Performance Analysis of a Porous Packed Bed Membrane Reactor for

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Performance Analysis of a Porous Packed Bed Membrane Reactor for Oxidative Coupling of Methane: Structural and Operational Characteristics H. R. Godini,*,† A. Gili,† O. Görke,*,‡ U. Simon,‡ K. Hou,† and G. Wozny† †

Chair of Process Dynamics and Operation, Berlin Institute of Technology, Strasse des 17. Juni 135, Sekr. KWT-9, D-10623 Berlin, Germany ‡ Institute for Material Science and Technologies, Berlin Institute of Technology, Hardenbergstrasse 40, D-10623 Berlin, Germany ABSTRACT: The performance of the Oxidative Coupling of Methane (OCM) reactions in a porous ceramic packed bed membrane reactor was experimentally investigated using an Mn−Na2WO4/SiO2 catalyst. A novel practical method was applied to modify the available commercial α-alumina membrane and shape it to the form of an inert fine oxygen distributor in an OCM membrane reactor. The characteristics of such modified membrane and the performance of the resultant OCM membrane reactor are reviewed in this paper. It was observed that establishing a 2 bar pressure gradient across the modified membrane ensures a safe and efficient oxygen-dosing along the OCM membrane reactor. Moreover, using a modified membrane with a descending permeation profile instead of a uniformed permeation profile improved the observed C2 selectivity (ethylene and ethane) by 10% in average. The efficient design of the membrane reactor setup and the stability of the prepared catalyst provided a robust operation and replicable results. In this experimental analysis, a very promising 25.5% C2 yield and 20.3% ethylene yield with 66% C2 selectivity were achieved under very low (20%) diluted reaction atmosphere for the methane-to-oxygen ratio 2. By proper exploiting the carbon dioxide instead of nitrogen dilution, the C2 yield was improved by 1−2% in average.

1. INTRODUCTION Investigating the performance of the Oxidative Coupling of Methane (OCM) reactions in various reactor configurations provides the understanding needed to identify and secure the desired thermal-reaction engineering characteristics in an efficient OCM reactor design. The main decision in this context is to choose a proper reactor feeding policy, which is capable not only to address the highly exothermic nature of this reaction but also to provide a selective methane conversion and high ethylene yield. In such an analysis, the performance of the catalyst is always a defining factor.1 Being selective and, more importantly, being stable are the main desired characteristics of a good OCM catalyst. Such a catalyst should then be exploited in a proper reactor. Each type of reactor enhances particular performance-aspect of the OCM reactions. For instance, a fluidized bed reactor operates close to the desired isothermal performance, but the obtained ethylene yield via this reactor is limited to below 20% due to the poor selectivity.2,3 Among the alternative reactor concepts, a membrane reactor, which properly employs the oxygen dosing policy,4 offers a selective performance toward ethylene production. In a membrane reactor structure, oxygen is distributed along the catalytic bed through an inorganic membrane. The catalysts particles are packed inside the tubular membrane where the permeated oxygen reacts with the separately fed methane. The main objectives for designing an efficient OCM membrane reactor are (1) establishing a fine oxygen distribution to obtain a high level of methane conversion with high selectivity toward ethylene, (2) avoidance of a significant contribution of the undesired gas phase reactions, and (3) prevention of the formation of hot-spots, which restricts the reactor’s performance. Above all, ensuring a safe © 2013 American Chemical Society

operation should be always given a priority. Therefore, the membrane diffusion characteristics should be designed in such a way that establishing an optimum pressure gradient across the membrane and a fine oxygen distribution along the porous Packed Bed Membrane Reactor (PBMR) are assured. This also embraces minimizing the back permeation of hydrocarbons into the shell side, which is a severe safety issue in this reactor. Among the few materials that can offer such characteristics under the harsh operating conditions of the OCM reactor, alumina membrane is the most recommended one for this application.5,6 However, in order to achieve the desired diffusion characteristics, the commercial α-alumina membranes should be modified5 and the available γ-alumina membranes should be stabilized before being implemented as an OCM membrane reactor.6 A new membrane modification method is presented in this paper and the characteristics of the resultant modified membrane are compared with the results of the other membrane modification methods recommended for this application.5 Moreover, the observed performance of the OCM membrane reactor, in which this modified membrane is exploited to finely distribute the oxygen along the catalytic bed, is also reported here. In this context, the effects of operating parameters such as methane-to-oxygen ratio, inert-gas dilution, and operating temperature profile, as well as the permeation profile along the membrane were experimentally investigated and are reported in the following sections. Received: October 10, 2013 Revised: December 7, 2013 Published: December 24, 2013 877

dx.doi.org/10.1021/ef402041b | Energy Fuels 2014, 28, 877−890

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Figure 1. (a) Membrane reactor setup equipped with multipoint thermocouple and cooler. (b) Drawing of the membrane reactor. (c) Representing the feeding policy in the coated membrane with BOTZ material. diffusion characteristics of this section of the original alumina membrane were modified to obtain a sharp concentration gradient of the species between the oxygen-rich gas stream in the shell-side and the tube-side gas stream. 2.1.1. Membrane Modification Methods. The silica-sol impregnation−calcination method has been employed to modify the membrane and develop a porous packed bed membrane reactor for OCM.5 This method of membrane modification successfully provides the desired amount of permeation. However, cracks will appear on the surface of such modified membrane and consequently the targeted homogeneous oxygen distribution cannot be established. Modifying the membrane with glassy materials is proposed here as an alternative membrane modification approach for making a crackfree coated layer over the porous membrane. In this research, the BOTZ glaze material available in market was utilized to coat the membrane and tailor the required membrane diffusion characteristics. The original 600 mm long tubular α-alumina microfiltration membrane purchased from “Fraunhofer Institute for Ceramic Technology” is composed of two layers. The tubular support layer has the specifications of 7/10 mm inner diameter/outer diameter (ID/ OD), 1.5 mm thickness, and 3 μm average pore size, and the membrane layer has a thickness of 10 μm and nominal pore size of 200 nm, which covers the inside of the porous ceramic tube. The pore sizes and pore volumes in this porous structure should be reduced in order to obtain the required diffusion characteristics and reduce the significant contribution of the original fresh membrane to the reactions, especially to the gas phase combustion reactions. A homogeneous suspension containing 150 mL of deionized water and 17 g of the BOTZ glaze was prepared to coat the ceramic membrane. The membrane was dipped in this solution. This procedure was repeated six times, each time followed by drying at

