Chemical Engineering Science 92 (2013) 4066

Contents lists available at SciVerse ScienceDirect

Chemical Engineering Science

0009-25

http://d

n Corr

E-m

journal homepage: www.elsevier.com/locate/ces

Review

Recent advances on membranes and membrane reactorsfor hydrogen production

Fausto Gallucci a,n, Ekain Fernandez b, Pablo Corengia b, Martin van Sint Annaland a

a Multiphase Reactors Group, Department of Chemical Engineering and Chemistry, Eindhoven University of Technology, 5612 AZ Eindhoven,

The Netherlandsb Tecnalia, Mikeletegi Pasealekua 2, 20009 Donostia, San Sebastian, Spain

H I G H L I G H T S

c Recent advances in hydrogen selective membranes are presented.c Commercial membranes for hydrogen production are discussed.c An overview of the membrane reactor concepts is highlighted.c The application of membrane reactors to different feedstock is reported.

a r t i c l e i n f o

Article history:

Received 22 October 2012

Received in revised form

17 December 2012

Accepted 6 January 2013Available online 23 January 2013

Keywords:

Membrane reactor

Hydrogen production

Packed bed

Fluidization

Separations

Membranes

09/$ - see front matter & 2013 Elsevier Ltd. A

x.doi.org/10.1016/j.ces.2013.01.008

esponding author. Tel.: 31 40 247 3675; faxail address: F.Gallucci@tue.nl (F. Gallucci).

a b s t r a c t

Membranes and membrane reactors for pure hydrogen production are widely investigated not only

because of the important application areas of hydrogen, but especially because mechanically and

chemically stable membranes with high perm-selectivity towards hydrogen are available and are

continuously further improved in terms of stability and hydrogen flux. Membrane reactors are

(multiphase) reactors integrating catalytic reactions (generally reforming and water gas shift reactions

for hydrogen production) and separation through membranes in a single unit. This combination of

process steps results in a high degree of process integration/intensification, with accompanying

benefits in terms of increased process or energy efficiencies and reduced reactor or catalyst volume.

The aim of this review is to highlight recent advances in hydrogen selective membranes (from

palladium-based to silica and proton conductors) along with the advances for the different types of

membrane reactors available (from packed bed to fluidized bed, from micro-reactors to bio-membrane

reactors). In addition, the application of membrane reactors for hydrogen production from different

feedstock is also discussed.

& 2013 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

2. Hydrogen separation membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

2.1. Membrane materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

2.2. Membrane configurations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

2.2.1. Geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

2.2.2. Unsupported and supported membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

3. Dense metal membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

3.1. General comments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

3.2. Progresses in commercialization of dense metal membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

3.2.1. CRI/Criterion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

3.2.2. ECN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

ll rights reserved.

: 31 40 247 5833.

www.elsevier.com/locate/ceswww.elsevier.com/locate/ceshttp://dx.doi.org/10.1016/j.ces.2013.01.008http://dx.doi.org/10.1016/j.ces.2013.01.008http://dx.doi.org/10.1016/j.ces.2013.01.008http://crossmark.dyndns.org/dialog/?doi=10.1016/j.ces.2013.01.008&domain=pdfhttp://crossmark.dyndns.org/dialog/?doi=10.1016/j.ces.2013.01.008&domain=pdfhttp://crossmark.dyndns.org/dialog/?doi=10.1016/j.ces.2013.01.008&domain=pdfmailto:F.Gallucci@tue.nlhttp://dx.doi.org/10.1016/j.ces.2013.01.008

F. Gallucci et al. / Chemical Engineering Science 92 (2013) 4066 41

3.2.3. Eltron Research, Inc. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

3.2.4. Green Hydrotec . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

3.2.5. Hy9 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

3.2.6. M&P . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

3.2.7. MRT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

3.2.8. Pall Corporation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

3.2.9. REB . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

3.2.10. Tokyo Gas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

3.2.11. UTRC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

4. Microporous membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

4.1. Zeolite membranes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

4.2. Metalorganic framework membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

4.3. Silica membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

4.4. Carbon membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

5. Proton conducting membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

5.1. Dense ceramic membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

5.1.1. Perovskite-type membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

5.1.2. Non-perovskite-type membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

5.2. Cermet membranes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

6. Advances in membrane reactors for hydrogen production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

6.1. Packed bed membrane reactors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52

6.2. Fluidized bed membrane reactors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55

6.3. Membrane micro-reactors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

6.4. Membrane bio-reactors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

7. Feedstock for hydrogen production. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

7.1. H2 production from methane in MR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

7.2. H2 production from other hydrocarbons in MR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

7.3. H2 production from biological related feedstock in MR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

8. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

Web references . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

1. Introduction

It is widely accepted that the solution of the global warmingproblem will be a combination of various contributions rangingfrom carbon capture and sequestration (CCS) to improved carbonefficiency of fossil fuels, to large use of renewable energy sources(in the long term). Among the various strategies proposed, CCSseems to be a good candidate for CO2 emissions mitigation;however, CCS has to be regarded as a mid-term solution as longas the energy economy is based on fossil fuels: CCS will be used aslong as other technologies such as large-scale exploitation ofrenewable sources and nuclear fusion are still under develop-ment. CCS consists of two challenging processes, viz. carbondioxide capture and its sequestration (mineralization or storage).In this review only carbon capture is discussed, while sequestra-tion (perhaps in off shore geological formations) is a process withits own technological and societal challenges. Three main tech-nological paths have been proposed for CCS from fossil-fuel firedpower stations, namely the post-combustion capture, the pre-combustion decarbonization and the oxyfuel combustion route(see Fig. 1).

This review focuses on separation and production of hydrogenthus can be applied to the decarbonization route. Hydrogen fromfossil fuels can also be used to increase the efficiency of thesystem. An example of increased efficiency is the conversion ofraw material into hydrogen and its use in fuel cells. In fact, forexample in automotive applications, electromotor combined withhydrogen powered fuel cells shows an overall efficiency (4055%)significantly higher than internal combustion engines (1330%).

Pt based catalyst are commonly used in polymer electrolytemembrane (PEM) fuel cells, the performance of the catalyst

decrease dramatically if carbon monoxide (a main product ofmost of the conversion technologies) or hydrogen sulfide ispresent. For this reason, the interest in the production of ultra-pure hydrogen has strongly increased in the last years. It has beendemonstrated that by using pure hydrogen produced by mem-brane reactors in co-generation units the total efficiency will beincreased by roughly 10% (Roses et al., 2010, 2011).

Traditionally, hydrogen is produced via steam reforming (SR)of hydrocarbons such as methane, naphtha oil or methanol/ethanol. But on industrial scale most of the hydrogen (more than80%) is currently produced by SR of natural gas carried out inlarge multi-tubular fixed-bed reactors. In small-scale applications,two other main alternatives are generally considered along withSR: partial oxidation reactions, with a significantly lower effi-ciency than SR, and auto-thermal reforming, where the partialoxidation (exothermic reaction) and SR (endothermic reaction)are carried out in the same reactor.

The main drawbacks of conventional SR, partial oxidation andauto-thermal conventional reactors are that all these reactionsare equilibrium limited and (even in case of complete fuelconversion) produce a hydrogen rich gas mixture containingcarbon oxides and other by-products. Consequently, in order toproduce pure hydrogen, these chemical processes are carried outin a number of reaction units (typically high temperature refor-mer, high and low temperature shift reactors) followed byseparation units (mostly pressure swing adsorption). The largenumber of different process steps decreases the system efficiencyand makes scale-down uneconomical. A typical reaction processscheme is reported in Fig. 2.

Using this process, high hydrogen yields are achieved, butcostly high temperature heat exchangers and complex energy

F. Gallucci et al. / Chemical Engineering Science 92 (2013) 406642

integration among different process units are required to obtainthe hydrogen at the desired high purity.

Among different technologies related to production, separa-tion and purification of H2, membrane technologies seem to be

Fig. 2. Conventional steam reforming reaction scheme. HT shift and L

Fig. 3. Membrane system and involv

Post-CombustionCapture

Pre-CombustionCapture

OxyfuelCombustion

Power & Heat

Air separatio

Gasification

Power &Heat

Air

Air/Steam

Air

O2

CoalGasBiomass

CoalBiomass

CoalGasBiomass

GasOil

Reforming+ CO2

separation

Fig. 1. The three main CO2 capture rout

the most promising and membrane separation is nowadays increas-ingly considered as a good candidate for substituting conventionalsystems. The specific thermodynamic constrains limiting traditionalreactors can be circumvented by using innovative integrated systems,

T shift are high and low temperature shift reactors, respectively.

ed gas streams (Lu et al., 2007).

CO2separation

n

CO2compression &

Dehydration

Power &Heat

N2 , O2

CO2

CO2

CO2

Air

N2

es (adapted from Metz et al., 2005).

F. Gallucci et al. / Chemical Engineering Science 92 (2013) 4066 43

such as the so-called membrane reactors (MRs), engineering systemsin which both reaction and separation are carried out in the samedevice (see Fig. 3).

In comparison to a conventional configuration in which areactor is combined with a downstream separation unit, the useof membrane reactors can bring various potential advantagessuch as reduced capital costs (due to the reduction in size of theprocess unit), improved yields and selectivities (due to theequilibrium shift effect) and reduced downstream separationcosts (separation is integrated).

The success of membrane reactors for hydrogen productiondepends crucially on: (i) the advances in the membrane produc-tion methods for the production of thin membranes with highhydrogen fluxes and high hydrogen perm-selectivities; (ii) thedesign of innovative reactor concepts which allow the integrationof separation and energy exchange, the reduction of mass andheat transfer resistances and the simplification of the housing andsealing of the membranes.

