Enhanced Methanol Yields from the Direct Partial Oxidation of

A few experiments were done with quartz-lined stainless steel tubes of 0.5- and 1.0-cm i.d. ... Safety features included a pressure-relief valve that ...
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Ind. Eng. Chem. Res. 2002, 41, 6528-6536

Enhanced Methanol Yields from the Direct Partial Oxidation of Methane in a Simulated Countercurrent Moving Bed Chromatographic Reactor Mark C. Bjorklund and Robert W. Carr* Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455

A laboratory-scale simulated countercurrent moving bed chromatographic reactor (SCMCR) for the direct, homogeneous partial oxidation of methane to methanol has been constructed and tested. Reaction conditions were evaluated from independent experiments with a single-pass tubular reactor. Separation was effected by gas-liquid partition chromatography with 10% Carbowax on Supelcoport. At the optimal reaction conditions of 477 °C, 100 atm, and feed methane-to-oxygen ratio of 16, the methane conversion in the SCMCR was 50%, the methanol selectivity was 50%, and the methanol yield was 25%. Factors affecting methane conversion were investigated. Possible ways to improve the methane conversion, and hence the methanol yield, are discussed. Introduction The direct partial oxidation of methane to methanol

CH4 + 1/2O2 f CH3OH

(1)

is a leading candidate for conversion of methane from remote natural gas reserves to a transportable fuel and chemical feedstock. The reaction has been extensively investigated from the early years of the 20th century onward, with increasing activity in the 1980s and 1990s. A number of reviews of the subject have been published.1-9 Reaction 1 can be carried out over solid catalysts or as a homogeneous gas-phase reaction. In either case, to optimize methanol selectivity and limit oxidation to CO and CO2, it is necessary to work with CH4/O2 sufficiently in excess of the stoichiometric ratio that single-pass methane conversion is limited to only a few percent. Methanol selectivity varies significantly with reaction conditions; literature values ranging from only about 0.2 or less to a high of 0.96110 have been reported. Because the best methanol selectivities occur at low methane conversions, methanol yields have been limited to less than 10%, and in most reports, they are significantly less than 10%. A successful commercial process would presumably require a large recycle stream and, hence, a significant capital equipment expense. No full-scale plant for direct partial oxidation of methane to methanol is currently in operation. It seems likely that novel reactor designs in which the methane conversion could be significantly increased might make process economics sufficiently favorable to spur commercialization efforts. The simulated countercurrent moving bed chromatographic reactor (SCMCR) is a candidate for this purpose. The SCMCR is a separative chemical reactor in which reaction and separation are integrated into a single process unit.11 The reaction can be catalyzed or not, and the separation of reactant(s) from product(s) is accomplished by solid adsorbents. The SCMCR is capable of taking the conversion of equilibrium-limited reactions and intrinsically low-conversion-

per-pass irreversible reactions to significantly higher levels than is possible in nonseparative reactors. In principle, complete conversion is possible provided that the SCMCR is carefully designed. The SCMCR achieves improvements in reactant conversion by separation of reaction products from reactants and removal of products from the reactor. The SCMCR generally influences selectivity only to the extent that variation in chemical composition, due to the separation, changes the reaction chemistry. It has been shown that, when the oxidative coupling of methane to ethylene and ethane is carried out in an SCMCR, the yield of the C2 products is significantly improved over single-pass conventional reactor performance.12,13 Methane partial oxidation is similar to oxidative coupling in the sense that, because of the high methane-to-oxygen ratio required, it is a low-conversionper-pass reaction. The investigation of methane partial oxidation reported in this paper was undertaken in anticipation that the SCMCR would provide substantially improved yields of methanol. In this work, reaction 1 was carried out homogeneously in the gas phase, as it has been demonstrated that there is no advantage in using solid catalysts.10,14,15 A three-section SCMCR configured for high-temperature reactions13 was employed. Experimental Apparatus Single-Pass Reactor. A single-pass tubular reactor capable of withstanding pressures in excess of 100 atm was used to characterize the partial oxidation reaction. The apparatus for these experiments consisted of gas cylinders to supply CP-grade O2 and CH4 and HP-grade He, which was used as the carrier gas in the SCMCR experiments. The O2 and CH4 flow rates were set with MKS high-pressure mass flow controllers. The three flows were passed through a preheated mixing volume and into a fused-silica-lined stainless steel tubular reactor fitted with Swagelok connectors. The flexible 1/16in.-o.d. reactor tubes were supplied by Supelco. The reactor was heated by heating tape and covered with

10.1021/ie0201617 CCC: $22.00 © 2002 American Chemical Society Published on Web 11/14/2002

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Figure 1. Schematic diagram of the simulated countercurrent moving bed chromatographic reactor for the direct partial oxidation of methane to methanol.

