Selective oxidation of ethane to ethylene in a dense tubular membrane reactor

Journal of Membrane Science 209 (2002) 457–467

Selective oxidation of ethane to ethylene in a dense tubular membrane reactor F.T. Akin, Y.S. Lin∗ Department of Chemical Engineering, University of Cincinnati, Cincinnati, OH 45221-0171, USA Received 18 December 2001; received in revised form 29 May 2002; accepted 10 June 2002

Abstract Selective oxidation of ethane (SOE) to ethylene was studied in a dense tubular ceramic membrane reactor made of oxygen ion conducting fluorite structured Bi1.5 Y0.3 Sm0.2 O3 (BYS) at temperatures 825–875 ◦ C. A dead-end tube and shell configuration was used where ethane was fed inside the tube and air was fed into the shell side of the membrane reactor system. At 875 ◦ C, per pass ethylene yield of 56% with ethylene selectivity of 80% was obtained in the membrane reactor. Under reaction conditions, the oxygen permeation flux through the dense membrane is over an order of magnitude higher than those under oxygen permeation conditions with He as the purge. Oxygen permeation mechanism switches from p- to n-type electronic conduction under SOE reaction conditions. The BYS membrane after 2 days of SOE experiments remained in good integrity with some impurity phases formed on the membrane exposed to ethane. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Ethane oxidation; Membrane reactor; Bismuth oxide; Ionic conductor

1. Introduction The chemical industry depends heavily on ethylene as a chemical feedstock. Hence, there is a very strong incentive to study methodologies for the conversion of ethane to ethylene. Partial oxidation of ethane is one route for its direct conversion to ethylene. The reaction proceeds through a bimodal pathway that consists of mostly sequential steps where ethyl radicals form first [1]. The ethyl radicals may subsequently experience further dehydrogenation to ethylene or be oxidized to carbon oxides. Compounds such as transition metal oxides, alkali metal oxides, rare earth oxides and perovskite-type mixed oxides have been ∗ Corresponding author. Tel.: +1-513-556-2761; fax: +1-513-556-3473. E-mail address: [email protected] (Y.S. Lin).

studied as catalysts for selective oxidation of ethane (SOE) to ethylene [1–5]. Unfortunately, ethane reaction with oxygen, over all known catalysts, results in the thermodynamically favored formation of carbon oxides, thereby decreasing the selectivity at a given conversion. In order to increase selectivity at a given conversion in reactions with parallel sequential network, such as (SOE), controlling the contact mode of reactants is necessary. Dense ceramic membrane reactors offer beneficial contact medium conditions for partial oxidation reactions such as SOE. Fig. 1 illustrates the principle of a dense catalytic membrane reactor: on the reaction side, ethane is oxidized by surface O2− and the surface oxygen is depleted; bulk O2− diffuse from oxygen rich side to fill in the oxygen vacancies. On the oxygen rich side, gaseous O2 is first reduced to O2− , which diffuses towards the reaction side. The driving force is the

0376-7388/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 6 - 7 3 8 8 ( 0 2 ) 0 0 3 6 3 - 0

458

F.T. Akin, Y.S. Lin / Journal of Membrane Science 209 (2002) 457–467

Fig. 1. Principle of a dense membrane reactor for ethane partial oxidation to ethylene.

oxygen partial pressure gradient across the membrane. The dense ceramic membrane inhibits the direct contact between the reactant and oxygen, and introduces oxygen not in the gaseous but in atomic form to the reaction chamber. This type of operation allows complete control over the contact mode of reactants with each other, and with the catalytically active surface. Membrane reactor operations are advantageous for various reasons: they offer in situ air separation; there is the possibility of increasing yield and selectivity across the thermodynamic limitations; they are energy efficient and relatively safe to operate; and they may avoid formation of hot spots as encountered in the co-feed reactor.

