Pyrolysis, flash product yields

In slow pyrolysis, the gas phase contains less methane and ethylene and more ethane and propane than hy flash pyrolysis (see Tables 10.4 and 10.7). The product yields obtained in the literature by different authors for the PE for slow pyrolysis (Pinto, Madorsky, Bockhom, Tsuji and Williams) and fast pyrolysis (Kaminsky, Williams, Scott and Conesa) are respectively presented in Eigures 10.2 and 10.3. 

The main product yield (after slow or flash pyrolysis) is the liquid phase and Sawagushi found that this is mainly composed styrene monomers, dimers and trimers. For a residence time of 60 min, increasing the temperature from 310 to 350°C increases the monomer fraction up to 78%. Table 10.16 shows the proportions of these three components in the liquid phase as a function of the temperature. 

Table 10.24 summarizes the product yield after flash pyrolysis of the simulated mixed plastics in relation to pyrolysis temperature. 

Differences between flash and slow pyrolysis have been pointed out, especially for PE and PP. Products yields by slow pyrolysis at high temperature (700°C) are similar to... 

Figure 18.4.1. Comparison between the pyrolysis products yields from a fluidized bed reactor (diamonds) and a filament heated flash pyrolysis (squares) [20] performed at 850° C for almond shells.

The aim of the flash pyrolysis of coal is the production of smaller molecules from it in a shortest possible particle residence time. Therefore, the objective of studying the process chemistry of coal pyrolysis is to investigate the experimental parameters that permit this aim to be achieved and to establish the optimum conditions that produce a favorable product slate. The basic process parameters that influence the prodnct yields dnring flash pyrolysis of coal are (1) reaction temperature, (2) gas pressure, and (3) residence times of coal particles and ensuing tar vapors. In addition to these major process parameters, product yields can be influenced by other factors such as the nature of the pyrolysis gas and its partial pressure and the gas-to-coal ratio. 

Pyrolysis of pyridine derivatives is a method for allylic and benzylic deamination the preparation of nitriles from aldehydes, and the preparation of isocyanates from acid chlorides or hydrazides Flash vacuum pyrolysis continues to yield interesting reactions and products. Recently, it has been reported that quite sensitive acetylene derivatives can be obtained by this method from 4-alkylideneisoxazol-5(4H)-ones by ring degradation. On the other hand, 2 carbamyl azide molecules cyclize under these conditions to form l,2,4-triazolidine-3,5-dione 1,2-ylids . 2H-Cyclohepta[b]furan-2-ones have been obtained by ring expansion of phenyl propiolates... 

The addition of phthalimidylnitrene (374) to simple alkynes affords 1-azirines in yields of 1-15% (Scheme 10). In this reaction, which is of no real preparative value, the symmetrical 2-azirines (375) were suggested as the most plausible intermediates and unequivocal proof of the existence of such species was demonstrated from a series of 1,2,3-triazole pyrolysis reactions <71CC1518). Extrusion of nitrogen from the regioisomeric 4,5-disubstituted 1,2,3-triazoles (376) during flash vacuum pyrolysis furnished identical product mixtures which included both regioisomeric 1-azirines (377). 

The first and to date only synthesis of the parent system 2 uses a flash-vacuum pyrolysis (FVP) of 7,8-diazapentacyclo[4.2.2.02-5.03 9.04 10]dec-7-ene (diazabasketene, 1). After condensation at -196 °C, the pyrolysis product is distilled in vacuum to give pure azocine in ca. 60% yield.12... 

In the case of flash vacuum pyrolysis, the reaction may be of some synthetic utility. For example, at 700 °C diphenyl sulphoxide produces diphenyl thiosulphonate in 40% yield, however, several other products are also formed. 

Methylene cyclopropene (5), the simplest triafulvene, is predicted to be of very low stability. From different MO calculations5 it has been estimated to possess only minor resonance stabilization ranging to 1 j3. Its high index of free valency4 at the exocyclic carbon atom causes an extreme tendency to polymerize, a process favored additionally by release of strain. Thus it is not surprising that only one attempt to prepare this elusive C4H4-hydrocarbon can be found in the literature. Photolysis and flash vacuum pyrolysis of cis-l-methylene-cyclopropene-2,3-dicarboxylic anhydride (58), however, did not yield methylene cyclopropene, but only vinyl acetylene as its (formal) product of isomerization in addition to small amounts of acetylene and methyl acetylene65 ... 

Flash vacuum pyrolysis of methyl imidazol-2-yl carboxylate 290 at 750°C gave 20% yield of 268 via the corresponding ketene 291. Similar pyrolysis of methyl imidazol-l-yl carboxylate 292 gave 20% of a 1 1 2 mixture of compounds 267,268, and 296. This fact can be rationalized by the pathway depicted in Scheme 71. Ketenes 291 and 295 may be intermediates formed from 293 and 294, respectively. They are products of rearrangement of 292. Similar pyrolysis of 4-imidazole carboxylic acid anilide performed at 800°C gave 267 in 20% yield (86JOC306). 

The photolysis of the furan derivatives 78 yielded the butadienals 79 as the main products [123], Further isomerizations leading to allenic esters used the radiation of a cyclopropene-1 -carboxylic acid ester [124] or applied flash vacuum pyrolysis to 3 -ethoxy cyclobut- 2-en-l-one[125]. 

Particularly good yields of the cydoadduct 329 are obtained if R1 = R2 = H is valid for the allenyl ketone 328 [165]. The Diels-Alder products 329 can undergo many chemical transformations, for example to the oximes 330, which yield the modified allenes 331 after a subsequent flash vacuum pyrolysis. The oximes 331 generated by retro-Diels-Alder reaction are not available from ketones 328 and hydroxylamine hydrochloride directly [122] (see also Scheme 7.19). 

Whereas enyne 429 is formed in excellent yield from allenyl sulfone 428 as a stable product of 1,4-elimination of water [118], short-lived butatrienones 431 can only be characterized by argon matrix infrared spectroscopy after 1,2-elimination of HX from precursors 430 by flash vacuum pyrolysis [373, 374]. 

The most interesting process is, therefore, the flash pyrolysis, because it leads to the maximum yield of the most valuable product, the oil. For this process, the key parameters are the char separation and the vapor residence time (determined by the quenching method). 

Wentrup and co-workers investigated the flash vacuum pyrolysis of isopropylidene (l-methyIpyrrolidin-2-ylidene)malonate (1261) (86CC369) (Scheme 51). When the pyrolysis was carried out at 450°C (10-4 torr, contact time 10 3 sec), and the product was condensed on a cold finger at -196°C, (pyrrolidinylidene)malonic anhydride (1262) could be identified. Malonic anhydride (1262) in chloroform solution at -20°C lost carbon dioxide to give methyleneketene (1263), which was reacted with a few drops of water or methanol to yield acrylates (1264). Flash vacuum pyrolysis of 1261 at higher temperature (800°C) gave pyrrolopyrrolone (1265). The products were characterized by IR and 13C-NMR data. 

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