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REVIEWS Hydrogen Membrane Separation Techniques Sushil Adhikari and Sandun Fernando* Department of Agricultural and Biological Engineering, Mississippi State UniVersity, Mississippi State, Mississippi 39762
About 80% of the present world energy demand comes from fossil fuels. Unlike using fossil fuels, using hydrogen as an energy source produces water as the only byproduct. Use of hydrogen as an energy source could help to address issues related to energy security including global climate change and local air pollution. Moreover, hydrogen is abundantly available in the universe and possesses the highest energy content per unit of weight compared to any of the known fuels. Consequently, demand for hydrogen energy and production has been growing in the recent years. Membrane separation process is an attractive alternative compared to mature technologies such as pressure swing adsorption and cryogenic distillation. This paper reports different types of membranes used for hydrogen separation from hydrogen-rich mixtures. The study has found that much of the current research has been focused on nonpolymeric materials such as metal, molecular sieving carbon, zeolites, and ceramics. High purity of hydrogen is obtainable through dense metallic membranes and especially palladium and its alloys, which are highly selective to hydrogen. Thin membranes would not only reduce the cost of materials but also increase the hydrogen flux. Metal alloys or composite metal membranes have been used for hydrogen purification. However, metallic membranes are sensitive to some gases such as carbon monoxide and hydrogen sulfide. Therefore, ceramic membranes, inert to poisonous gases, are desirable. Inorganic microporous membranes offer many advantages over thin-film palladium membranes. More importantly, in microporous membranes, the flux is directly proportional to the pressure, whereas in palladium membranes, it is proportional to the square root of the pressure. The paper also discusses the advantages and disadvantages of different hydrogen separation membranes. Also, the paper reports performance of selected membranes in terms of hydrogen selectivity and permeability. 1. Introduction Use of hydrogen as an energy source could address issues related to global climate change, energy security, and local air pollution. Demand for hydrogen has grown continuously in recent years, which has motivated research into improving methods of hydrogen production, separation, and purification.1-3 There are many byproducts associated with the production of hydrogen, especially when thermochemical means are used.4 Therefore, separation of hydrogen from other gases is an important step in the hydrogen production process.5 Hydrogen can be purified through several techniques, such as pressure swing adsorption (PSA), cryogenic distillation, or membrane separation. PSA and cryogenic distillation processes are commercially available separation techniques. However, they are energy intensive.6 Membrane-related processes are considered to be one of the most promising technologies for the production of high-purity hydrogen.7 It can provide an attractive alternative to PSA and cryogenic distillation, depending upon the purity and scale of production.8 Furthermore, membrane separation processes consume less energy with the possibility of continuous operation.9 Membrane separation techniques are important because catalytic reforming could be impregnated into the membrane so that the reversible reforming reaction shifts to the right, increasing hydrogen conversion. During the 1990s and * Corresponding author. Tel.: (662) 325-3282. Fax: (662) 325-3853. E-mail:
[email protected].
