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Sulfur-iodine cycle
 
 
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The sulfur-iodine cycle (S-I cycle) is a series of thermochemical
Thermochemistry

In thermodynamics and physical chemistry, thermochemistry is the study of the energy evolved or absorbed in chemical reactions and any physical transformations, such as melting and boiling....
 processes used to produce hydrogen
Hydrogen production

Hydrogen is commonly produced by extraction from hydrocarbon fossil fuels via a chemical path. Hydrogen may also be extracted from water via Biological hydrogen production in an algae bioreactor, or using electricity , chemicals or heat ; these methods are less developed for bulk generation in comparison to chemical paths derived from hydroc...
.

The S-I cycle consists of three chemical reaction
Chemical reaction

A chemical reaction is a process that always results in the interconversion of chemical substances. The substance or substances initially involved in a chemical reaction are called reactants....
s whose net reactant is water
Water

Water is a common chemical substance that is essential for the survival of all known forms of life. In typical usage, water refers only to its liquid form or States of matter, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam....
 and whose net products are hydrogen
Hydrogen

Hydrogen is the chemical element with atomic number 1. It is represented by the chemical symbol H. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly combustion and explosive Diatomic molecule gas with the molecular formula H2....
 and oxygen
Oxygen

Oxygen no O2 produced; 2) O2 produced, but absorbed in oceans & seabed rock; 3) O2 starts to gas out of the oceans, but is absorbed by land surfaces and formation of ozone layer; 4-5) O2 sinks filled and the gas accumulates]]...
. All other chemicals are recycled. The S-I process requires an efficient source of heat.

  • I2
    Iodine

    Iodine , is a chemical element that has the symbol I and atomic number 53. Naturally-occurring iodine is a single isotope with 74 neutrons....
     + SO2 + 2 H2O ? 2 HI + H2SO4 (120°C)
  • 2 H2SO4
    Sulfuric acid

    Sulfuric acid, hydrogen2sulfuroxygen4, is a strong mineral acid. It is soluble in water at all concentrations. Sulfuric acid has many applications, and is one of the top products of the chemical industry....
     ? 2 SO2
    Sulfur dioxide

    Sulfur dioxide is the chemical compound with the formula SO2. It is produced by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide....
     + 2 H2O
    Water (molecule)

    File:Blue-water-pool.jpgWater is the most abundant molecule on Earth's surface, constituting about 70% of the Earth's surface in liquid, solid, and gaseous states....
     + O2
    Oxygen

    Oxygen no O2 produced; 2) O2 produced, but absorbed in oceans & seabed rock; 3) O2 starts to gas out of the oceans, but is absorbed by land surfaces and formation of ozone layer; 4-5) O2 sinks filled and the gas accumulates]]...
     (830°C)
  • 2 HI ? I2 + H2
    Hydrogen

    Hydrogen is the chemical element with atomic number 1. It is represented by the chemical symbol H. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly combustion and explosive Diatomic molecule gas with the molecular formula H2....
     (450°C)
    • Net reaction: 2 H2O ? 2 H2 + O2


    The sulfur
    Sulfur

    Sulfur or sulphur is the chemical element that has the atomic number 16. It is denoted with the symbol S. It is an abundant Valence non-metal....
     and iodine
    Iodine

    Iodine , is a chemical element that has the symbol I and atomic number 53. Naturally-occurring iodine is a single isotope with 74 neutrons....
     compounds are recovered and reused, hence the consideration of the process as a cycle.






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    Encyclopedia


    The sulfur-iodine cycle (S-I cycle) is a series of thermochemical
    Thermochemistry

    In thermodynamics and physical chemistry, thermochemistry is the study of the energy evolved or absorbed in chemical reactions and any physical transformations, such as melting and boiling....
     processes used to produce hydrogen
    Hydrogen production

    Hydrogen is commonly produced by extraction from hydrocarbon fossil fuels via a chemical path. Hydrogen may also be extracted from water via Biological hydrogen production in an algae bioreactor, or using electricity , chemicals or heat ; these methods are less developed for bulk generation in comparison to chemical paths derived from hydroc...
    .