glass wool insulation. The temperature was measured and controlled using a Digisense temperature controller with a thermocouple probe placed between the outer wall of the reactor and the heating tape. Heating tape was also used to keep connecting and analysis lines heated to avoid condensation of liquid products. The reactor effluent passed through a capacitance manometer, a pressure-relief valve, and a metering valve that was set to control the total flow rate. Downstream from the metering valve, the pressure was reduced to just above 1 atm. A few experiments were done with quartzlined stainless steel tubes of 0.5- and 1.0-cm i.d. The emerging low-pressure stream was split, and a small, constant flow was taken for analysis by a Varian 6000 gas chromatograph (GC) equipped with flame ionization and thermal conductivity detectors. The products were analyzed using a 6 ft × 1/8 in. o.d. analytical column packed with 80/100 mesh Poropak QS, followed by a 6 ft × 1/8 in. o.d. analytical column packed with 80/100 mesh Carbosphere (Alltech). All of the carbon-containing species from the analytical columns were reduced to methane by admixture with H2 and passage over a 2% nickel catalyst supported on Chromosorb G (obtained from Varian) at 350 °C before entering the detectors. With this methanizer, it was necessary to calibrate the GC only for CH4. A Gateway P5-120 computer controlled the GC sample valve and quantified and displayed the chromatographic results using Labtech Notebook (v8.1) software. Separation. The separative characteristics of various granular adsorbents were tested with multicomponent samples that were typical reaction product mixtures. An experimental arrangement in which a single stainless steel column, packed with a granular solid for the separation, was inserted between the reactor and capacitance manometer of the single-pass reactor arrangement described above was employed for these experiments. No other changes were made to the singlepass reactor configuration. The SCMCR. A schematic of the SCMCR is shown in Figure 1. It consisted of a 1.8 m × 1 mm i.d. fused-

silica-lined tubular reactor connected to the gas supplies and high-pressure mass flow controllers as described above and three 1.8 m × 5.3 mm i.d. cylindrical stainless steel packed columns. All components were connected by 1-mm-i.d. fused-silica-lined tubing that was heated to prevent condensation. Six high-pressure, multiposition rotary valves purchased from Valco Instruments Co., Inc., were used to switch the internal flows among the packed columns at timed intervals (the switching time or switching interval). Safety features included a pressure-relief valve that was typically set to release above 110 atm and programming of the PC to shut down the flow controllers if the pressure recorded at the capacitance manometer went above or below certain thresholds. For some of the last SCMCR experiments, the CH4 flow controller ceased to control the flow, so a metering valve was placed before the controller, which then functioned only as a flow meter. The GC analysis system was identical to the arrangement of the single-pass reactor studies. The GC could be set up to sample reactor effluent streams automatically at a preset rate. Typically, sampling was done at 40-s intervals. The GC peak heights were averaged over many switching times. They were then integrated over the switching interval to determine stream compositions. Results Single-Pass Reactor Studies. CP-grade CH4, O2, and N2 were used in all experiments. The principal reaction products were CH3OH, CO, CO2, and H2O. The carbon-containing products were quantitated after reduction to CH4. Experiments were carried out to determine the effects of pressure, temperature, CH4/O2 feed ratio, and percent carrier gas on the selectivities for CH3OH, CO, and CO2 and the methane conversion. Other factors such as preheating, premixing, and surfaceto-volume ratio in the reactor were also addressed. The residence time in most experiments was about 10 s. Table 1 shows the experimental results. The data entries are not individual points but rather averages of several points taken at various times during the investigation. The particular conditions listed were chosen to illustrate the effect of changing conditions on the base case, which is the 1.8 m × 1 mm i.d. reactor under the conditions eventually used for the SCMCR experiments and listed as run 1 in Table 1. All experiments listed were conducted under conditions such that the O2 consumption was 100%. The methane conversion, XCH4, was determined from

XCH4 ) [FCH4,0 - FCH4]/FCH4

(2)

where FCH4,0 is the inlet flow rate of CH4 and FCH4 is the outlet CH4 flow rate, obtained by GC analysis and the total exit flow rate. The selectivity for the ith chemical species, Si, was determined from the GC response of the ith reaction product, Rip, divided by the sum of the responses of all products detected

Si ) Rip/ΣRip

(3)

Equation 3 was applied only to carbon-containing species detected as methane by the GC. In addition to the major products CH3OH, CO, CO2, and H2O, other GC peaks due to minor products were seen. The total yield these products was generally less

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Table 1. Reactor-Only Experiments run no.a

T (°C)

pressure (psig)

CH4/O2

He (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25c 26d 27e 28f

477 460 465 470 475 480 485 490 477 477 477 481 489 499 515 535 445 461 481 490 499 513 530 554 477 477 495 488

1450 1450 1450 1450 1450 1450 1450 1450 1450 1450 1450 1250 1000 750 500 250 1450 1450 1450 1450 1450 1450 1450 1450 1450 1450 1450 1450

16.3 16.3 16.3 16.3 16.3 16.3 16.3 16.3 12 20 25 16.3 16.3 16.3 16.3 16.3 16.3 16.3 16.3 16.3 16.3 16.3 16.3 16.3 16.3 16.3 16.3 16.3

63 63 63 63 63 63 63 63 63 63 63 63 63 63 63 63 0 20 40 50 60 70 80 90 63 63 63 63

XCH4c

SCO

SCO2

SMeOH

6.1 0 5.56 5.97 6.05 6.1 6.03 5.95

34.6 0 30 33.5 34.1 36 37.5 38.1 38.2 30.7 26.2 33.8 34.5 34.1 33.8 34.1 25.1 27.5 28.3 32.1 34 34.9 33.2 0 35.3 35.7 27.4 29

