performed by means of relevant equipment. Table 5 lists the equipment type utilized in each of the required processes as well as their respective main functions.
Table 5: A description on the function of each process and the equipment used for the
|Air compression||Pressurize air feed lines in order to achieve required production||Compressor|
|Air purification||Purifying the air by removing water and CO2||by Reversible exchangers/sieve absorbers|
|Heat exchanging||Cooling of incoming air feed by means of cold recovery from exiting products||Heat exchanger|
|Distillation||Partial separation of air into its constituents||Distillation columns|
|Product compression||Pressurize outgoing product feed in to meet user requirement||Compressor|
In figure 1, incoming air is pressurized by the air compressors and enters the process with a certain flowrate, as determined by the demand of air product at the output of the plant. The air pre-treatment section removes impurities like water, carbon dioxide and hydrocarbons and is necessary because it prohibits vapor condensation, liquid water solidification and gaseous CO2 condensation from occurring within the heat-exchanger, thus ensuring continuous operation of a train. To obtain the required conditions for air distillation, the air first needs to be cooled down and this is done by means of heat exchanging between the air and outgoing product streams. The distillation process is at the heart of the overall process; this process performs the actual separation of air into its constituents. These air products are produced with a certain purity, which is defined as the ratio of the quantity of 100% pure air product to the quantity of total of air product at the output. The outputs from the distillation process are fed back through the heat exchanger to realize the cold recovery process that is needed for the cooling of the incoming air feed lines. The main air product is at low pressure when exiting the distillation process and is, for this reason, usually compressed to the client’s pressure specification prior to delivery. The system, incorporating these processes, is referred to as an oxygen or nitrogen train, whatever the main product may be. Several such trains are usually cascaded to produce a high-tonnage air product output and is then collectively referred to as an oxygen or nitrogen plant.
- Oxygen Production Using Solid Electrolysis
Adsorptive air separation for oxygen production is based on the ability of several types of zeolites to adsorb nitrogen with a higher selectivity than oxygen [[xviii]]. Most modern processes use 13X molecular sieve modified by ion exchange. The highest selectivity’s are achieved with so called low-silica LiX molecular sieve, where almost all Na+ cations are exchanged for Li+ cations and the ratio of Si to Al is close to one. On this material the amount of nitrogen adsorbed is by a factor of six or more higher than the amount of oxygen adsorbed under standard conditions. In contrast, argon and oxygen have very similar adsorption properties on zeolites, which limits the oxygen purity of a one-stage adsorptive air separation process to approximately 95%; the rest is made up by argon. For economic reasons, the purity of the process is often chosen in the range of 90–93%, allowing for some breakthrough of nitrogen. This increases the recovery rates and thus reduces the specific energy consumption [[xix]].
Adsorptive air separation works with a relatively low ratio between adsorption and desorption pressure often in the range of 2–4. Even if a higher-pressure ratio would reduce the necessary amount of adsorbent, with no pressurized air as the feed gas both an increased adsorption pressure and a reduced desorption pressure contribute to higher power consumption and ask for more complex machinery. The low pressure ratio is the reason for relatively simple step sequences: There is no potential, nor is there a need for a multitude of pressure equalization steps as, for example, in adsorptive hydrogen purification processes or other applications characterized by pressurized feed gas [[xx]]. The overall sequence remains simple, and in the case of a modern VPSA process it mainly consists of the following steps:
1. Co-current pressurization with compressed feed air
2. Production (oxygen released)
3. Co-current depressurization
4. Counter-current depressurization
5. Counter-current evacuation
6. Counter-current evacuation + purge gas (from product buffer or another absorber)
7. Counter-current pressurization (for example with released gas from absorber in step3) [[xxi]].
Many smaller modifications to this basic procedure are described in the patent literature, all with the objective of improved bed utilization and reduced specific energy consumption. Typically, these modifications apply to the phase of pressure reduction after the production step and the usage of the release gas during this step. The described steps are typically realized with three, two or even only one absorber plus product buffer. The usage of machinery, feed air compression, and vacuum regeneration must be implemented in a proper way, which may give additional restrictions. Prevention or at least reduction of idle time is one important factor when it comes to optimized utilization of the installed power. More complex systems with four absorbers or more have been proposed, but it remains doubtful if the increased complexity is justified by a significant economic advantage when comparing such systems with a simple doubling of a two-absorber plant [[xxii]].
