Energy & Fuels 2002, 16, 687-693
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Non-Oxidative Coupling of Methane to C2 Hydrocarbons under Above-Atmospheric Pressure Using Pulsed Microwave Plasma Jun-qi Zhang,† Yong-jin Yang,† Jin-song Zhang,*,† Qiang Liu,† and Ke-rong Tan‡ Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, China, and Department of High-Tech. Development & Industrialization Ministry of Science and Technology of P. R. C. 15B, Fuxing Road, Beijing, 100862 Received August 16, 2001
A conventional high voltage wire-like plasma can be enhanced and spread by pulsed microwave to form an umbrella-like plasma which is continuously distributed in a reactor. Using such enhanced plasma, methane was directly converted to C2 hydrocarbons with higher yield than using conventional plasma at atmospheric or higher pressure. There was hardly any detectable trace of ethane in the product. The effects of pressure, microwave power, pulse duration, H2/CH4 mole ratio, and flow rate on methane conversion were investigated. With flow rate of 300 mL/ min, H2/CH4 ratio of 2, pressure of 0.13MPa, peak microwave power of 120 W, pulse duty factor of 400/400 ms, conversion of methane and yield of C2 had reached up to 59.2% and 52.1%, respectively. The ratio of C2H2/C2H4 in the product could be adjusted by changing the pressure.
1. Introduction The amount of discovered natural gas reserves has increased rapidly in recent years worldwide due to the advances in exploration techniques and a steady decrease in petroleum reserves. Therefore, it can be expected that natural gas will play an increasingly important role in energy and chemicals supplies in the 21st century. At present, natural gas is mainly utilized by producing synthesis gases that could be converted to liquid hydrocarbons and other chemical products. These indirect methods have drawbacks of longer processing time and higher cost which restrict their application. It is thermodynamically unfavorable for coupling natural gas directly to C2 hydrocarbons because methane molecules have higher stability. However, there are no significant advances or break-throughs in the use of conventional catalytic methods for natural gas conversion. Therefore, developing new techniques and processes of direct conversion of natural gas to higher hydrocarbons has become a challenging research topic. It is known that plasmas can activate methane molecules effectively in natural gas conversion. Particularly, low-temperature nonequilibrium plasmas can improve the selectivity of products and inhibit carbon deposition, and their research has become more active lately. Some of the works focused on the study of wirelike plasma and corona discharge. For instance, Gong et al. used pulsed corona plasmas combined with catalysts to convert methane.1 Liu et al. studied the * Corresponding author. Tel/Fax: +86-24-23906640 (direct). Email:
[email protected]. † Institute of Metal Research, Chinese Academy of Sciences. ‡ Department of High-Tech. Development & Industrialization Ministry of Science and Technology of P. R. C.
conversion of CH4/H2/O2, CH4/O2, CH4/H2O, and CH4/ CO2 systems via DC corona discharge in the presence or absence of heterogeneous catalysts such as zeolite. The highest yield of C2 hydrocarbons of 32% was obtained at the lowest flow rate tested (10 sccm, residence time ∼2.3 s).2-5 Because the wire-like or corona plasmas are discontinuously distributed in the space, they have such low activation and reaction area/ volume that higher conversion and yields are not feasible. A significant amount of research has been conducted in the conversion of natural gas via microwave (MW) plasma, as it has a wider discharge spectrum, higher energy density, and a higher ratio of electron temperature to ion temperature than conventional plasmas. The research of MW plasma methane conversion can be divided into two groups: one is under low-pressure, as shown by the work of Suib et al. in which MW plasma was combined with a catalyst to convert methane to ethane, ethylene, and acetylene at pressure of 1.33-6.65 KPa, with a total yield of 19%.6 In addition, Huang et al. studied the coupling of methane under MW plasmas,7 and Onoe et al. converted methane selectively to acetylene.8 All these methods have low energy efficiency because the conversion had (1) Gong, W. M.; Zhu, A. M.; Zhou, J.; Shi, H.; Ruan, G. S.; Zhang, B. A. The catalytic coupling of methane under pulsed corona plasma. In 13th International Symposium on Plasma Chemistry, Beijing; edited by Wu, C. K., Ed.; Peking University Press, 1997; pp 1578-1583. (2) Liu, C. J.; Marafee, A.; Hill, B.; Xu, G. H.; Mallinson, R.; Lobban, L. Ind. Eng. Chem. Res. 1996, 35, 3295. (3) Marafee, A.; Liu, C. J.; Hill, B.; Xu, G. H.; Mallinson, R.; Lobban, L. Ind. Eng. Chem. Res. 1997, 36, 632. (4) Liu, C. J.; Marafee, A.; Xu, G. H.; Mallinson, R.; Lobban, L. Appl. Catal. A 1997, 164, 21. (5) Liu, C. J.; Mallinson, R.; Lobban, L. J. Catal. 1998, 179, 326. (6) Suib, S. L.; Zerger, R. P. J. Catal. 1993, 139, 383. (7) Huang, J.; Suib, S. L. J. Phys. Chem. 1993, 97, 9403. (8) Onoe, K.; Fujie, A.; Yamaguchi, T.; Hatano, Y. Fuel 1997, 76, 281.
