Ind. Eng. Chem. Res. 1997, 36, 1401-1409
1401
Homogeneous Oxidation of Methane to Methanol: Effect of CO2, N2, and H2 at High Oxygen Conversions Anand S. Chellappa, Surajit Fuangfoo, and Dabir S. Viswanath* Department of Chemical Engineering, University of MissourisColumbia, Columbia, Missouri 65211
The effect of increasing levels of CO2, N2, and H2 in the feed on the homogeneous partial oxidation of methane was studied at reaction conditions giving high oxygen conversions. The reaction was carried out in a quartz lined tubular reactor at 643-703 K, 30-50 bar and residence times of 30-100 s. Air was used as the oxidizing agent and the CH4/O2 ratio in the feed was varied from 8 to 16. Kinetic modeling studies were also carried out by using a free radical scheme consisting of a set of 46 elementary reactions. The results predicted by the model are in fair agreement with the experimental data. Increasing CO2 levels in the feed (3-15%) did not have an adverse effect on methane conversion and methanol selectivity but increased HCHO selectivity to a small extent. Increasing O2 concentrations in the feed was found to increase the HCHO/ CH3OH ratio as well as temperature required for high conversions. The need for higher temperatures is probably due to the increase in N2 levels in the feed. The methane conversion of 7% and a methanol selectivity of 54% at 703 K and 34 bar obtained in this study compare well with the values reported in the literature. The model predicts that increasing H2 levels in the feed at high oxygen conversions decreases methanol selectivity without affecting methane conversion. Introduction The direct conversion of methane to methanol (partial oxidation) has been investigated for over 6 decades resulting in many published articles and quite a few patents. Research involving methane and its use has been stimulated mainly by the growing need for a more effective use of the enormous natural gas reserves, of which only around 7% is being used for commercial production of chemicals (Poirier et al., 1991). A compact plant located near the gas wells and producing methanol using a single step process would reduce the costs involved in the transportation of natural gas. The current two step process (ICI) for converting natural gas to methanol has limited scope for improvement due to equilibrium limitations and low thermal efficiency. Interest in the single step production of methanol is further sustained by the need to improve alternate routes to gasoline production like the MTG (methanol to gasoline) process. Fox et al. (1990) have evaluated the direct methane conversion processes and predict that a 25% reduction in methanol plant cost would result in a reduction of 77% for the MTG process as used in New Zealand today. Reviews by Pitchai and Klier (1986), Geerts et al. (1990), Brown and Parkyns (1991), and Srivastava et al. (1992) provide an overall view of the progress made in this field. Pressures in the range 30-50 bar and temperatures in the range 673-773 K have been found to favor the formation of methanol. Several reports discuss methane conversion and product selectivity trends, with residence times of 1-20 s. Methane conversions of 2-5% and methanol selectivities of about 40-50% seem to represent the best results reported so far. Notable exceptions are the results of Gesser and co-workers (1987, 1991) and, more recently, that of Feng et al. (1994). These two groups have reported methanol selectivities greater than 70% at methane conversions * Address correspondence to this author. Tel, (573) 8824281; fax, (573) 884-4940; e-mail, viswanath@ ecvax2.ecn.missouri.edu. S0888-5885(96)00666-5 CCC: $14.00
greater than 8%. However, these results do not agree with much of the experimental and kinetic modeling work. Under similar conditions, Burch et al. (1989) were able to achieve methane conversion of about 5% and methanol selectivities of 40%. Such discrepancies were attributed to temperature/heat transfer effects, localized flow patterns in the reactor and errors in the oxygen mass balance (Brown and Parkyns, 1991; Srivastava et al., 1992). As a result, the partial oxidation route has not been scaled up for commercial use because of the inability to achieve economical and repeatable results. Investigations have also addressed the use of different reactor materials. Burch et al. (1989) found that metal surfaces like stainless steel and copper reduced methanol selectivity and have recommended that quartz or Pyrex glass lined reactors be used. Gesser et al. (1991) studied the influence of different packing materials on methanol selectivities and found inert materials to be promising. However, Thomas et al. (1992) showed that increasing the surface to volume ratio using packing materials decreased methane conversions and attributed this to the possible increased rate of destruction of free radicals in the reaction. The influence of inert additives on product selectivities has also been studied (Chun and Anthony, 1993a; Feng et al., 1994; Omata et al., 1994). The effect of CO2 becomes an important factor if unreacted CH4 is recycled without complete separation of CO2. As low single pass methane conversions are necessary to achieve high methanol selectivities, operating the reactor by using a recycle scheme would be an attractive option, if the partial oxidation reaction is scaled up for commercial use. It is also possible that CO2 would have a beneficial effect on methanol formation, by acting as a promoter as well as a collisional third body in the free radical reaction scheme. The studies dealing with the effect of CO2 addition on conversion and selectivities yield conflicting results. Feng et al. (1994) report methanol selectivities of about 40% and methane conversions of 6% at pressures as low as 10 bar. They © 1997 American Chemical Society
1402 Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997
Figure 1. Schematic diagram of the experimental setup.
