chemistry

Sulfur–iodine cycle

Sulfur–iodine cycle is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Sulfur–iodine cycle rather than just read about it. In short: The sulfur–iodine cycle (S–I cycle) is a three-step thermochemical cycle used to produce hydrogen. The S–I cycle consists of three chemical reactions whose net reactant is water and whose net products are hydrogen and oxygen.

Key takeaways

Sulfur–iodine cycle belongs to chemistry; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Sulfur–iodine cycle to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Sulfur–iodine cycle from memory before moving on to harder problems.

Reference excerpt

The sulfur–iodine cycle (S–I cycle) is a three-step thermochemical cycle used to produce hydrogen. The S–I cycle consists of three chemical reactions whose net reactant is water and whose net products are hydrogen and oxygen. All other chemicals are recycled. The S–I process requires an efficient source of heat.

Process description

The three reactions combined to produce hydrogen are the following:

I2 + SO2 + 2 H2O - heat→ 2 HI + H2SO4 (120 °C (250 °F)) (Bunsen reaction) The HI is then separated by distillation or liquid/liquid gravitic separation. 2 H2SO4 + heat→ 2 SO2 + 2 H2O + O2 (830 °C (1,530 °F)) The water, SO2 and residual H2SO4 must be separated from the oxygen byproduct by condensation. 2 HI + heat→ I2 + H2 (450 °C (840 °F)) Iodine and any accompanying water or SO2 are separated by condensation, and the hydrogen product remains as a gas. Net reaction: 2 H2O → 2 H2 + O2 The sulfur and iodine compounds are recovered and reused, hence the consideration of the process as a cycle. This S–I process is a chemical heat engine. Heat enters the cycle in high-temperature endothermic chemical reactions 2 and 3, and heat exits the cycle in the low-temperature exothermic reaction 1. The difference between the heat entering and leaving the cycle exits the cycle in the form of the heat of combustion of the hydrogen produced.

Characteristics

Advantages All fluid (liquids, gases) process, therefore well suited for continuous production High thermal efficiency predicted (about 50%) Completely closed system without byproducts or effluents (besides hydrogen and oxygen) Suitable for application with solar, nuclear, and hybrid (e.g., solar-fossil) sources of heat – if high enough temperatures can be achieved More developed than competing thermochemical processes Scalable from relatively small scale to huge applications No need for expensive or toxic catalysts or additives More efficient than electrolysis of water (~70-80% efficiency) using electricity derived from a thermal power plant (~30-60% efficiency) combining to ~21-48% efficiency Waste heat suitable for district heating if cogeneration is desired

Disadvantages Very high temperatures required (at least 850 °C (1,560 °F)) – unachievable or difficult to achieve with current pressurized water reactors or concentrated solar power Corrosive reagents used as intermediaries (iodine, sulfur dioxide, hydriodic acid, sulfuric acid); therefore, advanced materials needed for construction of process apparatus Significant further development required to be feasible on large scale At the proposed temperature range advanced thermal power plants can achieve efficiencies (electric output per heat input) in excess of 50% somewhat negating the efficiency advantage In case of leakage, corrosive and somewhat toxic hydroiodic acid is released to the environment If hydrogen is to be used for process heat the required high temperatures make the benefits compared to direct utilization of heat questionable Unable to use non-thermal or low-grade thermal energy sources such as hydropower, wind power or most currently available geothermal power

Research The S–I cycle was invented at General Atomics in the 1970s. The Japan Atomic Energy Agency (JAEA) has conducted successful experiments with the S–I cycle in the helium cooled High Temperature Test Reactor, a reactor which reached first criticality in 1998, JAEA have the aspiration of using further nuclear very high-temperature generation IV reactors (VHTR) to produce industrial scale quantities of 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, General Atomics and Sandia National Laboratories are jointly developing the sulfur-iodine process. Additional research is taking place at the Idaho National Laboratory, and in Canada, Korea and Italy.

Material challenge The S–I cycle involves operations with corrosive chemicals at temperatures up to about 1,000 °C (1,830 °F). 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, silicon carbide (SiC), glass, silicon nitride (Si3N4), and others. Recent research on scaled prototyping suggests that new tantalum surface technologies may be a technically and economically 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. It does not require hydrocarbons like current methods of steam reforming but requires heat from combustion, nuclear reactions, or solar heat concentrators.

See also Cerium(IV) oxide–cerium(III) oxide cycle Copper–chlorine cycle Hybrid sulfur cycle High-temperature electrolysis Iron oxide cycle Zinc–zinc oxide cycle

Footnotes

References Paul M. Mathias and Lloyd C. Brown "Thermodynamics of the Sulfur-Iodine Cycle for Thermochemical Hydrogen Production", presented at the 68 th Annual Meeting of the Society of Chemical Engineers, Japan 23 March 2003. (PDF). Atsuhiko TERADA; Jin IWATSUKI, Shuichi ISHIKURA, Hiroki NOGUCHI, Shinji KUBO, Hiroyuki OKUDA, Seiji KASAHARA, Nobuyuki TANAKA, Hiroyuki OTA, Kaoru ONUKI and Ryutaro HINO, "Development of Hydrogen Production Technology by Thermochemical Water Splitting IS Process Pilot Test Plan", Journal of Nuclear Science and Technology, Vol.44, No.3, p. 477–482 (2007). (PDF).

External links Hydrogen: Our Future made with Nuclear (in MPR Profile issue 9) Use of the modular helium reactor for hydrogen production (World Nuclear Association Symposium 2003)

Illustrations

Worked examples

Example 1 — a first encounter with Sulfur–iodine cycle

Start with the simplest possible case. Write down what Sulfur–iodine cycle claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Sulfur–iodine cycle before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Sulfur–iodine cycle ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Sulfur–iodine cycle

In research: Sulfur–iodine cycle appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Sulfur–iodine cycle in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Sulfur–iodine cycle is common in secondary-school and first-year university syllabi. It links to neighbouring topics Hydrogen production, Inorganic reactions, so understanding it makes those chapters shorter.
In everyday life: Look for Sulfur–iodine cycle outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Sulfur–iodine cycle in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Sulfur–iodine cycle means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Sulfur–iodine cycle out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Sulfur–iodine cycle in simple terms?

Why does Sulfur–iodine cycle matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Sulfur–iodine cycle?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Sulfur–iodine cycle.