2. MATERIALS, METHODS, AND EXPERIMENTAL SETUP The selected membrane material should be structurally stable under the OCM operating temperature and should not contribute to the reactions. To fulfill such requirements, a thermally stable α-alumina membrane was modified not only with regard to its permeability characteristics but also concerning its reactivity potential under the OCM reaction conditions. The first specific aim was to prepare a stable small pore-size membrane structure, which creates a significant pressure drop when oxygen flows through it from the shell side into the tube side. As a result, the chance of convective methane backpermeation from the tube side into the shell side oxygen-rich stream and the risk of explosion are reduced. 2.1. Membrane Modification. Figure 1(a,b) shows the structure, operating concept, and a photo of the membrane reactor module used in the experimentation. The concept of the operation in the OCM membrane reactor has been represented in Figure 1c. As is seen in Figure 1, the ceramic membrane divides the inner volume of the metal reactor module into the shell and tube sides. Oxygen and nitrogen are normally fed into the shell side and pass through the membrane to be distributed along the catalytic bed. In the tube side, the whole stream of methane and nitrogen is axially fed into the catalytic bed. According to the specifications of the reactor module, the pores should be completely blocked in the beginning part (105 mm) and the end part (270 mm) of the membrane. This is needed to restrict the permeable section to the area which is filled with catalyst. As seen in Figure 1, the catalyst is implemented in the middle section where the pores are not completely blocked. The permeability in this section, 225 mm in the middle part, also should be significantly reduced in comparison to the permeability of the fresh membrane. Therefore, the 878


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Figure 2. Micro- (SEM) and macro-scale pictures of the BOTZ glaze coating over the membrane samples calcined at 950 °C (left) and 1040 °C (right): (a) SEM picture of the membrane coated layer, (b) SEM picture of the surface and support layer (The small cracks appeared later when the sample was broken to take the SEM picture from the edge of the membrane for representing the surface and internal part of the porous structure together.), (c) pictures of the outer surface of the samples in the scale of membrane diameter. 75 °C for 15 min. As a result, a thin and homogeneous layer was formed over the membrane surface. The membrane was then calcined at 950 °C for 30 min, with preheating/after-cooling rates of 3 °C/min. In order to make a full enamel layer and completely block the pores in the first and last segments of the membrane, extra coatings were applied and the membrane was calcined in the temperature range 1020−1060 °C for 45 min. This coating procedure can be repeated until reaching the desired characteristics. Figure 1 shows the resulted modified membrane. The temperature applied to calcine the modified membrane in the middle section was designed to be lower than the temperature used to calcine the fully enameled parts of the membrane. The effect of the calcination temperature on the characteristics of the coated layer is seen in Figure 2, where comparable pictures of two membrane samples calcined at 950 °C (left) and 1040 °C (right) are shown in both macro- and micro-scale. It can be concluded therefore that the calcination temperature has a significant impact on the homogeneity and the permeability of the resultant coating layer.

It was observed that using the higher calcination temperature (1040 °C) enables one to completely block the pores and prepare an impervious coated surface. The results of the permeability tests over several modified samples confirmed that this had been achieved. It was generally observed that the BOTZ glaze forms a homogeneous coated layer over the membrane surface and does not penetrate deep into the pore volume inside the porous structure. On the other hand, the membrane pore volume significantly contributes to the gas phase combustion reactions because the methane and oxygen first meet each other there where the oxygen concentration is usually high. In order to reduce the pore volume inside the membrane, especially in the middle section, a combination of the silica-sol impregnation modification method and the BOTZ glaze coating method (calcination at 950 °C) was applied in this section. In order to impregnate the membrane with silica, the fresh alumina membrane was immersed in an aqueous suspension of nanoparticle (14 nm) SiO2 followed by drying and calcination. The calcination was performed in three steps: a heating step 25 °C →800 °C (6 h), an isothermal period at 800 °C (6 879


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Table 1. Mass Deposited on the Membrane in Reference to the Number and Type of Modification Steps modification step

type of modification

total mass (g)

mass increase (g)

% mass increase (relative to the total deposited mass)

% mass increase (relative to the total sample weight in the previous step)

0 (O.M.)a 1 2 3 4 5 1b 2b 3c

SiO2 impregnation

66.3 69.7 70.9 71.1 71.2 71.3 72.6 73.4 74.9

3.4 1.3 0.2 0.1 0.1 1.3 0.9 1.5

39.4 14.8 2.2 0.7 0.7 15.3 10.1 16.8

5.1 1.8 0.3 0.1 0.1 1.9 1.2 2

BOTZ coating

a

O.M. represents the specifications of the original fresh membrane sample. bFirst and second enamel layers to completely block the pores at the end parts of the membrane using calcination at 1040 °C. cSingle coating on the middle section of the membrane to reduce the permeation using calcination at 950 °C.

Figure 3. (a) Pore volumes of the membrane support structure is partially filled with silica particles, (b) the SEM picture of the nonhomogeneous silica-sol modified membrane surface, (c) the coke formation inside the support layer as a measure for the intensity of reactions inside the support layer (1 and 2: view of the longitudinal and redial cross section of the membrane in the reaction area, 3: view of the membrane cross section outside the reaction zone). membranes in the range 700−1000 (cm3 cm−2 min−1 bar−1) was reduced to the final established permeation in the range 1−15 (cm3 cm−2 min−1 bar−1). Similar range of permeation has been reported by Coronas et al. for the same application.4 The BOTZ glaze coated layer is more homogeneous than the modified membrane surface with silica-sol impregnation−calcination method. The BOTZ coated layer not only restricts the permeation but also reduces the reactivity of the membrane surface. This was confirmed by testing the crashed form of the fresh membrane material and the BOTZ glaze material in a fixed bed reactor under the OCM conditions. 6% methane conversion and 23% C2 selectivity was observed for the fresh alumina sample and less than 1% methane conversion was observed for the calcined BOTZ glaze material under similar operating conditions.