- Oxygen Production from Membrane Air Separation
The separation of oxygen from air is accomplished mainly by semipermeable polymeric membranes and, more recently, also by dense ceramic oxygen transport membranes, particularly mixed ionic–electric conducting membranes.
6-1- Polymeric membranes: for air separation are thin films of rubbery or glassy polymers on porous support structures. Oxygen and other gases, particularly nitrogen, are transported through the membrane by a solution-diffusion mechanism. Commercially available polymeric membranes have selectivity of 4–8 barriers with corresponding permeability coefficients < 10 barriers. This limits the concentration of oxygen in the permeate in a single-stage process to < 70% and allows under economic aspects to produce rather oxygen-enriched air with 30–50% oxygen than high-purity oxygen [[xxiii]].
Oxygen enriched air generators with polymeric membranes are used for several smaller applications, e.g., medical breathing air concentrators delivering 1–10 L/min air with 30–40% oxygen. To produce oxygen enriched air on an industrial scale, membranes with a selectivity of at least 6–8 and oxygen permeabilities > 200 barriers are necessary to approach competitive cost targets [[xxiv]].
6-2- Oxygen Transport Membranes: Oxygen separation from air by ceramic oxygen transport membranes (OTM) is a comparatively new technology. The separation of oxygen is achieved by mixed-metal oxide ceramic-based membranes which are gas impermeable, but can transport selectively oxygen ions and, in case of so called mixed ionic/electric conducting (MIEC) membranes, electrons through the ceramic lattice at high temperatures > 700°C.
Materials of primary interest as active layer for OTMs are perovskites (ABO3), fluorites (AB2), brownmillerites (A2B2O5), and members of Riddlesden–Popper-series (An+1BnO3n+1), where A are typically lanthanide or alkaline earth metal ions and B are transition metal ions. The oxygen transport through OTMs is driven by the difference in oxygen partial pressure on the feed and permeate side. On the feed side, oxygen is reduced dissociative on the membrane surface into two oxide ions (O2). The ions are transported through the membrane via oxygen vacancies in the lattice to the permeate side where the recombine to molecular oxygen. Simultaneously four electrons are transported in the opposite direction to maintain the charge balance. This exclusive oxygen transport mechanism results in a very high selectivity for oxygen over nitrogen that approaches infinity for defective free membranes [[xxv]].
7- Oxygen (O2) Applications and Uses:
7-1- Industrial Uses of Oxygen [[xxvi]]
Many industries use oxygen for a variety of reasons. In this section, oxygen use in the glass, gasification and gas-to-liquid industries will be addressed. The primary focus will be on oxygen use in the municipal solid waste and coal gasification applications as well as the gas to liquid industry. Current research in metals oxidation will also be addressed.
7-1-1- Oxygen Use in the Glass Industry: Oxygen’s use in the glass industry has been well-documented. Oxygen enriched air fed into the furnace has proven to reduce fuel consumption, lower NOx emissions, and improve glass quality
7-1-2- Oxygen Use in Coal Gasification: One of the most efficient applications of syn gas is in Integrated Gasification Combined Cycle (IGCC) plants. Here, the syn gas is used to run an electric turbine which produces electricity. The waste heat from the turbine and gasification plant is used to make steam, which in turn is run through a steam turbine, thus producing more electricity. Traditionally, cryogenic air separators were used to create the oxygen for direct gasification. Cryogenics enabled operators to produce the volume and purity of air desired
7-1-3- Oxygen Use in Municipal Solid Waste Gasification and Gas-to-Liquid Systems: Municipal Solid Waste (MSW) is everyday trash created by people. Originally land filled, MSW is now most incinerated in the United States in lieu of diminishing land fill space. Incineration reduces the volume of trash; however, it does produce greenhouse gases and toxic emissions that require costly clean-up technologies. The systems today are also producing energy. For example, the Essex County Resource Recovery facility manages 1 million tons of trash per year and produces 45,000 MW in the process. However, as mentioned above, incineration of trash can lead to very pollutants that become airborne in the incineration process
[ii] – W. MELLOR, A Comprehensive Treatise on Inorganic and Theoretical Chemistry, Vol. 1, pp. 344-51, Longmans, Green, 1922. History of the discovery of oxygen.
