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Manuscript Number: FUPROC-D-07-00038R2 Title: Kinetic modeling of plasma methane conversion in a dielectric barrier discharge Article Type: Research Paper Section/Category: Keywords: Plasma, dielectric barrier discharge, methane, kinetic reaction Corresponding Author: Dr. Antonius Indarto, PhD Corresponding Author's Institution: Korea Insitute of Science and Technology First Author: Antonius Indarto, PhD Order of Authors: Antonius Indarto, PhD Manuscript Region of Origin:

Cover Letter

Plasma-Catalyst Chemical Process Laboratory Clean Technology Research Center Korea Institute of Science and Technology P.O. Box 131, Cheongryang, Seoul 130-650, Korea -----------------------------------------

February 10, 2007 Editor in Chiefs J. of Fuel Processing Technology

Dear Sir and Editor, Please find our manuscript of Kinetic modeling of plasma methane conversion in a dielectric barrier discharge by Antonius Indarto, Nowarat Congwanitwong, Jae-Wook Choi, Hwaung Lee, Hyung Keun Song. I would like to have this manuscript reviewed by the Journal of Fuel Processing Technology as a short communication. In this manuscript, we report our study on the kinetic of plasma methane conversion. Global kinetic model was applied by introducing eight different hydrocarbons to the plasma reactor. Analysis and calculation of the products distribution are presented in the manuscript. This simple model will be useful to understanding the global pathway of complex plasma reactions. I warmly welcomed your information as well as questions regarding to the submission process of this manuscript and those can be directed to my electronic mail at [email protected]. Finally, thank you very much for your consideration. Sincerely, Antonius Indarto Plasma-Catalyst Chemical Process Laboratory, Clean Technology Research Center, Korea Institute of Science & Technology, P.O. Box 131, Cheongryang, Seoul 130-650, Korea

Detailed Response to Reviewers

Kinetic modeling of plasma methane conversion in a dielectric barrier discharge Manuscript id: FUPROC-D-07-00038 Author response of the reviewer Question: The authors were very kind to address so many of my comments in my earlier review.

I believe the paper is much improved. However, one item remains that still must be

addressed:

the conversion column listed in Table 1 is incorrect.

The values listed as

conversion are all equal to the concentration in the products of each respective reactant, which is clearly not conversion. For example, in the first row of the table (with methane as the reactant), the conversion based on carbons in the products should be about 11%, not 86.2% as listed. Answer: Thank you very much for the correction. The reviewer is correct that the conversion values should not be similar to the reactant concentration in the products. Thank you for the correction and we put the correct value in the revision. Question: on my print copy, Table 2 appears to show "i" as the subscript in the heading of both the rows and the columns. If this is not just my printer, please make the column heading a "j", if that is correct. Answer: We put the different ‘i’ and ‘j’ in the revision. It looks similar but it is correct in the revision. Regarding the editor additional comment, I put a yellow highlight on the sentences or words as the correction of the previous manuscript. We tried the best to make the manuscript understandable and give a clear explanation for the readers. We hope this current format will be better than the previous one. Thank you very much.

* Manuscript

Kinetic modeling of plasma methane conversion in a dielectric barrier discharge

Antonius Indarto1,2,*, Nowarat Congwanitwong2, Jae-Wook Choi1, Hwaung Lee1, Hyung Keun Song1

1

Clean Technology Research Center, Korea Institute of Science and Technology, PO Box 131, Cheongryang, Seoul, Korea

2

School Environmental Resources Development, Asian Institute of Technology, PO Box 4, Klong Luang, Pathumthani, Thailand

Address: Clean Technology Research Center, Korea Institute of Science & Technology, P.O. Box 131, Cheongryang, Seoul 130-650, Korea Phone:

+82-10-2296-3748

Email: [email protected]

1

Abstract- Methane conversion by plasma offers a promising route to produce higher value-added products. As plasma reaction is relatively complex process, kinetic modeling is necessary to obtain a general pattern of the complex interaction on the basis of chemical reaction and products. In this paper, we present a method to obtain the kinetic rate coefficient (k) from the experimental data. Although plasma reaction was classified as chemically complex interaction, the reactions showed a certain pattern of the mechanism. In pure methane injection, the decomposition of methane by plasma could initiate coupling reactions and produce C2H6, C3H8, and C4H10. Dehydrogenation of C2H6 into C2H4 and then to C2H2 could be clearly seen by the higher value of the reaction rate constant of C2Hn+2 to C2Hn-2. Using the rate constant values (k) obtained by this method, the pathways of the methane conversion by a dielectric barrier discharge can be drawn.

