플라즈마와 촉매 반응에 의한 천연가스로부터 C2탄화수소,  플라즈마 Black 및 수소 제조에 관한 연구

조원일, 오영삼, 박달령, 백영순, 방효선, 김영채*

한국가스공사 연구개발원, 한양대학교 화학공학과*

The Study on Manufacture of C2 hydrocarbons, Plasma black, and Hydrogen from Natural Gas by Plasmic Catalysis

Wonihl Cho, Youngsam Oh, Dalryung Park, Youngsoon Baek, Hyosun Pang, Youngchai Kim*

Div. of Gas Util., KOGAS R&D Center

Dept. of Chem. Eng., Hanyang Univ.

Introduction

Direct conversion of natural gas to C2 hydrocarbons has been attracted in the natural gas industry. Therefore many researches are carried out with the aim to use it as the gas-to-liquid(GTL) processes in order to produce the synthesis gas and C2 hydrocarbons, ethane, ethylene and acetylene. In addition, a quantity of hydrogen in the world produces from methane. For decades, steam reforming process of natural gas has been known the most efficient and widely used process for the production of hydrogen.

However, the methane activation of the strong and stable bond between carbon and hydrogen is the crucial step involved in the reaction. Many methods have been developed for this activation, such as reaction with super-acids, coordination to metal complexes, and selective oxidation catalyzed by metal oxides.

Recently, the new idea of manufacture valuable chemicals is available by the plasma and catalyst reactions. In a previous paper, we have reported the experimental results obtained by the microwave plasmic catalysis.

In this study, the low temperature plasma system with catalytic reaction to the selective conversion of methane to C2 hydrocarbons, plasma black and hydrogen was investigated.

Experimental

The experimental apparatus used in this work is a low temperature plasma system which have two difference type, a microwave and a radio frequency generator. A microwave generator equipment use in this work operates at 2.45GHz(ASTEX Co.), which is the most common operating frequency for microwave devices. The microwave power available to the sample is possible to generated maximum power of 120W.

Fig 1. Schematic diagram of microwave plasma catalysis apparatus

Figure 1 is a  schematic diagram of microwave plasma reaction system. And a radio frequency was carried out a solid state power generator(Model OEM-12A) manufactured ENI Co., Inc. operating at a frequency of 13.56MHz and a power of 1.2kW. A mass flow controller connected to the plasma reactor controls the flow rate of methane (99.99%, Methason co.). Before reaction, the system was evacuated by a turbo-molecular pump down to 10-7 torr.  Plasma quartz reactor with a 25mm O. D. is connected to a microwave/R.F cavity. The forward and reflected powers of plasma generator were monitored during the experiments.  The plasma phase was initiated with a Tesla coil while evacuated through the reactor. The catalyst is selected to take part in methane activation. Also, the catalyst play an important role in improving the conversion of natural gas to C2 hydrocarbons because of absorbing the microwave radiation in order to provide the necessary kinetic energy.

Results and discussion

1. Microwave plasma and catalyst reaction

The reactions with catalyst were carried out at the same experimental conditions (pressure : 10torr, plasma power : 40-120W, flow rate : 2 - 40ml/min).

In table 1, the productivities and selectivities of C2 hydrocarbons are summarized under various condition. In particular, we used the electric field inductance (EFI) in order to evolve the high density of microwave plasma. The EFI locates near the plasma region. With the Pd-Ni catalyst, C2 hydrocarbons reaches 63.7% in conversion at a voltage of 8V, a current of 2A, a natural gas flow rate of 20ml/min and the plasma zone of 100W.  When the dehydrogenation Pt-Sn catalyst was used, the conversion was 52.8% which is lower than that of the Pd-Ni catalyst.

