Application of Technical Kinetics for Macroscopic Analysis of Ozone

ARTICLE pubs.acs.org/IECR

Application of Technical Kinetics for Macroscopic Analysis of Ozone Synthesis Process Szawomir Jodzis*,† †

Warsaw University of Technology, Faculty of Chemistry, Noakowskiego 3, 00-662 Warszawa, Poland ABSTRACT: The method of measuring the technical rate of ozone synthesis process, and the manner of utilizing the process rate for determining the boundary ozone contents in the reacting system has been presented. On the basis of macroscopic analysis of the process the reason causing the obtaining of the ozone concentration lower than that theoretically possible, has been pointed out. It is shown that an increase in temperature in the discharge gap does not inhibit the ozone-forming rate. The decrease in ozone concentration observed is a result of ozone intensive decomposition. However, under typical conditions of the process, the ozone decomposition is a result of the temperature in the microdischarge channels and not that of the gas or the cooling liquid.

1. INTRODUCTION In theoretical considerations on the kinetics of the ozone synthesis process, the assumption that the reactions inside the discharge gap occur under stable gas temperature conditions is made. The acceptance of such an assumption simplifies the kinetics calculations but omits the effect of diversified process conditions in the ozonizer gap. This is not in agreement with the commonly accepted mechanism of the process,13 according to which the temperature (energy) in various parts of the reaction space is diversified. When studies of laboratory as well as industrial ozonizers are carried out, the aim of the work is to correlate the effects obtained (ozone concentration, ozone yield, process efficiency) with the basic parameters of the process (gas flow rate, energy consumed, temperature of a cooling liquid), which are easy to permanently control. Sometimes this causes simplified interpretation of the phenomena occurring in the discharge gap. An example of such simplification is the opinion that the “temperature” influences the ozone concentration, the ozone yield, etc. Such an opinion results from the direct observation of the process, i.e. lower and lower ozone concentrations are obtained when the temperature of the liquid cooling the electrodes walls increases. The ozone concentration decreases also at high specific energy values, when an increase in the gas temperature inside the discharge gap is observed. The observations presented above correctly illustrate the practical aspect of the ozone synthesis process. However, the ambiguous understanding of the “temperature” term could raise doubts. This question will be widely discussed in the further part of this study. The aim of this work is to explain the actual reasons of the low ozone concentration obtained at higher temperature using the process rate measurement with the differential method. 1.1. Mechanism of the Ozone Synthesis Process. Ozone synthesis with the use of barrier-type discharges is a specific chemical process, which is initiated by electrons with a high energy (>6 eV), which flow through the microdischarge channels of the discharge gap. These channels are a special part of the interelectrode space. In the channels the fast reactions with the electrons (phase I) and the subsequent reactions, in which the atomic oxygen is involved (phase II), occur. The temperature Tch in the channels is much higher than that of the gas around them (Figure 1). When oxygen is the only r 2011 American Chemical Society

substrate, there are many, i.e. several dozen,410 reactions which lead to the formation of ozone. These reactions take place between various species in different energy states. The basis of the occurring phenomena is explained by a simplified process mechanism, which comprises the five most important subsequent reactions:2,3 Phase I :

O2 þ e f O þ O þ e

9

k1 ¼ 2:8 10 ðfor 120 TdÞ k1 ¼ 3:6 109 ðfor 140 TdÞ

ðR1Þ Phase II :

O þ O 2 þ M f O 3 þ M f O3 þ M

k2 ¼ 6:4 1035 expð663 Tch 1 Þ

ðR2Þ

O þ O þ M f O2 þ M k3 ¼ 3:8 10

30

Tch 1 expð 170 Tch 1 ÞðM ¼ O, O2 , or O3 Þ

ðR3Þ Phase III :

diffusion; heat exchange

The ozone present in the discharge gap before the beginning of the discharge phase is to a certain degree decomposed according to the following simultaneous fast reactions, which occur in discharge channels: in phase I :

O3 þ e f O 2 þ O þ e

in phase II :

k4 ¼ 3k1

ðR4Þ

O3 þ O f O 2 þ O 2

k5 ¼ 1:8 1011 expð 2300 Tch 1 Þ

ðR5Þ

Received: December 31, 2010 Accepted: March 29, 2011 Revised: March 22, 2011 Published: March 29, 2011 6053

dx.doi.org/10.1021/ie102603z | Ind. Eng. Chem. Res. 2011, 50, 6053–6060

Industrial & Engineering Chemistry Research

ARTICLE

Figure 2. The method used for determining the differential reaction rate of ozone synthesis. Figure 1. Temperatures in ozonizer; Tg average temperature of a gas, Tch gas temperature in the microdischarge channel, Tg0 inlet gas temperature, Tc cooling liquid temperature.