h) and a final cooling step 800 °C →25 °C (6 h). This procedure was repeated 5 times, as reported in Table 1. The declining trend of the observed deposited mass throughout the sequential modification steps implies that the membrane’s porevolume is gradually filled. In order to further decrease the permeation in the 225 mm middle section and to provide an inert and homogeneous coating layer over the membrane surface, the membrane was extra coated with BOTZ glaze. The amounts of the impregnated silica and the BOTZ glaze material coated over the membrane surface are reported in Table 1. For the original 66.3 g membrane, a total mass in the amount of 8.6 g was deposited and membrane permeability was reduced by more than 100 times. This typical trend was repeatedly observed by testing several fresh and modified membranes. For instance, due to the applied membrane modifications, the observed permeation of the fresh 880


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On the other hand, the pore volume in the BOTZ modified membrane is not reduced as much as its observed reduction due to the silica-sol impregnation−calcination method. The results of the BET test show that using the silica-sol impregnation, finally, the specific surface area and pore-volume in amounts of 3.9 m2/g and 1.2 cm3/g, respectively, were achieved. Significant reduction of the pore volume was also confirmed via microscanning of the internal structure of the highly porous mechanical support layer, as shown in Figure 3a. Figure 3a shows that using the silica sol modification, the pores in the membrane support layer are filled with the silica-clusters. The SEM picture in Figure 3b shows the surface of the silica-sol modified membrane with some cracks over it. This is in contrast to the observations after implementing the BOTZ glaze coating. For instance, the SEM pictures of the sample BOTZ glaze coated membranes shown in Figure 2 can be compared with the reported pictures in Figure 3. According to these typical observations, it became clear that the BOTZ glaze stays mostly on the surface as a homogeneous layer and the membrane pore volume is not affected significantly by this modification. Therefore, the porous structure of the membrane, specially the structure of the mechanical support layer with the bigger pore volume, remains available for the undesired gas phase reactions. The observed intense coke formation in the support layer, as shown in Figure 3c, confirms the significant contribution of this porous structure to the reaction. Such an intense coke formation was not observed in the silica-sol modified membrane. In conclusion, a combination of BOTZ coating and silica-sol modification was shown to be a promising modification approach. 2.1.2. Permeability Test. The permeability test was performed using the dead-end configuration in which the outlet of the shell side is closed. In this manner, the whole shell-side nitrogen gas stream in flow rate of 3600 μLs1− (216 cm3min−1) passes through the membrane, which encounters a certain amount of pressure drop across the membrane. In order to assimilate the normal operation of the membrane reactor, an extra 3000 μLs1− (180 cm3 min−1) nitrogen flow rate was introduced into the tube side while the temperature was increased from 25 to 750 °C. The observed pressure gradient across the membrane was considered as an indicator for the performance of the applied membrane modification. This factor was recorded as a function of operating temperature and demonstrated in a graph, as shown in Figure 4.

By further modifying the membrane and reducing its permeability, even pressure drop of 4 bar was achieved across the membrane. However, it was observed that such a high pressure gradient increases the probability of breakage of the membrane when the membrane reactor is exposed to a physical or thermal shock. 2.1.3. Blank Test of the Modified Membranes. The reactivity and possible contribution of the BOTZ-coated membrane to the OCM and combustion reactions was further investigated in a blank reactor test. The modified membrane was assembled in the reactor module and tested under the OCM reaction conditions without using a catalyst. The amount of the observed methane and oxygen conversion and carbon oxide selectivity were then analyzed. It is important to emphasize that the gas phase reactions occurring in an empty tube are responsible for some level of the observed methane conversion.7,8 However, in the current analysis, we attempted to qualitatively distinguish the effect of the bulk gas phase reactions from the effect of the reactions triggered by the membrane structure. This was investigated under various oxygen feeding scenarios in an empty membrane reactor. In this manner, three different feeding scenarios including (a) the cofeeding of oxygen and methane, (b) the partial oxygen-dosing feeding via the membrane, and (c) the complete oxygen-dosing feeding via the membrane were investigated. The observed methane conversion in the cofeeding of the reactants represents the amount of the bulk gas phase reactions inside the tube plus the surface induced reactions. By shifting to the partial and complete oxygen dosing policy, the impact of the membrane induced reactions on the observed methane conversion is intensified. Although for a fresh membrane, a significant amount of methane conversion and carbon dioxide production has been observed,9 the tested BOTZ coated membrane was observed to be rather inactive under the OCM reaction conditions. This is one of the reasons why usually higher C2 selectivity and therefore higher C2 yield have been achieved by implementing the BOTZ coated membrane in comparison to the other cases where membranes have been modified by other methods. After all, in the blank tests of the BOTZ coated membrane below 3% methane conversion was observed. 4% methane conversion was measured in similar test where the membrane was packed with inert particles under low feed flow rate and methane-to-oxygen ratio, which constitute a more intensified gas phase reaction atmosphere. 2.2. Catalyst Preparation. An Mn−Na2WO4/SiO2 catalyst was selected to be tested in the membrane reactor mainly because of its stability10 and also in order to continue the comprehensive efforts devoted by our research group (UniCat) to further developing this promising catalyst.3,11 In the current research, the Mn−Na2WO4/SiO2 catalyst was prepared using incipient wetness impregnation method. Amorphous SiO2 with grain sizes ranging from 35 to 60 mesh and specific surface area of 480 m2/g (Silica Gel Davisil grade 636, Sigma-Aldrich CASNo. 112926-00-8) was used as the catalyst support material. The silica gel was coated with an impregnating aqueous solution of a Mn(NO3)2·4H2O in an appropriate concentration.12 After drying in air under temperature of 130 °C for at least 5 h, the granules were impregnated with a second aqueous solution containing Na2WO4· 2H2O to achieve 1.9% Mn−5% Na2WO4/SiO2 catalyst. The samples were then dried for 8 h at 130 °C and annealed at 800 °C for 8 h under air. All annealing procedures were performed using a preheating rate of 10 °C/min. A quartz washed-granular with proven inert activity was purchased from Merck (CAS-No. 14808-60-7) and used as inert packing to dilute the catalytic bed and fix its position by filling the tube side in the area outside the reaction zone. In the experiments, 3−4 g of Mn− Na2WO4/SiO2 catalyst and 2−5.5 g of inert quartz granular were implemented in the catalytic packed bed. 2.3. Membrane Reactor Setup. The membrane reactor module was assembled using several fittings-connections. For instance, the shell and tube side feed streams are introduced into the reactor module via two Swagelok Tee-connections, as shown in Figure 1. This structure allowed simple loading and replacement of the ceramic membrane and the catalyst. The shell side of the reactor was made of alloy 800 (Nicrofer 3220 H, Fe/Cr alloy). Right after the reaction

Figure 4. Pressure drop across the membrane (ΔPN2) as a function of temperature. Depending on the feed flow rate, a pressure drop of 1−3 bar was achieved across the membrane during the experimentation under the OCM reaction conditions. The obtained pressure drop across the BOTZ coated membrane is enough to maintain a safe and efficient oxygen dosing performance. In order to establish the desired oxygen distribution along the reactor, not only the pressure drop across the membrane should be tuned but also the effect of pressure drop along the catalytic bed should be taken into consideration. Therefore, it was expected and experimentally observed that a higher pressure gradient across the membrane marginalizes the effect of pressure drop along the bed and promotes the uniform dosage of oxygen along the reactor. 881


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Figure 5. Picture of the membrane reactor module inside the experimental setup.

