Hexadecane Cracking in a Hybrid Catalytic Pulsed Dielectric Barrier

The reactor performance increases significantly when the discharge zone is ..... by heterogeneous catalysts, compared to streamers in the gas space (a...
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Hexadecane Cracking in a Hybrid Catalytic Pulsed Dielectric Barrier Discharge Plasma Reactor Navid Hooshmand,† Mohammad Reza Rahimpour,*,†,‡,§ Abdolhosein Jahanmiri,† Hamed Taghvaei,† and Meisam Mohamadzadeh Shirazi† †

Department of Chemical Engineering, School of Chemical and Petroleum Engineering, Shiraz University, Shiraz 71345, Iran Department of Chemical Engineering and Materials Science, University of California, Davis, One Shields Avenue, Davis, California 95616, United States

‡

ABSTRACT: In the present work, cracking of a model heavy hydrocarbon (hexadecane) in a nanosecond pulsed catalytic dielectric barrier discharge (DBD) plasma reactor has been investigated. The effect of different commercial catalyst materials based on alumina, titania, and silica has been considered on the reactor performance and products distribution. The reactor performance increases significantly when the discharge zone is packed with catalyst granules. Energy efficiency and hydrogen concentration in the produced gas vary between 36.98 and 194.44 lit/kWh and 17.7% and 63.7%, respectively. The highest energy efficiency was achieved when the plasma was packed with Mo−Ni/Al2O3 catalyst for 52.3 W power input. In this condition, the production rate and concentration of hydrogen have been 108.03 mL/min and 63.7%, respectively. The breakdown voltage is decreased significantly when the reactor is packed with TiO2 based catalyst.

1. INTRODUCTION Cracking is one of the principal refining processes in which long chain hydrocarbon molecules are converted into smaller and more worthy products. There are generally two types of cracking processes: thermal cracking and catalytic cracking. Catalytic cracking is similar to thermal cracking except that catalysts smooth the progress of cracking reactions. Typical temperatures in catalytic cracking processes are from 510 up to 550 °C at a pressure range of 1−2 atm.1 Due to the high operational temperature and low ratio of hydrogen to carbon in such processes coke is deposited on the catalyst as undesired product and renders it less catalytically active.2 Also higher operational temperature in this type of process increases equipment cost in addition to the requirement of a severe process control. The use of traditional catalysis is often limited by a time delay based on heating the catalyst to the required temperature which is the other drawback of these methods.3 In response to the above-mentioned challenges, significant advantages are currently expected from nontraditional approaches to the catalysis. One of these new approaches is to combine heterogeneous catalysis with the chemical activation of reactants by an electric gas discharge or plasma. Some advantages of catalytic plasma processing are the following: no catalyst deactivation due to coke deposition,4 high process rate (almost no time delay),3 high sulfur tolerance, and compatibility with a wide range of fuels including heavy hydrocarbons.5,6 The energy dispersed in a nonthermal plasma process is mostly used to excite the electrons and is not expended on heating the entire gas stream7 in comparison with thermocatalytic processes. Subsequently, gaseous species are chemically excited or dissociated directly by electron impact while the gas temperature remains relatively low. Consequently, product distributions may be obtained far from chemical equilibrium. Plasma processes have the ability to induce gas phase reactions; © 2013 American Chemical Society

however, they are often less selective than catalytic processes. On the other hand, catalytic reactions can give high selectivity while they require an active catalyst with its strict control.8 The energetic electrons in the plasma are highly efficient in producing radicals that can react with the hydrocarbon molecules and crack the C−C and C−H bonds. When the catalysts are placed directly inside the discharge region other active species generated in the plasma can contribute as well to crack the hydrocarbons.7 Therefore, the combination of plasma activated media with a selective catalytic material seems to be a challenge and a promising way to enhance traditional catalytic processes. Through the lower catalyst operating temperature and better end-products selectivity, the combination of plasma and catalysis is found to be beneficial compared to traditional thermal catalysis.9 Wallis et al.10 established that a variety of catalysts which were originally inspected in the field of thermal catalysis show similar synergy when combined with nonthermal plasma. Since the dielectric barrier discharge (DBD) allows direct insertion of a dielectric material into the discharge gap, it has attracted significant interest for combining the plasma reactors with heterogeneous catalysis under atmospheric pressure and relatively low temperatures.11 In order to have an efficient radical formation in catalytic reactions, the radical formation has to occur near the catalyst surface. A reactor concept working in this way is the packed bed DBD reactor, where high electric fields cause plasma generation near the surface of the dielectric packing.12 The surface of dielectric packing may be catalyst itself or coated catalyst. Received: Revised: Accepted: Published: 4443

