Catalytic reforming and Isomerization Flashcards
What is catalytic reforming used for?
a process used to convert low-octane naphthas into high-octane gasoline blending components
What are the primary feed components for catalytic reforming?
Paraffins, naphthenes, and aromatics.
What are the key reactions in catalytic reforming?
- Dehydrogenation of naphthenes to aromatics.
- Isomerisation of paraffins and naphthenes.
- Dehydrocyclisation of paraffins to aromatics.
- Hydrocracking and dealkylation.
What factors affect reforming catalyst performance?
Contaminants like sulfur, nitrogen, water, and metals such as arsenic, copper, and lead poison the catalyst.
What advantages do bimetallic catalysts provide in catalytic reforming?
Longer catalyst lives.
Operation at lower hydrogen partial pressures for higher reformate yields.
Lower recycle gas rates, saving energy.
Combination of the above.
What is Continuous Catalyst Regeneration (CCR)?
A mechanical concept allowing continuous catalyst regeneration, maintaining high activity under severe reforming conditions.
What happens to paraffins during dehydrocyclisation?
Paraffins cyclize to naphthenes, which are subsequently dehydrogenated to aromatics.
What dual functions must reforming catalysts have?
- Metal function (Pt): Promotes dehydrogenation, hydrogenation, hydrocracking, and rapid hydrogenation of coke precursors.
- Acid function (Cl): Promotes isomerisation, initial hydrocracking, and paraffin dehydrocyclisation.
What are common catalyst poisons, and why are they problematic?
- Sulfur: Forms H₂S, affecting hydrogenation.
- Nitrogen: Forms NH₃, neutralizing acid sites.
- Metals like As, Cu, Pb: Poison catalysts even at trace levels.
- Water: Strips halogens off catalysts.
What is the purpose of isomerisation in refining?
To convert straight-chain alkanes into branched isomers with higher octane numbers.
Which catalysts are used for paraffin isomerisation?
Initially, aluminium chloride with HCl, and later, platinum-based catalysts.
What is the significance of thermodynamic equilibria in isomerisation?
Lower reaction temperatures favor the formation of highly branched isomers
What is the main reaction type in isomerisation?
Reversible first-order reactions limited by thermodynamic equilibria.
Why is feedstock preparation critical for isomerisation processes?
Contaminants like sulfur, water, and benzene deactivate the catalyst and must be minimized.
What are the limitations of aluminium chloride catalysts in isomerisation?
- Sublimation at reaction temperatures.
- Solubility in liquid phase causes reactor plugging and system failure.
What is the main goal of aromatic isomerisation?
To maximize the production of para and ortho xylenes, essential raw materials in the chemical industry.
What is the Octafining process?
A process for C8 aromatic isomerisation, operating in the temperature range of 370–480°C.
What are the primary reactions in aromatic isomerisation?
- Isomerisation of naphthenes and aromatics.
- Hydrogenation and dehydrogenation of aromatics and naphthenes.
What determines the selectivity of isomers in aromatic isomerisation?
- Reaction temperature: Lower temperatures favor o-xylene and ethylbenzene.
- Catalyst acidity: Balances hydrogenation/dehydrogenation with acidic functions.
What are the main challenges in the separation of C8 aromatics?
The boiling points of ethylbenzene, p-xylene, and m-xylene are very close, requiring complex separation techniques like superfractionation.
Why is catalyst regeneration essential in these processes?
Over time, catalyst activity declines due to coke formation, necessitating periodic regeneration to maintain efficiency.
