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Thermal decomposition of tetrachloroethylene with excess hydrogen
Journal of Industrial and Engineering Chemistry 15 (2009) 510–515
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Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Thermal decomposition of tetrachloroethylene with excess hydrogen Yang-Soo Won Department of Environmental Engineering, Yeungnam University, Gyeongsan City 712-749, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 2 September 2008 Accepted 23 January 2009
Tetrachloroethylene (C2Cl4) was used as a model of highly chlorinated carbon to investigate the thermal decomposition process of chlorocarbons in pyrolytic reaction environment. The pyrolytic reaction of tetrachloroethylene has been conducted in an isothermal tubular flow reactor at 1 atm. total pressure with reaction temperatures 600–850 8C between 0.3 and 2.0 s under excess H2 reaction atmosphere. Complete decay (99%) of the parent reagent was observed at temperature near 850 8C with 1 s reaction time. Major products observed were C2HCl3, C2H2Cl2, C2H3Cl and HCl at below 775 8C. The number and quantity of chlorinated products decrease with increasing temperature. Formation of non-chlorinated hydrocarbons, C2H4, C2H6, CH4 and C2H2 increases as the temperature increases above 800 8C. The poor carbon mass balances (<50%) were given over a wide temperature range, although the decomposition rate of C2Cl4 increases rapidly at higher temperature. They are as expected due to formation of high molecular weight compounds, highly chlorinated species and soot which were not detected by GC analysis of this work quantitatively. The important pyrolytic reaction pathways to describe the important features of reagent decay, intermediate product distributions and carbon mass balances, based upon thermodynamic and kinetic principles, were suggested. The main reaction pathways for formation of major products along with preliminary activation energies and rate constants were given. ß 2009 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Keywords: Pyrolysis Thermal decomposition Tetrachloroethylene Reaction pathway
1. Introduction The incineration can affect the total conversion of hazardous organic compounds to safe, innocuous, thermodynamically controlled, end-products, such as carbon dioxide and water, plus compounds like HCl, which maybe easily scrubbed with existing pollution control equipment. In practice, total conversion to innocuous materials is not easily achieved without considerable effort, and with an incinerator of less than optimum design or operating conditions, stable components in the waste feed may not be totally decomposed [1]. The overall incineration process is complex and involves interactions of chemistry, heat transfer, and fluid dynamic phenomena. Louw et al. [2], for example, note that operating conditions of 1000 K and several minutes are needed to limit incinerators with certain hazardous feeds from emitting intolerable amounts of polychlorinated dibenzodioxins (PCDD). The emission of hazardous organic compounds from poorly designed or inadequately controlled incinerators presents a significant concern to the environment. Hazardous organic compounds are also subject to thermal (pyrolytic) degradation in droplets or in solids feed to incinerators and in other sources not specifically designed or regulated for their disposal [3]. It is
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important to understand the fundamental processes in both oxidative and pyrolytic thermal decomposition kinetics. This will permit development of an optimum combustion process at an ideal level, that can then be utilized for improvements in commercial scale units. One important family of hazardous wastes is the halogenated hydrocarbons. Such wastes include chlorinated methanes and ethanes, vinyl chloride, polychlorinated biphenyls (PCBs) and DDT (dichlorodiphenyltrichloroethylene) and others. In order to utilize incineration more effectively, and to better assess the applicability and limitation of the incineration process, the chemical kinetic steps involved in chlorinated hydrocarbon combustion must be understood in more detail [4,4a]. In addition, the manufacturing of chemicals by the controlled oxidation and pyrolysis of chlorinated hydrocarbons may be possible through the detailed knowledge of their reaction pathways. It is also important to discern if combustion of one and two carbon chlorinated species, which are common solvents can result in formation of higher molecular weight chlorinated aromatic species. These chloro-aromatics could serve as precursors to polychlorinated dibenzofurans (PCDFs) and PCDDs. PCDDs and PCDFs are thought to be possible cancer hazards and known to have non-cancer health effects on humans. Emission of these compounds from incinerators to the atmosphere is the dominant source [5–9]. These chlorinated compounds are known to inhibit hydrocarbon combustion processes, increase the levels of
1226-086X/$ – see front matter ß 2009 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jiec.2009.01.007
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carbon monoxide, and form high molecular weight compounds and soot in flames [10,11]. Chlorocarbons can interact differently with hydrocarbon combustion in each of the two stages of the combustion process. They can serve to accelerate the first pyrolysis and initiation stages by providing a very active chlorine atom (radical accelerates propagation). Chlorocarbons also inhibit oxidation of CO to CO2 in the later-burnout combustion phase. Investigation into the thermal decomposition of chlorinated hydrocarbons has received significant attention in recent years, due to concern for the environmental impact from results of burning these materials. Specifically, there is a consistent observation of known or suspected toxic/carcinogenic chlorocarbons or chloro-oxy carbon species in the effluent from waste incinerators [12]. In this study, tetrachloroethylene (C2Cl4) was used as a model chlorocarbon system to investigate hydrodechlorination processes of highly chlorinated carbon under excess hydrogen atmosphere. This work has focused on intermediate product distributions and the reaction pathways to form various products based on fundamental thermochemical and kinetic principles for pyrolytic C2Cl4 reaction. This work characterized the reactant loss, intermediate product formation as functions of time and temperature to describe the reaction process, and to investigate the feasibility of the formation of the light hydrocarbons, e.g. CH4 or C2H4 from the pyrolytic reaction of C2Cl4.
sampling valves and exhaust. All gas lines to the analytical equipment were held at 170 8C to limit condensation. The quartz tube reactor of 8 mm ID was housed within a three-zone electric tube furnace 32 in. in length equipped with three independent temperature controllers. The actual temperature profile of the tubular reactor was obtained using a type K thermocouple probe moving coaxially within reactor under steady state flow. Tight temperature control resulted in temperature profiles isothermal within 3 8C over 75% of the furnace length for all temperature ranges of this study. A HP 5890II on-line GC with FID was used to determine concentrations of the reaction products. The GC used a 5 ft long by 1.8 in. o.d. stainless steel column packed with 1% Alltech AT-1000 on graphpac GB as the column. A six-port gas sample valve was used to inject sample and was maintained at 170 8C and 1 atm. pressure. Quantitative analysis of HCl was performed for each run. The samples for HCl analysis were collected independent of GC sampling. Reactor effluent was diverted through to bubbler trains containing 0.01 M NaOH before being exhausted to a fume hood. The HCl produced was then calculated based on titration of the bubbler solution with 0.01 M HCl to its phenolphthalein end point.
2. Experimental method
Fig. 2 illustrates the parent reactant, C2Cl4 decay and product distributions identified by GC analysis in hydrogen excess environment. Carbon mole fraction of products relative to initial number of carbon mole of parent injected (C/C0) are plotted versus reaction temperature at 1 s reaction time. Complete destruction (99%) of the C2Cl4 was observed at temperature near 850 8C with residence time 1 s. Major products observed were C2HCl3, C2H2Cl2, C2H3Cl and HCl at below 775 8C. The formations of these as intermediate products increase subsequently to the decrease in C2Cl4, indicating that these chloroethylenes are the main stable products in thermal reaction of C2Cl4 with H2. Formation of C2HCl3 as primary product increases with increasing temperature to a maximum near 725 8C and then drops slowly. Formations of C2H2Cl2 and C2H3Cl also show the same trend with one of C2HCl3 but a maximum around 750 and 775 8C respectively. The concentrations of C2H2Cl2 and C2H3Cl drop as temperature increases up to near 825 8C, where C2H4 increases.