Keywords: Plasma, dielectric barrier discharge, methane, kinetic reaction

1. Introduction

The use of natural gas as a feedstock in the chemical and pharmaceutical industry is an alternative to crude oil whose supplies might run out in the next century. Methane (CH4), the major component of natural gas, is widely distributed at sites around the world and have a potential to be used as the precursor to produce higher value-added products, for example acetylene and ethylene. There are mainly three approaches to converting CH4 into higher hydrocarbons in a high temperature process: the direct oxidative methane conversion (OMC), the Fischer–Tropsch process via syngas, and the non-oxidative methane conversion [1]. Over the last decades, many researches have mainly focused on oxygen-containing 2

processes, either indirect ones as in the Fischer–Tropsch process or direct ones as in the oxidative methane conversion. In the Fischer–Tropsch processes, CH4 is converted into hydrocarbons, via synthesis gas, which is subsequently hydrogenated, e.g., to methanol [2]. In the presence of oxygen, the direct CH4 conversion to C2H6, C2H4 (one of the most important raw materials in industrial production cycles), C3Hn, C4Hn is thermodynamically feasible (exothermic), whereas the oxygen-free or dehydrogenative conversion is endothermic (two-step polymerization). OMC has been investigated extensively [3,4] and is still most frequently used due to its higher methane conversion although suffering from low selectivity of C2+ [5]. The oxygen-free methane conversion, however, has only been studied by a few groups [6–9]. The state of the art for this process was recently summarized by Guczi et al. [10]. A major disadvantage of the OCM is its low selectivity towards C2+, because considerable amounts of CH4 are consumed forming CO and CO2. In contrast to OCM, the non-oxidative methane conversion (NOMC) by thermal process gives numerous amounts of solid carbon and hydrogen [11] with less C2+ production. Higher hydrocarbon production via methyl coupling reactions occurred only with very short time and high temperature reaction [12]. Non-oxidative plasma methane conversion (NOPMC) offers a high selectivity towards C2+ products [11,13]. The reason for focusing on oxygen-free methane conversion is therefore to maintain the high C2+ selectivity, while simultaneously enhancing the CH4 conversion and the C2+ yield, respectively. The optimization of the NOPCM necessitates a detailed knowledge of the gas-phase reactions that occur and their kinetics. In this present work, we develop a kinetic model for plasma methane conversion to higher hydrocarbons. By finding the kinetic rate constant (k), the global pathway of the reactions could be drawn easily. The model includes the gas-phase transformation 3

of molecules that exist in the reactant and the products. Based on the comparison between experimental and simulation data, it was found that the proposed kinetics well accounted for the reaction products of C2H2, C2H4, C2H6, C3H6, C3H8, i-C4H10, and n-C4H10. The constructed model was applicable for the plasma reaction using dielectric barrier discharge at atmospheric pressure (c.a. 1 atm) and ambient temperature (c.a. 25oC). The presented kinetic model is a part of efforts encompassing the oxygen-free plasma chemistry for methane conversion.

2. Experimental setup and algorithm Figure 1

The schematic diagram of the experimental setup is shown in Figure 1. Details of each part of the system are described in the following sections.