[iv] -J. R. PARTINGTON, Λ History of Chemistry, Vol. 3, Macmillan, London, 1962; Scheele and the discovery of oxygen (pp. 219-22); Priestley and the discovery of oxygen (pp. 256-63); Lavoisier and the rediscovery of oxygen (pp. 402-10)
[vi] – Lemmon, E.W., Jacobson, R.T., Penoncello, S.G., and Friend, D.G. (2000) Thermodynamic Properties of Air and Mixtures of Nitrogen, Argon, and Oxygen from 60 to 2000 K at Pressures to 2000 MPa. Journal of Physical and Chemical Reference Data, 29 (3), 331–385.
[vii] – Battino, R., Rettich, T.R., and Tominaga, T. (1983) The Solubility of Oxygen and Ozone in Liquids. Journal of Physical and Chemical Reference Data, 12 (2), 163–178.
[viii] – Cleeff, A. van. and Diepen, G.A. (1965) Gas hydrates of nitrogen and oxygen. II. Recueil des Travaux Chimiques des Pays Bas, 84 (8), 1085–1093.
[ix] – Muromachi, S., Ohmura, R., and Mori, Y.H. (2012) Phase equilibriumfor ozone-containing hydrates formed froman (ozone + oxygen) gas mixture coexisting with gaseous carbon dioxide and liquid water. Journal of Chemical Thermodynamics, 49, 1–6.
[x] – Lemmon, E.W. and Jacobsen, R.T. (2004) Viscosity and Thermal Conductivity Equations for Nitrogen, Oxygen, Argon, and Air. International Journal of Thermophysics, 24 (1), 21–69.
[xi] – Steudel, R. and Wong, M.W. (2007) Dark-Red O8 Molecules in Solid Oxygen: Rhomboid Clusters, Not S8-Like Rings. Angewandte Chemie International Edition, 46 (11), 1768–1771.
[xii] – Freiman, Y.A. and Jodl, J.H. (2004) Solid oxygen. Physics Reports, 401 (1–4), 1–228.
[xiii] – Gates, B.C., Katzer, J.R., and Schuit, G.C.A. (1979) Chemistry of Catalytic Processes, McGraw-Hill, New York, p. 329.
[xv] – R. L. KUMP and L. J. TODD, J. Chem. Soc, Chem. Commun., 292-3(1980).
[xviii] – Sircar, S. and Myers, A.L. (2003) Gas Separation by Zeolites, in Handbook of Zeolite Science and Technology (eds. Auerbach, S.M. et al.), Marcel Dekker, New York.
[xix] – Sircar, S. and Myers, A.L. (2003) Gas Separation by Zeolites, in Handbook of Zeolite Science and Technology (eds. Auerbach, S.M. et al.), Marcel Dekker, New York.
[xx] – Grahl, M. (2010) Zeolites in industrial adsorption processes, 22. Deutsche Zeolith-Tagung, 03.–05.03.2010, München.
[xxi] – Praxair Technology (2010) US 7 763 100 (M.S.A Baksh, A.C. Rosinski).
[xxii] – Geisler-Kahlert, T. and Shahani, G. (2016) Oxygen is key Hydrocarbon Engineering, 21 (2), 50–56.
[xxiii] – Baker, R.W. (2002) Future Directions of Membrane Gas Separation Technology Industrial & Engineering Chemistry Research, 41, 1393–1411.
[xxiv] – Belaissaoui, B., Le Moullec, Y., Hagi, H. and Favre, E. (2014) Energy efficiency of oxygen enriched air production technologies: Cryogeny vs membranes Separation and Purification Technology, 125, 142–150.
[xxv] – Oyama, T.S. and Stagg-Williams, S. (2011) Inorganic Polymeric and Composite Membranes – Structure, Function and Other Correlations Elsevier, Amsterdam.
[xxvi] – Prakash Rao and Michael Muller, Center for Advanced Energy Systems, Rutgers, the State University of New Jersey. Industrial Oxygen: Its Generation and Use.