ACETYLENE FROM METHANE BY GLIDING ARC PROCESSING

Transcription

1 ACETYLENE FROM METHANE BY GLIDING ARC PROCESSING Krzysztof Krawczyk, Joanna Ruszniak, Jan Sentek, and Krzysztof 6FKPLGW6]DáRZVNL a Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, Warszawa, POLAND Abstract. Basing on thermodynamic data, the equilibrium composition of products formed from the mixtures CH 4 +H 2 was examined within a wide temperature range at pressure of 1 and 10 bar. The limits of solid carbon stability were indicated and temperature of the maximum acetylene concentration was found. In the experiments with methane processing by gliding arc discharges, the influence of CH 4 /H 2 ratio on methane conversion and selectivity of the acetylene production was investigated. 1. INTRODUCTION Among a number of studies on methane processing under plasma conditions, promising results were obtained using the gliding arc (Glid-Arc) discharges at atmospheric pressure [1-4]. Czernichowski [1] studied the methane Glid-Arc assisted pyrolysis at atmospheric pressure in a flat metal-and-glass reactor of capacity 0.11 l and in a tubular steel reactor of capacity 1.35 l. The gas flow rate was up to 2.3 m 3 /h STP, and temperature was kept up to 500 C. The methane conversion reached 34%, acetylene and hydrogen being the primary products. The formation of some carbon was also observed, but the reaction selectivity into acetylene was generally high: 70 to 90 %. The Glid-Arc reactor was successfully used [2,3] for the conversion of NG into syngas in studies of methane processing in mixtures with steam, carbon dioxide, and oxygen at moderate temperature about C. Interesting results were also obtained investigating the partial oxidation of methane with oxygen enriched air (containing 45% O 2 ). A complete conversion of methane into hydrogen and carbon monoxide (H 2 /CO = 2) has been attained at rather low energy cost. Conversion of methane in Glid-Arc was also investigated by Czech [5]. To sum up, it may be concluded that up till now the most favourable conditions for methane conversion were obtained using non-equilibrium plasma generated in Glid-Arc discharges. Under conditions of non-equilibrium plasma, as that being generated in gliding-arc discharges, stable methane molecules may be activated at relatively low temperatures owing to the high energy of electrons. The kinetics of the reactions thus initiated, as well as the generated products, depend mostly on the discharge conditions, because the E/n value (electric field strength E divided by the total gas density n) determines the average energy of electrons. However, it should be taken into account that the reactions initiated by the electron impact may be accompanied by many other transformations which can lead the reacting gas mixture toward the thermodynamic equilibrium state. Generally, the equilibrium state is not attained under the discharge conditions, but nevertheless the latter group of reactions may affect the final product yields, particularly in cases where the residence time of gas reagents in the reaction zone is sufficiently long. When converting methane by pyrolysis, and also in electrical discharges, carbon soot formation is often observed. As it is an undesirable reaction, if the process is aimed to produce hydrocarbons or other organic compounds, this problem should be taken into consideration. a Electronic address: kss@ch.pw.edu.pl

2 2. EQUILIBRIUM IN THE CH 4 +H 2 MIXTURES Basing on the available thermodynamic data, methane conversion values in the equilibrium state, involving main reaction products, have been calculated. Use was made of a computer program Astra developed by Russian Centre of Physicochemical Modelling CVD. The equilibrium state for the initial gas mixtures CH 4 +H 2 of different molar ratio of C/H (from 1:4 - like in methane, up to 1:32) were examined within the temperature range K at pressures 1 and 10 bar. It was found that methane, solid carbon and acetylene are the only components in the equilibrium state within the wide temperature range of up to 3000 K, with negligible amounts of other products (Fig. 1 and 2). At the pressure of 1 bar carbon forms a solid phase thermodynamically stable in temperature range K for the molar ratio C/H = 1:4 (pure methane) and for C/H = 1:32 in temperature range K. It is hence evident that the initial contribution of hydrogen in the reaction mixture has a substantial influence on the equilibrium composition. The presence of hydrogen has also a strong bearing on the maximum equilibrium methane conversion to acetylene (Y A ) being calculated from the formula: 2n[C2H 2] YA n [CH ] (1) 0 4 where: n[c 2 H 2 ] moles C 2 H 2 generated n 0 [CH 4 ] moles CH 4 introduced. The maximum value of Y A is 53.2% for molar ratio of C/H = 1:4 (at 3400 K) and 95.6% for C/H=1:32 (at 2900 K). Hence, in the mixtures CH 4 +H 2, the equilibrium methane conversion to acetylene Y A can reach large values, but only in a narrow range of process parameters (temperature and hydrogen concentration). The solid carbon, being the main product of the methane conversion, occurs in a much wider range of parameters. However, it should be noted that the temperature range of solid carbon occurrence is evidently diminished by the increase of pressure to 10 bar. For example, at C/H = 1:32, carbon is stable between 1300 K and 2900 K. On the other hand, a pressure increase extends the range of temperatures at which considerable values of Y A are attained (up to maximum 97% at 2900 K). Hence, it may be concluded that: 1. In the equilibrium state of products formed from the mixtures CH 4 +H 2, methane, acetylene, and solid carbon are the main components in temperatures 3000 K. 2. In such mixtures solid carbon is stable within a wide temperature range 3000 K. 3. The equilibrium methane conversion to acetylene (Y A ) attains large values (>90%) at temperatures K.