2.1. Plasma reactor and power system The reactor used in the experiments is a cylindrical pyrex tube (ID of 7.5 mm) with 2 parallel-straight wires (0.2 mm diameter, stainless steel) as the inner metal electrode and silver film coated at the outer side of tube as the outer electrode. The effective volume and length of the reactor were 8.8 ml and 200 mm, respectively. A high frequency alternating current (AC) power supply with maximum voltage of 20kV was connected to the electrodes. The supplied power to the reactor was maintained constant at 60W, measured by a watt meter (Metex model M-3860M). In order to maintain the similarity of the reactor configuration, e.g. electrodes gap distance, the reactor capacitance was checked by an RCL meter (Fluke model PM6304) before and after experiments. The reactor capacitance was in the range of 9.0-10.0 pF at ambient air condition. 4

2.2. Input gas and measurement system All experiments were carried out by introducing eight different hydrocarbons: CH4, C2H2, C2H4, C2H6, C3H6, C3H8, i-C4H10, n-C4H10. CH4 has purity of 99.97% while other gases have purity of above 99.9%. Those gases were provided by Kogas Corp. (Korea Gas Corp.). A Gas Chromatography (YoungLin M600D, column: 30ft of Hayesep D) has been used to quantitatively analyze the amount of products by flame ionization detection (FID) and thermal conductivity detector (TCD). The FID system was capable to detect compounds in hundreds ppm level. Considering the volume expansion and the compression of products, the output line was connected to a bubble flow meter to measure the flow difference of the products before and after reaction. The products concentration and reactants conversion were formulated as:

Conversion of C x H y =

moles of C x H y converted

 100

(1)

moles of C m H n produced  100 total moles products

(2a)

m  moles of C m H n produced  100 moles of C x H y converted

(2b)

Concentration of C m H n =

Selectivity of C m H n =

moles of initial C x H y

2.3. Model algorithm In the kinetic model, the possibility of molecular transformation to any possible compounds that exist in the products or the reactants was calculated. The reaction rate (dC/dt) of single hydrocarbon was measured experimentally by injecting pure compound to the reactor. Then, the products distribution was calculated and the rate of molecular transformation converted to other molecules could be obtained. From the experimental analysis, eights hydrocarbons existed in the products and those were (1) CH4, (2) C2H2, (3) C2H4, (4) C2H6, (5) C3H6, (6) C3H8, (7) i-C4H10, and (8) n5

C4H10. As we assume that the reaction will occur in 1st order reaction, the calculation of each compound follows: n n dCi =  k ij Ci +  k ji C j dt j=1 j=1

(3)

where n is the maximum value of i and j for k-set which is 8 and kij denotes the reaction rate constant of i molecule converts into specific j molecule. The assumption was chose to simplify the calculation and make the data comparable each other. The value of kij will be 0 if i=j.  dC 1  dt      dC 8  dt

    k 11  k 18  C 1  C 8  + k 11                     k 81  k 88  C 1  C 8  k 18   

 k 81  C 1        k 88  C 8

 C1        C 8 

(4)

By solving Eq. 4, the calculation will result a set of kinetic rate constant (k) which can be arranged as: k 12    k 21 0      k 81  k 87

k= 0

(5)

k 18   k 28      0 

The value of k was obtained by an error minimization between model data and experimental data. In this particular case, the minimization calculation was done employing Matlab sub-routine module ‘fminsearch’ after simultaneous integration of Eq. 4 using ‘ode23s’. The uniqueness of the result was measured by calculating the mean-squared error of each set value of kij obtained from eight different input gases which is expressed as:

s=

 k  k 

2

i

(6)

n 1

where n is the maximum size of k-set which is eight, k is the mean value of k-set.

6

3. Results and discussion Table 1

In order to obtain the rate constant k in Eq. 5, eight hydrocarbon gases: CH4, C2H2, C2H6, C3H6, C3H8, iC4H10, and nC4H10 were introduced separately to the plasma reactor at atmospheric pressure (~1 atm) and room temperature (~25oC). The flow rate and supplied power were maintained similar at 30 ml/min and 60 Watt, respectively. These values were obtained from the optimization of the previous research [14] in order to avoid large amount of unknown products and carbon deposition which were difficult to be measured. Fragmentation reactions of methane into smaller molecules were more favorable than synthesis reactions of higher hydrocarbons at lower input flow rates and higher supplied power [11,15,16]. As higher hydrocarbons are more useful and valuable products, increasing flow rates and lowering supplied power can be the way to avoid above problems. Table 1 shows the conversion rate of input gas and the distribution of products after treated by dielectric barrier discharge at flow rate of 30 ml/min and supplied power of 60 W. Except for the experiments initiated by C2H2 (run 2) and C3H6 (run 5), the total C and H atom balance between the reactants and products of all plasma process approximately closed to 100% which means that the molecular transformation of reactants to the products were almost perfectly identified. Small amounts of carbon deposition on the wall and inner electrode were found but, as the values were relatively small, those could be neglected in the calculation. It was a good achievement, since to obtain correct calculation of the kinetics, the C and H balance between reactants and products should be equal. In the case when C2H2 and C3H6 were used as the reactant, the reaction produced a lot of solid carbon and/or soot. We could not confirm whether the solid product was coming from the polymerization 7

