<<

J. Ind. Eng. Chem., Vol. 13, No. 3, (2007) 400-405

Thermal Stability and Reaction Mechanism of in Excess Hydrogen Atmosphere

Yang-Soo Won†

Department of Environmental Engineering, Yeungnam University Gyeongsan City 712-749, Korea

Received September 11, 2006; Accepted January 16, 2007

Abstract: The four chlorinated methanes, methyl (CH3Cl), methylene chloride (CH2Cl2), (CHCl3), and tetrachloride (CCl4), were used as model chlorocarbon systems with Cl/H ratios of 0.021 to 0.083 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 0.3∼2.0 s at temperatures between 525 and 900 oC. The thermal stabilities of the chloromethanes, o o i.e., the temperatures for 99 % destruction within a reaction time of 1 s were 875 C for CH3Cl, 780 C for o o CH2Cl2, 675 C for CHCl3, and 635 C for CCl4. The less-chlorinated were more stable, with CH3Cl the most stable chlorocarbon in this reaction system. This work focused on pyrolysis of CH3Cl in an ex- cess-hydrogen reaction atmosphere. The observed hydrodechlorinated products were CH4, C2H4, and C2H6 at o temperatures above 850 C in the CH3Cl/H2 reaction system. The number and quantities of intermediate chlori- nated products decreased with increasing temperature; the formation of non-chlorinated hydrocarbons in- creased as the temperature rose. One of main pathways for hydrodechlorinated products resulted from H 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, , methyl chloride, methylene chlor- ide, chloroform,

Introduction generally incinerated in an oxygen-rich environment, 1) with relative small amounts of hydrogen [5]. It is re- Chlorinated organic compounds are widely used in syn- ported that the combustion of chlorinated hydrocarbons thesis and in the chemical industry. Thermal treatment of under severe conditions converts all carbon to CO2 [6,7]. these chlorinated hydrocarbons provides a source of It is important to understand both the pyrolysis and oxi- in the initial stages of the process; they dation of these compounds. are thought to be associated with the formation of ar- When oxygen is involved in the process, oxygen and Cl omatics, such as dibenzodioxins and dibenzofurans in in- both compete for the available fuel hydrogen, which is cinerators, which have gained much attention due to the one reason why chlorinated hydrocarbons serve as flame fact that some are toxic or, indeed, carcinogenic [1-4]. inhibitors [8]. Also, C-Cl bonds may persist in an oxy- Different technologies have been developed for the safe gen-rich system of limited hydrogen atmosphere [6,9], so destruction of chlorinated hydrocarbons. Thermal de- that the emission of toxic chlorinated organic products struction of organic pollutants in an oxygen-rich atmos- persists during oxygen-rich incineration, in which carbon phere is the method most often used in the chemical species are one of the more stable sinks for chlorine waste disposal industry. Chlorinated hydrocarbons are atoms. To obtain quantitative formation of HCl, as one of the desired and thermochemically favorable products, from † To whom all correspondence should be addressed. chlorinated hydrocarbons, one might use a straightfor- (e-mail: [email protected]) Thermal Stability and Reaction Mechanism of Chloromethanes in Excess Hydrogen Atmosphere 401 ward thermal conversion of these compounds under a file of the tubular reactor was obtained using a type-K more reductive atmosphere of hydrogen. The “non-oxy- thermocouple probe moving coaxially within the reactor gen” methods were developed to avoid the formation of under steady state flow. The temperature profiles shown undesired oxy-containing products, such as and with varied flow rates in Figure 2 resulted from carefully dioxins [10,11]. The chlorocarbon and hydrogen system adjusting the heat flux to the reactor at each different contains only C, H, and Cl elements; it is expected to flow rate. Tight temperature control resulted in iso- lead to the formation of light hydrocarbons and hydrogen thermal temperature profiles within ±3 oC over 75 % of chloride at the temperatures where complete reaction the furnace length for all of the temperature ranges of occurs. Under such a system, carbon can be converted to this study. CH4, C2H2, C2H4, and C2H6 [12,13]. In this study, pure-compound chloromethanes were used as a model chlorocarbon system to investigate the thermal stability and hydrodechlorination of chlorocar- bons with excess hydrogen. This work focused on the in- termediate 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 (CH3Cl) with excess hydro- gen. 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., CH4 or C2H4, from the pyrolytic reaction of CH3Cl. Figure 1. Schematic diagram of the experimental system.

