Kinetics of soot formation in tetrachloromethane pyrolysis

Kinetics of soot formation is studied in tetrachloromethane pyrolysis behind shock waves. The time dependences of macrokinetic characteristics of soot particle growth (the induction period, the soot yield, and the apparent rate constant of soot particle growth) are determined. Based on the experimental data, the quantitative model of soot formation is developed for tetrachloromethane pyrolysis behind shock waves. Special attention is paid to the thermal effects in CC1⁴ pyrolysis.     

High-temperature pyrolysis of polymers

The experimental results on the pyrolysis of thermoplastic polymers under high-temperature heating and burning conditions are discussed. The reaction of polymer degradation in this case proceeds in the same way as in a low-temperature region.

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Zusammenfassung Die experimentellen Ergebnisse der Pyrolyse von thermoplastischen Polymeren unter Erhitzungs- und Brehnbedingungen hoher Temperatur werden besprochen. Die Reaktion der Polymerzersetzung verläuft in diesem Falle genau so wie in der Niedrigtemperaturzone ab.

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Characteristics of soot particles formed by diesel pyrolysis

This experiment involving diesel fuel pyrolysis was performed to study the process of soot formation without oxidation. The effects of temperature, residence time, and lubricating oil presence on soot formation were investigated through measurement of particle size distribution, morphology, and C/H ratio as well as through thermal analysis. The results show that the formation of soot during diesel pyrolysis depended strongly on both temperature and residence time. The critical temperature for the creation of soot with a primary particle diameter of 20 nm was about 1100 °C. Greater temperatures and residence times resulted in diesel soot particles that were more mature, i.e., with a higher C/H ratio, larger particle size, and higher ignition temperature. The carbonization of diesel soot through pyrolysis was also weakly affected by the addition of 5% lubricating oil to the diesel fuel. The results of this experiment provide information for modeling the formation of diesel soot without oxidation as well as for developing soot generators for after-treatment systems.     

Influence of oxygen on soot formation during acetylene pyrolysis

Kinetic modeling of pyrolysis of acetylene diluted with argon showed a strong influence of small additives of oxygen on the routes of formation of soot nuclei. The influence of oxygen on various channels of formation and consumption of propargyl radicals C₃H₃, which are important precursors of soot formation, as well as the fundamental possibility of controlling the process of soot formation and its properties are considered.     

Electrokinetic effects of adsorbed neutral polymers

On the basis of a previously developed hydrodynamic model for adsorbed polymers the charge flow along a charged interface with adsorbed (uncharged) polymer is calculated. An effective electrokinetic layer thickness is defined and its dependence on the characteristics of the adsorbed polymer and the ionic strength is studied. It is found that tails are very important for the hydrodynamic effects considered because they effectively screen the solvent flow from inner parts of the absorbed layer. The electrokinetic layer thickness increases with decreasing ionic strength, and tends to a limit equal to the hydrodynamic thickness at very low ionic strength.     

Morphology of adsorbed polymers and solid surface wettability

The adsorption of a polyacrylamide (MW 14600) and two polysaccharides (MW 9260 and 706 x 10(3)) onto model silica surfaces of different hydrophobicities was investigated. In all cases, adsorption adhered to the Freundlich isotherm, reflecting the heterogeneous character of the solid substrates. The latter strongly influenced the character of the adsorbed polymer, with morphologies from chainlike structures to thin films and patches being observed. Surface roughness, polymer type, and molecular weight also play roles in controlling adsorbed polymer morphology. Surface wettability is strongly influenced by the thickness of the adsorbed layer.     

