Chapter 3
Materials for Hydrogen
Separation and Purification
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Hydrogen production technologies have the potential to nearly eliminate
carbon emissions and dependency on oil. However, current technology
options for hydrogen production and CO2 separation are typically more
expensive than traditional energy production.
 Different methods are used but the most promising technique for
removing unwanted contaminants uses dense thin-metal membrane
purifiers which are compact, relatively inexpensive and simple to use.
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Hydrogen separation membrane technologies have the potential to play an
important role in near-zero-emission plants because membranes can produce
hydrogen economically, at large scale, and with very low levels of impurities.
Hydrogen separation membranes are commercially available, but most
developments have sprung from advancements in hydrogen separation
from steam methane-reforming plants or refineries.
 Most membranes used today are susceptible to contaminants commonly
found in coal-derived syngas, such as sulfur, ammonia, mercury, and
trace metals.
 Gas cleanup technologies will minimize many of these contaminants, but
trace amounts will break through, and system upsets will inevitably occur.
 Considering that most membrane materials are very expensive,
optimizing and demonstrating resistance to common contaminants is
needed.
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Conventionally, cold-gas cleanup methods have been employed to
remove contaminants from coal gasification syngas streams.
Methods such as Rectisol® or Selexol® are commercially available
and are very effective at removing contaminants but also have high
capital and operational costs.
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Significant economic benefits can be realized by utilizing warm-or
hot-gas-cleaning techniques.
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The Department of Energy (DOE) has reported that thermal
efficiency increases of 8% over conventional techniques can be
realized by integrating warm-gas cleanup technologies into
integrated gasification combined-cycle (IGCC) plants.
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Hydrogen separation membranes typically operate at warm-gas
cleanup temperatures, so they are a good match for IGCC projects
employing warm-gas cleanup and carbon capture.
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Conventional Hydrogen Separation Processes
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Pressure swing adsorption (PSA)
 PSA is the most common method used today for hydrogen separation.
 PSA is based on an adsorbent bed that captures the impurities in the syngas stream
at higher pressure and then releases the impurities at low pressure.
 Multiple beds are utilized simultaneously so that a continuous stream of hydrogen at
purities up to 99.9% may be produced.
 PSA is used for the removal of carbon dioxide (CO2) as the final step in the large-
scale commercial synthesis of hydrogen. It can also remove methane, carbon
monoxide, nitrogen, moisture and in some cases, argon, from hydrogen.
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Temperature swing adsorption (TSA) is a variation on PSA, but it is not widely
used because of the relatively long time it takes to heat and cool sorbents.
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Electrical swing adsorption (ESA) has been proposed as well, but it is
currently in the development stage.
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Cryogenic processes also exist to purify hydrogen, but they require extremely
low temperatures and are, therefore, relatively expensive.
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Pressure swing adsorption (PSA)
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Pressure swing adsorption (PSA)
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The use of the Pressure Swing Adsorption (PSA) process
has seen tremendous growth during the last decades mainly
due to its simplicity and low operating costs.
Major applications have been the recovery of high purity
hydrogen, methane and carbon dioxide as well as the
generation of nitrogen and oxygen.
In addition, it has gained significance for the bulk removal of
carbon dioxide from direct reduction top-gases.
Capacities range from a few hundred Nm³/h to large scale
plants with more than 400,000 Nm³/h.
The hydrogen product meets every purity requirement up to
99.9999 mol-% and is achieved at highest recovery rates.
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Pressure swing adsorption (PSA) – The
process

Separation by adsorption
 The Pressure Swing Adsorption (PSA) technology
is based on a physical binding of gas molecules to
adsorbent material. The respective force acting
between the gas molecules and the adsorbent
material depends on the gas component, type of
adsorbent material, partial pressure of the gas
component and operating temperature. A
qualitative ranking of the adsorption forces is
shown in the figure.
 The separation effect is based on differences in
binding forces to the adsorbent material. Highly
volatile components with low polarity, such as
hydrogen, are practically non-adsorbable as
opposed to molecules as N2, CO, CO2,
hydrocarbons and water vapour. Consequently,
these impurities can be adsorbed from a hydrogencontaining stream and high purity hydrogen is
recovered.
