STUDIES OF NITROUS OXIDE CONVERSION IN GLIDING ARC DISCHARGES

Transcription

1 STUDIES OF NITROUS OXIDE CONVERSION IN GLIDING RC DISCHRGES K. Krawczyk a, and M. Wieczorkowski Warsaw University of Technology, Faculty of Chemistry, ul. Noakowskiego 3, -664 Warszawa, POLND bstract. The aim of this work was develop, design and check new type of the reactor for N 2 O conversion to NO, nitrogen and oxygen in gliding arc discharge. The reactor was made of stainless steel. Experiment were carried out using the nitrous oxide and oxygen mixtures. It has been shown that both the nitrous oxide conversion to NO and overall conversion of nitrous oxide depend on the linear flow rate of gas mixture passed through a nozzle. The obtained results showed a clear effect of applied arrangement of discharge space on the conversion of nitrous oxide to NO 1. INTRODUCTION High concentrations of nitrogen and sulphur oxides, methane, carbon dioxide, and freons are responsible for the occurrence of the so-called greenhouse effect and for the destruction of the ozone layer. lso nitrous oxide belongs to the group of stable compounds having a negative effect on the natural environment. Much research work is being done to reduce the emission of nitrous oxide to the atmosphere. Thermal and catalytic decompositions to nitrogen and oxygen are among the methods most often used for destruction of nitrous oxide [1]. The method has been utilised commercially for purification of waste gases in the manufacture of adipic acid [2]. Conversion of N 2 O to NO is the most desirable method used. NO obtained in this process can be oxidised to NO 2 and used in the manufacture of nitric acid. In recent years studies have been carried out on the use of gliding arc discharge (Glid-rc) for decomposition of nitrous oxide. This type of discharge is characterised by a low temperature of the gas mixture, as compared with the temperature of the electrons, and a high productivity of the oxidizers. The Glid-rc reactors enable to carry out the reaction under high flow rates. nother advantage of using this type of discharge for decomposition of N 2 O is the possibility of obtaining large quantities of NO. The studies of nitrous oxide decomposition in gliding arc discharge were carried out in a reactor fitted with three pairs of electrodes and in a two-electrode system. The studies enabled to obtain high conversion of N 2 O to NO, which was of the order of 2 37% in the system with two electrodes. Such results were obtained with flow rates of 15, 2, 3 or 4Nl/h of gas mixtures having initial concentration of nitrous oxide of 2.5, 5 or 1% by vol. The overall conversion of N 2 O reached about 8% max [3, 4]. The conversions of nitrous oxide were also studied with the use of phenomena of heterogeneous catalysis [4]. The packing materials used were TiO 2, SiO 2, γ-l 2 O 3 and metal oxides deposited on a γ-l 2 O 3. It has been found the use of a solid packing material in the reaction space increases the conversion of N 2 O. The highest activity of the nitrous oxide conversion was observed in the processes in which nickel, iron, or copper oxides deposited on γ-l 2 O 3 were used. clear increase of the overall conversion of N 2 O was observed with all the catalysts used. The highest conversion of nitrous oxide (9%) was obtained with the use of CuO. However, no significant effect of the catalysts used on the conversion of N 2 O to NO was observed. a Electronic address: kraw@ch.pw.edu.pl

2 The influence of the gliding arc discharge space organisation on the conversion of nitrous oxide was also investigated. The conversion of N 2 O to NO about 5% was obtained in a reactor with reduced reaction zone and two electrodes forming an acute angle. The overall conversion of N 2 O was about 8%. These results were much higher than those obtained under identical conditions in a reactor of much larger cross-section and knife-shaped electrodes. These results have shown that the process of conversion of nitrous oxide depends on the layout and organisation of the reaction space, in which the electric discharge is effected, and on the hydrodynamic conditions of gas flow through the plasma generation zone. The aim of this work was to study the parameters influencing the conversion of nitrous oxide in gliding arc discharge and to determine the effects of interelectrode gap width, linear gas flow velocity, volume of reaction space, and electric power applied on the kinetics of conversion of nitrous oxide. 2. EXPERIMENTL The studies on conversion of nitrous oxide were carried out in a stainless steel reactor with two electrodes. The electrodes were connected to 5 Hz C supply (see Fig. 1) from a high voltage transformer (6). n oscilloscope (8) enabling the observation of voltage runs in the discharge and evaluation of performance of the reactor was connected to the transformer through a voltage probe (7). FIGURE 1. Setup of apparatus for studies on conversion of nitrous oxide. 1 autotransformer, 2 voltmeter, 3 resistor, 4 ammeter, 5 electric energy meter, 6 high voltage transformer, 7 voltage probe, 8 oscilloscope, 9 gas sampling fittings, 1 reactor, 11 gas flow control, 12 oxygen and nitrous oxide. The conversion of nitrous oxide was studied in oxygen atmosphere. Nitrous oxide was introduced to the reactor through a system of gas flow controlling instruments (11) and a quartz nozzle. The body of the reactor (Fig. 2) was made of stainless steel. Knife-shaped stainless steel electrodes were mounted inside the reactor. quartz glass nozzle was installed within the stainless steel body. The distance between the nozzle outlet and the narrowest interelectrode separation was 5 mm. The volume of the reaction space was reduced by placing ceramic elements on both sides of the electrodes.