In this review we will first discuss recent advances in (hightemperature) membranes to be applied in hydrogen productionunits, categorized into dense metal membranes, microporousmembranes and proton conducting membranes. An overview ofthe most permeable and selective membranes is given togetherwith a collection of commercial membranes for hydrogen produc-tion. Subsequently, recent developments in membrane reactorsdesign will be highlighted and the application of membranereactors for hydrogen production from different feedstock willbe discussed.

2. Hydrogen separation membranes

Membranes are basically barriers that allow the flow of somecomponents of a feed gas mixture stream. The stream containingthe components that permeate through the membrane is calledpermeate and the stream containing the retained components iscalled retentate, as shown in Fig. 3. Membranes for hydrogenseparation should have the following characteristics:

1.

TabCom

M

M

T

H

H

T

St

P

C

High selectivity towards hydrogen.

2. High flux.3. Low cost.4. High mechanical and chemical stability.

Fig. 4. Solutiondiffusion mechanism of hydrogen permeation through a densemetal membrane (Yun and Oyama, 2011).

There have been many applications of catalytic inorganicmembrane reactors for reactions involving hydrogen, such as

le 1parison of membrane types for hydrogen separation (Kluiters, 2004; Liu et al., 201

embrane type Polymeric Microporous ceramic

aterials Polymers: polyimide, cellulose

acetate, polysulfone, etc.

Silica, alumina, zirconia, titan

metal-organic frameworks (M

emperature (1C) o100 2006002 selectivity Low 5139

2 flux

(103 mol m2

s1) at DP1 bar

Low 60300

ransport

mechanism

Solution-diffusion Molecular sieving

ability issues Swelling, compaction,

mechanical strength

Stability in H2O

oisoning issues HCl, SOx, CO

ost Low Low

hydrogenation and dehydrogenation (Dittmeyer et al., 2001;Gimeno et al., 2009), methane steam reforming (Matsumura andTong, 2008), and watergas shift (Bi et al., 2009).

2.1. Membrane materials

Regarding the type of materials, hydrogen separation mem-branes may be classified into the following categories: polymericmembranes, porous membranes, dense metal membranes andproton conducting membranes. Table 1 shows a comparisonbetween the different membrane types for hydrogen separation.The most important parameters when comparing membranes arethe perm-selectivity, the flux and the temperature range at whichthe membranes can be applied.

In order to obtain high purity hydrogen-permeate streams,dense metal membranes (mainly palladium alloys) and denseceramic membranes are currently the most suitable materialsdue to their high hydrogen selectivity. Pdalloys (mainly PdAg,PdCu and PdXAu) are used to decrease the embrittlementproblem and to decrease the poisoning of the membrane when incontact with H2S and other pollutants (such as CO). Microporousceramic membranes are promising materials for high purityhydrogen production and separation. However, these membranesseparate hydrogen by size exclusion and thus selectivity is stilllimited compared to more expensive dense inorganic membranes.Nowadays, the selective layers are thin (for instance, Pd mem-branes are now produced with thicknesses in the order of 1 mm)and therefore the cost of the selective layer is low or moderatecompared to the entire cost of the membrane.

0)

Porous carbon Dense metallic Proton conducting

dense ceramic

ia, zeolites,

OF)

Carbon Palladium alloys Perovskites (mainly

SrCeO3d, BaCeO3d)

500900 300700 600900

420 41000 4100010200 60300 680

Surface diffusion,

molecular sieving

Solution-diffusion Solution-diffusion

Brittle, oxidizing Phase transition

(causes

embrittlement)

Stability in CO2

Strong adsorbing

vapors, organics

H2S, HCl, CO H2S

Low Moderate Low

F. Gallucci et al. / Chemical Engineering Science 92 (2013) 406644

2.2. Membrane configurations

2.2.1. Geometry

Membrane geometries may be planar, tubular (that includestubes, capillaries and hollow fibers), plate and frame and spiralwound. Currently the most commonly used geometries for gasseparation are planar and tubular. The planar membranes are oftenused in earlier laboratory research and development studies, whilefor medium scale and industrial scale the tubular membranes arethe most preferred option (due to their higher surface area-to-volume ratio in comparison to planar membranes).

2.2.2. Unsupported and supported membranes

The membranes may be classified into unsupported andsupported membranes.

Unsupported membranes need to be thick self-standing films(450 mm thick) in order to have a minimum mechanical stability.The main drawback of these membranes is their low hydrogenpermeance. Moreover, in the case of using an expensive mem-brane material the cost of the whole membrane will be sharplyincreased by increasing the membrane thickness. Thus there is athreshold between membrane mechanical stability and mem-brane thickness (and thus flux and costs). For this reason it isforeseen that the first industrially available membranes will besupported membranes.

Supported membranes consist of a thin selective film depo-sited onto a support that provides mechanical stability. Thus, thehydrogen permeance of the membrane will be higher so that lessmembrane area is required and the whole membrane cost will belower than that for unsupported membranes. However, in thetotal membrane cost, the cost of the support also becomesimportant. Especially when very thin film membranes areselected, the support pore size should be much lower and thesurface much smoother, so that its cost will increase.

There are mainly two types of porous support materials:metallic and ceramic.

Ceramic supports typically have better surface quality provid-ing membranes with thinner selective layers. However, they aremore fragile. Currently, the ceramic supports that are commer-cially available are tubes and hollow fibers. As an example, Inoporproduces and offers ceramic tubular supports of different ceramicmaterials and pore sizes (http://www.inopor.com/). In general,ceramic tubular supports consist of a porous ceramic substrateprocessed by extrusion and a porous low roughness ceramic layerdeposited onto the support by, for example, solgel techniques.On the other hand, hollow fibers are usually prepared by spinning(Li, 2007b).

Metallic supports are more robust than ceramic but thecommercial ones, mainly tubular, have lower surface qualities.This is due to the fact that they are not used as membranesupports but rather as particle filters. GKN (http://www.gkn-filters.de/), Mott (http://www.mottcorp.com/), and Pall (http://www.pall.com) are known metallic support suppliers.

Some research groups are developing hollow fiber supports(both ceramics and metallic) in order to increase the surface areato volume ratio and also improve the surface quality (Luiten-Olieman et al., 2012).

In the case in which both the selective layer and the supportare metallic and the entire membrane is used at temperaturesabove the Tamman temperature of one of the metallic parts, it isnecessary to position an inter-metallic diffusion barrier layerbetween the metallic support and the metallic selective layer.

This is a common situation in dense metal composite mem-branes used for high temperature reforming reactions. In this casethe materials used as barrier layer are: ZrO2 (Tarditi et al., 2013;

Li et al., 2008a), YSZ (Sanz et al., 2011, Li et al., 2007a), TiO2(Li et al., 2008b), CeO2 (Qiao et al., 2010) and Al2O3 (Dardas et al.,2009). The typical deposition technologies used are: atmosphericplasma spraying (APS) (Sanz et al., 2011; Huang and Dittmeyer,2007), wet powder spraying (WPS) (Zhao et al., 2004) and powdersuspension suction (Chi et al., 2010).

3. Dense metal membranes

3.1. General comments

Dense metal membranes are commonly used for high purityhydrogen production. The mechanism of hydrogen permeationthrough dense metal membranes has been extensively studied. Itis well known that it generally follows a solutiondiffusionmechanism. The steps involved in hydrogen transport from ahigh to a low pressure gas region are the following (Lewis, 1967;Fig. 4): (a) diffusion of molecular hydrogen to the surface of themetal membrane, (b) reversible dissociative adsorption on themetal surface, (c) dissolution of atomic hydrogen into the bulkmetal, (d) diffusion of atomic hydrogen through the bulk metal,(e) association of hydrogen atom on the metal surface, (f) desorptionof molecular hydrogen from the surface, (g) diffusion of molecularhydrogen away from the surface.

Common dense metal layer deposition technologies includephysical vapor deposition (PVD, including magnetron sputtering,thermal evaporation or pulsed laser evaporation), chemical vapordeposition (CVD or MOCVD), electroless plating (ELP), electro-plating and diffusion welding (Iniotakis et al., 1987). Each tech-nology has its strengths and weaknesses; therefore, there is atrend of tailoring the deposition technology to the features of thesupport in order to obtain a suitable composite membrane.

The most widely used preparation technology for dense metallayers is ELP due to its ability of covering supports with complexgeometries, the simplicity of the required equipment and its lowcost (absence of electrodes or electrical source) (Mallory andHajdu, 1990).

On the other hand, PVD magnetron sputtering is a veryattractive deposition technology because it could provide thinneruniform layers (down to only few nanometers), much lower thanthe thickness achieved with the ELP technique, with a controlledmicrostructure and composition across of these coatings. Otherimportant advantage of PVD versus ELP is its environmentalfriendly operation, without producing waste liquids from chemi-cal baths (Klette, 2005).

Among dense metal materials used for hydrogen purification,developments in Pd and Pd alloys have been carried out for a longtime (Buxbaum, 1999; Grashoff et al., 1983; Holleck, 1970). Themost important problem associated with the use of pure Pdmembranes is the hydrogen embrittlement phenomenon. Opera-tion with hydrogen at a temperature below 300 1C and a pressurebelow 2 MPa, leads to the nucleation of the b-hydride phase fromthe a-phase resulting in severe lattice strains. In this case a purepalladium membrane becomes brittle after a few a2b cycles(Hsieh, 1989).