12.2 0 9.7 10.9 11.5 13.6 16.9 24 17.5 10.4 6.9 12.8 13.9 14.1 14.5 15.9 7.9 8.5 9.7 11.5 11.9 12.6 12.9 0 13.4 14.1 10.3 11.4

50.1 0 57.1 52.3 51.1 47.5 42.3 34.9 40.4 54.9 62.8 48.1 45.9 45.2 42.3 35.1 63.4 60.9 58.8 52.9 50.5 47 35.2 0 48.4 46.8 57.2 54.6

6.3 6.25

a Runs 1-25 in 180 cm × 1 mm i.d. fused-silica-lined reactor. Conversion was not measured where blank. c Without preheat. d Without premixer. e 1-cm-i.d. quartz-lined stainless steel reactor. f 0.5-cm-i.d. quartz-lined stainless steel reactor. b

than 5% of the yield of major products. The minor products were not positively identified, but they were quantitated as CH4 by the GC analysis. Under conditions similar to those used in this work, Gesser et al.16 performed detailed analyses, finding small amounts of H2, C2H6, C2H4, C3H8, formaldehyde, acetaldehyde, acetaldehyde, methyl formate, formic acid, dimethoxymethane, acetone, dimethyl ether, and ethanol from the homogeneous partial oxidation of methane. Several other investigators have reported small yields of formaldehyde,17-21 acetaldehyde,17-21 ethanol,21 C2H6,19-21 and C2H4.20,21 Formation of these products can be explained by the free-radical chain mechanism by which this low-temperature oxidation occurs.1,8,17,22 The selectivities of CH3OH, CO, and CO2 sum to less than 100% for all of the runs reported in Table 1. (Not shown in the table.) For runs carried out at near-optimal conditions, the sum varied from about 95 to 97%, and it was smaller for runs at higher temperatures. Because eq 3 and its analogues for CO and CO2 give selectivities based on the total amount of carbon converted, the failure to sum to 100% must be due to the formation of other carbon-containing products. These are expected, according to the literature quoted above, and evidence for them was seen in the minor peaks observed by GC. Figure 2 shows the dependence of XCH4 and SMeOH on temperature for runs 1-8. Methane conversion is undetectable at temperatures lower than 460 °C. At 460 °C, the reaction rate accelerates, and XCH4 and SMeOH increased rapidly. Methane conversion jumps to about 90% of its maximum (corresponding to XO2 ≈ 90%) and then increases slowly up to complete O2 consumption, where XCH4 ) 6.1%. All experiments exhibit similar behavior, but the temperature for the onset of fast reaction depends on the experimental conditions. The observed value of XCH4 depends on the selectivities of

Figure 2. Dependence of methane conversion and methanol selectivity on temperature at 100 atm in the single-pass reactor.

the different products, as different amounts of methane are consumed per mole of O2 consumed for each product formed. Thus, there is a slight decrease in XCH4 as SMeOH decreases at higher temperatures. Methanol selectivity tends to be favored at lower temperature, just above the onset of fast reaction. In the SCMCR, it is preferable to run at ∼100% O2 conversion to minimize oxidation of CH3OH. Thus, the SCMCR experiments described below were conducted at 477 °C instead of lower temperatures that give slightly higher values of SMeOH but lower XO2. Experiments 12-16 in Table 1 show that SMeOH increases with increasing pressure. This trend has been reported without exception in the literature. The selectivities shown are optimized for temperature at each pressure. As pressure increases, the temperature of the optimal SMeOH decreases. The temperature for the onset of fast reaction in these experiments is somewhat higher than that found by most other workers. This difference can be attributed to the approximately 60% dilution with He here. Chelappa et al.25 found that, with air as the oxidant, higher temperatures are needed for high conversion, probably because of the N2 present. The dependence of SMeOH on dilution with carrier gas is shown by experiments 1724 in Table 1. As the percentage of He is increased, higher temperatures are required to achieve complete O2 consumption, and SMeOH decreases. At 90% He, for example, a temperature of 554 °C was insufficient to ignite the reaction. A further increase in the temperature to achieve ignition was not attempted because it was apparent that this would cause SMeOH to be unacceptably low. Table 1 also shows that SMeOH increases with increasing CH4/O2, in accord with other literature reports.1,8,21,23-25 Thomas et al.26 reported that a low surface-to-volume ratio favors SMeOH. This finding appears to be borne out by runs 27 and 28 in Table 1. The 1-cm-i.d. quartz-lined reactor gives slightly better SMeOH values than the 0.5cm-i.d. quartz-lined reactor for otherwise similar conditions, and both give better SMeOH values than the 1-mmi.d. reactor. However, the effect is not very pronounced, as the selectivity decreased by only about 12% over the 10-fold increase in surface-to-volume ratio. Separation Studies. The behavior of granular, solid packing material toward typical reaction product mixtures was investigated by means of the single-pass tubular reactor/single adsorbent bed in series arrange-

