Thermal Stability and Reaction Mechanism of Chloromethanes in Excess Hydrogen Atmosphere

Transcription

1 J. Ind. Eng. Chem., Vol. 13, No. 3, (2007) Thermal Stability and Reaction Mechanism of Chloromethanes in Excess Hydrogen Atmosphere YangSoo Won Department of Environmental Engineering, Yeungnam University Gyeongsan City , Korea Received September 11, 2006; Accepted January 16, 2007 Abstract: The four chlorinated methanes, methyl chloride (CH 3 Cl), methylene chloride (CH 2 Cl 2 ), chloroform (CHCl 3 ), and carbon tetrachloride (CCl 4 ), were used as model chlorocarbon systems with Cl/H ratios of to to investigate the thermal stability and hydrodechlorination of chloromethanes in excess hydrogen. The pyrolytic reactions were studied in an isothermal tubular reactor at a total pressure of 1 atm with reaction times of s at temperatures between 525 and 900 o C. The thermal stabilities of the chloromethanes, i.e., the temperatures for 99 % destruction within a reaction time of 1 s were 875 o C for CH 3 Cl, 780 o C for CH 2 Cl 2, 675 o C for CHCl 3, and 635 o C for CCl 4. The lesschlorinated hydrocarbons were more stable, with CH 3 Cl the most stable chlorocarbon in this reaction system. This work focused on pyrolysis of CH 3 Cl in an excesshydrogen reaction atmosphere. The observed hydrodechlorinated products were CH 4, C 2 H 4, and C 2 H 6 at temperatures above 850 o C in the CH 3 Cl/H 2 reaction system. The number and quantities of intermediate chlorinated products decreased with increasing temperature; the formation of nonchlorinated hydrocarbons increased as the temperature rose. One of main pathways for hydrodechlorinated products resulted from H atom cyclic chain reaction by abstraction. Product distributions along with preliminary activation energies and rate constants are reported. The pyrolytic reaction pathways that describe the important features of reagent decay and intermediate product distributions, based upon thermochemical and kinetic principles, are suggested. Keywords: pyrolysis, thermal stability, reaction mechanism, chloromethane, methyl chloride, methylene chloride, chloroform, carbon tetrachloride Introduction 1) Chlorinated organic compounds are widely used in synthesis and in the chemical industry. Thermal treatment of these chlorinated hydrocarbons provides a source of chlorine atoms in the initial stages of the process; they are thought to be associated with the formation of aromatics, such as dibenzodioxins and dibenzofurans in incinerators, which have gained much attention due to the fact that some are toxic or, indeed, carcinogenic [14]. Different technologies have been developed for the safe destruction of chlorinated hydrocarbons. Thermal destruction of organic pollutants in an oxygenrich atmosphere is the method most often used in the chemical waste disposal industry. Chlorinated hydrocarbons are To whom all correspondence should be addressed. ( yswon@yu.ac.kr) generally incinerated in an oxygenrich environment, with relative small amounts of hydrogen [5]. It is reported that the combustion of chlorinated hydrocarbons under severe conditions converts all carbon to CO 2 [6,7]. It is important to understand both the pyrolysis and oxidation of these compounds. When oxygen is involved in the process, oxygen and Cl both compete for the available fuel hydrogen, which is one reason why chlorinated hydrocarbons serve as flame inhibitors [8]. Also, CCl bonds may persist in an oxygenrich system of limited hydrogen atmosphere [6,9], so that the emission of toxic chlorinated organic products persists during oxygenrich incineration, in which carbon species are one of the more stable sinks for chlorine atoms. To obtain quantitative formation of HCl, as one of the desired and thermochemically favorable products, from chlorinated hydrocarbons, one might use a straightfor

