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Molecular sieve

A molecular sieve is a synthetic zeolite adsorbent with pores of uniform molecular size. It adsorbs small polar molecules strongly. In hydrogen PSA units it removes carbon monoxide, nitrogen and methane; in TSA dryers it removes water to low levels.

Also written as: zeolite, mol sieve

Molecular sieve explained

Common types are designated 3A, 4A, 5A and 13X by pore size. Molecular sieve holds water so strongly that water reaching a PSA sieve layer is not removed by pressure swing, and capacity is lost. That is why a guard layer of alumina or silica gel sits upstream of it.

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    A hydrogen dryer takes wet hydrogen from an electrolyser, which also contains a little oxygen, and delivers dry, oxygen-free hydrogen. A catalytic DeOxo reactor converts the…

  • PSA hydrogen purification

    Pressure swing adsorption (PSA) purifies hydrogen by passing a hydrogen-rich gas through beds of adsorbent at high pressure. Impurities are held on the adsorbent while hydrogen…

  • TSA hydrogen purification

    Temperature swing adsorption (TSA) removes water and other strongly held trace impurities from a gas by adsorbing them at ambient temperature and releasing them by heating…

  • Hydrogen purification

    Hydrogen purification removes the other gases from a hydrogen-rich stream until it meets the purity its user needs. Four technologies do most of this work: pressure…

Adsorbent
An adsorbent is a porous solid with a very large internal surface that holds selected gas molecules on it. In gas purification, beds of adsorbent remove impurities from a stream and are then regenerated by lowering pressure or raising temperature.
Activated carbon
Activated carbon is a porous carbon adsorbent with large internal surface and low affinity for water. In hydrogen PSA units it forms the middle layer of the bed, where it removes carbon dioxide, methane and heavier hydrocarbons from the feed.
Activated alumina
Activated alumina is a porous aluminium oxide adsorbent with a strong affinity for water and good mechanical strength. It is used as the first, guard layer in hydrogen PSA beds and as a drying adsorbent in temperature swing adsorption units.
Breakthrough
Breakthrough is the point in an adsorption step at which the impurity being removed first appears in the product above its allowed level. It shows that the usable capacity of the bed has been reached and it must be regenerated.

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