3350
Ind. Eng. Chem. Res. 1998, 37, 3350-3357
Hydrogenation of Carbon Dioxide to Methanol with a Discharge-Activated Catalyst Baldur Eliasson, Ulrich Kogelschatz,* Bingzhang Xue, and Li-Ming Zhou† ABB Corporate Research Ltd., 5405 Baden, Switzerland
To mitigate greenhouse gas CO2 emissions and recycle its carbon source, one possible approach would be to separate CO2 from the flue gases of power plants and to convert it to a liquid fuel, e.g., methanol. Hydrogenation of CO2 to methanol is investigated in a dielectric-barrier discharge (DBD) with and without the presence of a catalyst. Comparison of experiments shows that this nonequilibrium discharge can effectively lower the temperature range of optimum catalyst performance. The simultaneous presence of the discharge shifts the temperature region of maximum catalyst activity from 220 to 100 °C, a much more desirable temperature range. The presence of the catalyst, on the other hand, increases the methanol yield and selectivity by more than a factor of 10 in the discharge. Experiment and numerical simulation show that methane formation is the major competitive reaction for methanol formation in the discharge. In the case of low electric power and high pressure, methanol formation can surpass methanation in the process. CO2 + 3H2 f CH3OH + H2O
Introduction Carbon dioxide, the major man-made greenhouse gas, is mainly emitted from combustion of fossil fuels (coal, oil, and natural gas) and transportation. About onethird of the global emissions of CO2 originates from power stations generating electricity. One possible approach for mitigating CO2 emissions and recycling this C-source would be to separate CO2 from the flue gases of power plants and to convert it with the aid of hydrogen to more useful compounds, e.g., a liquid fuel like methanol. It has already been demonstrated that high-purity CO2 can be separated from the flue gas of a coal-fired power plant (Eliasson, 1994). As far as the overall CO2 reduction is concerned, this scheme depends on a source of hydrogen not causing additional CO2 emissions to the atmosphere. CO2-free hydrogen can be obtained by using solar energy, hydro energy, nuclear energy, or biomass and reforestation. Promising new applications for methanol are the direct use as a transportation fuel (either undiluted or blended with gasoline) and as a feedstock for the methanol-to-gasoline (MTG) process and for DME (dimethyl ether) synthesis. As an alternative transportation fuel, methanol can be used in cars without major changes to the engine. Recent announcements of major car manufacturers propagate on-board methanol reforming to hydrogen for feeding fuel cells in zeroemission vehicles. Background Conversion of CO2 to methanol by catalytic partial hydrogenation has been extensively investigated over different heterogeneous catalysts (Highfield, 1995). * To whom correspondence should be addressed. E-mail:
[email protected]. Fax: +41-56-493 45 69. Telephone: +41-56-486 81 67. † On leave from Xi’an Jiaotong University, Xi’an, People’s Republic of China.
(∆Ho220 °C ) -58.1 kJ mol-1, ∆Go220 °C ) 41.5 kJ mol-1) (1) Reaction (1) is an exothermic process which does not require external energy once the process is initiated. The required energy for CO2 hydrogenation is contained in the hydrogen. With the aid of catalysts at proper conditions this reaction can proceed at moderate temperatures. The best catalyst for CO2 hydrogenation to methanol is supported copper (Highfield, 1995). On a CuO-ZnO-Al2O3-Cr2O3 catalyst, 79% selectivity and 20% yield for methanol production with 25% CO2 conversion were obtained at the conditions of about 70 bar and 250 °C in a feed of H2:CO2 ) 3:1 (Arakawa et al., 1992). On a CuO-ZnO-Cr2O3 catalyst, 14% methanol yield and 24% CO2 conversion were also reported at a pressure of 50 bar and a temperature of 250 °C (Fujiwara et al., 1994). Maximum catalytic activity around 220 °C reveals that the activation energy barrier for the methanol reaction over copper-based catalysts can only be surmounted at elevated temperature. Therefore, alternative schemes for methanol synthesis at low or ambient temperature are of considerable interest. A nonequilibrium discharge is an effective tool to generate energetic electrons, which can initiate a series of plasma chemical processes such as ionization, dissociation, and excitation. Its generation and potential applications were reviewed, e.g., by Eliasson and Kogelschatz (1991a,b). Corona discharge and dielectricbarrier discharge (DBD) techniques are two of the commonly used methods for producing nonequilibrium plasmas at atmospheric pressure. Recent experiments (Zhou et al., 1998) show that methanol can be partially oxidized in DBDs with a yield as high as 3%. In other experiments, a corona discharge was used for oxidative coupling of methane (Liu et al., 1996). An enhancement of catalytic activity was also achieved by a dc corona discharge at relatively low temperature (Marafee et al., 1997). As far as the mechanism of methanol synthesis
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Ind. Eng. Chem. Res., Vol. 37, No. 8, 1998 3351
Figure 1. Dielectric-barrier discharge configuration and experimental setup (MFC, mass flow controller; BPV, backpressure valve; FM, flowmeter; P, pressure gauge).