of C2H2 and C3H6 or agglomeration of monomolecular carbon. However, C2H2 and C3H6 are well-known as soot precursor in the pyrolisis or combustion process [17]. In this case, the similar phenomenon is likely occurred in the plasma process. From table 1, it can be figured out that when methane was the reactant, the products were dominated by ethane (C2H6), with selectivity of 37.5%, propane (C3H8), 25.6%, and butane (normal- and iso-C4H10), 25.4%. This product composition has a similarity with Thanyachotpaiboon et al.[18] result, used DBD system and AC power supply similar to ours, with slightly higher selectivity of propane and butane. Higher value of the selectivity was due to the method different in selectivity calculation. Thanyachotpaiboon et al. was using fractional method instead of carbon balancebased calculation that we used. Higher ethane selectivity could be obtained by coupling reaction of methyl radical which also suggested by Zhao et al.[19]. Abundant amounts of methyl radical could combine with C2 molecules to form C3 hydrocarbons or with C3 molecules to form C4 hydrocarbons [20]. In the case of other products, e.g. acetylene and ethylene, those compounds could be produced due to stepwise dehydrogenation reaction of ethane. The tendency of acetylene formation from ethylene and ethylene from ethane by dehydrogenation reactions was clearly seen from run-4 of Table 1. When ethane was the reactant, the selectivity for ethylene was 29% which is two times higher compared when acetylene was used as the reactant. A similar phenomenon also occurred in the case of acetylene formation. The selectivity of acetylene production reached 27.7% when ethylene was the reactant and only 3.2% of the selectivity when methane was the reactant. An interesting phenomenon occurred when acetylene was used as the reactant which could be an important part to describe the kinetic pathways. Acetylene was converted mostly into methane, 40%, and at the same time, it also produced ethylene (39%), and 8

n-butane (15%). Those products required hydrogen to react with acetylene. C2H2 + 3 H2  2 CH4

(7)

C2H2 + H2  C2H4

(8)

2 C2H2 + 3 H2  i-C4H10

(9)

2 C2H2 + 3 H2  n-C4H10

(10)

In order to supply H2, fragmentation of C2H2 as the intermediate reaction was needed. C 2 H2  2 C + H 2

(11)

As the total C atom of gaseous products was only 87% compared to the total C atom of reactant, reaction 11 high possibly occurred in plasma zone and responsible for the formation of molecular solid carbon formation. The proposed reaction 11 was also supported by hydrogen atom ratio of the products to the reactant which closed to 1 means there was no losing hydrogen atom in the process. Coupling reactions between two C2 compounds could form butane and the long chain type (n-C4H10) was much favorable than the iso- type (i-C4H10). It was simply because the adduct reaction of two C2 moieties occurred without any molecular steric hindrance although later on, it converted to iso- form as iso- form is thermodynamically more stable than normal- type. C2H4 + C2Hx + (8-x)/2 H2  n-C4H10

(12)

In the case of C3H6 consumption process, the reactions occurred by two main parallel ways: C3H6 + H2  C3H8

(13)

C3H6  C2H4 + C + H2

(14)