2 Thermal Stability and Reaction Mechanism of Chloromethanes in Excess Hydrogen Atmosphere 401 ward thermal conversion of these compounds under a more reductive atmosphere of hydrogen. The nonoxygen methods were developed to avoid the formation of undesired oxycontaining products, such as phosgene and dioxins [10,11]. The chlorocarbon and hydrogen system contains only C, H, and Cl elements; it is expected to lead to the formation of light hydrocarbons and hydrogen chloride at the temperatures where complete reaction occurs. Under such a system, carbon can be converted to CH 4, C 2 H 2, C 2 H 4, and C 2 H 6 [12,13]. In this study, purecompound chloromethanes were used as a model chlorocarbon system to investigate the thermal stability and hydrodechlorination of chlorocarbons with excess hydrogen. This work focused on the intermediate product distributions and the major reaction pathways to form various products based on fundamental thermochemical and kinetic principles for the pyrolytic reaction of methyl chloride (CH 3 Cl) with excess hydrogen. We characterized the reactant loss and intermediate product formation as functions of time and temperature to describe the reaction process, and to investigate the feasibility of formation of light hydrocarbons, e.g., CH 4 or C 2 H 4, from the pyrolytic reaction of CH 3 Cl. The experimental apparatus and the procedures used in this study were similar to those used in our earlier studies [8,9,12]. Therefore, only a brief summary of these subjects is given. Pure chloromethanes were reacted with hydrogen (in the absence of O 2 ) in an isothermal tubular reactor at 1 atm. The products of such thermal degradation were analyzed systematically by varying the temperature from 525 to 900 o C and the residence time from 0.3 to 2.0 s. A diagram of the experimental system is shown in Figure 1. Hydrogen gas was passed through a multisaturator train held at 0 o C to ensure saturation with chlormethane at a constant reference temperature for accurate vapor pressure calculation. A second (dilutent) stream of hydrogen gas was used to maintain throughout the desired mole fraction of 4 % chloromethanes the experiment. The reagent with hydrogen gas was fed continuously into the tubular flow reactor in the vapor phase. The mixture was preheated to ac. 200 o C before entering the reactor to improve isothermal temperature control. The reactor effluent was passed through heated transfer lines to the gas chromatograph sampling valves and exhaust. All gas lines to the analytical equipment were held at 170 o C to limit condensation. The quartz tube reactor (8 mm ID) was housed within a threezone electric tube furnace (32 in. long) equipped with three independent temperature controllers. The actual temperature profile of the tubular reactor was obtained using a typek thermocouple probe moving coaxially within the reactor under steady state flow. The temperature profiles shown with varied flow rates in Figure 2 resulted from carefully adjusting the heat flux to the reactor at each different flow rate. Tight temperature control resulted in isothermal temperature profiles within ±3 o C over 75 % of the furnace length for all of the temperature ranges of this study. Figure 1. Schematic diagram of the experimental system. Experimental Method Figure 2. Reactor temperature profiles with tight control. An HP 5890II online GC with FID was used to determine the concentrations of the reaction products. The GC used a 5ftlong by 1.8in. o.d. stainlesssteel column packed with 1 % Alltech AT1000 on graphpac GB as the column. A sixport gas sample valve with a 0.5mL volume loop was used to inject the sample. Quantitative analysis of HCl was performed for each run. The samples for HCl analysis were collected independent from GC sampling. Reactor effluent was diverted to bubbler trains containing 0.01 M NaOH before being exhausted to a fume hood. The amount of HCl produced was then calculated based on titration of the bubbler solution with 0.01 M HCl to its phenolphthalein end point.

