Temperature in the middle section of the catalytic cracking riser
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Why can’t the temperature in the middle section of the catalytic cracking riser be too low? Here, it must remain above 470°C; the outlet temperature is generally kept between 490°C and 530°C. I’m new to this process – could someone please explain?
If the temperature is too low, coking is likely to occur in the lift pipe.
I read the replies; there are three different statements. Which one is correct? !
The catalytic cracking reaction process is actually a gas-phase reaction, during which the feedstock is vaporized and enters the pores of the catalyst. If the feedstock cannot be completely vaporized, then the reactions that occur in the riser are not catalytic cracking reactions; instead, it may be thermal cracking and condensation reactions, etc. Therefore, it is very important to study the dry point of the feedstock to ensure that it is completely vaporized inside the riser.
All of them are correct. It means the perspective on the problem is different. When the feedstock in the form of an oil mist comes into contact with a high-temperature catalyst, if the catalyst-to-oil ratio is too low, low temperatures will occur in the middle section of the riser. The catalytic cracking reaction is an endothermic process; without sufficient heat to serve as reaction energy, the reaction cannot proceed properly. The catalyst acts as a carrier and a heat transfer medium during this reaction process. When the temperature is low (the temperature of the heavy oil being below 460 degrees), the reaction stops; the hydrocarbons get adsorbed on the surface of the catalyst, where they neither react nor vaporize and leave. Coking occurs in the lower parts of the riser, primarily above the nozzles and at the swirl separation area.
The explanation is really thorough! I learned it! Thank you for the guidance!