found that increasing the total pressure to 50 bar using CO2 (keeping the methane/air partial pressure to 10 bar) increases methanol selectivity to 70%, but the corresponding methane conversion levels with CO2 addition are not provided. At similar pressures, but using oxygen instead of air, Chun and Anthony (1993a) found that cofeeding CO and CO2 mixtures with the methane/ oxygen feed had negligible effect on methanol selectivities and methane conversions at 50 bar and temperatures of 700-730 K. The effects on the selectivities to other products, especially HCHO, are not known. Omata and co-workers (1994) found that adding 10% CO2 and 10% N2 to the feed increases CO formation to a greater extent than methanol formation. In this study, the effect of increasing levels of CO2, N2, and H2 in the feed was investigated at similar oxygen conversion levels and at 34 bar. Emphasis was placed on high oxygen conversions (about 90%), as this translates to higher methane conversions and possibly high product yields. The flow rates used were in the range 10-26 cm3/s (STP). Kinetic modeling studies were also carried out using a free radical scheme consisting of a set of 46 elementary reactions, and the predicted conversion and product selectivities were compared with the experimental data. A primary reason for using air as the oxidizing agent in this study, in preference to pure oxygen, was to simulate the use of natural gasswhich has 1-20% N2sas starting feed material. By increasing oxygen levels in the feed at constant total flow rate, the effect of increasing nitrogen levels and decreasing methane levels in the feed on methane conversion and product selectivity was followed at 34 bar. It is important to account for hydrogen formation, in order to prevent unexpected hazards, especially if the recycle scheme is employed. Although many experimental studies on the homogeneous oxidation have been
carried out, the presence of hydrogen in the reactor outlet has been reported only in a recent work by Lødeng et al. (1995). This may be due to inadequacies in the analysis of the reactor outlet streams. In this study H2 was detected during the experimental trials. The model was used to predict the influence of increasing H2 levels in the feed, and the results are compared with the experimental work of Omata et al. (1994). They found that adding 4% H2 to CH4/O2/N2 feed at 41 bar slightly increased methanol yields and suppressed CO2 formation at temperatures less than 733 K. The objectives of this study were to (1) determine the effect of increasing O2 in the feed on product distribution using the reaction conditions at which methanol selectivities and methane conversions approached the range of economic interest, (2) determine the effect of adding different concentrations of CO2 to the feed and to compare the effect of CO2 addition with that of increased N2 addition in the feed, (3) determine the effect of adding hydrogen to the feed on methane conversion and product selectivity, and (4) develop a kinetic model using a free radical scheme of reactions and compare the results predicted by the model with the experimental data. Experimental Methods The reaction was studied by using a quartz lined tubular reactor in the temperature range 643-743 K and in the pressure range 30-50 bar and with a residence time of 36-100 s. A schematic diagram of the experimental setup used is shown in Figure 1. The reactor consisted of a 13-mm i.d. × 534-mm-long stainless steel (SS316) tube into which a close fitting quartz tube of 12-mm i.d. was inserted. Reactant gases were introduced separately into the reactor and the inlet tubes extended 130 mm into the reactor. At about 30
Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997 1403
Figure 2. Reaction matrix used to model the partial oxidation of methane.