also at present, much research has been directed to identify improved hydrogen selective membranes.10 The main objective of this study is to critically review membrane separation techniques that are used for hydrogen separation from a mixture of gases. Moreover, it presents the performance of the membranes as reported elsewhere. This paper also presents some of the membranes that have shown significantly higher permeance and selectivity. The paper is organized as follows: Section 2 presents a brief discussion on hydrogen separation techniques, Section 3 describes membrane construction techniques, and Section 4 discusses hydrogen separation membranes, followed by the conclusion (Section 5) of the study. 2. Hydrogen Separation Process In this section, three major hydrogen separation processes, namely PSA, cryogenic distillation, and membrane separation techniques, are discussed briefly for introductory purposes. The PSA process, by far, is the most extensively used industrial process to separate H2 from a mixture of gases. The PSA units are based on the capacity of adsorbents to adsorb more impurities at high gas partial pressure than at low gas partial pressure. These systems are most commonly used in the chemical and petrochemical industries. Extensive studies have been done in this area since the 1970s.6 Although the process is operated in batches, continuous flow systems can be achieved by using multiple adsorbers. The process is mainly divided into five steps: (i) adsorption, (ii) cocurrent depressurization, (iii) countercurrent depressurization, (iv) purge, and (v) countercur-
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rent pressurization. More details of the PSA process can be found elsewhere.6 Two major advantages of the PSA process are its ability to bring undesirable impurities down to a low level and to produce high-purity, up to 99.99%, hydrogen.11 The amount of hydrogen recovered is dependent on inlet pressure, purge gas pressure, level of impurities, and hydrogen concentration. Although temperature swing adsorption (TSA) could be used to remove the adsorbed impurities in the mixture of gases, it has the disadvantage that the number of cycles obtained in any given time is limited by slow heating and cooling steps.12 Because of this reason, TSA is limited to the removal of small quantities of impurities as compared to that of PSA. The cryogenic distillation process continues to be another widely used method of separating fluid mixtures. However, it consumes a considerable amount of energy.13 The cryogenic process is a low-temperature separation process which uses the difference in boiling temperatures of the feed components to effect the separation.14 Hydrogen has a high relative volatility compared to hydrocarbons. However, if the feed contains significant amounts of carbon monoxide (CO) and carbon dioxide (CO2), a methane wash column is required to reduce the levels of these gases. Higher hydrogen recovery at moderate hydrogen purities (95% or less) is possible with a cryogenic system; however, very high hydrogen purity is not practical.15 Other than PSA and cryogenic distillation, membrane separation techniques have attracted the widest interest. Membranes are barriers which, because of their physical nature, allow only selected materials to permeate across them. Some advantages of the membrane separation process over mature and commercially available technologies such as PSA and cryogenic distillation are as follows: ease of operation, low investment cost, low energy consumption, and cost effectiveness even at low gas volumes.16 Broadly, gas separation can be attributed to four mechanisms: (i) Knudsen diffusion, (ii) molecular sieving, (iii) solution-diffusion, and (iv) surface diffusion.8,17 Separations based on Knudsen diffusion occur when the pore diameter of the effective barrier layer is smaller than the mean free path for the gas being separated. Separations based on molecular sieving operate on a size-exclusion principle. Surface diffusion can occur in parallel with Knudsen diffusion. Gas molecules are adsorbed on the pore walls of the membrane and migrate along the surface. Having the pore size of 2.89 A° would be the best for hydrogen diffusion. The solution-diffusion separations are based on both solubility and mobility of one species in a solid effective barrier.17 Gas transport mechanism through most metals can be divided into two types: (i) solution-diffusion for dense and metallic membranes such as Pd based membranes and (ii) Knudsen diffusion or combined mechanism of Knudsen diffusion and surface diffusion for porous metal membranes such as porous stainless steel.9 Hydrogen transport through Pd includes the following: (i) dissociatively adsorbing H2 onto the metal surface, (ii) diffusing atomic H through bulk metal, and (iii) associatively desorbing H2 from the metal surface. Similarly, hydrogen transport through ceramic membranes is based on solution-diffusion (dense ceramic) and molecular sieving (microporous), whereas in the case of polymer membranes, gas transport is through the solution-diffusion mechanism. The performance of a membrane can be discussed in terms of flux (or permeance) and selectivity. The flux is the total transport of material through the membrane and can be expressed as mass or mole per unit time per unit area. Similarly, permeance is defined as the flux per unit pressure difference between upstream (retentate) and downstream (permeate) sides. Hydrogen flux can
