Scholarly article on topic 'A comparison of carbon tetrachloride decomposition using spark and barrier discharges'

A comparison of carbon tetrachloride decomposition using spark and barrier discharges Academic research paper on "Chemical sciences"

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Academic research paper on topic "A comparison of carbon tetrachloride decomposition using spark and barrier discharges"

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Bogdan Ulejczyk*, Krzysztof Krawczyk, Michat Mtotek, Krzysztof Schmidt-Szatowski, tukasz Nogal, Bolestaw Kuca

A comparison of carbon tetrachloride decomposition using spark and barrier discharges

Abstract: The decomposition of CCl4 in air was investigated at atmospheric pressure in two discharges. Reactors used to generate electrical discharges were powered by the same electric power supply. In both reactors, nearly 90% conversion of CCl4 was obtained. All chlorine was in the form of Cl2 in the process carried out in the barrier discharge, while in the spark discharge, COCl2 was formed. The conversion of CCl4 to COCl2 ranged from 2 to 12%. NO was formed in both discharges but the NO content in the gas leaving the reactors was 1.7-2.7% for the spark discharge and 0.045-0.06% for the barrier discharge. O3 was produced only in the barrier discharge and its content ranged from 0.1 to 0.2%.

Keywords: plasma, gas cleaning, decomposition, DBD, spark discharge

DOI: 10.1515/chem-2015-0059

received January 14, 2014; accepted May 30, 2014.

1 Introduction

Non-equilibrium plasma technology is one of the rapidly developing world science disciplines. New reactors and electric power supplies are designed and tested extensively. Non-equilibrium plasma is currently being applied in an industrial scale for ozone synthesis, gas dedusting, and decomposition of nitrogen oxides and sulfur. Plasma methods (including those implemented) are still studied and improved [1,2]. Reducing the energy consumption is

Corresponding author: Bogdan Ulejczyk: Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warszawa, Poland, E-mail: bulejczyk@ch.pw.edu.pl Krzysztof Krawczyk, Michat Mtotek, Krzysztof Schmidt-Szalowski: Faculty of Chemistry, Warsaw University of Technology, Noakowskiego 3, 00-664 Warszawa, Poland tukasz Nogal, Bolestaw Kuca: Faculty of Electrical Engineering, Warsaw University of Technology, Pl. Politechniki 1, 00-661 Warszawa, Poland

the main aim of the study. New constructions of reactors and power supplies are designed to reduce energy consumption.

The non-equilibrium plasma can be generated in various electric discharges, e.g., corona [3], barrier [4-6], spark [7], gliding [8-10], and microwave [11]. Each of these discharges has its own specific properties, but all of them can be used to decompose volatile organic compounds. Results obtained in different discharges are difficult to compare because they include various process parameters. The goal of a present work was to compare the decomposition of carbon tetrachloride (CCl4) in different discharges operating at atmospheric pressure in air. The experiments were also aimed at revealing the effects of the discharge type on products of CCl4 decomposition. In the experiments, the same gas flow rate and power supply system were used. Only the reactors were different.

2 Experimental procedure

CCl4 (extra pure 99%) is used as a model compound for experiments on the destruction of chlorinated organic compounds because of the stability of its structure. Other compounds, for example CHCl3, are more reactive and easier to decompose than CCl4 [8,12,13]. Thus, it is anticipated that methods for efficient CCl4 conversion can also be useful in the destruction of chlorine compounds with less stable structures.

CCl4 decomposition processes were carried out under the followed conditions:

- gas mixture of CCl4 and air (H2O < 10 ppm)

- the CCl4 content was 0.1% vol.,

- total gas flow rate was 10 L h-1.

2.1 Reactors

Barrier discharge was generated in a reactor as shown in Fig. 1. The grounded electrode was made from silver paste deposited on the outside of the quartz tube. The

I© 2015 Bogdan Ulejczyk et al., licensee De Gruyter Open.

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 License.

Figure 1: Schema of the barrier reactor: 1 - high-voltage electrode, 2, 3 - internal channels, 4 - discharge zone, 5 - dielectric barrier, 6 - grounded electrode.

2.2 Electrical measurements

The reactors were powered by one pulsed power supply system of 30-kHz pulse repetition frequency. The frequency, the current, and the voltage waveforms were recorded by using oscilloscope Tektronix TDS 3052 with a Tektronix P6015A voltage probe and a Tektronix TCP 312 current probe with a Tektronix TCP A300 amplifier.

The power of spark discharge was calculated according to the formula:

P = f JI(t)U(t)d(t)

Figure 2: Schema of the spark reactor: 1,8 - plastic covers, 2 - bolts, 3,7- stainless steel fence, 4,6 - electrode rods, 5 - quartz tube.

outer diameter of the quartz tube was 54 mm and the wall thickness was 2 mm. A high-voltage electrode was grooved. The distance between the high-voltage electrode and grounded electrode was 1.5 and 5 mm for top and bottom of the grooves respectively.