is concerned, the function of the catalyst for methanol synthesis is supposed to be the dissociation of the source gas molecules at the catalyst surface into atoms and radicals, thus initiating the methanol synthesis reaction (Chinchen et al., 1990). The idea of using DBDs for CO2 hydrogenation was proposed by Eliasson et al. (1993). The intention is to make use of excited molecules, atoms, and chemically active radicals to overcome the activation energy barrier of the CO2 hydrogenation reaction. Experiments performed in our laboratory showed that methanol can be formed in DBDs at room temperature and atmospheric pressure (Kogelschatz and Eliasson, 1996; Eliasson et al., 1996; Bill et al., 1997). On the basis of these investigations, our present efforts have been directed toward combining the positive effects of DBDs with those of a catalyst. Experimental Section The experimental configuration is shown in Figure 1. The DBD reactor is the same as that used before (Bill, 1997; Bill et al., 1997). It can be operated at pressures up to 10 bar and controllable wall temperatures up to 400 °C. An outer steel cylinder of 54-mm inner diameter serves as the ground electrode, and an alternating high voltage of peak voltages up to 20 kVpp with a frequency of about 30 kHz is applied to the other electrode inside a quartz tube mounted coaxially in the steel tube. A high-voltage power supply (Arcotec Corona generator CG 20) can feed between 100 and 1000 W into the discharge by adjusting the voltage amplitude. The dielectric-barrier discharge is maintained in the annular discharge gap of 1-mm width and 310-mm length, resulting in a discharge volume of about 50 mL. The catalyst chosen for this study is a commercial methanol synthesis catalyst (Haldor Topsøe S/A, MK-101), which is identical with that used in our catalytic packed-bed reactor (Bill, 1997; Bill et al., 1996, 1997). It is a CuO/ ZnO/Al2O3 catalyst normally supplied in pellets with a copper content of about 440 kg/m3. To introduce it into the narrow discharge gap, it was ground to a size range of 250-500 µm and then positioned between two thin sheets of glass fiber fleece which were wrapped around the quartz tube serving as the dielectric barrier. The feed gases CO2 and H2 are introduced into the reactor from high-pressure bottles via mass flow controllers (MFCs). The total gas flow can vary from 0.1 to 4 NL/min. A backpressure valve (BPV) at the exit of the reactor controls the pressure in the discharge independently of the mass flow. The temperature of the steel electrode can be controlled with a closed loop of
Figure 2. Methanol yield versus wall temperature for the three different cases (pressure, 8 bar; power, 500 W (no power in the case of catalyst only); flow rate, 0.5 NL/min; input, H2:CO2 ) 3:1).
