Hydrogen purification is any technology used to purify

hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...

. The impurities in hydrogen gas depend on the source of the H2, e.g., petroleum, coal, electrolysis, etc. The required purity is determined by the application of the hydrogen gas. For example, ultra-high purified hydrogen is needed for applications like proton exchange membrane fuel cells.

Purification technologies

Low temperature methods

The default large-scale purification of H₂ produced in

oil refineries An oil refinery or petroleum refinery is an industrial process plant where petroleum (crude oil) is transformed and refined into products such as gasoline (petrol), diesel fuel, asphalt base, fuel oils, heating oil, kerosene, liquefied pet ...

exploits its very low

boiling point The boiling point of a substance is the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid and the liquid changes into a vapor. The boiling point of a liquid varies depending upon the surrounding envi ...

of −253 °C. Most impurities have boiling points well above this temperature. Low temperature methods can be complemented by scrubbing to remove particular impurities.

Palladium membrane hydrogen purifiers

Hydrogen can be purified by passing through a membrane composed of

palladium Palladium is a chemical element; it has symbol Pd and atomic number 46. It is a rare and lustrous silvery-white metal discovered in 1802 by the English chemist William Hyde Wollaston. He named it after the asteroid Pallas (formally 2 Pallas), ...

and

silver Silver is a chemical element; it has Symbol (chemistry), symbol Ag () and atomic number 47. A soft, whitish-gray, lustrous transition metal, it exhibits the highest electrical conductivity, thermal conductivity, and reflectivity of any metal. ...

. Permeability of the former to hydrogen was discovered back in the 1860s. An alloy with a ca. 3:1 ratio for Pd:Ag is more structural robust than pure Pd, which is the active component that allows the selective diffusion of H₂ through it. Diffusion is faster near 300 °C. This method has been used for purification of hydrogen on a laboratory scale, but not in industry. Silver-palladium membranes are unstable toward alkenes and sulfur-containing compounds. Dense thin-metal membrane purifiers are compact, relatively inexpensive and simple to use.

Pressure swing adsorption

Pressure swing adsorption is used for the removal of

carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...

(CO₂) as the final step in the large-scale commercial synthesis of

. It can also remove

methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The abundance of methane on Earth makes ...

carbon monoxide Carbon monoxide (chemical formula CO) is a poisonous, flammable gas that is colorless, odorless, tasteless, and slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the si ...

nitrogen Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...

, moisture and in some cases,

argon Argon is a chemical element; it has symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as abu ...

, from hydrogen.

Applications

Metalorganic vapour phase epitaxy

Hydrogen purifiers are used in metalorganic vapour phase epitaxy reactors for

LED A light-emitting diode (LED) is a semiconductor device that emits light when current flows through it. Electrons in the semiconductor recombine with electron holes, releasing energy in the form of photons. The color of the light (corresp ...

production.

PEM fuel cells

Fuel cell electric vehicles commonly use polymer electrolyte membrane fuel cells (PEMFC) that are susceptible to a range of impurities. Impurities impact PEMFC using a range of mechanisms, these may include

poisoning Poisoning is the harmful effect which occurs when Toxicity, toxic substances are introduced into the body. The term "poisoning" is a derivative of poison, a term describing any chemical substance that may harm or kill a living organism upon ...

the anode hydrogen oxidation reaction catalysts, reducing the ionic conductivity of the ionomer and membrane, altering wetting behaviour of components or blocking porosity in diffusion media. The impact of some impurities like

formic acid Formic acid (), systematically named methanoic acid, is the simplest carboxylic acid. It has the chemical formula HCOOH and structure . This acid is an important intermediate in chemical synthesis and occurs naturally, most notably in some an ...

, or

formaldehyde Formaldehyde ( , ) (systematic name methanal) is an organic compound with the chemical formula and structure , more precisely . The compound is a pungent, colourless gas that polymerises spontaneously into paraformaldehyde. It is stored as ...

is reversible with PEMFC performance recovering once the supply of impurity is removed. Other impurities, for example sulphurous compounds, may cause irreversible degradation. The permissible limits of hydrogen impurities are shown below. Efforts to assess the compliance of hydrogen supplied by hydrogen refuelling stations against the ISO-14687 standard have been performed. While the hydrogen was generally found to be 'good' violations of the standard have been reported, most frequently for nitrogen, water and oxygen.

Combustion engines and appliances

Combustion applications are generally more tolerant of hydrogen impurities than PEFMC, as such the ISO-14687 standard for permissible impurities is less strict. This standard has itself been criticised with revisions proposed to make it more lenient and therefore suitable for hydrogen distributed through a repurposed gas network.

Sources of impurities

The presence of impurities in hydrogen depends on the feedstock and the production process. Hydrogen produced by

electrolysis In chemistry and manufacturing, electrolysis is a technique that uses Direct current, direct electric current (DC) to drive an otherwise non-spontaneous chemical reaction. Electrolysis is commercially important as a stage in the separation of c ...

of water may routinely include trace oxygen and water. Hydrogen produced by reforming of hydrocarbons contains carbon dioxide and carbon monoxide as well as sulphur compounds. Some impurities may be added deliberately, for example odorants to aid detection of gas leaks.

Methods for analysis

As the permissible concentrations for many impurities are very low this sets stringent demands on the sensitivity of the analytical methods. Moreover, the high reactivity of some impurities requires use of a properly passivated sampling and analytical systems. Sampling of hydrogen of is challenging and care must be taken to ensure that impurities are not introduced to the sample and that impurities do not absorb on or react within the sampling equipment, there are currently different methods for sampling but rely on filling a gas cylinder from the refuelling nozzle of a refuelling station. Efforts are underway to standardise and compare sampling strategies. A combination of different instruments is needed to assess hydrogen samples for all of the components listed in ISO 14687-2. Techniques suitable for individual impurities are indicated in the table below. Techniques such as electrochemical sensors and mass spectrometry.

References

External links

3 industrial hydrogen purifier systems

Power and Energy, Inc.Setting a new benchmark for hydrogen delivery
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