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ment described in the Experimental Section. A flow of reactants and carrier gas was introduced into the reactor, and the emerging reaction mixture entered the packed bed. The flow continued until all components of the mixture had broken through the bed. Then, the reactant flows were shut off, and the He flow was left on until all components had desorbed. The shape of the breakthrough and desorption curve could be determined by taking samples for GC analysis at short time intervals. Methane concentrations could be measured every 40 s. Other products such as CH3OH need longer analysis times, so many experiments had to be done, with samples taken at different times to determine the breakthrough. In later experiments, the SCMCR was modified to conduct adsorption/desorption experiments. For these experiments, the feed and carrier were turned on until all components had broken through the first section; then the flows were switched to the second section, and the reactant flow (but not the He flow) was simultaneously shut off. All species were allowed to flow through the second section, and the breakthrough and desorption curves of CH4 and CH3OH were determined. Such adsorber dynamics studies are essential in understanding SCMCR performance. Activated carbon was the first adsorbent tested, but it was found to adsorb CH3OH very strongly and to give unacceptably broad CH4 desorption profiles. This would cause unreacted CH4 to be purged from the SCMCR, reducing XCH4 and hence the CH3OH yield. This effect has been previously discussed in connection with CH4 oxidative coupling in an SCMCR.13 Furthermore, it seemed likely that any high-surface-area adsorbent would be unsatisfactory because, at the high pressures required for CH3OH production, diffusion from pores would be slow and would cause broadening of desorption curves and consequent CH4 loss. To avoid tailing, packings for gas-liquid partition chromatography were tested. Such packings consist of a liquid stationary phase distributed on an inert solid support. Separation occurs by partitioning of components between the mobile (gas) phase and the liquid phase. A satisfactory packing was found to be 80/100 mesh Supelcoport coated with 10 wt % Carbowax. The polar liquid was effective for CH4-CH3OH separation, with CH4 passing through essentially unretained. Independent GC experiments gave equal retention times for N2 and CH4 on Supelcoport/10% Carbowax, confirming the absence of any significant interaction of CH4 with this packing. Figure 3 compares CH4 breakthrough and elution profiles for activated carbon and the Supelcoport/10% Carbowax packing. The latter gives a nearly ideal block wave form for CH4, and the former shows the tailing that would lead to diminished SCMCR performance. Figure 4 shows the adsorption/desorption characteristics of a typical reaction mixture consisting of CH4, CO, CO2, CH3OH, and H2O on a column packed with Supelcoport/10% Carbowax. The reaction mixture is divided into two fractions, one consisting of CH4, CO, and CO2, and the other of CH3OH and H2O. In experiments at various temperatures, it was found that the CH4 block wave was unaffected by temperature, that the CH3OH breakthrough time decreased with increasing temperature, and that the shape of the CH3OH block wave was not dependent on temperature. Thus, the breakthrough times could be effectively adjusted by

Figure 3. Comparison of methane breakthrough and desorption profiles for activated carbon and for 10% Carbowax on 80/100 mesh Supelcoport.

Figure 4. Elution of CH4, CO2, CO, CH3OH, and H2O from 10% Carbowax on 80/100 mesh Supelcoport.

controlling the temperature. Figure 4 shows the desired case27 where the breakthrough time for a more strongly retained component (CH3OH here) is slightly greater than twice the breakthrough time for the less strongly retained one (CH4 here). Thus, if the CH3OH breakthrough time is less than 2 times the CH4 breakthrough time, some of the CH3OH will be lost because it will break through the packed bed before flows are switched, and this lead portion will be returned to the reactor and be further oxidized. Furthermore, if the CH3OH retention time is much greater than 2 times the retention time of CH4, the extra carrier flow rate must be

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increased to remove all of the CH3OH during one switching time. SCMCR Experiments. The SCMCR configuration of Figure 1, with the 1.8 m × 1 mm i.d. silica-lined tubular reactor and 1.8 m × 5.3 mm i.d. stainless steel columns packed with 10% Carbowax on Supelcoport, was used for all experiments. For SCMCR operation, all valves labeled 1 in Figure 1 are initially open, and CH4 and O2 are introduced into the tubular reactor with CH4/O2 ) 16.3. The reactor effluent is directed to the first packed bed on the left. At the end of the first switching interval, when CH4 breaks through the first bed, all no. 1 valves are closed, and all no. 2 valves are simultaneously opened so that the reactor effluent now enters the middle bed. At the same time, the reactor feed composition is changed to the makeup ratio, CH4/O2 ) 2, to replace the quantities of CH4 and O2 reacted. The makeup feed ratio is maintained during all subsequent switching periods. At the end of the second switching period, all no. 2 valves are closed, and all no. 3 valves are opened, allowing the reaction mixture to enter the right-hand bed. At the end of the third switching period, all no. 3 valves are closed, all no. 1 valves are opened, and the cycle is repeated. The He carrier gas is introduced one packed bed upstream of the bed taking the reaction mixture so that unreacted CH4 is returned to the tubular reactor. Extra carrier is introduced into the bottom of the second upstream bed to strip the reaction products from the SCMCR by reverse flow. These streams can be cycled through the SCMCR indefinitely. In preliminary experiments for a particular SCMCR run, the CH4 profiles were monitored by taking samples for GC analysis every 40 s. The He flow rate or the switching time could then be adjusted to ensure that the flows were switched an instant before the CH4 front breaks through a packed bed. The CH3OH profiles in the product stream were also monitored to determine whether the separation column temperature and extra carrier flow rate needed adjusting. If CH3OH were breaking through too soon, the temperature could be reduced. If the extra carrier were not purging all of the CH3OH, its flow rate could be increased. Optimal reactor conditions could be set by observing the methane and methanol GC peaks after a switch had been made and then making necessary adjustments. When appropriate conditions were found, the system was allowed to reach a periodic steady state, typically requiring 1 h or more. Then, the purge and product streams were sampled and analyzed for all products. The GC peak heights were calibrated and averaged over many switching times, and then, they were integrated over the switching time to determine the quantities of CH4 and CH3OH in each stream. CO and CO2 were handled similarly. During analysis for CO and CO2, the carbosphere analytical column was used in place of the Poropak QS column in the GC. CO and CO2 are retained only slightly on the Supelcoport/10% Carbowax packing and therefore travel with (or slightly behind) methane. Thus, in the SCMCR, CO and CO2 are not well separated from CH4. They therefore tend to accumulate along with CH4. It was found that CO and CO2 ultimately reached a periodic steady state after several cycles. Water is more strongly adsorbed than methanol, and it was found to accumulate in the system until it also reached a periodic steady state.