    The S-I cycle consists of three chemical reaction
    Chemical reaction

    A chemical reaction is a process that always results in the interconversion of chemical substances. The substance or substances initially involved in a chemical reaction are called reactants....
    s whose net reactant is water
    Water

    Water is a common chemical substance that is essential for the survival of all known forms of life. In typical usage, water refers only to its liquid form or States of matter, but the substance also has a solid state, ice, and a gaseous state, water vapor or steam....
     and whose net products are hydrogen
    Hydrogen

    Hydrogen is the chemical element with atomic number 1. It is represented by the chemical symbol H. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly combustion and explosive Diatomic molecule gas with the molecular formula H2....
     and oxygen
    Oxygen

    Oxygen no O2 produced; 2) O2 produced, but absorbed in oceans & seabed rock; 3) O2 starts to gas out of the oceans, but is absorbed by land surfaces and formation of ozone layer; 4-5) O2 sinks filled and the gas accumulates]]...
    . All other chemicals are recycled. The S-I process requires an efficient source of heat.

    Process Description


    The three reactions that produce hydrogen are as follows:
    1. I2
      Iodine

      Iodine , is a chemical element that has the symbol I and atomic number 53. Naturally-occurring iodine is a single isotope with 74 neutrons....
       + SO2 + 2 H2O ? 2 HI + H2SO4 (120°C)
      • The HI is then separated by distillation
        Distillation

        Distillation is a method of separation process mixtures based on differences in their Volatility in a boiling liquid mixture. Distillation is a unit operation, or a physical separation process, and not a chemical reaction....
        . Note that concentrated H2SO4 may react with HI, giving I2, SO2 and H2O (backward reaction). Many chemical process
        Chemical process

        In a "Process " sense, a chemical process is a method or means of somehow changing one or more chemicals or chemical compounds. Such a chemical process can occur by itself or be caused by somebody....
        es are reversible reaction
        Reversible reaction

        A reversible reaction is a chemical reaction that results in an chemical equilibrium mixture of reactants and Product . For a reaction involving two reactants and two products this can be expressed symbolically asA and B can react to form C and D or, in the reverse reaction, C and D can react to form A and B....
        s, such as ammonia production
        Ammonia production

        Because of its many uses, ammonia is one of the most highly-produced inorganic chemicals. There are numerous large-scale ammonia production plants worldwide, producing a total of 109,000,000 metric tons of ammonia in 2004....
         from N2 and H2, but removing the desired product will shift equilibrium
        Equilibrium

        For the opposite, see disequilibrium.Equilibrium is the condition of a system in which competing influences are balanced and it may refer to:...
         to the right. This reaction is sometimes referred to as Bunsen Reaction
        Bunsen Reaction

        The Bunsen Reaction is a chemical reaction that describes water, sulfur dioxide, and iodine reacting to form sulfuric acid and hydrogen iodide. This reaction is the first step in the Sulfur-iodine cycle to produce hydrogen....
        .
    2. 2 H2SO4
      Sulfuric acid

      Sulfuric acid, hydrogen2sulfuroxygen4, is a strong mineral acid. It is soluble in water at all concentrations. Sulfuric acid has many applications, and is one of the top products of the chemical industry....
       ? 2 SO2
      Sulfur dioxide

      Sulfur dioxide is the chemical compound with the formula SO2. It is produced by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide....
       + 2 H2O
      Water (molecule)

      File:Blue-water-pool.jpgWater is the most abundant molecule on Earth's surface, constituting about 70% of the Earth's surface in liquid, solid, and gaseous states....
       + O2
      Oxygen

      Oxygen no O2 produced; 2) O2 produced, but absorbed in oceans & seabed rock; 3) O2 starts to gas out of the oceans, but is absorbed by land surfaces and formation of ozone layer; 4-5) O2 sinks filled and the gas accumulates]]...
       (830°C)
      • The water, SO2 and residual H2SO4 must be separated from the oxygen byproduct by condensation. See Sulfur dioxide#Temperature dependence of aqueous solubility
        Sulfur dioxide

        Sulfur dioxide is the chemical compound with the formula SO2. It is produced by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide....
         for temperatures.
    3. 2 HI ? I2 + H2
      Hydrogen