Figure 6. Visualization of the equipment and the control system for the experimental OCM membrane reactor setup. membrane inside the metallic module and ensure an efficient sealing of the membrane. As shown in Figure 1, the ceramic membrane separates the shell side with an inner diameter of 20 mm and the tube side with an inner diameter of 7 mm. Within the tubular membrane, a WIKA k-type

zone, a cooling mechanism was utilized to immediately drop the temperature of the gas stream. In this manner, the reaction zone is practically restricted to the area packed with catalyst. The installed Teflon rings at both ends of the ceramic tube, where temperature does not exceed 200 °C, provide the possibility to fix the ceramic 882


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Figure 7. Performance of the OCM membrane reactor in terms of the C2 yield and C2 selectivity as a function of reaction-temperature for a very low methane-to-oxygen ratio (1.55), low total nitrogen dilution (11%), and methane flow rate = 2 cm3[STP]/s, nitrogen flow = 0.2 cm3 [STP]/s and 87% oxygen content in the shell gas stream. multipoint thermocouple (with 3 mm OD stem) forms an annular space which is filled with catalyst and inert packing. In this manner, a very thin catalytic bed (2 mm thick) was formed. This ensures an efficient contact between the reactants in such a small annular crosssectional area along the catalytic bed, an excellent heat transfer characteristics and avoidance of heat accumulation inside the bed. The shell side was also filled with inert quartz packing mainly to ensure a safer operation and efficient radial heat transfer. Figure 5 shows the membrane reactor module inside the experimental setup where the operating temperature is controlled using a two-zone electrical tube furnace. The first part of the tube, which is filled with inert packing, is considered as a short preheating zone. In this preheating zone, reactants are heated before entering the catalytic bed. The permeability of different segments of the ceramic membrane is modified according to the position and length of the permeable area. In this manner, it becomes possible to change the length of reaction zone and use the entire first heating zone for preheating task. The applied on/off heating mechanism via the electrical tube furnace allowed maintenance of the temperature in the desired range. The local operating temperatures were monitored in 10 points along the reaction zone. Two separate thermocouples measure the temperatures in the tube furnace. By assigning the desired set-points of these temperatures, the heat duty of each electrical heating element is tuned. Considering the effect of generated heat of reaction, the temperature inside the catalytic bed might be higher than the temperatures recorded by the thermocouples positioned behind the furnace’s shield. This shield separates the electrical heating elements from the surrounding insulation material and intensifies the radiation heat transfer to the reactor. Through the medium of the online user-interface of the control system (SIMATIC PCS-7 Siemens), the feed flow rates, operating pressures, and the applied temperature of the electrical heaters were tuned and monitored. Figure 6 shows a screenshot of the userinterface-visualization for a typical operating state of the membrane reactor setup. The equipment and control strategies in the experimental setup are clearly shown in this user-interface-visualization. The steady performance of the system is also demonstrated via the displays in which temperature, pressure, and outlet concentration of the species are recorded over time. The flow rate of oxygen, methane, and nitrogen entering the shell and tube sides of the membrane reactor was controlled using precise Mass Flow Controllers (MFC) with 0.8% precision range around the measured value. The operating pressure in the inlet and outlet of the shell and tube sides was monitored using four pressure indicators. In this manner, the pressure drop along the reactor and across the membrane are indicated. Four magnetic (on/off) valves were installed to direct each of the shell and tube outlet flow streams toward the Infrared Red (IR) SICK-S700 gas analyzer or the flare. The IR gas analyzer monitors the concentration of ethylene, ethane, methane,

carbon dioxide, and oxygen in the reactor outlet gas stream. The precise measurements of the IR were confirmed using GC sampling. All these equipment, measuring, and control scenarios have been also shown in Figure 6. To establish a dead-end feeding configuration in the experimentation, the outlet of the shell-side gas stream is completely closed. In this manner, the entire shell-side inlet gas stream containing oxygen passes through the membrane. As a result, back permeation of the hydrocarbon from the tube side into the shell side reduces significantly.

3. PERFORMANCE OF THE OCM MEMBRANE REACTOR The values of the methane conversion, C2 selectivity, C2 yield, and temperature profile along the reactor were recorded and analyzed as the reaction and thermal performance indicators of the OCM membrane reactor. 3.1. Effect of Reaction Temperature. It is not an exaggeration to claim that thermal engineering remains to be the main challenging task in designing an OCM reactor. Establishing the desired temperature profile along the reactor is crucial but very complicated. This factor is affected by several design and operating parameters. It should be emphasized that the average value of the observed local temperatures along the reactor does not fully represent the thermal performance of the OCM reactor. The impacts of the distribution of reaction intensity along the reactor and physical parameters of the OCM membrane reactor, including its dimensional and heat transfer characteristics, are better reflected via the recorded temperature profile along the reactor. In support of the discussion in the preceding paragraph, it is reasonable to highlight that observing a 100 °C local temperature rise in the packed bed membrane reactor is not unusual but has a crucial impact on the reactor performance. However, the contribution of the hot spot formation to the reactor performance is not reflected only in the recorded average reaction temperatures. The permeation pattern and distribution of oxygen along the membrane will be also affected by the local hot spots. It is therefore difficult to fairly represent and compare the effect of average reaction temperature on the performance of OCM membrane reactor. However, the significant effect of the operating temperature on the reactor performance can be better visualized when lower methane-tooxygen ratios and low nitrogen dilutions are employed. This is clearly observed in Figure 7 where typical trends of the C2 selectivity and C2 yield as a function of average reaction 883