10.1021/ef010217u CCC: $22.00 © 2002 American Chemical Society Published on Web 03/19/2002
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Figure 1. A schematic of the pulsed MW enhanced high voltage discharge plasma reaction apparatus.
to be done under very low pressure and flow rate, which are not suitable for industrial application. The other group is conversion under atmospheric pressure MW plasmas. Exciting an MW plasma at atmospheric pressure requires such high intensity of electromagnetic field that it is turned into a high temperature arc-light plasma at the instant the plasma is excited. This kind of plasma has exorbitant temperature and energy density, which tend to result in total dissolution of methane into acetylene and coking. Therefore, if it is possible to separate the plasma excitation and sustentation, i.e., the plasma is excited at atmospheric pressure by “ignition”, and it is maintained using low intensity electromagnetic field, high yield of conversion can be achieved at low cost and high rate. Achieving a low-temperature nonequilibrium MW plasma at atmospheric pressure is very advantageous for the effective and selective conversion of natural gas. Through a compatibility study of conventional high voltage wire-like plasmas and MW cavity, a conventional wire-like plasma enhanced and spread by pulsed MW was chosen. On one hand, microwave can spread a wire-like plasma to form an umbrella-like plasma, whose area (or volume) is much expanded and the activation of plasma is enhanced; on the other hand, it inhibits abrupt transition from deficient density to excessive density, which helps sustain a low temperature nonequilibrium plasma. A plasma chemical reaction apparatus was manufactured in which high voltage discharged wire-like plasma was enhanced by pulsed microwave. Using this device, the non-oxygen conversion of natural gas directly to C2 hydrocarbons at atmospheric and higher pressure was investigated. 2. Experimental Section The experimental apparatus contains six main parts: a gas flow controller, a pulsed microwave generator, a waveguide system, a pulsed plasma reactor, a pressure controller. and a product analyzer. The flow rate of feed gas was measured by a D07 mass flow controller and a D08 display unit (Beijing Jianzhong Machinery Factory), which were calibrated by a soap film meter. The frequency of the pulsed MW generator is 2450 MHz, with a maximum output power of 600 W and a pulse duration between 10 and 999 ms which is continuously adjustable. The pulsed MW plasma reactor is schematically illustrated in Figure 1, in which the high voltage is applied to the internal conductor. When the wire-like plasma is formed in the space between internal conductor and inner walls of the outer conductor, a pulsed microwave is introduced into the
