High-Temperature Selective Membranes for Hydrogen Separation E. Gobina', R. Hughes'*, D. Monaghan' and D. Arnel12 'Department of Chemical Engineering 'Department of Aeronautical and Mechanical Engineering University of Salford, Salfonl M5 4W UK

Palladium-silver alloy films (5-8 p a ) were deposited onto porous 'Vycor' glass tubes by the process of magnetron sputtering. The films were characterised by electron probe microanalysis (EPMA), scanning electron microscopy (SEM), relative light transmission and electrical resistivity respectively. A membrane reactor capable of operating at high temperatures (573-773K) was used to study the permeability characteristics of the membranes in relation to hydrogen gas from a 60% H2:40% N2 feed gas mixture. The membranes were found to be 100% selective for hydrogen with adequate permeation flu. A limiting purge rate of 500 cm-1 (STP)lmin was found to be sufficient to double the hydrogen permeation rate for a total feedrate of 350 cm3(STP)lmin.

Introduction In principle, an equilibrium shift of a reversible chemical reaction can be achieved by in-situ separation of the product species in a membrane reactor. There is currently a renewed interest in the application of inorganic membranes for the separation, concentration and purification of chemical species, particularly hydrogen, from a gaseous mixture.

Ceramics such as alumina and silica with pore sizes of about 40 x have been produced commercially, and they are being employed to effect equilibrium shift [la]. Microporous membranes with this pore size are governed by Knudsen diffusion mechanisms and therefore have low separation selectivities. A typical example is the H2:CO pair, having an ideal separation factor of only 3.74 based on Knudsen diffusivities. Such a separation factor is not large enough produce a significant industrial separation.

However, palladium and its alloys are known to be highly selective and permeable to hydrogen. Itoh [5 ] obtained enhanced conversion of cyclohexane to benzene from an equilibrium value of 14% (473 K and 1 am) to almost 99% by the selective removal of hydrogen from the product stream using a 25 pm palladium membrane. Commercially available palladium membranes are either in the form of foils (25 p) or thick-walled tubes (53 p). The hydrogen flux through a membrane is inversely proportional to the thickness of the film, and therefore these membranes give low hydrogen fluxes due to their relatively large thickness. Alternatively, mechanical strength depends directly on the membrane thickness. A thick film can withstand high pressure differentials while a thin film will rupture under these circumstances.

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Author for correspondence.

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E. Gobina, R. Hughes, D. Monaghan and D. Arne11

Therefore, it is necessary to make a compromise between mechanical strength and enhanced flux. This requires a mechanically stable porous substrate (support) onto which a thin metal film can be deposited to form a stable, strong composite capable of operation at high temperatures and pressures. Adhesion of the film to the porous support is also important, therefore the support must possess excellent surface integrity.

The process of magnetron sputtering can be used to deposit very thin fims on almost any porous substrate. It involves ion bombardment of the target, thus ejecting atoms which then adhere onto the porous substrate. By exposing the substrate onto a radio frequency (RF) glow discharge (plasma), the adhesion strength of the film can be enhanced.

In this investigation, a study of the preparation and characterization of thin Pd-Ag (7723 wt%) films on a porous 'Vycor' glass support is presented. The membranes were tested for the permeability of hydrogen using a 60% H,:40% N2 gas mixture at various temperatures and pressure differentials.

Experimental Details Porous 'Vycor' glass tubes (code 7930; Coming Glass Inc., USA) 10 mm 0.d x 7.8 mmoi.d. x 200 mm long were used as the substrate. These had an average pore size of 40 A. Prior to sputtering, the tubes were carefully cleaned with acidified distilled water and then activated in an oven for about 2 hours at 5"C/min to 450°C with a flow of dry nitrogen gas. After cooling to room temperature, the tubes were recovered and stored in a dessicator until the commencement of the sputtering runs. Table 1 shows the deposition parameters for the magnetron sputtering process. In the deposition runs, samples of porous glass were used for the characterization of the process parameters including estimation of the film thickness and adhesion strength.

Table 1. Pd-Ag alloy deposition parameters for the magnetron sputtering process.

System Pressure: 0.67 Pa

Substrate Temperature: < 300°C

Substratelkget distance: 70 mm RF Frequency: 13.56 M H z

RF Power to Substrate

- during deposition 10 w 110 w

RF power to target 150 W

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High.Temperatwe Selective Membranes for Hydrogen Separation

Figure I . Schematic diagram of experimental set-up. (1. Mass flow controller: 2. Fine control valve; 3. Flow meter; 4. Membrane reactor; 5. Needle valve; 6. Cooling bath: 7. Dryer).