recirculating oil from a thermostat. A MTI (Microsensor Technology Inc.) dual-module micro gas chromatograph (MTI 200H) is used to analyze the gaseous products. It is connected to the reactor by a heated line to avoid condensation. A Poraplot Q column (8 m, 0.32 mm) and a molecular sieve 5 A plot column (10 m, 0.32 mm) are used, both with a TCD (thermal conductivity detector). This arrangement allows us to measure the gases H2, N2, O2, CH4, CO, CO2, H2O, CH3OH, CH3OCH3, and C2H5OH within 2 min. Occasionally, an additional gas chromatograph (HP 5890A) with a Poraplot Q column and a FID detector was used to examine additional hydrocarbons in the product stream. The condensable products are collected in a cold trap with ice for further analysis. The mass balance was checked by adding a small amount of nitrogen downstream of the DBD reactor as a reference gas. Since nitrogen does not react with the products, the N2 GC signal can be used to determine the change of volume flow due to chemical reactions in the reactor (Figure 1). In all cases the deviation of the carbon mass balance was within (3%. The computation of yields and selectivities is based on carbon. Experimental Results 1. Temperature Effect Comparison. Methanol formation was first compared in the three cases of catalyst only, discharge only, and catalyst together with discharge in the temperature range from 60 to 250 °C. Other conditions were kept constant: pressure, 8 bar; flow, 0.5 NL/min; power, 500 W; mixing ratio, H2:CO2 ) 3:1. The obtained methanol yields are plotted in Figure 2 as a function of temperature. Catalyst Only. The catalyst, sandwiched between two sheets of glass fiber fleece, was inserted in the discharge gap. It was reduced in a gas mixture of 1.5% H2, 0.5% CO, 4% CO2, and 94% N2 at 4 bar running a special temperature program. With this activated catalyst, methanol formation in a mixture of H2/CO2 ) 3:1 at 8 bar starts at about 150 °C and its yield peaks around 220 °C, reaching a maximum of 2.2% with a selectivity of 31%. This yield is already close to the indicated equilibrium value of 3%. Besides methanol, CO, H2O, and traces of CH4 are also formed. With rising temperature, the methanol selectivity significantly decreases and the CO selectivity correspondingly increases. Above 200 °C, the CO selectivity is higher than the methanol selectivity. Methanation starts at about 250 °C. It is apparent that low temperature is prefer-
3352 Ind. Eng. Chem. Res., Vol. 37, No. 8, 1998 Table 1. Comparison of Experimental Results and Methanol Equilibrium Yields (Gas Pressure, 8 bar; Power, 500 W; Flow Rate, 0.5 NL/min; Input, H2:CO2 ) 3:1) selectivity (%) gas temp (°C) catalyst only discharge only discharge + catalyst catalyst only discharge only discharge + catalyst
100 220
convn of CO2 (%)
CO
CH4
CH3OH
exp yield of CH3OH (%)
0 12.4 14 6 13.3 11
0 96 76-80 67 96 84
0 3.2 12-13 0 3.7 14.3
0 0.4 7-10 31.3 0.47 1.9
0 0.06 0.8-1.0 2.2 0.06 0.22
able for methanol formation, even though CO2 conversion is small in this temperature range. CO is presumably produced by the reverse water-gas shift (RWGS) reaction
CO2 + H2 f CO + H2O (∆Ho220 °C ) 39.8 kJ mol-1, ∆Go220 °C ) 20.4 kJ mol-1) (2) Reaction (2) is an endothermic reaction, which is much faster at higher temperatures. It is the main competitive reaction for methanol formation in this case (Bill et al., 1996). This measurement is in agreement with our previous measurements in a tubular packedbed reactor (Bill, 1997; Bill et al., 1997). Therefore, it can be concluded that the glass fleece used as a support for the catalyst grains has no negative influence on catalyst activity. Discharge Only. Experiments were performed in the DBD reactor without catalyst in the discharge gap. The major products are CO and H2O, produced in similar amounts. The CO selectivity of more than 90% slowly decreases with rising temperature. About 0.30.4% CO2 conversion to CH4 is observed, and CH4 selectivity lies between 3 and 4%, both slightly rising with increasing temperature. The measured methanol yield is around 0.06% and does not depend on temperature. The methanol selectivity is 0.4-0.5% over the temperature range under investigation, which is roughly a factor of 7 lower than the CH4 selectivity. The fleece was inserted in the discharge gap as a catalyst support. We noticed an increased stability of the discharge especially at higher pressure. Apparently the presence of the fleece facilitates discharge initiation by introducing inhomogeneities of the electric field. It also improves the heat conduction in the gap. In the range of 80-220 °C, methanol formation is similar to that with discharge only (Eliasson et al., 1997), suggesting that the glass fleece has no noticeable catalytic effect on methanol formation. Discharge and Catalyst. The most important experiments were performed with the catalyst placed in the discharge zone and the discharge running. As shown in Figure 2, a new peak in methanol yield is now obtained at about 100 °C with a peak value of 0.8-1%, about 10 times higher than the value obtained in the discharge without catalyst at 100 °C and about half of what can be obtained with only the catalyst at 220 °C. At 220 °C an ignition of the discharge reduces the methanol output from 2.2% to less than 0.3%. It appears that the discharge lowers the catalyst activity at high temperature and gives rise to a new activity maximum at about 100 °C, at which no catalytic activity existed without discharge (Table 1). In other words, the discharge shifts the region of maximum catalyst activity by more than 100 °C, indicating a substantial reduction