Zhang et al. cavity. As a result, the wire-like plasma is enhanced and spread, forming an umbrella-like plasma which is continuously distributed in space. A short quartz tube, which is attached to the end of the coaxial conductors, can be used to force the feed gas to flow across the plasma zone. The pressure in the reactor is adjusted by a needle valve placed at the exit of the reactor. The reaction products are analyzed with a gas chromatograph equipped with a TCD (thermal conductivity detector), with ultrapure H2 as carrier gas. The experiments were carried out at both atmospheric and higher pressures. The natural gas used as reactant was obtained from Panjin oil and gas field in Liaoning Province, with methane content more than 99%. For calculation of the carbon balance, the ultrapure nitrogen is mixed into the products after the exhaust gas from the reactor flowed through a filter and a condenser to remove the solid and condensable material; the mole number of N2 is a known quantity and expressed as n0N2. The mole percent of products were evaluated directly from the GC peak arntotal ) n0N2/CN2eas. The material balances in the products can be calculated with following expressions:
ntotal ) n0N2/CN2 nCH4 ) ntotal × CCH4 ) CCH4 × n0N2/CN2 nC2H2 ) ntotal × CC2H2 ) CC2H2 × n0N2/CN2 nC2H4 ) ntotal × CC2H4 ) CC2H4 × n0N2/CN2 nC2 ) nC2H2+ nC2H4 carbon number of higher hydrocarbons ) n0CH4 - (nCH4 + 2 × nC2) where ntotal, nCH4, nC2H2, nC2H4, nC2 are the mole number of total gas products, CH4, C2H2, C2H4, and C2 hydrocarbons in the products, respectively; CN2, CCH4, CC2H2, and CC2H4, are the mole percent of N2, CH4, C2H2, and C2H4 in the gas exhaust, respectively; and n0CH4 is the inlet mole number of CH4. Presuming that all the polymerizates and condensable material are summed up as higher hydrocarbons, the conversion of methane, selectivity and yield of each product are defined as:
CH4 conversion ) (1 - nCH4/n0CH4) × 100% Selectivity of C2H2 ) [2nC2H2/(n0CH4 - nCH4)] × 100% Selectivity of C2H4 ) [2nC2H4/(n0CH4 - nCH4)] × 100% Selectivity of higher hydrocarbons ) (selectivities of C2H2, C2H4) 1-
∑
Yield of C2 hydrocarbons ) CH4 conversion ×
∑ (selectivities of C H , C H ) × 100% 2
2
2
4
Yield of higher hydrocarbons ) CH4 conversion × [1 -
∑ (selectivities of C H , C H )] × 100% 2
2
2
4
3. Results and Discussion 3.1. Effect of Enhanced and Spread Wire-Like Plasma with Pulsed MW. Using the developed device, pulsed MW can enhance and spread conventional wirelike plasma to form a continuously distributed umbrella-
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Table 1. Comparison to Other Plasma Processes plasma mode feedstock tested catalyst operating conditions pressure (kPa) residence time (ms) incident power (W) CH4 flow rate (sccm) CH4 conversion (%) C2 yield (%)
this research pulsed microwave CH4 + H2 without
Chang-jun Liu5 DC corona H2 + CH4 + O2 NaY zeolite
Steven L. Suib6 continuous microwave CH4 without
Shigeru Kado9 DC pulse discharge CH4 without
130 275-640 120 (peak) 60-140 52-72.5 47-52
101 230-2300 6.5 2-20 26-63.5 10-32.6
1.3∼13.1 10-25 40-80 50-500 3-38 3-38
101
Figure 2. Conventional wire-like and improved pulsed MW enhanced plasmas.