Spark discharge was generated in a reactor as shown in Fig. 2. Electrodes were made of 2 mm rods of platinum. The distance between electrodes was 12 mm. A case was made of a quartz tube of 9-mm diameter and 40-mm length.

P - power, W f - frequency, Hz I - current, A U - voltage, V

t - time of the pulse start and end, s

The Lissajous-figure method was used to measure the discharge power of barrier discharge:

P = f ■ C ■ S (2)

C - capacitance of the measuring capacitor, 20 nF

S - enveloped area of Lissajous-figure

The specific energy was calculated according to the

formula:

= ^ (3)

SE - specific energy, kJ g-1 of gas G - total gas (air and VOCs) flow rate at the inlet, g h-1

2.3 Gas analysis

The content of CCl4 was analyzed using a Hewlett-Packard HP 6890 with an FID detector and packed column with 5% Fluorocol on 60/80 Carbopack B. Chromatographic measurement error was less than 5%. This method allowed for determination of the content of other organic compounds containing chlorine.

Cl2 and COCl2 were analyzed using titration analysis. Gas was periodically passed by two bubblers with solution of KI (10 mmol) in water (100 mL). In this solution Cl2 reacted with KI to form I2 and KCl. COCl2 reacted with H2O to form HCl and CO2. The solution was titrated with a solution of Na2S2O3 (2.5 mmol) in water (50 mL) to determine the amount of I2 and with a solution of NaOH (5 mmol) in water (50 mL) to determine the amount of HCl. Titration analysis error was less than 3%.

The content of O3 was determined using a BMT 961TC ozone analyzer.

The content of NO was determined using a URAS 10P industrial photometer.

The CCl4 conversions were calculated according to formulas:

Wo[CCl4]-W [CCl4]

X = —------ •lOO

Wo[CCl 4]

X - overall CCl4 conversion, %

WJCClJ - CCl4 flow rate at the inlet, mol h1

W[CCl] - CCl4 flow rate at the outlet, mol h1

=WTO! • 1M

C0CU 2 • W0[CCl4]

CCl4 conversion to COCl2, %

Differences in voltage and current waveforms for the two types of reactors are strongly related to the construction of the reactors. Barrier discharge reactor in the general case can be considered a cylindrical capacitor, whose capacity can be calculated by the equation:

2pee„ d

C - capacity, F

£ - relative dielectric permittivity (dielectric constant) £0 - permittivity of vacuum, 8.854187 • 1012 F m1 d - length of the electrodes, m R - radius of the outer electrode, m r - radius of the inner electrode, m

Voltage and current waveforms for the circuit of the barrier discharge reactor are similar to the charging and discharging of the capacitor in a capacitive circuit.

Capacity of the spark reactor can be calculated by the equation:

C = ££o ^ (8)

Se - surface of the electrode top, m2 D - distance between electrodes, m

The spark reactor system has a low capacitance because the S is very small (~3.14 • 10-6 m2). The spark reactor operated at several times lower voltage than the barrier reactor, because there was no additional dielectric barrier impeding electrical breakdown.

W[COClJ - COCl2 flow rate at the outlet, mol h1 X„ - CCL conversion to CL, %

Cl2 4 2'

W[Cl] - Cl2 flow rate at the outlet, mol h-1

3 Results and Discussion

3.1 Electrical characteristics

The waveforms of the discharge current and voltage for each power supply system are shown in Figs. 3 and 4. These waveforms are different, despite the fact that they are powered by the same power supply. Changing the current-voltage characteristics was due to the differences in properties of the reactors.

3.2 Mechanism of CCl4 decomposition

The main product of CCl4 decomposition was Cl2 for both discharges (Figs. 5 and 6). This result corresponded with the results obtained in other discharges and other models of reactors [13-16]. A small amount of COCl2 was formed only in spark discharge (Fig. 5). New organic compounds, like CCL or CCL, were not formed. Based on these results and

2 6 2 4

on the literature data, the mechanism of decomposition of CCl4 in these conditions can be proposed.

CCl4 decomposition can be initiated by reactions with electrons or radicals of oxygen, chlorine, nitrogen or hydroxide [10,17-20]:

CCL + e ^ CCL + Cl +e

CCL + O ^ CCL + ClO

CCL + Cl ^ CCL + CL

3 0 -3 -6 -9

r r f '

0.3 0.2 0.1

-0.1 -0.2 -0.3

100 150 Time,

Figure 3: Traces of pulses voltage and current for the barrier reactor and Lissajous-figure.

i 0 ni

-0.5 -1 -1.5

I I1 II J \

w 'u \ nJ

If H m J

0.03 0 02 0.01 0

CCl, + N ^ CCL + NCl

CCl, + OH ^ CCL + HClO

-0.01 -0.02 -0.03

The CCl3 radical takes part in many reactions [13,21-26], where M is any molecule that exchanges energy:

CCl3 + Cl + (M) ^ CCl4 + (M)

100 Time,

CCL + CL ^ CCL + Cl

CCl3 + O ^ COCl2 + Cl

Figure 4: Traces of pulses voltage and current for the spark reactor.