3 VROLG VROLG 1.a. 1.b. FIGURE 1. The equilibrium state for the initial gas mixtures CH 4 +H 2 of molar ratio of C:H=1:4 within the temperature range K at different pressures: a 1bar, b 10 bar. Y methane conversion into C 2 H 2, C 2 H 4, C 2 H 6, C 2 H, C 3, solid C and other products (and contribution of remaining CH 4 ), (Y values were calculated in the same way as Y A ). VROLG VROLG 2.a. 2.b. FIGURE 2. The equilibrium state for the initial gas mixtures CH 4 +H 2 of molar ratio of C:H=1:32 within the temperature range K at different pressures: a 1bar, b 10 bar.

4 3. EXPERIMENTAL The experiments were carried out in a reactor (Fig. 3) made of quartz glass tube 4 of diameter 40 mm and length 260 mm, comprising two knife-shaped stainless-steel electrodes 2 [6]. The row gas mixture was introduced to it through a nozzle 1 of diameter 0,75 mm disposed at the axis of the tube. The process was carried out under atmospheric pressure. The temperature in the reactor (Table 1) was measured by means of thermocouple 5 located 4 cm under the electrodes. Electric current of frequency 50 Hz was supplied to the reactor by means of a system consisting of autotransformer, resistors, and high voltage transformer. The voltage, current, and power were measured in the primary, low voltage circuit of the transformer. The gases of commercial grade: methane, hydrogen, and argon were applied with the overall flow rate 300 Nl/h. The reaction products were analysed by means of 2 gas chromatographs: 1) Chrompack CP 9002 supplied with a column Poraplot Q and detector FID, and 2) a Willy Giede chromatograph supplied with a column Carboxen 1000 and detector TCD. HV FIGURE 3. Scheme of Glid-Arc reactor: HV-high voltage, 1-nozzle, 2-electrodes, 3-discharge zone, 4- glass tube, 5-termocouple. 4. RESULTS Acetylene was the main reaction product in the experiment performed, although small amounts of ethylene were also found. Carbon deposits were observed in reaction products of CH 4 +H 2 mixtures with initial methane concentration exceeding 20% by vol. At constant current value (in the low voltage circuit) the discharge power was scarcely influenced by the initial CH 4 concentrations. The methane conversion to C 2 hydrocarbons (Y C2 ) and the total methane conversion (Y M ) depended on the discharge power and on initial methane concentration (C o ) in the initial mixture CH 4 +H 2. A maximum value of Y C2 ( ) was observed at C 0 20%, whereas Y M was still increasing within the higher range of C o, because of the higher methane conversion into solid carbon. TABLE 1 Initial methane conc. / % 2,11 2,67 4,04 4,31 8,76 13,46 14,51 25,92 36,21 10A, Temp. / C A, Temp. / C

5 3RZHU: $ $ 0HWKDQHFRQYHUVLRQ 0$ $ 0$ $,QLWLDOPHWKDQHFRQFHQWUDWLRQ,QLWLDOPHWKDQHFRQFHQWUDWLRQ FIGURE 4. Discharge power vs. methane conc. FIGURE 5. Methane conversion vs. methane conc. in the initial mixture CH 4 +H 2. in the initial mixture CH 4 +H 2. Overall flow rate 300 Nl/h, Overall flow rate 300 Nl/h, current in low voltage circuit 10A current in low voltage circuit 10A and 20 A. and 20 A. Y M total CH 4 conversion Y C2 conversion into C 2 hydrocarbons ACKNOWLEDGMENT The work was granted by the State Committee for Scientific Research; Project Nr PZB/KBN/018/T09/99 REFERENCES [1] Czernichowski A., Czernichowski P., and Ranaivosoloarimanana A., Plasma Pyrolysis of Natural Gas in Gliding Arc Reactor. In: Proc. 11 th World Hydrogen Energy Conference, Stuttgart, Germany, 1996, pp [2] Czernichowski A., Karbo-energochemia-ekologia, 11, (1998) [3] Czernichowski A., Electrically Assisted Conversion of Natural Gas into Syngas. In: Gas To Liquids Processing, San Antonio, Texas (1998) [4] Fridman A., Nester S., Kennedy L.A., Saveliev A., Mutaf-Yardimci O., Progr. Energy Comb. Sci., (1999) [5] Czech T., Doctor Thesis, Institute of Fluid Flow Machinery, Polish Academy of Sciences, Gdansk (1997) [6] Schmidt-6]DáRZVNL.Krawczyk K., Ruszniak J., Processing of Methane by Gliding Arc Discharges. In: Proc. 15 th Int. Symp. on Plasma Chemistry, Orleans, France, 2001, 2933