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Ind. Eng. Chem. Res. 2001, 40, 2236-2242
Further Studies of the Oxidative Coupling of Methane to Ethane and Ethylene in a Simulated Countercurrent Moving Bed Chromatographic Reactor Mark C. Bjorklund, Alexey V. Kruglov, and Robert W. Carr* Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Avenue SE, Minneapolis, Minnesota 55455
An experimental and mathematical modeling study of a simulated countercurrent moving bed chromatographic reactor for oxidative coupling of methane to ethane and ethylene is reported. The experiments were done in a laboratory-scale three-section simulated moving bed reactor. Operating conditions were deduced from independent experiments in a microcatalytic fixedbed reactor and from adsorber dynamics studies. With a YBa2Zr3O9.5 catalyst and activated charcoal adsorbent, the sum of the ethane and ethylene yields at optimum simulated moving bed operating conditions was 45%. Numerical simulations of the reactor performance were done and compared with experimental results. Introduction The oxidative coupling of methane (OCM) to form ethane and ethylene (C2) has been intensively investigated in recent years.1 The reaction, which typically occurs by passing CH4 and O2 over suitable oxide catalysts, is an inherently low conversion per pass reaction, because to minimize formation of carbon oxides, a significant excess of CH4 over O2 is required. As a consequence, in conventional single-pass chemical reactors O2 is the limiting reagent, and the C2 yield varies from less than 10%/pass to at most about 2025%/pass. These yields are insufficient for a commercially viable process. To improve the overall yield, a number of recycle/separation arrangements have been reported.2,3 Separative chemical reactors, in which reaction and separation are integrated, have also been applied to this problem.4,5 This paper reports recent work6 on the further development of a separative reactor, the simulated countercurrent moving bed chromatographic reactor (SCMCR), for OCM. The SCMCR employs adsorptive separation of C2 products from unconverted CH4, taking advantage of simulated countercurrent operation to effect the separation in a flow reactor. It is capable of maintaining a high CH4/O2 ratio in the catalyst bed, which enables high C2 selectivity. It is also capable of attaining methane conversions that are substantially larger than conversions in conventional chemical reactors because of the separation of reactants from products. The merit of the SCMCR for inherently low conversion per pass reactions, such as OCM, is the ability to operate at optimum selectivity, which is governed by the chemistry, while the mode of operation of the reactor provides conversions that can, in principle, be 100%. Previous experimental work on OCM in a SCMCR demonstrated up to 65% methane conversion, 80% C2 selectivity, and better than 50% C2 yield with a powdered Sm2O3 catalyst.4,7 The primary goal of the present research was to experimentally evaluate the performance of a modified * To whom correspondence should be addressed. E-mail:
[email protected]. Fax: (612) 626-7246.
Figure 1. Schematic diagram of the SCMCR. All valves labeled | are on at once, then all || valves, and then all ||| valves.
SCMCR design. Numerical simulations of the SCMCR performance were also done with the aid of a mathematical model, and comparisons between model predictions and observed performance were made. Experimental Section Figure 1 is a schematic diagram of the SCMCR used in these studies. It is derived from the type introduced by Tonkovich et al.4 for high-temperature reactions, in which each SCMCR section consists of a fixed-bed reactor followed by a lower temperature adsorption bed. The two-temperature arrangement is necessary because at OCM temperatures, ca. 1000 K, adsorbents either thermally degrade or do not provide selective adsorption. Each reactor-adsorber pair serves the same function as integrated reactor/separator columns in which
10.1021/ie000775g CCC: $20.00 © 2001 American Chemical Society Published on Web 04/17/2001
Ind. Eng. Chem. Res., Vol. 40, No. 10, 2001 2237
