Direct thermolysis

FIGURE 9.5 Different available recovery options for plastics waste. 

Non-catalytic pyrolysis is carried out at 650-900°C and is designed to maximize the yield of fuel oils (Tukker et al., 1999). The process yields low-grade gasoUne or oil and is relatively inefficient. One of the most important of these, the BP process for instance, based on a fluidized bed reactor, is operated at SOO C in the absence of air. About 80% of the plastic is converted into a liquid mix of hydrocarbons (oil) and the remaining (8-10%) into a gas rich in monomers, under these pyrolysis conditions. A broad range of hydrocarbons ranging from C, to results from high-temperature thermolysis. 

The most studied and already demonstrated pyrolytic technique is fluidized-bed pyrolysis of waste (Conesa et al., 1997 Williams and Williams, 1999). The process is also carried out in two-steps where the plastics are initially thermally processed 

FIGURE 9.6 Schematic representation of a pyrolysis process for plastics. 

TABLE 9.3 Yield of Products from Pyrolysis of Mixed Plastic Waste at 440°C (dehydrochlorination Step was at 300°C for 30Min) 

---

Crucially, it was found22 that upon reaction of the vinylketene complex 42.a with 1-pentyne, the same product distribution was seen as for the direct thermolysis of 1-pentyne with the precursor carbene 43.a. The analysis was simplified by reduction of the crude reaction mixture with McMurry s reagent to produce a mixture of the isomeric indanols 44-46. 

The thermolysis of certain members of the carbomethoxy series (146.C and 145.c) results in a decarbonylation reaction that affords the exclusively E -diene complexes 150 or the E/Z-diene complex mixtures 151, respectively. These products may be formed by direct thermolysis of the precursor cyclopropene (144) in the presence of diiron nonacarbonyl, isolated in 80% overall yield and in a approximate ratio of 3 2 1 (150 Z-151 -151), a product distribution matching that obtained from the isolated vinylketene... 

The side product is formed by the addition of a [Fe(CO)3] unit to the postulated -vinylcarbene (182), generated in situ from the decarbonyla-tion of a vinylketene complex. Indeed, such a diiron species may be synthesized by direct addition of diiron nonacarbonyl to the vinylketene precursor 178.a, or by direct thermolysis of 178.a in toluene. 

In a process called direct thermolysis, at a high enough temperature thermal energy is sufficient to split water into hydrogen and oxygen. Only one reaction is involved in this process, that of equation (2.3.1). In Fig. 2.6, if the input water and output gas mixture are at the same temperature To, the minimum work input required to effect water splitting at temperature Tr can be written [60]... 

Similarly, a vinylcyclopropyl ketoester can be converted to a substituted 4-cyclohep-tenone as outlined in equation 173. Direct thermolysis via the ketoester enol requires somewhat higher temperatures and occurs with moderate yield. ... 

Thermochemical production of hydrogen involves the separation of water into hydrogen and oxygen through chemical reactions at high temperatures. Ideally, water can be separated directly (thermolysis) however this process requires temperatures in excess of 2 500°C. 

Recent studies of the chemistry of 2,5-disubstituted tetrazoles have been mainly concerned with reactions that involve loss of nitrogen and generation of nitrilimine intermediates. Examples of this have already been seen in the photolysis of these compounds and in the thermolysis of the unstable 2-acyltetrazoles obtained from acylation of 5-substituted tetrazoles. Direct thermolysis of 2,5-diaryltetrazoles141 142,442 also leads to nitrilimine intermediates [Eq. (39)]. These undergo a wide variety of... 

Hydrogen can also be produced by the direct thermolysis or thermocatalytic decomposition ( cracking ) of methane or other hydrocarbons. The energy requirement per mole of methane is in fact less than that for steam reforming (although only half as much hydrogen is produced) and the process is simpler. 

In addition, a useful by-product - clean solid carbon in the form of soot is produced. Obviously, this can be captured and stored more easily than gaseous carbon dioxide. Whereas thermocatalytic cracking offers the benefit of operating at a much lower temperature than direct thermolysis, it does suffer from progressive catalyst deactivation through carbon build-up. Moreover, reactivation would result in unwanted emissions of carbon dioxide. 

Since a direct thermolysis of water, which requires temperatures of > 2500 °C, is not practicable under normal circumstances, the splitting process is subdivided into different partial reactions, either one running on a lower temperature level. The principle is given by the following cycle ... 

The first two precursors in this category to be discovered (see Table 5), 112 and 113, contain the relatively weak C-I and C-Hg bonds which undergo either photolysis or thermolysis to yield benzyne (1). The latter process is noteworthy as one of the first generations of this intermediate in the gas phase. Similar o-iodoorganometallic intermediates may be involved in the formation of arynes from o-diiodobenzene derivatives in the presence of zinc or copper. Both direct thermolysis and photolysis of 114 also lead to arynes, presumably via a stepwise mechanism involving o-iodophenyl radicals (47). The extent to which the latter go on to arynes depends not only on the nature of the aryl residue but, as mentioned in Section ILl.C, on the source of the radical center. Pyrolysis of dibromo (115), iodonitro (116), nitrobromo (117), and dinitro (118) aromatic compounds in a mass spectrometer also produces the corresponding arynes and didehydro compounds. 

Ureas are usually intermediately converted to the corresponding carbamates, although direct thermolysis has been reported in at least one case [17], The reason for which ureas are not usually directly thermolysed may rely in the fact that, for symmetrical ureas, the boiling point of the isocyanate and amine products are much closer to each other than the ones of an isocyanate and a lower alcohol. Moreover, the rate of recombination of isocyanate with an amine appears to be fester than the one with an alcohol (see later and Chapter 6). With mixed ureas, a mixture of isocyanates and amines deriving from the two different aryl (or alkyl) groups is to be expected (eq. 3) ... 

However, in the only example we are aware of direct thermolysis of an asymmetric urea [17], mixed ureas containing an arylamino and a diethylamino groups selectively decomposed to the arylisocyanate and diethylamine. Since the reaction is catalysed by acids, it is likely that protonation of the more basic diethylamino nitrogen is the first step of the thermolysis, thus addressing the reaction towards the production of the arylisocyanate. This kind of process may become of much grater importance in the future, since the synthesis of ureas appears to be easier than the one of either isocyanates or carbamates with several catalytic systems. 

---