Contracting Organization : Worcester Polytechnic Institute 100 Institute Road Worcester, MA Contract Number : DE-AC22-92PC92113

Transcription

1 Contracting Organization : Worcester Polytechnic nstitute 1 nstitute Road Worcester, MA 169 Contract Number : DEAC2292PC92113 Title of the Report : Quarterly Technical Progress Report Contract Title of the Project: Methane Coupling by Membrane Reactor Reporting Period : Project Manager : Professor Yi Hua Ma Department of Chemical Engineering Worcester Polytechnic nstitute 1 nstitute Road Worcester, MA 169 Date of Report :

2 DSCLAMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any spccific commercia1 product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

3 TABLE OF CONTENTS ABSTRACT PROJECT OBJECTVE SYNTHESS OF MnWNalSiO, CATALYST 4 2. METHANE OXDATVE COUPLNG REACTONS N A CONVENTONAL PACKEDBED REACTOR WTH THE MnWNalSiO, CATALYST 4 3. COATNG OF MnWNa/SiO, CATALYST ON THE NSDE WALL OF THE DENSE MEMBRANE TUBE AND TS OXYGEN PERMEANCE MEASUREMENT 5 4. METHANE COUPLNG WTH THE MnWNalSiO, CATALYST N THE DENSE MEMBRANE REACTOR 6 5. MODFCATON OF A STABLZED GAMMA ALUMNA MEMBRANE TUBE WTH SLCA 6 FUTURE WORK 7 1

4 ABSTRACT A new catalyst, 1.9%(wt) Mn5%(wt) Na,WO,/SiO, was synthesized by the incipient wetness impregnation method. Xray diffraction studies of the catalyst calcined at 8 C showed that cristobalite, Na,WO,, and Mn23were the three phases that coexisted in the catalyst. Calcination at 1 C resulted in the formation of a new SiO, phase (tridymite). The new catalyst was characterized by running the methane oxidative coupling reactions in a conventional packedbed reactor. The highest C, yield obtained was 25 %. The MnWNa/SiO, catalyst was coated on the inside wall of the SrFeCo,,3~, dense membrane tube. A methane oxidative coupling experiment was conducted with the new catalyst in a dense membrane reactor. The oxygen permeance of the dense membrane tube coated with the catalyst was about.5 cc/min/cm2 at 85 C. By cofeeding methane and oxygen to the tube side of the reactor, C, yields up to 7 % were observed in these runs. To prepare the membrane tube for the radialflow reactor, silica was deposited on a lanthanumstabilized gamaalumina membrane tube and gas permeances were measured after the membrane tube was calcined at different temperatures. The pressure drop across the membrane was high enough for the membrane to be used in the radialflow reactor. 2

5 .. PROJECT OBJECTVE The goal of this research is to improve the hydrocarbon yield from oxidative coupling of methane by using a catalytic inorganic membrane reactor. A specific target is to achieve conversion of methane to C, hydrocarbons at very high selectivity and relatively higher yields than in a fixed bed reactors by controlling the oxygen supply through the membrane. A membrane reactor has the advantage of precisely controlling the rate of delivery of oxygen to the catalyst. This property permits balancing the rate of oxidation and reduction of the catalyst. Membrane reactors could also produce higher product yields by providing better distribution of the reactant gases over the catalyst than the conventional plug flow reactors. 3

6 QUARTERLY REPORT Report for the Period :9/25/9612/24/96 1. Synthesis of NaMnW/SiO, Catalyst The NaMnW/SiO, catalyst was prepared by incipient wetness impregnation, at 885"C, of a silica gel support (Davison, grade 643) with aqueous solutions with appropriate concentrations of Mn(NO,), (Aldrich, 98 %) and Na,WO, (Alfa, 98 %). Then the catalyst was dried at 13 C for 8 h, calcined at 85 C for 8 h, pelletized, calcined at 1 C for 4 hours, crushed and sieved to 356 mesh size., The catalysts calcined at two different temperatures (85 C and 1 C) were studied by Xray diffraction technique. Figures 1 and 2 show the XRD spectra of the catalysts calcined at 85 C and 1 C respectively. Three crystal phases (cristabolite, Na,WO,, and Mn,3) were observed in the catalyst calcined at 85 C. An extra phase was observed when the catalyst was calcined at 1 C. Calcination at 1 C resulted in the partial transformation of cristabolite into a new SiO, phase tridymite. 2. Methane Oxidative CouDling Reactions in a Conventional Packedbed Reactor with the MnWNa/SiO, Catalvst The MnWNa/SiO, catalyst was characterized by running the methane coupling reactions in a nonporous alumina, packedbed reactor. A quartz thermowell (D=l mm; OD=3 mm) was placed concentrically in the nonporous alumina tube (D=7 mm) gram catalyst was packed in the annular space between the alumina tube and the thermowell. The length of the packedbed was 14 cm. The reaction temperature was kept at 78 C during these runs. The flow rates of oxygen and helium were 4.1 cc/min 4