Tetrachloroethylene (C2Cl4) was reacted with excess hydrogen (in the absence of O2) in an isothermal tubular reactor at 1 atm. pressure. The products of such thermal degradation were analyzed systematically by varying the temperature from 525 to 900 8C and the reaction time from 0.3 to 2.0 s. A diagram of the experimental system is shown in Fig. 1. Hydrogen gas was passed through a multi-saturator train held at 0 8C to insure saturation with C2Cl4 at a constant reference temperature for accurate vapor pressure calculation. A second (dilutent) stream of hydrogen gas was used to achieve the desired mole fraction of 4% C2Cl4 that was maintained through whole experiment. The reagent with hydrogen gas was fed continuously into tubular flow reactor in vapor phase. The mixture was preheated to about 200 8C before entering the reactor to improve isothermal temperature control. The reactor effluent was passed through heated transfer lines to the gas chromatograph
3. Results and discussion 3.1. Major product distributions in C2Cl4/H2 reaction system
Fig. 1. Schematic diagram of experimental system.
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The major product, C2H4 is then produced from further reaction of C2HCl3, C2H2Cl2 and C2H3Cl as intermediate products with H2 bath gas. In summary, the highest concentrations of intermediate products were observed as 725 8C for C2HCl3, 750 8C for C2H2Cl2 and 775 8C for C2H3Cl. The one less chlorinated ethylene than parent increases with temperature rise subsequently. This implies the less chlorinated ethylenes are more stable. It is consistent with the bond strengths of C–Cl bonds on chlorinated hydrocarbons which increase with decreasing chlorination [13]. The number and quantity of chlorinated products decrease with increasing temperature. Formation of non-chlorinated hydrocarbons, C2H4, C2H6, CH4 and C2H2 increases as the temperature increases. The only non-chlorinated hydrocarbons are observed at 850 8C. The formations of C2H4 and C2H6 increase rapidly at temperatures above 800 8C where the carbon mass balances increase. The reaction pathways based on experimental results and kinetics will be discussed in next part. The important hydrodechlorination processes of chloroethylenes result from H atom cyclic chain reaction by abstraction and addition displacement reactions [3,4]. The overall reaction scheme based on product distributions of highest concentration chloroethylenes can be illustrated as follows: Cl
Cl
Cl
þH2
þH2
þH2
ð1Þ C2 Cl4 !C2 HCl3 !C2 H3 Cl!C2 H4
ðplus HCl in each stepÞ
3.2. Carbon and chlorine mass balances and trace species distributions in C2Cl4/H2 reaction system Fig. 3 shows the carbon and chlorine mass balances identified by GC analysis and HCl titration as a function of temperature at 1 s reaction time. The poor carbon mass balances are given over a wide temperature range, although the decomposition rate of C2Cl4 increases rapidly at higher temperature. The below 50% carbon mass balances are shown between 650 and 800 8C. The carbon mass balances increase up to 66% at 850 8C with increased formation of non-chlorinated compounds (CH4, C2H4 and C2H6). The poor carbon mass balances between 650 and 800 8C are as expected due to formation of high molecular weight compounds (including PAH and tar), highly chlorinated species and soot (char and fixed carbon) which were not detected by GC analysis of this work quantitatively. The black deposit (film) resulting from formation of tar and soot as a thermodynamic end product was observed in the post-reactor zone and sampling line filter. The carbon mass balances increase where the formations of C2H4 and C2H6 through hydrodechlorination reaction increase rapidly at temperatures above 800 8C. As demonstrated in Fig. 3, the 90–80% of chlorine (plus HCl) mass balances are given over wide range of reaction temperatures.
Fig. 2. Product distribution vs. temperature in C2Cl4/H2 reaction system.
The chlorine mass balances are better than the carbon ones. The C2Cl4 in chlorine mass balances for low (<625 8C) temperature range where is low decomposition of C2Cl4 are about 75%. The HCl in chlorine mass balances exhibits more than 65% at temperatures above 725 8C. A possible explanation of chlorine loss is that highly chlorinated heavy molecular weight species are formed which condense in the post reactor, preventing their detection. The observed product yields from C2CCl4 pyrolysis with excess hydrogen are presented in Table 1. The observed reaction intermediates can be divided into three classes: Class (I) high yield (>5%), Class (II) low yield (<5%), Class (III) undetected compounds by GC analysis method in this work. Class (I) includes C2HCl3, CH2CCl2, C2H3Cl, CH4, C2H4 and C2H6. Class (II) contains C2H2, C2HCl, C2Cl2 and CHClCHCl. The highly chlorinated species (more than 6 chlorine compounds) and higher molecular weight compounds (PAH, tar and soot) belongs to Class (III). The poor carbon mass balances in C2Cl4 reaction system result from Class (III). Previous studies [14,15] attributed the high sooting propensity of chlorinated hydrocarbons to a weaker C–Cl bond compared with C–H bond in pyrolytic reaction atmosphere. Frenklach et al. [15] observed that for chlorinated hydrocarbons with high Cl/H ratio, the formation of C2 species and their contribution to soot formation became significant. The experimental results on carbon mass balance were in good agreement with previous studies [14,16,17] about thermolysis of highly chlorocarbons. The various C2 chlorinated hydrocarbons as main products were detected in Table 1. The chlorinated hydrocarbons such as C2H2Cl2, C2H3Cl, C2H4Cl2, C2HCl and C2Cl2 were observed at lower reaction temperature range. The formation of less chlorinated compounds increases with increasing temperature through the hydrodechlorination processes under excess H2 reaction environment. The small
Table 1 Carbon mass balance in C2Cl4/H2 reaction system (C2Cl4: H2 = 4: 96, reaction time = 1 s). Species
CH4 C2H2 C2H4 C2H6 C2HCl C2H3Cl CH2CCl2 CHClCHCl C2HCl3 C6H6 C2Cl4 Total
Reaction temp. (8C) 600
625
650
675
700
725
750
775
800
850
0.4 ND 0.4 ND 0.3 ND 0.2 ND 1.6 ND 68.8 71.7
0.6 ND 1.3 ND 0.7 ND ND ND 0.8 ND 63.0 66.4
0.7 ND 1.1 0.2 0.5 ND 0.3 ND 1.7 ND 47.5 52.0
0.9 ND 0.9 0.2 0.3 ND 0.8 ND 3.0 0.1 34.3 40.6
1.1 0.1 0.7 0.2 0.3 0.4 2.1 0.3 5.2 0.1 27.1 37.2
1.4 0.2 1.5 0.4 0.3 2.2 4.9 0.6 6.7 0.1 21.5 39.9
2.3 0.9 4.7 1.5 0.4 4.4 6.1 0.7 3.8 0.3 9.7 34.9
3.3 0.9 9.9 3.7 0.4 5.7 6.2 0.6 1.7 0.3 8.8 41.5
4.1 2.5 18.6 8.2 0.6 4.3 3.7 0.3 ND 0.5 7.6 50.2
8.7 0.2 37.5 19.5 ND ND ND ND ND ND 0.1 66.0
ND: less than 0.1% carbon mole.
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Table 2 Kinetic parameters for unimolecular decomposition of C2Cl4. Reaction
Reaction rate parameter A
C2Cl4 ! C2Cl3 + Cl C2Cl4 ! CCl2 + CCl2 a b c
Fig. 3. Carbon and chlorine material balance vs. temperature in C2Cl4/H2 system.
quantities of chlorinated acetylenes, C2HCl and C2Cl2 were found over a wide temperature range, while C2H2Cl2 and C2HCl3 were decomposed through C–C double bond 4-member ring HCl elimination reaction of chloroethylenes which required relatively high energy barrier [18]. Also, the trace amounts (<0.1%) of chloroethanes, C2H5Cl and C2H4Cl2 were found at temperature between 575 and 750 8C. The chloroethanes were decomposed to C2H4 and C2H3Cl via C–C single bond 4-member ring HCl elimination reaction of chloroethanes with lower energy barrier [19,20]. The formation rates of C2H4 and C2H3Cl resulted from decomposition of C2H5Cl and C2H4Cl2 are fast. Benzene formation was observed above 675 8C where C2H2 concentration increased slightly from that at 625 8C. The trace amounts of olefinic C3 and C4 products were detected above 650 8C. The formation of benzene might be due to a ring closure mechanism with olefinic and acetylenic species as intermediates [15,19]. For chlorocarbon pyrolysis, the process involved degradation to smaller and more unsaturated species because Cl atoms were extremely reactive in presence of additive fuels (H2, natural gas or light hydrocarbons). They rapidly abstract hydrogens and were added to unsaturated sites [4,21–23]. The unsaturated molecules then began to condense, leading ultimately to high molecule structures or soot in this pyrolysis process [14,15]. The carbon mass balances increase upto 66% at 850 8C with increased formation of non-chlorinated compounds. Fig. 4 shows the product profiles in the pyrolysis of C2Cl4 as function of reaction time at 725 8C under excess hydrogen atmosphere. The formations of C2HCl3 and C2H2Cl2 increase with a reaction time from 0 to 0.5 s, where C2Cl4 drops quickly. With
a
3.4EI6 2.0E11
86.8 79.4
Ref.
