Pyrolytic coke

Pyrolytic coke R49 Encapsulation of catalyst pellet, deposits on tube wall high temperature, long residence time, presence of olefins, sulphur poisoning... 

The formation of pyrolytic coke is complex [200] [499]. In general, it is initiated by gas-phase reaction forming imsaturated molecules and... 

Figure 5.35 Pyrolytic coke formed in catalyst bed. Z1O2/K catalyst for catalytic steam cracking (refer to Section 4.3.3).

Pyrolytic coke is dense with a very small surface area. Hence, it is not possible to remove after ageing by steaming [499] and it may still be difficult to bum off in air. Steam crackers are regenerated by Ifequent on-line decoking with steam [374] instead of off-line decoking with steam and air. 

The TEM images of deposits observed on Catalyst I used for the steam reforming of naphthalene are shown in Fig. 5. Two types of deposits were observed and they were proved to be composed of mainly carbon by EDS elemental analysis. One of them is film-like deposit over catalysts as shown in Fig. 5(a). This type of coke seems to consist of a polymer of C H, radicals. The other is pyrolytic carbon, which gives image of graphite-like layer as shown in Fig. 5(b). Pyrolytic carbon seems to be produced in dehydrogenation of naphthalene. TPO profile is shown in Fig. 6. The peaks around 600 K and 1000 K are attributable to the oxidation of film-like carbon and pyrolytic carbon, respectively [11-13]. These results coincide with TEM observations. 

According to the results of TPO analyses for all tested catalysts (not shown here), most of the coke existed as film-like carbon over the catalysts whose lifetime were short. On the other hand, pyrolytic carbon existed over all catalysts. These results show that the deactivation of Co/MgO is caused by film-like carbon deposition. 

Historically aromatic compounds were produced from hard coal by coking. The polyaromatics present in coal are released under the pyrolytic conditions and are absorbed in oil or on activated charcoal to separate them from the other coal gases. The components are freed by codistillation with steam or by simple distillation. The contaminant nitrogen- and sulfur-containing compounds are removed by washing with sulfuric acid or by hydrogenation. 

A—Pyrolytic carbon showing ribbon-like structure in vitrinoid bands. B—Faint gray lines define compression cracks in a bright micrinoid particle. C—Pyrrhotite (white) formed by the thermal decomposition of pyrite impregnating semifusinoids (gray). D—Bright coke particles in a baked-bone coal layer... 

This influences the structural features of the mesophase which remains more disordered, a point made by Cranmer et al. (43). Stadelhofer (107) found that the presence of QI did not change rates of formation of mesophase. Romovacek et al. (108) consider that pyrolytic particles in pitch (primary QI) retard the development of mesophase and suppress coalescence. Decrease in size of optical texture, as brought about by mechanical modification as distinct from chemical modification of pitch properties can increase both the strength and reactivity to oxidising gases of the resultant coke, as recently put forward by Markovic et al. (109). ... 

The desulfurization chemistry appears to depend on the presence of free sulfur in the contacting gas and works in conjunction with organic sulfur pyrolytically released from the coke. Gases that decompose to form free sulfur or react to form free sulfur are suitable desulfurization agents and include, for example, refinery sour gas, pure and dilute S, mercaptans, mixtures of CO and SO2 or COS and H2O. 

Organic materials undergo pyrolytic decomposition when heated in an inert atmosphere. Polyaromatic ring structures are developed in the early stages of carbonization. As the heat-treatment temperature (HT1) is increased the solid char or coke begins to acquire short-range order with the formation of distorted graphitic lamellae. In addition, localized and anisotropic densification leads to the development of free space between the lamellae. 

The Likun Process (China) uses a two-stage cracking process under normal pressures where the waste plastics are first pyrolyzed at 350-400°C in the pyrolysis reactor and then the hot pyrolytic gases flow to a catalyst tower where they undergo catalytic reforming over zeolite at 300-380°C. By having the catalyst in the second stage this overcomes the problems of rapid catalyst deactivation from coke deposits on the surface of the catalyst. 

High-temperature pyrolysis (650-800°C) of plastic waste, fed into the rotary kiln via a screw feeder. Solid cokes and pyrolytic vapours are sent to further treatments in gasification or hydrogenation plant... 

As a result of the thermal cracking, solid coke and pyrolytic vapours are produced. The coke is removed from the reactor and utilized further, while the vapour products are passed into a two-step condensation system. Wenning [8] states that, in order to prevent... 

If biomass is subjected to the ASTM D 3172 procedure for determination of fixed carbon, chemical transformation of a portion of the organic carbon in biomass into carbonaceous material occurs as described here. All of the fixed carbon determined by the ASTM procedure is therefore generated by the analytical method. Furthermore, the amount of fixed carbon generated depends on the heating rate used to reach biomass pyrolysis temperatures and the time the sample is subjected to these temperatures. Nevertheless, such analyses are valuable for the development of thermal conversion processes for biomass feedstocks. But application of the ASTM procedures to biomass might more properly be called a method for determination of pyrolytic carbon or coking yields. In the petroleum industry, the Conradson carbon (ASTM D 189, differ-... 

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