Experimental Method

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 sub- jects is given. Pure chloromethanes were reacted with hy- drogen (in the absence of O2) in an isothermal tubular re- actor at 1 atm. The products of such thermal degradation were analyzed systematically by varying the temperature from 525 to 900 oC 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 multi-satu- Figure 2. Reactor temperature profiles with tight control. rator train held at 0 oC to ensure saturation with chlor- at a constant reference temperature for accurate An HP 5890II on-line GC with FID was used to de- calculation. A second (dilutent) stream of termine the concentrations of the reaction products. The hydrogen gas was used to maintain throughout the de- GC used a 5-ft-long by 1.8-in. o.d. stainless-steel column sired mole fraction of 4 % chloromethanes the experi- packed with 1 % Alltech AT-1000 on graphpac GB as ment. The reagent with hydrogen gas was fed con- the column. A six-port gas sample valve with a 0.5-mL tinuously into the tubular flow reactor in the vapor phase. volume loop was used to inject the sample. Quantitative The mixture was preheated to ac. 200 oC before entering analysis of HCl was performed for each run. The samples the reactor to improve isothermal temperature control. for HCl analysis were collected independent from GC The reactor effluent was passed through heated transfer sampling. Reactor effluent was diverted to bubbler trains lines to the gas chromatograph sampling valves and containing 0.01 M NaOH before being exhausted to a exhaust. All gas lines to the analytical equipment were fume hood. The amount of HCl produced was then calcu- held at 170 oC to limit condensation. The quartz tube re- lated based on titration of the bubbler solution with 0.01 actor (8 mm ID) was housed within a three-zone electric M HCl to its phenolphthalein end point. tube furnace (32 in. long) equipped with three indepen- dent temperature controllers. The actual temperature pro- 402 Yang-Soo Won