In reaction R5, part of the ozone formed in reaction R2 is decomposed. The first phase lasts only a few nanoseconds. As a result of reaction R1, atomic oxygen is formed. These atoms are completely used up in the reactions of phase II. Due to the fact that the rate of the reactions which occur in phases I and II are very high, it is assumed that the products of the reactions remain in the space formerly taken up by the microdischarge channels, even for approximately few microseconds1 after completion of the discharge phase. In phase III the diffusion of the reagents into the space around the former microdischarge channels occurs. Moreover, the temperature of the gas becomes homogeneous. Despite the fact that energy is consumed only in the microdischarge channels (phase I), its effect is noted in the entire discharge zone (also in phases II and III). It can be stated that the energy is used in two essential ways, namely in ozone formation and in the heating of the reaction space. The change of the electric energy into heat is, of course, an undesired effect in ozone synthesis, however, this is difficult to avoid. The temperature (in the channels) influences not only the values of the reaction rates, but also the local concentration of the reagents in the reaction space. Therefore, the kinetic models of the process should take into consideration the dynamics of the temperature changes in the microdischarge channels and its influence on the reagent concentration. 1.2. Kinetics of Ozone Synthesis Process (Macroscopic Description of the Model). Due to the fact that the process mechanism is very complex, some simplifications in the macroscopic description of the phenomenon need to be made. In the simplified descriptions the basic process parameters such as the energy uptake, the gas flow rate and the temperature of the cooling medium, are used. Then, the following chemical equation is the basis of the analysis: rf

3O2 sf rs 2O3 rd

ðR6Þ

where two competitive processes (reactions) occur with the rates rf and rd. The convention assumed allows the analysis of the process using the macroscopic: formation kf and decomposition kd rate constants. The simple model of the process, in which the stable gas temperature in the system (independently on the energy consumed) is assumed. The so-called “specific energy,” i.e. the ratio of active power and gas flow rate (P/V) is commonly used. The ozone concentration obtained is correlated with the specific energy. The relationship between ozone concentration cO3 and the P/V ratio is described by the equations below: Becker’s one, which was

derived for the model of a reactor with perfect mixing: P kf V cO3 ¼ P 1 þ kd V

ð1Þ

and VasilevKobozevEremin (VKE) one, which was derived for the plug flow reactor model: P ð2Þ cO3 ¼ cO3 1 exp kd V where c*O3 is the ozone concentration of the reacting system which increases when the discharge energy increases. Similar equations were proposed also by Monge,11 Bes,12 Held:13 kf P c O3 ¼ 1 exp ðkf þ kd Þ ð3Þ V kf þ kd and Skalny:14 c O3 ¼

2kf c0O2 P 1 exp ðkf þ kd Þ V 3ðkf þ kd Þ

ð4Þ

A useful modification of VKE equation is the relation:15,16 " !# kþ c ð5Þ a ¼ ae 1 exp Ev ae which was applied in researches concerning the catalytic effect of the silica packing placed inside the discharge gap.17,18 Another way of correlating the effects of the process with its parameters, could be an experimental determination of the technical rate of the process. This procedure is not directly connected with the macrokinetics eqs 15, but it can be used for the analysis of the process macrokinetics when the ozone boundary concentrations are experimentally determined. 1.3. Technical Reaction Rate of Ozone Synthesis. The technical reaction rate of a process is a parameter usually expressed in units which are used in actual industrial systems, e.g. weight of ozone produced in a given volume of an ozonizer in a unit of time (g O3/m3 3 s) or the number of moles of ozone produced in a given volume in a unit of time (mol O3/m3 3 s). Figure 2 depicts the method used for determining the process rate r (mol O3/m3 3 s) as a function of the process extent, i.e. the conversion degree x. In order to calculate the dependence of the process rate r and the conversion x, the following assumptions have been made: 1. The process takes place under constant pressure (p = const) in pure oxygen. The oxygen flow rate in the inlet stream is V0 (Ndm3/h; normal conditions: 0 °C, 1 bar). 2. In ozonizer Oz-1 some of the oxygen reacts to form ozone, with the ozone mole fraction equal to a1. 6054