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placed on the axis of the reactor. The outer surface of the inner electrode is polished to avoid localization of the electric field and to provide a uniformly distributed discharge. Aluminum foil (3 cm long) is wrapped around the outer surface of the quartz glass as the grounded electrode. The discharge zone which is between inner electrode and dielectric is packed with 2.2 cm3 of different commercial granules. The different catalyst types have been employed to investigate the effect of catalyst material on the reactor performance. Characteristics of these catalysts have been tabulated in Table 1. Prior to use, all the catalysts were dried by heating at 1 K/min (from room temperature to 150 °C, for about 2 h). The electrical circuit is also shown in Figure 1. A high voltage pulse generator is used to produce voltage pulses up to 11 kV, with a rise time and fall time of less than 80 ns, pulse width of less than 50 ns, and an adjustable pulse repetition frequency up to 18 kHz. This system is designed by using power switching technique and with bridge topology, which has high efficiency and contains four subsystems: 13 kV switched-mode power supply, resonant charger, modulator, and trigger. Since the pulse duration is much smaller than time lag between pulses, the average power input is low. Details about the employed power supply and electric behavior of the discharge have been delivered in our previous works.27−29 The average power delivered to the reactor (Pavg) can be calculated using eq 1 where f, Vi, and Ii are pulse repetition frequency, instantaneous applied voltage, and current, respectively:

Various publications reported different plasma reactors to produce hydrogen and light hydrocarbons from heavy hydrocarbon.4,13−17 Moreover, Biniwale et al. have explored isooctane cracking through a catalytic tubular DBD reactor. They employed three types of catalyst (Ni−W, Ni−Mn, and Rh−Ce) and illustrated that Rh−Ce catalyst is the best one in viewpoint of hydrogen production.18 Catalytic cracking of hexane and octane have been carried out in a catalytic plasma reactor by Xing et al.19,20 They used iron and nickel as the catalyst, and it has been found that nickel is better for lighter hydrocarbons production. Recent studies about the effect of pulse forms of applied voltage on DBD reactor’s performance have shown superior results when using pulse voltages.21 As a matter of fact, using fast rising pulse voltage for generating DBD can avoid local overheat of microdischarges.21−25 This results in the possibility to create a highly nonequilibrium discharge with small total discharge power and improves discharge efficiency by shortening pulse rise time and width to microsecond or nanosecond scales. However, since the duration of each pulse is much smaller than the time lag between pulses, the average power input is low (e.g., it is on the order of a few Watts for the setup used in this study).26 In our previous works, the plasma cracking of heavy naphtha through pulsed DBD reactor has been studied.27,28 However, the particular research on the continuous hydrogen production through nanosecond pulse catalytic discharge plasma reactors has not been documented in the literature. In this paper, we present continuous cracking of hexadecane (as a heavy hydrocarbon model) in a catalytic DBD reactor at relatively low temperatures using nanosecond pulsed high voltage power supply. Instantaneous hydrogen and light hydrocarbons production through plasma process is studied. The effect of catalyst material is discussed on the energy efficiency, quantity, and quality of products.

n ⎛ V + Vi + 1 ⎞⎛ Ii + Ii + 1 ⎞ ⎟(t ⎟⎜ Pavg = f ∑ ⎜ i − ti) ⎝ ⎠⎝ 2 ⎠ i + 1 2 i=0

(1)

In a typical experiment the methane gas as a carrier is entered into the reactor at a flow rate of 50 mL/min under the control of Alicat Scientific volumetric flow controller (MC-1 SLPM-D). This device also measures the entering gas temperature. Normal hexadecane, n-C16H34, was obtained from Merck corporation and used as the feed which enters into the reactor as it is shown in Figure 1. Its flow rate is fixed at 0.35 mL/min using liquid flow meter, which is calibrated beforehand. Table 2 shows the physicochemical properties of n-C16H34. Average flow rate and temperature of the produced gas are measured via a volumetric flow meter (MS-1 SLPM-D). This device transmits data to the computer with the sample frequency rate of 10 Hz. Average volumetric flow rate of product gas is calculated using instantaneous data. The pressure of reaction system is fixed at 90.7 kPa (barometric pressure). At the end of each experiment, 500 μL of produced gas is analyzed using a gas chromatograph (SPSIC GC112A) equipped with TCD detector (4 mm φ stainless steel packed column: Propak Q, 2 m length) for the analysis of hydrogen, carbon dioxide and light hydrocarbons.

