Hydrogen Separation and Purification Technologies, Exercises of Chemical Thermodynamics

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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 CO

2

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.



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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Conventional Hydrogen Separation

Processes



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 (CO 2

) 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.



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.



Electrical swing adsorption (ESA) has been proposed as well, but it is currently in the

development stage.



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) – 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 N 2

, CO, CO

2

, hydrocarbons and

water vapour. Consequently, these impurities can be

adsorbed from a hydrogen-containing stream and

high purity hydrogen is recovered.

Department of Mechanical Engineering, Yuan Ze University

Pressure swing adsorption (PSA) – The

process



Adsorption and regeneration

 The PSA process works at basically constant temperature and uses the effect of

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.

Department of Mechanical Engineering, Yuan Ze University

Department of Mechanical Engineering, Yuan Ze University

Pressure swing adsorption (PSA) – The

PSA sequence

Pressure equalization (step E1)

Provide purge (step PP)

Dump (step D)

Purging (regeneration)

Repressurization (steps R1/R0)

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.

 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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Types of Membranes

Department of Mechanical Engineering, Yuan Ze University

Hydrogen Separation Membranes, Energy & Environmental Research Center’s (EERC’s) National Center for

Hydrogen Technology (NCHT), Technical Brief, May 2010.

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 H

2

O, NH

3

, and CO

2

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 low-

temperature 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 Pd-

based membranes) is that it has a very low tolerance to H

2

S and

HCl, both of which are commonly found contaminants in coal-derived

syngas.

Department of Mechanical Engineering, Yuan Ze University

Tokyo-Gas Co.

http://www.tokyo-gas.co.jp/techno/challenge/014_e.html

Figure 1 The principle of a hydrogen separation

reformer

Figure 2. A 40 Nm

3 /h-class hydrogen separation

reformer and a CO 2

separation and recovery unit

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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.

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http://www.pecj.or.jp/english/technology/technology06.html

Japan Petroleum Energy Center

Palladium Membrane Purification (Johnson

Matthey)

Department of Mechanical Engineering, Yuan Ze University

http://pureguard.net/cm/Library/Palladium_Membrane_Purification.html

 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 O 2

, H

2

O,

CO, CO

2

, N

2

and all hydrocarbons

(THC) including methane (CH 4

 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.

References

 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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