3 402 YangSoo Won Results and Discussion Decay of Pure Compound Chloromethanes Figure 3 depicts thermal degradation profiles of chloromethanes for each pure compound as functions of temperature at 1s reaction times under an excess hydrogen atmosphere. The parent thermal stabilities (defined by the temperature required for 99 % destruction) were 875 o C for CH 3 Cl, 780 o C for CH 2 Cl 2, 675 o C for CHCl 3, and 635 o C for CCl 4. The bond dissociation energies of chloromethanes for CH and CCl are listed in Table 1 [14,15]. These characteristics of chlorinated hydrocarbon thermal decomposition can be attributed in part to the weaker CCl bond strengths relative to CH bonds. The low strength of the CCl bond means that thermal unimolecular fission of Cl from CCl compounds occurs orders of magnitude faster than the similar loss of H from CH. The trend of weakening CCl bond strengths occurred with increased substitution of chlorine for hydrogen. The 99 % destructions of each pure chloromethane were in agreement with the Least Bond Dissocia tion Energy (LBDE) trend shown in Table 1. This situation implies that the lesschlorinated methanes are more stable, consistent with the bond strengths of CCl bonds on chlorinated hydrocarbons, which increase with decreasing chlorination. Figure 3. Thermal stability of chloromethanes in excess H 2. However, close inspection of Figure 3 indicates that CHCl 3 was initially less stable than CCl 4. The reason is that a low activation energy of threecenter HCl elimination reaction (1a) is responsible for the rapid decomposition of CHCl 3 at fairly low temperatures (< 570 o C), although the LBDE of CHCl 3 is larger than that of CCl 4. Transition State Theory [15,16] for a simple bond cleavage reaction (1b) estimates a loose configuration and Arrhenius factor that is higher than that of the threecenter HCl elimination (1a), which is significantly lower than the simple bond cleavage. Previous studies [1720] have suggested that reaction (1a) dominates reaction (1b). We also feel strongly that: CCl 2 + HCl is the dominant initiation decomposition path for CHCl 3 from experimental results based upon product distributions [8]. The decomposition of CCl 4 was more sensitive to increasing temperature, and the decay curve of CHCl 3 crossed at 570 o C (40 % destruction) with CCl 4 more easily degraded above 570 o C, as illustrated in Figure 3. Table 1. Bond Dissociation Energies for Chloromethanes [14,15] CH Bond Energy Energy CCl Bond (kcal/mole) (kcal/mole) CH 3H CH 2ClH CHCl 2H CCl 3H CH 3Cl CH 2ClCl CHCl 2Cl CCl 3Cl Table 2. Kinetic Parameters for Decomposition of Choloroform Reaction CHCl 3 :CCl 2+HCl CHCl 3 CHCl 2+Cl A (s 1 ) 2.5E16 1.6E14 Reaction rate parameter Ea (kcal/mole) k (at 600 o C) ref. rzn no. 8 8 (1a) (1b) The acceleration of CCl 4 decomposition with increasing temperature resulted from several combinated effects: 1) The CCl bond dissociation energy (70 kcal/mol) of CCl 4 is lower than that (77 kcal/mol) of CHCl 3, leading to much more efficient Cl atom formation from CCl 4 thanchcl 3. 2) The Cl atom has a high Arrhenius A factor and low activation energy for abstraction of H from H 2 [reaction (3)]. 3) The H atom generated rapidly undergoes abstraction reaction (4), which rapidly regenerates H atoms [reaction (5)] and continues the chain reactions. 4) The Cl atom generated from reaction (2) is more reactive than dichlorocarbene (:CCl 2 ) generated from reaction (1a). Therefore, chain reactions were easier in reaction systems of CCl 4 than in those of CHCl 3. 5) Particularly, in the chloroform reaction system, :CCl 2 from the dominant initiation reaction of CHCl 3 reacted with H 2 bath gas to form stable CH 2 Cl 2 through the insertion termination reaction shown in reaction (6). CCl 4 CCl 3 + Cl Cl+H 2 H + HCl CCl 4 + H CCl 3 + HCl CCl 3 + H 2 CHCl 3 + H 2.6E [14, 15] (2) 4.8E [21] (3) 1.2E12 5.4E [21] [21] (4) (5) CCl 4+H 2 CHCl 3+HCl [overall rxn (4) & (5)] :CCl 2+H 2 CH 2Cl 2 5.0E [8] (6) (A units: (1/s) for unimoecular reaction, (cm 3 /mol s) for bimolecular reaction; Ea units: kcal/mol.)