Reaction no.
[18,22,24] [18,24,25]
(1a) (1b)
c
k (600 8C)
6.5 106 2.6 109
A unit: 1/s for unimoecular reaction, cm3/mol s for bimolecular reaction. Ea unit: kcal/mole k unit: mol/cm3 s.
reaction time rise, the C2HCl3 and C2H2Cl2 decrease to be maximum around 0.5 s and the formations of C2H3Cl, C2H4, C2H6 and CH4 increase. Product profiles against reaction time as shown in Fig. 4 demonstrate the similar trend to those against reaction temperature as shown in Fig. 2. The thermodynamically stable products, chloroethylenes were destructed and hydrodechlorinated by H atom cyclic chain abstraction and addition displacement reactions [3,4]. 3.3. Important reaction pathways for products in C2Cl4/H2 reaction system The reaction rate parameters for decomposition of C2Cl4 are listed in reactions (1a) and (1b). The reaction rate constant for reaction (1a) at 600 8C is by three orders of magnitude faster than for reaction (1b). Transition state theory [19] for a simple bond cleavage reaction (1a), estimates a loose configuration and Arrhenius A factor that is much higher than reaction (1b) which has a tight transition state. The decomposition rate increases rapidly at higher temperatures, although not as rapidly as has been observed for other chlorinated compounds. The experimental results are as expected due to formation of high molecular weight compounds, highly chlorinated species and soot resulting from reaction (1a) (Table 2). The initial decay of C2Cl4 was occurred due to Cl simple unimolecular dissociation of parent C2Cl4 to form C2Cl3 radical and Cl atom (reaction (1a)). The C2Cl3 radicals react with H2 bath gas to form primary product, C2HCl3 as listed in reaction (4). The H atom served to accelerate the decomposition of C2Cl4 by a series of chain reactions which resulted in formation of HCl and chlorocarbons, usually with a smaller number of chlorines than that of initial reagent. The H atom is produced from reaction of Cl with H2 bath gas as like reaction (2). The Cl atom from initiation decay of C2Cl4 (reaction (1a)) reacts with H2 to form H and HCl as like reaction (2). The H atom accelerates decomposition of C2Cl4 by Cl abstraction reaction (3). In reaction (3), the H atom is consumed, but the H atom is produced in reaction (5). So, the H atom is not consumed apparently as listed in overall reaction (5). The H cyclic chain reaction plays a catalytic role in the acceleration of C2Cl4 decomposition. This paper does not consider the formation of highly chlorinated species (more than 6 chlorine compounds) and higher molecular weight compounds unidentified by GC analysis of this work.
Cl + H2 ! H + HCl C2Cl4 + H ! C2Cl3 + HCl C2Cl3 + H2 ! C2HC3 + H C2Cl4 + H2 ! C2HCl3
Fig. 4. Product distribution vs. temperature in C2Cl4/H2 reaction system.
Ea
b
A
Ea
Ref.
Reaction no.
4.8E13 1.2E12 4.0E11
5.0 15.0 7.5
[26] [10,18] [3,26]
(2) (3) (4) (5) Overall reactions ((3) and (4))
The other pathway for C2Cl4 decay is H addition displacement reaction. The highly chlorinated ethylenes convert to less chlorinated ethylene by H atom addition. The chloroethylenes are hydrodechlorinated by H addition displacement reactions which are important channels for reducing the chlorine content of
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unsaturated chlorocarbons [3,4,27]. Atomic H can add to C2Cl4 to form [C2HCl4]# where designates the chemically activated adduct. The activated complex [C2HCl4]# and stabilized C2HCl4 can unimolecularly decompose to lower energy product channel (C2HCl3 + Cl) by C–Cl simple bond cleavage of radical. Finally, the one less chlorinated compound, C2HCl3 is produced through H addition displacement reaction. C2 Cl4 þ H $ ½C2 HCl4 # ! C2 HCl3 þ Cl
(6)
As a result of thermochemical consideration [19], one may expect sufficient Cl atom concentration in C2Cl4 pyrolysis reaction system, because the C–Cl bond energy is lower than the C–H bond energy [4,13,18]. The decay of C2HCl3 results from the abstraction (7) by H of Cl from C2HCl3. Reactions (7) and (8) can be represented in a cyclic pathway, driven by unimolecular decomposition (1a) and abstraction reaction (2). The H atoms react with C2HCl3 to form HCl and C2HCl2 (reaction (7)). The C2HCl2 radicals then react with H2 bath gas to regenerate H atoms and to produce a chloroethylene molecule with one less Cl than the parent reaction (8)). Also, H is not consumed apparently as listed in overall reaction (9) as like reaction (5). The H cyclic chain reaction plays a catalytic role in the acceleration of C2HCl3 decomposition.
C2HCl3 + H ! C2HCl2 + HCl C2HCl2 + H2 ! C2H2Cl2 + H C2HCl3 + H2 ! C2H2Cl2 + HCl
A
Ea
Ref.
Reaction no.
3.6E12 8.5E11
9.2 6.7
[26,28] [3,26]
(7) (8) (9) Overall reactions ((7) and (8))
The C2HCl3 converts to less chlorinated ethylene, C2H2Cl2 by H atom addition displacement reaction. The H atom can add to C2HCl3 to form C2H2Cl3 radical as shown in reaction (10). The [C2H2Cl3]# complex is initially ‘‘hot’’ since, in addition to the thermal energy, it contains energy resulting from the formation of the stronger C–H bond relative to p bond broken. Prior to stabilization, it may dissociate back to reactants, become a stabilized radical or b scission (radical simple unimolecular dissociation reaction) to C2H2Cl2 + Cl [18,19,29]. C2 HCl3 þ H $ ½C2 H2 Cl3 # ! C2 H2 Cl2 þ Cl
(10)
The decay of C2H2Cl2 was occurred due to Cl abstraction of C2H2Cl2 by H atom as reaction (11). The H atoms react with C2H2Cl2 and form HCl and C2H2Cl radical. The C2H2Cl radical then reacts, rapidly with H2 to regenerate H atoms and to produce C2H3Cl molecule with one less Cl than the parent compound as like reaction (12). This process is exothermic and will continue on both the parent and product chlorocarbons until only hydrocarbons (and HCl) remain as described in acceleration of C2HCl3 destruction (reactions (7) and (8)).
C2H2Cl2 + H ! C2H2Cl + HCl C2H2Cl + H2 ! C2H3Cl + H C2H2Cl2 + H2 ! C2H3Cl + HCl
A
Ea
Ref.
Reaction no.
1.2E13 6.2E11
5.5 6.0
[26,28] [18,30]
(11) (12) (13) Overall reactions of ((11) and (12))
The C2H3Cl is produced from further reaction of C2H2Cl2 via H addition reaction. This H addition reaction results in formation of C2H3Cl and Cl atom through Cl kick out reaction. C2H2Cl2 is decomposed similar to decomposition of C2HCl3. C2 H2 Cl2 þ H $ ½C2 H3 Cl2 ! C2 H3 Cl þ Cl
(14)
The C2H4 as final product of C2Cl4 reaction system is also formed from further reaction of H abstraction cyclic chain reactions (15) and (16), and C2H3Cl via H addition displacement reaction (18). The
C2H3Cl decay as illustrated in reactions (11)–(14) is explained by a mechanism similar to loss of C2H2Cl2. Finally, the C2H4 as final hydrodechlorinated hydrocarbon is produced in an elevated temperature range.