be straightforwardly derived into an expression known as Sievert’s law
N H2 )
n n - PH,perm ) FH2(PH,feed
l
(1)
where NH2 is the hydrogen flux; FH2 is the hydrogen permeability; l is the membrane thickness; and PnH,feed and PnH,perm are the hydrogen partial pressures in the retentate and permeate sides, respectively. It is noted that the Sievert’s law strictly pertains to metallic membranes. Permeability is a fundamental property of the material and is independent of the membrane thickness.17 Furthermore, permeability can be expressed as the product of the diffusion coefficient and the solubility constant and is temperature dependent. The permeability can be represented by the Arrhenius equation
FH2 ) K exp
(-E RT )
(2)
where K is the preexponential factor constant, E is the activation energy, R is the gas constant, and T is the absolute temperature. As can be seen from eq 1, membrane flux is inversely proportional to the thickness of the membrane. However, reduction in the thickness would reduce mechanical stability of the membranes. The exponential dependence of hydrogen flux on the pressure is represented with the exponent “n” and sometimes deviates from the ideal value of 0.5.18 Similarly, the ability of a membrane to separate gases is characterized by the selectivity of the membrane.19 Ideal selectivity (or separation factor) is defined as the ratio of the permeability of the penetrants of interest. The selectivity of hydrogen over nitrogen can be expressed as
RH2/N2 )
PH 2 PN2
(3)
where RH2/N2 is the ideal selectivity of the hydrogen to the nitrogen gas and PN2 is the nitrogen permeability. 3. Membrane Construction Techniques Several methods are available for the formation of hydrogen separation membranes, such as phase inversion, rolling, electroless-plating, chemical vapor deposition (CVD), sputtering, spray pyrolysis, coatings, metal deposition, sol-gel, and so on. A general description of fabrication methods for inorganic membranes can be found in review articles and books.17,20,21 Phase inversion is the most frequently used method for commercially available polymer membrane preparation. In this method, a polymer in a liquid state is transformed into a solid state, creating a membrane.8 This can be used to prepare both porous and nonporous membranes. The rolling method is a mature technique that has been used to produce self-supported pure and alloyed palladium membranes.22 Thickness of the membrane is dependent on the alloy used, but the membranes could be built in thicknesses on the order of microns.23 Primarily, three methods (electroless-plating, CVD, and sputtering) have been developed to coat thin metal films on porous metallic or ceramic supports.24,25 Electroless-plating is a method of metal plating by autocatalytic chemical reduction of the corresponding metal ions with simultaneous oxidation of a reducing agent. This technique can be applied for forming metal coatings even on nonconductive supports such as porous ceramic or glass. Electroless-plating became the most popular technique for the
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preparation of supported Pd based membranes, because Pd has sufficient autocatalytic activity.21 In CVD, a volatile component of coating materials is thermally decomposed on the surface of the heated substrate to form a thin film or coating. In sputtering, a thin continuous film is deposited on the porous substrate by the bombardment with high-energy particles on the support.24 Sputtering is especially useful when ternary or quaternary alloys are desired.23 Spray pyrolysis is a process in which a slurry solution containing metals to be deposited on a heated substrate is thermally decomposed to form a film. This method can be used to prepare silica and carbon molecular sieves from silicone rubbers and thermosetting polymers, respectively.8 Coating methods are especially important for dense polymeric and inorganic composite membranes. A number of coating procedures in use are dip coating, plasma polymerization, interfacial polymerization, and in situ polymerization.8 In the metal deposition process, the solid material to be deposited is first evaporated in a vacuum system using physical techniques. Thin to medium thickness film is subsequently condensed and deposited on the cooler substrate.22 The sol-gel process is able to produce pore sizes in the range of nanometers for ceramic membranes. In this process, an alkoxide precursor is formed into a gel via either the colloidal suspension route or the polymeric gel route.8 According to Kluiters,8 the most difficult step is drying the gel and can result in the formation of crack. Sintering is the final step in the process. 