the adsorbent and catalyst are mixed. However, with the reactor-adsorber pair arrangement, reaction with separation is no longer locally integrated, so differences in single-pass conversion are expected. The design in Figure 1, using only one fixed-bed reactor instead of the four of Tonkovich and Carr,7 has recently been discussed.8 The single fixed-bed reactor avoids reactor-toreactor catalyst activity variations and reduces reactor energy and construction costs upon scale-up. It is successively coupled to each downstream adsorber section by the SCMCR flow switching operations. This arrangement appears to be a single reactor followed by a separator, but it is in fact an SCMCR that is functionally identical to one with a fixed-bed reactor for each adsorber and is closely related to the fully integrated reactor/separator. A similarity of the SCMCR with the recycle reactor described by Jiang et al.2 is the use of an adsorbent to separate products and reactants. However, the SCMCR is quite different since because of the adsorber dynamics it operates in a periodic steady state rather than in a continuous flow mode. The SCMCR used here has only three sections, the theoretical minimum,9 rather than the conventional four sections to reduce complexity and solids inventory. For SCMCR operation, all valves labeled | in Figure 1 are open simultaneously. After a period of time (a switching period), all valves | are closed and valves labeled || are opened simultaneously for an identical switching period, at the end of which valves || are closed and valves ||| are opened. At the end of the third switching period, valves ||| are closed and valves | are opened to repeat the sequence. Makeup feed is introduced into the catalytic reactor section (CRS), which is the section of the SCMCR currently incorporating the fixed-bed reactor. Carrier gas is introduced one separation section upstream of the CRS to carry the unreacted methane from the previous switching period back through the CRS during the current switching period. Extra carrier flow is introduced two sections behind the CRS to strip reaction products from the SCMCR. The products are taken off at an intermediate point in the adsorber because they are significantly more strongly adsorbed than methane. The adsorbers are 43.2 cm long overall. They are configured as two packed columns separated by a three-way valve. Each pair of adsorbers is 12.7 and 30.5 cm long and is packed with 5.7 and 12.4 g of activated charcoal, respectively. Connections are made using 1/8 in. stainless steel or Teflon tubing. The fixed-bed reactor is a 40 cm long × 0.5 cm i.d. quartz tube heated by a tube furnace. The 24/40 mesh catalyst (30-70 mg) is packed halfway along the quartz tube and is held in place by 1 cm plugs of quartz wool. A 0.4 cm diameter quartz rod is inserted immediately downstream of the catalyst/quartz wool plug to increase the removal rate of the reaction product mixture from the heated zone and thereby reduce further oxidation to CO and CO2. The granular YBa2Zr3O9.5 catalyst,10 previously tested in this laboratory,11 was used in all experiments. It gives a considerably smaller pressure drop than the Sm2O3 powder catalyst previously used.4,7 A granular Sm2O3 catalyst was previously found to have poorer C2 selectivity than either powdered Sm2O3 or granular Yba2Zr3O9.5.11 MKS mass flow controllers, supplied by compressed methane and oxygen tanks, introduce the reactants into the carrier gas stream. A metering valve at the outlet of the SCMCR controls the total flow rate. The pressure