C2H3Cl + H ! C2H3 + HCl C2H3 + H2 ! C2H4 + H C2H3 + H2 ! C2H4 + HCl
A
Ea
Ref.
Reaction no.
1.0E13 5.0E11
6.5 7.3
[26,28] [26]
(15) (16) (17) Overall reactions ((15) and (16)) (18)
C2H3Cl + H $ [C2H4Cl] ! C2H4 + Cl
The overall reaction scheme based on analysis of major chloroethylene products and thermochemical kinetics estimation can be illustrated as follows: Cl
Cl
Cl
Cl
þH2
þH2
þH2
þH2
C2 Cl4 !C2 HCl3 !C2 H2 Cl2 !C2 H3 Cl!C2 H4 The discussions extend only up to C2 compounds in this study. A limited number of reactions to account for C4 and C6 chlorocarbons formation can be added, but our goal is to understand the lower molecular weight formation reaction. This paper does not consider the formation of highly chlorinated species (more than 6 chlorine compounds) and higher molecular weight compounds beyond experimental results. The formation of C2H6 increases at above 775 8C where the C2H4 rapidly increases as shown in Fig. 2. This implies that the formation of C2H6 is associated with C2H4. The addition H to C2H4 occurs at a higher rate because of lower barrier to the addition [29], and results in C2H5. The C2H5 radical reacts with H2 bath gas to produce C2H6 by reaction (20).
C2H4 + H ! C2H5 C2H5 + H2 ! C2H6 + H
A
Ea
Ref.
Reaction no.
4.0E13 3.5E12
2.6 13.2
[29,30] [26]
(19) (20)
The formation of CH4 increases at above 800 8C where the C2H6 rapidly increases. This indicates that the formation of CH4 results from decomposition of C2H6 (reaction (21)). The C–C bond energy for CH3CH3 (89 kcal/mole) is lower than the C–H bond energy (98 kcal/mole) [19]. The formation of CH3 radical is themodynamically favorable over one of C2H5 + H. Also, the reaction (21) can be occurred in high temperature range, even though it requires relatively high-energy barrier [30]. The CH3 radical reacts with H2 bath gas to produce CH4 by reaction (22).
CH3CH3 ! CH3 + CH3 CH3 + H2 ! CH4 + H
A
Ea
Ref.
Reaction no.
7.9E16 3.3E12
89.4 12.5
[19,29] [26]
(21) (22)
The small quantities of C2HCl were detected over a whole reaction temperature range (650–900 8C), while the CH2CCl2 was decomposed through 4-member ring HCl elimination reaction (23) as described in early part of discussion. Another reaction pathway for formation of C2HCl is reaction (24), and the C2HCl is formed from further reaction of CH2CCl via b scission (25). From Fig. 2, the C2H2 increases with increasing temperature to a maximum near 800 8C and then drops quickly. C2H2 was produced from 4-member ring HCl elimination of C2H3Cl (reaction (26)) and b scission of C2H3 (reaction (27)).
CH2CCl2 ! C2HCl + HCl CH2CCl2 + H ! CH2CCl + HCl CH2CCl ! C2HCl + H C2H3Cl ! C2H2 + HCl C2H3Cl + H ! C2H3 + HCl C2H3 ! C2H2 + H
A
Ea
Ref.
Reaction no.
1.1E14 1.2E13 6.2E12 5.3E13 1.0E13 1.6E12
69.1 5.5 38.5 68.7 6.5 38.3
[18,24] [26,28] [24,30] [31,30] [26,28] [24,29]
(23) (24) (25) (26) (27) (28)
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near 850 8C with residence time 1 s. Major products observed were C2HCl3, C2H2Cl2, C2H3Cl and HCl at below 775 8C. The formations of these as intermediate products increase subsequently to the decrease in C2Cl4. The number and quantity of chlorinated products decrease with increasing temperature. Formation of non-chlorinated hydrocarbons, C2H4, C2H6, CH4 and C2H2 increases as the temperature increases above 800 8C. The poor carbon mass balances (<50%) are given over a wide temperature range, although the decomposition rate of C2Cl4 increases rapidly at higher temperature. They are as expected due to formation of high molecular weight compounds, highly chlorinated species and soot which were not detected by GC analysis of this work quantitatively. The carbon mass balances increase up to 66% at 850 8C with increased formation of non-chlorinated compounds (CH4, C2H4 and C2H6). The main formation pathways for hydrodechlorinated products resulted from H atom cyclic chain reactions by abstraction and addition replacement. The important pyrolytic reaction pathways to describe the important features of reagent decay, intermediate product distributions and carbon mass balances, based upon thermochemical and kinetic principles, were suggested. The results of this work provided a better understanding of pyrolytic decomposition processes which occur during the pyrolysis of C2Cl4 and similar chlorinated hydrocarbons. Acknowledgement This research was supported by the Yeungnam University research grants in 2007. References [1] [2] [3] [4] Fig. 5. Outline of reaction pathways for intermediate products in C2Cl4/H2 reaction system.
Formations of benzene and chlorobenzene were observed above 750 8C, where C2H2 known as the precursor of benzene increases slightly from that at around 725 8C and then decreases as more benzene is formed as listed in Table 1. The unsaturated molecules begin to condense, leading ultimately to high molecules structure, aromatics or soot in pyrolysis [14,15]. Fig. 5 summarized the main reaction pathways to form hydrodechlorinated products from C2Cl4. This overall reaction scheme based upon analysis of the observed product distributions and thermochemical kinetics estimation was illustrated in Fig. 5. 4. Conclusions The thermal decomposition of C2Cl4 in excess hydrogen atmosphere was studied to investigate the pyrolytic reaction pathways of chlorocarbon. The reactions were conducted in an isothermal tubular reactor at a total pressure of 1 atm. with reaction times of 0.3–2.0 s between 600 and 850 8C. Complete destruction (99%) of the parent, C2Cl4 was observed at temperature
[5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31]
J.I. Baker, R.A. Hites, Environ. Sci. Technol. 34 (2000) 2879. R. Louw, J.H.M. Dijks, P. Mulder, Chem. Ind. (2004). Y.P. Wu, Y.S. Won, J. Ind. Eng. Chem. 9 (2003) 775. (a) Y.P. Wu, Y.S. Won, J. Hazard. Mater. B105 (2003) 63; (b) S. Maken, A. Gaur, N. Verma, K.C. Singh, S. Lee, J.W. Park, J. Ind. Eng. Chem. 13 (2007) 1098; (c) S.C. Oh, W.T. Kwon, S.R. Kim, J. Ind. Eng. Chem. 15 (2009) 438. B.K. Gullett, A. Touati, C.W. Lee, Environ. Sci. Technol. 34 (2000) 2069. K. Jay, L. Stieglitz, Chemosphere 22 (1991) 987. J.W. Bozzelli, Y.P. Wu, E.R. Ritter, Chemosphere 23 (1991) 1221. L.C. Dickson, D. Lenoir, Eviron. Sci. Technol. 26 (1992) 1822. Special Report: EPA Dioxin Risk Assessment, 24A (2005). W.D. Chang, S.B. Karra, S.M. Senkan, Combust. Flame 49 (1986) 107. S.B. Karra, D. Gutman, S.M. Senkan, Combust. Sci. Technol. 60 (1988) 45. W.A. Rubey, B. Dellinger, D.L. Hall, Chemosphere 14 (1985) 1483. Y.S. Won, J. Ind. Eng. Chem. 13 (2007) 400. D.A. Tirey, P.H. Taylor, J. Kasner, B. Dellinger, Combust. Sci. Technol. 74 (1990) 137. M. Frenklach, D.W. Clary, T. Yuan, Combust. Sci. Technol. 50 (1986) 79. M. Xieqi, B. Cicek, S.M. Senken, Combust. Flame 94 (1993) 131. G.P. Miller, Combust. Flame 101 (1995) 101. Y.S. Won, J.W. Bozzelli, Combust. Sci. Technol. 85 (1992) 345. S.W. Benson, Thermochemical Kinetics, John Wiley and Son, 1976. Y.S. Won, J.W. Bozzelli, Am. Soc. Mech. Eng. HTD 104 (1988) 131. P.H. Taylor, B. Dellinger, D.A. Tirey, Int. J. Chem. Kinet. 23 (1991) 1051. W. Tsang, Combust. Sci. Technol. 74 (1990) 99. M. Weissman, S.W. Benson, J. Phys. Chem. 87 (2) (1983) 243. D. Allara, R. Shaw, Phys. Chem. Ref. Data 9 (1981) 523. S.C. Chuang, J.W. Bozzelli, Environ. Sci Technol. 20 (1986) 568. J.A. Kerr, S.J. Moss, Handbook of Bimolecular and Termolercular Gas Reactions, CRC Press, Florida, 1981. W. Chang, S.M. Senkan, Environ. Sci. Technol. 23 (4) (1989) 442. NIST, Chemical Gas Kinetics Database, Version 7.0 (2006). A.M. Dean, J. Phys. Chem. 89 (1985) 4600. Y.P. Wu, Y.S. Won, Combust. Flame 122 (2000) 312. F. Zabel, Int. J. Chem. Kinet. 9 (1977) 651.
