Liquid-phase pyrolysis, modelling

By combining all of the possible TEM modes, we are able to explain the behavior of carbonaceous materials primarily in terms of the local molecular orientations established in the final stages of liquid-phase pyrolysis. The models established from these observations are supported by the results of other techniques, such as infrared analyses (33), optical microscopy (27), X-ray diffraction (24), and Raman spectroscopy (22). 

Our data can be used to estimate the effective temperatures reached in each site through comparative rate thermometry, a technique developed for similar use in shock tube chemistry (32). Using the sonochemical kinetic data in combination with the activation parameters recently determined by high temperature gas phase laser pyrolysis (33), the effective temperature of each site can then be calculated (8),(34) the gas phase reaction zone effective temperature is 5200 650°K, and the liquid phase effective temperature is 1900°K. Using a simple thermal conduction model, the liquid reaction zone is estimated to be 200 nm thick and to have a lifetime of less than 2 usee, as shown in Figure 3. 

Brus et al. prepared isolated silicon particles by high temperature pyrolysis of disilane with a subsequent passivation of the surface by oxidation [33]. The particles of various size are then processed by high-pressure, liquid-phase, size exclusion chromatography to separate sizes and obtain various fractions of monosize particles. Such particles represent an almost ideal model of silicon quantum dots. 

Independently of the adopted approach for the mechanism formulation, the estimation of the rate constants for all the reaction classes involved in the pyrolysis process of PE, PP, PS and PVC is of critical importance in model development. As already mentioned, the kinetic parameters of the condensed-phase reactions are directly derived from the rate parameters of the analogous gas-phase reactions, properly corrected to take into account transposition in the liquid phase (e.g., Section II.D). 

Fig. 38. Total weight loss and main species in the liquid phase during dynamic thermal decomposition. (a) Pyrolysis of polystyrene (Anderson and Freeman, 1961) (5°C/min, 1 mmHg). (b) Thermal decomposition of polypropylene (Ranzi et al, 1997a) (10°C/min, 1 atm), (c) Pyrolysis of polyethylene (Ranzi et al, 1997a) (10°C/min, 1 atm). Discrete model (Dashed) and Moment model (-).

The set of equations which describes the pyrolysis contains the differential mass balances for at least 10 gaseous components and additionally for the liquid and the solid phase, the transport equations (Dusty-Gas-Model, transport of the liquid phase), a heat balance, the sorption isotherm for water and up to 37 reaction equations. In the example given below only the equations that describe the drying in the initial phase of pyrolysis are given since during drying the temperture of the particle is too low to result in thermal degradation of the solid. 

A model describing one-dimensional heat transfer in parous material [S] is used and modified to study drying and pyrolysis of large wood particles. The model solves the conservation equations of energy, solid phase, liquid water, bound water and the concentrations of species in the gas phase of the porous material. The phases are assumed to be in thermal equilibrium and the variables are calculated as volume averages [6]. 

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