Another important problem is the palladium surface poison-ing, which can be more significant for thin metal membranes, bysulfur compounds (Edlund and Pledger, 1994), CO (Amandussonet al., 2000), H2O (Li et al., 2000), chlorine, carbon, unsaturatedhydrocarbons, etc.. In order to avoid hydrogen embrittlement,poisoning and reduce membrane cost, palladium can be alloyedwith other metallic elements such as Ag, Cu, Fe, Ni, Pt and Y(Bryden and Ying, 2002; Qiao et al., 2010; Uemiya et al., 2007) orthe palladium grains can be used in nanometer sized (PachecoTanaka et al., 2006).

http://www.inopor.com/http://www.gkn-filters.de/http://www.gkn-filters.de/http://www.mottcorp.com/http://www.pall.comhttp://www.pall.com

F. Gallucci et al. / Chemical Engineering Science 92 (2013) 4066 45

On the other hand, there is a group of metals called refractorymetals (e.g., vanadium, niobium, and tantalum) that have muchhigher permeability at these lower temperatures in comparison topalladium. These metals are much cheaper and have greatertolerances to high temperatures compared to palladium. Never-theless, producing membranes with only these metals (as purecomponents) is hindered by the high embrittlement produced atroom temperature in the presence of 6.9 barhydrogen. On thecontrary, several alloys of these metals are also non-embrittling,making them good candidates for hydrogen purification mem-branes (Dolan, 2010; Dolan et al., 2011; Ishikawa et al., 2011;Paglieri et al., 2011; Wolden et al., 2013). These metals are notnormally used alone for hydrogen permeation also because ofpoor surface properties (particularly formation of surface oxidelayers) that reduce hydrogen transport. When palladium orpalladium alloys are applied over the surface of refractory alloys(Makrides et al., 1967; Paglieri et al., 2009), forming metalmetalmatrix membranes, the surface barriers are removed and hydro-gen permeation follows the trend shown in Fig. 5.

As the number of possible combinations of these metals isenormous, a modeling approach can help in identifying the mostsuitable combination of metals for producing hydrogen mem-branes. An interesting approach towards the definition of suitablemodeling tools for this kind of study is given by the application ofdensity functional theory (DFT) calculations to predict the beha-vior of binary or ternary alloys with pure hydrogen or with gasmixtures. For example the group of Sholl published a series ofpapers showing the potentiality of DFT in predicting the solubi-lity/diffusivity of hydrogen in different alloys, resulting in inter-esting guidelines for the future membrane production. In theirworks on PdCu based ternary alloys (Semidey-Flecha et al., 2010,Kamakoti and Sholl, 2003, 2006) the authors applied DFT tofind an effective ternary additive to the PdCu alloy to retain thegood poisoning resistance of the binary alloy while increasing thetotal permeability of the resulting membrane. It was found thatamong different additives (such as Ni, Au, Ag, Rh, Pt) an additionof Au or Ag can increase the permeability of the alloy of up to5 times. They also found out that for this PdCuAg alloys, theincrease of solubility due to the increase of Ag is more dominanton the increase of permeability compared to the effect ofdiffusivity. This means that for PdCu ternary alloys an additivethat increases the solubility of hydrogen is to be preferred.

1/T, 1/K0.0005 0.0010 0.0015 0.0020 0

Perm

eabi

lity,

mol

/m s

Pa1

/2

1e+01e-31e-61e-91e-121e-151e-181e-211e-241e-271e-301e-331e-361e-391e-421e-451e-48

Fig. 5. Hydrogen permeability through several metals

Solubility of hydrogen is also important for binary alloys;Sonwane et al. (2006) have shown by using DFT calculations thatthe maximum solubility for PdAg is found at 30% Ag while forPdAu the maximum is found at 20% Au. For PdAg (30% Ag) thehydrogen solubility has been found to be 10 times higher than forpure Pd. Hao and Sholl (2011) also applied DFT to evaluate thepermeability of amorphous metal membranes (such as ZrCubased membranes). The results in this case show that thepermeability is not directly related to the solubility of hydrogen,giving guidelines also for the design of amorphous metal mem-branes. DFT calculations have also been applied to evaluate theeffect of contaminants in the gas mixture on the hydrogenpermeability (Ling and Sholl, 2009; Ozdogan and Wilcox, 2010;Gallucci et al., 2007a,b); it has been found that CO affects thepermeation of hydrogen especially because there is a preferentialadsorption of CO molecules on the same sites (hollow and bridge)used by H2; moreover CO can jump between the two sites andthis is why a small amount of CO is enough to deteriorate thehydrogen permeability (Gallucci et al., 2007a,b). Ozdogan andWilcox showed that the adsorption of H2S on the surface of themembrane depends on the metal used to alloy palladium. Inparticular they found a H2S binding tendency such as CuoPdoNb. This is why PdCu have are more resistant to H2S poisoning.Ling and Sholl (2009) showed that any sulfide formation on themembrane will decrease enormously the permeability of themembrane especially due to the extremely slow diffusion ofH through the sulfide layer.

Regarding the experimental works on membranes, some of themost relevant current developments on dense metal membranesfor hydrogen production are presented afterwards.

PdAg alloy membranes are usually prepared by electrolessplating using a sequential deposition of the two metals. Thisprocedure gives the deposition of bi-layered metal films andhence a high temperature and longer time is required for thecomplete inter-phase diffusion of two metals, in addition theprecise control of Pd/Ag ratio in the membrane is difficult toobtain.

At AIST-Japan, Pacheco Tanaka et al. (2005) developed amethod for the simultaneous plating of Pd-Ag with the desiredcomposition of metals. This was achieved by the uniform deposi-tion of nano-particles of Pd nuclei on the surface of the substrateand the careful control of the composition of the plating solution.

.0025 0.0030 0.0035 0.0040

Al Be Co (eps) Co (alfa) Cu Ge Au Fe Mo Ni Nb Pd Pt Si Ta W V Ti

as a function of temperature (Basile et al., 2008).

F. Gallucci et al. / Chemical Engineering Science 92 (2013) 406646

Alloying of Pd and Ag was possible under lower temperature andshort heat treatment. The same group at AIST also prepared thinPdAu alloy membranes by electroless plating of Au on a Pdmembrane; the amount of Au in the alloy was controlled byadjusting the concentration of gold in the plating solution(Okazaki et al., 2008a). Okazaki and Pacheco Tanaka also studiedthe effect of the substrate on the permeation of hydrogen at hightemperature, they demonstrated that at temperatures higher than650 1C alumina reacts with hydrogen and the PdAl alloy formeddecrease the permeation of hydrogen in that extent that at 850 1Cno permeation occurs. On the other hand, YSZ is more stable andcan be used until 750 1C. (Okazaki et al., 2008b, 2009). The samegroup in Japan, developed hydrogen permeable membrane ofnovel configuration (pore fill type membrane); in this configura-tion, palladium particles are filled in the nano-size pores ofceramic (g-Al2O3) layer located under the top surface (PachecoTanaka et al., 2006). This pore fill configuration providesadvantages of improved toughness on handling since palladiumlayer is not exposed on the surface. Moreover nano-size palla-dium grains confined in the nanopore substantially suppress theinternal stress associated with ab phase transition. This Pdmembrane is tolerable below the critical temperature, wherefatal damage occurs for usual. Almost constant hydrogen permea-tion and selectivity were maintained for more than 24 h at 50 1Cand 100 kPa pressure difference. At 300 1C and 4 bar of pressuredifference, the hydrogen permeance and H2/N2 ideal selectivity ofthis membrane were 1.3106 mol m2 s1 Pa1 and 41000,respectively. Due to the difference of expansion coefficientbetween Pd and alumina, nanoporous alumina filled with Pdhas low stability at temperatures higher than 350400 1C. Toovercome this problem, a membrane was prepared by fillingpalladium into the nano pores of YSZg-Al2O3. The PdYSZAl2O3 composite membrane revealed excellent thermal stabilityallowing long-term operation at elevated temperature (500 1C).This has been attributed to improved fracture toughness ofYSZAl2O3 layer and matching of thermal expansion coefficientbetween palladium and YSZ (Pacheco Tanaka et al., 2008).At 425 1C and 4 bar of pressure difference, the hydrogen per-meance and H2/N2 ideal selectivity of this membrane were1.6106 mol m2 s1 Pa1 and 300, respectively.

Various Pd alloys have been developed and tested as mem-brane by the Colorado School of Mines (CSM). Supported mem-branes have been obtained with PdAu, PdCu and PdRu alloysupported on commercial alumina and zirconia-coated porousstainless steel tubular substrates using an ELP technique (Roaet al., 2009). The most promising result has been obtained byusing an alumina supported PdAu membrane. A H2 flux of482 ml min1 cm2 (STP) and a H2/N2 ideal selectivity of 1000at a temperature of 400 1C and a partial pressure difference ofaround 7 bar. On the other hand, a 2.3 mm thick PdAu membranesupported on zirconia-coated porous stainless steel achieved a H2flux of 1.01 mol m2 s1 and a H2/N2 ideal selectivity of 82,000 at1.38 bar and 400 1C (Hatlevik et al., 2010). At these experimentalconditions no flux reduction was observed for this PdAu mem-brane for WGS mixture compared to a pure H2 feed gas. Thismeans that no CO adsorption on the membrane surface occurs atthese temperatures, which makes the membrane very attractivefor WGS reactions or pre-combustion CO2 capture plants.

A great research effort on novel membranes has been devotedin China where the Dalian Institute of Chemical Physics (DICP) isdeveloping supported composite PdAu alloy membranes onalumina substrates using ELP technique (Goldbach and Xu,2011). Very thin membranes with high fluxes have been producedby DICP. They reported a remarkable H2 permeability of1.3108 mol m m2 s1 Pa0.5 at 400 1C attained by a 5 mmPdAu composite membrane. The H2/N2 ideal selectivity was

around 1100 at 500 1C. The H2 flux achieved with a 23 mm thickPdAu composite membrane was 0.62 mol m2 s1 at 500 1C. TheH2/N2 ideal selectivity was around 1400. Moreover cycling inhydrogen between 250 and 450 1C had no significant effect onhydrogen and nitrogen permeation rates showing that mechan-ical stress caused by the differing thermal expansion of the joinedmaterials is within the tolerance of the metal/ceramic compositemembrane up to those temperatures (Shi et al., 2010).