like plasma. Photographs of a wire-like plasma and a plasma enhanced by pulsed MW are shown in Figure 2. It is easy to see that a discontinuous wire-like discharge plasma was spread to form a continuously distributed plasma, and that the plasma spread by the pulsed MW has an increased area (or volume). Here, it must be pointed out that this broadening of volume refers to the visible portion of the plasma, but it needs be proved in the experiments whether the zone of chemistry is similarly broadened. By introducing the pulsed MW a specific electromagnetic field is formed along the coaxial line: the electric field is axisymmetric with respect to the internal conductor and spreading out in the radial direction, the magnetic field is also axisymmetric with respect to the internal conductor, but perpendicular to the electric field. Such an electromagnetic field could accelerate the electrons and ions formed in the conventional wire-like discharge plasma, and greatly enhance the activation of plasma; on the other hand, the ions and electrons could spin in the electromagnetic field. Such accelerated electrons with higher kinetic energy could collide and excite the feed gas molecules, which would result in the spread of the wirelike plasma and form the continuous umbrella-like plasma. The conversion of CH4 + H2 system has been studied by using conventional wire-like plasma and pulsed MW enhanced plasma, respectively. The results have shown that when using conventional wire-like plasma there was hardly any conversion of CH4, but with enhanced and spread plasma by pulsed MW, the reaction has reached 59.2% conversion of methane and 52.1% single pass yield of C2 hydrocarbons, where the single pass yield of acetylene reached 42.7%. A comparison of operating conditions (pressure, residence times, incident power), % conversion, feedstocks, plasma mode, presence or absence of catalyst, % conversion, and yields for our method and other plasma processes of Chang-jun Liu,5 Steven L. Suib,6 and Shigeru Kado9 are (9) Kado, Shigeru; Sekine, Yasushi; Fujimoto, Kaoru Chem. Commun. 1999, 2485.
25 10 40.6 39.9
given in Table 1. The peculiarity of experiments reported here is related to higher pressure, high flow rate, higher conversion, and C2 yield. Therefore, it is effective for converting the natural gas by using conventional wire-like plasma enhanced and spread by pulsed MW. 3.2. Effect of Hydrogen and Reaction Mechanism. Some investigations on oxidative conversion of methane by gas discharges in the absence10 and presence5 of heterogeneous catalysts have shown that cold plasma methane conversion proceeds via steps involving free radicals, in which the formation of methyl radicals is the rate-controlling step. According to the analyses using mass spectra and a simulation of plasma reaction of methane, it was concluded that methyl radicals are responsible for the methane coupling to C2 hydrocarbons with low temperature nonequilibrium plasmas.8,11-13 The oxygen used under those conditions induces significant oxidation of methane and hydrocarbon products to produce carbon oxides (mostly carbon monoxide). The selectivity of higher hydrocarbons is thereby reduced. Methyl radicals can be produced by electron-methane collisions,10 and it has been shown that electronically excited CH4 [S1(9.6 and 10.4 eV) and S2(11.7 eV)] can be the precursor of radicals CH3, CH2, and CH:9
e( > 10eV) + CH4 f CH4(S1,S2) + e
(1)
CH4(S1,S2) f CH3 + H
(2)
CH3 f CH2 + H
(3)
CH2 f CH + H
(4)
CH f C + H
(5)
CH4(S1,S2) f CH2 + 2H
(6)
CH4(S1,S2) f CH + 3H
(7)
CH4(S1,S2) f C + 4H
(8)
Methyl radicals may also be generated from the reaction between methane and hydrogen radicals:10
CH4 + H f CH3 + H2
(9)
Hydrogen radicals are produced relatively easily in cold plasmas.14 In the experiments, hydrogen was first ejected into the coaxial reactor tubes and then excited (10) Oumghar, A.; Legrand, J. C.; Diamy, A. M.; Turillon, N. Plasma Chem. Plasma Process. 1995, 15, 87. (11) Ivanov, Yu. A. In Mechanisms of plasma chemical reactions of hydrocarbons and hydrogen-containing molecules; Institute of Petrochemical Synthesis, Russian Academy of Sciences, Moscow, 1987. (12) Ivanov, Yu. A.; Soldatova, I. V. In Physicochemical processes in low-temperature plasma; Institute of Petrochemical Synthesis, Russian Academy of Sciences, Nauka, Moscow, 1985. (13) Ivanov, Yu. A. Khim. V ysok. Energ. 1988, 22, 152 (in Russian).