dx.doi.org/10.1021/ie102603z |Ind. Eng. Chem. Res. 2011, 50, 6053–6060


ARTICLE

3. In ozonizer Oz-2 the mole fraction of ozone changes from a1 to a2, and the conversion degree from x1 to x2. 4. Into the differential part of the reaction space volume dVR, a mixture of oxygen and ozone is introduced with conversion equal to x, whereas the conversion in the outlet mixture is x þ dx. 5. In the volume dVR ozone synthesis occurs with the reaction rate r (mol O3/(m3 3 s)). The balance equation of ozone (mol O3) in volume dVR can be written as:

Table 1. Oxygen Contaminations (vol. ppm) H2O

CO

CO2

H2

N2

CnHm

1.3

e0.1

1

e1

e5

e0.5

2 V0 ð1 a1 Þ x Δτ þ r 3 dVR Δτ 3 3 22:4 3 3600 3 3 ¼

2 V0 ð1 a1 Þ ðx þ dxÞΔτ 3 3 22:4 3 3600

ð6Þ

The individual terms of an equation means, the number of ozone moles entering into the volume dVR, the number of ozone moles being formed in a volume dVR and the number of ozone moles leaving the volume dVR during the time Δτ, respectively. After simplification we obtain: r ¼

V0 ð1 a1 Þ dx 120960 dVR

ð7Þ

After the separation of variables and integration of eq 7 within the limits of 0 to VR and x1 to x2, the following equation is obtained: Z V0 ð1 a1 Þ x2 dx ð8Þ VR ¼ 120960 x1 r For minute changes of conversion in ozonizer Oz-2 it can be assumed that the reaction rate of the process is constant,hr. Hence the following is true: 120960 3 VR 1 ¼ ðx2 x1 Þ V0 ð1 a1 Þ r

ð9Þ

The average reaction rate equals: r ¼

V0 ð1 a1 Þ ðx2 x1 Þ 120960 3 VR

ð10Þ

and it can be ascribed to the average concentration of ozone in the reaction volume, that is the average conversion hx = (x1 þ x2)/2. Conversions can be obtained from the following equation: xi ¼

3 3 ai 2 þ ai

Figure 3. Outlet ozone content a2 vs initial ozone content a1 and active power; temperature of cooling liquid Tc = 25 °C; a1 = 0, 1.9, 3.2, 4.3, and 5.4 mol %.

oxygenozone mixture, which feeds the measuring ozonizer Oz-2. At the oxygen flow rate of V0 = 4 Ndm3/h, it allows the mixture containing up to ∼6 mol % of ozone to be obtained. The mixtures containing 0, 1.9, 3.2, 4.3, and 5.4 mol % of ozone were produced. The second ozonizer19 was made of borosilicate glass (Pyrex). The discharge gap (1.5 mm width) was double-side thermostatted. The length of the thermostatted part of an ozonizer was ∼25 cm, and the length of the discharge gap was 10 cm. The gas residence time in the both discharge and thermostatted zone was ∼12 and 30 s, respectively. The measurements were carried out at the temperature of cooling liquid 0, 25, and 50 °C. The measuring ozonizer was supplied with ac current (up to ∼11 kV, 1 kHz) by the inverter. The concentrations of ozone were measured with BMT961TC and BMT-963 VENT (Berlin Messtechnik) ozone meters operating with the ozone-specific UV 253.7 nm absorption line. The energy consumed by the Oz-2 ozonizer was measured (as an active power) using the TDS3032 (Tektronix) oscilloscope equipped with P6015A high-voltage probe and the TCP-312/TCPA3000 system for measuring the current.

ð11Þ

If the concentration changes are small, the average reaction rate has a local character and is the actual (real) rate. In the case of greater concentration changes, the obtained value is the average reaction rate. The reaction rate correlates with the temperature of the process. In practice, it is more convenient to measure the temperature of the cooling/thermostatting liquid. If the process is performed with relatively low specific energies, the discrepancy between these two values is negligible.