2. EXPERIMENTAL SECTION An overall scheme of the experimental apparatus is shown in Figure 1. The plasma is generated in a tubular reactor made of quartz glass, with the outer diameter of 13 mm and 1.5 mm wall thickness, acting as a dielectric separating inner and outer electrodes. A copper rod with 2.68 mm diameter is selected as the inner electrode which is connected to the high voltage and

3. RESULTS AND DISCUSSION The main goal of this study is to determine the best type of selected catalysts for highest possible process efficiency of hydrogen and light hydrocarbons (in the range of C1 to C4) production through the catalytic DBD reactor. The term C1 denotes methane and C2 is a hydrocarbon such as ethane, ethylene, or acetylene, which have two carbon atoms in each molecule. The terms C3 and C4 are defined similarly. For air removal, carrier gas is flowing through the reactor for 5 min

Figure 1. Overall schematic diagram of the packed DBD reactor. 4444

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Table 1. Characteristics and Chemical Properties of Utilized Catalysts active agents Pt-acid agent as Cl TiO2 Al2O3 WO3−NiO MoO3−CoO Pt−Re MoO3−NiO

weight percent 0.29 100 100 30−34 13.6 0.3 10−19

1−1.2

22−26 3.4 0.4 1−3

support

commercial name

manufacturer

Al2O3 TiO2 Al2O3 Al2O3/SiO2 Al2O3 Al2O3 Al2O3

R-134 CRS-31 CR-3S HC-102 DC-130 RG-682 RN-412

UOP Axens Axens Criterion Criterion Axens Criterion

In spite of heavy hydrocarbon cracking through conventional process and corona discharge, which leads high coke formation and process discontinuity,14 the DBD reactor used in this research produces negligible amount of coke on the high voltage electrode and catalyst surface. This does not interfere with the continuous plasma process. Nevertheless, after each experiment the inner electrode surface is polished to provide same process conditions. A reference test has been defined to survey the effect of catalyst material on the reactor performance. Since the liquid distribution and gas residence time in the glass packed discharge is similar to packed catalytic experiments, glass packed discharge has been selected as the reference test. The conditions of the reference experiment are reported in Table 3.

Table 2. Physicochemical Properties of n-Hexadecane property

value

boiling point (°C) vapor pressure (kPa @ 20 °C) melting point (°C) flash point (°C) density (kg/m3) viscosity (mPa s) molecular weight

285 1.7 × 10−4 18 135 773.1 3.18 226.44

before applying high voltage in our experiments. Then by applying high voltage between two electrodes, the filamentary discharges generate inside the catalytic plasma chamber in blue color. These filaments are randomly distributed over the entire electrode and catalyst surfaces. After 70 s, hexadecane is fed into the reactor. Once the liquid feed enters the plasma zone, reactions occur and liquid hydrocarbon decomposes to hydrogen and lighter hydrocarbon compounds. Abundant smoke is produced immediately which indicates the plasma cracking process rate is high. As plasma is generated, a pressure disturbance occurs in the reactor but it decreases during one minute before hexadecane is fed to the reactor. The plasma cracking process lasts for 180 s. During the experiment, the volumetric flow rate of produced gas increased as shown in Figure 2. The reason can be attributed to the fact that more energy will be lost into the catalyst and gas as time goes on so, the temperature increases and the reactor will be activated more. Once the applied voltage is turned off, the reactions stop immediately and the produced gas disappears quickly.

Table 3. Reference Experiment Conditions parameter

value

packing applied voltage (kV) pulse frequency (kHz) inner electrode diameter (mm) inner electrode material carrier gas flow rate (mL/min) feed flow rate (mL/min)