Table 1 summarizes the results of ethane selective oxidation to ethylene on some catalysts. The operating conditions of these materials tested all differ; comparison is based on the best results obtained in these systems. High conversion and selectivity were obtained on perovskite-type chloro-oxide SrFeO3−δ Clx [15] and YBa2 Cu3 O7−0.21 F0.16 [16] by the same research group. A study using a porous membrane reactor conducted by Tonkovich et al. [11] shows the benefit of distributed oxygen feed on the reaction. The membrane used in their study is an inert porous ␣-alumina membrane packed with MgO doped with Li and Sm2 O3 . Though this reactor configuration system does not change the reaction mechanism, it could improve the selectivity beyond the thermodynamic limitations by selectively adding one of the reactants. Dense ceramic membrane reactors have been studied for oxidative coupling of methane (OCM) and partial oxidation of methane to syngas extensively [6–9]. It is well recognized that the dense ceramic membrane reactors are candidate reactor configurations for partial oxidation reactions. SOE is another representative partial oxidation reaction but there has been only few study reported on SOE to ethylene in a dense catalytically active membrane reactor [18]. The objective of this paper is to study the feasibility of a dense membrane reactor configuration for SOE to ethylene with improved selectivity and yield. For this purpose, we used a dense catalytically active ceramic membrane reactor made of Bi1.5 Y0.3 Sm0.2 O3 (BYS), which was previously studied for OCM. The present study will also allow a comparison of the two important partial oxidative reactions (OCM and SOE) on the same membrane reactor.

Table 1 Comparison of catalysts studied for SOE Catalyst

Reactor configuration

Temperature (◦ C)

Ethane conversion (%)

Ethylene selectivity (%)

Pt-impregnated alumina [10] MgO/LiO/Sm2 O3 [11] Li-MgO [1] Ba-Ce-O [12] B2 O3 -impregnated alumina [13] Mo-V-Al-Ti oxide [14] SrFeO3−δ Clx [15] BaCl2 /Y2 O3 [16] YBa2 Cu3 O7−0.21 F0.16 [17]

Tube shell porous membrane reactor Tube shell porous membrane reactor Packed bed reactor Packed bed reactor Packed bed reactor Packed bed reactor Packed bed reactor Packed bed reactor Packed bed reactor

600 600 600 750 550 380 680 640 680

46 95 55 60 19 20 90 72 84

96 53 57 59 95 54 70 73 82

F.T. Akin, Y.S. Lin / Journal of Membrane Science 209 (2002) 457–467

2. Experimental 2.1. Membrane preparation and characterization The BYS powder was prepared using the citrate method. In this method, stoichiometric amounts of Bi(NO3 )3 ·5H2 O (98%, Fischer), Y(NO3 )3 ·6H2 O (99.5%, Alfa) and Sm(NO3 )3 ·6H2 O (99.5%, Alfa) were first fully dissolved in a 10 vol.% nitric acid solution, followed by addition of citric acid. The obtained solution was heated while stirring to 100 ◦ C. After the system was isothermally refluxed and stirred for about 3 h, water was evaporated until a gel-like material was formed. This gel-like substance was then self-ignited at 400 ◦ C and calcined at 600 ◦ C for 5 h. The obtained powder was then ground in an agate mortar before used for the processing of dead-end tubular membranes. Dead-end tubular BYS membranes were prepared by cold isostatic pressing (CIP) with green machining method. The BYS powder was first pressed into cylindrical rods, 3–5 cm high and 6–8 mm in diameter. Carbide bits of 4–4.2 mm diameter was used to drill the rods into dead-end geometry. Dead-end tubes

459

were then sintered at 1050 ◦ C for 20 h and annealed at 850 and 600 ◦ C for 2 h. Sample characterization was performed by XRD (Philips Analytical Xpert, Cu K␣) with a slit size of 1 mm for X-ray beam, on the inner and outer surfaces of a membrane piece taken from the middle section of a BYS membrane after used in the membrane reactor for SOE reaction for 2 days. The samples were measured with a 2θ scan from 20 to 70◦ with steps of 0.05. 2.2. Membrane reactor set-up and ethane oxidation experiments The experiments were carried out in a laboratory membrane reactor. The dead-end BYS membrane tube was sealed onto the mullite tube (13 mm o.d., 5 mm i.d., Coors Ceramics Co.) in which a smaller dense alumina tube (3.3 mm o.d., Alfa) was coaxially placed. The whole system was sited in a quartz tube (15 mm i.d., Custom Glassblowing of Louisville), which served as the shell side in the reactor operation as shown in Fig. 2. The 4 mm step in the mullite tube serves as a holder for both the seal material and the membrane. The sealing material consisted of

Fig. 2. Schematic diagram showing the dead-end tubular membrane.