Surface diffusion and relaxation of partially adsorbed polymers

We studied by lattice simulation the surface diffusion and relaxation of isolated, self‐avoiding polymers partially adsorbed onto a flat surface. The key parameters describing the system are the number of segments in the chain, N, the adsorption energy of a segment, expressed as a dimensionless surface temperature T_s, and the segmental friction factor on the surface relative to that in the bulk, ζ_s/ζ_b. The simulation data indicate Rouse scaling of the surface diffusion coefficient, D_∥, and in‐plane relaxation time, τ, versus N for all values of T_s and ζ_s/ζ_b studied. A simple application of the Rouse model to a partially adsorbed chain, which ignores fluctuations in adsorbed trains, yields a formula for D_∥ with the correct N‐scaling. It can account for the effects of T_s when ζ_s/ζ_b is finite (≲10), but it fails when ζ_s/ζ_b diverges, predicting no surface diffusion at all, whereas simulations indicate finite surface mobilities facilitated by a caterpillar‐like motion. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1146–1154, 2000     

Thermoanalytical studies of the pyrolysis of organic polymers

Some applications of thermoanalytical techniques (TG and DTA) to the study of organic polymer pyrolysis are described. It is shown that in order to obtain meaningful kinetic parameters strict control of environmental conditions is required. Experimental results are given for selected ethylene-propylene co- and ter-polymers, for halogenated polymers including polyvinyl chloride and neoprene in admixture with ferric oxide, and for other materials.     

Conformational behavior of polymers adsorbed on nanotubes

The importance of hydrophobic interactions in determining polymer adsorption and wrapping of carbon nanotubes is still under debate. In this work, we concentrate on the effect of short-ranged weakly attractive hydrophobic interactions between polymers and nanotubes (modeled as an infinitely long and smooth cylindrical surface), neglecting all other interactions apart for chain flexibility. Using coarse-grained Monte Carlo simulation of such simplified systems, we find that uniform adsorption and wrapping of the nanotube occur for all degrees of chain flexibility for tubes with sufficiently large outer radii. However, the adsorbed conformations depend on chain stiffness, ranging from randomly adsorbed conformations of the flexible chain to perfect helical or multihelical conformations (in the case of more concentrated solutions) of the rigid chains. Adsorption appears to occur in a sequential manner, wrapping the nanotube nearly one monomer at a time from the point of contact. Once adsorbed, the chains travel on the surface of the cylinder, retaining their helical conformations for the semiflexible and rigid chains. Our findings may provide additional insight to experimentally observed ordered polymer wrapping of carbon nanotubes.     

Initial stages of soot formation in thermal pyrolysis of acetylene. II. A model for the incipience and growth of soot particles

A model is developed which describes simultaneously occurring processes of the initial hydrocarbon pyrolysis, nucleation, surface growth, and coagulation of soot particles. The model permits one to find the size distribution of the primary soot particles up to size 30–40 nm using a relatively small set of equations. The computed time dependence of soot particle concentration agrees satisfactorily with available experimental data. The existence of two limiting stages of the soot formation is revealed.     

Simple models of adsorbed polymers: Vibrational properties

The vibrational properties of polymers adsorbed on surfaces are investigated within the frame of two simple models. First the surface is modelled as a one-dimensional infinite harmonic chain. Polymeric “ molecules ” are represented as harmonic chains of finite length. The coupling between the polymers and the surface is treated within the frame of the Interface Response Theory. Upon adsorption of polymers, resonant vibrational modes appear as well defined peaks in the variation of the density of states of the system. The effect of the interaction between adsorbed polymers via phonons is investigated and shown to lead to antiresonances in the spectral density of states and the formation of gaps in the density of states as the surface coverage is increased. A second more realistic model is introduced where the same finite harmonic chains (polymers) are grafted on a two dimensional [001] surface cut through a cubic harmonic crystal. Again the variation in density of states exhibits resonant modes between the polymers and the substrate. However, in this case, the interaction between polymers is short range. Adsorption of a film of polymers produces resonant modes which remain well-defined features within the substrate bulk band.     