glass packing 11 18 2.68 copper 50 0.35

In our previous research about the effect of applied voltage and pulse frequency on the reactor performance,27,29 it has been shown that increasing of these parameters lead to higher energy efficiency. So in this research, the applied voltage and pulse repetition frequency have been set on their maximum values (11 kV and 18 kHz). Also, we have found that the optimum diameter of the inner electrode is 2.68 mm for this reactor geometry. Additionally, the lower flow rate of carrier gas and liquid feed results in higher efficiencies.28 The behavior of reactor input power and total gas production rate versus catalyst type are presented in Figure 3. It could be found that average input power, generally, decreases when the reactor is packed with catalyst. In addition, using empty discharge and glass packed discharge consume almost equal power. Integrating catalyst into the plasma reactor increases the externally applied field and creates stronger microdischarge.3 Therefore, there are more energetic electrons and ions in the plasma medium. This fact could lead to more collisions between electrons and hydrocarbons which are the main reactions in the reactor. Thus, more bonds are broken and more activated radicals are produced, which increase the production rate of lighter components, as shown in Figure 3. The interaction between catalyst type and production rate is significantly high so that the production rate with Mo−Ni/ Al2O3 catalyst is about 169.5 mL/min which is 1.68 times

Figure 2. Gaseous product flow rate vs time for 400 s of continuous operation for 52.3 W power input. 4445

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applied field by a factor of 10 to 250 depending on the shape, porosity, and dielectric constant of the granules.3 Furthermore, Hensel et al.36 and Holzer et al.37 stated that microdischarges might be generated within the catalyst pores, resulting in more discharge per volume and increasing the average energy density of the discharge. As a consequence, it is predictable that the plasma cracking process will be improved near catalysis. In actual fact, enhanced ionization has been perceived in the presence of additional surface provided by heterogeneous catalysts, compared to streamers in the gas space (a process which is attributed to photoelectron emission from the surface).38−42 In general, it can be postulated that the cracking process becomes more efficient when the catalyst is integrated into the discharge. Breakdown voltage is the minimum applied voltage which causes generation of plasma. Therefore, beneath this voltage no chemical reaction occurs in the reactor. For the practical application of nonthermal plasma methods, the breakdown voltage is an important design parameter of the gas discharge reactor. Figure 5 shows that when the discharge volume is

Figure 3. Input power and total gas production rate vs catalyst material.

higher than Pt-Acid/Al2O3 (63.2 mL/min) and 3.3 times more as compared to glass packed plasma (39.3 mL/min). The Mo− Ni/Al2O3 catalyst thus shows maximum activity for hexadecane cracking through catalytic DBD plasma reactor. In addition, as illustrated in Figure 3, it is clear that the production rate is lower when the reactor is packed with glass packing compared to plasma alone with the same input power. Indeed, using glass packing in the discharge zone results more capacitance of the reactor, so more power is scattered in the packing and less cracking is occurred with the same level of power.30 Energy efficiency (ratio of the total volumetric flow rate of produced gas to the corresponding input power) is well-known in plasma chemistry research. This parameter follows the reactor performance and was selected in several studies as a criterion to find process optimum conditions.19,20,27−29,31−35 Figure 4 shows the energy efficiency vs catalyst type. As

Figure 5. Breakdown voltage vs catalyst material.

packed, the breakdown voltage is decreased. The decreases in breakdown voltage were attributed to the stronger internal field which is the consequence of refraction of electrical field by catalyst granules. Moreover, it has to be noticed that the breakdown voltage is reduced considerably when discharge zone is packed with TiO2 catalyst which can be assigned to the high refractive index of TiO2. Through nonthermal catalytic plasma operation, gas temperature increases due to inelastic electron−molecule collisions, indirectly (due to relaxation processes converting inner excitation and ionization of molecules into heat). As a result, higher catalyst surface temperatures are often measured in hybrid plasma catalyst arrangements.43 Variation of gas bulk temperature is demonstrated in Figure 6. Temperature increase during plasma operation is able to thermally activate catalyst materials. Catalyst activation is also possible by photon irradiation. Figure 7 illustrates the catalysts surface temperature after 150 s plasma processing. As it is shown, catalyst surface temperature more than 260 °C is measured for all catalysts. Nevertheless, the macroscopic gas temperature is often too moderate to justify thermal catalytic activation.37 It is presumed that hot spots can be created on the catalyst surface. These hot spots are moderately small zones which were uniformly scattered within the catalyst bed. Since ions, neutrals and active species still have much lower temperatures relative to the

Figure 4. Energy efficiency vs catalyst material.

mentioned, the glass packing, with a higher dielectric constant compared to empty discharge, increases the net dielectric constant of the discharge, maybe cause a decrease in discharge efficiency. Packing also significantly reduces the gas residence time in the reactor, resulting in lower process efficiency as discussed in our previous research.28 As it can be seen, the process efficiency increases using catalyst in comparison with glass packing and plasma alone. Fridman declares that catalyst granules refract the electric field, making it nonuniform and also stronger than an externally 4446