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Figure 8. Effect of methane-to-oxygen ratio on the C2 yield, methane conversion, and C2 selectivity (methane flow rate = 2.2 cm3[STP]/s, nitrogen dilution = 50%).

equally distributed between the shell and tube sides unless otherwise mentioned. 3.3. Effect of Gas Dilution. Using a proper amount of gas dilution is also an important factor in the reactor-scale as well as in the process-scale analysis of the OCM performance. Nitrogen, helium, carbon dioxide, steam, and excess methane have been exploited to dilute the OCM reaction environment. However, different researchers have reported different observations with regard to the effect of diluting the feed stream in OCM reactors. The effect of gas dilution on the OCM reactor performance has been claimed to be insignificant in some reports,15 while mostly a positive impact of this parameter on improving the C2 yield has been observed.16 Nevertheless, there is a general agreement that using higher dilution increases the C2 selectivity since it reduces the partial pressure of the reactants in the gas phase and consequently the potential for the undesired gas phase reactions. Having considered this well-accepted mechanism through which the dilution affects the C2 selectivity, it is also reasonable to systematically analyze the impact of dilution on methane conversion and C2 yield. For interpreting the observed trend of the C2 yield in various levels of dilution, special attention should be devoted to the effect of dilution on the thermal performance of an OCM reactor. For instance, under the isothermal performance of the OCM fluidized bed reactor, it has been observed that introducing higher amounts of nitrogen dilution does not have a significant impact on the C2 yield.3 Interpreting the results of the OCM membrane reactor in this regard is more complicated and is described here for the cases of using nitrogen and carbon dioxide as a diluent. 3.3.1. Nitrogen Dilution. Introducing nitrogen to dilute the reaction atmosphere in the OCM packed bed membrane reactor enhances the C2 selectivity due to reducing the gas phase reactions and hot spot formation. On the other side, higher nitrogen dilution usually reduces the methane conversion due to the shortening the contact time of methane and lowering the reaction temperature. Therefore, the effect of nitrogen dilution on the C2 yield depends on its relative impacts on the C2 selectivity and methane conversion. This relative impact may be different under various reaction environments in which different intensity of the generated reaction heat, operating temperatures, feed flow rates, and methane-to-oxygen ratios are employed. For instance, the effects of various levels of nitrogen dilution on the C2 yield and

temperature have been demonstrated for one of the lowest level of gas dilution and methane-to-oxygen ratio applied in this experimentation. It can be generally concluded that by increasing the reaction temperature, the observed C2 selectivity is increased due to the known favorable effect of temperature on the catalytic coupling reactions. However, after increasing the temperature above certain level, the possibility of hot spot formation increases and the combustion of the desired products is intensified. As a result, C2 selectivity and yield are reduced at very high temperature. This has been also reported and discussed in details elsewhere.13 This typical observed behavior reflects the nature of the OCM reactions rather than representing the performance of any specific catalyst or reactor. The experimental data represented in Figure 7 have been recorded in repeated experiments. The trend tendencies of the results have been highlighted in this figure in order to better visualize the effects. Here the C2 selectivity and C2 yield follow the same qualitative trend and show a maximum value around 825 °C. Therefore, for improving the C2 yield, temperature can be regulated only in certain range. 3.2. Effect of Methane-to-Oxygen Ratio. Methane-tooxygen ratio is another important factor to be investigated in this analysis. Increasing the methane-to-oxygen ratio increases the C2 selectivity and decreases methane conversion due to less available stoichiometric oxygen as is shown in Figure 8. This is a general observation and has been reported for most of the OCM reactor concepts. Figure 8 shows that in the area where the C2 selectivity reaches its plateau (e.g., 3 < CH4/O2 < 5), using lower methane-to-oxygen ratio and increasing the methane conversion becomes a crucial factor to improve the C2 yield. Particularly, it was observed that using the methane-to-oxygen ratios bigger than 4 slightly increases the C2 selectivity but significantly reduces the C2 yield. In this context, the impact of securing a high C2 selectivity on the downstream units and the costs of treating the unreacted methane, which are dictated by the applied methane-to-oxygen ratio, should also be analyzed in the scale of the whole process.14 In this manner, the optimum operating methane-to-oxygen ratio is not determined only by considering the performance of the reactor unit alone but through a process scale analysis and concurrently considering the performance of the reactor and downstream units. In the experimentation and for most of the experimental results reported in this paper, inert gas dilution has been used and 884


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Figure 9. Effect of nitrogen dilution and methane-to-oxygen ratio on the performance of the OCM membrane reactor (methane feed flow rate = 1.8 cm3 [STP]/s), Left: Analysis of the C2 yield. Right: Analysis of the C2 selectivity.

Figure 10. Effect of nitrogen dilution and methane-to-oxygen ratio on the performance of the OCM membrane reactor (methane feed flow rate = 2.2 cm3 [STP]/s). Left: Analysis of C2 yield. Right: Analysis of C2 selectivity.

C2 selectivity in different methane-to-oxygen ratios are represented in Figure 9. These three-dimensional graphs and the rest of the similar three-dimensional graph-representations in this manuscript were developed using 36−40 experimental data for each graph. The experimental data have been collected in 12−15 h long daily experimentations and the results have been recorded at least after 20 min observing the steady performance of the system for each set of operating conditions. The experimental results have been also demonstrated on the graphs. These graphs enabled us to represent the full factorial observations for the effect of nitrogen dilutions and methane-to-oxygen ratios. As seen in Figure 9, nitrogen dilution significantly enhances the C2 selectivity. Because of this, despite of the observed moderate reduction in methane conversion, the C2 yield is usually improved by increasing the nitrogen dilution. The C2 selectivity can be also improved using excessmethane provided by higher methane-to-oxygen ratios. Therefore, usually, the lowest C2 selectivity is achieved by using the lowest nitrogen dilution and methane-to-oxygen ratios. On the other side, methane conversion decreases in higher methaneoxygen ratios. This is a general trend, but depending on the range of methane-to-oxygen ratios, the relative impact of this parameter on the C2 yield is different. For instance, increasing the methane-to-oxygen ratio in the range of 2−3, significantly increases the C2 selectivity without affecting the methane conversion drastically. This is specially the case for less diluted reaction atmosphere. In this range, the intensity of generated