in the SCMCR is about 10 psig. Gas stream switching is accomplished by microcomputer-controlled two- and three-way Skinner solenoid electric valves and has been previously described.12 Chemical analysis was done with a Varian 6000 gas chromatograph (GC) equipped with flame ionization (FID) and thermal conductivity (TCD) detectors and a six-port rotary gas-sampling valve. The SCMCR exit stream is split with only a fraction passing first through a 300 cm3 stirred tank and then to the GC. The tank averages concentrations and ensures constant pressure and flow rate to the GC. Analysis is done with either a 3 m × 3.2 mm diameter column packed with 60/80 mesh Poropak QS or a 3 m × 3.2 mm diameter column packed with 60/80 mesh Carbosphere. The Poropak column can separate all carbon-containing reaction products and methane and water. The Carbosphere column is used to quantify O2. The PC controls GC sampling, and the chromatogram is recorded, analyzed, and displayed by the PC using Labtech Notebook software in conjunction with a BASIC program. GC samples could be taken at 20 s intervals. The GC was calibrated as necessary using a standard mixture of CH4, C2H4, C2H6, CO2, and CO. Results Microreactor Studies. Fixed-bed catalytic reactor studies were done to characterize single-pass performance of the YBa2Zr3O9.5 catalyst and to select operating conditions for the SCMCR. This catalyst was previously reported to have better than 90% selectivity for C2H6 and C2H4 at 850 °C.11 Experiments were conducted by introducing the reactants into the He carrier gas stream, passing the mixture through the fixed-bed reactor, and routing the effluent directly to the GC for analysis. The run temperature was determined by increasing the microreactor temperature until the O2 conversion was complete, that is, until the O2 peak from the GC became unobservable. Table 1 summarizes the data. In Table 1, C2 selectivity is reported as (moles of C in ethane and ethylene)/(total moles of C in all products), and conversion is reported as total moles of C in products per mole of CH4 fed. The selectivity data were obtained after the stated number of hours on stream. For all but the first few minutes of each run, the catalyst was found to be exceptionally stable. In run 46 the C2 selectivity was 0.98 immediately after catalyst regeneration. The initial high selectivity decreased rapidly over the first few minutes of a run, after which it was very stable for up to 30 h, the maximum test time tried. Figure 2 shows the effect of the CH4/O2 ratio on C2 selectivity, CH4 conversion, and C2 yield. These data indicate that the SCMCR should be operated at CH4/O2 ) 25 to optimize the C2 selectivity. The C2 selectivity was found to be insensitive to the mole fraction of CH4 from 0.20 to 0.33, the range covered in the SCMCR experiments. The C2H4/C2H6 ratio depends quite strongly on CH4/O2, as shown in Figure 3, which indicates that to optimize production of the more valuable product, C2H4, the SCMCR should be operated at lower CH4/O2. It is apparent that there are choices to make between operating the SCMCR at high CH4/ O2 to minimize the formation of COx, operating at low CH4/O2 to favor C2H4 over C2H6, or operating at an intermediate CH4/O2 to find an optimum. Adsorption/Desorption Studies. Studies of adsorption and desorption of methane and reaction products were done on three different adsorbents that were
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Table 1. Microcatalytic Reactor Resultsa run no.
catalyst
He flow rate (sccm)
temp (°C)
CH4 flow rate (sccm)
41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66
Y1Ba2Zr3O9.5
95
950
24.8
99 121 97
37.2 24.8
83
34.7
43.2
100
24.8
CH4/O2 43.9 14.6 29.5 10.6 43.9 30.5 42.4 11 21.9 10.6 21.9 16.4 11 5.5 7.3 21.9 16.4 11 5.5 7.3 43.9 21.9 16.3 11 5.5 7.3
hours on stream 0 24 25 26 30 regen 3 4 5 6 n/a
C2 selectivity
methane conversion
0.87 0.84 0.89 0.81 0.9 0.98 0.89 0.81 0.86 0.79 0.88 0.85 0.8 0.68 0.74 0.88 0.85 0.79 0.68 0.73 0.92 0.9 0.85 0.81 0.7 0.75
0.059 0.096 0.12 0.16 0.24 0.2
a Same catalyst formulation. Different batches sent from Amoco. regen means regenerated for 28 h. Selectivity exhibited for only a few minutes after regeneration. Blank entries are the same values as the entry above.
Figure 2. Dependence of C2 selectivity, CH4 conversion, and C2 yield on CH4/O2 in the microcatalytic reactor.
Figure 3. Dependence of C2H4/C2H6 on CH4/O2 for three CH4 mole fractions in the microcatalytic reactor.
selected from a survey of the literature. A brief description of some of the results on the three adsorbents, which are activated charcoal, carbon molecular sieve,
Figure 4. Breakthrough and desorption of several species in an adsorber packed with activated charcoal.
