Q2. What is the effect of transition metals on the cracking of zeolites?
Due to their electron configuration, transition metals are able to form chemical bonds with neutral molecules and it has been observed that the incorporation of transition metals creates new Lewis acid sites in HZSM-5 zeolites, leading to an increase of the acid activity of zeolites and influencing directly the selectivity of the catalytic cracking products.
Q3. What is the effect of a decrease in zeolite-to-matrix?
At constant catalyst activity, it is observed that a decrease in zeolite-to-matrix results in an increase of LCO, coke, dry gas yields, and a decrease in bottoms yields.
Q4. Why are intermediate products considered in the FCC process?
Because the FCC process involves successive reactions, the desired products such as olefins and gasoline are considered intermediate products.
Q5. What are the main reactions that take place over weaker sites?
64 While cracking reactions require the presence of strong acid sites, other reactions such as isomerization, cyclization and hydrogen transfer take place over weaker acid sites.
Q6. What is the effect of back mixing of solids inside riser reactors?
Back mixing of solids inside riser reactors results in a wide variation in residence times for the solids, as well as clustering and radial solids segregation.
Q7. What is the impact of the dealuminated zeolite on the hydrogen transfer?
The degree of dealumination of the zeolitic component in the FCC catalysts has an important impact on the hydrogen transfer: the more dealuminated the zeolite, the less extended the reaction.
Q8. What is the common modification method used for improving the acidity of zeolite?
The first usual and most widely used modification method applied for improving HZSM-5 zeolites performance is the incorporation of phosphorus in the zeolite framework.
Q9. What can be done to maximize light olefins yield?
the acidity of catalytic materials, as well as their strength and density can be adjusted to maximize light olefins in the product stream.
Q10. Why was propylene to ethylene ratio so low?
Yet due to the thermal nature of the process, propylene to ethylene ratio was bound to remain low and relatively inflexible, while propylene demand increases faster than ethylene.
Q11. What is the way to remove droplets from the hydrocarbon/steam mixture?
Liquid droplets in vaporized crude oil, which may contain materials with high coking tendency, have to be carefully removed from the hydrocarbon/steam gas mixture.
Q12. What is the way to reduce the amount of olefins produced?
Attempts to operate the FCC unit at higher temperature to produce more light olefins should be economically evaluated as incremental light olefins will come at the expense of secondary cracking of gasoline and in excessive dry gas (C2 and lighters) yields.
Q13. What is the name for the naphtha stream?
The naphtha stream produced from the process is highly aromatic, and thus may be used as a high octane, low olefin blend component for motor gasoline, or separated to valorise BTX.
Q14. What is the main reason for the higher yield of ethylene and propylene?
the main reason for the higher yield of ethylene and propylene is due to the decrease of the rate of the bimolecular reactions which is negatively affected by the decrease of readsorption of the basic compounds of the cracking products, such as ethylene, propylene, and butenes.
Q15. What was the effect of using residual oil as quench media?
It was found however that the use of residual oil, which is rich in heavy aromatics, was effective as quench media although temperature drop was minimal after quench.
Q16. What is the maximum amount of coke and dry gas that can be handled?
It is important to have in mind that, in a determined industrial unit, the amount of coke and dry gas that can be handled has a maximum fixed by due to compressor and blower capacity, as well as regenerator mechanical limits.
Q17. What is the probability of finding paired acid sites in the FCC catalyst?
Al atoms in the rare earth exchanged zeolitic compound of the FCC catalyst increases the probability to find paired acid sites which have a lower acidic strength than isolated acid sites (Figure 7) but which are suitable for carrying out isomerization, cyclization and/or hydrogen transfer reactions.
Q18. What is the way to reduce the residence time in a riser reactor?
Downer reactors are ideally suited for this, while it may result difficult to lower residence time to sub-second range in riser reactors, as the catalyst has to be pushed upwards.
Q19. What is the effect of the residence time on the reaction time in a conventional FCCU?
Table 10 gives an example of the impact of the residence time on the dry gas and light olefins production in a conventional FCCU and illustrates the fact that it exists an optimum residence time which is dependent on the feedstock nature and the catalytic system used.
Q20. Why is propylene production increasing faster than ethylene?
From the point of view of market demand, propylene production is creating new opportunities because: • Propylene demand is growing faster than ethylene demand.