4 Thermal Stability and Reaction Mechanism of Chloromethanes in Excess Hydrogen Atmosphere 403 As a result of thermochemical considerations, one may expect a sufficient Cl atom concentration in the CCl 4 pyrolysis reaction system, because CCl 4 has the lowest CCl bond energy of these chloromethanes. The acceleration of CCl 4 decay results from the abstraction (4) by H of Cl from CCl 4, and from reaction (2). H is produced from the reaction of Cl with H 2 bath gas, as shown in reaction (3). The Cl atom from the initiation reaction of CCl 4 [reaction (2)] reacts with H 2 to form H and HCl as reaction (3). The H atom accelerates decomposition of CCl 4 by Cl abstraction reaction (4). In reaction (4), H is consumed, but H atoms are produced in reaction (5). Thus, H is not consumed apparently as listed in the overall reaction. The H cyclic chain reaction plays a catalytic role in the acceleration of CCl 4 decomposition. Product Distribution in CH 3 Cl/H 2 Reaction System Figure 4 presents the parent reactant CH 3 Cl loss and product distributions identified by GC analysis in a hydrogenexcess environment as a function of temperature at a 1s reaction time. Complete destruction (99 %) of the CH 3 Cl was observed at temperatures near 875 o C with residence time at 1 s reaction time. The decomposition of CHCl 3 dropped quickly as the temperature increases up to 850 o C, where CH 4 and HCl increase. The products observed were CH 4, C 2 H 4, C 2 H 6, and HCl above 875 o C with almost complete conversion of CH 3 Cl. The formation of the major product, CH 4, as the primary product increased proportionally to the decrease in CH 3 Cl below 875 o C. Small amounts of C 2 H 4 (2 %) and C 2 H 6 (1 %) as secondary products were detected at temperatures above 850 o C. C 2 H 4 and C 2 H 6 were then produced from further reaction of CH 4 with the H 2 bath gas. The nonchlorinated hydrocarbons (CH 4, C 2 H 4, and C 2 H 6, including a small amount of C 6 H 6 ) were observed above 875 o C, while complete destructions of CH 3 Cl, as shown in Figure 4, occured. These findings clearly demonstrated that nonchlorinated hydrocarbons were more stable than chlorinated hydrocarbons. Figure 5 shows the product distribution in the pyrolysis of CH 3 Cl as a function of the reaction time at a 850 o C reaction temperature under an excesshydrogen atmosphere. The formation of CH 4 increased as the reaction time rose up to 1.0 s, where CH 3 Cl drops quickly. Small amounts of C 2 H 4 and C 2 H 6 were found over wide reaction times at 850 o C. Product distributions plotted against reaction times in Figure 5 demonstrate a similar trend to those plotted against the reaction temperature in Figure 4. CH 4 CH 3Cl HCl C 2H 4 C 2H 6 Figure 5. Product distribution vs. reaction time for CH 3Cl/H 2. Reaction Pathways for Products in CH 3 Cl/H 2 Reaction System The reaction pathways in the CH 3 Cl/H 2 system, based on analysis of product distributions and thermochemical kinetics [15] estimations, will be discussed as follows. The possible initiation reaction is the unimolecular decomposition of CH 3 Cl, as listed below. It is estimated from the listed kinetics that reaction (7a) dominates the other pathways by more than three orders of magnitude at 800 o C, based on the reaction rate constants (k 800 ) of initiation reactions (7a), (7b), and (7c). Reaction (7a) initiates the dechlorination process. The major reactions effecting CH 3 Cl loss are simple unimolecular dissociation and abstraction reactions, which have low energy barriers [15,16]. The decay of CH 3 Cl occurred due to the simple unimolecular dissociation reaction (7a) of parent CH 3 Cl to form the CH 3 radical and Cl atom. CH 3Cl CH 3 + Cl CH 3Cl CH 2Cl + H CH 3Cl CH 2 + HCl 2.6E15 5.9E15 1.6E [15, 19] [15] [21] (7a) (7b) (7c) CH 4 CH 3Cl HCl C 2H 4 C 2H 6 Figure 4. Product distribution vs. reaction temperature for CH 3Cl/H 2. The acceleration of CH 3 Cl decay results from the abstraction reaction (9) by H of Cl from CH 3 Cl. The H is