100 80 60 40 20 0

CCl3 + N ^ ClCN + 2 Cl

NCl takes part in the following reactions [26]: NCl + N ^ N2 + Cl NCl + O ^ NO + Cl

The ClO radical takes part in the following reactions [27-29]:

ClO+O ^ O2 + Cl

1.0 1.2 1.4 1.6 1.8 2.0 Specific energy, kJ/g

• overall "to chlorine Ato phosgene

Figure 5: The dependence between the CCl4 conversion and specific energy for the spark discharge.

ClO+Cl ^ Cl2 + O

2 ClO ^ O2 + Cl2 2 ClO ^ CO2 + Cl

HClO takes part in the following reactions [30,31]:

HClO + Cl ^ Cl2 + OH

C o 60

7 9 11 13 15

Specific energy, kJ/g

• overall "to chlorine

Figure 6: The dependence between the CCl4 conversion and specific energy for the barrier discharge.

HClO + O2 ^ ClO2 + OH

COCl2 may be formed under high temperature according following reaction [32]:

CCl, + CO ^ 2 COCl,

COCl2 and ClO2 may be decomposed by reactions with the Cl radical [13,21,33]:

Scheme 1: Pathway of CCL decomposition [10,13,17-35].

ClO2+Cl ^ Cl2 + O2

COCl2 + Cl ^ COCl + Cl2

COCl is not a stable compound and decomposes in the following reaction [13,21-23]:

COCl + Cl ^ CO + Cl2

ClCN and NCO may be decomposed in the following reactions [34,35]:

ClCN + O ^ NCO + Cl

NCO + O ^ CO + NO

The mechanism described above is shown in Scheme 1.

3.3 Nitrogen monoxide and ozone

in the gas leaving the reactor was 1.7-2.7% for the spark discharge and 0.045-0.06% for the barrier discharge. O3 was produced only in the barrier discharge (Fig. 8). Its content ranged from 0.1 to 0.2%. Figs. 7 and 8 show that the NO and O3 contents increase with the increasing specific energy.

The presence of NO and O3 in the post-reaction gases is undesirable, and these compounds should be removed from the gas.

Ozone decomposed relatively easy to O2 at elevated temperatures. To decompose the ozone, an oven operating at 200°C was placed downstream of the reactor. The residence time of the reactants in the oven was 0.5 s. Ozone removal was necessary because its reaction with KI interfered with the titration. NO did not disturb the analysis of gas composition, and therefore it was not removed from the gas. The removal of NO from gases is complicated. Catalytic reaction with ammonia is the most common method for removing NO.

The process was carried out in air. The components of 3.4 Spark discharge air in the conditions of the plasma were reactive reagents

which can form nitrogen monoxide and ozone. NO was High CCl4 conversion (almost 90%) was reached in the

formed in both discharges (Fig. 7). The content of NO spark discharge (Fig. 5). In the spark reactor, the main

□ | □

0 3 6 9 12 15

Specific energy, kJ/g

A spark □ barrier

Figure 7: The dependence between the NO content and specific energy for the barrier and spark discharges.

7 9 11 13

Specific energy, kJ/g

Figure 8: The dependence between the O3 content and specific energy for the barrier discharge.

product was Cl2, but COCl2 was also produced (Fig. 5). COCl2 formation is undesirable because it is toxic. Additionally, NO was produced in the spark reactor. NO content after the reaction ranged from 1.7 to 2.7% (Fig. 7). O3 was not produced in the spark reactor.

The spark discharge consists of individual streamers. Streamer channels have a diameter of ~ 0.1 mm, and their duration is 300-500 ns [36,37]. The average density of electrons decreases from 1015 to 1011 cm-3 with the duration of streamer [36]. The energy of electrons ranged from 0.1 to 4 eV in the spark discharge [36,38]. The maximum temperature of the gas reaches 5000-6000 K in the streamer channel [36]. Electrons have too low energy to cause dissociation of O2, for which electrons with energies above 6 eV are needed. However, binding energy of C-Cl is 3.6 eV. This means that the electrons have sufficient energy to cause dissociation of this bond.