and aluminosilicate, has been reported.11 The studies were done by elution analysis of the response of an adsorption bed to a step function increase in the adsorbates, followed at an arbitrary later time by a step decrease. The experimental arrangement consisted of the fixed-bed catalytic reactor, described above, a single adsorber bed, and the analytical GC, all connected in series. Experiments were conducted by first flowing He through the system, then introducing a known flow of CH4/O2 into the He, and commencing GC sampling of the adsorber effluent. After approximately 1 min, the CH4/O2 flow was stopped, and sampling was continued until all components had eluted from the adsorber. Figure 4 shows the chromatogram obtained from a 30 cm × 0.32 cm diameter stainless steel column packed with activated charcoal, at 100 °C and 60 sccm. GC analyses were done as often as possible, limited by the analysis time. Although the concentration waveforms are not accurately determined, they are sufficient to show whether satisfactory separations are possible. A chromatogram for carbosieve has been reported previ-
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seems unlikely that the experimental error could be more than a few percent given the two semi-independent (methane fed is used in both calculations) ways of calculating the conversion. All of the methane fed could be accounted for, to within 5%, by the sum of the C2H4, C2H6, CO, and CO2 formed, plus CH4 lost in the purge. An initial set of experiments was done to optimize the CH4/O2 ratio (runs 1-5). The optimal switching period (or interchangeably the carrier flow rate) was found from the CH4 breakthrough time, which was then tuned to minimize the methane lost in the purge stream. After the optimal CH4/O2 ratio was found in runs 1-5, a set of experiments was performed to verify that the chosen switching time (based on visual inspection) was indeed the optimal switching time. The model will show that the optimal C2 selectivity and yield coincide with the optimal conversion, which occurs at the optimal switching time. Therefore, in these experiments only the methane loss in the purge was measured in order to calculate conversion. Experiments 6-11 verified that 390 s was the optimal switching time. Comparison with model results and further discussion of these results can be found in the next section. Runs 12 and 13 used an adaptive feed to try to improve performance. In these experiments, rather than feed a constant flow of methane in the makeup feed for the whole switching time, 3 times the flow rate was applied for the last one-third of the switching time. This was done in order to replace the methane lost in the purge stream. The ideal makeup feed would exactly replace the methane lost. However, the experiments showed no improvement over those without this rudimentary adaptive feed control, probably because control of the mass flow controller was not good enough to reliably follow this program.
Figure 5. Methane breakthrough and desorption for CMS (Carbosphere), activated charcoal, and aluminosilicate (Valfor, PQ Corp.).
ously.11 These data were used to design the SCMCR adsorbers. It is apparent that, with activated charcoal, CO2 is not separated from the C2 products and that CO and CH4 are not separated. A key factor in the SCMCR design is the amount of “tailing” on the reactant desorption curve, which can lead to reactant loss by elution from the SCMCR and hence to conversions that are less than the ideal value of 100%. Figure 5 compares the elution waveform of CH4 for the three adsorbents tested. The most satisfactory one in this respect is aluminosilicate, but it gave poor separations of C2 and CH4 and had a low adsorption capacity for CH4. The carbosieve is capable of good separations, but it is expensive and is not available in large quantities. Thus, activated charcoal was selected for the SCMCR design. Its desorption characteristics are not quite as good as carbosieve but were judged satisfactory. SCMCR Experiments. Table 2 summarizes the SCMCR experimental results. For all runs the reactor temperature was 850 °C and the adsorbers were all at 100 °C. The carrier gas (He) flow rate was 60 sccm, and the extra carrier, also He, was 260 sccm. The methane flow rate in the makeup feed was set at 1.5 sccm before correction by a mass flowmeter calibration factor. Note that there are two conversions listed. The conversion based on methane lost ) (CH4 in - CH4 out)/(CH4 in). The conversion based on products formed and the C2 selectivity are defined above. The two conversion values for each experiment are within a few percent of each other, providing a check on the carbon atom balance. They also give an indication of experimental error. It
Mathematical Model A mathematical model of the SCMCR was developed. The model consists of two parts: one to describe the conversion and selectivity of the CRS and the other to describe the adsorber dynamics. Where available, experimental data were used as input for model calculations; otherwise, literature values were used. Numerical solutions of the model equations were obtained and compared with experimental results. Adsorber Model. It was found from experiments that the methane adsorption and desorption dynamics play a central role in the overall reactor performance, so particular attention was given to simulating the
Table 2. Experimental Results in the SCMCRa % conversion
run no.
switching time (s)
CH4/O2 ratio (makeup)
methane
1 2 3 4 5 6 7 8 9 10 11 12 13
390 390 390 390 390 439 424 405 390 365 353 390 390
2.16 1.76 1.32 1.32 1 1.32 1.32 1.32 1.32 1.32 1.32 1.32 1.32
56 69.7 77.5 79.9 84.2 72 75 76.7 79.3 76.7 73.8 78 76.9
a
Blank cells were not measured.