From these two reactions, we would obtain less amount of total C atom number in the gaseous products due to carbon solid formation of Eq. 14. It was confirmed by experimental result that the total C atom in the gaseous products was only 89% compared to the original reactant. Instead of above conversion reactions, C3H6 could 9

be formed by dehydrogenation of C3H8 (reverse reaction of Eq.13) or fragmentation of i-C4H10. This idea was supported by results of pure C3H8 and i-C4H10 injection. When C3H8 and i-C4H10 were the reactant, the selectivity of C3H6 reached 44% and 17%, respectively. The proposed reactions mechanism follows: C3H8  C3H6 + H2

(15)

i-C4H10  C3H6 + CH4

(16)

i-C4H10  C3H6 + CH2 + H2

(17)

C3H6 produced from Eq. 16 could be suffered from reverse reaction of n-C4H10 formation C3H6 + CH4  n-C4H10

(18)

In this case, the probability of Eq. 17 is higher than Eq. 16 as high concentration of H2 was found in the product stream. Moreover, coupling reaction of ‫׃‬CH2 diradical could form C2H4, as its selectivity reaches 14%, follows [21]: CH2 + CH2  C2H4

(19)

Taken from all above explanation, the reaction pathways of methane conversion in a dielectric-barrier discharge at flow rate of 30 ml/min and supplied power of 60W are shown in Figure 2. Figure 2

4. Kinetic model Table 2

In order to quantify the rate of the reaction, a kinetic model was built according to the algorithm (section 2.3). Some assumptions were used to reduce the complexity of the reactions: (i) all the reactions followed the first order reaction; (ii) the formation of coke and hydrogen was negligible in the model. All calculations were performed using Matlab by exploiting ‘ode23s’ to solve a set of differential equations in Eq. 4. The reaction rate constants, k (shown in Eq. 5) were obtained by least-square 10

calculation of the calculated values and the experimental results using ‘fminsearch’ module. The estimated reaction rate constants (k) are listed in Table 2. Higher k value means the reactant transformation to certain product is easier. From table 2, the value of the k-set supports our previous statements that the decomposition of methane will produce C2H6 (k14), C3H8 (k16), and C4 (k17 and k18). Low value of k13 (CH4C2H4) and k12 (CH4C2H2) shows that acetylene and ethylene was not coming from the direct methane conversion. Losing two H’s and coupling with another triplet ‫׃‬CH2 to form ethylene or losing three Hs and coupling with duplet ·CH required many steps. There was also a possibility to be attacked by other species before completing the radical coupling reaction. Acetylene and ethylene were formed by stepwise dehydrogenations of ethane. The dehydrogenation reactions of C2H6, occurred in the plasma reaction, could be detected by k43 (C2H6C2H4), 1.03 min-1, then k32 (C2H4C2H2), 1.79 min-1. The trend of the result was similar to Jeong et al. [22] which also suggested by Kozlov et al. [23] that pathways of C2 dehydrogenation reactions will follow: CH 4

 C2 H 6

 C2 H 4

 C2 H 2

The above stepwise dehydrogenation reaction was also identified in the thermal coupling of methane [24]. Interestingly, n-butane hydrocarbon could be formed from any hydrocarbons species, confirmed by higher value of kx8, >0.5 min-1, except for methane. This result could answer the high yield production of C4 compounds that usually produced in DBD using AC power supply. This result confirmed the previous research that the addition of higher hydrocarbons, e.g. ethane, on plasma methane conversion would increase the selectivity of butane [18]. Later on, intermolecular transformation of n-butane to ibutane occured as i-butane is thermodynamically more stable than n-butane, showed by k87 (1.38 min-1). In a parallel way, homolytic dissociation of n-butane to C2 11

hydrocarbons (acetylene and ethylene) confirms by k82 (1.07 min-1) and k83 (2.27 min1