equil yield of CH3OH (%) 34 3
of the catalyst activation energy for methanol formation inside the discharge plasma. Since it may be argued that this temperature shift can be explained by an increase of the temperature in the gap due to dissipation of electric power, we have to quantify the temperature rise caused by the discharge. These relations have been carefully studied in another application of DBDs, the generation of ozone which depends strongly on the temperature in the discharge gap (Eliasson et al., 1987; Kogelschatz and Eliasson, 1995). The average increase of gas temperature ∆Tg is determined by a balance of the dissipated discharge power and heat removal by radial heat conduction to the cooled steel electrode
∆Tg )
1dP 3λF
(3)
where d is the gap spacing, λ the heat conductivity of the feed gas, and P/F the power density referred to the electrode area. If the power, on a time average, is evenly dissipated in the gap volume, the resulting radial temperature profile is a half-parabola with its peak value at the (uncooled) quartz tube. The wall temperature Tw of the steel tube is determined by the cooling fluid. The average temperature in the discharge gap is given by
Tg ) Tw + ∆Tg ) Tw +
1dP 3λF
(4)
With typical values for the operating parameters (d ) 0.001 m, λ ) 0.18 W/mK, 500 W power or P/F ) 9.5 kW/m2), we arrive at an average temperature increase ∆Tg of about 18 °C in the gap. The peak value at the quartz tube reaches approximately Tw + 26 °C. For this estimate we used the heat conductivity of pure hydrogen. It will be lowered somewhat by the presence of 25% CO2. On the other hand, the heat flux will be drastically improved by the presence of solid materials (fleece, catalyst) in the discharge gap. These considerations show that the observed shift of the temperature range for maximum catalyst activity of 120 °C can never be explained by an increase of gas temperature due to thermal effects by power dissipation of the discharge. Presumably, it is caused by the presence of atoms, free radicals, and excited molecules at the catalyst surface generated in this nonequilibrium discharge. It could be argued that the discharge leads to CO formation with subsequent catalytic CO conversion to methanol. This explanation can be excluded from measurements of catalytic methanol synthesis in CO/H2 mixtures using the same catalyst (Bill, 1997). In this case the temperature zone of maximum activity was about 280 °C. At 100 °C and 8 bar, the theoretical equilibrium yield of methanol is about 34% (Table 1). Obviously, the
Ind. Eng. Chem. Res., Vol. 37, No. 8, 1998 3353
Figure 3. Selectivity of methanol and methane versus wall temperature in the case of discharge and catalyst (pressure, 8 bar; power, 500 W; flow rate, 0.5 NL/min; input, H2:CO2 ) 3:1).
obtained methanol yield at this temperature is far from the equilibrium value. So, in principle, there is much more room for improvement. To better understand this finding, the selectivities of methanol, CO, and CH4, which are the main products containing carbon atoms, are compared. The CO selectivity slowly decreases and the CH4 selectivity significantly increases with rising temperature. This selectivity competition among the products may result in the observed methanol peak around 100 °C. The competition between CH4 and methanol occurs over the whole temperature range investigated in this combination and also in the case of discharge only. As compared to the situation in the case of discharge only, this catalyst in the discharge not only enhances methanol formation but also gives significant improvement for methanation. In general, higher temperatures favor methanation and lower temperatures tend to favor the methanol synthesis (Figure 3). The two peaks of methanol yield observed at 220 and 100 °C, respectively (Figure 2), are caused by a significant change of the selectivities among the products. In the case of catalyst only, the CO selectivity is about 2 times higher than that of methanol at 220 °C. The decline of the methanol yield at temperatures higher than 220 °C is accompanied by CH4 formation. In the case of combined catalyst and discharge, the temperaturedependent selectivities of methanol and CH4 exhibit opposite tendencies as Figure 3 shows. This clearly indicates that the relatively low peak value at about 100 °C is due to strong methanation. The CH4 formation from CO2 + H2 and CO + H2 reactions is highly exothermic.
