Barry Dellinger

The homogeneous, gas-phase formation of chlorinated and brominated dibenzo-p-dioxin from 2,4,6-trichloro- and 2,4,6-tribromophenols☆

Abstract The homogeneous, gas-phase formation of polychlorinated dibenzo-p-dioxins (PCDD) and polybrominated dibenzo-p-dioxins (PBDD) has been observed from the high-temperature thermal decomposition of 2,4,6-trichlorophenol (2,4,6-TCP) and 2,4,6-tribromophenol (2,4,6-TBP), respectively. Experiments were conducted in a 1.0-cm-i.d. flow reactor over a temperature range of 300°–800°C with reactant concentrations of ∼ 3.0 × 10−7 mol/L in a reaction atmosphere of dry air. The 1,3,6,8- and 1,3,7,9-tetra chlorinated isomers were the dominant PCDDs observed from the thermal oxidation of 2,4,6-TCP with maximum yields of 0.05% each. The corresponding tetrabrominated isomers were observed from the thermal oxidation of 2,4,6-TBP; however, the maximum yields were approximately 500 times higher. The observed PCDD/PBDD yields and the temperature of their formation can be readily accounted for using a modified form of the original gas-phase formation model of Shaub and Tsang, if the activation energy for the formation of diphenyl ether by displacement of Cl Br from halophenol by phenoxy is decreased from 26 to 19.5 and 8.8 kcal/mol, for the chlorinated and brominated systems, respectively. This suggests that gas-phase formation reactions make a significant contribution to observed dioxin and furan yields in full-scale incinerator.

Mechanisms of Dioxin Formation from the High-Temperature Oxidation of 2-Chlorophenol

The homogeneous, gas-phase oxidative thermal degradation of 2-chlorophenol was studied in a 1 cm i.d., fused silica flow reactor at a concentration of 88 ppm, reaction time of 2.0 s, over a temperature range of 300 to 1000 degrees C. Observed products in order of yield were as follows: 4,6-dichlorodibenzofuran (4,6-DCDF) > dibenzo-p-dioxin (DD) > 1-monochlorodibenzo-p-dioxin (1-MCDD), 4-chlorodibenzofuran (4-MCDF), dibenzofuran (DF), naphthalene, chloronaphthalene, 2,4-dichlorophenol, 2,6-dichlorophenol, phenol, chlorobenzene, and benzene. In contrast to pyrolysis, 4,6-DCDF is the major product rather than DD, and 1-MCDD and naphthalene are formed at temperatures as low as 400 degrees C. Under oxidative conditions, .OH and Cl. are the major carriers, which favors 4,6-DCDF formation over DD or 1-MCDD through abstraction of H. through diketo- and ether- intermediates. It is proposed that below 500 degrees C, unimolecular tautomerization/HCI elimination and CO elimination/isomerization reactions result in the formation of 1-MCDD and naphthalene, respectively.

High-temperature degradation of polybrominated flame retardant materials

Abstract This experimental study examined gas-phase oxidative and pyrolytic thermal degradation of three brominated flame retardant materials in a high-temperature flow reactor. Br-benzenes, Br-phenols, PBDDs and PBDFs were formed at intermediate temperatures and subsequently destroyed by 800°C.

GAS PHASE FORMATION OF CHLORINATED AROMATIC COMPOUNDS FROM THE PYROLYSIS OF TETRACHLOROETHYLENE

Abstract The thermal degradation of tetrachloroethylene (C2C14.,) has been compared to that of ethylene (C2H2) using a high-temperature flow reactor system. Kinetic studies were conducted in an oxygen-Tree helium carrier for reaction times of 2,0 s. Exposure temperatures for this reaction time were varied from 300-1050° C. Significant yields of perchlorinatcd olcfinic and aromatic species were observed from C2C14 with similar yields of non-chlorinated aromatic species from C2H4. Acetylene (C2H4) was the major organic product from C2H4 while hexachlorobenzene (C6Cl6) was the major organic product from C2Cl4. Dichloroacetylene (C2C14) was not observed from C2C14, which is attributed to a rapid polymerization pathway. Chlorine atom displacement reactions involving perchlorinated olefinic radical attack on per-chlorinated olefins and acetylenes are proposed as key steps in molecular growth pathways.