% C2 yield
products
% C2 selectivity
C2H4/C2H6 ratio
methane
products
54 62.2 79.9 76.5 84.9
49 63.9 57.2 56.5 47.8
1.47 2.27 1.78 2.05 2.57
27.4 44.5 44.3 45.1 40.2
26.5 39.7 45.7 43.2 40.6
72.3 74.7
55.3 56.9
1.5 1.7
43.1 43.8
40.0 42.5
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adsorption sections of the SCMCR. The flow through the adsorbers is described by the axial dispersed plugflow model for a constant-pressure column with variable mobile phase velocity (due to adsorption).13 A linear driving force approximation is employed, and adsorption is described by a multicomponent (competitive) Langmuir isotherm.
ν
∂ci ∂2ci ∂ν ∂ci 1 - ∂qi + ci + + ) DL 2 ∂z ∂z ∂t ∂t ∂z NC
∑i ci ) C Figure 6. SCMCR model calculations of CH4 loss as a function of the switching time.
∂ν 1 - ∂qi + )0 C ∂z ∂t
S ) 0.555R0.143 E ) 4.24YCH4-0.279YO20.526
∂qi ) Ki(qi/- qi) ∂t qi/) qio
bici
1+
NC
∑i bici
The Langmuir isotherm parameters were determined from breakthrough curves of pure components at different pressures. Initially, axial diffusivity in the bed and internal mass-transfer coefficients were determined from the literature.14 These values were then tuned using the results from adsorption dynamics experiments in a single fixed bed. To cope with moving step gradients in the concentration profiles, a third-order quadratic upstream difference scheme was implemented to approximate the convective term.15 The system of differential-algebraic equations was solved using the method of lines.16 Reactor Model. Whereas convection, adsorption, and mass transfer in a bed of granulated adsorbent are rather well understood, the physical and chemical rate processes taking place in the catalyst bed are not. The OCM is a complex mix of heterogeneous and homogeneous reactions, with literally hundreds of elementary steps possible, all accompanied by pore diffusion and axial dispersion. Because many of the rate constants and activation energies are not well-known, a reliable, predictive reactor model is not possible. Hence, recourse was made to empirical fitting of microreactor data given in Table 1. The results showed that the reaction over the YBa2Zr3O9.5 catalyst may be described by the following overall reactions:
2CH4 + 1/2O2 f C2H6 + H2O
(1)
2CH4 + O2 f C2H4 + 2H2O
(2)
CH4 + 3/2O2 f CO + 2H2O
(3)
CH4 + 2O2 f CO2 + 2H2O
(4)
No other products were detectable via gas chromatography with a TCD. It was found that at a temperature of 1123 K the reaction could be characterized by the empirical relationships11
where S is the C2 selectivity, R is the CH4/O2 mole ratio, E is the ethylene-to-ethane ratio in the products, and YCH4 and YO2 are the mole fractions of methane and oxygen, respectively, in the feed. It is assumed and expected, based on catalytic reactor experiments, that there is ∼100% oxygen conversion. In numerical simulations, experimental values of R and Yi from independent catalytic reactor experiments were used to evaluate S and E (see Figures 2 and 3). These were then used to compute the inlet composition for the adsorber bed, and the adsorber model was used to compute the timevarying adsorber composition. Values of Ki, Di, and mass-transfer coefficients were determined from independent adsorber dynamics experiments, as described above (see Figure 4). These parameters were incorporated into the SCMCR model and were not thereafter adjusted to fit the experiments. There are no adjustable parameters in these simulations. Results from Model Calculations and Comparison with Experimental Results. The CH4 breakthrough time of 390 s is stated above to be the optimal switching time, and the model calculations shown in Figures 6-8 bear this out. Figure 6 shows model predictions of CH4 loss from the SCMCR in the purge stream and in the product stream. The purge stream loss is due to incomplete purging of the second adsorption column in the carrier section, due to CH4 tailing. CH4 appears in the product stream because of early breakthrough of methane in the second adsorption column of the feed section when ts is set at a time longer than the CH4 breakthrough time. The model predicts that the total CH4 loss is minimal at the optimal breakthrough time. The 20% predicted loss by the combined loss mechanisms is in good agreement with the approximately 80% CH4 conversion observed, as reported in Table 2 and shown in Figure 7 at ts ) 390 s. This clearly shows that all of the CH4 that does not escape the SCMCR is converted. Figure 7 compares the conversion predicted by the model with some experimental results. There is good agreement in terms of predicting the optimum switching time and the conversion at that switching time. The experimentally measured conversion vs ts relationship appears to be more sharply peaked than the model prediction, so actual CH4 losses must be more severe
Ind. Eng. Chem. Res., Vol. 40, No. 10, 2001 2241
Figure 7. Conversion vs switching time in the SCMCR.