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sequently, the catalyst material has represented a significant impact on hydrogen concentration in the produced gas. Chavadej et al.44 realized that employing TiO2 in the plasma zone results to an acceleration of the superoxide radical anion (O2−) generation. Wallis et al.10 also reported that due to plasma catalyst interactions, less parent Ti−O bonds are found on TiO2 surfaces after plasma processes. Therefore, formation of CO2 is expected when a TiO2-based catalyst is used in the discharge zone. Experimental results also confirm that carbon dioxide is produced (less than 1.5 mol %) when discharge volume is packed with TiO2 or Al2O3 catalysis, as shown in Figure 8. Figure 9 also illustrates that major hydrocarbon components are CH4 and C2. The C2 products are mainly ethylene produced from the dehydrogenation process of hexadecane, with a small amount of ethane. This suggests that hydrogen and ethylene components will be directly generated from the dehydrogenation process. Methane constitutes over 50% of produced gas when TiO2 catalyst is used. In addition, C2 components are major product in empty plasma discharge. Overall, hydrocarbons mole percent decreased when packed DBD reactor is employed instead of empty discharge. In hydrocarbon containing plasma reactors, the generation of energetic electrons (which have mean energy of 5 eV in DBDs45) is the initial step of the process. A part of these electrons which have enough energy collide with hydrocarbon molecules and break C−C and C−H bonds (which have energy around 3 eV45). Collisions between electrons and hydrocarbons lead to the production of small activated ions and radicals which could be recombined and produce hydrogen and lighter hydrocarbons such as alkanes and olefins. The bond dissociation energy for single carbon−carbon bonds is 347 kJ/mol C−C; so, the theoretical maximum energy efficiency for C−C bonds cracking is 2.882 × 10−3 mol C−C/ kJ (= 1/(347 kJ/mol)). The highest energy efficiency achieved during plasma cracking experiments is 194.44 L of produced gas/kWh for 52.3 W power input. Taking into account the produced gas composition in this condition, the net reaction stoichiometry, in mol/h, is as below:

Figure 6. Outlet gas temperature vs time for 400 s of continuous operation.

Figure 7. Catalyst surface temperature vs catalyst material.

expedited electrons, the used discharge is still of the nonthermal plasma type. As discussed beforehand, catalytic processes are beneficial in viewpoint of better selectivity of desired products. In order to investigate the effect of catalyst material on the quality of the produced gas, hydrogen, CO2, and light hydrocarbons mole percents are shown in Figure 8 and Figure 9. As demonstrated in Figure 8, hydrogen mole percent in the produced gas varies from 17.7% to 63.7% when discharge volume is packed with Mo−Ni/Al2O3 catalysis compared to empty plasma. Con-

0.01524C16H34 → 0.22665H 2 + 0.07983CH4 + 0.02028C2H4 + 0.01107C2H6 + 0.00162C3H8 + 0.01011C3H6 + 0.00134C3H4 + 0.00185C4 H10 + 0.00282C4 H6

Considering above reaction stoichiometry, 0.01524 mol/h of hexadecane is converting for 52.3 W power input which corresponds to 8.093 × 10−5 mol feed/kJ. Considering 15 C−C bonds in C16H34, process efficiency is 1.214 × 10−3 mol C−C/ kJ (15 × 8.093 × 10−5 mol feed/kJ). The ratio of this practical energy efficiency to the theoretical maximum energy efficiency is 42.1%. Performing the same calculations for the case of empty plasma and glass packed discharge results 22.7% and 16.7%, respectively.

4. CONCLUSION Conversion of a model hydrocarbon (C16H34) in a catalytic plasma reactor has been verified to be an effective method for instant hydrogen and light hydrocarbons production, at room temperature and atmospheric pressure. Methane was used as

Figure 8. Hydrogen and CO2 concentration in the produced gas vs catalyst material. 4447