SINTEF (Norway) has developed a two-step technique formanufacturing very thin defect free palladium-based hydrogenseparation membranes. First, a defect-free Pdalloy thin film isprepared by PVD magnetron sputtering onto a silicon wafer(surface with minimum roughness). In a second step, the film isremoved from the wafer and used as membrane. These films mayeither be used as self-supported membrane or be integrated withvarious supports of different pore sizes and geometries. Thisallows the preparation of thin (12 mm), high-flux membranessupported on macroporous substrates avoiding all the masstransfer resistances given by microporous supports or problemswith interdiffusion layers. It must be highlighted that according tothe DFT calculations of Ling and Sholl, for membranes of 1 mm, thedesorption resistance completely dominates the permeation attemperatures below 230 1C and it has an important contributionat temperatures below 450 1C. Thus, at these temperaturessmaller thicknesses will not improve the permeation flux of themembrane while will surely deteriorate the stability of themembrane.

In pure hydrogen, and at 400 1C and a H2 pressure difference of25 bar, the H2 permeance and H2/N2 ideal selectivity were1.46102 mol m2 s1 Pa0.5 and 2900. (Peters et al., 2011a).In WGS conditions (57.5% H2, 18.7% CO2, 3.8% CO, 1.2% CH4 and18.7% steam) SINTEF membranes have shown a H2 permeance of1.1103 mol m2 s1 Pa0.5 at 400 1C and 26 bar feed pres-sure. No membrane failure was detected operating the membranefor more than 1 year using WGS and H2N2 feed mixtures(Iaquaniello et al., 2011). SINTEF is also working in the develop-ment of Pd-based binary and ternary alloy membranes for H2separation (Peters et al., 2011b, 2012).

Research on self-supported membrane foils has been carriedout at Southwest Research Institute (SwRI). The institute used amagnetron sputtering technique to develop PdAu and PdAuPtfoils. SwRI tested PdAuPt (702010%) foils at 400 1C, 12.7 barwith feed gas composition of 50% H2, 30% CO2, 19% H2O, and 1%CO (Coulter et al., 2012), and a H2 flux was approximately0.212 mol/m2/s. Currently, SwRI is developing amorphousZr-based alloys, specially focusing on ZrCu based ternary alloys.The reason for the development of these alloys is that theyshowed good results (i.e., Zr30Cu60Ti10) in previous computationalcalculations predicting H2 flux through ternary alloys as functionsof operating temperature, H2 feed pressure, membrane thickness,and trans-membrane pressure drop to advance testing (CO2Handbook, 2011).

Important advances on membrane preparation have beenachieved at the center for inorganic membranes of the WorcesterPolytechnic Institute (WPI). The group directed by Prof. Ma isdeveloping Pd-based membranes on tubular stainless steel oralloy supports (such as, SS 316 L, Hastelloy and Inconel) by theELP technique. WPI achieved a H2 flux of 0.098 mol m

2 s1 and aH2/He selectivity of around 4500 at 450 1C and 1.03 bar DP with a7 mm thick Pd membrane with an Inconel support shown in Fig. 6(http://www.hydrogen.energy.gov/pdfs/progress09/ii_d_1_ma.pdf). WPI is also developing supported molten metal membranes,consisting of low-melting non-precious metals and its alloys (e.g.,Sn, In, Ga, Bi) as supported thin films (CO2 Handbook, 2011).

Moreover, WPI has also built an engineering-scale prototypemembrane with 8.8 mm thickness, 2 in. outer diameter, and 6 in.

http://www.hydrogen.energy.gov/pdfs/progress09/ii_d_1_ma.pdfhttp://www.hydrogen.energy.gov/pdfs/progress09/ii_d_1_ma.pdf

Fig. 6. Pd/Inconel hydrogen separation membranes manufactured by WPI (http://www.netl.doe.gov/publications/factsheets/project/Proj481.pdf).

Fig. 7. High temperature Pd-based composite commercial hydrogen separationmembranes manufactured by CRI/Criterion (http://www.cricatalyst.com/).

Fig. 8. Hysep 1308 hydrogen separation module manufactured by ECN.

F. Gallucci et al. / Chemical Engineering Science 92 (2013) 4066 47

length. WPI has demonstrated long-term membrane testingwith total test duration of 63 days at 450 1C, 1.03 bar DP,0.303 mol m2 s1H2 flux, 99.99% purity. This flux is equivalentto approximately 1.286 mol m2 s1 at 6.9 bar DP (Hydrogenfrom Coal Program, 2010).

Currently, WPI is performing long-term tests with Pd-basedmembranes under mixed gas streams (Guazzone et al., 2012;Augustine et al., 2012).

3.2. Progresses in commercialization of dense metal membranes

There are different companies working on the commercializa-tion of dense metal membranes for hydrogen separation/produc-tion. In the following, the main progresses achieved are presented.Some of these commercial membranes have been successfully usedin fluidized membrane reactors for methane steam reforming andin membrane separators in a pilot plant for 20 m3/h hydrogenproduction as described below.

3.2.1. CRI/Criterion

CRI/Criterion (a company owned by Shell) is in the process ofcommercializing Pd and Pdalloy membranes on sintered porousmetal supports as published in a report (Shell Impact, 2010). CRI/Criterion has produced membranes of this type as large as 2 in.OD by 48 in. L, by welding two separate 24 in. L sections. Thesupport is polished with a robotized machine before the Pddeposition. The H2 permeance of these membranes varies in arange of 4070 Nm3 m2 h1 bar0.5. Both hydrogen flux andseparation selectivity are stable at temperatures of 300500 1Cand differential pressures of 2642 bar. H2 purity of 499% hasbeen demonstrated for periods exceeding 4000 h in high tem-perature gas separations. In Fig. 7 some manufactured mem-branes are shown.

3.2.2. ECN

The Energy research Centre of the Netherlands (ECN) producesand offers a line of hydrogen separation modules (Hysep) on apre-commercial basis for evaluation purposes as described in itswebsite (http://www.hysep.com/). There are three modules avail-able: Hysep 108 (area of 0.04 m2), Hysep 308 (0.1 m2) and Hysep1308 (0.5 m2) (Fig. 8). The Hysep modules use palladium compo-site membranes composed by a 39 mm thick palladium layerdeposited by ELP onto a porous ceramic alumina support. Thenominal capacity of the largest membrane module (Fig. 5) equals3.56 Nm3 h1, based on the obtained hydrogen flux applyingreformate with 33% H2, an inlet pressure of 25 bar and H2 outletpressure of 4 bar. Lifetimes of several thousands of hours havebeen shown under different conditions and purities that can reachthe range from 99.5% to 99.995% depending on the initialcomposition. ECN is also developing novel Pd alloy compositemembranes (e.g. PdAg, PdCu; Acha et al., 2011).

3.2.3. Eltron Research, Inc.

Eltron Research, Inc. has developed alloy-based membranesand has developed a separator unit rated to produce 6.8 kg/day ofhydrogen (Hydrogen from Coal Program, 2010; CO2 Handbook,2011). Eltrons best alloy membrane has demonstrated a H2 fluxrate of 809 mL min1 cm2 (STP) at 400 1C and 6.9 bar DP withH2 pure gas feeding. Eltron has tested the membranes under WGSfeed stream conditions; tubular membranes were successfullytested for greater than 300 h with a feed gas composition of 50%H2, 29% CO2, 19% H2O, 1% CO, and 1% He.

3.2.4. Green Hydrotec

As shown on its website (www.grnhydrotec.com), GreenHydrotech has developed Pd and PdCu membranes on porousstainless steel tubes (Fig. 9) with an extremely high H2/N2 perm-selectivity (4100,000) and these membranes provide in situpurification of hydrogen in a steam reformer for high purityhydrogen (99.996%).

3.2.5. Hy9

Hy9 is commercializing HPSTM hydrogen purifiers based onplanar palladium alloy metal membranes (Fig. 10), as describedon its website (http://hy9.com/). Hy9s membrane purifiers

http://www.hysep.com/www.grnhydrotec.comhttp://hy9.com/http://www.netl.doe.gov/publications/factsheets/project/Proj481.pdfhttp://www.netl.doe.gov/publications/factsheets/project/Proj481.pdfhttp://www.cricatalyst.com/

Fig. 9. Pd-based composite membrane module manufactured by Green Hydrotech.

Fig. 10. HPS 689 hydrogen purifier composed by PdCu alloy planar membrane.

Fig. 11. Pd alloy composite tubular membrane manufactured by Pall Corporation(active surface area: 15 cm2).

F. Gallucci et al. / Chemical Engineering Science 92 (2013) 406648

tolerate a wide variety of feed stocks and gas mixtures, havingbeen successfully deployed with systems utilizing methanol,natural gas, diesel, liquefied petroleum gas, ammonia borane,and a variety of hydrides. The optimal operating temperatureconditions range from 300 to 350 1C and at inlet pressures from3.4 to 17.2 barg depending on specific requirements.

3.2.6. M&P

Media and Process Technology, Inc. (M&P) (Liu et al., 2010) hasdeveloped a Pd-based membrane module that is capable ofproducing a H2 flow of 0.575 mol m

2 s1 and H2/N2 selectivityof over 1000. The Pd membrane composite membranes consist ofa 2 mm thick Pd layer deposited onto a porous ceramic supportwith a H2 flux of 0.575 mol m

2 s1 at 1.38 bar. Currently, testsinvolving a WGS membrane reactor with this module are beingcarried out.

3.2.7. MRT

Membrane Reactor Technologies (MRT) is developing a rangeof hydrogen purifiers to provide high-purity hydrogen and torecover hydrogen from mixed gas streams, as described on itswebsite (http://www.membranereactor.com/).

MRT produces membranes either as rolled foils or as depositedthin films (815 mm).