In the catalytic activity, the natural gas conversion was represented by 16.8% of the  Pt-Sn and 27.8% of the Pd-Ni catalyst. The selectivity of C2+ was shown to be 91.3% of ethane for the Pt-Sn and 84% of ethane, 3% of acetylene for Pd-Ni catalyst as shown in Table 1. It should be noted that the best catalytic activity for natural gas conversion is the redox catalyst of the Pd-Ni bimetal rather than the dehydrogenation catalyst of the Pt-Sn.

In this system, catalysts in the plasma process play the role of absorbing any produced  radicals with the C-H bond and act as helpers for producing the ethyl and acetyl radical but restraining the methyl radical on the catalyst surfaces. Therefore, it can be seen that ethane is converted to the formation of ethylene and acetylene in a microwave plasma catalysis.

Table 1. Summary of Microwave Plasma and Catalysis

Catalyst	Reactors	Plasma Power(W)	dc Power	Productivity (%)	Products Selectivity(%)
					Ethylene	Ethane	Acetylene
Pt-Sn	Plasma	100		46	29.2	46.7	24.1
	Catalyst			16.8	-*	91.26	-*
	Plasma+Catalyst	80		46.3	22.3	50.4	27.3
	Catalyst+Plasma	80		48.2	25.7	54.3	20
	Plasma+EFI	100	8V, 2A	45.5	28.9	49.6	21.5
	Plasma+EFI+Catalyst	100	8V, 2A	49.4	32.4	33.2	34.4
	Catalyst+EFI+Plasma	100	8V, 2A	52.8	24.3	47.4	28.3
Pd-Ni	Plasma	100		46	29.2	46.7	24.1
	Catalyst			27.8	0.5	84	3
	Plasma+Catalyst	80		51.1	30.2	45.9	23.9
	Catalyst+Plasma	80		52.7	27.2	48.7	24.1
	Plasma+EFI	100	8V, 2A	45.5	28.9	49.6	21.5
	Plasma+EFI+Catalyst	100	8V, 2A	58.9	42.1	44	13.9
	Catalyst+EFI+Plasma	100	8V, 2A	63.7	37.4	40.2	22.4

The symbol * means NOT detected.

2. Radio frequency plasma and catalyst reaction

The conversion of natural gas increases from 33% to 63% with increasing the power of R.F discharge without catalyst as shown by Fig. 2. It is observed that the conversion increases with plasma black(C1 to C3 formation) formation above 600W. Up to 600W, the selectivity of ethane decreases with increasing the power while the selectivities of ethylene and acetylene increase. Above 600W, the selectivities of all component are constant  with the change of the power.

Fig. 3 shows the conversion of natural gas ranges from 41% to 73% depending on the power of radio frequency plasma with catalytic reaction. It is observed a maximum value of 73% at 900W and 20ml/min. The plasma black increase up to 600W. Fig. 4 shows the TGA-DTA curves for thermal properties of plasma black. The TGA curve shows entire weight loss up to 650℃. Exothermic reaction is observed above 480℃. The plasma black , C-H bond,is transformed to CO or CO2.

The first-order rate expression for the disappearance of methane may be written as

(1)

where [CH]4 is the concentration of methane at time t, and k1 denotes the overall rate coefficient for first-order disappearance of methane. Fig. 5 shows that the disappearance of methane in RF discharge appears to follow first-order kinetics rather precisely.

In the microwave discharge, the flow rate does not affect the selectivity and  only the power affect it. However, in the R.F discharge the selectivity is affected simultaneously  by the flow rate and the power(Fig. 6).

Up to 600W, the plasma power is  more dominant than the flow rate in the C2+ formation and above 600W the flow rate accelerates the rate of reaction.

Reference

1. Wan, J. K. S. U.S. Patent 4,574,038 (Mar. 4, 1986)

2. Suib, S. L. and Zerger, R. P., J. Catal, 139, 383 (1993)

3. W. Cho, Y. Baek, D. Park, Y.C. Kim, and M. Anpo, Res. Chem. Intermed.,

Vol. 24, No. 1, 55 (1998)