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by a conventional AC high voltage to form a hydrogen wire-like plasma. Second, the pulsed MW was introduced to enhance and spread the hydrogen plasma, which forms a large number of H-radicals and highenergy electrons. In general, hydrogen plays an important role in the formation of methyl radicals, which is thought to be responsible for the initiation of radical reactions leading to C2 hydrocarbon production. The resultant products formed by subsequent reactions are: Ethane formation:
CH3 + CH3 f C2H6
(10)
CH4 + CH2 f C2H6
(11)
Ethylene formation:
CH3 + CH2 f C2H4 + H
(12)
CH2 + CH2 f C2H4
(13)
Acetylene formation:
C2H4 f C2H2 + H2
(14)
CH2 + CH2 f C2H2 + H2
(15)
CH + CH f C2H2
(16)
CH2 + CH f C2H2 + H
(17)
Because the experimental results of this work show that there is hardly any ethane in the products, it can be concluded that the reactions 10 and 11 are negligible, so there are less excited molecule CH4 (S) and radical CH3 in the pulsed MW plasma zone as a result of insufficient activation of plasma. It needs further research for an explanation about the formation of ethane only at trace levels in the present work. There are also some small amount of high hydrocarbons and polymerizates during methane coupling in the pulsed MW plasma. These high hydrocarbons and polymerizates can be produced heterogeneously via: M
nC2H2 + e 98 (-C)CH-C)CH-)X + mH2
(18)
where “M” indicates inner conductor surface or quartz reactor wall, and “e” represents the high-energy electrons. In addition, the hydrogen radicals are assumed to be an essential ingredient for the removal of undesired carbon deposits that may have a negative effect on the stability of plasmas:
C(s) + H f CH
(19)
CH(s) + H f CH2
(20)
3.3. Effect of H2/CH4 Ratio. Figure 3 shows the effects of the H2/CH4 mole ratio in the feed on the yields of C2 and undesired products (Figure 3a) and methane conversion and selectivity (Figure 3b). When the H2/CH4 mole ratio is 2/1, the yield of C2 hydrocarbons is about (14) Hollahan, J. R.; Bell, A. T. Techniques and Applications of Plasma Chemistry; Wiley: New York, 1974.
Figure 3. Effects of the H2/CH4 mole ratio on the reactions: (b: C2 yield, 0: yields of higher hydrocarbons, O: methane conversion, 2: C2H2 selectivity, 1: C2H4 selectivity). Conditions: total flow rate ) 300 mL/min; peak power (MW) ) 120 W; pressure ) 0.13 MPa; pulse duty factor ) 400/400 ms.
52%, which is the upper limit, while that of higher hydrocarbons and other polymerizates are the minimum. This could be explained by reactions 14, 15, and 18. H2 prevents the radical reactions from happening as its proportion in the feed gas increases, and the polymerization of C2H2 is also deteriorated with increasing hydrogen feed concentration. Figure 3b indicates that between the H2/CH4 mole ratio in the feed of 1/1 and 5/1, the selectivity of both acetylene and methane conversion reaches their maximum at H2/CH4 ) 2/1, while the selectivity of ethylene increases from 12 to 22%. This observation supports the reaction mechanism described in reaction 14, i.e., increasing the proportion of H2 in the feed could inhibit the reaction C2H4 f C2H2 + H2, which results in increasing selectivity of C2H4. The effect of H2/CH4 mole ratio in the feed on methane conversion is probably related to the electronic properties of pulsed MW plasmas, which are under investigation. Thus it can be concluded that it plays a very important role in the conversion of methane that H2 is activated by the pulsed MW plasma to form a large number of active H-radicals and electrons. 3.4. Effect of Pressure. At atmospheric and higher pressure, low temperature, nonequilibrium pulsed MW plasma was achieved and maintained using the device. This is the main characteristic of the device, and it is essential to achieve high efficiency and selective conversion of natural gas with MW plasmas at atmospheric and higher pressure. The change of pressure has a significant effect on the coupling reactions of methane.
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Figure 5. Effects of pulse duration on the reactions: (O: methane conversion, b: yield of C2, 0: yield of higher hydrocarbons). Conditions: total flow rate ) 300 mL/min; H2/ CH4 ) 2/1; peak power (MW) ) 120 W; pressure ) 0.13 MPa; pulse duty ) 1/1.