reaction heat is enough to maintain the required reaction temperature for converting a significant portion of methane. In highly diluted gas streams, contact time, operating temperature, and, consequently, methane conversion become the defining factors to determine the trend of C2 yield versus methane-to-oxygen ratio. As a result, by increasing the methane-to-oxygen ratio, both methane conversion and C2 yield are continually reduced. All these trends are shown in Figure 10. It can be generally concluded that the quantitative impact of the inert dilution on improving the C2 yield depends on the current range of C2 selectivity, the intensity of the generated reaction heat and the possibility to control the reaction temperature and maintain the desired level of methane conversion. For example, the effect of nitrogen dilution might be different when it is applied to improve the C2 yield from 10% to 15% in comparison to the case when it is applied to improve the C2 yield from 20% to 25%. Observing the relative impact of increasing the methane-tooxygen ratio and nitrogen dilution on the reactor performance is another important outcome of this analysis. Higher methaneto-oxygen ratio reduces the partial pressure of oxygen, while higher nitrogen dilution reduces the partial pressure of both reactants (methane and oxygen) and significantly improves the C2 selectivity. At the same time, exploiting higher methane-to-oxygen ratio causes an extra duty for separating the unreacted methane. Employing nitrogen dilution has even stronger negative impacts because more extreme utilities, operating conditions, and 885


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Figure 11. Effect of nitrogen dilution and methane-to-oxygen ratio on the performance of the OCM membrane reactor in terms of ethylene yield and ethylene-to-ethane products ratio (methane feed flow rate = 2.2 cm3[STP]/s). Left: Analysis of the ethylene yield. Right: Analysis of the ethylene-to-ethane ratio.

In fact, the effect of exploiting the carbon dioxide under the OCM reaction atmosphere has been already investigated and its potential as well as its possible limitations have been highlighted.18,19 Here, in this section, some other aspects of utilizing carbon dioxide as a diluent will be also discussed. 3.3.2. Carbon Dioxide As a Diluent. Similar to the observed effects of nitrogen dilution, reduction of methane conversion and increasing the C2 selectivity are the expected consequences of using carbon dioxide dilution. This was experimentally observed in the current research and has been observed in other researches as well.19 At the same time, there are other aspects such as thermal effect of exploiting the carbon dioxide dilution, which should be also considered in the analysis. In order to have a comparable base for the final analysis, carbon dioxide and nitrogen were introduced to dilute the feed stream of the OCM reactor by 50%. The performance of the OCM reactor under various compositions of carbon dioxide/ nitrogen is represented in Figure 12. As is seen in Figure 12, gradually substituting the nitrogen with carbon dioxide as the diluent agent, usually improves the C2 yield. However, the observed values might be quantitatively different in higher or lower dilution rates, since the dilution and thermal effects are highly interconnected.

expensive equipment are needed to separate it in the downstream units. Eng et al. have reported that the return of investment (ROI) for the OCM process in the case of methane dilution is 14%, while for the nitrogen dilution case, it reduces to 3% ROI.17 This is mainly because of the operating cost in demethanizer which drastically increases by introducing nitrogen dilution. For instance, using methane-to-oxygen ratio 3 and air as the feed composition, 50% nitrogen has to be separated in the downstream units. This increases the operating cost of the cryogenic distillation by factor 4 in comparison to processing the nondiluted gas stream. This will increases the whole operating cost in the OCM process by 20−30% and makes this process economically much less attractive.14 Therefore, the observed trade-off with regard to using the gas dilution should be always taken into consideration through a process-scale analysis. To be more specific in the analysis, the effects of these parameters on the ethylene yield should be highlighted as one of the most important indicators for the performance of an OCM reactor. Typical observed trends of the ethylene yield and the ethylene/ethane products ratio are represented in Figure 11 showing that the methane-to-oxygen ratio has a crucial effect on the ethylene yield in a diluted reaction atmosphere. Figure 11 shows that using lower partial pressure of methane increases the rate of ethylene production. Therefore, the impact of this aspect should be seriously taken into account in concurrent analyzing the performances of the OCM membrane reactor and the whole OCM process. For instance, the type and amount of the required energy in cryogenic distillation to separate the unreacted methane, nitrogen, and ethane are among the main sources of operating costs in the whole OCM process.14 Even if other types of unit operations such as adsorption are used in the downstream of the OCM process, further processing the highly diluted gas stream is very costly and challenging. Therefore, there are several practical constraints on the exploitation of nitrogen or excess methane as the diluent agents in the industrial scale OCM process. After reviewing the challenges associated with using nitrogen and excess methane as the diluent agents, it is also reasonable to look for an alternative for diluting the gas stream inside the OCM reactor. Carbon dioxide is a promising alternative diluent, which can be processed easily in the adsorption and absorption units and even can be later exploited as the reactant in complementary reactions such as dry methane reforming.14

Figure 12. Relative effect of CO2 and N2 dilution on the performance of the OCM fixed bed reactor in terms of C2 yield in selected range of methane-to-oxygen ratio and 50% dilution (methane feed flow rate = 2 cm3 [STP]/s). 886


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Figure 13. Simultaneous observation of the thermal and reaction performance of the OCM membrane reactor in terms of the temperature profile and C2 yield (methane/oxygen = 2; N2 dilution = 51%; nitrogen flow in the tube side = 2 cm3 [STP]/s, nitrogen flow in the shell side = 1.5 cm3 [STP]/s, 42% oxygen content in the shell side, methane feed flow rate = 2.2 cm3 [STP]/s, T1 = oven 1; T2 = oven 2).