mm from the outlet, a stainless steel sheathed K-type (chromel-alumel) thermocouple was mounted into the reactor by using Swagelok fittings. This thermocouple was gold plated to minimize its influence on product distributions. The reactor skin temperatures were monitored with three thermocouples located equidistant on the reactor. Some investigators (Feng et al., 1994; Burch et al.,1989) have encountered difficulties in measuring HCHO and H2 in the product gases due to their presence in trace amounts. In this work, the product gases were cooled and bubbled through water for the duration of the run at steady state (typically 3 h) to absorb methanol and formaldehyde formed. This resulted in consistent analyses of even trace amounts of methanol and formaldehyde in the solution. The sensitivity of the thermal conductivity detector was also increased to detect H2 even at low concentrations of less than 0.4% (vol) in the product gases. The reactants used were methane (AG Co., UHP grade with 99.99% CH4), air (Midwest Air Inc., USP grade with 20-21% O2, and CO, CO2, and H2O as impurities). A flow of argon gas was maintained during the initial heating of the preheater coils and reactor to reaction temperature. Further details about the experimental setup, procedure, and analysis of the products using gas chromatography (GC) can be found elsewhere (Chellappa and Viswanath, 1995). In this study, better separation of formaldehyde present in the aqueous sample solution was achieved by using a Porapak-T column in the GC, instead of a Porapak-QS column, which was used in our earlier work. The residence time in the reactor was calculated at reaction temperature and pressure and was based on the total heated reactor volume. The methane conversion (x) was calculated from the difference between the amount of carbon in the feed and products and the
oxygen conversion from the difference between the oxygen in the feed and unreacted oxygen in the reactor outlet stream. Selectivities were calculated as the amount of a particular product as a percentage of the total amount of the products formed. The overall carbon balance closures (calculated as carbon in reactor outlet/ carbon in feed) were within (2%. Reaction Model The reaction matrix for modeling the partial oxidation reaction is shown in Figure 2, and the set of elementary reactions chosen to account for the consumption and reaction of each species is listed in Table 1. The expression for the rate constants and corresponding uncertainty factors are taken mostly from Combustion Chemistry Data Bases for methane related species (Tsang and Hampson, 1986) and for methanol related species (Tsang, 1987). The uncertainty factors are a measure of the accuracy of the rate parameters, which were obtained from direct experimental measurements (e.g. reactions 4, 10, and 21), theoretical calculations, or empirical evaluations (e.g., reactions 18 and 19). For instance, a factor of 1.3 indicates that the estimated accuracy is (30%. The kinetic modeling studies (Vedeneev, 1988 a,b; Thomas et al.,1992; Chun and Anthony, 1993b; Lødeng et al., 1995) include hydrogen peroxide in the reaction scheme, and the predicted hydrogen peroxide concentrations are generally comparable to that of methanol (Vedeneev, 1988a). The absence of hydrogen peroxide in the product stream during experimental studies was attributed to the rapid decomposition of hydrogen peroxide to water and oxygen on the walls of the reactor. Thomas et al. (1992) assumed that such decompositions are limited by the rate of diffusion to the surface and
1404 Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997 Table 1. Set of Elementary Reactions Used for Modeling the Partial Oxidation Reactiona 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46
reaction
A
CH4 + O2 ) CH3 + HO2 CH3 + O2 ) CH3OO CH3O + O2 ) CH2O + HO2 CH3O + CH3 ) CH4 + CH2O 2CH3OO ) CH2O + CH3OH + O2 CH3OO + HO2 ) CH3OOH + O2 CH4 + CH3O ) CH3 + CH3OH CH4 + CH3OO ) CH3 + CH3OOH CH4 + OH ) CH3 + H2O CH4 + H ) CH3 + H2 CH3OOH ) CH3O + OH CH3OH + CH3 ) CH2OH + CH4 CH3OH + CH3OO ) CH2OH + CH3OOH CH3OH + H ) CH2OH + H2 CH2O + CH3 ) HCO + CH4 CH2O + CH3O ) HCO + CH3OH CH2O + OH ) HCO + H2O CH2O + H ) HCO + H2 HCO + O2 ) CO + HO2 HCO + M ) CO + H + M CO + CH3O ) CO2 + CH3 CO + HO2 ) CO2 + OH CO + OH ) CO2 + H CH3OH + H ) CH3O + H2 CH2O + CH2OH ) HCO + CH3OH CH2O + O2 ) HCO + HO2 CH3OH + CH3 ) CH3O + CH4 CH3OH + OH ) CH3O + H2O CH3OH + OH ) CH2OH + H2O CO + H + M ) HCO + M CH3OO + CO ) CH3O + CO2 H + O2 ) OH + O CH3 + O2 ) CH3O + O CH3OH + O ) OH + CH2OH HCHO + O ) OH + HCO HO2 + O ) OH + O2 CO + O + M ) CO2 + M O + O + M ) O2 + M OH + O ) H + O2 CO2 + H ) CO + OH H2O + H ) H2 + OH H2O + O ) 2OH CH3 + H2O ) CH4 + OH H2 + O ) OH + H OH + H2 ) H + H2O CH3 + H2 ) CH4 + H