While palladium-based foils of common compositions areavailable commercially, MRT has developed its own alloy compo-sitions for added performance and robustness. In addition, a

patent-pending bonding technique has been developed to perma-nently attach membranes to support modules with a perfect,hydrogen-tight seal.

For membranes thinner than 15 mm, MRT uses a proprietarycoating technique (Iaquaniello et al., 2011). MRT achieved a H2flux of 95 N m3 m2 h1 with a 5 mm thick Pd membranesupported on alumina-coated porous stainless steel tubular sub-strate at 550 1C and hydrogen pressure difference of 3.4 bar(Li et al., 2007c).

3.2.8. Pall Corporation

Pall Corporation produces and offers Pdalloy compositetubular membranes on a pre-commercial basis (Fig. 11), aspublished on its website (Pall.com). The support of this mem-brane is composed of YSZ inter-diffusion barrier layer depositedonto a porous stainless steel tube. Pall has a couple of techniquesto deposit its 15 mm PdAu layer. Palls current membraneflux performance and H2/Ar ideal selective are 295354 mL min1 cm2 (STP) and 10,00020,000 at a temperatureof 400 1C and feed pressure of 1.38 barg (with permeate atmo-spheric) (CO2 Handbook, 2011, Pall.com, Damle, A. 2010). Devel-opment of a Pd-alloy composition tolerant up to 100 ppm H2S iscurrently ongoing (Bredesen et al., 2011).

3.2.9. REB

REB Research and Consulting produces and offers metalmembranes made of 1.51.6 mm thick Pd-coated refractory metaltubes (3/8 in. OD 0.007 in. wall) or palladiumsilver alloy tubes(1/8 in. OD 0.003 in.), as described on its website (REBre-search.com). Palladiumsilver alloy membranes are availableplain, coated with palladium-grey, or coated with palladiumcopper alloy.

The average membrane permeabilities of VTi based mem-branes are 0.15 mol m m2 s1 Pa0.5 at 600 1C and0.015 mol m m2 s1 Pa0.5 at 300 1C. For VNb membranes,the permeabilities are 0.2 mol m m2 s1 Pa0.5 at 429 1C and0.19 mol m m2 s1 Pa0.5 at 340 1C (REBresearch.com, Buxbaum2008). On the other hand, REB is developing metalmetal matrixmembranes which consist of a high permeable H2 metal layercoated with Pdalloys on each side, with the aim of reducingmembrane cost and increasing durability. A H2 flux of0.2 mol m2 s1 was achieved with a 0.5 mm PdCu coated oneach side of a high permeable metal membrane (B2) at 400 1C and3.03 bar DP with feed gas mixture containing H2, CO, CO2, CH4and H2S. These membranes have resisted poisoning from 50 hwith 100 ppm H2S. These novel membranes will be integratedin a disc membrane reactor to be tested with coalgas(CO2 Handbook, 2011).

REB membranes have also been successfully tested in fluidizedbed membrane reactors for hydrogen production as discussed inthe following sections. The double membrane layer was able to

http://www.membranereactor.com/

F. Gallucci et al. / Chemical Engineering Science 92 (2013) 4066 49

assure a high selectivity for long time under bubbling fluidizationconditions (Roses et al.). It should also be highlighted that thesemembranes have been successfully tested at temperatures up to650 1C without deteriorating the perm-selectivity of the mem-branes which is probably the higher temperature used so fare forPd-based membranes and it is also the temperature window thatallow an effective integration of Pd membranes into methanereformers (Gallucci et al., 2008a,b; Roses et al.).

3.2.10. Tokyo Gas

Tokyo Gas Co., Ltd. has been developing membrane reformers(MRF) for hydrogen production with Mitsubishi Heavy Industries,Ltd. since 1992. Recently, Tokyo Gas developed a MRF test systemwith a hydrogen production of 40 Nm3/h, hydrogen purity of99.99%, and hydrogen production efficiency of 70% (Shirasakiet al., 2009). The reformer has 112 reactor tubes, each of whichhas two planar-type membrane modules composed of stainlesssteel supports and PdY(Gd)Ag alloy films of less than 20 mmthick (Sakamoto et al., 1992). The longest operation time for thismembrane reformer was 3000 h. An interesting evolution of themembrane modules introduced by Tokio Gas company is whatthey called membrane-on-catalyst (MOC) module. This is a Pd-based membrane prepared on the porous surface of the tubularstructured catalyst that has catalytic activity for steam reformingreaction. The important aspect to be taken into account whenpreparing MOCs is the thermal expansion of the catalyst that hasto match with the membrane material, while also selecting aproper porosity, mechanical strength and thermal conductivity ofthe catalyst (Yasuda et al., 2006). This concept simplifies enor-mously the membrane module design while also reducing themass transfer resistances otherwise affecting the packed bedmembrane modules used in their hydrogen generator.

3.2.11. UTRC

United Technologies Research Center (UTRC) is developing apalladium copper (PdCu) trimetallic alloy hydrogen separator forcentral H2 production from coal gasification-derived syngas (CO2Handbook, 2011). UTRC has tested five separators using PdCuTMalloy which showed increased surface stability in bench-scaletests (Hydrogen from Coal Program, 2010). UTRCs current mem-brane flux performance is approximately 0.230 mol m2 s1 at atemperature of 400 1C and feed pressure of 6.9 bar). This mem-brane has also shown pressure capability up to 27.6 bar, sulfurtolerance of 20 ppmv, CO tolerance and the production of at least99.5% pure H2.

Three types of commercial membranes (MRT, ECN and one froma not specified Japanese company) are being tested it a 20 m3/hhydrogen production plant at Chieti (Italy) by Technimont-KT(De Falco et al., 2011). Preliminary experimental tests haveconfirmed the potential of membrane technology with an overallconversion of approximately 57.3% was achieved at 600 1C, 26%higher than what was achieved in a conventional reformer. Thisconversion is expected to be even increased up to 62.8% bydoubling the membrane surface by a factor of two. Moreover, theeffect of operating temperature and gas mixture space velocity havebeen evaluated over a period of 1000 h without detrimental effectson the membranes performance.

4. Microporous membranes

Microporous membranes are referred to as those with a porediameter smaller than 2 nm. Regarding the structure, the micro-porous membranes for H2 separation may be classified intocrystalline (zeolites and MOF) and amorphous (such as silica,carbon, etc.).

4.1. Zeolite membranes

Zeolites are microporous crystalline aluminosilicates withuniform molecular sized pores. The unique properties of zeolitemembranes are: size and shape selective separation behaviorand thermal and chemical stabilities. Due to their crystallinity,zeolites have a well-defined pore size. The size of the channels istypically 310 A, in the range of molecular dimensions. Therefore,the hydrogen permeation through zeolite membranes relies onmolecular sieve effect and/or competitive diffusion mechanisms(Dong et al., 2000).

Regarding material and structure, the most extensively studiedzeolite membranes for hydrogen separation are mainly MFI (ZSM(Hong et al., 2005), silicate-1 (McLeary et al., 2006), LTA (NaA(Xu et al., 2000), DDR (Lin and Kanezashi, 2007) and CHA (SAPO-34(Hong et al., 2008).

The most commonly used technologies for zeolite processingare in situ hydrothermal synthesis (Coronas and Santamaria,2004), secondary (seeded) growth synthesis (Xomeritakis et al.,2000) and vapor phase transport synthesis (Chiang and Chao,2001). Other various techniques have been applied, such asconventional heating, microwave heating (Li et al., 2006), synth-esis under centrifugal field (Tiscareno-Lechuga et al., 2003),moved-synthesis (Richter et al., 2003). Often, pre-treatment ofsupports (Berg et al., 2003) and post-treatment of membranes(Yan et al., 1997) (e.g. for pore size reduction and defects removal)are carried out to improve the quality of the as-synthesizedmembranes.

Some of the most relevant current developments on zeolitemembranes for hydrogen separation are presented below.

Lai et al. (Lai and Gavalas, 2000) developed ZSM-5 zeolitemembranes by hydrothermal synthesis using a template free gelin order to avoid calcination step. The H2/N2 ideal selectivity and H2permeance obtained were 109 and 1.2107 mol m2 s1 Pa1,respectively.

Welk et al. (2004) from Sandia National Laboratories (SNL)have studied the potential applications of zeolite membranes forhydrogen separation from reforming streams. ZSM-5 membraneswere prepared by hydrothermal synthesis and the testing resultsshowed that the H2 purity was enriched from 76% to more than a98% after a single pass through this membrane.

Tsapatsis and co-workers from University of Minnesota haveprepared 1 mm thick MCM-22/silica layer onto porous homemadealumina support discs. The H2/N2 ideal selectivity and H2 per-meance were 50 and 7108 mol m2 s1 Pa1 at 200 1C,respectively (Choi and Tsapatsis, 2010). Besides, the same groupis developing membranes consisting of ex-foliated MCM-22 layersonto commercial stainless steel tubular support to their laterintegration in WGS reactors and integrated gasification combinedcycle plants (CO2 Handbook, 2011).

Recently, Zhang et al. (2012) from Nanjing University havedeveloped a MFI zeolite membrane on porous a-alumina porousdiscs. The membrane was tested for the separation of H2/CO2mixture containing 20 cm3 (STP)/min H2, 20 cm

3 (STP)/min CO2and 5 cm3 (STP)/min He under atmospheric pressure. The perme-ate side was swept by He stream with the flow rate of 30 cm3

(STP)/min. The H2/CO2 separation factor and the H2 permeancewere 42.6 and 2.82107 mol/m2/s/Pa at 500 1C.