7 . and 44.3 cc/min, respectively. The feed ratio of methane to oxygen was varied by changing the flow rate of methane fed to the reactor inlet. Figure 3 shows the C, selectivity and C, yield as functions of methane conversion. Figure 4 gives the methane conversions, C, selectivities, and C, yields obtained at different methane to oxygen feed ratios. The highest C, yield observed was 25.2% with a C, selectivity of 55%. t is obvious that the newly synthesized catalyst is much better than the catalysts (Sm23 and La/MgO) used in the previous studies, in which the C, yields obtained were less than 11 %. At a methane conversion of about 37 %, 24 % C, yield and 65 % C, selectivity was obtained. Even at a high C, selectivity (85%), a reasonably high C, yield (12%) was achieved. 3. Coatin? of MnWNa/SiO,Catalvst on the nside Wall of the Dense Membrane Tube and ts Oxvgen Permeance Measurement Before coating the catalyst on the SrFeCo,,,O,, dense membrane tube, the inner surface of the membrane was pretreated with 1 N hydrochloric acid for 2 minutes at room temperature, washed with diluted sodium hydroxide solution and deionized water, and dried at 12 C for half an hour. Then the colloidal silica was coated on the inner surface of the membrane tube, and the membrane tube was dried at 9 C for 1 h, and 11 C for 2 hours. The silica gel was impregnated with appropriate amounts of aqueous solutions of Mn(NO,), and Na,WO, and dried at 13 C for 4 hours. Finally, the membrane tube was calcined at 85 C for 8 h. The MnWNa/SiO, catalystcoated dense membrane tube was fabricated in a membrane reactor module and its oxygen permeance was measured at 85 C. With air fed to the shell side (35 cc/min) and helium to the tube side (54 cc/min), the measured oxygen permeance was.54 cc/min/cm2. This value was about five times higher than that of the perovskitecoated dense membrane tube. 5

8 4. Methane Couplinp with the MnWNa/SiO, Catalyst in the Dense Membrane Reactor The methane oxidative coupling reaction was conducted in the MnWNa/SiO,catalyst coated dense membrane reactor. The same catalyst was also packed inside the membrane tube (catalyst loading was 1.95g). Figure 5 shows the experimental results obtained by cofeeding methane and oxygen (diluted with helium) to the tube side of the reactor. The flow rates of oxygen and helium were kept at 2 cc/min and 54 cc/min. The variation of methane conversion was realized by changing the flow rate of methane fed to the reactor. As can be seen in Figure 5, C, selectivity decreases as methane conversion increases. The C, yields in these runs were less than 7%, which is much less than those obtained in the conventional packedbed reactor as shown in Figures 3 and 4.The low C, yields may have resulted from the fact that the reaction mixture was still exposed to the dense membrane surface, which was not completely covered by the catalyst coated on the inner surface of the membrane tube. 5. Modification of a Stabilized Gamma Alumina Membrane Tube with Silica As shown by our previous results, after modification and thermal treatment at 9 C for 5 hours, the nitrogen permeance of the gammaalumina membrane tube was lower than that of the fresh gammaalumina membrane tube. Due to its smaller pore size in the top layer, the modified gamma alumina membrane tube has much higher resistance to the gas flow across the membrane than that of the alpha alumina membrane tube, which was used in our previous radialflow reactor setup. The use of a radialflow reactor with the La(NO,),modified gammaalumina membrane tube for methane oxidative coupling reactions will allow us to achieve better distribution of catalyst in the membrane toplayer and control of the reaction mixture flow through the membrane. The gamma alumina membrane tube was first stabilized by the treatment with 6

9 La(NO,), solution followed by calcination at 9 C. Since the catalyst that will be used in the radialflow reactor is supported on silica, the stabilized gamma alumina membrane tube was treated with tetramethyl orthosilicate (TMOS), and calcined at different temperatures. Figure 6 shows the gas permeances of the gamma alumina tube before and after TMOS modification. Even after 8 C calcination, the gas permeance was about 1 order of magnitude lower than that of the Lastabilized gamma alumina membrane tube before the TMOS modification. For a twocentimeterlong membrane tube (7 mm D) after 8 C calcination, the pressure drop across the membrane was 85 psi for a nitrogen flow rate of 8.2 cc/min through the membrane. Therefore, the pressure drop across the membrane is high enough to control the flow rate of gas through the membrane in the crossflow reactor. FUTURE WORK The stability of the catalyst and the effects of other factors on the catalyst performance (such as dilution gas flow rate and temperature) is being investigated. Both dense membrane reactor and porous gamma alumina membrane reactor configurations will be tested with the catalyst to evaluate their performances compared to the conventional packedbed reactor. A radialflow reactor with the La(NO,),modified gammaalumina membrane will be used to study the methane oxidative coupling reactions. Due to its smaller pore size in the top layer, the modified gamma alumina membrane tube has much higher resistance to the gas flow across the membrane than that of the alpha alumina membrane tube, which was used in our previous radialflow reactor setup. This will allow us to achieve better distribution of catalyst in the membrane toplayer and control of the reaction mixture flow through the membrane. 7

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11 d *2 x, X 2 2

12 s O methane conversion, % Fig. 3. Variations of C2 selectivity and yield with methane conversion for methane oxidative coupling with a WMn Na/SiO*catalyst in nonporous alumia packedbed reactor (T=78 C; He=44.3 cclmin; 2=4.1 cc/min; Wcat=l.95 gm)

13 .. Methane conversion,c, yield and selectivity, % a h) )o CD 1 x L cr, s V Q Q) l

14 t methane conversion, % 45 Fig. 5. Methane oxidative coupling in a dense membrane reactor setup operated in cofeed mode

15 1E+2 ko OOOOOOOOH~ E lo 5.2 La stabilized gamma alumina membrane (after 9 C calcination) a E a 2 after TMOS modification 1 O af 6E H, (8OOOC) 1 A A a tij Q 4 H,(6 C) A A A N, (8OOOC) A N, (6OOOC) a n A A H, (4OOOC) *ON pressure difference across the membrane, psi 1 Fig. 6. Gas permeances at 15OOC) through TMOS modified Lastabilized gamma alumina membrane calcined at different tem perat ures

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