) or into propene by k85 (1.05 min-1). Usually, decomposition of higher hydrocarbon

to lower hydrocarbons results higher reaction rate constant k (around 0.5-2 min-1) compared to the formation of higher hydrocarbons from the lower ones. It was acceptable since the dissociation reaction, e.g. decomposition of one butane molecule, will produce double C2 molecules or more if it decomposed to C1 molecules. Instead of coming from the intermolecular transformation of n-butane, the formation of i-butane could be formed from C3 compounds, k58 = 0.54 min-1 and k68 = 0.7 min-1. Although i-butane will be consumed in further reaction mostly to n-butane, by k78 = 2.54 min-1, we have to consider the conversion comparison between n-butane and ibutane. The conversion rate of i-butane is 10% lower than that of n-butane means that i-butane is more stable than n-butane. The reverse reactions to produce methane was mostly coming from acetylene, confirmed by k21 (1.31 min-1) which is higher than other reactants. From this data, we are also be able to figure out that the decomposition of higher hydrocarbons will not produce CH4 as the product and those can be ·CH3, ‫׃‬CH2, or ·CH. The existence of C1 radical hydrocarbons could help to increase the selectivity of C2, C3, and C4 compounds by coupling reactions. Unfortunately, as this method is just counting the tendency of the pathway reaction, the present of intermediate species could not be confirmed scientifically. Following the k values combination, it shows that C2H4 has an important role on the pathway traffic of methane conversion (Fig. 2). C2H4 has a function as a branching agent to diverse the reaction mechanism. It was acceptable idea because C2H4 was easier to be produced than other C2 compounds from the cracking of higher hydrocarbons (C3 and C4). To form C2H6, the reactions should be terminated by recombination reaction with H2. On the other hand, C2H2 formation required further 12

dehydrogenation reaction to remove two hydrogen atoms. Instead of dehydration reaction into C2H2, coupling reaction of C2H4 could form n-C4H10 (Eq. 12) or react with CH2 radical to form C3H8. In order to check the correctness of the model, a binary mixture of methane and acetylene was injected to the reactor at the total flow rate of 30 ml/min and supplied power of 60W. As shown in Figure 3, the model calculation was found to be in good agreement with the experimental data. It also means that the proposed values of reaction rate constant, k, as well as the model can rationalize the plasma mechanism of methane and/or hydrocarbon conversion.

Conclusions

The kinetics of methane conversion in a dielectric barrier discharge at supplied power of 60W was studied. It shows that C2H4 has an important role on the global mechanism, having function as a branching molecule which able to diverse the global reaction

mechanism.

Similar

to

thermal

methane

decomposition

reaction,

dehydrogenation of C2H6 was also occurred and producing C2H4 then continued by formation of C2H2. Coupling reaction of C2H4 with other C2 hydrocarbons would result butane which usually has long-chain molecule (normal- form). Using this method which is relatively simple, the pathway pattern of CH4 conversion could be obtained. It will be a good advantage to predict or optimize the end products of direct methane conversion by a dielectric barrier discharge. Further model improvement is still necessary to be able to conduct the calculation in various conditions, e.g. different flow rates, which did not cover in this research.

Acknowledgments 13

The authors would like to thank the Korea Institute of Science and Technology (KIST) and the Korea University (KU) for the study supports. The first author would like to express his appreciation to the Università Degli Studi di Torino for the support during study period in Torino, Italy.

References [1]

Gradassi, M.J., Green, N.W., 1995. Economics of natural gas conversion processes. Fuel Process. Technol. 42, 65-83.

[2]

Bell, A.T., 1984. Mechanisms of Fischer-Tropsch synthesis, in: NTiS report number: DE84014608.

[3]

Zanthoff, H., Baerns, M., 1990. Oxidative coupling of methane in the gas phase. Kinetic simulation and experimental verification. Ind. Eng. Chem. Res. 29, 2-10.

[4]

Baerns, M., van der Wiele, K., Ross, J.R.H., 1989. Catal. Today 4, 471.

[5]

Labinger, J.A., 1993. Quimica 48.

[6]

Belgued, M., Amariglio, H., Pareja, P., Amariglio, A., Saint-Just, J., 1992. Catal. Today 13, 437.

[7]

Koerts, T., van Santen, R.A., 1993, New frontiers in catalysis, in: Guczi, L., et al. (Eds.), Proc. 10th Int. Congress on Catalysis. Elsevier, Amsterdam, p. 1065.

[8]

Belgued, M., Amariglio, A., Pareja, P., Amariglio, H., 1996. J. Catal. 159, 441.

[9]

Solymosi, F., Erdöhelyi, A., Cserenyi, J., 1992. Catal. Lett. 16, 399.