      Hydrogen is the chemical element with atomic number 1. It is represented by the chemical symbol H. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly combustion and explosive Diatomic molecule gas with the molecular formula H2....
       (450°C)
      • Iodine and any accompanying water or SO2 are separated by condensation
        Condensation

        Condensation is the change of the physical state of aggregation of matter from gaseous phase into liquid phase. When the transition happens from the gaseous phase into the solid phase directly, bypassing the liquid phase the change is called Deposition , which is the opposite of sublimation....
        , and the hydrogen product remains as a gas.
    Net reaction: 2 H2O ? 2 H2 + O2


    The sulfur
    Sulfur

    Sulfur or sulphur is the chemical element that has the atomic number 16. It is denoted with the symbol S. It is an abundant Valence non-metal....
     and iodine
    Iodine

    Iodine , is a chemical element that has the symbol I and atomic number 53. Naturally-occurring iodine is a single isotope with 74 neutrons....
     compounds are recovered and reused, hence the consideration of the process as a cycle. This S-I process is a chemical heat
    Heat

    In physics and thermodynamics, heat is any transfer of energy from one body or thermodynamic system to another due to a difference in temperature....
     engine. Heat enters the cycle in high temperature endothermic
    Endothermic

    In thermodynamics, the word endothermic "within-heating" describes a process or reaction that absorbs energy in the form of heat. Its etymology stems from the Greek prefix endo-, meaning ?inside? and the Greek suffix ?thermic, meaning ?to heat?....
     chemical reactions 2 and 3, and heat exits the cycle in the low temperature exothermic
    Exothermic

    File:Explosion1.JPG In thermodynamics, the term exothermic describes a process or reaction that releases energy usually in the form of heat, but also in form of light , electricity , or sound....
     reaction 1. The difference between the heat entering the cycle and the heat leaving the cycle exits the cycle in the form of the heat of combustion
    Heat of combustion

    The heat of combustion is the energy released as heat when one mol of a compound undergoes complete combustion with oxygen. The chemical reaction is typically a hydrocarbon reacting with oxygen to form carbon dioxide, water and heat....
     of the hydrogen produced.

    Advantages and disadvantages

    The characteristics of the S-I process can be described as follows:
    • All fluid (liquids, gases) process, therefore well suited for continuous operation;
    • High utilization of heat predicted (about 50%), but very high temperatures required (at least 850 deg C);
    • Completely closed system without byproducts or effluents (besides hydrogen and oxygen);
    • Corrosive reagents used as intermediaries (iodine, sulfur dioxide, hydriodic acid, sulfuric acid); therefore, advanced materials needed for construction of process apparatus;
    • Suitable for application with solar, nuclear, and hybrid (e.g., solar-fossil) sources of heat;
    • More developed than competitive thermochemical processes (but still requiring significant development to be feasible on large scale).


    Research


    The S-I cycle was invented at General Atomics
    General Atomics

    General Atomics is a nuclear physics and defense contractor headquartered in San Diego, California. Among other things, it is the manufacturer of the RQ-1 Predator unmanned aerial vehicle ....
     in the 1970s. The Japan Atomic Energy Agency (JAEA) has conducted successful experiments with the S-I cycle with the intent of using nuclear high-temperature generation IV reactor
    Generation IV reactor

    Generation IV reactors are a set of theoretical nuclear reactor designs currently being researched. Most of these designs are generally not expected to be available for commercial construction before 2030, with the exception of a version of the Very High Temperature Reactor called the Next Generation Nuclear Plant ....
    s to produce hydrogen. (The Japanese refer to the cycle as the IS cycle.) Plans have been made to test larger-scale automated systems for hydrogen production. Under an International Nuclear Energy Research Initiative (INERI) agreement, the French CEA
    Commissariat à l'Énergie Atomique

    The Commissariat ? l??nergie atomique or CEA, is a France ?public establishment related to industrial and commercial activities? whose mission is to develop all applications of atomic energy, both civilian and military....
    , General Atomics and Sandia National Laboratories
    Sandia National Laboratories

    Sandia National Laboratories, which is managed and operated by the Sandia Corporation , is a major United States Department of Energy research and development United States Department of Energy National Labs with two locations, one in Albuquerque, New Mexico, New Mexico and the other in Livermore, California, California....
     are jointly developing the sulfur-iodine process. Additional research is taking place at the Idaho National Laboratory
    Idaho National Laboratory

    The Idaho National Laboratory is an 890-square-mile complex located in the desert land of eastern Idaho, between the town of Arco, Idaho and the city of Idaho Falls, at ....
    , in Canada, Korea and Italy.