selectivity and yield. This also affects the reaction intensity and, consequently, the temperature profile along the reactor. Typical temperature profiles and their corresponding reactor performance in terms of C2 yield are shown in Figure 13. In order to tune the temperature profiles along the catalytic bed, the applied heat-duties in the first and second heating zones were varied by setting different oven temperatures. In this manner, not only the temperature profile but also the distribution of oxygen along the catalytic bed are affected. In order to be more specific, it should be noted that because of the observed unavoidable pressure drop along the catalytic bed, the pressure gradient across the membrane at the end of the membrane reactor is always higher than in the beginning of reactor. As a result, more oxygen permeates into the membrane at the end of the catalytic bed, which leads to the undesired combustion of the already formed C2 products. Therefore, in a desired oxygen-dosing profile, more oxygen should permeate into the tube side in the beginning of the permeable zone. Such a desired oxygen distribution was established by reducing the temperature in the first heating zone. A lower applied temperature practically leads to a lessviscous gas-flow resistance and more oxygen flux across the membrane in that area. As a result, by applying an ascending temperature profile, a better balanced oxygen distribution along the bed was achieved. In this manner, the combination of an improved operating temperature profile and oxygen permeation profile along the reactor can significantly enhance the reactor performance, as shown in Figure 13. In the experimentation, by stepwise reducing the set-point of the oven temperature in zone-I from 695 to 410 °C and increasing the set-point of the oven temperature in zone-II from 700 to 740 °C, the corresponding values of the observed C2 yield were recorded. The observed C2 yield was increased more than 50% in this case by reducing 270 °C from the oven temperature in zone-I and adding 40 °C to the oven temperature in zone-II. In the final analysis, it should be highlighted that the direct effect of preheating on the reaction temperature and the indirect effect of temperature via affecting the oxygen permeation along the membrane are correlated. In one side, the reaction temperature profile changes as a direct result of changing the external heating/cooling duty applied by the

There is also, however, a further point to be considered in the analysis. The diluting effect of the generated CO2 via the OCM reactions should be also considered in this analysis. This is an important factor especially in lower methane-to-oxygen ratio (e.g., CH4/O2 = 2 in Figure 12) where even by implementing 50% nitrogen dilution more than 6% carbon dioxide is generated in the reaction atmosphere, which partially acts as a diluting gas. This factor will be even more crucial when undiluted or low inert diluted feed stream is employed. For example, diluting the feed stream composed of 67% methane and 33% oxygen by adding 20% nitrogen reduces the amount of produced CO2 from 15.9% to 11.4%. As a result, the impact of the generated carbon dioxide in diluting and cooling the reaction atmosphere is reduced. In the already diluted gas stream, either by N2-dilution or CO2-dilution, the C2 yield is usually improved by further increasing the dilution. This is also shown in Figure 12. Beside the safety reasons, this is one of the reasons why in the publications, most of the best OCM results have been reported for the highly diluted feeds. In short, it is important to highlight the existing limitations in interpreting the observed effects of diluting the feeds on the performance of an OCM membrane reactor. Under some operating conditions, the injected CO2 and N2 cools down the reaction temperature significantly and/or the adsorbed CO2 over the catalyst surface causes a reduction in methane conversion and consequently the generated reaction heat. Therefore, dilution might have a bigger impact on reducing the methane conversion rather than improving the C2 selectivity. As a result, sometimes the C2 yield might be reduced by applying higher levels of CO2 or N2 dilution and under some other conditions with lower C2 selectivity, higher reaction intensity and temperature, it may be improved or stay constant. Generally, in view of the impact of gas dilution on the performance of the whole OCM process, using carbon dioxide as a gas diluent provides more advantages in comparison to using nitrogen dilution or excess methane. This is mainly due to the fact that carbon dioxide is handled in the downstream units relatively easier and cheaper than methane or nitrogen. 3.4. Analysis of the Thermal Engineering Aspects in an OCM Membrane Reactor. Exploiting various levels of inert dilution and methane-to-oxygen ratio affect the products 887


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Figure 14. Conceptual schematic of the modified membrane with the uniformed and descending permeation profile.

Figure 15. Effect of nitrogen dilution and methane-to-oxygen ratio on the performance of the OCM membrane reactor using the membranes with the uniformed and descending permeation profiles.

3.5. Comparative Performance Analysis of the OCM Membrane Reactors Using the Uniformed and Descending Permeation Profile. Two ceramic membranes were modified to tailor a uniformed permeation profile or a descending permeation profile along their permeable lengths. Comparing the performance of the OCM membrane reactors, in which these two types of modified membranes are employed, enables better visualizing the impacts of the thermal and diffusion parameters. The observed effects of the nitrogen dilution and methane-to-oxygen ratio on the performance indicators of these two membrane reactors are shown in Figure 15. This figure highlights the relative effects of the nitrogen dilution and methane-to-oxygen ratio on the observed C2 selectivity and C2 yield of the membrane reactors when a uniformed or descending membrane permeation profile is exploited. As is seen in Figure 15, using the descending permeation profile always results in a higher C2 selectivity in comparison to the uniformed modified membrane. This has been also reported by Coronas et al.4 However, comparing the C2 yields is not so straightforward because of the challenge of fixing the level of methane conversion in both cases. By taking this into consideration, the comparable results of these two membrane reactors in terms of the C2 yield are also shown in Figure 15. Here again, the potential of exploiting a descending membrane

surrounding tube furnace. On the other side, tuning the temperature in different zones of the tube furnace affects the distribution of the oxygen and therefore the intensity of reaction along the reactor. As a step toward analyzing the direct and indirect effect of the applied temperature profile, the performance of an OCM membrane reactor with a descending permeation profile was analyzed. The local permeability in the beginning of such membrane reactor is higher and segment by segment its permeability becomes lower along the reaction section, as schematically shown in Figure 14. In order to establish this permeation profile, the amount of oxygen permeation in each local segment of the membrane is tuned by accordingly modifying the diffusion characteristics of the membrane in that segment. The criteria for such a design are to have a big portion of oxygen permeation in the beginning of membrane while too much oxygen permeation in the beginning of reactor and consequently the cofeeding of the reactants are avoided. At the end part of the membrane, the aim is to reduce the permeation but not such that the local methane conversion reduces drastically or the products inside such an almost impermeable tube get overheated for a long time. In general, the length and the permeation profile of the membrane reactor are designed in a way that the coupling reactions remain favored in competition with the combustion and the reforming reactions. 888


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*Tel: +49 30 31424976. Fax: +49 30 31428534. E-mail: oliver. [email protected].

permeation profile and the thermal effect of distributing the reaction intensity along the OCM membrane reactor are highlighted.

Notes

The authors declare no competing financial interest.