Results and Discussion dominant initiation decomposition path for CHCl3 from experimental results based upon product distributions [8]. Decay of Pure Compound Chloromethanes The decomposition of CCl4 was more sensitive to in- Figure 3 depicts thermal degradation profiles of chloro- creasing temperature, and the decay curve of CHCl3 o methanes for each pure compound as functions of tem- crossed at 570 C (40 % destruction) with CCl4 more o perature at 1-s reaction times under an excess hydrogen easily degraded above 570 C, as illustrated in Figure 3. atmosphere. The parent thermal stabilities (defined by the temperature required for 99 % destruction) were 875 Table 1. Bond Dissociation Energies for Chloromethanes o o o [14,15] C for CH3Cl, 780 C for CH2Cl2, 675 C for CHCl3, and o Energy Energy 635 C for CCl4. The bond dissociation energies of chlo- C-H Bond C-Cl Bond (kcal/mole) (kcal/mole) romethanes for C-H and C-Cl are listed in Table 1 CH -H 105 - - [14,15]. These characteristics of chlorinated 3 CH2Cl-H 103 CH3-Cl 84 thermal decomposition can be attributed in part to the CHCl2-H 101 CH2Cl-Cl 81 weaker C-Cl bond strengths relative to C-H bonds. The CCl3-H 96 CHCl2-Cl 77 low strength of the C-Cl bond means that thermal unim- - - CCl3-Cl 70 olecular fission of Cl from C-Cl compounds occurs or- ders of magnitude faster than the similar loss of H from Table 2. Kinetic Parameters for Decomposition of Choloroform C-H. The trend of weakening C-Cl bond strengths oc- Reaction rate parameter curred with increased substitution of chlorine for hydro- Reaction A Ea k -1 o ref. rzn no. gen. The 99 % destructions of each pure chloromethane (s ) (kcal/mole) (at 600 C) -3 were in agreement with the Least Bond Dissocia- tion CHCl3→ :CCl2+HCl 2.5E16 74.6 5.3×10 8 (1a) Energy (LBDE) trend shown in Table 1. This situation CHCl3→CHCl2+Cl 1.6E14 56.0 1.53 8 (1b) implies that the less-chlorinated methanes are more sta- ble, consistent with the bond strengths of C-Cl bonds on The acceleration of CCl4 decomposition with increasing chlorinated hydrocarbons, which increase with decreas- temperature resulted from several combinated effects: ing chlorination. 1) The C-Cl bond dissociation energy (70 kcal/mol) of CCl4 is lower than that (77 kcal/mol) of CHCl3, leading to much more efficient Cl atom formation from CCl4 thanCHCl3. 2) The Cl atom has a high Arrhenius A factor and low activation energy for abstraction of H from H2 [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 re- active than (:CCl2) generated from re- action (1a). Therefore, chain reactions were easier in re- action systems of CCl4 than in those of CHCl3. Figure 3. Thermal stability of chloromethanes in excess H2. 5) Particularly, in the chloroform reaction system, :CCl2 from the dominant initiation reaction of CHCl3 re- However, close inspection of Figure 3 indicates that acted with H2 bath gas to form stable CH2Cl2 through the CHCl3 was initially less stable than CCl4. The reason is insertion termination reaction shown in reaction (6). that a low activation energy of three-center HCl elimi- nation reaction (1a) is responsible for the rapid decom- o A Ea ref. rxn no. position of CHCl3 at fairly low temperatures (< 570 C), CCl4 → CCl3 + Cl 2.6E16 68.3 [14, 15] (2) although the LBDE of CHCl3 is larger than that of CCl4. Cl+H2 → H + HCl 4.8E15 5.0 [21] (3) Transition State Theory [15,16] for a simple bond cleavage reaction (1b) estimates a loose configuration CCl4 + H → CCl3 + HCl 1.2E12 5.0 [21] (4) and Arrhenius factor that is higher than that of the CCl3 + H2 → CHCl3 + H 5.4E12 14.3 [21] (5) three-center HCl elimination (1a), which is significantly CCl4+H2→CHCl3+HCl [overall rxn (4) & (5)] lower than the simple bond cleavage. Previous studies :CCl2+H2→CH2Cl2 5.0E12 14.0 [8] (6) [17-20] have suggested that reaction (1a) dominates re- (A units: (1/s) for unimoecular reaction, (cm3/mol⋅s) for bimo- action (1b). We also feel strongly that: CCl2 + HCl is the lecular reaction; Ea units: kcal/mol.) Thermal Stability and Reaction Mechanism of Chloromethanes in Excess Hydrogen Atmosphere 403

As a result of thermochemical considerations, one may Figure 5 shows the product distribution in the pyrolysis o expect a sufficient Cl atom concentration in the CCl4 py- of CH3Cl as a function of the reaction time at a 850 C rolysis reaction system, because CCl4 has the lowest reaction temperature under an excess-hydrogen atmo- C-Cl bond energy of these chloromethanes. The accel- sphere. The formation of CH4 increased as the reaction eration of CCl4 decay results from the abstraction (4) by time rose up to 1.0 s, where CH3Cl drops quickly. Small H of Cl from CCl4, and from reaction (2). H is produced amounts of C2H4 and C2H6 were found over wide re- o from the reaction of Cl with H2 bath gas, as shown in re- action times at 850 C. Product distributions plotted action (3). The Cl atom from the initiation reaction of against reaction times in Figure 5 demonstrate a similar CCl4 [reaction (2)] reacts with H2 to form H and HCl as trend to those plotted against the reaction temperature in reaction (3). The H atom accelerates decomposition of Figure 4. CCl4 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 over- all reaction. The H cyclic chain reaction plays a catalytic role in the acceleration of CCl4 decomposition.