2. EXPERIMENTAL SECTION The kinetic measurements were carried out in pure oxygen (Table 1) in a setup consisting of two DBD-type ozonizers connected in series. The idea of the measurements has been presented in Figure 2. The first ozonizer (Oz-1) generated the

3. RESULTS AND DISCUSSION Figures 3 and 4 depicts the basic experimental data, which were used for determining the rate of the ozone synthesis process: ozone content a2 in the postreaction gas as a function of the discharge active power, initial ozone content a1 and the cooling liquid temperature Tc. Figure 3 depicts exemplary characteristics obtained at Tc = 25 °C. As can be seen, independently of the initial ozone content, the energy is consumed at first for the increase in ozone content. The concentrations attain certain maximal values and then decrease systematically. Therefore, the energy is consumed less and less effectively. This phenomenon is commonly observed.11,12,1921 Too high concentrations of both electrons and ozone in microdischarge channels could cause undesirable reactions between them (reaction R4). This question will be described in greater depth in a further part of this paper. 6055



ARTICLE

Figure 5. Ozone synthesis process rate vs active power; temperature of cooling liquid Tc = 25 °C; initial ozone content a 1 = 0, 1.9, 3.2, 4.3, and 5.4 mol %.

Figure 4. Outlet ozone content a2 vs initial ozone content a1 and active power; temperature of cooling liquid Tc = 0, 25, and 50 °C; a1 = 0, 1.9, 3.2, 4.3, and 5.4 mol %. For clarification of the graph the experimental points have been omitted.

Figure 4 depicts the effect of the cooling liquid temperature and the active power on the ozone concentration. The diversification of the maximal concentration levels depending on the cooling liquid temperature is evident. The concentrations obtained at high powers are equal, independent of the initial ozone contents. It is worth to notice the characteristics obtained at 50 °C. As can be seen, at the highest initial ozone content a1 = 5.4 mol % and the power of 0 W, the ozone concentration in the postreaction gas is equal to that initial one (a2 = a1). Therefore, relatively high temperature of the gas in the discharge gap, which was produced by the thermostatting system, does not cause the ozone destruction. However, the systematic ozone decomposition starts already at very low discharge power. Because in such conditions the increase in gas temperature is equal zero (ΔT ≈ 0, see: Figure 10), it indicates that the decrease in ozone concentration must result from the reactions occurring in the microdischarge channels. The analogous effect of the energy delivered on the ozone decomposition can be expected at the other temperatures; however, for the sake of the range of concentrations generated by the Oz-1 ozonizer, it was not experimentally confirmed. 3.1. Measurements of the Technical Rate of the Process. Figure 5 depicts the determined rates of the process as a function of discharge active power, which were obtained at 25 °C for the mixture containing 0, 1.9, 3.2, 4.3, and 5.4 mol % of ozone. Analogous dependences (not presented) were obtained at the temperature of cooling liquid of 0 and 50 °C. The process rates decrease when both the ozone content in the gas and discharge power increases. 3.1.1. The Boundary Ozone Contents. Figure 6 depicts the process rates as a function of conversion degree, obtained at the cooling liquid temperature 25 °C in the mixtures containing 0, 1.9, 3.2, 4.3, and 5.4 mol % of ozone. The process rates decrease when both the conversion degree and the initial ozone content increases. The lines connecting the experimental points obtained at the constant (low) power determines the maximum conversion degree (x = 0.105) theoretically possible to be obtained in such conditions. That x value corresponds to the ozone content in the gas at the level of 7.25 mol %. It is the boundary ozone

Figure 6. Ozone synthesis process rate vs conversion degree and low active powers; temperature of cooling liquid Tc = 25 °C; initial ozone content a1 = 0, 1.9, 3.2, 4.3, and 5.4 mol %.

content (ae) at 25 °C. The boundary ozone contents at 0 and 50 °C, determined analogously, amount to ∼9.1 and 4.8 mol %, respectively. In practice, those values are not obtained, because suitably high energy should be delivered, which could result in unavoidable warming of the reaction space. 3.2. Ozone Losses in the Real Process. Figure 7 depicts the experimentally determined relations between ozone content and the energy factor Ev (the equivalent of the specific energy of the process), which were obtained at 0, 25, and 50 °C, as well the analogous dependences calculated on the basis of the formula (5), in which the former determined boundary ozone contents ae were taken into account. The calculations were carried out using the (Ev,a) values, obtained for the low power of the discharge, i.e. under conditions when the effect of power on the gas temperature is negligible. As can be seen, the difference between the actual and calculated characteristics considerably deepens above the specific energy of ∼1 W 3 h/N dm3, which corresponds to the power of 4 W. Figure 8 depicts the energy efficiencies of the process vs the ozone content, which were experimentally obtained at the cooling liquid temperatures of 0, 25, and 50 °C, as well the corresponding calculated relations (dashed lines). At all temperatures, the energy efficiency rapidly decreases when the ozone content increases. At the 6056



ARTICLE

Figure 9. Macroscopic rate constants (formation kþ and decomposition k) of the process vs the temperatures of the cooling liquid Tc = 0, 25, and 50 °C.