Figure 10. Dependence of C2 yields on CH4/O2 in the makeup feed.
Figure 11. Elution of CH4 from the SCMCR after an experiment. Figure 8. SCMCR model calculations of CH4 conversion, C2 selectivity, and C2 yield vs switching time in the SCMCR.
Figure 9. Effect of makeup CH4/O2 on the CH4 conversion and C2 selectivity in the SCMCR.
than the model prediction when ts is not optimal. The difference is more pronounced at longer switching times. Figure 8 shows that the overall conversion, C2 selectivity, and C2 yield predicted by the model all vary with switching time, and they are maximized at the optimal switching time. The selectivity in the SCMCR is determined by the actual CH4/O2 ratio in the reactor sections (which is not constant in time). At the optimal switching time, the majority of the methane originally fed to the SCMCR is retained in the system and is recycled, maintaining the high CH4/O2 ratio that is responsible for the high overall selectivity. At nonoptimal switching times, methane is eventually lost from the system through one of the two ways mentioned above, resulting in low conversion and low CH4/O2 ratios which give low C2 selectivity, and therefore low C2 yield. Another parameter that strongly affects the SCMCR performance is the CH4/O2 ratio in the makeup feed. Figure 9 shows the effect of this ratio on CH4 conversion and C2 selectivity, and Figure 10 shows the effect on C2 yield. Also in these figures are the results of model
calculations. The decrease in conversion and increase in selectivity with increasing CH4/O2 are readily understood. When CH4/O2 in the makeup feed is larger than the stoichiometric ratio, CH4 accumulates in the SCMCR until some of the excess CH4 elutes with either the product or purge stream, and the net conversion becomes smaller. Selectivity, on the other hand, is known to increase with increasing CH4/O2. The C2 yield, which is expressed as the product of conversion and selectivity, is seen in Figure 10 to have a maximum at CH4/O2 ≈ 1.4. At higher CH4/O2 ratios the low conversion limits the yield, whereas at lower CH4/O2 ratios low selectivity limits the yield. The model and experiment are in good agreement for conversion, but there are discrepancies between predicted and observed C2 selectivity and predicted and observed C2 yield. Discussion The observed C2 selectivity in the SCMCR varies from 48% to 64% for CH4/O2 ratios in the feed in the range 1-1.8 (see Table 2 and Figure 9), which almost certainly encompasses the stoichiometric ratio. With nonseparative reactors, feed ratios in this range result in very high selectivity for CO and CO2. The SCMCR achieves high C2 selectivity at these feed conditions by maintaining a large internal excess of CH4, with the feed replacing only the CH4 and O2 that has been consumed. The discrepancies between the model predictions and experiment in Figures 9 and 10 are most probably due to differences between the shape of the CH4 concentration profile calculated by the model and the actual CH4 profile. The CH4 concentration profile at the end of a run was determined by turning off the makeup feed, stopping the flow switching, and monitoring the CH4 as it is pushed out of the SCMCR by the He carrier. Figure 11 shows the result. The observed variability of the CH4 concentration means that the CH4/O2 ratio in