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Pressure swing adsorption (PSA) – The
process
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Adsorption and regeneration
 The PSA process works at basically constant temperature and uses the effect of
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alternating pressure and partial pressure to perform adsorption and desorption.
Since heating or cooling is not required, short cycles within the range of minutes are
achieved. The PSA process consequently allows the economical removal of large
amounts of impurities.
Adsorption is carried out at high pressure (and hence high respective partial pressure)
typically in the range of 10 to 40 bar until the equilibrium loading is reached. At this
point in time, no further adsorption capacity is available and the adsorbent material
must be regenerated.
This regeneration is done by lowering the pressure to slightly above atmospheric
pressure resulting in a respective decrease in equilibrium loading. As a result, the
impurities on the adsorbent material are desorbed and the adsorbent material is
regenerated.
The amount of impurities removed from a gas stream within one cycle corresponds
to the difference of adsorption to desorption loading. After termination of regeneration,
pressure is increased back to adsorption pressure level and the process starts again
from the beginning.
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Pressure swing adsorption (PSA) – The
process
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The figure illustrates the pressure swing adsorption process. It shows the adsorption
isotherms describing the relation between partial pressure of a component and its
equilibrium loading on the adsorbent material for a given temperature.
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Pressure swing adsorption (PSA) – The
PSA sequence
Pressure equalization (step E1)
Provide purge (step PP)
Dump (step D)
Purging (regeneration)
Repressurization (steps R1/R0)
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Principles of Hydrogen Separation Membranes
Most hydrogen separation membranes operate on the principle that only
hydrogen can penetrate through the membrane because of the inherent
properties of the material.
 The mechanism for hydrogen penetration through the membrane
depends on the type of membrane in question.
 Most membranes rely on the partial pressure of hydrogen in the feed
stream as the driving force for permeation, which is balanced with the
partial pressure of hydrogen in the product (permeate) stream.
 Figure 1 illustrates the basic operating principles of hydrogen separation
membranes for use in coal-derived syngas.
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This figure shows a tubular membrane, but plate and frame-style membranes have also
been developed.
The “syngas in” stream refers to the feed gas into the membrane module.
The permeate stream, which in this case is made up of mostly hydrogen, has
permeated through the membrane wall.
The remaining gases (raffinate stream) are what is left of the feed stream once the
permeate is separated.
A sweep gas such as nitrogen may be used on the permeate side to lower the partial
pressure and enable more hydrogen to pass through the membrane.
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Hydrogen Separation Membranes, Energy & Environmental Research Center’s (EERC’s) National Center for
Hydrogen Technology (NCHT), TechnicalDepartment
Brief, May
2010.
of Mechanical Engineering, Yuan Ze University
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Types of Membranes
Hydrogen Separation Membranes, Energy & Environmental Research Center’s (EERC’s) National Center for
Hydrogen Technology (NCHT), Technical
Brief, May
2010. Engineering, Yuan Ze University
Department
of Mechanical
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Commercially Available Membranes
Air Liquide has technology called MEDAL™ that is typically used in
refinery applications for hydrotreating. The membrane is selective to
components other than hydrogen, including H2O, NH3, and CO2 and,
therefore, would probably not be a good fit in most coal gasification
applications.
 Air Products offers a line of hydrogen recovery membranes referred
to as PRISM® membrane systems. The PRISM membrane is
intended for separations in hydrocracker and hydrotreater systems
or for CO purification in reformer gases. The systems are lowtemperature and not intended for processing on coal-derived
syngas.
 Wah-Chang offers small-scale Pd–Cu membranes for commercial
sale that are capable of producing an ultrapure stream of hydrogen
from syngas. The one drawback of the membrane (like many Pdbased membranes) is that it has a very low tolerance to H2S and
HCl, both of which are commonly found contaminants in coalderived syngas.