4.2. Metalorganic framework membranes

Metalorganic frameworks are microporous crystalline hybridmaterials consisting of metal cations or cationic oxide clustersthat are linked by organic molecules. Pore size tailorabilitycombined with tunable sorption behavior provides promisingpossibilities for the applications of MOFs as membranes for gas

F. Gallucci et al. / Chemical Engineering Science 92 (2013) 406650

separation applications. It is thought that MOF can solve pro-blems that zeolites have for their industrial use, such as, long-term stability, T-cycling, regeneration and difficult housing(Gascon and Kapteijn, 2010).

MOF are mechanically less stiff and brittle and have lessenergy intensive synthesis conditions compared to zeolites (Tanand Cheetham, 2011; Shah et al., 2012).

The zeolitic imidazolate frameworks (ZIFs) form a subfamily ofMOFs that are promising candidates as gas separation membranesdue to their thermal and chemical stability in combination withtheir small pores (generally less than 5 A) (Shah et al., 2012).Other MOF materials that have been used for gas separation areMOF-type (Yoo et al., 2009; Bux et al., 2009), HKUST-type(Guerrero et al., 2010; Guo et al., 2009) and SIM-type (Aguadoet al., 2011a), among others.

The most commonly used technologies for MOF preparationare in situ growth synthesis (Liu et al., 2010; Huang et al., 2010;Guo et al., 2009), secondary (seeded) growth synthesis (Liu et al.,2009; Ranjan and Tsapatsis, 2009; Yoo et al., 2009) and liquidphase epitaxy (Shekhah et al., 2011). Besides, postsyntheticmodifications of MOF membranes have been reported, forinstance, introducing side groups to give functionality to themembranes (Huang and Caro, 2011; Aguado et al., 2011b).

Some of the most relevant current developments on MOF mem-branes for hydrogen separation are presented afterwards. Li and co-workers have prepared 2 mm thick ZIF-7 membrane deposited on anasymmetric alumina disc by secondary (seeded) growth technique.The H2 permeance and H2/N2 ideal selectivity of the membrane at220 1C and at 1 atm were 4.55108 mol m2 s1 Pa1 and 20.7,respectively (Li et al., 2010).

Recently, the same group in Hannover (Huang and Caro, 2011)has prepared a 20 mm thick ZIF-90 membrane on a-Al2O3 porousdiscs by solvothermal reaction. After synthesis the membrane hasbeen modified using ethanolamine and its H2 permeance hasslightly decreased from 2.5 to 2.1107 mol m2 s1 Pa1 andthe H2/N2 ideal selectivity has considerably increased fromaround 7 to 17.5 at 200 1C and 1 atm.

(McCarthy et al. (2010) from Texas A&M University haveprepared a ZIF-8 membrane (20 mm thick) by in situ solvothermalgrowth on a-Al2O3 support after its surface modification. The H2permeance and H2/N2 ideal selectivity of the membrane at 25 1Cand at 1 atm were 1.73107 mol m2 s1 Pa1 and 11.6,respectively.

The same group from Texas (Guerrero et al., 2010) hasprepared a 25 mm thick HKUST-1 membrane deposited on aporous alumina support disc via secondary (i.e., seeded) growthmethod, using thermal seeding in order to anchor HKUST-1 seedcrystals on the support. The H2 permeance and H2/N2 idealselectivity of the membrane at 190 1C and at 1 atm were1.1106 mol m2 s1 Pa1 and 7.3, respectively.

Permeation data about other reported MOF membranes can befound in the review prepared by Shah et al. (2012).

4.3. Silica membranes

Silica membranes are the most important representatives ofamorphous microporous membranes, because they can be moreeasily prepared as ultra- or super-microporous thin layers incomparison to other metal oxides (such as alumina, titania orzirconia) and these can be used for molecular sieving applications.

The most widely used technologies for deposition of silicalayers on porous substrates are solgel (Tsuru, 2008) and CVD(Nagano et al., 2008). Silica membranes with pore diameters lessthan 1 nm can be prepared by CVD which provides high hydrogenselectivity (molecular sieving) but, consequently, a lower perme-ability. Moreover, the CVD method requires substantial capital

investment and well defined and controlled deposition condi-tions. On the contrary, solgel derived membranes generallyachieve lower selectivities but higher permeabilities. These mem-branes are much easier to produce with the possibility ofcontrolling the pore size of the silica membranes, but theprepared membranes still suffer of low reproducibility whichmakes their industrial exploitation less attractive.

Akamatsu et al. (2008) prepared and tested silica membranesvia CVD technique. The test results showed an excellent H2permeance at 600 1C of the order of 107 mol m2 s1 Pa1,and a high H2/N2 selectivity of over 1000.

Yoshino et al. (2005) reported silica membranes prepared bysolgel technique with a permeance at 600 1C of7107 mol m2 s1 Pa1 and H2/N2 selectivity around 100.This group (Yoshino et al., 2006) also fabricated a membranemodule with a membrane area of 0.05 m2.

The main problem of microporous silica membranes, as in thecase of titania and alumina, is that they are not stable at hightemperatures, especially in the presence of steam, leading to lossof permeability. This is due to closure of pore channels bydensification which is catalyzed by humidity, particularly at hightemperatures (Lin, 2001). Furthermore, this phenomenon maycause silica film embrittlement with the subsequent loss inseparation properties.

In order to improve the stability of silica membranes, differentapproaches have been proposed in the literature. The firstapproach used is the doping of silica with inorganic oxides (e.g.,titania, zirconia and alumina) (Kanezashi and Asaeda, 2006;Sekulic et al., 2002).

Kanezashi and Asaeda(2006) prepared Ni-doped silica mem-branes with a permeance of 4.6106 m3 (STP) m2 s1 kPa1

for H2 with a H2/N2 selectivity of 400 even after being kept insteam (steam pressure: 90 kPa) at 500 1C for about 6 days. Tsuruet al. (2011) prepared Co-doped silica membranes (approx. 50 nmthick layer) with a H2 permeance of approximately 1.8107 mol m2 s1 Pa1 and a H2/N2 selectivity of 730 even after60 h of exposure to steam (steam: 300 kPa) at 500 1C. Doping ofmetals into silica membranes has been investigated in order toincrease the hydrothermal stability at high temperatures and fortheir possible application to membrane reactors, such as steamreforming of methane (Battersby et al., 2009).

Another approach to obtain hydrothermal stable silica mem-branes is to incorporate methyl groups in the silica microstruc-ture as proposed by Campaniello et al. (2004). Recently, hybrid(organicinorganic) silica membranes are widely being studiedbecause the presence of organic groups in silica networks couldimprove the hydrothermal stability of silica structures and helpcontrolling the pore size of the membrane (Duke et al., 2004).Castricum et al. (2008) prepared hybrid silica membranes derivedby co-polymerization of methyltriethoxysilane and bis(triethox-ysilyl)ethane with high hydrothermal stability. Kanezashi et al.(2009) reported a hybrid silica membrane using BTESE as pre-cursor with a high H2 permeance around 10

5 mol m2 s1 Pa1

but low H2/N2 selectivity of around 10 at 200 1C.

4.4. Carbon membranes

Carbon membranes are promising candidates for hydrogenseparation due to their high separation performance and excellentthermal and chemical resistance (Ismail and David, 2001; Korosand Mahajan, 2000). Based on the pore size, carbon membranesare generally divided in carbon molecular sieve membranes(CMS) and selective surface flow (SSF) membranes. CMS mem-branes could allow the transport of small molecules through thepores, avoiding the passage of larger molecules. Due to their smallpores, CMS membranes possess a high selectivity for separation of

F. Gallucci et al. / Chemical Engineering Science 92 (2013) 4066 51

gas mixtures containing small gas species. In this case, a precisecontrol of pore sizes near to molecular sieving size is required. SSFmembranes have pores larger than the dimensions of the mole-cules and thus the separation is based on the preferentialadsorption of some components in gas mixture followed bysurface diffusion in the carbon matrix (Seo et al., 2002). Thissection will be focused on CMS membranes due to their proper-ties for high H2 separation in comparison to SSF membranes.

CMS membranes are the result of the pyrolysis of polymericprecursors under an inert or vacuum atmosphere. The most impor-tant parameters that would influence the final properties of CSMmembranes are: (i) type of polymeric precursors such as polyimides,polyfurfuryl alcohol, phenolic resins, polyvinylidene chloride, poly-acronitrile, cellulose derivates, and polyetherimide (Saufi and Ismail,2004); (ii) pyrolysis conditions (i.e., heating rate, atmosphere andfinal temperature) (Geiszler and Koros, 1996; Centeno et al., 2004);and (iii) modifications (i.e., pre- or post-treatment such as stabiliza-tion, activation or oxidation, and CVD) (Barsema et al., 2004; Leeet al., 2008; Li et al., 2008c; Wang et al., 2002).

The most commonly used techniques for depositing CSMlayers onto supports may be dip coating (Hayashi et al., 1997),ultrasonic deposition (Shiflett and Foley, 2000), vapor deposition(Wang et al., 2000), spin coating (Tseng et al., 2009), and spraycoating (Acharya and Foley, 1999).

Some of the most relevant current developments on carbonmembranes for hydrogen separation are presented afterwards.

Hosseini and Chung (2009) from National University of Singa-pore reported carbon membranes prepared from blends of PBIand polyimides with a high H2/N2 selectivity of 460480 and H2permeability of 60180 barrer (1 barrer11010 cm3(STP)cm cm2 s1 cm Hg1). Campo et al. (2010) have developedCMS membranes from cellophane paper with very interestinghydrogen permeation properties: H2 permeability of 39.3 barrer,H2/N2 and H2/CO2 selectivities of 1310 and 58.7, respectively(Celo600 Sample). Grainger and Hagg (2007) reported CMS fromcellulose base based precursors with a H2/N2 selectivity of 740and H2 permeability of 1110 barrer at 90 1C and at a feed pressureof 6 bar. Media and Process Technology, Inc. is commercializingCSM composite membranes prepared by the pyrolysis of a PEIprecursor deposited onto a ceramic porous substrate (Abdollahiet al., 2010; Sedigh et al., 2000).

