Preventing Coking

Scraped surface heat exchangers (SSHE) have been used as tubular reactors for plastics pyrolysis. SSHE overcome coking and carbon deposits forming on heat exchanging surfaces when the plastic pyrolyzes to hot gases. A tubular reactor with a special internal screw mixer has been developed in Poland [5]. The purpose of the specially shaped internal mixer is to mix the molten plastic and to scrape coke from the internal surface 

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The Pt-Re system has been studied extensively since the 1970s because adding Re to AhOs-supported platinum catalysts increases the resistance to deactivation of the catalysts used in naphtha reforming by preventing coke deposition. By using carbonyl precursors, well-defined bimetalhc species have been prepared. A proper characterization of these species allowed a relationship to be established between their structure and their catalytic behavior. Table 8.3 shows several Pt-Re bimetaUic catalytic systems prepared using different carbonyl species in which Pt-Re interactions determine the catalytic behavior. 

The technical development of petroleum coking has been inseparable from the development of thermal cracking. Untold millions have been spent on research and development trying to eliminate the formation of petroleum coke. Added millions have been spent learning how to prevent coke from forming in heating coils and making the coke deposit where it could be removed most conveniently (18). 

Several technologies differing in the method of heat supply have been developed.191-193 When superheated steam is used, it provides the necessary energy for the endothermic dehydrogenation, acts as a diluent to achieve favorable equilibrium conditions, and prevents coke formation by transforming carbon to carbon oxides. A new oxidative dehydrogenation process (Styro-Plus) is under development.194... 

In the process (Figure 9-8), the feedstock and recycle hydrogen gas are heated to reactor temperature in separate heaters. A small portion of the recycle gas stream and the required amount of additive are routed through the oil heater to prevent coking in the heater tubes. The outlet streams from both heaters are fed to the bottom of the reactor. 

With the introduction of microreactors, transient reactor operations became interesting due to their low internal reactor volume and, thus, fast dynamic behavior. In 1999, Liauw et al. presented a periodically changing flow to prevent coke development on the catalyst and to remove inhibitory reactants in an IMM microchan-nel reactor [58]. This work was preceded by Emig in 1997, of the same group, who presented a fixed-bed reactor with periodically reversed flow [59]. In 2001, Rouge et al. [14] presented the catalytic dehydration of isopropanol in an IMM microreactor. 

The reactor temperature required to prevent coke formation varies considerably for the different processes. Table 2.1 summarizes the values calculated assuming thermodynamic equilibrium for 2,2,4-trimethylpentane reforming. Generally, the coking tendency increases in the following order at constant O/C ratio SR > ATR > POx. These calculations demonstrate that at steam to carbon ratios (S/C) > 2 and reaction temperatures > 600 °C, which is very common for hydrocarbon fuel processors, coke seems to be an unstable species especially under the conditions of steam reforming. 

Table 2.1 Reactor temperature required to prevent coke formation for 2,2,4-trimethylpentane [11],...

Bimetallics prevent coke deposition and decrease unsaturated products adsorption. Re suppresses coke formation and cause coke redistribution on catalyst components.79 ... 

Supercritical fluid extraction can be used to remove carbonaceous material from spent catalysts. De Filippi and Robey (2) used supercritical carbon dioxide extraction to regenerate adsorbents. Abel (3) tried supercritical carbon dioxide extraction to regenerate a certain catalyst. Tiltscher et al. (4,5) studied the isomerization of 1-hexene on an alumina catalyst and showed that under supercritical conditions, 1-hexene was able to remove the oligomeric compounds (C -C q) from t ie catalyst surface and prevent coking. 

Both use a paddle agitator that also serves to prevent coking. 

The Veba Oel pyrolysis processes was originally designed for the upgrading of heavy oil hydrogenation residues (such as coal and petroleum residues). The pyrolysis process uses a rotary kiln reactor with spheres and crossform bodies to prevent coke deposition. The kilns used in this process were operational until 1964. However, it was found that these kilns had three disadvantages [8] ... 

The dehydrochlorinated molten polymer in the molten polymer vessel is qnantitatively fed into the pyrolysis reactor (Figure 26.11), which is of rotary kiln type. It has a hot air jacket and ceramic balls inside to prevent coking problems, as shown on the Fignre 26.12. Thermal degradation is at 400°C and 5 kPa overpressure. 

Consider, for instance, ethylbenzene dehydrogenation to styrene. The traditional plant used in the process industry [32] is based on an fixed-bed catalytic reactor to which a preheated mixture of ethylbenzene and steam, which prevents coke formation, is fed. The reaction products then normally undergo a rather complex separation scheme, mostly based on distillation columns, aimed at recovering styrene (the desired product), benzene, toluene and H2 (by products), and a certain amount of unconverted ethylbenzene, which has to be recycled. The overall conversion per pass is typically around 60%, whereas selectivity is close to 90%. 

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