460

F.T. Akin, Y.S. Lin / Journal of Membrane Science 209 (2002) 457–467

Fig. 3. Schematic diagram of membrane reactor system.

45 wt.% BYS, 25 wt.% SrCe0.95 Tb0.05 O3 , 10 wt.% Pyrex glass, 10 wt.% NaAlO2 and 10 wt.% B2 O3 . After the seal was dried in room atmosphere for 2 h, the support tube with the membrane was installed into the permeation/reaction system as shown in Fig. 3. The membrane reactor was surrounded by a clam-shell-type furnace allowing a maximum temperature of 1000 ◦ C (Fig. 3). The sealant remained intact approximately 2 days under experimental conditions. In the membrane mode, nitrogen/oxygen mixture and ethane/helium mixtures were respectively introduced to both sides of the membrane, as illustrated in Fig. 2. Ethane oxidation experiments in the BYS membrane reactor were conducted at different temperatures (825–875 ◦ C), C2 H6 /He feed flow rates (50–100 cm3 /min), and C2 H6 partial pressure in the tube side (0.05–0.15 atm). Air was fed to the shell side of the membrane reactor system in all the experiments. Table 2 summarizes the operating conditions of SOE in the BYS dense membrane reactor and physical dimensions of the membrane used in this study. Fixed bed experiments were carried out in a packed bed reactor made of dense alumina tube (Coors Ceramic) of 0.63 cm i.d., 56 cm long. The BYS pellets of 1–3 mm in sizes with irregular shapes were

made from crashing a sintered BYS membrane. A 1.0 g BYS pellets were packed in the reactor supported by quartz beads of 0.5 mm average size from both sides. Fixed bed experiments were carried out at 900 ◦ C, with C2 H6 /O2 ratio of 0.5. The PC2 H6 was kept at 0.1 atm and the balance of the feed stream was done by He. The total flow rate was kept at 90 ml/min. The effluents from the reactors were intermittently sampled by a six-port rotary valve (Valco Instruments), and analyzed by a GC (Perkin-Elmer 8500, 80/100 Carbosphere Column) that could detect oxygen with Table 2 BYS membrane reactor dimension and SOE reaction conditions Parameters

Value

BYS membrane inner diameter (mm) BYS membrane outer diameter (mm) BYS inner tube length (mm) BYS membrane inner surface area (cm2 ) Membrane surface to reaction volume ratio ( cm−1 ) Reaction temperature (◦ C) Downstream C2 H6 /He feed flow rate (cm3 /min) Upstream air feed flow rate (cm3 /min) PC2 H6 in feed (methane side) (atm) PO2 in feed (oxygen side) (atm)

3.9 6.3 35 3.8 25 825–875 50–100 100 0.05–0.15 0.21

F.T. Akin, Y.S. Lin / Journal of Membrane Science 209 (2002) 457–467

461

a concentration as low as 200 ppm. Oxygen concentrations in both the shell and tube side chambers were also monitored by a yttria stabilized zirconia oxygen sensor (6000 Oxygen Analyzer, from Illinois Instruments). Gas leakage through the sealant, if occurred, could be detected by monitoring nitrogen concentration in the tube side. Ethane oxidation reactions were performed in the membrane reactor with the feeds of O2 /N2 and C2 H6 /He mixtures. The ethane conversion was calculated by: 2YC H C =1−
2 6 ni Yi

(1)

where ni is the number of carbon atoms in the molecule of the carbon containing product i and Y is molar fractions in the effluent of the reaction chamber. The selectivity for the carbon containing product i is the percentage of reacted ethane that forms product i and was calculated by: Si =


ni Yi ni Yi − 2YC2 H6

(2)

The ethylene yield is the percentage of total ethane that forms ethylene, which is calculated from the product of ethane conversion and C2 H4 selectivity. In both the fixed bed and the membrane reactor mode experiments, the difference of carbon mass balance between the inlet and the outlet was found to be less than 5%.

3.1. Effect of flow rate and temperature

Fig. 4. Ethane conversion and ethylene yield in the BYS membrane reactor at different tube side flow rates. Conditions: T = 825–875 ◦ C, PO2 = 0.21 atm, F shell = 100 ml/min, F tube = 50–100 ml/min, PC2 H6 = 0.1 atm.