Figure 7. Ozone content a2 vs active power; temperature of cooling liquid Tc = 0, 25, and 50 °C; initial ozone content a1 = 0 mol %. Experimental data (solid lines) and characteristics calculated on the basis of eq 5 (dashed lines).

Figure 10. Schematic diagram of the temperatures in the microdischarge channel under stationary state conditions in the discharge gap (Tc, P, V0 = const). The increase of temperature in the microdischarge channel has no influence on the average temperature of the gas Tg.

describing the relation between the rate constants and the ozone boundary contents ae in the gas (denotations as in the formula 5): 1 k ¼ kþ 1 ð12Þ ae

Figure 8. Energy efficiency vs ozone content; temperature of cooling liquid Tc = 0, 25, and 50 °C; initial ozone content a1 = 0 mol %. Experimental data (solid lines) and characteristics calculated on the basis of eq 5 (dashed lines).

cooling liquid temperatures of 0 and 25 °C, the essential difference between the course of the actual and calculated characteristics is evident when the power exceeds 5 W. At 50 °C that difference appears already at ∼2 W. The effect of the insufficient efficiency of the heat from the gap removal in the range of higher powers (over 10 W, i.e. the specific energy equal 2.5 W 3 h/N dm3) is evident at all temperatures of the cooling liquid. 3.3. Macrokinetics of the Process. The experimental data (Ev,a) and the determined boundary ozone contents ae in the gas leaving the ozonizer were used for the calculation of ozone formation kþ and decomposition k macroscopic rate constants. The calculations were made using formula 5 and the dependence

As can be seen in Figure 9, in the range of the cooling liquid temperature of 050 °C the ozone formation rate constant is independent of temperature, whereas the value of the ozone decomposition rate constant clearly increases. It shows the main reason of ozone concentration decrease. The ozone is decomposed as a result of the reaction, which at the higher temperature occurs faster (in the microdischarge channels). Obviously, the temperature of the cooling liquid used is only an easy to control equivalent parameter. The chemical reaction occurs in the discharge gap at temperatures suitably higher (see Figure 10). The change of the cooling liquid temperature corresponds, however, to the change of both the gas and microdischarge channels temperature levels. At the steady-state conditions the gas temperature Tg is by ΔT higher than that of the cooling liquid Tc (when the energy delivered is low, then ΔT ≈ 0, i.e. Tg ≈ Tc). Independently of the cooling liquid temperature, the temperature Tch in the channel during the short time of its life is high, and after the channel decay (after the occurrence of reactions R1R5) decreases to the Tg level (phase III of the process mechanism). 3.4. Ozone Decomposition. As was shown in Figure 4 (a1 = 5.4 mol % O3, Tc = 50 °C, P = 0 W), under the high temperature of the cooling liquid conditions the ozone outlet concentration is equal to the inlet one. The concentrated ozone is not decomposed despite the long residence time of the gas inside the heated reactor (∼30 s). Its decomposition starts while the discharges 6057