Figure 5. Distribution of methane in the separation column preceding the reaction section of the SCMCR at the conclusion of a run.

All SCMCR experiments were performed with the following flow rates, reported at STP: He carrier gas, 410 cm3 min-1; extra He carrier gas, 1140 cm3 min-1; CH4 (first switching time only), 190 cm3 min-1; and O2 (first and all succeeding switching times), 10 cm3 min-1. The adsorber temperature was 102 °C. It was desired to run the reaction at the optimum 16:1 CH4-to-O2 feed ratio found for the single-pass reactor. The CH4/O2 ratio in the SCMCR was determined by purging and quantitating the unconsumed CH4 that would normally be returned to the tubular reactor from the adsorber upstream of the feed section. The CH4 profile obtained by this procedure is presented in Figure 5. The average methane mole fraction from Figure 5 is 0.35, giving a CH4 flow rate of 143 cm3 min-1. The total flow rate in this adsorber was 410 cm3 min-1. Adding the CH4 makeup flow rate of 20 cm3 min-1 gives 163 cm3 min-1 of CH4 entering the reactor, along with 10 cm3 min-1 of O2 from the makeup feed, for the 16:1 ratio. The effects of reactor temperature, pressure, CH4/O2 ratio in the makeup feed, and switching time on SCMCR performance were investigated. The results are listed in Table 2, which shows that the SCMCR increases XCH4 more than 8-fold, from the approximately 6% of the single-pass reactor to more than 50% in favorable cases. The reactor does this while maintaining comparable SMeOH values, resulting in CH3OH yields up to 24.5%. A carbon material balance on run 2 showed that 96% of the carbon fed as CH4 was accounted for in the CH3OH, CO, CO2, and unconverted CH4 in the product and purge streams. The remaining 4% is due to the formation of minor oxidation products and possibly also to undetermined CH4 losses in small leaks. Experiments 1-3 show that the reactor section temperature has been optimized, with run 2 giving the best CH3OH yield at 477 °C. This is the temperature that gives the maximum XCH4 for the single-pass reactor under these conditions. Comparing experiment 4 with experiment 2 shows that lowering the pressure leads to lower CH3OH yields, in accord with the trend of lower yields with lower pressure found for the single-pass reactor experiments. Figure 6 shows more clearly than can be discerned from Table 2 that there is an optimum switching time. The maximum yield of CH3OH occurs at the maximum conversion of CH4 and the maximum SMeOH. This is

Ind. Eng. Chem. Res., Vol. 41, No. 25, 2002 6533 Table 2. Experimental Results for Direct Partial Oxidation of Methane in the SCMCR run no.

reactor section temperature (°C)

pressure (psig)

CH4/O2ratio in makeup feed

switching time (s)

XCH4

SMeOH

YMeOH

1 2 3 4 5 6 7 8 9 10 11 12

483 477 472 477 477 477 477 477 477 477 477 477

1450 1450 1450 1400 1450 1450 1450 1450 1450 1450 1450 1450

2 2 2 2 2 2 2 2 1.5 1.75 2.25 2.5

440 440 440 440 420 430 450 460 440 440 440 440

51.3 49.7 46.2 49.1 43.7 45.2 47.5 44.1 54.8 51.7 44.2 35.3

42.5 49.3 50.5 46.6 42.5 44.8 47.3 43.7 32.2 43.1 53.1 54.7

21.8 24.5 23.3 22.8 18.4 20.2 22.5 19.3 17.6 22.3 23.5 19.3

Figure 8. Distribution of methanol in the SCMCR product stream at periodic steady state. Figure 6. Dependence of methane conversion, methanol selectivity, and methanol yield on SCMCR switching time.

Figure 9. Distribution of methane in the SCMCR product stream at periodic steady state.

Figure 7. Dependence of methane conversion, methanol selectivity, and methanol yield on makeup CH4/O2 ratio in the SCMCR.

because the optimal switching time minimizes the CH4 lost, leading to high conversion, and therefore keeps the concentration of CH4 high in the SCMCR, leading to high selectivity. Figure 7 compares how the CH4/O2 ratio affects conversion, selectivity, and yield. The maximum value of YMeOH in this case is found at neither the highestconversion conditions nor the highest-selectivity conditions. At low CH4/O2 ratios, not enough CH4 is fed to replace both the CH4 converted and that purged from

the SCMCR. The partial pressure of CH4 decreases, and this causes SMeOH to decrease. At high CH4/O2 ratios, more CH4 is fed than can be consumed by the available O2. The CH4 accumulates, increasing SMeOH, but the increased CH4 levels cause increasing losses, and a periodic steady state is eventually reached. However, the increased CH4 loss rate leads to lower values of XCH4, and the CH3OH yields suffer. Figures 8-10 show the “average” methane and product concentration profiles in the purge and product streams. These data were obtained by controlling the sampling valve via the computer during SCMCR experiments and analyzing for the appropriate component on the GC.