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Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Thermal decomposition of tetrachloroethylene with excess hydrogen Yang-Soo Won Department of Environmental Engineering, Yeungnam University, Gyeongsan City 712-749, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 2 September 2008 Accepted 23 January 2009
Tetrachloroethylene (C2Cl4) was used as a model of highly chlorinated carbon to investigate the thermal decomposition process of chlorocarbons in pyrolytic reaction environment. The pyrolytic reaction of tetrachloroethylene has been conducted in an isothermal tubular flow reactor at 1 atm. total pressure with reaction temperatures 600–850 8C between 0.3 and 2.0 s under excess H2 reaction atmosphere. Complete decay (99%) of the parent reagent was observed at temperature near 850 8C with 1 s reaction time. Major products observed were C2HCl3, C2H2Cl2, C2H3Cl and HCl at below 775 8C. The number and quantity of chlorinated products decrease with increasing temperature. Formation of non-chlorinated hydrocarbons, C2H4, C2H6, CH4 and C2H2 increases as the temperature increases above 800 8C. The poor carbon mass balances (<50%) were given over a wide temperature range, although the decomposition rate of C2Cl4 increases rapidly at higher temperature. They are as expected due to formation of high molecular weight compounds, highly chlorinated species and soot which were not detected by GC analysis of this work quantitatively. The important pyrolytic reaction pathways to describe the important features of reagent decay, intermediate product distributions and carbon mass balances, based upon thermodynamic and kinetic principles, were suggested. The main reaction pathways for formation of major products along with preliminary activation energies and rate constants were given. ß 2009 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Keywords: Pyrolysis Thermal decomposition Tetrachloroethylene Reaction pathway
1. Introduction The incineration can affect the total conversion of hazardous organic compounds to safe, innocuous, thermodynamically controlled, end-products, such as carbon dioxide and water, plus compounds like HCl, which maybe easily scrubbed with existing pollution control equipment. In practice, total conversion to innocuous materials is not easily achieved without considerable effort, and with an incinerator of less than optimum design or operating conditions, stable components in the waste feed may not be totally decomposed [1]. The overall incineration process is complex and involves interactions of chemistry, heat transfer, and fluid dynamic phenomena. Louw et al. [2], for example, note that operating conditions of 1000 K and several minutes are needed to limit incinerators with certain hazardous feeds from emitting intolerable amounts of polychlorinated dibenzodioxins (PCDD). The emission of hazardous organic compounds from poorly designed or inadequately controlled incinerators presents a significant concern to the environment. Hazardous organic compounds are also subject to thermal (pyrolytic) degradation in droplets or in solids feed to incinerators and in other sources not specifically designed or regulated for their disposal [3]. It is
E-mail address: [email protected].
important to understand the fundamental processes in both oxidative and pyrolytic thermal decomposition kinetics. This will permit development of an optimum combustion process at an ideal level, that can then be utilized for improvements in commercial scale units. One important family of hazardous wastes is the halogenated hydrocarbons. Such wastes include chlorinated methanes and ethanes, vinyl chloride, polychlorinated biphenyls (PCBs) and DDT (dichlorodiphenyltrichloroethylene) and others. In order to utilize incineration more effectively, and to better assess the applicability and limitation of the incineration process, the chemical kinetic steps involved in chlorinated hydrocarbon combustion must be understood in more detail [4,4a]. In addition, the manufacturing of chemicals by the controlled oxidation and pyrolysis of chlorinated hydrocarbons may be possible through the detailed knowledge of their reaction pathways. It is also important to discern if combustion of one and two carbon chlorinated species, which are common solvents can result in formation of higher molecular weight chlorinated aromatic species. These chloro-aromatics could serve as precursors to polychlorinated dibenzofurans (PCDFs) and PCDDs. PCDDs and PCDFs are thought to be possible cancer hazards and known to have non-cancer health effects on humans. Emission of these compounds from incinerators to the atmosphere is the dominant source [5–9]. These chlorinated compounds are known to inhibit hydrocarbon combustion processes, increase the levels of
1226-086X/$ – see front matter ß 2009 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jiec.2009.01.007
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carbon monoxide, and form high molecular weight compounds and soot in flames [10,11]. Chlorocarbons can interact differently with hydrocarbon combustion in each of the two stages of the combustion process. They can serve to accelerate the first pyrolysis and initiation stages by providing a very active chlorine atom (radical accelerates propagation). Chlorocarbons also inhibit oxidation of CO to CO2 in the later-burnout combustion phase. Investigation into the thermal decomposition of chlorinated hydrocarbons has received significant attention in recent years, due to concern for the environmental impact from results of burning these materials. Specifically, there is a consistent observation of known or suspected toxic/carcinogenic chlorocarbons or chloro-oxy carbon species in the effluent from waste incinerators [12]. In this study, tetrachloroethylene (C2Cl4) was used as a model chlorocarbon system to investigate hydrodechlorination processes of highly chlorinated carbon under excess hydrogen atmosphere. This work has focused on intermediate product distributions and the reaction pathways to form various products based on fundamental thermochemical and kinetic principles for pyrolytic C2Cl4 reaction. This work characterized the reactant loss, intermediate product formation as functions of time and temperature to describe the reaction process, and to investigate the feasibility of the formation of the light hydrocarbons, e.g. CH4 or C2H4 from the pyrolytic reaction of C2Cl4.
sampling valves and exhaust. All gas lines to the analytical equipment were held at 170 8C to limit condensation. The quartz tube reactor of 8 mm ID was housed within a three-zone electric tube furnace 32 in. in length equipped with three independent temperature controllers. The actual temperature profile of the tubular reactor was obtained using a type K thermocouple probe moving coaxially within reactor under steady state flow. Tight temperature control resulted in temperature profiles isothermal within 3 8C over 75% of the furnace length for all temperature ranges of this study. A HP 5890II on-line GC with FID was used to determine concentrations of the reaction products. The GC used a 5 ft long by 1.8 in. o.d. stainless steel column packed with 1% Alltech AT-1000 on graphpac GB as the column. A six-port gas sample valve was used to inject sample and was maintained at 170 8C and 1 atm. pressure. Quantitative analysis of HCl was performed for each run. The samples for HCl analysis were collected independent of GC sampling. Reactor effluent was diverted through to bubbler trains containing 0.01 M NaOH before being exhausted to a fume hood. The HCl produced was then calculated based on titration of the bubbler solution with 0.01 M HCl to its phenolphthalein end point.