High gas temperature in the streamer channel results in the formation of NO and COCl2 with no O3. The high gas temperature in the streamer channel causes ionization and thermal dissociation of the molecules. Thermal dissociation and ionization concerns mainly O2 and N2, which are found in the largest quantities in the reactor. The equilibrium concentration of oxygen and nitrogen atoms is ~20 and ~15% at 6000 K respectively. These atoms can initiate the decomposition of CCl4. However, the dominant reaction should be the formation of NO. NO is formed during cooling to ~3000 K. During further cooling to ~1000 K, NO decomposes into O2 and N2. At a temperature less than 1000 K, the NO decomposition reaction is inhibited due to the high activation energy of the reaction. As a result of all these processes, NO concentration in the exit gas ignition is high. At high temperatures, ozone does not occur, because its decomposition reactions are faster than the reactions of its formation.

The reaction of COCL formation from CCl and CO is an

endothermic reaction; AH is 70 kJ mol1 [32]. Therefore, this reaction will occur at high temperature. Consequently, the conversion of CCl4 to COCl2 was high in spark discharge, up to 12% (Fig. 5).

3.5 Barrier discharge

High CCl4 conversion (almost 90%) was reached in the barrier discharge (Fig. 6). The most important advantage of the barrier reactor was the possibility of running CCl4 decomposition process in such a way that all the chlorine was present in the Cl2. This is a significant advantage because Cl2 can be easily neutralized; for example, it reacts with NaOH to form NaClO. NaClO is often used for water disinfection. O3 was formed (Fig. 8) from the oxygen (in air) in the barrier discharge, and is easily decomposed. Small amounts of NO were also formed from the oxygen (Fig. 7). These are different from the results obtained in the spark discharge. This is due to the different properties of the plasma produced in the different discharges.

Barrier discharge consists of a number of microdischarges. Duration of a single microdischarge is 0.1-10 ns, the diameter of the microdischarge channel is ~0.1 mm, the density of electrons in the microdischarge channel ranges from 1014 to 1015 cm-3, the electron energy is 1-10 eV [39], and the temperature of gas in the microdischarge channel reaches 400 K [40]. The electron energy is sufficient to initiate dissociation of all of the reactants fed into the reactor (CCl4, O2, N2, H2O). In contrast, low temperature prevents thermal ionization and dissociation from occurring. Consequently, all of the

reactions should be initiated by the electrons. The low temperature also prevents COCl2 synthesis, from CCl4 and CO2, from occurring. In contrast, low temperature causes the O3 decomposition reactions to be slower than its synthesis reactions, and O3 was in the gases at the outlet of the barrier reactor. NO was formed in the reaction between oxygen and nitrogen radicals and in the sequence of reactions initiated by the collision of CCl4 and N radical.

3.6 Influence of specific energy

As seen in Figs. 5 and 6, the CCl4 conversion increased with the increasing specific energy for both reactors. This result corresponded with the results obtained in other discharges [3,8-10]. The specific energy was changed by changing the ratio of the pulse duration to the interval between discharge pulses. Discharge pulse consisted of a number of microdischarges for barrier discharge and streamers for barrier and spark discharge. Extending the duration of the pulse discharge resulted in increased numbers of microdischarges and streamers. As a result, the effect of the most important factors influencing chemical processes increased in each discharge.

The most important factor was the temperature in the spark discharge. Therefore the importance of the temperature-dependent processes increases with increasing specific energy. The importance of undesirable processes increased the most, to wit:

- an increase in the content of NO in post-reaction gases from 1.7 to 2.7% (Fig. 7),

- an increase the conversion of CCl, to COCK from 2 to

12% (Fig. 5).

The desired decomposition of CCl4 to Cl2 does not depend on the specific energy. The CCl4 conversion to Cl2 was 72-73.5% for tested specific energies in spark discharge (Fig. 7).

The number of electrons was important in barrier discharge. Rates of the reactions initiated by electrons increased with the number of electrons. The increase of the specific energy was related to an increase in the number of discharges. Increase in the number of electrons was the result of increasing the number of discharges. Therefore, the increase of the specific energy caused:

- a slight increase of the content of NO in post-reaction gases (Fig. 7),

- a two-fold increase, from 0.097 to 0.201%, the content of O3 in post-reaction gases (Fig. 8),

- an increase in the conversion of CCl4 to Cl2 (Fig. 6).

4 Conclusions

The results obtained for the two different reactors powered

by the same power supply system may be summarized as

follows:

- Both reactors effectively decompose CCl4.

- In the barrier reactor, phosgene does not occur.

- In the barrier reactor, nitrogen monoxide was produced in a very small amount.

- The temperature-dependent processes, such as synthesis of NO and COCl2, were important in the spark discharge.

- The processes initiated by high-energy electrons were the most important in the barrier discharge. The processes requiring high temperatures were not important.

Acknowledgements: This work was financially supported

by Warsaw University of Technology.

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