Figs. 4 and 5 show the effect of tube side flow rate on SOE to ethylene at temperatures 825–875 ◦ C. The ethane conversion is highest at higher temperatures (875 ◦ C) and at lower flow rates (50 ml/min). Conversion increases as the flow rate of ethane decreases due to the higher contact time of ethane with the catalytically active membrane surface. Ethylene selectivity increases with increasing flow rates and becomes higher at higher temperatures. At higher flow rates possibly intermediate specie, ethylene, does not have enough time to get further oxidized to CO2 leading to higher ethylene selectivity. Also shown in Fig. 4a, the conversion increases with temperature. Oxygen flux

through BYS tubular membrane increases with temperature [19]. Therefore, at higher temperatures, since the reoxidation of vacant oxygen sites in the BYS lattice would be faster, there is more oxygen available for the reaction, resulting in a higher ethane conversion. However, the CO2 selectivity is greater at lower temperatures indicating a certain dependence of reaction pathways on temperature (see below). As the flow rate increases, the CO2 selectivity too decreases, as shown in Fig. 5b. In all the experiments, CH4 selectivity was less than 5% and smaller (∼1%) at lower temperatures.

3. Results and discussion

462

F.T. Akin, Y.S. Lin / Journal of Membrane Science 209 (2002) 457–467

Fig. 5. CO2 and C2 H4 selectivity in the BYS membrane reactor at different tube side flow rates. Conditions: T = 825–875 ◦ C, PO2 = 0.21 atm, F shell = 100 ml/min, F tube = 50–100 ml/min, PC2 H6 = 0.1 atm.

At the highest residence time (lowest flow rate, i.e. 50 ml/min), ethylene selectivity goes through a maximum at 850 ◦ C. Ethylene selectivity is high at lower ethane conversion and decreases as the conversion increases, which is consistent with the trend seen with a parallel series reaction network. It is interesting to see an increase in ethylene selectivity with temperature. Depending on the temperature, the reaction network may involve two parallel reaction pathways [20] whose relative rates determine selectivity. Both paths include the formation of ethyl radicals (C2 H5 • ), however, one path leads to the formation of ethoxide and its oxidation, which is favored at lower temperatures

[19]. It has been demonstrated using various catalysts that selectivity for ethylene increases with temperature in ethane oxidation [21], suggesting that rate constant of ethane oxidation is comparable to rate constant of ethylene oxidation and at lower temperatures ethane oxidation rate is lower than that of ethylene oxidation. The same trend is observed in the dense BYS tubular membrane reactor for ethane oxidation as shown in Fig. 5a. As the temperature decreases, the ethylene selectivity decreases due to the formation of CO2 , suggesting an increase in the oxidation rate of ethylene. Partial oxidation of methane to ethane and ethylene was studied in the same dense BYS tubular membrane reactor [22]. The C2 (C2 H6 + C2 H4 ) selectivity in methane partial oxidation to ethane and ethylene decreases when the temperature is increased, which is different from what is observed in this study with ethane oxidation. However, the reaction rates for oxidation of ethane and ethylene are higher than that of methane [23]. Therefore, increase in temperature results in lower C2 selectivity in the methane oxidation reaction, an opposite trend of C2 selectivity–temperature dependence compared to SOE. The ethylene yield increases with increasing temperature as shown in Fig. 4b because of increased ethane conversion and ethylene selectivity. At 875 ◦ C, when the flow rate is 50 ml/min, per pass ethylene yield is 56% and the ethylene selectivity is 80%. At shorter residence times, yield decreases slowly, and at 825 ◦ C within the flow rates examined the ethylene yield stays almost constant. At temperatures above 670 ◦ C, thermal pyrolysis of ethane, C2 H6 (C2 H4 + H2 ), is thermodynamically favored. However, ethane conversion is determined by the reaction kinetics and reactor conditions. Since we have studied SOE at such high temperatures (>825 ◦ C), ethane conversion through the thermal pyrolysis route was of concern. Based on the available kinetic data (reaction order and rate constant) [24] and flug flow reactor model, we have estimated that the thermal pyrolysis of ethane at 850 ◦ C with residence time of 0.07–0.1 s (conditions for the present membrane reactor) would result in less than 0.2% ethane conversion. We have obtained much higher ethane conversions indicating that the thermal pyrolysis route is not significant in our experiments.