4.035E+10 7.227E+08 6.624E+07 2.409E+10 1.265E+08 4.637E+07 1.560E+08 1.807E+08 1.927E+02 2.246E+01 3.981E+15 3.200E-02 1.807E+08 1.696E+04 5.540E+00 1.024E+08 3.433E+06 2.192E+05 5.119E+10 5.119E+18 1.565E+10 1.506E+11 4.400E+03 4.240E+03 5.480E+00 2.048E+10 1.450E-02 6.310E+14 1.024E+11 1.686E+14 1.988E+15 1.000E+10 1.807E+10 1.747E+10 6.167E+08 1.886E+07 4.517E+11 3.000E+11 6.204E+04 4.578E+06 4.818E-01 1.084E+01 6.384E+03 2.891E-01
n
un facb
5.6913E+04 0.0E+00 2.6030E+03 0.0E+00 0.0E+00 -2583.00 8.8430E+03 1.8480E+04 2.1060E+03 8.7550E+03 4.3000E+04 7.1710E+03 1.3711E+04 4.8690E+03 5.8620E+03 2.9800E+03 -447.00 3.0010E+03 1.6890E+03 2.0422E+04 1.1803E+04 2.3647E+04 -740.00 4.8690E+03 5.8620E+03 38949.00 6.9350E+03
0 0 0 0 0 0 0 0 2.4 3 0 3.2 0 2.1 2.81 0 1.18 1.77 0 -2.14 0 0 1.5 2.1 2.8 0 3.1
3.6880E+03 2.3899E+04 1.7386E+04 2.9229E+04 4.6850E+03 3.0800E+03 -4.0000E+02 3.0000E+03 -1.7883E+03 5.9610E+01 2.5712E+04 1.8410E+04 1.7098E+04 1.4863E+04 5.9220E+03 2.9600E+03 8.7110E+03
-1.82 0 -0.9 -1.57 0 0 0 0 0 -0.5 0 1.9 1.3 2.9 2.8 2 3.12
5.00 1.30 2.00 5.00 2.00 5.00 3.00 10.00 1.25 1.30 c 1.40 1.30 1.50 2.00 3.00 1.25 1.30 1.50 5.00 5.00 3.00 d 1.50 3.00 2.00 1.40 1.10e 1.60e 3.00 f 1.30 3.00
E, cal/mol
1.50 1.25 1.30 1.40 g 2.50 2.50 1.60 1.60 1.40 1.50
a k ) ATn exp(-E/RT), L, mol, s). b un fac: uncertainty factor. c Lødeng et al. (1995). d Warnatz (1990). e k 28 + k29 ) 6.62e01 × exp(-483/T)T2.5; k28/k29 ) 3.7 exp(-1020/T). f Vedeneev et al. (1988b). g Baulch et al. (1976).
(1)
of the jth species, and ki is the rate constant of the ith reaction. In this case m ) 46 and n ) 18 (Table 1 and Figure 2). The negative sign in the summation term of eq 1 indicates consumption of species. The differential equations were integrated numerically by using the DDASAC (double precision differential algebraic sensitivity code) package developed by Caracotsios and Stewart (1985). The integration scheme for the model was verified by comparing solutions at different step sizes. The input concentrations of reactants were chosen to match the experimental feed concentrations. Isothermal integration was carried out, as the temperature deviations measured within the reactor was within ( 10 °C. The frequency factors of the rate expressions were used as found in the data sources and were not adjusted to obtain better comparisons with experimental data. Reactions 31-46 were included in the model set to account for possible interactions of product species but did not significantly influence the prediction of conversion and selectivity values.
where aji are the stoichiometric coefficients of the jth species in the ith reaction, (cj) is the molar concentration
Results and Discussion The reaction conditions yielding high oxygen conversions were first identified by carrying out a series of
calculated rate constants using molecular diffusivities and the surface to volume ratio of the reactor. These wall contributions were neglected by Lødeng et al. (1995), even though hydrogen peroxide was included in the scheme. Chun and Anthony (1993b) achieved better comparisons between experimental and predicted results by taking into account the temperature profile along the reactor length. In this work, the possible combination of reaction species which were not considered are indicated in Figure 2, and the reasons for such omissions are highlighted by using different legends. A set of ordinary differential equations were written to account for the formation and consumption of each species. In general, for the reaction set consisting of m elementary reactions and n species, the production rate rj of the jth chemical species was written as n
m
rj )
∑ i)1
aji(ki
(cj)a ) ( j ) 1, 2, ..., n), (i, 2, ..., m) ∏ j)1 ji
Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997 1405 Table 2. Conversion and Selectivity Values at 34 and 50 bar selectivity, % runa
T, K
P, bar
time, s
x(CH4), %
1
623
50
96
0.09
2 3 4 5 6
643
34 34 34 50 50
36 60 96 60 96
7 8 9 10 11
673
34 34 34 50 50
12 13 14 15 16
703
17 18
673
a
x(O2), %
CO
CO2
CH3OH