CO2 + 4H2 f CH4 + 2H2O (∆Ho220 °C ) -174.82 kJ mol-1, ∆Go220 °C ) -76.0 kJ mol-1) (5) CO + 3H2 f CH4 + H2O (∆Ho220 °C ) -214.6 kJ mol-1, ∆Go220 °C ) -96.4 kJ mol-1) (6) The fact that in our experiment’s high temperature favors mathanation indicates that the strong methanation is due to active radical reactions. It can be expected that an appropriate catalyst with alkali promoters (e.g., Li+, K+, Pb+, Cs+) known from catalytic methanol synthesis (Highfield, 1995) may inhibit methanation in methanol synthesis also in the discharge. Nevertheless,
Figure 4. Selectivity of methanol and methane versus the gas pressure in the reactor (power, 500 W; flow rate, 0.5 NL/min; wall temperature, 80 °C; input, H2:CO2 ) 3:1).
one should note that in these experiments maximum yield and maximum selectivity for methanol formation simultaneously appear around 100 °C. The methanol selectivity and yield has been simultaneously improved by the presence of a catalyst in the discharges already at fairly low temperature. This is esteemed a considerable advantage over the traditional thermal catalytic reaction, in which an increase of the selectivity of methanol always results in a decrease of its yield below 220 °C. 2. Other Effects in Combining Catalyst and Discharge. As already indicated, methanation is the main competitive reaction for methanol synthesis in the presence of a catalyst in the discharge. Since hydrogen is more expensive than methane, methanation is not the best option for CO2 hydrogenation. In further experiments, other physical parameters (pressure, mixing ratio, electric power, and flow rate) and the influence of CH4 addition were investigated. Special attention is paid to the competition between methanol formation and methanation in the following experiments. Pressure Effect. Figure 4 shows the CH3OH and CH4 selectivities as a function of the gas pressure in the reactor. Increasing pressure causes a linear increase of methanol selectivity. Below 10 bar, CH4 is more readily produced than methanol. However, at 10 bar, a higher selectivity for methanol (about 6%) than that for CH4 is obtained. It is apparent that high pressure favors methanol formation in this combined experiment. CO2 Concentration Effect. The effect of CO2 content in the feed is shown in Figure 5 in the range from 10 to 100%. With increasing CO2 content in the feed CO formation increases. The selectivities shift rapidly from CH4 and CH3OH to CO. At 10% CO2 in the feed, methanol selectivity is about 14%, which is about 2.5 times lower than that of CH4. A little higher methanol selectivity than CH4 selectivity is observed when the CO2 content is over 50%. Electrical Power Effect. In DBDs the electrical power determines the number of electrons generated and furthermore influences the subsequent chemical processes. As increasing power leads to an increase in the average temperature of the discharge gap, we used a thermostat to control the wall temperature. Considering the narrow discharge gap, the good heat conduction of hydrogen, and the fleece, the average temperature in the reactor was estimated to be reasonably close to the wall temperature. At a total flow rate of 0.5 NL/
3354 Ind. Eng. Chem. Res., Vol. 37, No. 8, 1998
Figure 5. Selectivity of methanol and methane versus CO2 content in the feed (pressure, 8 bar; flow rate, 0.5 NL/min; wall temperature, 80 °C; power, 500 W).
Figure 6. Selectivity of methanol and methane versus the electric power (pressure, 8 bar; flow rate, 0.5 NL/min; wall temperature, 80 °C; input, H2:CO2 ) 3:1).
min, the experimental results are shown in Figure 6 as a function of electrical power from 200 to 800 W. With increasing power, the methanol selectivity slightly decreases despite an increase of CO2 conversion, while the CH4 selectivity increases much faster. At 200 W the methanol selectivity is higher than that of CH4, indicating that relatively more methanol is formed at lower electric input power and that higher power strongly favors methanation. Apparently, lower electric power leads to a better performance of methanol synthesis. With regard to the exothermic characteristics of methanol synthesis, reaction (1) does not require an external energy input. The obtained results indicate that already a small amount of electric energy is enough to activate the source gases and catalyst for initiating methanol synthesis (eq 1). Flow Rate Effect (Influence of Residence Time). The flow rate was varied from 0.1 to 1 NL/min, corresponding to residence times from 200 to 20 s based on the volume of the empty reactor. If the volumes of fleece and catalyst are included, the residence time can be estimated to be at least 2 times less. The selectivities of CH3OH and CH4 are given in Figure 7. The methanol selectivity increases slightly with flow rate up to 0.5 NL/ min and then saturates around 4.5%. In contrast, the CH4 selectivity decreases quickly with increasing flow rate up to 1 NL/min. The CO selectivity increases correspondingly. Apparently, the selectivity shifts from CO to CH4 rather than to methanol when the flow rate decreases. Earlier experiments with a residence time of 600 s showed that the methane selectivity and yield
Figure 7. Selectivity of methanol and methane versus the flow rate of the feed gas (pressure, 8 bar; power, 500 W; wall temperature, 80 °C; input, H2:CO2 ) 3:1).
