1. Chapter 3 Materials for Hydrogen Separation and Purification
Department of Mechanical Engineering, Yuan Ze University

2. Department of Mechanical Engineering, Yuan Ze University
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.
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.
Department of Mechanical Engineering, Yuan Ze University

3. Department of Mechanical Engineering, Yuan Ze University
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.
Significant economic benefits can be realized by utilizing warm-or hot-gas-cleaning techniques.
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.
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.
Department of Mechanical Engineering, Yuan Ze University

4. 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 (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.
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.
Department of Mechanical Engineering, Yuan Ze University

5. Pressure swing adsorption (PSA)
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6. Pressure swing adsorption (PSA)
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 mol-% and is achieved at highest recovery rates.
Department of Mechanical Engineering, Yuan Ze University

7. 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 hydrogen-containing stream and high purity hydrogen is recovered.
Department of Mechanical Engineering, Yuan Ze University

8. 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

9. Department of Mechanical Engineering, Yuan Ze University
Pressure swing adsorption (PSA) – The process
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.
Department of Mechanical Engineering, Yuan Ze University

10. 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)
Department of Mechanical Engineering, Yuan Ze University

11. 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.
Department of Mechanical Engineering, Yuan Ze University

12. 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.
Department of Mechanical Engineering, Yuan Ze University

13. Department of Mechanical Engineering, Yuan Ze University
Types of Membranes
Hydrogen Separation Membranes, Energy & Environmental Research Center’s (EERC’s) National Center for Hydrogen Technology (NCHT), Technical Brief, May 2010.
Department of Mechanical Engineering, Yuan Ze University

14. 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 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 H2S and HCl, both of which are commonly found contaminants in coal-derived syngas.
Department of Mechanical Engineering, Yuan Ze University

15. 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 2. A 40 Nm3/h-class hydrogen separation reformer and a CO2 separation and recovery unit
Figure 1 The principle of a hydrogen separation reformer
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17. 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.
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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 % pure hydrogen.
Unlike distillation which involves gas-liquid 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
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19. 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.
Department of Mechanical Engineering, Yuan Ze University

20. Catalytic recombination or deoxygenation purifier
Catalytic recombination or deoxygenation is used to remove oxygen (O2) impurities. The process is also known as a 'deoxo' process.
The oxygen reacts with the hydrogen to form water vapor, which can then be removed by a dryer if necessary.
The catalysts that are used are based on platinum group metals (PGM).
A typical system could handle up to 3% O2 in H2 in the feed, and reduce the O2 content to less than 1ppm.
Department of Mechanical Engineering, Yuan Ze University

21. Department of Mechanical Engineering, Yuan Ze University
References
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.
Department of Mechanical Engineering, Yuan Ze University