3 FIGURE 2. Stainless steel reactor for the study of nitrous oxide conversion in gliding arc discharge. a) - cross-section view from above, b) - cross-section of face view, c) C-C cross-section of side view. 1 - electrodes, 2 ceramics, 3 quartz nozzle, 4 - thermocouple The studies were carried out with constant nitrous oxide and oxygen flow rates equal 2 Nl/h. The initial N 2 O concentration was 5% by volume. Three linear velocities of gas at the outlet from the quartz nozzle were applied: 45, 7 or 125 m/s by using nozzles with inner diameter of 1.25, 1 or.75mm, respectively. The width of the interelectrode gap was 1, 1.5, 2.25, 2.75, or 3.25mm. Studies with a reactor with modified reaction space volume were also performed using the same interelectrode distance and nozzle diameter of.75 or 1.25mm. The conversion of N 2 O to NO and the overall conversion of N 2 O were determined as a function of the electric power applied. The composition of the gas mixture was monitored throughout the experiment. The concentration of nitrous oxide before and after the reaction was determined by gas chromatography. The amount of nitric oxide formed was determined by titrimetric and gravimetric methods [5]. 3. RESULTS ND DISCUSSION The width of the interelectrode gap had no significant effect on the overall conversion of N 2 O and on conversion of N 2 O to NO. t higher gap widths the arc initiation occurred at higher voltage values. t the highest value of interelectrode gap width (3.25 mm) the overall conversion was higher by 1% than that observed at the other gap widths. It should be pointed out, however, that the 8%

4 conversion was obtained for the discharge power of about 47 W, which was by 17 W higher than that applied in the other experiments. The course of the process was much more influenced by the linear velocity of the gas mixture entering into the reactor (Fig. 3). Overall conversion of N 2 O, % a C Conversion of N 2 O to NO, % C b FIGURE 3. Effect of discharge power on overall conversion of nitrous oxide (a) and on conversion of nitrous oxide to NO (b). Interelectrode gape width 1 mm, initial nitrous oxide concentration 5% by vol. Nozzle with inner diameter:.75, 1, C 1.75mm. The overall conversion of nitrous oxide increased when the linear gas flow velocity was changed from 45m/s to 125m/s (quartz nozzle diameter from 1.25 to.75 mm, respectively). The change of the nozzle diameter (and the resulting change of linear gas velocity) changes the hydrodynamic conditions of gas flow through the plasma of gliding arc discharge and that could have increased the overall conversion. of nitrous oxide and conversion of N 2 O to NO. The maximum conversion of N 2 O to NO was about 22%, and the overall conversion of N 2 O was about 7% (Fig. 3). The studies carried out in the reactor with modified reaction space (with ceramics Fig. 2) were performed at constant interelectrode gap width (1 mm). Two nozzles used had the inner diameter of.75 mm or 1.25 mm, respectively. The decrease of the reaction space volume resulted in a change of hydrodynamic conditions of gas flow through the plasma formation zone. The conversion of nitrous oxide to NO was smaller than that in experiments with original reaction space volume (Fig. 4). The fact may be due to the higher temperature in the reactor with modified reaction space, resulting from the increase of power density within the reactor. In the reactor with modified reaction space the temperature was several dozen higher than in the reactor without ceramics. The increase of temperature can accelerate the decomposition of the NO formed. The observed values of overall conversion of N 2 O are only slightly smaller than those obtained in the reactor before the modification of the reaction space. The disclosed effect of discharge power on the overall conversion of nitrous oxide and on the conversion of N 2 O to NO (Fig. 4) shows that the linear gas flow velocity has a bearing on both the conversion of N 2 O to NO and the overall conversion of nitrous oxide. For lower powers applied the values of conversion of N 2 O to NO and overall conversion of nitrous oxide were higher in experiments where the linear gas velocity was 125m/s (quartz nozzle diameter.75 mm). For higher powers identical values were obtained for overall conversion of nitrous oxide and for conversion of N 2 O to NO. The results of the experiments performed have shown that in decomposition of nitrous oxide maximum amounts of NO are obtained when the process is carried out in a reactor with higher crosssection area. When high linear gas flow velocities were applied maximum conversion of N 2 O to NO was about 22%. The conversion of N 2 O to NO decreased by about 5% when the volume of the reaction space was reduced.

5 Overall conversion of N 2 O, % a Conversion of N 2 O to NO, % b FIGURE 4. Effect of discharge power on overall conversion of nitrous oxide (a) and on conversion of nitrous oxide to NO (b). Reactor with modified reaction space. Interelectrode gap width 1 mm. Initial concentration of nitrous oxide 5% by vol. Nozzle with inner diameter:.75, 1.75mm. REFERENCES [1] Kapteijn F., Rodriguez-Mirasol J., and Moulijn J.., ppl. Catal. : Environ., 9, (1996) [2] Reimer R.., Slaten C.S., Seapan M., Koch T.., and Tomlinson P.E., Development of Technologies for Control of N 2 O Emission ssociated with dipic cid Manufacture, In: Proc. of the 6th Int. Workshop on Nitrous Oxide Emission, Turku, 1995, [3] Krawczyk K., 5XV]QLDN-áRWHNCzernichowski., and 6FKPLGW6]DáRZVNL.., Pol. J. ppl. Chem., XLII, (1998) [4] Krawczyk K., DQGáRWHNppl. Catal. : Environ., 3, (21) [5] Krawczyk K., Petryk J., and 6FKPLGW6]DáRZVNL.ppl. Catal. : General,, 175, (1998)