[10] Guczi, L., van Santen, R.A., Sarma, K.V., 1996. Catal. Rev. Sci. Eng. 38, 249. [11] Indarto, A., Choi, J.W., Lee, H., Song, H.K., 2006. Methane conversion using dielectric barrier discharge: comparison with thermal process and catalyst effects. J. Natur. Gas Chem. 15, 87-92. [12] Holmer, A., Olsvik, O., Rokstad, O.A., 1995. Fuel Process. Tech. 42, 249 [13] Yang, Y., 2003. Direct non-oxidative methane conversion by non-thermal plasma: 14

experimental study. Plasma Chem. Plasma Process. 23. [14] Kim, S.S., Lee, H., Na, B.K., Song, H.K., 2003. Reaction pathways of methane conversion in dielectric-barrier discharge. Korean J. Chem. Eng. 20, 869-872. [15] Indarto, A., Choi, J.W., Lee, H., Song, H.K., 2005. Kinetic modeling of plasma methane conversion using gliding arc plasma. J. Natur. Gas Chem. 14, 13-21. [16] Indarto, A., Choi, J.W., Lee, H., Song, H.K., 2006. Effect of additive gases on methane conversion using gliding arc discharge. Energy 31, 2650-2659. [17] Frenklach, M., 2002. Reaction mechanism of soot formation in flames. Phys. Chem. Chem. Phys. 4, 2028-2037. [18] Thanyachotpaiboon, K., Chavadej, S., Caldwell, T.A., Lobban, L.L., Mallinson, R.G., 1998. Conversion of methane to higher hydrocarbons in AC nonequilibrium plasmas. AIChE J. 44, 2252-2257. [19] Zhao, G.-B., John, S., Zhang, J.-J., Wang, L., Muknahallipatna, S., Hamannb, J.C., Ackerman, J.F., Argyle, M.D., Plumb, O.A., 2006. Methane conversion in pulsed corona discharge reactors. Chem. Eng. J. 125, 67-79. [20] Kraus, M., Egli, W., Haffner, K., Eliasson, B., Kogelschatzb, U., Wokaun, A., 2002. Investigation of mechanistic aspects of the catalytic CO2 reforming of methane in a dielectric-barrier discharge using optical emission spectroscopy and kinetic modeling. Phys. Chem. Chem. Phys. 4, 668–675. [21] Kim, H.S., Oh, S.J., 1998. A study of the synthesis of C2-hydrocarbons using methane activation. J. Ind. Eng. Chem. 4, 12-18. [22] Jeong, H.K., Kim, S.C., Han, C., Lee, H., Song, H.K., Na, B.K., 2001. Conversion of methane to higher hydrocarbons in pulsed DC barrier discharge at atmospheric pressure. Korean J. Chem. Eng. 18, 196-201. [23] Kozlov, K.V., Michel, K., Wagner, H. E., 2000. Synthesis of organic compounds from mixtures of methane with carbon dioxide in dielectric-barrier discharges at atmospheric 15

pressure. Plasma Polymer. 5, 129-150. [24] Choudhary, T.V., Aksoylu, E., Goodman, D.W., 2003. Nonoxidative activation of methane. Catal. Rev. 45, 151-203.

16

Ga

AC power supply

MF

Bubble flow meter

Plasma Reactor

G

Figure 1. Schematic diagram of experimental set up.

17

i-C4H10 CH4

C2H6

C2H4

C3H8 C2H2

Figure 2. The pathway reaction of methane conversion.

18

C3H6 n-C4H10

50

Experimental Model

Selectivity (%)

40 30 20 10 0 H4 C2

H6 C2

C3

H6

H8 C3

4H i- C

10

n- C

1 4H

0

Compounds

Figure 3. The comparison between experimental and model simulation results of binary mixture methane + acetylene at the flow rate of 30 ml/min and supplied power of 60 W.