Figure 4. Effects of pressure on the reactions: (O: methane conversion, 2: C2H2 selectivity, 1: C2H4 selectivity, b: C2 yield, 0: higher hydrocarbons yields). Conditions: total flow rate ) 300 mL/min; H2/CH4 ) 2/1; peak power (MW) ) 120 W; pulse duty factor ) 400/400 ms.
The effect of pressure on methane conversion and selectivity of C2 is shown in Figure 4a. With increase of pressure from 0.12 to 0.15 MPa, the conversion of methane gradually decreases, and the selectivity of C2H2 decreases in a small degree. However, the selectivity of C2H4 increases significantly. When pressure increases, the plasma is “compressed” and the volume of plasma decreases; as a result, the quantity of feed gas flowing across the plasma zone is reduced. Meanwhile, the mean free path of electrons and gas molecules shrinks, the density of gas molecules increases, so the radical recombination “quenching” is enhanced, the energy of electrons is absorbed effectively by gas molecules and changed into heat energy, and as a result, the electrons cannot be accelerated enough by the electromagnetic field. Therefore, the dissociation of methane molecules is weakened, and methane conversion decreases. Further research is needed for an explanation about the fact that the selectivity of C2H4 increases with increasing pressure. Figure 4b indicates that the C2 yield (total yield of C2H2 and C2H4) does not change significantly when pressure increases, unlike that of high hydrocarbons and polymerizates which decreases significantly. From reactions 8 and 18, it can be seen that the carbon deposition and polymerization of acetylene will produce a large number of hydrogen. Increasing the pressure can inhibit these reactions moving to the undesirable direction, i.e., it inhibits the production of carbon and polymerizates. In addition, analyzing the deposits on
the inner walls of quartz tube placed at the end of the coaxial line and the oily products filtrated at the end of the reactor exit with GCQ GC/MS system (produced by Finnigan Corporation) showed that their main components are polymeric compounds of benzene ring and acetylene. Therefore, the dehydrogenation and polymerization of C2H2 are mainly responsible for C2 loss, and the “carbon deposition” in this paper basically refers to higher hydrocarbons and polymerizates. Thus, it can be concluded that proper reaction pressure could inhibit formation of higher hydrocarbons and polymerizates, and that changing the pressure could adjust the C2H2/C2H4 ratio in products. 3.5. Effects of Pulse Frequency. Introducing pulsed MW to enhance conventional plasmas increases the effective area and intensifies the activation of plasma; in addition, using pulse mode can effectively prevent the abrupt transition of microwave plasma from deficient density to excessive density. This is the key to sustain a low-temperature nonequilibrium MW plasma. The pulse duty factor was set at 1/1, other parameters were set constant in the experiments, and the effect of changing pulse duration (corresponding to microwave working time and intermitting time, respectively) on the reactions was investigated. The pulse duration was set as 100, 200, 300, 400, 500, and 600 ms. The effect of pulse frequency on methane conversion and C2 yield of reactions is shown in Figure 5. The methane conversion has a minimum value at 300 ms of pulse duration; the highest yield of C2 has been found at the pulse duration of 400 ms. At a constant microwave power density, the residence time of feed gas in the plasmas is an important parameter in methane conversion. The inner diameter of the quartz tube placed at the end of the coaxial line is 22 mm, as shown in Figure 1. When the flow rate of feed gas passing across the plasma is 300 mL/min, the linear gas velocity is 13 mm/s. Viewed from the side, the thickness of the umbrella-like plasma in the experiments is about 5 mm, so the residence time of feed gas in the plasma is about 300 ms. Considering the factor that abrupt volume expansion of feed gas in the pulsed MW plasma would result in a backflow toward the plasma zone, the time in which the unit mass of feed gas passing through the plasma zone is longer than 300 ms. With a pulse duty factor of 1/1, a long pulse duration
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Figure 6. Effects of flow rate on the reactions: (O: methane conversion, b: yield of C2, 0: yield of higher hydrocarbons). Conditions: H2/CH4 ) 2/1; peak power (MW) ) 120 W; pressure ) 0.13 MPa; pulse duty factor ) 400/400 ms.