2. Experimental method
Fig. 2 illustrates the parent reactant, C2Cl4 decay and product distributions identified by GC analysis in hydrogen excess environment. Carbon mole fraction of products relative to initial number of carbon mole of parent injected (C/C0) are plotted versus reaction temperature at 1 s reaction time. Complete destruction (99%) of the C2Cl4 was observed at temperature near 850 8C with residence time 1 s. Major products observed were C2HCl3, C2H2Cl2, C2H3Cl and HCl at below 775 8C. The formations of these as intermediate products increase subsequently to the decrease in C2Cl4, indicating that these chloroethylenes are the main stable products in thermal reaction of C2Cl4 with H2. Formation of C2HCl3 as primary product increases with increasing temperature to a maximum near 725 8C and then drops slowly. Formations of C2H2Cl2 and C2H3Cl also show the same trend with one of C2HCl3 but a maximum around 750 and 775 8C respectively. The concentrations of C2H2Cl2 and C2H3Cl drop as temperature increases up to near 825 8C, where C2H4 increases.
Tetrachloroethylene (C2Cl4) was reacted with excess hydrogen (in the absence of O2) in an isothermal tubular reactor at 1 atm. pressure. The products of such thermal degradation were analyzed systematically by varying the temperature from 525 to 900 8C and the reaction time from 0.3 to 2.0 s. A diagram of the experimental system is shown in Fig. 1. Hydrogen gas was passed through a multi-saturator train held at 0 8C to insure saturation with C2Cl4 at a constant reference temperature for accurate vapor pressure calculation. A second (dilutent) stream of hydrogen gas was used to achieve the desired mole fraction of 4% C2Cl4 that was maintained through whole experiment. The reagent with hydrogen gas was fed continuously into tubular flow reactor in vapor phase. The mixture was preheated to about 200 8C before entering the reactor to improve isothermal temperature control. The reactor effluent was passed through heated transfer lines to the gas chromatograph
3. Results and discussion 3.1. Major product distributions in C2Cl4/H2 reaction system
Fig. 1. Schematic diagram of experimental system.
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The major product, C2H4 is then produced from further reaction of C2HCl3, C2H2Cl2 and C2H3Cl as intermediate products with H2 bath gas. In summary, the highest concentrations of intermediate products were observed as 725 8C for C2HCl3, 750 8C for C2H2Cl2 and 775 8C for C2H3Cl. The one less chlorinated ethylene than parent increases with temperature rise subsequently. This implies the less chlorinated ethylenes are more stable. It is consistent with the bond strengths of C–Cl bonds on chlorinated hydrocarbons which increase with decreasing chlorination [13]. The number and quantity of chlorinated products decrease with increasing temperature. Formation of non-chlorinated hydrocarbons, C2H4, C2H6, CH4 and C2H2 increases as the temperature increases. The only non-chlorinated hydrocarbons are observed at 850 8C. The formations of C2H4 and C2H6 increase rapidly at temperatures above 800 8C where the carbon mass balances increase. The reaction pathways based on experimental results and kinetics will be discussed in next part. The important hydrodechlorination processes of chloroethylenes result from H atom cyclic chain reaction by abstraction and addition displacement reactions [3,4]. The overall reaction scheme based on product distributions of highest concentration chloroethylenes can be illustrated as follows: Cl
Cl
Cl
þH2
þH2
þH2
ð1Þ C2 Cl4 !C2 HCl3 !C2 H3 Cl!C2 H4
ðplus HCl in each stepÞ
3.2. Carbon and chlorine mass balances and trace species distributions in C2Cl4/H2 reaction system Fig. 3 shows the carbon and chlorine mass balances identified by GC analysis and HCl titration as a function of temperature at 1 s reaction time. The poor carbon mass balances are given over a wide temperature range, although the decomposition rate of C2Cl4 increases rapidly at higher temperature. The below 50% carbon mass balances are shown between 650 and 800 8C. The carbon mass balances increase up to 66% at 850 8C with increased formation of non-chlorinated compounds (CH4, C2H4 and C2H6). The poor carbon mass balances between 650 and 800 8C are as expected due to formation of high molecular weight compounds (including PAH and tar), highly chlorinated species and soot (char and fixed carbon) which were not detected by GC analysis of this work quantitatively. The black deposit (film) resulting from formation of tar and soot as a thermodynamic end product was observed in the post-reactor zone and sampling line filter. The carbon mass balances increase where the formations of C2H4 and C2H6 through hydrodechlorination reaction increase rapidly at temperatures above 800 8C. As demonstrated in Fig. 3, the 90–80% of chlorine (plus HCl) mass balances are given over wide range of reaction temperatures.
Fig. 2. Product distribution vs. temperature in C2Cl4/H2 reaction system.
The chlorine mass balances are better than the carbon ones. The C2Cl4 in chlorine mass balances for low (<625 8C) temperature range where is low decomposition of C2Cl4 are about 75%. The HCl in chlorine mass balances exhibits more than 65% at temperatures above 725 8C. A possible explanation of chlorine loss is that highly chlorinated heavy molecular weight species are formed which condense in the post reactor, preventing their detection. The observed product yields from C2CCl4 pyrolysis with excess hydrogen are presented in Table 1. The observed reaction intermediates can be divided into three classes: Class (I) high yield (>5%), Class (II) low yield (<5%), Class (III) undetected compounds by GC analysis method in this work. Class (I) includes C2HCl3, CH2CCl2, C2H3Cl, CH4, C2H4 and C2H6. Class (II) contains C2H2, C2HCl, C2Cl2 and CHClCHCl. The highly chlorinated species (more than 6 chlorine compounds) and higher molecular weight compounds (PAH, tar and soot) belongs to Class (III). The poor carbon mass balances in C2Cl4 reaction system result from Class (III). Previous studies [14,15] attributed the high sooting propensity of chlorinated hydrocarbons to a weaker C–Cl bond compared with C–H bond in pyrolytic reaction atmosphere. Frenklach et al. [15] observed that for chlorinated hydrocarbons with high Cl/H ratio, the formation of C2 species and their contribution to soot formation became significant. The experimental results on carbon mass balance were in good agreement with previous studies [14,16,17] about thermolysis of highly chlorocarbons. The various C2 chlorinated hydrocarbons as main products were detected in Table 1. The chlorinated hydrocarbons such as C2H2Cl2, C2H3Cl, C2H4Cl2, C2HCl and C2Cl2 were observed at lower reaction temperature range. The formation of less chlorinated compounds increases with increasing temperature through the hydrodechlorination processes under excess H2 reaction environment. The small
Table 1 Carbon mass balance in C2Cl4/H2 reaction system (C2Cl4: H2 = 4: 96, reaction time = 1 s). Species
CH4 C2H2 C2H4 C2H6 C2HCl C2H3Cl CH2CCl2 CHClCHCl C2HCl3 C6H6 C2Cl4 Total
Reaction temp. (8C) 600
625
650
675
700
725
750
775
800
850
0.4 ND 0.4 ND 0.3 ND 0.2 ND 1.6 ND 68.8 71.7
0.6 ND 1.3 ND 0.7 ND ND ND 0.8 ND 63.0 66.4
0.7 ND 1.1 0.2 0.5 ND 0.3 ND 1.7 ND 47.5 52.0
0.9 ND 0.9 0.2 0.3 ND 0.8 ND 3.0 0.1 34.3 40.6
1.1 0.1 0.7 0.2 0.3 0.4 2.1 0.3 5.2 0.1 27.1 37.2
1.4 0.2 1.5 0.4 0.3 2.2 4.9 0.6 6.7 0.1 21.5 39.9
2.3 0.9 4.7 1.5 0.4 4.4 6.1 0.7 3.8 0.3 9.7 34.9
3.3 0.9 9.9 3.7 0.4 5.7 6.2 0.6 1.7 0.3 8.8 41.5
4.1 2.5 18.6 8.2 0.6 4.3 3.7 0.3 ND 0.5 7.6 50.2
8.7 0.2 37.5 19.5 ND ND ND ND ND ND 0.1 66.0
ND: less than 0.1% carbon mole.