Detailed Modeling of the Pyrolysis of Trichloroethene: Formation of Chlorinated Aromatic Species

Comprehensive product yield determinations from the high-temperature, gas-phase pyrolysis of trichloroethene (C 2 HCl 3 ) using two fused silica tubular flow reactors coupled to in-line gas chromatographic-mass spectrometric analyses are reported. Initial decomposition was observed at 1000 K with formation of HCl and C 2 Cl 2 . Pronounced molecular growth was observed at higher temperatures as evidenced by the formation of C 2 Cl 4 , C 4 Cl 4 , and C 6 Cl 6 (cy) as major (≤5 mole%) products and C 4 Cl 2 , C 4 Cl 6 , C 6 HCl 5 (cy), C 8 Cl 6 (cy), C 8 Cl 8 (cy), C 10 Cl 8 (cy), and C 12 Cl 8 (cy) as minor (≤5 mole %) products. The effects of reactor surface area to volume (S/V) ratio were evaluated by conducting detailed product analyses with 0.1 cm i.d. and 1.0 cm i.d. reactors. Under the higher S/V ratio, C 2 HCl 3 decomposition was increased by an order of magnitude and product distributions suggested that radical-radical and radical-atom recombination rates were enhanced. Product yields under reduced S/V ratio indicated that yields of perchlorinated aromatic and perchlorinated PAH species were a factor of 10 larger than observed for higher S/V ratios. A detailed reaction mechanism is presented for the 1 cm i.d. reactor data describing molecular growth up to the formation of C 8 Cl 6 (cy) and C 8 Cl 8 (cy). Comparison of predicted versus experimental major and minor species profiles are presented, with generally good agreement. Important radical-molecule addition reactions leading to molecular growth are identified using sensitivity analysis and production rate calculations

A detailed kinetic model of the high-temperature pyrolysis of tetrachloroethene

Abstract Comprehensive product yield determinations from the high-temperature, gas-phase pyrolysis of tetrachloroethene (C 2 Cl 4 ) using two fused-silica tubular flow reactors are reported. The effects of reactor surface area to volume (S/V) ratio were evaluated by conducting detailed product analyses with 0.1-cm i.d. and 1.0-cm i.d. reactors. Under low S/V ratio, initial decomposition was observed at 1123 K with formation of dichloroacetylene (C 2 Cl 2 ) and hexachlorobenzene (C 6 Cl 6 (cy)). Molecular chlorine was also observed as a product at higher temperatures. Under high S/V ratio, C 2 Cl 4 decomposition was initiated at 973 K. Reaction products included Cl 2 , carbon tetrachloride (CCl 4 ), hexachlorobutadiene (C 4 Cl 6 ), and C 6 Cl 6 (cy). Product yields under low S/V ratio indicated that yields of C 6 Cl 6 (cy) were a factor of 4 larger than observed for high S/V ratios. A previously published detailed pyrolysis mechanism for trichloroethene (C 2 HCl 3 ) was incorporated into a C 2 Cl 4 pyrolysis model to provide predictions of the high-temperature reaction behavior of C 2 Cl 4 . The model predictions are in much better agreement with product distributions obtained using the 1 cm i.d. reactor versus the 0.1 cm i.d. reactor. Minor revisions of the C 2 HCl 3 model significantly improved comparisons with the observed C 2 Cl 4 product distributions without compromising previous agreement with C 2 HCl 3 product distributions. Important radical-molecule addition reactions leading to C 6 Cl 6 (cy) are identified using sensitivity analysis and production rate calculations.