can be more than 2 times higher than that of CO in cases with or without catalyst (Eliasson et al., 1993). With shorter residence time, as demonstrated here, the selectivity favors methanol formation. This tendency is consistent with the normal case with catalyst only (Lee et al., 1993). Influence of Injecting CH4. To obtain a better understanding of the influence of methane on methanol synthesis, additional experiments were conducted with injecting small amounts of CH4 in the feed in two cases, i.e., in the presence of the catalyst without and with igniting the discharge. Basically, no influence of CH4 addition on methanol synthesis was observed. This confirms that the observed negative influence of CH4 on methanol formation has to be ascribed to competitive intermediate reactions between methanation and methanol formation rather than to the presence of CH4 itself. 3. Chemical and Discharge Modeling. Computations Simulating Methanol Synthesis in the Discharge. Dielectric-barrier discharges at about atmospheric pressure consist of a large number of microdischarges. A simplified microdischarge model is employed to calculate the methanol yield in the discharge (Eliasson and Kogelschatz, 1991b; Eliasson et al., 1994). In this model, the microdischarges are considered as sources of atoms that initiate chemical reactions. The microdischarges generate energetic electrons of average energies in the range of 1-10 eV (Eliasson et al., 1987; Eliasson and Kogelschatz, 1991b), certainly high enough to dissociate CO2 and H2 molecules with dissociation energies of 5.45 and 4.48 eV, respectively.
e + H2 f H + H + e
(7)
e + CO2 f CO + O + e
(8)
In addition, generated CO will be dissociated into C and O atoms. By integrating the electronic dissociation reactions, we obtain the number of atoms generated in the discharge.
[H] ) R[H2]
(9)
[CO] ) β[CO2] - γ[CO]
(10)
[C] ) γ[CO]
(11)
Ind. Eng. Chem. Res., Vol. 37, No. 8, 1998 3355 Table 2. Main Reactions in the Kinetic Modela rate coefficient (cm3 molecule-1 s-1)
reaction
temp (K)
ref
CO + H + M f HCO + M HCO + H f CH2O CH2O + H f CH3O CH3O + H f CH3OH
Key Reactions Leading to CH3OH Formation 5.46 × 10-32(T/298)-1.82 exp(-1856/T)b 800-2500 7.77 × 10-14 exp(2285/T) 1500-1900 1 × 10-10 1 × 10-10
Tsang and Hampson, 1986 Tsuboi et al., 1981 estimated estimated
CO2 + e f CO + O + e H2 + e f H + H + M O + H2 f OH + H OH + H2 f H + H2O
Other Important Reactions for CH3OH Production (β, γ) (R) 1.52 × 10-13(T/298)2.8 exp(-2980/T) 400-1600 9.4 × 10-13(T/298)2 exp(-1490/T) 240-2400
adjusted, see text adjusted, see text Tsang and Hampson, 1986 Tsang and Hampson, 1986
H + H + M f H2 + M HCO + H f CO + H2 H + CH3OH f H2 + CH2OH O + CH3OH f OH + CH2OH OH + CH3OH f H2O + CH2OH CH2OH + H f CH3 + OH CH3 + H f CH4
Reactions Slowing Down CH3OH Production 9.11 × 10-33(T/298)-1.3 77-2000 2 × 10-10 300 1.66 × 10-11 exp(-2766/T) 500-680 6.11 × 10-13(T/298)2.5 exp(-1550/T) 1000-2000 6.19 × 10-13(T/298)2.5 exp(483/T) 300-2000 1.6 × 10-10 300-2500 2.05 × 10-10(T/298)-0.4 300-2500
Tsang and Hampson, 1986 Sarkisov et al., 1984 Hoyermann et al., 1981 Herron, 1988 Tsang, 1987 Tsang, 1987 Tsang and Hampson, 1986
HCO + H f CH2O HCO + H f CO + H2
Branching of the Two Reactions 7.77 × 10-14 exp(2285/T) 2 × 10-10
Tsuboi et al., 1981 Sarkisov et al., 1984
a
1500-1900 300
All temperatures in degree Kelvin (K). b Units: cm6 molecule-2 s-1.