19

A

Table 1. The reactant conversion and products distribution of plasma process Reactan

Conversion [ %]

CH4

C2 H2

C2H4

Products concentration [ %] C2 H6 C3 H6

C3 H8

iC4H10

Atom balance [ 1] Cout/ Cin Hout/ Hin 0.95 1.01

CH4

13.8

86.20

0.17

0.45

2.04

0.00

0. 93

0. 26

0. 43

C2 H2

12.0

3.03

81.95

1.45

0.00

0.14

0. 00

0. 00

0. 28

0.87

1.13

C2 H4

15.7

2.67

3.31

84.30

0.68

0.38

0. 67

0. 67

1. 84

0.95

0.98

C2 H6

9.4

0.96

1.27

2.10

90.57

0.16

0. 89

0. 05

0. 85

0.97

0.98

C3 H6

9.8

0.70

1.60

2.48

0.46

80.16

2. 23

0. 99

0. 57

0.89

1.01

C3 H8

11.7

2.31

0.47

2.03

0.95

2.54

88.27

0. 09

0. 00

0.97

0.96

iC4H10

16.7

2.36

2.35

4.02

1.65

3.19

1. 39

83. 33

5. 94

1.04

1.00

nC4H10

8.4

1.27

1.08

2.29

1.28

1.36

1. 46

2. 33

91. 63

1.03

0.98

Note: [1]The calculation of H balance includes the H2 in the output and the selectivity of component follows: selectivity of C m H n  were obtained at flow rate of 30 ml/min and supplied power of 60 Watt.

E

nC4H10

n moles of C m H n produced  . All data 4 moles of CH 4 converted

A

Table 2. reaction rate coefficient (k)

kij

i

(1) CH4 (2) C2H2 (3) C2H4 (4) C2H6 (5) C3H6 (6) C3H8 (7) i- C4H10 (8) n- C4H10

(1) CH4

(2) C2H2

(3) C2H4

j

(4) C2H6

(5) C3H6

(6) C3H8

0

( 0. 0

)

0.06

( 5. 3

)

0.18

( 11. 6 )

0. 5

( 6. 5

)

0. 02

( 7. 5

)

0. 3

1. 31

( 8. 1

)

0

( 0. 0

)

0.17

( 12. 0 )

0.18

( 8. 4

)

0. 31

( 38.6 )

0. 08

( 8. 5

)

1.79

( 8. 6

)

0

( 0. 0

)

0.01

( 18.9 )

0. 22

( 6. 2

0. 11

( 17. 3 )

0. 3

( 6. 1

)

1.03

( 9. 2

)

0

( 0. 0

)

0. 07

0. 09

( 21. 8 )

0.23

( 35. 1 )

0.33

( 7. 8

)

0.14

( 7. 0

)

0

0. 06

( 13. 5 )

0.97

( 11. 3 )

0.16

( 28. 2 )

0.49

( 8. 1

)

0. 48

0. 06

( 20. 6 )

0.67

( 36. 2 )

1.28

( 7. 7

)

0.89

( 21.7 )

1. 28

( 8. 2

0. 74

( 7. 2

1.07

( 21. 8 )

2.27

( 6. 4

)

0.39

( 8. 0

1. 05

)

)

( 9. 8

(7) i- C4H10

(8) n- C4H10

)

0. 13

( 10. 7 )

0. 29

( 11. 1 )

0. 06

( 20. 7 )

0. 14

( 16. 1 )

1. 47

( 9.1

)

)

0. 02

( 15. 6 )

0. 25

( 9.1

)

1. 01

( 9.9

)

( 18.0 )

0. 67

( 21. 7 )

0. 15

( 4.7

)

0. 94

( 13. 7 )

( 0. 0

)

0. 53

( 10. 9 )

0. 54

( 8.5

)

0. 53

( 7.1

( 10.8 )

0

( 0. 0

)

0.7

( 7.9

)

0. 97

( 13. 3 )

)

1. 45

( 8. 3

)

0

( 0.0

)

2. 45

( 8.9

)

( 18.3 )

0. 33

( 23. 8 )

1. 38

( 7.7

)

0

( 0.0

)

)

Note: The value of kij refers to the value of reaction rate coefficient of molecule i to molecule j. All k values are in mol min-1; the standard deviation values inside the parenthesis are in % unit.

ABIC