4. Types of Hydrogen Separation Membranes Broadly, hydrogen selection membranes (based on the materials used) can be categorized into four types: (i) polymer (organic), (ii) metallic, (iii) carbon, and (iv) ceramic.8 The latter three are called inorganic membranes. Inorganic membranes can be classified into two groups from the viewpoint of the raw material-metal membranes and ceramic membranes.21 Also, it could be divided into porous (meso- and microporous) and nonporous (dense) membranes.9 Microporous inorganic membranes are those porous membranes which have a pore diameter 0.1 mm for hydrogen separation became unattractive because of their high cost, low permeance, and low chemical stability. Consequently, dense inorganic membranes are generally prepared as thin films on porous support, which provides mechanical support but offers minimum mass resistance.25 4.1. Polymer Membranes. Gas separation from polymer membranes is already an existing technology.26 It is used industrially for hydrogen separation from gaseous mixtures that consist of nitrogen, CO, or hydrocarbons.19 Polymer membranes are the dense type and can be further divided into glassy and rubbery polymeric membranes. The former have higher selectivity and lower flux, whereas the latter have higher flux but lower selectivity.8 According to Kluiters,8 operating temperatures for polymer membranes are ∼373 K. Good ability to cope with high pressure drops and low cost are key advantages of polymer membranes. However, limited mechanical strength, relatively high sensitivity to swelling and compaction, and susceptibility to certain chemicals such as hydrochloric acid (HCl), sulfur
Table 1. Hydrogen Permeabilities and Selectivities of the Selected Polymer Membranes selectivity
polymer
H2 permeabilitya, Barrer
H2/N2
H2/CH4
H2/CO2
polysulfone polystyrene polymethyl methacrylate polyvinylidene fluoride
12.1 23.8 2.4 2.4
15.1 39.7 2.0 3.4
30.3 29.8 4.0 1.8
2.0 2.3 4.0 2.0
a Note: 1 Barrer ) 10-10 cm3(STP)cm/(cm2 s cmHg) where STP ) standard temperature and pressure.26
oxides (SOx), and CO2 make polymeric membranes less attractive. Polymer membranes used for separation processes operate according to the solution-diffusion mechanism.19 Indepth study on polymer membranes can be found elsewhere.17 Table 1 gives hydrogen permeabilities (at 300 K temperature and 30 psi feed gas pressure) for the selected polymer membranes and the selectivities over nitrogen (N2), methane (CH4), and CO2. The polystyrene shows the best combination of hydrogen permeability and selectivities over N2, CH4, and CO2.27 4.2. Inorganic Membranes. Much of the research has been focused on nonpolymeric materials19 such as metal, molecular sieving carbon, zeolites, and ceramics. High-purity hydrogen could be available through dense metallic membranes and especially through Pd and its alloys.23 Pd alloy membranes can be used to produce hydrogen for practical purposes with a purity of up to 99.99%.9 Metallic membranes for hydrogen separation could be of many types, such as (i) pure metals: Pd, V, Ta, Nb, and Ti; (ii) binary alloys of Pd: Pd-Cu, Pd-Ag, Pd-Y, and also Pd alloyed with Ni, Au, Ce, and Fe; (iii) complex alloys: Pd alloyed with 3-5 other metals; (iv) amorphous alloys: typically Group IV and Group V metals; and (v) coated metals: Pd over Ta, V, etc.28 Figure 1 presents hydrogen permeability for selected metals based on data presented elsewhere.9,29 Body centered cubic (BCC) metals such as Nb and V have higher permeability than face centered cubic (FCC) metals such as Pd and Ni.9 Hydrogen permeability decreases with increasing temperature in the case of Nb, V, and Ta. This phenomenon is due to the decrease of hydrogen solubility more rapidly than the increase of the diffusion coefficient.29 Although Nb, Ta, and V have higher permeability on the order of 1015× greater than that of Pd, these metals form oxide layers and are difficult to use as hydrogen separation membranes.9,24
Figure 1. Hydrogen permeability as a function of temperature for the selected metals.
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Table 2. Selected Hydrogen Separation Metallic Membranes and Their Performance method
support
permeance (10-6 mol m-2 s-1 Pa-1)
selectivity
CVD (Rh) CVD (Ir) CVD (Pd) electroless (Pd) electroless (Pd) electroless (Pd-Cu) electrodeposition (Pd-Ni) electrodeposition (Pd-Cu) microfabrication (Pd-Ag)
R- alumina R- alumina γ- alumina titania/ceramic alumina and stainless steel R-alumina/zirconia s. steel Ni-porous stainless steel silicon wafer
1.58 at 773 K 1.81at 773 K 0.1-0.2 at 573 K 6.3 at 773 K 0.22 at 623 K 0.023 at 723 K 6.7 at 723 K 8.4 at 723 K ≈ 45 at 723 K
H2/N2 ) 80 H2/N2 ) 93 H2/He ) 200-300 H2/N2 ) 1140 H2/N2 ) 110 H2/N2 ) 1 150 H2/N2 ) 3 000 H2/N2 > 10 000 H2/N2 ) 4 000
However, this problem could be overcome by coating a thin layer of Pd on the surface of the aforementioned metals.9 Moreover, Nb, Ta, and V metals are also cheaper than Pd,30 and they could meet the cost target of the system. Because of the presence of a two-phase region below the critical temperature, Pd membranes are not suitable for use at low temperatures (1 000 6-80 stability in CO2 H2S proton-conducting ceramics solution-diffusion
surface diffusion, molecular sieving
10-3 mol m-2 s-1 at pressure diff. ) 1 bar.