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Figure 10. Composition of the SCMCR purge stream at periodic steady state.

Discussion The product selectivities observed in the SCMCR compare favorably with the selectivities from the singlepass reactor studies. Methane and O2 enter the reactor section in a 16:1 ratio, which, in the single-pass work, gives selectivities of 0.5, 0.35, and 0.12 for CH3OH, CO, and CO2, respectively. For the same reactor conditions, the optimal CH3OH, CO, and CO2 selectivities observed in the SCMCR are 0.5, 0.32, and 0.1, respectively, in excellent agreement with the single-pass results. Because the separation of CH4, CO, and CO2 is incomplete (Figure 4), only a part of the CO, CO2, and H2O produced is removed from the SCMCR during each cycle, and the balance of these species contaminate the CH4 and O2 entering the reactor. This contrasts with the single-pass reactor, where no reaction products are in the feed. Their presence throughout the SCMCR has no observable influence on the CH3OH selectivity and very little influence on the CO and CO2 selectivities. Although, in principle, the SCMCR is capable of converting all of the CH4 fed, the experiments show that XCH4 was only about 0.5. Unreacted CH4 was found in both the purge and product streams, and these losses account for the failure to attain unit conversion. Consideration of the overall stoichiometry and the makeup feed composition permits an estimate of the CH4 consumption that is in good accord with the experimental determination of XCH4. Taking into account the selectivities of the three major products in the stoichiometric reactions 4-6, approximately equal amounts of CH4 and O2 are predicted to be consumed.

0.50CH4 + 0.25O2 ) 0.5CH3OH

(4)

0.32CH4 + 0.48O2 ) 0.64H2O + 0.32CO

(5)

0.10CH4 + 0.20O2 ) 0.20H2O + 0.10CO2

(6)

If the the stoichiometric coefficients of CH3OH, CO, and CO2 are set equal to their observed selectivities of 0.5, 0.32, and 0.10, respectively, in reactions 4-6, the sum of the CH4 stoichiometric coefficients is 0.92, and the sum of the O2 stoichiometric coefficients is 0.93. Because the 10 cm3 min-1 of O2 fed is completely consumed, nearly half of the 20 cm3 min-1 makeup CH4 is consumed. Thus, 10 cm3 min-1 of the total of 163 cm3 min-1 fed (calculated above) is consumed, and XCH4 (per pass) is predicted to be 0.06, in excellent agreement with the results of single-pass reactor studies. Because only half

of the makeup CH4 is consumed per pass in the SCMCR and CH4 does not accumulate at periodic steady state, 10 cm3 min-1 of CH4 must be eluted from the SCMCR. This predicts that the overall XCH4 in the SCMCR is 0.5, in good agreement with the experimental results. The 50% methane loss was confirmed from experimentally measured flow rates and composition measurements.28 Improvement of the CH3OH yield could be achieved by increasing either or both the CH4 conversion and the CH3OH selectivity. The selectivity is governed by reaction conditions, and significant improvement appears difficult at best when the literature is consulted. However, the SCMCR affords the possibility of significantly improved CH4 conversion. In this work, XCH4 was limited by losses in the product and purge streams. In the section below, we consider how these losses arise and how they might be avoided as the key to improved reactor performance. On Methane Loss. In work on CH4 oxidative coupling, it was shown that a broad CH4 desorption profile, due primarily to the favorable adsorption isotherm of CH4 on activated carbon, was the primary cause of CH4 loss.13 Figure 3 compares the waveforms of CH4 on activated carbon and 10% Carbowax on Supelcoport. Appreciable tailing occurs with the activated carbon but not with the Supelcoport/Carbowax packing. Furthermore, Figure 5 shows that, in the SCMCR, neither dispersion nor gas-liquid partition distort the CH4 waveform appreciably, and broadening of the CH4 profile is not the principal reason for CH4 loss from the SCMCR when this packing is used. Rather, flow rate differences account for this loss. To understand how to address the CH4 loss problem, it is helpful to consider the development of concentration profiles in the SCMCR. The methane and CH3OH profiles are qualitatively illustrated in Figure 11. The profiles were deduced from Figure 5 and Figures 8-10. During the first switching time, a rectangular wave of CH4 is fed, indicated by the dotted line at the beginning of the first switching time. (In the figure, it appears that the CH4 passes through the packed column upstream from the reactor, but it actually is delivered directly to the reactor from the CH4 cylinder.) At the end of the first switching time, as CH4 is breaking through the first column downstream from the reactor, the flows are switched, and FCH4 and FO2 are set to their makeup feed rates. The CH3OH produced has advanced about halfway along the packed column at this point. The next pair of profiles shows CH4 and CH3OH at the beginning and end of the second switching period. At the flow switch, all of the material in the reactor and connecting tubing is taken along with the reactor as it is placed between the second and third packed columns. The CH4 left in the middle column at the end of the first switching time is swept ahead by the He flow, and the CH3OH produced during the first switching time penetrates further into the second adsorber. The slight jump in methane concentration is from the introduction of the makeup feed. It is very important to recognize that the flow rate in the column downstream from the reactor is faster than that in the column upstream, with the result that all of the CH4 in the upstream column is not swept out during a switching period. The CH4 remaining behind is removed from the SCMCR in the product stream and can account for the majority of the CH4 lost. In the experiments, the switching time was