5 404 YangSoo Won produced from reaction of Cl with H 2 bath gas, as in reaction (8). The Cl atom from the initiation reaction of CH 3 Cl [reaction (7a)] reacts with H 2 to form H and HCl as in reaction (8). The H atom reacts with CH 3 Cl and rapidly forms HCl and the CH 3 radical. In reaction (9), H is consumed, but H atoms are produced in reaction (10). Thus, H is not consumed apparently as listed in the overall reaction. The H cyclic chain reaction plays a catalytic role in the acceleration of CH 3 Cl decomposition. This process is exothermic and will continue on chlorocarbons until hydrocarbons (and HCl) remain. CH 3 radicals from reactions (7a) and (9) react with the H 2 bath gas to produce the primary product CH 4, as listed in reaction (10). Cl + H 2 HCl + H 4.8E (8) The other formation pathway for C 2 H 4 is beta scission (simple unimolecular dissociation) of the C 2 H 5 radical. The C 2 H 5 radical from reactions (15b) and (16) can undergo beta scission to C 2 H 4 +H. This beta scission has a lower energy barrier compared with the simple unimolecular dissociation of a stable compound [15,26,27,28]. C 2H 6 + Cl CH 3CH 2 + HCl CH 3CH 2 C 2H 4 + H 2.7E13 5.0E [14,22] [26] (16) (17) Figure 6 summarizes the reaction pathways for the formation of the hydrodechlorinated products in the CH 3 Cl/ H 2 reaction system. This overall reaction scheme, based on analysis of the observed products and estimation of thermochemical kinetics, is illustrated in Figure 6. CH 3Cl + H CH 3 + HCl CH 3 + H 2 CH 4 + H 1.0E14 3.2E CH 3Cl + H 2 CH 4 + HCl (overall rxn (9) & (10)) (9) (10) CH 3Cl + Cl CH 2Cl + HCl 1.3E [22] (11) As shown in Figure 4, a small amount of C 2 H 6 (ca. 1%) was observed at temperatures above 800 o C. This C 2 H 6 formed as a consequence of two CH 3 radicals [from reactions (7a), (9), and (12)] undergoing radical+radical combination reactions (13). The combination process requires no energy barrier, resulting in fast reaction. The small amount of C 2 H 6 was detected because of the low concentration of CH 3 radicals, even though the combination reaction was fast. CH 4+ Cl CH 3 + HCl 3.1E13 CH 3+ CH 3 CH 3CH 3 3.0E [24] (12) (13) The combination reaction of the CH 3 and CH 2 Cl radicals gave the energized complex [C 2 H 5 Cl] #. This reaction can produce C 2 H 5 Cl by stabilization, but then further decomposed through reactions (15a) and (15b). As shown in reactions (15a) and (15b), the rate constant for the C 2 H 4 +HCl process dominated over the C 2 H 5 +Cl process. They are both endothermic, but reaction (15a) is the thermodynamically favorable channel. C 2 H 5 Cl was not observed at temperatures above 650 o C because the rate of four centered HCl elimination (15a) for chloroethanes is very fast [15,24,25]. The C 2 H 4 is produced in this reaction system through reaction (15a). CH 3+ CH 2Cl C 2H 5Cl C 2H 5Cl C 2H 4 + HCl C 2H 5Cl C 2H 5 + Cl 5.0E12 3.2E13 2.2E [14,24] [15,25] [24] (14) (15a) (15b) Figure 6. Reaction pathways in the CH 3Cl/H 2 reaction system. Conclusions The reaction of excess hydrogen with purecompound chloromethanes, methyl chloride, methylene chloride, chloroform, and carbon tetrachloride, has been studied in an isothermal tubular flow reactor at a pressure of 1 atm and in the temperature range o C. The parent thermal stabilities on the basis of the temperature required for 99 % destruction at 1 s reaction time were 875 o C for CH 3 Cl, 780 o C for CH 2 Cl 2, 675 o C for CHCl 3, and 635 o C for CCl 4. Chloroform was thermally less stable than CCl 4 at fairly low temperatures (< 570 o C), due to the low activation energy of the threecenter HCl elimination reaction of chloroform. The decomposition of CCl 4 became more sensitive to increasing temperature, such that CCl 4 was degraded more easily than CHCl 3 at temperatures above 570 o C. The lesschlorinated hydrocarbons were relatively more stable, with CH 3 Cl the most stable chlorocarbon in this reaction system. This work

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