F.T. Akin, Y.S. Lin / Journal of Membrane Science 209 (2002) 457–467

463

3.2. Effect of ethane partial pressure Fig. 6a shows the ethylene formation rate as a function of PC2 H6 at temperatures 825–875 ◦ C. The ethylene formation rate increases with PC2 H6 and the increase in the ethylene formation rate is steeper at higher temperatures. Although there are only three data points, the formation rate of ethylene was regressed according to RC2 H4 = k[PC2 H6 ]n , where k and n are the regression constants. Since the regressions were done using three data points, the quantitative apparent reaction rate orders may not be reliable. However, they are useful in the qualitative assessment of the effect of ethane partial pressure on the reaction. The apparent reaction rate of ethylene formation is first-order dependent on PC2 H6 at 875 ◦ C, while the apparent ethylene formation reaction rate order is 1.4 and 1.5 with respect to PC2 H6 at 850 and 825 ◦ C, respectively. The CO2 formation rate as a function of PC2 H6 at temperatures 825–875 ◦ C is shown in Fig. 6b and was regressed according to RCO2 = k[PC2 H6 ]n . The apparent CO2 formation rate is 3.5th-order dependent on PC2 H6 at 875 ◦ C, and the apparent CO2 formation rate order is 1.5 and 1.3 at 850 and 825 ◦ C, respectively. To compare and illustrate how the ethylene and CO2 formation rates change, the C2 H4 formation rate to CO2 formation rate ratio in BYS membrane reactor as a function of ethane partial pressure at temperatures 825–875 ◦ C is plotted in Fig. 6c. At 0.05 atm PC2 H6 and 875 ◦ C temperature, the C2 H4 formation rate/CO2 formation rate is approximately five to six times larger than those obtained at lower temperatures. This difference gets smaller as the ethane partial pressure is increased. On the other hand, the ratio of C2 H4 formation rate to CO2 formation rate, as is shown in Fig. 6c, changes with PC2 H6 at 875 ◦ C and decreases as the ethane partial pressure increases. At lower temperatures, since the dependence of both ethylene and CO2 formation rate orders with respect to PC2 H6 are similar (∼1.5), as shown in Fig. 6a and b, the change of C2 H4 formation rate/CO2 formation rate with PC2 H6 is not significant. Fig. 7a–c shows the effect of ethane partial pressure at temperatures 825–875 ◦ C on the reaction. Increasing the ethane partial pressure from 0.05 to 0.15 atm results in increasing conversion and decreasing ethylene selectivity. Again, higher ethane conversion and ethylene selectivity were obtained at

Fig. 6. C2 H4 formation rate to CO2 formation rate ratio for SOE in the BYS membrane reactor as a function of PC2 H6 at temperatures 825–875 ◦ C. Conditions: PO2 = 0.21 atm, F shell = 100 ml/min, F tube = 100 ml/min, PC2 H6 = 0.05–0.15 atm.

464

F.T. Akin, Y.S. Lin / Journal of Membrane Science 209 (2002) 457–467

875 ◦ C. Such increase in selectivity and decrease in conversion with a decreasing ethane partial pressure were not observed for partial oxidation of methane in the same reactor. In partial oxidation of methane at 900 ◦ C, the methane conversion decreases and C2 selectivity increases with an increase in methane partial pressure. Since oxygen supply across the membrane is practically constant, the reaction rate is controlled by oxygen supply. As the oxygen supply decreases, deep oxidation reactions do not take place leading to higher C2 selectivity, but lower methane conversion, for constant surface catalytic properties. In order to understand this phenomenon, it is useful to see how the oxygen permeation is affected by different reactions taking place in the tube side. 3.3. Oxygen permeation during SOE Fig. 8 compares the oxygen fluxes through the BYS tubular membrane in SOE, OCM and when the membrane is exposed to He. The oxygen permeation flux in O2 –N2 /CH4 –He is 1.5–3.5 times that in O2 –N2 /He. On the other hand, the oxygen permeation flux in O2 –N2 /C2 H6 –He is over an order of magnitude higher than that in O2 –N2 /CH4 –He. Oxygen permeation through the 1–2 mm thick ionic conducting BYS membrane is controlled by electronic conduction in the bulk membrane phase, and the oxygen permeation flux JO2 (mol/cm2 s) can be correlated to the upstream and downstream oxygen partial pressures PO 2 and PO 2 (atm) by Eq. (3) [25]: JO2 = 37.3(PO 0.27 − PO 2 0.27 ) 2