Evidence for a unified pathway of dioxin formation from aliphatic hydrocarbons

Acetylene is readily converted to perchlorinated gas-phase intermediates including hexachlorobenzene, hexachlorobutadiene, and tetrachloroethylene and heavier perchlorinated species via heterogeneous gas-solid reactions with HCl and cupric oxide on borosilicate under postcombustion conditions. Experiments were conducted using an integrated gas-solid flow-reactor and analytical system at temperatures ranging from 150 to 500°C for gas-phase residence times of 2.0 s and total reaction times of 60 min. Chlorine addition and chlorine net substitution mechanisms mediated by the conversion of Cu(II)Cl2 to Cu(I)Cl are proposed to account for the observed or inferred C2 reaction products including tetrachloroethylene, trichloroethylene, and dichloroacetylene. The formation of condensation products including tetrachlorovinylacetylene, hexachlorobutadiene, and hexachlorobenzene are proposed to be catalyzed by copper chloride species and involve the following steps: (1) chemisorption of a chlorinated ethylene or acetylene by HCl elimination or 1,2-Cu−Cl addition, respectively: (2) physisorption of additional chlorimated ethylenes or acetylenes followed by cis-insertions: and (3) carbon-to-copper chlorine transfer followed by desorption of the molecular growth product. The mechanism accounts for product isomer distributions and branching desorption of the higher molecular weight products, and regeneration of the copper chloride catalyst.

High-temperature gas-phase formation and destruction of polychlorinated dibenzofurans☆

The high-temperature gas-phase decomposition of a PCB isomer was studied using a tubular flow reactor. The formation and destruction of various PCDFs were investigated in atmospheres containing different concentrations of O2.

Pyrolysis and molecular growth of chlorinated hydrocarbons

Abstract The high-temperature pyrolysis of chlorinated hydrocarbons is reviewed with a primary focus on the authors’ research on the gas-phase molecular growth chemistry and elementary reaction mechanisms leading to the formation of chlorinated benzenes and chlorinated polycyclic aromatic hydrocarbons. The authors’ recent heterogeneous mechanistic studies of the chlorination and condensation of acetylene at lower temperatures are also summarized. A pathway diagram suggesting a link between these heterogeneous reactions and the formation of polychlorinated dibenzo- p -dioxins and furans under post-combustion conditions is presented and discussed.

The high-temperature pyrolysis of 1,3-hexachlorobutadiene

Abstract Comprehensive product yield determinations from the high-temperature, gas-phase pyrolysis of 1,3-hexachlorobutadiene (C 4 Cl 6 ) using two fused-silica tubular flow reactors are reported. The effects of reactor surface-area-to-volume (S/V) ratio were evaluated by conducting detailed product analyses with 0.1 cm i.d. and 1.0 cm i.d. reactors (high and low S/V ratio, respectively). Under low S/V ratio, initial decomposition was observed at 1023 K with formation of tetrachlorovinylacetylene (C 4 Cl 4 ), tetrachloroethene (C 2 Cl 4 ), and carbon tetrachloride (CCl 4 ), and molecular chlorine (Cl 2 ). Hexachlorobenzene (C 6 Cl 6 (cy)), C 8 Cl 8 (cy), and C 12 Cl 8 (cy) were also observed as products at higher temperatures. Under high S/V ratio, C 2 Cl 4 decomposition was initiated at 873 K. In addition to the products observed under low S/V ratio, C 3 Cl 4 , C 5 Cl 6 , C 12 Cl 10 , and C 16 C 10 were also observed. As in the case of C 2 chlorinated hydrocarbons we have studied, organic product yields were higher for the low S/V ratio experiments. Previously published detailed pyrolysis mechanisms for trichloroethene (C 2 HCl 3 ) and C 2 Cl 4 were used to provide predictions of the high-temperature reaction behavior of C 4 Cl 6 . Minor revisions of the C 2 Cl 4 model produced reasonable agreement with observed C 4 Cl 6 product distributions without compromising previous agreement with C 2 Cl 4 product distributions. The model predictions were in better agreement with product distributions obtained using the lower S/V ratio reactor, a result also observed for C 2 HCl 2 and C 2 Cl 4 pyrolysis. Plausible radical-molecule addition reactions leading to C 2 Cl 4 , C 4 Cl 4 , C 6 Cl 6 (cy), C 8 Cl 8 (cy), and C 12 Cl 8 (cy) formation were identified with the assistance of sensitivity analysis and production rate calculations.