ARTICLE

occur in the gap. As can be seen, therefore, in the range of temperatures occurring in the actual ozonizers, even when the ozone concentration is high (here ∼120 g/Nm3), its decomposition is a result of the discharges and not the gas temperature. It cannot, however, be ruled out that if the ozone concentration were considerably higher the ozone decomposition at 50 °C could be noticeable (without the discharges participation). The widespread enough opinion that the ozone decomposition is of a thermal nature is not sufficiently accurate. Ozone thermal destruction, i.e. O3 þ M f O2 þ O þ M k7 ¼ 1:65 109 expð 2270=ðRTch ÞÞ ð½M ¼ 0:06½O þ 0:44½O2 þ ½O3 þ 0:44½OÞð6Þ ðR7Þ actually occurs at high temperatures. In devices for ozone thermal (noncatalytic) destruction the process of ozone decomposition is conducted at the temperatures somewhat exceeding 200 °C. The results of investigations on ozone synthesis, in which the ozone concentrations decrease when the cooling liquid temperature rises, have deepened the conviction about ozone thermal decomposition. Meanwhile, under DBD ozone synthesis conditions, the gas temperature in the gap is generally so low that reaction R7 has no practical meaning. Typically, the increase in gas temperature over that of the cooling liquid equals a few degrees,1,22 and it depends on the energy delivered and the heat removal efficiency. As was reported in the Introduction, both the ozone-forming and decomposition reactions occur in the distinct parts of the discharge space (microdischarge channels), in which (during their lives) the temperatures higher than that of the gas surrounding them appear. In phase III of the process the temperatures of the gas in the channels and their surroundings are averaged. Since the gas in the gap is heated by the small volume of the gas being present in the microdischarge channels, in order to heat the gas to a few degrees over the temperature of the cooling liquid, the temperature increase in the channels should be suitably high (at least dozens of degrees). In other words, the temperature in the microdischarge channel (where the reaction occurs) significantly differs from those of both the cooling liquid and the gas. Among the reactions, the occurrence of which causes a drop in the ozone concentration with a temperature increase, especially reaction R5 is mentioned.23,24 Assuming that an increase in temperature in the microdischarge channel equals 100 °C (from 273 K up to 373 K), the rate constants can be calculated and the temperature dependence on the reaction rate effect can be estimated. For example, the k5 constant rate of reaction R5 increases as much as 10 times. It points to the sufficient participation of this reaction in the ozone decomposition. The ozone decomposition rate is higher when its concentration in the channel is high. According to Eliasson25 the occurrence of the other reaction is probable, in which the vibrationally excited ozone molecule participates:

O3 þ O f O2 þ O2

k8 ¼ 4 1012ð4Þ

ðR8Þ

This reaction is competitive for R2 reaction. In the discussed temperature range, the reaction R8 rate constant can be from 20 up to 1500 times larger than the k5 rate constant. If the O/3 molecule will not be quickly energetically stabilized, it could be destroyed. Obviously, this results in reduction of a total energy efficiency of the ozone synthesis process.

In the mechanisms describing the ozone decomposition process under electric discharge conditions, attention is also focused on reaction R4. The rate constant k4 depends on the reduced electric field E/N. Usually, it is assumed that it is 3-fold higher than the rate constant of R1 reaction, in which the electrons are also consumed. The rate constants of both the reactions are temperature independent. The balancing effect of the occurrence of these reactions depends on both the local O2 and O3 concentrations in the microdischarge channel during its short lifetime. However, it is worth noticing that there is no direct experimental proof confirming that reaction R4 actually occurs. Its presence in the mechanism of the process arose from the necessity to obtain an agreement between experimental and numerical results.25 Actually, in the microdischarge channels (phase I) the dissociative electron attachment reactions occur,26,27 in which the O3 particles react with the low-energy electrons. As a result the ozone is decomposed: O 3 þ e f O þ O2 O 3 þ e f O2 þ O

ðR9Þ ðR10Þ

Apart from that, other reactions can occur, in which O and O2 anions are also formed: O2 þ e f O þ O

ðR11Þ

O2 þ O2 þ e f O 2 þ O2

ðR12Þ

O2 þ e þ M f O 2 þ M

ðR13Þ

The reactions above are competitive for R1. There are highenergy electrons consumed in them. The occurrence of the R9R13 reactions is the reason for the low energy efficiency of ozone synthesis process. The anions being formed in the microdischarge channels make it necessary to supplement the main mechanism of the process with additional reactions (in phase II): O þ O2 þ M f O3 þ M

ðR14Þ

O2 þ O3 f O3 þ O2

ðR15Þ

O 2 þ O f O3 þ e

ðR16Þ

O3 þ M f O3 þ M þ e

ðR17Þ

O3 þ e f O3 þ e

ðR18Þ

O3 þ e f anions þ neutral products

ðR19Þ

Therefore, the mechanism of the ozone synthesis (and decomposition) process is actually much more complicated. However, the detailed analysis of the ozone decomposition mechanism was not the aim of the work.