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Palladium membrane hydrogen
purifiers

The palladium membrane is typically a metallic
tube of a palladium and silver alloy material
possessing the unique property of allowing only
monatomic hydrogen to pass through its crystal
lattice when it is heated above 300°C.
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Tokyo-Gas Co.
Figure 1 The principle of a hydrogen separation
reformer
Figure 2. A 40 Nm3/h-class hydrogen separation
reformer and a CO2 separation and recovery unit
http://www.tokyo-gas.co.jp/techno/challenge/014_e.html
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Japan Petroleum Energy Center
A high-efficiency process for purifying hydrogen with new hybrid separation
membrane is being developed for application to hydrogen production units
at refineries. The 99.99% pure hydrogen produced in this process will be
used for fuel cell vehicles.
http://www.pecj.or.jp/english/technology/technology06.html
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Making high-purity hydrogen with palladium
 Metal membrane made of palladium can be used to sieve hydrogen from mixed gas. Its
separation mechanism is shown in Figure 1.
 A hydrogen molecule is dissociated into two atoms on the membrane surface, dissolved
within the membrane, diffused by weaving through metal atoms, recombining on the other
side of the membrane and thus permeating through it.
 Molecules other than hydrogen cannot permeate the membrane as it is difficult for them to
dissociate, dissolve and diffuse.
 In manufacturing semiconductor and
LED, super high-purity hydrogen is
needed, and palladium membrane is
used to produce 99.9999999 % pure
hydrogen.
 Unlike distillation which involves gasliquid transformation, this separation
process has good energy efficiency,
and is thought promising for
hydrogen production for fuel cells.
 By supplying high-purity hydrogen
obtained through metal membrane,
the use of platinum as fuel cell
catalyst can be reduced.
Figure 1: Principle of hydrogen separation through metal
membrane
http://www.aist.go.jp/aist_e/aist_today/2008_29/feature/feature_03.html
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Palladium Membrane Purification (Johnson Matthey)
 Palladium membrane hydrogen purifiers operate via pressure driven diffusion across
palladium membranes. Only hydrogen can diffuse through the palladium.
 The palladium membrane is typically a metallic tube comprising a palladium and silver alloy
material possessing the unique property of allowing only monatomic hydrogen to pass
through its crystal lattice when it is heated above nominally 300◦ C.
 The hydrogen gas molecule coming into contact with the palladium membrane surface
dissociates into monatomic hydrogen and passes through the membrane.
 On the other surface of the palladium membrane, the monatomic hydrogen is recombined
into molecular hydrogen – the ultrapure hydrogen used in the semiconductor process.
 Palladium purifiers provide <1 ppb
purity with any inlet gas quality.
Impurities removed include O2, H2O,
CO, CO2, N2 and all hydrocarbons
(THC) including methane (CH4).
 Maximum operating pressure is 250
psig at 300 to 400◦C; high pressure
vessels can be designed as well.
 Normal life expectancy of a
palladium membrane purifier is 5
years and no routine maintenance
required.
http://pureguard.net/cm/Library/Palladium_Membrane_Purification.html
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Catalytic recombination or
deoxygenation purifier
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Catalytic recombination or deoxygenation is used to remove
oxygen (O2) impurities. The process is also known as a
'deoxo' process.
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The oxygen reacts with the hydrogen to form water vapor,
which can then be removed by a dryer if necessary.
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The catalysts that are used are based on platinum group
metals (PGM).
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A typical system could handle up to 3% O2 in H2 in the feed,
and reduce the O2 content to less than 1ppm.
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References
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http://www.tokyo-gas.co.jp/techno/challenge/014_e.html
http://www.pecj.or.jp/english/technology/technology06.html
http://www.aist.go.jp/aist_e/aist_today/2008_29/feature/feature_03.
html
http://pureguard.net/cm/Library/Palladium_Membrane_Purification.h
tml
Hydrogen Separation Membranes, Energy & Environmental
Research Center’s (EERC’s) National Center for Hydrogen
Technology (NCHT), Technical Brief, May 2010.
Hydrogen Recovery by Pressure Swing Adsorption, Linde,
Germany.
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