Catalytic cracking and Hydrotreating Flashcards
What is the main objective of catalytic cracking?
to convert low-value heavy petroleum fractions into a high yield of motor gasoline.
Why is catalytic cracking preferred over thermal cracking?
Catalytic cracking is more selective and produces higher-quality gasoline with fewer unwanted by-products compared to thermal cracking.
What is a Fluidized-Bed Catalytic Cracker (FCC unit)?
It is a modern catalytic-cracking process where the catalyst is continuously regenerated by burning off coke deposits.
What is the role of zeolites in catalytic cracking?
Zeolites replace older clay catalysts and offer higher activity, improving gasoline yields and octane ratings.
- Zeolites have higher activity, better selectivity for gasoline production, and improve product quality by yielding higher octane gasoline.
What are the primary and secondary reactions in catalytic cracking?
- Primary Reactions: Initial carbon-carbon bond scission producing paraffins and olefins.
- Secondary Reactions: Include isomerization, aromatization, and polymerization, enhancing fuel quality.
What are the characteristics of paraffin cracking?
- High production of C3 and C4 hydrocarbons.
- Isomerization leads to branched structures.
- Formation of aromatic hydrocarbons due to secondary reactions.
Describe the catalytic cracking mechanism involving carbocations.
Reactions proceed via carbocation intermediates, which undergo cracking, isomerization, or dealkylation before chain termination.
How is coke formed during catalytic cracking, and how is it removed?
Coke forms when heavy hydrocarbons decompose on the catalyst’s surface. It is removed by burning it off in the regenerator using air.
What are the advantages of using a fluidized-bed system in catalytic cracking?
It ensures efficient mixing of feedstock and catalyst, enhances heat transfer, and allows continuous regeneration of the catalyst.
What is hydrotreating, and what are its objectives?
Hydrotreating catalytically stabilizes petroleum products and removes objectionable elements like sulfur, nitrogen, and trace metals by reacting them with hydrogen.
What is catalytic dehydrosulfurization?
It removes sulfur by converting sulfur compounds (e.g., mercaptans, thiophenes) into hydrogen sulfide (H2S) and hydrocarbons.
What factors affect the severity of hydrotreating?
- Feedstock boiling point: Higher boiling fractions require more severe treatment.
- Catalyst deactivation: Heavier feedstocks cause faster catalyst fouling.
How are sulfur compounds removed in hydrotreating?
Sulfur compounds react with hydrogen to form H2S, which is separated from the product stream.
What catalysts are used in hydrotreating?
Mixed oxides of cobalt and molybdenum on synthetic alumina are commonly used. They are prepared through impregnation, co-precipitation, or wet mixing
What are the effects of temperature and pressure in hydrotreating?
-Increased temperature enhances desulfurization but can lead to undesirable hydrocracking.
- Higher pressure improves sulfur removal but increases hydrogen consumption.