5. Proton conducting membranes

Proton conducting membranes can be classified into twogroups: dense ceramic membranes and composite ceramic metal(cermet) membranes. Both have their advantages and disadvan-tages in terms of membrane flux, perm-selectivity and stability. Inthe following section both types of membranes are discussed.

5.1. Dense ceramic membranes

Dense ceramic membranes can recover a very high purity H2stream due to a proton transport mechanism, but they have tooperate at temperatures as high as 900 1C. The hydrogen fluxthrough these membranes is proportional to the ambipolarprotonicelectronic conductivity. It is necessary to have highvalues for protonic and electronic conductivities to obtain a highhydrogen flux. The dense ceramic membranes may be classifiedinto two sub-categories: perovskite-type and non-perovskite-type membranes.

5.1.1. Perovskite-type membranes

The protons are embedded in perovskites in the electron cloudof an oxygen ion, forming hydroxide defects. They migrate by

hopping between oxide ions (Grotthuss mechanism). As for theoxygen conductors, sufficiently high temperatures are requiredfor high proton conductivity.

The general formula of perovskite-type oxides is AB1xMxO3d, where the A element is taken from the group consistingof (Ca, Sr, B); the B element is taken from the group consisting ofbe Ce, Tb, Zr, Tl; M element is taken from the group consisting ofTi, Cr, Mn, Co, Ni, Co, Al, Y, Ga; x is less than the upper limit ofsolid solution formation range (usually less than 0.2) and d is theoxygen deficiency per unit cell.

The most extensively studied high temperature perovskite-type oxides are SrCeO3 (Higuchi et al., 2004; Song et al., 2002),BaCeO3 (Guan et al., 1997; Ma et al., 1998) and BaZrO3 (Tetsuoet al., 2002). These perovskites have high protonic conductivitybut their electronic conductivity is poor. In order to improve thelatter conductivity it is common to dope these perovskites withcations, such as Y (Sammes et al. 2004), Eu (Song et al., 2003) andGd (Shima and Haile, 1998).

The common methods for perovskite power synthesis are solgel (Selvaraj et al., 1991), spray drying (Varma et al., 1994),hydrothermal synthesis (Zheng et al., 1997) and combustion ofpolymerized complexes (Liu et al., 2002). On the other hand,typical methods for perovskite film deposition onto ceramic ormetal supports are CVD (Ngamou and Bahlawane, 2009), EVD(Pal and Singhal, 1990) and PVD (Ma et al., 2008).

Some of the most relevant current developments onperovskite-type membranes for hydrogen separation are pre-sented afterwards.

Yuan et al. (2010) have prepared SrCe0.75Zr0.20Tm0.05O3dmembranes with a H2 permeation flux up to 0.042 mL min

1 cm2

at H2 partial pressure of 0.4 atm and at a temperature of 900 1C. TheZr doping can increase mechanical stability of the membrane andthe resistance to reduction.

Yazdi et al. (2009) have developed BaCe1xYxO3d films by DCmagnetron sputtering at room temperature. Ceramatec (Paglieriand Way, 2002, CO2 Handbook, 2011) is producing a prototypemembrane for hydrogen separation from coal-derived syngasusing perovskite-type membranes. The pevoskite used is a mix-ture of barium cerate (BaCeO3) and ceria, where the former isproton conductor and the latter is electron conductor. The pre-liminary tests showed a perfect H2 separation (100%) from H2/CO2mixture.

5.1.2. Non-perovskite-type membranes

The non-perovskite-type membranes used for hydrogenseparation are mainly doped rare earth metal oxides andfluorite-structured metal oxides. There are lots of different dopedrare earth metal oxides that may be interesting for hydrogenseparation, such as, Tb2O3, Ln2Ti2O7 and Er2Ti2O7.

Haugsrud and Norby (2006) prepared a 10 mm thick film ofacceptor-doped LaNbO4 (i.e. La0.99Ca0.01NbO4) that has a H2 flux of0.1 mL (STP) cm2 min1 at a temperature of 1000 1C and adifference in pressure of 10 bar. This group also reported thatLn6WO12 membranes have sufficient mixed electronproton con-ductivity at intermediate temperatures (Haugsrud, 2007).

Escolastico et al. (2011) have developed Nd5LaWO12 mem-branes and the results showed that the addition of La to Nd6WO12increases the hydrogen flux from 0.03 to 0.05 mL min1 cm2 at1000 1C and difference pressure of 0.5 bar.

Among fluorite-structured metal oxides CeO2, YSZ and Y2O3are the most used.

Nigara et al. (2003) have reported hydrogen permeation throughdoped CeO2 and YSZ (Nigara et al., 2004) membranes, but unfortu-nately H2 permeabilities of these membranes are very low. Serraet al. (2005) studied H2 permeation through commercial alumina

F. Gallucci et al. / Chemical Engineering Science 92 (2013) 406652

tubes obtaining as a result a H2 flux of 4108 mol s1 m2 at1400 1C.

5.2. Cermet membranes

Cermet membranes consist of a combination of a ceramicphase and a metallic phase. The former is a pure proton con-ductor, while the latter is a highly electron conductor. Combiningthese two phases together may provide high H2 permeationbecause both proton and electron conductivities become high,resulting in high hydrogen permeation.

Due to the hydrogen transport by both metals and oxides,mainly three different combinations with respect to functionalproperties of both phases can be realized: (1) a metal with lowhydrogen conductivity in combination with a highly protonconductive oxide, (ii) a metal or an alloy with high hydrogenpermeability (i.e., Pd, Pd/Ag, Pd/Cu, Nb, Ta, V) combined with aceramic of low hydrogen permeability and (3) a combinationwhere both the metallic and ceramic phases conduct hydrogen.

Some of the most relevant current developments on cermetmembranes for hydrogen separation are presented in the following.Balachandran et al. (2006) from Argonne National Laboratory (ANL)developed Pd/YSZ composite membranes and the highest H2 fluxwas 20.0 cm3 (STP) min1 cm2 for a 22 mm thick membraneat 900 1C using 100% H2 as feed gas. Recently, Zhu et al. (2011) havedeveloped a 30 mm thick NiBZCY (Ba(Zr0.1Ce0.7Y0.2)O3d) cermetlayer onto ceramic support that exhibited a maximum H2 flux of2.4107 mol cm2 s1 at 900 1C using 80% H2/N2 (with 3% ofH2O) as feed gas and dry high purity argon as sweep gas.

Park et al. (2011) have developed a 0.5 mm thick Ta/YSZ mem-brane and the highest H2 flux obtained was 1.2 ml min

1 cm2 at300 1C using 100% H2 as the feed gas and Ar as the sweep gas.

Jeon et al. (2011a) have developed a 0.5 mm cermet mem-brane composed of Pd embedded in proton conducting ceramicmatrix (CaZr0.9Y0.1O3d) and its H2 flux was around 2.3 cm

3

(STP) min1 cm2 at 900 1C. This group also has developedPdGDC (Ce0.8Gd0.2O2d) cermet membranes for high sulfurresistance and no sharp drop in hydrogen permeation flux wasobserved using feed gases with 220 ppm H2S (Jeon et al., 2011b).

The best performances of membranes investigated in thevarious research organizations are summarized in Table 2.Besides, membranes on the way to commercialization commen-ted before are shown in Table 3. It is shown that relatively highfluxes with good perm-selectivities can be now achieved withhydrogen membranes which make this technology closer to thereal market.

6. Advances in membrane reactors for hydrogen production

The application of membrane reactors for dehydrogenationreactions has been first proposed to the scientific community byProf. Gryaznov in the late 60s (see for example Gryaznov et al.,1970). Removing hydrogen through a thick membrane resulted ina shift of the equilibrium reaction towards the product of interest.Membrane reactors in dehydrogenation reactions were a scien-tific curiosity until around 1996 with few papers published peryear. With the increasing interest into hydrogen as a possibleclean energy carrier, the scientific attention towards membranereactors as high efficient hydrogen production systems sharplyincreased in the last years (Fig. 12). Probably the echo producedby the two books The Hydrogen Economy (Rifkin, 2002;National Academy of Engineering, 2004) was also reflected inthe great increase of number of papers in the last 5 years.However, our research will focus on the progresses on membranereactors in the last 10 years. Accordingly with the increase of

number of papers, the patents awarded on hydrogen productionin membrane reactors also increased with time and in the last yearsthis number is increasing rapidly. In fact, most of the patents havebeen awarded in the last 10 years (Gallucci et al., 2009).

Different types of membrane reactors for hydrogen productionhave been proposed in the literature. Most of the previous workhas been performed in packed bed membrane reactors (PBMR);however, there is an increasing interest in novel configurationssuch as fluidized bed membrane reactors (FBMR) and micro-membrane reactors (MMR) especially because better heat man-agement and decreased mass transfer limitations can be obtainedin these reactor configurations. In the following, these reactorconfigurations, along with membrane bio-reactors (MBR) andcatalytic membrane reactors (CMR) for hydrogen production willbe discussed in detail.

6.1. Packed bed membrane reactors

The packed bed membrane reactor configuration is the firstand most studied configuration for hydrogen production inmembrane reactors. This is because, the first studies on mem-brane reactors focussed on the effect of the hydrogen permeationthrough membranes on the reaction system. Thus it was straight-forward to compare two packed bed reactors (avoiding thecomplication of complex fluid dynamics such as in fluidizedbed) in one of which a membrane was used.