Fig. 7. Results of SOE in the BYS membrane reactor at different PC2 H6 . Conditions: T = 825–875 ◦ C, PO2 = 0.21 atm, F shell = 100 ml/min, F tube = 100 ml/min, PC2 H6 = 0.05–0.15 atm.

(3)

If the same permeation mechanism applies to oxygen permeation during OCM or ethane partial oxidation, the above equation would indicate that oxygen permeation flux with (O2 –N2 /CH4 –He) and (O2 –N2 /C2 H6 –He) should be only less than 50–60% higher than that with (O2 –N2 /He). The larger-than-the-expected increase in oxygen permeation flux in OCM and ethane oxidation as compared to that in O2 –N2 /He indicates a contribution of n-type electron transport during methane and ethane oxidation reactions. In this case, an additional term −1/4 [kn (PO 2 −1/4 − PO2 )] appears on the right-hand side of Eq. (3). Although the electronic transport in the Bi2 O3 -based ceramics is primarily of p-type in

F.T. Akin, Y.S. Lin / Journal of Membrane Science 209 (2002) 457–467

465

Fig. 8. Comparison of oxygen fluxes in SOE reaction (O2 –N2 /C2 H6 –He), OCM reaction (O2 –N2 /CH4 –He) and in oxygen permeation experiments (O2 –N2 /He). Conditions in SOE: PO2 = 0.21 atm, F tube = 100 ml/min, PC2 H6 = 0.1 atm. Conditions in OCM: PO2 = 0.21 atm, F tube = 15 ml/min, PCH4 = 0.1 atm. Conditions in oxygen permeation: PO2 = 0.17 atm, F tube = 30 ml/min.

a large PO2 range, n-type electronic transport may appear at low oxygen partial pressure such as in CH4 . 3.4. Catalytic activity Zeng et al. [26] reported that perovskite-type ceramics are more active and selective for methane at high oxygen partial pressures (p-type electronic conductor). The same material could become less selective for OCM in lower PO2 when the material is present as an n-type electronic conductor. Our results suggest that there is a similar phenomenon observed in ethane selective oxidation to ethylene. It has been observed that many catalysts active for OCM also catalyzes the ethane partial oxidation to ethylene [27]. The activity for ethane oxidation is higher if the OCM yielded high ethylene to ethane ratios, so it would be reasonable to state that a similar activity—p-type conduction dependence—is valid in ethane selective oxidation to ethylene. Clearly, the n-type electronic conduction is more predominant in O2 –N2 /C2 H6 –He than in O2 –N2 /CH4 –He as shown in Fig. 8, where

the oxygen flux at 875 ◦ C is 13 times larger than that in OCM. In the membrane reactor, the oxygen partial pressure in the gas phase adjacent to the reaction membrane surface is determined by oxygen permeation flux, ethane partial pressure, flow rate of the reactants and temperature. The results suggest that the membrane surface is becoming catalytically less active and selective for ethane partial oxidation at higher ethane partial pressure, which gives lower oxygen partial pressure in the gas phase. For comparison, SOE was performed in fixed bed reactor packed with the BYS pellets. The reaction gives 11% ethylene selectivity with 99% ethane conversion resulting in 11% ethylene yield at steady state at 900 ◦ C. A strict comparison between fixed bed and membrane mode experiments cannot be made due to the differences in operating conditions. The temperature and C2 H6 /O2 were slightly different in fixed bed reactor. C2 H6 /O2 is controlled by the amount of oxygen permeating through the membrane and is 2–3.5 in membrane reactor while this ratio was kept at 0.5 in the fixed bed reactor. The oxygen amount higher

466

F.T. Akin, Y.S. Lin / Journal of Membrane Science 209 (2002) 457–467

Fig. 9. XRD pattern of the BYS membrane: (a) before SOE, (b) outside BYS tube after SOE and (c) inside BYS tube after SOE. (x) Fluorite structure, (y) Bi1.73 Sm0.27 O3 (PDF41-0306) and (z) Bi2 O2.33 (PDF27-0051).

