Thermal cracking temperature effects

Crude oil is heated to temperatures at which it thermally cracks into gasoline and distillate products and lower-molecular-weight gases. This liquid cracking process is referred to as visbreaking. A schematic of the process and the effect of operating variables on performance is shown in Fig. 19-12. 

Finally, it should be mentioned that some thermal cracking on the ceramic packing material was observed at temperatures above 850°C. Whenever such an effect was detected, an attempt was made to correct for it in the coking data. A typical value for an experimental run at 810°C for 160 minutes was 35 mg coke on 5 gram of reforming catalyst, or about 0.7 percent by weight. 

Gravimetric Results of Catalytic Cracking. Experiments were conducted to assess the effects of temperature, cat-to-oil ratio, and feedstock composition. In addition to the effect of variables on product yields, it was also important to identify the relative influence of thermal reactions, since free-radical reactions may adversely affect product quality. A series of experiments was conducted in the temperature range of 412°-415°C because this is the temperature of maximum increase in production from thermal cracking and catalytic vs. thermal effects are more easily discernible at this temperature. 

It has been demonstrated that oil recovered from PVC-containing plastic feedstock can be used as a fuel. Cost is the main obstacle since it requires a large amount of slaked lime to neutralize the HCl gas that is produced by the thermal cracking. Table 15.1 shows the effect that slaked lime has on reducing the chlorine content of fuel oil derived from the pyrolysis of PE (55 wt%), PP (28 wt%) and PS (17 wt%) at a decomposition temperature of 420°C. 

Further, each catalyst had a significant effect on the position of double bonds and olefinic content. The catalyst also affects the other properties of liquid fraction such as density. A decrease in density of the liquid indicates low average molecular weight. They also observed that the pour points were lower with ZSM-5 and FCC catalyst than with CRT. Maximum distillation temperatures of liquid products were lower after catalytic cracking than for thermal cracking. 

The effect of the different components present in used tyres on the generation of free radicals on coal has been studied by Ibrahim and Seehra22 through ESR experiments. Figure 6.10 shows that the addition of carbon black or SBR to coal at 420 °C leads to a significant increase in the free radical intensity. Likewise, it was observed that the tyre tread and its components lower the temperature of coal thermal cracking, favouring the formation of free radicals on the coal. 

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