In the following table the main investigators working withPBMR for hydrogen production are summarized (source Scopus):

The different authors showed that the concept of Pd-basedmembrane reactor can be used to carry out and intensify differentdehydrogenation reactions such as reforming of methane anddifferent alcohols. In particular, packed bed membrane reactorshave been used for producing hydrogen via reforming of methane(Gallucci et al., 2006; Matsumura and Tong, 2008), reforming ofalcohols (Kikuchi et al., 2008; Tosti et al., 2009), autothermalreforming (Simakov and Sheintuch, 2009), partial oxidation ofmethane (Tan and Li, 2009), etc. The results of these studies areinteresting to show that indeed the Pd membranes are notpoisoned by the different alcohols and the products of thereactions such as CO2 or higher hydrocarbons. The only poisoningcan be due to the presence of H2S as said above or CO (if lowtemperature o300 1C is used).

In a packed bed membrane reactor the catalyst is confined infixed bed configuration and in contact with a perm-selectivemembrane. The most used packed bed configuration is thetubular one where the catalyst may be packed either in themembrane tube (Fig. 13a) or in the shell side (Fig. 13b), while thepermeation stream is collected in the other side of the membrane(in case of hydrogen selective membranes) or one reactant is feedon the other side of the membrane (in case of oxygen selectivemembrane Jin et al., 2000).

For multi-tubular membrane reactor configurations the cata-lyst in tube configuration can be preferred especially for con-struction reasons and for the extent of bed-to-wall mass and heattransfer limitations which can be very detrimental when thecatalyst is positioned in shell configuration.

Often, a sweep gas can be used in the permeation side of themembrane in order to keep the permeation hydrogen partialpressure as low as possible for minimizing the membrane arearequired for the hydrogen separation. This practice is for examplevery useful if hydrogen for ammonia plant is being produced, inwhich case an amount of nitrogen can be used for sweeping thepermeation side producing a synthesis stream (N2/H2 1/3)ready for the final reaction step. If a sweep gas is used in thepermeation side then a packed bed membrane reactor can beused in both co-current and counter current modes. Using a

Table 2Permeation data of different hydrogen separation membranes reported in the literature.

Institute Membrane material Preparation method Selective

layer

thickness

(mm)

T (1C) PermeanceH2 [10

8

(mol/m2 s Pa)]a

Permeability

H2 [1013

(mol m/m2 s Pa)]

Ideal

selectivity

Sulfur

tolerance

(ppm)

Durability

(h)

AIST (Pacheco Tanaka et al., 2006) Pd/Al2O3 Pore filling 5 300 170 41,000 (H2/N2) AIST (Pacheco Tanaka et al., 2008) Pd/YSZ Pore filling 5 425 210 300 (H2/N2)

CSM (Hatlevik et al., 2010) PdAu/YSZ/PSS ELP 2.3 400 710 160 82,000 (H2/N2)

DICP (Goldbach and Xu, 2011) PdAu/Al2O3 ELP 23 500 620 160 1400 (H2/N2)

SINTEF (Peters et al., 2011a)b PdAg/PSS PVD-MS 2.8 400 1,500 420 2900 (H2/N2) 2000

SwRI (Coulter et al., 2012)b PdAuPt PVD-MS 25 400 54 130

WPI (Ma, 2009) Pd/Inconel ELP 7 450 96 67 4,500 (H2/He) 2,200CALTECH (Li et al., 2000) ZSM-5/Al2O3 Hydrothermal synthesis 67 150 12 7.8 109 (H2/N2)

SNL (Welk et al., 2004) ZSM-5/Al2O3 Hydrothermal synthesis 10 25 68 68 16.6 (H2/CO2, CO,CH4)

Nanjing University (Zhang et al., 2012) MFI/Al2O3 Hydrothermal synthesis 500 28 42.6 (H2/CO2)

Liebniz University Hannover (Li et al., 2010) ZIF-7/Al2O3 Secondary (seeded) growth 2 220 4.5 0.9 20.7 (H2/N2)

Liebniz University Hannover (Huang and

Caro, 2011)

ZIF-90/Al2O3 In situ growth (modified after synthesis

with ethanoldiamine)

20 200 21 42 17.5 (H2/N2)

Texas A&M University (McCarthy et al.,

2010)

ZIF-8/Al2O3 In situ growth (support surface modified) 20 25 17 42.5 11.6 (H2/N2)

Texas A&M University (Guerrero et al.,

2010)

HKUST-1/Al2O3 Secondary (seeded) growth 25 190 110 275 7.3 (H2/N2)

University of Minnesota (Choi and

Tsapatsis, 2010)

(MCM-22)-SiO2/Al2O3 Layer-by-layer deposition 1 200 7 0.7 50 (H2/N2)

The University of Tokyo (Akamatsu et al.,

2008)

SiO2/Al2O3 CVD 600 1014 4 1,000 (H2/N2) Total

Noritake Company (Yoshino et al., 2005) SiO2/Al2O3 Sol-gel 0.02 600 70 0.14 100 (H2/N2)

Hiroshima University (Kanezashi and

Asaeda, 2006)

NiSiO2/Al2O3 Solgel 0.3 500 20 0.6 400 (H2/N2) 144

Hiroshima University (Tsuru et al., 2011) CoSiO2 Solgel 0.05 500 18 0.09 730 (H2/N2) 60

Hiroshima University (Kanezashi et al.,

2009)

Hybrid silica (BTESE as

precursor)

Solgel 0.5 200 1,000 50 10 (H2/N2)

National University of Singapore (Hosseini

and Chung, 2009)

Carbon (from PBI and

polyimide blends)

Pyrolisis 35 0.20.27 460480 (H2/N2)

University of Porto (Campo et al., 2010) Carbon (from cellophane) Pyrolisis 9.22 29.5 0.14 0.13 1,310 (H2/N2)

NTNU (Grainger and Hagg, 2007) Carbon (from cellulose-

based precursors)

Pyrolisis 90 3.7 740 (H2/N2) 350

South China University of Technology (Yuan

et al., 2010)

SrCe0.75Zr0.20Tm0.05O3-d Liquid citrate method 900 0.64

University of Oslo (Haugsrud and Norby,

2006)

La0.99Ca0.01NbO4 Pressing 10 1000 0.22 0.22

ITQ- UPV (Escolastico et al., 2011) Nd5LaWO12 Solgel 900

1000

0.63

ANL (Balachandran et al., 2006) PdYSZ Pressing 22 900 150 330 University of Science and Technology of

China (Zhu et al., 2011)

Ni-BZCY/ceramic Pressing 30 900 2.9 8.7

Korea Institute of Energy Research (Park

et al., 2011)

TaYSZ Pressing 500 300 8.9 450

Chonnam National University (Jeon et al.,

2011a)

PdCZY Pressing 500 900 7.8 390

Chonnam National University (Jeon et al.,

2011b)

PdGDC Pressing 282 900 19 540 220

a Permeance values have been calculated for a H2 partial pressure of 1 bar.b Self-supported membranes.

F.G

allu

cciet

al.

/C

hem

ical

En

gin

eering

Science

92

(20

13

)4

0

66

53

Table 3Major investigators on packed bed membrane reactors

Investigator

name

Institution Number

of papers

Basile A. Institute on Membrane Technology (Italy) 35

Tosti S. ENEA (Italy) 29

Rahimpour,

M.R

Sharaz University (Iran) 27

Itoh N. Utsunomiya University (Japan) 12

Lombardo

E.A.

Instituto de Investigaciones en Catalisis y

Petroqumica (Argentina)

11

Nomura N. University of Tokyo (Japan) 7

Fig. 12. Number of papers on hydrogen production in membrane reactors peryear. Database Scopus (www.scopus.com). Keywords membrane reactor and

hydrogen production11 December 2012

Fig. 13. Membrane reactor catalyst in tube (A) and catalyst in shell(B) configurations.

F. Gallucci et al. / Chemical Engineering Science 92 (2013) 406654

counter-current mode leads to completely different partial pres-sure profiles in reaction and permeation sides with respect to theco-current mode (independently on the reaction system consid-ered) (Gallucci et al., 2008c).

Although the tube in tube configuration is quite useful to workin lab scale and for proof of principle of membrane reactors, forindustrial scale some other configurations need to be used inorder to increase the membrane area per volume of vessel used.In fact, the amount of hydrogen produced is directly related to theamount of membrane area installed in the reactor. Starting for thetube in tube configuration, a straightforward way to increase themembrane area in packed bed is the tube in shell configuration(Buxbaum, 2002; Tosti et al., 2008). An example of multi-tubemembrane housing has been patented by Buxbaum (2002) andreported in Fig. 14. In this case the catalyst is loaded in the shellside of the reactor while the membrane tubes are connected to acollector for the pure hydrogen. In particular, in the figure thepossibility to use a catalyst in a separate chamber is shown. Incase of reforming reactions, this chamber acts as a pre-reformingzone where the greatest temperature profiles are concentrated. Inthis way the membranes will work at an almost constanttemperature.

The second way to increase the membrane area per volume ofreactor is adopting the hollow fiber configuration. For example, incase of perovskite membranes the membrane flux is generallyquite low and the hollow fiber configuration is quite interesting.The main investigators of hollow fiber membrane reactors aresummarized in Table 4.

These studies confirmed that the membrane preparation proce-dure can be also intensified to produce hollow fiber membraneswith similar selectivities of the planar or tubular membranes.

Kleinert et al. (2006) studied for example POM in a hollowfiber membrane reactor. The perovskite membranes used by theauthors were produced from Ba(Co,Fe,Zr)O3d (BCFZ) powder viaphase inversion spinning technique. A tube in tube configurationhas been used while the catalyst was packed in the shell side ofthe reactor.

In their paper the authors show that the membrane was ableto give quite interesting results with a methane conversion of 82%and a CO selectivity of 83%. Moreover the membrane was quitestable under the reactive conditions investigated. Finally, thecombination of steam reforming and POM was studied by feedingsteam along with methane in order to suppress the carbonformation. Even in these conditions the membrane reactorshowed good stability.

The membrane area required for the separation can be reducedby incr