than the membrane reactor might have resulted in almost complete conversion observed in fixed bed reactor. Nevertheless, the difference in ethylene selectivity obtained in both reactors (11% in fixed bed and up to 95% in membrane operation) is very significant and is an indication of the effect of membrane reactor and surface reactions in the membrane mode. 3.5. Phase stability of BYS membrane under SOE conditions Fig. 9a shows the XRD pattern of a fresh sintered BYS membrane powder. Fig. 9b and c are the XRD patterns of tube and shell side surfaces of BYS membrane after SOE experiments that lasted for 2 days at temperatures 825–875 ◦ C. The membrane was quenched from SOE conditions to room temperature. The data shows that the fresh BYS membrane and shell side surface (air side) of the BYS tube after SOE reaction are in the fcc fluorite structure. The tube side surface (ethane side) of the BYS membrane after SOE still maintains mainly the fluorite structure, with some amount of impurity phase identified as Bi1.73 Sm0.27 O3 (JCPDS 41-0306) and Bi2 O2.33 (JCPDS 27-0051). Among the oxides of the three metal elements in BYS, bismuth oxide and yttrium

oxide is respectively least and most thermodynamically stable. Thus, under the reducing condition bismuth ion on the inner membrane surface (methane side) is more likely reduced to bismuth element which may not be XRD-detectable under the present conditions. This reduction is accompanied with formation of bismuth samarium oxide (Bi1.73 Sm0.27 O3 ) and bismuth oxide (Bi2 O2.33 ). The inner membrane surface remains catalytically active and selective for SOE and the fluorite structure still remains to be detectable by XRD.

4. Conclusions Ethane selective oxidation to ethylene is studied in a dense tubular membrane reactor made of BYS. At 875 ◦ C, when the flow rate is 50 ml/min, per pass ethylene yield of 56% with a ethylene selectivity of 80% is obtained. The oxygen flux during ethane oxidation reactions is over an order of magnitude higher than those obtained under O2 –N2 /He conditions. The larger than the expected increase in oxygen flux indicates the p- to n-type switch of electronic conduction mechanism switch accompanied with a decrease in the catalytic activity for ethane oxidation. In a fixed

F.T. Akin, Y.S. Lin / Journal of Membrane Science 209 (2002) 457–467

bed reactor packed with BYS pellets, at 900 ◦ C, 11% ethylene yield with 11% ethylene selectivity is obtained at steady state. The membrane reactor operation is much beneficial for achieving high selectivity and yield. Acknowledgements This project was supported by the National Science Foundation through grant (CTS-9502437). References [1] E. Morales, J.H. Lunsford, Oxidative dehydrogenation of ethane over a lithium-promoted magnesium oxide catalyst, J. Catal. 118 (1989) 255. [2] M. Loukah, G. Coudurier, J.C. Vedrine, M. Ziyad, Oxidative dehydrogenation of ethane on V- and Cr-based phosphate catalysts, Micropor. Mater. 4 (1995) 345. [3] A.D. Eastman, J.P. Guillory, C.F. Cook, J.B. Kimble, Oxidative dehydrogenation and cracking of paraffins with a promoted cobalt catalyst, US Patent 4,497,971 (1985). [4] S. Bernal, G.A. Martin, P. Moral, V. Perrichon, Oxidative dehydrogenation of ethane over lanthana: actual nature of the active phase, Catal. Lett. 6 (1990) 231. [5] Y. Wang, K. Otsuka, Partial oxidation of ethane by reductively activated oxygen over iron phosphate catalyst, J. Catal. 1 (1997) 171. [6] J.E. ten Elshof, B.A. Van Hassel, H.J.M. Bouwmeester, Activation of methane using solid oxide membranes, Catal. Today 25 (1995) 397. [7] Z. Shao, H. Dong, G. Xiong, Y. Cong, W. Yang, Performance of a mixed-conducting ceramic membrane reactor with high oxygen permeability for methane conversion, J. Membr. Sci. 183 (2001) 181. [8] G. Tsai, A.G. Dixon, W.R. Moser, Dense perovskite membrane reactors for partial oxidation of methane to syngas, AIChE J. 43 (1997) 2741. [9] Y. Lu, A.G. Dixon, W.R. Moser, Y.H. Ma, U. Balachandran, Oxygen permeable dense membrane reactor for the oxidative coupling of methane, J. Membr. Sci. 170 (2000) 27. [10] A.M. Champagnie, T.T. Tsotsis, R.G. Minet, I.A. Webster, A high temperature catalytic membrane reactor for ethane dehydrogenation, Chem. Eng. Sci. 45 (1990) 2423. [11] A.L.Y. Tonkovich, J.L. Zilka, D.M. Jimenez, G.L. Roberts, J.L. Cox, Experimental investigation of inorganic membrane reactors: a distributed feed approach for partial oxidation reactions, Chem. Eng. Sci. 51 (1996) 789.