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the catalyst bed must also be variable. In fact, it seems that CH4/O2 is smaller than desired during part of the reaction, and thus the C2 selectivity suffers, explaining the discrepancy between the experimental data and the model predictions shown in Figure 9. The model uses a Langmuir isotherm to calculate the CH4 profile. The shape predicted by this favorable isotherm is a sharp leading edge, a period of nearly constant CH4 concentration, and a broadened tail. It is this shape that is in the model calculations. If the 80-90% C2 selectivity of the microreactor experiments could be maintained in the SCMCR at the observed 80% CH4 conversion, then 64-72% C2 yields could be realized. The key to higher C2 selectivity is to maintain a higher CH4/O2 ratio. This might be done by adaptive control, in which the CH4 losses are monitored and replaced in the feed. Furthermore, placing additional adsorber sections between the feed and product takeoff points might significantly reduce CH4 losses. If both these aims could be achieved, very high C2 yields would be attained. Acknowledgment This work was supported by a grant from the Amoco Chemical Co. Amoco kindly donated samples of YBa2GeO3.5 and YBa2Zr3O9.5 prepared by Dr. M. P. Kaminsky. Literature Cited (1) Lunsford, J. H. The Catalytic Oxidative Coupling of Methane. Angew. Chem., Int. Ed. Engl. 1995, 34, 970. (2) Jiang, Y.; Yentekakis, I. V.; Vayenas, C. G. Methane to Ethylene with 85 Percent Yield in a Gas Recycle Electrocatalytic Reactor-Separator. Science 1994, 264, 1563. (3) For several papers on recycle reactors, see: Natural Gas Conversion V; Parmaliana, et al., Eds.; Elsevier: New York, 1998. (4) Tonkovich, A. L.; Carr, R. W.; Aris, R. Enhanced C2 Yields from Methane Oxidative Coupling by Means of a Separative Chemical Reactor. Science 1993, 262, 221.
(5) Hall, R. B.; Myers, G. R. Effects of Product Separation on the Kinetics and Selectivity of Oxidative Coupling. In Methane and Alkane Conversion Chemistry; Bhasin, M. M., Slocum, D. W., Eds.; Plenum Press: New York, 1995; p 123. (6) Bjorklund, M. C. Ph.D. Thesis, University of Minnesota, Minneapolis, MN, 1999. (7) Tonkovich, A. L. Y.; Carr, R. W. A Simulated Countercurrent Moving-Bed Chromatographic Reactor for the Oxidative Coupling of Methane: Experimental Results. Chem. Eng. Sci. 1994, 49, 4647. (8) Emerging Separation and Separative Reaction Technologies for Process Waste Reduction; Radecki, P. P., Crittenden, J. C., Shonnard, D. R., Bulloch, J. L., Eds.; CWRT-AIChE: New York, 1999; pp 109-113. (9) Fish, B. B.; Carr, R. W.; Aris, R. Design and Performance of a Simulated Countercurrent Moving-Bed Separator. AIChE J. 1993, 39, 1783. (10) Kaminsky, M. P.; Kleefisch, M. S.; Huff, G. A.; Washecheck, D. M.; Barr, M. K. Hydrocarbon Conversion. U.S. Patent 5,245,109, 1993. (11) Kruglov, A. V.; Bjorklund, M. C.; Carr, R. W. Optimization of the Simulated Countercurrent Moving-Bed Chromatographic Reactor for the Oxidative Coupling of Methane. Chem. Eng. Sci. 1996, 51, 2945. (12) Fish, B. B.; Carr, R. W.; Aris, R. Computer Aided Experimentation in Countercurrent Reaction Chromatography and Simulated Countercurrent Chromatography. Chem. Eng. Sci. 1988, 43, 1867. (13) Ruthven, D. M.; Farooq, S.; Knaebel, K. S. Pressure Swing Adsorption; VCH Publishers: New York, 1994; p 175. (14) Butt, J. B. Reaction Kinetics and Reactor Design; PrenticeHall: Englewood Cliffs, NJ, 1980; p 274. (15) Leonard, B. P. A Stable and Accurate Convective Modeling Procedure Based on Quadratic Upstream Interpolation. Comput. Methods Appl. Mech. Eng. 1979, 19, 59. (16) Brennan, K. E.; Campbell, S. L.; Petzold, L. R. Numerical Solutions of Initial-Value Problems in Differential-Algebraic Equations; Elsevier: New York, 1989.
Received for review August 24, 2000 Revised manuscript received March 12, 2001 Accepted March 14, 2001 IE000775G