Nanosil) prepared by Prabhu and Oyama48 exhibited significantly higher hydrogen selectivity with respect to CH4. There are two types of carbon membranes based on the transport mechanisms: molecular sieving and surface diffusion membranes. Molecular sieving membranes are seen as promising, both in terms of separation properties as well as reasonable flux and stabilities, but are not yet commercially available at a sufficiently large scale.8 Carbon molecular sieving (CMS) membranes can be prepared in two ways: (i) unsupported CMS membranes such as flat membranes, capillary tubes, or hollow fibers, and (ii) supported membranes on a macroporous material.56 According to Wang and Hong,56 the former might suffer from the problem of brittleness, and the latter is relatively difficult to prepare. Carbon membranes can be used in nonoxidizing environments with temperatures in the range of 773-1173 K. Carbon membranes are brittle and can be difficult to package if the membrane surfaces become large.8 Furthermore, carbon membranes are still expensive. The performance of the membranes will deteriorate severely if feed streams contain organic traces or other strongly adsorbing vapors such as H2S, NH3, or chlorofluorocarbons (CFCs). The performance of the CMS membranes is dependent on pyrolysis temperature.57 The gas permeance decreased with an increase in temperature because membranes became denser and their pore size decreased.58 Wang and Hong59 have reported that the H2/N2 selectivity of carbon molecular sieve membranes is ∼100 with extremely high permeance around 2 × 10-6 mol m-2 s-1 Pa-1 at 423 K. Table 4 illustrates the selected properties of the hydrogen separation membranes. The major properties listed are operating temperature range, hydrogen selectivity, hydrogenflux, stability, and poisoning issues. Details can be found elsewhere.8 Although there might be some variations in the data for each type of membrane as seen in Tables 2 and 3, Table 4 presents the general characteristics of different types of membranes. On the basis of the membrane’s operating temperature, polymer membranes are mainly suitable for temperatures 823 K because of the tin present at the interface of Pd and the support.21 The dense-metallic and ceramic membranes are highly selective for H2 permeation. 5. Conclusion Different types of hydrogen separation membranes have been discussed. Currently, inorganic membranes have attracted wider interest than organic membranes. Inorganic membranes can tolerate harsher conditions than organic membranes. Pd and its alloys are the major metallic membranes studied. Since Pd is an expensive material, ceramic membranes are becoming more attractive. Membranes produced from microfabrication techniques performed best for permeance, whereas electrodeposition showed higher H2 selectivity over N2 in metallic membranes. Similarly, sol-gel provides good hydrogen permeability and selectivity in the case of ceramic membranes. Furthermore, modified Vycor prepared from CVD exhibited significantly higher hydrogen selectivity with respect to CH4. This paper attempts to compare the performance of the selected hydrogen separation membranes. However, comparing multiple reports to identify the best membranes is challenging, since there are variations in the parameters used like temperature, pressure, and membrane thickness. In the case of metallic membranes, a challenge is that the nth order term in Seivert’s law varies between 0.5 and 1 as membrane thickness decreases. Originally, studies utilized 10-100 micron thick membranes which were consistently 1/2-order dependent. In the case of metallic membranes, more recent studies are focused on membrane thicknesses ranging from 0.2 to 10 microns, which deviates from Sievert’s law’s 1/2-order dependence. There have been some variations in how different studies report their data and compare it with 1/2-order dependent data. Literature Cited (1) Rosen, M. A.; Scott, D. S. Comparative efficiency assessment for a range of hydrogen production processes. Int. J. Hydrogen Energy 1998, 23, 631-640.
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ReceiVed for reView June 2, 2005 ReVised manuscript receiVed August 22, 2005 Accepted November 30, 2005 IE050644L
