Step pyrolysis

The stirred tank reactor, possibly with external heating loop and/or reflux cooler, is widely proposed as a plastics liquid phase pyrolysis reactor. Both BASF [15] and Professor Bockhom [6] have used a cascade of well-mixed reactors to produce a step-by-step pyrolysis of resin mixtures. 

Mitsubishi Heavy Industries pioneered a two-step pyrolysis of mixed plastics. A primary vessel is used for decomposing PVC at 300°C. Then, HCl is scrubbed out from the effluent. Molten material is fed into a second vessel operating at 400-500°C. The same concept was much later applied by BASF at Ludwigshafen, and extended to achieve a selective pyrolysis, making use of differences in thermal stability of the different resins. 

After these preheat treatment processes, the two treated (cross-linked) PMS resins were pyrolyzed at 1273 K. Resin recovery after preheat treatment and ceramic yield of these cross-linked resins at 1273 K are summarized in Table 19.2. Use of the reflux system is effective to increase resin recovery and ceramic yield. Figure 19.4 shows the overall ceramic yield of the starting PMS with different thermal histories. The dotted line indicates the intrinsic ceramic yield from PMS by direct pyrolysis up to 1273 K (about 30%). The filled circle indicates the ceramic yield at 1273 K with two-step pyrolysis, in which reflux treatment is the first step. Overall ceramic yield begins to increase at 423 to 523 K, and is saturated (approximately 75%) at 623 K. Even when a preheat treatment step on PMS is a simple heat treatment in an open argon gas flow (open circle), the overall... 

FIGURE 19.4 Ceramic yield of PMS after two-step pyrolysis. ... 

For a simple one step pyrolysis and char combustion with oxygen, an example for the conversion of wood particles assembled to a one dimensional pile is given in Fig.8. 

The maximum phenolic compounds which could be produced and identified was about 2.5 wt. % on an anhydrous feedstock basis during the one-step pyrolysis approach in the temperature range of 25-550 C. Interestingly, 4.4 wt, % phenols were identified following the fractional pyrolysis approach in a lower temperature range of 25-450 C. One can conclude that the removal of volatile phenols during the fractional approach prevents some secondary thermal decomposition reactions which occur during the one-... 

In rather restrained pyrolysis conditions, phenolic compounds produced at low temperatures were decomposed as they were exposed to high temperatures in the one-step pyrolysis (Table 4). Moreover, phenolic compounds in the acidic medium polymerize with aldehydes to form resins. The result was a progressive decline in the phenolic content. Under both fractional and one-step pyrolysis conditions, the oil yield was about the same (Table 1). 

There are several procedures to perform pyrolysis flash pyrolysis (pulse mode), slow gradient heating pyrolysis (continuous mode), step pyrolysis, etc. Commonly, the pyrolysis for analytical purposes is done in pulse mode. This consists of a very rapid heating of the sample from ambient temperature, targeting isothermal conditions at a temperature where the sample is completely pyrolysed. Controlled slow temperature gradients are also possible in pyrolysis, but their use in analytical pyrolysis is limited. Step pyrolysis heats the sample rapidly but in steps, each step following a plateau of constant temperature kept for a limited time period. 

A very interesting subject is the application of analytical pyrolysis for the study of biomarkers in extraterrestrial samples [2], Several meteorites and lunar samples were studied using this technique. Also, Viking Lander used a Py-GC/MS system to explore the Martian atmosphere and surface [74], Commonly, a stepped pyrolysis technique has been used in these studies to determine organic components in an inorganic matrix [75], The procedure involves a set of four or five temperatures that allow the analysis of trapped gases, analysis of small volatile molecules, and the performance of true pyrolysis on macromolecules. 

Clearly the most elegant chemical synthesis of chiral acetate is the route reported by Townsend et al. (33). The method, outlined in Scheme 2, has the advantage that tritium is introduced at a late stage in the synthesis, and both of the remaining steps, pyrolysis and Kuhn-Roth oxidation, proceed in high... 

The drawbacks of the Curie-point cells include the necessity to work at strictly fixed temperatures, which means that step-by-step pyrolysis is impossible. Also, until recently no provision was made in known Curie-point pyrolysers for heating the cell walls to prevent possible condensation of heavy pyrolysis products on the cold walls, nor was any attention paid to ensuring conditions for rapid entry of the pyrolysis products into the chromatographic column. 