Product Distribution in CH3Cl/H2 Reaction System Figure 4 presents the parent reactant CH3Cl loss and product distributions identified by GC analysis in a hy- drogen-excess environment as a function of temperature at a 1-s reaction time. Complete destruction (99 %) of the o CH3Cl was observed at temperatures near 875 C with residence time at 1 s reaction time. The decomposition of CHCl3 dropped quickly as the temperature increases up o to 850 C, where CH4 and HCl increase. The products o observed were CH4, C2H4, C2H6, and HCl above 875 C CH4 CH3Cl HCl C2H4 C2H6 with almost complete conversion of CH3Cl. The for- Figure 5. Product distribution vs. reaction time for CH3Cl/H2. mation of the major product, CH4, as the primary product increased proportionally to the decrease in CH3Cl below Reaction Pathways for Products in CH3Cl/H2 Reaction o 875 C. Small amounts of C2H4 (2 %) and C2H6 (1 %) as System secondary products were detected at temperatures above The reaction pathways in the CH3Cl/H2 system, based o 850 C. C2H4 and C2H6 were then produced from further on analysis of product distributions and thermochemical reaction of CH4 with the H2 bath gas. The non-chlori- kinetics [15] estimations, will be discussed as follows. nated hydrocarbons (CH4, C2H4, and C2H6, including a The possible initiation reaction is the unimolecular de- o small amount of C6H6) were observed above 875 C, composition of CH3Cl, as listed below. while complete destructions of CH3Cl, as shown in It is estimated from the listed kinetics that reaction (7a) Figure 4, occured. These findings clearly demonstrated dominates the other pathways by more than three orders that non-chlorinated hydrocarbons were more stable than of magnitude at 800 oC, based on the reaction rate con- chlorinated hydrocarbons. stants (k800 ℃) of initiation reactions (7a), (7b), and (7c). Reaction (7a) initiates the dechlorination process. The major reactions effecting CH3Cl loss are simple unim- olecular dissociation and abstraction reactions, which have low energy barriers [15,16]. The decay of CH3Cl occurred due to the simple unimolecular dissociation re- action (7a) of parent CH3Cl to form the CH3 radical and Cl atom.

A Ea ref. rxn no.

CH3Cl → CH3 + Cl 2.6E15 81.6 [15, 19] (7a) CH3Cl → CH2Cl + H 5.9E15 100.4 [15] (7b) CH3Cl → CH2 + HCl 1.6E14 130.9 [21] (7c)

CH4 CH3Cl HCl C2H4 C2H6 Figure 4. Product distribution vs. reaction temperature for The acceleration of CH3Cl decay results from the ab- CH3Cl/H2. straction reaction (9) by H of Cl from CH3Cl. The H is 404 Yang-Soo Won

produced from reaction of Cl with H2 bath gas, as in re- The other formation pathway for C2H4 is beta scission action (8). The Cl atom from the initiation reaction of (simple unimolecular dissociation) of the C2H5 radical. CH3Cl [reaction (7a)] reacts with H2 to form H and HCl The C2H5 radical from reactions (15b) and (16) can un- as in reaction (8). The H atom reacts with CH3Cl and dergo beta scission to C2H4+H. This beta scission has a rapidly forms HCl and the CH3 radical. In reaction (9), H lower energy barrier compared with the simple unim- is consumed, but H atoms are produced in reaction (10). olecular dissociation of a stable compound [15,26,27,28]. Thus, H is not consumed apparently as listed in the over- all reaction. The H cyclic chain reaction plays a catalytic A Ea ref. rxn no. role in the acceleration of CH3Cl decomposition. This C2H6 + Cl → CH3CH2 + HCl 2.7E13 3.6 [14,22] (16) process is exothermic and will continue on chlorocarbons CH3CH2 → C2H4 + H 5.0E13 40.9 [26] (17) until hydrocarbons (and HCl) remain. CH3 radicals from reactions (7a) and (9) react with the H2 bath gas to pro- Figure 6 summarizes the reaction pathways for the for- duce the primary product CH4, as listed in reaction (10). mation of the hydrodechlorinated products in the CH3Cl/ H2 reaction system. This overall reaction scheme, based A Ea ref. rxn no. on analysis of the observed products and estimation of Cl + H2 → HCl + H 4.8E16 5.0 [22,23] (8) thermochemical kinetics, is illustrated in Figure 6.