Pd-Ag thin fib

Pmge gaa h y c i r o g e n L

Figure 2 . Principle of membrane reactor operation.

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E. Gobina, R. Hughes, D. M o ~ g h a n and D. Arne11

A schematic diagram of the experimental flow system used to test the permeability of the prepared membranes is shown in Figure 1. It consisted of a feed flow system, a membrane reactor, and data acquisition equipment. The membrane reactor had a shell-and-tube configuration with the feed gas mixture entering via the shell side. A pressure differential maintained across the membrane resulted in the selective removal of hydrogen and permeation into the tube space; details are given in Figure 2.

Results and Discussion (a) Characterization of Film Film thickness (and hence depositon rates) were estimated using additional samples of porous 'Vycor' glass in the deposition chamber placed adjacent to the actual substrate of interest. Direct measurement of the film thickness on these samples was carried out using scanning electron microscopy (SEM). A typical SEM of one such sample is shown in Figure 3. The membrane thickness in this investigation was 6 pm. Film thickness uniformity was measured at various locations along and across the samples by electrical resistivity. The variation along the membrane averaged 6.7%. Film stoichiometry was determined by the electron probe microanalysis technique (EPMA). A total of 50 analyses was carried out over a randomly selected area, and an average value obtained. There was very little difference between the Pd:Ag content in the target (77:23 wt%), and that on the substrate (23Swt8 Ag). There was no light transmission across the membranes indicating a dense structure. Over an extended period of use in the membrane reactor, there was no physical peeling from the substrate. This is indicative of the good adherence strength of the film.

Figure 3. Scanning electron microscopy of Pd-Agfilm on porous 'Vycor' glass.

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High-Temperature Selective Membrane for Hydrogen Separation

(b) Permeability The specific characteristics of the membrane were studied for a 60% H2:40% N2 gas mixture, varying the driving force and the temperature in order to obtain quantitative information regarding the permeability of the membranes. If the permeate rate of hydrogen gas through the PdAg membrane (G; cmVs) is assumed to obey the half- power pressure law [61, then:

where Qo is the permeability constant of hydrogen gas through the membrane (cm3- cm thickness/cm2.s.atm0); A is the membrane area available for flow (cm,); and d, is the film thickness of the membrane (cm). Figure 4 shows a plot of QH VSPP and e’)at various operating temperatures. The permeation rate of hydrogen gas is directly proportional to the difference in the square roots of the upstream and downstream hydrogen partial pressures. This is consistent with Seven’s Law [7], and indicates that permeation through the bulk of the metal is the rate limiting step. A closer inspection of Figure 4 also reveals that there is a temperature effect on the permeability constant. This was further investigated by maintaining a constant

I

Driving farce (plom5 - atm

Figure 4. Effect of pressure on the permeation rate of H2.

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pressure difference while varying the temperature. The results are presented in Figure 5 correspond to the Arrhenius law. The permeability constant (Qo) of hydrogen gas through the film can then be expressed as follows:

Qo = 7.174 x 10'5 exp (-6.380/RT) (2)

where R is the ideal gas constant (kJ/mol.K) and T the absolute temperature (K). The activation energy of 6.38 kJ/mol obtained in this investigation compares well with reported values of 5.73 kJ/mol [8], 6.60 kJ/mol [9], and 5.86 kJ/mol [lo]. In these previous investigations, similar Pd-Ag alloy membranes were employed.

Permeability constants for a membrane thickness of 6 pm were available [l 11 to be compared with those obtained in this investigation. Table 2 compares the values of the permeability constant at two operating temperatures showing good agreement with the literature values. Equation (2) was also used to predict permeabilities at

10.9 1.4 1.6 1 e

Temperature (K'l x lov3)

Figure 5. Effect of temperature on the permeability constant.

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High-Temperature Selective Membranes for Hyctogm Separaiwn

conditions other than those investigated in this study. For example at 780K (407OC), a permeability constant of 2.34 x 10-5 cm3cm/cm2.s.atmo~5 was calculated for this work. A calculated value of 1.30 x 10-5 cm3-cm/cm2-s.am0~5 and an observed value [ll] of 1.51 x cm3-cm/cm2-s.atm~~~ show reasonable agreement considering the variety of techniques employed and the range of experimental conditions.

It is of interest to evaluate the potential hydrogen recovery rate from a feedstream in industrial practice. Using Equations (1) and (2) it is possible to estimate this recovery rate for various hydrogen upstream partial pressures. For example, with a pressure drop of 7 bar a recovery rate of 90% of the hydrogen present can be attained in the present system. Such a recovery rate is high enough to fonn the basis of an economic membrane separation unit.