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4. CONCLUSIONS The novel BOTZ-coating method described in this paper was successfully applied to modify an α-alumina membrane and shape it to the form of an inert fine distributer of oxygen along the OCM packed bed membrane reactor. It was observed that by reducing the pore volume of the membrane, formation of the low methane-to-oxygen ratio atmosphere inside the membrane porous structure and the subsequent gas phase combustion reactions are prevented. For further improving the C2 selectivity, a descending permeation profile along the modified membrane was established. It was generally observed that the combination of the thermal and reaction engineering parameters crucially affects the performance of the OCM membrane reactor. Operating temperature not only directly affect the reactor performance but also via affecting the permeation and distribution of oxygen along the membrane. Implementing a combination of an ascending temperature profile and a descending permeation profile along the membrane reactor leads to a significant increase in the C2 yield. Based on the performed analysis, several structural and operating parameters should be properly exploited to fulfill the following requirements: (1) establishing an optimum heat removal mechanism to prevent the unacceptable level of hot spot formation but maintain the required level of local reaction temperatures to ensure homogeneous distribution of reaction intensity along the reactor and secure the total methane conversion in the range of higher than 35%, (2) establishing a descending oxygen permeation profile along the membrane while reforming of the products at the last section of the membrane are prevented, (3) establishing an ascending temperature profile along the reactor and aim to have a preheating zone, which not only facilitates permeation of the colder oxygen in the beginning of reactor but also heats up the reactants to reach 750 °C before hitting the catalytic bed. Higher methane-to-oxygen ratio reduces the partial pressure of oxygen and increases the C2 selectivity in the reactor. On the other hand, it reduces the cost of CO2 removal but increases the cost of expensive operations in the downstream units to separate the unreacted methane. Using CO2 as a diluent agent also improves the C2 selectivity in the reactor but only increases the load of the CO2 removal in the downstream units. Carbon dioxide is therefore suggested to be a better diluent, especially when its exploitation also addresses the thermal characteristics of operation in the OCM reactor. Therefore, the proper level of methane-to-oxygen ratio and the carbon dioxide dilution are determined with respect to their impacts on both thermal and reaction performance of the OCM reactor as well as on the whole OCM process performance. A promising 25.5% C2 yield (20.3% ethylene) with 66% C2 selectivity were achieved using the 1.9% Mn−5% Na2WO4/ SiO 2 catalyst under very low (20%) diluted reaction atmosphere which is one of the highest ethylene yields so far reported under such industrially relevant operating conditions.

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ACKNOWLEDGMENTS The authors acknowledge the scientific and technical advices of Professor Coronas and professor Menendez from University of Zaragoza and financial support from the Cluster of Excellence “Unifying Concepts in Catalysis” coordinated by the Technische Universität Berlin and funded by the German Research FoundationDeutsche Forschungsgemeinschaft.

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NOMENCLATURE C2 = ethane + ethylene Descending permeation profile = modified membrane with nonuniformed membrane permeation pattern (the permeability of the membrane reduces along the reactor) FBR = fixed bed reactor ID = inner diameter methane conversion = portion of the inlet methane converted to the desired and undesired products OCM = oxidative coupling of methane OD = outside diameter PBMR = packed bed membrane reactor Selectivity = portion of the whole consumed methane which appears in the desired products STP = standard temperature and pressure (298 K and 1 bar) T = temperature (°C or K) UniCat = “Unifying Concepts in Catalysis”: a research group in Berlin Uniformed modified membrane = modified membrane with uniformed membrane permeation pattern (the local permeability of the membrane is constant all along the membrane) Yield = portion of the inlet methane, which appears in the desired products REFERENCES

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Corresponding Authors

*Tel: +49 30 31421619. Fax: +49 30 31426915. E-mail: hamid. [email protected]. 889


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and Catalytic Oxidative Coupling over Mn/Na2WO4/SiO2 Catalyst. Ind. Eng. Chem. Res. 2006, 45, 7090−7095. (9) Coronas, J. Sintesis de Hidrocarburos por Acoplamiento Oxidativo de Metano, Utilizacion de Reactores de Membrana. PhD Thesis, University of Zaragoza, March 1994. (10) Hou, S.; Cao, Y.; Xiong, W.; Liu, H.; Kou, Y. Site Requirements for the Oxidative Coupling of Methane on SiO2-Supported Mn Catalysts. Ind. Eng. Chem. Res. 2006, 45, 7077−7083. (11) Simon, U.; Görke, O.; Berthold, A.; Arndt, S.; Schomäcker, R.; Schubert, H. Fluidized Bed Processing of Sodium Tungsten Manganese Catalysts for the Oxidative Coupling of Methane. Chem. Eng J. 2011, 168, 1352−1359. (12) Wang, D.; Rosynek, M. P.; Lunsford, J. H. Oxidative Coupling of Methane over Oxide-Supported Sodium Manganese Catalysts. J. Catal. 1995, 155, 390−402. (13) Lee, J. Y.; Jeon, W.; Choi, J. -W.; Suh, J. -W.; Ha, J. -M.; Suh, D. J.; Park, Y. -K. Scaled-up Production of C2 Hydrocarbons by Oxidative Coupling of Methane over Pelletized Na2WO4/Mn/SiO2 Catalysts: Observing Hot Spots for the Selective Process. Fuel 2013, 106, 851− 857. (14) Godini, H. R.; Xiao, S.; Jašo, S.; Stünkel, S.; Salerno, D.; Nghiem Xuan, S.; Song, S.; Wozny, G. Techno-economic Analysis of Integrating the Methane Oxidative Coupling and Methane Reforming Processes. Fuel Process. Technol. 2013, 106, 684−694. (15) Czechowicz, D.; Skutil, K.; Taniewski, M. Nitrogen-Rich Natural Gases as a Potential Direct Feedstock for Some Novel Methane Transformation Processes. Part 1: Oxidative Processes. Energy Fuels 2009, 23, 4438−4448. (16) Tye, C. T.; Mohamed, A. R.; Bhatia, S. Modeling of catalytic reactor for oxidative coupling of methane using La2O3/CaO catalyst. Chem. Eng. J. 2002, 87, 49−59. (17) Eng, D.; Chiang, P.-H.; Stoukides, M. Methane Oxidative Coupling: Technical and Economic Evaluation of a Chemical Cogenerative Fuel Cell. Energy Fuels 1995, 9, 794−801. (18) Wang, S.; Zhu, Y. H. Catalytic Conversion of Alkanes to Olefins by Carbon Dioxide Oxidative Dehydrogenations. Energy Fuels 2004, 18, 1126−1139. (19) Smith, K. J.; Galuszka, J. Effect of CO2 on Methane Oxidative Coupling Kinetics. Ind. Eng. Chem. Res. 1994, 33, 14−20.

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Performance Analysis of a Porous Packed Bed Membrane Reactor for

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