CH3Cl + H → CH3 + HCl 1.0E14 7.6 [22,23] (9) CH3 + H2 → CH4 + H 3.2E12 12.5 [22,23] (10)

CH3Cl + H2 → CH4 + HCl (overall rxn (9) & (10))

CH3Cl + Cl → CH2Cl + HCl 1.3E14 3.6 [22] (11)

As shown in Figure 4, a small amount of C2H6 (ca. 1%) o was observed at temperatures above 800 C. This C2H6 formed as a consequence of two CH3 radicals [from re- actions (7a), (9), and (12)] undergoing radical+radical combination reactions (13). The combination process re- quires no energy barrier, resulting in fast reaction. The small amount of C2H6 was detected because of the low concentration of CH3 radicals, even though the combina- tion reaction was fast. Figure 6. Reaction pathways in the CH3Cl/H2 reaction system. A Ea ref. rxn no. CH4+ Cl → CH3 + HCl 3.1E13 3.6 [22,23] (12) Conclusions CH3+ CH3 → CH3CH3 3.0E13 0.0 [24] (13) The reaction of excess hydrogen with pure-compound The combination reaction of the CH3 and CH2Cl radi- # chloromethanes, methyl chloride, methylene chloride, cals gave the energized complex [C2H5Cl] . This reaction chloroform, and carbon tetrachloride, has been studied in can produce C2H5Cl by stabilization, but then further de- an isothermal tubular flow reactor at a pressure of 1 atm composed through reactions (15a) and (15b). As shown and in the temperature range 525∼900 oC. The parent in reactions (15a) and (15b), the rate constant for the thermal stabilities on the basis of the temperature re- C2H4+HCl process dominated over the C2H5+Cl process. quired for 99 % destruction at 1 s reaction time were 875 They are both endothermic, but reaction (15a) is the ther- o o o C for CH3Cl, 780 C for CH2Cl2, 675 C for CHCl3, and modynamically favorable channel. C2H5Cl was not ob- o o 635 C for CCl4. Chloroform was thermally less stable served at temperatures above 650 C because the rate of o than CCl4 at fairly low temperatures (< 570 C), due to four centered HCl elimination (15a) for is the low activation energy of the three-center HCl elimi- very fast [15,24,25]. The C2H4 is produced in this re- nation reaction of chloroform. The decomposition of action system through reaction (15a). CCl4 became more sensitive to increasing temperature, such that CCl4 was degraded more easily than CHCl3 at A Ea ref. rxn no. temperatures above 570 oC. The less-chlorinated hydro- CH + CH Cl → C H Cl 5.0E12 0.0 [14,24] (14) 3 2 2 5 were relatively more stable, with CH3Cl the most C2H5Cl → C2H4 + HCl 3.2E13 56.6 [15,25] (15a) stable chlorocarbon in this reaction system. This work C2H5Cl → C2H5 + Cl 2.2E15 79.5 [24] (15b) Thermal Stability and Reaction Mechanism of Chloromethanes in Excess Hydrogen Atmosphere 405