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Figure 11. Illustration of the development of methane and methanol concentration profiles in the SCMCR.

set to be slightly longer than the breakthrough time, so that approximately the same amount of CH4 is lost because of early breakthrough as because of flow rate difference. During the third switching time, CH3OH is swept out of the middle column with the extra carrier gas and into the product stream. Because of the flow rate difference, some CH4 gets purged from the SCMCR with CH3OH in the product stream. All succeeding switching periods look qualitatively the same as this last pair of profiles, which, after a sufficient number of switching periods, corresponds to the periodic steady state. If the per-pass conversion were larger and the loss remained the same, the overall conversion would be larger. This point is illustrated in the SCMCR experiments on the oxidative coupling of methane (OCM),13 where the makeup feed rate was small enough that the percentage increase in the flow rate at the reactor inlet was very small and losses from the flow rate difference were not important. In the OCM studies, the dominant CH4 loss mechanism was the desorption tail caused by the favorable isotherm of CH4 on the activated charcoal adsorbent. Improved Reactor Design. It is apparent that the main factor limiting XCH4, and hence the yield of methanol in the SCMCR, is the loss of methane resulting from the difference in flow rates ahead of and behind the reactor. Reducing lost CH4 would lead not only to higher conversion but also to higher CH4 concentrations in the reactor and greater values of SMeOH.

To reduce the flow rate differences and increase XCH4, one could decrease the makeup feed flow rate or increase the carrier flow rate. (Decreasing the carrier flow to produce a more concentrated product stream would adversely affect XCH4.) Decreasing the makeup feed decreases the productivity while increasing XCH4. There is presumably an optimum condition if this option is pursued. On the other hand, increasing the carrier without starting with a higher methane concentration leads to dilution of reactants, which requires increasing operating temperature and reduces SMeOH (see Table 1). Another approach would be to decrease CH4/O2 in the makeup feed, thus reducing the amount of unconsumed CH4 and hence the CH4 loss rate. Table 2 shows that the effect of decreasing CH4/O2 in the makeup feed is indeed to increase XCH4, but SMeOH decreases as a result, and the yield of CH3OH is reduced. A solution to the CH4 loss problem might be realized by using a four-section SCMCR. Referring to Figure 11, we see that, with three sections, the CH4 that is lost in the product stream comes from the second adsorber upstream from the tubular reactor. A four-section SCMCR could be configured so that the product could be taken from the third upstream adsorber if the CH3OH were strongly enough retained. Adjusting the adsorber temperature would accomplish this. Because CH4 retention is not much affected by temperature, CH4 would still be in the second upstream adsorber and would not be eluted with the product. Thus, CH4 conversions approaching 1 might be expected. If SMeOH

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were maintained at 0.5, CH3OH yields approaching 50% would be expected. Further improvements in SCMCR performance would have to come from increases in SMeOH. There are reports of SMeOH values greater than 0.5. Feng et al. reported values ranging from 0.55-0.70,20 Casey et al. gave values from 0.5 to 0.60,24 Gesser et al. reported 0.64,16 and Chelappa et al. gave 0.54.25 The best SMeOH value that we found with He carrier present (single-pass) is about 60%. Thus, a four-section SCMCR might be capable of giving CH3OH yields of as much as 60%. If further gains are to be obtained, conditions for increased values of SMeOH must be found. Acknowledgment This research was funded by the U.S. Environmental Protection Agency (U.S. EPA) and the National Center for Clean Industrial and Treatment Technologies (CenCITT). It has not been subjected to the U.S. EPA’s required peer and policy review and therefore does not necessarily reflect the views of the U.S. EPA or CenCITT and no official endorsement should be inferred. Literature Cited (1) Gesser, H. D.; Hunter, N. R. The Direct Conversion of Methane to Methanol by Controlled Oxidation. Chem. Rev. 1985, 85, 235. (2) Foster, N. R. Direct Catalytic Oxidation of Methane to Methanol-A Review. Appl. Catal. 1985, 19, 1. (3) Pitchai, R.; Klier, K. Partial Oxidation of Methane. Catal. Rev.-Sci. Eng. 1986, 28, 13. (4) Brown, M. J.; Parkyns, N. D. Progress in the Partial Oxidation of Methane to Methanol and Formaldehyde. Catal. Today 1991, 8, 305. (5) Mackie, J. C. Partial Oxidation of Methane: The Role of the Gas-Phase Reactions. Catal. Rev.-Sci. Eng. 1991, 33, 169. (6) Srivistava, R. D.; Zhou, P.; Stiegel, G. J.; Rao, V. U. S.; Cinquegrane, G. Direct Conversion of Methane to Liquid Fuels and Chemicals. In Catalysis, A Specialist Periodical Report; The Royal Society of Chemistry: London, 1992; Vol. 9, p 183. (7) Foulds, G. A.; Gray, B. F. Homogeneous Gas-Phase Partial Oxidation of Methane to Methanol and Formaldehyde. Fuel Process. Technol. 1995, 42, 129. (8) Arutyunov, V. S.; Basevich, V. Ya.; Vedeneev, V. I. Direct High-Pressure Gas-Phase Oxidation of Natural Gas to Methanol and Other Oxygenates. Russ. Chem. Rev. 1996, 65, 197. (9) Gesser, H. D.; Hunter, N. R. A Review of C-1 Conversion Chemistry. Catal. Today 1998, 42, 183. (10) Hunter, N. R.; Gesser, H. D.; Morton, L. A.; Yarlagadda, P. S. Methanol Formation at High Pressure by the Catalyzed Oxidation of Natural Gas and by the Sensitized Oxidation of Methane. Appl. Catal. 1990, 57, 45. (11) Carr, R. W.; Dandekar, H. Adsorption with Reaction. In Reactive Separation Processes; Kulprathipanja, S., Ed.; Taylor and Francis: New York, 2001; p 115. (12) Tonkovich, A. L.; Carr, R. W. A Simulated Countercurrent Moving-Bed Chromatographic Reactor for the Oxidative Coupling of Methane: Experimental Results. Chem. Eng. Sci. 1994, 49, 4647.