(e.g., 400, 500, and 600 ms), makes it sufficient for the pulsed MW plasma adequately to the methane flowing across the plasma zone, which is the precondition of achieving high conversion of methane. When the pulse duration is the short (e.g., 100 and 200 ms), a unit mass of feed gas flowing across the plasma could experience 2-3 pulses of time of MW plasma although a single pulse duration is shorter, so the methane conversion could also reach a high value. However, the C2 yield could decrease because a part of the products could probably experience a repetitious pulsed plasma, which may result in the formation of higher hydrocarbons and polymerizates. It is also shown that the C2 yield decreases when the pulse duration is longer than 400 ms, which is due to that the residence time of products in plasma zone is prolonged and polymerization occurs. On the contrary, when the pulse duration is 300 ms, a unit mass of feed gas flowing across the plasma could be exposed to the MW plasma for only one pulse period, which results in that part of the feed gas has passed the plasma zone without being activated and the conversion of methane would decrease markedly. Therefore, for different flow rates of feed gas, it is important to select appropriate pulse durations to keep high conversion of methane. 3.6. Effect of Flow Rate on Reactions. The flow rate of feed gas directly affects the residence time in the plasma zone. On a constant microwave power, as shown in Figure 6, the methane conversion and higher hydrocarbons production decrease with increase of total flow rate from 180 mL/min to 420 mL/min. The highest conversion of methane (72.5%) was observed at the flow rate of 180 mL/min. However, the highest C2 yield (52.1%) was found at the flow rate of 300 mL/min. A low flow rate corresponds to a long residence time within a pulsed plasma zone, so high hydrocarbons production and polymerizate formation are easily induced. Therefore, at low flow rates the C2 yield is low, although the methane conversion is high. These results also suggest that the effect of flow rate on reactions is also dependent on the pulse frequency, i.e., at different flow rates, a proper corresponding pulse frequency is essential to achieving high yields of C2 hydrocarbons. 3.7. Effect of Microwave Power. The effect of MW power on reactions is shown in Figure 7. The conversion of methane increases gradually as the MW power increases from 100 to 180 W. When the power is higher than 120 W the yield of C2 begins to decrease, while
Zhang et al.
Figure 7. Effects of peak power of MW on the reactions: (O: methane conversion, b: yield of C2, 0: yield of higher hydrocarbons). Conditions: total flow rate ) 300 mL/min; H2/ CH4 ) 2/1; pressure ) 0.13 MPa; pulse duty factor ) 400/400 ms.
the production of higher hydrocarbons and polymerizates increase with the MW power. As the MW power increase, the intensity of the electromagnetic field in the coaxial line is improved, so the electron energy distribution is changed and the number of higher energy electron increases, which result in improving the dissociation proportion of methane molecules. Therefore, the conversion of methane increases. However, a further increase of MW power raises the possibility of the C-H bonds of methane molecules breaking down entirely. The probability that C2 in the products colliding with high energy electrons to form polymerizates also increases, which is bound to result in decreasing C2 yield. Therefore, it is very important to use appropriate MW power in order to achieve high yields of C2 hydrocarbons. 3.8. Power Consumption And Energy Efficiency. It has been described in Section 3.7 that the conversion of methane increases with increasing the MW power, but the power consumption and the energy efficiency are unknown. The power consumption is defined as the methane conversion per unit power. In our experimental data, the power of high voltage wire-like plasma is 5 W, and the peak power of microwave is 120 W. Considering that the pulse duty factor of MW power is 1/1, the average power of MW used for the reaction should be 60 W. So the total average power used for the conversion of methane is 65 W. At a flow rate of 100 mL/min methane and 59.2% of conversion, the converted molecules per second is
(100 × 59.2%) × 6.02 × 1023/(60 × 1000 × 22.4) ) 2.65 × 1019 So the methane conversion per unit power is
Power consumption ) 2.65 × 1019/65 ) 4.1 × 1017(molecule/joule) According to these fashions, the power consumption of the work reported by Suib et al.6 was 1.42 × 1017(molecule/joule); in the work of Shigeru Kado,9 it was 0.73 × 1017; and in the work of Zhu A. M.,15 it was 1.5 × 1017. Thus, the power consumption of this research is relatively low, but the coupling method of microwave (15) Zhu, A. M.; Gong, W. M.; Zhang, X. L.; Zhang, B. A. Science In China (series B) 2000, 30, 167.