    Material challenge

    The S-I cycle involves operations with corrosive chemical at temperatures up to about 1000 °C. The selection of materials with sufficient corrosion resistance under the process conditions is of key importance to the economic viability of this process. The materials suggested include the following classes: refractory metals, reactive metals, superalloys, ceramics, polymers, and coatings. Some materials suggested include tantalum alloys, niobium alloys, noble metals, high-silicon steels, several nickel-based superalloys, mullite
    Mullite

    Mullite, or porcelainite, is a rare clay mineral aluminosilicate. It can form two stoichiometric forms 3Al2O32SiO2 or 2Al2O3SiO2....
     , silicon carbide
    Silicon carbide

    Silicon carbide is a Chemical compound of silicon and carbon bonded together to form ceramics, but it also occurs in nature as the extremely rare mineral moissanite....
     SiC, glass, silicon nitride
    Silicon nitride

    Silicon nitride is a hard, solid substance. It is the main component in silicon nitride ceramics, which have good shock resistance and other mechanical and thermal properties as compared to other ceramics....
     Si3N4, and others. Recent reasearch on scaled prototyping suggests that new tantalum surface technologies may be a technical and economical feasible way to make larger scale installations.

    Hydrogen economy


    The sulfur-iodine cycle has been proposed as a way to supply hydrogen for a hydrogen-based economy
    Hydrogen economy

    The hydrogen economy is a proposed system of meeting energy needs by using hydrogen as a fuel source that could be generated from alternative fuels or other energy sources that don't give off greenhouse gases....
    . With an efficiency of around 50% it is more efficient than electrolysis
    Electrolysis

    In chemistry and manufacturing, electrolysis is a method of separating Chemical bond chemical compound by passing an electric current through them....
    , and it does not require hydrocarbons like current methods of steam reforming
    Steam reforming

    Steam reforming , hydrogen reforming or catalytic oxidation, is a method of producing hydrogen from hydrocarbons. On an industrial scale, it is the dominant method for producing hydrogen....
     but requires heat from combustion, nuclear reactions, or solar heat concentrators. Considerable additional research must occur before the sulfur-iodine cycle can become a viable source of hydrogen. The first commercial generation IV reactors are expected around 2030.

    See also

    • Cerium(IV) oxide-cerium(III) oxide cycle
      Cerium(IV) oxide-cerium(III) oxide cycle

      The cerium oxide-cerium oxide cycle or CeO2/Ce2O3 cycle is a two step Thermochemistry process based on cerium oxide and cerium oxide for hydrogen production....
    • Copper-chlorine cycle
      Copper-chlorine cycle

      The copper-chlorine cycle is a sequence of processes used for hydrogen production at a temperature of at least about 590 degrees Celsius.The Cu-Cl cycle involves four chemical reactions for water splitting whose net reaction decomposes water into hydrogen and oxygen....
    • Hybrid sulfur cycle
      Hybrid sulfur cycle

      The hybrid sulfur cycle is a two-step water splitting process intended to be used for hydrogen production. Based on sulfur oxidation and reduction, it is classified as a hybrid thermochemistry cycle because it uses an electrochemical reaction for one of the two steps....
    • High-temperature electrolysis
      High-temperature electrolysis

      High-temperature electrolysis is a method currently being investigated for the production of hydrogen from water with oxygen as a by-product....
    • Iron oxide cycle
      Iron oxide cycle

      The iron oxide cycle is a Thermochemistry process proposed for use for hydrogen production....
    • Zinc zinc-oxide cycle


    External links


    • (in MPR Profile issue 9)


    Footnotes