[O] ) β[CO2] + γ[CO]
(12)
The coefficients R, β, and γ are functions of the electron density generated by individual microdischarge pulses and the reaction rates of the dissociation reactions (Eliasson et al., 1987). They depend on the parameters of a microdischarge, i.e., gas composition, pressure, temperature, dielectric, gap width, and the reduced electric field or mean electron energy. In this paper we try to determine the coefficients R, β, and γ in such a way that the measured product concentrations are approximated. The calculated values of [H], [CO], [C], and [O] are the initial particle densities for the subsequent chemical reactions. Our modeling simulates chemical reactions in the gas phase. The kinetic scheme includes 60 species and 348 reactions (Eliasson et al., 1993). Thermochemical data have been taken from the NIST database (Westley et al., 1992). Using the values 2 × 10-4, 5 × 10-2, and 1 × 10-6 for R, β, and γ, respectively, the calculated concentrations for CO2, H2, O2, and CO are in reasonable agreement with experimental values in the case without a catalyst (Figure 8). The main products are CO and H2O. For CH4 and CH3OH, the amounts are quite small and the agreement is not so good. The important reactions leading to methanol formation and also those slowing down methanol formation in this kinetic system are listed in Table 2. The radical reaction mechanism of methanol formation from CO2 and H2 in DBDs can be expressed as e
H
H
H
H
CO2 98 CO 98 CHO 98 CH2O 98 CH3O 98 CH3OH (13) This chemical route is similar to that proposed in the catalytic reaction (Coteron and Hayhurst, 1994). The produced methanol will further react with the radicals H, O, and OH to form CH4, slowing down CH3OH formation. H, O, OH
H
H
CH3OH 98 CH2OH 98 CH3 98 CH4
(14)
Equilibrium Calculations. Computations of equilibrium methanol yields were also performed for this
Figure 8. Comparison of calculated and measured concentrations in the case of discharge only (dotted line, computed value; symbol connected by line, experimental value; pressure, 4 bar; power, 400 W; wall temperature, 80 °C; flow rate, 1.0 NL/min).
feed gas mixture. Maximum methanol yields were determined for different temperatures, pressures, and component systems. These equilibrium calculations are based on the minimization of Gibbs free energy (Heuze et al., 1985). CO2 and H2 are chosen as the basic input components. The maximum methanol yield is shown in Figure 9 and also in Figure 2 for comparison with experimental results. When only the five components H2, CO, CO2, H2O, and CH3OH are considered in the chemical process, the computed equilibrium yield is close to the measured methanol yield in the temperature
3356 Ind. Eng. Chem. Res., Vol. 37, No. 8, 1998
Figure 9. Computed methanol equilibrium yield with and without including CH4 (pressure, 8 bar; input, H2:CO2 ) 3:1).
range of 220-250 °C in the case of only using the catalyst (Figure 2). With rising temperature the equilibrium methanol yield strongly decreases. The major reactions involved in the methanol synthesis from CO2 + H2 with catalysts are (Elvers et al., 1990; Wagialla and Elnashaie, 1991)
CO + 2H2 f CH3OH
(15)
CO2 + 3H2 f CH3OH + H2O
(16)
CO2 + H2 f CO + H2O
(17)
In the experiments with discharge, we found that methane is also a main product in addition to CO, H2O, and CH3OH. When CH4 is included in these equilibrium calculations, the methanol equilibrium yield is significantly decreased and the temperature dependence of the methanol yield is also changed. In this case, the methanol yield increases with rising temperature and the value of the equilibrium yield is much lower, several orders of magnitude, than the yield in the calculations without considering CH4. The system was further extended to a total of 19 components including H2, CO2, CO, H2O, CH3OH, CH4, C2H6O, C2H4O2, H, O, O2, O3, CH, CH3, C2H2, C2H4, C2H6, C3H8, and C4H10 although most of them were not detected in the experiments. The results show that the methanol equilibrium yield is approximately the same as that in the computation with six components (H2, CO, CO2, H2O, CH3OH, CH4) (Eliasson et al., 1997). In addition to reactions (15)(17), reactions (5) and (6) may be involved in the process. Again, this indicates that the formation of CH4 in the discharge is the main obstacle limiting the methanol yield. Conclusions From the measurements and the numerical simulations described above we conclude the following: (1) A dielectric-barrier discharge can shift the temperature range of maximum catalyst activity toward lower temperature by an amount of about 100-120 °C. This creates the theoretical possibility of much higher CO2 conversion to methanol, because the equilibrium value at 100 °C and 8 bar is about 34%, roughly 10 times higher than that at 220 °C. (2) Compared to the case without catalyst, the experiments with a catalyst inserted in the discharge produced