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Table 2 Kinetic parameters for unimolecular decomposition of C2Cl4. Reaction
Reaction rate parameter A
C2Cl4 ! C2Cl3 + Cl C2Cl4 ! CCl2 + CCl2 a b c
Fig. 3. Carbon and chlorine material balance vs. temperature in C2Cl4/H2 system.
quantities of chlorinated acetylenes, C2HCl and C2Cl2 were found over a wide temperature range, while C2H2Cl2 and C2HCl3 were decomposed through C–C double bond 4-member ring HCl elimination reaction of chloroethylenes which required relatively high energy barrier [18]. Also, the trace amounts (<0.1%) of chloroethanes, C2H5Cl and C2H4Cl2 were found at temperature between 575 and 750 8C. The chloroethanes were decomposed to C2H4 and C2H3Cl via C–C single bond 4-member ring HCl elimination reaction of chloroethanes with lower energy barrier [19,20]. The formation rates of C2H4 and C2H3Cl resulted from decomposition of C2H5Cl and C2H4Cl2 are fast. Benzene formation was observed above 675 8C where C2H2 concentration increased slightly from that at 625 8C. The trace amounts of olefinic C3 and C4 products were detected above 650 8C. The formation of benzene might be due to a ring closure mechanism with olefinic and acetylenic species as intermediates [15,19]. For chlorocarbon pyrolysis, the process involved degradation to smaller and more unsaturated species because Cl atoms were extremely reactive in presence of additive fuels (H2, natural gas or light hydrocarbons). They rapidly abstract hydrogens and were added to unsaturated sites [4,21–23]. The unsaturated molecules then began to condense, leading ultimately to high molecule structures or soot in this pyrolysis process [14,15]. The carbon mass balances increase upto 66% at 850 8C with increased formation of non-chlorinated compounds. Fig. 4 shows the product profiles in the pyrolysis of C2Cl4 as function of reaction time at 725 8C under excess hydrogen atmosphere. The formations of C2HCl3 and C2H2Cl2 increase with a reaction time from 0 to 0.5 s, where C2Cl4 drops quickly. With
a
3.4EI6 2.0E11
86.8 79.4
Ref.
Reaction no.
[18,22,24] [18,24,25]
(1a) (1b)
c
k (600 8C)
6.5 106 2.6 109
A unit: 1/s for unimoecular reaction, cm3/mol s for bimolecular reaction. Ea unit: kcal/mole k unit: mol/cm3 s.
reaction time rise, the C2HCl3 and C2H2Cl2 decrease to be maximum around 0.5 s and the formations of C2H3Cl, C2H4, C2H6 and CH4 increase. Product profiles against reaction time as shown in Fig. 4 demonstrate the similar trend to those against reaction temperature as shown in Fig. 2. The thermodynamically stable products, chloroethylenes were destructed and hydrodechlorinated by H atom cyclic chain abstraction and addition displacement reactions [3,4]. 3.3. Important reaction pathways for products in C2Cl4/H2 reaction system The reaction rate parameters for decomposition of C2Cl4 are listed in reactions (1a) and (1b). The reaction rate constant for reaction (1a) at 600 8C is by three orders of magnitude faster than for reaction (1b). Transition state theory [19] for a simple bond cleavage reaction (1a), estimates a loose configuration and Arrhenius A factor that is much higher than reaction (1b) which has a tight transition state. The decomposition rate increases rapidly at higher temperatures, although not as rapidly as has been observed for other chlorinated compounds. The experimental results are as expected due to formation of high molecular weight compounds, highly chlorinated species and soot resulting from reaction (1a) (Table 2). The initial decay of C2Cl4 was occurred due to Cl simple unimolecular dissociation of parent C2Cl4 to form C2Cl3 radical and Cl atom (reaction (1a)). The C2Cl3 radicals react with H2 bath gas to form primary product, C2HCl3 as listed in reaction (4). The H atom served to accelerate the decomposition of C2Cl4 by a series of chain reactions which resulted in formation of HCl and chlorocarbons, usually with a smaller number of chlorines than that of initial reagent. The H atom is produced from reaction of Cl with H2 bath gas as like reaction (2). The Cl atom from initiation decay of C2Cl4 (reaction (1a)) reacts with H2 to form H and HCl as like reaction (2). The H atom accelerates decomposition of C2Cl4 by Cl abstraction reaction (3). In reaction (3), the H atom is consumed, but the H atom is produced in reaction (5). So, the H atom is not consumed apparently as listed in overall reaction (5). The H cyclic chain reaction plays a catalytic role in the acceleration of C2Cl4 decomposition. This paper does not consider the formation of highly chlorinated species (more than 6 chlorine compounds) and higher molecular weight compounds unidentified by GC analysis of this work.
Cl + H2 ! H + HCl C2Cl4 + H ! C2Cl3 + HCl C2Cl3 + H2 ! C2HC3 + H C2Cl4 + H2 ! C2HCl3
Fig. 4. Product distribution vs. temperature in C2Cl4/H2 reaction system.
Ea
b
A
Ea
Ref.
Reaction no.
4.8E13 1.2E12 4.0E11
5.0 15.0 7.5
[26] [10,18] [3,26]
(2) (3) (4) (5) Overall reactions ((3) and (4))
The other pathway for C2Cl4 decay is H addition displacement reaction. The highly chlorinated ethylenes convert to less chlorinated ethylene by H atom addition. The chloroethylenes are hydrodechlorinated by H addition displacement reactions which are important channels for reducing the chlorine content of
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unsaturated chlorocarbons [3,4,27]. Atomic H can add to C2Cl4 to form [C2HCl4]# where designates the chemically activated adduct. The activated complex [C2HCl4]# and stabilized C2HCl4 can unimolecularly decompose to lower energy product channel (C2HCl3 + Cl) by C–Cl simple bond cleavage of radical. Finally, the one less chlorinated compound, C2HCl3 is produced through H addition displacement reaction. C2 Cl4 þ H $ ½C2 HCl4 # ! C2 HCl3 þ Cl
(6)
As a result of thermochemical consideration [19], one may expect sufficient Cl atom concentration in C2Cl4 pyrolysis reaction system, because the C–Cl bond energy is lower than the C–H bond energy [4,13,18]. The decay of C2HCl3 results from the abstraction (7) by H of Cl from C2HCl3. Reactions (7) and (8) can be represented in a cyclic pathway, driven by unimolecular decomposition (1a) and abstraction reaction (2). The H atoms react with C2HCl3 to form HCl and C2HCl2 (reaction (7)). The C2HCl2 radicals then react with H2 bath gas to regenerate H atoms and to produce a chloroethylene molecule with one less Cl than the parent reaction (8)). Also, H is not consumed apparently as listed in overall reaction (9) as like reaction (5). The H cyclic chain reaction plays a catalytic role in the acceleration of C2HCl3 decomposition.
C2HCl3 + H ! C2HCl2 + HCl C2HCl2 + H2 ! C2H2Cl2 + H C2HCl3 + H2 ! C2H2Cl2 + HCl
A
Ea
Ref.
Reaction no.