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Figure 9. Hydrocarbons concentration in the produced gas vs catalyst material. (7) Magureanu, M.; Mandache, N. B.; Parvulescu, V. I.; Subrahmanyam, Ch.; Renken, A.; Kiwi-Minsker, L. Improved performance of non-thermal plasma reactor during decomposition of trichloroethylene: Optimization of the reactor geometry and introduction of catalytic electrode. Appl. Catal., B 2007, 74, 270−277. (8) Pietruszka, B.; Heintze, M. Methane conversion at low temperature: the combined application of catalysis and nonequilibrium plasma. Catal. Today 2004, 90, 151−158. (9) Blackbeard, T.; Demidyuk, V.; Hill, S. L.; Whitehead, J. Ch. The effect of temperature on the plasma-catalytic destruction of propane and propene: A comparison with thermal catalysis. Plasma Chem. Plasma Process. 2009, 29, 411−419. (10) Wallis, A. E.; Whitehead, J. Ch.; Zhang, K. Plasma-assisted catalysis for the destruction of CFC-12 in atmospheric pressure gas streams using TiO2. Catal. Lett. 2007, 113, 29−33. (11) Istadi, I.; Amin, N. A. S. Co-generation of synthesis gas and C2+ hydrocarbons from methane and carbon dioxide in a hybrid catalyticplasma reactor: A review. Fuel 2006, 85, 577−592. (12) Hammer, Th.; Kappes, Th.; Baldauf, M. Plasma catalytic hybrid processes: gas discharge initiation and plasma activation of catalytic processes. Catal. Today 2004, 89, 5−14. (13) Li, M. W.; Liu, C. P.; Tian, Y. L.; Xu, G. H.; Zhang, F. C.; Wang, Y. Q. Effects of catalysts in carbon dioxide reforming of methane via corona plasma reactions. Energy Fuels 2006, 20, 1033−1038. (14) Prieto, G.; Okumoto, M.; Takashima, K.; Mizuno, A.; Prieto, O.; Gay, C. R. A plate-to-plate plasma reactor as a fuel processor for hydrogen-rich gas production. The 2001 IEEE industry applications society conference; The 36th IAS Annual Meeting, IL, September 30− October 4, 2001; pp 1099. (15) Prieto, G.; Okumoto, M.; Takashima, K.; Katsura, S.; Mizuno, A.; Prieto, O.; Gay, C. R. Nonthermal plasma reactors for the production of light hydrocarbon olefins from heavy oil. Braz. J. Chem. Eng. 2003, 20, 57−61. (16) Matsui, Y.; Kawakami, S.; Takashima, K.; Katsura, Sh.; Mizuno, A. Liquid-phase fuel reforming at room temperature using nonthermal plasma. Energy Fuels 2005, 19, 1561−1565. (17) Khani, M. R.; Razavi Barzoki, S. H.; Sahba Yaghmaee, M.; Hosseini, S. I.; Shariat, M.; Shokri, B.; Fakhari, A. R.; Nojavan, S.; Tabani, H.; Ghaedian, M. Investigation of cracking by cylindrical dielectric barrier discharge reactor on the n-hexadecane as a model compound. IEEE Trans. Plasma Sci. 2011, 39, 1807−1813. (18) Biniwale, R. B.; Mizuno, A.; Ichikawa, M. Hydrogen production by reforming of iso-octane using spray-pulsed injection and effect of non-thermal plasma. Appl. Catal., A 2004, 276, 169−177. (19) Xing, Y.; Liu, Zh.; Couttenye, R. A.; Willis, W. S.; Suib, S. L.; Fanson, P. T.; Hirata, H.; Ibe, M. Processing of hydrocarbons in an AC discharge nonthermal plasma reactor: An approach to generate

the carrier gas in this study. The influence of different commercial catalysis in the reactor has been investigated, and the following conclusions obtained: (1) Increased process efficiency and H2 concentration were accomplished when catalyst was used as a packing material. (2) Catalyst material has a significant impact on both process efficiency and gas composition. (3) The highest energy efficiency was achieved by Mo−Ni/Al2O3, with roughly 42% of the energy consumed by the reactor used for cracking single carbon−carbon bonds. (4) Production rate and energy efficiency decreased when discharge zone is packed with glass packing, compared with empty discharge. (5) In the case of TiO2 catalyst, the breakdown voltage was decreased significantly. (6) Other major products apart from hydrogen are C1 to C4 alkanes and alkenes.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +98 711 2303071. Fax: +98 711 6473180. E-mail: [email protected]. Present Address §

Mohamad Reza Rahimpour: sabbatical at the University of California, Davis. Notes

The authors declare no competing financial interest.

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REFERENCES

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dx.doi.org/10.1021/ie3022779 | Ind. Eng. Chem. Res. 2013, 52, 4443−4449