In view of the above, the development of gas thermorefractography, in which the pyrolysis products are separated by gas chromatography, is of great interest. Step-by-step pyrolysis may be regarded as a simple analogue of gas thermorefractography (see, for example, refs. 110 and 111). 

As was shown by McCormick [104], step-by-step pyrolysis can be used to distinguish statistical copolymers from mechanical mixtures of homopolymers. 

Construct and demonstrate a bench-scale two-step pyrolysis reforming system. 

One of the drawbacks of porous carbons is the poor vohunetric capacity which can be reached with these materials. Carbons obtained by one-step pyrolysis of organic precursors are very interesting, because they present a higher density than activated carbons. In this case, the lack of porosity available for charging the electrical double layer must be compensated by pseudo-faradaic properties. For this objective, the pyrolysis of nitrogen carriers seems to be very promising. 

Figure 9-2. Kinetics of the double-step pyrolysis of hydrocarbons in plasma-chemical jet reactor. Step 1, methane pyrolysis inplasmajet of hydrogen Step 2, propane pyrolysis, injection of propane is delayed with respect to start of the process (1) 0.17 ms, (2) 0.37 ms, (3) 0.73 ms. Propane injection temperature 293 K propane-to-methane flow rate ratio 1 2.

Resistively heated-filament pyrolyzers offer the most versatility of the available units. They allow a wide range of programmed temperature and time profiles including stepped pyrolysis. This allows the elucidation of the thermal stability profile of the sample it provides data to allow the kinetic analysis of polymer degradation and may facilitate the identification of unknown samples. 

Mixtures, formulated blends, or copolymers usually provide distinctive pyrolysis fragments that enable qualitative and quantitative analysis of the components to be undertaken, e.g., natural rubber (isoprene, dipentene), butadiene rubber (butadiene, vinylcyclo-hexene), styrene-butadiene rubber (butadiene, vinyl-cyclohexene, styrene). Pyrolyses are performed at a temperature that maximizes the production of a characteristic fragment, perhaps following stepped pyrolysis for unknown samples, and components are quantified by comparison with a calibration graph from pure standards. Different yields of products from mixed homopolymers and from copolymers of similar constitution may be found owing to different thermal stabilities. Appropriate copolymers should thus be used as standards and mass balance should be assessed to allow for nonvolatile additives. The amount of polymer within a matrix (e.g., 0.5%... 

Product distribution, variable or stepped pyrolysis and kinetic studies have all been used to expose mechanistic events during polymer degradation. For example, the thermal degradation of polystyrene has been elucidated using the block copolymer poly(styrene-b-styrene-dg). Hybrid and homo monomers (styrene) and dimers (2,4-diphenylbut-l-ene) were detected but without hybrid trimers (2,4,6-triphenylhex-l-ene) (Figure 10). The amount of hybrid dimer far exceeds that which might arise from adjacent residues, while similar proportions of dimers were obtained when the two homopolymers were pyrolyzed together. Data indicate intermolecular reaction rather than the previously proposed 1,3-transfer. 

The change in free radical concentration as a function of the pyrolysis temperature (Figure 5) shows that free radicals are formed and remain entrapped in the fibers [47]. The curve for the E-beam cured fibers can be decomposed into two peaks at 625"C and 1025 C, the former being more intense than the latter (whereas it is the reverse for the oxygen cured fibers). There is a correlation between the gas evolution (Figure 3) and the entrapped radical concentration (Figure 5), suggesting a two step pyrolysis process. 

Activated carbon SCN (the abbreviation stands for spherical carbon with nitrogen) was produced (Institute of Sorption and Problems of Endoecology, Kiev, Ukraine) by a step pyrolysis of a mesoporous vinylpyridine-divinylbenzene (VP-DVB) copolymer containing 10% of DVB at 350°C-900°C followed by steam activation at 900°C with bum-off up to 85%, bulk density 0.2 g/cm (Table 6.7). In these conditions, carbon with the turbostratic structure is formed (Figueiredo and Mouliin 1986). 

Wu L, Nabae Y, Mraiya S, Matsubayashi K, Islam NM, Kurold S, Kakimoto M, Ozaki J, Miyata S (2010) Pt-liee cathode catalysts prepared via multi-step pyrolysis of Fe phthalocya-nine and phenolic resin for fuel cells. Chem Commun (Camb) 46(34) 6377-6379... 

---