more than 10 times higher methanol yield with 10-20 times higher selectivity at a temperature of about 100 °C. (3) Experiments and simulations demonstrate that methanation is the main competitive reaction for methanol formation in the presence of a catalyst in the discharge. (4) The experiments indicate that the methanol selectivity can be enhanced up to a level over that of CH4 selectivity through system optimization like the use of low power and high pressure. The electric power used in these experiments would be prohibitive for industrial methanol production because the resulting yield is too low. Dielectric-barrier discharges will only find technical applications in this field if the specific energy can be lowered substantially, which might be achieved by using better catalysts and by using short discharge pulses. Acknowledgment Thanks are due to Eric Killer for help with the experiments, to W. Egli for help with the computations, and to Haldor Topsøe S/A for supplying the catalyst. L.M.Z. thanks ABB Corporate Research Ltd. for providing the opportunity to stay as a visiting scientist for 10 months during 1997. B.X. thanks Prof. H. C. Siegmann for his encouragement and support. Literature Cited Arakawa, H.; Dubois, J. L.; Sayama, K. Selective conversion of CO2 to methanol by catalytic hydrogenation over promoted copper catalyst. Energy Convers. Manage. 1992, 33, 521. Bill, A.; Eliasson, B.; Killer, E. Hydrogenation of carbon dioxide in a packed bed reactor. 11th World Hydrogen Energy Conference (HYDROGEN’96), Stuttgart, Germany, June 1996; Proc. Vol. II, p 1989. Bill, A. Carbon Dioxide Hydrogenation to Methanol at Low Pressure and Temperature. Ph.D. Thesis No. EPFL 1726, Federal Institute of Technology, Lausanne, Switzerland, 1997. Bill, A.; Wokaun, A.; Eliasson, B.; Killer, E.; Kogelschatz, U. Greenhouse gas chemistry. Energy Convers. Manage. 1997, 38, S415. Chinchen, G. C.; Mansfield, K.; Spencer, M. S. The methanol synthesis: How does it work? CHEMTECH 1990, 20, 692. Coteron, A.; Hayhurst, A. N. Kinetics of the synthesis of methanol from CO + H2 and CO + CO2 + H2 over copper-based amorphous catalysts. Chem. Eng. Sci. 1994, 49, 209. Eliasson, B. CO2 Chemistry: An Option for CO2 Emission Control? In Carbon Dioxide Chemistry: Environmental Issues; Paul, J., Pradier, C. M., Eds.; The Royal Society of Chemistry: Cambridge, U.K., 1994. Eliasson, B.; Kogelschatz, U. Nonequlilibrium volume plasma chemical processing. IEEE Trans. Plasma Sci. 1991a, 19, 1063. Eliasson, B.; Kogelschatz, U. Modelling and Applications of Silent Discharge Plasmas. IEEE Trans. Plasma Sci. 1991b, 19, 309. Eliasson, B.; Hirth, M.; Kogelschatz, U. Ozone synthesis from oxygen in dielectric-barrier discharges. J. Phys. D: Appl. Phys. 1987, 20, 1421. Eliasson, B.; Simon, F. G.; Egli, W. Hydrogenation of CO2 in a Silent Discharge. In Non-Thermal Plasma Techniques for Pollution Control; NATO ASI Series G; Springer: Berlin, Germany, 1993; Vol. 34B. Eliasson, B.; Egli, W.; Kogelschatz, U. Modelling of dielectric barrier discharge chemistry. Pure Appl. Chem. 1994, 66, 1275. Eliasson, B.; Kogelschatz, U.; Killer, E.; Bill, A. Hydrogenation of carbon dioxide and oxidation of methane in an electrical discharge. 11th World Hydrogen Energy Conference (HYDROGEN’96), Stuttgart, Germany, June 1996; Proc. Vol. III, p 2449. Eliasson, B.; Kogelschatz, U.; Xue, B.; Zhou, L. M. Application of dielectric-barrier discharges to the decomposition and utilisation of greenhouse gases. 13th International Symposium on Plasma Chemistry, Beijing, Aug 1997; Vol. IV, p 1784.
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Received for review December 30, 1997 Revised manuscript received May 25, 1998 Accepted May 26, 1998 IE9709401