(c) Effect of Purge Flowrate From Figure 4, an increase in the partial pressure difference (while maintaining all other factors constant) produces an increase in the permeation rate. From Equation (1). by reducing P2 an increase in the driving force can be achieved. In the present investigation this was achieved by using a purge gas on the permeate side. This reduces the partial pressure of hydrogen in the tubeside, and hence increases the driving force. Figure 6 shows the effect of purge flowrate on the permeation rate of hydrogen at 639K. and an upstream hydrogen partial pressure of 2.2 am. There is a two-fold increase in the permeation rate at the limiting purge flowrate of 500 cm3 /min. This has potential application in dehydrogenation reactions where the selective separation of the hydrogen gas from the product stream has the desired effect of shifting the equilibrium towards increased product yields. However as shown in Figure 7, under these conditions there is no change in the permeability constant.

Table 2. Typical data for membrane permeability constant.

Permeability Constant (cm3-cm/cm2.s.atm Oa5)

Resent Investigation Govind et al. [ll] (observed) (calculated) (observed) (639K) (639K) 643K)

2.16 10-5 1.15 10-5 1.36 x 10-5

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Figure 6. Effect of purge jlowrate on the hydrogen permeation rate.

Conclusions Near stoichiometric thin films (5-8 pm) of palladium/silver alloy (77:23 wt%) have been produced on tubular porous 'Vycor' glass by the process of magnetron sputtering. Quantitative and qualitative information has been obtained regarding the feasibility of using these composites in a membrane reactor at high temperatures for the selective removal of hydrogen from a gas mixture. Using Equations (1) and (Z), we estimate a recovery rate of 90% of essentially pure hydrogen at a pressure drop of 7 bar in our system. The principles investigated can be applied to a membrane reactor for in-situ reaction and separation.

Acknowledgements The authors wish to thank the SERC for financial support for this research. We are also grateful to Johnson Matthey Research Centre, Sonning Common, UK, for the EPMA analysis.

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High-Temperawe Selective Membranes for Hy&ogen Separation

3 Purge flowrate (cm (STP)/min)

Figure 7 . Effect of purge jlowrate on the permeability constant.

References 1.

2.

3.

4.

5. 6.

7.

8.

Sun. Y.M. and Khang. S.J. 1988. Catalytic membranes for simultaneous chemical reaction and separation applied to dehydrogenation reaction. I d . Eng. Chem. Rer., 27(7), 1136- 1142. Shinji, O., Misono, M. and Yoneda, Y. 1982. The dehydrogenation of cyclohexane by the use of a porous-glass reactor. Bull. Chem. Soc. Japan., 55(9), 2760-2764. Champagnie, A.M.. Tsotsis, T.T., Minet R.G., and Webster, I.A. 1990. A high temperature membrane reactor for ethane dehydrogenation. Chem. f i g . Sci., 45(8). 2423- 2429. Keizer. K., and Burggraaf, A.J. 1988. Porous ceramic materials in membrane applications. Sci. Cer.. 14.83-93. Itoh, N. 1987. A membrane reactor using palladium. AIChE J., 33(9), 1576-1578. Bohmholdt, G., and Wicke, E. 1967. Diffusion von H and D in Pd und Pd-Legierungen. 2.

Sieverts. A.. and Kumbhaar. W. 1910. Solubility of gases in metals and alloys. Ber. Deut. Chem. Ges.. 43,893-900. Yoshida. H.. Konishi. S.. and Naruse. Y. 1983. Effects of impurities on hydrogen permeability through palladium alloy membranes at comparatively high pressures and temperatures. J. Less Common Metals, 89,429436.

Phy~ik Chem. N.F., 56(3-4), 133-154.

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9. Ackerman, F.J.. and Koskinas. G.J. 1972. Permeation of hydrogen and deuterium through palladium-silver alloys. J . C h m . Eng. Data, 17(1), 51-55.

10. Chabot, J.. Lecomte. J.. Grumet, C. and Smier, J. 1988. Fuel clean-up system: Poisoning of palladium-silver membranes by gaseous impurities. F u b n Technology, 14,614-618.

11. Govind, R. and Atnoor, D. 1991. Development of a composite palladium membrane for selective hydrogen separation at high temperature. I d . Eng. C h . Res.. 30(3), 591-594.

Received 3 March 1993; Accepted after revision: 27 August 1993.

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