467

[12] J.E. Miller, A.G. Sault, D.E. Trudell, T.M. Nenoff, S.G. Thoma, N.B. Jackson, Oxidation reaction of ethane over Ba-Ce-O-based perovskites, Appl. Catal. A 201 (2000) 45. [13] G. Colorio, J.C. Vedrine, A. Aurox, B. Bonnetot, Partial oxidation of ethane over alumina-boria catalysts, Appl. Catal. A 137 (1996) 55. [14] N.F. Chen, K. Oshihara, W. Ueda, Selective oxidation of ethane over hydrothermally synthesized M-V-Al-Ti oxide catalyst, Catal. Today 64 (2001) 121. [15] H.X. Dai, C.F. Ng, C.T. Au, The catalytic performance and characterization of a durable perovskite-type chlorooxide SrFeO3−δ Clσ catalyst selective for the oxidative dehydrogenation of ethane, Catal. Lett. 57 (1999) 115. [16] H.X. Dai, Y.W. Liu, C.F. Ng, C.T. Au, The performances and characterization of BaO- and BaX2 - (X = F, Cl and Br)-promoted Y2 O3 catalysts for the selective oxidation of ethane to ethene, J. Catal. 187 (1999) 59. [17] H.X. Dai, C.F. Ng, C.T. Au, YBa2 Cu3 O7−δ Xσ (X = F and Cl): highly active and durable catalysts for the selective oxidation of ethane to ethene, J. Catal. 193 (2000) 65. [18] W. Yang, P. Yang, Z. Shao, Y. Cong, G. Xiong, H. Li, L. Li, High selectivity of ethylene for oxidative dehydrogenation of ethane in a mixed conducting membrane reactor, in: Proceedings of the 4th International Conference on Catalysis in Membrane Reactor, Zaragoza, Spain, 3–5 July 2000, pp. 35–36 (Abstracts). [19] F.T. Akin, Y.S. Lin, Y. Zeng, Oxidative coupling of methane on fluorite-structured samarium-yttrium-bismuth oxide, Ind. Eng. Chem. Res. 40 (2001) 5908. [20] H.H. Kung, Oxidative dehydrogenation of light (C2 –C4 ) alkanes, Adv. Catal. 40 (1994) 1. [21] A. Erdohelyi, F. Solymosi, Oxidation of ethane over silicasupported alkali-metal vanadate catalysts, J. Catal. 129 (1991) 497. [22] F.T. Akin, Y.S. Lin, Oxygen ionic conducting ceramic membrane reactor for oxidative coupling of methane with high yields, AICHE J., in print. [23] D.J. Hucknall, Chemistry of Hydrocarbon Combustion, Chapman & Hall, London, 1984. [24] A.M. Brodsky, R.A. Kalienko, K.P. Lavrosky, The principles governing high temperature ethane cracking, J. Chem. Soc. (1960) 4446. [25] Y. Zeng, Y.S. Lin, Oxidative coupling of methane on improved fluorite-structured bismuth oxide membrane reactors, AIChE J. 47 (2000) 436. [26] Y. Zeng, Y.S. Lin, S.L. Swartz, Perovskite-type ceramic membrane: synthesis, oxygen permeation and membrane reactor performance for oxidative coupling of methane, J. Membr. Sci. 150 (1998) 87. [27] E.M. Kennedy, N.W. Cant, Comparison of the oxidative dehydrogenation of ethane and oxidative coupling of methane over rare earth oxides, Appl. Catal. 75 (1991) 321.