Nonisothermal effects in the process of soot formation in ethylene pyrolysis behind shock waves

The nonisothermal nature of hydrocarbon pyrolysis explains the differences in the critical temperatures of soot formation in the experimental studies of these processes. When reaction heats are taken into account, the critical temperatures become close to 1600 K for all the systems studied. The estimated standard enthalpy of carbon atom formation in the composition of soot particles is δH_{f, z}. ≈ 11 ±6 kJ/mol. A kinetic model is proposed for soot formation in ethylene pyrolysis that describes the experimental data. The calculated temperature of soot particles may differ substantially depending on the choice of a model for energy exchange in collisions.     

Initial stages of soot formation in thermal pyrolysis of acetylene. I. Mechanism for homogeneous pyrolysis of acetylene

A probable mechanism for the homogeneous pyrolysis of acetylene, using carbene reactions, is considered. Analysis of the energetics for the probable mechanism of the initiation reactions shows the rearrangement C₂H₂CCH₂ to be the most probable. Using the energetic barriers for simple carbene reactions and formation enthalpies for more complicated carbenes, we evaluated the activation energies for the reactions mechanism. The vibrational excitation of the products of carbene reactions is taken into account. Calculations of the acetylene conversion kinetics and yields of the main gas-phase pyrolysis products, based on the carbene molecular mechanism, show significantly better agreement with available experimental data as compared to those based on traditional radical mechanisms. The calculated time for the appearance of aromatic products is close to the measured induction times for the appearance of soot particles.     

Interactions observed during the pyrolysis of binary mixtures of textile polymers

The techniques of differential thermal analysis (DTA), thermogravimetry (TG), large scale pyrolysis (LSP) and hot-stage microscopy (HSM) have been used to determine the pyrolysis behaviour of three binary polymer systems: wool/Terylene, wool/Courtelle and Terylene/Courtelle. Pyrolysis was carried out in a flowing nitrogen atmosphere at a heating rate of approximately 10°C min^?1.Evidence from DTA and TG indicates that the thermal stability of the polyester fibre Terylene is reduced when pyrolysed in the presence of either wool or Courtelle. It is considered that this reduction in thermal stability is the result of chemical interactions between Terylene and degradation products arising from the breakdown of the second polymer present. Unexpectedly high residual yields (at 1213 K) have been observed from LSP experiments on the wool/Courtelle and Terylene/Courtelle systems.HSM observations for these two systems indicate the formation of a coating of the fusing polymer around the non-fusing polymer during pyrolysis. TG studies indicate that this coating of fused polymer may be effective in retaining volatile degradation products from the non-fusing polymer within the solid residue. The eventual chemical bonding of these degradation products into the solid residue thus accounts for the unexpectedly high yields of solid residue observed by LSP.     

Bound fractions of methacrylate polymers adsorbed on silica using FTIR

The H‐bonding of carbonyl groups on a series of methacrylate polymers with silanols on fumed silica was studied with transmission FTIR. The set included poly(alkyl methacrylates) with alkyl groups, (n‐C_nH_2n+1) of n = 1, 2, 4, and 12 and poly(benzyl methacrylate). Shifts in the vibrational frequencies for bound carbonyl groups (of ～20 cm^?1 lower than those found in the bulk) were observed in the adsorbed polymer samples. A series of samples with different adsorbed amounts (varying from 0.5 to 2.0 mg m^?2) of each polymer was prepared to determine the effect of the side chain on the H‐bonding. The fractions of bound carbonyls, p, for each of the methacrylate polymers studied, were calculated from a model based on the ratios of the absorption coefficients of the bound to free carbonyl resonances, X (= α_b/α_f). The X values were determined from linear regressions of the ratios of the free to bound carbonyl intensities as a function of the amounts of adsorbed polymer, M_t. The bound fractions, p, were observed to decrease with increase in adsorbed amounts and with increase in the lengths of the side chains of the methacrylate polymers, except for poly(lauryl methacrylate) (PLMA). PLMA has a very low glass transition temperature (T_g) and is likely rubbery on the surface, whereas the other polymers are likely glassy at ambient temperature. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1911–1918, 2010