3.6E12 8.5E11
9.2 6.7
[26,28] [3,26]
(7) (8) (9) Overall reactions ((7) and (8))
The C2HCl3 converts to less chlorinated ethylene, C2H2Cl2 by H atom addition displacement reaction. The H atom can add to C2HCl3 to form C2H2Cl3 radical as shown in reaction (10). The [C2H2Cl3]# complex is initially ‘‘hot’’ since, in addition to the thermal energy, it contains energy resulting from the formation of the stronger C–H bond relative to p bond broken. Prior to stabilization, it may dissociate back to reactants, become a stabilized radical or b scission (radical simple unimolecular dissociation reaction) to C2H2Cl2 + Cl [18,19,29]. C2 HCl3 þ H $ ½C2 H2 Cl3 # ! C2 H2 Cl2 þ Cl
(10)
The decay of C2H2Cl2 was occurred due to Cl abstraction of C2H2Cl2 by H atom as reaction (11). The H atoms react with C2H2Cl2 and form HCl and C2H2Cl radical. The C2H2Cl radical then reacts, rapidly with H2 to regenerate H atoms and to produce C2H3Cl molecule with one less Cl than the parent compound as like reaction (12). This process is exothermic and will continue on both the parent and product chlorocarbons until only hydrocarbons (and HCl) remain as described in acceleration of C2HCl3 destruction (reactions (7) and (8)).
C2H2Cl2 + H ! C2H2Cl + HCl C2H2Cl + H2 ! C2H3Cl + H C2H2Cl2 + H2 ! C2H3Cl + HCl
A
Ea
Ref.
Reaction no.
1.2E13 6.2E11
5.5 6.0
[26,28] [18,30]
(11) (12) (13) Overall reactions of ((11) and (12))
The C2H3Cl is produced from further reaction of C2H2Cl2 via H addition reaction. This H addition reaction results in formation of C2H3Cl and Cl atom through Cl kick out reaction. C2H2Cl2 is decomposed similar to decomposition of C2HCl3. C2 H2 Cl2 þ H $ ½C2 H3 Cl2 ! C2 H3 Cl þ Cl
(14)
The C2H4 as final product of C2Cl4 reaction system is also formed from further reaction of H abstraction cyclic chain reactions (15) and (16), and C2H3Cl via H addition displacement reaction (18). The
C2H3Cl decay as illustrated in reactions (11)–(14) is explained by a mechanism similar to loss of C2H2Cl2. Finally, the C2H4 as final hydrodechlorinated hydrocarbon is produced in an elevated temperature range.
C2H3Cl + H ! C2H3 + HCl C2H3 + H2 ! C2H4 + H C2H3 + H2 ! C2H4 + HCl
A
Ea
Ref.
Reaction no.
1.0E13 5.0E11
6.5 7.3
[26,28] [26]
(15) (16) (17) Overall reactions ((15) and (16)) (18)
C2H3Cl + H $ [C2H4Cl] ! C2H4 + Cl
The overall reaction scheme based on analysis of major chloroethylene products and thermochemical kinetics estimation can be illustrated as follows: Cl
Cl
Cl
Cl
þH2
þH2
þH2
þH2
C2 Cl4 !C2 HCl3 !C2 H2 Cl2 !C2 H3 Cl!C2 H4 The discussions extend only up to C2 compounds in this study. A limited number of reactions to account for C4 and C6 chlorocarbons formation can be added, but our goal is to understand the lower molecular weight formation reaction. This paper does not consider the formation of highly chlorinated species (more than 6 chlorine compounds) and higher molecular weight compounds beyond experimental results. The formation of C2H6 increases at above 775 8C where the C2H4 rapidly increases as shown in Fig. 2. This implies that the formation of C2H6 is associated with C2H4. The addition H to C2H4 occurs at a higher rate because of lower barrier to the addition [29], and results in C2H5. The C2H5 radical reacts with H2 bath gas to produce C2H6 by reaction (20).
C2H4 + H ! C2H5 C2H5 + H2 ! C2H6 + H
A
Ea
Ref.
Reaction no.
4.0E13 3.5E12
2.6 13.2
[29,30] [26]
(19) (20)
The formation of CH4 increases at above 800 8C where the C2H6 rapidly increases. This indicates that the formation of CH4 results from decomposition of C2H6 (reaction (21)). The C–C bond energy for CH3CH3 (89 kcal/mole) is lower than the C–H bond energy (98 kcal/mole) [19]. The formation of CH3 radical is themodynamically favorable over one of C2H5 + H. Also, the reaction (21) can be occurred in high temperature range, even though it requires relatively high-energy barrier [30]. The CH3 radical reacts with H2 bath gas to produce CH4 by reaction (22).
CH3CH3 ! CH3 + CH3 CH3 + H2 ! CH4 + H
A
Ea
Ref.
Reaction no.
7.9E16 3.3E12
89.4 12.5
[19,29] [26]
(21) (22)
The small quantities of C2HCl were detected over a whole reaction temperature range (650–900 8C), while the CH2CCl2 was decomposed through 4-member ring HCl elimination reaction (23) as described in early part of discussion. Another reaction pathway for formation of C2HCl is reaction (24), and the C2HCl is formed from further reaction of CH2CCl via b scission (25). From Fig. 2, the C2H2 increases with increasing temperature to a maximum near 800 8C and then drops quickly. C2H2 was produced from 4-member ring HCl elimination of C2H3Cl (reaction (26)) and b scission of C2H3 (reaction (27)).
CH2CCl2 ! C2HCl + HCl CH2CCl2 + H ! CH2CCl + HCl CH2CCl ! C2HCl + H C2H3Cl ! C2H2 + HCl C2H3Cl + H ! C2H3 + HCl C2H3 ! C2H2 + H
A
Ea
Ref.
Reaction no.
1.1E14 1.2E13 6.2E12 5.3E13 1.0E13 1.6E12
69.1 5.5 38.5 68.7 6.5 38.3
[18,24] [26,28] [24,30] [31,30] [26,28] [24,29]
(23) (24) (25) (26) (27) (28)
Y.-S. Won / Journal of Industrial and Engineering Chemistry 15 (2009) 510–515
515
near 850 8C with residence time 1 s. Major products observed were C2HCl3, C2H2Cl2, C2H3Cl and HCl at below 775 8C. The formations of these as intermediate products increase subsequently to the decrease in C2Cl4. The number and quantity of chlorinated products decrease with increasing temperature. Formation of non-chlorinated hydrocarbons, C2H4, C2H6, CH4 and C2H2 increases as the temperature increases above 800 8C. The poor carbon mass balances (<50%) are given over a wide temperature range, although the decomposition rate of C2Cl4 increases rapidly at higher temperature. They are as expected due to formation of high molecular weight compounds, highly chlorinated species and soot which were not detected by GC analysis of this work quantitatively. The carbon mass balances increase up to 66% at 850 8C with increased formation of non-chlorinated compounds (CH4, C2H4 and C2H6). The main formation pathways for hydrodechlorinated products resulted from H atom cyclic chain reactions by abstraction and addition replacement. The important pyrolytic reaction pathways to describe the important features of reagent decay, intermediate product distributions and carbon mass balances, based upon thermochemical and kinetic principles, were suggested. The results of this work provided a better understanding of pyrolytic decomposition processes which occur during the pyrolysis of C2Cl4 and similar chlorinated hydrocarbons. Acknowledgement This research was supported by the Yeungnam University research grants in 2007. References [1] [2] [3] [4] Fig. 5. Outline of reaction pathways for intermediate products in C2Cl4/H2 reaction system.
Formations of benzene and chlorobenzene were observed above 750 8C, where C2H2 known as the precursor of benzene increases slightly from that at around 725 8C and then decreases as more benzene is formed as listed in Table 1. The unsaturated molecules begin to condense, leading ultimately to high molecules structure, aromatics or soot in pyrolysis [14,15]. Fig. 5 summarized the main reaction pathways to form hydrodechlorinated products from C2Cl4. This overall reaction scheme based upon analysis of the observed product distributions and thermochemical kinetics estimation was illustrated in Fig. 5. 4. Conclusions The thermal decomposition of C2Cl4 in excess hydrogen atmosphere was studied to investigate the pyrolytic reaction pathways of chlorocarbon. The reactions were conducted in an isothermal tubular reactor at a total pressure of 1 atm. with reaction times of 0.3–2.0 s between 600 and 850 8C. Complete destruction (99%) of the parent, C2Cl4 was observed at temperature
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