4. CONCLUSIONS The method of the measurement of the process rate has allowed not only characterizing the kinetics of the process but also observing the gas rich in ozone behavior under diversified conditions. Due to the elimination of the effect of electric energy on both the gas in the discharge gap and the cooling liquid temperature it was found (the measurements carried out at the active power equal to 0 W) that the opinion stating that the 6058


Industrial & Engineering Chemistry Research ozone destruction is caused by temperature is not sufficiently correct. As shown in Figure 4, even at the high temperature (50 °C) the ozone does not decompose until it is exposed to the action of both electrons and atomic oxygen. The results presented and discussed above allow concluding that: 1. Independently on the conditions of the ozone synthesis process occurrence its rate decreases when the ozone concentration increases. It is not important whether the high ozone concentration is generated in the process occurring in the main ozonizer or the ozone is introduced from another source. As a result of the competitive ozone formation and decomposition reactions the steady-state concentration is obtained, the level of which depends on the conditions in the discharge zone. 2. The ozone formation rate is temperature independent. 3. The ozone decomposition occurs in the microdischarge channels (it competes with the ozone-forming reaction). If the ozone concentration in the discharge space is high, its decomposition is possible also outside the discharge channels; however, the gas residence time in the interelectrode space should be suitably long, and its temperature should be especially high. In practice, when the discharge gap is cooled, the ozone decomposition by the action of the “high” average gas temperature can be omitted, because the main part of the ozone is destroyed under the significantly higher temperature, which prevails in the low volume of the microdischarge channel. Although the duration of such conditions is short (phases I and II of the process), the effect of those actions cannot go un-noticed.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ NOMENCLATURE a = ozone molar fraction in postreaction gas a1, a2 = ozone molar fraction: input and output, respectively ae = maximum (boundary) ozone molar fraction c = molar concentration of the reagents (mol/Nm3) c0O2 = initial concentration of the oxygen (g/Nm3) cO3 = ozone concentration (g/Nm3) c*O3 = ozone boundary concentration (g/Nm3) Δτ = duration time (s) ΔT = increase in gas temperature (°C) Ev = energy factor (kJ/mol); energy stream and mass stream ratio (the equivalent of the specific energy, Ev = 3.6 3 P/w) η = energy efficiency (g/kW 3 h) kd = ozone decomposition constant rate (cm3/J) kf = ozone formation constant rate (cm3/J) kþ = ozone formation constant rate (Nm3/J) k = ozone decomposition constant rate (Nm3/J) P = discharge active power (W) P/V = specific energy (W 3 h/Ndm3) r = ozone synthesis process rate (mol O3/m3s) rd = ozone synthesis rate (molecule/s) rf = ozone decomposition rate (molecule/s) Tc = temperature of the cooling liquid (°C) Tch = temperature of the gas in the microdischarge channel (°C) Tg = temperature of the gas (°C) Tw = temperature of the electrodes walls

ARTICLE

V = gas flow rate (Ndm3/h) V0 = oxygen flow rate in the inlet stream (Ndm3/h; normal conditions: 0 °C, 1 bar) VR = volume of the reaction space (m3) x = conversion degree x1, x2 = oxygen conversion degree: input and output, respectively w = oxygen molar flow rate (mol/h)