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power into plasma remains to be improved, which is expected to reduce the power consumption more. The definition of energy efficiency is the ratio of the enthalpy for the reaction producing C2 hydrocarbons from methane and the actual power consumption. Regarding the methane molecule as thermodynamic initial state, a balanced chemical equation for the coupling reaction to reduce C2 hydrocarbons can be given below:
CH4 f 0.5n1C2H6 + 0.5n2C2H4 + 0.5n3C2H2 + (2 - 1.5n1 - n2 - 0.5n3)H2 (21) where n1 is the mole percent of C2H6 in C2 products, n2 is the mole percent of C2H4 in C2 products, and n3 is the mole percent of C2H2 in C2 products. Reaction 21 is endothermic, and the enthalpy (∆H) for this reaction is
∆H ) n1∆H1 + n2∆H2 + n3∆H3 Here, ∆H1 ) 32.55 kJ/mol, ∆H2 ) 101.15 kJ/mol, ∆H3 ) 188.25 kJ/mol.16 Therefore,
∆H ) 32.55n1 + 101.15n2 + 188.25n3
(22)
Thus,
Energy efficiency )
∆H‚YC2‚F P
× 100%
(23)
Compare the energy efficiency of this work with those of Steven L. Suib6 and Zhu A. M.15 In this work, n1 ) 0, n2 ) (52.1 - 42.7)/52.1 ) 0.18, n3 ) 0.82. So energy efficiency is 10.3%; in the literature,6 the power ) 60 W, flow rate ) 50 mL/min, the methane conversion is 38%, selectivity of C2H6 is 50%, selectivity of C2H4 is 25%, selectivity of C2H2 is 25%. According to equations 22 and 23, the energy efficiency is 2.1%; for the work of Zhu,15 the energy efficiency reported is 3.2%. Thus it can be seen, that the energy efficiency of these plasmas (16) Oumghar A.; Legrand J. C.; Diamy A. M. Plasma Chem. Plasma Process. 1994, 14, 229.
are still on the lower level. For utilizing this pulsed MW plasma effectively, further research to improve its energy efficiency is necessary. Conclusion The following conclusions can be drawn on the basis of the findings of this work: (1) By the pulsed microwave enhancing and sustaining approach, a noncontinuously distributed and less activated wire-like plasma which is excited by conventional high voltage discharge can be spread and enhanced to form an umbrella-like plasma with a continuous distribution in the space. This approach has not only expanded the effective area (or volume) of plasmas, but also greatly intensified the activation of plasmas. (2) The methane non-oxidative coupling production of C2 hydrocarbons (acetylene and ethylene) via pulsed MW plasma has been confirmed in this experimental study. Furthermore, there is hardly any ethane in the products. The yields of products and methane conversion are affected by the H2/CH4 mole ratio, pressure, pulse frequency, flow rate of feed gas, and MW peak power. (3) With a flow rate of 300 mL/min, H2/CH4 ratio of 2, pressure of 0.13 MPa, microwave peak power of 120 W, pulse duty factor of 400/400 ms, single pass conversion of methane and single pass yield of C2 hydrocarbons could reach up to 59.2% and 52.1%, respectively. In which the yield of acetylene is 42.7%. (4) By using conventional wire-like plasma enhanced and spread by pulsed MW, the power consumption for methane conversion is 4.1 × 1017(molecule/joule). Energy efficiency of the plasma reaches up to 10.3%, which needs further research to improve more. Acknowledgment. The financial support for this work was provided by the High-Tech Development & Industrialization Ministry of Science and Technology of P.R.C. through a key task project in scientific and technological research (“9th Five-Year Plan of P.R.C.” #96-544). EF010217U