focused on the pyrolysis of CH3Cl in an atmosphere of nol., 85, 345 (1992). excess hydrogen. The observed hydrodechlorinated prod- 9. S. C. Chuang and J. W. Bozzelli, Environ. Sci. ucts were CH4, C2H4, and C2H6 at temperatures above Technol., 20, 568 (1986). o 850 C in the CH3Cl/H2 reaction system. The number and 10. E. T. Oppelt, J. Air Pollution Control Assoc., 37, quantities of intermediate chlorinated products decreases 558 (1987). with increasing temperature; the formation of non-chlori- 11. W. Tsang, Combust. Sci. Technol., 74, 99 (1990). nated hydrocarbons increased as the temperature rose. 12. Y. P. Wu and Y. S. Won, J. Hazard. Mater., B105, One of main pathways for hydrodechlorinated products 63 (2003). resulted from H atom cyclic chain reaction by abstrac- 13. J. A. Manion and R. Louw, J. Chem. Soc. Perkin. tion. Product distributions along with preliminary activa- Trans., 2, 1547 (1988). tion energies and rate constants are reported. Pyrolytic 14. M. Weissman and S. W. Benson, J. Phys. Chem., 87, reaction pathways that describe the important features of 243 (1983). reagent decay and intermediate product distributions, 15. S. W. Benson, Thermochemical Kinetics, John based upon thermochemical and kinetic principles, are Wiley and Son (1976). suggested. The results of this work provide a better un- 16. Y. P. Wu and Y. S. Won, J. Ind. Eng. Chem., 9, 775 derstanding of the pyrolytic decomposition processes that (2003). occur during the pyrolysis of chlorinated hydrocarbons. 17. F. E. Kung and W. E. Bissinger, J. Org. Chem., 29, 2739 (1964). 18. S. W. Benson and G. N. Spokes, 11th Symposium References (international) on combustion, 95 (1966). 19. K. P. Schug, H. G. Wagner, and F. Zabel, Ber. 1. J. I. Baker and R. A. Hites, Environ. Sci. Technol., Bunsenges Phys. Chem., 83, 167 (1979). 34, 2879 (2000) 20. I. P. Herman, F. Magnotta, R. J. Buss, and Y. T. Lee, 2. B. K. Gullett, A. Touati, and C. W. Lee, Environ. J. Chem. Phys., 79, 1789 (1983). Sci. Technol., 34, 2069 (2000). 21. D. W. Setser and T. Lee, J. Am. Chem. Soc., 89, 3. A. M. Mastral and M. S. Callen, Environ. Sci. Tech- 5799 (1985). nol., 34, 3051 (2000). 22. J. A. Kerr and S. J. Moss, Handbook of Bimolecular 4. (a) K. Li, E. M. Kennedy, B. Moghtaderi, and B. Z. and Termolecular Gas Reactions, CRC Press, FL Dlugogorski, Environ. Sci. Technol., 34, 584 (2000); (1981). (b) Y. Eom, S. Kim, S. S. Kim, and S. H. Chung, J. 23. NIST, Chemical Gas Kinetics Database, Version 5.0 Ind. Eng. Chem., 12, 846 (2006) (2003). 5. L. Mason and S. Unget, US EPA 600/2.79.198, 24. D. Allara and R. Shaw, Phys. Chem. Ref. Data, 9, NTIS PB 80-131964 (1979) 523 (1981). 6. M. R. Booty, J. W. Bozzelli, W. Ho, and R. S. 25. Y. S. Won and J. W. Bozzelli, Am. Soc. Mech. Eng., Magee, Environ. Sci. Technol., 29, 3059 (1995). HTD 104, 131 (1988). 7. R. Louw, H. Dijks, and P. Mulder, Chem. Ind., 23, 26. A. M. Dean, J. Phys. Chem., 89, 4600 (1985). 759 (1983). 27. Y. S. Won, J. Korean Ind. Eng. Chem., 17, 638 8. Y. S. Won and J. W. Bozzelli, Combust. Sci. Tech- (2006).