’ REFERENCES (1) Eliasson, B.; Hirth, M.; Kogelschatz, U. Ozone synthesis from oxygen in dielectric barrier discharges. J. Phys. D: Appl. Phys. 1987, 20, 1421. (2) Kogelschatz, U.; Eliasson, B.; Hirth, M. Ozone generation from oxygen and air: Discharge physics and reaction mechanism. Ozone: Sci. Eng. 1988, 10, 367. (3) Kogelschatz, U. A numerical model of ozone generation in an oxygen discharge. Proc. 16th Int. Conf. Phenom. Ionized Gases 1983, 240. (4) Hadj-Ziane, S.; Held, B.; Pignolet, P.; Peyrous, R.; Coste, C.; Benas, J. M. Ozone production study in an oxygen-fed wire to cylinder ozonizer. Proc. 10th IOA Ozone World Congr. 1991, 1, 15. (5) Akishev, Yu. C.; Deryughin, A. A.; Karal’nik, W. B.; Kochetov, I. W.; Napartovich, A. P.; Trushkin, N. I. Experimental study and numerical modeling of DC glow discharge under atmospheric pressure conditions (in Russian). Fiz. Plazmy 1994, 20 (6), 571. (6) Sabadil, H.; Bachmann, P.; Kastelewicz, H. Reaktionskinetik der Ozonbildung in der Sauerstoffglimmentladung. Beitr. Plasmaphys. 1980, 20 (4), 283. (7) Lee, H. M.; Chang, M. B. Kinetic modeling of ozone generation via dielectric barrier discharge. Ozone: Sci. Eng. 2004, 26, 551. (8) Soria, C.; Pontiga, F.; Castellanos, A. Plasma chemical and electrical modeling of a negative DC corona in pure oxygen. Plasma Sources Sci. Technol. 2004, 13, 95. (9) Kitayama, J.; Kuzumoto, M. Analysis of ozone generation from air in silent discharge. J. Phys. D: Appl. Phys. 1999, 32, 3032. (10) Pontiga, M. F.; Soria, C.; Castellanos, A.; Skalny, J. D. A study of ozone generation by negative corona discharge through different plasma chemistry models. Ozone: Sci. Eng. 2002, 24, 447. (11) Monge, C.; Peyrous, R.; Held, B. Optimization of a corona wireto-cylinder ozone generator. Comparison with economical criteria. Part I: Oxygen. Ozone: Sci. Eng. 1997, 19, 533. (12) Bes, R. S.; Mora, J. C.; Qederni, A.; Coste, C.; Benas, J. M. The use of the thermal and electronic effect in a cold plasma reactor for ozone synthesis. Ozone: Sci. Eng. 1987, 9, 247. (13) Held, B.; Peyrous, R. Analytic calculations of ozone concentration in an oxygen-fed wire-to-cylinder ozonizer and comparison with the Vasil’ev relation. Eur. Phys. J.: Appl. Phys. 1998, 4, 73. (14) Skalny, J. D.; Mikoviny, T.; Mason, N. J.; Sobek, V. The effect of gaseous diluents on ozone generation from oxygen. Ozone: Sci. Eng. 2002, 23, 329. (15) Schmidt-Szazowski, K.; Jodzis, S. Kinetic characteristics of the catalytic ozone synthesis in electrical discharges. Proc. Eur. Ozone Conf. IOA 1990, 125. (16) Schmidt-Szazowski, K.; Jodzis, S. Kinetic characteristics of the ozone synthesis in ozonizer with silica packing. Acta Phys. Univ. Comenianae 1993, 34, 19. (17) Schmidt-Szazowski, K.; Jodzis, S. Influence of dielectric packing and temperature on characteristics of ozone generation. Acta Phys. Univ. Comenianae 1994, 35 (1), 75. (18) Schmidt-Szazowski, K.; Jodzis, S.; Krawczyk, K.; Mzotek, M.; Gorska, A. Non-equilibrium plasma processes in heterogeneous systems at atmospheric pressure. Curr. Top. Catal. 2006, 5, 39. (19) Jodzis, S. Effect of silica packing on ozone synthesis from oxygen-nitrogen mixtures. Ozone: Sci. Eng. 2003, 25, 63. (20) Yagi, S.; Tanaka, M. Mechanism of ozone generation in air-fed ozonisers. J. Phys. D: Appl. Phys. 1979, 12, 1509. 6059



ARTICLE

(21) Peyrous, R.; Monge, C.; Held, B. Optimization of a corona wireto-cylinder ozonizer. Critical comparison with other authors results. Part II: Air and N2 þ O2 mixtures. Ozone: Sci. Eng. 1998, 20, 317. (22) Ozonek, J.; Fijazkowski, S.; Czerwinski, J. Thermodynamic aspects of equilibrium ozone generation. Plasma Processes Polym. 2007, 4, 701. (23) Deryughin, A.; Napartovich, A.; Gorse, C.; Paniccia, F.; Capitelli, M. Ozone production and de-NOx modeling in dry air electrical discharges. Plasma Chem. Plasma Process. 1997, 17, 79. (24) Chang, M. B.; Wu, S.-J. Experimental study on ozone synthesis via dielectric barrier discharge. Ozone: Sci. Eng. 1997, 19, 241. (25) Samoilovich, V. G.; Gibalov, V.; Kozlov, K. Physical Chemistry of the Barrier Discharge; Deutscher Verlag f€ur Schweisstechnik: Dusseldorf, 1997. (26) Skalny, J. D.; Mikoviny, T.; Matejcik, S.; Mason, N. J. An analysis of mass spectrometric study of negative ions extracted from negative corona discharge in air. Int. J. Mass Spectrom. 2004, 233, 317. (27) Cicman, P.; Francis, M.; Skalny, J. D.; Mikoviny, T.; Matejcik, S.; Mark, T. D. The rate constant for dissociative electron attachment to ozone revisited. Czech. J. Phys. 2003, 53, 181.

6060


Application of Technical Kinetics for Macroscopic Analysis of Ozone

Recommend Documents