(13) Bjorklund, M. C.; Kruglov, A. V.; Carr, R. W. Further Studies of the Oxidative Coupling of Methane to Ethane and Ethylene in a Simulated Countercurrent Moving Bed Chromatographic Reactor. Ind. Eng. Chem. Res. 2001, 40, 2236. (14) Burch, R.; Squire, G. D.; Tsang, S. C. Direct Conversion of Methane into Methanol, J. Chem. Soc., Faraday Trans. 1 1989, 85, 3561. (15) Gesser, H. D.; Hunter, N. R. The Direct Conversion of Methane into Methanol (DMTM). In Direct Conversion by Oxidative Processes. Fundamental and Engineering Aspects; Wolf, E. E., Ed.; Rheinhold-Van-Nostrand: New York, 1992; p 403. (16) Gesser, H. D.; Hunter, N R.; Shigapov, A. N. Some Characteristics of the Partial Oxidation of CH4 to CH3OH at High Pressures. In Methane and Alkane Conversion Chemistry; Bhasin, M. M., Slocumb, D. W., Eds., Plenum Press: New York, 1995; p 271. (17) Onsager, O. T.; Lødeng, R.; Søraker, P.; Anundskaas, A.; Helleborg, B. The Homogeneous Gas-Phase Oxidation of Methane and the Retarding Effect of Basic/Inert Surfaces. Catal. Today 1989, 4, 355. (18) Chun, J.-W.; Anthony, R. G. Partial Oxidation of Methane, Methanol, and Mixtures of Methane and Methanol, Methane and Ethane, and Methane, Carbon Dioxide, and Carbon Monoxide. Ind. Eng. Chem. Res. 1993, 32, 788. (19) Foulds, G. A.; Gray, B. F.; Miller, S. A.; Walker, G. S. Homogeneous Gas-Phase Oxidation of Methane Using Oxygen as Oxidant in an Annular Reactor. Ind. Eng. Chem. Res. 1993, 32, 780. (20) Feng, W.; Knopf, F. C.; Dooley, K. M. Effects of Pressure, Third Bodies, and Temperature Profiling on the Noncatalytic Partial Oxidation of Methane. Energy Fuels 1994, 33, 784. (21) Omata, K.; Fukuoka, N.; Fujimoto, K. Methane Partial Oxidation to Methanol. 1. Effects of Reaction Conditions and Additives. Ind. Eng. Chem. Res. 1994, 33, 784. (22) Chun, J.-W.; Anthony, R. G. Free Radical Kinetic Model for Homogeneous Oxidation of Methane to Methanol. Ind. Eng. Chem. Res. 1993, 32, 796. (23) Yarlagadda, P. S.; Morton, L. A.; Hunter, N. R.; Gesser, H. D. Direct Conversion of Methane to Methanol in a Flow Reactor. Ind. Eng. Chem. Res. 1988, 27, 252. (24) Casey, P. S.; McAllister, T.; Foger, K. Selective Oxidation of Methane to Methanol at High Pressures. Ind. Eng. Chem. Res. 1994, 33, 1120. (25) Chelappa, A. S.; Fuangfoo, S.; Viswanath, D. S. Homogeneous Oxidation of Methane to Methanol: Effect of CO2, N2, and H2 at High Oxygen Conversions. Ind. Eng. Chem. Res. 1997, 36, 1401. (26) Thomas, D. J.; Willi, R.; Baiker, A. Partial Oxidation of Methane: The Role of Surface Reactions. Ind. Eng. Chem. Res. 1992, 31, 2272. (27) Huang, S.; Carr, R. W. A Simple Adsorber Dynamics Approach to Simulated Countercurrent Moving Bed Reactor Performance. Chem. Eng. J. 2001, 82, 87. (28) Bjorklund, M. C. Ph.D. Thesis, University of Minnesota, Minneapolis, MN, 1999; pp 134 and 135.

Received for review February 27, 2002 Revised manuscript received September 26, 2002 Accepted October 2, 2002 IE0201617